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Analysis by FT-IR spectroscopy of SiO2-polycrystalline structures used in micromechanics: Stress measurements
Sensors and Acrualors
A, 32 (1992)
347-353
347
Analysis by FT-IR spectroscopy of SiO,-polycrystalline in micromechanics: stress measurements
structures used
J. Samitier, S. Marco, 0. Ruiz and J. R. Morante Deparramento Fisica Aplicada i Electrbnica, Universitat de Barcelona, Avda. Diagonal 645-647, E-08028 Barcelona (Spain)
J. Esteve-Tinto
and J. Bausells
Centro National de Microelectrdnica-CSIC,
Campus Universitat Authzoma de Barcelona, 08193 Behterra
(Spain)
Abstract The evolution of the intrinsic stress with the annealing temperature in silicon oxide layers thermally grown and deposited by the low-pressure chemical vapour method has been analysed by the reflection-absorption Fourier transform infrared (FT-IR) technique. The results show that the annealing process in the low-pressure chemical vapour deposition (LPCVD) silicon oxide produces a high compressive stress in the wafer plane, due to a decrease of density in the jayer. Moreover, for overlying structures used in micromechanics, such as polycrystalline silicon-silicon oxide, we,have pointed out the existence of induced stress between the layers. The stress evolution in the silicon oxide layers has been compared with the results obtained using warpage and X-ray diffraction measurements.
Introduction
The fabrication of micromechanical and microsensor devices involves the use of thin-film deposition techniques. Low pressure chemical vapour deposition (LPCVD) silicon oxide and polycrystalline silicon thin layers are currently used as sacrificial, passivation or structural layers in micromechanical devices based on silicon technology. It is well known that tensile or compressive stress distributions are almost always present in these deposited films. A first stress contribution is due to the different thermal expansion coefficients between the thin layer and the substrate. However, the measured stress values do not usually agree with the stress behaviour envisaged by the thermal mismatches and, in consequence, other stress contributions must be taken into account. These stresses may be intrinsic to the film, caused by impurities, voids and dislocation distributions, or they may arise from morphology changes within the films, such as grain growth, recryatallization or phase changes. Moreover, the final stress distribution of a thin layer can be affected by the intrinsic stress values of the substrate, or other thin films in the case of multilayered structures. O924-4247/92/%5.00
This mechanical residual stress causes great problems in the fabrication of micromechanical structures, such as wrinkling in thin diaphragms and deflection in cantilevers. In these applicatiolis the control of the structural and mechanical properties of the layers is critical, because they affect the performance of the device and even its feasibility. The goal of this work is to study the intkmal stress evolution of silicon oxide/polysilicon structures deposited over a silicon substrate as a function of the deposition and annealing temperatures, in order to obtain improvements in micromechanical devices, and the relations between the macroscopic stress behaviour observed and the structural and technological characteristics of the thin layers used. Generally, the multilayered structure stress can be obtained by warpage measurements using, for example, laser deflection or profilometers. Although these techniques allow the final average stress values to be obtained, it is difficult to extract the physical mechanisms which are the origin of the stress state observed. The microscopic stress effects in silicon oxide and the influence of the annealing treatments and polysilicon layer deposition have been analysed by Fourier transform infrared spectroscopy (FT-IR), which is sensitive to @ 1992 -
Elsevier Sequoia.
All rights reserved
348
modifications of the bond structure. The obtained stress evolution in t.he silicon oxide layers has been compared with the intrinsic stress in the polysilicon layers measured by X-ray diffraction and with the macroscopic results obtained by warpage measurements.
Samples
Polysilicon and silicon oxide films were deposited in an LPCVD reactor using pure silane and silane and oxygen, respectively. In order to analyse the LPCVD silicon oxide behaviour and its possible influence in the polysilicon layer, we have compared the LPCVD oxide measurements with the ones obtained with high-temperature thermal oxide grown in dry oxygen. Two inch (100) lightly doped silicon wafers were divided into three groups: (i) 1.0 and 1.5 um thick LPCVD silicon oxide films were deposited at 430 “C. The silane-oxygen flow ratio used was 1.6 with a pressure of 200 mTorr. (ii) 1.8 urn thick polysilicon films were deposited at 630 “C with a pressure of 80 mTorr, over 1.5 pm thick LPCVD silicon oxide deposited at 430 “C. (iii) 4500 A thick polysilicon films were deposited over 1000 8, of thermal oxide grown at T = 950 “C in dry oxygen atmosphere at temperatures between 550 and 630 “C and’80 mTorr pressure. Wafers from each group were annealed in a nitrogen atmosphere at temperatures between 800 and 1150 “C for 30 min.
Experimental
method
FT-IR spectroscopy is an interesting non-destructive technique for the bond analysis of multilayered structures [ 11. It has also been used in microsensor technology to measure the thickness of either thin layers deposited over silicon substrates or silicon diaphragms obtained by chemical etching [2, 31. The FT-IR bond characterization of microelectronic dielectric layers, such as silicon oxide, is performed in the middle infrared spectral region
(wave number range 400 cm-’ to 5000 cm-‘). In this range, the dielectric function of the oxide shows its poles and zeros, which are due to the different transverse and longitudinal optical vibrational modes of the bond unit. They determine the presence of absorption peaks related to these modes in the transmission or reflection spectra. Changes in stoichiometry, density and structure in the oxide layers affect the position and shape of these peaks, corresponding to the rocking mode near 450 cm-‘, bending near 800 cm-’ and stretching vibration near 1075 cm-‘. Theoretical calculations based on the force constant model for the vibrational properties of amorphous silica [4,5] predict that the frequency of the bond stretching vibration, v, is correlated to the silicon-silicon interatomic distance, dsi_si, through the Si-0-Si bond angle, 28 [6, 71: v = v0 sin(e)
with v0 a constant value
(1)
and d,i _si = 2ro sin 0
where r. is the Si-0 bond length (2)
Then a change of the angle 28, which can originate from internal and/or induced stress, produces a modification of the absorption stretching frequency. In this sense, the Si-Si interatomic distance obtained from eqn. (2) can be used as a measurement of the thickness averaged strain, E,, in the solution oxide layer, via E, =
(v/bed
-
1
(3)
where v,,~ is the stretching frequency value of a fully relaxed SiOZ layer. However, it must be noted that modifications in the angle 28 could also originate from mechanisms other than intrinsic or applied stress, possibly related with changes in the internal structure of the silicon oxide random network. Then, as the microscopic structure of the silicon oxide network is not completely known, it is necessary to carry out an accurate analysis in order to determine the corresponding Si02 fully relaxed frequency value. The reported values [7] for the extensively analysed high-temperature thermally grown SiOz corresponding to eqns. (1) and (2) are 20 = 144”, vo= 1134cm-‘, ro= 1.6 A and v,~ = 1078 cm-‘.
349
On the other hand, the absorption peak positions obtained in the transmittance or reflectance FT-IR spectra also depend on the optical geometrical features of the samples, such as thicknesses and refraction indices, so it is necessary to take into account these perturbative effects by means of a simulation program. Moreover, the fitting of the interference path observed in the FT-IR spectra allows both the thickness and the reflection index of layers to be determined. With these values, we can estimate the theoretical absorption peak position for a given geometry, V,im. The experimental set-up used for the FT-IR measurements has been a BOMEM DA3 spectrometer, which allows variable-angle transmittance and reflectance measurements to be made with a resolution of 0.02 cm-‘, and a BOMEN MB-120 spectrometer interfaced with the IRPLAN infrared spectroscope [8], which allows an infrared light spot of about 25 pm to be obtained using masking apertures and can be used to test mechanical microstructures in situ. The polysilicon films were analysed by X-ray diffraction. This technique has been commonly used to determine the texture (preferred orientation) [9], the degree of crystallinity and crystallite size [lo] of polysilicon layers. However, it is well known that X-ray diffraction can be used as a method of stress measurement in polycrystalline layers. The samples were measured in a Siemens D-500 diffractometer. All the X-ray work was carried out with Cu Ka and Fe Ka radiation (penetration depths 71 and 38 pm, respectively). A maximum of six peaks is found in the 28 range explored. However, the only peaks with enough intensity to be used in the stress results were {111) and (220). The most usual method of determining the stress in polycrystalline materials using X-ray diffraction is measuring the slope of the lattice parameter versus sin’ 0 plots [ 11, 121. However, the configuration of our goniometer imposes a geometric limitation on the maximum 8 angle available, so we have measured the deviation of the X-ray diffraction peak from the theoretical value. The accuracy of the measurement of the scattering angle is of major importance in the determination of residual stress via diffraction peak shift, This determines the accuracy of the interplanar
spacing and hence the strain. The most commonly used method for determining peak positions is a parabolic fit near the top of a peak. However, when the peaks are weak, we can obtain better precision by fitting with pseudo-Voigt functions plus a linear baseline than with parabolae [ 131. The developed routine also furnishes the error in the estimated peak position, which was always smaller than 0.01”. In order to obtain the biaxial stress values, u, from the measured strain in the z direction, E=, a suitable elastic constant must be calculated taking into account the crystal anisotropy. The values obtained are: grains
(11 l},
G = -5.17
x 10”ez dyn/cm’
grains
{ 1lo>,
c = - 13.16 x lO’*e, dyn/cn?.
Results and discussion
First we have analysed the strain behaviour in the silicon oxide layers using FT-IR spectroscopy. Figure 1 shows both the experimental data corresponding to an FT-IR absorbance spectrum of 950 “C thermally grown oxide in dry oxygen and the simulated spectrum obtained taking into account the oxide thickness and the Si02 theoretical dielectric function for high-temperature thermally oxidized SiOz [ 141. In both spectra, the maximum of the stretching vibrational mode is at 1072 cm-‘.
Wovenumbers
km-‘)
Fig. 1. (a) Simulated spectrum (dashed line) taking into account the thermal oxide dielectric function [ 141. (b) Experimental spectrum (continuous line) corresponding to a transmission absorbance spectrum obtained at 0”. of 100 nm thermal silicon oxide grown at 950 “C.
A LPCVD oxide as
B fhermal
oxide simulation
\x,
I 1350
1300
1250
Wavenumbers
1200
1150
1100
I 1050
(cm-‘)
Fig. 2. (A) Reflection spectrum obtained at 28” in the as-deposited 1.5 urn LPCVD silicon oxide layer. (B) Simulated spectrum taking into account the thermal oxide dielectric function [ 141 and the geometrical configuration.
lllOsA
1200
T annealing(C)
Fig. 3. Shift of TO, peak position in samples of LPCVD silicon oxide subjected to an annealing process. The IR spectra were obtained by reflection at 28”. The as-deposited LPCVD peak position is 1105 cm-‘.
Nevertheless, LPCVD silicon oxide shows a different behaviour (Fig. 2). The reflectance spectrum obtained at 28” in the as-deposited layers present a TO, peak position significantly lower than the value obtained by simulation taking into account the thermal oxide dielectric function and the geometrical configuration. This shift indicates a diminution of the Si-0-Si angle. After the annealing process, we observe a strong evolution of the TO9 absorption peak towards higher values (Fig. 3). So, the Si-0-Si angle increases to reach the value corresponding to a thermal grown oxide.
For the thermally grown oxides an annealing process at a temperature higher than the growth one produces a small shift in the TO3 peak position towards higher values. It is well known that thermal oxides have a non-uniform compressive stress across their thickness, having higher values near the Si-SiOz interface [6]. Theoretical models of the growth process for thermal Si02 suggest that the lack of free volume during growth to accommodate the density differences between SiO, and Si should lead to stresses in the layer. Annealing processes carried out at temperatures higher than the growth one partially relax this stress, although the stress value at the interface remains nearly constant. The assumed relaxation mechanism considers the viscous flow of a Maxwell solid [ 151. For our annealing temperatures the maximum TO3 peak position shifts are about 2 cm-‘. Comparing the above results, it can be concluded that the as-deposited LPCVD silicon oxide presents a high compressive stress, which relaxes after a high-temperature annealing. However, the warpage measurements of the LPCVD oxide layers over the silicon substrate point out the existence of a compressive stress in the as-deposited SiOz layers that strongly increases with the annealing temperature (Fig. 4). This last result is in agreement with the curvature measurements previously reported by Shioma and Maeda [ 161 in phosphosilicate glasses. In consequence, the strong IR shift in the asdeposited LPCVD silicon oxide cannot be produced only by an internal compressive stress and
c
e iii
800
900
1000
no0
1
T annealing(C)
Fig. 4. Strain in samples of LPCVD silicon oxide subjected annealing process obtained by warpage measurements.
to an
351
could originate from a different network structure to that of thermally oxide grown. This structure would be more compact, because the shift towards lower wavenumbers involves a diminution of the Si-Si distance (see eqn. (2)); in consequence there will be an increase of density. From the above hypothesis, the increase of the compressive stress with the annealing temperature in LPCVD silicon oxide can be explained as follows: when the LPCVD silicon oxide is submitted to a high-temperature anneal, the Si-0-Si angle tends to increase, and also the distance between silicon atoms. This increase produces a volume expansion. However, the layer can exbecause in pand only in the z-direction, the x-y plane the layer is attached to the substrate wafer. This process produces a compressive stress in this plane, which increases as the network relaxes towards the thermal silicon oxide structure. This hypothesis is in agreement with the results reported by several authors [ 17, 181 in plasmaenhanced chemical vapour deposition (PECVD) silicon oxide films. In this case, they have observed a similar IR peak evolution with annealing temperature, suggesting also that low-temperature PECVD silicon oxide has a network structure composed of densified amorphous Si02, which can relax with a high-temperature annealing. From the former results, we can conclude that the low temperature as-deposited silicon oxide has a denser network structure than the high-temperature thermally grown silicon oxide, which gives a different stress evolution with the annealing temperature. This densification would not be due to the existence of a high compressive stress. Once the mechanisms and evolution of the silicon oxide stress behaviour are known, we can consider the influence of the polysilicon layer over the silicon oxide. In Fig. 5 the evolution with the annealing temperature of the TO3 peak position in a polysilicon-thermal silicon oxide-silicon structure for three different polysilicon deposition temperatures is shown. As the shift found is higher than the maximal shift for the annealing effects in thermal oxides, we can conclude that these changes are produced by the polysilicon layers by means of an induced stress effect. A shift in the IR peak position before and after deposition of layers over the thermal silicon oxide has been reported previously [ 191.
BOO
900
1000
1100
1200
Anneal Temp (deg)
Fig. 5. TO, absorption peak position vs. annealing temperature for the polysilicon-thermal silicon oxide structures. The polysilicon layers were deposited at 550 “C ( l ), 590 “C (t) and 630 “C ( n ). The dashed line corresponds to the theoretical value obtained by simulation taking into account the thermal silicon oxide dielectric function and the geometrical features of structure.
30“E
20.
”
‘c z
o
*2 si z v)
lo-
0 --
-1o-
L L
Gi
-2o-
‘307~oo Anneal Temp (deg)
Fig. 6. Stress measurement by X-ray diffraction for 1 lo-oriented polysilicon crystallites in the same polysilicon-thermal silicon oxide structures as in Fig. 5.
In order to corroborate the induced stress in the SiOz layer originated by the polysilicon deposition, we have compared the observed evolution with the results obtained using X-ray diffraction analysis, which allows the grain strain values in the polysilicon layers to be determined. Figure 6 shows the biaxial stress versus the annealing temperature. The polysilicon samples deposited at 590 “C were also analysed using Raman scattering. Figure 7 shows the strain evolution determined by the shift of the Raman peak position and corroborates that the 590 “C deposited polysilicon layers are in a tensile stress state, which vanishes as
352 521.0
Unstrained
1 _
520.5-
position
----
7 E ” 7;
520.0-
: a
800
PO0
,140
low
T annealing
(“C )
Fig. 7. Shift of Raman peak position in samples of polysilicon-thermai silicon oxide. The polysiiicon was deposited at 590 “C. The dashed line corresponds to unstrained silicon.
thermal oxide. However, there is a shift towards tensile strain values. This effect can be due to the different behaviour of the two oxide types with the annealing temperature. The high compressive stress evolution in the LPCVD oxide would be the origin of a tensile stress component in the polysilicon. The different evolution observed with annealing temperature between polysilicon layers deposited at 630 “C and lower temperatures could be due to the different structure of the as-deposited layers. The layers deposited at 630 “C are polycrystalline, whereas the others are amorphous. This behaviour has been observed clearly in the evolution of preferential orientation.
Conclusions o.olz 1
Palysilicon layers deposited ot 63O’C LPCVD
-
o.0087;to Anneal
oxide
1244
temp f*Cf
Fig. 8. Strain measurement by X-ray diffraction for IIO-oriented potysilicon crystallites. (+) corresponds to polysilicon-LPCVD silicon oxide and ( n) to polysilicon-thermal silicon oxide.
the annealing temperature increases. This evolution is in agreement with that obtained from X-ray analysis. Comparing the FT-IR peak shift evolution in the thermal oxide with the stress in the polysilicon layer observed by X-rays shows a complementary behaviour. Therefore, this result seems to indicate that the internal stress of the polysilicon layer modifies the strain values of the thermal oxide, inducing stresses of the opposite sign in the silicon oxide region near the interface. For the polysilicon layers deposited at 630 ‘C over the LPCVD silicon oxide, we observe by X-ray diffraction (Fig. 8) a similar evolution to that previously obtained in polysilicon over the
We have analysed polysilicon-SiO, -silicon structures by different physical techniques such as FT-IR spectroscopy and X-ray diffraction. These microscopic techniques give info~ation about the physical mechanisms that are the origin of the stress state observed by macroscopic techniques such as warpage measurements. In this sense, from the experimental results obtained, it can be deduced that the annealing process in the LPCVD silicon oxide produces a high compressive strain in the wafer plane, due to a decrease of density in the layer. Moreover, the deposition of polysilicon layers over silicon oxide induces stresses of the opposite sign over it, as observed by a shift in the TOLt peak position. Finally, we remark the utility of FT-IR spectroscapy as a non-destructive technique for analysing the intrinsic or induced stress behaviour in silicon oxide layers.
References 1 J. R. Ferraro and K. Krishan (eds.), Practical Fourier Transform Infkared Spectroscapy, Academic Press, New York, 1989. 2 J. Bausells, J. Esteve, J. Samitier and J. R. Morante, Micromechanics activities of CNM and UB, Tech. Dig@, Micromechanics Europe, 1990, pp. 169- 174. 3 D. Gallem, P. Kopystynski and E. Obermeier, Thickness measurement of silicon diaphragms by transmission interference spectroscopy, Tech. Digest, Micromechanics Europe, 1990, pp. 69-74. 4 P. N. Sen and M. F. Thorpe, Phonons in AX, glasses: from molecular to band-like modes, P/I~s. Rev. B, lS( 1977) 4030-4038. 5 F. L. Galeener, Band limits and the vibrational spectra of tetrahedral glasses, Phys. Reu. B, 19 (1979) 4292-4297.
353
6 C. H. Bjorkman, J. T. Fitch and G. Lucovsky, Correlation between midgap interface state density and thickness-averaged oxide stress and strain at Si/SiO, interface formed by thermal oxidation of Si, Appl. Phys. Lett., 46 (1990) 1983-1985. 7 G. Lucovsky, M. J. Manitini, J. K. Srivastava and E. I. Irene, Low-temperature growth of silicon dioxide films: A study of chemical bonding by ellipsometry and infrared spectroscopy, J. Vat. Sci. Tech&. B, 5 (1987) 530-537. 8 IRPLANT”, made by Spectra-tTech, Norwalk, CT, USA. 9 T. 1. Kamins, M. M. Mandurah and K. S. Saraswat, Structure and stability of low pressure chemically vapor deposited silicon films, J. Electrochem. Sm., 125 (1978) 927-932. 10 G. Ha&eke, L. Krausbauer, E. F. Steigmeier, A. E. Widmer, H. F. Kappert and G. Neugebauer, Growth and physical properties of LPCVD polycrystalline silicon films, J. Electrochem. Sm., I31 (1984)
675-682.
11 A. J. Perry, M. Jagner, W. D. Sproul and P. J. Rudnik, Residual stress and strain distribution anomalies in TIN films deposited by physical vapor deposition, Surf. Coat. Technol., 42 (1990) 49-68. 12 B. D. Cullity, Elements of X-ray Diffraction, Addison-Wesley, Reading, MA, 1978, Ch. 16.
I3 H. Toraya, Whole-powder-pattern fitting without reference to a structural model: application to X-ray powder diffractometer data, J. Appl. Crust., 19 (1986) 440-447. 14 M. L. Naiman, C. T. Kirk, R. J. Aucoin, F. J. Terry, P. W. Wyatt and S. D. Senturia, Effect of nitridation of silicon dioxide on its infrared spectrum, J. Electrochem. Sm., 131 (1984) 637-640. 15 E. A. Irene, E. Tierney and J. Angilello, A viscous flow model to explain the appearance of high density thermal SiO, at low oxidation temperatures, J. Electrochem. Sot., 129 (1982) 25942597. 16 Y. Shioya
and M. Maeda, Comparison of phosphosilicate glass films deposited by three different chemical vapor deposition methods, J. Electrochem. Sm., 133 (1986) 1943- 1950. 17 R. A. B. Devine, On the structure of low temperature PECVD silicon dioxide films, J. Electron. Mater., 19 (1990) 1299-1301. 18 F. Ferrieu and R. A. B. Devine, Densification and porosity in low-temperature deposited oxide, J. Non-tryst. Solids, 113 (1989) 100-102. 19 J. Neal Cox and R. Rastogi, The effects of overlying layers on the FTIR spectra of thin thermal oxides on silicon, Marer. Rex Sot. Symp. Proc., JO5 (1988) 157-162.
A, 32 (1992)
347-353
347
Analysis by FT-IR spectroscopy of SiO,-polycrystalline in micromechanics: stress measurements
structures used
J. Samitier, S. Marco, 0. Ruiz and J. R. Morante Deparramento Fisica Aplicada i Electrbnica, Universitat de Barcelona, Avda. Diagonal 645-647, E-08028 Barcelona (Spain)
J. Esteve-Tinto
and J. Bausells
Centro National de Microelectrdnica-CSIC,
Campus Universitat Authzoma de Barcelona, 08193 Behterra
(Spain)
Abstract The evolution of the intrinsic stress with the annealing temperature in silicon oxide layers thermally grown and deposited by the low-pressure chemical vapour method has been analysed by the reflection-absorption Fourier transform infrared (FT-IR) technique. The results show that the annealing process in the low-pressure chemical vapour deposition (LPCVD) silicon oxide produces a high compressive stress in the wafer plane, due to a decrease of density in the jayer. Moreover, for overlying structures used in micromechanics, such as polycrystalline silicon-silicon oxide, we,have pointed out the existence of induced stress between the layers. The stress evolution in the silicon oxide layers has been compared with the results obtained using warpage and X-ray diffraction measurements.
Introduction
The fabrication of micromechanical and microsensor devices involves the use of thin-film deposition techniques. Low pressure chemical vapour deposition (LPCVD) silicon oxide and polycrystalline silicon thin layers are currently used as sacrificial, passivation or structural layers in micromechanical devices based on silicon technology. It is well known that tensile or compressive stress distributions are almost always present in these deposited films. A first stress contribution is due to the different thermal expansion coefficients between the thin layer and the substrate. However, the measured stress values do not usually agree with the stress behaviour envisaged by the thermal mismatches and, in consequence, other stress contributions must be taken into account. These stresses may be intrinsic to the film, caused by impurities, voids and dislocation distributions, or they may arise from morphology changes within the films, such as grain growth, recryatallization or phase changes. Moreover, the final stress distribution of a thin layer can be affected by the intrinsic stress values of the substrate, or other thin films in the case of multilayered structures. O924-4247/92/%5.00
This mechanical residual stress causes great problems in the fabrication of micromechanical structures, such as wrinkling in thin diaphragms and deflection in cantilevers. In these applicatiolis the control of the structural and mechanical properties of the layers is critical, because they affect the performance of the device and even its feasibility. The goal of this work is to study the intkmal stress evolution of silicon oxide/polysilicon structures deposited over a silicon substrate as a function of the deposition and annealing temperatures, in order to obtain improvements in micromechanical devices, and the relations between the macroscopic stress behaviour observed and the structural and technological characteristics of the thin layers used. Generally, the multilayered structure stress can be obtained by warpage measurements using, for example, laser deflection or profilometers. Although these techniques allow the final average stress values to be obtained, it is difficult to extract the physical mechanisms which are the origin of the stress state observed. The microscopic stress effects in silicon oxide and the influence of the annealing treatments and polysilicon layer deposition have been analysed by Fourier transform infrared spectroscopy (FT-IR), which is sensitive to @ 1992 -
Elsevier Sequoia.
All rights reserved
348
modifications of the bond structure. The obtained stress evolution in t.he silicon oxide layers has been compared with the intrinsic stress in the polysilicon layers measured by X-ray diffraction and with the macroscopic results obtained by warpage measurements.
Samples
Polysilicon and silicon oxide films were deposited in an LPCVD reactor using pure silane and silane and oxygen, respectively. In order to analyse the LPCVD silicon oxide behaviour and its possible influence in the polysilicon layer, we have compared the LPCVD oxide measurements with the ones obtained with high-temperature thermal oxide grown in dry oxygen. Two inch (100) lightly doped silicon wafers were divided into three groups: (i) 1.0 and 1.5 um thick LPCVD silicon oxide films were deposited at 430 “C. The silane-oxygen flow ratio used was 1.6 with a pressure of 200 mTorr. (ii) 1.8 urn thick polysilicon films were deposited at 630 “C with a pressure of 80 mTorr, over 1.5 pm thick LPCVD silicon oxide deposited at 430 “C. (iii) 4500 A thick polysilicon films were deposited over 1000 8, of thermal oxide grown at T = 950 “C in dry oxygen atmosphere at temperatures between 550 and 630 “C and’80 mTorr pressure. Wafers from each group were annealed in a nitrogen atmosphere at temperatures between 800 and 1150 “C for 30 min.
Experimental
method
FT-IR spectroscopy is an interesting non-destructive technique for the bond analysis of multilayered structures [ 11. It has also been used in microsensor technology to measure the thickness of either thin layers deposited over silicon substrates or silicon diaphragms obtained by chemical etching [2, 31. The FT-IR bond characterization of microelectronic dielectric layers, such as silicon oxide, is performed in the middle infrared spectral region
(wave number range 400 cm-’ to 5000 cm-‘). In this range, the dielectric function of the oxide shows its poles and zeros, which are due to the different transverse and longitudinal optical vibrational modes of the bond unit. They determine the presence of absorption peaks related to these modes in the transmission or reflection spectra. Changes in stoichiometry, density and structure in the oxide layers affect the position and shape of these peaks, corresponding to the rocking mode near 450 cm-‘, bending near 800 cm-’ and stretching vibration near 1075 cm-‘. Theoretical calculations based on the force constant model for the vibrational properties of amorphous silica [4,5] predict that the frequency of the bond stretching vibration, v, is correlated to the silicon-silicon interatomic distance, dsi_si, through the Si-0-Si bond angle, 28 [6, 71: v = v0 sin(e)
with v0 a constant value
(1)
and d,i _si = 2ro sin 0
where r. is the Si-0 bond length (2)
Then a change of the angle 28, which can originate from internal and/or induced stress, produces a modification of the absorption stretching frequency. In this sense, the Si-Si interatomic distance obtained from eqn. (2) can be used as a measurement of the thickness averaged strain, E,, in the solution oxide layer, via E, =
(v/bed
-
1
(3)
where v,,~ is the stretching frequency value of a fully relaxed SiOZ layer. However, it must be noted that modifications in the angle 28 could also originate from mechanisms other than intrinsic or applied stress, possibly related with changes in the internal structure of the silicon oxide random network. Then, as the microscopic structure of the silicon oxide network is not completely known, it is necessary to carry out an accurate analysis in order to determine the corresponding Si02 fully relaxed frequency value. The reported values [7] for the extensively analysed high-temperature thermally grown SiOz corresponding to eqns. (1) and (2) are 20 = 144”, vo= 1134cm-‘, ro= 1.6 A and v,~ = 1078 cm-‘.
349
On the other hand, the absorption peak positions obtained in the transmittance or reflectance FT-IR spectra also depend on the optical geometrical features of the samples, such as thicknesses and refraction indices, so it is necessary to take into account these perturbative effects by means of a simulation program. Moreover, the fitting of the interference path observed in the FT-IR spectra allows both the thickness and the reflection index of layers to be determined. With these values, we can estimate the theoretical absorption peak position for a given geometry, V,im. The experimental set-up used for the FT-IR measurements has been a BOMEM DA3 spectrometer, which allows variable-angle transmittance and reflectance measurements to be made with a resolution of 0.02 cm-‘, and a BOMEN MB-120 spectrometer interfaced with the IRPLAN infrared spectroscope [8], which allows an infrared light spot of about 25 pm to be obtained using masking apertures and can be used to test mechanical microstructures in situ. The polysilicon films were analysed by X-ray diffraction. This technique has been commonly used to determine the texture (preferred orientation) [9], the degree of crystallinity and crystallite size [lo] of polysilicon layers. However, it is well known that X-ray diffraction can be used as a method of stress measurement in polycrystalline layers. The samples were measured in a Siemens D-500 diffractometer. All the X-ray work was carried out with Cu Ka and Fe Ka radiation (penetration depths 71 and 38 pm, respectively). A maximum of six peaks is found in the 28 range explored. However, the only peaks with enough intensity to be used in the stress results were {111) and (220). The most usual method of determining the stress in polycrystalline materials using X-ray diffraction is measuring the slope of the lattice parameter versus sin’ 0 plots [ 11, 121. However, the configuration of our goniometer imposes a geometric limitation on the maximum 8 angle available, so we have measured the deviation of the X-ray diffraction peak from the theoretical value. The accuracy of the measurement of the scattering angle is of major importance in the determination of residual stress via diffraction peak shift, This determines the accuracy of the interplanar
spacing and hence the strain. The most commonly used method for determining peak positions is a parabolic fit near the top of a peak. However, when the peaks are weak, we can obtain better precision by fitting with pseudo-Voigt functions plus a linear baseline than with parabolae [ 131. The developed routine also furnishes the error in the estimated peak position, which was always smaller than 0.01”. In order to obtain the biaxial stress values, u, from the measured strain in the z direction, E=, a suitable elastic constant must be calculated taking into account the crystal anisotropy. The values obtained are: grains
(11 l},
G = -5.17
x 10”ez dyn/cm’
grains
{ 1lo>,
c = - 13.16 x lO’*e, dyn/cn?.
Results and discussion
First we have analysed the strain behaviour in the silicon oxide layers using FT-IR spectroscopy. Figure 1 shows both the experimental data corresponding to an FT-IR absorbance spectrum of 950 “C thermally grown oxide in dry oxygen and the simulated spectrum obtained taking into account the oxide thickness and the Si02 theoretical dielectric function for high-temperature thermally oxidized SiOz [ 141. In both spectra, the maximum of the stretching vibrational mode is at 1072 cm-‘.
Wovenumbers
km-‘)
Fig. 1. (a) Simulated spectrum (dashed line) taking into account the thermal oxide dielectric function [ 141. (b) Experimental spectrum (continuous line) corresponding to a transmission absorbance spectrum obtained at 0”. of 100 nm thermal silicon oxide grown at 950 “C.
A LPCVD oxide as
B fhermal
oxide simulation
\x,
I 1350
1300
1250
Wavenumbers
1200
1150
1100
I 1050
(cm-‘)
Fig. 2. (A) Reflection spectrum obtained at 28” in the as-deposited 1.5 urn LPCVD silicon oxide layer. (B) Simulated spectrum taking into account the thermal oxide dielectric function [ 141 and the geometrical configuration.
lllOsA
1200
T annealing(C)
Fig. 3. Shift of TO, peak position in samples of LPCVD silicon oxide subjected to an annealing process. The IR spectra were obtained by reflection at 28”. The as-deposited LPCVD peak position is 1105 cm-‘.
Nevertheless, LPCVD silicon oxide shows a different behaviour (Fig. 2). The reflectance spectrum obtained at 28” in the as-deposited layers present a TO, peak position significantly lower than the value obtained by simulation taking into account the thermal oxide dielectric function and the geometrical configuration. This shift indicates a diminution of the Si-0-Si angle. After the annealing process, we observe a strong evolution of the TO9 absorption peak towards higher values (Fig. 3). So, the Si-0-Si angle increases to reach the value corresponding to a thermal grown oxide.
For the thermally grown oxides an annealing process at a temperature higher than the growth one produces a small shift in the TO3 peak position towards higher values. It is well known that thermal oxides have a non-uniform compressive stress across their thickness, having higher values near the Si-SiOz interface [6]. Theoretical models of the growth process for thermal Si02 suggest that the lack of free volume during growth to accommodate the density differences between SiO, and Si should lead to stresses in the layer. Annealing processes carried out at temperatures higher than the growth one partially relax this stress, although the stress value at the interface remains nearly constant. The assumed relaxation mechanism considers the viscous flow of a Maxwell solid [ 151. For our annealing temperatures the maximum TO3 peak position shifts are about 2 cm-‘. Comparing the above results, it can be concluded that the as-deposited LPCVD silicon oxide presents a high compressive stress, which relaxes after a high-temperature annealing. However, the warpage measurements of the LPCVD oxide layers over the silicon substrate point out the existence of a compressive stress in the as-deposited SiOz layers that strongly increases with the annealing temperature (Fig. 4). This last result is in agreement with the curvature measurements previously reported by Shioma and Maeda [ 161 in phosphosilicate glasses. In consequence, the strong IR shift in the asdeposited LPCVD silicon oxide cannot be produced only by an internal compressive stress and
c
e iii
800
900
1000
no0
1
T annealing(C)
Fig. 4. Strain in samples of LPCVD silicon oxide subjected annealing process obtained by warpage measurements.
to an
351
could originate from a different network structure to that of thermally oxide grown. This structure would be more compact, because the shift towards lower wavenumbers involves a diminution of the Si-Si distance (see eqn. (2)); in consequence there will be an increase of density. From the above hypothesis, the increase of the compressive stress with the annealing temperature in LPCVD silicon oxide can be explained as follows: when the LPCVD silicon oxide is submitted to a high-temperature anneal, the Si-0-Si angle tends to increase, and also the distance between silicon atoms. This increase produces a volume expansion. However, the layer can exbecause in pand only in the z-direction, the x-y plane the layer is attached to the substrate wafer. This process produces a compressive stress in this plane, which increases as the network relaxes towards the thermal silicon oxide structure. This hypothesis is in agreement with the results reported by several authors [ 17, 181 in plasmaenhanced chemical vapour deposition (PECVD) silicon oxide films. In this case, they have observed a similar IR peak evolution with annealing temperature, suggesting also that low-temperature PECVD silicon oxide has a network structure composed of densified amorphous Si02, which can relax with a high-temperature annealing. From the former results, we can conclude that the low temperature as-deposited silicon oxide has a denser network structure than the high-temperature thermally grown silicon oxide, which gives a different stress evolution with the annealing temperature. This densification would not be due to the existence of a high compressive stress. Once the mechanisms and evolution of the silicon oxide stress behaviour are known, we can consider the influence of the polysilicon layer over the silicon oxide. In Fig. 5 the evolution with the annealing temperature of the TO3 peak position in a polysilicon-thermal silicon oxide-silicon structure for three different polysilicon deposition temperatures is shown. As the shift found is higher than the maximal shift for the annealing effects in thermal oxides, we can conclude that these changes are produced by the polysilicon layers by means of an induced stress effect. A shift in the IR peak position before and after deposition of layers over the thermal silicon oxide has been reported previously [ 191.
BOO
900
1000
1100
1200
Anneal Temp (deg)
Fig. 5. TO, absorption peak position vs. annealing temperature for the polysilicon-thermal silicon oxide structures. The polysilicon layers were deposited at 550 “C ( l ), 590 “C (t) and 630 “C ( n ). The dashed line corresponds to the theoretical value obtained by simulation taking into account the thermal silicon oxide dielectric function and the geometrical features of structure.
30“E
20.
”
‘c z
o
*2 si z v)
lo-
0 --
-1o-
L L
Gi
-2o-
‘307~oo Anneal Temp (deg)
Fig. 6. Stress measurement by X-ray diffraction for 1 lo-oriented polysilicon crystallites in the same polysilicon-thermal silicon oxide structures as in Fig. 5.
In order to corroborate the induced stress in the SiOz layer originated by the polysilicon deposition, we have compared the observed evolution with the results obtained using X-ray diffraction analysis, which allows the grain strain values in the polysilicon layers to be determined. Figure 6 shows the biaxial stress versus the annealing temperature. The polysilicon samples deposited at 590 “C were also analysed using Raman scattering. Figure 7 shows the strain evolution determined by the shift of the Raman peak position and corroborates that the 590 “C deposited polysilicon layers are in a tensile stress state, which vanishes as
352 521.0
Unstrained
1 _
520.5-
position
----
7 E ” 7;
520.0-
: a
800
PO0
,140
low
T annealing
(“C )
Fig. 7. Shift of Raman peak position in samples of polysilicon-thermai silicon oxide. The polysiiicon was deposited at 590 “C. The dashed line corresponds to unstrained silicon.
thermal oxide. However, there is a shift towards tensile strain values. This effect can be due to the different behaviour of the two oxide types with the annealing temperature. The high compressive stress evolution in the LPCVD oxide would be the origin of a tensile stress component in the polysilicon. The different evolution observed with annealing temperature between polysilicon layers deposited at 630 “C and lower temperatures could be due to the different structure of the as-deposited layers. The layers deposited at 630 “C are polycrystalline, whereas the others are amorphous. This behaviour has been observed clearly in the evolution of preferential orientation.
Conclusions o.olz 1
Palysilicon layers deposited ot 63O’C LPCVD
-
o.0087;to Anneal
oxide
1244
temp f*Cf
Fig. 8. Strain measurement by X-ray diffraction for IIO-oriented potysilicon crystallites. (+) corresponds to polysilicon-LPCVD silicon oxide and ( n) to polysilicon-thermal silicon oxide.
the annealing temperature increases. This evolution is in agreement with that obtained from X-ray analysis. Comparing the FT-IR peak shift evolution in the thermal oxide with the stress in the polysilicon layer observed by X-rays shows a complementary behaviour. Therefore, this result seems to indicate that the internal stress of the polysilicon layer modifies the strain values of the thermal oxide, inducing stresses of the opposite sign in the silicon oxide region near the interface. For the polysilicon layers deposited at 630 ‘C over the LPCVD silicon oxide, we observe by X-ray diffraction (Fig. 8) a similar evolution to that previously obtained in polysilicon over the
We have analysed polysilicon-SiO, -silicon structures by different physical techniques such as FT-IR spectroscopy and X-ray diffraction. These microscopic techniques give info~ation about the physical mechanisms that are the origin of the stress state observed by macroscopic techniques such as warpage measurements. In this sense, from the experimental results obtained, it can be deduced that the annealing process in the LPCVD silicon oxide produces a high compressive strain in the wafer plane, due to a decrease of density in the layer. Moreover, the deposition of polysilicon layers over silicon oxide induces stresses of the opposite sign over it, as observed by a shift in the TOLt peak position. Finally, we remark the utility of FT-IR spectroscapy as a non-destructive technique for analysing the intrinsic or induced stress behaviour in silicon oxide layers.
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