CA1239785A - Capacitance height gage applied in reticle position detection system for electron beam lithography apparatus - Google Patents

Abstract

Abstract Disclosed is a highly accurate capacitance height gauge which is applied in the presently preferred embodiment in a reticle position detection system for an electron beam lithography apparatus. The capacitive height gauge circuitry includes a hybrid circuit substrate which carries four measuring capacitor circuits and two reference capacitor circuits.
The driven plates of the four measuring capacitors are disposed on the bottom of the substrate opposite to the object surface to be measured.
The driven plates of the two reference capacitors are disposed on the top surface of the substrate under caps which support oppositely disposed grounded capacitor plates at a nominal distance from the driven plates.
The object surface comprises the grounded plate for the four measuring capacitors. The reference capacitors provide voltage regulation to ensure that a stable signal drives the measuring capacitors. The reference cap-acitors also set zero points for the measurements from the four measuring capacitors. These zero points are set at the nominal distance from the measuring capacitor sensors. The four measuring capacitors provide readings with respect to the object surface to determine the position of the object surface with respect to the nominal zero points. This information can be used by a suitable control system to manipulate the object surface or tools used to work on the surface in response to the position information as appropriate to the task being performed.

Description

rL'his invention relates to a capacitance height gauge applied in a retitle position detection system for electron beam lithography apparatus and to calibration apparatus for cay pistons height gauges.
There is a great need for high accuracy height measuring gauges for a wide variety of applications. One such application is in determining the position of a retitle in an electron beam lithography apparatus.
Electron beam lithography is rapidly becoming the method of choice for exposing ultrahigh accuracy retitles in the product lion of very large scale integrated circuits. An electron beam lithography apparatus typically includes an electron beam optics housing positioned over a vacuum chamber in which a retitle is installed. The retitle is a glass plate covered by a layer of chromium, with a layer of electron beam resist deposited o'er the layer of chromium. The retitle is mounted on a stage which moves the retitle in the X and Y directions under the control of the control system while the electron beam writes on, or exposes, the beam resist layer -to produce the desired circuit pattern on the retitle. The control system jot only moves the retitle to the desired I

coordinate for the slave position being exposed, but also, controls the beam deflection angle to control the point on the retitle at which the electron beam strikes.
In the past one of the more difficult problems in this art has been to determine the precise position of the retitle with respect to the electron beam optics. The precise position of the retitle must be determined in order to properly deflect the electron beam in order to accurately write on the retitle.
It is extremely important that any system used to determine the position of the retitle be vacuum compatible, compact, and non contacting.
In addition, accurate methods for calibrating capacitive height gauges are needed. In the past, capacitance gauges have been calibrated by means of mechanically measured distances, such as by means of micrometer measurements.
One aspect of the present invention comprises a highly accurate capacitance height gauge which is applied in the pro-sentry preferred embodiment in a retitle position detection system for an electron beam lithography apparatus.
The capacitive height gauge circuitry includes a hybrid circuit substrate which carries four measuring capacitor circuits and two reference capacitor circuits. The driven plates of the four measuring capacitors are disposed on the bottom of the substrate opposite to the object surface to be -1 measured. The driven plates of the two reference capacitors are disposed on the top surface of the substrate under caps 3 which support oppositely disposed grounded capacitor plates at 4 ¦ a nominal distance from the driven plates. The object surface comprises the grounded plate for the four measuring 6 capacitors. The reference capacitors provide voltage 7 regulation to ensure that a stable signal drives the measuring 8 capacitors. The reference capacitors also set zero points for 9 the measurements from the four measuring capacitors. These zero points are set at the nominal distance from the measuring 11 capacitor sensors. The four measuring capacitors provide 12 readings with respect to the object surface to determine the 13 position of the object surface with respect to the nominal zero 14 points. This information can be used by a suitable control system to manipulate the object surface or tools used to work 16 on the surface in response to the position information as 17 appropriate to the task being performed.

19 In the presently preferred embodiment, the capacitance height gauge is employed as an integral part of a retitle 21 position detection system for an electron beam lithography I apparatus. The "normal angle" of deflection of the electron 23 beam is first calibrated with respect to a calibration plate.

24 The four measurement sensors of the capacitance gauge are then utilized to detect the position of the calibration plate and to 26 input this position information into the electron beam control 27 system to define the plane which the calibration plate lies in 28 which is denoted as the "calibrated plane" for the retitle.
29 the retitle is then moved under the electron beam optics.

Riding are taken prom the four measuring sensors of the ~L~3~7~3~

capacitance gauge to detect the position of the retitle. This retitle position information is then also input -to the control system. The control system determines the deviation, if any, of the reticula from -the calibrated plane and appropriately adjusts the deflection angle of the electron beam in response to the detected deviation to write at the desired point of the retitle with a very high degree of accuracy.
In addition, a calibration apparatus for capacitance height gauges is disclosed which is far superior to the present mechanical measurement methods.
Thus, in accordance with one broad aspect of the invention, there is provided a capacitance height gauge for detecting the position of an object surface, comprised of:
a hybrid substrate supported proximately to said object surface;
a reference capacitor circuit supported on said substrate, said circuit including a reference capacitor having capacitor plates separated by a nominal distance, said reference capacitor having a capacitance, said reference capacitance circuitry generating a reference capacitance signal represent native of said capacitance of said reference capacitor;
a signal source circuit providing a driving signal;
said driving signal being controlled by said reference capacitor signal;
said reference capacitor circuit being driven by said driving signal;
a measuring capacitor circuit supported on said substrate, said measuring capacitor circuit including a sensor plate supported on said substrate opposite to said object surface, ... .

I

said sensor plate and said object surface comprising a measuring capacitor whereby the capacitance of said measuring capacitor varies as the separation between said sensor plate and said object surface varies, said measuring capacitor circuit try generating a measuring capacitor signal representative of said capacitance of said measuring capacitor, said measuring capacitor circuit being driven my said driving signal;
means for comparing said measuring capacitor signal with a reference signal, including means for generating a measure-mint signal representative of said comparison, said measurement signal representing the deviation in plate separation of said plates of said measuring capacitor from said nominal distance;
said reference capacitor plates and said measuring capacitor plates being separated by a dielectric, said dielectric being comprised of the measuring environment in which said hybrid substrate is located whereby said dielectric of said reference capacitor changes in the same respects as said dielectric of said measuring capacitor as said measuring environment varies; and a plurality ox sensor plates driven by said driving signal and disposed on the bottom of said substrate opposite to said object surface said sensor plates and said object surface comprising a plurality of measuring capacitors, each of said measuring capacitors having an associated measuring capacitor circuit for generating a measuring capacitor signal for each measuring capacitor, said comparing means comparing each of said measuring capacitor signals with said reference-signal to generate a measurement signal for each of said !`
measuring capacitors, said measuring signal representing the deviation in plate separation of each of said measuring capacitors from said nominal distance.

a-.
,. ...
_",. ~-~

I

In accordance with another broad aspect of the invention there is provided a method for detecting the position of a chrome-on-glass retitle in an electron beam lithography apparatus which includes an electron beam source in an electron beam housing, further comprised of a control system for controlling the angle of deflection of said electron beam, and a circuit substrate secured to a bottom of said electron beam housing, comprising the steps of:

positioning a calibration plate under said electron beam housing said calibration plate having an overlying conductive layer and a grid of spaced nonconductive points of known geometry formed into said conductive layer, one of said nonconducting points comprising an origination point, said origination point of said grid being aligned with the center line of said optics housing;
varying said deflection angle to deflect said electron beam from one of said nonconductive points of said grid to the next until said beam strikes a desired point, said desired point being spaced a specified distance from said origination point, the angle of deflection of said beam with respect to the center line of said optics housing when said beam strikes said desired point comprising a normal deflection angle;
measuring the distance between at least three sensor plates disposed on the bottom of said substrate and said calibration plate, each of said sensor plates being coupled to an associated measuring capacitor circuit, each of said sensor plates comprising the driven plate of a measuring capacitor and said calibration plate comprising the grounded plate of each of said measuring capacitors, each of said measuring capacitor circuits A b I
generating a measuring capacitor signal. representative of -the distance between each sensor plate and said calibration plate;
processing each of said measuring capacitor signals in-to information defining the position of said calibration plate with respect to said sensors, said position of said calibration plate comprising a calibrated position for said reticula;
positioning said retitle under said electron beam housing and aligning an origination point on said retitle with said center line of said housing;
measuring the distance between each of said sensors and said retitle wherein each of said sensors comprises -toe driving plate of a measuring capacitor and said retitle comprises the grounded plate of said measuring capacitors, each of said measuring capacitors generating a measuring capacitor signal representative ox the distance between each sensor plate and said retitle;
processing each of said measuring capacitor signals into information defining the position of said retitle with respect to said sensors, said position comprising a retitle position;
comparing said retitle position with said calibrated position to determine the deviation of said retitle from said calibrated position; and adjusting the angle of deflection of said electron beam from said normal deflection angle to a corrected deflection angle in response to said deviation in formation so that said electron beam is deflected said specified distance from said origination point on said retitle to strike said desired point on said retitle.

r 4 I

, _... .

Having described the invention in its presently preferred embodiment in brief overview, the advantages, features and novel aspects of -the invention will become apparent from -the more de-tailed description of the invention which follows taken in conjunction with the accompanying figures, in which:
Figure lo shows a schematic overview of the capacitance height gauge.
Figure lo shows a bottom view of the hybrid circuit substrate.

Figure lo shows an elevation Al view of the hybrid circuit substrate of the capacitance height gauge overlying the object surface.

Ed-~L~3~37~

1 Figure 2 shows the oscillator circuitry and other

2 circuitry driving the reference capacitor Crew l including

3 the reference capacitor feedback loops which stabilize the

4 driving signal of the gauge 6 Figure 3 shows an ele~ational view of the linearization of 7 the electric field lines of the driven plate of reference 8 capacitor Clef l under the influence of the guard ring.

Figure 4 shows a simplified circuit diagram of the measuring capacitor circuitry and measurement signal generation 12 circuitry for the +X sensor.

14 Figure 5 is a schematic diagram showing the environment of the capacitance height gauge as applied in a retitle position 16 detection system for an electron beam lithography apparatus.

18 Figure 6 show the calibration of the electron beam using 19 a calibration plate.

21 Figure 7 shows the adjustment made by the control system 22 to the deflection angle where the retitle is positioned above 23 the calibrated plane Figure 8 shows a top view of the hybrid circuit substrate 26 of the capacitance height gauge.

Figure 9 is a simplified circuit diagram of the 29 twerp lure control circuitry of the prevent invention.

321 s 1 1 ~3~7~
2 I Figure 10 shows a elevation Al view of the capacitance 3 height gauge calibration fixture.

41 Figure 11 shows a plan view of the fixture.

Figure 12 shows a perspective view of the interferometer 8 use support member, with the capacitive height gauge substrate 9 secured to its forward face.

Figure 13 shows a partially schematic, functional bloc 11 diagram of the calibration system.

13 figure 14 shows a functional block diagram of the 14 calibration fixture electronics.

foggier 15 is a schematic diagram showing the test set-up or the capacitance height gage.
I

DISCRETION OF THE PRESENTLY PREFERRED EMBODIMENT
OFF THE INVENTION
foggier lo shows a schematic diagram of the capacitance 23 weight gauge circuitry of the present invention.

The signal source circuitry 10 includes an oscillator 15 27 which provides a sinusoidal signal. The sinusoidal signal is 28 split between a pair of voltage regulating circuits 20, 24.
the upper signal goes through a phase 1 transformer 30 Chile he lower signal goes through a phase 2 transformer 32 to 30 enroot 18~ t of phase iguanas Thea of these driving 32 iguana no! fed to yard circuitry carried on a substrate later described) which overlies the object surface being ensured. Thy hybrid circuitry it comprised of reference 1 ¦ capacitor clrc~itry 40, 42 for OR f 1 and OR f I and 2 measuring capacitor circuitry 5Q, 52, 54, 56 for the Ox 3 ¦ Cry/ C X and C y measuring capacitors. The hybrid 4 ¦ circuit substrate 110 is shown in Figs. lo, LO and 8. Fits. lo

5 ¦ and lo show that only the OX, MY, -X and -Y sensors 112, 114,

6 ¦ 116, 118 and their associated guard rings 120, 122, 124, 126 71 are disposed on the bottom of the substrate opposite to the 81 object surface to be measured. The remainder of the bottom 9¦ surface is ground plane Fig. lo shoals that both reference I¦ Ref. 1 and Crew 2 are disposed on the top of 11 ¦ Ref. 1 and Crew 2 are comprised of driven 12 I plates 130 and oppositely disposed ground plates 134, which are 13 ¦ disposed on the internal surface of the caps 140 at a fixed 14 ¦ nominal distance from driven plates 130. See Fig. 3. The 15 ¦ measuring capacitors are comprised of OX, MY, -X, -Y driven 16 ¦ sensor plates 112, 114, 116, 118 and a grounded plate comprised 17 ¦ of the object surface 145 to be measured. See Fig. lo.
18 ¦ Crew 1 and Crew 2 include feedback loops 60, 62 to 19 ¦ regulate the voltage of the oscillator to provide a stable 20 ¦ signal to the measuring capacitors 50, 52, 54, 56. See 21 ¦ Rev. 1 and Crew 2 circuits 40, 42 also 22 ¦ assist in providing nominal zero points for each of the 23 ¦ measuring capacitor as will later be explained more fully.
241 The outputs of measuring capacitor circuits 50, 52, 54, 56 are 25¦ coupled to associated measurement signal generating ~61 circuitry 70, 72, 74, 76 to provide signals to the control 271 system 80 indicating the position of the object surface 145 281 with respect to the four sensors 112, 114, 116, 118.

32 _ _ ;~Q3~3~

1 The I and Cry circuits 50, 52 and C X and C y 2 circuits 54, 56 are driven by 180 out of phase signals to 3 ensure that there is no net current flow through the object 4 surface 145 to ground. This feature may be important where 5 object surface 145 is a retitle in an electron beam lithography 6 apparatus as will be explained later on.

8 Having described the capacitive height gauge in brief 9 overview, reference is now made to Fig. 2 which shows the signal source circuitry 10 and the circuitry 40 associated with 11 Crew 1 in greater detail. This circuitry provides a clean, 12 symmetric, sinusoidal driving signal to the measuring 13 capacitors as will now be explained.

With reference to Fig. 2, a TTL programmable 16 oscillator 150 generates a high frequency square wave signal 17 which is converted by the tuned circuit 152 into a sinusoidal 13 signal. The signal undergoes a unity voltage gain, current 19 amplification at amplifier 154 and is split at node 156 between the two symmetrically arranged circuits shown in the upper and 21 lower half of the figure. To simplify the description of the 22 circuitry, only the upper portion of the circuitry will be 24 described initially. The signal flows upwardly through R
Rand through the FRET 1 to ground. FRET 1 together with Al 25 comprise a voltage divider. The resistance of FIT 1 varies 26 with its gate signal as will later be explained more fully.
27 from node 158 the signal goes through a voltage amplification 28 Nat amplifier 160 and author unity voltage gain, current amplification at amplifier 1~2. the signal is then input over 30 pa trueness no 1~4 Jo to primmer coil of a step-up Lowe 1 transformer ]70 which is tuned to the oscillator frequency by a 2 variable capacitor 77~ connected in parallel with the secondary 3 coil 174. The transformer 170 acts as a band pass filter and a 4 low distortion, geometrically symmetric, sinusoidal signal at the oscillator frequency is thereby produced and transmitted 6 over a transmission line 180 to the hybrid substrate

7 circuitry 40 associated with reference capacitor Crew 1 and

8 the measuring circuitry 50, 52 associated with measuring

9 capacitors Ox Kiwi respectively. The circuit 40 for Crew 1 will be described first. Ignoring diodes Do and 11 Do and the associated guard ring 136 for the moment, the 12 signal is split at node 190 and passes through DC blocking 13 capacitors Of and C2. Diodes Do and Do are connected 14 between capacitors Of and C2 in a half wave rectifier arrangement- Reference capacitor Crew 1 is connected 16 between diodes Do and Do The plates of Crew 1 are 17 separated by a fixed distance which is equal to the "nominal 18 distance". The nominal distance is the desired distance 19 between sensor plates 112, 114, 116, 118 of the measuring capacitors and object surface 145. The driven plate 130 of 21 Crew 1 is supported on the upper surface of substrate 110 23 while grounded plate 134 is supported on the interior surface of ceramic cap 140 which is secured to substrate 110 over 24 driven plate 130 as shown in Fig. 3. Inductors Lo and Lo are connected between Of and Do, and C2 and Do, 26 respectively, as shown. Filter capacitors C3 and C4 are 7 connected between Lo and Lo, respectively, and ground. The 28 signal from coil Lo leaves the hybrid circuit via shielded coaxial cable 194 and is grounded through a coil Lo. The 32 signal from coil Lo leaves he hybrid circuit via a shielded _ 9 _ 1 coaxial cable 196 and is connected through a coil Lo to the 2 inverting input of transres1stance amplifier 200.
3 Transresistance amplifies 200 has a grounded non inverting input 4 and a feedback resistor R3 as shown.

6 Diodes Do and Do are connected in a half wave 7 rectifier arrangement, as noted, so that one diode is "off"
8 while the other diode is "on" and vice versa. As the 9 sinusoidal input signal increases from its maximum negative lo value to its maximum positive value, Do it forward biased and if conducts to charge capacitor Crew 1 Conversely, as the I signal falls from its maximum positive value to its maximum 13 negative value, Do is forward biased to drain the charge on 14 capacitor Crew 1 We use the current produced in Lo by the discharging of capacitor Crew l to regulate and 16 stabilize the signal supplied to the measuring capacitors Ox 17 and yo-yo as will become apparent. We have grounded the signal 18 passing through Lo in that for the purpose of voltage regulation we need only to monitor the Lo current.

21 As Crew 1 discharges through Do, C2 blocks all DC

23 current and passes the AC component so that a very small DC
current passes through coil Lo. Lo i 5 a large coil on the 24 order of 470 micro henries to block the AC component of the unrent. any small remaining AC component of the current is 26 filtered out by filter capacitor C`4. The small DC current 27 asses from coil Lo Thor toil Lo and is converted by 8 transresistance amplifier ~00 into a voltage representative of 29 the cusre~t rough Lo. The current through Lo which is 30 input to transresistance amplifier 200 is multiplied by the f -lo-1 value of resistance R3 to generate the output voltage. This 2 output ~r~ltage is they'll imposed across resistor I and 3 produces a current IT flowirlg to the right out of node 204.
4 A +7 volt reference voltage is imposed on the resistor R5 to produce a current flowing left to right into node 204 in 6 Figure 2.

8 It has been previously mentioned that the gate signal to 9 FRET l controls the drain to source resistance of the FETE If

10¦ we assume as an initial condition that just before start-up the

11¦ circuit power is "on" but the oscillator is "off", the 7 volt

12¦ reference voltage will be applied across R5 with zero voltage '31 being applied across R4. Consequently, a maximum current 14¦ will be applied through R5 to the inverting input of 15¦ differential amplifier 210 and differential amplifier 210 will 6¦ have its maximum negative output of -15 volts, for example, 171 which will shut FRET l "off" and present a maximum resistance to 18 ¦ ground. Consequently, the current flow through, and voltage 19 ¦ drop across, Al will be the minimum and the voltage at 20 1 node 158 will be a maximum. If we assume that R5 = 35K ohms 221 ¦ and that VRef l = 7 volts, then IT = 200 micro amperes flowing through R5 into the inverting pin just before start-up.

24 ¦ Just after start-up this maximum voltage at node 156 will 25 ¦ produce maximum current flow through Lo and to the right from 27 1 node 204 through R4. The current flow through R4 will 28 ¦ initially exceed the 200 micro ampere current through R5 which will make the output of differential amplifier 210 become less 29 l 1 Navaho and rise Jo lo volts, for example. The resistance of 32 FRET wit accordingly drop proportionately reducing the ~3~7~

2 voltage at node 158 and therefore the current through Lo and 31 R40 This cycle will repeat itself until the current flowing from node 204 through R4 equals the 200 micro ampere current flowing through R5 into node 210. At this point zero current will flow into the inverting input of differential 61 amplifier 210 and the system will consequently lock.

81 We know IT is constant at 200 micro amperes as calculated Al 5 above. To make IT equal to 200 micro amperes, if we assume R4 =
11 20K ohms a voltage of V = I R = (-200 micro amperes) 12 (20K ohms) = -4 volts must be imposed across R4 to lock the

13 system. Consequently the output of transresistance

14 amplifier 200 must be -4 volts at this point. We have already noted that the output of transresistance amplifier 200 is the 16 product of R3 and the input current I . Consequently, IL
17 VTRlR3, and if we assume that R3 = 50K ohms, then IL
18 -4 volts/50K ohms = -80 micro amperes. Therefore the system 19 stabilizes when the current through Lo = -80 micro amperes fox 0 the illustrative parameters used. Note that this 21 -80 micro ampere current through Lo corresponds to the capacitance of Crew 1 which has a plate separation of the 23 nominal distance. The current through Lo is denoted the 24 reference capacitor signal for Crew 1 The Crew circuit 40 together with the signal source circuit 10, thus, 26 together stabilize the driving signal supplied to the Ox and 27 Cry measuring capacitor circuits 50, 52. If the capacitance 28 of Crew 1 varies, the voltage of the driving signal will 29 correspondingly vary; however, the current flowing through Lo will remain stable and unchanged.

I
1 ¦ Note that the DC loop for the current through Lo, 2 comprising the reference capacitor signal, flows from ground 3 ¦ g Ref. 1' Do Lo Lo R3 and into the output 4 of transresi~tance amplifier 200 which has a very low 5 impudency and therefore, functions as a ground to complete the 6 sloop. The composition of this loop it important as will later 81 become apparent 9¦ Before describing the circuits for the measuring 10¦ capacitor Ctx and Kiwi mention should be made of the guard 11¦ ring circuitry of the circuit which has to this point been 12 ¦ ignored. The guard ring is simply an annular capacitor 13 ¦ plate 136 insulated from the driven plate 130 of Crew 1 and 14 ¦ disposed between the driven plate 130 and the substrate 110 as

15 ¦ best shown in Figure I The guard ring 136 is disposed

16 ¦ opposite to the grounded plate 134 and comprises the driven

17 ¦ capacitor plate of the capacitor, Guard 1 I ¦ encircles and overlaps the driven plate 130 of Crew 1 As 19 ¦ shown in Figure 3, guard ring 136 linearizes the electric field 20 ¦ lines at the edges of driven plate 130 of Crew 1 to 22 ¦ eliminate the fringe effects of the field lines which would 23 ¦ otherwise occur at the plate edges. By linearizing the field ¦ lines of Crew 1' guard ring 136 equalizes Crew 1 with the 24 ¦ measuring capacitors Ox and I as will be better I explained later on 26 l I ¦ Another important function of the guard ring 136 is to 28 ¦ shield Crew 1 from stray capacitances which would otherwise 29 be prevent button the driven plats of Crew 1 and other 3~17~

1 charged or conductive surfaces in the vicinity. These stray 2 ¦ capacitances could add to the current flowing from OR f 3 ¦ and lead to measurement inaccuracies if not shielded.
4 l 5 ¦ Guard ring 136 is driven through half-wave rectifying 6 ¦ diodes Do, Do in the same way as driven plate 130 is driven 7 ¦ through diodes Do, Do. Do, Do are identical to and 8 ¦ provide the same voltage drops as Do, Do, and accordingly, 9 ¦ the potential of the guard ring 136 at all times matches the 10 ¦ potential of driven plate 130. By maintaining this voltage 11 ¦ match, current flow between guard ring 136 and driven plate 130 12 ¦ is prevented and the field lines at the edge of the driven 13 ¦ plate 130 are maintained perpendicularly to grounded 14 ¦ plate 134. Capacitors Of and C2 also act as DC blocking 15 ¦ capacitors for the guard ring circuit.
16 l 17 ¦ The stabilized signal produced by the Crew 1 circuit is

18 ¦ used to drive the Ox and Cry circuits 50, 52 as will be

19 ¦ now explained. The Ox circuitry 50 will be described as

20 ¦ representative of the operation of the Cry circuitry 52 to

21 ¦ avoid duplicity in explanation.

22 l

23 ¦ Ox measuring circuitry So and measurement signal

24 ¦ generation circuitry 70 are shown in Figure 4. Again, the

25 ¦ guard ring 120 and associated diodes D12 and D13 will be

26 ¦ ignored initially. Measuring circuitry 50 of Figure 4 is

27 ¦ identical to the referenced capacitor circuit 40 as a

28 ¦ comparison of Fig 2 with Fig 4 will reveal. Consequently, as

29 ¦ the driving signal increases prom its maximum negative value to

30 ¦ its maximum positive value, Duo charges capacitor Ox which

31 l 397~

l is comprised of sensor plate 112 supported on the bottom of 2 substrate 110 as the driven plate, and object surface 145 as 3 the grounded plate. See Fig. 2C. As the signal falls from its 4 maximum positive value to its maximum negative value, Dull 51 conducts to drain charge from Ox This alternate switching 61 on and off of Duo and Dull causes small equal and oppositely 71 valued DC currents to flow through Lo and Lit as follows:
l When Duo is conducting, current flows from the output of 9¦ transresistance amplifier 220 which is effectively grounded due 10 It its very low impedance, through R12, L12, Lo, Duo 12 Rand Ox back into ground to complete the 13 sloop Thus the current through Lo/ IL
14 heft in Figure 4 and is considered to be positive. When D
15 Conduct , current flows from ground through Ox Dull, 16 Ill, L13, R13 and into the low impedance output of Itransresistance amplifier 230 to complete this loop. Hence, 18 the current through Lit, IL is left to right in Figure 4 19 Rand is considered to be negative. Note that these loops are 20 symmetric with respect to one another in that each of the loops 21 ware comprised of equivalent components. Consequently, the 22 Current values through Lo and Lit while opposite in 23 polarity will be equal in magnitude. The equivalency in 24 magnitude is also the result of the fact that the driving I Signal is a geometrically symmetric sinusoidal signal as 26 Previously mentioned. These small DC currents comprise the 27 measuring capacitor signal for Ox We previously noted that 28 the DC current loop for the reference capacitor signal through 29 Lo flowed from ground through OR f I D2l Lo, Lo and 30 IRK, and back into the output of transresistance 31 ~mpli~ie~ . We Lowe noted that the 80 mIcroampere current

32~
- lo I

1 through Lo corresponds to a separation between the plates of 2 Crew 1 of the nominal distance. Furthermore, we know that 3 the driven plate 112 of Ox is identical to the driven 4 plate 130 of Clef 1 and is fitted with an identical guard ring 120 to linearize field lines at the edge of plate 112.
6 The effect of the linearization of the field lines at the edges 7 of driven plates 112, 130 is to equate the grounded plates 145, 8 134 of the capacitors Ctx, Crew 1 Y
9 dimensions effectively identical. The driven plates 112, 130 for both capacitors are 1/4 inch in diameter. The grounded 11 plate for Ox is object surface 145 which is effectively an I in invite plate. The grounded plate for Crew 1 is the 13 interior surface 134 of the cap 140 having a diameter of 1/2 14 inch. Since the field lines at the edges of driven plates 112, 130 of both of these capacitors are vertical, however, the 16 effective diameter of both of the grounded plates 145, 134 is 17 1/4 inch. Hence, due to the effect of the guard rings 136, 18 120, at a plate separation of the nominal distance Ox is 19 electrically identical to Crew }I Consequently, assuming that the separation between the sensor 112 and object 21 surface 145 is the nominal distance, we will get equal and 22 opposite 80 micro amp currents through Lo and Lit since the 23 DC current loops for Lo and Lit are identical to the DC
24 current loop for Lo. As will become apparent, this 80 micro amp current level, as set by Crew 1 circuit 40 and 26 signal source circuit 10, and a reference signal plater 27 explained), is the "zero point" for the height of sensor 112 29 above object surface 145.

32 - lo-3 aye 1 To demonstrate that the 80 ~icroamp current level 2 corresponds to the zero point for measuring capacitor Ox as 3 a first condition let us assume that the object surface 145 is 4 lying at precisely the nominal distance below the sensor 112.
If this is the case, the current through Lo will be 6 +80 micro amps as discussed above. In Fig. 4, this current will 7 flow into measurement signal generation circuit 70.
8 Consequently, if R12 = 50K ohms, the inverting input to 9 transresistance amplifier 220 will be +80 micro amperes and its output will be I volts at node 235. We have a fixed reference voltage VRef 2 of 7 volts injecting a "reference signal"
121 current into node 235 through the resistor Rio which it a 13 ¦43.75K ohm resistor here. A -160 micro ampere current is 14 generated by this VR~f 2 reference voltage through Rio, 15 Rand assuming R12 = 50K ohms, this current imposes a 16 ¦(50K ohm)(-160 micro ampere) = -8 volt potential at node 235.
17 the net potential at node 235 is therefore 4 volts - 8 volts =
18 1-4 volts. This -4 volt potential is present at the inverting 19 input to differential amplifier 240. The equal and opposite 21 ¦-80 micro ampere current through Lit is converted by 22 ¦transresistance amplifier 230 to an output voltage of 23 TRY - IL R13 = (-80 micro amperes) (50K ohms = -4 volts, acumen Rl3 = 50K ohms. This -4 volt signal is imposed at I the non inverting input to differential amplifier 240.

26 differential amplifier 240 subtracts the -4 volt signal at the 27 inverting input from the -4 volt signal at its non inverting input to get a zero output. This zero output corresponds to 28 l 29 the "zero point" for the +X sensor in that it indicates to the 31 control system 80 that the sensor is the nominal distance above 1 the oh~ect surface. I've VRef 2 reference signal together 2 with thy roving signal generated by rough l circuit 40 and 3 signal source circuit lo combine to set this zero point.
4 Note that the overall operation of the circuitry is to compare the measuring capacitor signal from Ox circl~itxy 50 to the 6 reference signal supplied by Vie 2 The VRef 2 7 reference signal imposed at node 235 is representative of the 8 nominal plate separation. Measuring signal generating 9 circuitry 70, thus, compares the measuring capacitor signal lo with a nominal plate separation reference signal to determine 11 the deviation of the plates of Ox from the nominal 12 distance. Here, since we have assumed that the Ox plates 13 are separated by the nominal distance, we generate a Nero 14 deviation signal from circuitry 70.

16 Assume now that the object surface 145 is positioned 17 closer than the nominal distance such that the capacitance of 18 Ox increases and +82 micro amperes flow through Lo while 19 -82 micro amperes flow through Lit. Transresistance amplifier 220 will have an output of VTR3 = (+82 micro amperes) 21 (~50K ohms) = 4.1 volts. At node 235 the potential will be 22 4,1 - - -3.9 volts. Thus, -3.9 volts will be present at the 3 inverting input to differential amplifier 240.
I
Transresistance amplifier 230 on the other hand will have 26 an output of Al volts which will be present at the 27 non inverting input to differential amplifier 240. Differential 28 amplifier 240 will subtract the difference between -4.1 volts 29 and -3.9 volts = -.2 volts and multiply that value by a gain of So, for example, to own a output ~42 ox Lo volts. This 3'7~
1 -10 volt output will be interpreted by the control system to 2 indicate that the object surface is a scaled distance closer to 3 the -OX sensor than the nominal distance.

Again, the circuitry compares the 82 micro ampere measuring 6 capacitor signal with the VRef 2 signal representative of 7 nominal plate separation The output generated by circuitry 70 8 is, therefore, representative of the deviation in plate 9 separation of the plates of measuring capacitor Ox from the nominal distance.

12 The circuitry for the Cry capacitor is identical to, and 13 operates in exactly the same manner as, the circuitry just 14 described for capacitor Ox Thus, circuit 40 for Crew and circuit 10, together with the VRef 2 reference signal, 16 set the same nominal zero point for the MY sensor 114, and in 17 exactly the same way.

19 The other two measuring capacitors C X and C y are driven via the lower half of the circuit shown in Figure 2.
21 With reference to Figure 2, an identical signal to the signal 22 passing from node 156 upwardly through R1, passes from 23 node 156 downwardly. The signal drives identical circuitry, 24 the only exception being that the secondary coil 184 of transformer 186 is wound in a direction opposite to its primary 26 coil 182t whereas the coils 172, 174 of transformer 170 are 27 both wound in the same direction. The result is that a 28 180 degree out of phase equal and opposite signal drives 29 Crew I C X and C I, than drives Crew I OX and Kiwi I otherwise provides voltage regulation I I

1 feedback in exactly the same way as described above to generate 2 ¦ a geometrically symmetric and stable sinusoidal driving signal 3 ¦ for the measuring circuits 54, 56, of C X' C ye The 4 ¦ Crew 2 circuit 42 and signal source circuit 10 produces I equal and opposite 80 micro ampere current levels through the 6 C X' C y circuits 54, 156 when the -X and -Y sensors 116, 7 118 are separated the nominal distance from the object 8 surface 145 in exactly the same manner as was described with 9 reference to the Crew 1 circuit 40.

11 By driving the Ox and Cry circuits 50, 52 with an 12 equal but opposite signal to the signal which drives the C X
13 and C y circuits 54, 56, a zero net current is passed through 14 the object surface 145 to ground by the measuring sensors at any given point in time. This constant maintenance of a zero 16 current flow through object surface 145 prevents object 17 surface 145 from developing a voltage potential with respect to lo ground due to its impedance to ground. This ensures that 19 object surface 145 remains at ground potential even if it has a finite nonzeros impedance. It also prevents the object 21 surface 145 from being slightly charged due to dielectric 22 absorption or other effect. Where the object surface 145 is a 23 retitle in an electron beam system, the maintenance of zero net 24 current to ground may be advantageous as will later be explained. In applications where it is not necessary to 26 maintain a net zero current flow from the retitle to ground, 27 the use of a two phase system would not be required and all 28 measuring capacitors could be driven from a single phase RF

397~, 1 signal. Consequently, the capacitance height gauge circuit of 2 the present i~v~ti~n is not intended to be limited to a two 3 phase system such as the one disclosed.

Whether or not a two phase system is used, the four sensor 6 measurements, after being processed by the associated 7 measurement signal generation circuitry, are input to the 8 control system 80 to indicate the position of the object 9 surface 145 with respect to the four nominal zero points. The control system can then, in response to this information, 11 either manipulate the position of the object surface, or the 12 position of the tool being used to worn on the object surface, 13 for example, as appropriate to the task being performed.

Having described the capacitive height gauge of the 16 present invention in its general form, it will now be explained 17 as an integral part of a system for detecting the position of a 18 retitle 250 in an electron beam lithography apparatus The 19 many advantages and special features of the gauge will become more apparent from the description of the use of the gauge in 21 this application.

23 Fig. 5 shows the environment of the capacitance height 24 gauge as applied in an electron beam lithography apparatus. In Fig. 5, the gauge is shown as comprised of the hybrid circuitry 26 on the substrate lo and the remainder of the circuitry which 27 is denoted as simply "capacitance gauge electronics" 254. The 28 hybrid circuit substrate lo is secured to the bottom of the 29 electron beam optic housing 258 in aye overlying relationship wit respect to the chrome on-glass retitle 250. The I `

1-~39'~
1 retitle 250 is supported on a stage 260 in the vacuum 2 chamber 262 of the apparatus for movement in the X and Y
3 directions under the control of the control system 80. A
4 layer 266 of electron beam resist is deposited over the chromium layer 268 of the retook as shown. The electron 6 beam 270 passes through a central aperture ill in the 7 substrate lo and patterns the beam resist layer 266 with the 8 desired VLSI circuit pattern as the retitle 250 is advanced in 9 the X or Y directions.

11 In addition to positioning the retitle 250, the control 12 system 80 controls the deflection angle of the electron beam to 13 ensure that it strikes the desired point on the retitle with 14 the high degree of precision required of VLSI circuitry.

16 Given the high precision required in this art, prior to 17 utilizing the electron beam to write on the retitle 250, the 18 beam 270 must first be calibrated with respect to the current 19 environmental conditions. A calibration plate 280 such as that shown in Fig. 6 is used for this purpose. The calibration 21 plate 280 is secured to the stage 260 adjacent to the 22 retitle 250, and directly below the electron beam optics 23 housing 258. A conductive layer 282 covers the calibration 24 plate 280. A grid 284 of very tiny nonconductive points 286 is formed into layer 282. The grid 284 is comprised of a 26 multitude of equally spaced and symmetrically arranged 27 conductive points 286~ Point 290 is centered with respect to 28 the centerline of optics housing 258 by movement of the stage 29 to specified X, Y coordinates. Point 290 is denoted the "origination point." With the cap gauge measuring circuitry I turned off, the electron beam 270 is turned on and is 21 sequentially advanced by the control system from one 3¦ nonconductive point on the grid to the next. The control 41 system 80 senses each sequential nonconductive point and stops 51 when it strikes the desired point 294. The deflection angle A
61 which is necessary to cause the beam to strike precise 71 point 294 is recorded by the control system. Point 294 is 81 located a specified distance "d" from origination point 290 9¦ since the geometry of grid 284 is known. The electron beam 270 10¦ is now turned off and the cap gauge circuitry is turned on to 11 take calibration plate position readings from the four 12 sensors 112, 114, 116, 118. The associated measurement signal 13 generation circuitry or each sensor determines the pOsitioll of 14 the calibration plate 280 with respect to the nominal zero 15 point for the sensor and enters this information into the 16 control system 80. A best fit plane is calculated from the 17 four points using well known mathematics. This plane defines 18 the "calibrated plane" for the retitle 250.
lug Once the calibrated plane has been determined, stage 260 21 is moved to position retitle 250 under electron beam optics 22 housing 258 as shown in Fig. 7. The point 298 on the 23 retitle 250 directly aligned with the center line of the 24 housing is again denoted as the origination point. The 25 point 300 which is the precise distance d from point 298 is the 26 desired point for writing on retitle 250. The retitle outlined 27 by dotted lines in Fig. 7 is located precisely in the 2B calibrated plane, and hence, the beam 270 would strike 29 joint 300 on the dotted retitle using the normal deflection Nile I. The actual potion of the retitle in Fig. 7, '1~3~7~3.3 1 however, is illustrated by retitle 250 outlined in solid 2 lines In this position the beam 270 would write at point 302 3 if normal deflection angle A were used.

To avoid such an error, prior to writing on the retitle, 6 retitle position readings are taken by the four sensors 112, 7 114, 116, 118. These readings are input to the associated 8 measurement signal generation circuits and then to the control 9 system. A best fit plane is calculated for the four points.
This plane is known as the "retitle plane." The control system 11 compares the retitle plane with the calibrated plane and 12 determines the deviation. In this case, the control system 13 will determine that the retitle is a precise distance above the 14 calibrated plane and will adjust the electron beam deflection angle from normal deflection angle A to a precise corrected 16 deflection angle B to ensure that the beam writes at the 17 precise point 300 notwithstanding the fact that the retitle 250 18 is positioned above the calibrated plane. The electron beam 19 then writes or patterns the segment of the circuit included within this stage position and advances the retitle to the next 21 stage position. The retitle position readings are again taken I and processed to determine the deviation of the retitle plane 23 from the calibrated plane and appropriate adjustments are made 24 to the beau deflection angle to account for any deviation.
251 This cycle is repeated at each stage position until the retitle 26 is completely patterned, or a change in environmental 27¦ conditions warrants recalibration of the electron beam.

29 l 9~35 1 Having described the capacitance height gauge as applied 2 ¦ in the reticula position detection system disclosed, various 3 ¦ advantageous features of the invention will now become more 4 ¦ apparent. Fig. 8 shows the top surface of the hybrid circuit 5 ¦ substrate 110. As was mentioned previously, all of the hybrid 6 ¦ circuitry except for sensor plates 112, 114, 116, 118 and the 7 ¦ associated guard rings 120, 122, 124, 126 are supported on the 8 ¦ upper surface of the substrate. Thus, the reference ¦ circuits 40, 42 and identical measuring capacitor circuits 50, 10¦ 52, 54, 56 are supported on the top of the substrate. Each of 11 ¦ these circuits is comprised of four capacitors, four pin diodes 12 ¦ and two coils, all of which are discrete components 13 ¦ interconnected by signal lines deposited on the substrate. The 14 ¦ four circuits 50, 52, 54, 56 feed through to the sensors 112, 15 ¦ 114, 116, 118, respectively, and their associated guard 16 ¦ rings 120, 122, 126, 128. The two circuits 40, 42 feed 17 ¦ directly to their respective driving plates 130 and guard 18 ¦ rings 136 on the upper surface of the substrate 110.
19 l I ¦ The hybrid circuitry carried on the top of the substrate 21 ¦ also includes the resistance heating strips 310, shown in 22 ¦ Fig. 8, which can be silk screened on the substrate. These 23 ¦ strips 310 together comprise the resistor R in s shown in 24 ¦ the simplified circuit diagram of Fig. 9 which shows the 25 ¦ substrate temperature control circuitry of the invention. The 26 ¦ function of these resistive strips is to heat the substrate to 27 ¦ a temperature of a few degrees above room temperature and 28 ¦ thereby maintain control over the temperature of the hybrid 29 ¦ socket components at all times. With reference to Fig. 9, a 30 ¦ Roy 312 is used to set the desired temperature of the 31 l I

~3~37~
1 substrate 110. This temperature setting is input as a 2 corresponding voltage volts for eagle on the inverting 3 spin of differential amplifier 314. A substrate temperature 4 sensor 316 scaled to one micro ampere per degree kelvin outputs 5 ¦ a current proportional to the substrate temperature. This 6 ¦ current signal is input to the non inverting input of 7 ¦ transresistance amplifier 3180 The inverting input of the 8 ¦ transresistance amplifier 318 is provided with an offset 9 signal, Vie 4, to convert the output of amplifier 318 to 10 ¦ degrees Celsius. The amplifier output is applied to the 11 non inverting input of differential amplifier 314 wherein it is 12 compared with the rheostat setting. Where the two inputs are 13 equal, the output voltage of the amplifier provides the 14 required voltage drop across the resistance heating elements Strip to maintain the substrate at the current temperature.
16 If the substrate temperature drops below the desired 17 temperature then the output voltage of differential 18 amplifier 314 decreases to increase the voltage drop across the 19 resistive strips and thereby increase the substrate temperature. Conversely, if the substrate runs too hot the 21 voltage drop across the resistive strips is reduced to cool the 22 substrate. The resistive strips 310 are evenly distributed 23 about substrate 110 to prevent temperature gradients.

By regulating the temperature of the substrate in this way 26 the identical components of the reference and measuring 27 capacitor circuits are maintained at the same operating 28 temperature and the accuracy of the gauge is protected from 29 temperature drifts Moreover, in that both the reference and measuring capacitor are in the same "measuring environment,"
I

1239~35 1 the dielectric between the plates of all of these capacitors 2 undergoes the same environmental changes, such as changes in 3 temperature and humidity. Consequently when environmental 4 changes do occur in the vacuunl chamber, resulting in a change in the capacitance of the reference and measuring capacitors, 6 the changes are automatically accounted for by the reference 7 capacitor circuits which maintain the current flow through the 8 reference and measuring capacitor circuits at a stable level 9 regardless of environmental changes Consequently, current flow through the measuring 12 capacitors varies only with plate separation in that 13 environmental effects are minimized. Another important 14 advantage of the hybrid circuit embodiment disclosed is that the reference and measuring capacitor circuits are not only 16 identical, and subject to the same environmental influences, 17 but in addition, they are subjected to the same environment 18 throughout their operating life. Hence, the circuit components tend to age together at the same rate and in the same manner which further ensures the accuracy of the gauge over time.

23 A further important aspect of the invention is the fact that the four diodes used in the reference and measuring 24 capacitor circuits are ultra low capacitance PIN diodes. These diodes function as extremely small capacitors in their reversed 26 biased state, and hence, do not contribute to the current 28 flowing through the measuring and reference capacitors. In 29 addition, the capacitance of these diodes remains very small Al even during wide temperature drifts.

I

1 Having described the invention in its presently preferred 2 embodiment, the manner in which the outputs of the four sensors 3 are correlated with plate separation will now be explained.

Figs 10 and 11 show the calibration fixture S00 on which 6 the sensors are calibrated. Fixture 500 has a base 502 and 7 three adjustable supporting legs 504. Legs 504 can be 8 thread ably adjusted, for example. Base 502 supports a laser 9 table 506, which in turn supports a laser 508. A vertically 10 upstanding member 510, supported by base 502, supports a laser 11 receiver 512. Laser receiver 512 is secured to member 510 by 12 screws 14, for example. A support block 516, supported by base 13 502, supports an interferometer cube support member 520, which 14 is best shown in Fig.. 12. Support member 520 supports the 15 interferometer cube, or prism, 522, behind a vertical wall 16 524. Wall 524 has a large aperture 526 through which the laser beam projects as will later become apparent. Four internally 18 threaded posts 528 project orthogonally from the front side of 19 wall 52^~, as shown. The cap gauge substrate 110 is thread ably 20 secured by screws 530 to the posts 528. Each screw is 21 counter-sunk within a precisely-machined washer 532. By 22 mounting the substrate 110 in this way, it is intended that the 23 substrate's surface 113 will lie in a plane substantially 24 perpendicular to the path of the laser beam. vote that the 25 central aperture 111 of substrate 110 aligns with the aperture 26 526 of wall 524 and also permits the passage of the laser 27 beam. It is noted that the portion of the calibration fixture 28 80 far described, comprised of the laser 508, the laser 29 receiver 512~ and the interferometer cube 522, together with layer electronics (liter disrobed are all commercially 31 available aye pace from Hewlett-Packard as a laser 37~

1 ¦ interferometer, Model No HP5501A, having a resolution of 5 -2 ¦ nanometers. These components together comprise the laser 3 ¦ interferometer assembly and can measure precise distances in a 4 ¦ manner later described.
S l 6 ¦ Note that all components of the laser assembly are rigidly 71 secured to the fixture OWE The portion of the calibration 8 ¦ fixture 500, which will next be described, comprises an 9 ¦ assembly which movably positions a mirror, or dummy retitle, 10 ¦ with respect to the laser assembly.

12 ¦ A support table 540 supports an air bearing 542. Air 13 bearing 542 supports a longitudinally directed shaft 544 for 14 very low friction rectilinear movement. End 546 of shaft 544 15 ¦ rigidly supports a mirror support member 548, which in turn 16 ¦ supports the mirror 550 in a vertical orientation, directly 17 ¦ opposite to the sensors 112, 114, 116, 118 of cap gauge 18 ¦ substrate 110. The mirror surface is electrically conductive 19 in that it functions as one plate of a measuring capacitor, as 20 ¦ will be later described A drive magnet 552 is supported by 21 ¦ shaft 544 on the opposite side of air bearing 542 from the I ¦ mirror 550. Magnet 552 is reciprocally driven by voice coil I ¦ 544, which encircles shaft 544 and is rigidly supported by 24 ¦ table 540~ A velocity damping magnet 556 is secured at the end 25 ¦ 558 of shaft 544. A velocity sensing coil 560 encircles the 26 ¦ shaft 544 adjacent to magnet 556 and is rigidly supported by 27 ¦ table 540. Drive coil 544 it energized in a manner later 28 ¦ described to drive the mirror 550 to the desired location with 2g ¦ respect to the sensors 112, 114, 116, 118. Velocity damping 30 ¦ Monet 556 moves with shalt 544~ and as it moves, it generates 31 l ~Z3~ 5 1 a voltage across the velocity sensing coil 560. This voltage 2 is used as a velocity damping signal in a manner which will 3 also be later described.

In order to provide vibration isolation for the 6 calibration fixture 500, the support legs 504 can rest upon a 7 massive granite block 562, which in turn can be supported by 8 air bags 564. In addition, the entire fixture can be installed ; 9 in a temperature controlled environment 566.

11 Having described the basic structure of the calibration 12 fixture, its operation will now be described with reference to 34 the partially schematic, functional bloc diagram shown in Fig. 13.

16 As noted above, the laser interferometer assembly, 17 comprised of the laser 508, interferometer cube 522, laser 18 receiver 512, and laser electronics 570, is commercially 19 available as a package to very precisely measure small distances. In this case, the laser interferometer assembly is 21 used to determine the distance between the sensors 112, 114, 22 116, 118 and the surface of the mirror 550. The interferometer 23 assembly is used in the present invention in that it is the 24 best "yardstick" available for determining the distance between I the sensors 112, 114, 116, 118 and the mirror 550.

27 Briefly, the laser beam generated by laser 508 passes 28 through interferometer cube 522 and it reflected off of mirror 29 550 Jack to the laser receiver 512. The laser receiver 512 indicates to laser electronics 570 the position of mirror 550.

` 37~ 1 1 Laser electronics 570 is advised, in a manner latter described, 2 of tile diehard position of the mirror and utilizes the input 3 from laser receiver S12 to determine the differential distance 4 between the actual position of mirror 5S0 and its desired 5 position. Laser electronics 570 then outputs this differential 6 distance as an error signal. The use of this error signal by 7 the present apparatus to control the position of mirror 550 8 will be described shortly.

With reference to Fig. 13, to begin the calibration 11 procedure, microprocessor 574 instructs the calibration fixture 12 electronics 576 to energize drive coil 554 to drive mirror 550 13 into abutment with the four machined washers 532, which are 14 secured to substrate 110, as previously described. The 15 thickness of washers 532 is very accurately machined so that 16 the mirror 550 is positioned 20 milliinches from the sensors 17 112, 114, 116, 118. Microprocessor 574 now tells laser 18 electronics 570 that the mirror should be moved an additional 19 40 milliinches away from thy sensors 112, 114, 116, 118. Laser 20 receiver 512 tells the laser electronics 570 where the mirror's 21 present position is, and laser electronics 570 generates an 22 error signal representative of the distance between actual 23 position of the mirror 550 and its desired location This 24 error signal it input to calibration fixture electronics 576, 25 which in response thereto, energizes drive coil 554 to drive 26 mirror 550 back towards the desired position. As the mirror 27 moves away from the sensors 112, 114, 116, 118, the laser 28 receiver 512 constantly monitors its position and the error 29 signal generated by laser electronics 570 is correspondingly 31 reduced as the mirror 550 moves closer arid closer to the 1 desired location. As the mirror moves, the velocity damping 2 magnet ~56 moves with respect to the velocity sensing coil 560, 3 and a voltage it generated across the coil 560 in proportion to 4 the speed of the magnet 556. This velocity signal is input to the calibration fixture electronics 576 as a velocity damping 6 signal to prevent oscillation of the mirror about the desired 7 location in a manner more fully described later on. Once the 8 mirror reaches the desired position, a "zero error signal" is output from laser electronics 570, and calibration fixture 576 controls drive coil 554 to stop mirror 550 at the desired 11 location. At this time, laser electronics 570 indicates to 12 microprocessor 574 that mirror 550 has reached the desired 13 location which is 60 milliinches from the sensors 112, 114, 14 116, 118 in this initial situation. Microprocessor 574 now takes voltage readings from the four sensors 112, 114, 116, 118 16 and stores this information for each sensor for this 17 initialization point of 60 milliinches. Microprocessor 574 18 then directs laser electronics 570 to move the mirror to a 19 first sample point 5 milliinches, for example, from the initialization point. Laser electronics 570 again generates an 21 error signal representative ox the 5 milliinch distance between 22 the actual position of the mirror and the new desired location 23 and this error signal is again used by calibration fixture 24 electronics 576 to drive mirror 550 to the new desired location.- When the location is reached, a zero error signal is 26 again generated by laser electronics 570 and in response 27 thereto calibration fixture electronics stops the mirror 550 28 and microprocessor 574 again samples the four sensors 112~ 114, 29 116, 118. The voltage reading for each sensor 112, 114, 116, 118 is stored by microprocessor S74 for this new location The 31 mirror is then If in accordance with the control program of ~3~'7~
1 microprocessor 574 to the next sample point by the same 2 procedure and these sensors are again sampled at this new 3 location. Obviously, sensor readings for any desired number of 4 sample points can be obtained in this way. Microprocessor 574 stores this information in the form of a table, listing the 6 voltage readings for each sensor at each sample point. In this 7 way, the sensors can be calibrated over a range of plate 8 separation distances Having described the overall operation of the calibration 11 system of Fig. 13, calibration electronics 576 will now be 12 described in somewhat more detail with reference to Fig. 14.

14 As shown in Fig. 14, the error signal is delivered to a digital-to-analog converter 580 in the form of a 12-bit digital 16 word directly from laser electronics 570. The analog signal 17 produced by digital-to-analog converter 580 is then scaled by 18 amplifier 582 to a 0-10 volt scale before being delivered to 19 polarity determining circuit 584. Laser electronics 570 also O outputs a "direction bit" which is delivered to polarity 21 determining circuit 584, as shown The direction bit indicates the polarity of the error signal. That is, it indicates which 23 side of the desired location mirror 550 is currently located on 24 so that mirror 550 can be driven in the proper direction The 25 direction bit is processed by polarity determining circuit 584 26 to assign the proper polarity to the error signal before it is input to amplifier 586. Amplifier 586 then scales the error 28 signal between 0 to +10 volts, if the assigned polarity is 29 positive, or button to -10 vote, if thy assigned polarity 31 is negative. Thy error seal is then input to position gain 32 amplify TV 'lye position gain is adjustable by position

- 33 -1 gain adjusting circuit 590, which may, for example, comprise a 2 potentiometer positioned in the feedback loop of amplifier 3 588. Additional damping is provided by velocity coil 560. The 4 signal generated by coil 560 is input to velocity gain amplifier S92. A velocity gain adjusting circuit 594 is 6 provided to permit adjustment of the gain characteristics of 7 amplifier S92, and may also comprise a potentiometer positioned 8 in the feedback loop of amplifier 5920 The gain adjusting 9 circuits 590 and S94 are adjusted together to stabilize the lo circuit and prevent oscillation of mirror 550, or ringing, 11 about the desired vocation. The signals from amplifiers 588 12 and 592 are summed before finally being filtered at noise 13 filter 596, and then finally amplified at power amplifier 598 14 to attain the necessary power level to drive the drive coil 554.

16 the entire calibration fixture 500 is contained in a 17 temperature-controlled environment which may also be under the 18 control of microprocessor 574. Likewise, the cap gauge 19 substrates temperature control circuit shown in Fig. 9 may also be under the control of microprocessor 574. Microprocessor 574 21 will generally maintain the environmental temperature a Jew 22 degrees cooler than the substrate temperature so that temperature gradients in the substrate are avoided. The 24 environment and substrate temperatures may be varied by microprocessor 574 according to its control program to generate 6 sets of calibration data at various temperatures. Zen the 28 capacitance gauge is theft later used in an electron beam apparatus, for example, the appropriate set of calibration data 2g for the temperature at which the apparatus wily be used can be utilized 1 An important aspect of the present invention is that after 2 a particular capacitance gauge has been out in the field for a 3 period of time, it can be recalibrated on calibration fixture 4 550. For recalibration, the substrate is simply reinstalled in the fixture 500, and run through the save calibration procedure 6 as just described. The results of the calibration process will 7 indicate whether the sensor readings have varied with time or 8 have remained the same.

It should be noted that the initialization point described 11 above may not be exactly 60 milliinches from the sensors, but 12 rather is nominally 60 mullions from the sensors 112, 114, 13 116, 118. The principal reason for this is that, although the 14 washers 532 are finally machined to a 20 milliinch thickness, in that they are a mechanical part, it is expected that there will be some actual variation from an exact 20 milliinch 17 dimension. Consequently, when the mirror is first placed into 18 abutment with the washers 532, it may or may not be precisely 19 20 milliinches from the sensors 112, 114, 116, 118. Therefore, when it is backed up 40 milliinches by the microprocessor 514, 21 the initialization point will only be a nominal 60 milliinches 23 from the sensors rather than necessarily an exact 60 milliinch 24 distance from the sensors. As the mirror is moved in 5 milliinch increments, however, the incremental distances will be separated by precisely S milliinches to the accuracy of the 26 laser interferometer assembly, which is far more accurate than 27 any mechanical measuring technique Consequently, although the initialization point may not be exactly 60 milliinches from the 29 sensors, we know that the subsequent test points can be stepped 31 off in precise 5 milliinch increments tug the accuracy of the 32 laser in~rerometex~ Xavier it is these "difference I - I _ 7 to 1 readings" between the various points, which are of most concern 2 to us This is particularity true in the present application, 3 for example, wherein the capacitance gauge is being used to 4 measure the difference between the distance to the reference plane and the distance to the retitle. Such distance 6 differences can be measured very accurately by the instant 7 apparatus, and accordingly, fine adjustments can be made to the 8 electron beam apparatus to correct for the actual position of 9 the retitle.

11 An alternative manner in which the outputs of the four 12 measuring sensors are correlated with plate separation will now 13 be explained with reference to Fig. 15.

The capacitance gage substrate 110 is rigidly affixed in a 16 test fixture 330 opposite to a dummy retitle 334. A laser 17 interferometer 338 is also supported on fixture 330 and 18 projects a loser beam through opening 111 in substrate 112 19 towards retitle 250 which reflects it back towards the laser.
Retitle 250 is supported by a member (not shown) winch is 21 supported in an air bearing (not shown) for low friction 22 rectilinear movement with respect to substrate 110. The 23 retitle support member is reciprocally movable by means of 24 retitle positioning mechanism 342. The fixture 330 is housed in a temperature controlled enclosure on a vibration isolating 26 granite block which rests on air bags. The mixture temperature 27 can be controlled by means of temperature control 28 electronics 346, fixture oven 350, and fixture thermometer 354.

I

according to the testing procedure, the microprocessor 360 2 directs the laser electronics 364 to move the retitle to a 3 specified distance from the sensors of substrate lo. Laser 4 electronic s 364 relays the command to test fixture 5 electronics 368 which initiates retitle positioning 6 mechanism 342 to move retitle 334 in the direction specified.
7 As retitle 334 moves towards substrate lo, laser 8interferometer 338 and laser electronics 364 monitor its 9 progress and laser electronics 364 commands test fixture 10 electronics 368 to stop the movement of retitle 334 once the 11 specified distance between retitle 334 and substrate 110 is 12 achieved. Velocity sensor 372 damps the movement ox 13 retitle 334 to prevent oscillation about the desired point.
14 Microprocessor 360 now samples the substrate sensors via the 15 capacitance gage electronics 376~ The temperature of cap gage 16 electronics 376 can be controlled by the oven 380, thermostat 384, and temperature control electronics 346. In 18 that microprocessor 360 knows the distance between the 19 substrate sensors and the retitle, since the laser 20 interferometer has measured the distance, it can correlate the 21 output of the sensors with that distance. Microprocessor 360 23 takes sensor readings at various distances and temperatures and stores the data generated.

Nat that various retitle positioning mechanisms could be 26 used. For example, two voice coil/magnet assemblies could he mounted coccal on the centerline of the moving parts. The 28 magnets could be attached to the moving air bearing slide, with the voice coils fixed in postural. One coil/magnet set could 30 be use fit oust sensor damping and the other for movel[lent 31 of the retitle.

~2~7~3~

1 The data awoke ted from the sensors can be fit to a 2 curve defined by the equation:

4 I = k/v-vo + Zoo A By TV, 6 where "I" is plate separation "Z" as a function of cap gage 7 output voltage "v" for the sensor, with K, vow Zoo A, B and C
8 being constants which can be determined by the microprocessor 9 for the particular cap gage being tested.

11 The I = k/v-vo + Zoo portion of the equation can be 12 derived from Maxwell ' 5 equations to give the theoretical 13 function. The polynomial A BY -I C was empirically 14 found by the inventors herein to be necessary to reduce errors to the noise level.

17 Consequently, the microprocessor tests the cap gage and 18 calculates the particular values of the constants for the gage 19 being tested to provide a formula relating cap gage output voltage to plate separation. This formula can be used by the 21 control system of the electron beam lithography apparatus to 22 calculate the calibration plate points necessary to define the 23 calibrated plane and the retitle position points necessary to 24 define the retitle plane. Having defined the planes, the control system can determine the deviation of the retitle from 26 the calibration plane and adjust the deflection angle of the electron beam accordingly.

- I -I

1 Having disclosed the presently preferred embodiments of 2 the invention, many modifications and variations thereof will 3 be obvious to those skilled in the art. Particularly, others 4 will recognize that the calibration fixed apparatus disclosed 5 for calibrating capacitance height gauges may be applicable to the calibration of other types of capacitive height gauges than 7 the one particularly disclosed herein. Accordingly, the 8 present invention is intended to be limited only by the scope 9 ¦ of t appended claims.

US

Claims (13)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A capacitance height gage for detecting the position of an object surface, comprised of:
a hybrid substrate supported proximately to said object surface;
a reference capacitor circuit supported on said substrate, said circuit including a reference capacitor having capacitor plates separated by a nominal distance, said reference capacitor having a capacitance, said reference capacitance cir-cuitry generating a reference capacitance signal representative of said capacitance of said reference capacitor;
a signal source circuit providing a driving signal;
said driving signal being controlled by said reference capacitor signal;
said reference capacitor circuit being driven by said driving signal;
a measuring capacitor circuit supported on said substrate, said measuring capacitor circuit including a sensor plate supported on said substrate opposite to said object surface, said sensor plate and said object surface comprising a measuring capacitor whereby the capacitance of said measuring capacitor varies as the separation between said sensor plate and said object surface varies said measuring capacitor cir-cuitry generating a measuring capacitor signal representative of said capacitance of said measuring capacitor, said measuring capacitor circuit being driven by said driving signal;
means for comparing said measuring capacitor signal with a reference signal, including means for generating a measurement signal representative of said comparison, said measurement signal representing the deviation in plate separation of said plates of said measuring capacitor from said nominal distance;
said reference capacitor plates and said measuring capacitor plates being separated by a dielectric, said dielectric being comprises of the measuring environment in which said hybrid substrate is located whereby said dielectric of said reference capacitor changes in the same respects as said dielectric of said measuring capacitor as said measuring environment varies; and a plurality of sensor plates driven by said driving signal and disposed on the bottom of said substrate opposite to said object surface said sensor plates and said object surface comprising a plurality of measuring capacitors, each of said measuring capacitors having an associated measuring capacitor circuit for generating a measuring capacitor signal for each measuring capacitor, said comparing means comparing each of said measuring capacitor signals with said reference signal to generate a measurement signal for each of said measuring capacitors, said measuring signal representing the deviation in plate separation of each of said measuring capacitors from said nominal distance.

2. The capacitance height gage of Claim 1 further comprising a voltage regulation means for maintaining said driving signal at as stable level, said voltage regulation means including a feedback loop connected between said reference capacitor circuit and said signal source circuitry, said reference capacitor signal controlling said driving signal to ensure that a constant current flows through said reference capacitor, and that the current flow through said measuring capacitor is insulated from temperature and humidity variations in the measuring environment and varies only with plate separation.

3. The capacitance height gage of claim 1, wherein a guard ring driven by said driving signal is disposed between each of said sensors and the bottom of said substrate, and wherein the portion of the bottom surface of said substrate not covered by said guard rings is comprised of ground plane, and wherein a guard ring driven by said driving signal is disposed between said driving plate of said reference capacitor and said upper surface of said substrate, said reference capacitor circuit being supported on the upper surface of said substrate, said measuring capacitor circuits, except for said measuring capacitors and associated guard rings, being supported on the upper surface of said substrate, said driving signal being fed to said measuring capacitor circuits and from said measuring capacitor circuits through said substrate to said sensors and said guard rings of said measuring capacitors.

4. A capacitance height gage for detecting the position of an object surface, comprised of:
a hybrid substrate supported proximately to said object surface;
a reference capacitor circuit supported on said substrate, said circuit including a reference capacitor having capacitor plates separated by a nominal distance, said reference capacitor having a capacitance, said reference capacitance circuitry generating a reference capacitance signal representative of said capacitance of said reference capacitor;

a signal source circuit providing a driving signal;
said driving signal being controlled by said reference capacitor signal;
said reference capacitor circuit being driven by said driving signal;
a measuring capacitor circuit supported on said substrate, said measuring capacitor circuit including a sensor plate supported on said substrate opposite to said object surface, said sensor plate and said object surface comprising a measuring capacitor whereby the capacitance of said measuring capacitor varies as the separation between said sensor plate and said object surface varies, said measuring capacitor circuitry generating a measuring capacitor signal representative of said capacitance of said measuring capacitor, said measuring capacitor circuit being driven by said driving signal;
means for comparing said measuring capacitor signal with a reference signal, including means for generating a measurement signal representative of said comparison, said measurement signal representing the deviation in plate separa-tion of said plates of said measuring capacitor from said nominal distance;
said reference capacitor plates and said measuring capaci-tor plates being separated by a dielectric, said dielectric being comprised of the measuring environment in which said bybrid substrate is located whereby said dielectric of said reference capacitor changes in the same respects as said dielectric of said measuring capacitor as said measuring environment varies; and means for heating said substrate and for maintaining said substrate at a selected temperature greater than the ambient temperature of the measuring environment.

5. The capacitance height gage of Claim 4 wherein said heating means comprises one or more resistive heating strips applied to the substrate and temperature control circuitry connected to said resistive strips, said temperature control circuitry comprising a means for sensing the temperature of said substrate and for varying the voltage drop across said resistive strips to maintain the substrate temperature at said selected temperature.

6. The capacitance height gage of Claim 5 wherein a pair of sensor plates are disposed on said substrate and comprise first and second sensor plates, said first and second sensor plates and said object surface comprising first and second measuring capacitors, one of said measuring capacitor circuits being associated with each of said first and second measuring capacitors, and wherein said signal source circuit comprises a means for generating equal and opposite 180° out of phase first and second driving signals from said driving signal, said first driving signal driving said first capacitor and said second driving signal driving said second capacitor.

7. The capacitance height gage of Claim 1 wherein said object surface is a chrome-on-glass reticle of an electron beam lithography apparatus, which has an electron beam source enclosed in an electron beam optics housing, said substrate being secured to the bottom of said electron beam optics hous-ing and having a centrally disposed aperture for allowing said electron beam to pass through.

8. The capacitance height gage of Claim 7, wherein said electron beam has an angle of deflection with respect to the center line of said optics housing, and wherein said apparatus includes a means for controlling the angle of deflection of said electron beam, said apparatus further including a means for determining a calibrated plane for said reticle and a normal deflection angle for said electron beam, wherein when said reticle is lying in said calibrated plane said electron beam strikes a desired point on said reticle using said normal deflection angle, further including means for detecting the position of said reticle with respect to said calibrated plane, means for determining the deviation of said reticle from said calibrated plane and means for varying said deflection angle in response to said deviation to ensure that said electron beam strikes said desired point on said recticle.

9. A method for detecting the position of a chrome-on-glass reticle in an electron beam lithography apparatus which includes an electron beam source in an electron beam housing, further comprised of a control system for controlling the angle of deflection of said electron beam, and a circuit substrate secured to the bottom of said electron beam housing, comprising the steps of:
positioning a calibration plate under said electron beam housing said calibration plate having an overlying conductive layer and a grid of spaced nonconductive points of known geometry formed into said conductive layer, one of said nonconducting points comprising an origination point, said origination point of said grid being aligned with the center line of said optics housing;

varying said deflection angle to deflect said electron beam from one of said nonconductive points of said grid to the next until said beam strikes a desired point, said desired point being spaced a specified distance from said origination point, the angle of deflection of said beam with respect to the center line of said optics housing when said beam strikes said desired point comprising a normal deflection angle;
measuring the distance between at least three sensor plates disposed on the bottom of said substrate and said calibration plate, each of said sensor plates being coupled to an associated measuring capacitor circuit, each of said sensor plates comprising the driven plate of a measuring capacitor and said calibration plate comprising the grounded plate of each of said measuring capacitors, each of said measuring capacitor circuits generating a measuring capacitor signal representative of the distance between each sensor plate and said calibration plate;
processing each of said measuring capacitor signals into information defining the position of said calibration plate with respect to said sensors, said position of said calibration plate comprising a calibrated position for said reticle;
positioning said reticle under said electron beam housing and aligning an origination point on said reticle with said center line of said housing;
measuring the distance between each of said sensors and said reticle wherein each of said sensors comprises the driving plate of a measuring capacitor and said reticle com-prises the grounded plate of said measuring capacitors, each of said measuring capacitors generating a measuring capacitor signal representative of the distance between each sensor plate and said reticle;
processing each of said measuring capacitor signals into information defining the position of said reticle with respect to said sensors, said position comprising a reticle position;
comparing said reticle position with said calibrated position to determine the deviation of said reticle from said calibrated position; and adjusting the angle of deflection of said electron beam from said normal deflection angle to a corrected deflection angle in response to said deviation information so that said electron beam is deflected said specified distance from said origination point on said reticle to strike said desired point on said reticle.

10. The capacitance height gage of Claim 4 further comprising a voltage regulation means for maintaining said driving signal at as stable level, said voltage regulation means including a feedback loop connected between said reference capacitor circuit and said signal source circuitry, said reference capacitor signal controlling said driving signal to ensure that a constant current flows through said reference capacitor, and that the current flow through said measuring capacitor is insulated from temperature and humidity variations in the measuring environment and varies only with plate separation.

11. A capacitance height gage comprising means for providing a plurality of alternating current signals half of which are out of phase with the other half; a plurality of measuring plates mounted on a surface, each one driven by one of said alternating current signals to thereby induce a net zero current in an object surface adjacent the measuring plates; and means responsive to the capacitance of the measuring plates for determining the distance of the surface from the measuring plates.

12. The gage of claim 11 wherein the means for providing the alternating current signals include a plurality of circuits, some of which have transformers with a non inverting secondary winding and some of which have transformers with and inverting winding.

13. The gage of claim 11 further including heating means for maintaining the measuring plates at a regulated temperature above ambient temperature to reduce capacitance drift due to temperature changes.