Fabrication and Characterization of High-Q Silicon Nitride Membrane Resonators

Our research aims to explore the unique optical and mechanical properties of freestanding silicon nitride thin films. Our fabrication procedure allows us to rapidly design and characterize these devices, which allows us to make quicker advancements in the field of nano mechanics. Looking ahead, our group plans on using these devices to study the quantum properties of angular sensing to develop on ship accelerometers and to design experiments for probing physics on this tabletop scale.

To begin, obtain a glass Petri dish containing a clean double-sided silicon nitride wafer. Place a few drops of HMDS along the edge of a glass Petri dish using a disposable capillary pipette. Under a fume hood cover the Petri dish.

Heat it on a hot plate at 90 degrees Celsius for one minute. to evaporate the HMDS and allow it to adhere to the silicon nitride. After priming, place the wafer inside a spin coder.

Now use a needle and syringe to extract no more than four milliliters of S1813 Photo resist. Gently flick the syringe and dispense a small amount of fluid to remove any air bubbles. Safely remove the syringe needle and attach a two micrometer filter.

Flick the syringe again and dispense three to five drops to remove air bubbles in the filter Deposit the resist drop by drop onto the center of the wafer to form a large pool. As the pool expands, continue depositing resist near its edge to keep it centered. Use the needle tip to pop any surface bubbles that appear before proceeding.

Now lock the spin coder lid and set it to spin to create a uniform 1.5 micrometer thick layer across the wafer. Remove the wafer and place it on a hot plate to soft bake. After one minute, remove the wafer from the hot plate.

To define the device patterns, load the coated wafer into a massless alignment photolithography machine. Center it to minimize global rotation and offset errors. Load the populated wafer file onto the computer and convert it to a compatible format for the MLA software.

Begin patterning the top side layer with resonator designs. After alignment, select the option to include global rotation error load beam exposure parameters by setting the beam intensity to 140 millijoules per square centimeter at a wavelength of 375 nanometers. Now fill a large dish with approximately 75 milliliters of MF319 developer and another with deionized water.

Once the photolithography exposure is complete, unload the wafer from the photolithography machine. Then transfer it into the MF 3 1 9 developer for 20 seconds. Gently swirl the dish in a circular motion to agitate and remove the exposed resist for an additional 40 seconds.

Then transfer the wafer to the deionized water dish to dilute off residual developer. Rinse both sides of the wafer using a spray gun, use a compressed nitrogen gas gun to remove remaining water from the wafer surface, applying low pressure and proper orientation to prevent damage. Load the developed wafer into a reactive ion etcher to transfer the resist patterns onto the silicon nitride film.

Run the etching process for five seconds per cycle with the given settings. Repeat the process until all wafer regions not protected by the resist layer turn silver indicating the substrate is exposed. Finally, remove the wafer from the reactive ion etcher for backside patterning.

To release the patterned resonators from their silicon substrate. The wafer is diced or cleaved and processed at the chip scale. Carefully peel the diced chips from their tape.

Dip the chips into a sonicated acetone bath for at least 30 seconds to remove resist and surface contaminants. Optionally, to thin the silicon nitride film or remove stuck on contaminants, dip the chip into a 10%hydrofluoric acid buffer solution to remove 1.55 nanometers of silicon nitride per minute. Immediately place the clean chip into a custom PTFE holder under a fume hood.

The chip holder is then gently lowered into a 45%solution potassium hydroxide bath maintained at 86 degrees Celsius. Cover the solution to maintain a uniform thermal gradient. After the silicon around the resonator is fully etched and the film is released, stop the reaction by removing the vessel from the hot plate.

Without exposing the chip gradually pipette out the potassium hydroxide solution and replace it with deionized water, always maintaining the liquid level above the chip to prevent shattering the device. To avoid cross-contamination, transfer the chip holder and a clean beaker into a fresh bath of deionized water. Then transfer the chip holder into the clean beaker containing deionized water.

After the transfer, remove both beakers. Gradually replace the deionized water with isopropyl alcohol in the same way, ensuring the liquid level always remains above the chip. Repeat with methanol.

After the last bath, remove the chip and dry it carefully along the device plane using a gentle nitrogen stream. The device is characterized in a vacuum chamber operating below 10 to the power of negative seven millibars with optical lever readout. To set up an optical lever, place a focusing lens after a fiber collimator and a beam splitter between the focusing lens and the sample.

Focus a collimated laser beam onto the sample with a spot size similar to the largest feature being measured. Align the laser beam close to normal incidents onto the sample by retro reflecting into an optical fiber, which simplifies later alignment. Place a balanced photo detector along the reflected beam path, positioned beyond the rail length of the beam to maximize sensitivity.

with the laser incident on the split or quadrant photo detector, the detector output is sent to a digitizer. Compute the real-time power spectral density of the signal using the Fast Fourier transform method. To identify specific mode peaks, compare the location of thermal noise peaks in the broadband signal to predicted Eigenfrequencies from simulations.

Take a root means square average of several power spectral density estimates to confirm the peaks. Begin energy ring down by tracking the integral under the power spectral density peak at the resonant frequency to measure stored energy. To drive the mode, apply energy either by placing a Piezo Actuator on the side or by sending a second laser beam to the sample, modulate the Piezo voltage or the beam intensity at the resonant frequency.

Once the drive is stopped, track the exponential decay of the oscillation energy to determine the damping rate. Then calculate the modal Q by dividing the resonant frequency by the damping rate COMSOL simulations of the ribbon geometry revealed a 53 kilohertz flexural mode and a 70 kilohertz torsional mode. The flexural mode has the largest angular displacement, halfway between the center of the ribbon and its fillets.

A GDS2 design was created with 37 chips of 12 square millimeters each, showing a variety of geometries for fabrication. Power spectral density analysis demonstrates the effectiveness of the optical lever in reading out the angular displacement of both modes.