Photoresist

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A photoresist (also known simply as a resist) 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 electronic industry. [1]

Contents

The 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 (either polymerized or cross-linked) 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.

Photoresist of Photolithography Photoresist of Photolithography.png
Photoresist of Photolithography

A BARC coating (bottom anti-reflectant coating) 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. [2] [3] [4]

Conventional photoresists typically consists of 3 components: resin (a binder that provides physical properties such as adhesion, chemical resistance, etc), sensitizer (which has a photoactive compound), and solvent (which keeps the resist liquid).


Definitions

Simple resist polarity

Positive: light will weaken the resist, and create a hole

Negative: 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 (I) is related to energy by E = I*t.

Positive photoresist

A positive photoresist example, whose solubility would change by the photogenerated acid. The acid deprotects the tert-butoxycarbonyl (t-BOC), inducing the resist from alkali insoluble to alkali soluble. This was the first chemically amplified resist used in the semiconductor industry, which was invented by Ito, Willson, and Frechet in 1982. Acid catalyze photoresist.tif
A positive photoresist example, whose solubility would change by the photogenerated acid. The acid deprotects the tert-butoxycarbonyl (t-BOC), inducing the resist from alkali insoluble to alkali soluble. This was the first chemically amplified resist used in the semiconductor industry, which was invented by Ito, Willson, and Frechet in 1982.
An example for single component positive photoresist Positive photoresist SO2.tif
An example for single component positive photoresist

A positive photoresist is a type of photoresist in which the portion of the photoresist that is exposed to light becomes soluble to the photoresist developer. The unexposed portion of the photoresist remains insoluble to the photoresist developer.


Some examples of positive photoresists are

PMMA (polymethylmethacrylate) single component

Two components DQN resists:

Negative photoresist

A negative photoresist 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.


Modulation transfer function

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

A crosslinking of a polyisoprene rubber by a photoreactive biazide as negative photoresist Polyisoprene negative photoresist.tif
A crosslinking of a polyisoprene rubber by a photoreactive biazide as negative photoresist
A radical induced polymerization and crosslinking of an acrylate monomer as negative photoresist Acrylate negative photoresist.tif
A radical induced polymerization and crosslinking of an acrylate monomer as negative photoresist

Differences between positive and negative resist

The following table [6] is based on generalizations which are generally accepted in the microelectromechanical systems (MEMS) fabrication industry.

CharacteristicPositiveNegative
Adhesion to siliconFairExcellent
Relative costMore expensiveLess expensive
Developer baseAqueousOrganic
Solubility in the developerExposed region is solubleExposed region is insoluble
Minimum feature0.5 µm7 nm
Step coverageBetterLower
Wet chemical resistanceFairExcellent

Types

Based on the chemical structure of photoresists, they can be classified into three types: photopolymeric, photodecomposing, photocrosslinking photoresist.

Photopolymeric photoresist is a type of photoresist, usually allyl monomer, which could generate free radical when exposed to light, then initiates the photopolymerization of monomer to produce a polymer. Photopolymeric photoresists are usually used for negative photoresist, e.g. methyl methacrylate.

Photopolymerization of methyl methacrylate monomers under UV that resulting into polymer Methyl methacrylate photoresist.tif
Photopolymerization of methyl methacrylate monomers under UV that resulting into polymer

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 (DQ).

Photolysis of a dizaonaphthoquinone that leads to a much more polar environment, which allows aqueous base to dissolve a Bakelite-type polymer. Dizaonaphthoquinone photoresist.tif
Photolysis of a dizaonaphthoquinone that leads to a much more polar environment, which allows aqueous base to dissolve a Bakelite-type polymer.

Photocrosslinking photoresist is a type of photoresist, which could crosslink chain by chain when exposed to light, to generate an insoluble network. Photocrosslinking photoresist are usually used for negative photoresist.

Chemical structure of SU-8 (a single molecule contains 8 epoxy groups) SU-8 .tif
Chemical structure of SU-8 (a single molecule contains 8 epoxy groups)
Mechanism of SU-8 for negative photoresist SU-8 for negative photoresist.tif
Mechanism of SU-8 for negative photoresist

Off-Stoichiometry Thiol-Enes (OSTE) polymers [7]

For self-assembled monolayer SAM 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 photoresist, which generates a photo-patterned sample in the irradiated areas. And finally developer is used to remove the designed part (could be used as both positive or negative photoresist). [8]

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. [9] 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.[ citation needed ] In the deep ultraviolet (DUV) spectrum, the π-π* electronic transition in benzene [10] or carbon double-bond chromophores appears at around 200 nm.[ citation needed ] 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) [11] 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. [12] 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. [13] 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.

Parameters

Physical, chemical and optical properties of photoresists influence their selection for different processes. [14]

The smaller the critical dimension is, the higher resolution would be.

Positive photoresist

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

Epoxy-based polymer

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. [15] Mechanism of epoxy-based polymer is shown in 1.2.3 SU-8.

Off-stoichiometry thiol-enes(OSTE) polymer

In 2016, OSTE Polymers were shown to possess a unique photolithography mechanism, based on diffusion-induced monomer depletion, which enables high photostructuring accuracy. The OSTE polymer material was originally invented at the KTH Royal Institute of Technology, but is now sold by Mercene Labs. Whereas the material has properties similar to those of SU8, OSTE has the specific advantage that it contains reactive surface molecules, which make this material attractive for microfluidic or biomedical applications. [14]

Hydrogen silsesquioxane (HSQ)

HSQ is a common negative resist for e-beam, but also useful for photolithography. Originally invented by Dow Corning (1970), [16] and now produced (2017) by Applied Quantum Materials Inc. (AQM). Unlike other negative resists, HSQ is inorganic and metal-free. Therefore, exposed HSQ provides a low dielectric constant (low-k) Si rich oxide. A comparative study against other photoresists was reported in 2015 (Dow Corning HSQ). [17]

Applications

Microcontact printing

Microcontact printing was described by Whitesides Group in 1993. Generally, in this techniques, an elastomeric stamp is used to generate two-dimensional patterns, through printing the “ink” molecules onto the surface of a solid substrate. [18]

Creating the PDMS master Creating the PDMS master.svg
Creating the PDMS master
rightInking and contact process Inking and contact process.svg
rightInking and contact process

Step 1 for microcontact printing. A scheme for the creation of a polydimethylsiloxane (PDMS) master stamp. Step 2 for microcontact printing A scheme of the inking and contact process of microprinting lithography.

Printed circuit boards

The manufacture of printed circuit boards is one of the most important uses of photoresist. Photolithography allows the complex wiring of an electronic system to be rapidly, economically, and accurately reproduced as if run off a printing press. The general process is applying photoresist, exposing image to ultraviolet rays, and then etching to remove the copper-clad substrate. [19]

A printed circuit board-4276 SEG DVD 430 - Printed circuit board-4276.jpg
A printed circuit board-4276

Patterning and etching of substrates

This includes specialty photonics materials, MicroElectro-Mechanical Systems (MEMS), glass printed circuit boards, and other micropatterning tasks. Photoresist tends not to be etched by solutions with a pH greater than 3. [20]

A micro-electrical-mechanical cantilever inproduced by photoetching MEMS Microcantilever in Resonance.png
A micro-electrical-mechanical cantilever inproduced by photoetching

Microelectronics

This application, mainly applied to silicon wafers/silicon integrated circuits is the most developed of the technologies and the most specialized in the field. [21]

A 12-inch silicon wafer can carry hundreds or thousands of integrated circuit dice 12-inch silicon wafer.jpg
A 12-inch silicon wafer can carry hundreds or thousands of integrated circuit dice

See also

Related Research Articles

In integrated circuit manufacturing, photolithography or optical lithography is a general term used for techniques that use light to produce minutely patterned thin films of suitable materials over a substrate, such as a silicon wafer, to protect selected areas of it during subsequent etching, deposition, or implantation operations. Typically, ultraviolet light is used to transfer a geometric design from an optical mask to a light-sensitive chemical (photoresist) coated on the substrate. The photoresist either breaks down or hardens where it is exposed to light. The patterned film is then created by removing the softer parts of the coating with appropriate solvents.

<span class="mw-page-title-main">Ultraviolet</span> Form of electromagnetic radiation

Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight, and constitutes about 10% of the total electromagnetic radiation output from the Sun. It is also produced by electric arcs and specialized lights, such as mercury-vapor lamps, tanning lamps, and black lights. Although long-wavelength ultraviolet is not considered an ionizing radiation because its photons lack the energy to ionize atoms, it can cause chemical reactions and causes many substances to glow or fluoresce. Consequently, the chemical and biological effects of UV are greater than simple heating effects, and many practical applications of UV radiation derive from its interactions with organic molecules.

<span class="mw-page-title-main">Electron-beam lithography</span> Lithographic technique that uses a scanning beam of electrons

Electron-beam lithography is the practice of scanning a focused beam of electrons to draw custom shapes on a surface covered with an electron-sensitive film called a resist (exposing). The electron beam changes the solubility of the resist, enabling selective removal of either the exposed or non-exposed regions of the resist by immersing it in a solvent (developing). The purpose, as with photolithography, is to create very small structures in the resist that can subsequently be transferred to the substrate material, often by etching.

Photoengraving is a process that uses a light-sensitive photoresist applied to the surface to be engraved to create a mask that protects some areas during a subsequent operation which etches, dissolves, or otherwise removes some or all of the material from the unshielded areas of a substrate. Normally applied to metal, it can also be used on glass, plastic and other materials.

Nanolithography (NL) is a growing field of techniques within nanotechnology dealing with the engineering of nanometer-scale structures on various materials.

<span class="mw-page-title-main">Dip-pen nanolithography</span> Scanning probe lithographic technique

Dip pen nanolithography (DPN) is a scanning probe lithography technique where an atomic force microscope (AFM) tip is used to create patterns directly on a range of substances with a variety of inks. A common example of this technique is exemplified by the use of alkane thiolates to imprint onto a gold surface. This technique allows surface patterning on scales of under 100 nanometers. DPN is the nanotechnology analog of the dip pen, where the tip of an atomic force microscope cantilever acts as a "pen," which is coated with a chemical compound or mixture acting as an "ink," and put in contact with a substrate, the "paper."

In semiconductor fabrication, a resist is a thin layer used to transfer a circuit pattern to the semiconductor substrate which it is deposited upon. A resist can be patterned via lithography to form a (sub)micrometer-scale, temporary mask that protects selected areas of the underlying substrate during subsequent processing steps. The material used to prepare said thin layer is typically a viscous solution. Resists are generally proprietary mixtures of a polymer or its precursor and other small molecules that have been specially formulated for a given lithography technology. Resists used during photolithography are called photoresists.

<span class="mw-page-title-main">Compact Disc manufacturing</span> Mass replication process for CDs

Compact disc manufacturing is the process by which commercial compact discs (CDs) are replicated in mass quantities using a master version created from a source recording. This may be either in audio form (CD-DA) or data form (CD-ROM). This process is used in the mastering of read-only compact discs. DVDs and Blu-rays use similar methods.

<span class="mw-page-title-main">SU-8 photoresist</span> Epoxy-based polymer

SU-8 is a commonly used epoxy-based negative photoresist. Negative refers to a photoresist whereby the parts exposed to UV become cross-linked, while the remainder of the film remains soluble and can be washed away during development.

Contact lithography, also known as contact printing, is a form of photolithography whereby the image to be printed is obtained by illumination of a photomask in direct contact with a substrate coated with an imaging photoresist layer.

<span class="mw-page-title-main">LIGA</span> Fabrication technology used to create high-aspect-ratio microstructures

LIGA is a fabrication technology used to create high-aspect-ratio microstructures. The term is a German acronym for Lithographie, Galvanoformung, Abformung – lithography, electroplating, and molding.

<span class="mw-page-title-main">Photopolymer</span>

A photopolymer or light-activated resin is a polymer that changes its properties when exposed to light, often in the ultraviolet or visible region of the electromagnetic spectrum. These changes are often manifested structurally, for example hardening of the material occurs as a result of cross-linking when exposed to light. An example is shown below depicting a mixture of monomers, oligomers, and photoinitiators that conform into a hardened polymeric material through a process called curing.

<span class="mw-page-title-main">Nanochemistry</span> Combination of chemistry and nanoscience

Nanochemistry is an emerging sub-discipline of the chemical and material sciences that deals with the development of new methods for creating nanoscale materials. The term "nanochemistry" was first used by Ozin in 1992 as 'the uses of chemical synthesis to reproducibly afford nanomaterials from the atom "up", contrary to the nanoengineering and nanophysics approach that operates from the bulk "down"'. Nanochemistry focuses on solid-state chemistry that emphasizes synthesis of building blocks that are dependent on size, surface, shape, and defect properties, rather than the actual production of matter. Atomic and molecular properties mainly deal with the degrees of freedom of atoms in the periodic table. However, nanochemistry introduced other degrees of freedom that controls material's behaviors by transformation into solutions. Nanoscale objects exhibit novel material properties, largely as a consequence of their finite small size. Several chemical modifications on nanometer-scaled structures approve size dependent effects.

The proximity effect in electron beam lithography (EBL) is the phenomenon that the exposure dose distribution, and hence the developed pattern, is wider than the scanned pattern due to the interactions of the primary beam electrons with the resist and substrate. These cause the resist outside the scanned pattern to receive a non-zero dose.

<span class="mw-page-title-main">Diazonaphthoquinone</span> Chemical compound

Diazonaphthoquinone (DNQ) is a diazo derivative of naphthoquinone. Upon exposure to light, DNQ converts to a derivative that is susceptible to etching. In this way, DNQ has become an important reagent in photoresist technology in the semiconductor industry.

<span class="mw-page-title-main">Hydrogen silsesquioxane</span> Inorganic compound

Hydrogen silsesquioxane(s) (HSQ, H-SiOx, THn, H-resin) are inorganic compounds with the empirical formula [HSiO3/2]n. The cubic H8Si8O12 (TH8) is used as the visual representation for HSQ. TH8, TH10, TH12, and TH14 have been characterized by EA), gas chromatography–mass spectroscopy (GC-MS), IR spectroscopy, and NMR spectroscopy.

<span class="mw-page-title-main">X-ray lithography</span> Lithographic technique that uses X-rays instead of light

X-ray lithography is a process used in semiconductor device fabrication industry to selectively remove parts of a thin film of photoresist. It uses X-rays to transfer a geometric pattern from a mask to a light-sensitive chemical photoresist, or simply "resist," on the substrate to reach extremely small topological size of a feature. A series of chemical treatments then engraves the produced pattern into the material underneath the photoresist.

<span class="mw-page-title-main">Chemistry of photolithography</span> Overview article

Photolithography is a process in removing select portions of thin films used in microfabrication. Microfabrication is the production of parts on the micro- and nano- scale, typically on the surface of silicon wafers, for the production of integrated circuits, microelectromechanical systems (MEMS), solar cells, and other devices. Photolithography makes this process possible through the combined use of hexamethyldisilazane (HMDS), photoresist, spin coating, photomask, an exposure system and other various chemicals. By carefully manipulating these factors it is possible to create nearly any geometry microstructure on the surface of a silicon wafer. The chemical interaction between all the different components and the surface of the silicon wafer makes photolithography an interesting chemistry problem. Current engineering has been able to create features on the surface of silicon wafers between 1 and 100 μm.

Three-dimensional (3D) microfabrication refers to manufacturing techniques that involve the layering of materials to produce a three-dimensional structure at a microscopic scale. These structures are usually on the scale of micrometers and are popular in microelectronics and microelectromechanical systems.

Peter Trefonas is a retired DuPont Fellow at DuPont, where he had worked on the development of electronic materials. He is known for innovations in the chemistry of photolithography, particularly the development of anti-reflective coatings and polymer photoresists that are used to create circuitry for computer chips. This work has supported the patterning of smaller features during the lithographic process, increasing miniaturization and microprocessor speed.

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