Strong, high-yield and low-temperature thermocompression silicon wafer-level bonding with gold

Abstract

A systematic variation of process parameters for wafer-level thermocompression bonding with gold is presented for the first time. The process was optimized for high bond strength and high bond yield. In addition, the impact of the process temperature was investigated. A bond strength of 10.7 ± 4.5 MPa and a bond yield of 89% was achieved when bonding a wafer pair at 298 °C applying 4 MPa pressure for 45 min. A total of ten wafer pairs were bonded in a custom-built bonding tool and tested to establish the optimal process parameters. The bonded interface was found to be strong and dense enough for MEMS applications. The bonds were characterized using pull tests, transmission electron microscopy (TEM) and energy dispersive x-ray spectroscopy (EDS). The TEM inspections indicated that it is possible to form hermetic seals by using the presented bonding method.

Wafer-level Au–Au bonding in the 350–450 °C temperature range

“…However, the applied bonding pressures used in the current study are probably lower than the pressures used by Tsau et al [17] and higher than the pressure used by Taklo et al [19]. The average bond strengths obtained on our three laminates were higher than the ~10 MPa reported by Taklo et al [19] and the ~ 20 MPa reported by Kurotaki et al [16]. Mean fracture loads of 20 MPa have been reported for glass frit bonds [3], indicating that 20 MPa can be a sufficient bond strength for device seals in industrial products.…”

“…However, in order to activate a getter material during the bonding process, a bonding temperature of 400 -450 °C can be demanded [18]. According to Park et al and Taklo et al, increased bonding temperature improves the bond quality [10,19], while Kurotaki et al did not find any relationship between bonding temperature and bond strength [16]. Several authors report that an increased bonding pressure increases bond quality [10,17,19], while the bonding time has been found to be of low importance for the final bond strength [10,17].…”

“…The helium leak rate of Au-Au thermocompression bonds was measured to be 2.74×10 -11 Pa m 3 /s by Pa m 3 /s by Xu et al, indicating that high quality hermetic packaging can be realized using this technology [10,20]. As diffusion barrier and adhesion layer Cr, Ti, W, TiW and NiCr have been used [10,12,16,17,19,20].The relatively large discrepancy in the reported bonding parameters for Au-Au thermocompression bonding indicates that the process is still not fully understood and described. This paper presents an investigation of Au-Au bonding in the temperature range 350 -450 °C.…”

Ultrasonic Transducers

“…During the past decades, many bonding schemes have been developed [7,[71][72][73][74][75][76][77][78][79][80][81][82][83][84], according to their bonding mechanisms, these processes can be classified as direct bonding, anodic bonding, and bonding with intermediate layer.…”

“…To protect the bonding tool from damaging, the bonder must be cleaned periodically. On the other hand, during bonding, the ions may also diffuse into the counter wafer, i.e., they will bring contamination to the silicon substrate [84]. To avoid this effect, an additional barrier layer (e.g.…”