Electronic Design

Interconnecting MOEMS ICs

Interconnecting micro-optoelectromechanical-system (MOEMS) ICs is a much more complicated task than interconnecting conventional MEMS ICs. Like a regular MEMS IC, a MOEMS IC requires metal leads for interconnecting and packaging. But since optics are involved, it also requires a light path where light beams can enter and exit. And, moisture levels must be kept extremely low so the optics don’t fog up and degrade performance.

Despite these technical challenges, Texas Instruments successfully produced a MOEMS-based IC using a hermetic package for its digital micromirror device, or DMD (see the figure). Such a device forms the backbone of today’s large-screen, high-definition projection TVs.

TI has made its MEMS DMD compatible with the company’s conventional CMOS manufacturing process since the mid-1980s. First, all of the metallization layers for interconnecting the transistors are laid down. A low-temperature process then is used to put a MEMS structure on top of the completed CMOS chip.

This process is implemented by choosing the right aluminum alloys for the mechanical elements and a conventional photoresist material to act as a sacrificial spacer. Because all of this is done at temperatures under 200°C, neither metallization nor the finished CMOS circuitry is affected after the MEMS structure is added to the chip.

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