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MEMS mirror array drivers

For actuating a MEMS device, dedicated drivers are used. These drivers are mostly designed for the specific usage of the MEMS, certainly when the MEMS unit is an array of individual devices. If the driver units are integrated with the MEMS device or when the MEMS device is an external component, this will impact the requirements for the driver.

For the tilting type of MEMS devices, we have created drivers for both the uniaxial and biaxial tilting versions. In all these cases, the accuracy of tilting and the high update rate where key parameters for the application.

Challenge

For the larger MEMS devices, the voltages involved can increase beyond 100V. For the driver we need a High Voltage device with enough protection to withstand MEMS failures (Both temporary as static).

Independent of operating voltage, the accuracy and settling time are one of the key parameters for designing the drivers. For smaller integrated MEMS structures the Signal Integrity becomes also a key design parameter, certainly when the MEMS and driver structures span a large area. Another issue mainly involved with larger area structures is the total power consumption.

For going into full production of the devices, additional testability features needs to be implemented to be able to check the electrical characteristics of the drivers even for an integrated MEMS device where the actual electrodes towards the MEMS are not accessible anymore from external.

Solutions

Multi Million integrated mirror drivers

Multi million integrated mirror driversThe driver ASIC for the integrated multi Million mirror array is the largest ASIC that we have created. It is 22mm by 45mm in size and contains more than 10 Million individual mirror drivers that are located underneath each MEMS mirror. The accurate tilt angle setting of the uniaxial mirrors is performed at a maximum update rate of 6 k updates/s. The equivalent data rate that is needed to provide the ASIC with sufficient data is 600 Gbit/s. The data path is fully optimized for both accuracy and power even within the constraints of the area.

This large device has been processed in a standard CMOS process, but because the extreme dimensions of the device, stitching was needed during wafer processing in the Fab. After the CMOS wafer processing the additional MEMS layers were added. To cope with the flatness rules of the MEMS device, additional flatness rules were taken into account during the design phase.

Drivers for biaxial tilting mirror array

Biaxial mirror driverAn example of a biaxial mirror driver is a flip-chip ASIC containing drivers for actuating 64 biaxial MEMS mirrors. In this case, the MEMS device is an external component that needs High Voltage driving signals up to 50V. Every individual driver output is short circuit protected for connection to ground or to a voltage up to 100V. The total rate to update all drivers is up to 10k updates/s while maintaining an equivalent 12 bits accuracy on the driver outputs.

For diagnostic purposes every individual output can be made observable in the application on a dedicated analog test bus. This is used as an analog boundary scan during final product assembly.

Further reading

A paper about the Multi Million integrated mirror drivers was published in the Journal of Microelectromechanical Systems.