Improving Yield through Upgraded Pneumatic Manifolds

Submitted by Sri Gavini || Valin Corporation
It is on the top of every decision maker’s mind in the semiconductor industry: yield. Operators are continually doing what they can to improve their throughput and maximize yield in terms of wafer production, all with keeping an eye on increasing profitability. Although profitability is affected by various other factors such as equipment effectiveness, labor costs, and material consumption, improving the operation’s yield is at the top of the list. Specifically, in the semiconductor industry, yield refers to the ratio of all the wafers that come out of the fab to the total number of wafers that were started in the fab.

There are many different components associated with the ultimate yield of a semi fab such as wafer fabrication yield, wafer sort yield, and packaging yield. The competition in the industry continues to intensify and operators are on a seemingly never-ending quest to cut costs and improve processes that result in a greater yield. A large part of this effort is put into ensuring that the equipment used in each station is as effective as possible, minimizing downtime and leading to the greatest yield possible. In most of the semi-fabs, the legacy CVD, PVD, and ETCH tools that are deployed 20+ years ago need to be constantly monitored and improved as these are no longer OEM supported. The costs driven by OEM to service these legacy tools are huge and thus push Semifabs to buy new equipment. The new equipment purchase is not always a viable solution because it would drive up the overall capital costs and fabs always try to reduce this. The only option left behind for the fabs is to keep these legacy tools running by maintaining them properly and finding the replacement parts to get the maximum throughput. Many of these tools have been in service for 20+ years and it becomes extremely difficult to find the spare parts. Either the spare parts are obsolete or the OEMs charge a marginally higher price.  The fabs are forced to source these spares from untrusted sources such as eBay and they are sometimes at the complete mercy of the manufacturer and OEM. 

Endura Manifold
Among various components in these tools, pneumatic manifolds play a vital role in supplying the required Compressed Dry Air (CDA) for various process valves, slit valve doors, gas boxes, chambers, and various other processes and applications. These pneumatic manifolds are also subject to obsolescence, older technology, frequent coil failures, and minimum support from either the manufacturer for the spares or the OEM that manufactured the tools. Pneumatic manifold failures can cause CDA leaks and raise the energy costs and CO2 footprint of the Semifabs. The best solution for the problem is to replace these legacy manifolds once and for all with new manifolds that could solve the CDA leaks, frequent coil failure problem, and increase the overall valve manifold life cycles significantly. In addition, reduced downtime for maintenance to swap the failed coils will be noticed and altogether the overall yield of a specific tool can be improved.

Not only can the issues that arise be costly and frequent, but the challenges inherent in performing regular maintenance on the equipment are daunting.  Furthermore, the stakes are particularly high when discussing the Semifab industry. If processes are not performing at an optimal level, the wafers produced can ultimately turn out to be defective, requiring them to be discarded. This is something that no Semifab wants to have happened at all, let alone regularly.  Avoiding wafer scrap is always a top priority for Semifabs, as each wafer goes through a series of processes over the course of several weeks to obtain a final product.  So, consequently, the stakes are very high to have equipment in tip-top shape.

Pneumatic manifolds have several components that need to remain in operating order for them to be as effective and efficient as possible.  One such component is the valve, more specifically in fact, a series of valves across the platforms of the manifold. These valves are known to leak compressed dry air (CDA) on a fairly regular basis, and these leaks are not something that should be left unaddressed.  As mentioned above, this problem can commonly lead to costly wafer damage, leading to discarded wafers.  As most operators are painfully aware, every discarded wafer represents a cost that cannot be recovered. 

Not only do leaks of CDA across all platforms of a pneumatic manifold happen all too often, but they are also an enormous challenge to address. To access the hard-to-reach manifolds, personnel typically need to squeeze themselves into a very confined place. Furthermore, to replace a single valve that is leaking CDA, all other valves need to be shut off. By doing this, maintenance personnel is protecting against the possibility of causing additional leaks.

Another common challenge associated with continually changing out these valves is the possibility that the manufacturer has discontinued the valve line being used. It’s not all that uncommon to find situations where the valves are more than 20 years old, forcing operators to either hunt down valves from a third-party source or rebuild the semiconductor equipment completely.
If this type of reactive maintenance needs to take place once, twice, or even three times a month, it can be extremely tedious and laborious. Semifabs are sometimes known to put off checking and changing out these leaky valves simply due to the challenge of the exercise.  This approach of not addressing the problem, however, can lead to that “avoid at all costs” issue of wafer scrap.

Solutions have Presented Themselves

Operators and maintenance personnel would be pleased to find out that there are solutions available to solve the issue of leaking CDA across their pneumatic manifolds. Upgrade kits are now on the market that works as provide a drop-in, plug-and-play solution both electronically and mechanically.

The kits allow the operator to isolate a valve to repair it. This eliminates the issue of needing to shut down the manifold air supply, which otherwise can cause process chamber conditions to change. This feature eliminates the possibility of harming the wafers, rendering them defective and requiring them to be discarded.

The fit, form, and function of the manifolds remain the same so there is no fabrication needed.  These manifolds come with easy-to-follow, plug-and-play installation instructions that take away all the frustration customers can anticipate during an upgrade.

These upgrades ultimately reduce tool downtime and eliminate CDA leaks.  The upgrades also improve the performance with the larger airflow coefficient or Cv up to 0.3. The higher Cv  will enable quicker actuation of the process valves, slit valve doors, and other air-aided applications. One of the applications in which the new retrofit manifolds solved a critical problem at the fab, the air being carrier gas for various expensive gases such as Argon, Helium, and Hydrogen to various processes. The lesser the airflow the higher the volume of high purity gases that need to be pumped to point of the use. This also causes disturbance in the gas mixture and the chemistry associated changes and could affect the wafer. As the carrier air volume is reduced at the pneumatic manifold due to the CDA leaks the customer ends up consuming higher than optimum amounts of high purity gas. The customer replaced the legacy manifolds with the retrofit manifolds and was able to stop the CDA leak and enable the required flow of air and bring the gas mixture to the right proportion.

There are a few other areas of concern that upgrading a manifold can ultimately address. Fab tool engineers are becoming more and more concerned with flow and response times. To address this challenge, tools are continually being tweaked to improve their throughput. However, as these outdated tools are retrofitted with newer technology parts, timing and flow become issues that need to be addressed. Without a viable solution, the tool operation can become too bogged down.  For example, mass flow controllers (MFC’s) on the tools now run much larger flows for the gas boxes, where they ultimately control the gases being released. Older manifolds must constrain flow to keep up with the downstream valve opening.  This causes delayed reaction times.

Many of the older systems were designed with outdated, slow analog flow controllers where valve actuation timing was not a serious problem. However, with the issue of flow bursting, valve timing is most definitely a concern.  In most cases, three or more valves need to be open when the MFC is ready to flow or the MFC will drive full open to attempt a given setpoint.  If any of the valves have not opened quickly enough, a flow burst of the process gas will occur once the delayed valve opens.  When these bursts occur, they can generate particles that may ultimately affect the outcome of the process. This, again, can lead to a considerable amount of scrap.  

There is a similar challenge when it concerns venting air.  There are risks where some incompatible gases need to be isolated from one another.  If the AV valves close to slowly, the dangerous gases can cross paths and react with the process gas line.  This causes dusting with the severity correlating directly with the amount of delay.  These issues both build over time and become so troublesome that a major disassembly and cleaning of the gas panel is required.

By implementing an upgraded manifold, all of these issues can be solved. With an upgraded manifold comes valves that have a larger flow.  This larger flow equates to improved response time. 

These are some of the more noticeable benefits inherent to upgrading a pneumatic manifold with the kits available on the market today:
  • Proven in the field with Fab testing
  • Quick, easy installation (less than 45 minutes)
  • Fit, form, & function remains the same
  • 150 million valve cycle life expectancy
  • The power consumption of each coil is 0.8w and a switching time of 18 ms
  • Higher Cv (new valve .46 Cv compared to old valve .21 Cv)
  • Complete drop-in replacement kit
  • No fabrication or other items needed
  • Ease of valve replacement (1 self-contained set screw and embedded gasket)
  • No loss of gaskets and screw
  • Individual valve isolation for servicing
  • Fully warrantied manifold to keep tools operational for many years to come

Whatever the current challenges a process is experiencing, or whatever goals are trying to be achieved, upgrading a pneumatic manifold is a more simple, beneficial process than it has ever been before. It’s important to consult with an expert in the field to determine whether an upgrade is warranted before making any commitment.

Article featured in Semiconductor Digest Magazine.
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