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Materials Science in Semiconductor Processing 8 (2005) 267–271

Spectroscopic techniques for characterization of high-mobilitystrained-Si CMOS

Jens Schmidta,�, Gunther Voggb, Frank Benschb, Stephan Kreuzerb,Peter Rammb, Stefan Zollnerc, Ran Liuc, Peter Wennekersa

aFreescale Halbleiter GmbH, TSO-EMEA, Am Borsigturm 130, D-13507 Berlin, GermanybFraunhofer IZM, HansastraX e 27d, D-80686 Munchen, Germany

cFreescale Inc., 2100 E. Elliot Road, Tempe, AZ 85284, USA

Available online 21 October 2004

Abstract

The application of Raman spectroscopy and spectroscopic ellipsometry (SE) for characterization of strained silicon

layers on SiGe virtual substrates is demonstrated. X-ray diffraction measurements (XRD) for calibration of Raman

results have been carried out on strained Si/SiGe structures. For the composition-dependent shift of the Si–Si vibration

in SiGe the relation oSi2Si ¼ 520:6� 68xGe is found, the strain shift coefficient for the longitudinal optical phonon in Si

is estimated as �750 cm�1. Three different samples with strained-Si layers on step-graded SiGe profiles with nominal

final Ge concentrations in the range from 10% to 24% were investigated by XRD, transmission electron microscopy,

Raman spectroscopy and SE to determinate the parameters Si cap thickness, strain in the Si layer, Ge content and

relaxation of the SiGe film. A good correspondance of the results from all techniques is found.

r 2004 Elsevier Ltd. All rights reserved.

Keywords: Strained silicon; CMOS; SiGe; Raman spectroscopy; Spectroscopic ellipsometry

1. Introduction

The ongoing scaling of design rules in advanced

CMOS leads to the introduction of many new materials

requiring the monitoring of new parameters like

composition, strain, etc. in the production process.

One of these materials is SiGe. In CMOS applications of

SiGe the Si active layer is replaced with a thin Si layer

deposited epitaxially on top of a thick SiGe ‘buffer’

layer. The resulting biaxial tensile strain in the Si top

layer leads to higher carrier mobilities and therefore

offers better transport properties compared to bulk Si.

Here, the most significant parameters are: thickness of

e front matter r 2004 Elsevier Ltd. All rights reserve

ssp.2004.09.095

ing author. Tel.: +49 (0) 30 6686 1065; fax:

2065.

ess: jens.schmidt@freescale.com (J. Schmidt).

the Si cap, strain in the top Si layer and composition and

degree of relaxation of the SiGe film.

In this work, the application of spectroscopic

techniques (Raman spectroscopy, spectroscopic ellipso-

metry (SE)) for characterization of strained-Si CMOS is

demonstrated. Both techniques are non-destructive, fast

and allow uniformity measurements on a wafer scale

and are therefore particularly suited for a production

environment. The results of Raman spectroscopy and

SE are calibrated and/or verified by comparison with X-

ray diffraction and transmission electron microscopy.

2. Sample preparation

Strain-relaxed SiGe buffers were grown on Si(0 0 1)

using a commercial ASM Epsilon 2000 CVD reactor.

d.

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5640

5660

5680

5700

5720

5740

Qy*

1000

0 (r

lu)

224 reflectionCuKalpha1

SiGe grading

Strained Si cap

Si substrate

J. Schmidt et al. / Materials Science in Semiconductor Processing 8 (2005) 267–271268

Growth was performed in a reduced pressure mode with

hydrogen carrier gas. The SiGe virtual substrates are

step-graded SiGe profiles with an increase in Ge

concentration of about 10%/mm and nominal final Ge

contents of 10%, 20% and 24%. Finally, a 2mm thick

relaxed constant concentration SiGe buffer and a 15 nm

thick strained-Si cap were deposited.

Additionally, calibration samples with SiGe films of

constant concentration and different degrees of relaxa-

tion covered with thin Si caps (5–20 nm) have been

prepared.

3920 3940 3960 3980 4000 4020 4040 4060

Qx*10000 (rlu)

5600

5620

SiGe buffer

-60 -40 -20 0 20 40

Qx*10000 (rlu)

5600

5620

5640

5660

5680

5700

5720

Qy*

1000

0 (r

lu)

004 reflection

Fig. 1. High-resolution XRD 004 and 224 reciprocal space

maps.

3. Results and discussion

3.1. X-ray diffraction

X-ray diffraction (XRD) allows the direct calculation

of the Ge content and the degree of relaxation for

heteroepitaxial SiGe films. For pseudomorphic or fully

relaxed films the Ge content can be calculated from a

simple y–2y scan of the symmetric 004 reflection. For

partially relaxed films additional information from an

asymmetric peak (usually 224) is necessary in order to

separate the influence of relaxation and Ge content on

the lattice constants and thereby the peak position. For

pseudomorphic growth, the 224 reflections of the

different layers in a reciprocal space map (RSM) are

located vertically above each other, so that the SiGe

peak is found directly underneath the Si substrate

reflection. With increasing relaxation the layer peak is

shifted into direction of the line connecting the origin of

the RSM with the substrate peak (see Fig. 1). For full

relaxation, the peak is located directly on this line.

However, a deviation may occur in case of a possible tilt

between the SiGe layer and the Si substrate, which can

be accounted for by additionally recording an RSM of

the symmetric 004 reflection.

High-resolution XRD as well as X-ray reflectrometry

(XRR) was carried out using a PANalytical X’Pert

MRD diffractometer in a triple-axis configuration. For

the calculation of Ge content and degree of relaxation of

the constant concentration SiGe buffer the elastic

constants of SiGe have been interpolated linearly

between Si and Ge, whereas a parabolic approximation

has been used for the dependency of the Ge content on

the relaxed SiGe lattice constants [1–3]. For the sample

in Fig. 1, from the positions of the 004 and 224

reflections in the corresponding RSMs, the Ge content

of the SiGe buffer has been determined to be 24.4% with

a degree of relaxation of 97.1%.

The thin strained Si layer can also be observed in the

RSMs (Fig. 1). In the 224 RSM, the respective

diffraction pattern is located directly above the SiGe

peak, i.e. the Si is pseudomorphic (fully strained) with

respect to the SiGe buffer with a common in-plane

lattice constant of 5.4799 A. This gives a strain of 0.0090

and a corresponding stress of 1.62GPa.

The results for the samples with 10 and 20% nominal

Ge concentration are summarized in Table 1.

3.2. Raman spectroscopy

Figs. 2(a) and (b) show Raman measurements for the

24% Si/SiGe sample (see Fig. 1) and a reference Si wafer

for excitation wavelengths of 325 and 632.8 nm,

respectively.

In Fig. 2(b) three distinct lines with strain and

composition-dependent energies are found which are

ascribed to [4] Si–Si vibrations (505.7 cm�1), localized

Si–Si vibrations perturbed by neighboring Ge

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Table 1

Comparison of measured Si/SiGe film parameters

Sample 1 Sample 2 Sample 3

Ge content (%)

XRD 24.4 20.7 10.9

Raman 22.4 17.3 8.8

SE 21.7 17.5 8.1

Relaxation (%)

XRD 97.1 96.3 89.1

Raman 95 100 100

Si cap thickness (nm)

XRR 13.8 13.9 14.3

TEM 13

SE 14.5 14.0 13.2

Strain Si cap

XRD 0.0090 0.0076 0.0037

Raman 0.0081 0.0069 0.0037

Fig. 2. Raman spectra for (a) 325 and (b) 632.8 nm excitation

wavelength. Results for a Si bulk sample are shown for

reference.

J. Schmidt et al. / Materials Science in Semiconductor Processing 8 (2005) 267–271 269

(431.1 cm�1) and Si–Ge (403.6 cm�1). Smaller peaks are

due to the strained-Si cap layer and the Si substrate.

The frequency shifts of the Si–Si and Si–Ge lines allow

the simultaneous calculation of composition and resi-

dual strain (relaxation) in the SiGe layer using empirical

relations for the composition- and strain-induced

frequency shift [5].

Fig. 3 shows results for the composition-dependent

shift of the Si–Si vibration at around 500 cm�1 for

relaxed and fully strained SiGe layers. Experimental

results from the literature [4–8] together with results for

calibration samples from this work are shown as open

and full symbols. The Ge fraction xGe for our samples is

an effective value calculated from Ge content and

relaxation of the SiGe layer measured by XRD. The

solid line marked as ‘relaxed’ is a fit to our measurement

results:

orelaxSi2Si ¼ 520:6� 68xGe: (1)

The ‘fully strained’ line is an empirical relation given

by [9]

ostrainedSi2Si ¼ 520:6� 31xGe: (2)

Combining (1) and (2) results in the following formula

for the frequency shift of the Si–Si vibration:

oSi2Si ¼ 520:6� 68xGe þ 37� (3)

with the normalized strain �:Experimental data for the frequency shift of the Si–Ge

mode together with empirical results for relaxed and

fully strained films are displayed in Fig. 4. The empirical

frequency shift is given by [10]

oSi2Ge ¼ 400:5þ 14:2xGe þ 24�: (4)

Fig. 3. Frequency of the Si–Si mode in SiGe vs. Ge

concentration. For details see text.

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Fig. 4. Frequency of the Si–Ge mode in SiGe vs. Ge

concentration. For details see text.

Fig. 5. Strain-induced frequency shift of the Si LO mode.

J. Schmidt et al. / Materials Science in Semiconductor Processing 8 (2005) 267–271270

Solving equation (3) and (4) simultaneously for

Fig. 2(b) results in a value of 22.4% for the Ge content

and 95% relaxation.

Due to the small probing depth (approx. 5 nm) at

325 nm in Fig. 2(a) only the Raman signal of the Si cap

layer is visible. The peak due to the longitudinal optical

(LO) phonon is shifted to lower frequencies compared to

the Si reference sample. Fig. 5 relates the (parallel) strain

in the Si cap layer given by

� ¼aSi � a xGeð Þ

a xGeð Þ(5)

with the observed frequency shift of the Si LO mode

using results from analogous measurements on the

calibration samples. In Eq. (5) aSi is the silicon lattice

constant and a(xGe) the lattice constant of the

unstrained SiGe alloy, with xGe from XRD (see above).

The solid line is the result of a linear fit to our

calibration samples, giving a strain-shift coefficient of

�750 cm�1 compared to the theoretical value of

�830 cm�1 calculated from the values for the phonon

deformation potentials in bulk Si. The observed shift in

Fig. 2(a) corresponds to a strain in the Si film of 0.0081

(i.e. 1.46GPa stress).

Raman results for Ge content and relaxation of the

SiGe layers and strain in the Si cap for the 10% and

20% Ge samples are shown in Table 1. A comparison of

these data with the XRD values shows that for the Ge

content and relaxation in the SiGe layer a satisfactory

agreement with the XRD results can be achieved. For

more accurate results more sophisticated methods for

calculation of xGe and � will have to be used (see [6]).

The calculated strain in the Si cap from Raman

measurements is very much dependent on the empirical

strain-shift coefficient. In the literature a wide range of

values from about �800 to �1050 cm�1 is found. One

should note that for the thin Si caps investigated in this

study the corresponding XRD peak is only very weak,

so that the given values are probably prone to some

error.

3.3. Spectroscopic ellipsometry

Ellipsometric spectra were acquired in the wavelength

range between 240 and 500 nm at an angle of incidence

of 70.51 using a KLA-Tencor UV-1280SE system.

These spectra were analyzed by a numerical least-

squares fit in order to extract the thickness of the Si cap

and the Ge content of the SiGe buffer. The fit is based

on a well-established mathematical model that com-

prises Fresnel’s equations for the description of light

reflection and propagation at interfaces between differ-

ent materials and Airy’s formalism that accounts for the

interference of multiply reflected and refracted light in

multilayer systems.

The film stack assumed for the model consisted of a

SiGe substrate of quasi-infinite thickness, a Si layer and

a native SiO2 layer on top.

Modeling of the underlying Ge grading could be

omitted by restricting the fit to the spectral range

between 240 and 500 nm (penetration depth is below

1mm in this wavelength range). Due to the limited

penetration length, the compositionally graded SiGe

layers do not significantly contribute to the SE signal

and therefore are not included in the theoretical model.

The evaluation of ellipsometric spectral data requires

a precise knowledge of the optical dispersion relation of

all involved materials. The wavelength dependence of

the complex refractive index of SiGe was published by

Zollner et al. [11]. These data were used within a so-

called lookup model to provide interpolated dispersion

relations for any Ge content between 0 and 0.2746 and

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Fig. 6. Measured and fitted ellipsometric spectra.

J. Schmidt et al. / Materials Science in Semiconductor Processing 8 (2005) 267–271 271

offers the Ge content as an additional fit parameter. For

the Si cap layer the data from Zollner et al. for vanishing

Ge content were used and the native oxide was described

by standard thermal SiO2 dispersion data.

A plot of the acquired ellipsometric spectrum together

with the numerical fit is shown in Fig. 6. The essential

features are well reproduced by the fit, suggesting that

the model is well suited for the description of the present

sample.

Regarding the fact that the reference data [11]

correspond to bulk Si and fully strained SiGe, one

may be concerned about using them in the present case

for relaxed SiGe and strained Si. It is well known that

the dielectric function of Si changes under stress [12],

especially near the critical points of the band structure.

However, the good agreement of the Si cap thickness

between SE, XRR, and TEM (Table 1) suggests that this

is of minor importance.

4. Summary

Results for SE and Raman spectroscopy on strained

Si/SiGe layer structures with Ge concentrations up to

0.4 have been compared to XRD measurements. It was

found that both spectroscopic techniques allow quick

and reasonably accurate wafer-scale measurements of

important layer parameters like thickness and strain in

the Si layer and composition and relaxation of the SiGe

film.

In particular, the Raman measurements give the

relation oSi2Si ¼ 520:6� 68xGe þ 37� for the frequency

shift of the Si–Si vibration in SiGe. For the shift of the

LO optical phonon in strained Si a strain-shift

coefficient of �750 cm�1 is found.

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