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Resist Process Catalog

Overview

Producing patterns in various materials relies on selective processing. Defined areas of the substrate must be protected from or exposed to pattern transfer processes (etching, deposition, etc.). Pattern definition takes place in the resist--a thin layer of polymeric material that is coated on the substrate. The resist is modified so that it remains in some areas and is removed in others. This is a two-step process:

  1. Exposure Incident radiation, particles

     

  2. Development Selective removal in solvent or base

     

The resist system used almost universally for optical microlithography today is the so-called DNQ system: novolak resin with a diazonaphthoquinone sensitizer. The novolak resin is rendered base-insoluble by the addition of the sensitizer, or photoactive compound (PAC). It remains insoluble until photo-exposure transforms the PAC into a base-soluble product. Thus the PAC acts as a dissolution inhibitor until exposure transforms it. The reaction product is indene carboxylic acid. The exact composition of resist differs from type to type and the spectral sensitivity of every resist is different.

  

 

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Substrate Cleaning Prior to Lithography

Cleaning a wafer before coating it may involve removal of the native oxide, or simply cleaning with solvents. A brand new wafer may only require an isopropyl or methanol rinse. If there is grease on the wafer, methylene chloride may be required to remove it. If the wafer has been coated with resist before, this should be stripped before recoating

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HMDS Priming

Unfortunately, the surfaces of many of the materials we want to put resist on oxidize very easily. The surface oxide forms long range hydrogen bonds with water adsorbed from the air. When the resist is spun onto such a surface, it adheres to the water vapor rather than to the surface, and poor adhesion results.

 

Hexamethyldisilazane (

HMDS) is a common primer used before spin coating that serves as an adhesion promoter for

photoresist. HMDS may be applied in two ways. Liquid priming is the process of spinning HMDS, diluted in solvent, onto a dehydrated wafer. We use 20% HMDS in PGMEA, a common resist solvent. This is often effective, but is not as good as vapor priming. At CNF we have a YES Vapor Priming Oven in which the samples can be primed. During a 35-minute, pre-programmed cycle, the oven pumps down to dehydrate the samples, and then fills with pure HMDS vapor, resulting in a much more efficient prime than is possible otherwise.

 

 

 

Adhesion of resist to a surface coated with HMDS. R. Dammel, Diazonaphthoquinone-based Resists, SPIE Press, 1993, p. 100.

 

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Manual Spin Coating

Spin coatingis used to get resist onto the substrate with the required uniform thickness. The physics of spinning is complicated, and depends strongly on the evaporation rate of the solvent used. This is why there are only a few solvent systems in use for resist. The process is simply to spin for a fixed time, usually 30 seconds, at a speed chosen to result in the desired thickness. Spin speed charts allow you to determine what speed to use.

CNF offers two manual spin coaters for applying standard photoresists. A third manual spin coater is available for SU-8, Polyimide,LOR and other nontraditional photolithography materials.

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Automated Spin Coating

In order to acheive greater reproducibility in photolithographic processing, automated tools for spin coating resist are typically used in industry. This allows more precise control over the amount of material dispensed onto the substrate and the speed at which thesubstrate is spun. This also allows for handling of larger substrates; manually spinning wafers larger than 150 mm can be problematic.

The CNF operates a Brewer CEE 6000 automated spin coat, bake and develop tool. This tool can handle wafers from 3" to 200 mm in size. more information on this tool can be found on the Brewer CEE 6000 equipment page.

CNF is in the process of installing 3 Karl Suss RC-8 automated spin coat units. These units are equipped with automated resist dispense, edge bead removal and backside rinse. Thse tools will be used primarily for SU-8 and polyimide coating.

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Soft Bake

A pre-exposure bake, or soft bake, is used to drive the solvent from the resist. This is a critical step in that failure to sufficiently remove the solvent will affect the resist profile, as will excessive baking, which destroys photoactive compound and reduces sensitivity. A typical bake is 1 minute on a 90C vacuum hot plate or 30 minutes in a 90C convection oven. Thick resists may benefit from a longer bake time. Consistency is important once you have characterized your exposure for a particular bake.

 

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Post Exposure Bake

A post-exposure bake, or PEB, is used to reduce standing waves in regular positive resist exposed on the steppers, or to thermally activate chemical processes such as image reversal. It will also affect the resist profile. (See the figure below.) A typical PEB used for OCG 620-7i or Shipley 1813 is 115C on the hotplate for one minute.

 

 

 

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Manual Developing

Development of optical resists takes place in an alkaline solution. Simple solutions of NaOH (Shipley 351), or KOH (Shipley 606) could be used, but because of the possibility of mobile ion contamination in MOS devices, metal ion free developers are often used. These are usually TMAH, tetra-methyl ammonium hydroxide (AZ 300 MIF, Shipley CD-26, MF-321, OCG 945). Some developers also contain surfactants to improve wetting properties.

 

Each developer used has a different dilution, and some require longer development times than others. Developers are generally matched to a type of photoresist. Though they may be interchangeable to some extent, changing the type of developer used in a process will usually change the exposure time necessary to resolve the pattern.

 

Manual developing is typically perfomed in one of the base/solvent hoods in the photolithography area.

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Automated Developing 

 

 

CNF provides three tools for performing automated development of wafers. The Brewer CEE 6000 is equipped with development and baking capabilities. This makes it possible to perform post exposure baking and developing in the same tool. This tool can automatically process up to 25 wafers in a single batch and loads from standard wafer cassettes. Substrates 3" to 200 mm can be processed using the Brewer.

Two Hamatech single wafer spin processors are also available for developing wafers. These tools are manually loaded and do not have baking capabilities. They provide a convenient and highly reproducible way to develop wafers.

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Hard Bakingand Curing

A post-develop bake, or post-bake, is sometimes used to improve a resist's wet and dry etch resistance by hardening it. However, it may make the resist more difficult to remove. In nearly all cases, temperatures above ~ 130C will cause the resist to flow. Consequently, a DUV curing exposure can be performed first to retain the profile. CNF has a JBA DUV Photoresist Curing Station for performing these exposures.

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Image Reversal and Lift-off

The resists used in microlithography today are virtually all positive tone. Some new negative resists have recently become available, but we do not yet have enough experience to discuss them. In order to obtain negative tone from our positive resist, we must resort to image reversal. Since we can make masks in either tone at CNF, there is usually no reason to use image reversal simply to give a negative tone. Instead, we use it specifically to generate an undercut profile for lift-off.

 

Suppose you wished to form a metallized pattern on a wafer. If the metal you wanted to use could be etched, you could evaporate or sputter the metal film onto the wafer first, and then pattern the resist. But suppose the metal can't be etched, or only wet etched, which isn't very precise for small features. The preferred technique is lift-off. In lift-off, the resist is patterned first, then the metal is evaporated over the resist. The resist is then dissolved away in a solvent, carrying the unwanted metal with it. However, the normal positive resist profile presents a problem becuase of it's slope. Metal deposition over this slope creates a continuous film making it difficult for the solvent to dissolve the resist (refer to figure below). If the resist were removed, the edges of the metal film can tear, or the whole pattern could be torn away.

The solution lies in the use of image reversal to create an undercut profile. Instead of exposing the feature where you want the metal, you expose around the feature. This means you use a negative rather than a positive mask. After reversal, the sidewall slope that worked against you in positive tone now forms the undercut profile, which is favorable for lift-off. When the metal is evaporated, the film is discontinuous over the desired features. Now the resist can be removed cleanly, leaving a well-defined metallization pattern behind. A good rule of thumb is to use a resist layer at least three times the thickness of the desired metal thickness.

 

 

 

 

Image reversal may be accomplished at the CNF using the YES Image-reversal oven. In the YES process, the wafers are placed in the oven after exposure, where an ammonia diffusion bake takes place. The ammonia diffuses into the resist, where it binds to the indene carboxylic acid that has been generated in the exposed areas. The exposed areas are now rendered insoluble, while the unexposed areas are not affected by the ammonia. Following the bake, a flood exposure is performed to expose the previously unexposed areas. This is shown schematically below.

    

 

 

A PDF file outlining the image reversal process at CNF can be downloaded here.

 

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Photoresist Stripping

After pattern transfer it is necessary to remove or strip the resist. There are several ways to accomplish this. The simplest is to dissolve the resist away in acetone, Shipley 1165 Remover or AZ 300T. The latter two chemicals are more effective than acetone and leave less of a residue. CNF provides a dedicated wet processing deck for stripping wafers using heated AZ 300T.

Resists can also be removed by oxygen plasma stripping. The plasma creates reactive oxygen radicals that chemically etch the resist polymer. This process can be performed in a variety of tools at the CNF dedicated for this purpose.

Sometimes a combination of soaking in remover and plasma stripping is required to remove stubborn resist (in fact, it is usually recommended to follow a wet strip with a brief plasma strip to remove resist residues). If this does not work, you can resort to Nanostrip or Hot Piranha. These are acid etches specifically designed for the removal of organics. Unfortunately, both of these processes will also etch many metals.

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Library of Standard Photoresist Processes

An overview of standard resist processes available at the CNF is provided here organized by the exposure tool. These tables provide guidelines for soft bake, exposure, post exposure bake and develop conditions for the most commonly used resists at CNF.

Processing information for specific resists is provided below.

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SU-8 Processes

Coming soon

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Polyimide Pocesses

Coming soon

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