The real world problem
AnSem's customer is building an advanced machine that is processing terabytes of data during operation. At the heart of this machine is an array with more than 3000 MEMS switches. These need to be controlled by a programmable high voltage that is symmetrical around the chip substrate.
The solution
AnSem designed an ASIC containing all the supporting circuitry for the MEMS. On this ASIC, the MEMS array is postprocessed after regular CMOS silicon processing. Each MEMS is programmed by a control voltage with a range between -15V and +15V relative to the silicon substrate.
The voltage at each MEMS is controlled by an 9b current steering DAC with current steps of 500nA, which are then converted in high voltage driving signals.
Furthermore a diagnosis is built-in that detects a faulty condition of the MEMS.
Distributed over the array, multiple temperature sensors are placed to monitor the temperature over the chip.
Due to the large size of the MEMS array, special attention needed to be given to the layout of the array:
- The control logic is placed in the neighborhood of each MEMS and fully symmetrical so that the routing to all MEMS is well-balanced.
- Analog signals are communicated via current between different locations on chip and between power domains
- Deep N-well and high voltage pockets are used to have isolation substrates per local power domain
- The digital signals are regenerated in each power domain.
- Special techniques are used to avoid IR drop
The ASIC is programmed through an SPI interface.
AnSem selected as technology 0.18µm 40V BCD CMOS GEN II from TSMC. The main interesting feature for this application is the double isolated high voltage pockets that allow operation of MOS devices at voltages below the substrate without injecting substrate currents.
AnSem manages the complete ASIC life cycle including design, prototyping, industrialization and supply chain for volume production.
Key technical statistics
- Dual supply rail support: +15V, -15V
- +-12V output voltage range
- Clock switched 9b steering capacitor DAC
- 5 built in, on chip temperature sensors
- 0.18µm 40V BCD CMOS GEN II from TSMC
