500°C process enables PZT MEMS integration on CMOS

Example applications include actuators for auto focus lenses on digital cameras and inkjet heads for printers. In future, higher performance, multi-functional, smaller piezoelectric MEMS devices for next-gen advanced sensor technology are expected, rapidly expanding applications by integration with CMOS devices.

Piezo-electric MEMS is one of the most practical MEMS devices available today, however, the high process temperature is an obstacle for integration of the MEMS device directly onto a CMOS device. A CMOS device due to its nature, can only withstand a process temperature of 500°C or lower. A typical crystallisation temperature for a PZT thin film is 600°C for sputtering and 700°C for Sol-Gel.

ULVAC has developed the world's first technology to allow integration of piezoelectric MEMS onto CMOS, thus achieving high piezoelectric performance and cycle performance. This is accomplished by utilising unique sputtering technology with process temperature below 500°C.

The piezoelectric device comprises five layers; an adhesion layer, a lower electrode layer, a buffer layer, a piezoelectric layer, and upper electrode layer. All the accumulated layers are formed sequentially, through one single sputtering system developed by ULVAC. This multi-chamber type sputtering system (model SME-200) allows for consistent process flow, optimising each individual layer inside each process chamber respectively, achieving highly stable repeatability of the stacked layer performance, and also improving throughput. This system is designed to achieve a highly uniform and stable process utilising 8" silicon wafers, the largest size substrate available for MEMS device mass production.

A maximum of seven process chambers are used, including DC and RF magnetron sputtering chambers, a rapid thermal annealing chamber to accelerate crystallization and a load-lock chamber. The PZT thin film is accumulated by crystal growth on a heated wafer. The sputtering chamber is specifically designed for dielectric material to allow a stable deposition process and lead composition control, an attribute required for highly volatile materials such as PZT.

ULVAC says that parameters it has verified are: piezoelectric constant (e31) - 17°C/m2 , dielectric strength voltage ±100V and cycle performance of over 1011 cycles.

This technology should open the window to new electrode materials, substrates, and manufacturing process for higher performance, multi-function, and smaller MEMS devices where high PZT deposition temperature had previously limited its functions, says ULVAC.