Wafer-Level Vacuum Packaging by Thermocompression Bonding Using Silver after Fly-Cut Planarization
Authors: Liu Cong , Hirano Hideki , Froemel Joerg , Tanaka Shuji 
Published: February 2020

With the development of micro-electromechanical systems (MEMS), the integration of MEMS and large scale integrated (LSI) circuits together with hermetic packaging is getting more important for the reduction of device footprint and the improvement of performance. Wafer bonding technology is a good choice for such integration because it allows separate design and fabrication of MEMS and LSI. As a kind of wafer bonding, thermocompression bonding can realize electric interconnection as well as hermetic sealing with a smaller bond frame and still keeps sufficient bonding strength. The thermocompression bonding based on metals, Au, Al and Cu is well known. However, Cu needs pretreatments for oxide removal; Al requires high temperature and pressure which may cause damage to temperature–sensitive devices such as LSI. Au can be bonded at lower temperature with minimal surface pretreatments. However, thermocompression bonding using thin Au film cannot achieve vacuum sealing for micro-structured or non-flat wafers such as MEMS.

Our previous work solved most of these problems using electroplated Au bonding frames after fly-cut planarization. However, for lower cost, Ag is proposed which is much cheaper and shows superior thermal and electrical conductivity. Whereas, the oxidation affinity of Ag may cause bonding problems related to a surface oxide layer. The diffusion of Ag into Si is also concerned for deteriorating device characteristics.

In this study, we developed a novel inexpensive hermetic thermocompression bonding method using electroplated Ag bonding frames planarized by fly cutting. A barrier layer was used to block the Ag diffusion into a Si substrate during the bonding process. Extremely high shear strength and vacuum sealing were demonstrated at relatively low bonding temperature and bonding pressure.

Usually minority carrier lifetime in LSI would be severely reduced due to the contamination of metal atoms. In order to avoid the diffusion of Ag atoms from the bonding areas into a Si substrate during the bonding process, a titanium nitride (TiN) barrier layer was adopted because of its excellent barrier property in conjunction with good thermal stability and low contact resistance. A layered structure (20nm Ti/ 100nm TiN/ 20nmTi/ 100nmAg) was deposited on Si substrate by sputtering at room temperature. This test sample was heated up to the bonding temperature, held for 1 h. Secondary ion mass spectroscopy (SIMS) was carried out through stacked layers. No detectable Ag atom concentration was found in the Si substrate (Fig 1), suggesting that the diffusion was stopped by TiN.

Next, we verified the possibility of Ag for thermocompression bonding. A Si substrate was coated with the stacked layer mentioned above. Then the Ag frames of 39μm width and 10μm height were electroplated using photoresist mold. On one substrate, 64 dies were fabricated. Subsequently the bump frames were cut to 4.3μm in height by fly cutting with a diamond bit.

The planarization step played a decisive role in our method. Generally the electroplated surface was rough. However after planarization, even on non-flat wafers, Ag bonding frame surface became smooth to establish enough contact surface for sufficient atom diffusion. Additionally under the shear strain of the diamond bit, small grain size was obtained which improved grain boundary diffusion. Moreover, native surface oxide or any other surface contamination was removed.

The substrate with the planarized electroplated Ag frames and another Si substrate with sputter deposited Ag thin film pads were treated by Ar plasma and bonded in vacuum at 350~400℃ with a loading pressure of 54MPa. After the bonded wafer was diced into dies, die-shear tests were conducted to evaluate the shear strength. The achieved mean shear strength was up to 200 MPa. The samples fractured across the bonding material and in no case at the bonding interface. The lateral expansion of bonding frames due to pressure was less than 5%. From the cross-sectional scanning electron microscopy (SEM) of the bonded area, there was no visible bonding interface anymore (Fig 2). This suggested the newly formed oxide after planarization didn’t affect the bonding significantly. The thin oxide layer probably diffused into the bulk through grain boundaries.

Additionally, vacuum sealing was successfully demonstrated by the deflection of Si thin diaphragms fabricated through an SOI wafer due to pressure difference across the diaphragms. After bonding process, the membrane’s clear deflection was observed at atmosphere and confirmed by topography measurement system (Fig 3). The proposed process using the planarized electroplated Ag frames was useful for both wafer-level heterogeneous integration and hermetic packaging and versatile for various microsystems.

Figure 1