Lithography

Lithography
Technology Details
Other Names	photolithography, patterning, pattern transfer, litho
Equipment	List of Lithography equipment

Lithography is a method of transferring a two-dimensional pattern to a flat substrate. The patterning is achieved through one of two base methods: directly writing the pattern, or transferring the pattern through a mask/stamp. The defined pattern can help to define features on the substrate (e.g. etching) or the features can be formed by the deposited pattern.

The pattern is defined via a computer-aided design (CAD). Most often, the features are formed with the use of a resist, and they can be defined using light (with a photoresist), electron beam (with an e-beam resist), or via physical stamping (no resist needed). The features can be transferred to another layer using techniques such as etching, electroplating or lift-off.

Training modules

This online course covers the fundamentals of optical lithography at the LNF. It is also required for checkout on most lithography tools, see the tools checkout procedure for more details.

Technologies

There are several different lithography methods depending on the desired features. The most common types are optical lithograph and e-beam lithography. The LNF also offers Soft and Direct Write Lithography.

The following table compares some of the common lithography methods available at the LNF. Batch processing refers to the ability to pattern an entire sample at once, such as through a photomask or with a stamp. The write method describes how the material is patterned, such as by UV light, electron beam, or direct mechanical contact.

Lithography type	Materials	Batch processing	Write method	Minimum feature (nm)	Uses
Optical lithography	Photoresist	Yes	UV exposure	2,000
Electron beam lithography	PMMA	No	E-beam exposure	10
Soft Lithography	PDMS	Yes	Mechanical	2,000
Direct Write Lithography	Photoresist/PMMA	No	Mechanical	30,000

Optical Lithography

Main article: Optical lithography

In optical lithography, photoresist is exposed with UV light through a photomask. This method can pattern a wide variety of features, but has limited resolution. To achieve higher resolution shorter wavelength light (G-line 435.8nm, H-line 404.7nm, I-line 265.4nm) is utilized. At the LNF we have two types of optical lithography systems:

• Contact Lithography: With the systems available at the LNF, the minimum feature is approximately 2µm with a minimum alignment tolerance of approximately 1µm.
• Projection Lithography: With our projection lithography system (5X optical stepper) we can achieve a 0.5µm minimum feature, with a 0.2µm alignment tolerance.

At the LNF both contact and projection lithography utilize masks to define the pattern on the sample.

Electron beam Lithography

Main article: Electron beam lithography

Instead of using a light source, such as in optical lithography, electron beam (e-beam) lithography utilizes an electron beam to generate the patterns on the sample. Because of the much shorter wavelength, we can achieve much higher resolution features; however, because it is a single electron beam writing the sample, it takes longer to generate the pattern on the sample. Using standard resists, the e-beam tool at the LNF can achieve a 7 nm minimum feature with a 1 nm alignment tolerance. This is a direct write technique.

Soft Lithography

Main article: Soft lithography

This uses a pre-generated mold as the base to create a 3-dimensional structure. A soft material such as polydimethylsiloxane (PDMS) is poured onto the mold and cured. As it is peeled from the surface it maintains a negative of the mold. The PDMS material is often attached to another layer such as glass or another layer of PDMS. Soft lithography is often associated with larger feature devices. Microfluidic systems that have features in the range of 20 to 5000 µm are often produced using soft lithography. Additionally, users of the LNF use this technique to produce nanostructures, through a technique called nanoimprint lithography.

Direct Write Lithography

Main article: Direct writing

If a pattern is only going to be used once, it may be more economical to write directly onto the substrate than to generate a mask which is used to create the pattern on the sample. Direct write is used to create masks for photo-lithography andcan also be used to generate different height or greyscale features. In the LNF we can do direct lithography writing with two different equipmentː with our Heidelberg Mask Maker (photolithography) or with our JEOL E-Beam system (e-beam lithography).

Applications

Lithography is used to pattern a sample before a process step that a user does not want to affect their whole sample, primarily deposition, or etching. Before etching lithography is used to create a protective layer of resist that will only leave material where there is resist (negative pattern). Before deposition lithography is used to perform lift-off, where after deposition the resist is lifted off, only leaving material where there was no resist (positive pattern).

A typical process:

1. Start with a clean substrate and mask. If there are particles on the mask or substrate these can cause non-uniform resist coverage, causing errors on many devices.
2. Dehydration bake the sample. This removes any moisture from the surface and will improve the adhesion to the surface. Often when there is still moisture on the surface the resist will bubble during baking.
3. Spin the resist (often after an adhesion promoter). This needs to coat the surface uniformly otherwise exposure will be inconsistent.
4. Soft bake the resist, this drives off the solvents from the resist. Too much softbake will reduce the sensitivity of the resist.
5. Expose the resist (see previous sections)
6. In some resists a post exposure bake (PEB) is required. This will distribute the acid within the resist that breaks the bonds. It results in straighter sidewall profiles in the resist.
7. Develop the resist. The type of developer is dependent on the resist, and the substrate. Automated developing systems will give better reproducibility than manual methods.
8. Some resist require a hard bake, while others do not. Make sure you are following the recommended practices for the resist you are using.

Figures of Merit

Feature size

This is typically referred to as the minimum feature size or critical dimension (CD), which is the smallest part of the design. The achievable CD is dependent on the type of lithography you are using and the topology of the surface you are patterning on.

Alignment

Alignment refers to the registration of 2 layers to each other and is very important in many designs. Good designs take misalignment into consideration when creating the designs to ensure that the device will still function when there is a misregistration of the 2 layers.

Pattern reproduction

Pattern reproduction refers to how many times this pattern will be reproduced. Is this a one time used pattern? If so, you may be able to directly write the pattern on the surface. However, if it needs to be reproduced thousands of times, direct write is inefficient and having a mask or a mold that is used to produce the pattern is much more efficient.

Resist Film Thickness

Film Thickness refers to how thick your layer of resist is. Typically this can be measured on the NanoSpec 6100, and is given in microns or Angstroms. The thickness of you resist layer is dependent on how fast you spin on the resist.

Selectivity

Selectivity refers to the comparison of reaction speed for different materials. If you are following lithography with an etch process, you will need to consider selectivity to determine your film thickness. If your resist has a similar etch rate to your substrate, then your process has low selectivity and you will need a larger film thickness.

Selectivity is also used to discuss development. If your exposed resist develops at a much higher rate than your unexposed resist, then you have high selectivity. The the selectivity of your developer can help you determine your development time.

Materials

Resist

Main article: Photoresist

Resist is a polymer suspended in a solvent. Depending on the type of resist, it can be selectively removed using UV light or an electron beam. All resist can be broadly categorized as positive or negative resist, with positive being the most common. In a positive resist, the area exposed is removed after developing the resist. In a negative resist, the area exposed is remains after developing the resist.

Different types of lithography use different types of resist, so you will need to check what resist is appropriate for your process.

Developer

Developer is a base used to etch away photoresist after exposure. There are several kinds of developer available at the LNF: AZ 726, AZ 300, AZ 400K, MF 319, and Microposit Developer. Developers can be used at Base Bench 63, Base Bench 91, the ACS 200, and at either CEE Developer. Check each tool for what developers are allowed or available.

Mask

Main article: Mask_making

Masks are used to block light during exposure, so only photoresist within your desired pattern is exposed. This is useful if you need the same pattern across several samples. Masks are a glass or fused silica substrate with a layer of Chromium and a layer of photoresist. The photoresist is exposed in your desired .pattern, then developed. Afterwards, a Chromium etch is used to set the pattern in the mask.

Further reading

• LNF Tech Talk for Lithography
• Lithography Handbook
• Lithography Workshop June 12, 2015
• Photolithography Wiki

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