SU-8 photoresist
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SU-8 is a commonly used epoxy-based negative photoresist. Negative refers to a photoresist whereby the parts exposed to UV become cross-linked, while the remainder of the film remains soluble and can be washed away during development.
As shown in the structural diagram, SU-8 derives its name from the presence of 8 epoxy groups. This is a statistical average per moiety. It is these epoxies that cross-link to give the final structure.
It can be made into a
SU-8 was originally developed as a photoresist for the microelectronics industry, to provide a high-resolution mask for fabrication of semiconductor devices.
It is now mainly used in the fabrication of
Composition and processing
SU-8 is composed of Bisphenol A
SU-8 absorbs light in the UV region, allowing fabrication of relatively thick (hundreds of micrometers) structures with nearly vertical side walls. The fact that a single
The processing of SU-8 is similar to other
The soft bake is the most important of the bake steps for stress formation. It is performed after spin coating. Its function is to remove the solvent from the resist and make the layer solid. Typically at least 5% of the solvent remains in the layer after the soft bake, however the thicker the coating, the harder it becomes to remove the solvent, as evaporating solvent through thick layers becomes increasingly difficult with coating thickness. The bake is performed on a programmable hot plate to reduce the skinning effect of solvent depletion at the surface creating a dense layer which makes the remainder of the solvent more difficult to remove. In order to reduce stress, the bake procedure is generally a two-step process made up of holding at 65 °C before ramping to 95 °C and holding again for a time dependent on the layer thickness. The temperature is then lowered slowly to room temperature.
When dry films are used, the photoresist is laminated rather than spin-coated. As this formulation is essentially solventless (less than 1% solvent remaining), it does not require a soft bake step and does not suffer stress or skinning. For enhanced adhesion, a post lamination bake can be added. This step is carried out in a similar way to the solution based resist - i.e. holding at 65 °C then 95 °C, the time dependent on film thickness.
After this stage the SU-8 layer can now be exposed. Typically this is through a photomask with an inverse pattern, as the resist is negative. The exposure time is a function of exposure dose and film thickness. After exposure the SU-8 needs to be baked again to complete the polymerization. This baking step is not as critical as the prebake but the rising of the temperature (again to 95 °C) needs to be slow and controlled. At this point the resist is ready to be developed.
The main developer for SU-8 is
After exposing and developing, its highly cross-linked structure gives it high stability to chemicals and radiation damage - hence the name "resist". Cured cross-linked SU-8 shows very low levels of outgassing in a vacuum.[11] [12] However it is very difficult to remove, and tends to outgas in an unexposed state.[13]
Newer formulations
SU-8 2000 series resists use cyclopentanone for the primary solvent and can be used to create films between 0.5 and 100 μm in thickness. This formulation may offer improved adhesion on some substrates versus the original formulation.[14]
SU-8 3000 series resists also use cyclopentanone for the primary solvent and are designed to be spun into thicker films ranging from 2 to 75 μm in a single coat.[14]
SU-8 GLM2060 series of low-stress photoresist consist of epoxy GBL and silica formulation CTE 14.[15]
SU-8 GCM3060 Series of GERSTELTEC conductive SU8 with nanoparticles of silver.[15]
SU-8 GMC10xx Series of GERSTELTEC colored SU8 Red, Bleau, Green, black and others.[15]
SU-8 GMJB10XX Series of GERSTELTEC low viscosities epoxy for inkjet applications.[15]
SU8 GM10XX Series of Classic GERSTELTEC epoxy.[16]
Its polymerization process proceeds upon photoactivation of a photoacid generator (triarylsulfonium salts, for example) and subsequent post exposure baking. The polymerization process it a cationic chain growth, which takes place by ring opening polymerization of the epoxide groups.
SUEX is a Thick Dry Film Sheet (TDFS) which is a solventless formulation applied by lamination. As this formulation is a dry sheet, there is high uniformity, no edge-bead[17] formation and very little waste. These sheets come in a range of thicknesses from 100 μm to over 1mm.[18] DJMicrolaminates also sell a thinner range, ADEX TFDS, which are available in thicknesses from 5 μm through to 75 μm.[18]
External links
- SU-8: Thick Photo-Resist for MEMS A webpage with a lot of material data and process tricks.
- http://www.gersteltec.ch/
- Microchem data sheet
- SU 8 Information Provides information on how to use SU 8 to create desired thicknesses.
- SU-8 Spin Speed Calculator Selects a SU-8 type and calculates RPM for a given thickness.
- Suppliers: The solution based SU-8 can be obtained from Microchem or Gersteltec ; the SUEX dry sheets are obtained from DJ Microlaminates , formerly known as DJ Devcorp
References
- ^ "SU-8 Resists: FAQs". MicroChem. Archived from the original on 17 May 2009. Retrieved 21 Jul 2011.
- S2CID 111062837.
- ^ Johnsona DW, Goettertb J, Singhb V, et al. (2012). "SUEX Dry Film Resist – A new Material for High Aspect Ratio Lithography" (PDF). Louisiana State University Proceedings.
- PMID 20126695.
- PMID 23910365.
- PMID 25029537.
- ^ "NANO SU-8: Negative Tone Photoresist - formulations 50-100" (PDF). Microchem.com. 2011. Archived from the original (PDF) on 2012-04-25. Retrieved 12 Jun 2019.
- S2CID 18148474.
- ISBN 9781138072381.
- ^ "SU-8 Developer". Lambers Wiki (Material Safety Data Sheet). 2005. Archived from the original on 11 December 2017. Retrieved 12 Jun 2019.
- ^ "SU-8 photosensitive epoxy". 2003. Archived from the original on 30 May 2012. Retrieved 12 Jun 2019.
- S2CID 137789516.
- ^ "SU-8 Photoresist Processing" (PDF). engineering.tufts.edu. 2007. Archived from the original (PDF) on 9 November 2009. Retrieved 12 Jun 2019.
- ^ a b "SU-8 2000 Permanent Epoxy Negative Photoresist Processing Guidelines" (PDF). Microchem. Archived from the original (PDF) on 15 April 2017.
- ^ a b c d "SU-8 Functional Polymer". Gersteltec Engineering Solutions. Retrieved 12 Jun 2019.
- ^ "SU8". Gersteltec Engineering Solutions. Retrieved 12 Jun 2019.
- ^ S. Arscott, ‘The limits of edge bead planarization and surface levelling in spin-coated liquid films', J. Micromech. Microeng. 30, 025003, (2020).
- ^ a b "SUEX". djmicrolaminates.com. Retrieved 15 Feb 2017.