In
semiconductor fabrication
Semiconductor device fabrication is the process used to manufacture semiconductor devices, typically integrated circuit (IC) chips such as modern computer processors, microcontrollers, and memory chips such as NAND flash and DRAM that are pres ...
, 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
Lithography () is a planographic method of printing originally based on the immiscibility of oil and water. The printing is from a stone (lithographic limestone) or a metal plate with a smooth surface. It was invented in 1796 by the German a ...
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
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
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
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 ...
are called
photoresist
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.
T ...
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 is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
or
aluminum
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...
) is inlaid in an
electrically insulating matrix (typically fluorinated
silicon dioxide
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
or another low-k
dielectric
In electromagnetism, a dielectric (or dielectric medium) is an electrical insulator that can be polarised by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the mate ...
). The metal patterns define multiple electrical circuits that are used to connect the microchip's
transistor
upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink).
A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
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
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 ...
-- 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
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 and the production of integrated circuits (ICs or "chips") in particular. Masks are used ...
, which is a specially prepared stencil formed of opaque and transparent regions - usually a
quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
substrate with a patterned
chromium
Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium metal is valued for its high corrosion resistance and hardne ...
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 (MEMS), also written as micro-electro-mechanical systems (or microelectronic and microelectromechanical systems) and the related micromechatronics and microsystems constitute the technology of microscopic devices, ...
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
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
beams produced in
scanning electron microscopes. This is the basis of
electron-beam direct-write lithography.
A resist is not always necessary. Several materials may be deposited or patterned directly using techniques like
soft lithography,
Dip-Pen Nanolithography
Dip pen nanolithography (DPN) is a scanning probe lithography technique where an atomic force microscope, 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 th ...
, evaporation through a shadow mask or
stencil
Stencilling produces an image or pattern on a surface, by applying pigment to a surface through an intermediate object, with designed holes in the intermediate object, to create a pattern or image on a surface, by allowing the pigment to reach ...
.
Typical process
# Resist Deposition: The precursor solution is
spin-coated on a clean (semiconductor) substrate, such as a silicon
wafer
A wafer is a crisp, often sweet, very thin, flat, light and dry biscuit, often used to decorate ice cream, and also used as a garnish on some sweet dishes. Wafers can also be made into cookies with cream flavoring sandwiched between them. They ...
, to form a very thin, uniform layer.
# Soft Bake: The layer is baked at a low temperature to evaporate residual solvent.
# Exposure: A ''latent image'' is formed in the resist e.g. (a) via exposure to ultraviolet light through a
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 and the production of integrated circuits (ICs or "chips") in particular. Masks are used ...
with opaque and transparent regions or (b) by direct writing using a laser beam or electron beam.
# Post-Exposure Bake
# Development: Areas of the resist that have (or have not) been exposed are removed by rinsing with an appropriate solvent.
# Processing through the resist pattern: wet or dry etching, lift-off, doping...
# Resist Stripping
See also
*
Electron beam lithography
Electron-beam lithography (often abbreviated as e-beam lithography, EBL) 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 b ...
*
Nanolithography
Nanolithography (NL) is a growing field of techniques within nanotechnology dealing with the engineering (patterning e.g. etching, depositing, writing, printing etc) of nanometer-scale structures on various materials.
The modern term reflects on ...
*
Photolithography
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 ...
External links
MicroChemShipley (now Rohm and Haas Electronic Materials) Clariantmicro resist technology{Dead link, date=September 2018 , bot=InternetArchiveBot , fix-attempted=yes
Semiconductor device fabrication