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CALTECH APH 162 - Optical Lithography

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August 25, 2003 Trainer: Ali Ghaffari Location: Clean room, Watson 173 CALIFORNIA INSTITUTE OF TECHNOLOGY Optical Lithography: From the mask to the mold By Frosso Seitaridou Notes before you begin • This protocol was written for normal geometry chips (i.e. push down configuration where the control layer will end up being at the top of the flow layer in the final chip) • You need to have permission to enter the Clean Room (training is required). Before entering, make sure you are appropriately dressed and sign in. • You should only bring the mask (transparency) and the chem.-glasses with you in the clean room. Nothing else should be brought in, not even paper (you can find paper and pens in the Clean Room, if you want to take notes). Making molds 1. Right after coming in: Turn the hotplates, located at the back corner of the room on by pressing the Power Button. Set one to 105°, by pressing 105 and then Enter. Similarly, set the other hotplate to 125°. They take about 15min to heat up to the designated temperature. Important note: The temperatures and the baking time (discussed later) will determine the height of the channels. The aforementioned temperatures were used in the case of designs where the channel widths were 100μm. Check the attached graphs for more details on how the channel height depends on the soft bake temperature and the soft bake time. Different channel widths require different channel heights (width to height ratio should be about 8:1) so the baking temperatures and times are important in order for the final chip to function properly. It is always a good advice to ask before you decide on these parameters. 2. Notice that next to the door of the clean room, on the right, there is a working bench with shelves and drawers that contain all the equipment (scissors, razor blades, etc) that might be needed. On one of the shelves there is a basket that contains bags needed for putting the chem.-glass in while working. 3. Clean the chem-glass basis with Ni and acetone. Check with light to see if they are clean. Use Q-tips with acetone if needed or just plain paper with acetone. 4. Make sure the mask (the transparency that has the channels) is face up (i.e. ink is always up).5. Center the mask on the chem.-glass and stick it using tape. Put the chem.-glass with the mask on in a sandwich bag Si wafer and photoresist 1. Take the Si wafer. Always use the tongue and grab from the flat part of the wafer (the reference point). 2. In order to enhance the stickiness of the photoresist on the wafer, put the wafer in HMDS vapor for 2 min. Open the chamber that has HMDS written on (located in fume hood 2) pour a few drops of HMDS in the orange cup (it is in the chamber) and place the wafer such that it is either leaning against the walls of the chamber (recommended if working on more than one wafers at a time) or is flat at the center. Set the timer for 2 min. 3. In order to spin the photoresist on the wafer, work with the chamber in fume hood 1. Turn on the switch for the vacuum pump and the switch for Duplex 1 (located at the top of the hood). 4. Use the display located next to the photoresist chamber to find the right program (use the program button) or to create your own (use F1 for that purpose). Important note: The program you choose depends on what you want the channel height to be. The rpm’s, the acceleration, and the spinning time, just like the soft bake temperature and time, will determine the thickness of the photoresist on the wafer and, thus, the height of the channels. Check the attached graph for important information on how the channel height depends on the spincoat rpm. Also, remember that you want the channel width to height ratio to be about 8:1 5. Put the wafer in the chamber and center. Rotate by 90° and center again. Keep rotating until it is perfectly centered 6. Press the vacuum button. The wafer should now be stuck on the base of the chamber. 7. Close lid and press the Run button. If the wafer wobbles, press Stop and then Vacuum (so as to release the vacuum). Repeat 5, 6 and 7 as necessary. 8. Put the photoresist on the wafer (2/3 of the wafer area should be covered). The photoresist to use is PHOTORESIST 5740. You can find small bottles with photoresist in the cabinet below the working bench, right next to the door. The Rob Phillips’s group can use Corinne’s bottle that has a green label with her name on it. 9. Close the lid and press Run. 10. After done with the photoresist chamber, clean it well using paper and acetone. 11. After spinning the photoresist, if there are shades in the middle of the wafer, it means that the wafer was not really centered. So let the wafer sit for 5min before soft baking. (In the case of channels with widths of 100μm, the soft bake temperature is 105° and the baking time is 90sec. See the important note on the first page.) Be consistent with the wafers (place at the same spot on the hotplate, etc). Optical Lithography1. Turn on the vacuum pump (switch is located next to the machine, close to the wall) 2. Put the chem-glass bases with the mask on the mask holder (a square with a square hole the size of the chem.-glass in the center). Make sure that the pattern is facing you (the ink is up). 3. Center the chem-glass on the mask holder. 4. Flip the vacuum switch located on the switch box (the word “mask” is written on top of the switch) up. 5. The separation handle (located at the side of the machine) should be close to the wall. 6. Mount the mask holder on the machine. (Flip and rotate the holder to mount). Turn the knobs to tighten (finger tight) 7. Mount the wafer on the sliding tray. Slide the tray in and center using the knobs. There are three knobs used for this purpose. One will move the tray right and left, the other back and forth and the last will rotate the wafer. 8. Make sure there is no gap between the wafer and the mask (the bottom (smallest) knob should be used in order to remove the gap between the wafer and the mask) and then push the other handle (located above the separation handle) all the way back. While you push the handle back, the wafer will be moving closer to the mask. Therefore, push the handle back gently and stop if the wafer comes too close to the mask because that might cause the wafer to break. If you notice that the wafer is getting too close to the mask while you are still moving the handle, return the handle to its initial


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