Published on Jan 23, 2020
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
where, k1 is the resolution factor, ? is the wavelength of the exposing radiation and NA is the numerical aperture.
As minimum linewidths have shrunk, the exposing wavelength has also periodically shrunk. Table 1 lists the year, minimum linewidth generation and exposure wavelength for state-of-the-art ICs since the mid eighties. At 1.2µm and larger linewidths, the G-line output of mercury lamps (λ = 436nm) was used, at the 0.8µm (800nm) generation the I-line output of mercury lamps (λ = 365nm) was introduced for critical layers and I-line use continued to the 350nm linewidth where early adopters began to use Krypton Fluoride (KrF) Excimer Lasers (λ=248nm) as the exposure source.
KrF use has surprised many observers by persisting through the 130nm linewidth generation. With 90nm linewidths now entering production, KrF is finally running out of stream and Argon Fluoride (ArF) Excimer lasers are being introduced (λ= 193nm). Beyond ArF there are Fluorine Excimer lasers (F2) with λ = 157nm, but there are still a number of technical challenges to overcome. Below the 157nm wavelength, the optical exposure systems must change to all reflecting optics due to high levels of absorption in refractive lens at shorter wavelengths. The introduction of an all reflective lens exposure system introduces a number of technical challenges.
At the same time that exposure wavelengths have been reduced, improvements in lens design has led to improvements in the NA of exposure systems lens, see figure 1. In the mid eighties an NA value of approximately 0.4 was typical, today 248nm exposure systems are available with an NA greater than 0.8. The physical limit to NA for exposure systems using air as a medium between the lens and the wafer is 1, the practical limit is somewhere around 0.9, with recent reports suggesting that an NA as high as 0.93 may be possible for ArF systems in the future
SHORTER WAVE-LENGTH ALTERNATIVES
Few others tricks were tried out by experts. These are called Resolution Enhancement Techniques (RETs). Reducing the wavelength used is not possible as they are absorbed by the focusing lens used. Another approach is to use soft X-rays, also called Extreme ultraviolet (EUV), with a wavelength of 13nm. EUV needs hardly any resolution enhancement. Unfortunately, this approach trades refractive lenses for relatively complex and experimental reflective lenses, and it is years away from commercial use.
But now, out of nowhere, a new technique has emerged that promises to breathe new life into optical lithography and put off until the next decade its replacement by extreme ultraviolet. And it seems to be just in time. In the semiconductor industry, where a good year's sales add up to a quarter-trillion dollars, a three-year reprieve that lets researchers work out the kinks in the still-experimental technology of extreme ultraviolet lithography might just turn out to be the most timely and valuable pause in business history.
The upstart technology is known as immersion lithography. It accomplishes its life-extending wizardry by adding a tiny film of water between the optical system's projection lens and the silicon wafer, allowing lithographic systems to print wires and spaces once thought impossibly narrow.
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