Photoresist is a light-sensitive material used in several industrial processes, such as photolithography and photoengraving to form a patterned coating on a surface. Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 Photolithography (also called optical lithography) is a process used in Microfabrication to selectively remove parts of a thin film (or the bulk of a substrate Photoengraving also known as photo-chemical Milling is a process of Engraving using Photographic techniques The most common type of photoengraving involves
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Photoresist tone
Photoresists are classified into two groups, positive resists and negative resists.
- A positive resist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes soluble to the photoresist developer and the portion of the photoresist that is unexposed remains insoluble to the photoresist developer.
- A negative resist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes relatively insoluble to the photoresist developer. The unexposed portion of the photoresist is dissolved by the photoresist developer.
Differences Between Tone Types
Note: This table is based on generalizations which are generally accepted in the MEMS fabrication industry. Microelectromechanical systems ( MEMS) is the technology of the very small and merges at the nano-scale into Nanoelectromechanical systems (NEMS and Nanotechnology
Absorption at UV and shorter wavelengths
Photoresists are most commonly used at wavelengths in the ultraviolet spectrum or shorter (<400 nm). Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays A nanometre ( American spelling: nanometer, symbol nm) ( Greek: νάνος nanos dwarf; μετρώ metrό count) is a For example, diazonaphthoquinone (DNQ) absorbs strongly from approximately 300 nm to 450 nm. The absorption bands can be assigned to n-p* (S0-S1) and p-p* (S1-S2) transitions in the DNQ molecule [2]. In the deep ultraviolet (DUV) spectrum, the π-π* electronic transition in benzene [3] or carbon double-bond chromophores [4] appears at around 200 nm. Benzene, or benzol, is an organic Chemical compound and a known Carcinogen with the molecular formula C 6 H 6 A carbon-carbon bond is a Covalent bond between two Carbon Atoms. A chromophore is part (or moiety) of a Molecule responsible for its Color. 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 (typically 8 eV) can also release electrons which are capable of additional exposure of the photoresist. The ionization potential, ionization energy or EI of an Atom or Molecule is the Energy required to remove an Electron From about 8 eV to about 20 eV, photoionization of outer "valence band" electrons is the main absorption mechanism[5]. Above 20 eV, inner electron ionization and Auger transitions become more important. Electron ionization ( EI, formerly known as electron impact) is an Ionization technique widely used in Mass spectrometry, particularly for The Auger effect (ˈɔːʒɚ or Oh' jeh is a phenomenon in Physics in which the emission of an Electron from an Atom causes the emission of a second Photon absorption begins to decrease as the X-ray region is approached, as fewer Auger transitions between deep atomic levels are allowed for the relatively 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. Carbon (kɑɹbən is a Chemical element with the symbol C and its Atomic number is 6 6 eV. Secondary electrons generated by primary ionizing radiation have energies sufficient to dissociate this bond, causing scission. Secondary electrons are electrons generated as ionization products Image talkNew_radiation_symbol_ISO_21482svg for details --> Ionizing radiation In addition, the low-energy electrons have a longer photoresist interaction time due to their lower speed. Scission breaks the original polymer into segments of lower molecular weight, which are more readily dissolved in a solvent. The molecular mass (abbreviated m of a substance, more commonly referred to as molecular weight and abbreviated as MW, is the Mass of one A solvent is a liquid or gas that dissolves a solid liquid or gaseous Solute, resulting in a Solution.
It is not common to select photoresists for electron-beam exposure. Electron beam lithography usually relies on resists dedicated specifically to electron-beam exposure. Electron beam lithography (often abbreviated as e-beam lithography) is the practice of scanning a beam of Electrons in a patterned fashion across a surface covered
DNQ-Novolac photoresist
One very common positive photoresist used with the I, G and H-lines from a mercury-vapor lamp is based on a mixture of (d)iazo(n)aphtho(q)uinone (DNQ) and novolac resin (a phenol formaldehyde resin). The earliest commercial synthetic Resin is based on a Phenol formaldehyde resin (PF with the commercial name Bakelite, and is formed from an Elimination The earliest commercial synthetic Resin is based on a Phenol formaldehyde resin (PF with the commercial name Bakelite, and is formed from an Elimination DNQ inhibits the dissolution of the novolac resin, however, upon exposure to light, the dissolution rate increases even beyond that of pure novolac. The mechanism by which unexposed DNQ inhibits novolac dissolution is not well understood, but is believed to be related to hydrogen bonding (or more exactly diazocoupling in the unexposed region). A hydrogen bond results from a Dipole-dipole force between an Electronegative atom and a Hydrogen atom bonded to Nitrogen, Oxygen Diazo refers to a type of Organic compound that has two linked nitrogen (azo compounds DNQ-novolac resists are developed by dissolution in a basic solution (usually 0. 26N tetra-methyl ammonium hydroxide in water).
Negative photoresist
One very common negative photoresist is based on epoxy-based polymer. In Chemistry, epoxy or polyepoxide is a Thermosetting Epoxide Polymer that cures (polymerizes and crosslinks when mixed with a A polymer is a large Molecule ( Macromolecule) composed of repeating Structural units typically connected by Covalent Chemical bonds The common product name is SU-8 photoresist. For the heavy Soviet ground attack aircraft see Sukhoi Su-8 SU-8 Photoresist description SU-8 is a commonly used negative Photoresist BB
DUV photoresist
Deep Ultraviolet (DUV) resist are typically polyhydroxystyrene-based polymers with a photoacid generator providing the solubility change. However, this material does not experience the diazocoupling. The combined benzene-chromophore and DNQ-novolac absorption mechanisms lead to stronger absorption by DNQ-novolac photoresists in the DUV, requiring a much larger amount of light for sufficient exposure. The strong DUV absorption results in diminished photoresist sensitivity.
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. In Computer science, ACID ( Atomicity Consistency Isolation Durability) is a set of properties that guarantee that Database transactions are 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. In acid catalysis and base catalysis a Chemical reaction is catalyzed by an Acid or a base. A protecting group or protective group is introduced into a molecule by chemical modification of a Functional group in order to obtain Chemoselectivity [1] Acid diffusion is important not only to increase photoresist sensitivity and throughput, but also to limit line edge roughness due to shot noise statistics. [2] However, the acid diffusion length is itself a potential resolution limiter. In addition, too much diffusion reduces chemical contrast, leading again to more roughness. [2]
References
- ^ US Patent #4,491,628 "Positive and Negative Working Resist Compositions with Acid-Generating Photoinitiator and Polymer with Acid-Labile Groups Pendant From Polymer Backbone" J. M. J. Fréchet, H. Ito and C. G. Willson 1985. [1]
- ^ a b D. van Steenwinckel et al. , J. Vac. Sci. Tech. B, vol. 24, 316-320 (2006).
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