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.
|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 – Can run pieces up to 6" wafers.
- 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
Exposure tool selection
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.
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.
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.
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.
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.
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.
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.
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.