US3472755A - Cathodic sputtering apparatus wherein the electron source is positioned through the sputtering target - Google Patents

Description

Oct. 14, 1969 R. M. MOSESON 3,472,755

CATHODIC SPUTTERING APPARATUS WHERE-IN THE ELECTRON SOUPCE UGH THE SPUTTERING TARGET IS POSITIONED THRO Filed July 18, 1966 4 Sheets-Sheet 1 ra m/e INVENTOR Oct. 14, 1969 R. M. MOSESON 3,472,755

CATHODIC SPUTTERING APPARATUS WHEREIN THE ELECTRON SOURCE 1s POSITIONED THROUGH THE SPUT'IERING TARGET Filed July 18, 1966 4 Sheets-Sheet 2 Aime/VH5.

3.472,755 CTRON SOURCE AR Oct. 14, 1969 CATHODIC S 15 Filed July 18, 1966 R. M. MOSESON PUTTERING APPARATUS WHEREIN THE ELE POSITIONED THROUGH THE SPUTTERING 'I GET Sheets-Sheet 5 INVENTOR. F941? 14 Mafia/v flf/WE/V f Oct. 14, 1969 I POSITI Filed July 18,

' R. M. MOSESON CATHODIC SPUTTER APPARATUS WHEREIN THE ECTRON SOURCE IS TARGET 1966 4 Shee 'D THROUGH THE SPUTTERI cs-Sheet 4 INVENTOR. F045? WMw Zm/ United States Patent 3,472,755 CATHODIC SPUTTERING APPARATUS WHERE- IN THE ELECTRON SOURCE IS POSITIONED THROUGH THE SPUTTERING TARGET Roger M. Moseson, Rochester, N.Y., assignor, by mesne assignments, to The Bendix Corporation, Detroit, Mich., a corporation of Delaware Filed July 1-8, 1966, Ser. No. 565,860 Int. Cl. C23c 15/00 US. Cl. 204-298 9 Claims ABSTRACT OF THE DISCLOSURE Cathodic sputtering apparatus for depositing thin films on the surface of a large area substrate. The target is provided with openings each of which are traversed by a source of electrons which cause the formation of the sputtering plasma. An electron deflecting means is positioned in the path of electrons to deflect the electrons back to the target establish the sputtering plasma along the surface of the target and cause the target to be sputtered.

This invention relates to apparatus for sputtering, and more particularly, to sputtering apparatus in which no limitation exists on the size and shape of the object to be sputtered or the substrate to be coated.

In recent years the technique of sputtering either to clean the sputtered surface or to coat a Substrate with the sputtered material has found ever increasing application in industry. A major factor for this is that neither the choice of the substance to "be sputtered nor the choice of the substrate to be coated is in any way limited as to type of material. As a result, insulators such as glass, as well as metals, can be sputtered or coated.

Low voltage sputtering apparatus is known in which a discharge or plasma of positive ions'with a plasma axis is established between an anode and a cathode in a partially evacuated bell jar. A target, i.e., a material to be sputtered, is located to the side of the ion discharge and is biased negatively with respect to the anode. A substrate to be coated is located on the other side of the ion discharge, diametrically opposite the target, such that the surface to be coated faces the target. Ions are then attracted to the target causing the target material to be sputtered upon the substrate.

The rate and uniformity of coating a substrate in such a sputtering system are a function of the size and shape of the target and the substrate and of the size, shape, and density of the ion discharge. For example, in order to obtain a uniform coating rate over the entire substrate surface the substrate and the target are about the same size and shape, and the ion discharge is, in cross section parallel to the target, at least as big as the target and substrate surfaces. These geometrical relationships between the target and substrate, on the one hand, and the ion discharge, on the other hand, have in the past imposed a definite limitation upon the size and shape of the target and substrate, because the size of the ion discharge is itself limited by the type of electron-emissive material employed for the cathode and the operating voltage of the anode-cathode system.

To a certain extent the distribution of the ion discharge can be controlled, i.e., made more uniform, concentrated, or expanded, by applying a magnetic field thereto or by providing the anode-cathode system with intermediate electrodes. But these measures are not sufii cient to produce an ion discharge suitable for targets and substrates of any size and shape.

According to the invention an anode-cathode arrangement for producing an ion discharge in a sputtering sysice tem is embodied in a unitary structure that can be situated at will within a sputtering chamber and that can be combined with other anode-cathode units to provide an ion plasma or mass suitable for targets and substrates of any size and shape. More particularly, each anodecathode unit has a cathode element at one end of a longitudinal axis and an anode element at the other end, positively biased with respect to the cathode. The anode end of the unit is open and the anode is peripherally disposed so that it does not block the opening. A baflie is spaced from and situated in front of the opening. An appreciable number of the electrons emitted by the cathode are accelerated within the unit along the longitudinal axis and then beyond the anode. After deflection by the bathe, these electrons travel outside of the unit a distance beyond the anode, preferably transverse to the longitudinal axis of the unit, causing ionization in their path.

To create an ion plasma or mass over a large target surface, one or more of these units can be employed in a sputtering chamber. The target has holes in it, one corresponding to each anode-cathode unit. The anode end of each unit is inserted in a hole in the target so that an ion discharge forms near the surface of the target. By selecting the number of anode-cathode units and their arrangement any desired distribution of ions can be formed. Thus, sputtered material from the target produces a coating on a substrate of unlimited size and shape according to any desired thickness distribution.

An even greater control over the ion plasma density and shape is obtainable by interconnecting the anodes and cathodes of the various units. In one such arrangement the anode of each unit of a row is positively biased with respect to the cathode of the adjacent unit of the row in a series-type connection to establish independent ion discharges that form a continuous ion plasma or mass extending the length of the row. In another such arrangement the anode of a first unit is biased with respect to the cathode of a second unit and the anode of the second unit is biased with respect to the cathode of the first unit to establish between the units opposing, independent ion discharges that form an ion plasma with a very uniform density. In still another arrangement the anodes of several units are each biased with respect to the cathodes of unadjacent units to establish crossed independent ion discharges forming an ion plasma with a tapered density distribution.

These and other features of the invention are considered further in the following detailed description taken in conjunction with the drawings in which:

FIG. 1 is a side elevation view in section of a sputtering chamber employing a plurality of anode-cathode units independently biased with respect to one another;

FIG. 2 is a side elevation view in section of a sputtering chamber in which each anode is biased with respect to the cathode of an adjacent unit;

FIG. 3 is a side elevation view in section of a sputtering chamber in which the anode element of each unit of an adjacent pair of units is biased with respect to the cathode of the other unit of the pair; I

FIG. 4 is a side elevation view in section of a sputtering chamber in which the anodes of several units are biased with respect to the cathodes of nonadjacent units;

FIG. 5 is a front elevation view in section of the sputtering chambers of FIGS. 1, 2, and 3; and

FIG. 6 is a front elevation view in section of a sputtering chamber employing plural targets.

In FIG. 1 anode- cathode units 10, 11, 12, 13, and 14 are shown attached to the outer wall of on airtight sputtering chamber 16. The units are identically constructed. Therefore, only the parts of unit 10 are labeled. Each unit comof a longitudinal axis 21 and an annular anode 22 forming an opening 32 at the other end of longitudinal axis 21. Filament 20 is mounted upon a base 23 made of an insulator. Terminals 24 and 25 of filament 20, which jut out of the other side of base 23, are connected to the secondary winding of a transofrmer 26. Transformer 26 couples an alternating-current source 27 to filament 20 for heating purposes. A housing 28 surrounding filament 20 is provided at one end with a flange 29 to permit an airtight attachment of the unit to the wall of sputtering chamber 16. Housing 28 is preferably made of metal in order to facilitate heat transfer to the outside of the unit. A tube 30 made of an insulator extends concentrically with longitudinal axis 21 between filament 20 and anode 22, surrounded by anode 22 at its one end. Tube 30 is provided to prevent absorption by housing 28 of electrons emitted from filament 20. A bafile 31 also made of an insulator, is spaced from and oriented parallel to the plane of opening 32 in the end of the unit. Supporting means for baflle 31 (not shown) could be attached to tube 30. Anode 22 is positively biased with respect to filament 20 by means of the circuit comprising a variable resistor 33 and a battery 34 connected in series. The connection to anode 22 is made through an airtight fitting in flange 29 and the connection to filament 20 is made at the center tap of the secondary winding of transformer 26. Electrons emitted from filament 20 due to heating it are accelerated along longitudinal axis 21 toward anode 22. Because of the peripheral arrangement of anode 22 an appreciable number of the electrons are accelerated beyond it and are deflected by bafiie 31 so that they leave the unit and travel in a path transverse to longitudinal axis 21. Most of these electrons then return to anode 22. The path of electrons is illustrated by lines 85 and 86. Satisfactory results are in some cases also achieved when tube 30 is placed outside of anode 22 so as to circumscribe it. This is especally the case, in the interconnected arrangements of FIGS. 2, 3, and 4 described below.

Inside of sputtering chamber 16 a target material 40 and a substrate 41 to be coated having very large surfaces are supported spaced from one another in an upright position by means not shown. Target 40 and substrate 41 have surfaces parallel and similar in shape to one another. Target 40 has a plurality of holes in it. The anode end of an anode-cathode unit protrudes through each hole. Bafile 31 of each anode-cathode unit reduces the possibility of bombardment of substrate 41 by electrons emanating from opening 32, which would create a negative charge to draw ions to the substrate thereby causing substrate 41 itself to be sputtered. A plurality of parallel, horizontal conducting rods 42, 43, 44, and 45, each placed along the surface of target 40 in proximity to two of the holes (FIG. are all electrically connected to the positive terminal of a battery 46. The negative terminal of battery 46 is connected by a variable resistor 47 to target 40. A pump 48 connected to sputtering chamber 16 at its bottom is first employed to evacuate the chamber to about torr. Then a controlled amount of ionizable gas, such as argon, is admitted from a tank into sputtering chamber 16 by a valve 50 until a pressure of about 10 torr is reached. Gas in sputtering chamber 16 located in the path of the electron flow from the anodecathode units ionizes. These ions, of positive charge, are drawn to the surface of target 40. Upon impact they cause target 40 to be sputtered and thereby coat the surface of substrate 41. The effect of the holes in target 40 upon the pattern of the sputtered material rapidly reduces with distance from target 40 so that the holes do not adversely affect the ability to control the uniformity of the coating of substrate 41. Rods 42, 43, 44, and 45 provide a convenient return path for the target current, in view of the large area from which ions are attracted to target 40. As an altenative, a return path for the target current could be provided by connecting the positive terminal of battery 46 to the anode of one of the units. Although the anode-cathode units and the holes in target 40 are shown in FIGS. 5 and 6 with circular cross sections perpendicular to longitudinal axis 21, this is not necessary. Oval, rectangular, or square cross sections could, for example, also be utilized.

The anode-cathode units can be inserted at will into openings provided in the wall of sputtering chamber 16 to provide an ion plasma or mass of any desired size and distribution. Openings in sputtering chamber 16 which are not occupied by anode-cathode units in a particular sputtering operation are blanked off by a plate (not shown) to maintain the chamber airtight. These anode-cathode units can be placed at various angles with respect to one another, as illustrated by the position of unit 14, so as to provide a desired distribution of ions. The placement of unit 14 perpendicular to the other anode-cathode units in this case improves the ion density near the edge of target 40. A second row of anode-cathode units similar to units 10, 11, 12, and 13 could be placed perpendicular thereto, each protruding through a hole in a verticallydisposed target. This addition would permit sputtering in two dimensions simultaneously.

In FIGS. 2, 3, and 4 different arrangements of anodecathode units interconnected with one another are shown. These different arrangements can be combined in a single sputtering chamber with each other and with the intracorinected units illustrated in FIG. 1 to form an ion plasma of any desired distribution. My application Ser. No. 541,807, filed on Apr. 11, 1966 and assigned to the assignee of the present application, describes further arrangements of plural anode-cathode pairs in a sputtering system.

In the arrangement of FIG. 2 a series-type connection of the anode-cathode units is illustrated. The cathode of a unit 60 is negatively biased with respect to the anode of a unit 61, the cathode of unit 61 is negatively biased with respect to the anode of a unit 62, and the cathode of unit 62 is negatively biased with respect to the anode of a unit 63. Electrons flow in a path from the filament of each unit to the anode of the adjacent unit as illustrated by lines 72, 73, and 74 to establish ion discharges that intermix so as to form a large, continuous, conglomerate ion plasma or mass in front of target 40. The ion discahrges are independent from one another in the sense that they result from the flow of electrons in independent circuits.

In FIG. 3 the cathode of a unit 64 is biased negatively with respect to the anode of a unit 65 while the cathode of unit 65 is biased negatively with respect to the anode of unit 64. The anodes and cathodes of units 66 and 67 are similarly biased with respect to one another. The flow of electrons between unit 64 and unit 65, shown as lines 75 and 76, oppose each other and the flow of electrons between unit 66 and unit 67, shown as lines 77 and 78, oppose each other. This arrangement establishes opposing independent ion discharges that intermix so as to form a very uniform conglomerate ion plasma or mass in the region of target 40 between the openings through which units 64 and 65 protrude and between the openings through which units 66 and 67 protrude.

FIG. 4 shows an arrangement of units, in which the cathode of a unit 68 is negatively biased with respect to the anode of a nonadjacent unit and the cathode of a unit 69 is negatively biased with respect to the anode of a nonadjacent unit 71. In this arrangement, target 40 is biased by variable resistor 47 and battery 46 with respect to the anode of unit 70. The other arrangements could also employ this type of bias for target 40. The flow of electrons between units 68 and 70, shown as line 79, establishes an ion discharge e.g. a plurality of positive ions forming an ion plasma that is independent of and crosses the ion discharge established by the flow of electrons between units 69 and 71, shown as line 80. In the crossover region between units 69 and 70 the ion discharges intermix so that the density of the conglomerate ion mass is increased about twofold. As a result a distribution of ions that tapers from :a maximum in the middle to a minimum at the sides of target 40, is established. To expand this arrangement to include six units in a row, the first and fourth, second and fifth, and third and sixth units would be connected. Such an expanded arrangement would further increase the ion density at the middle of target 40.

The use of anode-cathode units according to the invention can be augmented by application of a magnetic field to the ion plasma if desired and also by application of a high frequency potential bias to target 40 instead of battery 46.

FIG. 6 shows an alternative to the single, large target in FIGS. 1 through 5. A plurality of separated targets 40a, 40b, 40c, and 40d are employed, each with its individual bias control circuit comprising a variable resistor (47a, 47b, 47c, 47d) and a battery (46a, 46b, 46c, 46d). Thus variations in the sputtering rate over the surface of substrate 41 are effected by controlling the bias of the various targets.

What is claimed is:

1. A sputtering apparatus for depositing thin films of material on a surface of a substrate by sputtering comprising:

an enclosure;

means for evacuating said enclosure and providing an ionizable atmosphere therein;

an ion target of said material in said enclosure, said target having at least one hole therein;

means for mounting said substrate in the enclosure with said surface of the substrate extending substantially parallel to, being spaced from and facing, a surface of the target; means for emitting electrons through at least one hole in said target;

means for deflecting said electrons in a path along the sputtering surface of the target to establish there an ion plasma; and

means for applying a negative potential to said target to attract ions from said plasma and cause the target to be sputtered.

2. The apparatus recited in claim 1 wherein the means for emitting electrons through the hole in the target comprises:

an anode cathode unit having an anode shaped to form an opening and exposed at one extremity of a longitudinal axis such that upon acceleration of electrons along the longitudinal axis toward the anode an appreciable number of electrons pass through the opening and beyond the anode to establish an ion plasma, the anode protruding through the hole in the target.

3. The apparatus of claim 1 wherein the means for emitting electrons is an anode-cathode unit comprising a hollow metal housing having a first open end and a second open end interconnected by a longitudinal axis, a base of insulator material placed over the first open end to form an airtight fitting with the housing, a cathode within the housing mounted upon the base, an annular anode member located along the longitudinal axis at a point substantially beyond the second open end of the housing, means for heating the cathode to cause emission of electrons therefrom, means for accelerating the emitted 6 electrons along the longitudinal axis toward the anode, means located at the second end of the housing capable of forming an airtight fitting with another surface, and a bafile for deflecting in a direction transverse to the longitudinal axis electrons accelerated through and beyond the anode member.

anode an appreciable number of electrons pass through the opening and beyond the anode, a cathode for emitting electrons located to permit acceleration of electrons along the longitudinal axis toward the anode, and means for deflecting in a direction transverse to the longitudinal axis electrons passing beyond the anode; means for biasing the anode of each anode-cathode unit positively with respect to the cathode of an anode-cathode unit so as to produce ion discharges in the region of the target located between the holes; and means for biasing the target so it draws ions from the discharge.

5. The sputtering apparatus of claim 4, in which the biasing means biases the anode of a first one of the anodecathode units positively with respect to the cathode of a second one of the anode-cathode units and biases the anode of the second anode-cathode unit positively with respect to the cathode of the first cathode unit.

6. The sputtering apparatus of claim 4, in Which the biasing means biases the anode of each anode-cathode unit positively with respect to the cathode of the adjacent anode-cathode unit so as to establish independent ion discharges forming a substantially continuous mass of ions extending between the holes.

7. The sputtering apparatus of claim 4, in which the biasing means biases the anode of one anode-cathode unit of nonadjacent pairs of anode-cathode units positively with respect to the cathode of the other anode-cathode unit of the pairs.

8. The sputtering apparatus of claim 4, in which the target comprises plural, separated parts and the means for biasing the target are individual to each part.

9. The sputtering apparatus of claim 8, in which the means for biasing the target are individually variable.

FOREIGN PATENTS 4/ 1960 Canada.

ROBERT K. MIHALEK, Primary Examiner US. Cl. X.R.

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