Electronic Design

Use MEMS Technology To Make Sense In An Analog World

There’s a very bright future for microelectromechanical-system (MEMS) sensors and actuators that will give us an unprecedented level of access to our largely analog world. That was the theme of many presentations at the 2008 MEMS Industry Group (MIG) Executive Congress meeting, held in Monterey, Calif., last November. Despite the downward spiral in the overall economy for 2009, analysts at the meeting projected a rosier picture for MEMS for 2010 and beyond, particularly in industrial, consumer electronics (including cell phones), and automotive applications.

MEMS sensors are inherently analog devices that use mechanical elements to produce analog signals. They sense a variety of analog signals like pressure, temperature, acceleration, flow, humidity, sound, light, and chemicals that are then conditioned and digitized for use by computers. Depending on the application, even the output MEMS actuator may produce an analog signal. Analog designers certainly understand that. As digital ICs increase in speed and density, analog issues become even more challenging and less forgiving.

MIG Congress keynote speaker Roger Meike, senior director of Area 51 at Sun Microsystems’ SunSPOT (Small Programmable Object Technology) project, discussed how MEMS sensors will provide us with an unlimited amount of awareness about our environment during his presentation. Essentially a Java-based inventor’s platform, the SunSPOT project puts MEMS and other sensor and transducer technologies into the hands of creative people in many fields, including education, consumer hobbies, the arts, science and R&D, sports and fitness, ecology, and computers (Fig. 1).

“We want to build a community of developers that will enable new devices and service and engage with new potential customers,” Meike said. “We need to inspire that community.”

Tapani Ryhänen, director and head of the Nokia Research Center at the Cambridge Laboratory, described a new paradigm of sensing, computing, and communication using cellular phones. He pointed to mobile services based on context information that will usher in new levels of context awareness in a data-rich environment (Fig. 2). Nokia is developing many of these types of projects together with leading academic institutions worldwide.

Panelists of all varieties addressed and debated business, venture capital, marketing, low-power, and energy conservation issues, as well as emerging MEMS applications and the convergence of MEMS in consumer electronics and mobile communications. Underlying these discussions was the search for ways to make MEMS ICs more cost-effective and lower in cost to meet potentially new application needs.

For example, panelists from Kavlico, Crossbow Technology, Texas Instruments, and Cymbet chimed in on how MEMS can enable low-power energy monitoring and conservation. This can include wireless crop monitoring, ambient energy harvesting, and powering portable electronic products.

Mark Denissen, TI’s vice president for worldwide strategic marketing, talked about the importance of low-power energy monitoring in emerging pico-projectors, a promising application space in which TI is successfully serving. He also drew a link between real-time sensors and data logging for healthcare.

“For maladies like heart disease and diabetes, MEMS-based data loggers are needed to sense key attributes like pressure, motion, flow rate, and chemicals. Lowering power levels supports data collection,” he explained.

Ralph Kling, Crossbow Technology’s chief architect for wireless products, described a future in which metrics for sensor power consumption and energy harvesters are improving. He said that we’ll reach a point of infinite battery life, an achievement that should delight consumers and manufacturers alike.

Another panel session of experts from Red Octane, Microsoft, NXP Semiconductors, Pixtronix, and VTI debated the ways MEMS has converged with consumer electronic items and portable devices, as well as the challenges that lie ahead.

“This is the decade of physical interactivity,” said Microsoft hardware researcher Mike Sinclair as he talked about the importance of gesture recognition at Microsoft. He explained that his company is very much interested in a slew of MEMS devices like microphones, accelerometers, gryroscopes, and RF switches, especially for mobile phones and displays. 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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