Developing MEMS for Marine Sensors
11/01/2006
BY SCOTT SAMSON, Center for Ocean Technology, University of South Florida AND MIKE MARTEL, J.P. Sercel Associates
MEMS is an enabling technology allowing for the development of smart products, augmenting the computational ability of microelectronics with the perception and control capabilities of microsensors and microactuators and expanding the space of possible designs and applications. It involves the integration of mechanical elements, sensors, actuators, and electronics on a common substrate through microfabrication technology. While the electronics are fabricated using integrated circuit (IC) process sequences, the micromechanical components are fabricated using compatible “micromachining” processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices.
Microelectronic ICs can be thought of as the “brains” of a system and MEMS augments this decision-making capability with “eyes” and “arms,” to allow microsystems to sense and control the environment. Sensors gather information from the environment through measuring mechanical, thermal, biological, chemical, optical, and magnetic phenomena. The electronics process the information and direct the actuators to respond by moving, positioning, regulating, pumping, and filtering, thereby controlling the environment for some desired outcome or purpose.
The University of South Florida’s Center for Ocean Technology in St. Petersburg has been working on the development of MEMS for use in marine sensor applications, among others. Engineers at the Center were looking for a technology that would allow them to micromachine micron-scale features on MEMS with precise accuracy and controllability. A MEMS device incorporating tiny movable mirrors that are approximately 100-µm wide, is key to these optical devices’ operation. At these dimensions and tolerances, mechanical material processing was no longer feasible, so UV laser (excimer) micromachining technology was considered.
 Figure 1. Scanning electron microscope image of moveable MEMS mirrors, prior to bonding the vertical mirrors. |
The project goal, in addition to developing the MEMS devices, was to develop reliable and practical methods for batch fabrication, and it became clear that traditional semiconductor processing equipment might not be adequate. Because the center is an R&D lab at the university, a broadly applicable tool that could be reconfigured, operate at different wavelengths, meet strong criteria, and that could process different materials was desired. An excimer micromachining system* configured to operate at two separate UV wavelengths - 248 or 193 nm - was the solution. It provides up to 20 watts of power on target at 248 nm, with up to 500Hz rep rate, and operates with a nitrogen-purged beam delivery system (BDS) at 193 nm, since shorter UV wavelengths are weakened or absorbed when transmitting through ordinary air. However, shorter wavelength UV is more readily absorbed by a wider range of materials, and thus able to process materials optimally that cannot be processed as well - or at all - using longer wavelengths or other technologies. This system is also being used for other applications including micromachining diamond.
Excimer (“excited dimer”) lasers are gas lasers that generate large-area square or rectangular laser beams, and are used for patterning and large-area processing, typically to irradiate a photomask, which is then demagnified and intensified onto a workpiece. They offer high average power compared to DPSS lasers; the DPSS laser’s power might be a few watts at 266 nm; or upwards of 5-10W at 355 nm. The most common excimer lasers are in the 50-100W range for industrial production use. Effective materials processing with excimer lasers occurs at relatively large areas of focus, i.e., a 500mJ UV beam at 1 J/cm2 target fluence theoretically can expose an area of up to 7 × 7 mm. The large beam of an excimer laser can be projected onto a mask to micromachine specific shapes and patterns; this is known as near field imaging. The features contained in a pattern are then etched into the target material at a magnification determined by the relative positioning of the optical elements.
 Figure 2. Completed MEMS corner cube retroreflector device created using laser-assisted packaging, and viewed through a final Pyrex package lid. |
Excimer lasers are available in various wavelengths. By adjusting or changing optics, gases, or other parameters, output wavelengths can be changed to 157, 193, or 248 nm, or others, making a number of discrete wavelengths achievable, including the same wavelengths usable with DPSS UV systems, e.g., 355 and 266 nm. Each type of laser offers different processing opportunities.
Laser micromachining begins with material removal through the laser photo-ablation process. Pulses of laser energy are absorbed by thin layers of material. The laser energy breaks the molecular bonds of the material, in effect “vaporizing” it. The material is ejected as plasma plume, and there is little or no heat-affected zone around the target area; heat is carried away by the plume. The repetition rate of the laser is rapid, and each pulse removes a controlled, measured, sub-micron-thick layer of material. Control of the repetition rate and number of pulses controls depth with sub-micron accuracy.
MEMS designed for use in sensors for marine applications, and for other environmental sensing and optical communications, incorporate tiny moving optical parts, including mirror structures. The eximer laser is used for post-process assembly of several components. The base silicon wafer contains arrays of small movable mirrors, built up using typical MEMS semiconductor process steps.1 Additional orthogonal vertical mirror surfaces are created using a different MEMS process** and bonded to the base wafer, in this case, to create a tiny corner cube retroreflector for data communications. Laser technology is used in conjunction with traditional MEMS fabrication methods that, though not as capable as the laser for more complex applications, are nonetheless cheaper and reliable for batch fabrication. In this case, the passive vertical mirror surfaces can be fabricated using potassium hydroxide etching, a wet etch on silicon process. Once the wet etching is done, the laser can be used to micromachine and fabricate more complex structures or elements. Another difference between etching and laser micromachining is the ability of the laser tool to create or assemble complex 3-D structures.
UV laser materials processing provides the ability to micromachine sub-micron features and drill holes in tolerances that simply cannot be achieved with mechanical drilling or machining tools. The lack of a heat-affected zone allows for clean, sharp, and precise processing without cracking, melting, burning, or other problems associated with some types of laser processing.
Conventional chemical or plasma etching techniques provide limited options for the geometries that they can produce.2 In addition, etching is often not possible or recommended for photonic applications where sensitive optical devices are integrated onto silicon wafers. Short-pulsed UV lasers provide an alternative to micromachine a wide range of features, because excimer lasers are capable of 3-D micromachining. Adequate beam illumination and projection techniques allow for sharp-edged and uniform energy density distribution on target. This leads to fine volume control of material removed per pulse leading to high machining-resolution and high surface-finish quality. Typical removal rates are between 0.05- to 1-µm per pulse, for repetition rates up to 400 Hz (for high-pulse energy lasers). When a single pattern must be repeated, the mask may contain an array of features, using the large beam cross section for simultaneously machining multiple features. The use of CAD conversion software minimizes set up time for new parts and allows efficient and high quality machining of virtually any geometry. Additionally, by adequately moving the mask and the workpiece in coordinated opposing motion (COMO) larger complex patterns can be created.
 Figure 3. Image of an assembled corner cube retro-reflector. |
The Center’s application required two mirror surfaces orthogonal to the MEMS mirror arrays. Wet chemical etching was used to fabricate one of these vertical mirrors onto a separate silicon wafer that was previously bonded to a Pyrex glass wafer. Then a metallization process is performed, where a titanium/gold layer is sputter-coated to make it more reflective. The gold also creates a metal-to-metal bonding surface, so the gold mirror tops are flipped upside-down, and compression-bonded onto a gold pad located where the vertical mirror will reside. The high-temperature thermo-compression bond fuses the gold on the MEMS structures with the gold on this vertical mirror structure. Now that the mirror is bonded to the MEMS, there is still a glass hold component that needs to be detached from the mirror. Because these parts are quite fragile, a process of laser micromachining, from the glass side of this stack through the Pyrex, was developed; the glass is partially transmissive at 248 nm. When the laser energy hits the interface between the glass and the silicon wafer, it de-bonds that interface, and runs along the length of the mirror where it’s bonded to the glass, and the glass wafer lifts off. Another similar mirror chip, which also includes an integrated package frame, is then bonded to the first parts. The process can be continued for additional components.
A challenge of UV laser micromachining is the potential for debris redepositing on the tiny MEMS structures from the micromachining process. The use of a water-soluble resist, which can be washed off with the debris after micromachining, can alleviate this problem. The potential exists for mirrors or parts to ‘stick down’ following water evaporation, although, for the time being, it is not a problem.
Conclusion
The ability to create 3-D structures with UV lasers such as tapered holes, cones, and other shapes and structures offers promise and potential, and there are a number of future applications, such as making micro-lenses and similar small optical devices, where this will be applied.
* J. P. Sercel Associates (JPSA).
** developed by graduate student Rahul Agarwal
References
- Scott Samson, Rahul Agarwal, Sunny Kedia, Weidong Wang, Shinzo Onishi, and John Bumgarner, “Fabrication processes for packed optical MEMS devices,” Proc. ICMENS 2005 Banff, Alberta, Canada, p. 113-118.
- “The Applications of Excimer Lasers in the Semiconductor Industry,” Marco Mendes, Sylvia Matos, Patrick Sercel, Jeff Sercel, presented at ALAC 2005.
SCOTT SAMSON, Ph.D., optoelectronic MEMS engineer, may be contacted at the Center for Ocean Technology, University of South Florida; 727/549-6649; E-mail: [email protected]. MIKE MARTEL may be contacted at J.P. Sercel Associates, 17D Clinton Drive, Hollis, NH 03049; 603/595-7048; E-mail: [email protected].