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ECE 344 PHOTORESIST PROCESSING 

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Contents

• Photoresist Chemistry - a discussion
• PREPARATION OF PHOTORESIST
• DEVELOPMENT
• PHOTORESIST REMOVAL

Photoresist Chemistry

AZ 5214 uses novolak polymer, propylene glycol monomethyl ether acetate (PGMEA) solvent, and diazonaphthoquinone as the sensitizer. On the next page is the chemical reaction used to create the novolak resin. Below that are the three distinguishable reactions undergone by the sensitizer attached to the novolak matrix during exposure. Note that some water is actually required.

For image reversal (negative) processing the indenecarboxylic acid is decomposed by a post exposure bake giving off carbon monoxide and rendering the sensitizer insoluble to the alkali developer. The previously unexposed sensitizer can then be made soluble by a subsequent flood exposure. Since there is no longer sensitizer in the previously exposed areas they will remain insoluble.

Most of the instructions for patterning with photoresist are included in this appendix, but the operation of the Kasper mask aligners is described in appendix G.  Some specific variables, such as etch time, which mask to use and wether to use the image reversal technique, are left to the experiment which references this appendix. The tasks presented here take place in three locations and, therefore, are split accordingly into directions suitable for posting at the spinner hood, the development station, and the PR removal station. All you have to memorize before going into the wetlab is:

• Which mask to use
• Exposure time or dose
• Positive or negative (image reversal) processing
• Which etch to use and for how long
• Etch stop condition (hot point probe usually)

PREPARATION OF PHOTORESIST

Before beginning, record the conditions of the PR room and equipment by making entries in your notebook. The wafer surface should be scrupulously clean before beginning this process.

1. Turn on the PR degreaser if it's not boiling and check that the developer is filled above the line. Ask your instructor for assistance if necessary.
2. Drive any moisture out of your wafer with a 2 minute bake on the bakeout hotplate (3 minutes if the room humidity is >60%). While you wait, check that the spinner is set for a 30 duration.
3. Allow your wafer to cool on the "cool block" for 20 seconds. Wipe off the spinner chuck with a kimwipe while waiting.
4. Center the wafer on the spinner chuck and start the spinner by momentarily pressing the front of the foot switch.
5. While the wafer is spinning, spray it with nitrogen from the N2 arm and check that it's spinning at 3000rpm.

Vacuum is applied to the chuck only while spinning. You may stop the spinner by pressing the back of the foot switch, but don't continue blowing nitrogen on it when it stops. The spinner will automatically stop after the preset time, usually 30 seconds.

6. While the spinner is spinning the wafer, drop three drops of hexamethyldisilazane (HMDS) onto the center of the wafer and stop the spinner as soon as the appearance of the wafer remains constant.

HMDS behaves as a surfactant, a wetting agent. It helps the photoresist adhere better. Think of it much like a detergent. In this case, the organic end (hexamethyl) of the molecule is similar to and binds well to the organic PR. The silazane end, being silicon based, sticks well to the the wafer/oxide surfaces. It even seems to help adhesion on aluminum as well.

7. Immediately place 12 to 14 drops of AZ 5214 positive resist on and around the center of the wafer using the filtered syringe. A pattern like that below works well for the first 9 drops. Use the last few to fill in any voids between the drops. Use extra drops if necessary to fill all interior dry spots within the PR puddle.



8. Wait at least 5 seconds after the drops completely flow together, then start the spinner. This time, let the spinner stop by itself.

A perfectly uniform and total coating is unnecessary because perfect processing would yield over 2,000 transistors which is almost 2,000 more than you would WANT to test. Don't worry about losing as much as 10% of your yield. You're not trying to make a profit from this anyway. A few bad transistors can sometimes be more educational than thousands of perfect ones.

9. Bake for 45 seconds on the Softbake hotplate.
10. Allow the wafer to cool for a few seconds on the "cool" plate.
11. The photoresist is now ready for exposure to ultraviolet light through a mask. Refer to the instructions for the mask aligners (Appendix G) which should be posted near the aligners.
12. If image reversal is desired, expose for a dose of 125mW cm^2/sec. Follow the exposure by a 60 second bake on the reversal bake hotplate and a 20 second flood exposure. This may reduce the development time unless more dilute developer is used.
13. Develop until pattern is sharp (see Development procedure to be posted on developer hood.)
14. Finally, complete the preparation of the PR for use as an etch stop by performing a 60 second hardbake on the hardbake hotplate (currently the same as the softbake hotplate).
15. Cool the PR for a few seconds on the "cool" plate before putting it in the wafer carrier.
16. The photoresist is now prepared for the etch (or deposition if using the lift-off technique).

DEVELOPMENT

A standard artists air brush will be used to continuously spray fresh developer on the wafer. Dip develpment has the advantage of not piting the surface of the PR, but the repeatability is low since it slows as it loads up with PR. In industry, 500rpm spinners beneath a low velocity nozzle are common.

1. It is stongly recommended that you change the D.I. rinses before beginning.
2. Open the faucet valve just barely enough to maintain a continuous stream (as opposed to a sequence of individual drops.)
3. Hold the wafer horizontally over the sink at the counter top height with tweezers from the side.
4. Pick up the sprayer and your wafer (use tweezers for your wafer of course.)
5. Note the time on the clock at the back of the hood so you can measure the development interval.
6. Begin spraying downward at a steep angle next to the wafer from a height of 8" to 10". Try to avoid getting any developer on your gloves.
7. Move the spray onto the wafer and employ a circular motion to make a uniform puddle form on the wafer.
8. As soon as the puddle covers the wafer, use a spiral motion to move in to a distance of 4" + 1". Let the developer puddle on your wafer. A radius of of motion of about .5" to .75" will help counteract the fact that the UV illumination was strongest in the center. Keep the wafer flat so a deep puddle protects the PR from the mechanical force of the sprayer at this distance. All the PR is soluble, it's just a matter of degree.
9. Spray until the pattern is sharp, approximately 35-40 seconds for the positive lithographic process. Pay most attention to the test areas. They will quickly cloud up with a pattern of multicolored blobs as the PR gets so thin that interference effects cause all the colors of the rainbow to be accentuated by the continuum of PR thicknesses. When they seem clear and uniform (or don't seem to change for 5 seconds), quit.
10. Quickly quench the developer in the first DI rinse and return the sprayer to its holder. Be careful not to break your wafer by swishing it too fast in the rinse.
11. Note the time.
12. Move to the FINAL RINSE tank for at least 10 seconds while you calculate the total time spent developing.
13. Gently rinse your wafer with DI from the faucet (near the nozle) and N2 dry (NO IPA.) Excessive water velocity can pit the surface of the PR. Although this is not usually a problem, it does look bad until the PR is removed. Fresh DI from the sprayer is used in case the rinse tank has enough PR in it from previous students to deposit a thin invisible film on the wafer. Students may change the DI rinse at their own discretion.
14. Return the wafer to the carrier face up.
15. Gloves and tweezers should be rinsed in D.I. if developer is suspected on them. Used developer is highly water soluble, but can be difficult to remove if left to dry on things.
16. Inspect the development under a UV filtered microscope. Pay particular attention to the smallest windows to be opened. They may take a little longer to develop because it's harder for fresh developer to reach the bottom to dissolve the PR. Gross under development will appear as splotches with multicolored rings in the larger areas which should be clear of PR. Why? Ignore the outer rim of the wafer because edge beading of the PR causes it to be too thick to expect proper patterning.
17. Repeat in 10 second intervals if necessary.

The solubility change during exposure of photoresist is a couple of orders of magnitude at best (although chemists are constantly improving it). Consequently, all the PR is will eventually dissolve in the developer. Before that happens the openings in the PR will widen and loose their sharpness. Therefore, development time should be minimized. The smallest windows which are to be opened are usually the limiting factor because fresh developer must diffuse down to the surface in order to do its job. This diffusion is slowed when the width of the window in the PR is comparable to its depth. A possible technique to minimize this effect would be to hold the wafer at an angle so the PR could not puddle. Unfortunately, PR adhesion and sprayer uniformity become greater problems (at least in ECE 344 lab). A compromise seems to be the best solution. Periodic tilting of the wafer to help change the developer in the small windows is suggested. Exactly, how often is optimum has not been determined. Record any useful observations you make which would help in your notebook.

PHOTORESIST REMOVAL

1. After the etch, the PR may be removed by ONE of the following three methods. Do not try one of the other methods before performing the microscope inspection step.
1. Use acetone, PGMEA, or other solvent to remove the majority of the PR by making a puddle of the solvent on the wafer while holding it level above the proper waste container. Pour the solvent off after 10-15 seconds and repeat until there is no significant improvement. Then degrease it in the "PR" degreaser for 1 minute, squirt with acetone, IPA, water, IPA again, and N2 dry.
2. Use the plasma asher. See http://fabweb.ece.uiuc.edu:1999/equipment/Asher/Instructi ons.html for instructions. This is the most reliable method, but also the most time consuming. Please try it at least once during the semester.
3. Use acetone, PGMEA, or other solvent to remove the majority of the PR as in the first method. Then use heated Posistrip, Microposit Remover, or other proprietary positive PR remover. DI rinse, N2 dry.
2. Inspect for residual PR under a microscope (preferably outside the wet lab).

Pay particular attention to the rim of the wafer where edge beading during the spin on process left extra thick PR. Removing the yellow UV filter from the illuminator will help you see PR, but be sure to return it. The residue will often look similar to slightly underdeveloped PR. Since such residues are likely to be very thin with respect to visible light wavelengths, they often take on a rainbow of colorations as the thickness variations cause different interference patterns. PR is a furnace contaminant and must be completely removed. The whole class is counting on you keep the furnaces clean.

3. If PR residue is detectable, go back to step 1. If there is only a little left, additional soaking with acetone will usually take care of it. Stubborn PR may need the plasma asher however. Occasionally, etched features will look like PR residue so consult with your instructor before going back to step 1 a third time.

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Warning! This is the archived 1999 Fabweb site! Here is the latest site