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The deposition of aluminum nitride on silicon by plasma-enhanced metal-organic chemical vapour deposition
ELSEVIER
DIAMOND AND RELATED MATERIALS
Diamond and Related Materials 5 (1996) 1210-1213
The deposition of aluminum nitride on silicon by plasma-enhanced metal-organic chemical vapour deposition Th. Stauden, G. Eichhorn, V. Cimalla, J. Pezoldt, G. Ecke Technische Universitat Ilmenau, Institut ffir FestkiJrperelektronik, PF 0565, D-98684 Ilmenau, Germany Received 8 November 1995; accepted 12 December 1995
Abstract Aluminium nitride (A1N) films were deposited by electron cycIotron resonance plasma-enhanced chemical vapour deposition using trimethylaluminum (TMA) as precurser at temperatures from 25 to 600 °C and at different concentrations of TMA. Adhesive layers with a thickness ranging from 100 to 200 nm and a refractive index of 1.65-2.0 are obtained. By Auger electron spectroscopy, reflectum high energy electron diffraction and eliipsometric measurements at 632 nm we found in all layers a high oxygen concentrations only slightly dependent on the deposition conditions. A quadrupole mass spectrometer was used to investigate the composition of the gas phase. The gas spectra did not indicate partial pressures of oxygen and water. Sputtering effects and plasma chemical etching of the microwave window made of quartz glass seem to be the source for the incorporated oxygen.
Keywords: Aluminium nitride; PE-CVD; Structural characterisation; AES
1. Introduction Aluminium nitride (A1N) has some outstanding physical properties, making this material interesting for a large variety of applications. A1N has a direct band gap of 6.2 eV, a room-temperature thermal conductivity of 3.2 W cm- 1 K - 1 and a velocity of surface acoustic waves of 6000 m s -1. With a microhardness of 15 GPa and non-toxic components it is suitable as hard coating for many applications. Some deposition techniques for the preparation of A1N are sputtering [-1], chemical vapour deposition (CVD) [2] and molecular beam epitaxy (MBE) [-3]. Plasma-enhanced methods become increasingly important for the low temperature deposition of A1N including the application of electron cyclotron resonance (ECR) plasma sources. The plasma-enhanced deposition of A1N on the basis of metal-organic materials has been reported less and it is the subject of our investigations.
Fig. 1. The final pressure in the Viton-sealed high vacuum chamber pumped by a 16001 s -I turbo pump is 7 x 10 -s mbar. An ECR plasma source (Roth & Rau Oberfl~tchentechnik) with a maximum microwave power of 800 W is fixed at the upper plate of the stainless steel chamber. The microwave power is coupled in via a
[]
[]
ECR plas(nasource ~ R antenna
IXII IXl nuartzw ,do,v
gas ring
2. Experimental details A1N film depositions were performed in a PLS 500 (Balzers Hochvakuum GmbH) plasma deposition apparatus. The layout of the process chamber together with the equipment for gas supply and analysis are shown in Elsevier Science S.A. PH S0925-9635 (96] 00512-2
substrai holder hedter Fig. 1. Schematic diagram of the ECR plasma metal-organic CVD system.
1211
Th. Stauden et al./Diamond and Related Materials 5 (1996) 1210-1213
quartz window into the ionization chamber of the source. Argon and nitrogen are introduced directly into the plasma source by a gas distribution ring near the quartz window, while the metal-organic compound trimethylaluminium (TMA) is separately introduced via a gas ring with a diameter of 200 mm located 80 mm above the substrate holder. TMA is stored in a bubbler with controlled temperature bath and transported by hydrogen carrier gas, The 4 in sample holder can be heated to 600 °C; d.c. or r.f. bias can be applied. A1N thin films were deposited on undoped Si(111) wafers cleaned with isopropanol. The microwave power was kept at 400 W for all processes. The substrate temperature was varied from 25 to 600 °C; the substrate was in 'self-bias mode', resulting in a bias of - 12 V. The depositions were carried out in Nz or N2-Ar plasma in a pressure range from 4 x 10 - 4 to 5 × 10 -3 mbar and deposition times of 10-30 min. By varying the bubbler temperature from 25 to 50 °C, the hydrogen carrier gas flow from 5 to 15 standard cm 3 rain -~ and the bubbler pressure from 1300 to 2500 mbar the TMA flow ranged from 0.05 to 0.8 standard cm 3 min -~, resulting in TMAto-N2 ratios of between 0.005 and 0.05.
3. Results
3.1. Ellipsometry By e11ipsometry measurements at 632 nm on layers with a good thickness homogeneity as shown in Fig. 2 we found an increasing refractive index for higher deposition temperatures. Above substrate temperatures of 400 °C, refractive indexes reached values greater than 2.0. The results shown in Fig. 2 agree with the results of Gordon and Riaz [-2]. Depositions made at a low substrate temperatures of 25 °C have already resulted in adhesive layers. The maximum deposition rate at a TMA flow of 0.8 standard cm 3 min -~ was 12.5 nm rain -~. Refractive indexes between 1.65 and 1.76 indicate incorporation of oxygen and hydrogen and non-
stoichiometric composition Of the A1N. Experiments were carried out to investigate the chemical stability of the low temperature deposition layers Fig. 3 shows that the refractive index changed drastically by exposing to the air a sample deposited at room temperature and transported to the ellipsometer under an N2 atmosphere. We estimate that the variation in refractive index is caused by the growth of a thin A1Ox layer on reaction with the atmospheric oxygen and moisture.
3.2. Auger electron spectroscopy The layers were investigated by Auger electron spectroscopy (AES). Fig. 4 shows a depth profile representative for A1N layers deposited at higher temperatures (d = 50 nm; TsuB= 300 °C). The ratio of A1 to N is nearly constant in the whole layer. The carbon content is close to the detection limit; this suggests that the CH3 groups of TMA are cracked and react with the hydrogen carrier gas to give methane, leading to no incorporation of carbon. Owing to the simple procedure of cleaning the sample surface, a thin region with higher concentrations of carbon and oxygen at the Si-A1N interface is detected. The oxygen peak falls near the interface to the detection limit and tends to increase steadily up to the surface. At the sample surface the analysis is strongly influenced by absorption layers. The comparison of the AES data of samples transported under nitrogen or air to the spectrometer showed no significant differences in oxygen content. This leads to the conclusion that the oxygen incorporation did not occur on the air-exposed surface. Also mass spectrometry analysis of the residual gas and the process gases gave no explanation for the fact that the high oxygen contents in our layers are nearly independent of the process parameters. As a result of our investigations it was suggested that sputtering of the microwave window (which was made of fused quartz) and plasma chemical etching during the presence of high partial pressures of hydrogen are the source of the oxygen contamination. The increasing
deposition
2,1 0 )4
•
II
2
P=400W,
¢-
•
temperature p=0,002
25 °C
mbar
At: 1 5 scorn, N~: 5 s c c m
0
H2:15
seem, T M A : 0 , 1 7 s c c m
>1,9 0 t~
o this paper • Gordon and Riaz[2]
& ~¢1,8 1,7
$
i
. . . . .
T ,
i
. . . . .
r
,
200 400 Deposition temperature °C
,
,
i
i
0
t
i
0
.
.
.
.
.
r
i
600
Fig. 2. Refractive index n vs. deposition temperature compared with the results of Gordon and Riaz [2].
500
1000
1500
2000
Time / s
Fig. 3. Change in the refractive index after the sample had been exposed to the air.
1212
Th. Stauden et aI./Diamond and Related Materials 5 (1996) 1210-1213
-
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AI60
--Q--C612
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~
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9
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30
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i
~
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60
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,
70
;
I
'
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93
s p u t t e r t i m e (rnin)
Fig. 4. AES depth profileof an AIN layer on Si(lll) (TDEp=300 °C; d,[N= 50 nm; PZCR=400W; TzMA= 50 °C; N2 flOWrate, 10 standard cma min-~; TMA flow rate, 0.8 standard cm~rain-i). temperature during the application of the plasma source leads in addition to desorption and increase in the interaction. Both effects can cause an increasing oxygen concentration in the plasma and the observed increase in oxygen peak height in the Auger spectra. We also observed high oxygen and silicon concentrations in Auger spectra of CNx layers prepared in the same device. Similar effects have been reported for the preparation of GaN in an MBE system where an ECR plasma source is used to generate atomic nitrogen. Already a low microwave power has led to oxygen, carbon and silicon concentrations in the 102°cm -3 range [41. Detailed investigation of oxygen contaminations in SiNx layers by Garcia et al. [5] demonstrated that sputtering of the quartz liner was the main source for the oxygen incorporation. 1000000
:ff
ff
3.3. G a s a n a l y s i s
Gas analysis measurements with a quadrupole mass spectrometer with an axial beam ion source and Faraday cup were carried out to improve the residual gas and the purity of all process gases. No partial pressures of oxygen or water were found which could be the source of the oxygen in the A1N layers indicated by the AES measurements. Fig. 5 shows a gas spectrum in the chamber without the presence of a plasma on introducing nitrogen at 4 standard cm 3 min- 1 and TMA at a bubbler temperature of 35°C in a hydrogen flow of 4 standardcm 3 min-1; the total pressure was 2 x 10 .4 mbar. Beside the peaks of nitrogen (28 amu) and hydrogen (2 amu) the pattern of TMA is clearly seen. Peaks at 57, 42 and 27 amu can be attributed to
"ff
,.~
O
"r <
100000
+¢,)
.4
'1-
<
10000 I/) c
i 000
10o lo
nI i ~, , ]lLIq~lIbl!q~,iF,lt
i ] [ .... ] . . . . . , , p, I ~ t [~PllP~il] I~ll~billlll,llq
I i [ , i i I I . I ~ L'IIIIP~]II
::[I::II:Iii,, {13 r~
m/e Fig. 5. Mass spectra in the absence of an ECR pIasma(p = 2 x 10-4 mbar; N2 flowrate, 4 standard cm3 min- 1; I-I2flowrate, 4 standard cm$ min-i; TTMA= 35 °C) (a.u., arbitrary units).
Th. Stauclen et aL/Diamond and Related Materials 5 (1996) 1210-1213
1213
The A1N layers were microcrystalline for deposition temperatures above 400 °C. The reflection high energy electron diffraction pattern showed a texture with the c axis nearly perpendicular to the substrate surface.
400 °C. AES depth profiling showed a high oxygen content, increasing as the surface was approached. It is suggested that the microwave window whict~ was made of fused quartz is the source of oxygen contaminations formed by plasma chemical etching under the influence of hydrogen partial pressures in the ECR source. To achieve lower oxygen incorporation A1N or BN should be substituted for the quartz or the quartz should be coated with a material containing no oxygen.
Conclusions
References
the decomposition of TMA. The methyl groups and the resulting cracking products are detected at 12-16 ainu.
3.4. Reflection high energy electron diffraction
A1N films were deposited on Si(111) wafers by plasmaenhanced metal-organic CVD using an ECR plasma source at a power of 400 W. The deposition temperature was varied from 20 to 600 °C. TMA transported by hydrogen carrier gas was used as the precursor and nitrogen as working gas in the plasma source. By ellipsometriy measurements we found refractive indexes greater than 2.0 for deposition temperatures above
1"1] H.-C. Lee, J.-Y. Lee and H.-J. Ahn, Thin Solid Films, 25I (1994) 136-140. J-2] G. Gordon and U. Riaz, J. Mater. Res., 7 (1992) i679-1684. I3] R.F. Davis, M.J. Paisley, Z. Sitar, D.J. Kester, K.S. Ailey and C. Wang, Microelectron. J., 25 (1994) 661-674. 1.4] R.-J. Molnar and T.D. Moustakas, J. Appl. Phys., 76 (1994) 4586-4595. 1.5] S. Garcia, J.M. Martin, M. Fernandez, I. Martil and G. GonzalesDiaz, J. Vac. Sci. Teehnol. A, 13 (1995) 826-830.
DIAMOND AND RELATED MATERIALS
Diamond and Related Materials 5 (1996) 1210-1213
The deposition of aluminum nitride on silicon by plasma-enhanced metal-organic chemical vapour deposition Th. Stauden, G. Eichhorn, V. Cimalla, J. Pezoldt, G. Ecke Technische Universitat Ilmenau, Institut ffir FestkiJrperelektronik, PF 0565, D-98684 Ilmenau, Germany Received 8 November 1995; accepted 12 December 1995
Abstract Aluminium nitride (A1N) films were deposited by electron cycIotron resonance plasma-enhanced chemical vapour deposition using trimethylaluminum (TMA) as precurser at temperatures from 25 to 600 °C and at different concentrations of TMA. Adhesive layers with a thickness ranging from 100 to 200 nm and a refractive index of 1.65-2.0 are obtained. By Auger electron spectroscopy, reflectum high energy electron diffraction and eliipsometric measurements at 632 nm we found in all layers a high oxygen concentrations only slightly dependent on the deposition conditions. A quadrupole mass spectrometer was used to investigate the composition of the gas phase. The gas spectra did not indicate partial pressures of oxygen and water. Sputtering effects and plasma chemical etching of the microwave window made of quartz glass seem to be the source for the incorporated oxygen.
Keywords: Aluminium nitride; PE-CVD; Structural characterisation; AES
1. Introduction Aluminium nitride (A1N) has some outstanding physical properties, making this material interesting for a large variety of applications. A1N has a direct band gap of 6.2 eV, a room-temperature thermal conductivity of 3.2 W cm- 1 K - 1 and a velocity of surface acoustic waves of 6000 m s -1. With a microhardness of 15 GPa and non-toxic components it is suitable as hard coating for many applications. Some deposition techniques for the preparation of A1N are sputtering [-1], chemical vapour deposition (CVD) [2] and molecular beam epitaxy (MBE) [-3]. Plasma-enhanced methods become increasingly important for the low temperature deposition of A1N including the application of electron cyclotron resonance (ECR) plasma sources. The plasma-enhanced deposition of A1N on the basis of metal-organic materials has been reported less and it is the subject of our investigations.
Fig. 1. The final pressure in the Viton-sealed high vacuum chamber pumped by a 16001 s -I turbo pump is 7 x 10 -s mbar. An ECR plasma source (Roth & Rau Oberfl~tchentechnik) with a maximum microwave power of 800 W is fixed at the upper plate of the stainless steel chamber. The microwave power is coupled in via a
[]
[]
ECR plas(nasource ~ R antenna
IXII IXl nuartzw ,do,v
gas ring
2. Experimental details A1N film depositions were performed in a PLS 500 (Balzers Hochvakuum GmbH) plasma deposition apparatus. The layout of the process chamber together with the equipment for gas supply and analysis are shown in Elsevier Science S.A. PH S0925-9635 (96] 00512-2
substrai holder hedter Fig. 1. Schematic diagram of the ECR plasma metal-organic CVD system.
1211
Th. Stauden et al./Diamond and Related Materials 5 (1996) 1210-1213
quartz window into the ionization chamber of the source. Argon and nitrogen are introduced directly into the plasma source by a gas distribution ring near the quartz window, while the metal-organic compound trimethylaluminium (TMA) is separately introduced via a gas ring with a diameter of 200 mm located 80 mm above the substrate holder. TMA is stored in a bubbler with controlled temperature bath and transported by hydrogen carrier gas, The 4 in sample holder can be heated to 600 °C; d.c. or r.f. bias can be applied. A1N thin films were deposited on undoped Si(111) wafers cleaned with isopropanol. The microwave power was kept at 400 W for all processes. The substrate temperature was varied from 25 to 600 °C; the substrate was in 'self-bias mode', resulting in a bias of - 12 V. The depositions were carried out in Nz or N2-Ar plasma in a pressure range from 4 x 10 - 4 to 5 × 10 -3 mbar and deposition times of 10-30 min. By varying the bubbler temperature from 25 to 50 °C, the hydrogen carrier gas flow from 5 to 15 standard cm 3 rain -~ and the bubbler pressure from 1300 to 2500 mbar the TMA flow ranged from 0.05 to 0.8 standard cm 3 min -~, resulting in TMAto-N2 ratios of between 0.005 and 0.05.
3. Results
3.1. Ellipsometry By e11ipsometry measurements at 632 nm on layers with a good thickness homogeneity as shown in Fig. 2 we found an increasing refractive index for higher deposition temperatures. Above substrate temperatures of 400 °C, refractive indexes reached values greater than 2.0. The results shown in Fig. 2 agree with the results of Gordon and Riaz [-2]. Depositions made at a low substrate temperatures of 25 °C have already resulted in adhesive layers. The maximum deposition rate at a TMA flow of 0.8 standard cm 3 min -~ was 12.5 nm rain -~. Refractive indexes between 1.65 and 1.76 indicate incorporation of oxygen and hydrogen and non-
stoichiometric composition Of the A1N. Experiments were carried out to investigate the chemical stability of the low temperature deposition layers Fig. 3 shows that the refractive index changed drastically by exposing to the air a sample deposited at room temperature and transported to the ellipsometer under an N2 atmosphere. We estimate that the variation in refractive index is caused by the growth of a thin A1Ox layer on reaction with the atmospheric oxygen and moisture.
3.2. Auger electron spectroscopy The layers were investigated by Auger electron spectroscopy (AES). Fig. 4 shows a depth profile representative for A1N layers deposited at higher temperatures (d = 50 nm; TsuB= 300 °C). The ratio of A1 to N is nearly constant in the whole layer. The carbon content is close to the detection limit; this suggests that the CH3 groups of TMA are cracked and react with the hydrogen carrier gas to give methane, leading to no incorporation of carbon. Owing to the simple procedure of cleaning the sample surface, a thin region with higher concentrations of carbon and oxygen at the Si-A1N interface is detected. The oxygen peak falls near the interface to the detection limit and tends to increase steadily up to the surface. At the sample surface the analysis is strongly influenced by absorption layers. The comparison of the AES data of samples transported under nitrogen or air to the spectrometer showed no significant differences in oxygen content. This leads to the conclusion that the oxygen incorporation did not occur on the air-exposed surface. Also mass spectrometry analysis of the residual gas and the process gases gave no explanation for the fact that the high oxygen contents in our layers are nearly independent of the process parameters. As a result of our investigations it was suggested that sputtering of the microwave window (which was made of fused quartz) and plasma chemical etching during the presence of high partial pressures of hydrogen are the source of the oxygen contamination. The increasing
deposition
2,1 0 )4
•
II
2
P=400W,
¢-
•
temperature p=0,002
25 °C
mbar
At: 1 5 scorn, N~: 5 s c c m
0
H2:15
seem, T M A : 0 , 1 7 s c c m
>1,9 0 t~
o this paper • Gordon and Riaz[2]
& ~¢1,8 1,7
$
i
. . . . .
T ,
i
. . . . .
r
,
200 400 Deposition temperature °C
,
,
i
i
0
t
i
0
.
.
.
.
.
r
i
600
Fig. 2. Refractive index n vs. deposition temperature compared with the results of Gordon and Riaz [2].
500
1000
1500
2000
Time / s
Fig. 3. Change in the refractive index after the sample had been exposed to the air.
1212
Th. Stauden et aI./Diamond and Related Materials 5 (1996) 1210-1213
-
--*--
AI60
--Q--C612
1,2. . . . . . . -
-,-~
N38~ ...................................................................................... ;~.......:...... , : _./'--...._ /:
--,.--
AI1394
i /
~
I - ° - si94"o~ I
'-
; ~
, ~"~i
/
\
/
e--
~
0,8. . . . . . . . . . . . . . . . . . . . . . . . . . .
-~ (g 0,6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ¢1.
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0,0-
~
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,
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,
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9
,
30
?
40
i
~
?
50
60
~
,
70
;
I
'
8o
93
s p u t t e r t i m e (rnin)
Fig. 4. AES depth profileof an AIN layer on Si(lll) (TDEp=300 °C; d,[N= 50 nm; PZCR=400W; TzMA= 50 °C; N2 flOWrate, 10 standard cma min-~; TMA flow rate, 0.8 standard cm~rain-i). temperature during the application of the plasma source leads in addition to desorption and increase in the interaction. Both effects can cause an increasing oxygen concentration in the plasma and the observed increase in oxygen peak height in the Auger spectra. We also observed high oxygen and silicon concentrations in Auger spectra of CNx layers prepared in the same device. Similar effects have been reported for the preparation of GaN in an MBE system where an ECR plasma source is used to generate atomic nitrogen. Already a low microwave power has led to oxygen, carbon and silicon concentrations in the 102°cm -3 range [41. Detailed investigation of oxygen contaminations in SiNx layers by Garcia et al. [5] demonstrated that sputtering of the quartz liner was the main source for the oxygen incorporation. 1000000
:ff
ff
3.3. G a s a n a l y s i s
Gas analysis measurements with a quadrupole mass spectrometer with an axial beam ion source and Faraday cup were carried out to improve the residual gas and the purity of all process gases. No partial pressures of oxygen or water were found which could be the source of the oxygen in the A1N layers indicated by the AES measurements. Fig. 5 shows a gas spectrum in the chamber without the presence of a plasma on introducing nitrogen at 4 standard cm 3 min- 1 and TMA at a bubbler temperature of 35°C in a hydrogen flow of 4 standardcm 3 min-1; the total pressure was 2 x 10 .4 mbar. Beside the peaks of nitrogen (28 amu) and hydrogen (2 amu) the pattern of TMA is clearly seen. Peaks at 57, 42 and 27 amu can be attributed to
"ff
,.~
O
"r <
100000
+¢,)
.4
'1-
<
10000 I/) c
i 000
10o lo
nI i ~, , ]lLIq~lIbl!q~,iF,lt
i ] [ .... ] . . . . . , , p, I ~ t [~PllP~il] I~ll~billlll,llq
I i [ , i i I I . I ~ L'IIIIP~]II
::[I::II:Iii,, {13 r~
m/e Fig. 5. Mass spectra in the absence of an ECR pIasma(p = 2 x 10-4 mbar; N2 flowrate, 4 standard cm3 min- 1; I-I2flowrate, 4 standard cm$ min-i; TTMA= 35 °C) (a.u., arbitrary units).
Th. Stauclen et aL/Diamond and Related Materials 5 (1996) 1210-1213
1213
The A1N layers were microcrystalline for deposition temperatures above 400 °C. The reflection high energy electron diffraction pattern showed a texture with the c axis nearly perpendicular to the substrate surface.
400 °C. AES depth profiling showed a high oxygen content, increasing as the surface was approached. It is suggested that the microwave window whict~ was made of fused quartz is the source of oxygen contaminations formed by plasma chemical etching under the influence of hydrogen partial pressures in the ECR source. To achieve lower oxygen incorporation A1N or BN should be substituted for the quartz or the quartz should be coated with a material containing no oxygen.
Conclusions
References
the decomposition of TMA. The methyl groups and the resulting cracking products are detected at 12-16 ainu.
3.4. Reflection high energy electron diffraction
A1N films were deposited on Si(111) wafers by plasmaenhanced metal-organic CVD using an ECR plasma source at a power of 400 W. The deposition temperature was varied from 20 to 600 °C. TMA transported by hydrogen carrier gas was used as the precursor and nitrogen as working gas in the plasma source. By ellipsometriy measurements we found refractive indexes greater than 2.0 for deposition temperatures above
1"1] H.-C. Lee, J.-Y. Lee and H.-J. Ahn, Thin Solid Films, 25I (1994) 136-140. J-2] G. Gordon and U. Riaz, J. Mater. Res., 7 (1992) i679-1684. I3] R.F. Davis, M.J. Paisley, Z. Sitar, D.J. Kester, K.S. Ailey and C. Wang, Microelectron. J., 25 (1994) 661-674. 1.4] R.-J. Molnar and T.D. Moustakas, J. Appl. Phys., 76 (1994) 4586-4595. 1.5] S. Garcia, J.M. Martin, M. Fernandez, I. Martil and G. GonzalesDiaz, J. Vac. Sci. Teehnol. A, 13 (1995) 826-830.