The lead section of this article may need to be rewritten.
This article is missing information about photoresist tone.
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.
|Other Names||UV lithography, photolithography|
|Equipment||List of lithography equipment|
- 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.
- ACS 200 cluster tool
- Manual spinners accept pieces up to 6" wafers
- GCA AS200 AutoStep – pieces up to 6" wafers, 5x reduction lens, 0.45 NA
- Contact Aligners
- MA/BA-6 Mask/Bond Aligner – 4" and 6" wafers with backside alignment capability
- MA6 Mask Aligner – 4" and 6" wafers
- MJB3-2 – pieces up to 3" wafers
- MJB 45S – SU-8 and PDMS exposure
- Direct Write
- Heidelberg µPG 501 Mask Maker – mainly for mask making but can also be used on samples up to 5" across
- ACS 200 cluster tool - Automated spray development of 4" and 6" wafers with AZ 300 and MF 319
- The CEE puddle developers may be used on pieces up to 6" wafers.
- Base Bench 63 – Beaker development is useful for developing thick photoresist like KMPR
Selection of type of optical lithography
This section may require copy editing for grammar, style, cohesion, tone, or spelling.
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
This section may be confusing or unclear to readers.
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
- Sample preparation
- Surface cleaning
- Adhesion promoter
- Photoresist application and soft bake
- Post exposure bake (PEB) (necessary with some resists)
- Hard bake (necessary with some resists)
- Plasma descum
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.
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
This section is missing information about why a PEB is used.
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.
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.
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.
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
The minimum feature resolution is generally a direct result of the type of resist, thickness, exposure time and develop time.
Also referred to as alignment accuracy, is critical to ensure 2 patterns are properly registered to each other.
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.
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.
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.