We studied the mobility change of MOSFETs in a chip-stacked multichip package. A 6-inch wafer that had the back ground to 50 μm thickness was diced into chips and a chip was cemented to a glass epoxy substrate with a nonconductive paste (NCP), and in addition, a second chip was bonded on the first chip in a flip chip arrangement. The interchip connection was made by thermocompression bonding of Au/Ni bumps formed on the upper and lower chips. The MOSFETs on the first chip had not shown changes in mobility before stacking the second chip, but after stacking the chip, the mobility of the pMOSFETs increased and the mobility of the nMOSFETs decreased. Before chip stacking, the first chip had been slightly convex, but after stacking it was deformed to a greatly concave shape. It is considered that the glass epoxy substrate or the NCP had undergone a plastic deformation during the thermocompression bonding of the chip and that the strain from this deformation remained after the thermocompression bonding process. As a consequence, bending compressive stress occurred in the first chip in the  direction, and it is considered that the mobility of the MOSFET changed due to the piezoresistive effect.
|Number of pages||8|
|Journal||Electronics and Communications in Japan, Part II: Electronics (English translation of Denshi Tsushin Gakkai Ronbunshi)|
|Publication status||Published - Jul 1 2006|
All Science Journal Classification (ASJC) codes
- Electrical and Electronic Engineering