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Lithography processing

Lithography processing is a series of processing steps used to pattern 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 Handbook. In addition, the lab offer a general Lithography 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.

Contents

Photolithography in the LNF

Creating a pattern with photolithography depends on the photoresist used, but typically involves the following steps:

  1. Cleaning and dehydrating the substrate
  2. Applying adhesion promoter
  3. Applying photoresist
  4. Soft baking the resist
  5. Exposing it to UV light
  6. Post-exposure bake (some resists only)
  7. Resist development
  8. Hard bake (some resists only)
  9. Plasma descum

Sample preparation

The following has been moved from Optical lithography and needs to be integrated into this section

Sample cleaning

Main article: Sample cleaning

For good photoresist adhesion, wafers should be clean and dry. While a simple process such as rinsing with Acetone and IPA and blowing dry with an N2 gun may suffice for clean wafers, typically a more aggressive clean should be used particularly after multiple processing steps. Suggested processes and equipment is listed below.

Processes

Piranha etch and Nanostrip are good organic cleans for removing residue on the surface of a wafer prior to processing. An oxygen plasma is another good alternative.

Equipment

Dehydration bake

Main article: Lithography processing § Dehydration bake

Because photoresist is hydrophobic, a dehydration bake is necessary to remove all moisture from the surface of the wafer. This can be done simply by placing the wafer on a hotplate but may also be included as part of the vapor prime step, discussed in the next section.

Adhesion promoter

Main article: Lithography processing § Adhesion promoter
For more details on HMDS application, see Vapor prime.

To promote adhesion, HMDS is often applied to a sample prior to applying the photoresist. The recommended method of application is with a vapor prime process in one of the tools listed below. It can also be applied in liquid form and spun on the surface, similar to spinning photoresist, but this has been shown to be less effective.

Equipment

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Substrate Cleaning and Surface Priming

For good photoresist adhesion, wafers should be clean and dry. Wafers that have just a few dust particles may only need to be blown clean with a N2 gun. Those with more particles or residue may be cleaned with solvents (rinse in acetone), then IPA). Wafers with stubbornly attached particles or organic residue can be cleaned using oxygen plasma (YES Plasma Stripper), in a Piranha or Nanostrip solution, or cleaned with the CL200 Megasonic Cleaner.

Dehydrating

Photoresist solvents are repelled by water, so any moisture at all on the wafer surface can cause adhesion problems. To remove moisture from their surface, wafers are typically given a dehydration bake. This can be done in an oven (e.g., 30 minutes at 115°C) or on a hotplate (e.g., 10 minutes at 115°C).

Adhesion Promoter

After the dehydration bake, priming the wafer surface with an adhesion promoter, hexamethyldisilazane (HMDS), is recommended for most resists. The recommended method for application combines both the dehydration bake and HMDS application using a vapor prime oven. These ovens heat the wafers, pump the chamber to vacuum, and inject HMDS vapor which creates a monolayer of HMDS on the wafer surface. This monolayer is important because the molecules will be aligned to provide even adhesion across the wafer. This can be done in either the Image Reversal Oven or the ACS 200 Cluster Tool. The pictures below demonstrate what can happen to a pattern without good adhesion. The wafer with HMDS was treated in the YES Image Reversal Oven.

  • Without HMDS

  • Wih HMDS

Photoresist application

The following has been moved from Optical lithography and needs to be integrated into this section

The most common method of applying photoresist to a sample is by spinning it on as a liquid and then baking the sample to remove the solvent. There are a variety of tools that can be used to apply the photoresist depending on the size of your sample and type of photoresist used.

Equipment

Soft Bake

Most photoresists require a softbake to bake off the remaining solvents. For many processes on the ACS 200 cluster tool, this bake is included automatically after the spin step. For manual spinning, this should be performed on a hotplate at the temperature recommended by the photoresist datasheet.

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Once the wafer is clean and primed, the photoresist can be dispensed. The main method of applying photoresist in the LNF is by spinning on the liquid resist and then baking out the solvents to dry it. The thickness of the photoresist layer is dependent on the viscosity of the resist, the spin speed, and the length of the spin. Typical spin speeds are from 1000-5000 rpm and last for 20-30 seconds. Often a slower “spread” is used before the final spin, at around 500rpm, to cover the wafer evenly with photoresist.

Spinning can be done by the ACS 200 cluster tool or manually. The LNF has two manual spinners in the 1440A/Silicon Bay, the Solitec Spinner and the CEE Spinner. The CEE has preprogrammed recipes for various spin speeds (see the list of recipes on the wall next to the spinner). The Solitec has manual controls for spin speed and time. The bowl that is commonly installed in the Solitec is for use with positive photoresists. SU-8 (which is not soluble in [Acetone (cmos)|acetone] and other non-standard materials such as polyimides and spin-on dopants should be spun in the [Wet Chemistry] lab. (Remember, if you need to spin a non-standard chemical, it must first be approved for use in the LNF.)

xxxxx When applying photoresist to the substrate the goal is to get an even layer across the full sample.

Spinning

Generally photoresist is applied with a photoresist spinner. XXXX

Spraying

It is also possible to apply photoresist with a micro-spray system. These can allow much high aspect ratio features to get coverage. The LNF is currently looking into a micro-spray system. xxxxx

Soft Bake

After application, photoresist is typically soft baked. Soft baking drives off some of the solvent in the photoresist and partially solidifies it, so that it is ready for exposure to UV light.

In the LNF, wafers can be baked in the ovens or on hotplates that are located in 1440A and 1440C. Hotplate baking has the advantage of shorter bake times, and is supposed to perform better than oven baking because solvent can escape from the top of the photoresist layer as heat is applied from below. 4”-6" diameter wafers can be baked in the ACS 200 cluster tool. Characterization data for the hotplates and ovens can be found on [Hotplate-characterization|Hotplate Characterization].


Exposure

The following has been moved from Optical lithography and needs to be integrated into this section

The LNF offers several types of exposure, explained in more detail on the lithography processing page. There are several contact aligners, one projection stepper, as well as direct write capabilities.

Equipment

[imageright|Photolithography exposure.|{UP(Public.Lithography)}Photoresist-small.jpg]

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After soft baking, the wafer is exposed to UV light, usually through a Chromium mask that has been patterned to allow light through certain areas. This is what creates the pattern on your wafer for directing the next processing step. 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 basic developer solution. With negative photoresist, the exposed polymer cross-links, making it impervious to the developer, which only removes the areas that are unexposed.

There are three main methods for exposing wafers: contact exposure, projection exposure, and direct write. The LNF has several different mask aligners, listed in [Lithography#Controlled_Exposure_Tools_17|Controlled Exposure Tools].

Contact Exposure

Contact exposure involves placing the wafer in direct contact or very close proximity (less than 100um) 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 lab can produce around 2um resolution.

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 projection exposure tool in the LNF has 5x reduction, so the features on your wafer will be 5 times smaller than those drawn on your mask. This tool has a resolution of 0.7um.

Direct Write

Direct write lithography does not use a mask. The UV image is drawn directly onto the wafer using the features specified in a CAD file loaded onto the tool. The [Heidelberg uPG 501] has capabilities for writing on a 4" (100mm) or smaller sample with 1um resolution. Direct write is also the most common way of creating the masks used in contact and projection lithography. Because of the time required to write a full wafer, this is not very common for writing on individual samples.


Post exposure bake

Some photoresists require a bake after the exposure and before the resist is developed. This bake assists the chemical reaction that occurs during the exposure. For some resists, like [SPR 220], it is not absolutely necessary, but will improve the sidewall roughness and profile. For other resists, like [SU-8 Series|SU-8] and [KMPR], the majority of the cross-linking reaction occurs during the post exposure bake. [1800 Series] resists do not require a post exposure bake.

Development

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. The remaining photoresist is what will be used to mask the subsequent processing step.

The most commonly used developer is a highly diluted [Tetramethylammonium Hydroxide|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 [Lithography#Developers_13|Materials>Developers]. There are also solvent-based developers for specific resists, like [SU-8 Series] and [PMMA].


Hard bake

The reason for hard baking resist is to 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 by 10-15%. Hard baking resist has been shown to have no effect or can be detrimental for RIE etching.

If you are doing wet etching and want to hard bake the resist always use [1800 series] resist (1813, 1822, 1827). DO NOT HARD BAKE [SPR 220], it will reflow and fill in small features and reduce the profile of the resist.

Original text:

Some photoresists recommend a hard bake, performed after the development of the sample. This is often recommended prior to performing wet etching processes. Please check the photoresist datasheet to determine if a hard bake should be used.

For some photoresists, like SPR 220, a hard bake should explicitly not be used, as it will reflow the resist and degrade the features.

Plasma descum

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, such as can be performed in the [YES Plasma Stripper] which etches the photoresist a small amount (on the order of 20-30nm). 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.

Original text:

Prior to subsequent processes, a plasma descum is highly recommended. This will remove any residue left after development that may degrade the performance of an etch or the adhesion of a deposition (e.g. when performing a lift-off. Specifically for wet etching, it will also make the photoresist surface hydrophilic, which will ensure even etching by the chemical.

Equipment

For descum, typically, an oxygen plasma step is performed using the YES-CV200RFS(E). Some RIE equipment also can perform the descum step, often immediately before beginning the etch process.

Materials

Photoresists

There are a variety of photoresists approved for use in the LNF. Please see the [photoresist] page for more details. The photoresists that are actively supported by the LNF are:

  • [SPR 220|SPR 220 3.0 and 7.0]
  • [1800 Series|Shipley 1813 and 1827]
  • [KMPR|KMPR 1010 and 1025]
  • [SU-8 Series|SU-8 2005, 2010, 2050 and 2100] (in [ROBIN only)

Many of the supported photoresists are plumbed into the photoresist dispense tools. Manual dispensing of photoresists can also be done and many photoresists can be purchased through the [LNF store] in personal syringes. For more information on how to manually dispense, please see our [Lithography Handbook].

Developers

The following developers are stocked and supported by the lab. The most common developers are plumbed into spin-developer tools and other developers should be used in the appropriate wet bench. Unless otherwise noted, the developers can be used with [SPR 220], [1800 Series|Shipley 1800], and [KMPR] series resists.

  • [AZ 726]
  • [AZ 300|AZ 300 MIF]
  • [MF 319|MF 319]
  • [CD-30]
  • ([1800 Series|S1800 series] only)
  • [AZ 400k] ([Photoresist, (AZ9260)] and [Photoresist, (AZ5214-E) (IR)])
  • [SU-8 Developer] ([SU-8 Series]) Note SU-8 resist and developer is only available in the [ROBIN] lab
  • [Methyl Iso-Butyl Ketone (cmos)|MIBK] (950k series PMMA, see [E-beam Resist (PMMA) Development])

Equipment

Vapor Prime Ovens

The vapor prime ovens can be used for both dehydration baking and HMDS application prior to coating a wafer with photoresist. The following tools can be used for this purpose.

  • [ACS 200 cluster tool] (most coating recipes have a vapor prime step built in)
  • [YES Vapor Prime Oven] (currently not online)
  • [YES Image Reversal Oven]

Photoresist Coating Systems

These systems are used for coating samples with photoresist, mainly by spinning the liquid photoresist at a set speed. For more information on the capabilities of each system, click on the appropriate link.

  • [ACS 200 cluster tool]
  • [CEE 200X PR Spinner 1]
  • [CEE 200X PR Spinner 2]
  • [CEE Spinner|CEE 100CB Manual Spinner]
  • [Manual Spinner] ([Solitec Spinner])


Controlled Exposure Tools

These tools are used for exposing the photoresist to high energy, either UV light or an electron beam, which causes a chemical reaction to either cure the resist or make it removable. For more information, see the [Lithography Handbook]. The lab has the following tools for exposure:

Projection Exposure

Contact Exposure

===Photoresist Development Tools===' These tools are used for developing the photoresist after exposure. There are several tools that automatically develop and rinse the wafer as well as a wet bench for manual soaking of wafers in the developer solution.

  • [ACS 200 cluster tool]
  • [CEE Developer 1]
  • [CEE Developer 2]
  • [Base Bench 1440B]

Mask Makers

Plasma Descum Tools

  • [YES Plasma Stripper]
  • [March Asher]


References

[Lithography Handbook]


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