- Email: [email protected]
Selective growth of tensily strained Si1−yCy films on patterned Si substrates
ARTICLE IN PRESS Materials Science in Semiconductor Processing 12 (2009) 34–39
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Selective growth of tensily strained Si1yCy films on patterned Si substrates A. Gouye´ a,, F. Hu¨e b, A. Halimaoui a, O. Kermarrec a, Y. Campidelli a, M.J. Hy¨tch b, F. Houdellier b, A. Claverie b, D. Bensahel a a b
STMicroelectronics, 850 rue Jean Monnet, 38926 Crolles Cedex, France CEMES-CNRS, 29 rue Jeanne Marvig, BP 94347, 31055 Toulouse Cedex 4, France
a r t i c l e i n f o
abstract
Available online 28 July 2009
Advanced structures with poly-Si gates, Si3N4 spacers, and shallow trench isolation (STI) areas were used for elaborating the selective growth of Si1yCy films into recessed source and drain (S/D). Selective Si1yCy films were grown by repeated cycles consisting of two distinct steps: a non-selective CVD growth of Si1yCy layers, and a chemical vapor etching with hydrochloric gas. This cyclic deposition/etching process has been experimented at 600 1C with a methylsilane/(methylsilane+trisilane+hydrogen) mass flow ratio (SiCH6 MFR) equal to 2.8 104 used for Si0.99C0.01 film deposition. Regarding etching step, a pure HCl gas/(hydrogen) mass flow ratio (HCl MFR) was about 4.3 101. We should note that the poly-crystalline Si1yCy layers are etched more rapidly than the monocrystalline layers. The etching rate ratio between poly and mono areas induces the capability, by cyclic process, to remove the deposited poly-crystalline Si1yCy layers on the dielectric areas (STI, spacers) selectively versus the recessed mono-crystalline Si1yCy layers. A global time process, of about 3 h, resulted in 50 nm thick Si0.99C0.01 films selectively grown into recessed S/D. The new TEM technique of dark-field holography was used to determine a mapping of the strain at transistor level (within Si channel among S/D). The tensile stress of about 0.2 GPa has been measured within Si channel (300 nm length) among recessed Si0.985C0.015 films. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Cyclic deposition/etching process Selective growth Low-temperature CVD Recessed Si1yCy films Strain measurements
1. Introduction For future CMOS technologies (32 nm and beyond), Si epilayers, SiGe alloy layers, and Si1yCy films are inten-
Corresponding author.
E-mail addresses: [email protected] (A. Gouye´), [email protected] (F. Hu¨e), [email protected] (A. Halimaoui), [email protected] (O. Kermarrec), [email protected] (Y. Campidelli), [email protected] (M.J. Hy¨tch), [email protected] (F. Houdellier), [email protected] (A. Claverie), [email protected] (D. Bensahel). 1369-8001/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2009.07.006
sively studied as possible source and drain (S/D) or channel materials. Tensily strained Si1yCy films are believed to be used as stressors for electron mobility enhancement into nMOS channel [1–3]. Nevertheless, the growth and moreover the integration of such layers are much more challenging than SiGe stressors used for hole mobility enhancement (pMOS devices) [3]. The growth of C-enriched films is not an easy task because of the high mismatch between the Si and C (b-SiC polymorph) lattice parameters (m ¼ (aSiab-SiC)/aSiE+20%) [4], the low solubility of C in Si [5], and various C incorporations into a Si matrix (either in substitutional or interstitial sites, randomly distributed, or in an ordered manner in the
ARTICLE IN PRESS A. Gouye´ et al. / Materials Science in Semiconductor Processing 12 (2009) 34–39
2. Experimental details Thin films were grown in an industrial ASM Epsilons RPCVD reactor, on 200 mm diameter wafers. In this study, the working pressure was in the range of 10–760 Torr. Silicon, and carbon source gases used in the experiment were trisilane (Silcores, Si3H8), and methylsilane (5% SiCH6 in H2), respectively. The carrier gas was purified hydrogen (H2). Pure hydrochloric gas (HCl) was used to conduct the etching experiments. Advanced structures with poly-Si gates, Si3N4 spacers, and shallow trench isolation (STI) areas were used for elaborating the selective growth of Si1yCy films into recessed source and drain (S/D). In order to evaluate the selectivity of deposited Si1yCy films on silicon versus insulating areas, we have used a KLA Tencor SE1280 spectroscopic ellipsometer. A scanning electron microscopy (SEM) allowed an efficient observation of insulating areas. Si1yCy-deposited thickness into recessed S/D has been evaluated by atomic force microscopy (AFM) used as high-resolution profilometer. AFM was performed in a tapping mode on a Digital Instruments DI3100 NanoScope III. Regarding Si1yCy films, the layers were analysed by secondary ion mass spectroscopy (SIMS) and high-resolution X-ray diffraction (HR-XRD). The SIMS measurements
were carried out on a CAMECA IMS 5F spectrometer. Cs+ primary ions were used to evaluate the total carbon incorporation in silicon. The X-ray diffraction measurements were performed on a Philips high-resolution diffraction (X’Pert MRD) system at a Cu Ka1 wavelength of 1.5406 A˚. A Philips XRD simulation program was used to fit the experimental profiles. We have adopted Vegard’s law with linear extrapolations between the lattice parameters and ratios of the elastic coefficients of Si and b-SiC [4]. The good agreement between the simulated and measured (0 0 4) HR-XRD o2y curves enabled us to determine the substitutional carbon contents in the Si1yCy films (assuming pseudomorphic Si1yCy/Si stacks). The strain measurements within Si1yCy films on Si surfaces either within Si channel among recessed Si1yCy films have been carried out using dark-field holography which is a new technique of strain measurements by transmission electron microscopy (TEM) [10,11]. TEM experiments were performed on the SACTEM-Toulouse (spherical aberration corrected TEM), a 200 kV Tecnai F20 (FEI) equipped with field emission gun (FEG), an objective lens aberration corrector (CEOS), a rotatable electron biprism (FEI) and a 2k CCD camera (Gatan). Electron holograms were analysed using a modified version of GPA Phase 2.0 software (HREM Research Inc.). 3. Results and discussion 3.1. Substitutional carbon incorporation in Si1yCy films We investigated Si1yCy films grown at low temperature (o650 1C) on Si (0 0 1) substrates. Trisilane and methylsilane were used for 30 nm-thick Si1yCy layers grown at 550 and 600 1C. Fig. 1 shows the comparison between the total carbon concentration, extracted from SIMS analysis, and the substitutional carbon concentration, extracted from X-ray diffraction measurements. A linear plot (Fig. 1) means a total carbon content absolutely incorporated into substitutional sites. For Si1yCy films grown at 550 1C, the total carbon content, increasing from 0.62% up to 1.51%, is mainly incorporated into substitutional sites [8]. However, for total carbon concentration 4.5 [C] subst. content (%)
form of precipitates [6–8]). The total C content in random substitutional sites is strongly limited by the temperature and other factors of growth [6,9]. To take advantage of the tensile stress Si1yCy/Si, the level of carbon into substitutional sites must be about 1% or 2%. Such levels of substitution require low growth temperatures (o650 1C). In addition, regarding the application to S/D, the growth of the Si1yCy films must be selective versus insulating areas (mainly nitride and silicon oxide). Given these specifications, the choice of the silicon precursor for the chemical vapor deposition (CVD) is of a great importance. The usual precursor, dichlorosilane (SiH2Cl2), for selective growth requires relatively high temperatures (Z700 1C) for thermal decomposition, which are weakly compatible with the carbon incorporation, particularly into substitutional sites. In contrast, trisilane (Si3H8) precursor (Silcores) is a promising candidate for low-temperature CVD growth of Si1yCy films. In the first part of this work, the substitutional carbon incorporation in Si1yCy films, grown at low-temperature (To600 1C) using trisilane and methylsilane precursors, is studied. The second part deals with the selective growth of mono-crystalline Si1yCy films on patterned Si substrates with recessed S/D, using trisilane, methylsilane, and pure hydrochloric gas. In the last part, the strain measurements are carried out using transmission electron microscopy (TEM) in a new powerful technique called dark-field holography [10,11]. We will show that using this technique, it is possible to map the strain in two dimensions, at nanometre spatial resolution, and over the scale of the transistor.
35
[C] subst. (XRD) = [C] total (SIMS) % C (550°C) % C (600°C)
4 3.5 3 2.5 2 1.5 1 0.5 0
0
1
2 3 [C] total content (%)
4
5
Fig. 1. Comparison between the total carbon concentration (as obtained from XRD) and the substitutional carbon concentration (from SIMS) in Si1yCy layers grown at 550 and 600 1C on Si (0 0 1) substrate.
ARTICLE IN PRESS A. Gouye´ et al. / Materials Science in Semiconductor Processing 12 (2009) 34–39
1M
Si substrate
counts / s
100K
· experim - simul
10K Si1-yCy film
1K 100 10 1 0.1 -2000
-1000
0
1000
2000
3000
ω - 2θ (s) Fig. 2. (0 0 4) HR-XRD o2y spectra (experimental: blue line, simulated: red line) of a 30 nm-thick Si0.99C0.01 layer grown at 600 1C on Si (0 0 1) substrate with a methylsilane/(methylsilane+trisilane+carrier gas) mass flow ratio (SiCH6 MFR) equal to 2.8 104.
higher than 1.51%, the substitutional carbon content is lower than the total carbon content, and is attributed to carbon incorporation into interstitial sites. The deposition rate of Si0.985C0.015 film grown at 550 1C was evaluated to be about 5.6 nm min1. For Si1yCy films grown at higher temperature (600 1C), the divergence between the total carbon content and the substitutional carbon content starts above a total carbon content of about 1%, indicative of a strong increase of interstitial carbon. The deposition rate of Si0.99C0.01 film grown at 600 1C was estimated to be 12 nm min1. These Si1yCy layers grown at 550 and 600 1C show that the carbon incorporation into interstitial sites increases for higher growth temperature. Low interstitial carbon concentrations require low growth temperatures. The trade-off between high growth rate and substitutional carbon strongly enriched layer, induces the growth of Si0.99C0.01 film at 600 1C with a methylsilane/(methylsilane+trisilane+hydrogen) mass flow ratio (SiCH6 MFR) equal to 2.8 104. The (0 0 4) HR-XRD o2y experimental spectrum obtained from 30 nm-thick Si0.99C0.01 layer, grown at 600 1C on Si (0 0 1) substrate, is presented in Fig. 2 (blue line). The presence of well defined Si0.99C0.01 layer peak is characteristic of a good crystalline quality layer with sharp Si(0 0 1) substrate/ Si0.99C0.01 interface. The Si1y Cy-related peak is shifted to higher o values with respect to Si due to a smaller (0 0 4) interplanar distance within the strained Si1yCy layer. The absolute value of this shift is related to the difference between (0 0 4) interplanar distances of Si and strained Si1yCy and gives the value of strain relative to Si. The best fit between the simulated (Fig. 2, red line) and experimental spectra was obtained for y ¼ 1%. Applying the Hooke’s law, the in-plane biaxial stress value is near 1 GPa, which is significant for integrating tensile Si1yCy films grown into recessed source and drain.
3.2. Selective growth of recessed Si1yCy films on patterned Si substrates Si0.99C0.01 films, grown at 600 1C with a methylsilane/ (methylsilane+trisilane+hydrogen) mass flow ratio (SiCH6
MFR) equal to 2.8 104 have been investigated on , patterned Si substrates. The mono-crystalline and the poly-crystalline Si1yCy films were deposited on Si (0 0 1) surfaces and insulating areas, respectively. The growth of Si0.99C0.01 films is not selective versus insulating areas (mainly nitride and silicon oxide). In order to achieve selective growth of Si1yCy films on patterned Si substrates, we have elaborated a chemical vapor etching (CVE) of the poly-crystalline layers. Regarding the etching step at 600 1C, a pure HCl gas/(hydrogen) mass flow ratio (HCl MFR) was about 4.3 101. Fig. 3 shows kinetics (chemical vapor deposition/etching) as functions of the process time. The deposition rate achieved on Si surfaces and insulating areas is equal to 11 and 16 nm min1, respectively. The etching rate reached for the polycrystalline Si1yCy layers on insulating areas is evaluated to be 4 nm min1. The mono-crystalline Si1yCy layers are weakly etched (about 0.3 nm min1). Thus, the polycrystalline Si1yCy layers are etched more rapidly than the mono-crystalline layers. We should note that the deposition rate ratio between poly and mono areas is lower than the etching rate ratio between poly and mono areas. This shape induces the capability to remove the deposited poly-crystalline Si1yCy layers on insulating areas, and to achieve the required-thick mono-crystalline Si1yCy films on silicon. For integrating 50 nm-thick recessed mono-crystalline Si1yCy films, the chemical vapor etching has to remove a thick deposited poly-crystalline Si0.99C0.01 layer. Moreover, this deposition/etching process induces damaged interfaces between the mono-crystalline Si1yCy films and insulating areas. Such issues prevent a probative growth of recessed Si1yCy films. In order to improve the selective growth of recessed Si1yCy S/D, Si0.99C0.01 films were grown by repeated cycles consisting of two distinct steps [12]: deposition and etching of Si1yCy layers, both during a short process time. Each cycle (deposition/etching process) removes the
18 16
CVD Poly Si:C 1%
14 Kinetics (nm / min)
36
CVD mono Si:C 1%
12
CVE Poly Si:C 1%
10
CVE mono Si:C 1%
8 6 4 2 0 -2 -4 -6
0
10 20 Process time (min)
30
Fig. 3. Kinetics (chemical vapor deposition (CVD)/etching (CVE)) as functions of the process time.
ARTICLE IN PRESS A. Gouye´ et al. / Materials Science in Semiconductor Processing 12 (2009) 34–39
deposited poly-crystalline layers on the dielectric areas (STI, spacers), and grows thin recessed mono-crystalline Si1yCy films on silicon. Many repeated cycles induce the capability to achieve thick recessed Si1yCy films. A global time process, of about 3 h, resulted in 50 nm-thick Si0.99C0.01 films selectively grown into recessed S/D. In Fig. 4, we show a scanning electron microscopy top-view image of deposited Si0.99C0.01 S/D separated by a high temperature oxide (HTO) layer deposited on poly-Si gate. All around Si0.99C0.1 S/D, we should note a SiO2 surface without deposited Si:C nuclei, which means a good selectivity of deposited Si0.99C0.01 S/D on silicon versus insulating areas. In Fig. 5, a transmission electron microscopy cross-sectional image of recessed Si0.99C0.01 film is presented. Si surface and Si channel side induce two directions for growing recessed Si1yCy film, and these directions create a bulb near interface between mono-crystalline Si0.99C0.1 film and Si3N4 spacer.
Fig. 4. Scanning electron microscopy top-view image of deposited Si0.99C0.01 S/D separated by a HTO layer deposited on poly-Si gate. No Si:C nuclei on SiO2 surface.
Fig. 5. Transmission electron microscopy cross-sectional image of 50 nm-thick recessed Si0.99C0.01 film.
37
3.3. Strain measurements Comparative techniques of strain measurements within Si1yCy films on a Si substrate have already been analysed [8]. In this work, patterned Si substrates are used. The strain measurements within recessed Si1yCy S/ D and within Si channel among recessed Si1yCy S/D have been carried out using dark-field holography method from transmission electron microscopy (TEM) [10,11]. The strain measurements have been determined for recessed Si0.99C0.01 films either Si0.985C0.015 films. In practice, this method consists in measuring the atomic plane displacement from the reference lattice using holographic moire´ fringes. The details of this method can be found in [11]. All the measurements have been performed on a halftransistor assuming symmetry. Regarding measurements within recessed 50 nm-thick Si1yCy layers on Si surfaces, the strain relative to Si, ezzSi ¼ (ca[Si])/a[Si], where c is the lattice constant of strained Si1yCy film in the [0 0 1] direction, and a[Si] is the lattice constant of Si, was obtained. A line profile from the strain map can be given (Fig. 6). The average value of strain, ezzSi, is equal to 0.55% and 0.78% for recessed Si0.99C0.01 layer and Si0.985C0.015 layer, respectively. We can deduce the reduced tensile strain within these Si1yCy layers, ezz ¼ a[Si](1+ezzSi)/a[Si1yCy]1, equal to 0.17% and 0.26% for recessed Si0.99C0.01 film and Si0.985C0.015 film, respectively. In-plane biaxial stress was evaluated to be 451 and 712 MPa for recessed Si0.99C0.01 layer and Si0.985C0.015 layer, respectively. These strain measurements within embedded Si1yCy S/D show that the tensile stress increases for higher carbon content. Finite element simulations [13] were carried out (COMSOL MultiPhysics) to compare the strain measurements. The line profile of the simulated strain,ezzSi, matches very well with the experimental one, for recessed Si0.99C0.01 film either Si0.985C0.015 film. Regarding measurements within Si channel among recessed Si1yCy S/D, the strain relative to Si, exxSi ¼ (aJ ¼ a[Si])/a[Si] ¼ exx, where aJ is the lattice constant of strained Si channel in the [11 0] direction, was obtained. Fig. 6 gathers results concerning longitudinal strain exx within Si channel. As the channel is rather long (300 nm), we are not expecting a high strained Si channel. Nevertheless, dark field holography allows us to measure very small deformations (o0.1%). The average value of tensile strain, exx, is equal to +0.08% and +0.15% for Si channel among recessed Si0.99C0.01 layers and among Si0.985C0.015 layers, respectively. We can deduce the uniaxial tensile stress which was estimated to be 104 and 195 MPa among recessed Si0.99C0.01 S/D and among Si0.985C0.015 S/D, respectively. These strain measurements within Si channel show that the tensile stress increases for higher carbon content into recessed Si1yCy S/D. Finite element simulations were carried out to be compared with the strain measurements. As in the case of the strain ezzSi, simulated and experimental results are similar. Note the same behaviour at the boundary between recessed Si1yCy layer and Si channel. The bulb of Si1yCy seems to deteriorate the experimental mapping of the strain exx, what explains the fluctuations and the
ARTICLE IN PRESS 38
A. Gouye´ et al. / Materials Science in Semiconductor Processing 12 (2009) 34–39
Fig. 6. Comparison between strain measurements by dark field holography and strain calculation by finite element simulations. Line profile of the strain components ezzSi and exxSi.
loss of signal (black spot in the longitudinal strain map, Fig. 6).
Si1yCy S/D is rather long (300 nm). A shorter length (50 nm) would improve the uniaxial tensile stress (near 1 GPa).
4. Conclusion A cyclic deposition/etching process has been used for a selective growth of Si1yCy films at 600 1C. A global process time, of about 3 h, resulted in 50 nm-thick Si0.99C0.01 films selectively grown into recessed source and drain. The strain measurements were carried out using darkfield holography methods. We have determined a mapping of the strain at transistor level. The uniaxial tensile stress of about 0.2 GPa has been measured within Si channel among recessed Si0.985C0.015 films. From used structures in this work, the channel among embedded
Acknowledgements This work has been carried out in the frame of CEA-LETI/STMicroelectronics collaboration, and partially supported by the European Commission through the PullNano project References [1] Ang K-W, Chui K-J, Bliznetsov V, Wang Y, Wong L-Y, Tung C-H, et al. IEDM Tech Dig 2005:503–6.
ARTICLE IN PRESS A. Gouye´ et al. / Materials Science in Semiconductor Processing 12 (2009) 34–39
[2] Ang K-W, Chui K-J, Chin H-C, Foo Y-L, Du A, Deng W, et al. Symp VLSI Technol Dig Tech Pap 2006:66–7. [3] Ang K-W, Chui K-J, Bliznetsov V, Tung C-H, Du A, Balasubramanian N, et al. Appl Phys Lett 2005;86:093102. [4] Berti M, De Salvador D, Drigo AV, Romanato F, Stangl J, Zerlauth S, et al. Appl Phys Lett 1998;72(13):1602–4. [5] Olesinski RW, Abbaschian GJ. Bull Alloy Phase Diagr 1984;5:486–9. [6] Tersoff J. Phys Rev Lett 1995;74:5080–3. [7] Simon L, Kubler L, Bischoff JL, Bolmont D, Faure´ J, Claverie A, et al. Phys Rev B 1996;54(15):10559–64.
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[8] Cherkashin N, Gouye´ A, Hu¨e F, Houdellier F, Hy¨tch MJ, Snoeck E, et al. MRS 2007 Fall Proceedings 1026-C07-03. [9] Lave´ant P, Gerth G, Werner P, Go¨sele U. Mater Sci Eng B 2002;89:241–5. [10] Hy¨tch MJ, Snoeck E, Houdellier F, Hu¨e F. Proce´de´ et syste`me de mesure de de´formations a` l’e´chelle nanome´trique. French Patent Application FR No. 07 06711. [11] Hy¨tch MJ, Houdellier F, Hu¨e F, Snoeck E. Nature 2008;453:1086–9. [12] Bauer M, Weeks D, Zhang Y, Machkaoutsan V. ECS Trans 2006;3(7):187–96. [13] Yeo Y-C, Sun JS. Appl Phys Lett 2005;86:023103.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Selective growth of tensily strained Si1yCy films on patterned Si substrates A. Gouye´ a,, F. Hu¨e b, A. Halimaoui a, O. Kermarrec a, Y. Campidelli a, M.J. Hy¨tch b, F. Houdellier b, A. Claverie b, D. Bensahel a a b
STMicroelectronics, 850 rue Jean Monnet, 38926 Crolles Cedex, France CEMES-CNRS, 29 rue Jeanne Marvig, BP 94347, 31055 Toulouse Cedex 4, France
a r t i c l e i n f o
abstract
Available online 28 July 2009
Advanced structures with poly-Si gates, Si3N4 spacers, and shallow trench isolation (STI) areas were used for elaborating the selective growth of Si1yCy films into recessed source and drain (S/D). Selective Si1yCy films were grown by repeated cycles consisting of two distinct steps: a non-selective CVD growth of Si1yCy layers, and a chemical vapor etching with hydrochloric gas. This cyclic deposition/etching process has been experimented at 600 1C with a methylsilane/(methylsilane+trisilane+hydrogen) mass flow ratio (SiCH6 MFR) equal to 2.8 104 used for Si0.99C0.01 film deposition. Regarding etching step, a pure HCl gas/(hydrogen) mass flow ratio (HCl MFR) was about 4.3 101. We should note that the poly-crystalline Si1yCy layers are etched more rapidly than the monocrystalline layers. The etching rate ratio between poly and mono areas induces the capability, by cyclic process, to remove the deposited poly-crystalline Si1yCy layers on the dielectric areas (STI, spacers) selectively versus the recessed mono-crystalline Si1yCy layers. A global time process, of about 3 h, resulted in 50 nm thick Si0.99C0.01 films selectively grown into recessed S/D. The new TEM technique of dark-field holography was used to determine a mapping of the strain at transistor level (within Si channel among S/D). The tensile stress of about 0.2 GPa has been measured within Si channel (300 nm length) among recessed Si0.985C0.015 films. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Cyclic deposition/etching process Selective growth Low-temperature CVD Recessed Si1yCy films Strain measurements
1. Introduction For future CMOS technologies (32 nm and beyond), Si epilayers, SiGe alloy layers, and Si1yCy films are inten-
Corresponding author.
E-mail addresses: [email protected] (A. Gouye´), [email protected] (F. Hu¨e), [email protected] (A. Halimaoui), [email protected] (O. Kermarrec), [email protected] (Y. Campidelli), [email protected] (M.J. Hy¨tch), [email protected] (F. Houdellier), [email protected] (A. Claverie), [email protected] (D. Bensahel). 1369-8001/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2009.07.006
sively studied as possible source and drain (S/D) or channel materials. Tensily strained Si1yCy films are believed to be used as stressors for electron mobility enhancement into nMOS channel [1–3]. Nevertheless, the growth and moreover the integration of such layers are much more challenging than SiGe stressors used for hole mobility enhancement (pMOS devices) [3]. The growth of C-enriched films is not an easy task because of the high mismatch between the Si and C (b-SiC polymorph) lattice parameters (m ¼ (aSiab-SiC)/aSiE+20%) [4], the low solubility of C in Si [5], and various C incorporations into a Si matrix (either in substitutional or interstitial sites, randomly distributed, or in an ordered manner in the
ARTICLE IN PRESS A. Gouye´ et al. / Materials Science in Semiconductor Processing 12 (2009) 34–39
2. Experimental details Thin films were grown in an industrial ASM Epsilons RPCVD reactor, on 200 mm diameter wafers. In this study, the working pressure was in the range of 10–760 Torr. Silicon, and carbon source gases used in the experiment were trisilane (Silcores, Si3H8), and methylsilane (5% SiCH6 in H2), respectively. The carrier gas was purified hydrogen (H2). Pure hydrochloric gas (HCl) was used to conduct the etching experiments. Advanced structures with poly-Si gates, Si3N4 spacers, and shallow trench isolation (STI) areas were used for elaborating the selective growth of Si1yCy films into recessed source and drain (S/D). In order to evaluate the selectivity of deposited Si1yCy films on silicon versus insulating areas, we have used a KLA Tencor SE1280 spectroscopic ellipsometer. A scanning electron microscopy (SEM) allowed an efficient observation of insulating areas. Si1yCy-deposited thickness into recessed S/D has been evaluated by atomic force microscopy (AFM) used as high-resolution profilometer. AFM was performed in a tapping mode on a Digital Instruments DI3100 NanoScope III. Regarding Si1yCy films, the layers were analysed by secondary ion mass spectroscopy (SIMS) and high-resolution X-ray diffraction (HR-XRD). The SIMS measurements
were carried out on a CAMECA IMS 5F spectrometer. Cs+ primary ions were used to evaluate the total carbon incorporation in silicon. The X-ray diffraction measurements were performed on a Philips high-resolution diffraction (X’Pert MRD) system at a Cu Ka1 wavelength of 1.5406 A˚. A Philips XRD simulation program was used to fit the experimental profiles. We have adopted Vegard’s law with linear extrapolations between the lattice parameters and ratios of the elastic coefficients of Si and b-SiC [4]. The good agreement between the simulated and measured (0 0 4) HR-XRD o2y curves enabled us to determine the substitutional carbon contents in the Si1yCy films (assuming pseudomorphic Si1yCy/Si stacks). The strain measurements within Si1yCy films on Si surfaces either within Si channel among recessed Si1yCy films have been carried out using dark-field holography which is a new technique of strain measurements by transmission electron microscopy (TEM) [10,11]. TEM experiments were performed on the SACTEM-Toulouse (spherical aberration corrected TEM), a 200 kV Tecnai F20 (FEI) equipped with field emission gun (FEG), an objective lens aberration corrector (CEOS), a rotatable electron biprism (FEI) and a 2k CCD camera (Gatan). Electron holograms were analysed using a modified version of GPA Phase 2.0 software (HREM Research Inc.). 3. Results and discussion 3.1. Substitutional carbon incorporation in Si1yCy films We investigated Si1yCy films grown at low temperature (o650 1C) on Si (0 0 1) substrates. Trisilane and methylsilane were used for 30 nm-thick Si1yCy layers grown at 550 and 600 1C. Fig. 1 shows the comparison between the total carbon concentration, extracted from SIMS analysis, and the substitutional carbon concentration, extracted from X-ray diffraction measurements. A linear plot (Fig. 1) means a total carbon content absolutely incorporated into substitutional sites. For Si1yCy films grown at 550 1C, the total carbon content, increasing from 0.62% up to 1.51%, is mainly incorporated into substitutional sites [8]. However, for total carbon concentration 4.5 [C] subst. content (%)
form of precipitates [6–8]). The total C content in random substitutional sites is strongly limited by the temperature and other factors of growth [6,9]. To take advantage of the tensile stress Si1yCy/Si, the level of carbon into substitutional sites must be about 1% or 2%. Such levels of substitution require low growth temperatures (o650 1C). In addition, regarding the application to S/D, the growth of the Si1yCy films must be selective versus insulating areas (mainly nitride and silicon oxide). Given these specifications, the choice of the silicon precursor for the chemical vapor deposition (CVD) is of a great importance. The usual precursor, dichlorosilane (SiH2Cl2), for selective growth requires relatively high temperatures (Z700 1C) for thermal decomposition, which are weakly compatible with the carbon incorporation, particularly into substitutional sites. In contrast, trisilane (Si3H8) precursor (Silcores) is a promising candidate for low-temperature CVD growth of Si1yCy films. In the first part of this work, the substitutional carbon incorporation in Si1yCy films, grown at low-temperature (To600 1C) using trisilane and methylsilane precursors, is studied. The second part deals with the selective growth of mono-crystalline Si1yCy films on patterned Si substrates with recessed S/D, using trisilane, methylsilane, and pure hydrochloric gas. In the last part, the strain measurements are carried out using transmission electron microscopy (TEM) in a new powerful technique called dark-field holography [10,11]. We will show that using this technique, it is possible to map the strain in two dimensions, at nanometre spatial resolution, and over the scale of the transistor.
35
[C] subst. (XRD) = [C] total (SIMS) % C (550°C) % C (600°C)
4 3.5 3 2.5 2 1.5 1 0.5 0
0
1
2 3 [C] total content (%)
4
5
Fig. 1. Comparison between the total carbon concentration (as obtained from XRD) and the substitutional carbon concentration (from SIMS) in Si1yCy layers grown at 550 and 600 1C on Si (0 0 1) substrate.
ARTICLE IN PRESS A. Gouye´ et al. / Materials Science in Semiconductor Processing 12 (2009) 34–39
1M
Si substrate
counts / s
100K
· experim - simul
10K Si1-yCy film
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-1000
0
1000
2000
3000
ω - 2θ (s) Fig. 2. (0 0 4) HR-XRD o2y spectra (experimental: blue line, simulated: red line) of a 30 nm-thick Si0.99C0.01 layer grown at 600 1C on Si (0 0 1) substrate with a methylsilane/(methylsilane+trisilane+carrier gas) mass flow ratio (SiCH6 MFR) equal to 2.8 104.
higher than 1.51%, the substitutional carbon content is lower than the total carbon content, and is attributed to carbon incorporation into interstitial sites. The deposition rate of Si0.985C0.015 film grown at 550 1C was evaluated to be about 5.6 nm min1. For Si1yCy films grown at higher temperature (600 1C), the divergence between the total carbon content and the substitutional carbon content starts above a total carbon content of about 1%, indicative of a strong increase of interstitial carbon. The deposition rate of Si0.99C0.01 film grown at 600 1C was estimated to be 12 nm min1. These Si1yCy layers grown at 550 and 600 1C show that the carbon incorporation into interstitial sites increases for higher growth temperature. Low interstitial carbon concentrations require low growth temperatures. The trade-off between high growth rate and substitutional carbon strongly enriched layer, induces the growth of Si0.99C0.01 film at 600 1C with a methylsilane/(methylsilane+trisilane+hydrogen) mass flow ratio (SiCH6 MFR) equal to 2.8 104. The (0 0 4) HR-XRD o2y experimental spectrum obtained from 30 nm-thick Si0.99C0.01 layer, grown at 600 1C on Si (0 0 1) substrate, is presented in Fig. 2 (blue line). The presence of well defined Si0.99C0.01 layer peak is characteristic of a good crystalline quality layer with sharp Si(0 0 1) substrate/ Si0.99C0.01 interface. The Si1y Cy-related peak is shifted to higher o values with respect to Si due to a smaller (0 0 4) interplanar distance within the strained Si1yCy layer. The absolute value of this shift is related to the difference between (0 0 4) interplanar distances of Si and strained Si1yCy and gives the value of strain relative to Si. The best fit between the simulated (Fig. 2, red line) and experimental spectra was obtained for y ¼ 1%. Applying the Hooke’s law, the in-plane biaxial stress value is near 1 GPa, which is significant for integrating tensile Si1yCy films grown into recessed source and drain.
3.2. Selective growth of recessed Si1yCy films on patterned Si substrates Si0.99C0.01 films, grown at 600 1C with a methylsilane/ (methylsilane+trisilane+hydrogen) mass flow ratio (SiCH6
MFR) equal to 2.8 104 have been investigated on , patterned Si substrates. The mono-crystalline and the poly-crystalline Si1yCy films were deposited on Si (0 0 1) surfaces and insulating areas, respectively. The growth of Si0.99C0.01 films is not selective versus insulating areas (mainly nitride and silicon oxide). In order to achieve selective growth of Si1yCy films on patterned Si substrates, we have elaborated a chemical vapor etching (CVE) of the poly-crystalline layers. Regarding the etching step at 600 1C, a pure HCl gas/(hydrogen) mass flow ratio (HCl MFR) was about 4.3 101. Fig. 3 shows kinetics (chemical vapor deposition/etching) as functions of the process time. The deposition rate achieved on Si surfaces and insulating areas is equal to 11 and 16 nm min1, respectively. The etching rate reached for the polycrystalline Si1yCy layers on insulating areas is evaluated to be 4 nm min1. The mono-crystalline Si1yCy layers are weakly etched (about 0.3 nm min1). Thus, the polycrystalline Si1yCy layers are etched more rapidly than the mono-crystalline layers. We should note that the deposition rate ratio between poly and mono areas is lower than the etching rate ratio between poly and mono areas. This shape induces the capability to remove the deposited poly-crystalline Si1yCy layers on insulating areas, and to achieve the required-thick mono-crystalline Si1yCy films on silicon. For integrating 50 nm-thick recessed mono-crystalline Si1yCy films, the chemical vapor etching has to remove a thick deposited poly-crystalline Si0.99C0.01 layer. Moreover, this deposition/etching process induces damaged interfaces between the mono-crystalline Si1yCy films and insulating areas. Such issues prevent a probative growth of recessed Si1yCy films. In order to improve the selective growth of recessed Si1yCy S/D, Si0.99C0.01 films were grown by repeated cycles consisting of two distinct steps [12]: deposition and etching of Si1yCy layers, both during a short process time. Each cycle (deposition/etching process) removes the
18 16
CVD Poly Si:C 1%
14 Kinetics (nm / min)
36
CVD mono Si:C 1%
12
CVE Poly Si:C 1%
10
CVE mono Si:C 1%
8 6 4 2 0 -2 -4 -6
0
10 20 Process time (min)
30
Fig. 3. Kinetics (chemical vapor deposition (CVD)/etching (CVE)) as functions of the process time.
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deposited poly-crystalline layers on the dielectric areas (STI, spacers), and grows thin recessed mono-crystalline Si1yCy films on silicon. Many repeated cycles induce the capability to achieve thick recessed Si1yCy films. A global time process, of about 3 h, resulted in 50 nm-thick Si0.99C0.01 films selectively grown into recessed S/D. In Fig. 4, we show a scanning electron microscopy top-view image of deposited Si0.99C0.01 S/D separated by a high temperature oxide (HTO) layer deposited on poly-Si gate. All around Si0.99C0.1 S/D, we should note a SiO2 surface without deposited Si:C nuclei, which means a good selectivity of deposited Si0.99C0.01 S/D on silicon versus insulating areas. In Fig. 5, a transmission electron microscopy cross-sectional image of recessed Si0.99C0.01 film is presented. Si surface and Si channel side induce two directions for growing recessed Si1yCy film, and these directions create a bulb near interface between mono-crystalline Si0.99C0.1 film and Si3N4 spacer.
Fig. 4. Scanning electron microscopy top-view image of deposited Si0.99C0.01 S/D separated by a HTO layer deposited on poly-Si gate. No Si:C nuclei on SiO2 surface.
Fig. 5. Transmission electron microscopy cross-sectional image of 50 nm-thick recessed Si0.99C0.01 film.
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3.3. Strain measurements Comparative techniques of strain measurements within Si1yCy films on a Si substrate have already been analysed [8]. In this work, patterned Si substrates are used. The strain measurements within recessed Si1yCy S/ D and within Si channel among recessed Si1yCy S/D have been carried out using dark-field holography method from transmission electron microscopy (TEM) [10,11]. The strain measurements have been determined for recessed Si0.99C0.01 films either Si0.985C0.015 films. In practice, this method consists in measuring the atomic plane displacement from the reference lattice using holographic moire´ fringes. The details of this method can be found in [11]. All the measurements have been performed on a halftransistor assuming symmetry. Regarding measurements within recessed 50 nm-thick Si1yCy layers on Si surfaces, the strain relative to Si, ezzSi ¼ (ca[Si])/a[Si], where c is the lattice constant of strained Si1yCy film in the [0 0 1] direction, and a[Si] is the lattice constant of Si, was obtained. A line profile from the strain map can be given (Fig. 6). The average value of strain, ezzSi, is equal to 0.55% and 0.78% for recessed Si0.99C0.01 layer and Si0.985C0.015 layer, respectively. We can deduce the reduced tensile strain within these Si1yCy layers, ezz ¼ a[Si](1+ezzSi)/a[Si1yCy]1, equal to 0.17% and 0.26% for recessed Si0.99C0.01 film and Si0.985C0.015 film, respectively. In-plane biaxial stress was evaluated to be 451 and 712 MPa for recessed Si0.99C0.01 layer and Si0.985C0.015 layer, respectively. These strain measurements within embedded Si1yCy S/D show that the tensile stress increases for higher carbon content. Finite element simulations [13] were carried out (COMSOL MultiPhysics) to compare the strain measurements. The line profile of the simulated strain,ezzSi, matches very well with the experimental one, for recessed Si0.99C0.01 film either Si0.985C0.015 film. Regarding measurements within Si channel among recessed Si1yCy S/D, the strain relative to Si, exxSi ¼ (aJ ¼ a[Si])/a[Si] ¼ exx, where aJ is the lattice constant of strained Si channel in the [11 0] direction, was obtained. Fig. 6 gathers results concerning longitudinal strain exx within Si channel. As the channel is rather long (300 nm), we are not expecting a high strained Si channel. Nevertheless, dark field holography allows us to measure very small deformations (o0.1%). The average value of tensile strain, exx, is equal to +0.08% and +0.15% for Si channel among recessed Si0.99C0.01 layers and among Si0.985C0.015 layers, respectively. We can deduce the uniaxial tensile stress which was estimated to be 104 and 195 MPa among recessed Si0.99C0.01 S/D and among Si0.985C0.015 S/D, respectively. These strain measurements within Si channel show that the tensile stress increases for higher carbon content into recessed Si1yCy S/D. Finite element simulations were carried out to be compared with the strain measurements. As in the case of the strain ezzSi, simulated and experimental results are similar. Note the same behaviour at the boundary between recessed Si1yCy layer and Si channel. The bulb of Si1yCy seems to deteriorate the experimental mapping of the strain exx, what explains the fluctuations and the
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Fig. 6. Comparison between strain measurements by dark field holography and strain calculation by finite element simulations. Line profile of the strain components ezzSi and exxSi.
loss of signal (black spot in the longitudinal strain map, Fig. 6).
Si1yCy S/D is rather long (300 nm). A shorter length (50 nm) would improve the uniaxial tensile stress (near 1 GPa).
4. Conclusion A cyclic deposition/etching process has been used for a selective growth of Si1yCy films at 600 1C. A global process time, of about 3 h, resulted in 50 nm-thick Si0.99C0.01 films selectively grown into recessed source and drain. The strain measurements were carried out using darkfield holography methods. We have determined a mapping of the strain at transistor level. The uniaxial tensile stress of about 0.2 GPa has been measured within Si channel among recessed Si0.985C0.015 films. From used structures in this work, the channel among embedded
Acknowledgements This work has been carried out in the frame of CEA-LETI/STMicroelectronics collaboration, and partially supported by the European Commission through the PullNano project References [1] Ang K-W, Chui K-J, Bliznetsov V, Wang Y, Wong L-Y, Tung C-H, et al. IEDM Tech Dig 2005:503–6.
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