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Boron nitride thin films by microwave ECR plasma chemical vapor deposition
30
Thin Solid Films, 235 (1993) 30-34
Boron nitride thin films by microwave ECR plasma chemical vapor deposition M. J. Paisley a, L. P. Bourget b and R. F. Davis a aDepartment of Materials Science and Engineering, Box 7919, North Carolina State University, Raleigh, NC 27695-7907 (USA) bApplied Science and Technology (ASTeX), Inc., 35 Cabot Road, Woburn, MA 01801 (USA)
(Received August 18, 1992; accepted May 14, 1993)
Abstract The objective of this study was the deposition of BN on Si (100) and (111) using microwave electron cyclotron resonance plasma chemical vapor deposition with borazine as the source material. Films were deposited and analyzed employing the techniques of ellipsometry, X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy and Fourier transform infrared spectroscopy. Results indicated that the deposited films were composed of microcrystals of c-BN and oriented hexagonal BN as well as amorphous BN.
I. Introduction The deposition of BN films has received much attention for the very wide variety of applications of all phases of BN. The literature contains a number of excellent reviews on each of the phases of BN: cubic BN [1-4], wurzitic BN [5] and hexagonal BN (h-BN)
[6]. Microwave electron cyclotron resonance (ECR) sources have been used successfully for the growth of a number of nitrides such as G a N [7], A I N - G a N [8], InN [9] and BN [10]. This study chose to utilize an ECR plasma source but also undertook a slightly different approach to BN deposition. The change in deposition strategy involved the use of a reactant gas that already contained B - N bonding. This was deemed an advantage, as the deposition would require only bond reordering, not bond formation. Borazine (B3N3H6, analogous to the carbon compound benzene), was selected since there are three direct B - N bondpairs, and the BN:H ratio is low (for borazine, BN:H is 1:2; for diborane and ammonia, BN:H is 1:6). Borazine has been used previously for deposition of BN films in chemical vapor deposition (CVD) reactors [11-13], but had not previously been tried in a microwave ECR plasma reactor.
2. Experimental procedure The depositions were conducted in two separate phases at ASTeX, Inc. The two deposition trials were largely the same, so the following description notes only the differences for the second set of depositions. A
microwave ECR plasma CVD system adapted for handling borazine was used. The changes to the system involved the installation of a special mass flow controller, calibrated for low vapor pressure gases, and a bottle of borazine liquid (50 g bottle, research grade, Callery Chemical Co., Pittsburgh, PA) into the gas handling system. Since the vapor pressure of borazine is ,-~9 kPa at room temperature, the bottle was connected directly to the mass flow controller and the vapor was drawn from the liquid at room temperature. The microwave ECR plasma CVD gas system consisted of a gas manifold and the associated mass flow controllers that fed gases directly into the plasma or downstream near the substrate. Depositions were performed with argon and nitrogen injected directly into the plasma, while the borazine was injected downstream. The 35 cm diameter chamber was evacuated by an 880 1 s -1 turbomolecular pump (base pressure: mid 10 -5 Pa) backed by a 85 m 3 h -1 mechanical pump that was vented to an exhaust system. For Phase I the substrate was r.f. induction heated and its temperature calibrated with a thermocouple placed on the substrate's surface. Phase II trials involved inverting the deposition chamber to minimize particulates. In addition, the substrate heating method was changed to infrared heating from a lamp package facing the rear surface of the wafer located outside the chamber. During both deposition trials, the substrate support was electrically isolated, making it possible to add r.f. biasing to the substrate to control ion energy and ion flux to the substrate independently. A schematic diagram of the ASTeX laboratory system during Phase I is shown in Fig. 1.
Elsevier Sequoia.
M. J. Paisley et al. / Boron nitride thin films by microwave ECR plasma CVD Symme~c
31
Applied r.f. power is often reported in the literature. However, this value is not nearly as descriptive as the induced voltage reported in Table 1, because when r.f. power is applied to the substrate, a d.c. bias voltage is induced on the substrate surface since the plasma behaves electrically like a leaky diode. This bias results in ions being accelerated to the substrate surface. The r.f. bias was measured simply by attaching a voltmeter between ground and the substrate holder with the addition of a low-pass filter to protect the voltmeter from r.f. voltages. After deposition, the samples were cooled and measured by a rotating polarizer type ellipsometer to obtain index of refraction data. The samples were further analyzed at NCSU using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fouriertransform infrared spectroscopy (FTIR) and crosssectional transmission electron microscopy (XTEM).
Fig. 1. Schematic diagram of microwave E C R plasma C V D system (in the Phase I configuration) used at ASTeX, Inc. to deposit BN films.
Substrates of Si (111) or Si (100) were placed into a load lock and evacuated before loading into the main deposition chamber. To investigate the effects of the substrate on the phase composition of the BN films, some Si (100) samples for phase II were coated on the surface for deposition with aluminum or nickel in an r.f.-plasma sputter deposition chamber before loading into the ECR system. Nickel and aluminum were chosen based on access to the targets and lattice match to BN. All samples were subsequently heated under vacuum until the deposition temperature was reached. Finally, nitrogen and argon gas flows were begun, the plasma was ignited and the borazine flow was started, which initiated deposition. In situations where substrate bias was used, the plasma was initiated and the bias conditions were established prior to deposition. The range of conditions used for deposition is shown in Table 1.
3. Results and discussion
The resulting refractive indices from both deposition trials varied among the films over the range 1.65-2.28, where n =2.12 for c-BN, n =2.02 (H c axis) and n =2.22 ( L c axis) for h-BN [1]. This indicates the presence of a mixture of phases, with porosity and/or hydrogen incorporation accounting for the lower values measured. An X-ray spectrum of a film from the first deposition trial with n = 2.01 is shown in Fig. 2. The spectrum shows only a broad band of reflections in the region of 420-50 ° (20), which covers a number of peaks from all phases of BN. The Si (111) Cu K/~ peak is exactly in the predicted location, but is unlikely to occur since the X-ray detector system has a graphite monochromator to eliminate that peak. The h-BN (002) peak is either obscured in the lower edge of the Si (111) peak, or shifted down slightly (i.e. the lattice
§i'
~i
(111)
§i IBNonSi(m) I
(222)
(333) 1000 W ~Wave
Is ~ c ~ b o ~ e
I
20 sccm ni
T A B L E 1. Deposition conditions used in E C R C V D of BN
Borazine flow Nitrogen flow A r g o n flow C h a m b e r pressure Microwave power Substrate temperature Substrate r.f.-induced voltage Growth time (BN) Deposited film thickness
Phase I
Phase II
0 . 6 - 4 . 0 sccm 0.0-20.0 sccm 0.0-50.0 sccm 0.1-0.65 Pa 500-1000 W 590-620 °C
2.5-5.0 sccm 10.0-60.0 sccm 20.0-40.0 sccm 0.1-0.32 Pa 750-1000 W R T - 5 0 0 °C
0 to - 65 V D C 3 - 3 0 min 50-2000 nm
0 to - 100 V D C 30 min 50-2000 n m
Ix-ray: CuK ~
s "~
I 20
]
(111 Cu I~
I
BN zegion
30
40
50
60
70
80
DiffractionAngle(2 O)
90
I 100
110
120°
Fig. 2. Typical X-ray spectrum from BN deposited (during Phase I) on an Si (11 l) substrate. Note BN region which cannot be resolved into individual peaks.
M. J. Paisley et al. / Boron nitride thin films by microwave ECR plasma CVD
32
~1
Si (1130)w/AI IxWave: 1000 Wlayer B3N3H6 flow:2.5 sore IN2 flow:10 sccm Ar flow:20 sccm Temp.:300°C if-bias: -15 VDC
h-BN - - (002)
Si (200) Si (200)
[ I
r%t.i (m) 'trt~, [ Si~
20
30
40
50
60
70
80
Diffraction Angle (2 O)
90
100
110
120
Fig. 3. Typical X-ray spectrum from BN deposited (during phase II) on an Si (100) substrate. Note BN region which cannot be resolved into individual peaks.
expanded), and is where the Cu Kfl peak should be located. The weak bonding between planes of h-BN makes this explanation more likely. If the latter case pertains, its relative intensity seems to indicate that a significant portion of the broad BN region is from phases other than h-BN. The fact that the region is continuous also indicates at least some amorphous component. An X-ray spectrum of a film from the second deposition trial is shown in Fig. 3. The spectrum shows only a broad band of reflections from < 2 0 ° to 40 ° (20), with the portion from < 2 0 ° to 30 ° being from the mount material. The origin o f the broad band from 30 ° to 40 ° is unclear, but does correspond to the forbidden c-BN (110) reflection. The Si (600) reflection was not observed on substrates without BN deposits. This reflection is also exactly where the c-BN (400) reflection should be located due to the relative sizes of the unit cells (Si/c-BN = 1.502). However, because (1) the K~,2 doublet is so well resolved, and (2) the c-BN (200) reflection is absent, this peak has been assigned to the Si (600) reflection. The Si (100) wafers with an Ni layer sputtered on prior to BN deposition showed no change in the X-ray spectra compared with the bare Si. This indicates that the closer lattice match of the Ni layer did not enhance the formation of c-BN, but also did not enhance the formation of any other BN phase. The samples with an AI layer sputtered on prior to BN deposition showed an increase in the h-BN component, with no other changes in the phase composition. This can be seen from the X-ray pattern shown in Fig. 4. Analysis of the representative XPS spectrum shown in Fig. 5 revealed that the surfaces of these films grown in Phase I were free from contamination other than carbon and oxygen. Most, if not all, of this contamination was due to atmospheric exposure. Since contamination levels were so low, XPS was not performed on
'~11
20
30
40
.-
1
(102) h-BN J [
I I h-BNI03) 50
/
| |
Si
60 70 80 Diffraction Angle (2 @)
90
100
110
120
Fig. 4. Typical X-ray spectrum from BN deposited (during Phase II) on an Si (100) substrate with an A1 sputtered layer. Note that many h-BN peaks are now present.
Nls XPS data, Mg anode Resolution: 2 eV [Substrate Si (111) ]
"~
am
KLL
]
Bls
~H~qI~l[~i~i~lqi~l~|~Èpi~|~i1~Hq~lI~1lIqH~aÈ~H~q~H~
1000 900
800
700
600 500 400 300 Binding Energy (eV)
200
100
0
Fig. 5. XPS spectrum from BN deposited (during Phase I) on Si ( l l l) substrate. Note that carbon and oxygen contamination peaks are probably due to atmospheric exposure.
B N on Si (III) ] Bot-alinedownstream| Nicolet SIOP FT-IR I ....
I
Scansav 'd-32 g " I
iiM-rt htretch
-I'~t"12
h-BN
2V
~2
4~,00
40110
3500
3000
2500
J h-BN I t [
20130
Wavenumber(cri~-I)
1500
I000
500
Fig. 6. FTIR spectra of B N on Si (III) (Phase I) showing h-BN and c-BN peaks and very littlehydrogen/argon incorporation (0 V bias, I0 sccm Ar, I000 W; spectrum corrected for SiO 2 peak at 615 cm-l).
the phase II samples. Calculations of nitrogen/boron rations using peak area calculations yielded a nominally stoichiometric BN composition. F T I R spectra of these films in Figs. 6 and 7 show hexagonal and/or amorphous (peaks at 1370 and 870cm -]) and cubic (peak at l l 0 0 c m -I) BN phases,
M. J. Paisley et al. / Boron nitride thin films by microwave ECR plasma CVD
33
r I
/
l•J
I
~
h-BN
h-BN t
,
.i
4500
4000
3500
3000 2500 _2~)00 Wavenumber(cm
1500
li'
:7"
Fig. 8. XTEM image of BN on Si (111) (Phase I) with z =[110]. Inset shows SAD pattern of Si with oriented polycrystallineh-BN.
I000
500
Fig. 7. FTIR spectra of BN on Si (111) (Phase I) under various processing conditions showing heavily distorted h-BN and c-BN
(almost invisible) peaks, as well as hydrogen/argonincorporation.
with very minor amounts of hydrogen incorporation under some processing conditions. The FTIR spectrum shown in Fig. 6 is fairly clean, with only very slight hydrogen/argon incorporation, as indicated by the almost undetectable peaks in the 3200-3500 cm -1 range. Calculation of the proportions of cubic and hexagonal/ amorphous phases using reported relative absorption factors [14] yields Vh/Vc= 1.27, thus the film contains 10% c-BN. The peak at 3240 cm -~ is probably a second N - H stretching mode, from a comparison with group frequency charts [ 15, 16]. This peak is apparently related to incorporation effects, as discussed below. Figure 7 shows spectra from several different depostion conditions in Phase I. A comparison of the spectra of Figs. 5 and 6 reveals variations in the intensity of the N - H stretch and 3240cm -1 peaks, as well as both peaks for h-BN. The spectra in Fig. 7 show strong peaks of various intensities in the 3200-3500cm -1 range, and an apparent broadening of the B - N stretching mode in the range from 1380 to 1550cm-L The latter relates to either a disordering effect from argon incorporation or perhaps the addition of an N - H bending mode that also occurs in that region [16]. It seems that Ar flow rates ~>10sccm, or microwave power levels > I 0 0 0 W , or bias conditions ~<-20 VDC, cause hydrogen and perhaps argon to begin to be incorporated into the growing film. FTIR spectra from Phase II showed very similar results, except for a slightly increased c-BN component at lower deposition temperatures. The c-BN component was calculated using the method discussed above to reach ,~15% at a substrate temperature estimated to be 400 °C. Samples analyzed with Raman spectroscopy from Phase II deposition trials did not reveal the presence of
c-BN, and revealed only a slight signal from the h-BN component, implying that no c-BN was present in the films. However, other researchers [17, 18] have observed evidence of c-BN in electron diffraction that showed no Raman peaks, which they attributed to small crystallite size in the sample, rather than no c-BN being present in the sample. Samples from the conditions yielding the most c-BN were prepared for cross-section TEM using standard techniques [19]. A bright-field micrograph of the crosssection region of the film is shown in Fig. 8. The measured film thickness was 0.7 ~tm, corresponding to a growth rate of 1.4 pm h -1. The inset shows the SAD pattern observed for this film and indicates that the deposited layers are oriented h-BN, the texture appearing to be h-BN (002) II si (111). While no c-BN was observed in the SAD patterns, this is not surprising given the low concentration of c-BN and the polycrystalline/amorphous nature of much of the film. In addition, many of the c-BN diffraction lines are very close to those of other phases of BN. This makes unambiguous phase identification virtually impossible unless the diffraction patterns are extremely sharp. Samples grown at high magnitude substrate bias conditions (/> - 50 VDC) delaminated from the Si substrate, presumably due to induced stress from ion bombardment. Pieces of the delaminated films were placed on nickel grids and examined by TEM. SAD patterns from a typical region of the BN layer showed very diffuse rings from a-BN. This was especially true for the highest bias conditions ( - 6 5 VDC).
4. Conclusions
Boron nitride thin films of nominal stoichiometric composition deposited on Si via microwave ECR CVD from borazine and nitrogen showed a mixed composition of amorphous, hexagonal and cubic phases of BN. Varying deposition conditions produced various amounts of c-BN phase in the films, the balance being
34
M. J. Paisley et al. / Boron nitride thin films by microwave ECR plasma CVD
either hexagonal or amorphous BN. Evidence of hydrogen/argon incorporation was observed at high deposition power and bias levels, as well as high argon flow rates. In addition, substrate biasing seemed to degrade the film quality, in contrast to the results reported by other workers [ 14, 20, 21]. Use of Si (100) substrates is preferred to Si (111) substrates for c-BN, as X R D showed that Si (111) substrates promoted h-BN formation. This is probably due to the six-fold coordination of the Si (111) surface. Substrate temperatures in the 400 °C range produced the highest amount of c-BN. Use of an Ni sputtered layer on the substrate did not appear to change the phase composition of the deposited BN. When an A1 sputtered layer was used, formation of h-BN was strongly enhanced.
Acknowledgments The authors would like to acknowledge the support for this work from the SDIO/IST, managed by O N R (Mr. Max Yoder, monitor) under contract N00014-86K-0686, the Kobe Steel Research Laboratories, USA, and from Applied Science and Technology (ASTeX), Inc. Additional thanks go to L. Smith and R. C. Glass for assistance with the XPS and X-ray analyses.
References 1 K. H. Hellwege (ed.), Landolt-B6rnstein: Numerical Data and Functional Relationships in Science and Technology, Series III, Springer Verlag, Berlin, 1987.
2 R. C. DeVries, Cubic Boron Nitride: Handbook o f Properties, Technical Information Series, General Electric Company, Corporate Research and Development, 72CRD178, 1972. 3 C. Deshpandey and R. F. Bunshah, Thin Solid Films, 163 (1988) 131. 4 L. Vel, G. Demazeau and J. Etourneau, Mater. Sci. Eng., lOB (2) (1991) 149. 5 R. R. Wills, Int. J. High Technol. Ceram., l (1985) 139. 6 A. Lipp, K. A. Schwetz and K. Hunold, J. Eur. Ceram. Soc., 5(1) (1989) 3. 7 S. Zembutsu and T. Sasaki, Appl. Phys. Lett., 48 (13) (1986) 870. 8 Z. Sitar, M. J. Paisley, B. Yan, R. F. Davis, J. Ruan and J. W. Choyke, Thin Solid Films, 200 (1991) 311. 9 W. E. Hoke, P. J. Lemonias and D. J. Weir, J. Cryst. Growth, 111 (1991) 1024. 10 Y. Osaka, M. Okamoto and Y. Utsumi, in Proc. Low Energy Ion Beam and Plasma Modification o f Materials Symp., J. M. E. Harper, K. Miyake, J. R. McNeil and S. M. Gorbatkin (eds.), Anaheim, CA, Materials Research Society, Pitsburgh, PA, 1991. 11 R. R. Rye, J. Vac. Sci. Technol., 9A (3) (1991) 1099. 12 J. Kouvetakis, V. V. Patel, C. W. Miller and D. B. Beach, J. Vac. Sci. Technol., 8A (6) (1990) 3929. 13 S. Shanfield and R. Wolfson, J. Vac. Sci. Technol., IA (2) (1983) 323. 14 H. Yokoyama, M. Okamoto and Y. Osaka, Jpn. J. Appl. Phys., 30 (2) (1991) 344. 15 K. Nakamoto, Infrared and Raman Spectra o f Inorganic and Coordination Compounds, Wiley, New York, 1986. 16 R. C. Weast (ed.), CRC Handbook o f Physics and Chemistry, CRC Press, Boca Raton, FL, 1988. 17 T. Ikeda, Y. Kawate and Y. Hirai, Kobelco Technol. Rev., 6 (1989) 1. 18 T. Ikeda, Y. Kawate and Y. Hirai, J. Vac. Sci. TechnoL, 8A (4) (1990) 3168. 19 J. C. Bravman and R. Sinclair, J. Electron Microsc. Tech., 1 (1984) 53. 20 C. Mitterer, P. Rodhammer, H. Stori and F. Jeglitsch, J. Vac. Sci. Technol., 7.4 (4) (1989) 2646. 21 K. Inagawa, K. Watanabe, K. Saitoh, Y. Yuchi and A. Itoh, New Diamond, (1988) 60.
Thin Solid Films, 235 (1993) 30-34
Boron nitride thin films by microwave ECR plasma chemical vapor deposition M. J. Paisley a, L. P. Bourget b and R. F. Davis a aDepartment of Materials Science and Engineering, Box 7919, North Carolina State University, Raleigh, NC 27695-7907 (USA) bApplied Science and Technology (ASTeX), Inc., 35 Cabot Road, Woburn, MA 01801 (USA)
(Received August 18, 1992; accepted May 14, 1993)
Abstract The objective of this study was the deposition of BN on Si (100) and (111) using microwave electron cyclotron resonance plasma chemical vapor deposition with borazine as the source material. Films were deposited and analyzed employing the techniques of ellipsometry, X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy and Fourier transform infrared spectroscopy. Results indicated that the deposited films were composed of microcrystals of c-BN and oriented hexagonal BN as well as amorphous BN.
I. Introduction The deposition of BN films has received much attention for the very wide variety of applications of all phases of BN. The literature contains a number of excellent reviews on each of the phases of BN: cubic BN [1-4], wurzitic BN [5] and hexagonal BN (h-BN)
[6]. Microwave electron cyclotron resonance (ECR) sources have been used successfully for the growth of a number of nitrides such as G a N [7], A I N - G a N [8], InN [9] and BN [10]. This study chose to utilize an ECR plasma source but also undertook a slightly different approach to BN deposition. The change in deposition strategy involved the use of a reactant gas that already contained B - N bonding. This was deemed an advantage, as the deposition would require only bond reordering, not bond formation. Borazine (B3N3H6, analogous to the carbon compound benzene), was selected since there are three direct B - N bondpairs, and the BN:H ratio is low (for borazine, BN:H is 1:2; for diborane and ammonia, BN:H is 1:6). Borazine has been used previously for deposition of BN films in chemical vapor deposition (CVD) reactors [11-13], but had not previously been tried in a microwave ECR plasma reactor.
2. Experimental procedure The depositions were conducted in two separate phases at ASTeX, Inc. The two deposition trials were largely the same, so the following description notes only the differences for the second set of depositions. A
microwave ECR plasma CVD system adapted for handling borazine was used. The changes to the system involved the installation of a special mass flow controller, calibrated for low vapor pressure gases, and a bottle of borazine liquid (50 g bottle, research grade, Callery Chemical Co., Pittsburgh, PA) into the gas handling system. Since the vapor pressure of borazine is ,-~9 kPa at room temperature, the bottle was connected directly to the mass flow controller and the vapor was drawn from the liquid at room temperature. The microwave ECR plasma CVD gas system consisted of a gas manifold and the associated mass flow controllers that fed gases directly into the plasma or downstream near the substrate. Depositions were performed with argon and nitrogen injected directly into the plasma, while the borazine was injected downstream. The 35 cm diameter chamber was evacuated by an 880 1 s -1 turbomolecular pump (base pressure: mid 10 -5 Pa) backed by a 85 m 3 h -1 mechanical pump that was vented to an exhaust system. For Phase I the substrate was r.f. induction heated and its temperature calibrated with a thermocouple placed on the substrate's surface. Phase II trials involved inverting the deposition chamber to minimize particulates. In addition, the substrate heating method was changed to infrared heating from a lamp package facing the rear surface of the wafer located outside the chamber. During both deposition trials, the substrate support was electrically isolated, making it possible to add r.f. biasing to the substrate to control ion energy and ion flux to the substrate independently. A schematic diagram of the ASTeX laboratory system during Phase I is shown in Fig. 1.
Elsevier Sequoia.
M. J. Paisley et al. / Boron nitride thin films by microwave ECR plasma CVD Symme~c
31
Applied r.f. power is often reported in the literature. However, this value is not nearly as descriptive as the induced voltage reported in Table 1, because when r.f. power is applied to the substrate, a d.c. bias voltage is induced on the substrate surface since the plasma behaves electrically like a leaky diode. This bias results in ions being accelerated to the substrate surface. The r.f. bias was measured simply by attaching a voltmeter between ground and the substrate holder with the addition of a low-pass filter to protect the voltmeter from r.f. voltages. After deposition, the samples were cooled and measured by a rotating polarizer type ellipsometer to obtain index of refraction data. The samples were further analyzed at NCSU using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fouriertransform infrared spectroscopy (FTIR) and crosssectional transmission electron microscopy (XTEM).
Fig. 1. Schematic diagram of microwave E C R plasma C V D system (in the Phase I configuration) used at ASTeX, Inc. to deposit BN films.
Substrates of Si (111) or Si (100) were placed into a load lock and evacuated before loading into the main deposition chamber. To investigate the effects of the substrate on the phase composition of the BN films, some Si (100) samples for phase II were coated on the surface for deposition with aluminum or nickel in an r.f.-plasma sputter deposition chamber before loading into the ECR system. Nickel and aluminum were chosen based on access to the targets and lattice match to BN. All samples were subsequently heated under vacuum until the deposition temperature was reached. Finally, nitrogen and argon gas flows were begun, the plasma was ignited and the borazine flow was started, which initiated deposition. In situations where substrate bias was used, the plasma was initiated and the bias conditions were established prior to deposition. The range of conditions used for deposition is shown in Table 1.
3. Results and discussion
The resulting refractive indices from both deposition trials varied among the films over the range 1.65-2.28, where n =2.12 for c-BN, n =2.02 (H c axis) and n =2.22 ( L c axis) for h-BN [1]. This indicates the presence of a mixture of phases, with porosity and/or hydrogen incorporation accounting for the lower values measured. An X-ray spectrum of a film from the first deposition trial with n = 2.01 is shown in Fig. 2. The spectrum shows only a broad band of reflections in the region of 420-50 ° (20), which covers a number of peaks from all phases of BN. The Si (111) Cu K/~ peak is exactly in the predicted location, but is unlikely to occur since the X-ray detector system has a graphite monochromator to eliminate that peak. The h-BN (002) peak is either obscured in the lower edge of the Si (111) peak, or shifted down slightly (i.e. the lattice
§i'
~i
(111)
§i IBNonSi(m) I
(222)
(333) 1000 W ~Wave
Is ~ c ~ b o ~ e
I
20 sccm ni
T A B L E 1. Deposition conditions used in E C R C V D of BN
Borazine flow Nitrogen flow A r g o n flow C h a m b e r pressure Microwave power Substrate temperature Substrate r.f.-induced voltage Growth time (BN) Deposited film thickness
Phase I
Phase II
0 . 6 - 4 . 0 sccm 0.0-20.0 sccm 0.0-50.0 sccm 0.1-0.65 Pa 500-1000 W 590-620 °C
2.5-5.0 sccm 10.0-60.0 sccm 20.0-40.0 sccm 0.1-0.32 Pa 750-1000 W R T - 5 0 0 °C
0 to - 65 V D C 3 - 3 0 min 50-2000 nm
0 to - 100 V D C 30 min 50-2000 n m
Ix-ray: CuK ~
s "~
I 20
]
(111 Cu I~
I
BN zegion
30
40
50
60
70
80
DiffractionAngle(2 O)
90
I 100
110
120°
Fig. 2. Typical X-ray spectrum from BN deposited (during Phase I) on an Si (11 l) substrate. Note BN region which cannot be resolved into individual peaks.
M. J. Paisley et al. / Boron nitride thin films by microwave ECR plasma CVD
32
~1
Si (1130)w/AI IxWave: 1000 Wlayer B3N3H6 flow:2.5 sore IN2 flow:10 sccm Ar flow:20 sccm Temp.:300°C if-bias: -15 VDC
h-BN - - (002)
Si (200) Si (200)
[ I
r%t.i (m) 'trt~, [ Si~
20
30
40
50
60
70
80
Diffraction Angle (2 O)
90
100
110
120
Fig. 3. Typical X-ray spectrum from BN deposited (during phase II) on an Si (100) substrate. Note BN region which cannot be resolved into individual peaks.
expanded), and is where the Cu Kfl peak should be located. The weak bonding between planes of h-BN makes this explanation more likely. If the latter case pertains, its relative intensity seems to indicate that a significant portion of the broad BN region is from phases other than h-BN. The fact that the region is continuous also indicates at least some amorphous component. An X-ray spectrum of a film from the second deposition trial is shown in Fig. 3. The spectrum shows only a broad band of reflections from < 2 0 ° to 40 ° (20), with the portion from < 2 0 ° to 30 ° being from the mount material. The origin o f the broad band from 30 ° to 40 ° is unclear, but does correspond to the forbidden c-BN (110) reflection. The Si (600) reflection was not observed on substrates without BN deposits. This reflection is also exactly where the c-BN (400) reflection should be located due to the relative sizes of the unit cells (Si/c-BN = 1.502). However, because (1) the K~,2 doublet is so well resolved, and (2) the c-BN (200) reflection is absent, this peak has been assigned to the Si (600) reflection. The Si (100) wafers with an Ni layer sputtered on prior to BN deposition showed no change in the X-ray spectra compared with the bare Si. This indicates that the closer lattice match of the Ni layer did not enhance the formation of c-BN, but also did not enhance the formation of any other BN phase. The samples with an AI layer sputtered on prior to BN deposition showed an increase in the h-BN component, with no other changes in the phase composition. This can be seen from the X-ray pattern shown in Fig. 4. Analysis of the representative XPS spectrum shown in Fig. 5 revealed that the surfaces of these films grown in Phase I were free from contamination other than carbon and oxygen. Most, if not all, of this contamination was due to atmospheric exposure. Since contamination levels were so low, XPS was not performed on
'~11
20
30
40
.-
1
(102) h-BN J [
I I h-BNI03) 50
/
| |
Si
60 70 80 Diffraction Angle (2 @)
90
100
110
120
Fig. 4. Typical X-ray spectrum from BN deposited (during Phase II) on an Si (100) substrate with an A1 sputtered layer. Note that many h-BN peaks are now present.
Nls XPS data, Mg anode Resolution: 2 eV [Substrate Si (111) ]
"~
am
KLL
]
Bls
~H~qI~l[~i~i~lqi~l~|~Èpi~|~i1~Hq~lI~1lIqH~aÈ~H~q~H~
1000 900
800
700
600 500 400 300 Binding Energy (eV)
200
100
0
Fig. 5. XPS spectrum from BN deposited (during Phase I) on Si ( l l l) substrate. Note that carbon and oxygen contamination peaks are probably due to atmospheric exposure.
B N on Si (III) ] Bot-alinedownstream| Nicolet SIOP FT-IR I ....
I
Scansav 'd-32 g " I
iiM-rt htretch
-I'~t"12
h-BN
2V
~2
4~,00
40110
3500
3000
2500
J h-BN I t [
20130
Wavenumber(cri~-I)
1500
I000
500
Fig. 6. FTIR spectra of B N on Si (III) (Phase I) showing h-BN and c-BN peaks and very littlehydrogen/argon incorporation (0 V bias, I0 sccm Ar, I000 W; spectrum corrected for SiO 2 peak at 615 cm-l).
the phase II samples. Calculations of nitrogen/boron rations using peak area calculations yielded a nominally stoichiometric BN composition. F T I R spectra of these films in Figs. 6 and 7 show hexagonal and/or amorphous (peaks at 1370 and 870cm -]) and cubic (peak at l l 0 0 c m -I) BN phases,
M. J. Paisley et al. / Boron nitride thin films by microwave ECR plasma CVD
33
r I
/
l•J
I
~
h-BN
h-BN t
,
.i
4500
4000
3500
3000 2500 _2~)00 Wavenumber(cm
1500
li'
:7"
Fig. 8. XTEM image of BN on Si (111) (Phase I) with z =[110]. Inset shows SAD pattern of Si with oriented polycrystallineh-BN.
I000
500
Fig. 7. FTIR spectra of BN on Si (111) (Phase I) under various processing conditions showing heavily distorted h-BN and c-BN
(almost invisible) peaks, as well as hydrogen/argonincorporation.
with very minor amounts of hydrogen incorporation under some processing conditions. The FTIR spectrum shown in Fig. 6 is fairly clean, with only very slight hydrogen/argon incorporation, as indicated by the almost undetectable peaks in the 3200-3500 cm -1 range. Calculation of the proportions of cubic and hexagonal/ amorphous phases using reported relative absorption factors [14] yields Vh/Vc= 1.27, thus the film contains 10% c-BN. The peak at 3240 cm -~ is probably a second N - H stretching mode, from a comparison with group frequency charts [ 15, 16]. This peak is apparently related to incorporation effects, as discussed below. Figure 7 shows spectra from several different depostion conditions in Phase I. A comparison of the spectra of Figs. 5 and 6 reveals variations in the intensity of the N - H stretch and 3240cm -1 peaks, as well as both peaks for h-BN. The spectra in Fig. 7 show strong peaks of various intensities in the 3200-3500cm -1 range, and an apparent broadening of the B - N stretching mode in the range from 1380 to 1550cm-L The latter relates to either a disordering effect from argon incorporation or perhaps the addition of an N - H bending mode that also occurs in that region [16]. It seems that Ar flow rates ~>10sccm, or microwave power levels > I 0 0 0 W , or bias conditions ~<-20 VDC, cause hydrogen and perhaps argon to begin to be incorporated into the growing film. FTIR spectra from Phase II showed very similar results, except for a slightly increased c-BN component at lower deposition temperatures. The c-BN component was calculated using the method discussed above to reach ,~15% at a substrate temperature estimated to be 400 °C. Samples analyzed with Raman spectroscopy from Phase II deposition trials did not reveal the presence of
c-BN, and revealed only a slight signal from the h-BN component, implying that no c-BN was present in the films. However, other researchers [17, 18] have observed evidence of c-BN in electron diffraction that showed no Raman peaks, which they attributed to small crystallite size in the sample, rather than no c-BN being present in the sample. Samples from the conditions yielding the most c-BN were prepared for cross-section TEM using standard techniques [19]. A bright-field micrograph of the crosssection region of the film is shown in Fig. 8. The measured film thickness was 0.7 ~tm, corresponding to a growth rate of 1.4 pm h -1. The inset shows the SAD pattern observed for this film and indicates that the deposited layers are oriented h-BN, the texture appearing to be h-BN (002) II si (111). While no c-BN was observed in the SAD patterns, this is not surprising given the low concentration of c-BN and the polycrystalline/amorphous nature of much of the film. In addition, many of the c-BN diffraction lines are very close to those of other phases of BN. This makes unambiguous phase identification virtually impossible unless the diffraction patterns are extremely sharp. Samples grown at high magnitude substrate bias conditions (/> - 50 VDC) delaminated from the Si substrate, presumably due to induced stress from ion bombardment. Pieces of the delaminated films were placed on nickel grids and examined by TEM. SAD patterns from a typical region of the BN layer showed very diffuse rings from a-BN. This was especially true for the highest bias conditions ( - 6 5 VDC).
4. Conclusions
Boron nitride thin films of nominal stoichiometric composition deposited on Si via microwave ECR CVD from borazine and nitrogen showed a mixed composition of amorphous, hexagonal and cubic phases of BN. Varying deposition conditions produced various amounts of c-BN phase in the films, the balance being
34
M. J. Paisley et al. / Boron nitride thin films by microwave ECR plasma CVD
either hexagonal or amorphous BN. Evidence of hydrogen/argon incorporation was observed at high deposition power and bias levels, as well as high argon flow rates. In addition, substrate biasing seemed to degrade the film quality, in contrast to the results reported by other workers [ 14, 20, 21]. Use of Si (100) substrates is preferred to Si (111) substrates for c-BN, as X R D showed that Si (111) substrates promoted h-BN formation. This is probably due to the six-fold coordination of the Si (111) surface. Substrate temperatures in the 400 °C range produced the highest amount of c-BN. Use of an Ni sputtered layer on the substrate did not appear to change the phase composition of the deposited BN. When an A1 sputtered layer was used, formation of h-BN was strongly enhanced.
Acknowledgments The authors would like to acknowledge the support for this work from the SDIO/IST, managed by O N R (Mr. Max Yoder, monitor) under contract N00014-86K-0686, the Kobe Steel Research Laboratories, USA, and from Applied Science and Technology (ASTeX), Inc. Additional thanks go to L. Smith and R. C. Glass for assistance with the XPS and X-ray analyses.
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