Fraunhofer Reduces Test Time by 6X for Wafer-Level MEMs Inertial Sensors

Dr. Oliver Schwarzelbach, Fraunhofer Institute of Silicon Technology

"One of the primary advantages of this NI PXI solution is the ability to achieve faster measurement speed without sacrificing measurement accuracy, which is largely because of FPGA onboard signal processing capabilities, as well as faster NI SMU measurement capabilities."

- Dr. Oliver Schwarzelbach, Fraunhofer Institute of Silicon Technology

The Challenge:

Reducing the test time, test system footprint, and overall cost for final-stage wafer- level testing of microelectromechanical systems (MEMs) inertial sensors while maintaining measurement quality.

The Solution:

Using the NI PXI platform to develop a system that simultaneously tests up to four sensors, supports 1D to 6D inertial measurement unit (IMU) measurements, and reduces the test time per wafer with 3,400 sensors to less than three hours.

Fraunhofer ISIT Company Background

Fraunhofer Institute of Silicon Technology (Fraunhofer ISIT) is one of Europe's leading microelectronics and microsystems technology research and development institutions. It operates a facility that houses fabrication capabilities scaling from research to industrial production. Fraunhofer ISIT works with partners that produce power electronics and microsystems with fine moveable structures for sensors such as pressure, movement, and biochemical analysis and for actuators such as valves, scanners, and mirror arrays in areas from device- and wafer-level design and fabrication to packaging technologies. Component and technology applications include automotive, consumer electronics, communication systems, medical devices, and more.


An Economical and Technical Challenge

Final-stage wafer level testing of MEMs inertial sensors, such as gyroscopes and accelerometers, involves making parametric measurements, including measuring parasitic leakage currents, and extracting mechanical characteristics such as resonance frequencies, environmental pressure, and mechanical coupling. Companies need a cost-effective way to test MEMs inertial sensors because they are typically used in high-volume consumer electronics or automotive markets.


The NI PXI Solution

We used an NI PXI-7854R multifunction RIO module for all mechanical characteristic measurements, including resonance frequencies, environmental pressure, and mechanical coupling, with the following configuration:

  • 2 X 2 analog output and input pairs for each axis (sinusoid + rectangular waves)
  • A field-programmable gate array (FPGA) configured with two synchronous demodulators per axis 
  • Peak detection and AC/DC measurements and digital signal processing algorithms written using the LabVIEW FPGA Module and deployed onboard the Xilinx Virtex-5 FPGA LX110 (onboard processing significantly reduced test times)


We used a 4-channel NI PXIe-4141 precision source measure unit (SMU) for all parametric measurements. Each SMU makes leakage current measurements between the signal pads. The signals are routed to the pads through a switch matrix. Each NI PXIe-4141 can support up to four 1D gyroscopes with four SMU channels.


NI PXI Solution Advantages

One primary advantage of this NI PXI solution is the ability to achieve faster measurement speed without sacrificing measurement accuracy, which is largely because of FPGA onboard signal processing capabilities and faster NI SMU measurement capabilities. Additionally, system cost is significantly less and the footprint occupies only a fraction of the rack space required by previous-generation test systems, which required multiple box instruments.


The total test time was 30,260 seconds (8.41 hours) per wafer, with four devices being tested in parallel, which equals well under three hours per wafer with 3,400 DUTs, which is nearly a 6X reduction in test time from the previous-generation test system. Additionally, the system retains the flexibility to extend by four parallel test sites should the need arise to test in an even greater parallel fashion.


Author Information:

Dr. Oliver Schwarzelbach
Fraunhofer Institute of Silicon Technology

Table 1. Results for 1D Gyroscope With 3,400 Devices Under Test (DUTs) per Wafer