Still sore from hiking up the Great Wall, a day in the lab was a welcome relief. We again donned our coveralls and face masks and stepped into the cleanroom, this time accompanied by PhD student Wei Wu. He walked us through the coating process, first breaking the large circular silicone wafer into several perfectly straight slices with practiced ease and then transferring the samples to the wetroom. Here he cleaned the slices with acetone, alcohol, and acidic solutions and then attached them to a plate. Now prepared for coating, they were submitted into a machine where they were covered in a layer of hafnium oxide. Because all coating processes take 30 minutes or more, while we waited for that process to finish we coated other wafer slices with titanium nitrate using an ion beam and tested the sample's resistivity in order to ensure it had the proper electrical properties to be used for the array. After our day of testing, Yilong took us to dinner at a nearby hot pot restaurant that specializes in lamb spine and we closed the night enjoying chrysanthemum tea, assorted mushrooms, and fried bean curd.
Continuing the process from the day before, the team arrived at the clean room early in the morning. Beginning the next step, in which Tantalum Oxide was added to the wafer, the machine befell technical difficulties (the power supply was not functioning) which delayed the experiment for several hours. Once the lab engineer was able to correct the power supply error, the team continued the Tantalum Oxide step and then moved on to adding a layer of Aluminum Oxide in a different machine. Each step took upwards of 40 mins to complete, and thus the was in-and-out of the lab all day. After the two steps were completed, the graduate student recommended we come back the next day to to the lithography steps, given that the lab was closing for the night.
Today the lab procedure moved to the dim yellow lights of the lithography room. After baking the wafer for 30 minutes to dry, its cleaned off with a spray of nitride. Then the sample is doused in a photoresistive coating in the form of a thick scarlet dye, and placed in a spinner to ensure an even layer. Next a masking pattern is heated to a hundred degrees celsius, and cleaned with acetone and alcohol. Then the pattern is transferred to the wafer, ready for etching tomorrow.
Lab work complete for the day, we hopped on the subway to check out the Beijing Auto Museum, which boasts 5 floors of familiar vintage classics as well as many Chinese models that we we had never seen before, such as Volgas and Hongqis that had been used by Chinese dignitaries and officials.
Day Four of making the crossbar array consisted of etching the layers of Hafnium Oxide, Tantalum Oxide, Titanium Mononitrogen, and Aluminum Oxide together using Silicon Oxide onto the aluminum wafer. The process took about two hours to complete both pieces. Afterwards, the team used the Dektak 150 in order to check if the density of the piece was correct. Once everything looked good, we used a photo solution to remove the photoresist, giving the piece a greenish tint. Finally, our little project was completed and the crossbar array was made.
Having concluded our lab work for the week, the team decided to visit the Forbidden City since we finally had our passports and new visas. Our host student Liya guided us to the center of Beijing, where we entered Tiananmen Square and the Forbidden City. We wandered the place for hours, but due to the excessive heat, left to find a refreshing place to eat and drink. Liya took us to a nearby noodle shop she adored for lunch, and afterwards the team left back to campus to escape the humidity and rest.
Saturday, we journeyed to the Temple of Heaven, a 15th century monument where emperors of old would journey to pray for favor and ask for rain and good harvest. To this day, people all over the world still visit the ancient site to offer up their own prayers, wishing for wealth, better grades, or good luck. In the afternoon, we walked through nearby Qianmen Street, which is home to dozens of tiny shops selling tea, jewelry, and silk scarves. We ate their for dinner, dipping into a busy restaurant to discuss next week's agenda over bowls of beef noodles and shared dumplings.
Today was a rest day so everyone was left to their own devices. most did laundry, some napped all day, and others studied for their future fall quarter classes. at the end of the day, however, we all gathered to have a movie night in the hotel, watching Totoro and Castle in the Sky.