Roger Allan
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ED Online ID #20543 |
January 20, 2009
A Systems Challenge
Despite impressive performance advances in MEMS sensors, the MEMS design community faces a major challenge: cost-effectively meeting promising future applications. By themselves, MEMS sensors are limited to what they can provide and “should be viewed as part of a larger system context,” said Roger Grace of Roger Grace Associates. “Designers need to think outside the chip, and that means taking into consideration signal conditioning, testing, and packaging steps in a MEMS-based product’s cycle that involve a lot of analog expertise, time, and cost.”
Integrating sensing and supporting circuitry all on the same piece of silicon and processing it on a standard CMOS fab facility would seem to be the ideal goal that would make MEMS-based products commercially competitive. However, MEMS transducers are mechanical elements and are very different from supporting electronic circuitry. As a result, they require different processing, testing, and packaging approaches.
Analog Devices has integrated signal-conditioning functions with a capacitive accelerometer on the same silicon chip. Memsic has done the same thing with its accelerometer. Akustica produces silicon MEMS microphones on a CMOS chip that integrates the transducer element and signal-conditioning circuitry. However, most MEMS sensor manufacturers use a multichip approach.
Generally, an ASIC holds the signal-conditioning circuitry. This partitioning of functions onto two or even three chips complicates the manufacturing process since each chip must be tested separately and then interconnected to the others. Finally, all the chips are packaged and the entire assembly must be tested again.
Some companies like Si-Ware Systems have recognized this challenge and provide design solutions and intellectual property (IP) for highly differentiated and demanding products, including ASICs. Si-Ware’s designers work with MEMS manufacturers early in the design stage, providing unique mixed-signal design and verification methodology from the design’s inception to successful tapeouts of complex chips (Fig. 3). The company also provides reusable IP for analog/mixed-signal and RF applications.
“We provide turnkey product development, including handling foundry and packaging logistics, test, and characterization, delivering working samples with evaluation boards, and characterization reports and data sheets,” said Hisham Haddara, Si-Ware’s CEO.
Si-Ware has developed a low-power, 10-bit, 4/8-Msample/s pipelined analog-to-digital converter (ADC) with impressive specifications. It was designed for cellular communication systems, handheld digital video broadcasting (DVB-H), and terrestrial integrated services digital broadcasting (ISDB-T).
Emphasizing The Package In MEMS
“If MEMS IC manufacturers would just put greater effort in the packaging process, at the beginning step of a MEMS IC design, this can save them at least a year in time-to-market,” said Chip Spangler, vice president of Aspen Technologies, which specializes in packaging complex MEMS and non-MEMS ICs and getting them ready quickly for the market. “There’s a great need to fully understand the issues of materials compatibility and thermal interactions in a MEMS chip when packaging it for a product to be successful in the market and get to the market on time.”
For example, Aspen has packaged a high-resolution micro-optical-electromechanical system device that’s used in a very high pixel-count display for flight simulators like those made by Evans and Southerland and planetariums like the Griffith Observatory in Los Angeles.
The device consists of a “driver” die flip-chip attached to a larger MEMS die. Each driver die has 576 solder bumps for a total of 4608 solder connections per device. The MEMS die has a linear array of ribbons in a grating light valve (GLV) configuration. After the dies are flip-chip attached, this subassembly is bonded to an aluminum-nitride plastic grid-array (PGA) with 470 pins (Fig. 4).
The MEMS die is then wire-bonded to the package with 532 wirebonds. Next, xenon-fluoride (XeF2) is used to release the MEMS ribbons. A lid with an optical window is seal seamed to the top of the package. Three such devices are used in a display, one each for red, green, and blue.
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