Brian Cunningham's recent posting suggests "gettering" as a method of for
keeping MEMS vacuum packages at good vacuum.

Gettering MEMS vacuum packages is a logical extension of proven, vacuum tube
technology. A distinction can be made between evaporable getters, such barium
films and non-evaporable types. Barium films are classic getters, a one-shot
release requiring a short heat cycle. Barium getters consist of an iron wire
with a hollow core filled with barium thus the material can be handled in air
then released in vacuum by resistive heating, either rf-coupled or applied via
feedthroughs.

I have worked with two types of non-evaporable getters. This was more than a
decade ago. However, SAES Getters Inc used to make several gettering alloy
depending on application. An alloy might be chosen for maximum surface area for
1X activation while another might fill the need of multiple pumping cycles.

I used ST101 alloy in vacuum tubes, lasers and in hydrogen lamps. ST101 is a
reactivateable alloy. From memory, this material is a Zr/Al alloy. Once the
surface is saturated, ST101 can be reactivated by heating. The reacted surface
is driven to equalibrium with the bulk alloy. The getter surface can thus be
refreshed 100's of time. In the reactivation process, most reacted gases are
not relased. H2 is an exception.

Hydrogen soluability in ST101 is a function of temperature. ST101 can store a
significant volume of hydrogen. Thus, hydrogen pressure in a sealed volume can
be controlled by alloy temperature. After reactivation, released hydrogen is
re-absorbed in the bulk alloy. I used this feature to build H2 discharge lamps
with extended lifetimes. Glow discharge lamps with sealed volumes are subject
to failure by gas depletion. The use of ST101 provided a large surplus of H2
with pressure control by alloy temperature.

SAES also manufactures "dispensors". Dispensors are temperature activated
sources of reactive material which allow handling in air. After installation in
a device, reactive materials such as Li and difficult to handle material such a
mercury can be dispensed in vacuum, at known volumes. This is how photocathodes
and thin-film batteries are made.

The address for SAES is:


    SAES Getters USA 1122 E. Cheyenne Mountain Blvd. Colorado Springs CO Zip
    80906 719-576-3200

Sincerely,
Bob Hamilton
o