A Better Test for MEMS Inertial SensorsMarch 1, 2006 By: John Rychcik, Solidus, Chris Nickerson, Solidus, Marc Straub, Solidus, Gary A. Morrell, Solidus, Hongyuan Yang, Solidus Sensors
Save time and money by verifying the dynamic mechanical behavior of your sensor elements before the packaging and assembly steps.
The mechanical response of MEMS inertial sensors (accelerometers and gyros) must be tested to verify the mechanical integrity and performance of the sensor element. One major obstacle to identifying inertial sensor defects early in the manufacturing process has been an inability to dynamically stimulate and sense the mechanical performance of the sensor at wafer level.
Instead, manufacturers have tested the sensor's static capacitance and leakage behavior. This methodology provides limited sensor characterization and cannot detect mechanical performance or process anomalies that are common in inertial sensors. Mechanical performance is therefore not fully determined until the sensor is assembled, packaged, and tested on a mechanically dynamic system such as a vibration or rate table. This is often costly and too late in the development or manufacturing process to respond to design or fabrication issues.
A Better Test Methodology
Drive Sense Technology (DST) is a test technology that can determine the sensor's mechanical behavior and response at wafer level. In contrast to conventional wafer-level test methodologies, DST dynamically stimulates and senses the mechanical movement of the sensor element (box). Sensor resonant frequency, damping ratio, –f3dB frequency, Q-factor, and spring constant are among the measurands, with significantly shorter test times than conventional methods.
The STI3000 wafer-level test system consists of a mixed-signal tester (Figure 1) interfaced to a device-specific element probe card. DST is based on three signals: analog drive, tuned AC source, and conditioned output. The STI9000 ATE (automated test equipment) unit generates an analog voltage drive stimulus that is further processed by the STI3000E probe card electronics to create the final tuned AC source signal used to drive the sensor element. The probe card measures the element response and conditions the output. This signal is then further processed and digitized by the ATE response subsystem. The processed data are a representation of the sense element mechanical position vs. time during mechanical excitation. As noted above, resonant frequency, damping ratio, –f3dB frequency and spring constant are just a few of the parameters that can be obtained with DST. Figure 2 illustrates the drive signal (step function) and the resultant conditioned dynamic response signal of a typical sensor element. The element response shows an exponential decay (damped natural frequency response).
> Figure 1. The STI3000 wafer-level test system consists of a mixed-signal tester interfaced to a device-specific element probe card
Figure 3 shows a typical DST test flow for a MEMS capacitive accelerometer. The first test measures the leakage current for each pad. High leakage currents can be caused by contamination or other processing defects.
Next, a step response test applies a DC step voltage input (positive or negative step) to the sense element and captures the mechanical position (response) of the sense element vs. time. During this test, the STI9000 test system calculates the resonant frequency, spring constant, and damping ratio from the element response data. In addition to testing the element performance, this characterization is also extremely useful for validating parametric design models.
> Figure 2. The drive signal (step function) and the resultant conditioned dynamic response signal of a typical sensor element
Following the step response test, a hysteresis test is performed in which a positive then a negative voltage ramp is applied to the sense element. The increasing voltage mechanical path is compared with the decreasing voltage mechanical path. Hysteresis, slopes, and nonuniform slope changes are captured and characterized.
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