SU-8 photoresist
Encyclopedia
SU-8 is a commonly used epoxy
-based negative photoresist
. It is a very viscous polymer
that can be spun
or spread over a thickness ranging from <1 micrometer
up to >300 micrometer
and still be processed with standard contact lithography
. It can be used to pattern high aspect ratio
(>20) structures. Its maximum absorption
is for ultraviolet
light with a wavelength
of 365 nm (it is not practical to expose SU-8 with g-line ultraviolet light). When exposed, SU-8's long molecular chains cross-link
causing the solidification of the material. SU-8 series photoresists use gamma butyrolactone as the primary solvent.
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 microfluidics
(mainly via soft lithography
, but also with other imprinting techniques such as nanoimprint lithography
) and microelectromechanical systems
parts. It is also one of the most biocompatible
materials known and is often used in bio-MEMS
.
SU-8 is highly transparent in the ultraviolet region, allowing fabrication of relatively thick (hundreds of micrometers) structures with nearly vertical side walls. After exposition and developing, its highly cross-linked structure gives it high stability to chemicals and radiation damage
. Cured cross-linked SU-8 shows very low levels of outgassing
in a vacuum
. However it is very difficult to remove, and tends to outgas in an unexposed state.
The main developer for SU-8 is 1-Methoxy-2-propanol acetate.
for the primary solvent and can be used to create films between 0.5 and 200 microns in thickness. This formulation may offer improved adhesion on some substrates verses the original formulation.
SU-8 3000 series resists also use cyclopentanone for the primary solvent and are designed to be spun into thicker films ranging from 4 to 150 microns in a single coat.
Epoxy
Epoxy, also known as polyepoxide, is a thermosetting polymer formed from reaction of an epoxide "resin" with polyamine "hardener". Epoxy has a wide range of applications, including fiber-reinforced plastic materials and general purpose adhesives....
-based negative photoresist
Photoresist
A photoresist is a light-sensitive material used in several industrial processes, such as photolithography and photoengraving to form a patterned coating on a surface.-Tone:Photoresists are classified into two groups: positive resists and negative resists....
. It is a very viscous polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
that can be spun
Spin coating
Spin coating is a procedure used to apply uniform thin films to flat substrates. In short, an excess amount of a solution is placed on the substrate, which is then rotated at high speed in order to spread the fluid by centrifugal force...
or spread over a thickness ranging from <1 micrometer
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
up to >300 micrometer
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
and still be processed with standard contact lithography
Contact lithography
Contact lithography, also known as contact printing, is a form of photolithography whereby the image to be printed is obtained by illumination of a photomask in direct contact with a substrate coated with an imaging photoresist layer.-History:...
. It can be used to pattern high aspect ratio
Aspect ratio
The aspect ratio of a shape is the ratio of its longer dimension to its shorter dimension. It may be applied to two characteristic dimensions of a three-dimensional shape, such as the ratio of the longest and shortest axis, or for symmetrical objects that are described by just two measurements,...
(>20) structures. Its maximum absorption
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
is for ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
light with a wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of 365 nm (it is not practical to expose SU-8 with g-line ultraviolet light). When exposed, SU-8's long molecular chains cross-link
Cross-link
Cross-links are bonds that link one polymer chain to another. They can be covalent bonds or ionic bonds. "Polymer chains" can refer to synthetic polymers or natural polymers . When the term "cross-linking" is used in the synthetic polymer science field, it usually refers to the use of...
causing the solidification of the material. SU-8 series photoresists use gamma butyrolactone as the primary solvent.
SU-8 was originally developed as a photoresist for the microelectronics
Microelectronics
Microelectronics is a subfield of electronics. As the name suggests, microelectronics relates to the study and manufacture of very small electronic components. Usually, but not always, this means micrometre-scale or smaller,. These devices are made from semiconductors...
industry, to provide a high-resolution mask for fabrication of semiconductor devices.
It is now mainly used in the fabrication of microfluidics
Microfluidics
Microfluidics deals with the behavior, precise control and manipulation of fluids that are geometrically constrained to a small, typically sub-millimeter, scale.Typically, micro means one of the following features:* small volumes...
(mainly via soft lithography
Soft lithography
200px|right|thumb|Figure 1 - "Inking" a stamp. PDMS stamp with pattern is placed in Ethanol and ODT solution200px|right|thumb|Figure 2 - ODT from the solution settles down onto the PDMS stamp. Stamp now has ODT attached to it which acts as the ink....
, but also with other imprinting techniques such as nanoimprint lithography
Nanoimprint Lithography
Nanoimprint lithography is a method of fabricating nanometer scale patterns. It is a simple nanolithography process with low cost, high throughput and high resolution. It creates patterns by mechanical deformation of imprint resist and subsequent processes. The imprint resist is typically a monomer...
) and microelectromechanical systems
Microelectromechanical systems
Microelectromechanical systems is the technology of very small mechanical devices driven by electricity; it merges at the nano-scale into nanoelectromechanical systems and nanotechnology...
parts. It is also one of the most biocompatible
Biocompatibility
Biocompatibility is related to the behavior of biomaterials in various contexts. The term may refer to specific properties of a material without specifying where or how the material is used , or to more empirical clinical success of a whole device in...
materials known and is often used in bio-MEMS
Bio-MEMS
Bio-MEMS is an abbreviation of biological microelectromechanical systems and refers to a special class of microelectromechanical systems where biological matter is manipulated to analyze and measure its activity under any class of scientific study. This class of devices belongs to one of the areas...
.
SU-8 is highly transparent in the ultraviolet region, allowing fabrication of relatively thick (hundreds of micrometers) structures with nearly vertical side walls. After exposition and developing, its highly cross-linked structure gives it high stability to chemicals and radiation damage
Radiation damage
Radiation damage is a term associated with ionizing radiation.-Causes:This radiation may take several forms:*Cosmic rays and subsequent energetic particles caused by their collision with the atmosphere and other materials....
. Cured cross-linked SU-8 shows very low levels of outgassing
Outgassing
Outgassing is the release of a gas that was dissolved, trapped, frozen or absorbed in some material. As an example, research has shown how the concentration of carbon dioxide in the Earth's atmosphere has sometimes been linked to ocean outgassing...
in a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
. However it is very difficult to remove, and tends to outgas in an unexposed state.
The main developer for SU-8 is 1-Methoxy-2-propanol acetate.
Newer formulations
SU-8 2000 series resists use cyclopentanoneCyclopentanone
Cyclopentanone is a colorless liquid organic compound with a peppermint-like odor. It is a cyclic ketone, structurally similar to cyclopentane, consisting of a five-membered ring containing a ketone functional group.-Safety:...
for the primary solvent and can be used to create films between 0.5 and 200 microns in thickness. This formulation may offer improved adhesion on some substrates verses the original formulation.
SU-8 3000 series resists also use cyclopentanone for the primary solvent and are designed to be spun into thicker films ranging from 4 to 150 microns in a single coat.
External links
- SU-8: Thick Photo-Resist for MEMS A webpage with a lot of material data and process tricks.
- SU-8 Forum
- Microchem data sheet
- SU 8 Information Provides information on how to use SU 8 to create desired thicknesses.
- Suppliers: Microchem , Gersteltec