Specific Process Knowledge/Lithography/SU-8

Resist description

SU-8 is an epoxy based negative i-line photoresist with a high contrast. More then 10:1 aspect ratio imaging with vertical sidewalls is one of the widely used properties of this resist. It also has high chemical, plasma and temperature resistance after curing which makes it well suited for permanent use applications.

Note: Since the SU-8 is epoxy-based, the user is required to take the mandatory epoxy course "Personlig sikkerhed ved arbejde med epoxy". Registration to this course can be found on Portalen under: Arbejdsmiljø kurser

In the cleanroom the SU-8 2000 product line is widely used, where the SU-8 2005, SU-8 2035, and SU-8 2075 are supplied by Nanolab. Other resist formulations can be used, but has to be ordered by the user.

Process recommendations:

• a standard process consist of spinning, soft bake, exposure, post-exposure bake and development. Please notice that the entire process must be done continuously, meaning that it is not recommendable to spin the resist and then wait until the next day before exposing
• exposure using radiation above 350 nm is recommended
• Controlled soft and post-exposure bakes are recommended and must be optimized for the specific application, but baseline parameters which can be used as a starting point will be given

Pretreatment

• To dehydrate the surface, bake in 250C oven at least 30 min - the longer the bake, the better. Baking overnight is recommended
• Another dehydration step can be done in the HMDS oven using recipe 9. This does not expose the wafers to HMDS but instead makes a bake-out at reduced pressure

Spinning

SU-8 resist is designed to produce low defect coatings over a very broad range of films thickness. Spin Coater: RCD8 is dedicated to spin all SU-8 resist formulations.

Safety rules:
To keep the cleanroom safe and odor free during the spinning, please follow the following rules:

• Always use point exhaust above the place, where you handle the resist
• When you move the wafers to the dedicated hotplates, always carry them in a closed container

Recommenced spin coating conditions:

• Dispense manually from bottle/syringe for the thin resists or use a syringe dispense system for the thick resists
• Use static dispense; approximately 1 ml of SU-8 per inch of substrate diameter
• For SU-8 layers thicker then 100 um use two-step spin coating procedure:
  • 5 second spread cycle 500 rpm, acceleration 100 rpm/s, following by:
  • thickness definition cycle with the final spin speed
  • layer uniformity of the thick films can be improved by using a rotation cover (Gyrset), but remember the speed limitation is then 3000 rpm
• For SU-8 layers thinner then 10 um the spread cycle can be omitted to improve the uniformity of the films

Spin curves: Spin curves for SU-8 2002 and SU-8 2005 (from the old KS Spinner): Spinning curves for SU-8 2002 and 2005.

• A rotation cover (Gyrset) can be also used in case you need to spin less than 1 um resist film
  • 3000 rpm, 300 rpm/s, 30 sec with gyrset gives approximately 850 nm film thickness, when using SU-8 2002
• Another way to get a thinner film is to dilute SU-8 2002 in cyclopentanone solution
  • 120 nm film: dilute 25 ml SU-8 2002 in 80 ml cyclopentanone, spin at 7000 rpm
  • 60-80 nm film: dilute 5 ml SU-8 2002 in 32 ml cyclopentanone, spin at 2000 - 7000 rpm

Baking

SU-8 baking can be divided in 2 steps:

1. Soft bake step after spinning
2. Post exposure bake after exposure

The goal of the soft bake step is to reduce the amount of solvent in the photoresist layer. The applied heat evaporates the solvent from the resist film. The soft bake step is be done on dedicated SU-8 hotplates, and is usually done in one or two steps with a 5-8 deg/min temperature ramp between the steps.
The soft bake temperature is typically between 20°C and 110°C, and the soft bake time depends on the film thickness. After soft bake the resist should be firm to the touch (not sticky), in order to be used in the exposure tools.

The goal of the post exposure bake is to increase the polymerization reaction (the cross-linking). The post exposure step can be done on the same hotplates as the soft bake step, with the temperature range between 20°C and 120°C, and with temperature ramping of 5-8 deg/min. NB: It is important to do the post exposure bake immediately after exposure to limit photoinitiator diffusion into non-exposed areas.

Exposure

The exposure step is usually done in near UV-radiation at aligners system which blocks the wavelengths below the i-line (365nm wavelength). KS Aligner and Aligner: MA6-2 both have i-line filters. Aligner: Maskless 01 (MLA1) has a 365nm source. Exposure on Aligner: Maskless 02 (MLA2) using the 375nm source works, but needs a higher dose, and the resist thickness is limited on this tool.

Duration of the UV-exposure depends of resist thickness, but notice that if the UV exposure becomes long then the heating at the interface with the mask can cause formation of hard skin at the surface of SU-8 so called T-topping phenomena. The the main rule is if the dose of the exposure exceeds 250-300 mJ/cm2 it is recommended to do a multiple exposure instead and have a waiting time between the steps to allow the resist cool down.

Developing

Developing of the SU-8 patterns is done in the special developing station placed in cleanroom C-1 at Danchip. Under developing the substrates are immersed in Propylene Glycol Methyl Ether Acetate (PGMEA) first in a bath marked "First" and after in a bath marked "Final".

The development time is dependent on the layer thickness. Minimum development time: 1 min per 20µm in "First".

After the development the substrate must be rinsed with Isopropyl Alcohol (IPA) on both side for 30 sec. White traces during rinsing indicate incomplete development of SU-8.

Finally the substrates are dried in air in the drying box or with a nitrogen gun.

PhD dissertations

• PhD Thesis by Thomas Anhøj (February 2007) on process optimization of 40µm thick SU-8 25 for lab-on-a-chip.
  • Fabrication of High Aspect Ratio SU-8 Structures for Integrated Spectrometers
  • Slides from defence