Fabrication of Large-area Free-standing Ultrathin Polymer Films

The overall goal of this procedure is to produce ultra thin polymer films of large diameter with thicknesses down to 10 nanometers and diameters of 13 centimeters. This is accomplished by first cleaning a silicone wafer in piana acid and increasing the concentration of hydroxyl groups on the surface. The second step is to modify the wafer by spin, coating it with a poly cation, drying it, and then rinsing off the excess to leave behind a near monolayer that strongly binds to the hydroxyl groups on the wafer surface.

Next poly vinyl formal is deposited onto the wafer by spin coating a solution of the polymer and ethyl lactate onto the wafer and then drying it. The final step is to remove the film from the wafer by floating off the film onto the surface of a water bath, and then capturing the film with a hoop and gently removing it from the water. The main advantage of this technique over existing methods, such as sacrificial layers, is that the liftoff agent is a monolayer that remains attached to the wafer.

Thus, there is no chance of contaminating the film during either deposition or liftoff. Generally, individuals new to this method will struggle because it's important to lift out the films at the right angle, especially when you're working with films that are less than 30 nanometers thick. Otherwise, the films might tear during liftoff.

Lifting the films out of the water bath requires a technique that is best communicated visually. After allowing the wafer to sit on the hot plate for 30 seconds, rinse the dried P dac, layer off with the ionized water and let the wafer dry on the clean bench. Filter 60 grams of ethyl lactate through a syringe fitted with a 0.2 micrometer syringe filter.

Place the solution into the oven at 50 degrees Celsius for 24 hours. After removing from the oven, shake the vial gently to see if the polymer has dissolved completely. Weigh one gram of PDAC in a 20 milliliter vial and fill the vial with 40 milliliters of filtered deionized water.

Swirl the solution gently for the substrate preparation. Pour 60 milliliters of concentrated sulfuric acid into a clean 250 milliliter beaker. Then slowly add 20 milliliters of 30%hydrogen peroxide.

After waiting until the fuming subsides, swirl the solution gently. Following this place a 150 milliliter Petri dish onto a hot plate and pour the acid into the dish. Then set the hot plate to 100 degrees Celsius.

Place a four inch silicone wafer into the acid with the polished side up. Gently push the wafer down in the middle with a pair of tweezers to make sure the entire surface is wedded. Bubbling indicates that surface organics are being oxidized.

After leaving the wafer and acid for 30 minutes, remove it with a pair of tweezers and rinse the front and back of the wafer thoroughly with the ionized water from a squirt bottle, a second rinse in the sink can be added. Dry the wafer on a clean bench. Next rinse, a disposable three milliliter syringe and a 0.2 micrometer syringe Filter first with the ionized water and then with the P dac solution by drawing the liquid into the syringe, mounting the filter, and then pushing the liquid out through the filter.

Mount the cleaned wafer onto a spin coer and center the wafer by its center of rotation. Draw up two milliliters of the P dac solution into the syringe and dispense it through the filter onto the middle of the wafer. Spin at 4, 000 RPM for 20 seconds.

When finished, transfer the wafer to a hot plate preheated to 50 degrees Celsius for one minute. After allowing the wafer to sit on the hot plate for 30 seconds, rinse the dried P dac layer off with the ionized water and let the wafer dry on the clean bench. At this point, place the dried P dac treated wafer onto the spin coder and center it After rinsing a disposable three milliliter syringe and a point 45 micrometer.

Filter with ethyl lactate twice. Deposit 2.5 milliliters of poly vinyl formal solution through the filter with the syringe in the middle of the wafer. Spin at 200 RPM for 10 seconds.

Then at 1, 700 RPM for three seconds to give a uniform liquid film on the wafer. After allowing the film to dry in the spin coder until it is visibly dry, place it on a hot plate preheated to 50 degrees Celsius for 10 minutes to dice the films into smaller squares. For liftoff, place the wafer on a square cutting template on which all edges are taller than the wafer.

To prevent the straight edge used to guide the blade during scribing from touching the wafer, then push the wafer against two edges to align it. After placing a straight edge along two alignment marks, draw a razor blade gently along the straight edge to scribe the film. Next, fill a 190 by 100 millimeter culture dish.

With deionized water, clamp the wafer by the large flat to a rack and pinion mounted to a tilt stage and slowly lower it into the deionized water. Continue lowering the wafer at a rate that gives the film enough time to separate from the wafer rather than pushing the liftoff interface below the waterline. When the first row of squares have detached from the wafer and is floating on the surface, pause the lowering of the wafer.

The floating film squares can be moved into a convenient liftoff position with the fiber. Following this, immerse the head of the film holder into the water and move it underneath the film. Line up the handle edge of the hoop with one of the film edges and touch the hoop with the film so that the film sticks to the hoop.

Retract the hoop slowly from the water at an angle of 35 degrees. Then lift the film out of the water very slowly. When the hoop is fully retracted, make sure the bottom of the hoop is free of drops before putting it down to dry and use a curved surface such as a wafer tray to avoid creating a liquid seal between hoop and surface.

If more scribed squares remain on the wafer, continue lowering it into the water and repeat the previous steps for the remaining squares. Films should be dried before use. A cover helps to protect the films from dust while they are drying.

To lift off a film in one piece from the wafer, scribe the wafer edge with a razor blade between the two flats on the wafer. Scribe a strip wide enough to clamp the wafer onto the rack and pinion. For a wafer size film, place the wire hoop underneath the scribed section just a centimeter away from the edge.

Make sure that the hoop is centered underneath the film before beginning the capture. Maintaining some distance between the film edge and the hoop edge so that the film can wrap around the hoop and fold back on itself. A 55 nanometer thick polyvinyl formal film mounted on a 13 centimeter diameter steel hoop over a large area is shown here.

The d lamination occurs over the large areas without introducing defects that lead to tearing of the film. Thus, the intrinsic strength of poly vinyl formal can be exploited even for very thin films. A 22 nanometer thick freestanding film strong enough to be loaded with a watch glass and copper beads that way more than the film is displayed here.

Spectroscopic ellips symmetry can be used to confirm the thickness of the freestanding film. Ellips symmetric data for an eight nanometer film is shown here. Silicon services treated with PDAC can be used multiple times for film delamination.

X-ray photo electron spectroscopy spectra show that once deposited, the p dac is robustly attached to the surface and is not removed during the liftoff procedure. Once mastered, the film production portion of this technique can be performed in less than 30 minutes per wafer due to etching and dry times. The wafer preparation can take longer, but it can be done in advance.

After watching this video, you should have a good understanding of how to make ultra thin polymer films and how to lift them off of the water. Don't forget that working with piana etching solution can be extremely hazardous and precautions such as a fume hood and proper PPE should always be taken when performing this procedure.