Metal-Assisted Electrochemical Nanoimprinting of Porous and Solid Silicon Wafers

Our protocol provides a new patterning method for silicon that enables for the creation of three-dimensional hierarchical microstructures that enable the design of metasurface-based microoptic elements and waveguide technologies. This protocol allows for the replication of 3D structures from polymeric and hard molds into monocrystalline and porous silicon wafers in a single step. And at the same time, providing sub 100 nanometer resolutions in all three directions.

Porous silicon and silicon itself are great materials for fabricating optical biosensors and infrared optical devices. However, we expect this technique to expand to the group III-V semiconductors and even beyond. Stamp to substrate tip and tilt alignment is extremely important for a uniform Mac-imprint.

Following this protocol, the alignment can be performed by mounting stamp to the holder while it is in contact with the substrate. To prepare a stamp for the Mac-imprint, firstly clean the silicon master mold using RCA-1 solution according to the steps described in the protocol and then place a clean silicon master mold into a plastic Petri dish inside of a desiccator. Use a plastic pipette to add a few drops of PFOCS to a plastic weigh boat and place the weigh boat next to the dish with the master mold.

Turn on the vacuum pump, open the desiccator valve, and apply vacuum for 30 minutes. While the vacuum is being applied, use a glass spatula to mix the base and curing agent from a silicon elastomer kit at a 10-to-one ratio for 10 to 15 minutes. At the end of the desiccation, remove the weigh boat from the desiccator and spacers from underneath silicon master mold, then carefully cover the master mold with a two to three millimeter layer of the freshly prepared PDMS.

To degas the PDMS after opening the desiccator valve, apply vacuum for an additional 20 minutes or until the bubbles disappear. At the end of the second desiccation period, transfer the dish onto an 80 degree Celsius hot plate. After two hours, use a scalpel to trim the edges of the cured PDMS inside the plastic Petri dish and use tweezers to carefully remove the PDMS mold from silicon master mold.

For photoresist UV nanoimprinting, use a scriber to cleave a 2.5 by 2.5 centimeter silicon chip out of the silicon wafer, cleaning it using RCA-1 solution according to the steps described in the protocol and then place the clean chip onto the vacuum chuck in a spin coater. Set the spin coating parameters as indicated to apply a 20 micrometer thick layer of photoresist onto the chip and press VAC ON to apply vacuum to the system. Pour 1.5 milliliters of SU-8 2015 photoresist onto the center of the chip and close the spin coater lid.

Then press Start to start the spin. At the end of the spin, press VAC OFF to turn off the vacuum and use tweezers to remove the photoresist-coated chip. Carefully place the PDMS mold onto the photoresist-coated silicon chip pattern side down and manually press the mold into the chip.

Place a UV transparent glass plate on top of the PDMS to apply 15 grams per square centimeter of pressure to the mold and chip and expose the setup to six watts of UV light for two hours. At the end of the irradiation period, use tweezers to slowly remove the mold from the chip in the direction parallel to the direction of the cured photoresist pattern. To deposit a 250 nanometer thick gold/silver alloy layer onto the chip, firstly deposit 20 nanometer thick layer of chromium and 50 nanometer thick layer of gold as it is described in the text protocol.

Next, click GUN 1 OPEN to open the gold and silver gun’s shutter. Set the time to process to 16.5 minutes and set the DC set point to 58 and RF set point to 150. Click Rotation and set the argon flow rate to 50 standard cubic centimeters per minute.

Click Argon, DC Supply and RF Supply. When the signal plateaus, set the argon control to five. Click Start in zero thickness to start the crystal thickness monitor and to tear the thickness respectively.

Click Timed Process to start the time-controlled process and click PLATEN SHUTTER Solid to open the platen shutter. Click zero thickness again. When the sputtering ends, click Solid to close the platen solid shutter and press Press to Vent to vent the magnetron sputter chamber.

To de-alloy the silver/gold alloy coated Mac-imprint stamp, first mix deionized water and nitric acid at a one-to-one ratio in a glass beaker and place the beaker onto a stirring hot plate. Submerge a perforated PTFE sample holder in the mixture and heat the solution up to 65 degrees Celsius with constant stirring at 100 revolutions per minute. When the solution has reached the target temperature, place the patterned gold/silver alloy-coated Mac-imprint stamp into the holder for two to 20 minutes.

After de-alloying, quench the Mac-imprint stamp in room temperature deionized water for one minute. To perform stamp to substrate alignment, place stamp facing down on top of silicon chip in electrochemical cell, add a droplet of SU-8 on the back of the stamp. Bring PTFE rod in contact with SU-8 and cure it in place in dry conditions under UV light for two hours.

The contact is about 86 millimeters from home position. To perform a Mac-imprinting operation, clean patterned silicon chip using RCA solution according to the steps described in the protocol and then place it into the center of an electrochemical cell and position the cell under a PTFE rod with the Mac-imprint stamp. Mix the etching solution of hydrofluoric acid and hydrogen peroxide at a 17-to-one ratio inside a PTFE beaker.

After five minutes, use a plastic pipette to add the etching solution to the electrochemical cell. To position the Mac-imprint stamp in contact with the pattern chip, bring the stamp down by about 86 millimeters and then move the stamp down from the contact position by about 300 to 1, 000 micrometers to achieve the desired contact force. Maintain the Mac-imprint stamp in contact with the chip from one to 30 minutes before clicking Home to return the rod to the home position.

Use a pipette to carefully aspirate the etching solution from the cell and rinse the imprinted silicon chip with isopropyl alcohol and deionized water. Then dry the chip with clean dry air. Scanning electron microscope images can be obtained to study the morphological properties of the gold-coated Mac-imprint stamps and the resulting imprinted silicon surfaces.

In this representative analysis, the cross-sectional profile of the imprinted solid silicon obtained by atomic force scan was compared to that of the used porous gold stamp. Pattern transfer fidelity and porous silicon generation during Mac-imprint were two major criteria to analyze the experimental success. The Mac-imprint was considered successful if the stamp pattern was accurately transferred onto the silicon and no porous silicon was generated during the Mac-imprint.

Here, the results of a suboptimal experiment in which a lack of pattern transfer fidelity and a porous silicon generation occurred during the Mac-imprint can be observed. It is important to remember that Mac-imprint of silicon involves hydrofluoric acid, which is life-threatening substance and following a safety protocol and wearing appropriate personal protective equipment are paramount. Once you utilize stamps coated in porous gold to facilitate mass transport.

Following this procedure, the composition of the etching solution or the contact force could be varied to study the kinetics of the process or the stamp durability.