Process notes for working with SU-8-2100



Process notes for working with SU-8-2100. Resulting structure heights vary from 100—200 µm. Be sure to obtain the appropriate SU-8 datasheet from .

NOTE: SU-8 procedures are notoriously empirical and these steps should be considered guidelines instead of fixed rules.

REMINDER: Before you go into the cleanroom be sure to get your SU-8-2100.

Substrate Pretreat

1) Before proceeding, make sure you are using nitrile gloves (acetone eats through vinyl). Also, be sure to use proper ventilation when working with SU-8 and developer chemicals.

2) Rinse a 3″ wafer with acetone then isopropyl alcohol (IPA) vigorously. The effluent acetone and IPA should go into an organic waste beaker which should then be disposed of properly.

3) Dry the wafer with nitrogen or air. Inspect the wafer to make sure it is clean. If not, rinse with acetone only and dry again. Use wafer tweezers to handle the wafer properly.

4) Place the wafer on a 125 ºC hotplate for a few minutes. To set the hotplate temperature, press SET and then PLATE TEMP followed by the temperature you want. Finally, press END.

Coat

5) While the wafer is on the 125 ºC hotplate, pour SU-8 from the reagent bottle into a smaller amber bottle (available in the chemical storeroom or look in a cleanroom drawer) until nearly full. The small amber bottle provides easy and controlled dispensing.

6) Remove the wafer from the hotplate. Set the hotplate to 65 ºC so it will be at the correct temperature after the wafer is spun.

7) Clean the spinner with acetone if dirty. Center wafer on spinner chuck. The best way to check if the wafer is centered is to turn on the vacuum and manually spin the wafer a couple of times by hand.

8) Once the wafer is centered, make sure the vacuum is on, and pour a puddle of SU-8 1.25–1.5″ in diameter onto the center of the wafer. Minimize trapping air bubbles in SU-8 by rotating the bottle while pouring.

9) Run the appropriate program (see How to Program the Spinner). Ten seconds at 500 RPM followed by 30 seconds at 2000 RPM is a good starting point.

10) When the program is done remove the edge bead. There are two ways: chemical or mechanical. Chemical is to spin the wafer at a very slow speed and squirt acetone on the edge of the wafer. Mechanical is to wipe the edge of the wafer with a cleanroom wipe or applicator stick while turning the wafer by hand.

11) Turn off the vacuum and remove the wafer from the spinner. Remove any extraneous SU-8 by running a BetaWipe around the edge of the wafer.

Soft Bake

12) Place a clean dummy 4″ wafer on the 65 ºC hotplate. Make sure the hotplate is level by using the bubble level on the 4″ dummy wafer. Please do not melt the bubble level by leaving it on the hotplate too long.

13) Place the spun 3″ wafer on top of the dummy 4″ wafer. The SU-8 should be facing up. Cover the hotplate with an aluminum foil top. Upside-down crystallization dishes may also be used—make sure they hang over the edge of the hotplate slightly to allow evaporated solvent to escape. Plastic (Petri dishes) is not a good idea since the solvent from the SU-8 will dissolve it.

14) Set the hotplate temperature to 95 ºC and the timer for 2:05 (h:m). This is much longer than the Microchem data sheet recommends. You should experiment to find out what works best for you. Use the AUTO OFF feature on the hotplate and allow wafer to cool to room temperature after the bake (about 1 h).

15) Before leaving the cleanroom during the soft bake, be sure to clean out the spinner with acetone as it tends to harden, and temporarily place the used BetaWipes in the fume hood (throw them away when you come back in to expose). Be sure to dump the contents of the organic waste beaker into an appropriate waste bottle.

Expose

16) Carefully separate the 3″ wafers from the 4″ wafers using a razor blade. Slice around the edge of the wafer horizontally instead of trying to pry it apart.

17) Adjust the height of the collimator to 10″ for standard 3″ wafers (see Novacure Setup section below for details). Also, make sure that the UV source is level by using the bubble level. (NOTE: The height should already be set at 10″).

18) Enter your program (4 programs can be stored in memory).

a. Go to SET UP.

b. Enter password (1111).

c. Select EXPOSURE.

d. Follow instructions on screen.

e. Select DOSE and enter the appropriate value (typically 150 J). Press OK.

f. Follow directions on the display for calibrating the light guide.

g. On the first Time screen, simply hit OK. This is the automatic time the unit has calculated it needs for your dose.

h. On the next screen, Max Time, enter a longer time than on the previous screen. The manual recommends 1.3( the automatic time. Press OK.

19) While wearing the UV safety glasses, place a dummy wafer on the base of the ringstand, run any registered program, and focus the spot until the edge is sharp. While the light is still on, use a pair of tweezers to center the wafer within the light circle. When done, press ABORT to turn light off. Press RE-TEST to get back to the exposure menu.

20) Clean off the mask you intend to use with nitrogen or air.

21) Place your SU-8 wafer on top of the dummy wafer, followed by the mask. Make sure that the emulsion side of the mask is down (alternatively, make sure any words or numbers on the mask are readable—i.e., not backwards). Adjust the position of the mask on the silicon to maximize the number of non-uniform SU-8 regions covered by the opaque regions of the mask.

22) Place the glass plate on top of the wafer and mask.

23) Wearing UV safety glasses, run the appropriate program.

Post Expose Bake

24) Place 3″ wafers back on the 4″ wafers and cover. Set the hotplate to 95 ºC and set the timer for 1:05 (h:m). Use AUTO OFF and allow the wafer to cool to room temperature after the bake (about 1 h).

Develop

25) ONLY USE DEVELOPER IN A HOOD OR UNDER A VENTILATION TUBE. Pour just enough developer into a large crystallization dish to cover the 3″ wafer. Gently swirl until all uncured SU-8 has been removed. Continue to develop for an additional 1–2 min.

26) Rinse wafer with acetone for about 10 s and then blow dry with nitrogen or air.

Hard Bake

27) Place the wafers on the hotplate and bake at 150 ºC for 35 min. Be sure to enable AUTO OFF.

Novacure setup

Originally written by H. Zeringue 3/19/02

Figure 1: The Novacure light guide is routed through the ring on the ring stand into the collimator. The size of the spot is adjusted by changing the collimator height (here, 10(). The spot can be focused with a knob on the collimator.

Intensity Calculation

The thickness of the SU-8 layer determines what UV dose should be used. The dose entered into the Novacure is what comes out of the end of the light guide. We have to take into account the height of the collimator in order to know the dose being delivered to the wafer.

Start with the equation

[pic] (1)

in which D is dose (J cm-2), I is intensity (lumen cm-2), and t is time (s). Recall that

[pic] (2)

where E is energy (J) and A is area (cm2). Assume that the energy reaching the end of the light guide (Elg) is the same as the energy reaching the wafer (Ew).

[pic] (3)

Using Eq. 2 to expand Eq. 3, we get

[pic] (4)

Assume the light guide diameter is 0.5 cm and the spot size diameter is 7.6 cm (at the ring stand). Then

[pic] (5)

and

[pic] (6)

which means

[pic] (7)

So to achieve doses of 400 mJ cm-2 and 1600 mJ cm-2 for a thin (20 µm) and thick (400 µm) SU-8 layer, we must set the Novacure for doses of approximately 92 J cm-2 and 370 J cm-2, respectively.

Update log

6-28-2004 – Incorporated suggestions by Bryan Gorman.

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