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High-resolution Rutherford backscattering spectrometry and the analysis of very thin silicon nitride layers
Nuclear Instruments and Methods 200 (1982) 499-504 North-Holland Publishing Company
499
HIGH-RESOLUTION RUTHERFORD BACKSCATI'ERING ANALYSIS OF VERY THIN SILICON NITRIDE LAYERS Y. T A M M I N G A ,
M.F.C. WILLEMSEN,
SPECTROMETRY
F.H.P.M. HABRAKEN
AND THE
and A.E.T. KUIPER
Philips Research Laboratories, Eindhoven, The Netherlands
Received 8 February 1982
High-resolution Rutherford backscattering spectrometry is shown to be a valuable technique for the analysis of low-pressure chemical vapor-deposited silicon nitride layers. The optimization of the backscattering geometry factors, such as channeling and low angle exit are shown to provide the best depth resolution. For nitride layers with a thickness in the order of 50 .A,, the depth resolution is proved to be mainly determined by the acceptance angle of the detector. By reducing this angle a depth resolution of about 10 A, was obtained for the thinnest layers.
I. Introduction
2. Sample preparation
The application of Rutherford backscattering spectrometry (RBS) for near surface analysis has been developed during the past two decades. By far, most of the applications are found in the semiconductor field and especially silicon and its compounds have been extensively investigated. The following conditions are often used for analysis: 2 MeV He ~ ions are normally incident on the sample to be analysed and the measured fraction of the backscattered He + particles has a scattering angle of 170 °. With the above-mentioned configuration, a surface barrier detector with an energy resolution of 13 keV provides a depth resolution of 300 ~, in silicon. With special techniques it is possible to improve the depth resolution. For example one can reduce the energy resolution by using a magnetic spectrometer [1] or an electrostatic analyser [2]. However these techniques are rather timeconsuming and require the development of special target chambers. A different method, which proves to be much easier, is the application of glancing angle scattering geometries as described by Williams [3]. He reports a depth resolution of 35-40 A in silicon. In our particular case we applied glancing angle backscattering of 2 MeV He + for the analysis of very thin low pressure chemical vapor-deposited (LPCVD) nitride layers with thicknesses varying between 13 ~, and 600 ,A [4]. Because of the low mass of nitrogen and oxygen, relative to the mass of silicon, we had to combine channeling and glancing angle conditions. The ultimate depth resolution in thin L P C V D nitride layers will be shown to be less than 10 A. 0167-5087/82/0000-0000/$02.75
© 1982 North-Holland
Low pressure chemical vapor-deposited (LPCVD) nitrides were prepared in a standard Tempress L P C V D reactor, operating at a pressure of 0.2 Torr and a temperature of 820°C. Three-inch syton-polished silicon substrates were covered with silicon nitride using a mixture of SiH2CI 2 and N H 3 in the ratio of 1:2.5. The growth rate is approximately 60 A / m i n . Before deposition the substrates were chemically cleaned in a H 2 S O 4 / H 2 0 2 mixture, subsequently rinsed thoroughly in de-ionized water and spun dry. The facility of the L P C V D reactor to apply a HCI etch prior to deposition was not used.
3. Equipment The Rutherford backscattering measurements were performed with 2 MeV He + particles, produced by the 2.5 MeV Van de Graaff accelerator (High Voltage Engineering) at Philips Research Laboratories. The accelerated He + particles were separated from ions with different mass by means of a 90 ° magnet with a radius of 1.5 m. A Bruker ( N M R 12) nuclear magnetic resonance regulation unit was used for both the measurement and stabilization of the magnetic field. The stability and reproducibility of the energy of the He ~ beam was measured to be smaller than 1 keV. Angular variations of the beam were kept smaller than 0.02 ° by placing two sets of diaphragms between the target chamber and the separation magnet. The energy of the backscattered He + particles was
500
Y Tamminga et aL / High-resolution RBS
measured with an O R T E C surface barrier detector at a distance of about 15 cm from the target. It was possible to cool the detector to - 2 0 ° C . The detector area was 25 mm 2 and the measured energy resolution of the detector was 13 keY fwhm (full width half maximum). Pulse height analysis was done with a computerized measuring system, the Canberra Scorpio system, which was modified for backscattering analysis. Target manipulation was performed by a McLean 3-axis manipulator, with steps of 0.01 o and the stepping motors inside the vacuum (Panmure Instruments Ltd.). The vacuum in the target chamber and the beam lines was in between l0 -7 and 5.10 7 Torr. The probed area under normal incidence (beam spot size) of the beam is 1 × I mm 2. For glancing angle analysis the spot size was matched to the geometry of the detector, target and beam in order to reduce the energy variations caused by the geometry. The He + current was about 100 nA. The energy loss tables as given by Ziegler [51 were used for the in-depth elemental analysis. The total amounts of silicon, nitrogen and oxygen were determined with a scattering angle 0 = 170 ° and a target tilt and rotation smaller than 5 °.
~woo
,
sbo
Fig. I. Aligned spectra for 2 MeV He" ions incident on a Si sample covered with layers of LPCVD nitride of different thicknesses. The dechanneling increases with layer thickness.
plotted in fig. 2 and the areas of both the oxygen peak and the nitrogen peak can easily be calculated. These areas were converted to the number of a t o m s / e r a 2 by means of the following formula [7]: Ax
Osi 8 ( E )
N x - RHs' °x 4. Experimental results and discussion
eb {keV]
Energy
I~1~',
atoms/cm2
(1)
Ax
peak area of atom X in counts, random height of silicon in counts, Rutherford cross-section in cm 2 of element X, OX channel width in eV, 8(E) number of X in a t o m s / c m 2, Nx stopping cross-section factor of silicon in e V / 1 0 is a t o m s / c m 2. The random height of silicon was calculated from the RBS spectra, which were measured with a tilt and rotation of the target of 5 ° from the incoming He ~ beam and by spinning the target during the measurement [8]. The stopping cross-section of He ~ is given by forRHsi
For analysis of SiaN 4 and SiO 2 layers on top of Si by means of Rutherford Backscattering Spectrometry (RBS) analysis, it is known that it is difficult to separate the small N and O signals from the high background of the random Si signal of the substrate. Reduction of the background was achieved by the application of channeling in the [1001 or [1111 direction, depending on whether the silicon substrate orientation was (100) or (111), respectively [6]. In this way the ultimate sensitivity for nitrogen is found to be about 2 × 10 t5 a t o m s / c m 2 and for oxygen i × 1015 a t o m s / c m 2. Fig. 1 shows the RBS spectra of low-pressure chemical vapor deposited (LPCVD) nitrides on "bare" silicon with near normal channeling incidence of the He + beam and a scattering angle of 170 °. The four peaks of the RBS spectra are due to He + collisions with silicon, oxygen, nitrogen and carbon atoms. The carbon peak has nothing to do with the L P C V D layer, but is due to the cracking of oil. This oil very probably originates from the stepping motors of the manipulator, which are inside the vacuum. For normal incidence the carbon covering of the sample hardly affects the spectra, but one has to be careful at glancing angle geometries. The areas of the oxygen and nitrogen peaks were calculated after background subtraction of the silicon signal. A third degree polynomial least squares fit of the silicon spectrum between 400 keV and 1000 keV provides a good approximation. After subtraction of this /east squares fit one obtains differential spectra as
I 2000
N
third degreepolynompalht
~'t,
0
1ooo
0 5'oo
~o [keV] ¢oo Energy
~0
Fig. 2. The same spectrum as in fig. 1, with background subtraction of the silicon signal. The silicon signal was approached with a third degree least squares polynomial fit.
501
Y. Tamminga et al. / High-resolution RBS
mula (2) [7]:
~(Eo) ~(K×eo) I~1~-- gx c o s , + Icos(0 +*)1 '
(2)
nitride layers, and an accurate in-depth analysis of the nitrogen and oxygen concentration is not possible with the scattering geometry used.
I~1~ stopping
cross-section factor of element X in matrix Y, K x elastic scattering constant of element X, ,# target tilt. 8 scattering angle. The results for the different LPCVD nitride layers are summarized in table 1. As can be seen in this table, the amount of oxygen present in the nitride layers is nearly constant. The layer thicknesses in A of "SiO 2" and "Si.~N a" were calculated from No and N N (number of a t o m s / c m 2) by assuming the density for SiO 2 to be Ps,o2 = 6.96 × 10 22 a t o m s / c m 2 and for Si3N 4 Ps,,~, = 10.29 × 10 22 a t o m s / c m 2. The Si3N 4 layer thickness is linearly dependent on the deposition time. However it takes about 0.6 rain before the deposition process starts. This may be caused by nucleation differences on SiO 2 and Si3N 4 or may simply be due to a delay in the gas supply to the substrates. However, the delay time seems to be rather long. A different method for the determination of layer thicknesses is the following: When the density p ( c m - 3 ) of the matrix through which the He* particles moves is known, the measured energy loss A E (eV) of the He* particles in a layer can be converted to a layer thickness by the formula [71: 10SAE .
t - - -
A.
5. G l a n c i n g a n g l e m e a s u r e m e n t s
For these cases we applied the principle of glancing angle measurements [3] and in order to keep the background signal of the silicon low we used a combination of the glancing angle method and channeling. The scattering configuration we used was different from the four extremes mentioned by Williams [3]. In his case the glancing angle conditions were created by a rotation of the detector around the target, which implies a smaller scattering angle 0 (fig. 3). Unfortunately at smaller scattering angles 9 the mass resolution decreases and the energy variation of the elastic collision constant K caused by solid angle variations increases. Therefore we decided to use a target tilt of 54.7 °, so that larger scattering angles could be used (54.7 ° is the largest angle between one of the main axes and the surface of (100) and (l l l) cut Si.) This means then that we performed channeling in the [ I l l ] and I l l 0 ] directions for (100) and (111) cut silicon, respectively (fig. 3). I
dete~rotation ofthedetector
(3)
However, with the very thin layers involved, the energy width of the different peaks is equal to the energy resolution of the detector. When this energy resolution is converted to a depth resolution for Si3N 4, a value of 190 ,A is obtained for near normal incidence of the He * beam and a scattering angle of 170 °. Thus the results obtained with formula (3) are meaningless for the thin
dEetector
,ec,or , /E
target~ Table 1 Sample number
Si3N 4 (A)
1
-
2 3 4 5 6 7 8
13 28 57 222 404 602
SiO2 (,A)
Deposition time in minutes
20 20 20 17 23 24 25 29
0.3 0.6 0.9 1.2 1.6 4.7 7.8 I 1.0
/2 Fig. 3. I Standard scattering geometry. II Rotation of the detector towards smaller scattering angles. The outgoing path length of the backscattered He + ions in the target is enlarged and provides a better depth resolution. Ill Scattering configuration with optimum conditions.
Y. Tamminga et aL / High-resolution RBS
502
In general one can state that a higher value for the stopping cross-section provides a better depth resolution. The maximum of the stopping power of He + ions in Si3N 4 is around 500 keV. With this in mind the use of 2 MeV He + seems to be far from optimal. However for glancing angle conditions this problem is irrelevant for the following reasons: Firstly, the outgoing path for our glancing angle geometry is much larger than the incoming path, so that the stopping cross-section is mainly determined by the energy loss on the outgoing path. The He ' energy after collision against nitrogen is about 600 keV, which means that we are quite close to the maximum of the stopping cross-section. Secondly, on the one hand, the angle between the outgoing beam and the sample surface a was chosen as small as possible to obtain maximum depth resolution. On the other hand the angle a was chosen in such a way that the signals of nitrogen at the surface and possible signals from oxygen in depth (for example at the interface between Si and SigN4) would not overlap. For a He + beam with an E 0 of 2000 keV the energy separation between He + particles scattered from N and O at the surface is 102.4 keV. This means that we have to match the backscattering geometry in such a way that the energy loss of the H e " caused by the Si3N 4 layer is smaller than 102.4 keV. With an energy resolution of 13 keV for the detector an energy loss in the Si3N 4 layer of 65 keV provides a useful separation between O and N signals in the energy spectrum. The requirement of an energy loss of 65 keV limits the depth resolution to 1 3 / 6 5 - - 1/5 of the layer thickness (detector resolution is 13 keV). Therefore if it is possible to rotate the detector through sufficiently small angles a for the outgoing He beam, the first requirement for maximum stopping power does not play any role at all. In that case a straightforward extrapolation is that the energy separation between O and N should be as large as possible for optimal depth resolution. This can easily be realized by choosing a .maximum energy E o for the incoming He + beam. However, in order to avoid deviations from the Rutherford cross-section, we did not apply energies E 0 above 2000 keV. The requirement of the 65 keV energy loss of the He + ion in the Si3N 4 layer means that a one-to-one correlation exists between the layer thickness of the nitride and the applied angle a. For our scattering configuration it was calculated that one degree ( a ) corresponds to 20 ,~, Si3N 4. To a first approximation the relation between a and layer thickness is linear for small angles (a < 10°). A different question is that if the detector is rotated to angles a smaller than 10 °, the influence of the detector acceptance angle Aa can no longer be neglected. Fig. 4 shows that these energy differences are directly related to the depth from which the H e - particle is scattered. For the surface no energy differences
= _:5?-
-
-
Fig. 4. Schematic plot to illustrate path length differences due to the detector acceptance angle. Path number I is much larger than number II.
due to path length differences can be encountered. The maximum and minimum path lengths of the He + through the complete nitride layer, implying a maximum energy difference AE,x,,, are indicated by path I and path II in fig. 4. A E.a,, is given by the formula:
AEa,,---AEll-sin("~an-~Aa)], which means that for the interface between nitride and oxide (65 keV energy loss) we have AEa,~ = 65[1
sin(~-~Aa) ] '
In fig. 5 AEz,, is plotted as a function of a and with Aa as parameter. Due to the other requirements a can directly be converted to a nitride thickness. To illustrate the consequence of the requirements and phenomena mentioned above, we consider by way of example: a nitride layer of 20/~. The optimum for a is one degree. so that for our detector of 25 mm 2 A Ea,, = 4 5 keV (fig. 4), which is very high compared with the 13 keV energy resolution of the detector. By placing a slit of 1 mm in front of the detector, this spread is reduced to 18 keV. A spread of 18 keV was found to be acceptable, because it is a maximum energy difference. This means that all layers could be measured with such a slit. Energy variations due to energy straggling of the He ~ ions can be precluded for the thin layers involved. Another difficulty is that the measurement of the integrated He + ion current at glancing angle configuration is irreproducible. Therefore the areas of the nitrogen and oxygen peaks were quantitatively interpretated by normalizing the area of the nitrogen peak to that measured at normal incidence of the beam.
503
Y. Tamrninga et aL / High-resolution RBS sample number 5
5O
o nitrogen oxygen
'E 5 1022
~o
3
~o 20
o
oC ,(degrees)
Fig. 5. Energy spread as a function of the angle between outgoing beam and target. The vertical scale is normalized to 65 keV energy loss, which corresponds to the nitride silicon interface. The applied angle a multiplied by 20 corresponds to the nitride layer thickness to be analysed. The curve number corresponds to the slitwidth in mm, while 1 mm corresponds to a & a = 0 . 4 °.
For the conversion of energy loss to depth with formula (3) we used the stopping cross-section of Si3N 4 and we neglected the presence of "SiO2". In this way the depth position of the oxygen signal from any oxygen that may be present between Si and the L P C V D nitride layer is normalized with respect to the nitrogen depth scale. On the one hand the height of the oxygen peak may be too high because of the use of a too high stopping cross-section, but on the other hand the height may be far too low because of the energy resolution of the detector. As an example of these consideration fig. 6 shows the results of the application of glancing-angle conditions to a nitride layer with a thickness of 60 ,~. The spectrum differs strongly from that obtained at normal incidence and 8 = 170 °. The most striking features are the flat-topped peaks for silicon and nitrogen,
sample numOer 5
400
depth(,~
6o
80
Fig. 7. Concentration versus depth profiles, calculated from the results shown in fig. 6.
while the oxygen peak is split into two parts. The measuring time was chosen such that the energy shift due to carbon built up was less than 5 keV. The silicon background was found to be very low. This is easily explained by the very short path length of the incoming He + particles in the amorphous layer which implies a low dechanneling of the He + beam. The two oxygen peaks are positioned at energies corresponding to the surface and silicon interface of the nitride layer. For all different thicknesses of the nitride layer, the interfacial low energy peak is found to be larger than the surface peak. The interracial peak is attributed to the native oxide present on silicon, while the oxygen peak at the surface shows that Si3N 4 is also slightly oxidized in the air. This oxy.gen peak at the surface always corresponds to about 8 A of SiO 2 [4]. The background subtraction and the determination of the peak contents were performed in the same way as mentioned in the foregoing, and so too was the energyto-depth conversion, in which we used the stopping cross section factor of stoichiometric Si3N 4. In this way concentration profiles are obtained for the oxygen and
57.~ Sl~N~
sample number3
t 4oc
2000~ev ~.ip" S,
~'o
'~.~
s'92
134 S§N
,/
s,o2
£" ~300
} .ol
,
g "~2oo
J,
N
0
700
500
750 Energy [keVj
1000
7250
Fig. 6. Aligned spectrum for 2 MeV He * ions with glancing angle exit. The LPCVD nitride layer thickness is 57 ,&.
~oo
¢5o
Energy (ke4/)
~o
7~
Fig. 8. Aligned spectrum for 2 MeV He + ions with glancing angle exit. The LPCVD nitride layer thickness is 13 ,&.
504
Y. Tamminga et al. / High-resolution RBS
nitrogen present in the nitride layers. Fig. 7 shows the results of such a conversion, which clearly confirms the foregoing interpretation of the oxygen peaks. The nitrogen concentration of the LPCVD nitride layers is close to the ideal value of stoichiometric Si3N 4 (5.88 X 10 22 atoms/cm3). For the region in between the two perfectly separated oxygen peaks, the oxygen concentration is less than 10 2~ a t o m s / c m 3. It is concluded that the native oxide present on top of the silicon is not mixed with the nitride during the deposition. The best depth resolution was obtained for a 13 ,~ nitride layer. The RBS spectrum is shown in fig. 8. With such a thin layer the use of a slit in front of the detector is absolutely necessary. In order to keep the count rate as high as possible we used a rectangular I mm slit perpendicular to the plane of rotation of the detector. The silicon signal of the RBS spectrum shows very interesting features. A very high peak is detected in the low energetic part, and with increasing energy a lower second peak is detected. This lower second peak is attributed to the silicon of the nitride layer, while the two neighbouring minima are attributed to the silicon of the two oxide layers mentioned. The very high part of the peak is the surface peak of silicon, which is caused by the lattice vibrations of the silicon single-crystal substrate atoms. The peak content is 1.2 × 1016 a t o m s / c m 2, which is in nice agreement with the value given by Stensgaard [9] and Jackman [10]. The equivalent thickness, amounting to 24 ,A, easily explains the large height of this surface peak. It should be remarked that figure 6 shows the same features. As can be seen in figure 8, even for the 13 ,~ nitride layer the oxygen peaks are sep.arated. The experimental depth resolution is about 10 A, whereas the theoretical value would be one fifth of the nitride layer thickness (2.6 ,A). The reason why we found a larger value for the depth resolution than expected is that the sum of the thickness of the oxide layers is much greater than the nitride layer thickness. This means that the total energy loss of the He + particles can no longer be approached
5omple n u m b e r 3
o nttrogen Cx),ger~
c_
5, 70;
depth{A)
~o
Jo
Fig. 9. Concentration versus depth profiles, calculated from the results shown in fig. 8.
by the energy loss in the SisN 4 alone. Thus the angle a under which the sample is measured is not smaller than one degree, as suggested by fig. 5 for a nitride layer of 13 ,A. Fig. 9 shows the depth profiles of oxygen and nitrogen. The depth scale was calculated in the way described earlier.
6. Conclusion RBS of 2 MeV He + particles combined with channeling and glancing angle scattering conditions proves to be a powerful technique for the analysis of LPCVD layers of silicon nitride with a layer thickness, smaller than 1430 A (65 X 22 ,A). It has been shown that the depth resolution can be improved by one fifth of the nitride layer thickness. The best depth resolution obtained for a 13 ,A nitride layer is about 10 A. The depth resolution is not determined by energy straggling, multiple scattering, beam divergence or kinematic recoil factor variations. The detector acceptance angle is shown to be the main cause of the energy spread of the measured fraction of the backscattered H e " ions. The detector acceptance angle accounts for path length differences in the measured fraction of the He + particles. When the layers are very thin the surface peak due to thermal vibrations of the underlying silicon single crystal is clearly resolved in the spectrum. In addition we have shown that it is possible to characterize very thin nitride layers in a vacuum system operating at pressures of about 5 × 10 7Torr. It has to be mentioned that the accelerator was designed by dr. J. Politiek. He is also acknowledged for helpful discussions We thank R. van Silfhout for computational assistance. F. Huizinga prepared the samples.
References [11 J.K. Hirvonen and GK. Hubler, Nucl. Instr. and Meth. 149 (1978) 457. [2] A. Feuerstein, H. Grahmann, S. Kalbitzer and H. Oetzman, Nucl. Instr. and Meth. 149 (1978) 471. [3] J. Williams, Nucl. Instr. and Meth. 149 (1978), p. 207. {4] F.H.P.M. Habraken, A.E.T. Kuiper, A. van Oostrom, Y. Tamminga and J.B. Theeten, J. Appl. Phys. 53 (1982) 404. [5] J. Ziegler, Helium: Stopping powers and ranges in all elemental matter (Pergamon Press, 1977). [6] W.K. Chu. J.W. Mayer, M.A. Nicolet, Backscattering spectrometry (Academic Press, 1978). [7] J.F. Ziegler, New uses of ion accelerators (Plenum Press, 1975). [8] P. Blood, Nucl. Instr. and Meth. 149 (1978) 225. [91 I. Stensgaard et al., Surf. Science 77 (1978) 513. [10] T.E. Jackman et al., Surf. Science 100 (1980) 35.
499
HIGH-RESOLUTION RUTHERFORD BACKSCATI'ERING ANALYSIS OF VERY THIN SILICON NITRIDE LAYERS Y. T A M M I N G A ,
M.F.C. WILLEMSEN,
SPECTROMETRY
F.H.P.M. HABRAKEN
AND THE
and A.E.T. KUIPER
Philips Research Laboratories, Eindhoven, The Netherlands
Received 8 February 1982
High-resolution Rutherford backscattering spectrometry is shown to be a valuable technique for the analysis of low-pressure chemical vapor-deposited silicon nitride layers. The optimization of the backscattering geometry factors, such as channeling and low angle exit are shown to provide the best depth resolution. For nitride layers with a thickness in the order of 50 .A,, the depth resolution is proved to be mainly determined by the acceptance angle of the detector. By reducing this angle a depth resolution of about 10 A, was obtained for the thinnest layers.
I. Introduction
2. Sample preparation
The application of Rutherford backscattering spectrometry (RBS) for near surface analysis has been developed during the past two decades. By far, most of the applications are found in the semiconductor field and especially silicon and its compounds have been extensively investigated. The following conditions are often used for analysis: 2 MeV He ~ ions are normally incident on the sample to be analysed and the measured fraction of the backscattered He + particles has a scattering angle of 170 °. With the above-mentioned configuration, a surface barrier detector with an energy resolution of 13 keV provides a depth resolution of 300 ~, in silicon. With special techniques it is possible to improve the depth resolution. For example one can reduce the energy resolution by using a magnetic spectrometer [1] or an electrostatic analyser [2]. However these techniques are rather timeconsuming and require the development of special target chambers. A different method, which proves to be much easier, is the application of glancing angle scattering geometries as described by Williams [3]. He reports a depth resolution of 35-40 A in silicon. In our particular case we applied glancing angle backscattering of 2 MeV He + for the analysis of very thin low pressure chemical vapor-deposited (LPCVD) nitride layers with thicknesses varying between 13 ~, and 600 ,A [4]. Because of the low mass of nitrogen and oxygen, relative to the mass of silicon, we had to combine channeling and glancing angle conditions. The ultimate depth resolution in thin L P C V D nitride layers will be shown to be less than 10 A. 0167-5087/82/0000-0000/$02.75
© 1982 North-Holland
Low pressure chemical vapor-deposited (LPCVD) nitrides were prepared in a standard Tempress L P C V D reactor, operating at a pressure of 0.2 Torr and a temperature of 820°C. Three-inch syton-polished silicon substrates were covered with silicon nitride using a mixture of SiH2CI 2 and N H 3 in the ratio of 1:2.5. The growth rate is approximately 60 A / m i n . Before deposition the substrates were chemically cleaned in a H 2 S O 4 / H 2 0 2 mixture, subsequently rinsed thoroughly in de-ionized water and spun dry. The facility of the L P C V D reactor to apply a HCI etch prior to deposition was not used.
3. Equipment The Rutherford backscattering measurements were performed with 2 MeV He + particles, produced by the 2.5 MeV Van de Graaff accelerator (High Voltage Engineering) at Philips Research Laboratories. The accelerated He + particles were separated from ions with different mass by means of a 90 ° magnet with a radius of 1.5 m. A Bruker ( N M R 12) nuclear magnetic resonance regulation unit was used for both the measurement and stabilization of the magnetic field. The stability and reproducibility of the energy of the He ~ beam was measured to be smaller than 1 keV. Angular variations of the beam were kept smaller than 0.02 ° by placing two sets of diaphragms between the target chamber and the separation magnet. The energy of the backscattered He + particles was
500
Y Tamminga et aL / High-resolution RBS
measured with an O R T E C surface barrier detector at a distance of about 15 cm from the target. It was possible to cool the detector to - 2 0 ° C . The detector area was 25 mm 2 and the measured energy resolution of the detector was 13 keY fwhm (full width half maximum). Pulse height analysis was done with a computerized measuring system, the Canberra Scorpio system, which was modified for backscattering analysis. Target manipulation was performed by a McLean 3-axis manipulator, with steps of 0.01 o and the stepping motors inside the vacuum (Panmure Instruments Ltd.). The vacuum in the target chamber and the beam lines was in between l0 -7 and 5.10 7 Torr. The probed area under normal incidence (beam spot size) of the beam is 1 × I mm 2. For glancing angle analysis the spot size was matched to the geometry of the detector, target and beam in order to reduce the energy variations caused by the geometry. The He + current was about 100 nA. The energy loss tables as given by Ziegler [51 were used for the in-depth elemental analysis. The total amounts of silicon, nitrogen and oxygen were determined with a scattering angle 0 = 170 ° and a target tilt and rotation smaller than 5 °.
~woo
,
sbo
Fig. I. Aligned spectra for 2 MeV He" ions incident on a Si sample covered with layers of LPCVD nitride of different thicknesses. The dechanneling increases with layer thickness.
plotted in fig. 2 and the areas of both the oxygen peak and the nitrogen peak can easily be calculated. These areas were converted to the number of a t o m s / e r a 2 by means of the following formula [7]: Ax
Osi 8 ( E )
N x - RHs' °x 4. Experimental results and discussion
eb {keV]
Energy
I~1~',
atoms/cm2
(1)
Ax
peak area of atom X in counts, random height of silicon in counts, Rutherford cross-section in cm 2 of element X, OX channel width in eV, 8(E) number of X in a t o m s / c m 2, Nx stopping cross-section factor of silicon in e V / 1 0 is a t o m s / c m 2. The random height of silicon was calculated from the RBS spectra, which were measured with a tilt and rotation of the target of 5 ° from the incoming He ~ beam and by spinning the target during the measurement [8]. The stopping cross-section of He ~ is given by forRHsi
For analysis of SiaN 4 and SiO 2 layers on top of Si by means of Rutherford Backscattering Spectrometry (RBS) analysis, it is known that it is difficult to separate the small N and O signals from the high background of the random Si signal of the substrate. Reduction of the background was achieved by the application of channeling in the [1001 or [1111 direction, depending on whether the silicon substrate orientation was (100) or (111), respectively [6]. In this way the ultimate sensitivity for nitrogen is found to be about 2 × 10 t5 a t o m s / c m 2 and for oxygen i × 1015 a t o m s / c m 2. Fig. 1 shows the RBS spectra of low-pressure chemical vapor deposited (LPCVD) nitrides on "bare" silicon with near normal channeling incidence of the He + beam and a scattering angle of 170 °. The four peaks of the RBS spectra are due to He + collisions with silicon, oxygen, nitrogen and carbon atoms. The carbon peak has nothing to do with the L P C V D layer, but is due to the cracking of oil. This oil very probably originates from the stepping motors of the manipulator, which are inside the vacuum. For normal incidence the carbon covering of the sample hardly affects the spectra, but one has to be careful at glancing angle geometries. The areas of the oxygen and nitrogen peaks were calculated after background subtraction of the silicon signal. A third degree polynomial least squares fit of the silicon spectrum between 400 keV and 1000 keV provides a good approximation. After subtraction of this /east squares fit one obtains differential spectra as
I 2000
N
third degreepolynompalht
~'t,
0
1ooo
0 5'oo
~o [keV] ¢oo Energy
~0
Fig. 2. The same spectrum as in fig. 1, with background subtraction of the silicon signal. The silicon signal was approached with a third degree least squares polynomial fit.
501
Y. Tamminga et al. / High-resolution RBS
mula (2) [7]:
~(Eo) ~(K×eo) I~1~-- gx c o s , + Icos(0 +*)1 '
(2)
nitride layers, and an accurate in-depth analysis of the nitrogen and oxygen concentration is not possible with the scattering geometry used.
I~1~ stopping
cross-section factor of element X in matrix Y, K x elastic scattering constant of element X, ,# target tilt. 8 scattering angle. The results for the different LPCVD nitride layers are summarized in table 1. As can be seen in this table, the amount of oxygen present in the nitride layers is nearly constant. The layer thicknesses in A of "SiO 2" and "Si.~N a" were calculated from No and N N (number of a t o m s / c m 2) by assuming the density for SiO 2 to be Ps,o2 = 6.96 × 10 22 a t o m s / c m 2 and for Si3N 4 Ps,,~, = 10.29 × 10 22 a t o m s / c m 2. The Si3N 4 layer thickness is linearly dependent on the deposition time. However it takes about 0.6 rain before the deposition process starts. This may be caused by nucleation differences on SiO 2 and Si3N 4 or may simply be due to a delay in the gas supply to the substrates. However, the delay time seems to be rather long. A different method for the determination of layer thicknesses is the following: When the density p ( c m - 3 ) of the matrix through which the He* particles moves is known, the measured energy loss A E (eV) of the He* particles in a layer can be converted to a layer thickness by the formula [71: 10SAE .
t - - -
A.
5. G l a n c i n g a n g l e m e a s u r e m e n t s
For these cases we applied the principle of glancing angle measurements [3] and in order to keep the background signal of the silicon low we used a combination of the glancing angle method and channeling. The scattering configuration we used was different from the four extremes mentioned by Williams [3]. In his case the glancing angle conditions were created by a rotation of the detector around the target, which implies a smaller scattering angle 0 (fig. 3). Unfortunately at smaller scattering angles 9 the mass resolution decreases and the energy variation of the elastic collision constant K caused by solid angle variations increases. Therefore we decided to use a target tilt of 54.7 °, so that larger scattering angles could be used (54.7 ° is the largest angle between one of the main axes and the surface of (100) and (l l l) cut Si.) This means then that we performed channeling in the [ I l l ] and I l l 0 ] directions for (100) and (111) cut silicon, respectively (fig. 3). I
dete~rotation ofthedetector
(3)
However, with the very thin layers involved, the energy width of the different peaks is equal to the energy resolution of the detector. When this energy resolution is converted to a depth resolution for Si3N 4, a value of 190 ,A is obtained for near normal incidence of the He * beam and a scattering angle of 170 °. Thus the results obtained with formula (3) are meaningless for the thin
dEetector
,ec,or , /E
target~ Table 1 Sample number
Si3N 4 (A)
1
-
2 3 4 5 6 7 8
13 28 57 222 404 602
SiO2 (,A)
Deposition time in minutes
20 20 20 17 23 24 25 29
0.3 0.6 0.9 1.2 1.6 4.7 7.8 I 1.0
/2 Fig. 3. I Standard scattering geometry. II Rotation of the detector towards smaller scattering angles. The outgoing path length of the backscattered He + ions in the target is enlarged and provides a better depth resolution. Ill Scattering configuration with optimum conditions.
Y. Tamminga et aL / High-resolution RBS
502
In general one can state that a higher value for the stopping cross-section provides a better depth resolution. The maximum of the stopping power of He + ions in Si3N 4 is around 500 keV. With this in mind the use of 2 MeV He + seems to be far from optimal. However for glancing angle conditions this problem is irrelevant for the following reasons: Firstly, the outgoing path for our glancing angle geometry is much larger than the incoming path, so that the stopping cross-section is mainly determined by the energy loss on the outgoing path. The He ' energy after collision against nitrogen is about 600 keV, which means that we are quite close to the maximum of the stopping cross-section. Secondly, on the one hand, the angle between the outgoing beam and the sample surface a was chosen as small as possible to obtain maximum depth resolution. On the other hand the angle a was chosen in such a way that the signals of nitrogen at the surface and possible signals from oxygen in depth (for example at the interface between Si and SigN4) would not overlap. For a He + beam with an E 0 of 2000 keV the energy separation between He + particles scattered from N and O at the surface is 102.4 keV. This means that we have to match the backscattering geometry in such a way that the energy loss of the H e " caused by the Si3N 4 layer is smaller than 102.4 keV. With an energy resolution of 13 keV for the detector an energy loss in the Si3N 4 layer of 65 keV provides a useful separation between O and N signals in the energy spectrum. The requirement of an energy loss of 65 keV limits the depth resolution to 1 3 / 6 5 - - 1/5 of the layer thickness (detector resolution is 13 keV). Therefore if it is possible to rotate the detector through sufficiently small angles a for the outgoing He beam, the first requirement for maximum stopping power does not play any role at all. In that case a straightforward extrapolation is that the energy separation between O and N should be as large as possible for optimal depth resolution. This can easily be realized by choosing a .maximum energy E o for the incoming He + beam. However, in order to avoid deviations from the Rutherford cross-section, we did not apply energies E 0 above 2000 keV. The requirement of the 65 keV energy loss of the He + ion in the Si3N 4 layer means that a one-to-one correlation exists between the layer thickness of the nitride and the applied angle a. For our scattering configuration it was calculated that one degree ( a ) corresponds to 20 ,~, Si3N 4. To a first approximation the relation between a and layer thickness is linear for small angles (a < 10°). A different question is that if the detector is rotated to angles a smaller than 10 °, the influence of the detector acceptance angle Aa can no longer be neglected. Fig. 4 shows that these energy differences are directly related to the depth from which the H e - particle is scattered. For the surface no energy differences
= _:5?-
-
-
Fig. 4. Schematic plot to illustrate path length differences due to the detector acceptance angle. Path number I is much larger than number II.
due to path length differences can be encountered. The maximum and minimum path lengths of the He + through the complete nitride layer, implying a maximum energy difference AE,x,,, are indicated by path I and path II in fig. 4. A E.a,, is given by the formula:
AEa,,---AEll-sin("~an-~Aa)], which means that for the interface between nitride and oxide (65 keV energy loss) we have AEa,~ = 65[1
sin(~-~Aa) ] '
In fig. 5 AEz,, is plotted as a function of a and with Aa as parameter. Due to the other requirements a can directly be converted to a nitride thickness. To illustrate the consequence of the requirements and phenomena mentioned above, we consider by way of example: a nitride layer of 20/~. The optimum for a is one degree. so that for our detector of 25 mm 2 A Ea,, = 4 5 keV (fig. 4), which is very high compared with the 13 keV energy resolution of the detector. By placing a slit of 1 mm in front of the detector, this spread is reduced to 18 keV. A spread of 18 keV was found to be acceptable, because it is a maximum energy difference. This means that all layers could be measured with such a slit. Energy variations due to energy straggling of the He ~ ions can be precluded for the thin layers involved. Another difficulty is that the measurement of the integrated He + ion current at glancing angle configuration is irreproducible. Therefore the areas of the nitrogen and oxygen peaks were quantitatively interpretated by normalizing the area of the nitrogen peak to that measured at normal incidence of the beam.
503
Y. Tamrninga et aL / High-resolution RBS sample number 5
5O
o nitrogen oxygen
'E 5 1022
~o
3
~o 20
o
oC ,(degrees)
Fig. 5. Energy spread as a function of the angle between outgoing beam and target. The vertical scale is normalized to 65 keV energy loss, which corresponds to the nitride silicon interface. The applied angle a multiplied by 20 corresponds to the nitride layer thickness to be analysed. The curve number corresponds to the slitwidth in mm, while 1 mm corresponds to a & a = 0 . 4 °.
For the conversion of energy loss to depth with formula (3) we used the stopping cross-section of Si3N 4 and we neglected the presence of "SiO2". In this way the depth position of the oxygen signal from any oxygen that may be present between Si and the L P C V D nitride layer is normalized with respect to the nitrogen depth scale. On the one hand the height of the oxygen peak may be too high because of the use of a too high stopping cross-section, but on the other hand the height may be far too low because of the energy resolution of the detector. As an example of these consideration fig. 6 shows the results of the application of glancing-angle conditions to a nitride layer with a thickness of 60 ,~. The spectrum differs strongly from that obtained at normal incidence and 8 = 170 °. The most striking features are the flat-topped peaks for silicon and nitrogen,
sample numOer 5
400
depth(,~
6o
80
Fig. 7. Concentration versus depth profiles, calculated from the results shown in fig. 6.
while the oxygen peak is split into two parts. The measuring time was chosen such that the energy shift due to carbon built up was less than 5 keV. The silicon background was found to be very low. This is easily explained by the very short path length of the incoming He + particles in the amorphous layer which implies a low dechanneling of the He + beam. The two oxygen peaks are positioned at energies corresponding to the surface and silicon interface of the nitride layer. For all different thicknesses of the nitride layer, the interfacial low energy peak is found to be larger than the surface peak. The interracial peak is attributed to the native oxide present on silicon, while the oxygen peak at the surface shows that Si3N 4 is also slightly oxidized in the air. This oxy.gen peak at the surface always corresponds to about 8 A of SiO 2 [4]. The background subtraction and the determination of the peak contents were performed in the same way as mentioned in the foregoing, and so too was the energyto-depth conversion, in which we used the stopping cross section factor of stoichiometric Si3N 4. In this way concentration profiles are obtained for the oxygen and
57.~ Sl~N~
sample number3
t 4oc
2000~ev ~.ip" S,
~'o
'~.~
s'92
134 S§N
,/
s,o2
£" ~300
} .ol
,
g "~2oo
J,
N
0
700
500
750 Energy [keVj
1000
7250
Fig. 6. Aligned spectrum for 2 MeV He * ions with glancing angle exit. The LPCVD nitride layer thickness is 57 ,&.
~oo
¢5o
Energy (ke4/)
~o
7~
Fig. 8. Aligned spectrum for 2 MeV He + ions with glancing angle exit. The LPCVD nitride layer thickness is 13 ,&.
504
Y. Tamminga et al. / High-resolution RBS
nitrogen present in the nitride layers. Fig. 7 shows the results of such a conversion, which clearly confirms the foregoing interpretation of the oxygen peaks. The nitrogen concentration of the LPCVD nitride layers is close to the ideal value of stoichiometric Si3N 4 (5.88 X 10 22 atoms/cm3). For the region in between the two perfectly separated oxygen peaks, the oxygen concentration is less than 10 2~ a t o m s / c m 3. It is concluded that the native oxide present on top of the silicon is not mixed with the nitride during the deposition. The best depth resolution was obtained for a 13 ,~ nitride layer. The RBS spectrum is shown in fig. 8. With such a thin layer the use of a slit in front of the detector is absolutely necessary. In order to keep the count rate as high as possible we used a rectangular I mm slit perpendicular to the plane of rotation of the detector. The silicon signal of the RBS spectrum shows very interesting features. A very high peak is detected in the low energetic part, and with increasing energy a lower second peak is detected. This lower second peak is attributed to the silicon of the nitride layer, while the two neighbouring minima are attributed to the silicon of the two oxide layers mentioned. The very high part of the peak is the surface peak of silicon, which is caused by the lattice vibrations of the silicon single-crystal substrate atoms. The peak content is 1.2 × 1016 a t o m s / c m 2, which is in nice agreement with the value given by Stensgaard [9] and Jackman [10]. The equivalent thickness, amounting to 24 ,A, easily explains the large height of this surface peak. It should be remarked that figure 6 shows the same features. As can be seen in figure 8, even for the 13 ,~ nitride layer the oxygen peaks are sep.arated. The experimental depth resolution is about 10 A, whereas the theoretical value would be one fifth of the nitride layer thickness (2.6 ,A). The reason why we found a larger value for the depth resolution than expected is that the sum of the thickness of the oxide layers is much greater than the nitride layer thickness. This means that the total energy loss of the He + particles can no longer be approached
5omple n u m b e r 3
o nttrogen Cx),ger~
c_
5, 70;
depth{A)
~o
Jo
Fig. 9. Concentration versus depth profiles, calculated from the results shown in fig. 8.
by the energy loss in the SisN 4 alone. Thus the angle a under which the sample is measured is not smaller than one degree, as suggested by fig. 5 for a nitride layer of 13 ,A. Fig. 9 shows the depth profiles of oxygen and nitrogen. The depth scale was calculated in the way described earlier.
6. Conclusion RBS of 2 MeV He + particles combined with channeling and glancing angle scattering conditions proves to be a powerful technique for the analysis of LPCVD layers of silicon nitride with a layer thickness, smaller than 1430 A (65 X 22 ,A). It has been shown that the depth resolution can be improved by one fifth of the nitride layer thickness. The best depth resolution obtained for a 13 ,A nitride layer is about 10 A. The depth resolution is not determined by energy straggling, multiple scattering, beam divergence or kinematic recoil factor variations. The detector acceptance angle is shown to be the main cause of the energy spread of the measured fraction of the backscattered H e " ions. The detector acceptance angle accounts for path length differences in the measured fraction of the He + particles. When the layers are very thin the surface peak due to thermal vibrations of the underlying silicon single crystal is clearly resolved in the spectrum. In addition we have shown that it is possible to characterize very thin nitride layers in a vacuum system operating at pressures of about 5 × 10 7Torr. It has to be mentioned that the accelerator was designed by dr. J. Politiek. He is also acknowledged for helpful discussions We thank R. van Silfhout for computational assistance. F. Huizinga prepared the samples.
References [11 J.K. Hirvonen and GK. Hubler, Nucl. Instr. and Meth. 149 (1978) 457. [2] A. Feuerstein, H. Grahmann, S. Kalbitzer and H. Oetzman, Nucl. Instr. and Meth. 149 (1978) 471. [3] J. Williams, Nucl. Instr. and Meth. 149 (1978), p. 207. {4] F.H.P.M. Habraken, A.E.T. Kuiper, A. van Oostrom, Y. Tamminga and J.B. Theeten, J. Appl. Phys. 53 (1982) 404. [5] J. Ziegler, Helium: Stopping powers and ranges in all elemental matter (Pergamon Press, 1977). [6] W.K. Chu. J.W. Mayer, M.A. Nicolet, Backscattering spectrometry (Academic Press, 1978). [7] J.F. Ziegler, New uses of ion accelerators (Plenum Press, 1975). [8] P. Blood, Nucl. Instr. and Meth. 149 (1978) 225. [91 I. Stensgaard et al., Surf. Science 77 (1978) 513. [10] T.E. Jackman et al., Surf. Science 100 (1980) 35.