Interface study of bonded wafers by digitized linear frequency modulated thermal wave imaging

Thermography is a whole field, non-contact and non-destructive characterization technique that is widely used for the investigation of sub-surface features in various kinds of solid materials (conductors, semiconductors, insulators and polymers). In this work, a new method is proposed and demonstrated for the interface study of the bonded wafers that are frequently used in MEMS and VLSI. For the demonstration, two oxidized wafers are direct bonded: one plane and the other having anisotropically etched cavities. To observe the effect of annealing on bonding, the bonded pair was annealed at different temperatures for a fixed period, and also for different time periods at a fixed temperature. Compared to conventional passive wave and pulse thermography techniques, the proposed digitized linear frequency modulated thermal wave imaging (DFMTWI) method overcomes certain drawbacks associated with conventional thermal imaging techniques: in comparison to lock-in thermography it requires very less experimentation time whereas in comparison with pulse thermography less peak power heat sources are sufficient. It is further demonstrated that DFMTWI, in principle, is capable of differentiating depths of various cavities, and the thermal phase images can lead to actual depth estimation also.