Nonevaporable getters have been proposed for use in maintaining vacuum inside hermetically sealed microelectromechanical systems (MEMS) that rely on vacuum for proper operation. A vibratory microgyroscope is an example of such a MEMS. The proposed getters could also be used in such vacuum components as cathode-ray tubes, microwave tubes, conventional electron tubes, plasma display devices, particle accelerators and colliders, vacuum thermal insulation, ultrahigh-vacuum systems for processing semiconductors, x-ray tubes, lamps, and field-emission display devices.
The need for getters arises as follows: Over time, the vacuum inside a sealed MEMS (or inside any other sealed vacuum device, for that matter) is degraded by outgassing of common atmospheric gases and packaging-material vapors from the surfaces of the vacuum chamber, and by diffusion and/or microleaking of these and other gases. Getters are materials that help maintain vacuum by chemically sorbing gases. Getters have been used in vacuum electron devices since the early years of electronic technology, but until now, there has been little systematic effort to incorporate them into MEMS.
The proposed getters would be components of the MEMS vacuum packaging in which they would be installed. They could be fabricated in simple planar shapes or in more complex three-dimensional shapes. They would be made from Zr-Al-Fe, Zr-V-Fe, or other suitable materials (SAES Getters USA Inc., CO, USA). They would be made highly porous to facilitate access of gases and to provide high active surface area for sorption. The getters would be required to be mechanically stable in the sense that they must endure any vibrations or shocks during use and must not shed particles (which could interfere with MEMS functions) at any time during fabrication, activation, or use.
In general, getters must be activated prior to use. Activation is an integral part of the process of fabrication of the device in which a getter is installed. A getter of the proposed type would be handled, installed, and activated according to the following instructions:
- A getter should be handled only with clean tools or with rubber or plastic gloves, and never with bare hands.
- A getter could be cleaned by ultrasonic agitation in highly pure isopropyl alcohol for a few seconds, then dried in an oven.
- For long-term storage, a getter should be placed in a clean, dry environment; e.g., a phosphorus pentoxide or silica gel dessicator or a dry nitrogen atmosphere.
- Weld the getter into the MEMS package according to the fabrication procedure for the particular MEMS design.
- Apply a vacuum pump to the MEMS until the pressure in the vacuum chamber in the MEMS is <10–6 torr (<10–4 Pa).
- Activate the getter by heating.
- The activation parameters are temperature, time, and method of heating; the amount of time spent at a constant activation temperature is very important.
- The temperature of the getter should be monitored by use of thermocouples, at least until the heating parameters (e.g., heater power and time) that yield the required activation temperature for the required time have been established.
- The rate of heating during activation should be controlled to prevent excessive outgassing.
- Monitor the maximum pressure during activation to obtain an indication of the gas content of the getter.
- Allow the getter to cool to its test temperature while still pumping.
- Carefully pinch off and seal the vacuum chamber in the MEMS package.
This work was done by Rajeshuni Ramesham of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category. NPO-20617
This Brief includes a Technical Support Package (TSP).
Nonevaporable getters to maintain vacuum in sealed MEMS
(reference NPO-20617) is currently available for download from the TSP library.
Don't have an account? Sign up here.