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:

  1. A getter should be handled only with clean tools or with rubber or plastic gloves, and never with bare hands.
  2. A getter could be cleaned by ultrasonic agitation in highly pure isopropyl alcohol for a few seconds, then dried in an oven.
  3. 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.
  4. Weld the getter into the MEMS package according to the fabrication procedure for the particular MEMS design.
  5. Apply a vacuum pump to the MEMS until the pressure in the vacuum chamber in the MEMS is <10–6 torr (<10–4 Pa).
  6. Activate the getter by heating.
    1. The activation parameters are temperature, time, and method of heating; the amount of time spent at a constant activation temperature is very important.
    2. 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.
    3. The rate of heating during activation should be controlled to prevent excessive outgassing.
    4. Monitor the maximum pressure during activation to obtain an indication of the gas content of the getter.
  7. Allow the getter to cool to its test temperature while still pumping.
  8. 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

Topics:
Elastomers Emissions Exhaust emissions Fabrication Gases Manufacturing processes Materials Materials handling Microelectromechanical devices Packaging Particulate matter (PM) Polymers Pressure Semiconductors Sensors and actuators Vacuum
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Nonevaporable getters to maintain vacuum in sealed MEMS

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NASA Tech Briefs Magazine

This article first appeared in the February, 2000 issue of NASA Tech Briefs Magazine (Vol. 24 No. 2).

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Overview

The document discusses the development and application of nonevaporable getters (NEGs) for maintaining vacuum in sealed microelectromechanical systems (MEMS), particularly focusing on a microgyro designed for space applications. The Jet Propulsion Laboratory (JPL) has been working on a microgyro for the Avionic Flight Experiment mission, which requires a vacuum level of 0.001 Torr or better. Achieving and maintaining this vacuum is critical for the microgyro's performance, as it must withstand the harsh environmental conditions of spaceflight.

The primary challenge addressed in the document is the loss of vacuum due to outgassing from packaging materials and microleaks in the vacuum container. Common gases such as water vapor, hydrogen, carbon monoxide, nitrogen, oxygen, and carbon dioxide can permeate through the packaging, compromising the vacuum. The document highlights that traditional getter materials, such as those previously used by Allied Signal, have not proven effective for ultrahigh vacuum applications, leading to failures in maintaining the necessary vacuum levels.

To solve this problem, the document proposes the use of a family of high porosity, non-evaporable getter materials, including zirconium-aluminum-iron, which have shown significant promise in literature for use in sealed devices. These getters are designed to chemically absorb gases, thereby reducing the outgassing problem and ensuring long-term vacuum stability in MEMS devices.

The novelty of this work lies in the application of these well-known getter materials to MEMS packaging, which is a new approach. The document emphasizes the importance of maintaining a suitable vacuum level inside MEMS packages to enhance their stability, reduce size, and lower costs. It also notes that even if a MEMS device is initially bonded in a high vacuum, maintaining that vacuum over time is critical, as outgassing can occur during the bonding process and throughout the device's operational life.

In summary, the document outlines a significant advancement in MEMS technology by proposing the use of non-evaporable getters to maintain vacuum integrity, thereby enabling the successful operation of sensitive devices like microgyroscopes in space applications. This innovative approach addresses critical challenges in vacuum packaging and enhances the reliability of MEMS devices for long-duration missions.