The multiorder effects of MEMS requires a well-crafted specification that is produced in a closed-loop practice shared between a MEMS device customer and an experienced MEMS manufacturer. Such a specification can spell the difference between producing a device with reasonable time-to-market and one that never reaches production.
In short, while many semiconductor foundry owners seek to leverage their older fab-line capacity by offering to manufacture MEMS devices for fabless MEMS developers, success goes way beyond the mechanics of manufacturing. A MEMS manufacturing partner must quantitatively understand the specifications required by an application, as well as the trade-offs between MEMS device specifications and end-user system design. The manufacturer must be able to construct an accurate model of a MEMS component to ensure repeatable manufacturability.
MEMS are designed and manufactured using many of the same techniques employed to produce ICs. However, MEMS are mechanical devices requiring a thorough understanding of the inevitable cause-and-effect relationship between the electrical and mechanical domains of device behavior.
Designing MEMS devices requires modeling and physical simulation of a broad set of physics (individual device phenomena) and coupled-physics (interdependent phenomena) domains like structural mechanics, electrostatics, magnetostatics, thermomechanics, electromechanics, electrothermal, piezoresistance-thermomechanics and thermoelectromechanics.
In one example, a customer prepared a specification for a manufacturer to construct a micromachined variable optical attenuator (VOA). MEMS-based VOAs are fast, small and low-cost. They allow gain-flattening of optical amplifiers, as well as power-level equalization of the multiple channels in dense wavelength-division multiplexing (DWDM) optical communications systems.
The MEMS VOA comprises a silicon-on-insulator (SOI) single-crystal shutter vane mounted on a micromachined drawbridge actuator and placed at the end-face of an optical communications fiber. When a driving voltage is applied to the actuator, it moves the shutter vane into the light path and blocks a portion of the laser beam, thereby attenuating the signal.
Among the highly scrutinized functional specifications for this application are polarization-dependent loss (PDL) and optical return loss (ORL). Of these, PDL is the ratio of the maximum and minimum output optical power with respect to all polarization states; ORL is the ratio (in dB) of the incident optical power to the reflected optical power.
The MEMS VOA device can affect both ORL and PDL by introducing small reflections, called backscatter, or Rayleigh scattering, caused by the reflection of light, the result of nonuniformities (like stray particles or physical imperfections) in the shutter vane itself. The reflected light can distort the modulation and spectral characteristics of the primary laser signal and affect data-transmission quality and information throughput.
In evaluating the crucial ORL and PDL restrictions for this application, it was important to know what aspects of the MEMS device contributed to ORL and PDL and how those aspects could be measured during the MEMS manufacturing. It was found that the contribution of the MEMS device to ORL and PDL was directly related to the amount of backscatter introduced by the material characteristics and roughness of the shutter vane surface. The original device specification was altered to minimize the number and density of defects and stray particles in the MEMS device. A special coating was also devised for the shutter vane surface to improve ORL and PDL.
The reliability and performance of MEMS devices is a key issue that can be completely addressed only by direct measurements on small specimens with dimensions on the same order of magnitude as the fabricated microdevices. Such properties depend on the manufacture and process conditions, including the type of substrate, deposition temperature, doping, annealing and chemical etching.
Another example could be a MEMS silicon microphone, a low-cost, high-performance and high-volume alternative to conventional electret condenser microphones. In a silicon microphone, tiny capacitive diaphragm plates vibrate in response to audio stimulus and that vibration generates an analogous electrical signal.
In this case, a correlation was found among the most important application specification, which is microphone sensitivity, and several basic, mechanical MEMS device specifications, namely diaphragm mass, thickness and flatness. Specifically, for high sensitivity, the silicon microphone device must have very thin, flat free-floating diaphragm plates. Ultimately, by understanding the application effect of these particular specifications in the MEMS device, a measurable and testable approach to manufacturing it was devised to have repeatable sensitivity.
The microacoustic MEMS silicon microphone is fashioned from polysilicon to take advantage of highly repeatable semiconductor manufacturing processes. The result is stable acoustic performance and flexibility for future design enhancements. But to ensure that application needs were met with manufacturable consistency, the microphone, like the VOA, required a fully engaged specification process that focused well outside the MEMS device.
The shared, closed-loop specification process not only facilitates the production of a successful device, but each step of the four-stage MEMS development process proof-of-concept, development, preproduction and production is shortened by this approach. The MEMS manufacturer must go to great lengths to correlate measurable and testable parameters in the MEMS device film thickness, stress gradients, material properties, fabricated dimensions, aspect ratios, side wall verticality, surface smoothness, particles with end-application requirements.
The final example is a microphotonics MEMS device for communications infrastructure equipment. This micromachined mirror array device drives a densely populated multiport fiber-optic cross-connect switch. In this device, the mechanical MEMS specification for mirror curvature radius directly affects optical insertion loss.
Ideal mirrors (i.e: perfectly flat with no defects or spray particles) add no insertion loss to the system, but any tiny curves or bows in the mirror surface start to scatter light and introduce insertion loss. Maintaining a very low insertion loss is a key differentiator for the customer's product line, and it is crucial to ensure that the specification of the MEMS device is consistent with this. Knowing that during the specification process gave emphasis to the radius-of-curvature measurement as a key part of the fabrication procedure.
In short, while many semiconductor foundry owners seek to leverage their older fab-line capacity by offering to manufacture MEMS devices for fabless MEMS developers, success goes way beyond the mechanics of manufacturing.
Ron Wages (email@example.com) is president and general manager of the Design and Manufacturing Services Business Unit of Memscap Inc. (Durham, N.C.).