EE 432 Lab 3 PMOS source/drain lithography and diffusion Group Leader: Yue Zhang Group Numbers: Yueyi Jiao, Yin Huang, Lafit Masud TA: Andy Hoyt Section: NO. 5 Introduction: In this lab, we will perform the second lithography step to create the openings needed for the PMOS source and drain regions. Then we will finish the boron diffusion that forms the source and drain regions of the PMOS transistors. The deposition half of the two-step diffusion was done by GT according to the SOPs. We will receive information regarding the soak (deposition) times for our wafers. Overview: The purpose of this lab is to finish the boron diffusion that forms the source and drain regions of the PMOS transistors. Overall, the purpose of the lab is to make sure that the surface concentration of our wafers are 2.5 x 10^18 cm-3 approximately and the junction depth is about 1.25 um.
Photolithography The second mask provides the PMOS diffusion pattern that can be used for our final device. It creates the highly doped p-type region in silicon wafer. There are mainly four parts of doing photolithography process. First is the preparation and setup for exposure machine. We first start the vacuum pumping and turn on the nitrogen and air switches of the aligner. Then, we opened the aligner match and pre heat the light bulb which means it could provides the even intensity UV light from the lamp. Then, we placed the mask on the holder by pushing the vacuum mask bottom. When we check the mask is settled on the holder correctly, we put our cleaned wafer aside. The second step is spinning photo resisters on the top of our wafers. We put our wafer in the spinning machine, and drop some HDMS onto the center of the wafer. We spinning our wafer, and after the machine stop by itself, we checked the layer of HDMS and do the same process by replacing HDMS as AZ, which is the photo resistor we need. In this process, once we got a bad result, we use acetone and methanol cleaned the top and repeat spinning process once more, until we reach a good result. Once we finished all of our wafers, we put them in the baking oven and set 88 degree for 25 min. While the 25 min baking is done, it s time to align the wafer and expose the wafer. We turn on the aligner s microscope light, located on the top of the mask aligner. we also choose soft contact mode and focus the microscope on the mask. By open to separation mode, we first rotate the wafer to make its edge parallel with the edge in mask. Then we find a corner on
mask and trying to find its corresponding order in the wafer. After we found it, by adjusting the knob for x, y, and z direction, we align the cross mark in mask with the cross marks in wafers together. From the biggest mark down to smallest mark, finally we get to the correct position. We closed the separation mode and checked the exposure time. We put on UV goggles and start to exposure for 90 seconds. Once we finished the exposure, the following step is developing photo resister and post bake. We put some AZ developing solution in the glass pitch dish and put the exposure wafers into pitch dish for about 50 seconds. After the developing is finished, we put the wafer in the holder and put it in water for more than three minutes. We also took pictures of those wafers. We observed that Yue s wafer is not developed enough so we put her wafer in developing solution again for several seconds and wash it again. Finally we saw the good result under microscope.
Boron deposition and drive 1. Purpose and Standard Clean The purpose of the standard clean is that didn t let to the chemistry mix and happen the reaction. Moreover, we use the measuring cylinder to measure chemical that the largest amount is 500ml each time. Firstly, we use the measuring cylinder to measure 2500ml DI H2O and put in the SC-1, and the measuring cylinder measure 500ml each time so we need measure five times. We clean the measuring cylinder by DI H2O. Secondly, we use the measuring cylinder to measure 500ml NH4OH and put in the SC-1. We clean the measuring cylinder by DI H2O. Thirdly, we use the measuring cylinder to measure 500ml H2O2 and put in the SC-1. We clean the measuring cylinder by DI H2O. Forth, we use the measuring cylinder to measure 3000ml DI H2O and put in the SC-2, and the measuring cylinder measure 500ml each time so we need measure six times. We clean the measuring cylinder by DI H2O. Fifth, we use the measuring cylinder to measure 500ml HCl and put in the SC-2. We clean the measuring cylinder by DI H2O.
Sixth, we use the measuring cylinder to measure 500ml H2O2 and put in the SC-2. We clean the measuring cylinder by DI H2O. Seventh, we waiting the temperature of SC-1 and SC-2 arrive to 80 C and keeping. Eighth, we open the cascade rinse and let the cistern to be full of DI H2O. Ninth, we put the wafers in the mix solution of SC-1 for 15 minutes. Tenth, we put the wafers in the cistern to cascade rinse for 3 minutes. Eleventh, we put the wafers in the HF solution at 15 minutes. Twelfth, we put the wafers in the cistern to cascade rinse for 1 minute. Thirteenth, we put the wafers in the mix solution of SC-2 for 15 minutes. Fourteenth, we put the wafers in the cistern to cascade rinse for 3 minutes. Finally, we use the drying machine to make the wafers to dehydrate. 2. the boron deposition process Figure 1: Shows the initial boron deposition in the p-well regions. Image taken from: http://archive.nanofab.utah.edu/technologylibrary/metalgatecmos/process/process2.htm
The purpose of this portion is to introduce source boron onto the surface of the silicon so that it can be driven in further in the latter drive step (see figure above). For this part of the lab, we essentially performed a P+ dopant diffusion. In our case, the p- type is the Boron, which doped the p-well regions of our wafers. The source wafer was in this case a ceramic wafer of Boron Nitride (BN), which has been oxidized to form surface layers of B 2 O 3 glass. When heated, this B 2 O 3 glass evaporates and condenses on the wafer, leading to the following chemical reaction: After the right amount of Boron has been deposited onto our wafers, we remove the source wafers and continue onto the Boron Drive step to diffuse the boron deeper into the Silicon wafer. To perform the Boron Deposition, we first perform a standard clean. In the next step, we insert our wafers adjacent to BN source wafers on the wafer boat. The wafer boat is then inserted into the furnace at a rate of about 1 inch per 12 seconds. Before inserting the wafers into the furnace, we set the furnace temperature to and the N2 gas flow to be 2 slpm. After the wafers have been inserted into the oven, we bring down the N 2 gas flow to 1 slpm and introduce 1slpm of O 2 gas. This gas flow mixture is continued for 20 minutes. When the 20 minute has passed, we introduce 40 sccm of hydrogen gas into the chamber to begin the sourcing step of the deposition, which takes 2 minutes. When the source step is completed, we perform the main soak step of the deposition. ccording to our calculations shown in the appendix we found that the
soak time should be minutes at a constant temperature of. or this step, we turned off the O 2 and H 2 gasses and introduced 2 lpm of N 2 gas. After the 70 minutes had passed, we pulled out the wafers at a rate of 1 inch per 12 seconds. To finish the deposition steps, we deglazed the wafers in the BOE tub for 30 seconds, followed by rinsing in the cascade rinse tub for 3 minutes and finally spin drying the wafers. 3. the boron drive process, including the LTO Mainly there are three steps of doing boron drive process. By using LTO to remove the boron skin, wet oxidation grows the oxide on silicon wafer and driving process pushing the dopant deeper into the wafer. After we cleaned the wafers, we checked the temperature of tube #3 goes to 800 C, and 1slpm dry N 2 flow. Then we gill the DI water into bubbler and check the bubble goes correctly inside. Then start the nitrogen gas flow through the bubbler with 200 sccm. Then we turn the power to bubbler temperature controller and set temperature of 98 C. We wait for about 15 mins for the heating. We put boron wafers in between of our silicon wafers, and load them into the tube. We push the boat into the center of furnace by using glass rod with the rate of 1 inch every 12 seconds. And we continue do it for totally 5 mins.
We checked the bubbler temperature is 98 C, and then turned off the dry N 2 flow. Then we flip the vent bypass switch to tube. And when the oxidation process goes about a half hour, we switch it back, and star to let the N 2 gas flow into the furnace. This is low temperature oxidation (LTO) process. After the LTO process, we are ready to unload our wafers. By using rod with the rate of one inch 12 seconds, we take off our silicon wafers. We put them in the plate and wait them to become cool. We etch the wafers in the BOE tub for 30 seconds and rinse for 3 mins. As usual, we spin to them dry enough. After we spin those wafers, we reload those wafers again it tube #3 with the same rate as before. We checked bubbler as before again and then we turn off the bubbler power and shut off the flow of nitrogen and turn the flow of dry nitrogen to the tube as 1slpm. We check again the temperature is 800 C and then leave our wafers in the furnace overnight ramp tp 600 C and set the nitrogen flow to 0.3 slpm.
Results 1. Pattern for PMOS source/drain: For this part, we used a mask level 2 to create a P-type source and drain in N-type well. Comparing to pattern for P-well, in this lab, we need to do alignment since it is essential to avoid making an overlap with P-well. We used microscopy to find a few cross marks in mask as well as in our wafer. By doing this, we can make sure that mask 2 has aligned well with our wafer. After doing the exposure, we took some pictures about our wafer. The picture shows as following: Figures taken in lab: figure 1 figure 2 The cross mark for making alignment After exposure, picture of a part of a die figure 3 Anonther picture of a part of a die
figure 4 figure 5 Another cross mark at a different position of wafer A figure shows a part of die firgure 5 Sometime the patter n confuses me. I hope that in class, it may be explained what it is. 2. Problem in this process: 1) During this lab process, the most difficult problem is to how match cross mark in mask and with cross mark in wafer perfectly. Since, we are not familiar with structure of pattern, sometimes we don t know where our wafer is. Even though we found the edge of a die in wafer, it is still a little of difficult to find a right direction to move wafer. 2) Another problem which is not so critical is that during the SiO 2 etch, whether there is water attaching to wafer or not is an important signal for us to stop etching. Sometimes, people forgot to observe this and follow standard operating procedure.
3. Boron deposition for PMOS source/drain: For this part, basically, we just did same step in lab for P-well diffusion. There are source and drain well created using boron wafer as a boron source. Professor has done first step in two step diffusion. According to his note, first step is one hour at 850 o C. When we did our lab, we use one hour at 1100 o C. Be careful, in this lab, the time for oxidation should be also taken into account for calculating depth. N(0) 1.77 10 Dt1 7.054 10 Dt2 6.134 10 dose 7.7712 10 depth 1.243 m cm 18 3 cm 13 2 cm 10 2 cm 13 2 Figure2, a profile of P source and drain
4. Problem in this process: 1) For this lab, there are not so many problems we met. There are something mentioned for how to control flow of N 2 and H 2 O. Beside, when we need to refill the DI water in bubbler, the TA shows us how to do it correctly in case of break bubbler.
Appendix 1. calculation: Constants for Boron Diffusion: EA = 3.50 ev, D0 = 1.0 cm 2 /s NB = 3.25 x 10 15 cm -3 kb = 8.617 x 10-5 ev/k T1 = 850 C or 1123 K T2 = 1100 C or 1375 K Diffusion Constants D = D0exp(-EA/kBT) D1 = (1.0)exp(-3.50/(8.617 x 10-5 )(1123)) = 1.9596 x 10-16 um 2 D2 = (1.0)exp(-3.50/(8.617 x 10-5 )(1375)) = 1.4824 x 10-13 um 2 t 1 =3600s t 2 =4320s Q 2 N / Dt 7.77 10 cm s 1 13 2 18 3 0 / / 1.77 10 N Q Dt cm 2 x 4 Dt In( N(0) / N ) 1.243 m j 2 B
2. Process Traveler:
Conclusion: From this lab, we learn how to perform the second lithography step to create the openings needed for the PMOS source and drain regions. Next we finished the boron diffusion that forms the source and drain regions of the PMOS transistors. Moreover we received information regarding the soak (deposition) times for our wafers.