EE432/532 CYMOS PROCESS PWELL LITHOGRAPHY AND DIFFUSION [Document subtitle] GROUP 4 GROUP 4 (TUESDAY AFTERNOON) GROUP LEADER: ANDREW MCNEIL GROUP MEMBERS: WENG HOONG LOO MARIO PEREZ ZHIHAO LIAO LAB INSTRUCTOR MATTHEW WEINSTEIN JANUARY 31 2017
1 Overview From the last lab of oxidation as shown in Figure 1, we carried on the process to build our CyMOS. In this week s lab, we were introduced to the lithography as shown in Figure 2 and Boron Deposition Process as shown in Figure 3 for the PWELL process. These are two important procedures that were required for a successful development of devices on a silicon wafer. Throughout these two processes we followed the CyMOS process traveler closely. After our procedures, Dr. Tuttle helped us with the LTO (Low Temperature Oxidation), PWELL boron drive and re-oxidation that results in Figure 4. The process traveler, the Standard Operating Procedures and Dr. Tuttle lead to the successful development of the P-Well. The procedures are explained in detail below in this report along with figures and calculations of our process. Figure 1: Oxidation from the lab before (taken from www.tuttle.merc.iastate.edu) Figure 2: Lithography for PWELL (taken from www.tuttle.merc.iastate.edu)
2 Figure 3: Boron deposition PWELL (taken from www.tuttle.merc.iastate.edu) Figure 4: Boron Drive and re-oxidation for PWELL (taken from www.tuttle.merc.iastate.edu) Photolithography This process begins with a thin layer of photoresist added to the wafer using a spinner. We first apply a layer of hexamethyldisilazane (HMDS), this is to secure the photoresist onto the wafers. The HMDS was applied to the wafer using a dropper. HDMS was placed on the center of the wafer where it was spread evenly through centrifugal force once the wafer begun to spin Next, the photoresist was applied to the wafer in the same fashion in the center of the wafer. This step also sets a layer of photoresist uniformly across the wafer. The wafers were then placed in the prebake phase and we began to set the mask aligner up for photolithography. Once the prebake was completed, each wafer was then placed in the Mask Aligner as shown in Figure 5, aligned and placed in contact with the mask. When we were satisfied with the alignment, the wafers and mask were exposed to UV light. The photoresist exposed in the open regions in the mask were diminished by the UV light, allowing us to remove it using a developer. The wafers were transferred to the lab s wet bench a chemical bath and rinse as shown in Figure 6, to remove the unwanted layer excess. We then dried the wafers and prepped them for a post bake at 120 C. The next step is to etch away the exposed layer of silicon dioxide. We took our wafers to the wet bench and started the buffered oxide etch (BOE), deglazing step. When the BOE bath and rinse
3 were completed, the wafers went to the acetone and methanol tub to remove any extra photoresist. After this chemical cleaning and rinse the wafers underwent a spin dry cycle. The wafers were then dried and placed under a microscope to verify our etching. At the end of this step, our wafers should look like Figure 1. Figure 5: The Mask Aligner used to perform etch the photoresist with UV to create the pattern needed for the next step (Diffusion). Figure 6: The wafers that have went through the developer, sitting in the cascade rinse tub while other wafers photoresists were being etched.
4 Boron Deposition and Drive Once the photolithography was completed and our wafers were etched, we then started the boron deposition process. In this process, boron atoms are deposited onto the wafers. This step is where we dope our wafers in order to create the PWELL. We began the process by calculating the temperature and time needed to dose our wafers as shown as Figure 8. In this deposition step of the process, we simply saturate the surface of the wafer with boron. In a separate step. we drive the dose deep into the wafers. We began this step with the standard clean process, this removes previous chemicals and excess dust or particles that may have collected on the surface of the wafers during previous processes. It is important to remove these impurities, because at high-temperatures these impurities may fuse into the surface of the silicon wafers. This can have adverse and detrimental effects on our wafers. The Standard Clean Process washes the wafers with ammonium hydroxide, hydrochloric acid, hydrofluoric acid, and deionized water. Each chemical has its own bath cycle and is followed by a rinse in the de-ionized water. After the chemical baths, have been applied, a final wash is done in the spin rinse and dryer. The deposition process consisted of 5 separate steps as shown in Figure 7. We began by pushing our wafers in the quartz boat at a rate of 1 inch every 12 seconds. This was to be a 5 minute process of pushing the boat all the way into the furnace. Our beginning settings were a temperature of 850⁰C. The nitrogen level being pumped into the furnace was to be at 2 slpm (standard liter per minute). The second step of this process was recovery. This was a set time and temperature of 20 mins at 850⁰C respectively. This was a process of allowing oxygen to begin flowing into the furnace. The recovery process of having mixed nitrogen and oxygen flow is needed before introducing hydrogen. Source was the third step in the deposition process. This was introducing nitrogen into the furnace. This was the shortest individual process in the deposition steps. 40 sccm (standard cubic centimeter per minute) of hydrogen began flowing for a 2 min process. The next process was the soak time. In this step, we were given the ambient gas values that were to be flowing into the furnace, which was 2 slpm of nitrogen. In this step, we needed to choose our own time and temperature depending on the values that were given to us. The dose time in this step, Q, needed to be between 7.5E13 cm -2 and 10E14 cm -2. We chose a dose value of 10E13 cm -2.
5 Figure 7: The simplified version of the Boron Deposition (taken from the process traveler) From the calculations attached in appendix Figure 12, by setting the temperature to 850⁰C to get the time. Thus, by solving, the deposition time was 76 mins and 20 seconds approximately based on our calculations. The final step in the process was pulling out. This was done at 1 inch every 12 seconds. The gases that were flowing during this process was just 2 slpm of nitrogen. The temperature was to continue being at 850⁰C. The reason we chose our soak temp to be 850⁰C was to keep the temperature constant throughout the entire deposition process. Once the process was complete and the wafers were removed and cooled, a deglaze process was done. This was a simple 3 step process of a BOE dip of 30 seconds, and a 3 minute cascade rinse followed. The last step was just a spin rinse/dry. After the boron deposition comes the process for the drive. We begin with the standard clean to clear the silicon wafers surface of impurities. If not cleaned, these impurities could diffuse into the silicon while in the furnace ruin our progress. The standard cleaning procedure is described earlier in the process. The wafers go into the furnace which is set to 800 C with a flow of N2 for the Low-Temperature-Oxide step (LTO). The LTO is a necessary step to remove the boronlayer that was created during the boron deposition. The growth of a thin oxide allows us to remove this layer and with it any potential impurities that have developed on top of the surface. A deglazing step is performed to remove the thin LTO oxide layer. Once completed, the wafers go back into the furnace overnight for a drive at high temperature. This step was completed by the professor due to the amount of time required.
6 Results Looking back through the Photolithography and Boron Deposition processes, it went pretty smoothly except for one of the wafers breaking in half as explained in the problem section later on. After the lithography step was done, we took a look at the wafers under the microscope as shown in Figure 8. From the inspection, we found that the wafers were aligned correctly. This is because there were no bubbles appearing on the wafers surfaces, the markings on the wafers were clear. The patterns on the wafers were same as the pattern on the mask and the coordinates obtained through the microscope matched the coordinates we counted without the microscope. Pictures of the patterns of the wafers were taken and shown in Figure 9. Figure 8: The microscope used to inspect the photoresist patterns. It is also connected with a camera to take pictures that were taken as the pictures in Figure 9.
Figure 9: The wafers in a microscopic view, we inspect the photoresist to determine if repetition of any photoresist process is needed. 7
8 Figure 10: The constant dose profile of boron obtained from the diffusion spreadsheet (obtain from www.merc.tuttle.iastate.edu)
9 Problem Discuss any problems that you may have encountered during the processing. The major problem that we faced during this lab is during the PWELL Diffusion was Weng Hoong Loo s wafer broke into half as shown in Figure 11. We noticed this after the boron deposition procedure. Once the soak step was done, during the pulling procedure, at first, we noticed that the wafer was not fitting correctly on the boat, as half of the wafer was dangling from the bottom of the boat, but still contained in the boat. After pulling it out, we inspected in on a plate and were pretty amazed by how it got broken right in the middle. By gathering information from our TA and Dr. Tuttle we have some explanations of why the wafer could have broken. One of the possibilities is there could have been micro scratches done by us, from handling it too rough, It could be from scratches from the microscope while we were inspecting it. Pulling the wafer out from the mask aligner too abruptly Clipping the wafers too hard from using the tweezers. Not placing it in the boat correctly, with pressure applying on the boat holder infused with the heat, the pressure was too great and caused the wafer to snap. Manufacturing defect. The heat absorbed throughout the wafer may not have been even, that caused one area to expand faster than other causing it to crack. By analyzing the different causes and discussing with the team members, we believed that it was due to the micro scratches. Thus, we will be more cautious and gentle towards our wafers from now on then. Figure 11: The broken wafer.
10 Appendix Figure 12: Calculations to get the temperature and time needed for the Boron Deposition by setting the temperature to be 850⁰C re-typed from Figure 13.
Figure 13: Calculations by hand with the formulas in Figure 14, blood, sweat and tears (done by Andrew McNeil) 11
12 Figure 14: Formula used for the calculations in Figure calculations. Figure 15: The measurement of the oxide thickness from the oxidation process of the previous lab. (taken from the process traveler.)
Process Traveler for PWELL Lithography and Diffusion 13
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