Germanium emitters questioned

10/01/1998

Germanium emitters questioned

I read with great interest "Deposits on Semiconductor Corona Emitters in Cleanroom and Simulated Air" (July, p. 249) by Dr. Charles Noll of Simco Corp. While I question the relevance to the semiconductor industry of testing in artificial atmospheres and Class 100 nonsemiconductor cleanrooms, I am writing about the inadvisability of introducing germanium ionizer emitter points into silicon manufacturing.

Ion Systems has actively researched emitter point materials for corona ionizers. Our research has taught us to disbelieve claims of zero particle production from any semiconductor production equipment [1]. Also, particulate generation is not the sole factor that must be considered when selecting emitter point materials.

Germanium affects silicon device performance. Similar to metallic emitter points, any particles produced by germanium points have potential to be "killer" particles. Single crystal silicon emitter points have avoided this possibility since 1992. Used in a well-controlled process, germanium can have benefits. Researchers are investigating the properties of the heterojunction bipolar transistor, combining silicon and germanium junctions to provide improved high frequency characteristics [2].

A silicon-based semiconductor with high charge-carrier mobility and saturation velocity can be designed through selective incorporation of germanium atoms in the lattice of silicon during the growth of silicon films.

Given their metallic-like qualities, germanium emitter points have the potential to change the properties of silicon in an uncontrolled fashion, introducing an unknown risk into the manufacturing process. Other industries using ionizers will need to evaluate the results of long term testing to see if germanium points can be substituted for the titanium, tungsten alloy, and silicon emitter points used today.

I would be very interested in comments from Dr. Noll and others in the semiconductor and FPD industries.

Arnold Steinman, M.S.E.E.Chief Technology OfficerIon Systems Inc.ph: 510/548-3640; [email protected]

1. A. Steinman, "Test Chamber Measures Particle Emissions from Ionizers," Cleanrooms, Nov. 1993, a follow-up to: M. Yost, "Apparatus for Measuring Ultrafine Particle Emissions from Air Ionization Equipment," Particles in Liquids and Gases, Vol. 2, Plenum Press, NY, 1990.

2. J.D. Cressler, "Re-engineering Silicon: Si-Ge Heterojunction Bipolar Technology," IEEE Spect., Vol. 32, No. 3, p. 49-55, 1995.