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Si3N4 double layers on silicon produced by chemical vapour deposition
Thin SolidFilms, 78 (1981) L63-L66
L63
Letter An ellipsometric study of SiO,/Si,N, vapour deposition
double layers on silicon produced by chemical
H. W. DINGES
Forschungsinstitut der Deutschen Bundespost beim FTZ, P.O. Box 5000, D-6100 Darmstadt (Received
February
2,198l;
accepted
February
(F.R.G.)
23,198l)
I.Introduction
Si,N, layers 100-300 nm thick were deposited onto silicon by the reaction of ammonia and silane at temperatures of 1000-1200 K ‘. These films showed extremely good interface state densities (lOlo crn2 eV-‘)2. Since S&N4 films on silicon usually show higher interface state densities, it was suspected that these good interface properties were due to an intermediate layer of Si02. An ellipsometric analysis confirmed the existence of such an intermediate layer. This agrees with a thermodynamic analysis of the chemical vapour deposition (CVD) process carried out by Wahl’, which shows that oxygen-containing impurities present in the gas phase are completely converted into SiO, because of the high reactivity of silicon and oxygen. 2. Experimental The measurements were carried out with an ellipsometer manufactured by Rudolph Research Co. (type 43603-200E) which has been described in ref. 3. The angle of incidence was 70”; the wavelengths used, 546.1 nm and 436.0 nm, were selected by interference filters of halfwidth 2 nm from the spectrum of a mercury lamp. The difficulties encountered in the determination of optical properties by ellipsometric measurements are well known 3*4.A sample consisting of a substrate covered with a homogeneous and absorption-free film has four optical constants, namely the index of refraction ii, (= n, - ik,) of the substrate, the index of refraction nF of the film and the thickness d of the film. The system of ellipsometric equations is never fully determined by ellipsometry alone since there are only two measured quantities d and Ycompared with four unknown parameters. Thus two parameters have to be determined independently. The cleaned and chemically etched’ silicon slices were therefore measured before the CVD process to determine the substrate properties. It is known that, immediately after chemical etching5, wafers of semiconductors are covered with a thin natural oxide layer. The values for k, = 0.028 6 and for nox = nF = 1.46 (SiO,) at I = 546.1 nm were taken from the literature. Then the real part of the refractive index of silicon was determined to be n, = 4.06 * 0.01. The lowest measured thickness d of the thin natural oxide layer was 0.7 nm. The other values for the thicknesses were in the range 0.8-1.5 nm, depending 0040-6090/81/0000-0000/$02.50
0 Elsevier Sequoia/Printed
in The Netherlands
L64
LETTERS
on the exposure time to air. For A = 436 nm the index of refraction of the substrate was 4.808 -0.115i. All determined values were in very good agreement with those reported in the literatures-‘. 3. Discussion With these optical constants for the substrate and for values between 1.9 and 2.1 for the refractive index of the S&N, films a ?P,A network was drawn in order to insert the measured u’,A values of the S&N,-covered slices. Because of the CVD apparatus’ used the coverage of the slices was not uniform in thickness; this could be seen by different interference colours on the slices. The profile of thickness was approximately wedge shaped. With a sufficiently small aperture (diameter, 0.3 nm) it was possible to measure a number of Y, A values on one slice and to insert them in the !P,A network shown in Fig. 1. If the silicon slices were covered only with pure homogeneous absorption-free S&N, layers2 the fits of the measured values should give a constant value for the refractive index of the S&N, layers. The measurements could not be fitted with a unique value of nr.
240
220
200
d hml
180 160
1LO
120
100
Fig. 1. Ellipsometer readings (+) for a covered silicon slice. The full curve is calculated for the system shown in the inset, with the thickness (in nanometres) of the Si,N, layer as a curve parameter. The broken curves are calculated for single layers of S&N, (n = 1.95) and SiO, (n = 1.456) on silicon.
L65
LETTERS
At both wavelengths the curves fitted through the measured values did not go through the zero point (see Fig. l), i.e. the Y, A values given by the optical constants of the bare substrate alone and for thickness d = 0 (or repeating thickness d, = 12/2(n2-sin2(p)“2). The fitted curves passed the zero point displaced by 2” towards greater Y values. If the assumption that the films are absorption free were wrong, the curves should pass the zero point at a smaller !P value than that of the zero point’. The assumption of an intermediate layer of SiO, produces the same deviations. Therefore the curve for nF = 1.456 at 546.1 nm and d = O-50 nm as a parameter was inserted in the !P,A network. The fit of the measurements for one film-covered slice crossed the SiO, curve at 37.5 nm SiO,. Thus it was concluded that for this slice the intermediate layer of SiO, had a thickness of 37.5 nm and was produced by oxygencontaining impurities present in the CVD apparatus at the beginning of the process. Destructive secondary ion mass spectrometry measurements made on the same slice gave the same result’. From the Fresnel formulae the ellipsometer equation for the system given in the inset of Fig. 1 was developed. The Y, A curves were calculated for the following values (at 546.1 nm): * = 4.06 - 0.028i nsi nsioz = 1.456 d SiOa
=
37.5 nm
nsi3N4= 1.9, 1.95,2.0 and 2.05 dsisN4= O-500 nm For nsisN4= 1.95 the ellipsometer readings and the calculated curve were in good agreement. At 436 nm the refractive index of this Si,N, layer was nSiaN4= 1.98. The results were confirmed by measurements of other slices with other thicknesses of the intermediate SiO, layer from 15 to 40 nm. The refractive index of the S&N, layers was in the range 1.95-1.98 at 546.1 nm. Using another CVD apparatus’ we prepared several Si/SiO,/Si,N, test structures with S&N, layers of uniform thicknesses. It was possible to produce definite intermediate layers of SiO, down to 2.5 nm, i.e. only 1 nm more than the thickness of the natural oxide layer. These structures were also investigated by ellipsometry. 4. Conclusions Si,N, layers prepared on silicon by CVD were analysed by ellipsometry. Before deposition of the layers the etched silicon slices have thin natural SiO, layers of thickness 0.7-1.5 nm. The deposited Si,N, layers showed good interface properties, which were, however, due to an intermediate layer of SiO,. This could be shown by ellipsometry. It was possible to reduce the thickness of the intermediate layer to 2.5 nm. The refractive index of the S&N, layers was in the range 1.95-1.98 at 546.1 nm. The author is indebted to A. Neidig for the preparation of the structures and to G. Weimann and D. Fritzsche for discussions.
L66
LETTERS
1 A. Neidig and G. Wahl, Chemical vapor deposition of SiO,/Si,N, double layers, to be published. 2 A. Neidig, Symp. on Solid State Device Technology, 1976, in Verh. Dtsch. Phys. Ges. 7 (1976) (paper HL 164). 3 F. L. McCrackin, E. Passaglia, R. R. Stromberg and H. L. Steinberg, Nail. Eur. Stand. (U.S.), 67A (1963) 363-377. 4 H. W. Dinges, Thin Solid Films, 50 (1978) L17-L20. 5 R. J. Archer, J. Opt. Sot. Am., 52 (1962) 970-977. 6 S. S. So and K. Vedam, J. Opt. Sot. Am., 62 (1972) 596-598. 7 K. Vedam, Surf. Sci., 56 (1976) 221-236.
L63
Letter An ellipsometric study of SiO,/Si,N, vapour deposition
double layers on silicon produced by chemical
H. W. DINGES
Forschungsinstitut der Deutschen Bundespost beim FTZ, P.O. Box 5000, D-6100 Darmstadt (Received
February
2,198l;
accepted
February
(F.R.G.)
23,198l)
I.Introduction
Si,N, layers 100-300 nm thick were deposited onto silicon by the reaction of ammonia and silane at temperatures of 1000-1200 K ‘. These films showed extremely good interface state densities (lOlo crn2 eV-‘)2. Since S&N4 films on silicon usually show higher interface state densities, it was suspected that these good interface properties were due to an intermediate layer of Si02. An ellipsometric analysis confirmed the existence of such an intermediate layer. This agrees with a thermodynamic analysis of the chemical vapour deposition (CVD) process carried out by Wahl’, which shows that oxygen-containing impurities present in the gas phase are completely converted into SiO, because of the high reactivity of silicon and oxygen. 2. Experimental The measurements were carried out with an ellipsometer manufactured by Rudolph Research Co. (type 43603-200E) which has been described in ref. 3. The angle of incidence was 70”; the wavelengths used, 546.1 nm and 436.0 nm, were selected by interference filters of halfwidth 2 nm from the spectrum of a mercury lamp. The difficulties encountered in the determination of optical properties by ellipsometric measurements are well known 3*4.A sample consisting of a substrate covered with a homogeneous and absorption-free film has four optical constants, namely the index of refraction ii, (= n, - ik,) of the substrate, the index of refraction nF of the film and the thickness d of the film. The system of ellipsometric equations is never fully determined by ellipsometry alone since there are only two measured quantities d and Ycompared with four unknown parameters. Thus two parameters have to be determined independently. The cleaned and chemically etched’ silicon slices were therefore measured before the CVD process to determine the substrate properties. It is known that, immediately after chemical etching5, wafers of semiconductors are covered with a thin natural oxide layer. The values for k, = 0.028 6 and for nox = nF = 1.46 (SiO,) at I = 546.1 nm were taken from the literature. Then the real part of the refractive index of silicon was determined to be n, = 4.06 * 0.01. The lowest measured thickness d of the thin natural oxide layer was 0.7 nm. The other values for the thicknesses were in the range 0.8-1.5 nm, depending 0040-6090/81/0000-0000/$02.50
0 Elsevier Sequoia/Printed
in The Netherlands
L64
LETTERS
on the exposure time to air. For A = 436 nm the index of refraction of the substrate was 4.808 -0.115i. All determined values were in very good agreement with those reported in the literatures-‘. 3. Discussion With these optical constants for the substrate and for values between 1.9 and 2.1 for the refractive index of the S&N, films a ?P,A network was drawn in order to insert the measured u’,A values of the S&N,-covered slices. Because of the CVD apparatus’ used the coverage of the slices was not uniform in thickness; this could be seen by different interference colours on the slices. The profile of thickness was approximately wedge shaped. With a sufficiently small aperture (diameter, 0.3 nm) it was possible to measure a number of Y, A values on one slice and to insert them in the !P,A network shown in Fig. 1. If the silicon slices were covered only with pure homogeneous absorption-free S&N, layers2 the fits of the measured values should give a constant value for the refractive index of the S&N, layers. The measurements could not be fitted with a unique value of nr.
240
220
200
d hml
180 160
1LO
120
100
Fig. 1. Ellipsometer readings (+) for a covered silicon slice. The full curve is calculated for the system shown in the inset, with the thickness (in nanometres) of the Si,N, layer as a curve parameter. The broken curves are calculated for single layers of S&N, (n = 1.95) and SiO, (n = 1.456) on silicon.
L65
LETTERS
At both wavelengths the curves fitted through the measured values did not go through the zero point (see Fig. l), i.e. the Y, A values given by the optical constants of the bare substrate alone and for thickness d = 0 (or repeating thickness d, = 12/2(n2-sin2(p)“2). The fitted curves passed the zero point displaced by 2” towards greater Y values. If the assumption that the films are absorption free were wrong, the curves should pass the zero point at a smaller !P value than that of the zero point’. The assumption of an intermediate layer of SiO, produces the same deviations. Therefore the curve for nF = 1.456 at 546.1 nm and d = O-50 nm as a parameter was inserted in the !P,A network. The fit of the measurements for one film-covered slice crossed the SiO, curve at 37.5 nm SiO,. Thus it was concluded that for this slice the intermediate layer of SiO, had a thickness of 37.5 nm and was produced by oxygencontaining impurities present in the CVD apparatus at the beginning of the process. Destructive secondary ion mass spectrometry measurements made on the same slice gave the same result’. From the Fresnel formulae the ellipsometer equation for the system given in the inset of Fig. 1 was developed. The Y, A curves were calculated for the following values (at 546.1 nm): * = 4.06 - 0.028i nsi nsioz = 1.456 d SiOa
=
37.5 nm
nsi3N4= 1.9, 1.95,2.0 and 2.05 dsisN4= O-500 nm For nsisN4= 1.95 the ellipsometer readings and the calculated curve were in good agreement. At 436 nm the refractive index of this Si,N, layer was nSiaN4= 1.98. The results were confirmed by measurements of other slices with other thicknesses of the intermediate SiO, layer from 15 to 40 nm. The refractive index of the S&N, layers was in the range 1.95-1.98 at 546.1 nm. Using another CVD apparatus’ we prepared several Si/SiO,/Si,N, test structures with S&N, layers of uniform thicknesses. It was possible to produce definite intermediate layers of SiO, down to 2.5 nm, i.e. only 1 nm more than the thickness of the natural oxide layer. These structures were also investigated by ellipsometry. 4. Conclusions Si,N, layers prepared on silicon by CVD were analysed by ellipsometry. Before deposition of the layers the etched silicon slices have thin natural SiO, layers of thickness 0.7-1.5 nm. The deposited Si,N, layers showed good interface properties, which were, however, due to an intermediate layer of SiO,. This could be shown by ellipsometry. It was possible to reduce the thickness of the intermediate layer to 2.5 nm. The refractive index of the S&N, layers was in the range 1.95-1.98 at 546.1 nm. The author is indebted to A. Neidig for the preparation of the structures and to G. Weimann and D. Fritzsche for discussions.
L66
LETTERS
1 A. Neidig and G. Wahl, Chemical vapor deposition of SiO,/Si,N, double layers, to be published. 2 A. Neidig, Symp. on Solid State Device Technology, 1976, in Verh. Dtsch. Phys. Ges. 7 (1976) (paper HL 164). 3 F. L. McCrackin, E. Passaglia, R. R. Stromberg and H. L. Steinberg, Nail. Eur. Stand. (U.S.), 67A (1963) 363-377. 4 H. W. Dinges, Thin Solid Films, 50 (1978) L17-L20. 5 R. J. Archer, J. Opt. Sot. Am., 52 (1962) 970-977. 6 S. S. So and K. Vedam, J. Opt. Sot. Am., 62 (1972) 596-598. 7 K. Vedam, Surf. Sci., 56 (1976) 221-236.