The Pressure is on

Pressure drop is a concern in semiconductor fabrication plants because it can limit process gas flow. Limited gas flow directly affects throughput and process yields that go right to the bottom line profitability. There are only two ways to overcome pressure drop limitations. One is to increase the upstream pressure; the second is to lower the pressure drop of the system.

Pressure drop has been a factor preventing the application of purifiers in corrosive gas use. Purifiers have repeatedly been shown to reduce gas system component failure and metallic contamination on the wafer. However, previous purifier technologies created a high pressure drop, so filters were used as an alternative. Recently, scientists have developed a low pressure drop media for HCl, HBr, and F₂ that makes gas purification a viable and desirable option.

WHAT IS IT AND HOW DOES IT WORK?

Particulate, moisture, and unwanted molecular contaminants can compromise product quality and impede manufacturing processes. By removing contaminants you increase yields, improve consistency, and lower the process variability. In addition, moisture in corrosive gas can lead directly to gas system component failure.

A gas purifier is different to a filter. Filters only remove solid particulate, while a purifier traps and binds molecular contaminates which may be smaller in size than the gas being purified. Purifiers may be ordered with filters integrated into the device to provide particulate removal in addition to gas purification. Inert gas purifiers utilise inorganic based media with very high surface area which, when activated, provide a media with which chemical reactions may occur. Initial activation at the factory removes all impurities within the media providing a clean, unsaturated surface ready for use. The purification process is based on a chemical process known as catalysis. This process allows several types of reactions to occur on the surface of the pellets to bind the impurities or contaminants being introduced.

MEDIA PROBLEMS

In using a corrosive gas purifier, the media is usually some form of Zeolite. The structure is formed around SiO₄ tetrahedral linked together by sharing of oxygen ions. The silicon can be replaced with an aluminum ion which creates a charge imbalance. The channels in the structure combined with the charge imbalance combine to form regular traps for contaminants. Zeolites have tremendous surface areas and are chosen based on the channel size relationship to the contaminant to be trapped.

Zeolites are normally powders. Powders make for poor filtration medias. They pack tightly generating high pressure drops as gas flows through it. As they wet up through normal use, they can clump up leading to channelling. Channelling is the process where small tunnels form in the media. This leads to premature breakthrough of the media and incomplete gas purification. This is especially troublesome when HCl or other corrosive substances are involved.

To overcome the powder issue, zeolites are commonly made into pellets with a binder. Most pellets are based on clay binders. However, in corrosive gas service, clay binders breakdown. Binder breakdown leads to particle generation and eventual contamination on the wafer. Most of the technologies that were previously used in corrosive gas services were either in a powder form that generated a very large pressure drop or were pelletised with an aluminum substrate. Alumina is known to react with HCl to form AlCl₃, a particulate that can form down stream of the purifier and affect wafer quality. HCl and HBr are commonly used semiconductor corrosive gases. When dry, these gases are relatively inert. However, they readily absorb moisture. The moisture enables the molecules to dissociate. Chlorine and bromine ions rapidly attack stainless steel. This leads to the release of metal contaminants (Fe, Cr, and Ni) from the steel into the process gases and leads to metal contamination on the wafer. This is costly both in equipment failure and product yield.

HBR GAS CABINET EXAMPLE

A major DRAM manufacturer was having frequent regulator failures in their HBr gas cabinets. The gas they purchased was guaranteed to be less than 1 ppm moisture. However, with every cylinder change they had to replace an $800 regulator. The problem was that moisture concentrated on the regulator due to the Joule-Thompsen effect: as a gas flows through an orifice there is a high pressure drop with an associated loss in temperature. The lower temperature leads to condensation of moisture. Concentrated moisture reacts with the corrosive gas and leads to frequent failures in valve seats and most commonly on the gas cabinet regulator seat.

To solve the problem the customer replaced the filter on their gas cabinet with an Aeronex 30K point of use purifier. Instead of replacing a regulator every cylinder change-out, they were able to get eight cylinder changes per purifier.

Using the SEMI model for cost of ownership, a cost of ownership study was conducted to determine the actual savings to the fab. We used 33 wafers per hour and 10 cylinders per year. The cost of the replacement corrosive gas regulator was $800. The time required to change the regulator was twelve hours (two people at six hours each). The expected life was ten years. The model showed a savings of $68,500 (81,209 Euros) per cabinet. This fab had 10 such cabinets leading to a total savings of $685,000 (812,092 Euros). That does not take into account the increased reliability of all other gas system components in the lines and the increased safety through reduced exposure of HBr to service engineers.

RELUCTANCE

Given the strong monetary and safety incentives to use purifiers in corrosive gases, why haven't they become more prevalent? The major reason preventing their application has been performance related. Owing to the high pressure drop associated with previous technologies, to get the same gas pressure at the point of use, the source pressure had to be increased substantially. This led to safety concerns from Environmental Health and Safety Groups and prevented their installation. In addition, higher pressure drop leads to increased gas condensation. Increased condensation leads to Joule-Thompson effect corrosion and an increased likelihood of liquification of the gas in the purifier. Liquification of corrosive gas will allow it to bypass the purifier function, defeating its purpose. Lower HCl pressures are safer and reduce process piping corrosion rates by minimising Joule-Thompson effects.

As having to change out regulators or entire gas cabinet delivery panels frequently is very expensive, in addition to process contamination and gas variability, more stringent quality and performance criteria demanded gas purification. Scientists needed to find a purifier media in a powder free form to avoid high pressure drop, but that would not break down under corrosive gas conditions and contaminate the process.

HOW HAS THIS PROBLEM BEEN RESOLVED?

Recently, engineers have created a new zeolite media. This high surface area zeolite is aluminum depleted and bound with a silica binder. It is measured to reduce the pressure drop by a factor of ten.

The zeolite is loaded with a halide salt to maximum its capacity for the specific halide to be purified: chlorine, fluorine or bromine. By eliminating the tendency for the process gas to channel through the media, the life expectancy was extended by five times, leading to a five fold increase in capacity without changing the package volume.

Changing to a low pressure drop media makes using a purifier instead of a filter a more viable option. However, switching from a filter to a purifier isn't an option in many gas panel applications because the purifier has to conform to such small space requirements that the capacity (volume) is sacrificed.

Due to the large quantity of corrosive gas cabinets in service, to make it possible to switch, a purifier was designed that was retrofitable. Most filters designed into these cabinets were only 7/8" in diameter. To meet the narrow back panel clearance, yet deliver low pressure drop and long life, Aeronex developed a patent pending offset package that has a centre line to base dimension of 10mm (0.375") that replaces all panel mount filters and purifiers. The innovative design allows for a fully swept internal to get maximum capacity out of the limited panel footprint. Now companies have the option of replacing filters with purifiers without having to replace or modify the entire gas panel.

The new corrosive media is available in point of use, area, and bulk applications so it can be used with tube trailers, gas cabinets, and at the process tool. Since lower pressure is needed to maintain the same process flows, facility managers can pipe HCl and other corrosive gases at lower pressures to the process tool.

This new low pressure drop gas purifier removes moisture and minimises the condensation issue. It eliminates regulator failures. In addition to reducing the pressure drop the new media has much larger capacity for moisture removal since channelling through the media is virtually eliminated. Moisture is removed down to less than 100 ppb in corrosive gas.

These new gas purifiers make it possible for wafer manufacturers to improve their process and yield, improve safety, and save money.