View source for Optical lithography

{{Infobox technology
|image =
|technology = Lithography
|names = UV lithography, photolithography
|equipment = [[:Category:Lithography equipment|List of lithography equipment]]
}}
{{about|optical (UV) lithography|information on e-beam lithography|Electron beam lithography}}
[[{{PAGENAME}}]] (also termed '''photolithograpy''' or '''UV lithography''') is the patterning of masks and samples with [[photoresist]] prior to other processing steps (e.g. deposition, etching, doping).  There are a variety of lithography processes that are available in the LNF.  <!--This page specifically talks about optical (UV) lithography.  For information on electron beam lithography, please see the [[Electron beam lithography]] page. For detailed information on optical lithography practices and common processes, see the [[Lithography processing]] page.-->The lab offers a general [[Lithography training session|training session]] for lithography processing including details of process steps and the tools available. This session is required for authorization on most lithography tools.
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==Equipment==
<onlyinclude>
===Exposure===
;Stepper
*[[GCA AS200 AutoStep]]{{spaced ndash}} Can run pieces up to 6" wafers.  
;Contact Aligner
*[[MA-BA-6_Mask-Bond_Aligner|MA/BA-6 Mask/Bond Aligner]]{{spaced ndash}}4" and 6" wafers with backside alignment capability
*[[MA6 Mask Aligner]]{{spaced ndash}}4" and 6" wafers
*[[MJB3-2]]{{spaced ndash}}pieces up to 3" wafers
*[[MJB 45S]]{{spaced ndash}}[[SU-8]] and [[PDMS]] exposure
;Direct Write
*[[Heidelberg µPG 501 Mask Maker]]{{spaced ndash}}mainly for [[mask making]] but can also be used on samples up to 5" across

===HMDS===
* [[TEL Mark Vz Coater]] - HMDS vapor prime is included in the standard spin processes in the TEL
* [[Image Reversal Oven]] - Batch process small pieces up to 25 6" wafers.
* HMDS can also be spun at the manual spinners.

===Photoresist Spinning===
* [[TEL Mark Vz Coater]]
** Automated spinning of [[SPR 220|SPR 220 (3.0)]] and [[SPR 955]]
** 4" and 6" wafers
* Manual spinners accept pieces up to 6" wafers
** [[CEE 200X photoresist spinner 1]] and [[CEE 200X photoresist spinner 2]] {{spaced ndash}}automatic dispense of [[SPR 220|SPR 220 (3.0)]], [[S1813]], and [[SPR_955|SPR 955]]
** [[Spinner CEE Apogee]]{{spaced ndash}}fully manual spinner for photoresist and other polymers

===Development===
* [[TEL Mark Vz Coater]] - Automated spray development of 4" and 6" wafers with [[AZ 300]] and [[AZ 726]]
* The CEE puddle developers may be used on pieces up to 6" wafers.
** [[CEE Developer 1]]{{spaced ndash}}[[AZ 726]]
** [[CEE Developer 2]]{{spaced ndash}}[[AZ 726]] and [[Microposit Developer]]
* [[Base Bench 63]]{{spaced ndash}} Beaker development is useful for developing thick photoresist like [[KMPR]]</onlyinclude>

==Training modules==
This [[Category:Pages with Rise modules]] [https://rise.articulate.com/share/cwWW6hFLG6g_FUYu9fzGEkE2SrhN7mcV online course] covers the fundamentals of optical lithography at the LNF. It is also required for checkout on most lithography tools, see the tools checkout procedure for more details.

==Exposure tool selection==
===Contact Aligner===
Contact lithography places the mask in direct contact with the photoresist layer and exposes the entire sample at once.  The minimum feature size will be larger than with projection lithography, and is governed by the wavelength and how much gap is between the mask and photoresist.  At the LNF we only recommend contact lithography for 2µm and larger features and 2µm or larger alignment tolerance.  The better the contact, the more often the mask will need to be cleaned as more particles and photoresist will transfer.    

===Stepper===
With project lithography light shines through the mask, goes through a reduction lens and projects onto the substrate.  Since the mask never comes into contact with the sample it stays cleaner.  At the LNF 500nm gratings will reliably print on [[SPR 955]], however the resolution will depend on the feature type and photoresist thickness.  As feature size decreases the depth of focus also decreases, so thinner resist must be used.  The [[GCA AS200 AutoStep]] has a max die size of 14.7mm x 14.7mm and a minimum alignment tolerance of 200nm for wafers, if running pieces the tolerance will be larger.  This also strongly depends on the accuracy of the artwork on the mask; mask plates produced with the [[Heidelberg µPG 501 Mask Maker]] will require larger tolerances because of this.

===Direct Write===
For prototyping and one-off jobs using the [[Heidelberg µPG 501 Mask Maker]] to directly expose a sample can be effective.

==Methods of operation==
===Dehydration and HMDS===
In order to get good adhesion the sample should be clean and free of moisture and usually have an adhesion promotor, such as HMDS, applied.  Normally this is accomplished by doing HMDS vapor prime, although some materials such as Ti will have good adhesion without HMDS.

===Photoresist application===
Typically photoresist is spun on the sample and the thickness is determined by the spin speed and viscosity of the resist.  During this spin a large amount of the solvent evaporates. It's also possible to apply photoresist using a spray-on tool although the LNF doesn't currently have this capability.  The photoresist needs to be thick enough to survive it's intended purpose, such as a RIE mask, but the thicker the resit the larger the minimum feature size will be.  At the LNF 3µm of SPR 220 and 0.97µm of SPR 955 are common thicknesses to use. Going thicker then 3µm (5µm or 10µm) will make the overall process more difficult.

===Softbake===
Next the resist is baked to reduce the solvent content.  Baking hotter and longer pulls out more solvent which reduces how fast non-exposed resist is attacked by the developer, however baking too hot and too long will start to decompose the photoactive compound in the resit, reducing its photo sensitivity.

===Exposure===
After the softbake the resist is exposed to UV light. In positive photoresist, 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.

===Post exposure bake===
When the resist is exposed to monochromatic light on a reflective substrate, such as Si, you will form standing waves in the resist of high and low light intensity.  These standing waves will show up in the sidewalls of the photoresist.  Baking the resist will help to diffuse the acid from the exposure, leveling out these standing waves.

===Development===
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, typically 2.38% TMAH.  At the LNF we typically use [[AZ 300]] for spray developing and [[AZ 726]] for puddle developing. There are also solvent-based developers for specific resists, like [[SU-8]] and [[PMMA]].

===Hard bake===
Some photoresists recommend hard-baking the resist after development. This can improve the adhesion and reduce the amount of undercut during wet chemical etching.

===Plasma descum===
Oxygen plasma treatment is recommended prior to [[wet etching]]. Photoresist is naturally [[Wikipedia:hydrophobic|hydrophobic]] and will repel water-based solutions, causing small features to not be etched. Exposure to oxygen plasma renders the surface hydrophilic and enables etching to occur.

==Figures of Merit==
===Resolution===
The minimum feature resolution is generally a direct result of the type of resist, thickness, exposure time, and develop time.

===Registration===
Also referred to as alignment accuracy, is critical to ensure 2 patterns are properly registered to each other.

===Thickness===
When photoresist is used as a mask for etching it will also be etched (usually at a significantly slower rate).  Therefore it is important to have enough thickness to last the length of the etch.  Generally it is good to have a reasonable margin of safety, usually ~25-50% is ok.  On the other hand it's easier to pattern smaller features in thinner resist.
 
===Sidewall Angle===
The sidewall angle of the photoresist is critical to many processes.  Vertical sidewalls are necessary for etching where as an outwards sloping sidewall makes [[Liftoff|liftoff]] easier.

===Defect Level===
The majority of defects in optical lithography are user created errors.  It's necessary to pay close attention to '''every''' step of the process.  A glove finger print may not show up initially on the wafer until the wafer is etched.  A drop of acetone from the mist created by an acetone squirt bottle can destroy part of a pattern.  Often it's possible to reduce both particulate and defects by using an automated tool such as the [[ACS 200 cluster tool]] to remove as much of the human skill needed as possible.

==See also==
* [[:Category:Lithography equipment|List of lithography equipment]]
* [[Electron beam lithography]]

==Further reading==
* [http://lnf-wiki.eecs.umich.edu/wiki/User_Resources#LNF_Tech_Talks_.28technology_seminar_series.29 LNF Tech Talk for Photolithography]
* [https://docs.google.com/document/d/1lwKiyFrUNtZ2j9MNi21j697glr_YwgbzE97o4q3xDRY/preview Lithography Handbook]
* [https://www.dropbox.com/s/ikouq6mxkde9mfu/2015-06%20LNF%20Tech%20Seminar%20Lithography.pdf?dl=0 Lithography Workshop June 12, 2015]
* [[Wikipedia:Photolithography]]

[[Category:Lithography]]