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Optical lithography

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==Methods of operation==
{{confusing|section}}The ability to focus an image into the sample is proportional to the distance from the focal plane (depth ==Methods of focus), and the amount of diffraction of the light. Both of these parameters are proportional to the wavelength of the light. The amount of diffraction is proportional to the wavelength, therefore to resolve finer features, shorter wavelengths are required. However, with shorter wavelengths the depth of focus is also lower, minimizing the amount of topology on the surface that is acceptable. There are a series of steps that are common to all types of optical lithography#Sample preparation#*Surface cleaning#*Dehydration#*Adhesion promoter#Photoresist application and soft bake#Exposure#Post exposure bake (PEB) (necessary with some resists)#Development#Hard bake (necessary with some resists)#Plasma descumoperation=====Sample preparationDehydration and HMDS==={{main|Lithography processing#Sample preparation}}Prior In order to photoresist application and exposure, get good adhesion the sample should be [[Cleaning|cleaned]] clean and free of [[Lithography processing#Dehydration bake|moisture]]. Additionally, and usually have an [[Lithography processing#Adhesion promoter|adhesion promoter]] promotor, such as [[HMDS]] may be , applied. Normally this is accomplished by doing HMDS vapor prime, although some materials such as Ti will have good adhesion without HMDS.
===Photoresist application===
{{main|Lithography processing#Photoresist application}}Once the sample is prepared, Typically photoresist is applied to spun on the sample. This and the thickness is most commonly achieved determined by spinning it on as the spin speed and viscosity of the resist. During this spin a liquid and then baking large amount of the sample solvent evaporates. It's also possible to remove apply photoresist using a spray-on tool although the solventLNF doesn't currently have this capability. The thickness of photoresist needs to be thick enough to survive it's intended purpose, such as a RIE mask, but the thicker the resit the layer is determined by larger the speed at which it is spunminimum feature size will be. Spin curves At the LNF 3µm of SPR 220 and bake times and temperatures can be found on 0.97µm of SPR 955 are common thicknesses to use. Going thicker then 3µm (5µm or 10µm) will make the [[photoresist]]s' datasheetsoverall process more difficult.
It ===Softbake===Next the resist is also possible baked to apply photoresist using a sprayreduce the solvent content. Baking hotter and longer pulls out more solvent which reduces how fast non-on tool or through {{em|what's it called when you dip exposed resist is attacked by the developer, however baking too hot and too long will start to decompose the wafer photoactive compound in a tub of resist and slowly draw it out? I'm pretty sure that's a thingthe resit, reducing its photo sensitivity...}}, although these methods are not currently available at Typically follow the recommended bake in the LNFdatasheet.
{{main|Lithography processing#Exposure}}After the photoresist is applied and baked, it softbake the resist is exposed to UV light to generate the desired pattern. The UV light causes a chemical reaction in the photoresist. In positive photoresist, the reaction PACs (photoactive compounds) makes the photoresist acidic, so that it will dissolve in a alkaline [[developer]] solution. With negative photoresist, the exposed polymer cross-links, making it impervious to the developer, which only removes the areas that are unexposed. The exposure may be [[direct writing|directly]] written on the mask with a laser, or the entire wafer can be exposed through a [[mask]]. The latter is a much faster and more cost-effective process when multiple samples are desired. Mask exposure can further be divided into two categories: {{em|contact}} and {{em|projection}} ; Contact exposureContact exposure involves placing the wafer in direct contact or very close proximity (less than 100{{nbsp}}μm) with the mask. This reduces diffraction through the mask to create a clear image in the photoresist. In this type of exposure, the pattern drawn on the mask will be directy transferred into the photoresist. Resolution is limited by the amount of diffraction and photoresist thickness. The majority of contact exposure tools in the LNF have a resolution of around 2{{nbsp}}μm. ; Projection exposureIn projection exposure, a lens is placed between the mask and the wafer, which focuses the image on the surface of this wafer. This allows for contact-less lithography, which can be cleaner and easier. Additionally, the lens typically reduces the size of the image from the mask, allowing for improved resolution. The [[GCA AS200 AutoStep|projection exposure tool]] in the LNF has 5x reduction, so the features on the wafer will be 5 times smaller than those drawn on the mask. This tool has a resolution of 0.7{{nbsp}}μm.
===Post exposure bake===
{{main|Lithography processing#Post exposure bake}}{{missing information|section|why a PEB When the resist is used}}Some photoresists recommend or require exposed to monochromatic light on a post exposure bakereflective substrate, such as Si, you will form standing waves in the resist of high and low light intensity. Like These standing waves will show up in the soft bake, this can be performed on a [[hotplate]] or in sidewalls of the [[ACS 200 cluster tool]]photoresist. Please check Baking the photoresist datasheet resist will help to determine if this is recommendeddiffuse the acid from the exposure, leveling out these standing waves.
{{main|Lithography processing#Development}}After exposure, the The photoresist is placed in a developer solution which dissolves parts of the photoresist on the wafer. For positive photoresist, the areas that were exposed dissolve, and for negative photoresist, the areas that were un-exposed dissolve. For most standard resists, this is performed by soaking the sample in a [[:Category:Bases|alkaline]] solution, although some use [[:Category:Solvents|solvent]] based developerstypically 2.38% TMAH. Check At the photoresist datasheet to determine the recommended developer. {{em|Optional alternative paragraph: The most commonly used developer is a highly diluted [[TMAH]] solution. The LNF recommends we typically use [[AZ 726300]], a developer with the same concentration of TMAH as for spray developing and [[AZ 300726]] plus surfactants to improve the uniformity of the developer during for puddle developing. Other supported developers are listed in on the [[developer]] page. There are also solvent-based developers for specific resists, like [[SU-8]] and [[PMMA]].}}
===Hard bake===
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