LITHOGRAPHY METHODS IN FABRICATION

lithography

What is Lithography?

Lithography(Greek word) means printing is done on stone. It was invented in 1796 by German author and actor Alois Senefelder as a cheap method of publishing theatrical works. Lithography can be used to print text or art work onto paper or other suitable material.

Lithography originally used an image drawn with oil, fat, or wax onto the surface of a smooth, level lithographic limestone plate. The stone was treated with a mixture of acid and gum Arabic , etching the portions of the stone that were not protected by the grease-based image. When the stone was subsequently moistened, these etched areas retained water; an oil based ink could then be applied and would be repelled by the water, sticking only to the original drawing. The ink would finally be transferred to a blank paper sheet, producing a printed page. This traditional technique is still used in some fine art printmaking applications.

In modern lithography, the image is made of a polymer coating applied to a flexible plastic or metal plate. The image can be printed directly from the plate (the orientation of the image is reversed), or it can be offset, by transferring the image onto a flexible sheet (rubber) for printing and publication.

As a printing technology, lithography is different from intaglio printing(gravure), wherein a plate is either engraved, etched, or stippled to score cavities to contain the printing ink; and woodblock painting or letterpress printing, wherein ink is applied to the raised surfaces of letters or images. Today, most types of high-volume books and magazines, especially when illustrated in colour, are printed with offset lithography, which has become the most common form of printing technology since the 1960s.

The related term “photolithography” refers to the use of photographic images in lithographic printing, whether these images are printed directly from a stone or from a metal plate, as in offset printing. “Photolithography” is used synonymously with “offset printing”. The technique as well as the term were introduced in Europe in the 1850s. Beginning in the 1960s, photolithography has played an important role in the fabrication and mass production of integrated circuits in the microelectronics industry.

Photolithography

Photolithography is the process of transferring geometric shapes on a mask to the surface of a silicon wafer.

Photolithography Steps:

1. Wafer Cleaning: In the first step, the wafers are chemically cleaned to remove organic, ionic, and metallic impurities.
2. Barrier Layer Formation: After cleaning, silicon dioxide, which serves as a barrier layer, is deposited on the surface of the wafer.
3. Photoresist Application: Photoresist is applied to the surface of the wafer by high-speed centrifugal spinning. This technique, known as “Spin Coating,” produces a thin uniform layer of photoresist on the wafer surface. In this process a liquid solution of photoresist is give out from the wafer by rapid spin and produce uniform thin layer (0.5µm to 2.5µm). Spin coating/ spinner typically runs at 1200rpm to 4800rpm for 30sec to 60sec. Chemicals commonly use as photoresist are;  Poly methyl methacrylate (PMMA)  Poly methyl glutarimide (PMGI)  Phenol formaldehyde resin (DNQ/Novolac)

a) Positive Photoresist: Positive photoresists is exposed to UV light, the underlying material is to be removed. In these resists, exposure to the UV light changes the chemical structure of the resist so that it becomes more soluble in the developer. The exposed resist is then washed away by the developer solution. In other words, “whatever shows, goes.”

b) Negative Photoresist: Negative photoresists behave in just the opposite manner. Exposure to the UV light causes the negative resist to become polymerized, and more difficult to dissolve. Therefore, the negative resist remains on the surface wherever it is exposed, and the developer solution removes only the unexposed portions.

1. Prebaking: Prebaking is the step during which almost all of the solvents are removed from the photoresist. The photoresist become photosensitive after prebaking. Photoresist is prebake at 90C o to 100C o for 5min to 30min.
2. Mask Alignment and Exposure: A mask or “photo mask” is a square glass plate with a patterned combination of metal film on one side pattern transferred onto the wafer surface. There are three primary exposure methods: contact, proximity, and projection. They are shown in the figure below.  Contact Printing: In contact printing wafer is brought into physical contact with photo mask. Because of the contact between the resist and mask, very high resolution is possible. The problem with contact printing is that fragments trapped between the resist and the mask, can damage the mask and cause defects in the pattern.  Proximity Printing: The proximity exposure method is similar to contact printing except that a small gap, 10 to 25 microns wide is maintained between the wafer and the mask. This gap minimizes (but may not eliminate) mask damage. Approximately 2 to 4 micron resolution is possible with proximity printing.  Projection Printing: Projection printing avoids mask damage entirely. An image of the patterns on the mask is projected onto the wafer, which is many centimetres away. To achieve high resolution, only a small portion of the mask is imaged it has about 1-micron resolution.
3. Development: Development is a process in which exposed/non-exposed area is dissolved by developer. Most commonly used developer is tetra methyl ammonium hydroxide is used in concentrations of 0.2–0.26. Developer is important in controlling the development uniformity. Therefore two methods are mainly used i.e. spin development and spray development. During spin development wafers are spun and developer is poured onto the rotating wafer. In spray development, the developer is sprayed rather than poured, on the wafer by using a nozzle that produces a fine spray over the wafer.
4. Hard-Baking: The hard bake is used to harden the final resist image at the temperature (120°C — 150°C), so that it will hold out the harsh environments of etching.
5. Etching: Etching is performed either using wet chemicals such as acids, or more commonly in a dry etching (by exposing the material to a bombardment of ions) . The photoresist will “resists” the etching and protects the material covered by the resist. When the etching is complete, the resist is stripped leaving the desired pattern.
6. Stripping: After the imaged wafer has been etched the remaining photoresist must be removed. There are two classes of stripping techniques; wet stripping and dry stripping. A simple example of stripper is acetone. Acetone tends to leave residues on the wafer. Most commercial organic strippers are phenol-based.

Applications of Photolithography

Main application: IC designing process 

Other applications: Printed electronic board, nameplate and printer plate.

Electron Beam Lithography 

Electron Beam Lithography is a specialized technique for creating extremely fine patterns. It is derived from the scanning electron microscope. Electron beams can be focused to a few nanometres in diameter.

The basic idea behind electron beam lithography is identical to optical lithography. The substrate is coated with a thin layer of resist, which is chemically changed under exposure to the electron beam, so that the exposed/non-exposed areas can be dissolved in a specific solvent.

Electron beam lithography is the most power full tool for the fabrication of feathers as small as 3nm to 5 nm.

The EBL system is normally referred to as the column. An EBL column (Fig. 4) typically consists of following components;

Electron source: Electrons may be emitted from a conducting material either by heating or by applying an electric field.

Stigmators: A stigmator is a special type of lens used for the alignment of e-beam. Stigmators may be either electrostatic or magnetic and consist of four or more poles.

Electron Lenses: Electron lenses can be made only to converge, not diverge. Electrons can be focused either by electrostatic forces or magnetic forces. 

Apertures: Apertures are small holes through which the beam passes on its way down the column. There are several types of apertures. A blanking aperture is used to turn the beam on and off. A beam limiting aperture has two effects: it sets the beam convergence angle through which electrons can pass through the system, controlling the effect of lens aberrations and thus resolution. 

Blanking Plates: Blanking plates are use to modify the e-beam, these are simple electrostatic deflector. One or both of the plates are connected to a amplifier with a fast response time. 

Advantages of EBL

1. Print complex patterns directly on wafers 
2. Eliminates the diffraction problem 
3. High resolution up to 20 nm(photolithography ~50nm) 
4. Flexible technique

Disadvantages of EBL

1. Slower than optical lithography (approximately 5 wafers / hour at less than 0.1 µ resolution). 
2. Expensive and complicated 
3. Forward scattering 
4. Backward scattering 
5. Secondary electrons

Applications of EBL

Electron beam Lithography (EBL) is used primarily for two purposes 

1. Very high resolution lithography. 
2. Fabrication of masks.

Nanoimprint lithography

(NIL) is a method of fabricating nanometer scale patterns. It is a simple nanolithography process with low cost, high throughput and high resolution. It creates patterns by mechanical deformation of imprint resist and subsequent processes.

X Ray Lithography

X-ray lithography, is a process used in electronic industry to selectively remove parts of a thin film. It uses X-rays to transfer a geometric pattern from a mask to a light-sensitive chemical photoresist, or simply “resist,” on the substrate.

• Irradiation
• Development
• Electroforming
• Mould insert
• Mould filling
• Mould releases

Advantages of X-Ray Lithography

•Short wavelength from X-rays 0.4–4 nm

•No lens

Disadvantages of X-Ray Lithography

•Distortion in absorber

•Cannot be focused through lens

•Masks are expensive to produce

Extreme ultraviolet lithography

A lithography system essentially is a projection system. An EUV system uses a high-energy laser that fires on a microscopic droplet of molten tin and turns it into plasma, emitting EUV light, which then is focused into a beam.

Thank You!

Authors: Anuradha Lohar, Namrata Magdum, Vivek Magre, Ashutosh Jha.