Characterisation, Measurement & Analysis
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Measuring Dynamic MEMS Devices

Introduction

Optical profilers are valuable tools for characterizing the surfaces of MEMS devices. Traditionally, an optical profiler such as the NewView 7300™ was used for measuring a wide range of surface features and device parameters. However, one fundamental requirement was that the surface and device be kept in a static condition during measurement. As industries such as MEMS manufacturing matured and began creating more dynamic devices, it also became more important to characterize and understand the functionality of the device while it is in motion. Advancements in optical profiling instruments now make this possible. Using the NewView 7300™ and the new Dynamic Metrology Module (DMM), it is now possible to synchronize the illumination source to the motion of the device, in essence freezing the sample motion and allowing the user to effectively measure as if the part were fixed in space.

Stroboscopic Illumination

When measuring a sample with a white light profiler (such as the NewView 7300™), the sample is typically held in a stationary position. Motion of the part will cause a blurred image, poor data integrity, or even loss of data. For samples such as MEMS devices, however, it can be extremely useful to characterize how the shape of the sample changes when the device is activated and therefore in motion.

Figure 1.

Figure 1 - Schematic of Dynamic Metrology Module (DMM) components

In a dynamic configuration, a strobed LED illuminator is synchronized with the drive signal of the device. By adjusting the strobe frequency of the light source, the motion can be effectively frozen, allowing precision scanning white light interferometer (SWLI) measurements to be made on the dynamic device. Adjustment of the illumination phase delay—the lag between the drive signal and the illuminator strobe—allows the device’s full range of motion to be examined. The DMM option includes MetroPro™ software which allows these parameters to be adjusted automatically to ‘sweep’ through ranges of frequencies and phase delays to fully characterize the device under test. The modularity of the Dynamic Metrology Module also makes it possible to upgrade existing NewView 7300™ systems with these enhanced capabilities.

Why Dynamic Measurements?

Dynamic measurement of a MEMS device is useful for both research and development and production quality control. Sweeping of device drive frequencies and illumination phase delays can be used by the researcher to validate design parameters and examine device resonances. As a quality check, dynamic measurement mimics the MEMS actual usage for a true functional test. Rapid characterization over the full range of motion and frequencies experienced is possible.

Making Measurements

ZYGO’s Dynamic measurement option makes the characterization of dynamic MEMS devices just as simple as measuring a static device. All standard MetroPro™ numerical and graphical results can be used to quantify the motion of a dynamic device. These results can also create resonance amplitude and nyquist plots, and all plots are available for creating a movie of the device motion. For power users, custom results can be generated with ZYGO’s MetroScript™, an easy-to-use scripting language.

For any dynamic measurement, the user needs to set up the drive parameters for the device including voltage, signal shape, and frequency. Depending on what metrology is required, one of four types of dynamic metrology can then be pursued.

 

Single measurement
A single measurement can be used to look at a device’s behavior at a particular drive signal and phase delay. One application where this would be particularly useful is a DC drive where a static offset would be expected based on the voltage applied. By examining the sample with the voltage turned on and off, the change in the device may be observed.

Frequency Sweep
A frequency sweep measures a device at a particular phase delay and steps through the desired frequency range. This can be a quick method for identifying which frequency ranges a device is most sensitive to. For devices whose responses are independent of phase delay, this can be an ideal approach to use for determining the resonant frequency of the device.

Phase Sweep
In a phase sweep, a device is driven at a single frequency while the phase delay of the light source is varied. The delay range and step size are user configurable from 0 to 360°, allowing the user to map a complete, single period of motion. If desired, the user can select a single measurement from the sweep to subtract from the subsequent measurements, showing shape change during motion. After the data is collected, the user can export the data as an .AVI movie that can be viewed through most standard media players.

Figure 2.

Figure 2 - Deformation of a cantilever at multiple resonance frequencies (75kHz, 450kHz, and 1.2MHz)

Phase & Frequency Sweep
The final measurement mode combines the previous two— sweeping and stepping both the drive frequency and the phase delay. The user can define the start, end, and step size for both frequency and phase. At each drive frequency in the sweep, a phase sweep as described above is performed. This is particularly useful for detection of system resonance.

Dynamic Measurement Specs

Parameter Specification
Measurement Range Out of Plane: Heights / deflections up to 5mm depending on slope
In Plane: microns to millimeters (FOV and Objective dependent)
Measurement Resolution Vertical resolution: <1 nm (velocity dependant)
Lateral resolution ≈.1 pixel (field-ofview/objective dependant)
Frequency Range DC / Static (White LED)
Dynamic: 400Hz to 10MHz (Green LED)
Frequency Resolution 0.01Hz
Frequency Resolution 0.01 Hz
Device Voltage Range 0 to 10 volts peak-to-peak
0 to ±200 volts peak-to-peak with supplied
High Voltage Amplifier
Phase Range 0-360 °
Phase Resolution
Waveforms DC, sine, triangular, square, or user-defined arbitrary waveform
Light Source and Controller High-output LED with integrated controller
Software Version Required MetroPro™ 8.1.5.3 and greater

Conclusion

Whether for quality control in production manufacturing, or for laboratory research, ZYGO’s NewView 7300™ with the DMM option provides all the tools necessary for inspection of both static and Dynamic MEMS. With best-in-class measurement range and speed, the NewView 7300™ and DMM Option are the ideal turnkey solution for dynamic MEMS measurement.