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.
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The main properties characterizing the photoresist types are:
Photoresists are classified into two groups: positive resists and negative resists.
Differences between tone types[1]
| Characteristic | Positive | Negative |
|---|---|---|
| Adhesion to Silicon | Fair | Excellent |
| Relative Cost | More Expensive | Less Expensive |
| Developer Base | Aqueous | Organic |
| Minimum Feature | 0.5 μm and below | ± 2 μm |
| Step Coverage | Better | Lower |
| Wet Chemical Resistance | Fair | Excellent |
Note: This table is based on generalizations which are generally accepted in the MEMS fabrication industry.
The most important light types include UV, DUV, and the H and I lines of a mercury-vapor lamp.
This particular parameter is closely related to the thickness of the applied photoresist, with thinner layers corresponding to shorter wavelengths, permitting a reduced aspect ratio and a reduced minimum feature size. This is important in microelectronics and especially the ITRS reduction in minimum feature size. Intel has semiconductor fabrication facilities currently operating at the 32 nanometer node.
Different chemicals may be used for permanently giving the material the desired property variations:
The above materials are all applied as a liquid and, generally, spin-coated to ensure uniformity of thickness.
Photoresists are most commonly used at wavelengths in the ultraviolet spectrum or shorter (<400 nm). For example, diazonaphthoquinone (DNQ) absorbs strongly from approximately 300 nm to 450 nm. The absorption bands can be assigned to n-π* (S0–S1) and π-π* (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. 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 be as low as 5 eV in condensed solutions)[5] 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.[6] 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.
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.[7] 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 (acids) which catalyze further scission reactions (see the discussion on chemically amplified resists below).
It is not common to select photoresists for electron-beam exposure. Electron beam lithography usually relies on resists dedicated specifically to electron-beam exposure.
One very common positive photoresist used with the I, G and H-lines from a mercury-vapor lamp is based on a mixture of diazonaphthoquinone (DNQ) and novolac resin (a phenol formaldehyde resin). DNQ inhibits the dissolution of the novolac resin, but 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). DNQ-novolac resists are developed by dissolution in a basic solution (usually 0.26N tetramethylammonium hydroxide (TMAH) in water).
Contrary to past types, current negative photoresists tend to exhibit better adhesion to various substrates such as Si, GaAs, InP and glass, as well as metals, including Au, Cu and Al, compared to positive-tone photoresists. Additionally, the current generation of G, H and I-line negative-tone photoresists exhibit higher temperature resistance over positive resists.
One very common negative photoresist is based on epoxy-based polymer. The common product name is SU-8 photoresist, and it was originally invented by IBM, but is now sold by Microchem and Gersteltec. One unique property of SU-8 is that it is very difficult to strip. As such, it is often used in applications where a permanent resist pattern (one that is not strippable, and can even be used in harsh temperature and pressure environments) is needed for a device.[8]
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.
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.[9] Acid diffusion is important not only to increase photoresist sensitivity and throughput, but also to limit line edge roughness due to shot noise statistics.[10] However, the acid diffusion length is itself a potential resolution limiter. In addition, too much diffusion reduces chemical contrast, leading again to more roughness.[10]
The following reactions are an example of commercial chemically amplified photoresists in use today:
The e− represents a solvated electron, or a freed electron that may react with other constituents of the solution. It typically travels a distance on the order of many nanometers before being contained;[13][14] such a large travel distance is consistent with the release of electrons through thick oxide in UV EPROM in response to ultraviolet light. This parasitic exposure would degrade the resolution of the photoresist; for 193 nm the optical resolution is the limiting factor anyway, but for electron beam lithography or EUVL it is the electron range that determines the resolution rather than the optics.
Dan Daly states that Shipley, acquired by Rohm and Haas, and Hoechst, now called AZ Electronic Materials, are two producers of microelectronic chemicals. Common products include Hoechst AZ 4620, Hoechst AZ 4562, Shipley 1400-17, Shipley 1400-27, Shipley 1400-37, and Shipley Microposit Developer. The resists mentioned are, generally, applied in a relatively thick layer—approximately 120 nm to 10 µm—and are used in the manufacture of microlens arrays. Microelectronic resists, presumably, utilize specialized products depending upon process objectives and design constraints. The general mechanism of exposure for these photoresists proceeds with the decomposition of diazoquinone, i.e. the evolution of nitrogen gas and the production of carbenes.