High density interconnect bonding of heterogeneous materials using non-collapsible microbumps at 10 μm pitch

This paper reports on a successful demonstration of the use and reliability of Cu/Sn microbumps for the fine pitch interconnection of heterogeneous semiconductor die. InP die have been bonded to Si substrates using a 6.4 mm × 5.12 mm area array of alloyed Cu/Sn microbumps on 10 μm pitch. After bonding, 256 separate channels, each connecting 1280 microbumps, were electrically tested on each die; the channel yield was used to estimate the operability of the 640 × 512 arrays, which was found to be greater than 99.99% for both InP-Si die pairs and Si-Si control die pairs. After 500 thermal cycles from -40 °C to 125 °C, no significant change in the channel yield was seen for the homogeneous Si-Si die pairs. The InP-Si die pairs underfilled with an experimental epoxy had a 2.8% decrease in channel yield while those die pairs without the underfill saw a 13.9% decrease. The channels that failed during thermal cycling were predominately located near the edges of the die pairs, indicating that the bump failures were due to the difference in thermal expansion between InP and Si. Cross-sectional SEM images taken from the edge areas of both underfilled and non-underfilled InP-Si die pairs were used to assess the failure mode in the non-underfilled samples and to demonstrate the positive effects of the use of the underfill.