PHOTORESIST CHEMISTRY

The ECE 344 lab currently uses AZ 5214 photoresist (PR) because it's safer than most and because it can be processed as either positive or negative PR. It is also the choice of many of many researchers. Since it costs ~$600 per gallon, it will never be competitive in large scale production, however. Luckily, the chemistries for all positive photoresists are similar so learning about this one will not be for naught. Photoresists all contain 3 basic components: an organic polymer (usually novolak resin) which is what "resists" etchants, a solvent carrier (usually an acetate) so it's a liquid for easy application by spinning, and a photoactive component often called a sensitizer (usually a diazamine), which causes the solubility to become dependent upon exposure to UV radiation.

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, and the operation of the Ultratech steppers is described in Appendix H.  Some specific variables, such as etch time, which mask to use and whether 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.

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. 

(to be posted at developer hood.)

Two methods of development will be presented in lab. A standard artists air brush will be used to continuously spray fresh developer on the wafer for 2" wafer development.. Dip develpment has the advantage of not piting the surface of the PR and is used for development of the 4" wafers, although careful inspections must be followed since development slow as the PR loads up after multiple wafers have been processed. 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 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.  

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 (if TCE is available), 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.