Electrochemical method simplifies MEMS fabrication

The problem lies not with the MEMS devices themselves, but with the semiconductor-based manufacturing techniques deployed to build them. Semiconductor wafer fabs excel at producing high-volume integrated circuits using standard CMOS processing. MEMS devices need to be manufactured in lower volumes, however, and with far more complex structures, such as moving three-dimensional micromirrors instead of planar transistors.

Optical MEMS companies have found that developing precision optical components using silicon micromachining is a slow and expensive process. For example, it was not unusual for a single MEMS prototyping run to last 12 weeks or more. And building a MEMS fabrication facility typically costs $50 million, a reach for even the most lavishly funded of startups. Consequently, optical MEMS companies could not corner the market when demand was high, then had difficulty adapting to meet rapidly changing market conditions when demand softened.

A new approach called Electrochemical Fabrication (EFAB) addresses the unusual requirements of micromechanical device manufacturing. EFAB is a solid free-form fab technology that creates complex, miniature three-dimensional shapes based on 3-D CAD data. Inspired by rapid prototyping techniques, EFAB generates complex shapes by stacking multiple patterned layers. Unlike rapid prototyping, however, EFAB is a batch process suitable for volume production of fully functional devices in engineering materials, not just models and prototypes.

EFAB is based on an in-situ patterning method called Instant Masking that simplifies the fabrication of micromachines. The Instant Mask consists of an insulator patterned on an anode. Instant Masking patterns a substrate by pressing the mask against the substrate, electrodepositing material through apertures in the insulator and then removing the mask from the substrate. The result is a layer rapidly deposited and patterned in a single step. The process is significantly faster than photolithography and makes it possible to fabricate MEMS devices with dozens of patterned layers in a single day, compared with several weeks for a conventional 3-layer MEMS device.

Each level of an EFAB part build comprises both structural and sacrificial material. The block of sacrificial material in which EFAB-built devices are temporarily embedded serves as mechanical support of structural material. Additional material can be deposited over the entire layer without constraint. Thus, the use of sacrificial material eliminates all geometrical restriction, allowing the structural material on a layer to overhang-and even be disconnected from-that of the previous layer. Such geometrical freedom also makes possible monolithically fabricated "assemblies" of discrete, interconnected parts. This eliminates the need for subsequent bonding or assembly steps.

In order to fabricate a multilayer device, the geometries of the layer cross sections are automatically determined based on the desired 3-D geometry using special software. One or more photomasks are generated, which include all the unique cross sections of the device. The photomask can then be used to fabricate the Instant Mask.

EFAB can be used to form structures from any electrodepositable metal or alloy. Its only constraint is that the accompanying sacrificial metal can be selectively etched after the layers are formed. Engineers are working to develop materials other than metal as well, including insulators to enhance the capabilities of the process.

It's possible to use the EFAB process to produce a range of devices such as switches, variable optical attenuators and tuning elements for lasers, as has been done with traditional MEMS. Instead of coating silicon mirrors with metal to increase reflectivity-which creates thermal mismatch warping-EFAB builds all-metal mirrors. The EFAB process is IC compatible and simplifies IC integration with MEMS.

In addition, EFAB is well suited to the manufacture of millimeter scale devices such as optical packages and fiber-alignment aids, a market niche that neither MEMS nor precision machining technology addresses adequately. The EFAB process runs in a single, fully automated machine that does not require a clean-room environment to operate. Therefore, it is now possible to make MEMS in a single machine tool rather than in a costly clean room.

Because EFAB allows unrestricted, 3-D CAD-driven geometry, optical engineers now have the ability to design devices optimally without being limited to flat silicon chips and the two-dimensional design constraints inherent in silicon planar processing.

Consider the following example: To build a micromirror using conventional silicon micromachining, an optical engineer would first have to design the device. Then a process expert would detail a suitable fabrication process flow, after which a set of two-dimensional photomasks would be developed based on the fab process parameters. Finally, a prototype would be built in about 12 weeks or longer, depending on the amount of custom process development required. Post-process assembly would be needed to elevate the mirror from the substrate. Once the testing is concluded, any design modifications would likely entail changes to the fabrication process and more process development, as well as a new 12-week prototyping run to test the new design.

By using EFAB technology, the same micromirror was fabricated from a 3-D CAD file. No process flow design or final assembly was required, and design revisions made to the CAD file were automatically incorporated without changing the fab process. The micromirror looked similar to other MEMS micromirrors, but it required no assembly and took only three days to complete, in a single automated machine located on a factory floor.