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X-ray reflectivity study of r.f.-sputtered thin SiO2 films
Thin Solid Films, 229 (1993) 29-32
29
X-ray reflectivity study of r.f.-sputtered thin
SiO 2
films
A. Bender Technisehe Hoehschule Wismar, Fakultiit Elektroteehnik, Philipp-Miiller-StraJ3e, Wismar 0-2400 (Germany)
Th. Gerber, H. Albrecht and B. Himmel Universitiit Rostock, Fachbereich Physik, PF 999 Universitiitsplatz 3, Rostock 0-2500 (Germany)
(Received November 5, 1992; accepted January 25, 1993)
Abstract X-ray reflectivity investigations have been performed on thin amorphous S i O 2 films obtained by r.f.-sputtering of fused quartz onto Si single crystals in an argon atmosphere. This is a non-destructive technique to determine thickness and density as well as the interface and surface roughness. For the SiOJSi system (small differences in density) the X-ray reflectivity is sufficiently sensitive to provide precise information on the occurence of an intermediate layer we obtained only on well-polished silicon wafers. The thickness of the films is compared with that obtained from ellipsometry data and mechanical scanning measurements (stylus method).
1. Introduction Because of their highly protective and dielectric properties, thin SiO2 films are widely used for passivation and insulation of silicon devices and integrated circuits. The r.f.-sputtering method allows the deposition of high quality SiOz films at low temperatures (nearly 420 K) on any semiconducting or metallic device without essential changes of their functions. In this paper X-ray reflectivity measurements are used to investigate SiO2 films obtained by r.f.-sputtering on polished silicon wafers. The results are correlated to ellipsometry data and mechanical thickness measurements. Three samples are selected for our discussion which are representative of all the observations we made on m a n y samples.
2. Experimental 2.1. Deposition technique
The S i O 2 films were deposited by r.f.-sputtering (2.9 M H z ) in a standard sputtering system type H Z M - 4 P (Manfred yon Ardenne, Dresden) equipped with an oil diffusion p u m p fitted with a cold trap enabling a residual pressure of 2 x l0 -4 Pa to be obtained. Argon was used as the working g a s ( p r e s s u r e 1.4 Pa). Two quartz targets (76 m m in diameter) weremounted on two permanent magnet planar magnetron sputtering sources. The sputtering power for each
0040-6090/93]$6.00
Magnetron was 1.7 kW. The t a r g e t - s a m p l e distance was about 50 mm. A deposition rate of about 0.28 nm s -1 was attained during intermittent deposition (8 min) at a substrate temperature of nearly 420 K. The substrates were on a floating bias potential ( - 80 V) during the deposition. Samples with different thicknesses were obtained by variation of the deposition time only. The stoichiometry was determined by Rutherford backscattering spectroscopy with a ratio of about 2.1:1 between oxygen and silicon. 2.2. X - r a y reflectivity
The X-ray reflectivity analysis of thin films is a non-destructive technique that requires no complicated sample preparation and can be applied under almost ambient conditions. The reflectivity is sensitive mainly to the film electron density distribution normal to the surface. Information can be obtained about the thickness, the electron density and the interface roughness. The main application is the detailed determination of the electron density profile of the film. It can be used to calculate the material density if the stoichiometry is known. It is well known that the refraction index n is less than one for X-ray radiation. That means total external reflection occurs at the surface (air/material interface) if the incidence angle of the beam is smaller than a critical angle ®c- The refraction index is given by n = 1 --c3 - - i f l
(1)
© 1993 - - Elsevier Sequoia. All rights reserved
30
A. Bender et al. / X-ray reflectivity study of r.f -sputtered th#l S i O 2 films
where 6 is the real part and fl is the imaginary part describing the absorption. It can be shown that 6 =~re(f+f')
(2)
and 22 fl = ~ re(f'') pM
(3)
where p is the mass density, re is the classical electron radius, M is the molar mass, and f and f " are correction factors for dispersion and absorption respectively, of the atomic form factor at the zero scattering angle. The parameters 6 and/3 reach values between 10-s and 10 -8. The critical angle of total external reflection ®c must be very small because n is nearly one. According to Kjaer et al. [1] the critical angle 0c is given by 0~=2.
~
(4)
where Pe is the electron density and 2 is the wavelength (Cu Kc~ radiation). A highly absolute accuracy (better than 0.005 °) for the determination of the incidence angle of reflection is required for exactly determining the material densities of the thin film monolayers. It is possible to distinguish the difference in densities of amorphous SiO2 (2.2-2.4 g cm -3) and crystalline silicon (2.33 g cm-3). Total reflection occurs if the incidence angle is smaller than the critical angle 0~. For incidence angles larger than 0~ the radiation penetrates deep into the material. The effects of X-ray reflectivity can be described exactly by the solution of the Maxwell equations. In the case of X-rays the Fresnel equation is also correct. This is the basis of the recursive formula given by Parratt [2]. Rn
- -
I ,tl
' + l +. .F. ". .- i n 7 ='-- = a2 l ( E f t - l ) 4 1[ R " ""-a,,_ [R,,..+,Fn_,.,, + 13 -
(5)
with ftl -- I - - f n
F,_,.,
f,_, +in
(6)
and in = ( 02 -- 2•n -- 2ifln) d2
(7)
The parameter a,, is an amplitude factor and given by
E, describes the incidence beam on the n th layer and E, R describes the reflected beam from this layer. The measured intensity can be obtained by the ratio of the incident and reflected beams ~lR2 = [R,I 2 i R = IE~. LI
(9)
Using this formula it is possible to construct layer models which describe the experimental results fairly accurately. A number of methods exist for treating interface roughness, two of which are described in detail in ref. 3. The first is based on the fact that the localized interface height follows a Gaussian distribution and results in a statistical Debye-Waller-like term which influences the reflectivity. The Gaussian method is successful only for description of the surface roughness. However, it is not possible to distinguish between interior interfaces of thin film systems consisting of more than one layer. The second method includes the possibility of the location of interfaces between neighbouring layers of the film system. By this method a continuous transition of the refraction index can be achieved. The real part of the complex refraction index 5 changes continuously from interface to interface and follows a hyperbolic tangent function [4]. The interface is split in a succession of ten thin layers whose density is arranged in steps following the real distribution tan h. In order to include the roughness term in our calculations and to obtain the possibility of distinction between interior interfaces, we have used the second method. 2.3. Measuring arrangenlent The X-ray reflection is measured using a modified horizontal axis ® - 2 ® diffractometer H Z G 4 equipped with a small angle device (Pr/izisionsmechanik Freiberg GmbH). The accuracy of angular measurement is 0.0025-0.0001 ° . The reflection curves are measured for incidence angles in the range between 0.0 ° and 2.5 °. Figure 1 shows the schematic view of the measuring arrangement. All investigations were made with Nifiltered Cu Kc~ radiation produced by a sealed tube having a line focus. This kind of monochromaticity was chosen because the same arrangement of measurement is also used for SAXS experiments. The intensity yield is higher in comparison with the use of a crystal monochromator. The beam is collimated by the two slits S1 and $2 as shown in Fig. 1 (diameters 0.52 mm and 0.04 mm respectively). The slit $3 is characterized by two sides which are adjustable independently of each other. The function of this slit is the shielding of the parasitic scattering of $2. The angular aperture on the side of the proportional counter is determined by the slits $4 (diameter 1.75 ram) and $5 (diameter 0.09 ram). A revolving filter changer containing ten nickel foils of different thicknesses were placed between slits S 1 and $2 in order to always guarantee the condition of proportionality of the counter. This way the attenuation of the Cu Kc~ radiation could be varied step by step with a factor of ten in each case. A personal computer was the controlling function for movements of the sample, the detector and the attenuation by nickel filters.
A. Bender et al. / X-ray reflectivity study of r.f.-sputtered thin SiO=flms
31
101 100
~
~
8.0~l _~ 7.5
10t
[-~
COU~TER ~-]
II
II ] CO.TRO. U.,T ]
Sa S1...S5 SOURCE DETECTOR DIFFRACTOMETER
-~ 7'°t
lo;
$5
•
~J~ 100.0
SAMPLE SLITS CuKu~ PROPORTIONAL COUNTER TETA-2TETA-DIFFRACTOMETER
,i
0.5
~o6~o4~"
"V~'l deg
s
""
'-o 6 5 1 ~ , / _ _ _ , _ _ ~ . ,
10"3
DI CTOMETER
~ o~-,i,m
/---~
-~
6
1.0
1.5
Fig. 3. Calculated ( ) and experimental ( ....... ) reflectivity curves of sample 2 (curves are nearly identical)
Fig. 1. Schematic view of the grazing incidence X-ray reflectometer.
I01 100
~
10;
.o 7.51
lo: " lo-"
3. Results and discussion
8'01[
Si02- fitm
Si
f---
-:7° I --,,~,,
- -
'-o 6 5 t ~ _ ~ ' 70 68 66
t.
2
0
-2
3. I. Reflectivity
Figures 2 - 4 show the logarithmic plot o f the reflectivity for the sputtered SiO2 thin films. The plot o f the & dependence on the sample thickness is shown in the right corner o f every figure. The Kiessig oscillations in the reflectivity curves are small because o f the small differences in density o f the SiOdSi system. The overall slope and the d a m p out o f the oscillations at larger angles are determined by the surface roughness. The model parameters used to fit the data in Figs. 2 - 4 are given in Table 1, which also contains the reflectivity parameters obtained by Heald et al. [5] for thermal oxides to make a c o m p a r i s o n possible. The interference oscillations for sample 1 in Fig. 2 show a d a m p e d behaviour near 1.0 ° with increasing amplitude at larger angles. The same observation for plasma-enhanced chemical v a p o u r deposition ( P E C V D ) o f SiO2 on Si was discussed by Heald et al. [5]. T h e y explained this behaviour by adding an additional, less dense SiO 2 layer near the interface. O u r model calculation using a less dense intermediate layer gives the best a d a p t a t i o n to our reflectivity curve. Heald et al. [5] suppose that the intermediate layer is a consequence o f the surface preparation technique used prior to the deposition.
10" 10c
[
Si02 -
8"0 /
t
10-1
': 7.0r
10"~
[
lilm
Si
"
105
dlnm
10"t 10"
16 o.o
o.s
1.o
"t~l~eg
1.s
20
Fig. 2. Calculated ( ) and experimental ( ....... ) reftectivity curves of sample 1 (curves are nearly identical)
.10'.
0II
0.5
Fig. 4. Calculated (
1.0
(:leg
1.5
2.0
2.5
) and experimental ( ....... ) reflectivity curves
of sample 3 TABLE 1. Reflectivity parameters obtained from model calculation to fit the data in Figs 2-4 and one reflectivity parameters obtained by Heald et al. for one thermal oxide (sample 4) Sample Layer Density Refraction Absorption Thickness Roughness no. (gem -3) (10 -6) (10 -7) (nm) (nm) I
SiO2 2.15 SiO2 2.11 Si 2.33
7.03 6.78 7.55
1.73 0.98 1.73
100.2 3.9 -
1.1 0.5 0.7
2
SiO2 2.15 Si 2.33
7.03 7.55
1.73 1.73
106.5 -
2.1 1.2
3
SiO2 2.15 Si 2.33
7.03 7.55
1.05 1.73
67.0 -
1.85 0.9
4
SiO2 2.26 Si 2.33
57.7 -
0.43 0.1
Figure 3 shows the reflectivity curve o f sample 2 without significant d a m p i n g o f the oscillations. The roughness for both the Si and the SiO2 surface, is higher c o m p a r e d with sample 1 (see Table 1). We assume that the less dense SiO2 layer near the Si interface in sample 1 is a result o f the sputter deposition process. D u r i n g the first second o f deposition no floating bias exists. The substrate temperature is less than 420 K (nearly 300 K). This results in a p o r o u s SiO2 layer o f a few nanometers. Because o f the packing density o f 0.96 the p o r o u s structure produces a SiO2 n e t w o r k which consists o f extended SiO2 areas ( m o r e
32
A. Bender et al. / X-ray reflectiviO, study of r f.-sputtered thin Si02 films
than 60 nm in diameter [6]). According to ref. 7 well-defined polyhedra are the topological building units of a continuous SiO2 network. These polyhedra are fully connected to each other in the above-mentioned SiO2 areas, but these extended SiO2 areas cannot be completely connected to each other so the existence of dangling bounds and inner surfaces is highly probable. The substrate temperature rises during further sputtering resulting in a better surface diffusion of atoms. The floating bias leads to a resputtering of loosely bound atoms. The consequence is a more dense SiO2 film, but the energy transition to the substrate is not high enough to make a volume diffusion possible. Hence the intermediate layer is preserved. Sample 2 (same deposition conditions as sample 1) with a rough Si surface does not have such an intermediate layer. A distinction between the interface roughness and the intermediate layer is not possible and a defined intermediate layer does not exist on rough Si surfaces. An interface region (often denoted as SiOx) of about 1 nm like that at the Si-SiO2 interface of thermally grown SiO2 (ref. 8) is not the cause of the thicker intermediate SiO2 layer described here. In general the investigated r.f.-sputtered films show less density and a greater surface roughness compared with thermal oxides (see Table 1) which can be related to the sputtering technique, the low substrate temperature and the Si surface roughness. Figure 4 shows the reflectivity curve of sample 3 containing a shoulder at 0.4 °. The model curve (best adaptation) differs from the experimental one at large angles. The deposition conditions were the same for samples 1-3. Thus we assume that the cause of the shoulder may be a result of the Si wafer (surface, stress, curvature) which influences the growth of the SiO2 film during deposition. A full discussion of this observation cannot be given at this time. 3.2. Optical and mechanical thickness measurement
The optical measurements are performed with an ellipsometer AFE 401, Z W G (wavelength 2 = 632.8 nm). The mechanical measurements were performed with a mechanical scanning measuring device ME-10 (Carl Zeiss Jena) at an edge prepared by etching with p-etch solution (60 ml H20, 2.15 ml H N O 3 (65%), 3.6 ml H F (38%)). These techniques are used for a local characterization of the thickness only, whereas the X-ray reflectivity measurement is an integral method. Therefore the ellipsometry and mechanical measurements are made at several points near the sample centre to make a comparison of the thickness with the results of X-ray reflectivity possible. The homogeneity of thickness shows differences of about 0.1% in the tangential as well as in the radial direction. Table 2 summarizes the results for the thickness of the three selected samples.
TABLE 2. Thickness d and refraction index measured for the three selected samples (1-3) Sample no.
1 2 3
Ellipsometry
Mechanical scanning d(0.5) (nm)
n
d(nm)
1.470 4- 0.005 1.470 + 0.005 1.470 _+ 0.005
104.1 104.0 103.5 105.1 70.2 69.0
Reflectivity d(0.1)(nm)
104.1 105.5 67.0
Sample 1 does not show any significant difference because of the low surface roughness. Sample 2 shows a higher thickness measured by mechanical scanning and by X-ray reflectivity compared with ellipsometry measurements. The cause is the higher surface roughness of the Si wafer and the SiO2 film (see Table 1) which is taken into account especially in X-ray reflectivity measurements. Sample 3 shows a higher ellipsometry thickness compared with the thickness determined by X-ray reflectivity. The maximum error of the refraction index is greater than the error of sample 1 and 2. This may be an indication of the influence of the Si wafer described in Section 3.1.
4. Conclusion X-ray reflectivity measurements are mainly sensitive to investigating the SiO2/Si system to provide useful information about the S i O 2 layer and the interface regions. The r . f . - s p u t t e r e d S i O 2 films discussed in this paper have a lower density and a higher surface roughness compared with thermal oxides investigated by Heald et al. [5]. Any significant differences in thickness cannot be detected for samples with low surface roughness by the methods used here. A full picture of the influence of the Si wafers as substrates on X-ray reflectivity curves cannot be given at this time.
Acknowledgments We are thankful to the Deutsche Forschungsgemeinschaft Bonn for the financial support (Project number: Ge 667/3-1) and also to the D F G for financial support to one of the authors (BH).
References 1 K. Kjaer, C. A. Helm and H. M6hwald, Thin Solid Films, 159(1988) 17-28. 2 L. G. Parratt, Phys. Rev., 95 (1954) 359-369. 3 L. Brfigemann, Ph.D. Thesis, University Kiel, 1989. 4 E. S. Wu and W. W. Webb, Phys. Rev. A, 8 (1973) 2065. 5 S. M. Heald, J. K. D. Jayanetti, A. A. Bright and G. W. Rubloff, J. Vac. Sci. Teehnol. A, 8 (1990) 2046. 6 A. Bender, Ph.D. Thesis, University Rostock, 1992. 7 Tb. Gerber and B. Himmel, J. Non-Cryst. Solids, 83(1986) 324-334. 8 R. Haight and L. C. Feldman, J. Appl. Phys., 53 (1982) 4884-4887.
29
X-ray reflectivity study of r.f.-sputtered thin
SiO 2
films
A. Bender Technisehe Hoehschule Wismar, Fakultiit Elektroteehnik, Philipp-Miiller-StraJ3e, Wismar 0-2400 (Germany)
Th. Gerber, H. Albrecht and B. Himmel Universitiit Rostock, Fachbereich Physik, PF 999 Universitiitsplatz 3, Rostock 0-2500 (Germany)
(Received November 5, 1992; accepted January 25, 1993)
Abstract X-ray reflectivity investigations have been performed on thin amorphous S i O 2 films obtained by r.f.-sputtering of fused quartz onto Si single crystals in an argon atmosphere. This is a non-destructive technique to determine thickness and density as well as the interface and surface roughness. For the SiOJSi system (small differences in density) the X-ray reflectivity is sufficiently sensitive to provide precise information on the occurence of an intermediate layer we obtained only on well-polished silicon wafers. The thickness of the films is compared with that obtained from ellipsometry data and mechanical scanning measurements (stylus method).
1. Introduction Because of their highly protective and dielectric properties, thin SiO2 films are widely used for passivation and insulation of silicon devices and integrated circuits. The r.f.-sputtering method allows the deposition of high quality SiOz films at low temperatures (nearly 420 K) on any semiconducting or metallic device without essential changes of their functions. In this paper X-ray reflectivity measurements are used to investigate SiO2 films obtained by r.f.-sputtering on polished silicon wafers. The results are correlated to ellipsometry data and mechanical thickness measurements. Three samples are selected for our discussion which are representative of all the observations we made on m a n y samples.
2. Experimental 2.1. Deposition technique
The S i O 2 films were deposited by r.f.-sputtering (2.9 M H z ) in a standard sputtering system type H Z M - 4 P (Manfred yon Ardenne, Dresden) equipped with an oil diffusion p u m p fitted with a cold trap enabling a residual pressure of 2 x l0 -4 Pa to be obtained. Argon was used as the working g a s ( p r e s s u r e 1.4 Pa). Two quartz targets (76 m m in diameter) weremounted on two permanent magnet planar magnetron sputtering sources. The sputtering power for each
0040-6090/93]$6.00
Magnetron was 1.7 kW. The t a r g e t - s a m p l e distance was about 50 mm. A deposition rate of about 0.28 nm s -1 was attained during intermittent deposition (8 min) at a substrate temperature of nearly 420 K. The substrates were on a floating bias potential ( - 80 V) during the deposition. Samples with different thicknesses were obtained by variation of the deposition time only. The stoichiometry was determined by Rutherford backscattering spectroscopy with a ratio of about 2.1:1 between oxygen and silicon. 2.2. X - r a y reflectivity
The X-ray reflectivity analysis of thin films is a non-destructive technique that requires no complicated sample preparation and can be applied under almost ambient conditions. The reflectivity is sensitive mainly to the film electron density distribution normal to the surface. Information can be obtained about the thickness, the electron density and the interface roughness. The main application is the detailed determination of the electron density profile of the film. It can be used to calculate the material density if the stoichiometry is known. It is well known that the refraction index n is less than one for X-ray radiation. That means total external reflection occurs at the surface (air/material interface) if the incidence angle of the beam is smaller than a critical angle ®c- The refraction index is given by n = 1 --c3 - - i f l
(1)
© 1993 - - Elsevier Sequoia. All rights reserved
30
A. Bender et al. / X-ray reflectivity study of r.f -sputtered th#l S i O 2 films
where 6 is the real part and fl is the imaginary part describing the absorption. It can be shown that 6 =~re(f+f')
(2)
and 22 fl = ~ re(f'') pM
(3)
where p is the mass density, re is the classical electron radius, M is the molar mass, and f and f " are correction factors for dispersion and absorption respectively, of the atomic form factor at the zero scattering angle. The parameters 6 and/3 reach values between 10-s and 10 -8. The critical angle of total external reflection ®c must be very small because n is nearly one. According to Kjaer et al. [1] the critical angle 0c is given by 0~=2.
~
(4)
where Pe is the electron density and 2 is the wavelength (Cu Kc~ radiation). A highly absolute accuracy (better than 0.005 °) for the determination of the incidence angle of reflection is required for exactly determining the material densities of the thin film monolayers. It is possible to distinguish the difference in densities of amorphous SiO2 (2.2-2.4 g cm -3) and crystalline silicon (2.33 g cm-3). Total reflection occurs if the incidence angle is smaller than the critical angle 0~. For incidence angles larger than 0~ the radiation penetrates deep into the material. The effects of X-ray reflectivity can be described exactly by the solution of the Maxwell equations. In the case of X-rays the Fresnel equation is also correct. This is the basis of the recursive formula given by Parratt [2]. Rn
- -
I ,tl
' + l +. .F. ". .- i n 7 ='-- = a2 l ( E f t - l ) 4 1[ R " ""-a,,_ [R,,..+,Fn_,.,, + 13 -
(5)
with ftl -- I - - f n
F,_,.,
f,_, +in
(6)
and in = ( 02 -- 2•n -- 2ifln) d2
(7)
The parameter a,, is an amplitude factor and given by
E, describes the incidence beam on the n th layer and E, R describes the reflected beam from this layer. The measured intensity can be obtained by the ratio of the incident and reflected beams ~lR2 = [R,I 2 i R = IE~. LI
(9)
Using this formula it is possible to construct layer models which describe the experimental results fairly accurately. A number of methods exist for treating interface roughness, two of which are described in detail in ref. 3. The first is based on the fact that the localized interface height follows a Gaussian distribution and results in a statistical Debye-Waller-like term which influences the reflectivity. The Gaussian method is successful only for description of the surface roughness. However, it is not possible to distinguish between interior interfaces of thin film systems consisting of more than one layer. The second method includes the possibility of the location of interfaces between neighbouring layers of the film system. By this method a continuous transition of the refraction index can be achieved. The real part of the complex refraction index 5 changes continuously from interface to interface and follows a hyperbolic tangent function [4]. The interface is split in a succession of ten thin layers whose density is arranged in steps following the real distribution tan h. In order to include the roughness term in our calculations and to obtain the possibility of distinction between interior interfaces, we have used the second method. 2.3. Measuring arrangenlent The X-ray reflection is measured using a modified horizontal axis ® - 2 ® diffractometer H Z G 4 equipped with a small angle device (Pr/izisionsmechanik Freiberg GmbH). The accuracy of angular measurement is 0.0025-0.0001 ° . The reflection curves are measured for incidence angles in the range between 0.0 ° and 2.5 °. Figure 1 shows the schematic view of the measuring arrangement. All investigations were made with Nifiltered Cu Kc~ radiation produced by a sealed tube having a line focus. This kind of monochromaticity was chosen because the same arrangement of measurement is also used for SAXS experiments. The intensity yield is higher in comparison with the use of a crystal monochromator. The beam is collimated by the two slits S1 and $2 as shown in Fig. 1 (diameters 0.52 mm and 0.04 mm respectively). The slit $3 is characterized by two sides which are adjustable independently of each other. The function of this slit is the shielding of the parasitic scattering of $2. The angular aperture on the side of the proportional counter is determined by the slits $4 (diameter 1.75 ram) and $5 (diameter 0.09 ram). A revolving filter changer containing ten nickel foils of different thicknesses were placed between slits S 1 and $2 in order to always guarantee the condition of proportionality of the counter. This way the attenuation of the Cu Kc~ radiation could be varied step by step with a factor of ten in each case. A personal computer was the controlling function for movements of the sample, the detector and the attenuation by nickel filters.
A. Bender et al. / X-ray reflectivity study of r.f.-sputtered thin SiO=flms
31
101 100
~
~
8.0~l _~ 7.5
10t
[-~
COU~TER ~-]
II
II ] CO.TRO. U.,T ]
Sa S1...S5 SOURCE DETECTOR DIFFRACTOMETER
-~ 7'°t
lo;
$5
•
~J~ 100.0
SAMPLE SLITS CuKu~ PROPORTIONAL COUNTER TETA-2TETA-DIFFRACTOMETER
,i
0.5
~o6~o4~"
"V~'l deg
s
""
'-o 6 5 1 ~ , / _ _ _ , _ _ ~ . ,
10"3
DI CTOMETER
~ o~-,i,m
/---~
-~
6
1.0
1.5
Fig. 3. Calculated ( ) and experimental ( ....... ) reflectivity curves of sample 2 (curves are nearly identical)
Fig. 1. Schematic view of the grazing incidence X-ray reflectometer.
I01 100
~
10;
.o 7.51
lo: " lo-"
3. Results and discussion
8'01[
Si02- fitm
Si
f---
-:7° I --,,~,,
- -
'-o 6 5 t ~ _ ~ ' 70 68 66
t.
2
0
-2
3. I. Reflectivity
Figures 2 - 4 show the logarithmic plot o f the reflectivity for the sputtered SiO2 thin films. The plot o f the & dependence on the sample thickness is shown in the right corner o f every figure. The Kiessig oscillations in the reflectivity curves are small because o f the small differences in density o f the SiOdSi system. The overall slope and the d a m p out o f the oscillations at larger angles are determined by the surface roughness. The model parameters used to fit the data in Figs. 2 - 4 are given in Table 1, which also contains the reflectivity parameters obtained by Heald et al. [5] for thermal oxides to make a c o m p a r i s o n possible. The interference oscillations for sample 1 in Fig. 2 show a d a m p e d behaviour near 1.0 ° with increasing amplitude at larger angles. The same observation for plasma-enhanced chemical v a p o u r deposition ( P E C V D ) o f SiO2 on Si was discussed by Heald et al. [5]. T h e y explained this behaviour by adding an additional, less dense SiO 2 layer near the interface. O u r model calculation using a less dense intermediate layer gives the best a d a p t a t i o n to our reflectivity curve. Heald et al. [5] suppose that the intermediate layer is a consequence o f the surface preparation technique used prior to the deposition.
10" 10c
[
Si02 -
8"0 /
t
10-1
': 7.0r
10"~
[
lilm
Si
"
105
dlnm
10"t 10"
16 o.o
o.s
1.o
"t~l~eg
1.s
20
Fig. 2. Calculated ( ) and experimental ( ....... ) reftectivity curves of sample 1 (curves are nearly identical)
.10'.
0II
0.5
Fig. 4. Calculated (
1.0
(:leg
1.5
2.0
2.5
) and experimental ( ....... ) reflectivity curves
of sample 3 TABLE 1. Reflectivity parameters obtained from model calculation to fit the data in Figs 2-4 and one reflectivity parameters obtained by Heald et al. for one thermal oxide (sample 4) Sample Layer Density Refraction Absorption Thickness Roughness no. (gem -3) (10 -6) (10 -7) (nm) (nm) I
SiO2 2.15 SiO2 2.11 Si 2.33
7.03 6.78 7.55
1.73 0.98 1.73
100.2 3.9 -
1.1 0.5 0.7
2
SiO2 2.15 Si 2.33
7.03 7.55
1.73 1.73
106.5 -
2.1 1.2
3
SiO2 2.15 Si 2.33
7.03 7.55
1.05 1.73
67.0 -
1.85 0.9
4
SiO2 2.26 Si 2.33
57.7 -
0.43 0.1
Figure 3 shows the reflectivity curve o f sample 2 without significant d a m p i n g o f the oscillations. The roughness for both the Si and the SiO2 surface, is higher c o m p a r e d with sample 1 (see Table 1). We assume that the less dense SiO2 layer near the Si interface in sample 1 is a result o f the sputter deposition process. D u r i n g the first second o f deposition no floating bias exists. The substrate temperature is less than 420 K (nearly 300 K). This results in a p o r o u s SiO2 layer o f a few nanometers. Because o f the packing density o f 0.96 the p o r o u s structure produces a SiO2 n e t w o r k which consists o f extended SiO2 areas ( m o r e
32
A. Bender et al. / X-ray reflectiviO, study of r f.-sputtered thin Si02 films
than 60 nm in diameter [6]). According to ref. 7 well-defined polyhedra are the topological building units of a continuous SiO2 network. These polyhedra are fully connected to each other in the above-mentioned SiO2 areas, but these extended SiO2 areas cannot be completely connected to each other so the existence of dangling bounds and inner surfaces is highly probable. The substrate temperature rises during further sputtering resulting in a better surface diffusion of atoms. The floating bias leads to a resputtering of loosely bound atoms. The consequence is a more dense SiO2 film, but the energy transition to the substrate is not high enough to make a volume diffusion possible. Hence the intermediate layer is preserved. Sample 2 (same deposition conditions as sample 1) with a rough Si surface does not have such an intermediate layer. A distinction between the interface roughness and the intermediate layer is not possible and a defined intermediate layer does not exist on rough Si surfaces. An interface region (often denoted as SiOx) of about 1 nm like that at the Si-SiO2 interface of thermally grown SiO2 (ref. 8) is not the cause of the thicker intermediate SiO2 layer described here. In general the investigated r.f.-sputtered films show less density and a greater surface roughness compared with thermal oxides (see Table 1) which can be related to the sputtering technique, the low substrate temperature and the Si surface roughness. Figure 4 shows the reflectivity curve of sample 3 containing a shoulder at 0.4 °. The model curve (best adaptation) differs from the experimental one at large angles. The deposition conditions were the same for samples 1-3. Thus we assume that the cause of the shoulder may be a result of the Si wafer (surface, stress, curvature) which influences the growth of the SiO2 film during deposition. A full discussion of this observation cannot be given at this time. 3.2. Optical and mechanical thickness measurement
The optical measurements are performed with an ellipsometer AFE 401, Z W G (wavelength 2 = 632.8 nm). The mechanical measurements were performed with a mechanical scanning measuring device ME-10 (Carl Zeiss Jena) at an edge prepared by etching with p-etch solution (60 ml H20, 2.15 ml H N O 3 (65%), 3.6 ml H F (38%)). These techniques are used for a local characterization of the thickness only, whereas the X-ray reflectivity measurement is an integral method. Therefore the ellipsometry and mechanical measurements are made at several points near the sample centre to make a comparison of the thickness with the results of X-ray reflectivity possible. The homogeneity of thickness shows differences of about 0.1% in the tangential as well as in the radial direction. Table 2 summarizes the results for the thickness of the three selected samples.
TABLE 2. Thickness d and refraction index measured for the three selected samples (1-3) Sample no.
1 2 3
Ellipsometry
Mechanical scanning d(0.5) (nm)
n
d(nm)
1.470 4- 0.005 1.470 + 0.005 1.470 _+ 0.005
104.1 104.0 103.5 105.1 70.2 69.0
Reflectivity d(0.1)(nm)
104.1 105.5 67.0
Sample 1 does not show any significant difference because of the low surface roughness. Sample 2 shows a higher thickness measured by mechanical scanning and by X-ray reflectivity compared with ellipsometry measurements. The cause is the higher surface roughness of the Si wafer and the SiO2 film (see Table 1) which is taken into account especially in X-ray reflectivity measurements. Sample 3 shows a higher ellipsometry thickness compared with the thickness determined by X-ray reflectivity. The maximum error of the refraction index is greater than the error of sample 1 and 2. This may be an indication of the influence of the Si wafer described in Section 3.1.
4. Conclusion X-ray reflectivity measurements are mainly sensitive to investigating the SiO2/Si system to provide useful information about the S i O 2 layer and the interface regions. The r . f . - s p u t t e r e d S i O 2 films discussed in this paper have a lower density and a higher surface roughness compared with thermal oxides investigated by Heald et al. [5]. Any significant differences in thickness cannot be detected for samples with low surface roughness by the methods used here. A full picture of the influence of the Si wafers as substrates on X-ray reflectivity curves cannot be given at this time.
Acknowledgments We are thankful to the Deutsche Forschungsgemeinschaft Bonn for the financial support (Project number: Ge 667/3-1) and also to the D F G for financial support to one of the authors (BH).
References 1 K. Kjaer, C. A. Helm and H. M6hwald, Thin Solid Films, 159(1988) 17-28. 2 L. G. Parratt, Phys. Rev., 95 (1954) 359-369. 3 L. Brfigemann, Ph.D. Thesis, University Kiel, 1989. 4 E. S. Wu and W. W. Webb, Phys. Rev. A, 8 (1973) 2065. 5 S. M. Heald, J. K. D. Jayanetti, A. A. Bright and G. W. Rubloff, J. Vac. Sci. Teehnol. A, 8 (1990) 2046. 6 A. Bender, Ph.D. Thesis, University Rostock, 1992. 7 Tb. Gerber and B. Himmel, J. Non-Cryst. Solids, 83(1986) 324-334. 8 R. Haight and L. C. Feldman, J. Appl. Phys., 53 (1982) 4884-4887.