Fabrication of High Contrast Gratings for the Spectrum Splitting Dispersive Element in a Concentrated Photovoltaic System

The overall goal of the following experiment is to demonstrate the fabrication of the high contrast spectrum splitting element in a concentrated photovoltaic system. This is achieved by performing PDMS nano imprint lithography over a titanium dioxide glass substrate covered with a layer of UV curable photo resist. As a second step, the residual nano imprint photo resist is etched away and a patterned chromium hard mask is produced using lifted off techniques.

Next reactive ion edges used to form the titanium dioxide and glass grading array based on measurement results from a spectrometer. The fabricated high contrast grading exhibits a broadband reflectance that can be used as dispersive elements in a concentrated photovoltaic system. We first had the idea for this method why discovered that unlike traditional deficient greetings, these Titan dioxide high contrast greetings exhibit an exotic proven reflectance profile.

Therefore, we can use it to build a spectrum splitting dispersive element that would greatly enhance energy absorption with low cost single junction solar cells. The main advantage of this technique over existing methods, such as using expensive multi junction solar cell technology to absorb broadband solar light is that the gratings offer higher absorbers efficiencies at lower fabrication cost, And imprint graphy is a low cost high yield fabrication technology that is suitable for the fabrication of large area high contrast greetings. To begin the microfabrication process, place a clean silicon wafer into a glass vacuum desiccate and add one drop of liquid trichlorethylene into a small beaker inside the desiccate.

Close the desiccate lid and evacuate the chamber until the gauge dips below one atmosphere. Then shut off the source vacuum regulator and allow the vaporized sine solution to coat the wafer surface for five hours under static vacuum. After the treatment, release the vacuum and take the wafer out of the desiccate.

Next, measure 10 grams of PDMS and one gram of PDMS curing agent In a medium sized beaker, mix both parts evenly with a glass rod and place the dish inside a vacuum desiccate for 20 to 30 minutes to degas all bubbles from the mixture. After degassing, place the wafer into a plastic Petri dish and slowly pour the PDMS onto the silicon wafer. Transfer the wafer into a vacuum oven and cure the PDMS for seven hours at 80 degrees Celsius.

Finally, take the solidified PDMS out of the oven and let the polymer film cool to room temperature. Starting with another piece of silicon wafer, place the substrate onto a spin coder. Chuck dispense 12 drops of UV curable photo resist on the wafer and spin coat the resist at 1500 RPM for 30 seconds while the photo resist is still wet.

Carefully remove a section of the cured PDMS from the previous wafer and gently press the PDMS onto the UV curable Resist. Allow the PDMS to remain in contact with a photo resist for five minutes and carefully peel the PDMS from the photo resist substrate to create an imprint of grading features onto the soft photo resist layer. Gently press the PDMS film onto an existing Silicon Master mold containing the negative relief features of interest.

Place the PDMS Silicon Master combination into a UV oven and turn on the UV lamp to cure the soft sandwiched photoresist layer for five minutes under a nitrogen atmosphere. After curing, peel the PDMS mold from the silicon master wafer. As a result, the PDMS surface should now contain a thin layer of cured photoresist with surface relief corresponding to the nano grading features.

Finally, treat the cured photoresist surface with a quick RF oxygen plasma clean. Then place the PDMS mold in a vacuum desiccate and incubate with one drop of trichlorethylene for two hours. The features on the PDMS photo resist nano imprinting mold are now ready to be pattern transferred onto any given substrate.

To begin place a fused silica device wafer complete with a 340 nanometer layer of sputtered titanium dioxide on its surface. Onto the spin coder, Chuck dispense eight drops of PMMA on the wafer and spin code the substrate at 3, 500 RPM for 50 seconds. Transfer the wafer onto a preheated hot plate, set at 120 degrees Celsius and hard Bake the PMMA for five minutes.

After cooling the wafer to room temperature, place the device substrate back onto the spin chuck. Apply eight drops of UV curable resist on the surface and spin coat. The second layer of resist at 1500 RPM for 30 seconds.

The resulting device wafer is ready for the nano imprinting transfer step while the UV resist is still wet. Gently placed the nano imprinted PDMS mold with the cured resist feature side facing down into contact with a soft UV resist surface. Place the entire sandwich into a UV oven and cure the wet resist layer under UV illumination for five minutes under a nitrogen atmosphere.

Once the UV resist is cured, peel the PDMS mold off the device wafer to remove the remaining UV resist residing within the recessed regions of the grading features. Transfer the wafer into an inductive coupled plasma etcher and run the UV resist etch recipe with oxygen plasma for two minutes at the end of this step. The PMMA underlay should now be exposed at the recessed regions of the nanostructure.

In order to remove the exposed PMMA layer and uncover the titanium dioxide film underneath, choose the PMMA etch program from the Etcher console and apply the oxygen plasma treatment for two minutes. At this moment, the device wafer is ready for metal deposition and patterning steps that would define the final features of the nano grading. To create the chromium etch mask on top of the titanium dioxide.

First, unload the wafer from the plasma etcher and transfer the substrate into an electron beam metal evaporator containing a chromium source. After chamber evacuation, melt the metal source and deposit 20 nanometers of chromium at a rate of 0.03 nanometers per second. Unload the wafer from the e-beam evaporator and immerse the substrate in an acetone filled glass container.

Then place the container inside an ultrasonic tank, actuate the ultrasonic tank for five minutes at room temperature. By applying ultrasonic agitation to the wafer, the solvent will quickly dissolve the underlying PMMA layer, thereby releasing any chromium that was not already an intimate contact with a titanium dioxide surface. As a result, the array of stable chromium patterns left on the wafer will define the final geometry of the grading using tweezers.

Take the device out of the acetone bath and remove all traces of PMMA and loose metal filings with a rinse sequence of acetone, methanol, and isopropanol. Dry the wafer with a gentle stream of nitrogen or compressed air and briefly check for any wafer defects under a microscope. Then load the wafer into the plasma etcher, commence the titanium dioxide etch recipe, unload the wafer after the etch, and verify the removal of the exposed titanium dioxide layer under the microscope to define the glass sub grading features underneath the titanium dioxide.

Reload the wafer into the plasma etcher and commence the silicon dioxide etch recipe. Unload the substrate and inspect the wafer for defects under the microscope. Finally, remove the chromium hard mask with a quick acid.

Etch the exposed titanium dioxide pattern and the glass recess trenches both represent the final structure of the high contrast grading. To measure the broadband reflectance of the titanium oxide grading. Begin by turning on the optical measurement system and placing a reference standard mirror on the sample holder.

Align the sample holder to the incident beam path and calibrate the detector such that the total measured reflectance off of the standard mirror is at a theoretical 100%level. Next, replace the standard mirror with a high contrast titanium oxide grading and commence the broadband reflectance measurement of the device. Save the data from the measurement and log out of the measurement system while elevating the substrate temperature.

During the metal sputtering process results in a larger titanium dioxide grain size and a higher refractive index. Its higher surface roughness makes it an undesirable top grading material when compared to metal oxide Films deposited at lower temperatures when viewed under the electron microscope. The aspect ratio sidewall angle, pitch, and line width of the dual layer grading structure can be readily seen compared with the reflectance data captured.

With a normal detector of small angle of acceptance. A spherical detector with a wider angle of acceptance can capture and characterize the grading reflectance and scattering from rougher metal oxide surfaces. As the data suggests, a slight increase in the index of refraction can induce a dramatic increase in the reflectance bandwidth.

Moreover, as the sidewall angle deviates from a perfect right angle, the reflectance bandwidth degrades dramatically. Once master, this technique can be done in several hours if it's performed properly. It's important to remember to control the defects during the imprint process.

After watching this video, you should have a good understanding of how to fabricate large areas, high contrast girding, using nano printing techniques, and apply them to photo botta system architecture.