2.4 Basic Steps for UV Lithography of SU-8 and Some Processing Tips
The standard lithography processing procedures of SU-8 include: (1) pretreat the substrate, spin-coat SU-8; (2) preexposure bake, UV exposure (320 to 450 nm); (3) postexposure bake; and (4) development. The process parameters determine the final quality of the microstructures. The curing process of SU- 8 is completed in two steps: formation of acid during optical exposure and thermal epoxy cross-linking during the postexposure bake. A flood exposure or controlled hard bake is recommended to further cross-link the exposed SU-8 microstructures if they are going to be used as parts of the final prod- ucts. Because most of the publications in the field do not provide detailed lithography conditions, beginners often have to learn from their own expe- riences and the learning curve can sometimes be exceptionally long. Some basic lithography conditions are provided here as guidelines for those read- ers who may need something to start from [21–24].
2.4.1 Pretreat for the Substrate
To obtain good adhesion for SU-8 on a substrates, the substrate needs to be cleaned with acetone, IPA, and DI water sequentially, and then dehydrated
FIGURE 2.8
Comb structures made using filtered light source and gap compensation with glycerin.
26 Bio-MEMS: Technologies and Applications at 120°C for 5 to approximately 10 minutes on a hotplate. The substrate may also be primed using plasma asher immediately before spin-coating the resist. In addition, an adhesion promoter may be used as needed. For the applications involving electroplating metals and alloys and stripping of cured SU-8, the vendor of SU-8, MicroChem, recommends using OmniCoat before coating of SU-8.
2.4.2 Spin-Coating SU-8
The thickness of SU-8 film is dependent on several factors: the viscosity of the SU-8 used, the spin speed, and the total number of turns. The vendor of SU-8, MicroChem, provides some spin-coating curves for different SU-8 formulations, such as SU-8 5, SU-8 50, and SU-8 100. Some research labs have also developed their own spin-coat curves based on the particular equipment used. Figure 2.9 shows some typical spin-coating curves of SU-8.
FIGURE 2.9
Selective SU-8 spin-speed vs. film thickness curve. (Courtesy of Mark Shaw, MicroChem Corp., Newton, MA.)
SU-8 spin speed curves
0 5 10 15 20 25 30 35 40 45
750 1000 1250 1500 1750 2000 2250 2500 2750 3000 3250 Spin speed (rpm)
Film thickness (microns)
SU-8-2 SU-8-5 SU-8-10 SU-8-25
SU-8 spin speed curves
0 50 100 150 200 250
750 1000 1250 1500 1750 2000 2250 2500 2750 3000 3250 Spin speed (rpm)
SU-8-50 SU-8-100 DK532X_book.fm Page 26 Tuesday, November 14, 2006 10:41 AM
UV Lithography of Ultrathick SU-8 27 Bubbles formed during the spin-coating step may lead to reduced lithogra- phy quality. To eliminate bubbles in resist film, the substrate should be placed on a flat and horizontal plate for 2 to approximately 10 hours before prebake.
This is an especially critical step for obtaining good quality of thick SU-8 film.
2.4.3 Soft Bake
The spin-coated sample needs to be soft baked to evaporate the solvent on a leveled hotplate or in convection ovens. The heat transfer condition and ventilation are different for the hotplate and the convection ovens, and the preferred soft baking times are therefore different as shown by the curves for measured soft baking times in Figure 2.10. Ramping and stepping the soft bake temperature is often recommended for better lithography results.
The glass temperature of the unexposed SU-8 photoresist is about 50 to approximately 60°C. Figure 2.11 shows a typical soft-baking temperature curve used in our laboratory. This soft-bake process consists of multiple steps of ramping up, dwell, and ramping down. The total cooling time is about 8
FIGURE 2.10
Soft bake time vs. SU-8 thickness.
FIGURE 2.11
A selected soft bake profile for 1100 mm–thick SU-8 film.
10 8 6
Bake time (hours) 4 2 0
0 200
Thickness (µm) Soft bake time vs. thickness
400 600 800 1000
Bake in oven Bake on hot-plate
Dwell at 50°C/4 hrs
Ramp to 50°C in 40 m Dwell at 75°C/15 ms
Ramp to 75°C in 40 m Ramp to 110°C in 30 m
Dwell at 75°C/15 ms Ramp to 75°C in 30 m
20°C/1~2 hrs Ramp to 20°C in 3 hrs
Dwell at 110°C/10 hrs
28 Bio-MEMS: Technologies and Applications to approximately 10 hours for a 1000 µm–thick SU-8 resist. For ultrathick SU-8 film (more than 1000 µm thick), a baking temperature of 110°C is used coated with Cr/Au film (as commonly used in the UV-LIGA process as the plating seed layer), a 110°C bake temperature is suggested instead of 96°C. At the same time, the bake time should be slightly reduced.
2.4.4 Exposure
A near UV (320 to 450 nm) light source is normally used for lithography of SU-8. As the wavelength of the light source increases, the absorbance of the light reduces and the transmission increases significantly. The transmission increases from 6% at λ = 365 nm to about 58% as the wavelength increases to 405 nm. SU-8 has high actinic absorption for wavelengths less than 350 nm, but is almost transparent and insensitive for above 400 nm wavelengths.
Because of the high absorption of SU-8 for light with shorter wavelengths, a light source dominated by shorter wavelength components often results in overexposure at the surface of the resist and underexposure at the bottom part of the resist layer. This is the main reason that UV lithography of SU using an i-line-dominated light source tend to produce microstructures with T-topping geometric distortions. Thickness of the resist is another key parameter that dictates the required dosage of the exposure. Figure 2.12 shows two curves of required exposure dosage and the thickness of SU-8. MicroChem, the vendor of SU-8, advises that the user filter out the light with a wavelength lower than 350 nm to improve lithography quality. After filtering the light components with wavelengths shorter than 350 nm from the light source of the Oriel UV station used in our laboratory, with its spectrum as shown in kept in a range of 1:7 to approximately 1:10 to achieve perfect vertical side- walls, especially for the SU-8 resist with thickness around 1 mm. For lithog- raphy of a very thick resist, multiple exposures are required to avoid
FIGURE 2.12
Exposure dosage vs. film thickness: the preferred exposure dosage should fall between the top and bottom curves. (Courtesy of Mark Shaw, MicroChem Corp., Newton, MA.)
800 600 400
Esposure energy (mJ/cmˆ2)
200 0
0 25 50 75 100 125 Film thickness (µm)
150 175 200 225 250 DK532X_book.fm Page 28 Tuesday, November 14, 2006 10:41 AM
as shown in Figure 2.11. To improve the adhesion of the SU-8 film on substrate
Figure 2.6, the total exposure dosage ratio between the i-line and h-line are
UV Lithography of Ultrathick SU-8 29 overheating, scattering, and diffusion on the surface of the resist. Typically, exposures need to be separated in 20-second (or less than 400 mJ/cm2 per time) intervals with 60-second waiting periods in between. For a highly reflective substrate, the effect of the reflection needs to be taken into account in estimating the total exposure time.
2.4.5 Postexposure Bake (PEB)
Postexposure bake (PEB) is performed to cross-link the exposed regions of the SU-8 resist. The cross-link, or the curing step of SU-8, can be achieved at room temperature. Postbaking at a raised temperature helps accelerate profile. For resist thickness up to a few hundred micrometers, postbake at 96°C for 15 to approximately 20 minutes is required either on a hotplate or in a convection oven. SU-8’s cross-link process may cause significant residual stress, which may cause cracks or debonding. In order to minimize possible residual stresses, wafer bowing, and cracking, rapid cooling from the PEB temperature should be avoided. For resist films with thicknesses more than 1000 micrometers, ramping the PEB temperature down from 96°C should take more than 8 hours. Another possible way to reduce postbake stress is to use lower PEB temperatures, such as 50°C or 55°C, but longer baking times. This method would result in much lower thermal stress in comparison with using a PEB temperature of 96°C.
2.4.6 Development
After exposure and postbake, the sample is then developed by SU-8 devel- oper. Recommended development times can be found in the catalog pro- vided by vendor of SU-8 or your lab’s experiment data. The development process can be optimized based on the experiment’s agitation rate, develop- ment temperature, and SU-8 resist processing conditions. After the sample is developed by SU-8 developer, it is sometimes dipped into a fresh SU-8 developer to rinse, then rinsed with isopropyl alcohol (IPA) for 3 to 5 min- utes. If white spots can be observed in the IPA, the SU-8 is underdeveloped.
FIGURE 2.13
A possible temperature profile to be followed in PEB for 1100 µm–thick SU-8 film.
Dwell at 50°C/4 hr
Ramp to 50°C in 30 m Dwell at 75°C/10 m
Ramp to 75°C in 30 m Ramp to 96°C in 20 m
Dwell at 75°C/10 m Ramp to 75°C in 30 m
Ramp to 20°C in 3 hr Dwell at 96°C/20 m
the polymerization process [20]. Figure 2.13 shows a typical PEB temperature
30 Bio-MEMS: Technologies and Applications The sample needs to be immersed into SU-8 developer or rinsed with fresh SU-8 developer to further development. After the sample is completely developed, it needs to be rinsed using fresh IPA. If possible, avoiding a deionized (DI) water rinse is preferred. Finally, the sample is dried naturally or by nitrogen gas blow.