- Email: [email protected]
Silicon nitride films prepared by high energy ion beam enhanced deposition
NOMB
Nuclear Instruments and Methods in Physics Research B 86 (1994) 293-297 North-Holland
Silicon nitride films prepared enhanced deposition
Beam Intonctions with Materials & Atoms
by high energy ion beam
J. Zemek a,*, F. &rnf b, M. ZKtovi a, M. Van6Eek a and V. ieleznJi a a Institute of Physics, Acad. Sci. of the Czech Republic, Cukrovamickn’ 10, 162 00 Prague 6, Czech Republic b Faculty of Mechanical Engineering,
Czech Technical lJniversi&
Technicka’ 4, 164 07 Prague 6, Czech Republic
Received 13 August 1993 and in revised form 11 November 1993
Amorphous silicon nitride films have been synthesized by a simultaneous electron beam evaporation of silicon and high energy nitrogen ion bombardment. Surface sensitive techniques, angular-resolved X-ray induced photoelectron spectroscopy (ARXPS) and Auger electron spectroscopy (AES), have been used for the surface characterization of air-exposed films and concentration depth profiling. Optical and photoelectrical properties have been investigated via infra-red (IR) transmission spectroscopy and photoconductivity measurements. A specific feature of high ion beam kinetic energies and high atomic arrival rate ratios of N/Si is that a part of the silicon nitride is grown inside the silicon substrates. It is suggested that ion beam bombardment enhanced diffusion of nitrogen from deeper regions can contribute to the silicon nitride growth.
1. Introduction
In the past decade many experiments were carried out in films preparation by simultaneous deposition and ion bombardment [l-8]. This method is usually called ion beam assisted deposition (IBAD) or ion beam enhanced deposition (IBED). Using this technology it is possible to easily control the composition, thickness and some basic properties of the films [3,4]. It is also of practical interest that films are deposited at relatively low temperature and can be considered as hydrogen free. Up to now most of the IBED experiments were carried out at low ion energy (less than 1000 eV) [3]. The highest ion beam energy, which was used in the IBED preparation of silicon nitride films, was 40 keV [6,7]. The nitrogen content and depth distribution in IBED silicon nitride films have been studied by RBS [5,6,8] and AES [5-81, the structure of the films by TEM [6], and the chemical bonding by IR measurements [5,6,8]. The depth distribution of nitrogen atoms has also been calculated by a simple model calculation and Monte Carlo simulation with reasonable fit to the experimental data [6,7]. However, there are no reports available on photoconductivity measurements and the composition and chemical bonding of the near-surface
* Corresponding 3123184.
author, tel. +42 2 8422419, fax +42 2
region of air-exposed IBED silicon nitride films frequently used for an investigation. The present paper reports on the formation and basic properties of IBED amorphous silicon nitride on silicon substrates in the high ion beam energy range up to 92 keV. At such high ion beam energies the silicon nitride growth could start even inside the silicon substrate (just beneath the original silicon surface), and hence excellent adhesion properties are expected.
2. Experimental
details
A scheme of the chamber where IBED is performed is shown in Fig. 1. The equipment was evacuated using a diffusion pump with a liquid nitrogen trap. Polished Si(ll1) wafers were mounted to a holder, fixed to a water-cooled manipulator. The Si wafers as well as the growing films were bombarded by energetic nitrogen ions with kinetic energy in the range 60-92 keV at an angle of 45” or 80” with respect to the sample normal. The substrate temperature was kept below 200°C during thin film growth. All silicon nitride films were found to be amorphous. Layer thicknesses were measured by a crystal microbalance thickness monitor. The ratio of fluxes of nitrogen to silicon atoms, evaluated from the nitrogen ion beam density and the layer thickness, was varied for various samples in the range from 0.1 to 4.4. X-ray photoelectron spectra were measured at room temperature by an ADES-400 spectrometer using
0168-583X/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDI 0168-583X(93)E0865-E
J. Zemek et al. / Nucl. Znstr. and Meth. in Phys. Res. B 86 (1994) 293-297
294
r
r-4
THICKNESS
etch rate was determined to be about 0.15 rim/s as determined by mechanical measurement of the step height after sputtering the sample for an extended period. IR spectra were measured with the FTIR spectrometer Bruker 113 in the 400-3000 cm-’ range at room and liquid-nitrogen temperatures. Spectral response of silicon nitride-silicon heterojunctions was measured in the range 420-1240 nm. Electrodes from colloid graphite were used in configuration: graphite-silicon nitride-silicon-graphite, with an illumination from the silicon nitride side. The primary photocurrent was measured for a constant number of photons with the help of a lock-in-amplifier, the light was chopped with frequency 3-150 Hz. Reflection measurements were used for a correction of the primary photocurrent spectrum.
M°FiToR
ION
SAMPLE
L-
SOURCE
I
I
EVAPORATOR _I
Fig. 1. A schematic view of the IBED preparation chamber.
MgKa radiation (1253.6 eV, 110 W). The energy analyzer was operated in the constant energy mode at 20, 50 or 100 eV pass energy. The apparatus was caliline at 83.8 eV. Angular rebrated by the Au4f,,, solved spectra were recorded in regions of Si2p, Nls, 0 1s and C 1s lines at fixed position of the sample by moving the electron analyzer around the sample, at take-off angles of O-50” relative to the sample normal. All spectra were charge corrected with respect to the C 1s line at 285.0 eV [9]. AES measurements were performed at room temperature using a Varian AES system equipped with a cylindrical mirror analyzer and the integral electron gun. The first derivative spectra of Si, N, 0 and C KLL transitions were recorded at the primary electron energy and current of 3000 eV and 1 X 10e6 A. The conversion of the line intensities to the atomic concentrations is based on a quantitative approach applying standards of Si, SiO, and Si,N, rods cleaved or broken under ultrahigh vacuum condition. Sputter-etching was accomplished with the help of a Xe ion beam characterized by an energy and current of 1000 eV and 1 X 10e4 A, of diameter 1 cm at the target. The mean
3. Results and discussion 3.1. Concentration
depth profiling
Concentration depth profiles of the IBED silicon nitride films reveal three different regions (see Fig. 2). The near-surface region is silicon-rich. This is expected considering the finite nitrogen ion ranges (see Table 1) and the fact that the film growth is completed when the silicon evaporation and nitrogen ion implantation are simultaneously interrupted. The effect has been recently reported [6,7]. The thickness of the silicon-rich region can be tailored by variation of the energy and angle of ion beam incidence. It may also be reduced by possible nitrogen transport from deeper regions of the sample, as discussed below. Beneath the near-surface region, nearly uniform layers of silicon nitride were synthesized. The layers were found to be stoichiometric (as shown in Fig. 2) or substoichiometric depending on the energy, angle of
Table 1 IBED silicon nitride samples analyzed by XPS: E - the nitrogen ion beam energy, A - the nitrogen ion beam incident angle with respect to the sample normal, R, and R, - the projected ranges of atomic nitrogen ions [lo] calculated for Si and Si,N, targets, corrected for the nitrogen ion beam incident angle, d - the film thickness, C$ - the atomic arrival rate ratio of N/Si fluxes of nitrogen atoms to fluxes of silicon atoms, Q - charging of the sample surface during the photoemission experiment. Sample no.
A
Rl
R2
d
:eV]
[dwl
bml
bl
bml
1 2 3 4 5 6 7
92 92 92 85 60 60 80
80 80 80 45 45 45 45
41 41 41 156 111 111 147
26 26 26 98 70 70 93
a Position of an envelope maximum.
178 89 79 92 190 186
Binding energy/FWHM
4
_ 0.4 0.3 2.5 0.2 0.3 0.7
[eV]
EVI
Si2p a
Nls
01s
1.5 1.3 1.4 2.4 3.0 3.7 4.2
101.5/3.0 101.9/2.3 102.6/3.1 102.9/2.3 103.5/1.7 103.4/2.2 103.3/2.1
397.3/1.5 397.6/U 397.6/1.6 398.3/2.0 - 398.6/2.1 398.7/2.1
532.2/2.0 532.6/1.9 532.5/1.8 532.9/2.0 532.9/1.7 532.9/2.0 532.7/2.0
J. Zemek et al./ Nucl. Instr. and Meth. in Phys.Rex B 86 (1994) 293-297
LAYER
295
[13], where a high dose nitrogen ion implantation Si wafers is investigated. An interface region between silicon nitride silicon is in all samples rather broad, reflecting finite nitrogen ion ranges at the beginning of deposition process (see Fig. 2 and Table 1).
: SUBSTRATE
into and the the
3.2. Photoelectron spectra 0
200 SPUTTERING
400
600
(set)
TIME
Fig. 2. Typical concentration depth profile of IBED silicon nitride film, deposited with the ion beam energy of 60 keV, the atomic arrival rate ratio d(N)/&%) = 4.4 and at a nitrogen ion incident angle of 45”. Thickness of the deposited layer and the mean etch rate used during depth profiling was
evaluated to be 28.9 nm and 0.15 rim/s..
ion beam incidence and the N/M ratios of atomic fluxes. Based on the mean value of the sputtering rate and the film thickness, the deposited silicon nitride layer of Fig. 2 is sputter-etched in about 200 s, which is about one third of the etching time spent for depth profiling of the whole silicon nitride. It means that in this case a non-negligible part of the silicon nitride is grown into the silicon substrate. The corresponding depth (position) of the silicon nitride/ silicon interface depends on the ion beam energy and the incident angle. The silicon nitride formation in the Si substrate is not mentioned for IBED films prepared under lower ion beam energy where only a nitrogen tail is observed in the corresponding substrates [6,7]. Also the simple model calculations and Monte Carlo simulations used in ref. [7] for a nitrogen ion energy of 40 keV anticipate only the nitrogen tail. The finding of silicon nitride beneath the original Si substrate surface is clearly consistent with the calculated projected ranges [lo]. For 60 keV, 4.5” and silicon (Si,N,) targets the ranges are, respectively: 111 nm (70 nm) for N+ and 54 nm (34 nm) for Nl dissociated at the target surface. Considering also the standard deviations, one expects that the nitrogen content in the grown layer of Fig. 2 (above the original surface of Si wafer) should not be sufficient for the Si,N, formation. Hence, some transport of nitrogen from the substrate regions with nitrogen concentration exceeding the stoichiometric limit [ll] is suggested. The transport mechanism is most likely athermal, because of the low deposition temperature used. This is concordant with the recent observation of anomalous diffusion of nitrogen in SiO, under ion bombardment [12], ascribed to the ion bombardment induced diffusion. Nitrogen transport towards the surface is also suggested in ref.
In this section the air-exposed surface and nearsurface regions of the IBED silicon nitride are characterized by means of ARXPS. It may be useful to note that the information depth of the method used is estimated to be 5-6 nm for Si2p electrons. Several important technological parameters, the projected ranges of atomic nitrogen ions, charging of the samples, binding energies (BE) and corresponding line widths (FWHM) of Si 2p, N 1s and 0 1s photoelectron lines are summarized in Table 1. Due to the peak overlapping, the BE of the Si2p lines is taken as an envelope maximum. Judging from numerous technological parameters the near-surface regions appear to be mainly influenced by the angle (A) of ion beam incidence: (1) surface charging of the sample during the photoemission experiment is substantially higher for A = 45”; (2) BEs (corrected for charging) of Si 2p, N 1s and 0 1s lines are shifted to higher values for A = 45”; (3) FWHM parameters are larger for Si 2p lines (A = 80”) and N 1s lines (45”), while the FWHM of 0 1s lines seems to be independent of the angle A. As mentioned above, concentration depth profile experiments reveal the existence of a silicon-rich nearsurface region. For A = 80” the region is thinner, contains a higher amount of nitrogen, and the chemical
100
105 Binding
(eV1 energy
Fig. 3. XPS Si2p spectra of IBED film surfaces (both taken at normal take-off angle): sample 4 deposited at an angle of nitrogen ion beam incidence of 45” with respect to the surface normal (full line), sample 3 deposited at 80” (dashed line).
J. Zemek et al./Nucl. Instr. and Meth. in Phys. Rex B 86 (1994) 293-297
A Si 2p
J\ la)
0’
50’
I
.
1
100
105
Binding
105
100
IeVl energy
IeV) energy
Binding
Fig. 4. XPS Si2p spectra taken at take-off angles 0” and 50” with respect to the sample normal: (a) sample 3 deposited at an angle of nitrogen ion beam incidence of go”, (b) sample 4 deposited at 45”.
bonding of silicon atoms is more complex. The different silicon bonding is illustrated in Fig. 3, where Si2p lines taken from surfaces of two IBED films are com-
atmosphere) a weak band in the vicinity of - 480 cm-’ appears [17].
pared. The wider line with a tail at the low binding energy side corresponds to the sample prepared under A = 80”. In this case the signal shape indicates Si-N and Si-0 chemical bonding states, the latter originating from different suboxides. A reasonable model for the near-surface region, deduced from the ARXPS data (see also Fig. 4a), consists of a SiO, top surface layer followed by the silicon nitride. For A = 45” the shape of the Si 2p line (Fig. 3) is almost symmetric with prevailing Si-0 bonds. Shifts in BEs and larger values of the FWHM for N 1s lines, shown in Table 1, indicate a silicon oxynitride formation. This is concordant with similar shifts reported by other authors [14,15]. In addition, photoelectron spectra taken at different take-off angles (see Fig. 4b) and hence at different information depths, indicate that the uppermost surface region is composed mainly of silicon dioxide followed by silicon oxynitride, SiN,O,, in a deeper region.
3.4. Photoelectrical spectra The samples exhibit a high photocurrent value in the wavelength region around 1000 nm (Fig. 6). This corresponds to homogeneous absorption in the bulk of the crystalline silicon (c-Si) wafer (this is the region where the optical absorption coefficient of c-Si is of the order of lo* cm-‘). The photocurrent minimum at about 600 nm corresponds to an absorption depth of about 1 pm (the absorption coefficient here is equal to lo4 cm-‘). We attribute this minimum to an increased recombination in the subsurface layer of c-Si, damaged by the energetic ions during the deposition of the silicon nitride layer.
70
’
I
I
I
60 -
3,
3.3. IR spectra The absorption bands belonging to the c-Si lattice vibrations and Si-0 bonds, present in all the curves shown in Fig. 5, originate from the absorption in the substrate, as obvious from a comparison with the spectrum of the c-Si substrate (curve 11. In addition the spectra of all our silicon nitride films display a strong and broad band at - 850 cm-‘. Absorption in this region is typical for the Si-N vibrations [16,17]. The large width of the band may be caused by the presence of additional absorption of compounds such as Sic, carbonates, SiO,, etc. After anneal (1 h at 900°C in Ar
2
-
1
20 10
I
500
I I 1000 1500 WAVENUMBERS
I 2000 tcrii’)
2500
Fig. 5. Typical IR transmission spectra of two IBED films and of c-Si substrate (11, &(N)/&(Si) = 2.4 (2) &N)/&Si) = 0.2 (3).
J. Zemek et aL/ Nucl. fnstr. and Meth. in Phys. Res. 3 86 @994) 293-297
297
angle of incidence and N/Si arrival rate ratio part of the silicon nitride can be grown into the Si substrate. This fact explains the exceIIent adhesion properties. - Photoconductiui~ measu~me~ts can be used for characterization of the density of recombination centers in the silicon nitride layer and in the spatially deeper region of the Si-rich interface.
Acknowledgements
The authors wish to express their thanks to A. Abraham for the reflectivity measurements. This work was supported by the Research Project 11032/GA/ CSAV. i50
mo
750
900
1050
1200
WAVELENGTH [m-n] Fig. 6. Spectral response of silicon nitride/silicon heterojunction for a constant number of impinging photons, corrected for reflection.
For shorter wavelengths the photocurrent starts to increase sharply again. We ascribe this to the absorption in the silicon nitride heterojunction region, which has a much lower density of recombination centers than the above described region of high recombination. This can give us some indication about the quality of the silicon nitride layer.
4. Summsry We would like to emphasize several specific feaof the IBED silicon nitride films deduced from our experimental data. - The near-surface regions of the silic5n nitride flms: These regions are silicon-rich. Composition and bondtures
ing in the near-surface regions of air-exposed films appear to be nitrogen ion incidence angle dependent. - Siticon nitride layers grown on silicon s~strates: Deposition conditions can be optimized for the stoichiometric silicon nitride growth, if desirable. There is evidence for athermal, ion bombardment induced diffusion of nitrogen from the substrate to the growing silicon nitride layer. - Silicon nitrides grown into the depth of the silicon
substrates: In dependence
on the ion beam energy,
References [l] D.M. Mattox, J. Vat. Sci. Technol. A 7 (1989) 1105. [2] M. Kiuchi, K. Fujii, T. Tanaka and M. Satou, Nucl. Instr. and Meth. B 33 (1988) 649. [3] J.J. Cuomo and S.M. Rossnagel, Nucl. Instr. and Meth. B 19/20 (1987) 963. [4] D. Van Vechten, G.K. Hubler and E.P. Donovan, J. Vat. Sci. Technol. A 6 (1988) 1934. [5] E.P. Donovan, D.R. Brighton, G.K. Hubler and D. Van Vechten, Nucl. Instr. and Meth. B 19/20 (1987) 983. [6] X. Liu, B. Xue, Z. Zheng, Z. Zhou and S. Zou, Nucl. Instr. and Meth. B 39 (1989) 185. [7] J. Zhou, D. Fink, J.P. Biersack, G. Yang, B. Jiang, X. Liu and S. Zou, Thin Solid Films 197 (1991) 381. [8] Y. Yu, X. Liu, Z. Fang and S. Zou, Appl. Surf. Sci. 40 (1989) 145. [91 Th. Gross, M. Ramm, H. Sonntag, W. Unger, H.M. Weijers and E.H. Adem, Surf. Interf. Anal. 18 (1992) 59. DOI J.F. Gibbons, W.S. Johnson and S.W. Mylroie, Projected Range Statistics (Stroudsburg, Pennsylvania, 1975). 1111 H.-J. Fusser and H. Oechsner, Surf. Coat. Technot. 48 (1991) 97. [121 I. Banejee and D. Kuzminov, Appl. Phys. Lett. 62 (1993) 1541. t131 E. Ya. Chemyak, Sov. Phys.-Tech. Phys. 36 (1991) 1184. D41 S. Hasegawa, L. He, T. Inokuma and Y. Kurata, Phys. Rev. B 46 (1992) 12478. iI51 J.A. Taylor, G.M. Lancaster, A. Ignatiev and J.W. Rabalais, J. Chem. Phys. 68 (1978) 1776. &I H. Watanabe, K. Katch and M. Yasui, Jpn. J. Appl. Phys. 21(1982) I-341. 1171 S. Fuita, H. Toyoshima, T. Ohishi and A. Sasaki, Jpn. J. Appl. Phys. 23 (1984) L268.
Nuclear Instruments and Methods in Physics Research B 86 (1994) 293-297 North-Holland
Silicon nitride films prepared enhanced deposition
Beam Intonctions with Materials & Atoms
by high energy ion beam
J. Zemek a,*, F. &rnf b, M. ZKtovi a, M. Van6Eek a and V. ieleznJi a a Institute of Physics, Acad. Sci. of the Czech Republic, Cukrovamickn’ 10, 162 00 Prague 6, Czech Republic b Faculty of Mechanical Engineering,
Czech Technical lJniversi&
Technicka’ 4, 164 07 Prague 6, Czech Republic
Received 13 August 1993 and in revised form 11 November 1993
Amorphous silicon nitride films have been synthesized by a simultaneous electron beam evaporation of silicon and high energy nitrogen ion bombardment. Surface sensitive techniques, angular-resolved X-ray induced photoelectron spectroscopy (ARXPS) and Auger electron spectroscopy (AES), have been used for the surface characterization of air-exposed films and concentration depth profiling. Optical and photoelectrical properties have been investigated via infra-red (IR) transmission spectroscopy and photoconductivity measurements. A specific feature of high ion beam kinetic energies and high atomic arrival rate ratios of N/Si is that a part of the silicon nitride is grown inside the silicon substrates. It is suggested that ion beam bombardment enhanced diffusion of nitrogen from deeper regions can contribute to the silicon nitride growth.
1. Introduction
In the past decade many experiments were carried out in films preparation by simultaneous deposition and ion bombardment [l-8]. This method is usually called ion beam assisted deposition (IBAD) or ion beam enhanced deposition (IBED). Using this technology it is possible to easily control the composition, thickness and some basic properties of the films [3,4]. It is also of practical interest that films are deposited at relatively low temperature and can be considered as hydrogen free. Up to now most of the IBED experiments were carried out at low ion energy (less than 1000 eV) [3]. The highest ion beam energy, which was used in the IBED preparation of silicon nitride films, was 40 keV [6,7]. The nitrogen content and depth distribution in IBED silicon nitride films have been studied by RBS [5,6,8] and AES [5-81, the structure of the films by TEM [6], and the chemical bonding by IR measurements [5,6,8]. The depth distribution of nitrogen atoms has also been calculated by a simple model calculation and Monte Carlo simulation with reasonable fit to the experimental data [6,7]. However, there are no reports available on photoconductivity measurements and the composition and chemical bonding of the near-surface
* Corresponding 3123184.
author, tel. +42 2 8422419, fax +42 2
region of air-exposed IBED silicon nitride films frequently used for an investigation. The present paper reports on the formation and basic properties of IBED amorphous silicon nitride on silicon substrates in the high ion beam energy range up to 92 keV. At such high ion beam energies the silicon nitride growth could start even inside the silicon substrate (just beneath the original silicon surface), and hence excellent adhesion properties are expected.
2. Experimental
details
A scheme of the chamber where IBED is performed is shown in Fig. 1. The equipment was evacuated using a diffusion pump with a liquid nitrogen trap. Polished Si(ll1) wafers were mounted to a holder, fixed to a water-cooled manipulator. The Si wafers as well as the growing films were bombarded by energetic nitrogen ions with kinetic energy in the range 60-92 keV at an angle of 45” or 80” with respect to the sample normal. The substrate temperature was kept below 200°C during thin film growth. All silicon nitride films were found to be amorphous. Layer thicknesses were measured by a crystal microbalance thickness monitor. The ratio of fluxes of nitrogen to silicon atoms, evaluated from the nitrogen ion beam density and the layer thickness, was varied for various samples in the range from 0.1 to 4.4. X-ray photoelectron spectra were measured at room temperature by an ADES-400 spectrometer using
0168-583X/94/$07.00 0 1994 - Elsevier Science B.V. All rights reserved SSDI 0168-583X(93)E0865-E
J. Zemek et al. / Nucl. Znstr. and Meth. in Phys. Res. B 86 (1994) 293-297
294
r
r-4
THICKNESS
etch rate was determined to be about 0.15 rim/s as determined by mechanical measurement of the step height after sputtering the sample for an extended period. IR spectra were measured with the FTIR spectrometer Bruker 113 in the 400-3000 cm-’ range at room and liquid-nitrogen temperatures. Spectral response of silicon nitride-silicon heterojunctions was measured in the range 420-1240 nm. Electrodes from colloid graphite were used in configuration: graphite-silicon nitride-silicon-graphite, with an illumination from the silicon nitride side. The primary photocurrent was measured for a constant number of photons with the help of a lock-in-amplifier, the light was chopped with frequency 3-150 Hz. Reflection measurements were used for a correction of the primary photocurrent spectrum.
M°FiToR
ION
SAMPLE
L-
SOURCE
I
I
EVAPORATOR _I
Fig. 1. A schematic view of the IBED preparation chamber.
MgKa radiation (1253.6 eV, 110 W). The energy analyzer was operated in the constant energy mode at 20, 50 or 100 eV pass energy. The apparatus was caliline at 83.8 eV. Angular rebrated by the Au4f,,, solved spectra were recorded in regions of Si2p, Nls, 0 1s and C 1s lines at fixed position of the sample by moving the electron analyzer around the sample, at take-off angles of O-50” relative to the sample normal. All spectra were charge corrected with respect to the C 1s line at 285.0 eV [9]. AES measurements were performed at room temperature using a Varian AES system equipped with a cylindrical mirror analyzer and the integral electron gun. The first derivative spectra of Si, N, 0 and C KLL transitions were recorded at the primary electron energy and current of 3000 eV and 1 X 10e6 A. The conversion of the line intensities to the atomic concentrations is based on a quantitative approach applying standards of Si, SiO, and Si,N, rods cleaved or broken under ultrahigh vacuum condition. Sputter-etching was accomplished with the help of a Xe ion beam characterized by an energy and current of 1000 eV and 1 X 10e4 A, of diameter 1 cm at the target. The mean
3. Results and discussion 3.1. Concentration
depth profiling
Concentration depth profiles of the IBED silicon nitride films reveal three different regions (see Fig. 2). The near-surface region is silicon-rich. This is expected considering the finite nitrogen ion ranges (see Table 1) and the fact that the film growth is completed when the silicon evaporation and nitrogen ion implantation are simultaneously interrupted. The effect has been recently reported [6,7]. The thickness of the silicon-rich region can be tailored by variation of the energy and angle of ion beam incidence. It may also be reduced by possible nitrogen transport from deeper regions of the sample, as discussed below. Beneath the near-surface region, nearly uniform layers of silicon nitride were synthesized. The layers were found to be stoichiometric (as shown in Fig. 2) or substoichiometric depending on the energy, angle of
Table 1 IBED silicon nitride samples analyzed by XPS: E - the nitrogen ion beam energy, A - the nitrogen ion beam incident angle with respect to the sample normal, R, and R, - the projected ranges of atomic nitrogen ions [lo] calculated for Si and Si,N, targets, corrected for the nitrogen ion beam incident angle, d - the film thickness, C$ - the atomic arrival rate ratio of N/Si fluxes of nitrogen atoms to fluxes of silicon atoms, Q - charging of the sample surface during the photoemission experiment. Sample no.
A
Rl
R2
d
:eV]
[dwl
bml
bl
bml
1 2 3 4 5 6 7
92 92 92 85 60 60 80
80 80 80 45 45 45 45
41 41 41 156 111 111 147
26 26 26 98 70 70 93
a Position of an envelope maximum.
178 89 79 92 190 186
Binding energy/FWHM
4
_ 0.4 0.3 2.5 0.2 0.3 0.7
[eV]
EVI
Si2p a
Nls
01s
1.5 1.3 1.4 2.4 3.0 3.7 4.2
101.5/3.0 101.9/2.3 102.6/3.1 102.9/2.3 103.5/1.7 103.4/2.2 103.3/2.1
397.3/1.5 397.6/U 397.6/1.6 398.3/2.0 - 398.6/2.1 398.7/2.1
532.2/2.0 532.6/1.9 532.5/1.8 532.9/2.0 532.9/1.7 532.9/2.0 532.7/2.0
J. Zemek et al./ Nucl. Instr. and Meth. in Phys.Rex B 86 (1994) 293-297
LAYER
295
[13], where a high dose nitrogen ion implantation Si wafers is investigated. An interface region between silicon nitride silicon is in all samples rather broad, reflecting finite nitrogen ion ranges at the beginning of deposition process (see Fig. 2 and Table 1).
: SUBSTRATE
into and the the
3.2. Photoelectron spectra 0
200 SPUTTERING
400
600
(set)
TIME
Fig. 2. Typical concentration depth profile of IBED silicon nitride film, deposited with the ion beam energy of 60 keV, the atomic arrival rate ratio d(N)/&%) = 4.4 and at a nitrogen ion incident angle of 45”. Thickness of the deposited layer and the mean etch rate used during depth profiling was
evaluated to be 28.9 nm and 0.15 rim/s..
ion beam incidence and the N/M ratios of atomic fluxes. Based on the mean value of the sputtering rate and the film thickness, the deposited silicon nitride layer of Fig. 2 is sputter-etched in about 200 s, which is about one third of the etching time spent for depth profiling of the whole silicon nitride. It means that in this case a non-negligible part of the silicon nitride is grown into the silicon substrate. The corresponding depth (position) of the silicon nitride/ silicon interface depends on the ion beam energy and the incident angle. The silicon nitride formation in the Si substrate is not mentioned for IBED films prepared under lower ion beam energy where only a nitrogen tail is observed in the corresponding substrates [6,7]. Also the simple model calculations and Monte Carlo simulations used in ref. [7] for a nitrogen ion energy of 40 keV anticipate only the nitrogen tail. The finding of silicon nitride beneath the original Si substrate surface is clearly consistent with the calculated projected ranges [lo]. For 60 keV, 4.5” and silicon (Si,N,) targets the ranges are, respectively: 111 nm (70 nm) for N+ and 54 nm (34 nm) for Nl dissociated at the target surface. Considering also the standard deviations, one expects that the nitrogen content in the grown layer of Fig. 2 (above the original surface of Si wafer) should not be sufficient for the Si,N, formation. Hence, some transport of nitrogen from the substrate regions with nitrogen concentration exceeding the stoichiometric limit [ll] is suggested. The transport mechanism is most likely athermal, because of the low deposition temperature used. This is concordant with the recent observation of anomalous diffusion of nitrogen in SiO, under ion bombardment [12], ascribed to the ion bombardment induced diffusion. Nitrogen transport towards the surface is also suggested in ref.
In this section the air-exposed surface and nearsurface regions of the IBED silicon nitride are characterized by means of ARXPS. It may be useful to note that the information depth of the method used is estimated to be 5-6 nm for Si2p electrons. Several important technological parameters, the projected ranges of atomic nitrogen ions, charging of the samples, binding energies (BE) and corresponding line widths (FWHM) of Si 2p, N 1s and 0 1s photoelectron lines are summarized in Table 1. Due to the peak overlapping, the BE of the Si2p lines is taken as an envelope maximum. Judging from numerous technological parameters the near-surface regions appear to be mainly influenced by the angle (A) of ion beam incidence: (1) surface charging of the sample during the photoemission experiment is substantially higher for A = 45”; (2) BEs (corrected for charging) of Si 2p, N 1s and 0 1s lines are shifted to higher values for A = 45”; (3) FWHM parameters are larger for Si 2p lines (A = 80”) and N 1s lines (45”), while the FWHM of 0 1s lines seems to be independent of the angle A. As mentioned above, concentration depth profile experiments reveal the existence of a silicon-rich nearsurface region. For A = 80” the region is thinner, contains a higher amount of nitrogen, and the chemical
100
105 Binding
(eV1 energy
Fig. 3. XPS Si2p spectra of IBED film surfaces (both taken at normal take-off angle): sample 4 deposited at an angle of nitrogen ion beam incidence of 45” with respect to the surface normal (full line), sample 3 deposited at 80” (dashed line).
J. Zemek et al./Nucl. Instr. and Meth. in Phys. Rex B 86 (1994) 293-297
A Si 2p
J\ la)
0’
50’
I
.
1
100
105
Binding
105
100
IeVl energy
IeV) energy
Binding
Fig. 4. XPS Si2p spectra taken at take-off angles 0” and 50” with respect to the sample normal: (a) sample 3 deposited at an angle of nitrogen ion beam incidence of go”, (b) sample 4 deposited at 45”.
bonding of silicon atoms is more complex. The different silicon bonding is illustrated in Fig. 3, where Si2p lines taken from surfaces of two IBED films are com-
atmosphere) a weak band in the vicinity of - 480 cm-’ appears [17].
pared. The wider line with a tail at the low binding energy side corresponds to the sample prepared under A = 80”. In this case the signal shape indicates Si-N and Si-0 chemical bonding states, the latter originating from different suboxides. A reasonable model for the near-surface region, deduced from the ARXPS data (see also Fig. 4a), consists of a SiO, top surface layer followed by the silicon nitride. For A = 45” the shape of the Si 2p line (Fig. 3) is almost symmetric with prevailing Si-0 bonds. Shifts in BEs and larger values of the FWHM for N 1s lines, shown in Table 1, indicate a silicon oxynitride formation. This is concordant with similar shifts reported by other authors [14,15]. In addition, photoelectron spectra taken at different take-off angles (see Fig. 4b) and hence at different information depths, indicate that the uppermost surface region is composed mainly of silicon dioxide followed by silicon oxynitride, SiN,O,, in a deeper region.
3.4. Photoelectrical spectra The samples exhibit a high photocurrent value in the wavelength region around 1000 nm (Fig. 6). This corresponds to homogeneous absorption in the bulk of the crystalline silicon (c-Si) wafer (this is the region where the optical absorption coefficient of c-Si is of the order of lo* cm-‘). The photocurrent minimum at about 600 nm corresponds to an absorption depth of about 1 pm (the absorption coefficient here is equal to lo4 cm-‘). We attribute this minimum to an increased recombination in the subsurface layer of c-Si, damaged by the energetic ions during the deposition of the silicon nitride layer.
70
’
I
I
I
60 -
3,
3.3. IR spectra The absorption bands belonging to the c-Si lattice vibrations and Si-0 bonds, present in all the curves shown in Fig. 5, originate from the absorption in the substrate, as obvious from a comparison with the spectrum of the c-Si substrate (curve 11. In addition the spectra of all our silicon nitride films display a strong and broad band at - 850 cm-‘. Absorption in this region is typical for the Si-N vibrations [16,17]. The large width of the band may be caused by the presence of additional absorption of compounds such as Sic, carbonates, SiO,, etc. After anneal (1 h at 900°C in Ar
2
-
1
20 10
I
500
I I 1000 1500 WAVENUMBERS
I 2000 tcrii’)
2500
Fig. 5. Typical IR transmission spectra of two IBED films and of c-Si substrate (11, &(N)/&(Si) = 2.4 (2) &N)/&Si) = 0.2 (3).
J. Zemek et aL/ Nucl. fnstr. and Meth. in Phys. Res. 3 86 @994) 293-297
297
angle of incidence and N/Si arrival rate ratio part of the silicon nitride can be grown into the Si substrate. This fact explains the exceIIent adhesion properties. - Photoconductiui~ measu~me~ts can be used for characterization of the density of recombination centers in the silicon nitride layer and in the spatially deeper region of the Si-rich interface.
Acknowledgements
The authors wish to express their thanks to A. Abraham for the reflectivity measurements. This work was supported by the Research Project 11032/GA/ CSAV. i50
mo
750
900
1050
1200
WAVELENGTH [m-n] Fig. 6. Spectral response of silicon nitride/silicon heterojunction for a constant number of impinging photons, corrected for reflection.
For shorter wavelengths the photocurrent starts to increase sharply again. We ascribe this to the absorption in the silicon nitride heterojunction region, which has a much lower density of recombination centers than the above described region of high recombination. This can give us some indication about the quality of the silicon nitride layer.
4. Summsry We would like to emphasize several specific feaof the IBED silicon nitride films deduced from our experimental data. - The near-surface regions of the silic5n nitride flms: These regions are silicon-rich. Composition and bondtures
ing in the near-surface regions of air-exposed films appear to be nitrogen ion incidence angle dependent. - Siticon nitride layers grown on silicon s~strates: Deposition conditions can be optimized for the stoichiometric silicon nitride growth, if desirable. There is evidence for athermal, ion bombardment induced diffusion of nitrogen from the substrate to the growing silicon nitride layer. - Silicon nitrides grown into the depth of the silicon
substrates: In dependence
on the ion beam energy,
References [l] D.M. Mattox, J. Vat. Sci. Technol. A 7 (1989) 1105. [2] M. Kiuchi, K. Fujii, T. Tanaka and M. Satou, Nucl. Instr. and Meth. B 33 (1988) 649. [3] J.J. Cuomo and S.M. Rossnagel, Nucl. Instr. and Meth. B 19/20 (1987) 963. [4] D. Van Vechten, G.K. Hubler and E.P. Donovan, J. Vat. Sci. Technol. A 6 (1988) 1934. [5] E.P. Donovan, D.R. Brighton, G.K. Hubler and D. Van Vechten, Nucl. Instr. and Meth. B 19/20 (1987) 983. [6] X. Liu, B. Xue, Z. Zheng, Z. Zhou and S. Zou, Nucl. Instr. and Meth. B 39 (1989) 185. [7] J. Zhou, D. Fink, J.P. Biersack, G. Yang, B. Jiang, X. Liu and S. Zou, Thin Solid Films 197 (1991) 381. [8] Y. Yu, X. Liu, Z. Fang and S. Zou, Appl. Surf. Sci. 40 (1989) 145. [91 Th. Gross, M. Ramm, H. Sonntag, W. Unger, H.M. Weijers and E.H. Adem, Surf. Interf. Anal. 18 (1992) 59. DOI J.F. Gibbons, W.S. Johnson and S.W. Mylroie, Projected Range Statistics (Stroudsburg, Pennsylvania, 1975). 1111 H.-J. Fusser and H. Oechsner, Surf. Coat. Technot. 48 (1991) 97. [121 I. Banejee and D. Kuzminov, Appl. Phys. Lett. 62 (1993) 1541. t131 E. Ya. Chemyak, Sov. Phys.-Tech. Phys. 36 (1991) 1184. D41 S. Hasegawa, L. He, T. Inokuma and Y. Kurata, Phys. Rev. B 46 (1992) 12478. iI51 J.A. Taylor, G.M. Lancaster, A. Ignatiev and J.W. Rabalais, J. Chem. Phys. 68 (1978) 1776. &I H. Watanabe, K. Katch and M. Yasui, Jpn. J. Appl. Phys. 21(1982) I-341. 1171 S. Fuita, H. Toyoshima, T. Ohishi and A. Sasaki, Jpn. J. Appl. Phys. 23 (1984) L268.