The Study of the Bonding Energy on Silicon-to-Glass Wafer Bonding. T.-K. Chuang, A. Usenko, and J. S. Cites

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1 / The Electrochemical Society The Study of the Bonding Energy on Silicon-to-Glass Wafer Bonding T.-K. Chuang, A. Usenko, and J. S. Cites Science & Technology, Corning Incorporated, Corning, New York 14831, USA The bonding energy plays a significant role in wafer bonding. The temperature dependence of the bonding energy has been studied to understand the physical/chemical interaction at the bonded Si/Glass interface for a variety of glasses. Several primary glass attributes, such as surface roughness, Young s modulus, strain point, CTE, and glass constituents, have been evaluated to correlate to the bonding energy. While the other attributes have been proven insignificant to the bonding energy, notable correlations with the glass surface roughness, the strain point of the glass, and the alkaline content in the glass have been discovered. Introduction A strong bond at the interface of the silicon and the glass substrates is desirable to facilitate the layer transfer process and assure the transfer quality. The bonding energy of two substrates is a crucial measure to gauge the integrity of the wafer bonding and subsequently predetermine the final film-transfer quality. Tong et al. reported the thinfilm silicon transferred onto the inexpensive silicate glass by wafer bonding in At that time, however, the quality of the film transfer was far from perfection due to the unpredictable surface morphology after chemical cleaning. An additional PECVD TEOS (Si(C 2 H 5 O) 4 ) layer had to be deposited on the glass surface to serve as the bonding layer to facilitate the bonding 1. Silicon bonded to crystal glass materials, such as Quartz and Sapphire, have also been demonstrated for specific applications like quartz crystal resonators and high-power microwave devices 2,3,4. However, Si/Quartz and Si/Sapphire bonding pose challenges primarily due to the large difference in the thermal expansion coefficients (CTEs) between the silicon and the glass substrates. The utilization of the high-temperature anneal to strengthen the bond is then prohibited because of the CTE mismatch. In order to achieve high bonding energy to improve the overall layer transfer quality, this work attempts to find the predominant factor(s), which governs the bonding energy and consequently affects the transfer process. It is attempted to understand the physical/chemical interaction occurring at the silicon/glass interface through the characterization of the bonding energy as a function of temperature for a variety of glasses. In hydrophilic silicon-to-silicon wafer bonding, it is known that the bond can be significantly strengthened at a temperature above 200 ºC, and the bond is completed at approximately 1100 ºC by forming covalent siloxane bonds 5. According to the experiments, in contrast, Si/Glass bonded pairs complete the bonds at lower temperatures 150 ºC to 350 ºC, depending upon the type of glass. The bond energies of the silicon wafers bonded to a variety of glasses, such as Soda Lime, Corning 1737, Corning EAGLE XG glass, Corning Jade TM glass, SCHOTT Borofloat, Corning Pyrex, and 501

2 Quartz, have been characterized as a function of annealing temperature. The primary glass attributes, such as Young s modulus, surface roughness, CTE, strain point, and glass constituents, have also been evaluated to clarify the correlation with the bonding energy. Experimental 6-inch silicon and glass wafers were used to make bonded pairs for the measurements. The silicon wafers were all prime (100) p-type and single-side polished. Some of the glass wafers were polished, while Corning 1737 glass, EAGLE XG glass, and Jade glass were non-polished due to the fact that they were originally formed in sheets by Corning s proprietary Fusion-Draw Process and cored into wafers afterwards. The surface roughness of the glass was characterized before and after the wafer clean process. Both silicon and glass wafers were cleaned respectively right before bonding with each other. The bonding energy was then characterized at room temperature. Afterwards, the bonded pair was annealed at elevated temperatures incrementally. In between each two annealing process, the bonding energy was re-measured until the razor blade broke the glass during the measurement. Wafer Clean The silicon wafers were cleaned by diluted HF to remove the native oxide, DI ozone water to remove organics, and diluted NH 4 OH/H 2 O 2 mixture to remove metal contaminants. The glass wafers were cleaned by diluted NH 4 OH/H 2 O 2 mixture and DI ozone water. Wafer Bonding As soon as the wafers were cleaned, the wafer bonding process was then followed to form the bonded pair. The bonding process was implemented under a controlled environment, i.e. class 100 cleanroom facility, at room temperature. Anneal After the silicon was bonded to the glass, the bonded pair was loaded on a customized atmosphere anodic bonder instead of a hot plate. The bonder consisted of top and bottom heating plates. During the annealing process, both plates were touching the bonded pair with moderate force which is just enough to hold the pair. Therefore, the temperature distribution across the bonded pair was very uniform, and the temperature ramps were very steady and controllable. The temperature over-shoot was always less than 3% of the target temperature. There was no voltage applied to the annealing process. Metrology The bonding energy was measured by the conventional crack-opening method 6,7 as shown in Figure 1 schematically. This technique is based on the balance of the elastic forces of two substrates and the bonding energy at the crack tip

3 Figure 1. Schematic side view of the crack-opening method. t b represents the thickness of the razor blade, whereas t w1 and t w2 are the thickness of the two substrates respectively. E 1 and E 2 are the Young s Moduli. L represents the crack-opened distance. For two dissimilar substrates, glass and silicon in this case, the bonding energy (γ) can be calculated by the following equation t b E1t w1e 2t w2 γ = [1] L( E t + E t ) 1 w1 2 w2 The surface roughness (RMS) was measured by the Zygo white-light profilometer. Totally four evenly-distributed spots were measured for each glass, and the reported RMS was the average of these four measurements. The measured area was about 120 µm x 120 µm for each spot. Results and Discussions Before the glass was cleaned by the wet chemistry, the surface roughness was measured by the non-contact Zygo profilometer. Only Corning 1737 glass, EAGLE XG glass, and Jade glass are fusion-drawn glasses. The other glasses are polished. According to the measurements, Quartz and Pyrex showed much higher surface roughness than the other glasses (Figure 2). After being cleaned, all glasses showed micro-roughening effect at the surfaces. In other words, an increased surface roughness was observed for all glasses (Figure 3). Pyrex remained the roughest after the cleaning process, and Quartz was still the second roughest. The chemistry to clean the glass surface seemed to have the most significant roughening effect on Pyrex, due to the fact that the RMS increased by 7 Å. In contrast, Quartz seemed to stay intact during the cleaning process. There was almost no roughening on the Quartz surface. Corning s fusion-drawn glasses showed better chemical resistance to the wet chemical cleaning process than the other glasses except the Quartz. It is known that RMS less than 5 Å is adequate for silicon wafer bonding at room temperature via hydrogen bonds 9. This criterion is also applicable to silicon-to-glass bonding. Typically the smoother the substrate surface is, the stronger the bond will be. After both the silicon and the glass wafers were cleaned, they were bonded at room temperature. The bonding energy was then measured for each bonded pair at room temperature, after annealing at 100 ºC, 150 ºC, 200 ºC, 250 ºC, and 300 ºC. For most of the bonded pairs, the bonding energy has exceeded the glass elastic energy below 300 ºC. As a result of that, the glass substrate broke as the razor blade was attempted to crackopen two substrates. In that case, the bond was considered completed with that certain glass thickness. The glass thickness was either 0.5 mm or 0.63 mm, and the bonding 503

4 energy was calculated accordingly. The last data point before the glass broke was the highest measurable bonding energy unless a thicker glass substrate was used. Figure 2. Root-mean-square surface roughness of glasses before the wet chemistry clean. Figure 3. Root-mean-square surface roughness of glasses after the wet chemistry clean. According to the bonding energy as a function of temperature, Soda Lime glass completed the bond with the silicon wafer at the lowest temperature, i.e. 150 ºC, and exhibited the highest bonding energy among all (Figure 4). The Borofloat glass and Corning 1737 glass had the second highest bonding energy. However, the Borofloat glass completed the bond at 200 ºC, while Corning 1737 glass did not complete the bond until 250 ºC. They had very similar bonding energy at lower temperatures but started to deviate at 150 ºC. Soda-lime float glass (i.e. conventional float glass or modern window glass) is a member of the alkali-alkaline-earth-silicate glass family. This glass does not 504

5 contain any boron compounds. Instead it has a % alkaline earth content and contains a higher proportion of alkalis (approx. 15% Na 2 O) than Borofloat. Among these three glasses with similar surface roughness, it is evident that the higher the alkaline content was in the glass, the higher bonding energy and the lower bond completion temperature. EAGLE XG glass and Jade glass also had about the same surface roughness as the previous three glasses but lower bond energies. The bonding energy curves of EAGLE XG glass and Jade glass are almost overlapped below 200 ºC, but EAGLE XG glass completed the bond at a higher temperature. It is known that both EAGLE XG glass and Jade glass have lower alkaline content than the Soda Lime, Borofloat, and Corning 1737 glass. Quartz had slightly rougher surface than the aforementioned glasses prior to bonding. Therefore, it is understandable that Quartz had lower bonding energy. The bonding energy curve of Quartz was in fact nearly overlapped with the curves of EAGLE XG glass and Jade glass below 100 ºC. While the temperature rose up to 150 ºC, the difference of the bond energies between the Quartz and the other two glasses started to become significant. The predominant factor might be alkaline content in the glass, which includes both alkali metals and alkaline earths. It is hypothesized that the alkaline constituents in the glass becomes mobilized as the anneal temperature increases, and that helps breaking down the hydrogen bonds and the formation of the siloxane bonds. As a result, the bond is strengthened. However, further study is needed to clarify the actual mechanism. The Quartz substrate broke in the process of the 200 ºC anneal due to the significant CTE mismatch between two substrates. This phenomenon was reported elsewhere by T. Abe, et al. 10 In the case of Si-to-Pyrex bonded pair, it is clear that the surface roughness was predominant to the bonding energy regardless the high alkaline content in the Pyrex glass. Figure 4. Bonding energy as a function of temperature for various Si/Glass pairs. Other glass attributes, such as the strain point, CTE, and Young s Modulus have been evaluated to justify the correlation with the bonding energy. Table I lists all three physical attributes of the glasses that are being studied. It is apparent that the bonding energy has no significant dependence on the Young s modulus or on the CTE. On the 505

6 other side, it is found that the strain point of the glass is inversely proportional to the bonding energy. The strain point and the alkaline content are known to be interrelated. Typically the strain point lowers down as the alkaline content in the glass increases. Pyrex is exceptional again because the surface roughness predominates the bonding energy. Table I. Physical attributes of glasses. SLS Borofloat Corning 1737 EAGLE XG Jade TM Quartz Pyrex Strain Point (ºC) CTE (ppm/ºc) Young s Modulus (GPa) Conclusions Very distinct bonding kinetics over the temperature for silicon-to-glass (Si/Glass) wafer bonding has been observed as compared to the silicon-to-silicon (Si/Si) wafer bonding. The hydrophilically bonded Si/Glass pairs form very strong bonds at lower temperature range of ºC, whereas the Si/Si bonded pair does not complete the bond until 800 ºC. The Young s modulus and CTE have been evaluated and found no contribution to the bonding energy. On the other hand, the surface roughness, the strain point, and the alkaline content in the glass, have been found to be significantly predominating the bonding energy. It is well known in Si/Si wafer bonding that the contribution of the surface roughness is significant. However, the mechanism of the alkaline content in the glass to enhance the bonding energy has not been studied. It can be concluded that the prerequisite factor to achieve the high bonding energy is the smooth surface, i.e. low surface roughness, and the alkaline content in the glass can further strengthen the bond as the temperature increases. The alkali metals and/or alkaline earths become mobilized as the substrate temperature rises. The hypothesis is that the mobilized alkaline at the surface of the glass catalyze the water removal process at the Si/Glass interface and the formation of siloxane bond (Si-O-Si). For instance, the mobilized alkaline could help the energy transportation to effectively break the hydrogen bonds or lower the activation energy barrier to break the hydrogen bonds in favor of the formation of the strong siloxane bonds. However, further study is needed to understand the fundamental mechanism of the interaction of the alkaline content and the interface water. References 1. Q.-Y. Tong, T.-H. Lee, W.-J. Kim, T. Y. Tan, U. Gösele, H. M. You, W. Yun, and J. K. O. Sin, Proc. of the IEEE Intl. SOI Conf., 1996, p K. Eda, A. Kanaboshi, T. Ogura, and Y. Tagichi, Proceedings of 2 nd International Symposium on Semiconductor Wafer Bonding: Science, Technology and 506

7 Applications, Vol , The Electrochemical Society, Pennington, NJ, 1993, p T. Abe, K. Ohki, A. Uchiyama, K. Nakazawa, and Y. Nakazato, Jpn. J. Appl. Phys., 33, 514 (1994). 4. G. P. Imthurn, G. A. Garcia, H.W. Walker, and L. Forbes, J. Appl. Phys., 72, 2526 (1992). 5. Q.-Y. Tong and U. Gösele, Semiconductor Wafer Bonding, John Wiley & Sons, Inc., M. S. Metsik, J. Adhes., 3, 307 (1972). 7. W. P. Maszara, G. Goetz, A. Cavilla, and J. B. McKitterick, J. Appl. Phys., 64, 4943 (1988). 8. G. Cha, R. Gafiteanu, Q.-Y. Tong, and U. Gösele, Proceedings of 2nd International Symposium on Semiconductor Wafer Bonding: Science, Technology and Applications, Vol , The Electrochemical Society, Pennington, NJ, 1993, p T. Abe and J. H. Matlock, Solid State Technol., Nov., 39 (1990). 10. T. Abe, K. Sunagawa, A. Uchiyama, K. Yoshizawa, and Y. Nakazato, Jpn. J. Appl. Phys., Vol. 32, pp , (1993). 507