Resist
Encyclopedia
In 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...

, 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
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...

 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
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...

 or its precursor and other small molecules (e.g. photoacid generators) that have been specially formulated for a given lithography technology. Resists used during 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...

 are called photoresist
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.-Tone:Photoresists are classified into two groups: positive resists and negative resists....

s.

Background

Semiconductor devices (as of 2005) are built by depositing and patterning many thin layers. The patterning steps, or lithography, define the function of the device and the density of its components.

For example, in the interconnect layers of a modern microprocessor, a conductive material (copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

 or aluminum) is inlaid in an electrically insulating
Electrical insulation
thumb|250px|[[Coaxial Cable]] with dielectric insulator supporting a central coreThis article refers to electrical insulation. For insulation of heat, see Thermal insulation...

 matrix (typically fluorinated silicon dioxide
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...

 or another low-k dielectric
Dielectric
A dielectric is an electrical insulator that can be polarized by an applied electric field. When a dielectric is placed in an electric field, electric charges do not flow through the material, as in a conductor, but only slightly shift from their average equilibrium positions causing dielectric...

). The metal patterns define multiple electrical circuits that are used to connect the microchip's transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...

s to one another and ultimately to external devices via the chip's pins.

The most common patterning method used by the semiconductor device industry is 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...

 -- patterning using light. In this process, the substrate of interest is coated with photosensitive resist and irradiated with short-wavelength light projected through a photomask
Photomask
A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern. They are commonly used in photolithography.-Overview:...

, which is a specially prepared stencil formed of opaque and transparent regions - usually a quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

 substrate with a patterned chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...

 layer. The shadow of opaque regions in the photomask forms a submicrometer-scale pattern of dark and illuminated regions in the resist layer -- the areal image. Chemical and physical changes occur in the exposed areas of the resist layer. For example, chemical bonds may be formed or destroyed, inducing a change in solubility. This latent image is then developed for example by rinsing with an appropriate solvent. Selected regions of the resist remain, which after a post-exposure bake step form a stable polymeric pattern on the substrate. This pattern can be used as a stencil in the next process step. For example, areas of the underlying substrate that are not protected by the resist pattern may be etched or doped. Material may be selectively deposited on the substrate. After processing, the remaining resist may be stripped. Sometimes (esp. during Microelectromechanical systems
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), the patterned resist layer may be incorporated in the final product. Many photolithography and processing cycles may be performed to create complex devices.

Resists may also be formulated to be sensitive to charged particles, such as the electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

 beams produced in scanning electron microscope
Scanning electron microscope
A scanning electron microscope is a type of electron microscope that images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern...

s. This is the basis of electron-beam direct-write 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...

.

A resist is not always necessary. Several materials may be deposited or patterned directly using techniques like soft lithography
Soft lithography
200px|right|thumb|Figure 1 - "Inking" a stamp. PDMS stamp with pattern is placed in Ethanol and ODT solution200px|right|thumb|Figure 2 - ODT from the solution settles down onto the PDMS stamp. Stamp now has ODT attached to it which acts as the ink....

, Dip-Pen Nanolithography, evaporation through a shadow mask or stencil
Stencil
A stencil is a thin sheet of material, such as paper, plastic, or metal, with letters or a design cut from it, used to produce the letters or design on an underlying surface by applying pigment through the cut-out holes in the material. The key advantage of a stencil is that it can be reused to...

...

Typical process

  1. Resist Deposition: The precursor solution is spin-coated
    Spin coating
    Spin coating is a procedure used to apply uniform thin films to flat substrates. In short, an excess amount of a solution is placed on the substrate, which is then rotated at high speed in order to spread the fluid by centrifugal force...

     on a clean (semiconductor) substrate, such as a silicon wafer
    Wafer (electronics)
    A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...

    , to form a very thin, uniform layer.
  2. Soft Bake: The layer is baked at a low temperature to evaporate residual solvent.
  3. Exposure: A latent image is formed in the resist e.g. (a) via exposure to ultraviolet light through a photomask
    Photomask
    A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern. They are commonly used in photolithography.-Overview:...

     with opaque and transparent regions or (b) by direct writing using a laser beam or electron beam.
  4. Development: Areas of the resist that have (or have not) been exposed are removed by rinsing with an appropriate solvent.
  5. Post-Exposure Bake
  6. Processing through the resist pattern: wet or dry etching, lift-off, doping...
  7. Resist Stripping

See also

  • Bottom Anti-Reflection Coating (BARC)
  • 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...

  • Lift-off Resist (LOR)
  • Nanolithography
    Nanolithography
    Nanolithography is the branch of nanotechnology concerned with the study and application of fabricating nanometer-scale structures, meaning patterns with at least one lateral dimension between the size of an individual atom and approximately 100 nm...

  • 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...


External links

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