Chipworks Looks Inside the Freescale HARMEMS Process

By St.J. Dixon-Warren, Manager, Process Engineering, Chipworks

The Freescale MMA6222AEG Accelerometer is targeted for automotive applications and is implemented in a relatively large 13.0 mm x 7.6 mm x 3.3 mm thick 20-LEAD SOIC package. The MMA6222AEG is fabricated with Freescale’s HARMEMS process using a thick SOI layer to form the mechanical layer of the MEMS structure. Chipworks has just completed a full MEMS Process Review analysis of this market leading technology.

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Freescale MMA6222AEG Package

Cross-sectional analysis of the package reveals two chips: a MEMS chip with a hermetic cap and a separate ASIC, both mounted upside down beneath the package lead frame. Unlike most MEMS device we analyze, the MEMS chip is encapsulated in silicone gel, likely to reduce packaging stress on the MEMS die.

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Freescale MMA6222AEG Package Cross Section

Detailed examination shows the MEMS layer, sandwiched between the MEMS die and the MEMS cap. The thick SOI layer provides increased stiffness and greater mass for the moving mechanical element, plus increased electrical capacitance. This should give increased sensitivity compared to Freescale’s standard surface micromachined process, which we found in the Freescale MMA7455L, where the mechanical poly 2 layer was only ~3 µm thick.

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Freescale MMA6222AEG MEMS Structure Cross Section

The HARMEMS SOI process uses a deposited polysilicon layer, with air bridges, to form the electrical interconnects for the MEMS die. The SOI process does not permit the use of a buried poly for the electrical interconnects. The poly air bridges would have been formed on a sacrificial layer (likely oxide) after the deep reactive ion etch (DRIE) used to form the MEMS structures. A timed etch is then used to release the MEMS structures.

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Freescale MMA6222AEG MEMS Structure Tilt View

The use of single crystal SOI should allow better control of the DRIE process, thus giving better consistency in the mechanical properties of the device; however, the SOI process limits the kinds of anchors that can be used. Essentially, released beams need to be narrow, while anchored structures need to be wide enough to ensure that enough oxide remains to provide a connection to the substrate. It is worth noting that, Freescale’s European competitors, Bosch, ST Microelectronics and SensorDynamics, have chosen to develop a thick polysilicon process which allows for greater flexibility in the formation of interconnect and in the formation of anchors.

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