Optical lithography is a process used to create integrated circuits on silicon wafers. It involves coating the wafer with light-sensitive resist, exposing it to focused beams of visible or UV light through photomasks, and etching the surface. The resist can be positive or negative, and photomasks can be used in contact printing, proximity printing, or projection printing. The final steps involve washing the wafer with developer solution and etching the exposed areas.
Optical lithography is a chemical process usually used in the production of computer chips. Flat wafers, often made from silicon, are etched with patterns to create integrated circuits. Typically, this process involves coating the wafers in a chemical resistant material. The resist is then removed to reveal the circuit diagram and the surface is etched. The way to remove the resist involves exposing the light sensitive resist to visible or ultraviolet (UV) light, hence the term optical lithography.
The main factor in optical lithography is light. Just like photography, this process involves exposing light-sensitive chemicals to beams of light to create a patterned surface. Unlike photography, however, lithography usually uses focused beams of visible – or more commonly, UV – light to create a pattern on a silicon wafer.
The first step in optical lithography is to coat the surface of the wafer with chemical resist material. This viscous liquid creates a light-sensitive film on the wafer. There are two types of resist, positive and negative. Positive resist dissolves in the developer solution in all areas where it is exposed to light, while negative resist dissolves in areas that have been shielded from light. Negative resist is more commonly used in this process, because it is less likely to distort in the developer’s solution than positive.
The second step in optical lithography is to expose the resist to light. The goal of the process is to create a pattern on the wafer so that the light is not emitted uniformly over the entire wafer. Photomasks, often made of glass, are typically used to block light in areas developers don’t want to be exposed. Lenses are also typically used to focus light on particular areas of the mask.
There are three ways photomasks are used in optical lithography. First, they can be pressed against the wafer to directly block light. This is called contact printing. Defects on the mask or wafer can allow light to penetrate the surface of the resist, thereby interfering with pattern resolution.
Second, the masks can be held close to the wafer, but not touching it. This process, called proximity printing, reduces interference from defects in the mask and also allows the mask to avoid some of the additional wear associated with contact printing. This technique can produce a slight diffraction between the mask and the wafer, which can also reduce the accuracy of the pattern.
The third, and most commonly used, technique for optical lithography is called projection printing. This process sets the mask further away from the wafer, but uses lenses between the two to direct light and reduce scattering. Projection printing typically creates the highest resolution pattern.
Optical lithography involves two final steps after the chemical resist has been exposed to light. Wafers are typically washed with developer solution to remove positive or negative resist material. Then, the wafer is typically etched into any areas where the resist no longer covers. In other words, the material ‘resists’ being engraved. This leaves parts of the wafer etched and others smooth.
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