To evaluate the performance of MEMS devices and develop strategies to improve the process, the characterization of their movement is now a necessity. MEMS devices now have additional integrated functionality, such as the incorporation of thin piezoelectric layers.
This has led to novel devices that require detailed measurements and characterizations in environmentally representative operating conditions. In addition, changes in residual stresses caused by manufacturing processes can result in a reduction in device performance, resulting in new measurement challenges. As such, a methodology is now required to evaluate the performance of these functional materials in extremely small sizes.
The national standards laboratory of the United Kingdom, the National Physical Laboratory (NPL), is an independent and internationally respected center of excellence for research and development, and the transfer of knowledge in measurement and materials science. The NPL Functional Materials group has integrated a Polytec micro-scan laser Doppler vibrometer with an environmental chamber to characterize nanoscale MEMS devices in a wide range of vacuum conditions. To examine the movement of MEMS devices over a wide range of pressures (from 1 bar (atmospheric), up to 10-5 mbar) developed a specialized Doppler laser vibrometer (LDV).
Figure 1: Photograph of the environmental chamber, which shows a sample attached to the XY stages compatible with vacuum. The long focal length lens can be seen above the sample, which is attached to the Polytec Micro Systems Analyzer differential vibrometer.
Figure 2: SEM image of the 4 mm diameter PZT circular membrane. The resonant modes are shown in the image to the right.
Benefits of the Polytec MSA-400 microsystem analyzer combined with a vacuum chamber
The NPL system uses a Polytec MSA-400-PM2-D microsystem analyzer (new MSA-600 up to 2.5 GHz now available) together with long-distance objective optical lenses. Such lenses can reduce the beam size to 1 μm. This allows the characterization of samples with lateral sizes of less than 10 μm. The Polytec system was chosen for the following reasons:
- Its small point size and with that an excellent lateral spatial resolution
- Its ability to measure through a vacuum window.
- Its differential measuring capacity
Broadband stimuli such as periodic screeching and white noise can be used to achieve device excitation. Alternatively, for specific frequencies, excitation can be achieved using single frequency waveforms such as sine or square. Speed or displacement profiles that show off-plane movement with a resolution greater than 10 pm can be produced by scanning the laser through the sample.
The scan of the sample in the normal operating mode is done through the internal optics of the MSA-400. NPL has incorporated a set of stages compatible with X-Y vacuum in the MEMS chamber for larger samples (up to 25 mm). Ordinarily used for the positioning and alignment of the sample, these steps can also be used to scan the sample and generate the movement profile. The study of devices in more realistic working conditions is carried out with the combination of a scanning unit with a vacuum chamber of flexible and dedicated design.
Sophisticated Data Processing
The speed signal from the vibrometer controller was imported into a blocking amplifier (Stanford Research Systems SR830). The reason for this was twofold. First, speed / displacement and phase information can be easily extracted. Second, much smaller excitation voltages are allowed, since the use of blocking techniques allows noise levels to be reduced by obtaining vibrational information on a single frequency. Initially, the position of the resonant peaks of a piezoelectric MEMS device was determined by recording a frequency sweep. The vibration mode was mapped by laser or sample scan.
Example: piezoelectric transducer membrane (PZT)
Tests have been performed on a 4 mm macroscale PZT membrane (Fig. 2) and an atomic force microscopy (AFM) tip (Fig. 3) to demonstrate the scale on which the scanning vibrometer system can operate. To compare the effects on their dynamic response, measurements were taken in the air and in a vacuum.
A sinusoidal voltage was applied directly between the upper (inner) and lower electrodes to excite the PZT membrane. This exhibited a fundamental mode of vibration at a frequency of 20.78 kHz in air and 17.81 kHz under a vacuum of 4 x 10-5 mbar (see figure 4). A sealed air gap that exists behind the sample membrane and the mounting substrate causes this reduction in frequency.
By reducing the pressure, an increase in the frequency of the “freely” vibrating samples is observed due to the reduction in the damping of the air acting on the sample. Other vibration modes can also be seen in addition to the expected higher order harmonics. This result would be more difficult to observe with a single point vibrometer. Compared to the physical scanning of the sample, laser scanning can save time for smaller devices and can also reduce the effects of lateral movement and reaction of the scanning stages. A full scan of out-of-plane movement can be obtained in several minutes depending on the spatial resolution and the required frequency range.
Figure 3: SEM image of the Veeco UltraleverTM AFM tip. The resonant modes are shown below.
Figure 4: Resonant modes of the PZT membrane obtained under a vacuum of 4 x 10-5 mbar (fundamental, first and second order).
Figure 5: Maximum amplitude resonance modes of the AFM tip obtained under a vacuum of 3 x 10-5 mbar
An oil-free environment is provided for the examination of samples with the use of a turbomolecular pump with diaphragm backing. Good RFI / EMI shielding is enabled with good optical clarity by directing the LDV laser beam through a glass window covered with special coating.
Computer control of the vacuum stages is allowed with the standard ports, even in the MEMS vacuum chamber (see Figure 1).
Sample excitation for dynamic analysis is provided through electrical supplies. The future update capability, including the possibility of studying the movement of the sample based on temperature and humidity, is configured with the sample chamber.
Example 2: AFM Council
The lateral resolution of the system is demonstrated through the results of the AFM tip (feature size ~ 20 μm) as shown in Figure 3. A V-shaped cantilever was scanned in both air and vacuum. The resonant frequency of the cantilever was 62.12 kHz in air and 62.59 kHz at a pressure of 3 x 10-5 mbar As shown in Figure 5, the amplitude of the vibration increased by a factor of approximately 50. At reduced operating air pressures, an increase in resonance frequency and an improved Quality factor were observed. This is because as the overhang resonates in your local environment, you experience a reduction in the dissipation forces of friction energy. Tools such as finite element analysis can be used later to compare this data with a software-modeled response. The calculation of the force constant is possible with the knowledge of the fundamental frequency of the tip. A comparison can then be made with the values provided by the manufacturers.
Example 3: SNOM gold probe
Figure 6a shows data taken from a 250 µm diameter gold probe used for a near field scanning optical microscope (SNOM) using the NPL system. An engraved gold probe is propelled in resonance with an amplitude of about 1 nm by a small piezoelectric element. This is necessary for the SNOM to function successfully. To reduce the area over which the information is obtained, the amplitude is kept small.
Scanning along a relatively long probe (approximately 6 mm in length) showed that the fundamental mode of vibration was, in fact, a higher order harmonic vibration. If the probe is excited at this higher frequency, this would complicate the operation of SNOM. Figure 6b shows the frequency response of the vertex of a shorter probe (approximately 2 mm in length). It was shown that the probe only required a voltage of about 0.6 mV to produce a tip displacement of 1 nm at a resonant frequency of 22.06 kHz.
Figure 6a: LDV scan of the SNOM gold probe.
Figure 6b: Frequency response of the 2mm long Au SNOM tip. The LDV image shows that the fundamental mode is being excited, rather than a higher order harmonic mode of vibration.
Conclusions and perspectives
This work focused on obtaining high resolution vibrometry scans of several samples of interest. The rapid acquisition and processing of data through FFT analysis are advantages offered by the NPL Polytec MSA-400-PM2-D (differential) microsystem analyzer. It also allows vibrational analysis in the plane. A unique characterization system is provided for out-of-plane and in-plane dynamic analysis of MEMS and macro-sized devices through a combination with the NPL vacuum chamber that operates up to 10-5 mbar with optional temperature / humidity control. A variety of other samples of active and passive MEMS devices have provided more speed and displacement profiles, some of which can be seen in animated movies at www.npl.co.uk/materials/functional/mpp1_3_4.html
This information has been obtained, reviewed and adapted from materials provided by Polytec.
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