Lithography

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Lithography

Lithography is a method of printing an image by applying patterned layers of color to paper with a series of etched metal or stone plates. This is the process used to print many newspapers and multi-colored lithographs. It is also the general name for the techniques used to fabricate integrated circuits (ICs).

Lithography in printing

The concept of lithography was developed by German actor and playwright Johann Alois Senefelder (1771–1834) between 1796 and 1798, while in Bohemia (what is now the Czech Republic). He used a stone slab with printed grease marks and dampened it with water. When a coating of ink was applied to the stone, it adhered to the grease marks and washed away from the wet areas. The ink was then transferred to paper by pressing the stone against it.

Senefelder’s method was perfected over time. Metal plates were soon used in place of stone slabs. Several chemical solutions that repelled water and adhered to ink better than grease were later used. Lithography was used with several different color inks to create color pictures, called lithographs, which were made famous by Currier and Ives.

Photolithography

The invention of photography in the twentieth century spurred the development of a new lithographic process called photolithography. In this method, the printer shines a bright light through a photo negative onto a thin plate coated with light-sensitive chemicals. The areas of the plate struck by the light harden into a reproduced image, serving the same function as the grease design in early lithography. Today, lithographic processes are the most widely-used printing methods.

Lithography and integrated circuits

The same lithographic concepts used to reprint text and pictures on paper can be used to manufacture integrated circuits. In this case, a polymer resist is used to repel the subsequently applied layers of metal conductors, semiconductor materials, and dielectric insulators, which are the “ink.” An integrated circuit is a tiny version of a conventional electrical circuit. Thin films of various materials act as insulators between conductive material and the silicon metal substrate, or protect existing layers from implantation of other atoms. These devices are built by coating a silicon wafer with patterned layers of material, designed to allow the insulators or protective barriers to be applied, or to leave holes in the barrier layer permitting electrical contact. Sophisticated circuits may require 20 or more layers. Small features and narrow lines must be precisely placed, and the absence of material in a given spot is as critical as the presence of it somewhere else.

There are several ways lithography is used to make integrated circuits, including visible and ultraviolet lithography (forms of photolithography), electron beam patterning, ion beam patterning, and x-ray lithography. The most common method is photolithography, which is well suited to high volume production of consumer electronics.

Making integrated circuits

In the manufacture of integrated circuits, the silicon wafer that acts as the base and a light-sensitive

KEY TERMS

Negative photoresist —A type of photosensitve polymer that leaves a barrier only where exposed to light.

Photolithography —A method of integrated circuit fabrication that uses a light-sensitive polymer to pattern a silicon wafer with other materials.

Photoresist —Photosensitive polymer that is used to pattern silicon wafers during integrated circuit fabrication.

Positive photoresist —A type of photosensitive polymer that leaves a barrier only where not exposed to light.

Reticle —A photomask used to print patterns on silicon wafers, typically made of chrome-patterned transparent glass.

Stepper —A lithographic system that exposes the wafer one small section at a time before ‘stepping’ to the next location.

Substrate —The foundation material on which integrated circuits are built; usually made of silicon.

Wafer —A very thin disk of silicon metal on which integrated circuits are built.

polymer material, called photoresist, are used to create the pattern of the circuit’s layers. Negative photoresists harden when exposed to light, adhering to the base through the developing process. Positive photoresists degrade when they are exposed to light and developed, leaving a depression. The coated wafer is then dried 10 to 30 minutes in an oven at 176 to 194°F (80 to 90°C).

After photoresist is applied to the silicon wafer, it is selectively exposed to light with the aid of a reticle. A reticle consists of a layer of patterned chrome on transparent glass; gaps in the chrome permit light to reach the resist-covered wafer. Exposure takes place in a device, called a stepper, that shines light on the wafer through the transparent regions of the reticle. Only a small region of the wafer is exposed at a time. Then, the wafer is moved, or stepped, forward and a new segment is exposed. After exposure, the wafer is put into developing solution where the positive and unex-posed photoresists are removed. It is hard-baked at temperatures between 248 to 356°F(120 to 180°C).

After the photoresist is in place, a layer of conducting, semiconducting, or insulating metal solution is applied to the wafer and adheres in the pattern opposite to the photoresist. The application and removal of photoresist and metal solutions is repeated 10 to 20 times in the manufacture of a single integrated circuit. In addition to the number of steps needed to make the circuit, the complexity of the task is increased by the necessity for precision in the manufacturing process. For instance, some circuits use printed features as small as 0.35 micron. A human hair, on the other hand, is about 100 microns in diameter.