JOURNAL OF APPLIED PHYSICS 98, 053502 共2005兲
Thermal stability of diamondlike carbon buried layer fabricated by plasma immersion ion implantation and deposition in silicon on insulator Zengfeng Di,a兲 Anping Huang, Ricky K. Y. Fu, and Paul K. Chub兲 Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
Lin Shao, T. Höchbauer, and M. Nastasi Los Alamos National Laboratory, Los Alamos, New Mexico 87545
Miao Zhang, Weili Liu, Qinwo Shen, Suhua Luo, Zhitang Song, and Chenglu Lin The Research Center of Semiconductor Functional Film Engineering Technology and State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology (SIMIT), Chinese Academy of Sciences (CAS), Shanghai 200050, People’s Republic of China
共Received 11 May 2005; accepted 22 July 2005; published online 2 September 2005兲 Diamondlike carbon 共DLC兲 as a potential low-cost substitute for diamond has been extended to microelectronics and we have demonstrated the fabrication of silicon on diamond 共SOD兲 as a silicon-on-insulator structure using plasma immersion ion implantation and deposition in conjunction with layer transfer and wafer bonding. The thermal stability of our SOD structure was found to be better than that expected for conventional DLC films. In the work reported here, we investigate the mechanism of the enhanced thermal stability. We compare the thermal stability of exposed and buried DLC films using Raman spectroscopy and x-ray photoelectron spectroscopy 共XPS兲. Our Raman analysis indicates that the obvious separation of the D and G peaks indicative of nanocrystalline graphite emerges at 500 ° C in the exposed DLC film. In contrast, the separation appears in the buried DLC film only at annealing temperatures above 800 ° C. Analysis of the XPS C1s core-level spectra shows that the 共sp3 + C – H兲 carbon content of the unprotected DLC film decreases rapidly between 300– 700 ° C indicating the rapid transformation of sp3-bonded carbon to sp2-bonded carbon combined with hydrogen evolution. In comparison, the decrease in the 共sp3 + C – H兲 carbon content of the buried DLC film is slower below 800 ° C. Elastic recoil detection results show that this superior thermal stability is due to the slower hydrogen out diffusion from the buried DLC film thereby impeding the graphitization process. We propose that the SiO2 overlayer retards the graphitization process during annealing by shifting the chemical equilibrium. © 2005 American Institute of Physics. 关DOI: 10.1063/1.2034651兴 I. INTRODUCTION
More than three decades have elapsed since the report on the production of amorphous carbon films,1 and diamondlike carbon 共DLC兲 continues to be a material attracting intensive research. DLC thin films have physical and chemical properties which approach those of diamond, for example, high hardness, high optical index, low friction coefficient, chemical inertness, high electrical resistivity, and high thermal conductivity. Recently, the use of DLC has been extended to microelectronics, especially in silicon-on-insulator 共SOI兲 technology. A buried silicon dioxide layer is used in conventional SOI to enhance the operating characteristics of siliconbased devices. However, its poor thermal conductivity leads to inefficient heat dissipation from the active silicon layer and it becomes an even more serious issue when the device size shrinks.2,3 Diamond which has excellent thermal cona兲
Also affiliated with The Research Center of Semiconductor Functional Film Engineering Technology and State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology 共SIMIT兲, Chinese Academy of Sciences 共CAS兲. b兲 Author to whom correspondence should be addressed; FAX: 85227889549 or 852-27887830; electronic mail:
[email protected] 0021-8979/2005/98共5兲/053502/5/$22.50
ductivity, high electric resistivity, and high electrical breakdown is a promising substitute for SiO2 and is expected to be less prone to the self-heating phenomenon.4–6 However, its rough surface prohibits direct bonding to silicon wafer in the fabrication of the silicon-on-diamond 共SOD兲 substrate without extensive polishing and surface treatment. DLC, which is a potential low-cost substitute for diamond, has been successfully used as the buried insulator instead of silicon dioxide by our group.7 In spite of the fabrication success, the acceptance of SOD by the semiconductor industry hinges on its thermal stability. DLC films are known to graphitize at relatively low temperature thereby losing the insulating characteristics. It is imperative that the DLC film in SOD can resist graphitization at elevated temperature in order to be compatible to microelectronics processing. We have observed enhanced thermal stability in our SOD structure compared with DLC films used as coatings. In this study, we investigated the reasons for the enhanced characteristics. We compared the thermal stability between uncovered and covered or buried DLC films and found that the buried film possessed much better thermal stability. The microstructures of two types of films annealed under different temperatures were studied by Raman spectroscopy and x-ray
98, 053502-1
© 2005 American Institute of Physics
053502-2
Di et al.
J. Appl. Phys. 98, 053502 共2005兲
photoelectron spectroscopy 共XPS兲. Elastic recoil detection 共ERD兲 was carried out to investigate the degree of hydrogen loss from the DLC films during annealing. Our results suggest that the enhanced thermal stability is related to the mitigated hydrogen loss and a chemical equilibrium model is proposed to explain the phenomenon. II. EXPERIMENTAL DETAILS
The DLC thin films were fabricated on p-type Si 共100兲 substrates using plasma immersion ion implantation and deposition 共PIIID兲.8 During deposition, a mixture of acetylene 关20 standard cubic centimeter per minute 共SCCM兲兴 and argon 共5 SCCM兲 was introduced into the vacuum chamber and the plasma was triggered using a radio-frequency 共rf兲 plasma source. Film deposition was carried out at a working pressure of 8 ⫻ 10−4 Torr at a constant rf power of 500 W. The applied voltage, repetition rate, and pulse width were −20 kV, 40 Hz, and 400 s, respectively. Some of the deposited samples were covered by a 40-nm-thick electronbeam-evaporated SiO2 layer in order to compare the thermal stability of exposed and covered DLC films. SiO2 was chosen as the covering layer because it is transparent in our Raman measurements. The exposed and covered DLC samples underwent furnace annealing at different temperatures in N2 ambient for 30 min. III. RESULTS AND DISCUSSION
Raman spectroscopy is commonly used to characterize DLC films. The graphitic band 共G兲 at approximately 1550 cm−1 and disordered band 共D兲 at approximately 1360 cm−1 are especially useful in elucidating the structure. The G band is assigned to the C–C stretching mode of graphite,9 whereas the D band is associated with in-plane vibrations resulting from structural imperfections.10 Figures 1共a兲 and 1共b兲 display the Raman spectra of the exposed and buried DLC films annealed at various temperatures, respectively. A prominent second-order Si substrate peak at ⬃960 cm−1 occurs in all the samples as the films are highly transparent. In the exposed DLC films, the typical broad asymmetric Raman peaks with declining background luminescence because of hydrogenation are visible, and their shape does not change significantly until the annealing temperature reaches 300 ° C. However, the rate of decline becomes smaller after annealing at 500 ° C, and it disappears from the film annealed at 700 ° C indicating significant hydrogen loss11 which has been correlated to the loss of sp3 bonding and resultant loss of diamondlike properties.12 Meanwhile, an obvious separation between the D and G peaks appears at 500 ° C and then the separated D and G peaks decrease in intensity with increasing annealing temperature up to 700 ° C, indicating that the DLC film is almost converted to nanocrystalline graphite at higher temperature.13,14 It can be deduced that the exposed DLC film gradually graphitizes and oxidizes even at temperature as low as 500 ° C. In contrast, the separation indicative of nanocrystalline graphite does not appear in the buried DLC film until the annealing temperature reaches about 800 ° C. The background luminescence continues to decline, indicat-
FIG. 1. Raman spectra acquired from 共a兲 the exposed DLC film and 共b兲 the buried DLC film after annealing at different temperatures.
ing that the emission of hydrogen from the DLC films is retarded by the covering SiO2 layer. This leads to slower graphitization of the DLC film. The detailed structural information of the buried DLC films annealed at various temperatures can be obtained from the Raman parameters derived from the combined Gaussian and Lorentzian curve fitting of the Raman spectra. Figure 2
FIG. 2. G peak position and ID / IG ratio derived from the combined Gaussian and Lorentzian curve fitting of the Raman spectra of the buried DLC film.
053502-3
Di et al.
shows the Raman parameters of the G peak position and the integrated intensity ratio of the D and G peaks 共ID / IG兲 of the buried DLC film. The G band of the annealed DLC films shifts towards higher wave numbers with increasing annealing temperatures, implying that the concentrations of sp2 carbon become higher and the graphitic component in the amorphous carbon increases. The ID / IG ratio exhibits a similar tendency like the variation in the G peak position. It suggests increases in the graphite size and number of the graphitic sp2 clusters.15,16 The Raman findings including G peak position and ID / IG ratio illustrate that the structural transformation of the buried DLC film happens gradually without any abrupt change below 800 ° C. This graphitization process is enhanced at the temperature between 800 and 1000 ° C, resulting in rapid conversion from DLC to nanocrystalline graphite. Compared with conventional DLC coatings that completely convert to nanocrystalline graphite at 450– 600 ° C,13,17 the graphitization process of the buried DLC film is significantly retarded. XPS analyses were performed on the exposed and buried DLC films annealed at various temperatures. The C1s spectra were taken at almost the same depth after removing 1 – 2 nm of the DLC layer from both the exposed and buried samples employing argon-ion sputtering. Figure 3 depicts the C1s spectra acquired from three different DLC films: 共a兲 asdeposited, uncovered DLC film 共same structure as the unannealed buried DLC film兲, 共b兲 buried DLC film after annealing at 500 ° C, and 共c兲 exposed DLC film after annealing at 500 ° C. The solid line shows the fitted results based on the combined Gaussian and Lorentzian function. Compared with the as-deposited DLC film, the binding energies 共BEs兲 of the two annealed samples, whether exposed or buried, shift to lower values. The corresponding C1s spectra can be deconvoluted into two peaks situated at approximately 284.4 and 285.3 eV. It is well known that the binding energy of graphite which consists of 100% sp2 bonds is 284.3– 284.5 eV,18 polycrystalline diamond after Ar+-ion etching is 285.5 eV,19 and C–H is 284.9– 285.5 eV.20 Obviously, it is hard to separate the sp3 C–C and C–H bonds because of the bindingenergy superposition. Here, the sp3 C–C and C–H bonds are considered to be one component. In the as-deposited sample, the 共sp3 + C – H兲 carbon content is as high as 62.3%. In Fig. 3共b兲, the peak corresponding to 共sp3 + C – H兲 carbon in the buried DLC after annealing at 500 ° C becomes smaller because of hydrogen loss and slight graphitization. In the exposed DLC film after annealing at 500 ° C, hydrogen loss is greatly enhanced. The 共sp3 + C – H兲 carbon component is significantly reduced along with the increase in the peak corresponding to sp2, as shown in Fig. 3共c兲. It is found that the exposed film becomes more graphite like after annealing at 500 ° C. Figure 4 shows the amounts of sp3 C–C and C–H bonds in the exposed and buried DLC films calculated based on the ratio of the peak areas plotted as a function of the annealing temperature. In the exposed DLC film, the 共sp3 + C – H兲 carbon content decreases slowly below 300 ° C, then significantly decreases between 300– 700 ° C. It is associated with the rapid transfer of sp3-bonded carbon to sp2-bonded carbon combined with hydrogen out diffusion. On the other hand,
J. Appl. Phys. 98, 053502 共2005兲
FIG. 3. Deconvoluted of C1s XPS spectra acquired from three different DLC films: 共a兲 as-deposited exposed DLC film or buried DLC film, 共b兲 buried DLC film after annealing at 500 ° C, and 共c兲 exposed DLC film after annealing at 500 ° C.
the buried DLC film shows a slower decrease in the 共sp3 + C – H兲 carbon content below 800 ° C, and then it rapidly decreases to about 23.4% at 1000 ° C indicative of almost complete graphitization. At annealing temperatures under 700 ° C 共the exposed DLC film is almost graphitized completely at 700 ° C兲, the 共sp3 + C – H兲 carbon content in the buried DLC film is much higher than that of the exposed DLC film implying retarded graphitization in the buried DLC film. Based on previous studies conducted on amorphous hydrogenated carbon and particularly DLC films,13,21–23 hydrogen loss in the form of H2, methane, or CHx causes irreversible graphitization of the DLC film during annealing. The relative hydrogen contents in the buried and exposed DLC
053502-4
Di et al.
J. Appl. Phys. 98, 053502 共2005兲
FIG. 6. Schematic presentation of a possible structural model for hydrogen evolution which yields methane and C v C during annealing.
FIG. 4. 共sp3 + C – H兲 carbon content as determined from the fitting of the XPS C1s peaks for both the buried and exposed DLC films, as a function of the annealing temperature.
films after 500 ° C annealing as determined by ERD are presented in Figs. 5共a兲 and 5共b兲, respectively. There is hydrogen loss from both the buried and exposed DLC films after annealing at 500 ° C. However, the hydrogen loss from the buried DLC film is slower and the hydrogen content of the 500 ° C buried sample is about 82% of the as-deposited
sample. This is in comparison to a value of 63% calculated from the exposed film under the same annealing conditions. The present results suggest that the covering SiO2 layer acts as a hydrogen diffusion barrier impeding hydrogen loss from the buried DLC film. Our findings are consistent with a similar phenomenon observed in exposed DLC films used as coatings. The film with a higher density exhibits relatively higher thermal stability because of retarded hydrogen evolution.22,23 Both the Raman and XPS results suggest that the buried DLC has superior thermal stability because of a slower graphitization process. In view of the hydrogen loss in the form of H2, methane, or CHx during DLC graphitization, the phenomenon can be explained by the shift in the chemical equilibrium. The evolution of H2, methane, or other CHx species from the annealed DLC film involves many complex reactions. To briefly explain the retardation of hydrogen out diffusion by the covering SiO2 layer, we can make use of one reaction example which yields methane and C v C formation as a demonstration in this paper: 4C共sp3兲 − H + 6C共sp3兲 − C共sp3兲 ⇔ 4C共sp3兲 − C共sp2兲 + C共sp2兲 = C共sp2兲 + CH4↑.
FIG. 5. Hydrogen concentrations as determined by ERD from 共a兲 the buried DLC film and 共b兲 the exposed DLC film after annealing at 500 ° C.
共1兲
A possible structural model for reaction 共1兲 is also shown schematically in Fig. 6. Any other similar reactions concerning graphitization and hydrogen evolution can be analogously analyzed.24 If reaction 共1兲 is truly in equilibrium, the partial pressure of methane will be at a steady-state value. In the exposed DLC film, hydrogen loss starts when the CH4 共or H2, or CHx兲 products diffuse out of the surface during annealing. Under a flowing N2 ambient, the partial pressure of CH4 共or H2, or CHx兲 on the surface of the exposed DLC film is about zero. Consequently, the position of the equilibrium in reaction 共1兲 will shift to the right to produce more methane 共or H2, or CHx兲, subsequently resulting in rapid hydrogen loss. It will continue until all the hydrogen atoms leave the DLC film resulting in a collapse in the carbon matrix and complete graphitization. In the buried DLC film, the overlying SiO2 layer as a hydrocarbon diffusion barrier will slow down the out diffusion of CH4 共or H2, or CHx兲 to the sample surface. This is analogous to the film having a higher density as aforementioned. The partial pressure of CH4 共or H2, or CHx species兲 between the covering SiO2 layer and DLC film is thus higher than that on the surface of the exposed film. Therefore, the tendency of the chemical equilibrium shifting to the right diminishes because of the residual CH4 共or H2, or CHx兲 at the DLC/ SiO2 interface. The corresponding transformation of sp3 carbon atoms to sp2 car-
053502-5
bon atoms is thus retarded. It should be noted that in SOD substrate which consists of a thick silicon surface layer on the buried DLC, the crystalline surface silicon layer is more effective in suppressing hydrogen loss than the thin SiO2 layer used in this experiment. Thus, the thermal stability of the buried DLC layer in SOD is enhanced and this phenomenon bodes well for SOD in microelectronics applications. IV. CONCLUSION
To investigate the cause of the high thermal stability of the buried diamondlike carbon layer in silicon on diamond, we compare the thermal stability between exposed DLC and buried DLC films. Both Raman spectroscopy and XPS results disclose that the buried DLC film has a higher thermal stability due to a slower graphitization process compared with the exposed DLC film. This superior thermal stability is believed to be due to the smaller hydrogen loss that is impeded by the covering SiO2 layer. ACKNOWLEDGMENTS
This work was jointly supported by City University of Hong Kong Strategic Research Grant 共SRG兲 No. 7001820, Shanghai Rising Star Program No. 04QMX1463, the Special Funds for Major State Basic Research Projects G2000036506, and the National Natural Science Foundation of China under Grant Nos. 90101012 and 60476006. The work at Los Alamos National Laboratory was supported by the DOE Office of Basic Energy Sciences. S. Aisenberg and R. Chabot, J. Appl. Phys. 42, 2953 共1971兲. W. Redman-White, M. S. Lee, B. M. Tenbroek, M. J. Uren, and R. J. T. Bunyan, Electron. Lett. 29, 1180 共1993兲. 3 M. Zhu, P. Chen, R. K. Y. Fu, Z. An, C. Lin, and P. K. Chu, IEEE Trans. Electron Devices 51, 901 共2004兲. 1 2
J. Appl. Phys. 98, 053502 共2005兲
Di et al. 4
S. D. Wolter, D.-A. Borca-Tasciuc, G. Chen, N. Govindaraju, R. Collazo, F. Okuzumi, J. T. Prater, and Z. Sitar, Diamond Relat. Mater. 12, 61 共2003兲. 5 S. D. Wolter, G. N. Yushin, F. Okuzumi, B. R. Stoner, J. T. Prater, and Z. Sitar, Diamond Relat. Mater. 11, 482 共2002兲. 6 G. N. Yushin, S. D. Wolter, A. V. Kvit, R. Collazo, B. R. Stoner, J. T. Prater, and Z. Sitar, Appl. Phys. Lett. 81, 3275 共2002兲. 7 M. Zhu, P. K. Chu, X. Shi, M. Wong, W. Liu, and C. Liu, Appl. Phys. Lett. 85, 2532 共2004兲. 8 P. K. Chu, S. Qin, C. Chan, N. W. Cheung, and L. A. Larson, Mater. Sci. Eng., R. 17, 207 共1996兲. 9 R. Dillon, J. Woollam, and V. Katkanant, Phys. Rev. B 29, 3482 共1984兲. 10 F. Tuinstra and J. L. Koenig, J. Chem. Phys. 53, 1126 共1970兲. 11 W. J. Yang, K. Niihara, and K. H. Auh, J. Ceram. Proc. Res. 5, 269 共2004兲. 12 B. Dischler, A. Bubenzer, and P. Koidl, Solid State Commun. 48, 105 共1983兲. 13 D. R. Tallant, J. E. Parmeter, M. P. Siegal, and R. L. Simpson, Diamond Relat. Mater. 4, 191 共1995兲. 14 S. Prawer, K. W. Nugent, Y. Lifshitz, G. D. Lempert, E. Grossman, J. Kulik, I. Avial, and R. Kalish, Diamond Relat. Mater. 5, 433 共1996兲. 15 A. A. Ogwu, R. W. Lamberton, S. Morley, P. Maguire, and J. Mclaughlin, Physica B 269, 335 共1999兲. 16 J. Turlo, D. Gan, and N. Gopi Nathan, Proceedings of the MRS Spring 1998 Meeting, San Francisco, CA, 13–17 April 1998 共unpublished兲. 17 W. J. Yang, Y.-H. Choa, T. Sekino, K. B. Shim, K. Niihara, and K. H. Auh, Thin Solid Films 434, 49 共2003兲. 18 J. F. Moulder, W. F. Stickle, P. E. Sobol, and K. D. Bomben, Handbook of X-ray Photoelectron Spectroscopy 共Perkin-Elmer, Eden Prairie, MN, 1992兲. 19 J. Shirafuji, Y. Sakamoto, A. Furukawa, H. Shigeta, and T. Sugino, Diamond Relat. Mater. 4, 984 共1995兲. 20 P. Yang, N. Huang, Y. X. Leng, J. Y. Chen, R. K. Y. Fu, S. C. H. Kwok, Y. Leng, and P. K. Chu, Biomaterials 24, 2821 共2003兲. 21 Y. Bounouh, M. L. Thèye, A. Dehbi-Alaoui, A. Matthews, and J. P. Stoquert, Phys. Rev. B 51, 9597 共1995兲. 22 M. Benlahsen, B. Racine, K. Zellama, and G. Turban, J. Non-Cryst. Solids 283, 47 共2001兲. 23 C. Wild and P. Koidl, Appl. Phys. Lett. 51, 1506 共1987兲. 24 H. Efstathiadis, Z. L. Akkerman, and F. W. Smith, J. Appl. Phys. 79, 2954 共1996兲.
