Final Design - Project Description

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.

Review of Existing Solutions

There is not a large and accessible market for purchasing testing devices for MEMS microphone production. One product, developed by Analog Devices and National Instruments, was quoted at $40,000 [1]. This system tests only two MEMS in parallel, and utilizes LabVIEW software coupled with PXI modular instrumentation [2]. However, this system also tests MEMS accelerometers and gyroscopes. A report also mentioned that the previous solution for such a system cost $450,000 [1].

There are some aspects of microphone testing which can be purchased in modules, such as a testing interface from Listen Inc. which would provide signal conditioning from the MEMS microphones, but this does not include a stimulus device or signal generation [3].

There is a report of a successful 32 device testing system developed by Jetek Technology. Jemmy Chen explains in a National Instruments (NI) case study that the system uses the “industry standard” PCI eXtensions for Instrumentation (PXI) platform coupled with LabVIEW software, to test 32 devices in parallel [4].

Statement of Requirements

• Design and build a modular MEMS microphone testing device.

• The device should be driven by a piezo ceramic stack actuator.

• The device’s test chamber should be pressure sealed and compact.

• The acoustic stimulus should be sent to at least four MEMS devices, in parallel.

• Device should be able to test both front and back ported MEMS.

• Each device should receive a sinusoidal stimulus with frequency ranging from 20Hz to 20kHz at approximately 130dB.

• Total harmonic distortion (THD) should be below 1% for all frequency inputs.

Project Deliverables

• Testing device (one or multiple)

• Assembly and calibration procedure

• Operators Manual

• Report and analysis of findings

• Bill of Materials

• Relevant software

• Relevant drawings and CAD files

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.