Key Points of Helium Mass Spectrometry Leak Testing for MEMS Package Hermeticity
MEMS devices (such as pressure sensors, accelerometers, gyroscopes, microphones, infrared/optical micro-structures, etc.) typically rely on micro-cavities, thin films, cantilevers, or resonant structures to perform sensing and signal transduction. Packaging is not only responsible for electrical interconnection and mechanical protection, but also directly determines the internal cavity pressure, gas composition, humidity level, and cleanliness. Once a micro-leak occurs, the cavity environment drifts over time, which can lead to zero-offset shift, sensitivity degradation, reduced Q-factor, increased noise, and ultimately device failure. Therefore, hermeticity verification is a critical component of MEMS reliability assessment. Helium mass spectrometry leak detection, with its high sensitivity, quantitative capability, and traceability, has become one of the primary methods for MEMS fine leak evaluation.
1. Why MEMS Micro-Cavities Are Highly Sensitive to Leakage
Compared with traditional ICs, MEMS performance is strongly coupled with the “packaged internal environment,” making the impact of micro-leaks faster, stronger, and more difficult to mitigate.
First, micro-cavity volumes are extremely small, typically at the microliter level or below. Even a very low leak rate can significantly alter cavity pressure and gas composition per unit volume, causing measurable drift within a relatively short time. Second, many MEMS devices require specific internal gas conditions. For example, resonant MEMS often rely on vacuum or low pressure to achieve high Q-factors, while some devices depend on inert or damping gases to tune dynamic response. Leakage changes damping, thermal transfer paths, and internal pressure equilibrium, thereby shifting frequency response and temperature behavior. Third, the effects of moisture and contamination are amplified in micro-cavities: adsorption, condensation, corrosion, or stiction of micro-structures can result in irreversible damage.
Hence, for MEMS devices, “qualified hermeticity” means not only survival, but also long-term stability of key parameters and batch-to-batch consistency.
2. Helium Mass Spectrometry Principles and Interpretation of Leak-Rate Metrics
Helium mass spectrometry uses helium as a tracer gas. The core procedure is to evacuate the test chamber under vacuum, then identify and quantify helium ions entering the system through a mass spectrometer, thereby obtaining the leak rate. Helium is particularly suitable for fine-leak testing because of its small molecular size, chemical inertness, low atmospheric background, and minimal interaction with packaging materials.
Leak-rate readings should be interpreted at the system level in two ways:
a) Leak rate represents the “strength of the leakage path” in a package, i.e., the amount of helium transmitted per unit time. It is a quantitative, comparable, and calibratable metric.
b) The same leak rate does not produce the same environmental drift across different cavity volumes. The smaller the micro-cavity, the faster a given leak rate changes internal pressure and gas composition. Therefore, leak-rate thresholds should be defined based on cavity volume, target lifetime, and environmental sensitivity, rather than directly adopting generic IC criteria.
3. Mainstream Helium Leak-Test Methods for MEMS Hermeticity
For MEMS micro-cavity structures, laboratories commonly use three helium leak-test approaches. The selection depends on package type, target leak-rate level, and whether leak localization is required.
Vacuum spray method (Vacuum Spray): The device is placed in a vacuum leak-test chamber, helium is sprayed onto suspected external regions, and any rise in helium partial pressure is monitored. This method offers fast response and good localization, making it suitable for packages with clear structural leak paths. However, for ultra-low leak rates or deep micro-cavity structures, sensitivity may be limited.
Pressurize-and-vacuum method (Pressurize + Vacuum Detect): The device is first exposed to pressurized helium for a defined soak period, allowing helium to penetrate through micro-leaks. It is then transferred to a vacuum chamber where helium outgassing is measured. This method provides high sensitivity to fine-path leaks and stable quantitative results, making it well suited for pre-production screening and batch-consistency analysis.
Bombing test (Bombing Test): The device is held in high-pressure helium for a certain duration so helium diffuses into the cavity, then transferred into a vacuum leak-test chamber to measure helium release. Bombing is the primary approach for MEMS fine-leak detection, especially for high-reliability ceramic, metal, or glass packages. For very small cavities or extremely low target leak rates, helium charging and outgassing kinetics can strongly affect measurement repeatability, so process windows must be validated carefully.
4. Process-Window Control: The Key to Reliable Results
In MEMS helium mass spectrometry testing, the process window directly determines result validity and repeatability. The following parameters are particularly critical:
Bombing pressure and soak time
These parameters define the amount of helium entering the cavity. Too low a pressure or too short a soak leads to insufficient helium charging and underestimated leak rates. Excessive pressure or overly long soak may increase material adsorption, background helium, or surface residues, resulting in overestimated leak rates. The window should be validated and fixed based on cavity volume, material permeability, and target leak-rate grade.
Transfer and dwell time
If the transfer time between bombing and measurement is too long, helium may escape prematurely and reduce measured leak rate. If too short, surface-adsorbed helium may cause false indications. A standardized transfer path and consistent dwell time should be defined, supported by blank parts or reference samples to confirm effectiveness.
Background helium and system cleanliness
Residual helium in the vacuum system, permeation through seals, and chamber memory effects can elevate the background. A background-stability monitoring plan and periodic calibration are required, using standard leaks or reference devices to confirm system sensitivity and repeatability.
Temperature control
Temperature changes influence material permeability and gas diffusion rates, shifting effective leak-rate readings. For stringent reliability programs, bombing and measurement should be conducted under controlled temperature conditions.
5. Data Interpretation and Common Sources of Deviation
For MEMS devices, leak-rate evaluation should not rely solely on single-point thresholds. A “statistical distribution + mechanism consistency” approach is recommended. Production leak-rate data frequently show long-tail distributions; analyzing batch distribution shapes helps identify process drift and latent failure risks. For highly stable devices, accelerated humidity-thermal or pressure-cycling tests can be used to verify the correlation between leak-rate thresholds and electrical-performance drift.
Common sources of deviation include:
a) Virtual leaks / surface adsorption: residual helium on surfaces or helium desorption from materials can cause false positives.
b) System micro-leak superposition: micro-leaks in fixtures, connectors, or tubing may raise measured values.
c) Micro-cavity kinetic effects: extremely small cavities may exhibit non-linear behavior such as insufficient charging or rapid outgassing; comparisons under different dwell strategies are needed to confirm that readings represent true internal leakage.
These effects can be significantly reduced through reference-sample comparison, blank verification, window re-validation, and process-capability analysis.
Hermeticity testing for MEMS packages is a system-level effort. It requires both an understanding of how micro-cavity structures respond sensitively to leakage and the establishment of a validated, repeatable process window and interpretation framework for helium mass-spectrometer leak detection. With an appropriate test method and rigorous control of key parameters—bombing pressure/time, dwell, background helium, and temperature—helium leak testing can provide reliable and traceable quantitative evidence for MEMS package integrity and long-term stability.
In practical projects, many teams work with third-party laboratories that have vacuum/mass-spectrometry leak-testing capability and solid MEMS reliability experience to jointly define process windows, verify thresholds, and improve lot-to-lot consistency. Professional platforms such as Rapid Rabbit Laboratory typically offer effective support in method selection, parameter optimization, and data interpretation, helping MEMS products complete reliability qualification more robustly.