Due to the Coronavirus, Ohio State has cancelled classes until March 30th. This has set me back in schedule since I was going to take final measurements after spring break. I will not be returning to Columbus until school resumes, therefore I will not be able to to collect measurements until after the April 1st deadline. There are three measurements that need to be taken. After the first measurement, the samples need to be etched and the machine takes time to start up/shut down. This takes time and all measurements should be done in the same day so Friday, April 3rd, is the best day when I can complete all data collection.

The photolithography process is complete. After checking the results under the microscope it was discovered that the resist had been removed in areas that were not supposed to be removed. This was the same for all three samples. Therefore it was assumed that the mask that was used was dirty. One of the samples also had black square markingings. After consulting with my supervisor it was determined that the black marks were from a defect in the crystal structure of the silicon wafer. Even though the samples aren’t perfect, they are still good to use in the etching process.

Timeline

Training:

• Photolithography- 3 hours
• Plasma Etcher- 3 hours
• Dektak and NMX- 1 hour

Wafer Preparation (3 hours): Three wafers will be prepared to etch. Two wafers are made of silicon and the third is silicon oxide.Preparing the wafers entails creating a pattern on the wafer using photoresist (resin). All the wafers will have photoresist 1813 on them. When etched, areas without photoresist will be engraved.

• Wash and Bake: All the wafers will be cleaned with isopropyl alcohol and blasted with nitrogen gas. This will remove any surface particles (dust) from the surface of the wafer. The wafers will then be loaded into a baker to bake for 20 minutes. The baker is an evaporator that will remove any water from the wafer which will help the photoresist stick to the wafer.
• Photolithography: A wafer is loaded into a coater and photoresist is placed onto the wafer using a pipet. The coater is closed and spins the wafer so that the photoresist spreads evenly across. The photoresist layer is less than a micron thick. This process is done for each wafer. The wafers are then placed on a hot plate so slightly harden the resist.
• Exposure: A mask and wafer are loaded into an aligner. A mask is a glass pane that has the negative image of a pattern. UV light passes through the clear areas of the mask and hits the photoresist underneath. The photoresist that is exposed to the light will change in chemistry. This change in chemistry will be different depending on the photoresist and will give different results in the next part.
• Lift Off: A chemical called MF 319 is placed in a glass dish. One at a time, a wafer is placed in the chemical. When removed, the wafer is placed under running water and then dried with nitrogen gas. Depending on the type of photoresist, the areas that was exposed to the light will either lift off (negative resist) or remain (positive resist).
• Microscope check: The wafers are placed under a microscope to make sure the liftoff was done completely.

Data Collection (3 hours):

• NMX: This machine will take the initial thickness of each wafer with photoresist.
• Plasma etch: A plasma ray of CF4 will be shot at each silicon wafer while CHF3 will be used for the silicon oxide wafer. This causes everything at the top to be removed at a certain rate. Areas of wafer that are under the photoresist are protected as the resist is removed first. Areas of exposed wafer will be etched before areas covers by photoresist. The etch lasts 5 minutes for each wafer.
• NMX: This machine will take the new thickness of each wafer with photoresist.
• Dektak: Will measure the depth of the etch.
• SEM: Cleave (break) wafer into smaller sample and place in electron microscope to see side view of wafer.

For  my capstone project I will be using an opportunity that I was offered at my job. I work as a student assistant at Ohio State’s Nanotech West Laboratory. The lab is a user facility which means that professors and companies pay a fee to be able to access the facility. My position is a part of the staff. I usually do chemical and supply inventory as well as basic machine cleaning. Yet I am also trained to use many of the machines in the lab which I can use for when I am sent to do a process.

The lab bought an etcher a few years ago and is missing some information therefore I was assigned to look into the etch rate ratio of silicon wafer to photoresist. The samples that are placed into the etcher are silicon wafers/discs which have a resin, called photoresist, on them. The resin forms a pattern on the wafer. When placed in the etcher, a plasma beam is shot at the sample causing all the material at the top of the sample to be removed. Areas of the silicon wafer that are under the photoresist are unaffected until all the resin has been removed.

It is important to know the relative etch rate of the silicon wafer to photoresist because that way users can determine how thick to make their photoresist to protect the wafer underneath and to make used the etch isn’t too long that the wafer is cut through.

This experiment will also need preliminary steps in order to prepare the wafers to be etched. This includes the photolithography process which is where photoresist is placed on the wafer.