Difference between revisions of "Optical lithography"

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{{missing information|photoresist tone}}
 
 
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{{about|optical (UV) lithography|information on e-beam lithography|Electron beam lithography}}
 
{{about|optical (UV) lithography|information on e-beam lithography|Electron beam lithography}}
 
{{details|Lithography processing|optical lithography practices and common processes}}
 
{{details|Lithography processing|optical lithography practices and common processes}}
[[{{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 several of the tools, but can be taken by anyone in the lab.
+
[[{{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.
 +
{{TOC|limit=2|clear=left}}
  
 
==Equipment==
 
==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===
 
===HMDS===
 
* [[ACS 200 cluster tool]] - HMDS vapor prime is included in the standard spin processes in the ACS
 
* [[ACS 200 cluster tool]] - HMDS vapor prime is included in the standard spin processes in the ACS
 
* [[Image Reversal Oven]] - Batch process small pieces up to 25 6" wafers.
 
* [[Image Reversal Oven]] - Batch process small pieces up to 25 6" wafers.
 +
* HMDS can also be spun at the manual spinners.
  
 
===Photoresist Spinning===
 
===Photoresist Spinning===
Line 20: Line 32:
 
** Automated spinning of [[SPR 200|SPR 220 (3.0)]] and [[SPR 955]]
 
** Automated spinning of [[SPR 200|SPR 220 (3.0)]] and [[SPR 955]]
 
** 4" and 6" wafers
 
** 4" and 6" wafers
* Manual Spinners accept pieces up to 6" wafers
+
* Manual spinners accept pieces up to 6" wafers
** [[CEE 200X photoresist spinner 1]]{{spaced ndash}}automatic dispense of [[SPR 220|SPR 220 (3.0) and (7.0)]] and SPR 955 0.9
+
** [[CEE 200X photoresist spinner 1]]{{spaced ndash}}automatic dispense of [[SPR_220|SPR 220 (3.0) and (7.0)]] and [[SPR_955|SPR 955]]
** [[CEE 200X photoresist spinner 2]]{{spaced ndash}}automatic dispense of [[SPR 220|SPR 220 (3.0)]] and [[1813]]
+
** [[CEE 200X photoresist spinner 2]]{{spaced ndash}}automatic dispense of [[SPR_220|SPR 220 (3.0)]] and [[1813]]
 
** [[CEE 100CB photoresist spinner]]{{spaced ndash}}fully manual spinner for photoresist and other polymers
 
** [[CEE 100CB photoresist spinner]]{{spaced ndash}}fully manual spinner for photoresist and other polymers
 
===Exposure===
 
;Stepper
 
*[[GCA AS200 AutoStep]]{{spaced ndash}}pieces up to 6" wafers, 5x reduction lens, 0.45 NA
 
;Contact Aligners
 
*[[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
 
  
 
===Development===
 
===Development===
Line 40: Line 41:
 
* The CEE puddle developers may be used on pieces up to 6" wafers.
 
* The CEE puddle developers may be used on pieces up to 6" wafers.
 
** [[CEE Developer 1]]{{spaced ndash}}[[AZ 726]]
 
** [[CEE Developer 1]]{{spaced ndash}}[[AZ 726]]
** [[CEE Developer 2]]{{spaced ndash}}[[AZ 726]] and [[Microposit developer concentrate]]
+
** [[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]]
+
* [[Base Bench 63]]{{spaced ndash}} Beaker development is useful for developing thick photoresist like [[KMPR]]</onlyinclude>
  
==Selection of type of optical lithography==
+
==Training modules==
{{copy edit|section}}
+
This [[Category:Pages with Rise modules]] [https://rise.articulate.com/share/cwWW6hFLG6g_FUYu9fzGEkE2SrhN7mcV online course] covers the fundamentals of optical lithography at the LNF and replaces the in-person group session that was in place before the CVODI-19 pandemic. It is also required for checkout on most lithography tools, see the tools checkout procedure for more details.
For optical lithography a physical pattern is required with clear and dark areas typically on a mask. Most often the mask is a glass plate with a UV opaque material on it such as chromium.
 
There are two types of optical lithography utilized in clean room environments:
 
#Contact lithography places the glass mask in direct contact with the sample. This has some distinct advantages and disadvantages:
 
#*Because it is in direct contact with the photoresist, it is subject to picking up particles and transferring them to all subsequent wafers.
 
#*It is capable of patterning an entire wafer with a single exposure
 
#*Minimum features are larger than projection lithography
 
#**At LNF 2µm features
 
#*Registration is limited to what can be seen (power of the microscope)
 
#**At LNF 1µm alignment tolerance
 
#Projection lithography shines light through the mask, a set of lenses and projects this onto the substrate.
 
#*Mask stays cleaner, less susceptible to particulates
 
#*Many steppers have reduction in the optics allowing for definition of smaller features
 
#**At LNF 0.5µm minimum features, 0.2µm minimum alignment tolerance
 
#*Will focus on different parts of the wafer, eliminating problems with warped wafers
 
#*Especially with a reduction, limited in die size
 
#**At LNF 15mm x 15mm
 
#*More limited depth of focus (step height)
 
  
==Methods of operation==
+
https://rise.articulate.com/share/cwWW6hFLG6g_FUYu9fzGEkE2SrhN7mcV
{{confusing|section}}
+
 
The ability to focus an image into the sample is proportional to the distance from the focal plane (depth 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.
+
==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.
  
There are a series of steps that are common to all types of optical lithography
+
===Direct Write===
#Sample preparation
+
For prototyping and one-off jobs using the [[Heidelberg µPG 501 Mask Maker]] to directly expose a sample can be effective.
#*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 descum
 
  
===Sample preparation===
+
==Methods of operation==
{{main|Lithography processing#Sample preparation}}
+
===Dehydration and HMDS===
Prior to photoresist application and exposure, the sample should be [[Cleaning|cleaned]] and free of [[Lithography processing#Dehydration bake|moisture]]. Additionally, an [[Lithography processing#Adhesion promoter|adhesion promoter]] such as [[HMDS]] may be applied.
+
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===
 
===Photoresist application===
{{main|Lithography processing#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.
Once the sample is prepared, photoresist is applied to the sample. This is most commonly achieved by spinning it on as a liquid and then baking the sample to remove the solvent. The thickness of the layer is determined by the speed at which it is spun. Spin curves and bake times and temperatures can be found on the [[photoresist]]s' datasheets.
 
  
It is also possible to apply photoresist using a spray-on tool or through {{em|what's it called when you dip the wafer in a tub of resist and slowly draw it out? I'm pretty sure that's a thing...}}, although these methods are not currently available at the LNF.
+
===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===
 
===Exposure===
{{main|Lithography processing#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.
After the photoresist is applied and baked, it 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 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 exposure
 
Contact 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 exposure
 
In 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===
 
===Post exposure bake===
{{main|Lithography processing#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 intensityThese standing waves will show up in the sidewalls of the photoresistBaking the resist will help to diffuse the acid from the exposure, leveling out these standing waves.
{{missing information|section|why a PEB is used}}
 
Some photoresists recommend or require a post exposure bakeLike the soft bake, this can be performed on a [[hotplate]] or in the [[ACS 200 cluster tool]]Please check the photoresist datasheet to determine if this is recommended.
 
  
 
===Development===
 
===Development===
{{main|Lithography processing#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% TMAHAt 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]].
After exposure, 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 developersCheck 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 [[AZ 726]], a developer with the same concentration of TMAH as [[AZ 300]] plus surfactants to improve the uniformity of the developer during 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===
 
===Hard bake===
{{main|Lithography processing#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.
Some photoresists recommend hard-baking the resist after development. This will densify the resist, improve the adhesion to the surface, and make it more resistant to wet chemical etching. It will reduce the undercut of the resist during wet chemical etching. As a rough approximation, it reduces the undercut for an [[Shipley 1800|1800 series]] mask by 10&ndash;15%. Hard baking resist has been shown to have no effect or can be detrimental for [[RIE]] etching.
 
 
 
{{note|The hard bake is not recommended for certain photoresists, such as [[SPR 220]]. Please consult the photoresist datasheet to determine if it is recommended.|reminder}}
 
  
 
===Plasma descum===
 
===Plasma descum===
{{main|Plasma ashing}}
+
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.
For many applications, a [[plasma descum]] step is performed after the lithography before further processing. This step typically consists of a short, low power [[oxygen]] plasma, which etches the photoresist a small amount (on the order of 20&ndash;30{{nbsp}}nm). The purpose of the descum is to remove any residue from the surface where the photoresist was developed, and to get rid of the "tail" that often occurs at the interface between the photoresist and the substrate, and improve the vertical profile of the features. A descum is strongly recommended before any [[RIE]] etching. The oxygen plasma can be replaced with a high power [[Argon]] plasma, which can be useful for etches where surface roughness and sidewall profile are critical.
 
 
 
Oxygen plasma treatment is also strongly 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==
 
==Figures of Merit==
 
===Resolution===
 
===Resolution===
The minimum feature resolution is generally a direct result of the type of resist, thickness, exposure time and develop time.
+
The minimum feature resolution is generally a direct result of the type of resist, thickness, exposure time, and develop time.
  
 
===Registration===
 
===Registration===
Line 139: Line 99:
 
===Defect Level===
 
===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.
 
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==
 
==See also==
Line 147: Line 106:
  
 
==Further reading==
 
==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://docs.google.com/document/d/1lwKiyFrUNtZ2j9MNi21j697glr_YwgbzE97o4q3xDRY/preview Lithography Handbook]
* [https://umich.box.com/s/buip2kslnb3ewvv9jewzmoi5hrvmgoaw Lithography Workshop June 12, 2015]
+
* [https://www.dropbox.com/s/ikouq6mxkde9mfu/2015-06%20LNF%20Tech%20Seminar%20Lithography.pdf?dl=0 Lithography Workshop June 12, 2015]
 
* [[Wikipedia:Photolithography]]
 
* [[Wikipedia:Photolithography]]
  
 
[[Category:Lithography]]
 
[[Category:Lithography]]

Latest revision as of 16:46, 5 August 2021

Optical lithography
Technology Details
Other Names UV lithography, photolithography
Technology Lithography
Equipment List of lithography equipment
This article is about optical (UV) lithography. For information on e-beam lithography, see Electron beam lithography.
For more details on optical lithography practices and common processes, see Lithography processing.

Optical lithography (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. The lab offers a general training session for lithography processing including details of process steps and the tools available. This session is required for authorization on most lithography tools.

Equipment

Exposure

Stepper
Contact Aligner
Direct Write

HMDS

  • ACS 200 cluster tool - HMDS vapor prime is included in the standard spin processes in the ACS
  • Image Reversal Oven - Batch process small pieces up to 25 6" wafers.
  • HMDS can also be spun at the manual spinners.

Photoresist Spinning

Development

Training modules

This online course covers the fundamentals of optical lithography at the LNF and replaces the in-person group session that was in place before the CVODI-19 pandemic. It is also required for checkout on most lithography tools, see the tools checkout procedure for more details.

https://rise.articulate.com/share/cwWW6hFLG6g_FUYu9fzGEkE2SrhN7mcV

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 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 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 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

Further reading