Photoresist
Encyclopedia
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.
Differences between tone types
Note: This table is based on generalizations which are generally accepted in the MEMS
fabrication industry.
.
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.
The above materials are all applied as a liquid and, generally, spin-coated to ensure uniformity of thickness.
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. In the deep ultraviolet (DUV) spectrum, the π-π* electronic transition in benzene
or carbon double-bond
chromophore
s 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) 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.
-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 (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.
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 diazo
coupling in the unexposed region). DNQ-novolac resists are developed by dissolution in a basic solution (usually 0.26N tetramethylammonium hydroxide
(TMAH) in water).
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.
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.
s 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:
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; 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.
, 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.
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
-sensitive material used in several industrial processes, such as photolithography
Photolithography
Photolithography is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical "photoresist", or simply "resist," on the substrate...
and photoengraving
Photoengraving
Photoengraving also known as photo-chemical milling is a process of engraving using photographic processing techniques. The full form of photoengraving is photo mechanical process in the graphic arts, used principally for reproducing illustrations. The subject is photographed, and the image is...
to form a patterned coating on a surface.
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 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 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
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...
fabrication industry.
Developing light wavelength
The most important light types include UV, DUV, and the H and I lines of a mercury-vapor lampMercury-vapor lamp
A mercury-vapor lamp is a gas discharge lamp that uses an electric arc through vaporized mercury to produce light. The arc discharge is generally confined to a small fused quartz arc tube mounted within a larger borosilicate glass bulb...
.
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
ITRS
ITRS or ItRS can refer to:* International Terrestrial Reference System for creating earth measurement reference frames.* International Technology Roadmap for Semiconductors, an international body for guiding the semiconductor industry....
reduction in minimum feature size. Intel has semiconductor fabrication
Semiconductor fabrication
Semiconductor device fabrication is the process used to create the integrated circuits that are present in everyday electrical and electronic devices. It is a multiple-step sequence of photolithographic and chemical processing steps during which electronic circuits are gradually created on a wafer...
facilities currently operating at the 32 nanometer node.
Chemical constituencies
Different chemicals may be used for permanently giving the material the desired property variations:- Poly(methyl methacrylate) (PMMA),
- Poly(methyl glutarimide) (PMGI)
- Phenol formaldehyde resinPhenol formaldehyde resinPhenol formaldehyde resins include synthetic thermosetting resins such as obtained by the reaction of phenols with formaldehyde. Sometimes the precursors include other aldehydes or other phenol. Phenolic resins are mainly used in the production of circuit boards...
(DNQ/Novolac), - SU-8.
The above materials are all applied as a liquid and, generally, spin-coated to ensure uniformity of thickness.
- Dry film – stands alone amongst the other types in that the coating already exists as a uniform thickness, semi-solid film coated onto a polyester substrate and the user applies that substrate to the workpiece in question by lamination.
Applications
- Fabrication of printed circuit boards. A common application includes applying photoresist, exposing to the image, followed by an etch step using iron chlorideIron(III) chlorideIron chloride, also called ferric chloride, is an industrial scale commodity chemical compound, with the formula FeCl3. The colour of iron chloride crystals depends on the viewing angle: by reflected light the crystals appear dark green, but by transmitted light they appear purple-red...
, cupric chloride or an alkaline ammonia etching solution to remove the copperclad substrate.
- Sand carving. Sand blasting materials after a photolithographically printed pattern has been applied as a mask.
- Microelectronics – mainly silicon wafers/silicon integrated circuits. This application is the most developed of the technologies and the most specialized in the field.
- Patterning and etching of substrates. This includes specialty photonics materials, MEMS, glass printed circuit boards, and other micropatterningMicropatterningMicropatterning is the art of miniaturisation of patterns. Especially used for electronics, it has recently become a standard in biomaterials engineering and for fundamental research on cellular biology by mean of soft lithography...
tasks.
Absorption at UV and shorter wavelengths
Photoresists are most commonly used at wavelengths in the ultravioletUltraviolet
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...
spectrum or shorter (<400 nm). For example, diazonaphthoquinone
Diazonaphthoquinone
Diazonaphthoquinone is a diazo derivative of naphthoquinone. Upon exposure to light, it undergoes a Wolff rearrangement to form a ketene...
(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. In the deep ultraviolet (DUV) spectrum, the π-π* electronic transition in benzene
Benzene
Benzene is an organic chemical compound. It is composed of 6 carbon atoms in a ring, with 1 hydrogen atom attached to each carbon atom, with the molecular formula C6H6....
or carbon double-bond
Carbon-carbon bond
A carbon–carbon bond is a covalent bond between two carbon atoms. The most common form is the single bond: a bond composed of two electrons, one from each of the two atoms. The carbon–carbon single bond is a sigma bond and is said to be formed between one hybridized orbital from each...
chromophore
Chromophore
A chromophore is the part of a molecule responsible for its color. The color arises when a molecule absorbs certain wavelengths of visible light and transmits or reflects others. The chromophore is a region in the molecule where the energy difference between two different molecular orbitals falls...
s 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
Ionization potential
The ionization energy of a chemical species, i.e. an atom or molecule, is the energy required to remove an electron from the species to a practically infinite distance. Large atoms or molecules have a low ionization energy, while small molecules tend to have higher ionization energies.The property...
of the photoresist (can be as low as 5 eV
Electronvolt
In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...
in condensed solutions) 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
Electron ionization
Electron ionization is an ionization method in which energetic electrons interact with gas phase atoms or molecules to produce ions...
and Auger transitions
Auger electron
The Auger effect is a physical phenomenon in which the transition of an electron in an atom filling in an inner-shell vacancy causes the emission of another electron. When a core electron is removed, leaving a vacancy, an electron from a higher energy level may fall into the vacancy, resulting in...
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 CCarbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
-C bond is 3.6 eV. Secondary electrons
Secondary electrons
Secondary electrons are electrons generated as ionization products. They are called 'secondary' because they are generated by other radiation . This radiation can be in the form of ions, electrons, or photons with sufficiently high energy, i.e. exceeding the ionization potential...
generated by primary ionizing radiation
Ionizing radiation
Ionizing radiation is radiation composed of particles that individually have sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, which are atoms or molecules containing unpaired electrons...
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
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
, 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
Electron beam lithography
Electron beam lithography is the practice of emitting a beam of electrons in a patterned fashion across a surface covered with a film , and of selectively removing either exposed or non-exposed regions of the resist...
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 lampMercury-vapor lamp
A mercury-vapor lamp is a gas discharge lamp that uses an electric arc through vaporized mercury to produce light. The arc discharge is generally confined to a small fused quartz arc tube mounted within a larger borosilicate glass bulb...
is based on a mixture of diazonaphthoquinone
Diazonaphthoquinone
Diazonaphthoquinone is a diazo derivative of naphthoquinone. Upon exposure to light, it undergoes a Wolff rearrangement to form a ketene...
(DNQ) and novolac resin (a phenol formaldehyde resin
Phenol formaldehyde resin
Phenol formaldehyde resins include synthetic thermosetting resins such as obtained by the reaction of phenols with formaldehyde. Sometimes the precursors include other aldehydes or other phenol. Phenolic resins are mainly used in the production of circuit boards...
). 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 diazo
Diazo
Diazo refers to a type of organic compound called diazo compound that has two linked nitrogen atoms as a terminal functional group. The general formula is R2C=N2. The simplest example of a diazo compound is diazomethane...
coupling in the unexposed region). DNQ-novolac resists are developed by dissolution in a basic solution (usually 0.26N tetramethylammonium hydroxide
Tetramethylammonium hydroxide
Tetramethylammonium hydroxide is a quaternary ammonium salt with the molecular formula 4NOH. It is used as an anisotropic etchant of silicon. It is also used as a basic solvent in the development of acidic photoresist in the photolithography process. Since it is a phase transfer catalyst, it is...
(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
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 polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
. The common product name is SU-8 photoresist
SU-8 photoresist
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 300 micrometer and still be processed with standard contact lithography. It can be used to pattern high aspect ratio structures...
, 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.
DUV photoresist
Deep Ultraviolet (DUV) resist are typically polyhydroxystyrene-based polymerswith 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, acidAcid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
s 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
Acid catalysis
In acid catalysis and base catalysis a chemical reaction is catalyzed by an acid or a base. The acid is often the proton and the base is often a hydroxyl ion. Typical reactions catalyzed by proton transfer are esterfications and aldol reactions. In these reactions the conjugate acid of the carbonyl...
many such 'deprotection
Protecting group
A protecting group or protective group is introduced into a molecule by chemical modification of a functional group in order to obtain chemoselectivity in a subsequent chemical reaction...
' 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 (193 nm) → acid cation + sulfonateSulfonateA sulfonate is a salt or ester of a sulfonic acid. It contains the functional group R-SO2O-.- Sulfonate salts:Anions with the general formula RSO2O− are called sulfonates. They are the conjugate bases of sulfonic acids with formula RSO2OH. As sulfonic acids tend to be strong acids, the...
anion - sulfonate anion + hν (193 nm) → e− + sulfonate
- e− + photoacid generator → e− + acid cation + sulfonate anion
The e− represents a solvated electron
Solvated electron
A solvated electron is a free electron in a solution. Solvated electrons occur widely although they are often not observed directly. The deep colour of solutions of alkali metals in ammonia arises form the presence of solvated electrons: blue when dilute and copper-colored when more concentrated...
, 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; 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
Electron beam lithography
Electron beam lithography is the practice of emitting a beam of electrons in a patterned fashion across a surface covered with a film , and of selectively removing either exposed or non-exposed regions of the resist...
or EUVL it is the electron range that determines the resolution rather than the optics.
Some common photoresists
Dan Daly states that ShipleyShipley
Shipley may refer to:Places in England*Shipley, Derbyshire*Shipley, West Sussex*Shipley, West Yorkshire**Shipley People*Ann Shipley, Canadian politician*Burton Shipley, first Maryland Tarrapins men's basketball coach...
, acquired by Rohm and Haas
Rohm and Haas
Rohm and Haas Company, a Philadelphia, Pennsylvania based company, manufactures miscellaneous materials. Formerly a Fortune 500 Company, Rohm and Haas employs more than 17,000 people in 27 countries, with its last sales revenue reported as an independent company at USD 8.9 billion. On July 10,...
, and Hoechst
Hoechst AG
Hoechst AG was a German chemicals then life-sciences company that became Aventis Deutschland after its merger with France's Rhône-Poulenc S.A. in 1999...
, now called AZ Electronic Materials
AZ Electronic Materials
AZ Electronic Materials plc is a British specialty chemicals company. It is listed on the London Stock Exchange.-History:The Company was established in the 1950s as a division of Hoechst . The name of the company is derived from the organic compound diazo...
, 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.