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Cost Effective Filtration

Adoption of Point-of-Use filtration within lithography exposure tools solves severe contamination problems with efficient and cost-effective means, avoiding tool downtime required for optical surface cleaning. Given the variations in environmental conditions, specific combinations of media may be required for each application. Reticle contamination requires a lot of attention where breather membrane solutions have been investigated. Paul Vannerem of Donaldson reports.
Adoption of Point-of-Use filtration within lithography exposure tools solves severe contamination problems with efficient and cost-effective means, avoiding tool downtime required for optical surface cleaning. Given the variations in environmental conditions, specific combinations of media may be required for each application. Reticle contamination requires a lot of attention where breather membrane solutions have been investigated. Paul Vannerem of Donaldson reports.

The negative affect on yield rates and tool up-time of acidic, basic and organic molecules in the gas phase is getting increasing attention within the semiconductor industry. While in deep ultra violet (DUV) lithography, the filtration of resist-poisoning amines has been carried out for a number of years, newer concerns include the degradation of optics, chemical contamination of reticles and changes in resist sensitivity. This all contributes to the evolution of air and gas purity requirements within exposure tools. Simultaneously in other areas, such as mask making, the use of chemical filtration has recently also become a necessity, due to the implementation of chemically amplified resists (CAR) and DUV wavelengths.

Using a controlled environment at the "Point-of-Use", the OEM or toolowner can be more specific in the type of environment required locally. This offers benefits in terms of tool uptime and yield rates, and also, by limiting the size of this environment, the cost of operation can be substantially reduced.

The advantages of Point-of-Use filtration are indeed numerous :

* The supply of high purity air or gas is restricted strictly to the targeted critical area

* Adapted solutions can be developed for existing designs, making it an ideal tool for retrofit situations

* Cost of Ownership is significantly reduced since the flow with the tightest specifications is reduced to a minimum

* Maintenance is spaced out, since a reduced flow also means an increased filter lifetime

Point-of-Use solutions can be developed at different scales: for process air inside lithography tools, typical airflows will be 300-3000m3/hour, for compressed air or N2, around 100litres/min. For breather applications with a limited flow in function of temperature and pressure fluctuations, passive-adsorptive filters can be used.

For both economic and technical reasons, one of the most recent and important issues on which the industry has concentrated for many years with Point-of-Use filtration is the protection of lens and other optical elements in lithography exposure tools. From a technical point of view, increased outputs as well as the higher energy of shorter wavelengths have resulted in increased pollution of optical surfaces. From an economic point of view, the driving forces have been reduction of tool downtime required for cleaning optical parts and improvement of yield rates. The spectrum of gas phase contaminants to be controlled has increased, while acceptable levels for species such as SOx, NOx, Si-based components, DiOctylPhtallate, Cl-, ... are now well below 1µg/m3.
In addition to the lens, the wafer and reticle stages, as well as the illuminator system, are new examples of zones that can benefit from Point-of-Use filter protection.

In each of these specific zones, the environmental conditions or physico-chemical characteristics of the air or gas stream can be very different. Filter efficiency will be highly dependent on those characteristics, which will dictate the choice of filtration media and the filters overall design. As an example, at the wafer level, the chemical filter must remove gas phase basic, acidic and organic contaminants in the presence of moisture (typically 50% relative humidity). However, at the reticle stage and within the illuminator, gas phase contamination control has to be achieved in an environment (compressed air or compressed nitrogen) that has extremely low levels of humidity (<-70¼C dew point). A comparative study of the behaviour of chemical filtration media in varying conditions of temperature, pressure and relative humidity is essential in order to select the media best suited to each individual application. In particular, relative humidity has been shown to have a large impact on filtration performance.

In high relative humidity conditions, physical adsorption of activated carbons is often reduced, the adsorption of water having a negative impact on its capacity for organics. This must be compensated for by careful selection of carbon material or modifications to the carbon surface.

For the removal of basic and acidic species, on the contrary, high relative humidity levels enhance the chemisorption filtration mechanism of chemically impregnated carbon while at extremely low levels of water content (dew point <-70¡C), removal efficiency is reduced, making other media such as ion exchange better suited.

For each specific application, it becomes essential to select or combine different media in order to cover effectively the range of contaminants of concern.
Clean dry air used for air bearings in critical process areas, nitrogen or clean air used for purging of optical elements in lithography tools are examples of Point-of-Use filtration that help address todays technologys challenges. Field tests have shown contamination levels of basic, acidic and organic contaminants downstream of these filters to be below detection limits of 0.1ppb.

Another area of interest is reticle protection similarly to disk drive and sensor applications, where breathers are used for protection against particulate contamination, moisture or acid gas contaminants, current pellicle frames protecting the reticles are fitted with breather membranes. So far, these membranes are for particulate filtration only, while chemical contamination issues of reticles, sometimes causing killer defects, are increasingly observed with the move to 193nm lithography and to 300mm wafers.

A possible solution is to design a breather membrane ensuring chemically clean as well as particulate-free air in the semi-closed environment between reticle and pellicle frame. Solutions similar to those developed for the disk drive industry may be of great help.

At the cleanroom level, the move from Class 1 to SMIF environments combined with tighter air purity requirements also brings chemical contamination challenges that will have to be tackled with Point-of-Use solutions.

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