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Design and Characterization Methodology for Efficient Wide Range Tunable MEMS Filters
Chapters
Summary February 4th, 2018
A protocol for a fixed-fixed beam design using a laser Doppler vibrometer (LDV), including the measurement of frequency tuning, modification of tuning capability, and avoidance of device failure and stiction, is presented. The superiority of the LDV method over the network analyzer is demonstrated due to its higher mode capability.
Transcript
Filters are very popular and widely used in receiver and transmitter sequence in wireless communication. In addition, gas sensors, biosensors and temperature sensors are the most popular application. These high demanding filters should be fabricated in CMOS MEMS process to support both a more reliable fabrication and low noise signal design by eliminating the extra wires between two separate chips.
Here, CMOS stands for complementary metal oxide semiconductor and MEMS stands for microelectromechanical systems and sensors. Moreover, the post process should be designed in a way to avoid stiction during the fabrication process. A well known method to measure the resonance of the MEMS resonators is using network analyzer but it is not as powerful method as laser doppler vibrometer technique due to following reasons.
One of the big challenge with network analyzer method is to eliminate the parasitic capacitance. I see design tool was used to blow up the frequency and phase response of the equivalent circuit for a 120 micron long beam. These two watt peak to peak value drastically decreased from 6 dB to 0.34 dB even when the parasitic capacitance increased from one femtofarad to 20 femtofarad.
That is why this requires one chip up to fire design just next to max resonators. Laser doppler vibrometer is another method that uses a laser to sense the vibration of the beams when they resonate. In contrast to network analyzer, laser doppler vibrometer technique eliminates the parasitic capacitance problem.
In addition, it can detect higher mode resonance that brings many advantages in different research areas such as biosensitive applications and can characterize much smaller resonators in contrast to network analyzer. This enables fast prototyping and more sensitive and accurate resonators, especially in biosensitive applications. The goal of this study is to provide a guideline to demonstrate after design, measure the frequency tuning, tune the tuning capability, avoid the dual stictional fixed-fixed beam by using laser doppler vibrometer.
The process starts by finding the optimum structure. Select, fixed-fixed beam on the second wide-range frequency tuning because fixed-fixed beam compared to other candidates enables wide-range tuning when it is heated due to its large temperature coefficient of frequency and individual thermal expansion constant. Design longer beam if the purpose is better tuning efficiency.
Design shorter beam if the purpose is frequency hopping or signal tracking applications. Design and create the 3D model for the MEMS feeder in a finite elemental based program. Reconstruct the same layout in an integrated circuit design tool layer by layer to create the GDS file.
Submit this GDS file to CMOS foundry for fabrication. Here, we use CMOS 0.6 micron technology. Once the CMOS process completed, the chips should come with polysilicon, aluminum and oxide layers.
The next step is to conduct the post-process steps. Conduct the CHF302 dry etch process via an ICPH system, which is silicon dioxide between aluminum layers to form the beams in the aspect ratio of 5.7. For this process, use the following parameters.
CHF3 at 40sccm, oxygen at 5sccm, pressure at 0.5 pascal, ICP power at 500 watts, sample power at 100 watts with the 56 minutes total etch time. Apply xenon fluoride etch process in the silicon substrate to create a nine micrometer depth cavity under the beams. For this process, use xenon fluoride etching system for three cycles at three torr for 60 seconds per cycle.
Characterize the devices under ECM to make sure they are properly fabricated. For this step, change the beam accelerating voltage to 2.58 kilovolts and working distance to 9.5 millimeters. Device testing consists of many steps including joule heating test, and frequency response test.
Locate the thermal camera on top of the chip and test ambient heaters to make sure they heat the beams. Connect the power supply to the chip package to apply a DC voltage on embedded heaters between 0 volts and 5.7 volts with small increments to increase the temperature throughout the beams. Record the temperature profile throughout the chip package with your thermal camera during the heating process and save the results in numerical completing program and plot the heating profile.
Locate the laser on top of the 120 micrometer long beams. Connect the power supply between the two 120 micron long beams to apply about seven seven volt DC and three AC voltage for the resonance operation. Connect additional DC bias voltage to the embedded heaters with a maximum of 5.7 volt to apply joule heating to the beams during the resonance operation.
Move the laser to a different spot on the beam to get a long less laser deflection. Make sure to increase the intensity of blue bar to decrease the noise. Divide the screen into multiple views to calibrate and start the measurement setup.
Go to acquisition settings. Set the measurement mode to FFT. Do not use any filter.
And set the bandwidth to two megahertz. Change the velocity that can support the maximum frequency of 2.5 megahertz. Use periodic chip waveform.
Here amplitude stands for AC voltage and offset stands for DC voltage. Start continuous measurement with this new setup. Update the acquisition settings by changing the DC voltage to one volt.
When Ref1 shows red alarm it means the signal is noisy. Decrease the applied bias voltage in acquisition settings window to fix the problem. Move laser to different spot on the beam to get further increase in the signal to noise ratio.
Sometimes you may find bad spots on the beam that causes red alarm on vibration bar. Just keep on searching for the best spot on the beam. Select the 68 micron long MEMS filter for the testing.
Apply 25 volt DC voltage and five volt AC voltage together between the two 68 micron long adjacent beams. Here DC voltage provides banding and AC voltage enables the resonance operation. Apply an additional DC voltage to the embedded heaters located in the 68 micron long beam and increase the voltage from zero volt to 5.7 volt with small step increments.
This will provide frequency tuning based on joule heating. Observe and record resonance frequency and phase response with respect to applied bias voltage at each step and summarize the results in a table. Here total frequency tuning for this sample is around 874 kilohertz when 5.7 volts DC voltage is applied to the embedded heater.
Push A/D button to go to acquisition settings window demonstrated in calibrating LDV and test setup section and change the velocity that can support very high frequencies. Measure the first and the second mode with their phase. Apply a one hertz square wave signal to solve the stiction problem resulted from a velocity charging from two adjacent beams.
Go to the generator tab and select a square wave form under the waveform dropdown menu. Go to offset box and set the DC voltage to one volt. Go to frequency box and set the frequency to one hertz.
Activate and apply these new setup on the beams. Observe the separation of the beams. Use an extra sample for the thermal stress test.
Increase the applied bias voltage on the embedded heater with small increment to find the maximum allowable voltage before device failure due to high thermal stress. Apply a 25 volt DC voltage and five volt AC voltage together between two 68 micron adjacent beams while increasing the applied bias voltage on the embedded heater from 0 volt to 5.7 volt to get a total of 661 kilohertz frequency shift. Increase the applied bias voltage from 25 volt to 35 volt to add additional softening effect between the two 68 micron long adjacent beams while applying one volt AC voltage and keeping the same bias voltage setup on the embedded heaters.
Record the 32%improvement in total frequency shift as it should increase from 661 kilohertz to 875 kilohertz coming from this additional softening effect. Wide range frequency tuning with the application of applied bias voltage to the embedded heaters is achieved and verified with laser doppler vibrometer. Higher volt resonance measurement is very crucial for the resonators as it offers promising results for the high sensitive and accurate biosensors.
Laser doppler vibrometer enables the high volt measurement that is almost not possible to read with network analyzer. The 5th mode was measured with laser doppler vibrometer by measuring multiple points on each beam. The measured mode shape for affecting matches with the finite element analyze based program results shown on the right corner.
This video teaches how to design, fabricate, and characterize long waves, wide range tunable CMOS MEMS filters. Wide range tunable MEMS filters are very demanding especially in signal tracking and frequency hopping applications. That is why after boosting tuning range while avoiding the failure it is successfully demonstrated, easy to apply, and repeatable.
Methods of avoiding common problems such as burning, and stiction are successfully demonstrated for the sake of reliability and low cost fabrication. For characterization purpose the superiority of laser doppler vibrometer or network analyzer is successfully demonstrated. to not only enable the fifth mode striping but also enable the cutting edge technology for portable biosensor and for early diagnosis such as HIV.
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