Cost Effective Filtration
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.
AngelTech Live III: Join us on 12 April 2021!
AngelTech Live III will be broadcast on 12 April 2021, 10am BST, rebroadcast on 14 April (10am CTT) and 16 April (10am PST) and will feature online versions of the market-leading physical events: CS International and PIC International PLUS a brand new Silicon Semiconductor International Track!
Thanks to the great diversity of the semiconductor industry, we are always chasing new markets and developing a range of exciting technologies.
2021 is no different. Over the last few months interest in deep-UV LEDs has rocketed, due to its capability to disinfect and sanitise areas and combat Covid-19. We shall consider a roadmap for this device, along with technologies for boosting its output.
We shall also look at microLEDs, a display with many wonderful attributes, identifying processes for handling the mass transfer of tiny emitters that hold the key to commercialisation of this technology.
We shall also discuss electrification of transportation, underpinned by wide bandgap power electronics and supported by blue lasers that are ideal for processing copper.
Additional areas we will cover include the development of GaN ICs, to improve the reach of power electronics; the great strides that have been made with gallium oxide; and a look at new materials, such as cubic GaN and AlScN.
Having attracted 1500 delegates over the last 2 online summits, the 3rd event promises to be even bigger and better – with 3 interactive sessions over 1 day and will once again prove to be a key event across the semiconductor and photonic integrated circuits calendar.
So make sure you sign up today and discover the latest cutting edge developments across the compound semiconductor and integrated photonics value chain.
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