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.

Photoresist categories

The main properties characterizing the photoresist types are:

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. 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

Characteristic	Positive	Negative
Adhesion to Si	fair	excellent
Relative cost	more	less
Developer Base	aqueous	organic
Minimum	0.5 μm	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.

Developing Light Wavelength

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 correspoding 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 45 nanometer node.

Chemical Constituencies

Different chemicals may be used for permanently giving the material the desired property variations:

1. PMMA. Polymethylmethacrylate

2. PMGI. PolyMethylGlutarimide

3. DNQ/Novolac. Phenol formaldehyde resin

4. SU-8.

5. Dry Film. Alone amongst the the other types, in that the coating already exists on a substrate and the user applies that substrate to the workpiece in question. The above materials are all applied as a liquid and, generally, spin-coated to ensure uniformity of thickness.

Applications

1. Printed Circuit Board Fabrication. A common application includes applying photoresist, exposing to the image, followed by an etch step using Ferric chloride to remove the copperclad substrate.

2. Sand Carving. Sand blasting materials after a photolithographically printed pattern has been applied as a mask.

3. Microelectronics - mainly silicon wafers/silicon integrated circuits. This application is the most developed of the technologies and the most specialized in the field.

4. Glass, or other substrate, patterning and etching. This includes specialty photonics materials, MEMS, glass printed circuit boards, and other micropatterning tasks.

Other Aspects of Photoresist Technologies

Absorption at UV and shorter wavelengths

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-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. 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. 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. 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 3.6 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. Scission breaks the original polymer into segments of lower molecular weight, which are more readily dissolved in a solvent.

It is not common to select photoresists for electron-beam exposure. Electron beam lithography usually relies on resists dedicated specifically to electron-beam exposure.

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 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).

Negative photoresist

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) is needed for a device. Futurrex, Inc., out of New Jersey, is a specialty chemical provider focused on negative-toned resists. Negative-tone photoresist product families for the fabrication of MEMS devices, biochips, optoelectronics and analog or mixed signal semiconductors include the NR2 (advanced adhesion), NR4 (advanced adhesion), NR5 (high temperature, extreme thickness), NR9 (advanced adhesion) and NR71 (high temperature) lines.

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. 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.

Some Common Photoresists

Dan Daly states that Shipley, acquired by Rohm and Haas, and Hoechst, now called AZ Electronic Materials, are two producers microelectronic chemicals. Common products include Hoechst AZ 4620, Hoechst AZ 4562, Shipley 1400-17, Shipley 1400-27, Shipley 1400-37, and Shipley Microposit Developer. It should be noted that the resists mentioned are, generally, applied in a relatively thick layer-approximately 120 um to 10 um--and are utilized 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.

External links

Rohm and Haas Company Website [6] Microelectronics photoresists and developers

AZ Electronic Materials Company Website [7] Microelectronic photoresists

Michrochem Company Website [8] Microelectronic photoresists. [9]

Gersteltec Company Website [10] Microelectronic photoresists. [11]

Film Photoresists [12] Sandcarving photoresist masks

Film Photoresists [13] Sandcarving photoresist masks

Film Photoresists [14] Printed circuit board photoresist-produced stencil

Fujifilm Company Website [15] Microelectronic photoresists. [16]