GB2449327A - Semiconductor growth reflectometry control method - Google Patents

Abstract

Controlling compound semiconductor growth in situ using reflectometry measurements and a multi-layer stack model. The method comprises: i. illuminating a semiconductor wafer 40 having one or more layers of different semiconductor materials in a state of growth within a reaction chamber 42 with a source of laser light 44, said growth having at least one controllable process variable such as temperature, pressure or flow rate, ii. detecting light 48 normally reflected from the surface of the semiconductor wafer, and from each underlying interface to generate a plurality of sets of reflectance data over discrete time periods, and iii. using the reflectance data sets in a matching procedure to determine one or more characteristics of the illuminated wafer 40. The matching procedure involves the processing of data in matrix form, wherein the matrices are of the order of the number of layers in the compound semiconductor wafer. A priori data (such as layer refractive indices or desired thicknesses) provide a measure of variation which is used via a feedback control signal to control the process variable. The wafer may rotate at a predetermined frequency and the data collected at a frequency dependent on the rotation frequency.

Description

SEMICONDUCTOR GROWTH CONTROL METHOD AND APPARATUS

This invention is concerned with a method and apparatus for controlling or regulating the growth of one or more semiconductor materials within a typical reaction chamber. More specifically, the invention concerns the control of growth reaction parameters such as temperature, pressure, volumetric flow rates, wafer rotation speeds and the like which can have a material affect on the resulting semiconductor wafer.

BACKGROUND

Reflectometry is the known art of analysing a beam of light reflected from a semiconductor wafer surface to determine certain physical characteristics of that seminconductor, e.g. refractive index, thickness, chemical composition, surface roughness and much reflectance data for a variety of different semiconductor materials already exists. Pyromtery is a similar technique, except that the parameter analysed is the thermal radiation emitted from the semiconductor wafer as it is being grown within a reaction chamber. The term In-situ" is thus commonly applied to these techniques to indicate that the processes occur as the semiconductor is being grown.

Although the following description is generally concerned with reflectometry, the reader should understand that the invention has application in the fields of both reflectometry and pyrometry.

Although the invention is mostly concerned with the improvement of in-situ ref lectometry techniques as they are applied to the measurement of Ga-N and Ga-Al-N semiconductor materials which have recently been more widely adopted in the semiconductor industry, those skilled in the art will appreciate that this invention is not restricted to such semiconductor materials, and indeed may have application to traditional and more modern semiconductor materials. Moreover, from the following, it will be appreciated that the invention may improve in-situ reflectometry for any material which may be grown using any of a number of deposition techniques, such as Chemical Vapour Deposition (CVD), Metal Organic Vapour Phase Epitaxy (MOVPE), Molecular Beam Epitaxy (MBE) and the like, or indeed any process where a reflectometry and/or pyromtery technique is required to assess the characteristics of a substance within a reaction chamber when the conditions within that reaction chamber during the growth of, or other chemical or physical alteration to, the substance are such that conventional measurement techniques techniques are impossible.

In modern semiconductor wafer growth, particularly where the wafer comprises multiple layers of different semiconductor materials, it is important to carefully monitor conditions and characteristics of the substrate at all times to achieve an acceptable degree of uniformity across all the wafers that may be grown during a particular process run, or in successive process runs. Specifically, the resulting characteristics of a wafer are highly dependent on the reaction conditions, particularly temperature and pressure, during growth, and currently, water growth of this type requires exceedingly skilled technicians to monitor the process and often make instinctive judgements as to whether certain process parameters should be changed In order to obtain wafers of the highest quality and uniformity. A real-time analysis of reflectance data and automatic control of the semiconductor growth process would thus be of great advantage.

One real-time semiconductor property characterization method in current use is known as reflection high electron energy deflection (RHEED), and this method is widely used in molecular beam epitaxy (MBE) to control two-dimensional growth, growth rates and composition of ternary layers. However, CVD and other techniques do not involve high vacuum conditions required for the use of electrons and therefore RHEED cannot be applied in these conditions.

Journal of Crystal Growth 174 (1997) 564-571, "In situ pre-growth calibration using reflectance as a control strategy for MOCVD fabrication of device structures" by Breiland et a!. discloses that a combination of in-situ normal incidence reflectance data and a virtual interface model may be used for commercial MOCVD reactors to measure growth rates of compound semiconductor films. The technique may serve as a pre-growth calibration tool analogous to the use of RHEED in MBE as well as a real-time monitor throughout the production run.

More specifically, the above method is alleged to overcome the current difficulty in the CVD production of semiconductor substrates wherein a number of calibration runs are required through the reaction chamber before an actual device structure is produced. The results of the calibration runs are then used in an iterative "dead reckoning" control strategy to make adjustments in the device structure recipe. However, new calibrations must be performed for every different substrate which is to be grown, and furthermore reactors are known to drift, requiring recalibration. Additionally, most calibrations are performed first with test structures and finally with repeated trials on the actual device structure using post-process, ex-situ analyses such as microscopy, photoluminescence, X-ray diffraction, but in practice these methods are not very successful in determining the origins of device inconsistencies or process deviations. The grower is often forced to guess what portion of the particular recipe needs alteration, and then test the guess with another production run. As will be immediately appreciated, not only is this expensive in terms of raw materials and time, producing semiconductor wavers with consistent physical properties is a process involving very highly skilled operators.

It is straightforward and known to model the reflectance from a smooth semiconductor substrate with an arbitrary number of smooth, homogeneous films deposited on it. The only parameters required are the complex refractive index for the substrate and the thickness and refractive index of each layer. If a growing film is monitored, reflectance interference oscillations are observed as the topmost film thickness changes with time.

This is also straightforward to model by expressing the topmost film thickness as the product of a growth rate and the time. There are, however, severe practical limitations that arise when such a straightforward approach is used to model the in situ reflectance waveform of a growing multiple-layer film. First, the deposition is performed at elevated temperatures. The optical constants of most semiconductor materials are not well known at the temperatures typical for CVD growth. This is particularly true for compound semiconductor alloys. Secondly, the thickness of a layer is not necessarily known precisely, particularly during a calibration run. In a multiple-layer reflectance model, errors in the refractive index and thickness of each layer contribute additively to uncertainties in the description of the growth of the topmost layer. This leads to a growth rate determination that is progressively less accurate with each additional calibration layer.

The solution to the multiple-layer reflectance modelling problem is to use a virtual interface concept, which is illustrated in Fig. 1. One chooses a virtual interface position (dashed line) that lies anywhere within the topmost film. It is then possible to rigorously describe the effects of all underlying layers as a single effective complex refractive index N, of a virtual interface effective substrate. The precise value of N, can, in principle, only be calculated from a complete knowledge of all the refractive indexes and thicknesses of the underlying layers. However, N is taken to be an unknown, it is always true that any always true that any multiple-layer structure requires only two parameters, the real and imaginary parts of Ny,, to describe the effects of all underlying layers below the virtual interface boundary.

Analysis of the topmost layer is thus made completely independent of the optical constants and interface positions of underlying layers. By choosing a new virtual interface position with each new layer, cumulative effects are eliminated in the analysis of a growing multiple-layer film structure. In their paper Journal App!. Phys. 78 (1995) 6726 Breiland and Killeen have also described the full formal virtual interface approach to semiconductor wafer growth. Herein, the time-dependent normal incidence complex reflectance r(t) of a multiple-layer growing film may rigorously be described by the following equation: r +r r(t)= 1re_i4mGl/2 The film thickness is expressed as the growth rate G times the time t where the virtual substrate interface is defined to be at t = 0. r. is the complex reflectance of an infinitely thick film of the topmost layer and is given explicitly by (1-N)/(1 + N), where N is the complex refractive index of the topmost growing film, N = n -ik, at the wavelength A. r, is the virtual interface complex reflectance just inside the topmost layer and is the ratio of backward and forward electric field amplitudes at the virtual interface boundary. It is formally related to the virtual interface effective refractive index by r, = (N -N1)/(N + N). For the purposes of the pre-growth calibration method, r is simply treated as an adjustable parameter in a least-squares fit to an observed reflectance waveform, and N is never calculated. Several features of the above equation make it particularly suitable for analyzing growing films. The expression contains five materials parameters: the growth rate G and the real and imaginary parts of N and ru,. These parameters are linearly independent and can thus be determined with a least squares fit to an observed reflectance waveform R(t) = Ir(t)12. The frequency of the reflectance oscillation is determined by the product NG. If the absolute reflectance is recorded, the r term effectively provides a measure of N independent of G, allowing one to separate the NG product. It is thus possible to determine the growth rate of a thin semiconductor film with in situ normal incidence reflectance without having to know the refractive index of the film during growth conditions. The accuracy of G is related directly to the accuracy of the absolute reflectance. A 1% error in reflectance results in approximately a 1% error in growth rate. It is thus very important to self-calibrate the reflectance at the beginning of each run and to maintain instrument stability throughout the run.

It is therefore possible to use a reflectance waveform having at least several extrema of oscillations to estimate the values of the parameters to be fit. This becomes extremely useful for routine pre- growth calibrations. The calibration procedure is as follows: A multiple-layer film is grown and a single-wavelength in situ reflectance interferogram is recorded. Each layer is grown thick enough to provide several extrema of interference oscillations. The order of the layers and timing is chosen to yield good contrast between the interferograms in each layer. For example, a GaAs growth calibration layer is done after an AlAs calibration layer is placed on the GaAs substrate. Segments of data from each layer are chosen for analysis. The starting time for each segment is arbitrary, provided that it does not include the transition from one layer to the next. The choice of starting time changes only the value of r, which is of no physical interest. Typically, the starting segment is chosen to be several seconds beyond the time at which the analyzed layer is known to have started. The stopping time is chosen to be a few seconds before the next layer is known to have started. The automated procedure is used to provide starting estimates for the five-parameter fit. A least-squares analysis is then done to fit the data segment to R(t) = Ir(t)12, and provide the growth rate for the layer. These can all be accomplished without prior knowledge about the growth rates or optical constants of the deposited materials. The process is then repeated for each layer in the calibration run. Fitting takes no more than a few seconds for each layer.

However, this method cannot be applied to real-time process control, exactly because fitting takes this length of time. Hence, while the above method is entirely adequate for calibration of a reactor and a particular desired process, the real-time control of that process and the production run through it is again controlled primarily manually by a skilled technician.

It is an object of this invention to provide an improved method and apparatus for achieving real-time semiconductor growth control, where the term "real-time", as it is applied to the control of the process, means that effective feedback is provided to the apparatus within the order of a few lOs of milliseconds or less.

BRIEF SUMMARY OF THE DISCLOSURE

In a first aspect of the invention there is provided a method of controlling compound semiconductor growth comprising the steps of: illuminating a semiconductor wafer having one or more layers of different semiconductor materials in a state of growth within a reaction chamber with a source of light, said growth having at least one controllable process variable, detecting light normally reflected from the surface of the semiconductor wafer, and from each discrete underlying semiconductor layer or growing substrate interface to generate a plurality of sets of reflectance data over discrete time periods, and using said reflectance data sets in a matching procedure to determine one or more physical characteristics of the illuminated wafer, characterised in that the matching procedure involves the processing of data in matrix form, wherein the matrices used in the matching procedure are of an order N 1 where N is the number of layers in the compound semiconductor wafer excluding the substrate on which it is being grown, said matching procedure being dependent on the provision of at least some a priori data for a semiconductor wafer having a first layer being the same as the first layer in the wafer so as to provide a measure of variation which is used to provide a feedback control signal to control the said at least one controllable process variable.

To clarify, the processing of data by mathematical matrix means provides a multi-layer stack model in which each semiconductor layer (and the growing substrate layer) in a wafer is accounted for in the data processing step, as opposed to the previously described virtual interface model in which an approximation is made.

Preferably, the process control variable is at least one of temperature, and/or pressure, volumetric flow rate of one or more of the compounds being delivered into the reaction chamber.

Preferably, the data processing includes the steps of compiling a digital representation of an interterogram from the reflectance data.

Most preferably, the processing of data using matrices of an order according to the number of layers in the wafer is conducted using the following relationships: The total anticipated reflectance from the transparent, multilayer structure is given as R = Where r is the reflectance amplitude and r is its conjugate, said reflectance amplitude being given by: n -Y r= no + Y Where n0 is the refractive index of the incident medium (=1 for air) and Y = -where B and C are defined by the matrix: C =[ cosSj iN,sinS1 cosS3 Where j refers to the layer (j=1 for the substrate, and m being the final layer in the wafer), N is the complex refractive index for the th layer, and S is given by the following: 2rNd S= 2' Where d is the thickness of the f layer, and A is the wavelength of laser light used for illumination.

Most preferably the matrix data processing is conducted by dedicated processing apparatus which delivers one or more feedback signals to process control apparatus, such processing apparatus being most preferably a suitably programmed PC.

By the term a priori is meant some base level data characterising the wafer layer structure which is to be grown. For instance, the simplest data required may be the particular semiconductor materials which are to constitute each layer to be grown, the desired thicknesses of each layer of material, together with the complex refractive indices for such materials. No previous set of reflectance data is needed in order to obtain a fit, but in a more advanced embodiment of the invention, some previously experimentally or empirically obtained reflectance data for each particular semiconductor material layer may be used, such being recursively improved as the growth process progresses and more data is gathered. Such data is also known as characterisation data for particular wafers and for the majority of wafers currently being grown for the semiconductor industries, static and growth characterisation data is already available.

In a most preferred embodiment, the growing substrate, and thus the semiconductor wafer is caused to rotate at a predetermined frequency, and the detection of normally reflected light (and thus the capture of data) from the surface of the wafer is conducted intermittently.

Most preferably, the intermittent capture of data is conducted by a PIG chip operating under the control of a basic computer programme, said PlC chip having been provided with the periodicity of the substrate rotation and a user-specified angle between 0 and 36O as determined from an arbitrary substrate rotation datum, said PIG chip having a clock and effectively converting the user-specified angle into a clock cycle count, after each such count (or multiple thereof), said PlC chip causes the data to be captured and forwarded to the dedicated processing apparatus.

Thus the PlC chip acquires the reflectance data for one or more fixed positions on the wafer, which may be the centre of the wafer in the simplest arrangement, or multiple locations around any locus traced on the substrate by the light impinging thereon as it rotates, and calculates a real time for when this measurement was made. Further, the background is re-calculated every rotation and subtracted from the reflectance signal.

This ensures that full account is taken of any changes such as window fogging from reaction products. This reflectance data as a function of position, wafer number and time is then delivered to the dedicated processing apparatus for matrix processing.

Reference to fixed positions on the wafer refers to a position fixed with respect to a frame of reference of the substrate; thus, as the substrate rotates, the frame of reference of the substrate rotates at the same angular frequency.

Thus, in all aspects of the invention it is preferable that the processing of received data is conducted in conjunction with the receipt of a time signal being the exact time at which the reflected light, i.e. the reflectance data, was captured, as compared relative to the start time of the growth operation. This allows the recordal of data points in real time on every interferogram by phasing within the rotation when each wafer is under the laser beam. It is essential for real time control that the actual time is recorded for each measurement and not just the time when each rotation cycle begins.

Thus the real time matrix-analyses conducted by this invention will enable useful growth parameters such as growth rate, thickness, roughening, etc. to be known at every point in the interference cycle and not just at the turning points as with the virtual interface model.

Embodiments of the invention have the advantage that one or more growth parameters may be determined for changes in thickness of less than a quarter of a wavelength. In some embodiments such parameters may be determined for changes in thickness much less than a quarter of a wavelength. In some embodiments changes in thickness of down to 1 nm are sufficient to enable the apparatus to determine the one or more growth parameters, depending on the material being analysed.

The invention has further advantages over the currently practised off line characterisation methods, including the virtual interface method mentioned above.

Specifically, the invention provides a complete mathematical fit using all the reflectance data (from the start of growth) and can cope with multiple thin films. The results of the matrix processing performed by the PC are: film thickness, growth rate, film roughening, complex refractive index, and these can be provided effectively in real-time with after every additional piece of data is delivered to the PC from the PlC chip/capture apparatus.

In a preferred arrangement, the dedicated processing apparatus is adapted to receive data from a known interferometer, such as the MIniEYE product available from the applicant herefor. Such interferometers capture the normally reflected light and convert this into signals representative of an interferogram. Specifically, interferometers: a) provide absolute reflectance measurements (automatically accounting for background light); b) are triggered to make a measurement at the centre of each water in multi-wafer configurations; C) are ultra-stable as the data presented to dedicated processing apparatus has to be raw data, not smoothed or massaged in any way; d) record time the absolute time at which each data point is recorded; e) capture substantially noise-free data; ideally a windowless diode is used to remove intensity variations due to precession of the reflected beam coupled with small variations in the thickness of the cover glass); most preferably large area diodes are used so there is no need to pass the reflected beam through a focusing lens (the percentage of light passing through a lens depends on the angle of the incident light, which in turn depends on the precession of the reflected beam); f) be substantially free of electronics in the optical head to comply with certain standards involving electronics in potentially explosive environments, i.e. the reaction chamber.

In terms of the further advantages provided by the invention, the following are relevant: a) allows calibration of a reactor immediately prior to growing a device structure; b) allows for continuous checking of the performance of the reactor during growth of the structure; c) Immediately identify wafers which have not been seated correctly (they precess more which results in large standard deviations in the fit' parameter; If the wafers are not seated properly, then the structure grown on them will be useless); d) Reduces or precludes the need for postgrowth analysis and characterisations, especially when wafers falling into (c) above are discarded; e) Allows for productivity enhancements of the reactor; the calibration means that the reactor is able to meet the ever tightening specifications imposed on structures in the semiconductor market; f) Allows for feed-back control of deposition plant; since it generates information and not data, which is then fed back to the control engine' (i.e. usually a PC, but possibly other dedicated hardware) of the reactor to keep the growth/deposition in line with requirements.

Preferably the method further comprises the step of correcting the reflectance data to compensate for a variation in reflected light intensity that is not due to a change in a thickness of the growing substrate.

The feature of correcting for varations in reflectance data to compensate for a variation in reflected light intensity that is not due to a change in a thickness of the growing substrate has the advantage that an accuracy with which a reflectivity of a given point on the surface of the wafer can be measured is increased.

In real world growth chambers, a window through which the source of light illuminating the wafer must pass can become fogged'. In other words, an amount of light transmitted by the window can decrease due to coating of the window with material present in the chamber such as material being deposited on the growing substrate.

Fogging can result in an increase in the intensity of light detected by the apparatus, resulting in a measurement of the amount of light that is greater than would be the case if the window had not become coated with material being deposited on the growing substrate. Other materials can also fog the window.

The present inventors have recognised that one of the factors affecting an accuracy with which a thickness of a film can be measured is a change in an amount of light reflected back towards the detector that is not due to the growing film, but due to factors including fogging of the window through which light enters the growth chamber. An increase in an amount of fogging can result in an increase in an amount of light reflected back to the detector by material depositing on the window. In some embodiments material depositing on the window may cause a decrease in an amount of light reflected back towards the detector. In some embodiments the material causing fogging absorbs the light. In some embodiments this material scatters the light away from the detector.

In some embodiments of the invention the present method overcomes this problem by measuring the amount of light detected by the detector when the substrate is being illuminated, and measuring the amount of light detected by the substrate when the substrate is not being illuminated and substantially no light that has passed beyond the window has been reflected back to the detector.

The light detected under these latter conditions is therefore partially due to reflection by the window itself (the amount of this can be separately determined by obtaining a measurement of the amount of light detected before growth begins) and partially due to reflection by material deposited on the window as a consequence of fogging.

In some embodiments of the invention the method includes correction of the amount of light detected by the detector when the substrate is being illuminated to compensate for an amount of light reflected by the window itself and by the material causing fogging, and taking account of the consequent decrease in an intensity of light incident on the substrate due to reflection due to fogging.

Preferably the method comprises the step of obtaining a background reflectance signal, the step of obtaining a background reflectance signal comprising the step of illuminating a surface other than the surface of the wafer, said other surface being arranged whereby any light reflected by said other surface is substantially not detected.

The wafer is preferably arranged whereby during a cycle of rotation of the wafer, the source of light illuminates said other surface.

Preferably said other surface corresponds to a surface of a holder on which the waler is mounted or a surface of one or more components or walls of the chamber in which the substrate is provided.

Said other surface may be any surface which does not reflect light back towards the detector. Said other surface may be one of a plurality of different surfaces illuminated during a course of growing a film.

Preferably the method comprises the step of measuring a value of an intensity of light from the source of light that is detected by the detector before and during growth of the substrate both when the light source is illuminating a predetermined point on the substrate and when the light source is illuminating said other surface.

Preferably the method comprises the step of periodically correcting a value of intensity of light reflected by the substrate based on the value of the intensity of light detected when the light source is illuminating said other surface.

The method may comprise causing a plurality of growing substrates, and thus a plurality of semiconductor wafers, to rotate at the predetermined frequency.

Said other surface may correspond to a surface of a substrate holder that is exposed to said light source as the substrate is rotated.

The method may comprise the step of correcting the reflectance data to compensate for a variation in reflected light intensity due to fogging of a window through which the light must pass.

In a second aspect of the invention there is provided apparatus for controlling growth of a compound semiconductor in a growth process havingat least one controllable process variable, the apparatus comprising: means for illuminating a semiconductor wafer having one or more layers of different semiconductor materials in a state of growth within a reaction chamber with a source of light; means for detecting light normally reflected from the surface of the semiconductor wafer, and from each discrete underlying semiconductor layer or growing substrate interface; and means for generating a plurality of sets of reflectance data over discrete time periods, wherein the apparatus is configured to use said reflectance data sets in a matching procedure to determine one or more physical characteristics of the illuminated wafer, the matching procedure comprising the steps of: processing the data in matrix form, wherein the matrices used in the matching procedure are of an order N 1 where N is the number of layers in the compound semiconductor wafer excluding the substrate on which it is being grown, said matching procedure being dependent on the provision of at least some a priori data for a semiconductor wafer having a first layer being the same as the first layer in the wafer so as to provide a measure of variation, the apparatus being operable to provide a feedback control signal based on said measure of variation to control the said at least one controllable process variable.

The apparatus is preferably arranged to process the data in matrix form using a multi-layer stack model in which each semiconductor layer (and the growing substrate layer) in a wafer is accounted for in the data processing step.

The process control variable may be at least one of temperature, pressure and volumetric flow rate of one or more of the compounds being delivered into the reaction chamber.

Preferably the apparatus is configured to compile a digital representation of an interferogram from the reflectance data.

The apparatus may be configured whereby the processing of data using matrices of an order according to the number of layers in the wafer is conducted using the following relationships: the total anticipated reflectance from the transparent, multilayer structure is given as R = rr where ris the reflectance amplitude and r is its conjugate, said reflectance amplitude being given by: lit -Y r= no + Y

B

where n0 is the refractive index of the incident medium (=1 for air) and Y = -where B and C are defined by the matrix: isinS ( j[n[ CO N zNsinS1 cosJ where j refers to the layer (j=1 for the substrate, and m being the final layer in the wafer), N is the complex refractive index for the j"1 layer, and S is given by the following: -2irN1d1 2 where d1 is the thickness of the jt' layer, and A is the wavelength of laser light used for illumination.

Preferaby the apparatus is configured whereby the matrix data processing is conducted by dedicated processing apparatus arranged to deliver one or more feedback signals to process control apparatus.

Preferably the processing apparatus comprises a suitably programmed computer.

The apparatus may be arranged to cause rotation of a growing substrate, and thus a semiconductor wafer, at a predetermined frequency, the apparatus being configured whereby intermittent capture of data corresponding to the normally reflected light is performed at a frequency dependent on the frequency of substrate rotation.

Preferably the data corresponding to the normally reflected light corresponds to an intensity of the normally reflected light.

The apparatus may be configured whereby the intermittent capture of data is conducted by a controller operating under the control of a computer program, said controller having been provided with data corresponding to the period of substrate rotation and a user-specified angle between 0 and 360 as determined from an arbitrary substrate rotation datum, said controller having a clock and being configured to effectively convert the user-user-specified angle into a clock cycle count, after each such count (or multiple thereof), said controller being arranged to cause the data to be captured and forwarded to the dedicated processing apparatus.

Preferably the controller comprises a programmable interface controller (PlC) chip.

The apparatus may be configured whereby the processing of received data is conducted in conjunction with the receipt of a time signal being the exact time at which the reflected light was captured, as compared relative to the start time of the growth operation.

Preferably the dedicated processing apparatus is adapted to receive data from an interferometer capable of a) providing absolute reflectance measurements (automatically accounting for

background light)

b) being triggered to make a measurement at the centre of each wafer in multi-wafer configurations c) recording time the absolute time at which each data point is recorded, and d) capturing substantially noise-free data, The interferometer may be substantially free of electronics in an optical head of the interferometer.

The apparatus may comprise said interferometer.

Preferably the interferometer is provided with a windowless diode for removing intensity variations due to precession of the reflected beam coupled with small variations in the thickness of the cover glass of the reaction chamber.

The apparatus may be further configured to correct the reflectance data to compensate for a variation in reflected light intensity that is not due to a change in a thickness of the growing substrate.

Preferably the apparatus is configured to obtain a background reflectance signal, the apparatus being arranged to illuminate a surface other than the surface of the wafer, said other surface being arranged whereby any light reflected by said other surface is substantially not detected by the apparatus.

The apparatus may be arranged whereby during a cycle of rotation of the wafer, the source of light illuminates said other surface.

Preferably said other surface corresponds to a surface of a holder on which the wafer is mounted or a surface of one or more components or walls of the chamber in which the substrate is provided.

Preferably the apparatus is arranged to obtain a value of an intensity of light from the source of light that is detected by the detector before and during growth of the substrate both when the light source is illuminating a predetermined point on the substrate and when the light source is illuminating said other surface.

Preferably the apparatus is arranged to periodically correct a value of intensity of light reflected by the substrate based on the value of the intensity of light detected when the light source is illuminating said other surface.

The apparatus may be arranged whereby a plurality of growing substrates, and thus a plurality of semiconductor wafers, are caused to rotate at the predetermined frequency.

Preferably said other surface corresponds to a surface of a substrate holder that is exposed to said light source as the substrate is rotated.

Preferably the apparatus is arranged to correct the reflectance data to compensate for a variation in reflected light intensity due to fogging of a window through which the light must pass.

A specific embodiment of the invention will now be provided by way of example with reference to the following drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a schematic for a physical compound wafer of ri layers as compared to the corresponding approximation for this water in the virtual interface model; Figure 2 shows a schematic representation of a prior art reactor and wafer growth monitoring arrangement; Figure 3 shows a schematic representation of the actual reflection of light from a compound semiconductor wafer when the incident light is non-normal; Figure 4 shows a schematic representation of the arrangement of different aspects of the invention, and Figure 5 provides a flowchart of one sequence of operations carried out by the PIG chip ideally used in this invention.

Referring to Figure 2 which shows a prior art MOVPE system equipped with a commercially available reflectometer and schematically indicated at 2, consisting of a white light source 4 and a CCD spectrometer 6 (Filmetrics F 30 or alternatively the EpiEVE device available from applicants herefor). The spectrometer is a 512-element photodiode array with a spectral range of 400 nm -1100 nm and a resolution of 2 nm.

The spectrometer is controlled by a computer 8 and the spectrometer software allows calculation of growth-variable semiconductor physical characteristics such as deposition rate, the refractive index n, the extinction coefficient k and reflectivity R. For these purposes, material data libraries are contained in the software executing on the computer.

As will be appreciated from Fig. 1, an optical access to the substrate with the nitride layer growing thereon in the MOVPE reactor is mandatory, and in the arrangement shown, a normal angle of incidence is used.

The reactor 10 comprises a liner tube 12 made of quartz glass. To the outside of the reactor, there is provided a water-cooled jacket 14, and to the outside of said jacket there is provided a radio-frequency heating coil 16 which acts to direct high intensity RE energy onto a susceptor 18 on top of which is positioned a substrate 20 which is most commonly made of sapphire. During use, a source of mixed metal organic gases, whose flow rates can be controlled, passes into the chamber through an inlet 22 and as a result of the controlled conditions within the reactor and the composition of the inlet gas, semiconductor material begins firstly to nucleate on the substrate, and subsequently grow thereon. A source of purging gas is also provided which flows around the liner tube and whose flow ultimately aids in the expulsion of the metal organic gas stream from the reactor in general. It is to be understood that the nature of the gaseous flows used in such reactors is often exceptionally toxic to humans, and that great care must be taken in how such gases are handled.

In use, due to the horizontal configuration of the reactor, the ceiling of the liner may become coated with Nitride or other elemental/molecular deposits during semiconductor growth, rendering it opaque to at least some extent. Therefore, a 5mm diameter hole is drilled in the liner ceiling. The liner is located inside a quartz cylinder (outer reactor tube), which is surrounded by the water cooling jacket made of quartz, too. The ref lectometer is mounted directly above the zenith of the usually cylindrical liner in which the hole is drilled so that, except for variations in the surface profile of the semiconductor, light incident thereon from the ref lectometer is reflected directly back towards the source of the light as generally indicated at 26. Both the incident and reflected light has to pass through all the quartz walls and the cooling water. Disturbing reflections from the quartz walls can be eliminated by reference measurements as generally the oscillatory characteristics of the quartz is not affected by reaction conditions.

The spectrometer and the light source are connected to the lens system 28 by optical fibers of a coaxial type, outer strands of which are intended to carry reflected light back to the spectrometer, and the inner strands of which are intended to carry white light from the white light source of the reflectometer. The reflectance of the sample surface, recorded during the growth process, is continuously monitored and recorded. After loading the substrate into the reactor, substrates are typically heated up to 950CC under a steady flow of a nitrogen / hydrogen mixture. Following this sapphire surface cleaning step, the substrate temperature is lowered to 520CC for the deposition of the low temperature nucleation layer. After the nucleation layer is deposited, reactor temperature is increased to 1050 C for growth of undoped bulk GaN.

Although a normal incidence angle is shown in the Figure, in the present invention, a non-normal angle of incidence can be used, such that the reflection is also non-normal.

The reason for this is that the reflection of polarised, e.g. laser light from a semiconductor wafer substrate results in a change in the polarisation components which render it simpler to detect the various different reflected rays, see Fig. 3, l, 12, 13, 14,.. .L, arid additionally generate the requisite interferogram.

In accordance with the invention, and referring to Figure 4, a compound semiconductor wafer 40 is being grown in a reaction chamber 42, various aspects of which can be controlled through a variety of known means. The controllable parameters A, B, C, .. .M may include temperature, pressure, inlet flow rates, and the like. A beam of polarised laser light 44 is directed towards the wafer 40 and is multiply reflected from the uppermost surface thereof, and the various other surface of the remaining layers in the wafer, and additionally from the uppermost surface of the substrate 46 on which the wafer is being grown. The various reflected beams of light 48 are directed towards a photodetector 50 which converts the various different reflected laser beams into electrical signals which are then delivered to the PIG chip 52, which has some limited processing capability 52A coupled with memory 52B, and some programmable firmware 52G.

It is to be mentioned that there may be a plurality of substrates and compound semiconductor wafers growing thereon within the reaction chamber 42, and furthermore that the laser light illuminating them may be intermittent, the frequency of the intermittency being equal to the period of rotation of each of said substrates in order to compensate for the effects of precessing of said substrates. In these circumstances, at least one of the instances of data capture must be arranged to be within a time when the substrate/semiconductor wafer is being illuminated. Common periods for the substrate rotation and laser illumination thereof are 6-lOs.

At the beginning of the growth process, the substrate is first nucleated with the desired semiconductor compound and the first layer of a particular chemical composition is grown. The various growing and physical characteristics can be determined from analysis of the successive sets of data derived from the reflected laser light, and in particular the growth rate, layer and wafer thickness, refractive index, chemical composition, extinction coefficients, and surface roughness may be determined. As mentioned above, base level characterisation data is required by the dedicated processing apparatus 54, such as the wafer & substrate composition, layer thickness (which may either be provided explicitly or calculated from the specific times during which particular layers are grown, such being information readily available from the PlC chip 52 -see below), and complex refractive indices of each of the materials which constitute the layers and the substrate.

At the commencement of the growing process, the PIG chip 52 will effectively capture light from the substrate before any semiconductor growth has commenced. This time can be thought of as t = 0.

As the growth of the first layer in the wafer commences over the substrate, it is most unlikely the time of each laser illumination will coincide exactly with the growth of said first (and each subsequent) layer, and therefore the time of start of growth of each particular layer must be determined as accurately as possible and also be provided to the processing apparatus during the growth process.

In terms of the basic data capture performed by the PlC chip 52, one feature is adequately explained with reference to Figure 5 which shows a flowchart of the PlC chip logic in delivering data signals captured at the correct time (i.e. at the correct rotational angle of the substrate) and delivering this data for subsequent processing as detailed below.

The conversion of angle (which may be anything between 0 and 360 in 0.01 increments) to time is performed by determining how many clock cycles of the processing apparatus is equivalent to the rotation period and then applying the corresponding multiplying factor to the proportion of 360 which is the requisite capture angle.

It can therefore be seen from the above that the PlC chip 52 can deliver the requisite data and time signals to the dedicated processing apparatus 54, which is ideally a PC or other suitable device which can make requisite adjustments, effectively as soon as each output from the processing apparatus is received, to the various growth control parameters A, B, C, . . . M. It will be appreciated that the reaction chambers considered for use in the present invention are essentially closed systems, and thus the detection of reflected laser light from the substrate and/or wafer growing thereon during the reaction is prone to the effects of reactants fogging a particular area or window of the internal surface of the reaction chamber through which the laser light must pass before being detected by the photodetector apparatus. As will also be understood, the effects of such fogging are dynamically changing as the reaction progresses, and furthermore may have absorptive, dispersive of scattering effects on the reflected laser light. Hence, although the inherent dispersive or absorptive qualities of the particular window may be negated by initial calibration of the apparatus, there has heretofore been no means of correcting or re-calibrating the raw data produced by the photodetector according to the extent to which the window has become coated with reactants or fogged.

In the present invention however, this difficult is neatly overcome in that, in a preferred arrangement, the method includes the steps, conducted in the software executing on the dedicating processing apparatus, of storing a first reflectance datum, by taking a background reflectance reading between substrates passing under the laser beam, representative of a zero-fogging condition prior to any wafer growth on the substrate, and then automatically recalibrating subsequently received data with the first reflectance datum to remove any discrepancies present in said subsequent data which arise from progressive fogging of said window.

It is to be appreciated that by compensating for fogging or obscuration to light transmission of the window through which light must pass in order to be normally reflected by a substrate in the growth chamber, an accuracy with which a change in thickness of a film at a prescribed location of the substrate may be significantly enhanced. Furthermore, the provision of apparatus configured to ensure that a measurement of thickness of a film on a wafer is made at substantially precisely the same location of the wafer despite the fact that the wafer is rotating further allows an accuracy with which a change in thickness of a wafer as a function of time can be measured to be increased.

It is to be understood that rotation of a wafer during growth may be required to be performed in order to provide an increase in a uniformity of thickness of the material being deposited.

In some embodiments a carousel supporting a plurality of wafers is rotated during growth.

Some embodiments of the invention are arranged whereby compensation for fogging of the window is performed by measuring the amount of light detected by the detector when the light source is illuminating a portion of the carousel between wafers, as the carousel rotates, or any other suitable surface that does not reflect or otherwise scatter light back towards the detector.

In some embodiments at least one reference datum is obtained prior to growth, the reference datum corresponding to an amount of light detected by the detector before fogging of the window begins. In some embodiments a reference datum is obtained when a prescribed position on the wafer surface is being illuminated with light and when the carousel or other surface not scattering light to the detector is being illuminated.

The one or more reterence datums are used to correct reflectance data obtained during growth by means of the detector.

In some embodiments of the invention in which wafer rotation is not performed, compensation for fogging may be performed by preventing light that has entered the chamber through the window from the light source from being reflected back out through the window. In some embodiments a shutter arrangement is provided whereby light impinging on the shutter may be absorbed and/or scattered in one or more directions other than back towards the detector. The shutter is introduced into a path of the light source when it is required to obtain a measurement of an amount of light being reflected back to the detector by the window and any material depositing on the window thereby fogging the window.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Claims (48)

1. CLAIMS: 1. A method of controlling compound semiconductor growth

   comprising the steps of: illuminating a semiconductor wafer having one or more layers of different semiconductor materials in a state of growth within a reaction chamber with a source of light, said growth having at least one controllable process variable, detecting light normally reflected from the surface of the semiconductor wafer, and from each discrete underlying semiconductor layer or growing substrate interface to generate a plurality of sets of reflectance data over discrete time periods, and using said reflectance data sets in a matching procedure to determine one or more physical characteristics of the illuminated wafer, characterised in that the matching procedure involves the processing of data in matrix form, wherein the matrices used in the matching procedure are of an order Ni-i where N is the number of layers in the compound semiconductor wafer excluding the substrate on which it is being grown, said matching procedure being dependent on the provision of at least some a priori data for a semiconductor wafer having a first layer being the same as the first layer in the waler so as to provide a measure of variation which is used to provide a feedback control signal to control the said at least one controllable process variable.
2. 2. A method according to claim 1 wherein the processing of data in matrix form is achieved using a multi-layer stack model in which each semiconductor layer (and the growing substrate layer) in a wafer is accounted for in the data processing step.
3. 3. A method according to any preceding claim wherein the process control variable is at least one of temperature, and/or pressure, volumetric flow rate of one or more of the compounds being delivered into the reaction chamber.
4. 4. A method according to any preceding claim wherein the data processing includes the steps of compiling a digital representation of an interferogram from the reflectance data.
5. 5. A method according to any preceding claim wherein the processing of data using matrices of an order according to the number of layers in the wafer is conducted using the following relationships: The total anticipated reflectance from the transparent, multilayer structure is given as R = rr Where r is the reflectance amplitude and r is its conjugate, said reflectance amplitude being given by: n -Y no + Y Where n0 is the refractive index of the incident medium (=1 for air) and Y = where B and C are defined by the matrix: isinö lB.1= in cosS N. LC) . (N.

   Nsinö cos, Where j refers to the layer (j=1 for the substrate, and m being the final layer in the wafer), N is the complex refractive index for the th layer, and is given by the following: -2,rN1d1

   A

   Where d is the thickness of the 1th layer, and A is the wavelength of laser light used for illumination.
6. 6. A method according to any preceding claim wherein the matrix data processing is conducted by dedicated processing apparatus which delivers one or more feedback signals to process control apparatus.
7. 7. A method according to claim 6 wherein the processing apparatus is a suitably programmed PC.
8. 8. A method according to any preceding claim wherein the growing substrate, and thus the semiconductor wafer, is caused to rotate at a predetermined frequency, and the detection of normally reflected light (and thus the capture of data) from the surface of the wafer is conducted intermittently at a frequency dependent on the frequency of substrate rotation.
9. 9. A method according to claim 8 wherein the intermittent capture of data is conducted by a PlC chip operating under the control of a basic computer programme, said PlC chip having been provided with the periodicity of the substrate rotation and a user-specified angle between 0 and 360 as determined from an arbitrary substrate rotation datum, said PlC chip having a clock and effectively converting the user-specified angle into a clock cycle count, after each such count (or multiple thereof), said PlC chip causes the data to be captured and forwarded to the dedicated processing apparatus.
10. 10. A method according to any preceding claim wherein the processing of received data is conducted in conjunction with the receipt of a time signal being the exact time at which the reflected light was captured, as compared relative to the start time of the growth operation.
11. 11. A method according to any preceding claim wherein the dedicated processing apparatus is adapted to receive data from an interferometer capable of a) providing absolute reflectance measurements (automatically accounting for

    background light)

    b) being triggered to make a measurement at the centre of each wafer in multi-wafer configurations c) recording time the absolute time at which each data point is recorded d) capturing substantially noise-free data, and which is substantially tree of electronics in the optical head.
12. 12. A method according to claim 11 wherein the interferometer uses a windowless diode for removing intensity variations due to precession of the reflected beam coupled with small variations in the thickness of the cover glass of the reaction chamber.
13. 13. A method according to any preceding claim further comprising the step of correcting the reflectance data to compensate for a variation in reflected light intensity that is not due to a change in a thickness of the growing substrate.
14. 14. A method according to claim 13 comprising the step of obtaining a background reflectance signal, the step of obtaining a background reflectance signal comprising the step of illuminating a surface other than the surface of the wafer, said other surface being arranged whereby any light reflected by said other surface is substantially not detected.
15. 15. A method as claimed in claim 14 depending through claim 8 wherein the wafer is arranged whereby during a cycle of rotation of the wafer, the source of light illuminates said other surface.
16. 16. A method as claimed in claim 15 wherein said other surface corresponds to a surface of a holder on which the wafer is mounted or a surface of one or more components or walls of the chamber in which the substrate is provided.
17. 17. A method as claimed in any one of claims 14 to 16 comprising the step of obtaining a value of an intensity of light from the source of light that is detected by the detector before and during growth of the substrate both when the light source is illuminating a predetermined point on the substrate and when the light source is illuminating said other surface.
18. 18. A method as claimed in claim 17 comprising the step of periodically correcting a value of intensity of light reflected by the substrate based on the value of the intensity of light detected when the light source is illuminating said other surface.
19. 19. A method as claimed in claim 13 depending through claim 8 or any one of claims 14 to 18 depending through claim 8 wherein a plurality of growing substrates, and thus a plurality of semiconductor wafers, are caused to rotate at the predetermined frequency.
20. 20. A method as claimed in claim 14 depending through claim 8 or any one of claims 15 to 19 depending through claim 8 whereby said other surface corresponds to a surface of a substrate holder that is exposed to said light source as the substrate is rotated.
21. 21. A method as claimed in claim 13 or any one of claims 14 to 20 depending through claim 13 comprising the step of correcting the reflectance data to compensate for a variation in reflected light intensity due to fogging of a window through which the light must pass.
22. 22. Apparatus for controlling growth of a compound semiconductor in a growth process having at least one controllable process variable, the apparatus comprising: means for illuminating a semiconductor wafer having one or more layers of different semiconductor materia(s in a state of growth within a reaction chamber with a source of light; means for detecting light normally reflected from the surface of the semiconductor wafer, and from each discrete underlying semiconductor layer or growing substrate interface; and means for generating a plurality of sets of reflectance data over discrete time periods, wherein the apparatus is configured to use said reflectance data sets in a matching procedure to determine one or more physical characteristics of the illuminated wafer, the matching procedure comprising the steps of: processing the data in matrix form, wherein the matrices used in the matching procedure are of an order N 1 where N is the number of layers in the compound semiconductor wafer excluding the substrate on which it is being grown, said matching procedure being dependent on the provision of at least some a priori data for a semiconductor wafer having a first layer being the same as the first layer in the wafer so as to provide a measure of variation, the apparatus being operable to provide a feedback control signal based on said measure of variation to control the said at least one controllable process variable.
23. 23. Apparatus according to claim 22 arranged to process the data in matrix form using a multi-layer stack model in which each semiconductor layer (and the growing substrate layer) in a wafer is accounted for in the data processing step.
24. 24. Apparatus according to claims 22 or 23 wherein the process control variable is at least one of temperature, pressure and volumetric flow rate of one or more of the compounds being delivered into the reaction chamber.
25. 25. Apparatus according to any one of claims 22 to 24 configured to compile a digital representation of an interferogram from the reflectance data.
26. 26. Apparatus according to any one of claims 22 to 25 configured whereby the processing of data using matrices of an order according to the number of layers in the wafer is conducted using the following relationships: the total anticipated reflectance from the transparent, multilayer structure is given as R = rr where r is the reflectance amplitude and r is its conjugate, said reflectance amplitude being given by: r= fl1fY no + Y where n0 is the refractive index of the incident medium (=1 for air) and Y = -where B and C are defined by the matrix: isinO.

    lB ft cosJ, N c) (N.

    iN sin cos ö where j refers to the layer (j=1 for the substrate, and m being the final layer in the wafer), N is the complex refractive index for the th layer, and is given by the following: 2,rN.d 5= where d1 is the thickness of the th layer, and A is the wavelength of laser light used for illumination.
27. 27. Apparatus according to any one of claims 22 to 26 configured whereby the matrix data processing is conducted by dedicated processing apparatus arranged to deliver one or more feedback signals to process control apparatus.
28. 28. Apparatus according to claim 27 wherein the processing apparatus comprises a suitably programmed computer.
29. 29. Apparatus to any one of claims 22 to 28 arranged to cause rotation of a growing substrate, and thus a semiconductor wafer, at a predetermined frequency, the apparatus being configured whereby intermittent capture of data corresponding to the normally reflected light is performed at a frequency dependent on the frequency of substrate rotation.
30. 30. Apparatus as claimed in claim 29 wherein the data corresponding to the normally reflected light corresponds to an intensity of the normally reflected light.
31. 31. Apparatus according to claim 29 or 30 configured whereby the intermittent capture of data is conducted by a controller operating under the control of a computer program, said controller having been provided with data corresponding to the period of substrate rotation and a user-specified angle between 0 and 360 as determined from an arbitrary substrate rotation datum, said controller having a clock and being configured to effectively convert the user-specified angle into a clock cycle count, after each such count (or multiple thereof), said controller being arranged to cause the data to be captured and forwarded to the dedicated processing apparatus.
32. 32. Apparatus according to claim 31 wherein the controller comprises a programmable interface controller (PlC) chip.
33. 33. Apparatus according to any one of claims 22 to 32 configured whereby the processing of received data is conducted in conjunction with the receipt of a time signal being the exact time at which the reflected light was captured, as compared relative to the start time of the growth operation.
34. 34. Apparatus according to any one of claims 22 to 33 claim wherein the dedicated processing apparatus is adapted to receive data from an interferometer capable of a) providing absolute reflectance measurements (automatically accounting for

    background light)

    b) being triggered to make a measurement at the centre of each wafer in multi-wafer configurations c) recording time the absolute time at which each data point is recorded, and d) capturing substantially noise-free data,
35. 35. Apparatus as claimed in claim 34 wherein the interferometer is substantially free of electronics in an optical head of the interferometer.
36. 36. Apparatus as claimed in claim 34 or 35 comprising said interferometer.
37. 37. Apparatus according to claim 36 wherein the interferometer is provided with a windowless diode for removing intensity variations due to precession of the reflected beam coupled with small variations in the thickness of the cover glass of the reaction chamber.
38. 38. Apparatus according to any one of claims 22 to 37 further configured to correct the reflectance data to compensate for a variation in reflected light intensity that is not due to a change in a thickness of the growing substrate.
39. 39. Apparatus according to claim 38 configured to obtain a background reflectance signal, the apparatus being arranged to illuminate a surface other than the surface of the wafer, said other surface being arranged whereby any light reflected by said other surface is substantially not detected by the apparatus.
40. 40. Apparatus according to claim 39 depending through claim 29 arranged whereby during a cycle of rotation of the wafer, the source of light Illuminates said other surface.
41. 41. Apparatus according to claim 40 wherein said other surface corresponds to a surface of a holder on which the wafer is mounted or a surface of one or more components or walls of the chamber in which the substrate is provided.
42. 42. Apparatus according to any one of claims 38 to 41 arranged to obtain a value of an intensity of light from the source of light that is detected by the detector before and during growth of the substrate both when the light source is illuminating a predetermined point on the substrate and when the light source is illuminating said other surface.
43. 43. Apparatus according to claim 42 arranged to periodically correct a value of intensity of light reflected by the substrate based on the value of the intensity of light detected when the light source is illuminating said other surface.
44. 44. Apparatus according to any one of claims 38 to 43 depending through claim 29 wherein a plurality of growing substrates, and thus a plurality of semiconductor wafers, are caused to rotate at the predetermined frequency.
45. 45. Apparatus according to claim 39 or any one of claims 40 to 44 depending through claim 39 whereby said other surface corresponds to a surface of a substrate holder that is exposed to said light source as the substrate is rotated.
46. 46. Apparatus according to claim 38 or any one of claims 39 to 45 depending through claim 38 arranged to correct the reflectance data to compensate for a variation in reflected light intensity due to fogging of a window through which the light must pass.
47. 47. A method substantially as hereinbefore described with reference to the accompanying drawings.
48. 48. Apparatus substantially as hereinbef ore described with reference to the accompanying drawings.