Low-Temperature Direct Bonding of Borosilicate, Fused Silica, and Functional Coatings

Plasma activation at atmospheric pressure has proved to be a suitable pre-treatment process for low-temperature direct bonding of silicon wafers [1]. The activation with a dielectric barrier discharge in oxygen process gas is known to form a porous silicon dioxide which apparently has beneficial structural properties for low-temperature bonding [2]. Activation in oxygen, nitrogen, or synthetic air increases bond strength and reduces accumulation of bond defects during annealing. In experiments using inert gas mixtures with hydrogen or ammonia, on the other hand, only a slight increase of bond strength was found. Furthermore the accumulation of bond defects during annealing was stronger than for the non-treated reference wafer [3]. New experiments on borosilicate glass and fused silica glass as well as silicon wafers covered by silicon dioxide, silicon nitride, and silicon oxynitride, resp., were done for low-temperature bonding. The substrates were activated either in a self-made setup of Fraunhofer IST or using the “Plasma Tooling” of SUSS MicroTec aligner. Before plasma activation, the substrates were cleaned from particle by a spin rinse step. After activation the substrates were bonded in ambient air whereas reference wafers were bonded without plasma activation. The transient development of bond strength during annealing of a wafer pair was characterized by dynamic surface energy measurements. Therefor we used a setup for crack length measurements in situ, consisting of selfmade fixtures for the wafer pair and the blade, and a motor, driving the blade through the bonded interface. The setup was placed in a furnace and pictures of the crack were taken by IR transmission photography during annealing [4]. In the following figures both the surface energy and the wafer temperature are plotted over the annealing time. Results for plasma activated borosilicate wafer are shown in Fig. 1.