Additions to the MEMS Toolkit
Paula Doe, SEMI, San Jose -- Semiconductor International, 6/15/2007
Developments in nanoscale processing provide some new options for the MEMS sector to consider, including diamond on insulator (DOI) wafers ready for sacrificial etching of structures with the hardness and thermal conductivity of diamond, and nanoimprint tools approaching production-level throughputs.
The low-cost process for nanoscale control of diamond growth could make diamond just another high-performance substrate material, such as silicon carbide (SiC) or sapphire, or so says Advanced Diamond Technologies (ADT, Romeoville, Ill.). To show off the possibilities of its DOI wafers, the company has been releasing diamond MEMS products of its own.
ADT developed deposition technology to convert methane to diamond in a chemical vapor deposition (CVD)-like batch process with standard semiconductor equipment onto off-the-shelf silicon wafers, so production should be easily scalable to meet demand. The company says it can control the process to produce a film with crystals of a desired size and roughness, although most interesting may be the 3-5 nm grain-size materials that create a mirror-smooth surface without need of polishing.
“There’s still a whole infrastructure and cycle of development that has to occur,” said Neil Kane, president of ADT. “But we’ve demonstrated — though not published yet — that the performance of the devices is as predicted from the properties of diamond.” He noted, however, that “using it as an add-on only takes you so far. You really get the results it can deliver only if you design with the properties of the diamond to start.”
MEMS structures made from diamond do not wear out as fast as silicon, and the naturally hydrophobic surfaces of these structures provide built-in antistiction. Mechanical diamond resonators also vibrate at very high frequencies, useful for improving the performance of RF devices, such as filters and time references operating at frequencies greater than 1 GHz, and for atomic force microscopy (AFM) probe tips used for dynamic mode imaging.
ADT has been working on exploiting the acoustic velocity of diamond for high-frequency RF devices for a rugged and broader-band wireless telecommunications in a project funded by DARPA. Kane argued that the price differential of using diamond for one of the 25-35 process steps would not be noticeable at the die level. “And because the diamond is so robust, it may even make up that cost with what it saves in packaging,” he suggested.
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| ADT is developing diamond RF MEMS resonators for use in broad-band wireless telecommunications systems. The device shown is the micron-scale equivalent of a tuning fork, but is designed to work at much higher frequencies (~1 GHz). (Source: Advanced Diamond Technologies) |
The high-frequency resonance also turns out to be useful for probes for some metrology applications, because the diamond probes can be larger than silicon ones of the same frequency, which get too small to handle the high frequencies needed for some measurements. Also attractive, of course, is the hardness of diamond, which could allow sturdier scanning probe tips to be etched out of the diamond on oxide wafers to outlast those made using silicon or silicon nitride. Diamond is also one of the best thermally conductive materials available, and may be promising for heat spreaders provided issues of integration, flatness and bonding can be worked out.
Nanoimprint lithography edges into production
Nanoimprint may not be ready for next-generation semiconductor lithography quite yet, but throughput is up to 30 wph, and one can now go out and buy a consumer product that contains an optical component patterned with an Obducat (Malmo, Sweden) system. Users are reportedly close to production with nanoimprint for creating structures on top of LEDs to increase light efficiency, and for optical media and hard disk substrates.
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| SEM micrograph of a 30 nm half-pitch pattern area on a Ni-master stamp. The pattern was mastered with an Obducat EBR-30kV TFE electron beam recorder. (Source: Obducat) |
Throughput and yields have been more problematic, however, with the slow e-beam writing required for the master stamp and the stamp having to be pressed repeatedly against the product. Obducat improves throughput and yield by using the costly master to make disposable stamps to be used for the actual imprinting, stamped out in volume on rolls of polymer liner. While one imprint head is stamping out the polymer stamps, a second head presses the polymer stamps into the substrate. Each polymer intermediary stamp is used only once, eliminating cleaning and preventing transfer of contamination to the substrate. Each polymer impression serves to clean the master, as any particles will stick to the soft polymer. And the master does not get damaged by particles pressed between the hard stamp and hard substrate. Obudcat says a 90 wph system is in the works, although hard disk applications would need up to 600 substrates per hour and a different configuration of the tool.
Obducat uses a full-wafer stamp instead of a step-and-repeat system. The full-area imprint enables a much shorter imprint time per wafer; which overall, even though it does take longer to pattern the stamp, offers a more cost-efficient solution. The company does offer an e-beam recorder that works rather like a CD burner, rotating the substrate and blanking the beam for faster writing of consistent patterns than the typical vector-scan approach. But it also argues that the near-term applications for imprint require patterns that cover larger areas for which the larger stamps are more appropriate.
To go with its larger stamp, Obducat uses a soft-press stamping system, which uses pressurized gas to apply pressure to the imprint stamp, allowing it to conform to uneven substrates and potentially apply even pressure across display-sized sheets.
ADT and Obducat are SEMICON West 2007 Technology Innovation Showcase winners, and will present their innovations on Wednesday, July 18, at SEMICON West at the Emerging Technologies & TechXPOT, located on the 2nd floor of Moscone West, along with other invited speakers. to address the latest in MEMS manufacturing and materials innovations. Visit www.semiconwest.org to register for SEMICON West 2007 , held July 16-20 at Moscone Center in San Francisco.
