Immersion Lithography

OPTICAL LITHOGRAPHY

The dramatic increase in performance and cost reduction in the electronics industry are attributable to innovations in the integrated circuit and packaging fabrication processes. ICs are made using Optical Lithography. The speed and performance of the chips, their associated packages, and, hence, the computer systems are dictated by the lithographic minimum printable size. Lithography, which replicates a pattern rapidly from chip to chip, wafer to wafer, or substrate to substrate, also determines the throughput and the cost of electronic systems. From the late 1960s, when integrated circuits had linewidths of 5 µm, to 1997, when minimum linewidths have reached 0.35 µm in 64Mb DRAM circuits, optical lithography has been used ubiquitously for manufacturing. This dominance of optical lithography in production is the result of a worldwide effort to improve optical exposure tools and resists.

A lithographic system includes exposure tool, mask, resist, and all of the processing steps to accomplish pattern transfer from a mask to a resist and then to devices. Light from a source is collected by a set of mirrors and light pipes, called an illuminator, which also shapes the light. Shaping of light gives it a desired spatial coherence and intensity over a set range of angles of incidence as it falls on a mask. The mask is a quartz plate onto which a pattern of chrome has been deposited.

It contains the pattern to be created on the wafer. The light patterns that pass through the mask are reduced by a factor of four by a focusing lens and projected onto the wafer which is made by coating a silicon wafer with a layer of silicon nitride followed by a layer of silicon dioxide and finally a layer of photo-resist. The photo resist that is exposed to the light becomes soluble and is rinsed away, leaving a miniature image of the mask pattern at each chip location.

Regions unprotected by photo resist are etched by gases, removing the silicon dioxide and the silicon nitride and exposing the silicon. Impurities are added to the etched areas, changing the electrical properties of the silicon as needed to form the transistors.

As early as the 1980s, experts were already predicting the demise of optical lithography as the wavelength of the light used to project the circuit image onto the silicon wafer was too large to resolve the ever-shrinking details of each new generation of ICs. Shorter wavelengths are simply absorbed by the quartz lenses that direct the light onto the wafer.

Although lithography system costs (which are typically more than one third the costs of processing a wafer to completion) increase as minimum feature size on a semiconductor chip decreases, optical lithography remains attractive because of its high wafer throughput.

RESOLUTION LIMITS FOR OPTICAL LITHOGRAPHY

The minimum feature that may be printed with an optical lithography system is determined by the
Rayleigh equation:
W=k1?
NA
where, k1 is the resolution factor, ? is the wavelength of the exposing radiation and NA is the numerical aperture.