Optical lithography

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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 several of the tools, but can be taken by anyone in the lab.

Equipment

HMDS

Photoresist Spinning

Exposure

Stepper
Contact Aligners
Direct Write

Development

Exposure tool selection

Contact Aligner

Contact lithography places the glass mask in direct contact with the sample. Because it is in direct contact with the photoresist, it is subject to picking up particles and photoresist then transferring them to subsequent wafers. The better the contact the more often the mask will need to be cleaned. However it is capable of patterning an entire wafer with a single exposure. 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.

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 AutoStep 200 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

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.

There are a series of steps that are common to all types of optical lithography

  1. Sample preparation
    • Surface cleaning
    • Dehydration
    • Adhesion promoter
  2. Photoresist application and soft bake
  3. Exposure
  4. Post exposure bake (PEB) (necessary with some resists)
  5. Development
  6. Hard bake (necessary with some resists)
  7. Plasma descum

Sample preparation

Prior to photoresist application and exposure, the sample should be cleaned and free of moisture. Additionally, an adhesion promoter such as HMDS may be applied.

Photoresist application

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 photoresists' datasheets.

It is also possible to apply photoresist using a spray-on tool or through 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.

Exposure

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 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: contact and projection

Contact exposure

Contact exposure involves placing the wafer in direct contact or very close proximity (less than 100 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 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 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 μm.

Post exposure bake

Some photoresists recommend or require a post exposure bake. Like 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

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 alkaline solution, although some use solvent based developers. Check the photoresist datasheet to determine the recommended developer.

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

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 1800 series mask by 10–15%. Hard baking resist has been shown to have no effect or can be detrimental for RIE etching.

The hard bake is not recommended for certain photoresists, such as SPR 220. Please consult the photoresist datasheet to determine if it is recommended.

Plasma descum

Main article: Plasma ashing

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 20–30 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 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