MEMS-LSI Chip Lateral Interconnects(High-Density Mounting Technology with Low-Temperature Chip Stacking)Mitsumasa Koyanagi, Professor, Graduate School of Engineering, Tohoku University
We are conducting research aimed at developing a new mounting technology using low-temperature chip
stacking for the high-density integration of heterogeneous devices and electronic parts, such as LSI chips, MEMS
chips, optical devices, and passive elements. This new technology fuses IC, MEMS, mounting, and micro-optic
technologies comprising 1) a technique for high-density mounting of MEMS and LSI chips on a flexible printedcircuit board using self-assembly, 2) a technique for high-density formation of microbumps on a flexible printed
circuit board, 3) a technique for forming crossover interconnects over chips mounted on a substrate, and 4) atechnique for forming passive elements on chips, such as resistors, capacitors, inductors, and coils. Here, I will
present our findings from investigations into MEMS-LSI mounting using self-assembly and the formation ofcrossover interconnects.
MEMS-LSI Mounting Using Self-Assembly
Numerous MEMS and LSI chips can be densely mounted together on a flexible printed circuit board using
the surface tension of liquid droplets. When mounting the chips facedown on a hydrophobic flexible printed
circuit board, as shown in Fig. 1, the underside surfaces of the multiple MEMS and LSI chips forming aplurality of microbumps are made hydrophilic and coated with an aqueous solution or liquid organic matter. At
the same time, hydrophilic areas are formed on the surface of the substrate for temporarily holding the chips.
These areas are also coated with an aqueous solution or liquid organic matter. The plurality of chips whoseunderside surfaces are coated with the aqueous solution or liquid organic matter are flipped so that theirunderside surfaces are facing downward and are simultaneously lowered onto the surface of the substrate sothat the aqueous solution or liquid organic matter on the underside surfaces merges with that on the surface ofthe substrate. The chips are aligned on the substrate through self-assembly owing to the surface tension of the_1 µm
liquid and are bonded in place. This technique enables the chips to be positioned with an accuracy of +within one second.
Crossover InterconnectsWe are developing a technique to form high-density lateral interconnects that cross over MEMS and LSI
chips mounted on a flexible printed circuit board through self-assembly. With this technique, it is important toform the copper interconnect patterns over the high chips so that the side surfaces of the chips are wellcoated, as shown in Fig. 2. To achieve this, we studied various techniques on insulating layer formation, hot
lithography, and copper electroplating. We succeeded in forming crossover interconnects with widths of 5–10µm and a pitch of 10 µm.

Copper wire
Seed layer
Insulating film
Flexible printed
circuit board

Chip
Bonding area
Droplet
LSI chip(1) Forming bonding areas (2) Applying droplets (3) Chip placementon the flexible printed
to the bonding
circuit board
areas

Substrate(4) Positioning byself-assembly(a) Cross-sectional structure

Chip

Cu Wiring
36 m

Substrate
Chip alignment

5 seconds after beginningalignment

10 seconds after beginningalignment

Fig. 1 MEMS-LSI mounting technology using self-assembly5(b) SEM image
Fig. 2 Crossover interconnect