- Email: [email protected]
SIMS study on the initial oxidation process of AlN ceramic substrate in the air
Applied Surface Science 148 Ž1999. 73–78
SIMS study on the initial oxidation process of AlN ceramic substrate in the air Ruifeng Yue a
c
a,)
, Yan Wang b, Youxiang Wang c , Chunhua Chen
c
NoÕel DeÕices Research DiÕision, Institute of Microelectronics, Tsinghua UniÕersity, Beijing 100084, China b Department of Electronic Engineering, Xi’an UniÕersity of Technology, Xi’an 710048, China State Key Laboratory of Surface Physics, Chinese Academy of Sciences, PO BOX 912, Beijing 100084, China Received 21 October 1998; accepted 15 February 1999
Abstract Secondary ion mass spectrometry ŽSIMS. and X-ray diffraction ŽXRD. measurement were employed to study the initial oxidation process of AlN ceramic substrate in the air at 850–11008C. The results show that there is already a very thin O-rich layer in the surface region of untreated AlN ceramic substrate. When the sample is annealed for 10 min, the O-rich layer becomes thicker rapidly with the increasing of annealing temperature. When it is annealed at 11008C for 20 min, a continuous oxide layer is formed. In the end, combined with chemical thermodynamics, the initial oxidation mechanism near the surface of AlN substrate is discussed. q 1999 Elsevier Science B.V. All rights reserved. PACS: 81.65.Mq; 81.05.Je Keywords: Secondary ion mass spectrometry; AlN ceramics; Oxidation
1. Introduction AlN ceramics has recently been receiving recognition as a promising material in microelectronic packaging. It has such advantages over other materials in packaging as high thermal conductivity, moderate dielectric constant and thermal expansion, matching that of silicon w1x. In its applied field and during its preparation, the common concern has been what effect can oxygen bring on AlN ceramics. Oxygen is the main impurity dissolved in the AlN lattice; the
)
Corresponding author. E-mail: [email protected]
resulting Al vacancy, which can scatter the phonon, is the main reason of lowering thermal conductivity of AlN ceramics w2,3x. How to reduce or eliminate oxygen completely has been the key factor to produce AlN ceramics with high thermal conductivity. On the other hand, AlN becomes unstable and easily oxidized in oxygen-containing atmosphere; the formed oxide film not only can impede the inner part to be oxidized further, thus improving the anti-corrosiveness, but also can increase obviously the adhesion strength between AlN and Cu w4,5x; therefore, the study on AlN oxidation has a practical meaning. There are two kinds of AlN materials used in the research of oxidation: powders and ceramic substrates. The former are the raw materials to prepare
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 1 2 8 - 2
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R. Yue et al.r Applied Surface Science 148 (1999) 73–78
the latter, and have very large ratio surface, so they are easier to be oxidized. AlN powders are still stable even after 60 h of thermal oxidation in the air at 7008C w6x. Usually, it is accepted that the initial surface oxidation temperature is 700–8008C for AlN and the final product is a-Al 2 O 3 w7,8x. Thermogravimetric analysis ŽTGA. is the most popular method to study the oxidation of AlN, especially AlN powders. The weight difference D M before and after oxidation is used to evaluate the oxide weight and the oxide layer thickness; it is also the characteristics of oxidability. There is no general recognition about the thermal oxidation process w3,6–11x and the main analytical method is bulk analytical method such as TGA. The detection sensitivity of TGA is comparatively low; therefore, in order to achieve the mass variation, the AlN substrate is usually oxidized in the air for several or several 10 h between 900–14008C, then the calculated thickness of oxide layer ranges from a few to several hundred micrometer. There are few systematic reports on the thermal oxidation of AlN ceramic substrate using surface analytical technique until now. In this paper, SIMS technique, together with XRD, is employed to research the initial oxidation process of AlN ceramic substrate.
2. Experimental AlN ceramic substrate was sintered using highpurity raw AlN powders with additives of 3.5% Dy2 O 3 and CaO at 18508C for 4 h. A substrate with diameter of 17 mm was cut into 7 pieces after it was finely polished with standard ceramographic techniques to a final polish of approximately 0.1 mm, then the samples were ultrasonically cleaned with acetone, ethanol and DI water. The thermal oxidation experiment in the air was conducted in quartz tube with open ends. The samples were pushed into the tube after the desired temperatures were stable, then taken out and quickly cooled to room temperature when the oxidation was finished. The thermal oxidation temperaturertime were 8508Cr10 min, 9508Cr10 min, 10508Cr10 min, 11008Cr10 min, 11008Cr20 min, 11008Cr40 min, respectively. One untreated sample, together with a piece of polished a-Al 2 O 3 Ž1102. Žsapphire. was used as reference
materials. SIMS analysis was carried out on Riber MIQ-156 Ion Microprobe, Csq was used as the primary ions with beam current of 0.06 mA, ion beam energy was 10 keV, and incident angle was 458. The beam was rastered over a 0.48 = 0.29 mm2 area, the secondary ions were detected from center 5% area of the scanning region, the background vacuum was 3.7 = 10y7 Pa. ACE576N electron gun was used to reduce the charging effect of AlN substrate under filament current regulation mode, and electron beam energy was 2 keV. XRD was performed on a Drmax-RB ŽRigaku. diffractometer; CuK a radiation was adopted with a scanning speed of 48rmin.
3. Results During SIMS analysis, we did some exploring work on which kind of secondary ion should be detected and how to adjust the operation parameters of the equipment. We have the following considerations: Ž1. to ensure the comparativity under the same experimental condition, the operation parameters are carefully chosen so that the intensity of secondary ions detected in each sample is moderate to allow the equipment to have enough dynamic range; Ž2. mass interference must be reduced, but the mass number of the detecting ions should be close in order to improve the mass resolution and to adjust the equipment parameters more easily; Ž3. to choose the type of cluster secondary ions properly may be beneficial to achieving structure information. There are two kinds of SIMS operating modes: positive and negative; the negative mode was chosen after comparison in our experiment. Fig. 1 is the negative SIMS mass spectra of surface of several samples. Intensity of secondary ion AlNy is quite high in the surface of untreated sample Žcurve 1., and decreases to noise background after being oxidized in the air at 11008C for 40 min Žcurve 2.. Compared with the sapphire surface spectrum Žcurve 3., AlOy in curve 2 is still noise 2 background. In SIMS depth profiling, Aly, Oy 2, AlNy and AlOy were chosen as detecting secondary ions according to the considerations above. Fig. 2 is the SIMS depth profiles of AlN samples. y There is a very thin Oy 2 -rich and AlO -rich layers in
R. Yue et al.r Applied Surface Science 148 (1999) 73–78
75
Fig. 1. Negative SIMS mass spectra of surface of several samples. 1: AlN, untreated; 2: AlN, oxidized at 11008C for 40 min; 3: Sapphire.
the surface of untreated sample, while the inner part has a uniformly distributed composition as shown in Fig. 2a. After the sample is oxidized at 8508C for 10 y min ŽFig. 2b., the intensity of Oy 2 , AlO signals near the surface increases obviously, while the corresponding AlNy signal decreases a lot. The Oy 2 and AlOy signals increase to a great extent after the sample is oxidized at 9508C for 10 min, Oy 2 -rich and AlOy-rich layer extends distinctly. If we keep the oxidation time, with the increasing of temperature, the main changes are as following: the thickness of y y Oy declining 2 -rich, AlO -rich layer, as well as AlN region increases obviously, and the transition layer y between Oy 2 -rich, AlO -rich layer and the inner part of AlN substrate becomes wider. When keeping the oxidation temperature at 11008C, with the increasing of oxidation time, Aly, AlOy and Oy 2 signals drop y while the thickness of Oy 2 -rich, AlO -rich and nonAlNy layer increases gradually. In Fig. 2g, the thickness of non-AlNy layer, according to our estimation by experience, may be about several hundred nanometer Ž- 1 mm.. As for the abnormal phey nomenon in Fig. 2g that Oy and Aly curves 2 , AlO rise after 90 min of sputtering, the reason is very
complicated, it might relate to the irregular microstructure, matrix effects of SIMS measurement and crater sidewall effects after long time sputtering etc.. We compare Fig. 2g with SIMS depth profile of sapphire in Fig. 3, there are big differences in their surface regions, indicating that there are big differences in their structures. The untreated sample is consisted mainly of poly y crystalline AlN and a little AlDyO 3 from curve 1 in Fig. 4. During the preparation of AlN ceramics, Dy2 O 3 was used as sintering additive because it could react with impurities such as Al 2 O 3 in AlN powders, thus was effective to reduce oxygen in AlN ceramics. The reaction products of Dy2 O 3 and Al 2 O 3 usually are: DyAlO 3 , Dy4 Al 2 O 9 , Dy3 Al 5 O 12 , and the ratio of AlrDy is 1r1,1r2,5r3, respectively. If the content of Dy2 O 3 is the same, the higher the AlrDy ratio is, the more Al 2 O 3 react with Dy2 O 3 , and the more oxygen is reduced in AlN. In our samples, only DyAlO 3 is found, showing that the oxygen is not removed thoroughly or the AlN powders are of high purity, and Dy2 O 3 is relatively too much. Curve 2 is the XRD of the sample oxidized in the air at 11008C for 40 min, three weak a-Al 2 O 3
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R. Yue et al.r Applied Surface Science 148 (1999) 73–78
diffractive peaks emerge, indicating a-Al 2 O 3 is produced in the oxide layer.
2AlN q 3O 2 Ž g . ™ Al 2 O 3 q N2 O 3 Ž g . DG 298.15 s y868.438 kJrmol
Ž 4.
2AlN q 4O 2 Ž g . ™ Al 2 O 3 q N2 O5 Ž g . DG 298.15
4. Discussion From the point of thermodynamics, AlN may react with oxygen in the air at room temperature as following: 4AlN q 3O 2 Ž g . ™ 2Al 2 O 3 q 2N2 Ž g . DG 298.15 s y2016.554 kJrmol
Ž 1.
2AlN q 2.5O 2 Ž g . ™ Al 2 O 3 q 2NO Ž g . DG 298.15 s y834.685 kJrmol
Ž 2.
2AlN q 3.5O 2 Ž g . ™ Al 2 O 3 q 2NO 2 Ž g . DG 298.15 s y905.384 kJrmol
Ž 3.
s y889.943 kJrmol
Ž 5.
According to kinetics, the above reactions will be more obvious at 700–8008C. We have known the initial oxidation process of AlN ceramics from the above experiments. If we keep the oxidation time Ž10 min., with the increase of temperature from 850– 11008C, although the thickness of oxygen-rich layer increases obviously and AlN content decreases violently in the same layer, AlNy signal can still be detected in the surface region, showing under above
Fig. 2. SIMS depth profiles of AlN samples. Ža. Untreated; Žb. 8508Cr10 min; Žc. 9508Cr10 min; Žd. 10508Cr10 min; Že. 11008Cr10 min; Žf. 11008Cr20 min; Žg. 11008Cr40 min.
R. Yue et al.r Applied Surface Science 148 (1999) 73–78
Fig. 2 Žcontinued..
condition, that the surface region of AlN ceramics is oxidized partially, and no continuous oxide layer formed. Under the condition of 11008Cr20 min, a continuous oxide layer with a certain thickness appears. When the oxidation time is extended to 40 min, the oxide layer thickness is evaluated to be several hundreds nanometer. Compared with other conditions, under the condition of 11008Cr40 min, the oxide layer is the thickest, but in Fig. 4, the three a-Al 2 O 3 diffractive peaks are very weak. In addition, the SIMS mass spectrum and depth profile of this sample are quite different from those of sapphire. Therefore, we can conclude that besides polycrystalline a-Al 2 O 3 in the oxide layer, there may exist some kind of amorphous or transition oxides. We can also deduce that in the initial stage of
Fig. 3. SIMS depth profile of sapphire sample.
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R. Yue et al.r Applied Surface Science 148 (1999) 73–78
Fig. 4. XRD spectra of AlN samples. 1: Untreated; 2: 11008Cr40 min.
oxidation in the air at 850–11008C, the surface of AlN is not oxidized to a-Al 2 O 3 directly, the transition oxides are formed at first, and with the increasing of oxidation temperature and time, they are transformed into amorphous products and then become crystalline a-Al 2 O 3 . From XPS results by Katnani and Papathomas w6x, there was transition oxide ŽAlON. appeared during the oxidation of AlN powders. As for AlN ceramic substrate, it has sintering additives in it and the microstructure is much more complicated, so other surface analytical methods must be employed to determine what transition oxides are formed during the oxidation.
5. Conclusion SIMS was used to probe the initial surface oxidation process of AlN ceramic substrate together with
XRD; the results show that a very thin oxygen-rich layer already exists in the untreated AlN sample. Under the oxidation condition of 850–11008Cr10 min in the air, the thickness of O-rich layer increases rapidly with the increase of oxidation temperature, but there is no continuous oxide layer formed until the oxidation time is extended to 20 min at 11008C. Within the above temperature range, AlN substrate is not oxidized to a-Al 2 O 3 directly, some transition oxides are formed at first and with the increase of oxidation temperature and time, the transition oxides are transformed into amorphous Al 2 O 3 gradually, and in the end, the a-Al 2 O 3 . The above experiment results also show SIMS is an effective tool to research the initial oxidation process of AlN ceramic substrate.
References w1x F. Miyashiro, N. Iwase, A. Tsuge, F. Ueno, M. Nakahashi, T. Takahashi, IEEE Trans. on CHMT 13 Ž2. Ž1990. 313. w2x K. Niwa, K. Hashimoto, Scripta Metallurgica et Materialia 31 Ž8. Ž1994. 1007. w3x A.V. Virkar, T.B. Jackson, R.A. Cutler, J. Am. Ceram. Soc. 72 Ž1989. 1763. w4x N. Iwase, K. Anzai, K. Shinozaki, Solid State Technology 29 Ž10. Ž1986. 135. w5x N. Iwase, K. Anzai, K. Shinozaki, O. Hirao, T.D. Thanh, Y. Sugiura, IEEE Trans. on CHMT 8 Ž2. Ž1985. 253. w6x A.D. Katnani, K.I. Papathomas, J. Vac. Sci. Technol. A 5 Ž4. Ž1987. 1335. w7x T. Sato, K. Haryu, T. Endo, M. Shimada, J. Mater. Sci. 22 Ž1987. 2277. w8x D. Suryanarayana, L.J. Matienzo, D.F. Spencer, IEEE Trans. on CHMT 12 Ž4. Ž1989. 566. w9x V.A. Lavrenko, A.F. Alexeey, Ceram. Int. 9 Ž3. Ž1983. 80. w10x D. Suryanarayana, J. Am. Ceram. Soc. 73 Ž4. Ž1990. 1128. w11x D. Robinson, R. Dieckmann, J. Mater. Sci. 29 Ž1994. 1949.
SIMS study on the initial oxidation process of AlN ceramic substrate in the air Ruifeng Yue a
c
a,)
, Yan Wang b, Youxiang Wang c , Chunhua Chen
c
NoÕel DeÕices Research DiÕision, Institute of Microelectronics, Tsinghua UniÕersity, Beijing 100084, China b Department of Electronic Engineering, Xi’an UniÕersity of Technology, Xi’an 710048, China State Key Laboratory of Surface Physics, Chinese Academy of Sciences, PO BOX 912, Beijing 100084, China Received 21 October 1998; accepted 15 February 1999
Abstract Secondary ion mass spectrometry ŽSIMS. and X-ray diffraction ŽXRD. measurement were employed to study the initial oxidation process of AlN ceramic substrate in the air at 850–11008C. The results show that there is already a very thin O-rich layer in the surface region of untreated AlN ceramic substrate. When the sample is annealed for 10 min, the O-rich layer becomes thicker rapidly with the increasing of annealing temperature. When it is annealed at 11008C for 20 min, a continuous oxide layer is formed. In the end, combined with chemical thermodynamics, the initial oxidation mechanism near the surface of AlN substrate is discussed. q 1999 Elsevier Science B.V. All rights reserved. PACS: 81.65.Mq; 81.05.Je Keywords: Secondary ion mass spectrometry; AlN ceramics; Oxidation
1. Introduction AlN ceramics has recently been receiving recognition as a promising material in microelectronic packaging. It has such advantages over other materials in packaging as high thermal conductivity, moderate dielectric constant and thermal expansion, matching that of silicon w1x. In its applied field and during its preparation, the common concern has been what effect can oxygen bring on AlN ceramics. Oxygen is the main impurity dissolved in the AlN lattice; the
)
Corresponding author. E-mail: [email protected]
resulting Al vacancy, which can scatter the phonon, is the main reason of lowering thermal conductivity of AlN ceramics w2,3x. How to reduce or eliminate oxygen completely has been the key factor to produce AlN ceramics with high thermal conductivity. On the other hand, AlN becomes unstable and easily oxidized in oxygen-containing atmosphere; the formed oxide film not only can impede the inner part to be oxidized further, thus improving the anti-corrosiveness, but also can increase obviously the adhesion strength between AlN and Cu w4,5x; therefore, the study on AlN oxidation has a practical meaning. There are two kinds of AlN materials used in the research of oxidation: powders and ceramic substrates. The former are the raw materials to prepare
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 1 2 8 - 2
74
R. Yue et al.r Applied Surface Science 148 (1999) 73–78
the latter, and have very large ratio surface, so they are easier to be oxidized. AlN powders are still stable even after 60 h of thermal oxidation in the air at 7008C w6x. Usually, it is accepted that the initial surface oxidation temperature is 700–8008C for AlN and the final product is a-Al 2 O 3 w7,8x. Thermogravimetric analysis ŽTGA. is the most popular method to study the oxidation of AlN, especially AlN powders. The weight difference D M before and after oxidation is used to evaluate the oxide weight and the oxide layer thickness; it is also the characteristics of oxidability. There is no general recognition about the thermal oxidation process w3,6–11x and the main analytical method is bulk analytical method such as TGA. The detection sensitivity of TGA is comparatively low; therefore, in order to achieve the mass variation, the AlN substrate is usually oxidized in the air for several or several 10 h between 900–14008C, then the calculated thickness of oxide layer ranges from a few to several hundred micrometer. There are few systematic reports on the thermal oxidation of AlN ceramic substrate using surface analytical technique until now. In this paper, SIMS technique, together with XRD, is employed to research the initial oxidation process of AlN ceramic substrate.
2. Experimental AlN ceramic substrate was sintered using highpurity raw AlN powders with additives of 3.5% Dy2 O 3 and CaO at 18508C for 4 h. A substrate with diameter of 17 mm was cut into 7 pieces after it was finely polished with standard ceramographic techniques to a final polish of approximately 0.1 mm, then the samples were ultrasonically cleaned with acetone, ethanol and DI water. The thermal oxidation experiment in the air was conducted in quartz tube with open ends. The samples were pushed into the tube after the desired temperatures were stable, then taken out and quickly cooled to room temperature when the oxidation was finished. The thermal oxidation temperaturertime were 8508Cr10 min, 9508Cr10 min, 10508Cr10 min, 11008Cr10 min, 11008Cr20 min, 11008Cr40 min, respectively. One untreated sample, together with a piece of polished a-Al 2 O 3 Ž1102. Žsapphire. was used as reference
materials. SIMS analysis was carried out on Riber MIQ-156 Ion Microprobe, Csq was used as the primary ions with beam current of 0.06 mA, ion beam energy was 10 keV, and incident angle was 458. The beam was rastered over a 0.48 = 0.29 mm2 area, the secondary ions were detected from center 5% area of the scanning region, the background vacuum was 3.7 = 10y7 Pa. ACE576N electron gun was used to reduce the charging effect of AlN substrate under filament current regulation mode, and electron beam energy was 2 keV. XRD was performed on a Drmax-RB ŽRigaku. diffractometer; CuK a radiation was adopted with a scanning speed of 48rmin.
3. Results During SIMS analysis, we did some exploring work on which kind of secondary ion should be detected and how to adjust the operation parameters of the equipment. We have the following considerations: Ž1. to ensure the comparativity under the same experimental condition, the operation parameters are carefully chosen so that the intensity of secondary ions detected in each sample is moderate to allow the equipment to have enough dynamic range; Ž2. mass interference must be reduced, but the mass number of the detecting ions should be close in order to improve the mass resolution and to adjust the equipment parameters more easily; Ž3. to choose the type of cluster secondary ions properly may be beneficial to achieving structure information. There are two kinds of SIMS operating modes: positive and negative; the negative mode was chosen after comparison in our experiment. Fig. 1 is the negative SIMS mass spectra of surface of several samples. Intensity of secondary ion AlNy is quite high in the surface of untreated sample Žcurve 1., and decreases to noise background after being oxidized in the air at 11008C for 40 min Žcurve 2.. Compared with the sapphire surface spectrum Žcurve 3., AlOy in curve 2 is still noise 2 background. In SIMS depth profiling, Aly, Oy 2, AlNy and AlOy were chosen as detecting secondary ions according to the considerations above. Fig. 2 is the SIMS depth profiles of AlN samples. y There is a very thin Oy 2 -rich and AlO -rich layers in
R. Yue et al.r Applied Surface Science 148 (1999) 73–78
75
Fig. 1. Negative SIMS mass spectra of surface of several samples. 1: AlN, untreated; 2: AlN, oxidized at 11008C for 40 min; 3: Sapphire.
the surface of untreated sample, while the inner part has a uniformly distributed composition as shown in Fig. 2a. After the sample is oxidized at 8508C for 10 y min ŽFig. 2b., the intensity of Oy 2 , AlO signals near the surface increases obviously, while the corresponding AlNy signal decreases a lot. The Oy 2 and AlOy signals increase to a great extent after the sample is oxidized at 9508C for 10 min, Oy 2 -rich and AlOy-rich layer extends distinctly. If we keep the oxidation time, with the increasing of temperature, the main changes are as following: the thickness of y y Oy declining 2 -rich, AlO -rich layer, as well as AlN region increases obviously, and the transition layer y between Oy 2 -rich, AlO -rich layer and the inner part of AlN substrate becomes wider. When keeping the oxidation temperature at 11008C, with the increasing of oxidation time, Aly, AlOy and Oy 2 signals drop y while the thickness of Oy 2 -rich, AlO -rich and nonAlNy layer increases gradually. In Fig. 2g, the thickness of non-AlNy layer, according to our estimation by experience, may be about several hundred nanometer Ž- 1 mm.. As for the abnormal phey nomenon in Fig. 2g that Oy and Aly curves 2 , AlO rise after 90 min of sputtering, the reason is very
complicated, it might relate to the irregular microstructure, matrix effects of SIMS measurement and crater sidewall effects after long time sputtering etc.. We compare Fig. 2g with SIMS depth profile of sapphire in Fig. 3, there are big differences in their surface regions, indicating that there are big differences in their structures. The untreated sample is consisted mainly of poly y crystalline AlN and a little AlDyO 3 from curve 1 in Fig. 4. During the preparation of AlN ceramics, Dy2 O 3 was used as sintering additive because it could react with impurities such as Al 2 O 3 in AlN powders, thus was effective to reduce oxygen in AlN ceramics. The reaction products of Dy2 O 3 and Al 2 O 3 usually are: DyAlO 3 , Dy4 Al 2 O 9 , Dy3 Al 5 O 12 , and the ratio of AlrDy is 1r1,1r2,5r3, respectively. If the content of Dy2 O 3 is the same, the higher the AlrDy ratio is, the more Al 2 O 3 react with Dy2 O 3 , and the more oxygen is reduced in AlN. In our samples, only DyAlO 3 is found, showing that the oxygen is not removed thoroughly or the AlN powders are of high purity, and Dy2 O 3 is relatively too much. Curve 2 is the XRD of the sample oxidized in the air at 11008C for 40 min, three weak a-Al 2 O 3
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R. Yue et al.r Applied Surface Science 148 (1999) 73–78
diffractive peaks emerge, indicating a-Al 2 O 3 is produced in the oxide layer.
2AlN q 3O 2 Ž g . ™ Al 2 O 3 q N2 O 3 Ž g . DG 298.15 s y868.438 kJrmol
Ž 4.
2AlN q 4O 2 Ž g . ™ Al 2 O 3 q N2 O5 Ž g . DG 298.15
4. Discussion From the point of thermodynamics, AlN may react with oxygen in the air at room temperature as following: 4AlN q 3O 2 Ž g . ™ 2Al 2 O 3 q 2N2 Ž g . DG 298.15 s y2016.554 kJrmol
Ž 1.
2AlN q 2.5O 2 Ž g . ™ Al 2 O 3 q 2NO Ž g . DG 298.15 s y834.685 kJrmol
Ž 2.
2AlN q 3.5O 2 Ž g . ™ Al 2 O 3 q 2NO 2 Ž g . DG 298.15 s y905.384 kJrmol
Ž 3.
s y889.943 kJrmol
Ž 5.
According to kinetics, the above reactions will be more obvious at 700–8008C. We have known the initial oxidation process of AlN ceramics from the above experiments. If we keep the oxidation time Ž10 min., with the increase of temperature from 850– 11008C, although the thickness of oxygen-rich layer increases obviously and AlN content decreases violently in the same layer, AlNy signal can still be detected in the surface region, showing under above
Fig. 2. SIMS depth profiles of AlN samples. Ža. Untreated; Žb. 8508Cr10 min; Žc. 9508Cr10 min; Žd. 10508Cr10 min; Že. 11008Cr10 min; Žf. 11008Cr20 min; Žg. 11008Cr40 min.
R. Yue et al.r Applied Surface Science 148 (1999) 73–78
Fig. 2 Žcontinued..
condition, that the surface region of AlN ceramics is oxidized partially, and no continuous oxide layer formed. Under the condition of 11008Cr20 min, a continuous oxide layer with a certain thickness appears. When the oxidation time is extended to 40 min, the oxide layer thickness is evaluated to be several hundreds nanometer. Compared with other conditions, under the condition of 11008Cr40 min, the oxide layer is the thickest, but in Fig. 4, the three a-Al 2 O 3 diffractive peaks are very weak. In addition, the SIMS mass spectrum and depth profile of this sample are quite different from those of sapphire. Therefore, we can conclude that besides polycrystalline a-Al 2 O 3 in the oxide layer, there may exist some kind of amorphous or transition oxides. We can also deduce that in the initial stage of
Fig. 3. SIMS depth profile of sapphire sample.
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R. Yue et al.r Applied Surface Science 148 (1999) 73–78
Fig. 4. XRD spectra of AlN samples. 1: Untreated; 2: 11008Cr40 min.
oxidation in the air at 850–11008C, the surface of AlN is not oxidized to a-Al 2 O 3 directly, the transition oxides are formed at first, and with the increasing of oxidation temperature and time, they are transformed into amorphous products and then become crystalline a-Al 2 O 3 . From XPS results by Katnani and Papathomas w6x, there was transition oxide ŽAlON. appeared during the oxidation of AlN powders. As for AlN ceramic substrate, it has sintering additives in it and the microstructure is much more complicated, so other surface analytical methods must be employed to determine what transition oxides are formed during the oxidation.
5. Conclusion SIMS was used to probe the initial surface oxidation process of AlN ceramic substrate together with
XRD; the results show that a very thin oxygen-rich layer already exists in the untreated AlN sample. Under the oxidation condition of 850–11008Cr10 min in the air, the thickness of O-rich layer increases rapidly with the increase of oxidation temperature, but there is no continuous oxide layer formed until the oxidation time is extended to 20 min at 11008C. Within the above temperature range, AlN substrate is not oxidized to a-Al 2 O 3 directly, some transition oxides are formed at first and with the increase of oxidation temperature and time, the transition oxides are transformed into amorphous Al 2 O 3 gradually, and in the end, the a-Al 2 O 3 . The above experiment results also show SIMS is an effective tool to research the initial oxidation process of AlN ceramic substrate.
References w1x F. Miyashiro, N. Iwase, A. Tsuge, F. Ueno, M. Nakahashi, T. Takahashi, IEEE Trans. on CHMT 13 Ž2. Ž1990. 313. w2x K. Niwa, K. Hashimoto, Scripta Metallurgica et Materialia 31 Ž8. Ž1994. 1007. w3x A.V. Virkar, T.B. Jackson, R.A. Cutler, J. Am. Ceram. Soc. 72 Ž1989. 1763. w4x N. Iwase, K. Anzai, K. Shinozaki, Solid State Technology 29 Ž10. Ž1986. 135. w5x N. Iwase, K. Anzai, K. Shinozaki, O. Hirao, T.D. Thanh, Y. Sugiura, IEEE Trans. on CHMT 8 Ž2. Ž1985. 253. w6x A.D. Katnani, K.I. Papathomas, J. Vac. Sci. Technol. A 5 Ž4. Ž1987. 1335. w7x T. Sato, K. Haryu, T. Endo, M. Shimada, J. Mater. Sci. 22 Ž1987. 2277. w8x D. Suryanarayana, L.J. Matienzo, D.F. Spencer, IEEE Trans. on CHMT 12 Ž4. Ž1989. 566. w9x V.A. Lavrenko, A.F. Alexeey, Ceram. Int. 9 Ž3. Ž1983. 80. w10x D. Suryanarayana, J. Am. Ceram. Soc. 73 Ž4. Ž1990. 1128. w11x D. Robinson, R. Dieckmann, J. Mater. Sci. 29 Ž1994. 1949.