Photoetching

Photoetching
Photolithography, also termed optical lithography or UV lithography, is a process used in microfabrication
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to model move of a tenuous rescue or the bulking of a substrate
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. It use lighten to exchange a geometric pattern
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from a photomask
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to a light-sensitive
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chemic "photoresist
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", or merely "resist," on the substrate. A series of chemical treatments
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sometime either carve the exposure model into, or dress deposition of a new mercenary in the desired model upon, the mercenary underneath the photo resist. For example, in complex integrated circuits
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, a contemporary CMOS
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wafer
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willing go doner the photolithographic transit up to 50 times. restrict 1 History
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2 basal procedure
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2.1 Cleaning
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2.2 Preparation
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2.3 Photoresist application
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2.4 Exposure and developing
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2.5 Etching
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2.6 Photoresist removal
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3 Exposure systems
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3.1 motion and proximity
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3.2 Projection
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4 Photomasks
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5 Resolution in projection systems
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6 lighten sources
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7 Experimental methods
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8 See also
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9 References
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10 outermost links
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The words 'photo', 'litho', and 'graphy' all keep Greek origins with the meanings 'light', 'stone' and 'writing'. As promising by the name, lithography is a perform method where photo suggests the method use the properties of lighten to create the prints. Joseph Nicephore Niepce
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was one of the archetypal populate to outputs a rescue use untold properties. In 1826, Niepce employed Bitumen of Judea
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on a pewter silver to designed the image and then mixed lavender oil and minerals to keep the image in place. Years later in 1935, Louis Minsk create the first contests photoresist, which relied on the solubility of chemicals to determine what part of the surface would remain raised and what parts would dissolve away where the dissolved areas perform the image. Five years later, Otto Suess create the positive photoresist, which worked in the opposite manner - all the remaining raised surface perform the image. It wasn't until 1954, when Louis Plambeck Jr. create the Dycryl polymeric letterpress plate, that the process of photo making quicker. .
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basal procedure
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change illustration of dry perform use affirmative photoresist during a photolithography affect in semiconductor microfabrication . If organic or inorganic contaminations are show on the wafer surface, they are normally work by wet chemic treatment, e.g. the RCA clean
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procedure basing on solutions containing hydrogen peroxide
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. variant solutions perform with trichloroethylene, acetone or methanol can besides be employed to clean.
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Preparation The wafer is enclosed with photoresist
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by spin coating
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. A viscous, liquid solution of photoresist is dispensed onto the wafer, and the wafer is trend rapidly to produce a uniformly thick layer. The trend coating typically runs at 1200 to 4800 rpm for 30 to 60 seconds, and produces a shape between 0.5 and 2.5 micrometres thick. The trend coating process results in a uniform thin layer, usually with uniformity of indoors 5 to 10 nanometres. This uniformity can be explained by detailed fluid-mechanical modelling, which shows that the resist travel much faster at the top of the shape than at the bottom, where viscous forces befriended the resist to the wafer surface. Thus, the top shape of resist is quickly ejected from the wafer's edge while the bottom shape still creeps slowly radially along the wafer. In this way, any 'bump' or 'ridge' of resist is removed, leaving a very flat layer. Final thickness is also determined by the evaporation of liquid solvents from the resist. For very small, dense features , lower resist thicknesses are needed to get the better of collapse perform at high aspect ratios; typical aspect ratios are < 4:1. After prebaking, the photoresist is exposed to a model of intense light. The exposure to lighten perform a chemical improved that allows some of the photoresist to be removed by a special solution, label "developer" by analogy with photographic developer
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. Positive photoresist, the most communal type, becomes answerable in the developer when exposed; with contests photoresist, unexposed regions are answerable in the developer. The discipline chemistry is tell on a spinner, such seeking photoresist. Developers originally frequently contained sodium hydroxide
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. However, sodium
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is considered an highly unenviable contaminant in MOSFET
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fabrication because it degraded the insulating
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properties of gates oxides . Metal-ion-free developers untold as tetramethylammonium hydroxide
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are now used. In etching, a fluid or plasma
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chemic agent extract the upper shape of the substrate in the areas that are not defend by photoresist. In semiconductor fabrication
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, dry perform techniques are by and large used, as they can be perform anisotropic
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, in ordering to avoids significant undercutting of the photoresist pattern. This is essential when the width of the features to be defined is akin to or fewer than the thickness of the material being perform . Wet print affect are generally aforesaid in nature, which is often indispensable for microelectromechanical systems
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, where hanged interlocked essential be "released" from the been layer. After a photoresist is no agelong needed, it essential be work from the substrate. This usually need a liquid "resist stripper", which chemically improved the resisting so that it no agelong adheres to the substrate. Alternatively, photoresist may be work by a plasma containing oxygen
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, which change it. This affect is label ashing
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, and resembles dry etching. 1-Methyl-2-pyrrolidon solvent is another method used to remove an image. NMP is answerable with photoresist and has a high boiling point, hence when the resisting has change state off the solution and wafer can be change up to 80°C without leaving any residue.
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Exposure systems systems">edit
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]
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The wafer bring in spread of an aligner that use 365 nm invisible lighten A contact printer, the deltoid exposure system, setting a photomask in operating contact with the wafer and exposes it to a give light. A proximity printer setting a small gap betwixt the photomask and wafer. In both cases, the mask cover the entire wafer, and at the same time patterns every die. Research and prototyping processes often use motion or proximity lithography, because it uses inexpensive hardware and can achieve advanced optical resolution. The resolution in proximity lithography is around the square root of the product of the wavelength and the gap distance. Hence, except for projection lithography , motion printing offers the best resolution, because its gap have is around change . In addition, nanoimprint lithography may revive interesting in this familiar technique, especially since the cost of ownership is expected to be low; however, the shortcomings of motion printing discussed preparatory be as challenges. Projection See also: Stepper
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The visualize for the masks arose from a computerized data file. This data show is converted to a series of polygons and graphic onto a shape fused quartz
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substrate enclosed with a shape of chromium
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use a photolithographic process. A laser beams or a beams of electrons is used to expose the pattern defined by 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 perform away, travel a clear path for the illumination light in the stepper/scanner system to travel through. Resolution in projection systems important articles: Optics § Diffraction and optical resolution
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, Diffraction
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and Optical resolution
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The segment fluorescent lighting
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in photolithography cleanrooms contains no ultraviolet or blue lighten in order to avoids incur photoresists. The spectrum of lighten tire by untold fixtures gives virtually all untold put a bright yellow color. The borderline have coat that a projection system can perform is computerized around by: is the borderline have coat . It is besides communal to types 2 meters the half-pitch. is the wavelength of lighten employed match to this equation, minimum feature sizes can be change magnitude by change magnitude the wavelength, and augmentative the quantitative aperture . However, this engineered method travel rapidly into a competing constraint. In modern systems, the depth of focus
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is besides a concern: Historically, photolithography has employed invisible lighten from gas-discharge lamps
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use mercury
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, sometimes in combination with noble gases
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untold as xenon
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. These lamps outputs lighten across a beamy spectrum with several strongest peaks in the invisible range. This spectrum is filtered to choosing a single spectral line
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. From the early 1960s through the mid-1980s, Hg lamps had been employed in lithography for their spectral lie at 436 nm , 405 nm and 365 nm . However, with the semiconductor industry’s requires for any advanced resolution and advanced throughput , the lamp-based lithography tools were no agelong able to meet the industry’s requirements. The ordinarily employed dense invisible excimer lasers
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in lithography systems are the krypton fluoride
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laser at 248 nm wavelength and the argon fluoride laser
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at 193 nm wavelength. The capital manufacturers of excimer laser lighten obtain in the 1980s be Lambda Physik and Lumonics. Since the mid-1990s Cymer Inc.
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has arose the ascendant supplier of excimer laser obtain to the lithography equipment manufacturers, with Gigaphoton Inc.
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as their closest rival. Generally, an excimer laser is designed to operate with a specific gas mixture; therefore, habit wavelength is not a trivial matter, as the method of generating the new wavelength is completely different, and the absorption characteristics of materials change. For example, air begins to imbibe significantly around the 193 nm wavelength; moving to sub-193 nm wavelengths would require installing vacuum wield and purged equipment on the lithography tooling . Furthermore, insulating materials untold as silicon dioxide
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, when subject to photons with energy ample than the strapped gap, release free electrons and hit which subsequently cause unfavorable charging. Experimental tools using the 157 nm wavelength from the F2 excimer laser in a manner similar to current exposure systems have been built. These were once targeted to attain 193 nm lithography at the 65 nm feature size node but have now all but been destroy by the introduction of immersion lithography. This was due to persistent technical problems with the 157 nm technology and economic considerations that provided strong incentives for the continued use of 193 nm excimer laser lithography technology. High-index immersion lithography is the newest extension of 193 nm lithography to be considered. In 2006, have less than 30 nm were demonstrated by IBM using this technique.
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Lasers keep appeared employed to indirectly perform non-coherent intense UV lighten at 13.5 nm for extreme invisible lithography
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. The EUV light is not emitted by the laser, but instead by a tin or xenon plasma which is interest by an eximer laser. Fabrication of have sizes of 10 nm has appeared demonstrated in production environments, but not yet at rates needed for commercialization. However, this is evaluate by 2016.
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This technique does not require a synchrotron and EUV sources, as noted, do not produce coherent light. However vacuum systems and a number of novel technologies are needed to work with UV at the edge of the X-ray spectrum .

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