Team 12 - Cohu SEG MEMS Acoustics

Comprehensive overview of project:

Project Background

Cohu Semiconductor Equipment Group is a leading supplier of test handling solutions used globally in the semiconductor industry. Semiconductor companies implement Cohu products into their business to test their products after manufacturing and before distribution. One class of products tested on Cohu handlers is MEMS, or micro-electro-mechanical systems. One function of the handlers is to provide the test stimulus to the MEMS being tested.

MEMS devices are applied in many common products that people use every day. They have a wide range of functions, such as accelerometers, gyros, pressure sensors, and microphones. They are popular in everyday portable devices such as cellphones, laptops, and headsets because of their low cost, high quality, and small size. Because they are produced on such a large scale, they need to be tested quickly and at low cost.

Cohu’s current MEMS microphone testing method is in need of improvement to increase throughput and efficiency. Although the MEMS cost only cents to manufacture, the cost to test them before distribution is more than two orders of magnitude greater. The present module lacks the ability to generate a similar or consistent signal for each MEMS device under testing (DUT). Further, the test signal is not transferred to the MEMS DUT effectively, making the test module inefficient and expensive. This method of testing multiple devices involves a speaker, with test performance lacking sufficient sound pressure level at high frequencies. Cohu is seeking a more effective way to test many devices in parallel to reduce cost and increase throughput. They hope that by using a smaller test chamber, the pressure distribution will be uniform and consistent between devices being tested simultaneously.

Objective

The project involves developing a concept of a module that is smaller and allows testing of more devices in parallel. This module needs to be compact and provide an acoustic stimulus to at least 4 devices. The module must be able to provide the stimulus to either side of the devices. Each device must receive a sinusoidal stimulus varying from as low as 20Hz. up to 20kHz. The amplitude of the acoustic stimulus can vary from device to device but the total harmonic distortion must be less than 1%.

Final Design Solution

The block diagram below shows the signal flow used to test the acoustic module. Initiation of the system was done via a custom LabView program. The LabView program interfaced with a function generator capable of a 50mV-10V amplitude sinusoidal signal. This signal was then amplified by the power amplifier to a voltage required by the piezo actuator. The piezo actuator was the driving component of the acoustic module. A plunger was fixed to the actuator’s free end. A reasonably compliant O-ring sealed the interface between the plunger membrane and the port layer. The small sinusoidal displacement of the actuator compressed the plunger onto the O-ring, creating a pressure wave throughout the chamber. It was crucial that contact between the O-ring and the plunger membrane was maintained at all times to maintain a sealed chamber. It was also crucial that the O-ring compression be repeatable between every assembly. To ensure both of these requirements were met, a micrometer positioning head was added to allow fine positioning of the piezo stack. The pressure wave was directed through four small holes in the port layer into the MEMS devices, where it stimulated the membranes internal to the MEMS. The MEMS then produced output voltage signals, based on the signal they received, that were conditioned and returned to the LabView program to be recorded. In conjunction with the three MEMS devices there was also a high-quality reference microphone for more accurate signal measurement. Matlab was then used to process the data and generate a meaningful display of results.

Test Block Diagram

1. Use Labview to control and generate signal

2. Signal goes to PZT and creates deformation

3. Deformation generates pressure wave in sealed chamber

4. Pressure waves hits MEMS

5. Read signal from MEMS

6. Compare signal from MEMS to signal from calibrated reference microphone

Summary of Performance Results