What is on-wafer / unpackaged MEMS testing?

Until recently companies who needed to test sensors and actuators had two possibilities: They could make limited electrical tests during the stages before packaging and they could test fully packaged devices with the non-electrical input and/or output the sensor needed to work.

An electrical test can only give very basic information about certain parameters of the die but cannot fully analyze the function of the sensors and actuators.

If the MEMS device is still in the design stage time to data is of the essence and a complete analysis at wafer level and throughout the various packaging stages provides a valuable insight into the design characteristics whilst also enabling process monitoring. In production, waiting to test until the sensor is packaged means that bad dies get taken right through the production process before being detected.

Identification of Known Good Dies (KGD) before packaging will most likely form the single most profitable activity in the manufacturing process when a new microsystem is launched into the market. It has been suggested that the production costs and the high price to market are the main reasons for the slow investment into the MEMS sector. Studies show that 80% of manufacturing costs are caused by the packaging process, by testing at earlier stages of production, the packaging costs for bad dies can be saved and thus both production costs and end product price can be reduced. By testing at several production stages, possible errors can be identified and eliminated resulting in an optimized process and higher yields. MEMS manufacturers who use foundries need to build in quality control mechanisms into the incoming wafers just as the foundries need to prove the quality of their products before shipment.

However vital and advantageous it seems, it is not altogether easy to test MEMS before they are packaged.

Typical cost breakdown of the manufacturing stages of an airbag accelerometer

MEMUNITY PAC150: A specially constructed vacuum probe system for probing at 70K

Currently MEMS testing is mainly performed using techniques and instrumentation designed for testing of CMOS devices and packages. However, these techniques cannot test specific MEMS-related issues such as moving parts, temperature, humidity, pressure, sound, particles, gases etc. Special chambers, probes, sample holders, test structures, detection systems, sample preparation techniques and electronics are required. Equally parameter analysis is not straightforward: Movable metallic parts, such as in RF-MEMS switches, may be prone to creep and fatigue..

There are many issues to be considered when testing MEMS which are substantially different to those for other electronic products:

• The environment at the point of sensing or actuation is often very harsh thus the test set-up needs to be able to test influences such as temperature, high-G vibrations, pressure and vacuum.
• The device often has openings through which the medium carrying the sensor/actuator signals is exposed directly to the microsystem chip inside the package. This means that the device is exposed to unwanted environmental influences and must be tested in a completely shielded environment such as a vacuum chamber.
• The materials and packaging techniques used for microsystems devices are normally device-specific or application-specific and tend to have failure mechanisms, which differ substantially from those of other electronic components and systems. This requires a high level of equipment customization.
• Many MEMS are used in safety-critical applications where long-term reliability is critical (e.g. medical, safety devices in cars, gas detection and aerospace applications).
• Substrate handling is often difficult as because common pick and place systems and pin drives could damage the micromechanical parts
• There is no standardization of substrates or technology

Price reduction through test is significant