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Hardware and software for in-situ process monitor and control using multiple wavelength ellipsometry
680
Surface and Coatings Technology, 62 (1993) 680—682
Hardware and software for in-situ process monitor and control using multiple wavelength ellipsometry Blame Johs, David Doerr, Shakil Pittal and John A. Woollam* J. A. Woollam Co., 650 J Street, Suite 39, Lincoln, NE 68508 (USA)
Ishwara Bhat and S. Dakshinamurthy Rensselaer Polytechnic Institute, Troy, NY 12180 (USA)
Abstract We have built and tested a low-cost ellipsometer which acquires data in the spectral range 400—800 nm at multiple (up to 44) wavelengths simultaneously in less than 1 s. A personal computer is used to acquire data and analyze the data in real time for studies of time dependent phenomena. In addition, this permits feedback control which we have demonstrated on epitaxial growth of CdTe on GaAs by metal organic chemical vapor deposition.
1. Introduction For many years variable angle spectroscopic ellipsometry (VASE®t) has been used ex situ to determine optical constants, layer thicknesses, void fractions, alloy compositions, surface roughnesses, and other microstructural properties of bulk, thin film, and multilayer materials [1]. The power of VASE® results from the ability to take data not only at a large number of wavelengths and angles, but equally importantly at the optimum conditions for sensitivity to the particular parameter or parameters of interest. The optimum conditions will depend strongly on the materials under investigation, their optical properties, thicknesses, surface roughness, etc. The advantage of spectroscopic ellipsometry as a diagnostic tool and potential instrument for controlling industrial processes is that the light beams are totally non-invasive. Even in cases where photosensitive materials are of interest, the ellipsometric light beam is extremely weak, stemming from the fact that absolute light intensities are not measured, only ratios of intensities. Another advantage of ellipsometry is the incredible sensitivity to layer thicknesses, typically as thin as one layer of atoms on a surface. For example, a film thickness of 150 nm can be known to better than half a percent. Using long wavelength light, films as thick as 50 j.tm can be examined by spectroscopic ellipsometry. The main serious drawback to ellipsometry as a *Author to whom correspondence should be addressed. tYASE® is a registered trademark (in the USA) of the J. A. Woollam Co., Inc.
0257—8972/93/56.00
process control sensor tool is that optical window ports need to be installed on process chambers at typically 70°or 75°to the normal of the material of interest. This can be done quite cost effectively by planning ahead of time, and even retrofits of existing chambers can be done quickly with minimal disruption in many cases. The purpose of this paper is to discuss the design and performance of a new low-cost ellipsometer designed especially for in-situ industrial applications [2]. In addition to the rugged compact hardware, software has been developed to convert the raw ellipsometric data into meaningful parameters of direct interest in real time [3]. Fast acquisition of raw ellipsometric data is not enough. Efficient algorithms permit fast real-time process control of layer thicknesses, alloy ratios, optical constants, and other material properties. An example of one industrial process which will benefit from this technology is semiconductor crystal growth by metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy. Another general category is in optical coatings deposition where the optical coating performance can be enhanced by having precise monolayer sensitive process control.
2. Basics of the instrument Figure 1 shows the general layout of the ellipsometer in place on a chemical vapor deposition reactor. The light source (which is not shown) consists of a simple compact xenon arc lamp and housing. In the case shown, light is brought to the input polarizer assembly via an
©
1993
—
Elsevier Sequoia. All rights reserved
B. Johs et al.
/
Multiple wavelength ellipsometry
681
exhaust
Quartz Reactor Tube \ to Xenon arc temp fused s1Uc~opt1c~t flat windows roteting analyzer
InPut Oases flit
~
I
Output Arm
~
optic tilt stage”
collimating tens
Input Arm
Fig. 1. General layout of the multiple wavelength ellipsometer in an in-situ environment. The distance from left to right extremes in the diagram is approximately 50 cm.
optical fiber. Light is then collimated and linearly polarized with an azimuthal angle under stepper motor control from the computer. This beam is then incident at an angle of 75° to the normal of the sample under investigation. After reflection, the generally elliptically polarized beam is polarization-state analyzed using a second rotating polarizer called the “analyzer”. In this instrument design, the light still contains all wavelengths which are dispersed by a grating contained in the box shown on the left. After dispersion, a diode array detector measures light intensity with up to 44 color multiplexed channels. Reflection from the grating changes (with a wavelength dependence) the polarization state of the light. This polarization altering effect is precalibrated, permitting accurate data to be acquired in all wavelength channels. A regression calibration of polarizer and analyzer azimuthal angles is initially performed [4]. Filtering and multiplexing electronics are connected to each element of the array, and signals are rapidly digitized at each wavelength as the analyzer rotates. Typical acquisition is done when averaging over 15 analyzer revolutions, acquiring data at all wavelengths in less than 1 s. Faster acquisition is possible, at the expense of signal to noise ratio, and this depends strongly on the particular material system under study. The most advanced personal computer available is used (presently an Intel 80486-based 66 MHz microprocessor) to control the measurement, and acquire and analyze data. Software runs in the Microsoft WindowsTM operating system environment, Fast and efficient data acquisition and analysis are done with sophisticated software (written in C + +)
using the exact Fresnel reflection coefficient expressions applied to multilayer thin film optical models for the material. The Levenberg—Marquardt regression algorithm is used to fit the model generated data with the experimental data. In the example given in this paper, data are analyzed in real time using a variation of the “virtual interface” approach of Aspnes et al., with a commonpseudosubstrate approximation [5]. Data acquired at two consecutive sampling times are used to determine the film growth rate and/or composition within this very short time interval. The data taken at the first sampling are converted to pseudodielectric functions, defining a “virtual interface” containing the previous history of the sample. At the next sampling time an overlayer has been grown (or deposited) and a calculation of its thickness and/or composition is made. This can be used quite well at repeated data samplings to provide growth rate, thickness, etc., in real time.
3. Example data The thickness of a CdTe layer grown by MOCVD on a GaAs substrate at a temperature of 350 °Cis shown in Fig. 2, where growth was initiated after 1 mm of ellipsometer time scan. Optical constants for the GaAs at growth temperature were determined in situ immediately before growth was initiated. Likewise the CdTe optical constants were determined in situ at the growth temperature after growth of approximately 100 nm of material.
682
B. Johs et al.
/
Multiple wavelength ellipsometry
lengths simultaneously in less than 1 s covering the 400—800 nm general spectral range. This ellipsometer can be used ex situ on a “bench top”, but was designed especially for in-situ users. It is capable of rapid realtime conversion of raw ellipsometric data into useful materials parameters, and has been demonstrated to be useful as a process control tool. Multiple wavelengths are necessary in order to solve the maximum number of uncorrected parameters with maximum accuracy. Many problems simply cannot be solved with single wavelength ellipsometry.
180
t60 140 120
-
100 80C
~
60-
1—~
40-
20 0 -20 0
1
2
3
4
5
6
7
8
Time (minutes) Fig. 2. Example data in which the growth of a CdTe layer on a GaAs substrate was controlled using an output from the computer to control the Cd carrier gas flow rate. Data in 12 of the 44 channels were used, covering 400—800 nm.
The kink in the data of Fig. 2 at 4.5 mm is due to turning on the growth control function, in which the Cd mass flow controller flow rate was controlled by an output from the computer. Thus this example shows CdTe growth at 0.28 nm s 1 being computer controlled using real-time ellipsometric data. —
4. Conclusions and discussion A low-cost, compact, rugged ellipsometer has been built and tested which acquires data at up to 44 wave-
Acknowledgments Research was supported by the Advanced Research Projects Agency (ARPA) Contract DAAHO1-92-C-R19l and US Army Contract No. DAABO7-92-C-K755. References I J. A. Woollam, P. G. Snyder, H. Yao and B. Johs, SPIE J., 1678 (1992) 246. 2 B. Johs, D. Doerr and S. Pittal. Proc. mt. Conf on Spectroscopic Ellipsometry, Paris, 1993, Thin Solid Films, 233 (1993) 293. 3 B. Johs, J. L. Edwards, K. T. Shiralagi, R. Droopad, K. Y. Choi, G. N. Maracas, D. Meyer, G. T. Cooney and J. A. Woollam, Mater. Res. Soc. Proc., 222 (1991) 75. 4 B. Johs, Proc. Int. Conf on Spectroscopic Ellipsometry, Paris, 1993, Thin Solid Films, 234 (1993) 395. 5 D. E. Aspnes, W. E. Quinn and S. Gregory, App!. Phys. Lett., 57 (1990) 2707.
Surface and Coatings Technology, 62 (1993) 680—682
Hardware and software for in-situ process monitor and control using multiple wavelength ellipsometry Blame Johs, David Doerr, Shakil Pittal and John A. Woollam* J. A. Woollam Co., 650 J Street, Suite 39, Lincoln, NE 68508 (USA)
Ishwara Bhat and S. Dakshinamurthy Rensselaer Polytechnic Institute, Troy, NY 12180 (USA)
Abstract We have built and tested a low-cost ellipsometer which acquires data in the spectral range 400—800 nm at multiple (up to 44) wavelengths simultaneously in less than 1 s. A personal computer is used to acquire data and analyze the data in real time for studies of time dependent phenomena. In addition, this permits feedback control which we have demonstrated on epitaxial growth of CdTe on GaAs by metal organic chemical vapor deposition.
1. Introduction For many years variable angle spectroscopic ellipsometry (VASE®t) has been used ex situ to determine optical constants, layer thicknesses, void fractions, alloy compositions, surface roughnesses, and other microstructural properties of bulk, thin film, and multilayer materials [1]. The power of VASE® results from the ability to take data not only at a large number of wavelengths and angles, but equally importantly at the optimum conditions for sensitivity to the particular parameter or parameters of interest. The optimum conditions will depend strongly on the materials under investigation, their optical properties, thicknesses, surface roughness, etc. The advantage of spectroscopic ellipsometry as a diagnostic tool and potential instrument for controlling industrial processes is that the light beams are totally non-invasive. Even in cases where photosensitive materials are of interest, the ellipsometric light beam is extremely weak, stemming from the fact that absolute light intensities are not measured, only ratios of intensities. Another advantage of ellipsometry is the incredible sensitivity to layer thicknesses, typically as thin as one layer of atoms on a surface. For example, a film thickness of 150 nm can be known to better than half a percent. Using long wavelength light, films as thick as 50 j.tm can be examined by spectroscopic ellipsometry. The main serious drawback to ellipsometry as a *Author to whom correspondence should be addressed. tYASE® is a registered trademark (in the USA) of the J. A. Woollam Co., Inc.
0257—8972/93/56.00
process control sensor tool is that optical window ports need to be installed on process chambers at typically 70°or 75°to the normal of the material of interest. This can be done quite cost effectively by planning ahead of time, and even retrofits of existing chambers can be done quickly with minimal disruption in many cases. The purpose of this paper is to discuss the design and performance of a new low-cost ellipsometer designed especially for in-situ industrial applications [2]. In addition to the rugged compact hardware, software has been developed to convert the raw ellipsometric data into meaningful parameters of direct interest in real time [3]. Fast acquisition of raw ellipsometric data is not enough. Efficient algorithms permit fast real-time process control of layer thicknesses, alloy ratios, optical constants, and other material properties. An example of one industrial process which will benefit from this technology is semiconductor crystal growth by metal organic chemical vapor deposition (MOCVD) or molecular beam epitaxy. Another general category is in optical coatings deposition where the optical coating performance can be enhanced by having precise monolayer sensitive process control.
2. Basics of the instrument Figure 1 shows the general layout of the ellipsometer in place on a chemical vapor deposition reactor. The light source (which is not shown) consists of a simple compact xenon arc lamp and housing. In the case shown, light is brought to the input polarizer assembly via an
©
1993
—
Elsevier Sequoia. All rights reserved
B. Johs et al.
/
Multiple wavelength ellipsometry
681
exhaust
Quartz Reactor Tube \ to Xenon arc temp fused s1Uc~opt1c~t flat windows roteting analyzer
InPut Oases flit
~
I
Output Arm
~
optic tilt stage”
collimating tens
Input Arm
Fig. 1. General layout of the multiple wavelength ellipsometer in an in-situ environment. The distance from left to right extremes in the diagram is approximately 50 cm.
optical fiber. Light is then collimated and linearly polarized with an azimuthal angle under stepper motor control from the computer. This beam is then incident at an angle of 75° to the normal of the sample under investigation. After reflection, the generally elliptically polarized beam is polarization-state analyzed using a second rotating polarizer called the “analyzer”. In this instrument design, the light still contains all wavelengths which are dispersed by a grating contained in the box shown on the left. After dispersion, a diode array detector measures light intensity with up to 44 color multiplexed channels. Reflection from the grating changes (with a wavelength dependence) the polarization state of the light. This polarization altering effect is precalibrated, permitting accurate data to be acquired in all wavelength channels. A regression calibration of polarizer and analyzer azimuthal angles is initially performed [4]. Filtering and multiplexing electronics are connected to each element of the array, and signals are rapidly digitized at each wavelength as the analyzer rotates. Typical acquisition is done when averaging over 15 analyzer revolutions, acquiring data at all wavelengths in less than 1 s. Faster acquisition is possible, at the expense of signal to noise ratio, and this depends strongly on the particular material system under study. The most advanced personal computer available is used (presently an Intel 80486-based 66 MHz microprocessor) to control the measurement, and acquire and analyze data. Software runs in the Microsoft WindowsTM operating system environment, Fast and efficient data acquisition and analysis are done with sophisticated software (written in C + +)
using the exact Fresnel reflection coefficient expressions applied to multilayer thin film optical models for the material. The Levenberg—Marquardt regression algorithm is used to fit the model generated data with the experimental data. In the example given in this paper, data are analyzed in real time using a variation of the “virtual interface” approach of Aspnes et al., with a commonpseudosubstrate approximation [5]. Data acquired at two consecutive sampling times are used to determine the film growth rate and/or composition within this very short time interval. The data taken at the first sampling are converted to pseudodielectric functions, defining a “virtual interface” containing the previous history of the sample. At the next sampling time an overlayer has been grown (or deposited) and a calculation of its thickness and/or composition is made. This can be used quite well at repeated data samplings to provide growth rate, thickness, etc., in real time.
3. Example data The thickness of a CdTe layer grown by MOCVD on a GaAs substrate at a temperature of 350 °Cis shown in Fig. 2, where growth was initiated after 1 mm of ellipsometer time scan. Optical constants for the GaAs at growth temperature were determined in situ immediately before growth was initiated. Likewise the CdTe optical constants were determined in situ at the growth temperature after growth of approximately 100 nm of material.
682
B. Johs et al.
/
Multiple wavelength ellipsometry
lengths simultaneously in less than 1 s covering the 400—800 nm general spectral range. This ellipsometer can be used ex situ on a “bench top”, but was designed especially for in-situ users. It is capable of rapid realtime conversion of raw ellipsometric data into useful materials parameters, and has been demonstrated to be useful as a process control tool. Multiple wavelengths are necessary in order to solve the maximum number of uncorrected parameters with maximum accuracy. Many problems simply cannot be solved with single wavelength ellipsometry.
180
t60 140 120
-
100 80C
~
60-
1—~
40-
20 0 -20 0
1
2
3
4
5
6
7
8
Time (minutes) Fig. 2. Example data in which the growth of a CdTe layer on a GaAs substrate was controlled using an output from the computer to control the Cd carrier gas flow rate. Data in 12 of the 44 channels were used, covering 400—800 nm.
The kink in the data of Fig. 2 at 4.5 mm is due to turning on the growth control function, in which the Cd mass flow controller flow rate was controlled by an output from the computer. Thus this example shows CdTe growth at 0.28 nm s 1 being computer controlled using real-time ellipsometric data. —
4. Conclusions and discussion A low-cost, compact, rugged ellipsometer has been built and tested which acquires data at up to 44 wave-
Acknowledgments Research was supported by the Advanced Research Projects Agency (ARPA) Contract DAAHO1-92-C-R19l and US Army Contract No. DAABO7-92-C-K755. References I J. A. Woollam, P. G. Snyder, H. Yao and B. Johs, SPIE J., 1678 (1992) 246. 2 B. Johs, D. Doerr and S. Pittal. Proc. mt. Conf on Spectroscopic Ellipsometry, Paris, 1993, Thin Solid Films, 233 (1993) 293. 3 B. Johs, J. L. Edwards, K. T. Shiralagi, R. Droopad, K. Y. Choi, G. N. Maracas, D. Meyer, G. T. Cooney and J. A. Woollam, Mater. Res. Soc. Proc., 222 (1991) 75. 4 B. Johs, Proc. Int. Conf on Spectroscopic Ellipsometry, Paris, 1993, Thin Solid Films, 234 (1993) 395. 5 D. E. Aspnes, W. E. Quinn and S. Gregory, App!. Phys. Lett., 57 (1990) 2707.