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Structural and optical properties of nitrided silicon oxide layers rapid thermally grown in a pure N2O ambient
IOURNA
1, O F
Journal of Non-Crystalline Solids 187 (1995) 380-384
ELSEVIER
Structural and optical properties of nitrided silicon oxide layers rapid thermally grown in a pure N 2 0 ambient E. H a r t m a n n s g r u b e r " ' * ,
U . R o s s o w b, A. H o y e r a, P. L a n g e a
a Fraunhofer-lnstitutJ~r Siliziumtechnologie, Dillenburger Str. 53, 14199 Berlin, Germany b lnstitutfdr Festkfrperphysik der TU Berlin, Sekr. PN6-1, Hardenbergstr. 36, 10623 Berlin, Germany
Abstract
Thin nitrided silicon oxide layers with thicknesses in the 3-12 nm range were grown in a rapid thermal processor in a pure N20 ambient. These layers were investigated by X-ray photoelectron spectroscopy, spectroscopic ellipsometry and wet etching experiments and compared with oxides grown in a 02 ambient. Information about the distribution and binding properties of nitrogen in the oxide and its interfacial properties are provided. The nitrogen concentration peaks at the interface and decreases towards the oxide bulk to an extent of approximately 5 nm for a film thickness of 9 nm. Compared with 02 grown oxides, the nitrided oxides show an enhanced saturation of Si-dangling bonds by nitrogen and a reduction of the suboxide regime. These facts may explain the up to now reported improved electrical performance of metal oxide semiconductor devices using N20 grown oxides as the gate dielectric.
1. Introduction
Thin nitrided silicon oxide layers rapid thermally grown in a N 2 0 ambient either directly on silicon or on a pregrown SiO2 film on silicon have attracted much attention recently. Their application as gate dielectrics in the complementary metal oxide semiconductor (CMOS) technology promise a better performance of ultra-large-scale integrated (ULSI) devices when gate oxide thicknesses below 10 nm are required. C o m p a r e d with conventional oxides the N 2 0 grown dielectrics profit from the benefits of nitrogen incorporation into SiO2 such
* Corresponding author. Tel: + 49-30 8299 8431. Telefax: +49-30 8299 8199.
as an increase of the dielectric strength and an enhanced resistance against hot carrier stress as well as impurity penetration I-1-4] without suffering from the problems related to hydrogen incorporation occurring from N H 3 thermal nitridation
1,-5]. Nevertheless, up to now the mechanisms of nitrogen incorporation into N 2 0 grown oxides have not been completely understood and therefore also the physical properties of these dielectrics have to be examined. Of significant interest is the structure of the Si/SiOxN r interfacial region, which is assumed to be particularly responsible for the resulting electrical properties of M O S devices. Most work which has been done in this field was focussed on concentration depth profiling of the incorporated nitrogen as obtained from secondary ion mass spectroscopy (SIMS) [2,4,6], Auger electron
0022-3093/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD1 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 1 6 8 - 9
E. Hartmannsgruber et al. /Journal of Non-Crystalline Solids 187 (1995) 380 384
spectroscopy (AES) [1,7], elastic recoil detection (ERD) [7], and X-ray photoelectron spectroscopy (XPS) [-8]. Results showed that the nitrogen is mainly located at the Si/SiOxN r interface with a concentration of a few a t . ° , which decreases towards the oxide bulk. The structural arrangement in the bulk of N 2 0 grown and conventionally grown oxides was found to be almost identical by Fourier transform infrared (FTIR) spectroscopy [-9]. Chemical analysis of XPS [8] and FTIR [9] spectra showed the formation of SiN bonds to be much more likely than that of N O bonds. The goal of this work is the presentation of new results concerning the incorporation of nitrogen at the interface and in the bulk of the oxide. The distribution of nitrogen was obtained from XPS and wet chemical etch rate measurements. Results from spectroscopic ellipsometry (SE) in the visible and UV spectral region will be discussed with respect to the extent and structure of the interfacial transition region of N 2 0 grown as compared to 02 grown oxides. For these investigations we prepared N 2 0 oxide samples with a relative high nitrogen content by directly oxidizing silicon substrates in a pure N 2 0 ambient in a R T P r e a c t o r . For comparison, non-nitrided oxides were grown in a pure 0 2 ambient in the same reactor.
2. Experimental procedure The R T P system (AST 100) is equipped with 21 tungsten halogen lamps for both sides illumination of single wafers with a 100 mm diameter lying inside a cold-wall quartz reactor. This arrangement is surrounded by a highly reflective gold-plated process chamber. The temperature is monitored by a pyrometer with a sensitivity maximum at 2.7 ~tm. An additional silicon reflection ring is placed around the wafer to enhance the temperature uniformity across the wafer. More details of the R T P equipment have been given in Ref. [-9]. The silicon substrates used were n-type, phosphorous-doped with a resistivity of 1-3 f~cm and (1 0 0 ) orientation. They were transferred from the storage box to the reactor without any precleaning procedure to avoid surface roughening which might result from
381
native oxide etching. Thin layers with thicknesses in the 2-12 nm range were grown either in a pure N 2 0 ambient (RTON) or, for comparison, in a pure 0 2 ambient (RTO) at 1150°C. The gas flow was parallel to the wafer surface with a rate adjusted to 51/min in both cases. A single wavelength ellipsometer (SWE) equipped with a H e - N e laser (632.8 nm) was used to determine the layer thicknesses directly after growth and the wet chemical etch rates of R T O N and RTO samples in a highly diluted (about 1 : 100) H F : H 2 0 solution. The refractive index used was fixed to a value of 1.475 assuming a SiO2 bulk layer, which is thinner than 20 nm [10]. In addition, a spectroscopic ellipsometer with rotating analyzer was used in the spectral range 1.8-5.5 eV. In this case the raw data were converted into an effective dielectric function ( e ) [11,12] which describes the effective response of the oxide layer and the silicon substrate. For the interpretation of the measured ( e ) modelling is necessary. For a single layer on a substrate a so-called three-phase model is appropriate [12]. In this model the three phases are vacuum, layer and substrate, assuming no interfacial layer. An effective dielectric function can be calculated with the well-known dielectric function of silicon [11] and that of the layer as well as the layer thickness as input. As a layer dielectric function that of SiOz [,13] was used for the non-nitrided as well as the nitrided oxides. This serves as a good approximation since the nitrogen content in the R T O N samples is small (lower than 5 a t . ° , see below) and the optical gaps of an amorphous SiOxNy compound are in the UV above 5.5 eV. Deviations between the measured and calculated effective dielectric function must be correlated for the interfacial or layer properties and can be compared between R T O N and RTO samples. XPS spectra were recorded using a commercially available XPS system (VG ESCALAB) using A1 K~ radiation (1486.6 eV) and a hemispherical photoelectron energy analyser. From the Si 2p, N ls and O ls core level peaks the binding energy and the concentration of these elements were derived. For concentration depth profiling argon ion sputtering was used.
E. Hartmannsgruber et al. / Journal of Non-Crystalline Solids 187 (1995) 380-384
382
100 90 ! 80
Si02
70
c-Si
10 '9 8
10
~,
7 *~
"-=60~
1:100 HF:H20
6
i6 N
0040 (3 30 .,_20 10 0
i ~F !
8 q
4 o 3 ~, z ,2
4
Q X
o
2
0 0 1 2 3 4 5 6 7 8 9 10 Ar-SputterTime [rain]
Fig. 1. XPS depth profile of an N 2 0 oxide layer with 10 n m thickness with the concentrations in at. % of silicon, oxygen, and nitrogen. The Si/SiOxN r interface is indicated by the dashed line. The solid lines are drawn as guides for the eye.
50
1O0 150 etch time [sec]
200
Fig. 2. Oxide thickness as a function of etching time in highly diluted HF. The dashed line indicates the region where the measurement is effected by the growth of native oxide. The solid lines are drawn as guides for the eye.
3. Results and discussion
3 - 4 nm due to the escape depth of the photoelectrons and sidewall effects of the sputtered crater.
3.1. XPS measurements
3.2. Wet etch experiments
A great deal of spectroscopic data for N 2 0 o x addressing the concentration of nitrogen at the interface, bulk and surface has been published. However, recently a discrepancy in the atomic percentage of interfacial nitrogen was reported [6]. For the purposes of comparison, we would like to discuss very briefly the XPS results that we obtained on our samples. F r o m the depth profiles for a R T O N sample of 10 nm thickness a nitrogen concentration of about 4 at.% at the interface was determined with decreasing concentration towards the bulk of the oxide (Fig. 1). This result corresponds well to that reported in Ref. [8]. According to the suggestions made therein for an evaluation of the measured binding energies we obtained 295.0 eV for the Si 2p (oxide) and N ls peak separation and 429.4 eV for the Si 2p (oxide) and O ls peak separation indicating that the nitrogen is solely bonded to silicon and not to oxygen. Depth profiling of the binding energies was not performed since argon ion sputtering induces damage of the chemical environment. The depth resolution of XPS for concentration profiling is restricted to
A more exact estimate of the extent of the nitrogen containing region is provided by the measurement of the wet chemical etch rate of the layer. This method is based on the fact that Si02 can easily be removed by H F while the etch rate for Si3N4 is rather low. Therefore, regions with a higher nitrogen content etch slower in H F than those with a lower nitrogen concentration [14]. Fig. 2 shows the measured film thickness over the etching time in highly diluted H F for a R T O N and, for comparison, for a R T O sample of nearly the same original thickness. In contrast to the constant etch rate of the R T O sample we observed for the R T O N sample a more or less continuous etch rate reduction. The onset of this effect is observable roughly 5 nm above the interface. Below this thickness a decrease of the slope of the curve takes place indicating an increasing amount of incorporated nitrogen. However, a quantification of the nitrogen content from this measurement appears difficult, because nitrogen-induced network relaxation effects can also contribute to the etch rate reduction. Nevertheless, it becomes clear that this method is very
ides
E. Hartmannsgruber et al. / Journal of Non-Crystalline Solids 187 (1995) 380-384
so I
,
.
~
~,
'
.
Re
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s.9 nm'~:'L
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o A
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2
- 10L i 1Prim %:ON/Si 40
20-
Re
ENERSY (eV)
02
-20
,
,
2
5
,
,
.
4
,
5
6
Fig. 4. Difference spectra of the imaginary part of the effective dielectric function A i m ( e ) as a function of energy for N 2 0 grown oxides (2.7nm solid line, 3.9nm dashed line) and 02 grown oxides (3.2 nm dotted dashed line and 4.6 nm dashed line).
FNERCY (eV)
Fig. 3. Energy dependence of the effective dielectric function (e,) for a 12 and a 2.7 nm layer of N20 grown oxide. The dashed lines indicate the calculated spectra.
effective to estimate the spatial extent of the nitrogen containing region. 3.3. S E m e a s u r e m e n t s
Finally, information about the electronic properties of the interface can be provided by applying spectroscopic ellipsometry as a non-destructive method. In Fig. 3 the energy dependence of the effective dielectric function ( e ) for a 12 and a 2.7 nm thick layer is displayed. The dashed lines represent the simulated spectra. The fundamental transitions are also indicated. The three-phase model fits very well the experimental spectra of both samples, although some deviations appear, which increase with decreasing layer thickness. This effect was also reported for conventionally grown SiO2 films [15] where the observed deviations were attributed to interfacial properties, which are of course more effective with decreasing layer thickness. To estimate any differences in this regard between nitrided and non-nitrided oxides a series of both oxide types with thicknesses below 10 nm was measured. For all measured samples modelling was performed and these data were
subtracted from the experimental ones. Fig. 4 shows the difference in the imaginary part of the effective dielectric function A i m ( e ) as a function of energy for two N 2 0 and two 0 2 grown oxides. In this figure three distinct features are observable, at the Ez-gap, at the E~), El-gap, and in the region around 3 eV. The two first-named features are of the same order of magnitude for all samples and can therefore not be related to the incorporated nitrogen in the R T O N samples. A deviation at the Ez-gap can be attributed to the surface roughness of silicon at the interface to the oxide [16] and a deviation at the E~), El-gap may be caused by a change of the silicon bulk properties. By contrast, the peak located around 3 eV observed in Fig. 4 shows a strong dependence on the layer thickness and on the type of growth process. Since the only difference between R T O N and R T O samples is the nitrogen appearance in the former this feature must be effected by the incorporated nitrogen. The smallest deviations at 3 eV in Fig. 4 are appearing for the nitrided samples. Since a good fit for R T O N was obtained with the properties of SiO2 and no consideration of nitrogen, it is most likely that no nitrogen-related transitions can contribute significantly to the spectrum. This assumption is also supported by the fact that the optical gap of an amorphous Si3N4 layer is in the UV region above 5.5 eV. If there is a deviation at 3 eV occurring, the question for the nature of these
384
E. Hartmannsgruber et al. / Journal of Non-Crystalline Solids 187 (1995) 380-384
t r a n s i t i o n s a p p e a r s . It has been r e p o r t e d [17 19] t h a t the s a t u r a t i o n of S i - d a n g l i n g b o n d s b y arsenic a n d h y d r o g e n leads to a b u l k t e r m i n a t i o n of silicon. This m e a n s t h a t the silicon states at the surface resemble m u c h m o r e those in the silicon b u l k t h a n in the case of a clean surface. C o n s e q u e n t l y , the dielectric function of a clean surface differs from t h a t where the d a n g l i n g b o n d s are saturated. W e are c o n v i n c e d t h a t a similar effect occurs for the system u n d e r investigation. T h e n i t r o g e n at the interface is s a t u r a t i n g Si-dangling b o n d s a n d leads to a b u l k t e r m i n a t i o n of silicon. T h e r e m a i n i n g small d e v i a t i o n s b e t w e e n fit a n d e x p e r i m e n t a l d a t a are d u e to the r e m a i n i n g n o n - s a t u r a t e d S i - d a n g l i n g bonds. In turn, the larger d e v i a t i o n s a p p e a r i n g for the R T O s a m p l e s are reflecting an increased n u m b e r of d a n g l i n g bonds. If the interfacial region, which c o n t a i n s the d a n g l i n g b o n d s , is viewed as a t r a n s i t i o n region, which c o n t a i n s also SixOy suboxides, a d d i t i o n a l a r g u m e n t s for the e x p l a n a t i o n a b o v e can be p r o vided. SiO, one species of the possible SixOy suboxides has a b a n d g a p a r o u n d 3 eV, thus optical t r a n s i t i o n s at this energy m a y c o n t r i b u t e to this spectrum. F r o m the m a g n i t u d e of d e v i a t i o n s in the spectra, it can be c o n c l u d e d t h a t in the case of nitrided oxides this s u b o x i d e c o n t a i n i n g i n t e r l a y e r is reduced.
4. Conclusion F r o m the results r e p o r t e d we c o n c l u d e t h a t the extent of the interfacial t r a n s i t i o n regime, which is m a i n l y caused by interface roughness, is very similar for N 2 0 a n d 0 2 g r o w n oxides. A p r o n o u n c e d S i - d a n g l i n g b o n d s a t u r a t i o n by n i t r o g e n occurs a n d the region in which s u b o x i d e s are formed, however, a p p e a r s significantly s m a l l e r for the N 2 0 oxides. These findings m a y c o n t r i b u t e to the exp l a n a t i o n of the i m p r o v e d electrical p e r f o r m a n c e of N 2 0 oxides as c o m p a r e d to 0 2 oxides. B e y o n d this, an effective m e t h o d has been p r e s e n t e d to
locate the d i s t r i b u t i o n of n i t r o g e n within the oxide layer b y wet chemical etching in highly diluted H F . F u r t h e r m o r e , X P S d a t a were p r e s e n t e d which correlate well with the results r e p o r t e d earlier by o t h e r research groups. P a r t of this w o r k was funded by the C E C E s p r i t Basic Research a c t i o n no. 6878 ' E A S I ' a n d b y the J E S S I B T 1 B / E S P R I T 7236 project ' A D E Q U A T ' .
References I-1] H. Hwang, W. Ting, D.L. Kwong and J. Lee, IEEE IEDM Tech. Dig. (1990) 421. 1,2] A. Uchiyama, H. Fukuda, T. Hayashi, T. Iwabuchi and S. Ohno, IEEE IEDM Tech. Dig. (1990) 425. 1,3] H. Fukuda, T. Arakawa and S. Ohno, IEEE Trans. Electron Dev. 39 (1992) 127. 1,4] Y. Okada, P.J. Tobin, R.I. Hedge, J. Liao and P. Rushbrook, Appl. Phys. Lett. 61 (1992) 3163. 1,5] T. Hori, T. Yasui and S. Akamatsu, IEEE Trans. Electron Dev. 39 (1992) 134. 1,6] Y. Okada, P.J. Tobin, P. Rushbrook and W.L. DeHart, IEEE Trans. Electron Dev. 41 (1994) 191. 1,7] A.E.T. Kuiper, H.G. Pomp, P.M. Asveld, W. Arnold Bik and F.H.P.M. Habraken, Appl. Phys. Lett. 61 (1992) 1031. 1,8] E.C. Carr and R.A. Buhrman, Appl. Phys. Lett. 63 (1993) 54. 1,9] P. Lange, H. Bernt, E. Hartmannsgruber and F. Naumann, J. Electrochem. Soc. 141 (1994) 259. 1,10] A. Kalnitsky, S.P. Tay, J.P. Ellul, S. Chongsawangvirod, J.W. Andrews and E.A. Irene, J. Electrochem. Soc. 137 (1990) 234. 1,11] D.E. Aspnes and A.A. Studna, Phys. Rev. B27 (1983) 985. 1,12] R.M.A. Azzam, N.M. Bashara, Ellipsometry and Polarized Light (North-Holland, Amsterdam, 1977). 1,13] E.D. Palik, ed., Handbook of Optical Constants of Solids (Academic Press, New York, 1985) 1,14] M.M. Moslehi and K.C. Saraswat, IEEE Trans. Electron Dev. ED-32 (1985) 106. [15] E.A. Irene, Thin Solid Films 233 (1993) 96. [16] U. Rossow, PhD thesis, TU Berlin (1993). [17] U. Rossow, U. Frotscher, W. Richter and D.R.T. Zahn, Surf. Sci. 287&288 (1993) 718. [18] M.K. Kelly, S. Zollner and M. Cardona, Surf. Sci. 285 (1993) 282. [19] A.B. Miiller, U. Rossow, U. Resch and W. Richter, Thin Solid Films 233 (1993) 19.
1, O F
Journal of Non-Crystalline Solids 187 (1995) 380-384
ELSEVIER
Structural and optical properties of nitrided silicon oxide layers rapid thermally grown in a pure N 2 0 ambient E. H a r t m a n n s g r u b e r " ' * ,
U . R o s s o w b, A. H o y e r a, P. L a n g e a
a Fraunhofer-lnstitutJ~r Siliziumtechnologie, Dillenburger Str. 53, 14199 Berlin, Germany b lnstitutfdr Festkfrperphysik der TU Berlin, Sekr. PN6-1, Hardenbergstr. 36, 10623 Berlin, Germany
Abstract
Thin nitrided silicon oxide layers with thicknesses in the 3-12 nm range were grown in a rapid thermal processor in a pure N20 ambient. These layers were investigated by X-ray photoelectron spectroscopy, spectroscopic ellipsometry and wet etching experiments and compared with oxides grown in a 02 ambient. Information about the distribution and binding properties of nitrogen in the oxide and its interfacial properties are provided. The nitrogen concentration peaks at the interface and decreases towards the oxide bulk to an extent of approximately 5 nm for a film thickness of 9 nm. Compared with 02 grown oxides, the nitrided oxides show an enhanced saturation of Si-dangling bonds by nitrogen and a reduction of the suboxide regime. These facts may explain the up to now reported improved electrical performance of metal oxide semiconductor devices using N20 grown oxides as the gate dielectric.
1. Introduction
Thin nitrided silicon oxide layers rapid thermally grown in a N 2 0 ambient either directly on silicon or on a pregrown SiO2 film on silicon have attracted much attention recently. Their application as gate dielectrics in the complementary metal oxide semiconductor (CMOS) technology promise a better performance of ultra-large-scale integrated (ULSI) devices when gate oxide thicknesses below 10 nm are required. C o m p a r e d with conventional oxides the N 2 0 grown dielectrics profit from the benefits of nitrogen incorporation into SiO2 such
* Corresponding author. Tel: + 49-30 8299 8431. Telefax: +49-30 8299 8199.
as an increase of the dielectric strength and an enhanced resistance against hot carrier stress as well as impurity penetration I-1-4] without suffering from the problems related to hydrogen incorporation occurring from N H 3 thermal nitridation
1,-5]. Nevertheless, up to now the mechanisms of nitrogen incorporation into N 2 0 grown oxides have not been completely understood and therefore also the physical properties of these dielectrics have to be examined. Of significant interest is the structure of the Si/SiOxN r interfacial region, which is assumed to be particularly responsible for the resulting electrical properties of M O S devices. Most work which has been done in this field was focussed on concentration depth profiling of the incorporated nitrogen as obtained from secondary ion mass spectroscopy (SIMS) [2,4,6], Auger electron
0022-3093/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD1 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 1 6 8 - 9
E. Hartmannsgruber et al. /Journal of Non-Crystalline Solids 187 (1995) 380 384
spectroscopy (AES) [1,7], elastic recoil detection (ERD) [7], and X-ray photoelectron spectroscopy (XPS) [-8]. Results showed that the nitrogen is mainly located at the Si/SiOxN r interface with a concentration of a few a t . ° , which decreases towards the oxide bulk. The structural arrangement in the bulk of N 2 0 grown and conventionally grown oxides was found to be almost identical by Fourier transform infrared (FTIR) spectroscopy [-9]. Chemical analysis of XPS [8] and FTIR [9] spectra showed the formation of SiN bonds to be much more likely than that of N O bonds. The goal of this work is the presentation of new results concerning the incorporation of nitrogen at the interface and in the bulk of the oxide. The distribution of nitrogen was obtained from XPS and wet chemical etch rate measurements. Results from spectroscopic ellipsometry (SE) in the visible and UV spectral region will be discussed with respect to the extent and structure of the interfacial transition region of N 2 0 grown as compared to 02 grown oxides. For these investigations we prepared N 2 0 oxide samples with a relative high nitrogen content by directly oxidizing silicon substrates in a pure N 2 0 ambient in a R T P r e a c t o r . For comparison, non-nitrided oxides were grown in a pure 0 2 ambient in the same reactor.
2. Experimental procedure The R T P system (AST 100) is equipped with 21 tungsten halogen lamps for both sides illumination of single wafers with a 100 mm diameter lying inside a cold-wall quartz reactor. This arrangement is surrounded by a highly reflective gold-plated process chamber. The temperature is monitored by a pyrometer with a sensitivity maximum at 2.7 ~tm. An additional silicon reflection ring is placed around the wafer to enhance the temperature uniformity across the wafer. More details of the R T P equipment have been given in Ref. [-9]. The silicon substrates used were n-type, phosphorous-doped with a resistivity of 1-3 f~cm and (1 0 0 ) orientation. They were transferred from the storage box to the reactor without any precleaning procedure to avoid surface roughening which might result from
381
native oxide etching. Thin layers with thicknesses in the 2-12 nm range were grown either in a pure N 2 0 ambient (RTON) or, for comparison, in a pure 0 2 ambient (RTO) at 1150°C. The gas flow was parallel to the wafer surface with a rate adjusted to 51/min in both cases. A single wavelength ellipsometer (SWE) equipped with a H e - N e laser (632.8 nm) was used to determine the layer thicknesses directly after growth and the wet chemical etch rates of R T O N and RTO samples in a highly diluted (about 1 : 100) H F : H 2 0 solution. The refractive index used was fixed to a value of 1.475 assuming a SiO2 bulk layer, which is thinner than 20 nm [10]. In addition, a spectroscopic ellipsometer with rotating analyzer was used in the spectral range 1.8-5.5 eV. In this case the raw data were converted into an effective dielectric function ( e ) [11,12] which describes the effective response of the oxide layer and the silicon substrate. For the interpretation of the measured ( e ) modelling is necessary. For a single layer on a substrate a so-called three-phase model is appropriate [12]. In this model the three phases are vacuum, layer and substrate, assuming no interfacial layer. An effective dielectric function can be calculated with the well-known dielectric function of silicon [11] and that of the layer as well as the layer thickness as input. As a layer dielectric function that of SiOz [,13] was used for the non-nitrided as well as the nitrided oxides. This serves as a good approximation since the nitrogen content in the R T O N samples is small (lower than 5 a t . ° , see below) and the optical gaps of an amorphous SiOxNy compound are in the UV above 5.5 eV. Deviations between the measured and calculated effective dielectric function must be correlated for the interfacial or layer properties and can be compared between R T O N and RTO samples. XPS spectra were recorded using a commercially available XPS system (VG ESCALAB) using A1 K~ radiation (1486.6 eV) and a hemispherical photoelectron energy analyser. From the Si 2p, N ls and O ls core level peaks the binding energy and the concentration of these elements were derived. For concentration depth profiling argon ion sputtering was used.
E. Hartmannsgruber et al. / Journal of Non-Crystalline Solids 187 (1995) 380-384
382
100 90 ! 80
Si02
70
c-Si
10 '9 8
10
~,
7 *~
"-=60~
1:100 HF:H20
6
i6 N
0040 (3 30 .,_20 10 0
i ~F !
8 q
4 o 3 ~, z ,2
4
Q X
o
2
0 0 1 2 3 4 5 6 7 8 9 10 Ar-SputterTime [rain]
Fig. 1. XPS depth profile of an N 2 0 oxide layer with 10 n m thickness with the concentrations in at. % of silicon, oxygen, and nitrogen. The Si/SiOxN r interface is indicated by the dashed line. The solid lines are drawn as guides for the eye.
50
1O0 150 etch time [sec]
200
Fig. 2. Oxide thickness as a function of etching time in highly diluted HF. The dashed line indicates the region where the measurement is effected by the growth of native oxide. The solid lines are drawn as guides for the eye.
3. Results and discussion
3 - 4 nm due to the escape depth of the photoelectrons and sidewall effects of the sputtered crater.
3.1. XPS measurements
3.2. Wet etch experiments
A great deal of spectroscopic data for N 2 0 o x addressing the concentration of nitrogen at the interface, bulk and surface has been published. However, recently a discrepancy in the atomic percentage of interfacial nitrogen was reported [6]. For the purposes of comparison, we would like to discuss very briefly the XPS results that we obtained on our samples. F r o m the depth profiles for a R T O N sample of 10 nm thickness a nitrogen concentration of about 4 at.% at the interface was determined with decreasing concentration towards the bulk of the oxide (Fig. 1). This result corresponds well to that reported in Ref. [8]. According to the suggestions made therein for an evaluation of the measured binding energies we obtained 295.0 eV for the Si 2p (oxide) and N ls peak separation and 429.4 eV for the Si 2p (oxide) and O ls peak separation indicating that the nitrogen is solely bonded to silicon and not to oxygen. Depth profiling of the binding energies was not performed since argon ion sputtering induces damage of the chemical environment. The depth resolution of XPS for concentration profiling is restricted to
A more exact estimate of the extent of the nitrogen containing region is provided by the measurement of the wet chemical etch rate of the layer. This method is based on the fact that Si02 can easily be removed by H F while the etch rate for Si3N4 is rather low. Therefore, regions with a higher nitrogen content etch slower in H F than those with a lower nitrogen concentration [14]. Fig. 2 shows the measured film thickness over the etching time in highly diluted H F for a R T O N and, for comparison, for a R T O sample of nearly the same original thickness. In contrast to the constant etch rate of the R T O sample we observed for the R T O N sample a more or less continuous etch rate reduction. The onset of this effect is observable roughly 5 nm above the interface. Below this thickness a decrease of the slope of the curve takes place indicating an increasing amount of incorporated nitrogen. However, a quantification of the nitrogen content from this measurement appears difficult, because nitrogen-induced network relaxation effects can also contribute to the etch rate reduction. Nevertheless, it becomes clear that this method is very
ides
E. Hartmannsgruber et al. / Journal of Non-Crystalline Solids 187 (1995) 380-384
so I
,
.
~
~,
'
.
Re
A
lO
V
0
s.9 nm'~:'L
j!}:
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o A
383
2
- 10L i 1Prim %:ON/Si 40
20-
Re
ENERSY (eV)
02
-20
,
,
2
5
,
,
.
4
,
5
6
Fig. 4. Difference spectra of the imaginary part of the effective dielectric function A i m ( e ) as a function of energy for N 2 0 grown oxides (2.7nm solid line, 3.9nm dashed line) and 02 grown oxides (3.2 nm dotted dashed line and 4.6 nm dashed line).
FNERCY (eV)
Fig. 3. Energy dependence of the effective dielectric function (e,) for a 12 and a 2.7 nm layer of N20 grown oxide. The dashed lines indicate the calculated spectra.
effective to estimate the spatial extent of the nitrogen containing region. 3.3. S E m e a s u r e m e n t s
Finally, information about the electronic properties of the interface can be provided by applying spectroscopic ellipsometry as a non-destructive method. In Fig. 3 the energy dependence of the effective dielectric function ( e ) for a 12 and a 2.7 nm thick layer is displayed. The dashed lines represent the simulated spectra. The fundamental transitions are also indicated. The three-phase model fits very well the experimental spectra of both samples, although some deviations appear, which increase with decreasing layer thickness. This effect was also reported for conventionally grown SiO2 films [15] where the observed deviations were attributed to interfacial properties, which are of course more effective with decreasing layer thickness. To estimate any differences in this regard between nitrided and non-nitrided oxides a series of both oxide types with thicknesses below 10 nm was measured. For all measured samples modelling was performed and these data were
subtracted from the experimental ones. Fig. 4 shows the difference in the imaginary part of the effective dielectric function A i m ( e ) as a function of energy for two N 2 0 and two 0 2 grown oxides. In this figure three distinct features are observable, at the Ez-gap, at the E~), El-gap, and in the region around 3 eV. The two first-named features are of the same order of magnitude for all samples and can therefore not be related to the incorporated nitrogen in the R T O N samples. A deviation at the Ez-gap can be attributed to the surface roughness of silicon at the interface to the oxide [16] and a deviation at the E~), El-gap may be caused by a change of the silicon bulk properties. By contrast, the peak located around 3 eV observed in Fig. 4 shows a strong dependence on the layer thickness and on the type of growth process. Since the only difference between R T O N and R T O samples is the nitrogen appearance in the former this feature must be effected by the incorporated nitrogen. The smallest deviations at 3 eV in Fig. 4 are appearing for the nitrided samples. Since a good fit for R T O N was obtained with the properties of SiO2 and no consideration of nitrogen, it is most likely that no nitrogen-related transitions can contribute significantly to the spectrum. This assumption is also supported by the fact that the optical gap of an amorphous Si3N4 layer is in the UV region above 5.5 eV. If there is a deviation at 3 eV occurring, the question for the nature of these
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t r a n s i t i o n s a p p e a r s . It has been r e p o r t e d [17 19] t h a t the s a t u r a t i o n of S i - d a n g l i n g b o n d s b y arsenic a n d h y d r o g e n leads to a b u l k t e r m i n a t i o n of silicon. This m e a n s t h a t the silicon states at the surface resemble m u c h m o r e those in the silicon b u l k t h a n in the case of a clean surface. C o n s e q u e n t l y , the dielectric function of a clean surface differs from t h a t where the d a n g l i n g b o n d s are saturated. W e are c o n v i n c e d t h a t a similar effect occurs for the system u n d e r investigation. T h e n i t r o g e n at the interface is s a t u r a t i n g Si-dangling b o n d s a n d leads to a b u l k t e r m i n a t i o n of silicon. T h e r e m a i n i n g small d e v i a t i o n s b e t w e e n fit a n d e x p e r i m e n t a l d a t a are d u e to the r e m a i n i n g n o n - s a t u r a t e d S i - d a n g l i n g bonds. In turn, the larger d e v i a t i o n s a p p e a r i n g for the R T O s a m p l e s are reflecting an increased n u m b e r of d a n g l i n g bonds. If the interfacial region, which c o n t a i n s the d a n g l i n g b o n d s , is viewed as a t r a n s i t i o n region, which c o n t a i n s also SixOy suboxides, a d d i t i o n a l a r g u m e n t s for the e x p l a n a t i o n a b o v e can be p r o vided. SiO, one species of the possible SixOy suboxides has a b a n d g a p a r o u n d 3 eV, thus optical t r a n s i t i o n s at this energy m a y c o n t r i b u t e to this spectrum. F r o m the m a g n i t u d e of d e v i a t i o n s in the spectra, it can be c o n c l u d e d t h a t in the case of nitrided oxides this s u b o x i d e c o n t a i n i n g i n t e r l a y e r is reduced.
4. Conclusion F r o m the results r e p o r t e d we c o n c l u d e t h a t the extent of the interfacial t r a n s i t i o n regime, which is m a i n l y caused by interface roughness, is very similar for N 2 0 a n d 0 2 g r o w n oxides. A p r o n o u n c e d S i - d a n g l i n g b o n d s a t u r a t i o n by n i t r o g e n occurs a n d the region in which s u b o x i d e s are formed, however, a p p e a r s significantly s m a l l e r for the N 2 0 oxides. These findings m a y c o n t r i b u t e to the exp l a n a t i o n of the i m p r o v e d electrical p e r f o r m a n c e of N 2 0 oxides as c o m p a r e d to 0 2 oxides. B e y o n d this, an effective m e t h o d has been p r e s e n t e d to
locate the d i s t r i b u t i o n of n i t r o g e n within the oxide layer b y wet chemical etching in highly diluted H F . F u r t h e r m o r e , X P S d a t a were p r e s e n t e d which correlate well with the results r e p o r t e d earlier by o t h e r research groups. P a r t of this w o r k was funded by the C E C E s p r i t Basic Research a c t i o n no. 6878 ' E A S I ' a n d b y the J E S S I B T 1 B / E S P R I T 7236 project ' A D E Q U A T ' .
References I-1] H. Hwang, W. Ting, D.L. Kwong and J. Lee, IEEE IEDM Tech. Dig. (1990) 421. 1,2] A. Uchiyama, H. Fukuda, T. Hayashi, T. Iwabuchi and S. Ohno, IEEE IEDM Tech. Dig. (1990) 425. 1,3] H. Fukuda, T. Arakawa and S. Ohno, IEEE Trans. Electron Dev. 39 (1992) 127. 1,4] Y. Okada, P.J. Tobin, R.I. Hedge, J. Liao and P. Rushbrook, Appl. Phys. Lett. 61 (1992) 3163. 1,5] T. Hori, T. Yasui and S. Akamatsu, IEEE Trans. Electron Dev. 39 (1992) 134. 1,6] Y. Okada, P.J. Tobin, P. Rushbrook and W.L. DeHart, IEEE Trans. Electron Dev. 41 (1994) 191. 1,7] A.E.T. Kuiper, H.G. Pomp, P.M. Asveld, W. Arnold Bik and F.H.P.M. Habraken, Appl. Phys. Lett. 61 (1992) 1031. 1,8] E.C. Carr and R.A. Buhrman, Appl. Phys. Lett. 63 (1993) 54. 1,9] P. Lange, H. Bernt, E. Hartmannsgruber and F. Naumann, J. Electrochem. Soc. 141 (1994) 259. 1,10] A. Kalnitsky, S.P. Tay, J.P. Ellul, S. Chongsawangvirod, J.W. Andrews and E.A. Irene, J. Electrochem. Soc. 137 (1990) 234. 1,11] D.E. Aspnes and A.A. Studna, Phys. Rev. B27 (1983) 985. 1,12] R.M.A. Azzam, N.M. Bashara, Ellipsometry and Polarized Light (North-Holland, Amsterdam, 1977). 1,13] E.D. Palik, ed., Handbook of Optical Constants of Solids (Academic Press, New York, 1985) 1,14] M.M. Moslehi and K.C. Saraswat, IEEE Trans. Electron Dev. ED-32 (1985) 106. [15] E.A. Irene, Thin Solid Films 233 (1993) 96. [16] U. Rossow, PhD thesis, TU Berlin (1993). [17] U. Rossow, U. Frotscher, W. Richter and D.R.T. Zahn, Surf. Sci. 287&288 (1993) 718. [18] M.K. Kelly, S. Zollner and M. Cardona, Surf. Sci. 285 (1993) 282. [19] A.B. Miiller, U. Rossow, U. Resch and W. Richter, Thin Solid Films 233 (1993) 19.