Photoresist
A photoresist is a light-sensitive material used in several processes, such as photolithography and photoengraving, to form a patterned coating on a surface. This process is crucial in the electronics industry.
The photoengraving process begins by coating a substrate with a light-sensitive organic material. A patterned mask is then applied to the surface to block light, so that only unmasked regions of the material will be exposed to light. A solvent, called a developer, is then applied to the surface.
In the case of a positive photoresist, the photo-sensitive material is degraded by light, and the developer will dissolve away the regions that were exposed to light, leaving behind a coating where the mask was placed.
In the case of a negative photoresist, the photosensitive material is strengthened by light, and the developer will dissolve away only the regions that were not exposed to light, leaving behind a coating in areas where the mask was not placed.
A BARC may be applied before the photoresist is applied, to avoid reflections from occurring under the photoresist and to improve the photoresist's performance at smaller semiconductor nodes.
Conventional photoresists typically consist of 3 components: resin, sensitizer, and solvent.
Simple resist polarity
Positive: light will weaken the resist, and create a holeNegative: light will toughen the resist and create an etch-resistant mask.
To explain this in graphical form, you may have a graph on Log exposure energy versus fraction of resist thickness remaining. The positive resist will be completely removed at the final exposure energy, and the negative resist will be completely hardened and insoluble by the end of exposure energy. The slope of this graph is the contrast ratio. Intensity is related to energy by E = I*t.
Positive photoresist
A positive photoresist is a type of photoresist in which a portion is exposed to light and becomes soluble to the photoresist developer. The unexposed portion of the photoresist remains insoluble in the photoresist developer.Some examples of positive photoresists are:
PMMA single-component
- Resist for deep-UV, e-beam, x-ray
- Resin itself is DUV sensitive
- Chain scission mechanism
- Common resists for mercury lamps
- Diazonaphthoquinone derivatives, or Diazoquinone ester 20-50% weight
- * photosensitive
- * hydrophobic, not water soluble
- Phenolic Novolak Resin
- * Frequently used for near-UV exposures
- * Water soluble
- * UV exposure destroys the inhibitory effect of DQ
- Issues: Adhesion, Etch Resistance
Negative photoresist
- Based on cyclized polyisoprene
- * variety of sensitizers
- * free radical initiated photo cross-linking of polymers
- Issues:
- * potential oxygen inhibition
- * swelling during development
- ** long narrow lines can become wavy
- ** swelling is an issue for high-resolution patterning
- Example: SU-8, Kodak Photoresist
MTF (modulation transfer function is the ratio of image intensity modulation and object intensity modulation and it is a parameter that indicates the capability of an optical system.
Differences between positive and negative resist
The following table is based on generalizations which are generally accepted in the microelectromechanical systems fabrication industry.| Characteristic | Positive | Negative |
| Adhesion to silicon | Fair | Excellent |
| Relative cost | More expensive | Less expensive |
| Developer base | Aqueous | Organic |
| Solubility in the developer | Exposed region is soluble | Exposed region is insoluble |
| Minimum feature | 0.5 μm | 7 nm |
| Step coverage | Better | Lower |
| Wet chemical resistance | Fair | Excellent |
Classification
Based on the chemical structure of photoresists, they can be classified into three types: photopolymeric, photodecomposing, and photocrosslinking photoresist.- Photopolymeric photoresist is a type of photoresist, usually allyl monomer, that generates free radicals when exposed to light, which then initiates the photopolymerization of the monomer to produce a polymer. Photopolymeric photoresists are usually used for negative photoresist, e.g. methyl methacrylate and poly/PAG blends
- Photocrosslinking photoresist is a type of photoresist that could crosslink chain by chain when exposed to light in order to generate an insoluble network. Photocrosslinking photoresists are usually used for negative photoresist.
- Photodecomposing photoresist is a type of photoresist that generates hydrophilic products under light. Photodecomposing photoresists are usually used for positive photoresist. A typical example is azide quinone, e.g. diazonaphthaquinone.
- For self-assembled monolayer photoresist, first a SAM is formed on the substrate by self-assembly. Then, this surface covered by SAM is irradiated through a mask, similar to other photoresists, which generates a photo-patterned sample in the irradiated areas. And finally, developer is used to remove the designed part.
Light sources
Absorption at UV and shorter wavelengths
In lithography, decreasing the wavelength of light source is the most efficient way to achieve higher resolution. Photoresists are most commonly used at wavelengths in the ultraviolet spectrum or shorter. For example, diazonaphthoquinone absorbs strongly from approximately 300 nm to 450 nm. The absorption bands can be assigned to n-π* and π-π* transitions in the DNQ molecule. In the deep ultraviolet spectrum, the π-π* electronic transition in benzene or carbon double-bond chromophores appears at around 200 nm. Due to the appearance of more possible absorption transitions involving larger energy differences, the absorption tends to increase with shorter wavelength, or larger photon energy. Photons with energies exceeding the ionization potential of the photoresist can also release electrons which are capable of additional exposure of the photoresist. From about 5 eV to about 20 eV, photoionization of outer "valence band" electrons is the main absorption mechanism. Above 20 eV, inner electron ionization and Auger transitions become more important. Photon absorption begins to decrease as the X-ray region is approached, as fewer Auger transitions between deep atomic levels are allowed for the higher photon energy. The absorbed energy can drive further reactions and ultimately dissipates as heat. This is associated with the outgassing and contamination from the photoresist.Electron-beam exposure
Photoresists can also be exposed by electron beams, producing the same results as exposure by light. The main difference is that while photons are absorbed, depositing all their energy at once, electrons deposit their energy gradually, and scatter within the photoresist during this process. As with high-energy wavelengths, many transitions are excited by electron beams, and heating and outgassing are still a concern. The dissociation energy for a C-C bond is 3.6 eV. Secondary electrons generated by primary ionizing radiation have energies sufficient to dissociate this bond, causing scission. In addition, the low-energy electrons have a longer photoresist interaction time due to their lower speed; essentially the electron has to be at rest with respect to the molecule in order to react most strongly via dissociative electron attachment, where the electron comes to rest at the molecule, depositing all its kinetic energy. The resulting scission breaks the original polymer into segments of lower molecular weight, which are more readily dissolved in a solvent, or else releases other chemical species which catalyze further scission reactions. It is not common to select photoresists for electron-beam exposure. Electron beam lithography usually relies on resists dedicated specifically to electron-beam exposure.Parameters
Physical, chemical, and optical properties of photoresists influence their selection for different processes. The primary properties of the photoresist are resolution capability, process dose and focus latitude;Resolution
;Contrast
;Sensitivity
;Viscosity
;Adherence
;Etching resistance
;Surface tension
Chemical amplification
Photoresists used in production for DUV and shorter wavelengths require the use of chemical amplification to increase the sensitivity to the exposure energy. This is done in order to combat the larger absorption at shorter wavelengths. Chemical amplification is also often used in electron-beam exposures to increase the sensitivity to the exposure dose. In the process, acids released by the exposure radiation diffuse during the post-exposure bake step. These acids render surrounding polymer soluble in developer. A single acid molecule can catalyze many such 'deprotection' reactions; hence, fewer photons or electrons are needed. Acid diffusion is important not only to increase photoresist sensitivity and throughput, but also to limit line edge roughness due to shot noise statistics. However, the acid diffusion length is itself a potential resolution limiter. In addition, too much diffusion reduces chemical contrast, leading again to more roughness.The following reactions are an example of commercial chemically amplified photoresists in use today:
- photoacid generator + hν → acid cation + sulfonate anion
- sulfonate anion + hν → e− + sulfonate
- e− + photoacid generator → e− + acid cation + sulfonate anion