Several fundamental characteristics of MEMS devices influence the overall structure of the high-volume test and calibration equipment. MEMS de-vices require electronic interfaces. Performance is highly dependent on the manufacturing process. Automated material handling solutions must be customized too.
With respect to the electronics contained in a MEMS device, there's a clear trend toward a common structure. The purpose of the electronics is to apply a calibration correction function, perform analog signal conditioning, and transmit the output signal. Therefore, the structure of each MEMS device can then be separated into three parts.
The first, the MEMS element, is the raw transducer or actuator. The next is the signal conditioning circuit, which may include amplifiers and filters. These circuits must be adjusted, or trimmed, on a per-device basis. The third part is memory, which is used to store the calibration constants determined during the trimming process.
Early versions of signal-conditioning circuits were laser trimmed. That gives only simple correction capability. Next came custom fixed-function circuits with memory registers. The trend has continued to include fixed-state machines with multiple register sets. Now we are seeing fully programmable microcontrollers. Eventually there will be full microprocessors. All of this is geared toward providing better performance. It's this performance advantage at a low price that determines which MEMS manufacturers will be successful. When all of these parameters are taken into consideration, the determination of requirements can commence.
Given the intense competition in any of the high-volume MEMS applications, manufacturers are faced with compressed development schedules. This means that production capability must be flexible to accommodate changing specifications and production rates.
Therefore, test requirements must be clearly defined for the life of the MEMS product and some consideration must be made for future requirements. Typical functional requirements categories for sensors include sensitivity correction, offset correction, and temperature-coefficient compensation. The electronic requirements categories for typical electronically trimmed sensors include the number of device pins, power supply, dc source and measure, ac source and measure, and digital I/O communication.
By providing the essential data for each of these requirements categories, the specification of the high-volume test system can be extracted. If specified correctly, there should be enough capacity and capability to support the production effort throughout the entire production life of the MEMS device.
One of the challenges to MEMS manufacturers is determining how much testing is required. In order to minimize cost, it's desirable to minimize the amount of testing as long as yield, performance, and quality levels can be maintained.
There are certain parameters that can be guaranteed by design and others that can be determined by sample-based testing. These generally apply to situations where statistical analysis reveals an acceptable level for the standard deviation in the production process.
If the degree of repeatability is too low, then each device must be tested explicitly on an individual basis. It's the nature of many MEMS devices to fall into this category. There's usually a high degree of variability that's an intrinsic characteristic of the MEMS element and its sensitivity to the mechanics of the production process.
One role of the production test equipment is to provide a mechanism to collect data for analysis. This is to determine if there are test optimizations that can be applied.
It's important to understand variability of the MEMS product on a continual basis. So, the capability must fully characterize devices and be able to change tests and measurements on an ongoing basis.
Although many solutions exist for testing standard semiconductor devices, which are strictly electronic, MEMS devices have both an electronic instrumentation element as well as a mechanical, optical, or chemical instrumentation element. This introduces a whole new perspective on physical instrumentation. Much of the existing physical instrumentation is geared for laboratory use and may not be easily scaled for high-volume use. These might exist as individual instruments, but may not be easily integrated into production machinery.
For example, as mentioned earlier, accelerometers and pressure sensors are well established high-volume MEMS devices. They can each be discussed to highlight how their differences affect the implementation of high-volume production test equipment.
Automated material handling systems for MEMS device production are a significant and costly element of any production system due to their high level of customization. The key consideration is how many devices must be processed at one time.
For pressure devices, the tendency is to manipulate as large a batch as possible, partly because of the dynamics of controlling the pressure stimulus. It also is due to the need to have robust pneumatic connections. Furthermore, it's quite possible to connect many pressure devices in parallel.
This is a sharp contrast to accelerometer devices. Because the effective area of the accelerometer stimulus is limited, only a small number of devices can typically be processed at one time.
The device I/O pins include all electrical contacts to the device. Both the pressure devices and the accelerometer devices share similar requirements. Each has power, ground, and, typically, a digital communication function and an output pin.
One important difference relates to the output pin. The accelerometer is a device whose operational signal bandwidth is high in comparison to that of a pressure device. How sophisticated the pin electronics and signal processing electronics must be is dictated by these differences.
Most calibration compensation circuits have provisions to compensate for temperature coefficients. The degree to which the temperature effects behave in a linear pattern determines how many temperature set points are required in the calibration process.
Temperature behavior is primarily determined by the intrinsic method of sensing—namely, resistive or capacitive. Normally, piezoresistive devices require more temperature compensation. Basically, because the typical pressure sensor is piezoresistive and the typical accelerometer is capacitive, that pressure sensor requires more complicated temperature compensation techniques.
The role of the device fixture is to provide electrical and mechanical connections to the device. The major concern for pressure fixtures is pressure leaks, while the major concern for accelerometer fixtures is mechanical resonance.
Another important yet subtle aspect of the fixture design is that the fixture itself becomes part of the signal measurement path. As a result, a wider range of technical disciplines is required to develop a robust production solution.
We have focused our attention on the testing of the currently popular MEMS devices, specifically pressure sensors and accelerometers. Major challenges still exist in the development of high-performance and cost-effective testing of these mature devices. As the commercialization of more-complex nonphysical input-stimulus devices becomes a reality, much-more-complex testing system and reliability analysis will be required.
The level of complexity of this problem will be reduced on one hand by the wealth of knowledge gained to date regarding the testing of physical sensors. Complexity will be dramatically increased by the difficulties in creating and calibrating systems associated with RF, chemical, and photonic input. Success for MEMS device manufacturers will depend on the ability to specify and procure the specialized equipment that meets their detailed requirements.
MEMS testing and reliability will be addressed at Commercialization of Microsystems 2000, to be held in Santa Fe, N.M., Sept. 5-9. Information on the conference is available at www.asm.unm.edu/mot/coms/COMS2000.htm.