In-situ observation of chemical vapour deposition growth of epitaxial SiGe thin films by reflexion supported pyrometric interferometry

J. . . . . . . .

ELSEVIER

CRYSTAL GROWTH

Journal of Crystal Growth 146 (1995) 119-124

In-situ observation of chemical vapour deposition growth of epitaxial SiGe thin films by reflexion supported pyrometric interferometry G . R i t t e r a,,, B. T i l l a c k a, M . W e i d n e r a, p. Z a u m s e i l H . M611er b

a, F . G . B 6 b e l b, B. H e r t e l b,

a Institut fiir Halbleiterphysik Frankfurt (Oder), Walter Korsing Strasse 2, D-15230 Frankfurt (Oder), Germany b . . . Fraunhofer lnstttut fiir. Integnerte Schaltungen IIS-A, Am Weichselgarten 3, D-91058 Erlangen, Germany

Abstract

Reflexion supported pyrometric interferometry (PYRITTE) has been used for the in-situ observation of Si(l_x)Gex heteroepitaxial growth on Si-wafers in a rapid thermal chemical vapour deposition (RTCVD)-reactor at 500°C. The thickness and the optical parameters of thin films at 500°C have been evaluated by real time computer fitting of the measured reflectivity data at the wavelength A = 650 nm. These parameters have been compared with those obtained ex-situ at room temperature by ellipsometry in the wavelength range 240-700 nm. The thickness of growing Si(l_x)Gex film depends on time linearly. The temperature coefficient of the real part of the refractive index has been found as 3 x 10 -4 K -1.

I. Introduction In-situ observation of growing films is a powerful method to control the deposition and to obtain information concerning the kinetics of the process. For this purpose different ellipsometric methods [1,6-9] and several other techniques, e.g. thermal wave techniques [2] have been used. The temperature is commonly measured by pyrometers. The pyrometric interferometry ( P Y R I T T E ) is an alternative tool for simultaneous temperature measurement and in-situ film analysis, recently

* Corresponding author.

developed for molecular beam epitaxy of heterostructures as G a A I A s / G a A s [3,4,10]. The new reflexion supported pyrometric interferometry (PYRITTE-RS) [10] has been used in this work for the first time for in-situ observation of heteroepitaxial growth processes of strained Si(]_x) Gex thin films on Si-wafers in a rapid thermal chemical vapour deposition (RTCVD)-reactor. Si(]_x)Ge~ films deposited at 500°C and in-situ analyzed by the P Y R I T T E have been characterized ex-situ at room temperature by X-ray diffractometry (Ge-content x, film thickness d) and ellipsometry (complex refractive index n, film thickness d, and Ge-content x). Based on the in-situ evaluated data for the optical constants and the film thickness the temperature depen-

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G. Ritter et al. /Journal of Crystal Growth 146 (1995) 119-124

120

dence of the refractive index and the growth kinetics of the Si(l_x)Ge~-films were available.

2. Experimental procedure 2.1. Epitaxial growth The strained Si(l_x)Gex-epitaxial layers recently incorporated in Si based structures are very important for high speed electronic devices and have promising potential for new optoelectronic devices. The Si(l_x)Ge x thin films have been deposited in a rapid thermal chemical vapour (RTCVD)single wafer reactor at the temperature of 500°C and the pressure of 200 Pa from the gas mixture of H 2, Sill4, and G e H 4. The reactor consists of water cooled stainless steel walls and two air cooled quartz windows. The deposition process has been controlled by the wafer temperature which could be varied abruptly by the radiation heat source (W-halogen lamps). Due to different G e H J S i H 4 flow ratios at the same total gas flow (2 slm) Si(t_x)Gex-films with different Ge-content x and different growth rate y have been deposited on 4 inch Si-wafers. The growth rates increase with increasing Gecontent [5]. The Ge-content x, the thickness d, the growth rates y, and the refractive index nf of thin Si(l_x)Gex-films obtained by different ex-situ methods are compared in Table 1 with those parameters evaluated in-situ by PYRITFE.

2.2. PYRITTE in-situ data evaluation The experimental configuration of PYRITTE and the basic physical theory of the method has been described in detail previously [3,10]. Light from an electronically modulated light emitting diode (LED) at 650 nm wavelength has been directed normally to the sample through a high vacuum proof quartz window, and the reflected signal has been measured by sensitive Si-detectors, and processed via lock-in amplifiers. The reflected intensity of the heated wafer depends on the temperature T, the thickness d, and the (temperature dependent) complex refractive index of both the growing film (nf + ikf) and the substrate (n s + iks). In our experiment the temperature was 500°C during the deposition process and has been measured by a pyrometer at 5 ~m wavelength. Because of interference effects of the multiply reflected light beam at the film surface and the interface between substrate and film the reflectance oscillates with growing film thickness with a period of A/2nf. The PYRITTE software is able to extract temperature, layer thickness, material composition, and optical parameters, respectively from the data. Two strategies are used to evaluate the data: - A one parameter search, to adapt the thickness of the film. - A multiple parameter fit, to adapt film thickness and optical parameters. The one parameter search is used in the first region of the growing layer, where thickness is typically smaller than 25 nm. In this region the

Table 1 Experimental data concerning thickness d, growth rate y, Ge-content x, and refractive index n of three Si(l_x)Gex-samples Nrs. 3, 4, and 5, obtained both in-situ by PYRITTE and ex-situ by ellipsometric measurements, X-ray diffractometry and Raman spectroscopy Sample Nr. 4

PYRITTE Spectr. ellipsometer Single wavel, ellipsom. X-ray diffraction

3

d (nm)

y (nm/min)

x (%)

58.8 56.2 61.4 58.1

1.18 1.12 1.23 1.16

4.081 5 1 . 1 < 17.0 3.919 47.2 51.2 16 48

a Measured by Raman spectroscopy.

n

d (nm)

5 3' (nm/min) 3.41 3.15 3.41 3.2

x (%) 23 23.4

n

d (nm)

y (nm/min)

4.107 113.4 13.61 3 . 9 6 3 118 14.16 120.7 14.48 120 14.4

x (%)

n

> 35 (37)

a

4.285 4.057

121

G. Ritter et aL /Journal of Crystal Growth 146 (1995) 119-124

multiple p a r a m e t e r fit will not work, because there is not enough information for the algorithm. The one p a r a m e t e r search looks for roots with respect to thickness d in an equation like

ters from a system of equations. These equations are of the form

Rn(d , F/s, ks, nf, k f ) = %(t),

t=

[T', T],

(1)

(2)

where rn(t) is the measured reflected signal at time t, and Rn(d) is the calculated reflectance function from Ref. [3], depending on layer thickness d. The index n indicates the fact that normalized values are used. The reflectance function depends further on the optical parameters of substrate and layer, respectively, i.e. refractive index of substrate n s and layer nf, and absorption index of substrate k S and layer kf. These optical parameters can be extracted from calibration measurements or have be provided by an external database. With this algorithm arbitrary functional dependencies of growth rate can be detected. The multiple p a r a m e t e r fit determines layer thickness and three out of four optical parame-

where the start of growth is assumed to be at t = T', and data are measured up to t = T. Additionally each parameter, which is determined, is supplied with a quantitative information about the quality of the fit, i.e. a standard deviation. The multiple p a r a m e t e r fit is based on a conventional numerical algorithm that was optimized for high speed applications, which led to real time capacity, so that thickness is updated every second and optical parameters at least every 5 seconds. When the optical parameters shall be determined a certain functional dependency of layer growth (commonly constant growth rate y) has to be assumed.

Rn(d ) -rn(/)

=0,

8

7---

Si L

6-

-

u

a

/

_

\

\

c 5-

(~i

_

\\\\

4-

• +~-

•--

3-

x i1) c

. . . .

•

i

. . . .

i

. . . .

i

. . . .

i

. . . .

.

Sll .xae ~ ~ 3 ~. 3 \ _ ~5 ~ 2~ ~ ' ~ x _ '~ F ~ ~'~J / E . .,~d ~ c ~//~a //Si .E_ 1~.~,,~,~_ j/ 2.5

3

~ ~

.

3.5 4 e n e r g y in e V

spectra: a:x=0.17 b:x=O.225 0:x=0.35 d:x=0.53 4.5

5

Fig. 1. Refractive index n and absorption index k of Sid_x)oe x in dependence on energy for different values of x (0.17, 0.225, strained [7], 0.35, and 0.53, relaxed [6], respectively) and experimental values obtained by spectroscopic ellipsometer at room temperature for three samples 3, 4, 5 (see Table 1).

122

G. Ritter et al. /Journal of Crystal Growth 146 (1995) 119-124

If only thickness shall be determined the one parameter search may be used during the whole deposition process, detecting also short term fluctuations of growth rate. In this experiment we used the one parameter search only in a region below 25 nm. Above the multiple parameter fit was used, with a linear dependency of layer thickness on time. The fitted parameters were thickness d, refractive index of layer nf, absorption index of the layer kf and the substrate k s. 2.3. Ex-situ ellipsometry

Both single-wavelength ellipsometer (PLASMOS SD 2302) and spectroscopic ellipsometer ( S O P R A ES4G) were used for ex-situ analysis of the heteroepitaxial Si(l_x)Gex layers immediately after extracting the wafers from the high vacuum RTCVD-reaction chamber. The spectroscopic ellipsometer consists of a rotating polarizer system with tracking analyzer. The angle of incidence was 75 °, near the Brewster angle of Si. The data acquisition of tan qr (amplitude ratio) and cos A (phase difference) was made as a function of wavelength in the h range 240-700 nm. A regression analysis of these spectra using several reference dielectric functions for Si, SiO 2, and Si(l_x)Ge x from the literature [6,7] provides the thickness of native SiO2 and the thickness of SiGe layer due to minimizing the least squares difference between the measured and calculated spectra. The fitting procedure for the SiO 2 thickness should be in the A-range 240-350 nm, in which the influence of the thin overlayers is very sensitive, while the influence of the thickness of Si(l_x)Ge, layer significantly occurs in the region with low absorption (h > 400 nm). The penetration depth in Si is there higher than 100 nm. The roughness of surface and interface were neglected. The experimental spectra were compared with different Si(l_x)Gex reference spectra with similar x values [6,7]. Optimal agreement has been reached for x = 0.17, and x = 0.225 ([7]), but little worse for x = 0.35 ([6]), respectively. The complex refractive index N = n + ik of the Si(l_x)Gex

layer has been calculated as a function of energy using a multilayer model with fitted layer thicknesses. The results are demonstrated in Fig. 1.

3. Results

The results of different ex-situ and in-situ measurements are shown in Table 1 for three different samples (Nrs. 3, 4, 5). The first two samples (x = 0.16, and 0.23, respectively) were strained, but the third one (x = 0.37) was relaxed to a degree of 20% due to the high values of x and d (evaluated by Raman spectroscopy and X-ray diffraction). A typical P Y R I T T E measurement curve of reflected h = 650 nm intensity during the Si(l_x)Ge x deposition of the sample Nr. 3 (x = 0.23) and the in-situ calculated layer thickness are demonstrated in Fig. 2. The change over from one parameter fit to multiple parameter search is visible at the thickness of 25 nm. The thickness dependence on the time is linear during the whole deposition process and the growth rate reaches 3.4 n m / m i n . The Si(l_x)Ge x film thickness and growth rate evaluated in-situ by P Y R I T T E agree sufficiently with those ex-situ obtained by the other methods.

I r e f l e c t e d au× [pR]

Thlckness [nm]

910 9OO

/ * ~ ~ /

88089/0 870 850

. i

.

. . time-10.3[s] 1.2

50 "~0 30 2O 10 0

i,~"

Fig. 2. Experimental data (intensity of reflected light with = 650 nm measured with PYRITI'E) and in-situ fitted thickness of the Si0.77Ge0.23-1ayerat 500°C of sample Nr. 3 obtained by one- and multiple-parameterfit.

G. Ritter et al. /Journal of Crystal Growth 146 (1995) 119-124

123

4.5 4.4e4.3q} .ID

.~-

4.2-

o 4.1

4

3.9 3.8

0

10

20

30 6o-¢ontont

40

50

60

x [%]

Fig. 3. Refractive index at A = 650 nm of Si(l_x)Gex in dependence on the Ge-content x at room temperature (filled squares) obtained by spectroscopic ellipsometry and evaluated by PYRITTE at 500°C(filled circles) in comparisonwith the data from Refs. [6,7] (black curve) and approximated curve (dotted) due to Ref. [8]. The best fit of P Y R I q T E data has been reached for the refractive index n S = 4.0, and the absorption index k s = 0.05 for the Si-substrate at 500°C. The evaluated data of layer absorption index kf is less reliable (standard deviation in the same order of magnitude as the value itself). Therefore it does not appear in the further comparison. But the data of film refractive index nf obtained by P Y R I T I ' E at A = 650 nm and T = 500°C is very precise, because the standard deviation was more than 3 orders of magnitude smaller than the value. The corresponding nf data obtained at room temperature by spectroscopic ellipsometry at the A = 650 nm are shown in Fig. 3 in comparison with the reference curve obtained from literature data for relaxed [6] and strained [7] Si d_x)Gex layers, respectively. In the same figure the approximated curve of tie at 500°C has been shown (due to Ref. [8] with a temperature coefficient obtained for Si of 3 x 10 -4 K - z and applied for Si d_x)Gex, too).

4. D i s c u s s i o n

The refractive index at A = 650 nm of Si(l_x) Ge x measured at room temperature and those

obtained in-situ by P Y R I T T E at 500°C agree sufficiently with the curves in Fig. 3. With exception of the value measured at room temperature in the relaxed sample with high Ge-content the differences are smaller than 1%. That means, that both the in-situ evaluated data at 500°C by the new P Y R I T I ' E method for A = 650 nm are suitable and that the temperature coefficient of 3 x 10 -4 K - l obtained in Ref. [8] for Si can be applied for Si{l_x)Gex, too. The relatively high difference of refractive index of the sample 5 measured at room temperature by spectroscopic ellipsometry may be due to the poor comparison with reference data for this sample.

5. C o n c l u s i o n s

The new method of reflexion supported pyrometric interferometry ( P Y R I T T E - R S ) has been used in this work for the first time as a powerful and successful tool for the in-situ observation of R T C V D Si d_x)Gex-epitaxial processes. The growth of hetero-epitaxial layers at 500°C could be in situ analyzed through the whole process. The film thickness depends on the time linearly.

124

G. Ritter et al. /Journal of Crystal Growth 146 (1995) 119-124

The refractive index of films have been evaluated at deposition temperature by real time computer fitting of the data. The temperature coefficient at A = 650 nm of 3 x 10 -4 K - l obtained by Ref. [8] for silicon can be applied to Si(l_x)Gex up to 500°C, too. Acknowledgement The autors wish to thank B. Dietrich for Raman spectroscopic measurements of the Ge-content and the degree of relaxation. References [1] C. Pickering, R.T. Carline, D.J. Robbins, W.Y. Leong, D.E. Gray and R. Greef, Thin Solid Films 223 (1993) 126.

[2] P.M. Patel, D.P. Almond and H. Reiter, Appl. Phys. B 43 (1987) 9. [3] F.G. B/Abel and H. M/Aller, IEEE Trans. Semicond. Manufacturing 6 (1993) 2. [4] H. Grothe and F.G. B6bel, J. Crystal Growth 127 (1993) 1010. [5] D. Dutartre, P. Warren, I. Berbezier and P. Perret, Thin Solid Films 222 (1992) 52. [6] G.E. Jellison Jr., T.E. Haynes and H.H. Burke, Opt. Mater., to be published. [7] C. Picketing and R.T. Carline, J. Appl. Phys. 75 (1994) 4642. [8] G. Vuye, S. Fisson, V. Nguyen Van, Y. Wang, J. Rivory and F. Abele~, Thin Solid Films 233 (1993) 166. [9] M. Weidner, P. Zaumseil and M. Eichler, Phys. Status Solidi (a) 136 (1993) 131. [10] F.G. B/Abel, H. M/Aller, A. Wowchak, B. Hertel, J. Van Hove, L.A. Chow and P.P Chow, J. Vac. Sci. Technol. B 12 (1994) 1207.