In situ spectroscopic ellipsometry: present status and future needs for thin film characterisation and process control

MATERIALS SCIENCE & ERWlNEERIWG ELSEVIER

B

-

Materials Scienceand EngineeringB37 (1996) 116-120

In situ spectroscopic ellipsometry: present status and future needs for thin film characterisation and process control Pierre Boher, Jean Louis Stehle SOPRA

S.A.,

26 rue Pierre

Joigneazrx,

92270

Bois-Colombes,

France

Abstract Principle and limitations of real time spectroscopic ellipsometry are presented. The technical characteristics of the SOPRA ES4G OMA system are summarised and illustrated by practical examples. With 1024 pixels in a spectral range of 260-1060 nm, a reproducibility better than 10- 3 and the possibility to record 40 spectra per second in groups of eight pixels, this instrument can fulfil a great part of the present in situ characterisation requirements. The next generation, which will be more dedicated to process control, will also be presented and discussed in terms of precision, speed and accuracy. The major importance of spectral range and resolution will be demonstrated in practical cases. Keywords:

Spectroscopic ellipsometry; Thin films; Real time; Film deposition

tion of the technique in terms of application examined.

1. Introduction

will be

The layers and thin films used in solid state physics, in microelectronics and optics become thinner as the

nanotechnologies are developed. The main deposition processes used nowadays, such molecular beam epitaxy, CVD or sputtering techniques, have reached a high degree of operation and now have the capability to deposit very complicated multilayer structures on the nanometre

scale. To

go further,

one critical

point

would be how to be able to control the deposition processes in real time with an accuracy better than a monatomic layer. Optical techniques have been extensively used for real time nondestructive characterisation of thin layer deposition particularly in UV, visible and near infrared for optical coatings where the reflectivity and the transmittance could be measured and optimised in real time. The optical technique is also used as an end point detector

to stop the deposition,

or etching,

when

a

pre-set value has been reached. In this paper, the different optical techniques will be presented and compared in term of precision, sensitivity and speed. The advantages of real time spectroscopic ellipsometry (RTSE) will be presented and the limita0921-5107/96/$15.000 1996- Elsevier ScienceS.A. All rights reserved S.TlIl

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2. The different in situ optical characterisations Measurement during deposition and growth (or etching) has the advantage of knowing in real time the different steps of the process; this information is lost when the sample is only measured a posteriori. A second advantage is that one can get the information before, during and after the deposition, and for data reduction, the time and deposition rate can be helpful. Finally, one can use the real time information to optimise, correct and stop the process when it is still possible. To achieve this level of real time process control, it is necessary to know the refractive indices of the materials in the conditions of deposition. In particular, the possible effects of thickness and temperature must be taken into account. An interlayer or a gradient must also be detected. All this information can be extracted and correlated with other nonoptical investigations, including electrical or destructive techniques, made a posteriori at room temperature. This procedure is called

P. Bohr, J.L. St&e / Materials Science arId Engineering B37 (1996) 116-120

monitoring and is a necessary step to achieving real time process control which is the ultimate goal at the atomic scale. Spatially, the optical characterisation is currently limited, in situ, to areas from 100 pm to a few millimetres in diameter. 2.1. The difSerent techniques Rejectometry. Made

at one wavelength using a laser, this gives only reflectance or transmittance vs. a reference beam and depends on the stability of the source. The intensity can be affected by the transmittance of the windows if some material is deposited on them during the process. Only one parameter can be measured per wavelength. This technique is widely used for optical coatings because the final quality of the sample is the transmission or the reflection. Another possibility using white light, is to determine the spectral position of the peaks for a filter for example. Nevertheless, when the sample is exposed to the air, the absorption of water can change the filter characteristics and alter its performance. This effect can be quantified and taken into account, but today, with implanted assisted deposition (IAD), the layers can be packed and will not change during air exposure. This was possible only because comparisons have been made in vacuum and under air. This simple example shows that real time in situ measurement can be useful. Interferometry. This technique

gives the same kind of information as above, but only the optical thickness at the HeNe laser line (N x T where N is the optical index and T the thickness) is measured and not only the intensity as with reflectometry. It is used for end point detection particularly in the case of etching. The time difference between the maxima of interferences gives the etching rate. So a close observation of the interferogram and knowing the initial layer thickness can be useful to find the end of etching. The two main disadvantages are: first, we need to have a non-negligible optical contrast between the layer under etch and the substrate, and second, that the etching process must be stable.

117

All the above detection techniques are sensitive to the intensity, so the transmittance of the window, the stability of the source, of the detector and the interfering stray light are critical. In addition a reference beam or a reference sample is needed. Single wavelength ellipsometry (SWE). Generally, this is made using a HeNe laser through two nonbirefregent windows. The signal is less sensitive to the intensity because it makes the ratio of amplitude continuously. This technique does not require reference beam or reference sample. It is sensitive to very thin layers (below 0.03 nm) as well as thick ones. There is no problem of order in the periodicity as one knows the starting thickness. The well-known shadow zone, (when the optical thickness is an antireflective coating), gives a signal which is the same as without a layer, so the extraction of data is difficult or impossible at this stage. Vasiable angle single wavelength ellipsometry (VAS WE).

To get additional information, one solution is, to make the same kind of measurement at different incident angles. Of course for in situ measurement this possibility is quite difficult to apply. In addition, in numerous cases when different physical parameters are changing at the same time, the angle variation is not very useful because the optical index contrast does not change. Multidiscrete wavelength ellipsometry (MD WE). A com-

promise to get additional useful information without going to real spectroscopic techniques can be to use different discrete wavelengths (two or three for example using lasers) in real time during growth. Even if this technique is obviously more interesting than SWE, in numerous cases, especially with semiconductor compounds, precise information on layer compositions cannot be obtained. Indeed, fine optical structure cannot be detected with only two or three wavelengths, even if they have been precisely selected. 2.2. The real time spectroscopic ellipsometry (RTSE) technique

RTSE has the advantages of the SWE without the limitations.

Pyrometry. This technique is based on the detection of

the emission of the sample in the near or far IR. It works only if the sample is heated during the process. Another point is that the sample rear face also contributes to the detected signal. The layers under growth are changing the transmittance through the sample and so the spectral response changes with the optical thickness of the layer. In the near and mid IR most of the layers are transparent and then this technique can be applied in a great number of cases, except for metallic materials.

2.2.1. Description of the instrument

The use of optical multichannel array (OMA) was first reported in 1987 [l]. The optical layout of the SOPRA RTSE system is shown in Fig. 1. The spectroscopic ellipsometer is based on the principle of a rotating polariser linked to an encoder [2]. The source is a short arc Xe lamp. The light is conducted in the polariser arm with a fibber optics. It is collimated to produce an image on the sample of diameter 5 mm and a divergence of 1 mrad. The rotating speed of the polar-

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P. Boher, J.L. Stehle / Materials Science and Engineering B37 (1996) 116-120

Monochromator

Fig. 1. Optical diagram of the RTSE mounted on a vacuum chamber.

iser can be programmed from 6 to 50 turns per second. The reflected beam passes through a fixed analyser and is transported to the entrance slit of a spectrograph via an optical fibber. The spectral range is limited by the lamp emission and the transmission of the optics, fibres, polarisers and by the sensitivity and spectral response of the detector. Generally, we use a Si diode array which can cover UV (260 nm) to near infrared (1060 nm). More detail on the instrument can be found elsewhere [3,4]. During the polariser rotation, the 1024 Si detectors are scanned in a maximum of 8 ms when grouped by four. A Hadamard transform of the signal is performed. During the reading of one pixel the other pixels integrate the light, thus the dead time is only 200 ns for 8 ms. Each pixel is integrated with 14 bits of dynamic range. The accuracy of the measurement is around lo- 3 on tanY and cosA parameters. For a good stability, maximum speed has been kept at 40 spectra of 128 channels per second. The repeatability is limited by the fluctuation of the Xe lamp, the electrical reading of the channels and the stability of the rotation. Nevertheless, it is very good and less than IOh in most cases. More difficult to read is the cross talk ratio due to scattering which appears on each pixel. For this reason, precise absolute measurements of dielectric constants are made with the scanning ES4G ellipsometer. 2.2.2. Description of timing diagram for the measurement and the data reduction A typical time diagram of the RTSE system is represented in Fig. 2. One measurement on 512 pixels integrated on one turn of the polariser and the

corresponding data transfer is performed in 200 ms. Then the regression on this set of data takes place during the next 200 ms in parallel with the next measurement step. So, the mean delay between one measurement and obtaining the results is about 300 ms. These results are available graphically on the RTSE computer and to the host computer via the RS232C port and SECSII protocol. 2.2.3. Adaptation of the speed of mecw~wm~tt to the process If the deposition rate is slow, the time of measurement can be long without losing any information. There is more time for averaging and increasing signal to noise ratio (SNR) and possibly the accuracy, thus the delay between the measurement and the results is not significant for the thickness of the layer. On the contrary, when the deposition rate is very fast, less time is available for the measurement and for the data processing; the SNR is reduced as well as the time for data reduction; the accuracy is decreased, and any delay will produce an error on the thickness. So when a fast deposition is needed, a faster and more sensitive value is required and it becomes more difficult to get them together at the same time; this is a paradox. Fig. 3 shows the sensitivity of RTSE for different deposition rates.

3. Applications for in situ processcontrol Measurements of thickness within 0.1 A of deviation, were obtained on an Sb deposition by MBE [5]. This thickness corresponds to a coverage factor of l/30.

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P. Boher, J.L. Stehle / Materials Science and Engineering B37 (1996) 116-120

Time diagram of RTSE system

Measurement 1 Data transfer 1

Regression 1 Communication

Measurement 2 Data transfer. 2

Jitter 30ms

4OOms

200ms

0

Time Fig. 2. Principle of the time diagram of the RTSE system.

Different deposition rates were measured, corresponding to the temperatures of the Knudsen source. The sample was also heated and the desorption rate could be measured and related to the substrate temperature. In ultra high vacuum the windows must not only be nonbirefregent but also bakeable to allow good quality base pressure. This has been achieved by using PVCF O-rings and free-silica windows to avoid stress. A vacuum better than IO- lo is achieved, since the windows can be heated to 200 “C, and eventually removed for cleaning. The MBE process enables stopping and scanning a full spectrum in 10 min. This is not possible for faster deposition rates ranging from 0.1 to 30 8, s- ‘. In this case one can only use a fast ellipsometer, with a single wavelength chosen at a sensitive position or better M.B.E.

using a simu1taneot.s multichannel spectroscopic ellipsometer. In this case there is a limitation to about 0.5 pm thick layer, because of the transmission of the layer, the number of pixels and the resolution of the spectrograph. Another example is the growth of Sic, -x,Ge, on Si by LPCVD(6). As shown in Fig. 4, dielectric functions of a 580 A Si,,,,Ge,,,, layer deposited on silicon at 610 “C have been measured with very good accuracy all through the deposition process. The integration time

sputtering

C.V.D.

--,180 ----,300

0.1

Energy

I Deposition

rate (ks)

Fig. 3. Comparison of measurement time vs. deposition rate.

set set

(eV)

Fig. 4. Real part of the optical index measured in situ during the growth of a Si,,,,Ge,,,, layer on silicon at 610 “C. From Ref. [6].

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P. Boher, J.L. Stehle / Materials Science and Engineering B37 (1996) 116-120

was fixed to 2 s. During the deposition the peak in the (n) spectrum moves from the Si substrate position (3.05 eV) towards the bulk Si,,,,Ge,,,, position (2.85 ev). In fact, more precisely for thicknesses above 200 A, the peak moves between Si and alloy positions before approaching the bulk alloy spectrum. This behaviour is consistent with growth of a uniform alloy layer, as shown in Ref. [6].

4. Future developments The actual use of RTSE is still for research and monitoring. This technique requires a relatively heavy investment in equipment and development. The results are very sensitive. The investigation is well adapted to the understanding of deposition conditions. This is a powerful tool for optimisation which, with the present simple control, can supply the user with efficient information; the RTSE can be removed from the chamber and used elsewhere to develop another process, but very rarely is it used ,for real time process control. This will change when the new application and the new equipment will prove their worth in improving the quality and the throughput of difficult samples. In the future it will be possible to improve on some points. The data reduction must take account of all the spectral range without delay because the speed of computer and the algorithm will be optimised. It must be possible to use a refined algorithm with neural networks, and artificial intelligence is required to avoid the need of a specialist to analyse data. Other real time information can be added and mixed with the SE data, e.g. spectral reflectivity, laser stray light, a quartz signal for thickness, temperature, angles of incidence and deflection. This information will be computed in real time to separate the information, to correlate it; to enhance the precision and the confidence of the results. Other applications can be addressed which are not required to be under vacuum, and if the price drops

with quantities and amortisation of development, the RTSE will be found in the next generation of deposition equipment.

5. Conclusion We have presented principles and limitations of RTSE compared with other nondestructive methods. In fact, up to now ellipsometry appears to be the technique of choice to measure in real time precise structural information on complex multilayer stacks under growth. In terms of measurement accuracy and speed, the present experimental systems, such as SOPRA ES4G OMA, have very good characteristics which can fulfil a great part of the experimental requirements. To make a real process control, real time analysis of all the measured information is also needed. In fact, with the great development of fast computers, this problem is on the way to being solved. RTSE will certainly be integrated in the next generation of deposition equipment.

Acknowledgements Dr. C. Pickering (DRA, UK) and Dr. C. Schneider are kindly acknowledged for useful discussion during the development of the RTSE.

References [l] M. Stehle,J.L. Stehleand 0. Thomas,Proc. NATO Sump., Cargese, May, 1987.

[2] D. Aspnes,J. Opt.Sot. A/n., 63(1974)5. [3] F. Ferrieu,J.L. Stehle,F. Bernouxand0. Thomas,Mater. Aes. Sot. Symp. Proc., Vol. 101,MaterialsResearch Society,Pittsburgh,PA, 1988. [4] J.P. Piel, J.L. Stehleand 0. Thomas,Thirl Solid Filmq 233 (1993)301. [5] S.Andrieu,F. FerrieuandF. Arnaudd’Avitaya,Appl. Phys. A, 49 (1989)719. [6] C. Pickering,J. Met., in presQ1995)