Why Are Getter Materials Used in MEMS Vacuum Packaging?

Micro-Electro-Mechanical Systems (MEMS) combine miniature mechanical structures with electronic components on a semiconductor substrate. Devices such as gyroscopes, accelerometers, pressure sensors, and micro-resonators often contain movable structures that must operate inside a carefully controlled environment.

For many of these devices, maintaining a vacuum inside the package is essential. A low-pressure environment reduces unwanted gas interactions, minimizes damping, and helps achieve stable device performance.

However, sealing a MEMS device inside a vacuum package does not guarantee that the vacuum will remain stable throughout its operating lifetime. Gas molecules can be released from internal surfaces, packaging materials, and device structures. Small leaks and gas permeation can also increase the pressure inside the cavity.

To address this challenge, manufacturers integrate getter materials into MEMS vacuum packages. These materials capture selected residual gas molecules and help maintain the required internal pressure over time.

What Are Getter Materials in MEMS Packaging?

Getter materials are specially selected substances that remove certain unwanted gas molecules from an enclosed environment through chemical or physical interactions.

In MEMS vacuum packaging, getters are usually integrated into the package cavity as thin films or small solid structures.

They function as internal gas-absorbing materials that help maintain the vacuum after the device has been hermetically sealed.

Common getter materials include:

• Titanium (Ti): A reactive metal commonly used in thin-film getter structures.
• Zirconium (Zr): Offers strong chemical affinity for several residual gases.
• Vanadium (V): Used in getter alloys and multilayer materials.
• Hafnium (Hf): Used in selected getter material systems.
• Zirconium-based alloys: Used to improve gas absorption capacity and activation characteristics.

Some commonly investigated non-evaporable getter compositions combine titanium, zirconium, and vanadium.

The material is selected according to its gas absorption capability, activation temperature, available surface area, and compatibility with the MEMS fabrication and packaging processes.

Why Is Vacuum Maintenance Important in MEMS Devices?

Many MEMS devices contain tiny moving structures such as proof masses, resonating beams, and suspended mechanical elements.

When gas molecules are present inside the package, they interact with these moving structures and can produce unwanted mechanical damping.

Maintaining a suitable vacuum helps reduce these interactions.

1. Reduces Mechanical Damping

In devices such as MEMS gyroscopes and resonators, gas molecules can resist the movement of mechanical structures. Lower gas pressure reduces this damping effect.

2. Improves Quality Factor (Q-factor)

The quality factor describes how efficiently a resonating system stores energy relative to its energy loss.

For vacuum-dependent resonators, reducing gas damping can improve the Q-factor and support more stable operation.

3. Improves Sensor Sensitivity

Unwanted gas damping can influence the movement of suspended structures. Maintaining an appropriate vacuum helps preserve the intended mechanical response.

4. Supports Long-Term Reliability

Changes in cavity pressure can alter the operating characteristics of MEMS devices. Getter materials help control residual gas levels and maintain a more stable internal environment.

The required vacuum level depends on the MEMS device design and its intended application.

What Types of Getter Materials Are Used in MEMS Vacuum Packaging?

Getter materials can be classified according to how they capture gases and how they are integrated into the package.

A. Non-Evaporable Getters (NEGs)

Non-evaporable getters are widely used in MEMS vacuum packaging because they can be integrated as thin films without releasing getter material into the cavity.

Common examples include titanium, zirconium, vanadium, and their alloys.

Their important characteristics include:

• High chemical reactivity after activation.
• Ability to capture several reactive residual gases.
• Compatibility with thin-film deposition and patterning.
• Potential for integration directly into wafer-level packages.

NEG thin films are particularly useful where package space is limited.

B. Evaporable Getters

Evaporable getters operate by heating a getter material until it releases reactive material that deposits onto internal package surfaces.

The deposited material then captures selected gas molecules.

Although this approach is used in some vacuum-sealed devices, it is generally less convenient for compact MEMS cavities because uncontrolled deposition can contaminate sensitive structures.

C. Moisture Getters

Moisture getters are designed to capture water vapor inside the package.

Water vapor can be released from package materials and internal surfaces, potentially affecting sensitive MEMS structures.

Depending on the packaging design, dedicated moisture-absorbing materials may be used alongside NEG films.

What Are the Challenges in Getter Integration?

Although getter materials help maintain vacuum conditions, integrating them into MEMS packages requires careful process control.

Some important challenges include:

1. Getter Activation Temperature

Most NEG materials require thermal activation to expose their reactive surfaces. The activation temperature must be compatible with the MEMS device, bonding materials, and packaging process.

2. Limited Package Space

MEMS packages have very small internal cavities. The getter must occupy minimal space without interfering with moving structures or electrical connections.

3. Limited Gas Absorption Capacity

Every getter has a finite gas absorption capacity. Once its available capacity is consumed, its ability to remove additional gas decreases.

4. Surface Passivation

Exposure to oxygen, moisture, and other contaminants can create a surface layer that reduces getter activity. Suitable deposition, storage, and activation conditions are therefore important.

5. Material Compatibility

Getter materials must not react undesirably with nearby package materials, electrical connections, or mechanical structures.

6. Particle Contamination

Getter particles or unwanted material release can interfere with moving MEMS components. Thin-film getters are often preferred because they can be patterned and integrated without placing loose getter particles inside the cavity.

These challenges make getter material selection and process integration important aspects of MEMS vacuum packaging design.

Why Are Getter Materials Important for Advanced MEMS Applications?

As MEMS devices become smaller and more sensitive, maintaining a stable internal environment becomes increasingly important.

Getter materials support several applications:

• MEMS Gyroscopes: Help maintain low cavity pressure to reduce mechanical damping and support stable sensing performance.
• MEMS Accelerometers: Help preserve the operating environment of suspended proof-mass structures.
• Micro-Resonators: Support high-Q operation by reducing unwanted gas damping.
• Infrared Sensors and Microbolometers: Help maintain vacuum conditions that reduce unwanted heat transfer through the cavity gas.
• RF MEMS Devices: Support stable mechanical and electrical performance in vacuum-dependent structures.

Getter integration is especially valuable in wafer-level vacuum packaging, where multiple MEMS devices can be sealed simultaneously at the wafer level.

By incorporating thin-film getters into the package cavity, manufacturers can improve vacuum stability without significantly increasing package size.

Conclusion

Getter materials are essential components in many MEMS vacuum packaging technologies because they help remove residual gases and maintain a stable low-pressure environment after sealing.

Non-evaporable getter films made from titanium, zirconium, vanadium, and related alloys are commonly investigated for this purpose. After suitable activation, these materials capture selected gas molecules released from internal surfaces or introduced through leakage and permeation.

By controlling cavity pressure, getter materials help reduce mechanical damping, support high-Q performance, and improve the long-term stability of vacuum-dependent MEMS devices.

As MEMS technology continues to advance, thin-film getter integration remains an important approach for achieving reliable wafer-level vacuum packaging.