Controlling pressure in microsystem packages by on-chip microdischarges between thin-film titanium electrodes

This article describes on-chip sputter-ion pumps and their use in the controlled reduction of pressure in large cavity packages. The devices utilize thin-film titanium electrodes patterned on glass substrates and exposed to the cavity environments. Microdischarges between the electrodes sputter the cathodes, resulting in the selective chemisorption of oxygen, nitrogen, and other titanium-reactive gases. dc and pulsed high voltage powering schemes are investigated. Using dc discharges, devices on a 1cm2 footprint remove 168Torr of air from a hermetically sealed commercial package with a volume of 6.33cm3. Starting at 200Torr, the removal rate of air is 7.2Torr∕h; for oxygen it is 11.5Torr∕h and for nitrogen 3.4Torr∕h. The relative humidity is reduced 6%∕h, starting with 61%. The pumps have been shown not to remove helium from the environment. A 5× increase in pump power efficiency is obtained through the use of 25ms pulsed discharges as opposed to dc discharges. The gas removal rates also depend on both initial pressure and discharge gap spacing. A theoretical model outlining the dependency of gas removal rates on microdischarge parameters is reported.