PhotolithographyPhotolithography is the process of transferring patterns on to a substrate by using light. By using a “mask”, light is used to project a pattern onto a light sensitive material called a resist. This light either adheres the resist onto the substrate or allows it to wash away in a chemical bath, leaving behind the desired pattern. This process is used every day in the manufacturing process of complex computer circuits and microelectromechanical devices. The phone in your pocket contains billions of circuits that were created using the process that you will explore in this laboratory experiment.

A mask/pattern will be provided to you in the form of a plastic transparency sheet. The image shown below on the left is the university’s seal. The image on the right is acquired from an optical profiler and will be similar to the one that you will use to analyze the pattern you create.

Objectives:In this experiment, you will:

- Transfer a pattern to a silicon wafer via photolithography.

- Take a few measurements with an optical profiler to analyze that pattern.

Materials:

mask (UVU logo) silicon wafer (d=5 cm)photoresist developer solutiondistilled water 2 petri dishesmethanol acetonedropper adhesive tapehot plate spin coaterUV light table Optical Profiler

SafetyGloves must be worn while chemicals are in use.

Procedure

1. Make sure you have the mask with the university seal printed on it. The seal should be no bigger than your wafer (d = 5 cm).

2. Obtain and wear gloves.

Steps 3 – 6 below are to apply the photoresist onto the silicon wafer. If time does not permit these steps to be completed, skip down to step 7, as your instructor will have already prepared your wafers.

3. Clean the wafer using acetone. Prepare a warm acetone bath (do not exceed 55 degrees C) and submerge the wafer for ten minutes. Place the wafer in a bath of methanol for 2-5 minutes. Use a petri dish to submerge your wafer in the acetone and methanol. You will only need enough to cover the wafer. After the methanol bath the wafer is rinsed with filtered DI water. The wafer is then dried with nitrogen. REMEMBER TO USE TWEEZERS TO HOLD THE WAFER. DO NOT USE YOUR HANDS.

4. Set the hotplate temperature to 115oC and place the wafer on the plate to remove any remaining water from the substrate. Remove the wafer after 2 minutes.

5. TURN OFF THE OVERHEAD LIGHTS AND TURN ON THE SAFELIGHTS. TIP: Also turn on the safelight on the UV light box. This will give you a little extra light while completing the next few steps.

Place the wafer on the pedestal of the spin coater. Add a few drops of AZ3312 photoresist with an eye dropper. The photoresist should be kept in the refrigerator. Add enough drops to cover about 80% of the wafer. The spin coater needs to be set to a two-step process. In the first step, the wafer spins at 500 rpm for 30 seconds. This causes even distribution of the resist across the wafer. In the second step we will use a spin speed of 1000 rpm for 40 seconds to achieve an average resist thickness of about 2.2 microns as shown in the chart below.

6. Follow the steps below to program the spin-coater:6.1. Press the “Program Select” button until a “B” is displayed in the upper right-hand

corner of the display screen. This indicates that program B has been selected.6.2. Press “F1” to edit the program. Note that a flashing cursor appears on the display,

and that the first line of the display will read “PGM 001/001”. This indicates that the screen is currently displaying step one-of-one. Remember, you will need to program a two-step process.

6.3. While the display shows “PGM 001/001” use the left/right arrow buttons to move the cursor under the “sec” label. Now use the up/down arrows to change the value. Set the value to 30.0 seconds. This is how long the spin coater will spin your wafer to distribute the photoresist.

6.4. Use the left/right arrows to move the cursor under the “rpm” label. Change this value to 500.

6.5. Use the left/right arrows to move the cursor to the right of the “ACL=” label and change its value to 002. This completes the programing of step one of the spin process.

6.6. Push the “Add Step” button. Now push the “Step” button. Notice the top line of the display now reads “PGM 002/002”, indicating that you are about to program step 2 of 2.

6.7. Using the same process as in programming step one, change the spin time to 40 seconds, at 1000 rpm, and the acceleration to 003.

6.8. Press “F1” to save the program.6.9. Now turn on the main power to the vacuum pump via a switch on its power cord

and press the “vacuum” button on the spin coater. This should create suction to hold the wafer as it spins. Now press “Run/Stop” to begin the 2-step spin process.

6.10. Once completed, the machine will stop spinning. Press the “vacuum” button to release our wafer and turn off the main power to the vacuum pump. Do not remove the wafer.

7. Post-Apply Bake (soft bake). Set a hotplate to a temperature between 90-105◦ C. YOU SHOULD STILL BE WEARING GLOVES. Open the lid to the spin coater and use tweezers to

remove your wafer. GRAB ONLY AN OUTSIDE CORNER TO AVOID DAMAGE TO THE COATING OF PHOTORESIST. Place the wafer carefully on the hotplate and heat the wafer for 60-90 seconds. This improves adhesion of the resist to the wafer and stabilizes the resist in preparation for exposure.

8. Reminder: THE OVERHEAD LIGHTS SHOULD BE OFF. THE SAFELIGHTS SHOULD BE ON. YOU SHOULD BE WEARING GLOVES. TIP: There is a safelight built in to the UV light box. Turn it on by pushing the button labeled “Safelight”. This will help you see while completing the next step.

9. You will now create the pattern on your wafer by exposing the mask and wafer to UV light. Place the mask onto the glass of the UV light box and center your wafer over the pattern so that the photoresist is facing down towards the lights and touching the pattern. Tape the wafer in place using scotch tape. The idea is that the UV light will cast a shadow of the pattern onto the resist.

10. Set the timer on the UV light box to 5 minutes. Close and LOCK the lid, push “vacuum”, then “Exposure” to begin the UV light exposure process. CAUTION: DO NOT OPEN THE LID WHILE THE UV LIGHTS ARE ON.

11. Post exposure bake: After exposure the wafer is again baked on a hotplate at 110-115 ◦C for 60-90 seconds.

12. Development: Use a petri dish to submerge the wafer in AZ300 developer for 60 seconds. This removes all soluble resist from the wafer surface.

13. Rinse the wafer with distilled water, and blow dry using nitrogen. It is now safe to turn the overhead lights back on.

Clean up

14. Dispose of the waste. Rinse the petri dishes.

MeasurementsNow that your pattern has been transferred to a silicon wafer, you will be able to take a few measurements to verify your work. This is an important part of the process of photolithography and can be completed in several ways. This time, you will be utilizing an Optical Profiler which measures surface “roughness”. With the available software you can measure this “roughness” and obtain an 3D image representing the surface of your silicon wafer and the exposed pattern you just created. If an Optical Profiler is not available, your instructor will advise you to take measurements using another method.

Optical profilers are interference microscopes, and are used to measure height variations, such as surface roughness, with great precision using the wavelength of light as the ruler. Optical interference profiling is a well-established method of obtaining accurate surface measurements.

Optical profiling uses the wave properties of light to compare the optical path difference between a test surface and a reference surface. Inside an optical interference profiler, a light beam is split, reflecting half the beam from a test material which is passed through the focal plane of a microscope objective, and the other half of the split beam is reflected from the reference mirror.

When the distance from the beam splitter to the reference mirror is the same distance as the beam splitter is from the test surface and the split beams are recombined, constructive and destructive interference occurs in the combined beam wherever the length of the light beams varies. This creates the light and dark bands known as interference fringes.

Since the reference mirror is of a known flatness – that is, it is as close to perfect flatness as possible – the optical path differences are due to height variances in the test surface. This interference beam is focused into a digital camera, which sees the constructive interference areas as lighter, and the destructive interference areas as darker. In the interference image (an "interferogram") below, each transition from light to dark represents one-half a wavelength of difference between the reference path and the test path.

If the wavelength is known, it is possible to calculate height differences across a surface, in fractions of a wave. From these height differences, a surface measurement – a 3D surface map, if you will – is obtained.

1. Start the computer on the optical profiler’s desk. Once running, click on the software application icon called “MetroPro”.

2. You will come to the main startup screen where you can select from several different mini-applications. Select “Micro.app”.

Once selected, you will see the application screen appear. This is where you will do your measurements and imaging.

Calibration process for selecting objective lense:

You will use the joysticks to move objective lense up and down, or stage along the x and y axis. This will help you focus and see the image in the live view screen shown on the left computer monitor.

3. Start by pushing forward on the z-axis control to separate the stage and the objective lense to make room for sample. You can change the speed of the z-axis movements pushing the “slow/fast” buttons shown in the picture above.

4. Place your wafer near the center of the stage and use the X/Y axis joystick to move it under the objective lense.

5. Once the wafer is under the lense, slowly move the objective lense closer to the wafer using the z-axis joystick. Look at live view while bringing objective lense close to wafer. IT IS POSSIBLE TO COLLIDE THE LENSE AND THE STAGE SO BE VERY CAREFUL!

6. The screen may turn red due light saturation while bringing stage and lense together. Hit F5 to auto adjust light. THis will allow you to see image. You will need to adjust the focus with higher precision once sample is in rough focus. Slow down the z-axis speed to do this.

7. Once the image is in focus and the light has been auto-adjusted, you should have an image in the live-view screen that looks something like the picture below. The picture to the right shows a jagged line that has been transferred to a silicon wafer through the same process that you just went through above. You will now be able to take a measurement. Click “Measure” on the application screen and a 2D and 3D representation of the live-view image will appear along with a measurement scale next to the images.

8. Save your image.

This measurement will verify the thickness of the resist that was applied to the wafer as well as the manufacturers specifications for this photoresist. The speed and process that was programmed into the spin coater was one that was specified by the manufacturer and needs to be accurate.

To get an accurate measurement, you will need to look at several different spots on the image to get an average thickness. Write down your measurements below. You will need 3.

____________ ______________ ______________

Now find the average and write it here: _______________

You will also need the standard deviation, which describes a “tolerance” or how spread out your measurements are from the average value. That is calculated with the formula below.

σ=√ 1N∑i=1

N

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For the purposes of this laboratory experiment the standard deviation will be calculated with Excel. To do this, type in each measurement into its own cell. In a fourth cell type “=StdDev(cell numbers)” and hit enter.

Record the standard deviation here: ______________