Lithography

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

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Lithography is a method of transferring a predefined pattern to a sample. That pattern is then used to create a shape into another material, or the shape becomes the structure. The patterning of the polymer is achieve through one of two base methods: Directly writing the pattern onto the material, or through a mask/stamp.

Transfer of a pattern to the sample is achieved most often by using a predefined computer-aided design (CAD) to create a pattern on the surface of a sample using a polymer (usually photoresist or beam resist). The pattern can be transferred using light, electron beam, or physical stamping. This transfered pattern or masking layer can then be used to transfer the pattern to another layer using techniques such as etching, electroplating or lift-off.

Technologies

There are several different lithography methods depending on the desired features. The most common is optical lithography, where photoresist is exposed with UV light through a photomask. This method can pattern a wide variety of features but has limited resolution. For nanoscale features, electron beam lithography or nanoimprinting can be used. Stamping (soft lithography) is used to pattern features in polymers such as PDMS.

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. In direct contrast is direct write lithography, where the pattern is drawn on the sample. 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	Direct write	Write method	Minimum feature (nm)	Uses
Optical lithography	Photoresist	Yes	Yes	UV exposure	2000
Electron beam lithography	PMMA	No	Yes	E-beam exposure	10
Stamping	PDMS	Yes		Mechanical
Nanoimprint lithography	?	Yes	No	Mechanical	10	Mold created using e-beam lithography
Inkjet printing	?	No	Yes	Mechanical	30,000
Dip pen nanolithography	?	No	Yes	Mechanical	50

Optical Lithography

Main article: Optical lithography

Optical lithography utilizes light to create patterns on the sample. 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 2 types of optical lighography 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 the 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 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 are able to 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 lithograpy 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.

Mask Making

Main article: Mask making

If the pattern is to be reproduced many times, a mask should be generated. Masks can come in several forms from a printed material on a transparent film, to chrome on glass plates. The different types of masks have advantages and disadvantages depending on the needs for the features.

Direct Write

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 can also be used to generate different height or grayscale features.

Lithography support equipment

Main article: General lithography

Because lithography is the foundation of generating patterns on the substrate, there are many tools in the clean room devoted to achieving better lithography. From high quality masks and clean substrates to uniform film thickness and developing, many factors go into achieving good lithography. Poor lithography techniques have a greater impact on yield of devices than most processing steps. The LNF has a wide range of equipment dedicated to the process of achieving optimal results. Skipping steps in this process will result in inferior devices, lower yields, and repeatability issues.

Applications

Lithography is the basis for most micro and nano fabrication procedures. It allows a wide range of materials to be shaped into 3 dimensional structures that create everything from transistors to optical devices and micro-electro-mechanical systems (MEMS) structures. The ability to transfer a pattern in mass across a large surface to an underlying material is the foundation of all integrated circuits and MEMS. Poor lithography is often the reason for devices not working rather than problems with the tools.

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 mis-alignment into consideration when creating the designs to ensure that the device will still function when there is a mis-registration 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

Main article: Resist

The type of resist that will be used will depend on how the material will be used, type of lithography, and thickness of the resist material required. Resist come in two different categories positive resist, and negative resist. Positive resist is a polymeric material where light or electron beam that is shown on the polymer will break the crosslinks so it is then soluble in a base solution (what is exposed will be developed away). Negative resist is a polymeric material where light or electron beam will crosslink the polymer so it is no longer soluble in a base solution (what is exposed will not be developed). The most common type of resist is positive.

The type of resist you will need to use will depend on what you are using the resist for. Different resists have different resistance to the method you are going to use to transfer the pattern into the underlying layer. This is called selectivity. Ideally you want a high selectivity to the etch you are going to perform, however in some situations high selectivity cannot be achieved. Therefore you need to determine thickness of the resist based on the selectivity.

There are also different resists for optical and beam lithography, as well as if the resist is going to remain as a structural material or will be removed after the pattern has been transferred to the underlying layer.

Materials

Lithography can be done on most types of surfaces. Depending on the material often the resist must be calibrated. For optical lithography the reflectivity of the material will affect the lithography. For e-beam, the conductivity will affect the generation of the pattern, and resolution.

Further reading

• Lithography Handbook
• Lithography Workshop June 12, 2015
• Photolithography Wiki