Filtration is an important step in the delivery of high purity liquids for manufacturing. The purity of acids, process chemicals, and other liquids used in semiconductor manufacturing is becoming more critical with the increasing trend towards higher device densities. Specifically, in ultra large scale integration (ULSI) manufacturing, it is crucial to make the surface of silicon wafers as clean as possible in order to realize the decreased device dimensions and increased performance and reliability of currently available technologies. Existing liquid distribution systems (LDS) used to distribute ultra-pure liquid to processing equipment for such cleaning processes are susceptible to a variety of particle contaminants. In particular, incoming liquid can damage critical components. In addition, liquid distribution systems themselves can be a potential source of particle debris.
FIG. 1 is a block diagram that depicts a simple example of a prior art processing system 10. The example processing system 10 includes a liquid distribution system 20 and a target product or process area 30. The liquid distribution system 20 includes a single array of various high precision components such as pressure regulators 22, flow control valves 24, flow meters 26 and check valves 28. It should be understood that typical systems can include several such component arrays, each forming a different liquid distribution channel. The example processing system 10 is shown without any type of filtration system, neither upstream or downstream of the liquid distribution system 20. Such a configuration will pass particles within the liquid onto the target process area 30 and potentially contaminate the components 22, 24, 26, 28 of the LDS 20.
Turning to FIG. 2, prior art systems teach that the ideal position for a filtration system 40 is as near to the final destination of the liquid as possible. This technique, termed "point-of-use" filtration, provides the cleanest possible liquid to the target process area 30. In this example pictured, the target process area 30 is the final destination of the liquid. Essentially, the output of the liquid distribution system 20 feeds the input of a post-filtration system 40, which then passes the liquid to the target 30. The region after the post-filtration system 40, but before the next point in which contaminants may be introduced into the liquid, in this case the target process area 30, is referred to as the filter zone 50. The chance of any impurities being introduced into the filtered liquid, before reaching the target 30, is minimized because the filter zone 50 immediately proceeds the target process area 30.
However, point-of-use filtration used alone, after a LDS 20 with in-line components 22, 24, 26, 28 has a significant drawback. Such a post-filtration system 40 leaves the components 22, 24, 26, 28 of the liquid distribution system 20 exposed and vulnerable to possible permanent damage by incoming contaminated liquid. It is not uncommon that the liquid entering an LDS 20 will contain colloidal silica, pyroxenes, bacteria, pyrogens (bacteria fragments), particulate debris, resin beads, and total organic carbon (TOC) that could permanently impair the precision LDS components 22, 24, 26, 28.
An inadequate prior art solution to the problem of contaminated incoming liquid damaging LDS components 22, 24, 26, 28 is to use a pre-filtration system 60 before the liquid distribution system 20 as shown in FIG. 3, instead of the post-filtration system 40 configuration of FIG. 2. Conventional liquid distribution systems 20 are normally filtered prior to the inlet port as shown in FIG. 3. The distance between the filter and the final output port is dependent on the number of LDS components 22, 24, 26, 28 the length of interconnecting tubing, and the liquid distribution design. In the example configuration pictured, the filter zone 70 extends from the filter to the beginning of the LDS 20.
As indicated above, although a pre-filtration system 60 positioned before an array of sensitive liquid distribution components 22, 24, 26, 28 will capture incoming particles and protect the components 22, 24, 26, 28 particles detrimental to the target process or product 30 can potentially release from the LDS 20 itself. In other words, this solution is inadequate because components 22, 24, 26, 28 that are positioned after the outlet port of the pre-filtration system 60 can shed detrimental particles due to mechanical fatigue, material deterioration, and friction. As a general rule, the greater the distance between the filter and final destination of process liquid, the probability for additional particle contamination increases exponentially.
Such detrimental particles can come from moving parts within the wetted area or inner portions of the liquid passages including valve seats, fittings, springs, o-ring seals, and diaphragm seals of the LDS components 22, 24, 26, 28 that fatigue over time and/or during mechanical operation. The release of particle matter becomes more of a problem after thousands of cycles or hours of operational use. As the system ages with use, the deterioration of the internal cavities of the component bodies, rubber seals, gaskets, and metal fittings comprise the majority of the detrimental particles released by the LDS 20. What is needed is a filtration system that will both protect the LDS components and retain the detrimental particles released by the LDS.
To summarize, the critical disadvantages of the prior art configurations depicted in FIGS. 2 and 3 are twofold. If incoming liquid is plagued with particle debris, components downstream of the liquid flow path can be permanently damaged or contaminated. If a filtration system is positioned upstream of the components, incoming liquid plagued with particle debris would be captured by the filter, however; the components still remain a source of particle generation.
A simple solution would be to use both a pre-filtration system 60 and a post-filtration system 40 simultaneously as depicted in FIG. 4. This configuration provides "before and after" filtration and protection. A pre-filtration system 60 is positioned upstream of a given array of LDS components 22, 24, 26, 28 in front of the input port of the liquid distribution system. A secondary, post-filtration system 40 is positioned downstream of the last component 28 of the LDS 20, connected at the output port of the last component 28 and as close to the target product or process area 30 as possible. The pre-filtration system 60 is used to capture gross particles prior to liquid entering the distribution system 20. The secondary filtration system 40 captures any particles released from the components during cycling (i.e. on-off) use or as a result of aging.
Still referring to the configuration of FIG. 4, the distance between the secondary filtration system 40 and the terminal destination of the output liquid, the target process area 30, is minimized. As indicated above, the position of the post-filtration system 40 relative to the liquid distribution system 20 is critical. Such point-of-use filtration provides the cleanest possible liquid by positioning the filtration system 40 as close as possible to the target product or process area 30.
While such a configuration does provide the benefits of "before and after" filtration and protection, the in-line filtration solution of FIG. 4 also includes a number of drawbacks that have rendered it commercially impractical, particularly where an existing LDS needs to be retrofitted with extra filtration systems. The use of two filtration systems in line with a LDS component array increases the total length of the LDS. In particular, when the LDS includes several distribution channels and components, the total length required increases substantially. In addition, the maintenance requirement of an LDS is significantly increased with the use of both pre-filtration and post-filtration. As with space requirements, the maintenance requirements increase proportionately with the number of liquid distribution channels, routes, and flow directions. In more complex systems, it can become difficult to perform maintenance on the filtration systems because it becomes difficult to access the various different filtration systems located at both ends of each distribution channel.
Additionally inexpensive and currently commercially available filtration systems do not have retention shut-off check valves. This means that residual liquid in the distribution system 20 will likely leak out of the line during filter maintenance. Liquid spills in the equipment chassis of the distribution system can be hazardous to an operator and eventually induce rust, corrosion, and contamination inside the liquid passages of the LDS. Clearly, this problem is aggravated by increasing the number of filtration systems within the LDS 20. Further, the potential for loose connectors and fittings increases as more filters are added to the system. This also increases the chances of the LDS releasing more detrimental particles into the liquid after each filter maintenance cycle.
Thus, for non-trivial liquid distribution systems that include several component arrays, it is impractical to insert in-line filtration systems before and after each component array. What is needed then, is a liquid distribution system in which the benefits of both point-of-use and before-and-after filtration are realized without the increased length and maintenance requirements of inserting a discrete filtration system (in-line with the main liquid channel) both upstream and downstream of each component array. What is further needed is a means for maintaining such a system without allowing liquid spills or an increased chance of contaminating the system. What is also needed is a means to easily add before-and-after filtration to existing liquid distribution systems that are lacking either a pre- or post-filtration system.
It is an object of the present invention to provide a liquid distribution system with the benefits of both point-of-use and before-and-after filtration and without increased space and maintenance requirements.
It is a further object of the present invention to provide a liquid particle reduction filtration system for existing liquid distribution systems.
It is a further object of the present invention to provide a point-of-use filtration system that can be easily adapted for use with existing liquid distribution systems requiring only a minimal degree of alteration of the existing system.
It is a further object of the invention to provide a filtration system that requires no maintenance other than scheduled replacement over a specific period of time.
It is a further object of the present invention to minimize footprint and space requirements when integrating the filtration system of the present invention into existing liquid distribution systems.
It is a further object of the present invention to provide an easy to install before-and-after filtration system for existing liquid distribution systems that improves the ease of maintenance and does not add complications or potential contaminants to the system.