University of California, Santa Barbara ME/ECE 141 B LAB 2: In this lab, you will create a microfluidic device with floating potential metal electrodes (in other words, electrodes that are not addressable externally). This device will be the practice device for your final project. For this lab, you will use masks design that we give you to. First, you will fabricate a hard mask (chrome) to etch a microchannel (10 µm), then remove the hard mask with chrome etch, and use liftoff to pattern a thin Ti-Au film for the electrodes. You will seal the channel with a ~3-5mm thick piece of PDMS that has access holes punched through it above the reservoirs of your microfluidic device. The success of the lab is a functional microfluidic device. Please consider all comments from Lab Report 1 when writing this lab report, and make sure to address all the questions asked in the last pages of this lab. DO NOT ATTEMPT ANY PROCEDURE BEFORE BEING PROPERLY TRAINED. Process Overview: Step 1: Hard mask deposition, lithography, metal etch, microchannel etch, and hard mask removal 1. Use E-Beam evaporator to deposit 500 Å of Chromium and 500 Å of Gold following the detailed procedure below. Bob will help you with the E-beam evaporator – Make sure you are properly trained. 2. Solvent clean your samples and spin positive tone photoresist, following the procedure described below. Expose photoresist with the microchannel pattern and develop. 3. Use Gold etchant to etch the Gold then Chrome etchant to etch the Chrome. a. From Etch Rates for Micromachining Processing- Part II (Williams, Gupta, Wasilik 2003) i. Gold Etch: AU-5 etches gold at ~660nm/min ii. Chrome Etch: CR-7 etches chrome at ~170nm/min iii. Chrome Etch: CR-14 etches chrome at ~93nm/min Ask Bob for further detailed instructions and safety precautions. 4. Inspect wafer (and take pictures) to ensure that you have etched all the way through the metal. 5. Solvent clean your sample once again using the procedure outlined below. 6. Etch your sample approximately 10 microns in Buffered HF (BHF). The etch rate of BHF for glass slides is ~1.13 µm/min. Immerse the chip in DI for 1 minute before you place it in HF. This prevents bubble formation and ensures uniform etching in HF. PLEASE READ SAFETY INFORMATION REGARDING HF ETCHING (on web) PRIOR TO THIS STEP. 7. Strip off Au using the Gold etch and Cr using Chrome etch. Step 2: Lithography using the metal mask pattern, metallization and lift-off 1. Solvent clean samples and spin positive tone photoresist (See below for process details). 2. Align the metal mask pattern (darkfield) to transfer the electrode pattern to the samples. See Below for the detailed process steps. Remember that there is an additional toluene soak step involved here. Also the exposure and develop times required are longer. 3. Prepare the evaporator to evaporate 500 Å of Titanium and 500 Å of Gold. Bob will help you with the E-beam evaporator. 4. Lift-off the undesired metal by placing the samples in Acetone. Be extremely gentle or all the metal may come off. 5. Strip off the photoresist in acetone and then rinse the samples in isopropyl alcohol and DI H2O.Step 3: PDMS preparation and channel sealing 1. Prepare a 10:1 mixture of PDMS base to curing-agent in a disposable metal tray. Prepare enough to cover a glass slide by 3-5mm. 2. Place a blank glass slide in another disposable tray and pour PDMS onto it. 3. Put into vacuum chamber to remove bubbles. 4. Place a cover to reduce evaporation and cure on the hot plate or oven at 80C for 10 minutes 5. Allow it to cool to room temperature. 6. Use a razor blade to cut out a rectangular piece larger than the fluidic pattern. 7. Use the biopsy punch to cut access holes in the PDMS above each of the microchannel reservoirs. 8. Descum with O2 plasma, align access holes to the reservoirs and bond. Detailed processing steps: Chip handling 1. Use gloves while handling the chip. 2. Using the diamond tipped scribe, make a small ‘G’ scratch at the edge on the back side of the chip. This is done because the chips are transparent and a mark helps in distinguishing the top side during processing. 3. Avoid scratching the front surface of the chip - always place the chip on a clean wipe. 4. It might take some time in getting used to holding the chips. Chip Cleaning 1. Place the chips to be cleaned in a chip holder and then immerse in a beaker containing Acetone( ACE ). Keep the beaker in the ultrasonic cleaner for 3 minutes. 2. Repeat for 3 minutes in beaker of Isopropyl alcohol ( ISO ) in the ultrasonic cleaner. 3. Rinse the chip in deionized (DI) water. 4. Blow dry with N2 . 5. Dehydrate in 125 deg C oven if going directly to lithography. Note: 1. The cleaning solutions can be reused several times. All the groups will share the same set of cleaning solvents, so be careful not to contaminate the solutions. 2. In case the existing cleaning solvents need to be replaced, dispose them in the appropriate container- Do not pour solvents down the drain. 3. All beakers in the lab must be labeled correctly. Beakers are color coded to prevent cross-contamination, please do not use for other purposes. Optical lithography The following detailed rocedures given here are for optical lithography using AZ 4110, which is a positive photoresist. Spin on Photoresist 1. Place cleaned chips on the aluminum tray, place the tray on the aluminum block in the furnace. Dehydration bake for 3 minutes at 120 °C.2. Place chips under the HMDS vapor deposition hood for 1-3 minutes. 3. Place a dummy chip on the spinner and set the spinner speed at 4000 RPM. This is to check if the spinner vacuum is on and other things are working as expected. 4. Place the actual sample on the spinner and put on a few drops of the resist till all of the chip is covered. 5. Spin the chip at 4000 RPM. 6. Pre-bake for 1 minute at 85 °C on the hot plate. Exposure using MJB-3 mask aligner 1. Turn on <POWER> 2. Slide out mask holder and put the correct mask in with the chrome side facing the chip. 3. Put your chip on the chuck and slide tray under mask 4. Rotate lever to raise chuck. Make sure you actually are in <CONTACT>. 5. Put chip in <SEPERATION>and align mask to your pattern, (if needed) 6. Set exposure <TIMER> (UV=7.5 mW/cm2) recommended for resist in use (17-25 seconds). For AZ 4110, this time is 20