Thermal Cycling Effects on Critical Adhesion Energy and Residual Stress in Benzocyclobutene-Bonded Wafers

Abstract

The effects of thermal cycling on critical adhesion energy and residual stress at the interface between benzocyclobutene ͑BCB͒ and silicon dioxide ͑SiO 2 ͒ coated silicon wafers were evaluated by four-point bending and wafer curvature techniques. Wafers were bonded using BCB in an established ͑baseline͒ process, and the SiO 2 films were deposited by plasma-enhanced chemical vapor deposition ͑PECVD͒. Thermal cycling was done between room temperature and a peak temperature. In thermal cycling performed with 350 and 400°C peak temperatures, the critical adhesion energy increased significantly during the first thermal cycle. The increase in critical adhesion energy is attributed to relaxation of residual stress in PECVD SiO 2 , which in turn is attributed to condensation reactions in those films. Thermal cycling also cures the BCB beyond the ϳ88% achieved in the baseline process, and the residual stress in the BCB is reset at a glass transition temperature corresponding to the increased BCB cure conversion. As more thermal cycles are performed, stress hysteresis in the BCB decreases as the cure stabilizes at 94-95%.

Adhesive wafer bonding

“…A curing temperature of 250°C requires a curing time of about 1 h. During the curing process, the BCB coating reaches a low viscous phase while cross-linking, compensating extremely well for topographic features on the wafer surfaces and achieving very strong and durable bonds. 13,60,72,192 Soft-baked and partially cured BCB coatings can be stored several days or weeks in particle-free environments before the actual wafer bonding is performed without compromising the bonding results. Substrates with diameters of up to 200 mm have been successfully bonded using BCB in combination with the presented bonding parameters.…”

“…Substrates with diameters of up to 200 mm have been successfully bonded using BCB in combination with the presented bonding parameters. 13,14,70,84,96,192 Figure 7 shows the bond interfaces of a 200-mm-diam bonded wafer pair in which the top wafer has been sacrificially removed to expose the BCB adhesive layer. The color fringes at the wafer surface result from small BCB thickness variations that result form BCB reflow during the wafer bonding process.…”

Three‐Dimensional (3D) Integration

“…Work using BCB as a bonding layer has shown that a strong bond (adhesion energy >30 J/m 2 as measured by four-point bending) can be achieved at a bonding temperature of 250 8C; this bond remains stable at temperatures as high as 400 8C [48,50]. The mechanism for bonding in this case is proposed to be cross-linking across the original bonding interface [72,75].…”

“…Via chains with specific contact resistance $5 Â 10 À6 O cm 2 were demonstrated [46] by integrating wafer-to-wafer alignment, bonding, HAR etching in inductively coupled plasma (ICP), HAR filling using CVD copper, and thinning to an etch stop. This approach has demonstrated that active devices and passive copper/low-k structures can survive this bonding process and an aggressive thinning process several times over [47], thermal cycling to 400 8C [48], liquid-to-liquid thermal shock, and autoclave tests [49,50], and that the advantageous stress-buffering features of BCB remain after bonding [50].…”

III‐V/silicon photonics for on‐chip and intra‐chip optical interconnects

“…Structured silicon wafers were bonded both to unpatterned silicon wafers and Pyrex wafers. Many other papers report on the excellent properties (bonding strength, residual stress, bonding quality) of DVS-BCB bonding [34][35][36].…”