Photolithography is a process used in semiconductor device fabrication to transfer a pattern from a photomask (also called reticle) to the surface of a substrate. Often crystalline silicon in the form of a wafer is used as a choice of substrate, although there are several other options including, but not limited to, glass, sapphire, and metal. Photolithography (also referred to as "microlithography" or "nanolithography") bears a similarity to the conventional lithography used in printing and shares some of the fundamental principles of photographic processes. Nasas Glenn Research Center clean room. ... As used in photolithography, a photomask is typically a transparent fused quartz blank covered with a pattern defined with chrome metal as the absorbing film. ... The word substrate can mean the following: In biochemistry, a substrate is a molecule which is acted upon by an enzyme. ... General Name, Symbol, Number silicon, Si, 14 Chemical series metalloids Group, Period, Block 14, 3, p Appearance dark gray, bluish tinge Atomic mass 28. ... An etched silicon wafer In microelectronics, a wafer is a thin slice of semiconducting material, such as a silicon crystal, upon which microcircuits are constructed by doping (for example, diffusion or ion implantation), etching, and deposition of various materials. ... This article refers to the material. ... Sapphire is the single-crystal form of aluminium oxide (Al2O3), a mineral known as corundum. ... Nanolithography — or lithography at the nanometer scale — refers to the fabrication of nanometer-scale structures, meaning patterns with at least one lateral dimension between the size of an individual atom and approximately 100 nm. ... Lithography is a method for printing on a smooth surface. ... The folder of newspaper web offset printing press Printing is a process for production of texts and images, typically with ink on paper using a printing press. ...

Photolithography involves a combination of:

• substrate preparation
• photoresist application
• soft-baking
• exposure
• developing
• hard-baking
• etching

and various other chemical treatments (thinning agents, edge-bead removal etc.) in repeated steps on an initially flat substrate. Etching is an intaglio method of printmaking in which the image is incised into the surface of a metal plate using an acid. ...

A part of a typical silicon lithography procedure would begin by depositing a layer of conductive metal several nanometers thick on the substrate. A layer of photoresist -- a chemical that hardens when exposed to light (often ultraviolet) -- is applied on top of the metal layer. The photoresist is selectively "hardened" by illuminating it in specific places. For this purpose a transparent plate with patterns printed on it, called a photomask or shadowmask, is used together with an illumination source to shine light on specific parts of the photoresist. Some photoresists work well under broadband ultraviolet light, whereas others are designed to be sensitive at specific frequencies to ultraviolet light. It is also possible to use other types of resist that are sensitive to X-Rays and others that are sensitive to electron-beam exposure. A nanometre (American spelling: nanometer) is 1. ... Photoresist is a light sensitive material used in several industrial processes, such as photolithography and photoengraving to form a patterned coating on a surface. ...

Generally most types of photoresist will be available as either "positive" or "negative". With positive resists the area that you can see (masked) on the photomask is the area that you will see upon developing of the photoresist. With negative resists it is the inverse, so any area that is exposed will remain, whilst any areas that is not exposed will be developed. After developing, the resist is usually hard-baked before subjecting to a chemical etching stage which will remove the metal underneath.

Finally, the hardened photoresist is etched using a different chemical treatment, and all that remains is a layer of metal in the same shape as the mask (or the inverse if negative resist has been used).

Lithography is used because it affords exact control over the shape and size of the objects it creates, and because it can create patterns over an entire surface simultaneously. Its main disadvantages are that it requires a substrate to start with, it is not very effective at creating shapes that are not flat, and it can require extremely clean operating conditions.

In a complex integrated circuit, (for example, CMOS) a wafer will go through the photolithographic area up to 50 times. For Thin-Film-Transistor (TFT) processing many fewer photolithographical processes are usually required. Optical Microscope image of an integrated circuit showing defects in the aluminium layer deposition. ...

Technology

A wafer is introduced onto an automated "wafertrack" system. This track consists of handling robots, bake/cool plates, and coat/develop units. The robots are used to transfer wafers from one module to another. The wafer is initially heated to a temperature sufficient to drive off any moisture that may be present on the wafer surface. Hexa-methyl-disilizane (HMDS) is applied in either liquid or vapor form in order to promote better adhesion of the photosensitive polymeric material, called photoresist. Photoresist is dispensed in a liquid form onto the wafer as it undergoes rotation. The speed and acceleration of this rotation are important parameters in determining the resulting thickness of the applied photoresist. The photoresist-coated wafer is then transferred to a hot plate, where a "soft bake" is applied to drive off excess solvent before the wafer is introduced into the exposure system. Photosensitivity is the amount to which an object reacts upon receiving photons of light. ... Polymer is a generic term used to describe a very long molecule consisting of structural units and repeating units connected by covalent chemical bonds. ... Photoresist is a light sensitive material used in several industrial processes, such as photolithography and photoengraving to form a patterned coating on a surface. ...

The desired pattern is then projected onto the wafer in either a machine called a stepper or scanner. The stepper/scanner functions similarly to a slide projector. Light from a mercury arc lamp or excimer laser is focused through a complex system of lenses onto a "mask" (also called a reticle), containing the desired image. The light passes through the mask and is then focused to produce the desired image on the wafer through a reduction lens system. The reduction of the system can vary depending on design, but is typically on the order of 4X-5X in magnitude. A stepper is a device, similar to a slide projector or a photographic enlarger, that is used in photolithography to transfer patterns that will become part of an integrated circuit (IC) onto the surface of a silicon wafer. ... A stepper is a device, similar to a slide projector or a photographic enlarger, that is used in photolithography to transfer patterns that will become part of an integrated circuit (IC) onto the surface of a silicon wafer. ... A carousel slide projector, the most common form of projector A slide projector is an opto-mechanical device to view photographic slides. ... General Name, Symbol, Number mercury, Hg, 80 Chemical series transition metals Group, Period, Block 12, 6, d Appearance silvery white Atomic mass 200. ... The 300,000-watt Plasma Arc Lamp in the Infrared Processing Center (IPC) at Oak Ridge National Laboratory An arc lamp is a device that produces light by the sparking (or arcing, from voltaic arc or electric arc) of a high current between two carbon rod electrodes. ... An excimer, originally short for excited dimer, was given as a short-lived molecule that bonds two molecules when in an electronic excited state. ... As used in photolithography, a photomask is typically an optically transparent fused quartz blank imprinted with a pattern defined with chrome metal. ...

When the image is projected onto the wafer, the photoresist material undergoes some wavelength-specific radiation-sensitive chemical reactions, which cause the regions exposed to light to be either more or less acidic. If the exposed regions become more acidic, the material is called a positive photoresist, while if it becomes less susceptible it is a negative photoresist. The resist is then "developed" by exposing it to an alkaline solution that removes either the exposed (positive photoresist) or the unexposed (negative photoresist). This process takes place after the wafer is transferred from the exposure system back to the wafertrack. For alternative meanings see acid (disambiguation). ... Development has meaning in several contexts: Science and Engineering Biological development of embryos in the context of developmental biology Child development (physical emphasis) or post-natal human development (pediatrics, etc) Software engineering, the methodology and process of development of computer software Technology development in industry, as in Software development New... The common (Arrhenius) definition of a base is a chemical compound that either donates hydroxide ions or absorbs hydrogen ions when dissolved in water. ...

Developers originally often contained sodium hydroxide (NaOH). However, sodium is considered an extremely undesirable contaminant in MOSFET fabrication because it degrades the insulating properties of gate oxides. Metal-ion-free developers such as tetramethyl ammonium hydroxide (TMAH) are now used. Flash point non flammable Supplementary data page Structure and properties n, εr, etc. ... General Name, Symbol, Number sodium, Na, 11 Chemical series alkali metals Group, Period, Block 1, 3, s Appearance silvery white Atomic mass 22. ... The metal oxide semiconductor field-effect transistor (MOSFET), is by far the most common field-effect transistor in both digital and analog circuits. ... // Definition An Insulator is a material or object which resists the flow of electric charge. ...

A post-exposure bake is performed before developing, typically to help reduce standing wave phenomena caused by the destructive and constructive interference patterns of the incident light. The develop chemistry is delivered in a similar fashion to how the photoresist was applied. The resulting wafer is then "hardbaked" on a bake plate at high temperature in order to solidify the remaining photoresist, to better serve as a protecting layer in future ion implantation, wet chemical etching, or plasma etching. A standing wave, also known as a stationary wave, is a wave that remains in a constant position. ... Interference of two circular waves - Wavelength (decreasing bottom to top) and Wave centers distance (increasing to the right). ... Ion implantation is a materials engineering process by which ions of a material can be implanted into another solid, thereby changing the physical properties of the solid. ... Wet etching is the removal of material by immersing the wafer in a liquid bath of chemical etchant. ... The word plasma has a Greek root which means to be formed or molded (the word plastic shares this root). ...

The ability to project a clear image of a very small feature onto the wafer is limited by the wavelength of the light that is used and the ability of the reduction lens system to capture enough diffraction orders from the illuminated mask. Current state-of-the-art photolithography tools use deep ultraviolet (DUV) light with wavelengths of 248 and 193 nm, which allow minimum resist feature sizes down to 65nm. The wavelength is the distance between repeating units of a wave pattern. ... Ultraviolet (UV) radiation is electromagnetic radiation of a wavelength shorter than that of the visible region, but longer than that of soft X-rays. ... A nanometre (American spelling: nanometer, symbol: nm) is 1. ...

Optical lithography can be extended to feature sizes below 65nm using 193nm and liquid immersion techniques. Also termed immersion lithography, this enables the use of optics with numerical apertures exceeding 1.0. The liquid used is typically ultra-pure, deionised water, which provides for a refractive index above that of the usual air gap between the lens and the wafer surface. This is continually circulated to eliminate thermally-induced distortions. Using water will only allow NA's of up to ~1.4 but higher refractive index materials will allow the effective NA to be increased. // Introduction In photolithography, immersion lithography is a resolution enhancement technique that interposes a liquid medium between the optics and the wafer surface, replacing the usual air gap. ... Numerical aperture is a technical term of multiple uses: Numerical aperture of optical telecommunication fiber Numerical aperture in microscopy This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ... The refractive index of a material is the factor by which the phase velocity of electromagnetic radiation is slowed relative to vacuum. ... The refractive index of a material is the factor by which the phase velocity of electromagnetic radiation is slowed relative to vacuum. ...

Tools using 157nm wavelength DUV in a manner similar to current exposure systems have been developed. These were once targeted to succeed 193nm at the 65nm feature size node but have now all but been eliminated by the introduction of immersion lithography. This was due to persistent technical problems with the 157nm technology and economic considerations that provided strong incentives for the continued use of 193nm technology. Beyond the 45nm node Extreme ultraviolet lithography may be required. EUV lithography systems are currently under development which will use 13.5nm wavelengths, approaching the regime of x-rays. // Introduction Extreme Ultraviolet Lithography (also known as EUV or EUVL) is a Next-Generation Lithography technology capable of creating nanometer-scale patterns for use in semiconductor processing. ... In the NATO phonetic alphabet, X-ray represents the letter X. An X-ray picture (radiograph) taken by Röntgen An X-ray is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz...

The image for the mask is originated from a computerized data file. This data file is converted to a series of polygons and written onto a square fused quartz substrate covered with a layer of chrome using a photolithographic process. A beam of electrons is used to expose the pattern defined in the data file and travels over the surface of the substrate in either a vector or raster scan manner. Where the photoresist on the mask is exposed, the chrome can be etched away, leaving a clear path for the light in the stepper/scanner systems to travel through. Fused quartz is a man-made material manufactured principally from sands. ... General Name, Symbol, Number chromium, Cr, 24 Chemical series transition metals Group, Period, Block 6, 4, d Appearance silvery metallic Atomic mass 51. ...

Optical lithography can be extended to a resolution of 15 nm by using the short wavelengths of 1 nm X-ray lithography for the illumination. The technique is developed to the extent of batch processing. The extension of the method relies on Near Field X-rays in Fresnel diffraction: a clear mask feature is "demagnified" by proximity to a wafer that is set near to a "Critical Condition." This Condition determines the mask-to-wafer Gap and depends on both the size of the clear mask feature and on the wavelength. The method is rapid because it uses broadband, and simple because it requires no lenses.

Work is in progress on an optical maskless lithography tool. This uses a digital micro-mirror array to directly manipulate reflected light without the need for an intervening mask. Throughput is inherently low, but the elimination of mask-related production costs - which are rising exponentially with every technology generation - means that such a system would be far more cost-effective for small-scale manufacturing applications. In maskless lithography the light used to expose the photosensitive emulsion (or photoresist) is confined to a narrow beam. ...

See also

• Nanolithography
• Soft lithography

Nanolithography — or lithography at the nanometer scale — refers to the fabrication of nanometer-scale structures, meaning patterns with at least one lateral dimension between the size of an individual atom and approximately 100 nm. ... In technology, soft lithography refers to a set of methods for fabricating or replicating structures using elastomeric stamps, molds, and conformable photomasks (in the words of Rogers and Nuzzo, p. ...

External links

• Semiconductor Lithography — Overview of lithography
• Optical Lithography Introduction — IBM site with lithography-related articles
• Immersion Lithography Article — Shows how depth-of-focus is increased with immersion lithography
• Photolithography — Article showing contact, proximity, and projection lithography