Getters in Electronics and MEMS Applications

M2N Technologies LLC

The term getter is used to discuss a material that absorbs an unwanted substance. This is most often applied to removing residual gases from a gas stream or vacuum chamber (1). Getter cartridges are used in gas purification, like particle filters, in wafer fabs to remove oxygen, water vapor and nitrogen. Solid state getters have also been used to removing unwanted contaminants from solids like transition metals from silicon, or particles from a microcavity. Silicon wafer getter solutions include precipitated oxygen, a mechanical damaged backside surface, a polysilicon layer and ion damage to the wafer backside (2). Transition metals will precipitate at the dislocations and grain boundaries instead of in the active device regions of the ICs. In this article we will focus on the type of getters used to reduce the cavity pressure of electronics systems and MEMS (MicroElectro Mechanical Systems). 

Both thin film and NonEvaporable Getters (NEGs) have been used in these electronics systems (3-6). Over a century ago the glass vacuum tube was the most advanced form of electronics technology. These electronic parts were sealed in a glass tube under vacuum to enable the hot metal filament to survive a reasonable amount of time without oxidation. It was quickly found that by flash evaporating a volatile metal on the inside of the glass tube, the reliability and lifetime of the product could be increased. The metal film, evaporated by electrically heating a wire after the tube was sealed under a low pressure, acted as a solid-state vacuum pump or getter. The fresh metal surface on the glass walls, which can be seen in Figure 1, absorbed and reacted with residual gas trapped in the glass tube during the sealing operation. Pure metals and alloys of Ba, Ti, Zr, V, Fe, Co and other reactive metals are used in cathode ray tubes, flat panel displays, particle accelerators, semiconductor processing equipment and other vacuum equipment to lower the pressure. These metals trap various gases through oxide, nitride and hydride formation and by simple surface adsorption.

The idea of using metal getters has been applied with MicroElectroMechanical System technology. Several MEMS devices, such as resonators, IR, optical, pressure sensor and field emission devices- like the vacuum tube and tunneling sensors, rely on vacuum packaging for improved performance. The unwanted gases dampen resonators, absorb light or radiation, or reduce tunneling efficiency. The getters improve device performance by dramatically reducing the concentration of these unwanted gas molecules in the package. This thin-film getter technology has been applied to chip-scale vacuum packaging of commercially available gyroscopes, accelerometers, pressure sensors, optical devices, infrared sensors, microfluidic density and chemical concentration meters, Pirani gauges, biological cell and particle detection, drug infusion systems RF MEMS resonators and micromachined Coriolis mass flow sensors.

Microsystem devices that operate under vacuum generally start development in a laboratory bell jar. If functionality is demonstrated, packaging often progresses to a solder or weld sealed ceramic or metal package. If the microsystem has high-volume applications or is exceptionally fragile then chip-level vacuum packaging, preferably performed at the wafer level is developed and integrated into the wafer process flow and chip design, as illustrated in Figure 2. Traditional vacuum wafer bonding methods include anodic, glass frit, eutectic, solder, reactive and fusion bonding. One problem encountered with existing wafer-to-wafer vacuum bonding is relatively high cavity pressures that change with time and temperature. Anodic bonding is known to generate oxygen and result in cavity pressures of 100 to 400 Torr. Solder, frit and eutectic bonding produces cavity pressures around 2 Torr due to surface desorption of gases. Baking wafers prior to solder reflow does reduce the amount of adsorbed water but only lowers the microcavity pressure from 2 Torr to 1 Torr. While the pressure of the vacuum wafer bonding system can get down to the microTorr level, ultimately wafer surface desorption after sealing limits the microcavity pressure of MEMS devices. 

Historically in large-scale vacuum systems, baking of surfaces are used in desorbing gases from surfaces. High temperature and time limits the effectiveness of this method in obtaining ultra-low cavity pressures in microsystems. Shallow BiCMOS junctions, thin film alloying, novel MEMS sensor materials and cantilever warpage limit prebonding bake temperatures. It is to overcome the surface desorption limit found with wafer bonding that getters have been employed. In the early 1990’s NonEvaporable Getters (NEGs) either in tablet or strip form were placed in an extra micromachined cavity or adjacent to the chip in a ceramic package (3). To maximize surface area, the NEG is often fabricated using powder metallurgy techniques in which the sintering of the metal particles is just initiated, leaving gaps between metal beads. A high temperature activation step in vacuum or a hydrogen containing reducing ambient is required to remove the surface oxide layer that forms on the metal particles during the sintering process. This activation step is accomplished by either through annealing the whole package or by Joule heating of the NEG strip. One problem encountered with sintered getters is particle generation. When NEG metal strips are employed, they are typically cut into small segment and hand placed into a microcavity prior to wafer bonding. The strips often bend during cutting, requiring additional manual handling to straighten the pieces. Particles are generated during handling and the cutting process. The 2 to 3 micron diameter metal particles can cause electrical shorts, impede motion and shift resonant frequencies.  A frequency shift occurs due to a mass change in the resonator caused by the attached particle. An opening in the silicon diaphragm separating the NEG from the resonator provides access between the NEG and resonator chamber. Particles that shed from the sintered NEG can also migrate through this opening to the resonating or tunneling element cavity.   For side-by-side cavity designs the die size area is essentially doubled, while for a vertical integration the chip thickness is increased. The size penalty and need for pick and place NEG loading also prevents the NEG method from finding use in high-volume MEMS products. Integrating a thin-film getter can reduce the complexity of the chip-scale package (4-6).  For the wafer bonding process, a capping wafer, generally either silicon or glass is patterned and etched to form both a cavity that will enclose the active micromachine and open up access to the electrical bond pads. Next the getter and the sealing material are placed on the cap wafer. The sealing material may be made up of a reflowed glass layer, gold or eutectic solder. The getter film is generally comprised of a sputtered or evaporated multi-layered or alloyed reactive thin-film metal. Since thin film deposition techniques are employed in a cleanroom environment, the getter is virtually particle free compared to conventional NEGs formed using powder metallurgy. Figure 3 shows the cap and active MEMS wafers prior to bonding. Vacuum wafer-to-wafer bonding is performed next in this process. Figure 3 shows the wafer stack after bonding as well as singulated chips and the cavity coated top cap chip. For many thin-film getters an activation step is not required. In the majority of vacuum packaged microsystems the capping wafer is part of a passive enclosure. Adding the thin-film getter does not impact the chip size. 

Wafer-to-wafer bonding and a thin film getter pushes the microcavity pressure under 1-10 milliTorr, resulting in Q values greater than 60,000 for silicon resonators (5). Without the thin film getter a cavity pressure of 1.4-2 Torr is obtained with the same wafer bond sealing due to squeeze-film damping caused by desorbed, trapped gas. A Q value of 40 is obtained for this wide resonator when vacuum packaged without a getter. The microcavity pressure has been reduced by more than 3 orders of magnitude through the use of the getter. The key to obtaining high Q values in a chip-scale package is the metallic getter placed in the cavity of the cap wafer as illustrated in Figure 2.

Since several vacuum sealing technologies exist and each can result in different residual gases of varying pressures, it would be ideal to have a flexible getter approach. Anodic bonding is known to generate relatively large amounts of oxygen as the sodium in the glass is ionized under bias at high temperature. A thicker getter layer, than needed for other bonding methods, is required to chemisorb the high concentration of oxygen produced by the anodic bond process. Glass frit and solder can outgas water or carbon dioxide, requiring a compound that absorbs hydrogen and carbon. Reactive sealing using an oxide, nitride or polysilicon deposition process would require the ability to getter hydrogen left behind after chemical vapor deposition processing; therefore different getter film combinations can be developed for these applications.  Care must be taken in MEMS design with reactive encapsulation to insure the getter surface is not coated during sealing. This thin film getter technology offers the flexibility to customize the getter layers used and the thickness employed which is of great advantage in changing the getter for different MEMS encapsulation processes. The deposition of getters is not limited to MEMS chips. Both MEMS die and conventional metal and ceramic vacuum packages, see Figure 4, can incorporate a patterned thin film on either the lid or other surfaces. Thin-film getters on wafers can be patterned with traditional photo-etch processes, lift-off resist and shadow masks. Metal photolithography gives the advantage of being able to make small feature sizes. Shadow masking can be used to place the getter in deep cavities but has course linewidth control. Figure 5 shows and example of shadow masks (6) that have been used with both wafers and metal lids employed with soldered or glass reflow sealed ceramic and welded metal packaging.

This new approach to obtaining ultrahigh vacuum in a microcavity has seen widespread use in the field of MEMS. The performance of many resonating devices has been improved through higher Q and gain. High-volume devices like gyroscopes, timing and pressure sensors as well as RF-MEMS chips can benefit from this technology. In addition to improved device performance, the long-term reliability of resonators, tunneling devices, optical and pressure sensors can be improved with lower cavity pressures. In automotive and aerospace applications, sensors and actuators are expected to maintain tight specifications for 5 to 10 years operating at temperatures between -40°C and up to 150°C. Long-term Q and sensitivity degradations, which were partially reversible and traceable to adsorbed gases have been observed with MEMS gyroscopes. Millions of absolute pressure sensors are produced each year for a variety of applications and they are also prone to temperature hysteresis due to packaging stress and from reversible microcavity gas desorption. Long-term reliability improvements have been documented in MEMS sensor performance when thin film getters are employed (7,8). Part to part variation in output and stability were improved through the use of getters. Thin film getters are seeing widespread use not just by the microsystem academic users but the industrial community as well, extending the life of thin-film getters technology that started with the vacuum tube, more than one hundred years ago. These getter films and gas purifiers are commercially available through companies like Materion (formerly NanoGetters), Hi-Rel, SAES and Entegris (formerly SAES Pure Gas). 

References

1. T.Giorgi, “Getters and gettering,” Jap. J. Appl. Phys. Suppl., Vol. 2, p.53-60, 1974.

2. D.Sparks, et al., "Anomalous diffusion and gettering of transition metals in silicon," Appl. Phys. Lett., Vol. 49, p.525, 1986.

3. M.Esashi, S.Sugiyama, K.Ikeda, Y.Wang and H.Miyashita, “Vacuum-sealed silicon micromachined pressure sensors,” Proc. IEEE, Vol. 86, No. 8, p.1627-1632, 1998. 

4. M.Moraja, M. Amiotti, “Getters films at wafer level for wafer to wafer bonded MEMS,” Design, Test, Integration and Packaging of MEMS/MOEMS 2003, IEEE, pp. 346-349, May 5-7, 2003.

5. D. Sparks, et al., “Chip-Level Vacuum Packaging of Micromachines Using NanoGetters,” IEEE Trans. Adv. Packaging, Vol. 26, No. 3, pp.277-282, August 2003.

6. D. Sparks, “Thin Film Getters: Solid-State Vacuum Pumps for Microsensors and Actuators: With MEMS Applications the Use of Thin Film Getters for Vacuum Packaging Passes the Century Mark,” Vacuum Technology & Coating, Vol. 11, No.4, pp.44-49 April 2010.

7. J.Mitchell, et al., “Long-term reliability, burn-in and analysis of outgassing in a Au-Si eutectic wafer-level vacuum packages,” Solid-State Sensor, Actuator and Microsystem Workshop, Hilton Head Island, SC, Jun 4-8,p.376-379, 2008.

8. D.Sparks et al., “Long-term evaluation of hermetically glass frit sealed silicon to Pyrex wafers with feedthroughs,” J. Micromech.&Microengr., Vol.15, p.1560-1564, 2005.