MEMS equipment diffuses into mainstream IC markets, finds ways to improve yields
Quicker 3D simulation, more practical dry etch, and automated die inspection
By Dr. Paula Doe, contributing editor
Though the MEMS market has remained essentially flat in 2008 and 2009, as plenty of new consumer and medical applications continued healthy growth (see page 6), equipment demand is another story. The MEMS equipment market (new specialty MEMS tools) likely reached less than $200 million in 2008, down from $330 million in 2007, says Yole Développement CEO Jean-Christophe Eloy, and 2009 looks about the same, though visibility is very limited. However, “several specialty MEMS tool suppliers are adding capacity in light of strong order intake so far this year,” he notes. “This growth is driven by the diffusion of MEMS technologies into other markets like 3D ICs, image sensors, and new applications for nanoimprint.”
Virtual fabrication of a MEMS accelerometer by the X-Fab SOI process.
Suppliers also see potential in offering new approaches to improve tricky MEMS yields in manufacturing faster, and at lower cost, with solutions for quicker simulation, more practical dry etch, and smarter package inspection.
One option for getting those MEMS devices to yield would be to find design problems by simulation. Some fabs are using Coventor Inc.’s 3D virtual fabrication software to find design flaws or process problems before fabrication, reports Stephen Breit, Coventor VP of product development. Using voxels (cubic 3D equivalents of pixels) instead of traditional compute-intensive CAD-like solid modeling techniques–plus some elegant compression algorithms–allows fast modeling of the entire process sequence. The virtual prototype that results shows real engineering information on how the 2D layout will translate to a 3D device, revealing things like gaps in film coverage or cavities in underlying layers.
Users specify the parameters that impact the MEMS device geometry, like layer thickness, snowfall or conformal deposition, or etch rate ratios between different materials, for each step in the sequence. The modeler then applies a series of strictly geometric operations to generate a realistic virtual prototype of the device. The parameters must be experimentally calibrated, but the simpler modeling process can then simulate the entire fabrication sequence over a large part of the die within a few hours on a desktop computer.
X-Fab now regularly uses the tool throughout design and process development to validate MEMS designs before tape out, saving test wafers, and speeding time to yield.
More practical dry etch
Though wet etch remains the workhorse technology for etching away sacrificial layers to release the functional MEMS structures, at smaller geometries the surface tension effects of trapped moisture tend to stick down the released structures. Dry etch processes prevent this stiction, but they’ve yet to see wide adoption in production. Vapor phase HF etchers still require careful control of the condensation from the water used as a catalyst, and have been relatively low throughput, and the option of using XeF2 is extremely expensive.
Primaxx proposes to avoid stiction at lower cost with a batch vapor HF system that better keeps water out of the process. It uses low cost anhydrous HF, with low-water electronic grade alcohol for the catalyst, to minimize the H2O content of reagents going in, control H2O byproducts and keep them in gas phase, and draw the H2O away from the etch interface with the alcohol properties. The process also runs at minimal power at 45°C. CEO Paul Hammond says etch rates range from 0.05-5µm/minute, depending on oxide type, and with <10% variability within wafer, wafer to wafer, and batch to batch, on 25-wafer batches of 200mm wafers.
Air Products and Chemicals, meanwhile, is bringing down the cost of using XeF2 by reclaiming the xenon. Increasing demand for xenon for new applications in flat-panel displays and other electronics is pushing prices up, but the rare gas is also simply costly to extract and distill. It occurs naturally in air only in minute quantities of 87 parts per billion–so manufacturing typically requires some 220 watt-hours to extract one liter of Xe from air, then further purification by cryogenic distillation. This energy-intensive extraction process only makes sense on very large air plant, mostly associated with steel mills, and with the economic downturn they’ve curtailed production, further tightening supplies.
Air Products’ system for the fab reclaims the xenon by-product of the XeF2 etch process, then sends it back to an Air Products facility for manufacturing into more XeF2, explains commercial development manager Eugene Karawacki.
Automated package die inspection
Printed circuit board inspection tool supplier Vi Technology is entering the MEMS market by applying its automated optical inspection expertise to automating the inspection of MEMS dies before encapsulation. First customer for the new product started full volume production in mid-May.
The tool replaces traditional inspection by an operator with a microscope with a quick, two-step automated process. The first pass uses a laser to measure the tilt of each die in the package, to make sure products like inertial sensors are correctly seated so they work. It also measures the exact focal distance of the die in the package, adjusting to make sure the optical system can see from the top to the bottom of the dimensional MEMS device. The second pass comes back with a high-end camera and telecentric lens to take pictures of the die with different fields of view. After stitching the pictures together to reconstruct an image, it compares that to a reference image to identify any differences, and flags the ones that are actually real defects, down to 3µm in size. Both passes take about 2-5 seconds per die, depending on size and types of defects.
“This ensures that only the known good dies go on to the next step, usually encapsulation, therefore saving costs,” says product line manager David Richard. “It also enables for the first time a real tilt measurement, which is a key functional criterion for accelerometers and gyros, and no electrical test can measure this.”
These ideas are some that will be discussed in the MEMS programs at SEMICON West in San Francisco, July 14-16. Yole will present it latest market forecast for the supply chain, and leading European development foundries IMEC, CEA Leti, and Silex Microsystems will discuss their progress in using standard processes to cut development time and costs. The sessions are part of a series on key developments in disruptive semiconductor technologies featured this year in the Extreme Electronics program. See www.semiconwest.org for details.