IEICE TRANS. ELECTRON., VOL.E90–C, NO.1 JANUARY 2007
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LETTER
Special Section on Microoptomechatronics
Low-Temperature Au-to-Au Bonding for LiNbO3/Si Structure Achieved in Ambient Air Ryo TAKIGAWA†a) , Nonmember, Eiji HIGURASHI† , Member, Tadatomo SUGA† , Nonmember, Satoshi SHINADA†† , and Tetsuya KAWANISHI†† , Members
SUMMARY A lithium niobate (LiNbO3 )/silicon (Si) hybrid structure has been developed by the surface-activated bonding of LiNbO3 chips with gold (Au) thin film to Si substrates with patterned Au film. After organic contaminants on the Au surfaces were removed using argon radiofrequency plasma, Au-to-Au bonding was carried out in ambient air. Strong bonding at significantly low temperatures below 100◦ C without generating cracks has been demonstrated. key words: low-temperature bonding, Au-to-Au bonding, surface-activated bonding, lithium niobate/silicon structure, hybrid integration
1.
Introduction
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Combining lithium niobate (LiNbO3 ) with silicon (Si) is attractive for the realization of highly integrated and functional optical devices because LiNbO3 is one of the most widely used electrooptic materials and Si is the most frequently used material for ICs, microelectromechanical systems, and optoelectronic packaging. Although heteroepitaxial methods have been used in the past for the fabrication of LiNbO3 thin films on Si, the films usually possess a lower crystalline and optical quality than bulk crystals. The bonding of dissimilar materials has, therefore, emerged as an interesting alternative. Several methods of fabricating LiNbO3 /Si hybrid structures have recently been reported. They include fusion bonding [1] and surface activated bonding (SAB) [2]. The former bonding method requires an annealing process to achieve strong bonding (usually performed above 400◦ C), which causes thermal stress that may often produce cracks during annealing. The latter, SAB, is carried out under ultrahigh vacuum to prevent the activated surface from reoxidation. However, a low vacuum would be preferable to decrease the process cost. In this study, the feasibility of the low-temperature bonding of LiNbO3 chips to Si substrates using Au films in ambient air is examined. Since Au surfaces do not oxidize, it is expected that the surface activation will be effective for the bonding of Au in ambient air.
Fig. 1 (a) SEM micrograph showing patterned Au on Si substrate. (b) Magnified view of (a).
Manuscript received August 1, 2006. Manuscript revised September 6, 2006. † The authors are with The University of Tokyo, Tokyo, 1138656 Japan. †† The authors are with National Institute of Information and Technology, Koganei-shi, 184-8795 Japan. a) E-mail:
[email protected] DOI: 10.1093/ietele/e90–c.1.145
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2.
Sample Information and Bonding Process
In our experiments, LiNbO3 chips (thickness: 500 µm, size: 6 mm × 6 mm) with 100-nm-thick Au thin film and Si substrates (thickness: 380 µm, size: 12 mm × 12 mm) with 2µm-thick patterned Au film were used. A thin titanium layer was used as an adhesion layer. In the case of metal thin film to metal thin film bonding, a large bonding load was required for the interatomic attraction to overcome surface asperities. Therefore, patterned Au film, which is easy to deform, was used to cause intimate Au-to-Au contact. The diameter and pitch of Au micropatterns on the Si substrates were approximately 6 µm and 25 µm, respectively, as shown in Fig. 1. The rms surface roughnesses of Au on the LiNbO3 chips and Si substrates, measured with an atomic force microscope, were about 1.3 nm and 10.5 nm, respectively. A flip-chip bonder combined with a surface activation process was used for the experiments [3]. After organic contaminants on the Au surfaces of the LiNbO3 chips and Si substrates were removed by the surface activation process using with argon (Ar) radio-frequency (RF) plasma, Au-Au bonding was carried out only by contact in ambient air with applied static pressure. The etching rate of Au film was about 30 nm/min (plasma power: 100 W). Results and Discussion
Bonding was carried out in ambient air at temperatures ranging from 25 to 150◦ C with the contact load maintained at 441 N and plasma irradiation for 150 s. Figure 2 shows the relationship between measured tensile strength and bonding temperature. Without plasma irradiation (bonding tem-
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IEICE TRANS. ELECTRON., VOL.E90–C, NO.1 JANUARY 2007
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Fig. 2
Tensile strength versus bonding temperature.
Fig. 4 strate.
Cross-sectional SEM image of bonded LiNbO3 chip on Si sub-
Fig. 3 Photograph of cracked LiNbO3 chip on Si substrate (bonding temperature: 150◦ C).
perature: 100◦ C), the tensile strength was about 0 MPa. This indicates that the surface activation process is effective for low-temperature bonding, and conventional Auto-Au thermocompression bonding cannot be applied under this condition (bonding temperature: 100◦ C, contact load: 441 N). The tensile strength (calculated by dividing the total cross-sectional area of the initial, undeformed micropatterns) shows an increase with bonding temperature and reaches 70 MPa at a bonding temperature of 100◦ C. The heating of the specimens is expected to soften the Au films, enhance the deformation of the Au films and remove surface barrier films, such as a layer of water molecules adsorbed on Au surfaces. However, when the bonding temperature was increased to be greater than 150◦ C, the LiNbO3 chips cracked during bonding, as shown in Fig. 3. This result indicates that the residual stress due to thermal expansion mismatch is a serious problem. Therefore, low-temperature bonding is essential for LiNbO3 /Si bonding. Figure 4 shows a scanning electron microscopy (SEM) image of the cross section of the bonded LiNbO3 /Si structure (bonding temperature: 100◦ C). The patterned Au films were deformed and the thickness was changed from 2 µm to about 1.2 µm after bonding. This deformation of the Au films is expected to lead to intimate contact such that interatomic attraction occurs. Figure 5 shows SEM micrographs of the fracture surface after a tensile test. From these SEM images, it can be observed that some parts of the Au thin film on the LiNbO3 chip were transferred to the Si substrate, which clearly indicates the strong Au/Au bonding achieved by the surfaceactivated bonding process.
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Fig. 5 SEM micrographs of fracture surface after tensile test. (a) LiNbO3 chip (100◦ C), (b) Si substrate (100◦ C), (c) LiNbO3 chip (25◦ C), (d) Si substrate (25◦ C).
4.
Conclusions
LiNbO3 chips were successfully bonded to Si substrates at a low temperature (< 100◦ C). The tensile strength of the bonded specimens was sufficient for use in optical applications. These results show the potential for producing highly functional optical devices. References [1] A. Namba, M. Sugimoto, T. Ogura, Y. Tomita, and K. Eda, “Direct bonding of piezoelectric crystal onto silicon,” Appl. Phys. Lett., vol.67, no.22, pp.3275–3276, 1995. [2] H. Takagi, R. Maeda, N. Hosoda, and T. Suga, “Room-temperature bonding of lithium niobate and silicon wafers by argon-beam surface activation,” Appl. Phys. Lett., vol.74, no.16, pp.2387–2389, 1999. [3] T. Suga, T. Itoh, Z. Xu, M. Tomita, and A. Yamauchi, “Surface activated bonding for new flip chip and bumpless interconnect systems,” Proc. Electronic Components and Technology Conference (52nd ECTC), pp.105–111, 2002.
