The overall goal of this procedure is to introduce carbon nano tube forests as a microfabrication material. Begin with the growth of patterned vertically aligned carbon nanotubes using capillary forming densify the carbon nano tube pillars to increase the robustness and modify their shapes after infiltrating with a polymer to form robust nano composite microstructures use replica molding to cast the polymer replicas. Ultimately scanning electron microscopy is used to confirm the fidelity of replication between the master mold and the polymeric replicas.
The main advantage of our technique in comparison to existing methods for microfabrication, like DPV and extra lithography, is that CNT based master molds have high toughness, can include features with high aspect ratios, and that the CNT microfabrication can be engineered to create complex three dimensional features. Also for the accurate design of the three dimensional CNT shape, the user CNT growth process needs to be carefully characterized and optimized. Visual demonstration of this method is critical.
As the carbon nanotube densification steps are difficult to master. This method can be used to fabricate microstructures out of a variety of polymers. Today we're going to demonstrate SU eight, but it could also be used for P-M-M-A-P-D-M-S polyurethane or perhaps even biomaterials Proof.
Perform these procedures on the polished side of a silicon wafer first spin coat a layer of HMDS to promote adhesion between the wafer and the photoresist. Similarly, spin coat a layer of SPR two 20 dash three. After baking the wafer on a hot plate, position the desired mask for catalyst patterning and expose the wafer to UV light in hard contact mode.
Now place the wafer on a hot plate at 115 degrees Celsius for 90 seconds. Proceed to develop the exposed photoresist for 60 seconds, followed by a one minute rinse in deionized water. Now load the wafer into the catalyst deposition system.
Then deposit 10 nanometers aluminum oxide followed by one nanometer iron using ebeam evaporation or sputtering. Manually scribe and break the wafer into pieces of of approximately 20 by 20 millimeters and transfer the wafer pieces into a one liter beaker containing 100 milliliters of acetone to lift off the photoresist place in an ultrasonic bath at power six for eight minutes. Refresh the acetone and repeat sonication.
Then soak the wafer pieces in isopropanol for two minutes using tweezers to remove the wafer pieces, dry each piece with a gentle nitrogen stream using a handheld nozzle before every CNT growth. The tube is baked. Air is flowed through the tube at 100 SCCM while the furnace is heated to 875 degrees Celsius and maintained at that temperature for 30 minutes.
Then load a boat with growth substrates into the tube using a stainless steel rod position the boat such that the leading edge is located 30 millimeters downstream of the furnace.Thermocouple. Connect the end caps, sealing the tube without disturbing the position of the boat or the pattern. Silicon pieces then program the software to flush the quartz tube with helium for five minutes at room temperature.
Next, set the temperature to ramp up to 775 degrees Celsius in 10 minutes. Under hydrogen and helium flow, hold the flows and temperature for 10 minutes. A kneeling causes the film to chemically reduce from iron oxide to iron and to dit into nanoparticles.
To grow the carbon nanotubes, adjust the hydrogen and helium flow rates while adding ethylene and maintaining the furnace at 775 degrees Celsius. The height of the CN Ts is controlled by the duration of the growth. Step To stop CNT growth and cool the sample manually slide the quartz tube downstream until the catalyst chips are located approximately one centimeter downstream of the furnace insulation.
Use care to maintain the same flows and furnace. Set point temperature for 15 minutes after flushing the tube with helium for five minutes, retrieve the samples and turn off the furnace. Apply a piece of double-sided tape to a 0.8 millimeter thick aluminum mesh with 6.25 millimeter diameter holes.
Check that the mesh is larger than the opening of a one liter beaker, and the tape is approximately centered on the mesh Mount the silicon wafer piece with CNTs on the tape. With CNT microstructures facing upward place 100 milliliters of acetone in the one liter beaker on a hot plate inside a fume hood. Set the hot plate to 110 degrees Celsius.
When the acetone starts boiling. Position the aluminum mesh on the beaker with mounted sample facing downwards. Once the vapor front approaches the top of the beaker, observe the apparent color changes on the surface of the silicon substrate.
Rainbow leg patterns will appear and sweep across the entire surface. Next, after removing the mesh from the beaker, carefully peel off the sample from the double-sided tape Using a razor blade. Utmost care should be taken at this step as it is easy to break the sample during removal.
To fabricate the CNT master mold pool, SU eight 2002 on the densified CNT microstructures. After spinning the sample, bake at 65 degrees for two minutes, then at 95 degrees Celsius for four minutes. Irradiate the sample for 20 seconds, then bake the sample.
If replicating delicate structures, place the master in a desiccate along with a glass vial of izing agent at 400 millitorr for 12 hours. After mixing approximately two grams of PDMS with a ratio of 10 to one monomer to crosslinker, place the CNT master in an aluminum foil dish and pour PDMS to submerge the sample. Place the sample in vacuum and degas below 20 tor for 15 minutes.
When bubbles form, periodically increase the pressure rapidly to burst large bubbles after curing the negative at 120 degrees Celsius for 20 minutes, peel back the aluminum foil and separate the master from the soft PDMS negative. If replicating delicate structures, place the negative and silent agent in a desiccate at 400 millitorr for 12 hours. Pour SU eight 2002 into the PDMS negative.
Evaporate the solvent from the thick layer of SU eight by baking it for six hours at 95 degrees Celsius. Now irradiate the sample, then bake at 95 degrees Celsius for eight minutes. Last, manually demold the SU eight replica from the PDMS negative, vertically aligned carbon nanotubes or CNT forests offer a new material for microfabrication.
These SEM images represent an array of cylindrical CNT pillars before and after densification. Note the alignment and density of the CNTs. You may notice that higher aspect ratio, CNG pillars are less straight and their straightness is progressively reduced during densification and replication.
Densification of the semi-circular pillars results in uniform incline structures over large areas. This comparison of C-N-T-S-U eight master and replica micro pillar structures indicates high fidelity replication of microscale shape and nanoscale texture over a large area. Current limits in terms of structure formation include high aspect ratio walls like this, honeycomb reran structures indicated in this sloped CNT micro well and twisted CNT micro pillars.
In this video, we demonstrated how to pattern grow and densify CNT Microstructures and subsequently used these as master molds to cast polymer replicas While replicating this procedure. Remember to take care not to contaminate the PDMS as this may result in incomplete curing or reduce fidelity during replication. It is also important to carefully control the amount of solvent that condenses on the substrate in order to avoid damage to the CNT microstructures.
Don't forget that working with carbon nanotubes may be hazardous, so appropriate precautions should be taken. When handling samples. You should always wear gloves, and if you're removing carbon nanotube forests from the substrate, you should always wear a respirator.
