Evan D. H. Green

Packaging of MEMS devices

This paper examines issues in the packaging of silicon-micromachined devices. Standard microelectonics packaging seeks to physically isolate the integrated circuits from harmful elements in the environment, to provide mechanical strength for the die, to facilitate thermal dissipation, and to sustain electrical communication with outside circuits. However, due to the many novel applications of MEMS devices, new sets of packaging requirements need to be met and further research on packaging technology to meet non-traditional requirements are needed.

Self-aligning spatial filter

In this paper we describe a prototype self-aligning spatial filter (SASF). We present studies of the design and the results of fabrication prior to the final processing step. The SASF consists of an electrostatically actuated platform on which an optical spatial filter (pinhole) has been fabricated. The pinhole is in the center of a four quadrant split-cell photodetector, which serves as the alignment gauge for the system. When a focused beam at the pinhole is aligned, all four detectors sense the same optical current. In future devices, this information from the photodetectors will be fed back to the electrostatic actuation system to push the platform and align the beam. The electrostatic actuators are formed from the parallel walls of vertical side- wall capacitors built between the silicon bulk and the movable platform. Electrical signal paths in the integrated system used diffused interconnects, while the photodetectors are simply reverse-biased p+n diodes. Fabrication techniques are similar to surface micromachining, except that a wafer bonding step is used to create single crystal structures.

An interdisciplinary approach to teaching and learning

An interdisciplinary course in semiconductor processing has been developed successfully introduced into the chemical, materials and electrical engineering curricula. Teams of students drawn from a range of engineering and scientific disciplines build microelectronic devices using a 5 mask PMOS metal gate process and perform open-ended experiments to improve this process. The team approach is set in the context of a start-up company culture and professional work environment to give students the opportunity to be actively engaged constructing their own knowledge. The wide spectrum of fields and experiences that students bring to the course mirror the highly interdisciplinary nature of microelectronics device processing which requires knowledge of physics, chemistry and chemical, materials and electrical engineering. The results of surveys given to the students taking the course over the past three years (1995-97) were used to assess the success of this approach and to deduce the most important features for effective teamwork in this context.

Self-aligning pinhole system

A practical self-aligning pinhole (SAP) system, capable of actively aligning a pinhole to an incident optical beam, has been demonstrated. The enabling technology for the SAP is a silicon micromachined pinhole (SiMP). The SiMP is an example of a simple optical element fabricated from silicon in order to take advantage of both the mechanical structure allowed by micromachining technology and the electrical structures allowed by semiconductor technology. To complete the transformation from an enabling technology to a working system, development was necessary in packaging, mechanical mounting and operation, and algorithms.

Highly stable six-axis alignment mechanism

One of the major challenges for typical opto-mechanical assemblies is that they require multiple degrees of freedom with large travel (several millimeters) but very small (sub-micron) resolution. After adjustment, assemblies must be stable to a few nanometers to survive environmental and mechanical shock over a lifetime of use. Using parts with engineered mating surfaces, we have developed a low-cost and robust set of components with demonstrated sub-50-nm adjustment resolution and comparable stability after multiple environmental stress events. For this work, we have adopted -30 to +70 C temperature cycling and 10 G (15 ms) half-sine shock as our environmental qualification standards. We apply the methodologies of reliability testing learned for Telcordia qualification of passive fiber optic components to opto-mechanical components and assemblies for capital equipment instruments. Demonstration of sub-50-nm resolution and stability for our developed opto-mechanical components requires a suitable test stand, which we have developed using scanning knife-edge beam profilers and a highly-repeatable kinematic loading base with a built-in reference. We use these test results to develop system error budgets in design and manufacture based on component, assembly, and measurement tolerances. The developed opto-mechanical assemblies have been demonstrated to have sub-50 nm stability in laboratory and field tests.

Combined split-cell and linear position-sensing diodes

By fabricating a discrete cell photodiode with multiple edge contacts, and allowing for operation under two different current sensing schemes, a diode which operates optionally as either a continuous or discrete cell position sensitive photodetector has been realized. Inclusion of on-chip CMOS switching circuitry enables a user of such a photodetector to change its operating mode by the use of a single digital input line. Experimental results of the dual mode photodiode with switching circuitry are presented in this paper.