Photoresist

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A photoresist 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]

Photolithography, also called optical lithography or UV lithography, is a process used in microfabrication to pattern parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a photosensitive chemical photoresist on the substrate. A series of chemical treatments then either etches the exposure pattern into the material or enables deposition of a new material in the desired pattern upon the material underneath the photoresist. In complex integrated circuits, a CMOS wafer may go through the photolithographic cycle as many as 50 times.

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

Electronics industry global industry

The electronics industry, especially meaning consumer electronics, emerged in the 20th century and has now become a global industry worth billions of dollars. Contemporary society uses all manner of electronic devices built in automated or semi-automated factories operated by the industry. Products are assembled from integrated circuits, principally by photolithography of printed circuit boards.

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

Definitions

Positive photoresist

A positive photoresist example, whose solubility would change by the photogenerated acid. The acid deprotects the tbutoxycarbonyl(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 Prof. Ito Prof. Willson and Prof. Frechet in 1982. Acid catalyze photoresist.tif
A positive photoresist example, whose solubility would change by the photogenerated acid. The acid deprotects the tbutoxycarbonyl(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 Prof. Ito Prof. Willson and Prof. 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.

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.

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 [3]

CharacteristicPositiveNegative
Adhesion to SiliconFairExcellent
Relative CostMore expensiveLess expensive
Developer BaseAqueousOrganic
Solubility in the developerExposed region is solubleExposed region is insoluble
Minimum Feature0.5 µm2 µm
Step CoverageBetterLower
Wet Chemical ResistanceFairExcellent

Note: This table is based on generalizations which are generally accepted in the Microelectromechanical systems (MEMS) fabrication industry.

Microelectromechanical systems technology of very small devices

Microelectromechanical systems is the technology of microscopic devices, particularly those with moving parts. It merges at the nano-scale into nanoelectromechanical systems (NEMS) and nanotechnology. MEMS are also referred to as micromachines in Japan, or micro systems technology (MST) in Europe.

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 [4]

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). [5]

Self-assembled monolayer

Self-assembled monolayers (SAM) of organic molecules are molecular assemblies formed spontaneously on surfaces by adsorption and are organized into more or less large ordered domains. In some cases molecules that form the monolayer do not interact strongly with the substrate. This is the case for instance of the two-dimensional supramolecular networks of e.g. perylenetetracarboxylic dianhydride (PTCDA) on gold or of e.g. porphyrins on highly oriented pyrolitic graphite (HOPG). In other cases the molecules possess a head group that has a strong affinity to the substrate and anchors the molecule to it. Such a SAM consisting of a head group, tail and functional end group is depicted in Figure 1. Common head groups include thiols, silanes, phosphonates, etc.

Self-assembly process in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, local interactions among the components themselves, without external direction

Self-assembly is a process in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, local interactions among the components themselves, without external direction. When the constitutive components are molecules, the process is termed molecular self-assembly.

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. [6] 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 [7] 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) [8] 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. [9] 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.

Diazonaphthoquinone 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.

Photon energy is the energy carried by a single photon. The amount of energy is directly proportional to the photon's electromagnetic frequency and thus, equivalently, is inversely proportional to the wavelength. The higher the photon's frequency, the higher its energy. Equivalently, the longer the photon's wavelength, the lower its energy.

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. [10] 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. [11]

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. [12] 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 photolitography 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. [13]

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. [14]

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. [15]

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, Micro-Electro-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. [16]

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. [17]

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

Related Research Articles

Photochemistry sub-discipline of chemistry

Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet, visible light (400–750 nm) or infrared radiation (750–2500 nm).

Electron-beam lithography

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.

Nanolithography is a growing field of techniques within nanotechnology dealing with the engineering of nanometer-scale structures. From Greek, the word can be broken up into three parts: "nano" meaning dwarf, "lith" meaning stone, and "graphy" meaning to write, or "tiny writing onto stone." Today, the word has evolved to cover the design of structures in the range of 10−9 to 10−6 meters, or structures in the nanometer range. Essentially, field is a derivative of lithography, only covering significantly smaller structures. All nanolithographic techniques can be separated into two categories: those that etch away molecules leaving behind the desired structure, and those that directly write the desired structure to a surface.

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.

SU-8 photoresist

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.

LIGA

LIGA is a German acronym for Lithographie, Galvanoformung, Abformung that describes a fabrication technology used to create high-aspect-ratio microstructures.

Color filter array

In photography, a color filter array (CFA), or color filter mosaic (CFM), is a mosaic of tiny color filters placed over the pixel sensors of an image sensor to capture color information.

Microcontact printing

Microcontact printing is a form of soft lithography that uses the relief patterns on a master polydimethylsiloxane (PDMS) stamp to form patterns of self-assembled monolayers (SAMs) of ink on the surface of a substrate through conformal contact as in the case of nanotransfer printing (nTP). Its applications are wide-ranging including microelectronics, surface chemistry and cell biology.

Lift-off process in microstructuring technology is a method of creating structures (patterning) of a target material on the surface of a substrate using a sacrificial material . It is an additive technique as opposed to more traditional subtracting technique like etching. The scale of the structures can vary from the nanoscale up to the centimeter scale or further, but are typically of micrometric dimensions.

Multiphoton lithography

Multiphoton lithography of polymer templates has been known for years by the photonic crystal community. Similar to standard photolithography techniques, structuring is accomplished by illuminating negative-tone or positive-tone photoresists via light of a well-defined wavelength. The fundamental difference is, however, the avoidance of reticles. Instead, two-photon absorption is utilized to induce a dramatic change in the solubility of the resist for appropriate developers.

Polymeric materials have widespread application due to their versatile characteristics, cost-effectiveness, and highly tailored production. The science of polymer synthesis allows for excellent control over the properties of a bulk polymer sample. However, surface interactions of polymer substrates are an essential area of study in biotechnology, nanotechnology, and in all forms of coating applications. In these cases, the surface characteristics of the polymer and material, and the resulting forces between them largely determine its utility and reliability. In biomedical applications for example, the bodily response to foreign material, and thus biocompatibility, is governed by surface interactions. In addition, surface science is integral part of the formulation, manufacturing, and application of coatings.

Chemistry of photolithography

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 microelectromechanical systems (MEMS). Photolithography makes this process possible through the combined use of hexamethyldisilazane (HMDS), photoresist, a spin or spray coater, 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 science has been able to create features on the surface of silicon wafers between 1 and 100 µm.

Perfluorodecyltrichlorosilane, also known as FDTS, is a colorless liquid chemical with molecular formula C10H4Cl3F17Si. FDTS molecules form self-assembled monolayers. They bond onto surfaces terminated with hydroxyl (-OH) groups, such as glass, ceramics, or SiO2 forming a regular covalent bond. It anchors on oxide surfaces with its tricholoro-silane group and attaches covalently.

Off-stoichiometry thiol-ene polymer

An off-stoichiometry thiol-ene polymer polymer is a polymer platform comprising off-stoichiometry thiol-enes (OSTE) and off-stoichiometry thiol-ene-epoxies (OSTE+).

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 DuPont Fellow at DuPont, where he works 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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