Specialty Gas Report • Third Quarter 201132 www.specialtygasreport.com

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Maintaining purity

Just 15 years ago, there were virtually no hydrocarbon gases used in the semiconductor industry. Gases were widely used in the manufacture of

semiconductors, but these were reactants such as silane and ammonia, etching gases, dopant gases, gases for various cleaning steps, and inerting agents like nitrogen, argon, and helium.

As technology progresses, other gases have come to be used in semiconductor manufacturing. Today, this includes many hydrocarbon gases such as those produced at Matheson’s Palm Plant in Pennsylvania.

One very important gas used today in semiconductor manufacturing is high purity propylene. Propylene is used in a process that creates what is commonly called an ‘ashable hard mask’. This is an enabling technology for the photolithography process that patterns the lines which become circuits in a chip. This method stretches available photolithography technology to allow the writing of lines below 65 nm with a 193 nm lithography tool.

Semiconductors use incredibly advanced printing techniques that project extremely-fine images onto a photo emulsion on the semiconductor substrate (wafer) surface. After developing the image, this photo pattern is etched into thin-film layers of conducting and insulating films resulting in high-value integrated circuit devices. These intricate images are very small – much finer than the wavelengths of light used to create them.

Propylene is used to create a low reflectivity amorphous carbon layer, known as an anti-reflective coating or ARC (Figure 1). Photoresist, a photo sensitive material applied to the surface of the semiconductor wafer, works in conjunction with the anti-reflective properties of this amorphous carbon layer, to produce a sharp pattern or image of the circuit. This pattern is etched away leaving what will become the circuitry of the chip. Through a clever sequence

Matheson explains why the use of hydrocarbon gases in the semiconductor industry has risen so rapidly in recent years.

Hydrocarbon gases in the semiconductor industry

Third Quarter 2011 • Specialty Gas Report 33

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Figure 1 – Use of an anti-reflective coating (ARC) made from propylene reduces reflected light to less than 0.05 percent. When combined with its improved etch resistance, this enables nanoelectronic engineers to manufacture the most advanced chips using conventional process equipment (Drawing courtesy of Matheson).

of processes, using a combination of several lower resolution images and a resist trimming process utilizing gases such as sulfur dioxide or carbonyl sulfide, along with careful quality control, ashable hard mask technology allows chip circuitry to be engineered smaller and faster than would be possible without hard mask.

Propylene is ideally suited to this task because it is inexpensive, easily applied to wafers in a high-throughput process known as chemical vapor deposition, and because it results in a film with exacting optical properties, with a thickness controlled to five nanometers.

The pace of change is relentless. Fine features are reduced in size (shrunk) by more than 25 percent per year. Nanoelectronic engineers have met this challenge by developing resolution enhancement tools such as anti-reflective coatings and ever-more-sensitive photo emulsions (called photoresist). These tools come with a penalty. These photoresist layers are now so thin that they are insufficient to withstand harsh etch processes. Hydrocarbons have leapt to the rescue.

ProgressionEvery few years we hear that Moore’s law will finally cease to define the rate of technology advancement in semiconductor manufacturing. In most cases, photolithography limitations are said to be the hurdles.

Hard mask is a technology which allows photolithography to leapfrog what should be a physical obstacle to writing smaller and smaller lines. This, in turn, enables smaller and smaller chips which can perform more functions, faster than would be otherwise possible.

By volume, propylene is the hydrocarbon gas most highly used by the semiconductor industry, but it is certainly not the only hydrocarbon being used in semiconductor manufacturing processes. Other high purity semiconductor grade gases include carbon monoxide, carbon dioxide, methane, ethylene, and ethane.

The common characteristics of all of these gases are that they must be very high purity, with low metals. Purity is extremely important in semiconductor manufacturing since even trace amounts of certain elements can have a large effect on semiconductor device yield. A chip failure can be caused by a defect that is smaller than two nm – 300 times smaller than a red blood cell.

The production of the highest quality hydrocarbon gases requires a combination of cryogenic and atmospheric fractional distillation, adsorption, and isomerization. The development and production team at Matheson’s Palm facility considers high purity hydrocarbon production a precious specialty, supported by equally sophisticated laboratory analysis to verify product purities.

Some of the key gas products, and their important applications, include:

Carbon dioxide• – Used in a new cleaning process. Supercritical carbon dioxide is injected onto the surface of the semiconductor wafers to clean them. Supercritical fluids have both gas and liquid properties; supercritical carbon dioxide has key advantages as it can penetrate better than a liquid and clean better than a gas.Ethylene• – Used to replenish the carbon in the dielectric layers of the chip, which may have been damaged during etch processes. Methane• – Used to modulate the etch rate in the etch process step. Methane mixtures are also used to etch compound semiconductors.Carbon monoxide• – Also used to modulate the etch rate in the overall etch process step, improving etch selectivity for greater control of etch profiles.Ethane• – Used in etch modulation as well as other processes. Ethane mixtures can be used to etch compound semiconductors.

The use of hydrocarbon gases by the semiconductor industry has progressed rapidly over the past four years; however, this rapid progress was preceded by many years of research and process development.

Propylene was first used in semiconductor R&D in the early part of the last decade. Serious production usage began in 2007. By mid-2008, propylene had become a major product for the industry and for MATHESON’s Palm Plant. Of the other gases mentioned above, some are still in the R&D phase, while others are used in semiconductor production. In addition to the gases listed in this article, many other hydrocarbon gases are being tested in semiconductor R&D laboratories. Only time will

tell whether these other gases become important parts of the semiconductor manufacturing process in the future.

It is easy to take for granted the technological innovations which the semiconductor industry has brought forth in the scant 64 years since three Bell Labs scientists created the very first semiconductor device. The latest technology enhancements we take for granted today have, to a very real extent, been made possible through the gritty research, development, and production technology coming out of the specialty gas business at places such as Matheson’s Palm Plant.

Brooks Hurd is general manager for Matheson’s Palm Plant and draws on over 36 years of gas industry experience. Established in 1993, Matheson’s Palm Plant (formerly Advanced Gas Technologies) has become one of the most comprehensive

suppliers of high purity hydrocarbons in the world.

Brooks Hurd

By volume, propylene is the hydrocarbon gas most highly used by the semiconductor industry

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SGR