By MATT FERRARO, Product Manager, Semiconductor, Swagelok Company, Solon, Ohio
Semiconductor wafer manufacturing is one of the most complex, challenging, and high-stakes industrial applications in the world today. Wafer fabricators must get everything just right to enable production of the cutting-edge semiconductors that are the critical enablers of countless digital tools and devices used by millions around the world. New technological advancements continue to drive performance demands ever higher, forcing the industry to keep pace.
Highly complex processes and volatile and expensive inputs require that extreme precision be maintained during manufacturing to ensure high quality outputs and production rates. There is no room for error, and heated competition raises the stakes even higher.
Semiconductor tool OEMs face similar challenges. Continuous improvement in tool design is a necessity so OEMs can differentiate their offerings and help their customers achieve more – and do so more efficiently – without sacrificing quality. Finding new ways to help fabs optimize their inputs (enabling the use of new precursor gases, for example) is also an important priority, as they work to develop higher-performance, next-generation semiconductor technology.
To these ends, atomic layer deposition (ALD) processes have continued to improve over the past several years, which has been critical to the success of both fabs and tool OEMs. Part of that ongoing optimization has involved the use of highly engineered ultrahigh-purity (UHP) valves for gas dosing (FIGURE 1). These components are no ordinary valves, as they have an outsized impact on the failure or success of a fab’s manufacturing processes.
ALD processes require the significantly higher performance and precision that UHP valves provide compared to their general industrial counterparts. Yet, semiconductor manufacturers constantly seek greater performance attributes from UHP valves so they can reach new levels of innovation and productivity.
Enabling that performance begins with understanding the current inefficiencies stakeholders face – such as thermal inconsistencies within valves and lower-than-ideal flow rates – and ends with advanced ALD valve technology that addresses those deficiencies. This article will review the latest technology advancements that are allowing fabs and tool manufacturers to experiment with different ALD chemistries and achieve the high-quality depositions they need without sacrificing process efficiency.
Identifying opportunities for higher performance
Innovation involves experimentation. And as that experimentation has continued among semiconductor fabs, a few common limitations associated with legacy UHP valve technologies have emerged, including:
Variable Thermal Stability: UHP valves must be heated to extreme temperatures during ALD processes to stop low-vapor pressure gases from solidifying prematurely and causing issues and dosing precision errors. However, actuators in traditional UHP diaphragm valves often cannot be immersed completely in the gas box and must be thermally isolated to maintain proper functionality.
The downside of thermal isolation is that it can cause temperature discrepancies between different valve components. As these discrepancies occur, gases moving through the valve may cool, as illustrated in Figure 2. This is an especially problematic occurrence when using precursors that require precise temperature stability to remain in a gaseous state prior to deposition. If the gases cool, molecules may remain inside the valve and stick to internal surfaces. The resulting unwanted residue buildup can lead to inconsistent dosing, which may affect wafer quality and production efficiencies.
Inconsistency is the enemy of repeatability – and repeatability is critical for semiconductor fabrication. Using a higher-performing UHP valve here could help eliminate the potential for variance stemming from thermal isolation and would be a welcome development for semiconductor makers.
Reduced Flow Rates: Current UHP valves suitable for ALD processes have limited flow capacities – another key challenge for both fabs and semiconductor tool OEMs as they seek to innovate.
While flow rates through traditional UHP valves tend to decline as they are heated, those rates have been acceptable to date. However, evolving processes have changed the demands. Fabs have discovered that boosting the flow rate capabilities of valves could potentially boost the rate at which semiconductor wafers can be produced. Higher flow rates would also contribute greater process flexibility to ensure the stability of precursor gases – and that’s good news for the bottom line.
Limited Contributions to Experimentation: Importantly, higher flow rates could enable fabs to become more experimental in their production processes. New highly reactive precursor gases hold promise for enabling the creation of tomorrow’s microchip technology, but these gases require higher flow rates to avoid pressure drop across the valve, which can cause low-vapor pressure gases to change states. Sufficient flow capacities are not presently available in existing UHP valve technology to make effective use of some precursor chemistries. Figure 3 shows the impact of flow rate on pressure drop in three different valves, demonstrating the variability found in the market.
Fabs can still use these new highly reactive precursor gases if they slow the flow rate in their processes enough to obtain the low pressure drop they require. However, doing so comes at the expense of overall system efficiency, and therefore productivity and profitability. As such, higher-flow-capacity UHP valves can enable greater experimentation and process optimization.
Improvements in UHP design and performance
Fortunately for semiconductor manufacturers and fabs, the next generation of ALD valves coming to market holds great promise for the future of wafer manufacturing. Various design improvements over existing ALD valve technology are providing a positive outlook on improved thermal consistency, higher flow capacity, and better overall performance.
Here are a few specific performance characteristics that are available in the next generation of ALD valves:
Full Immersibility: Eliminating the risk for buildup or deposition inconsistencies in ALD processes is essential for high repeatability and reliability. New ALD valve technology is designed to allow the entire valve to be heated to 200°C (392°F). In new valves, the actuator does not need to be isolated to maintain its integrity or dosing precision. This enables semiconductor fabs to be confident that gases flowing through their valves will be exposed to uniform temperatures, eliminating one source of potential condition variability. An ideal state of thermal stability is illustrated in Figure 4 – an improvement over the varied temperatures shown in Figure 2.
Higher Flow Rates: While limited flow rates are a challenge in current ALD valve technologies, newer valve designs are enabling greater flow rates without compromising cleanliness or component longevity.
Consider this example: An existing valve might offer a 0.6 Cv flow coefficient, whereas new valves can double that flow to 1.2 Cv in the same footprint. This enables tool manufacturers to provide greater output without any required retooling or other major process changes. Meanwhile, if a semiconductor fab has the design flexibility to use new ALD valves with a slightly larger footprint (1.75 in.), they can achieve a flow capacity that nearly triples that of traditional valves, with coefficients reaching 1.7 Cv.
New ALD valves that enable these substantial improvements feature a bellows design instead of a traditional diaphragm design, helping to enable higher flow rates. Additionally, the bellows themselves are polished to a 5 μin Ra finish to deliver UHP performance that manufacturers expect. Figure 5 illustrates one such bellows design and how it combines the optimal features of traditional and emerging valve technologies into a single UHP valve with a very high cycle life.
Increased opportunity for innovation: Due to the above performance improvements, deploying new ALD technology can allow forward-thinking semiconductor players to experiment and innovate without traditional constraints.
The temperature and flow improvements will enable semiconductor manufacturers to work in new areas of the periodic table of elements with more exotic gases. Low-vapor pressure precursor gases have shown particular promise in delivering higher performance than the gases typically used in today’s ALD processes. In addition, new ALD valves are offered in highly corrosion-resistant materials such as Alloy 22, thereby enabling more aggressive chemistries to be deployed for processing without the potential for crevice or pitting corrosion, which can cause serious issues.
Enhanced performance for a constantly evolving industry
Semiconductor tool OEMs or semiconductor fabs looking to sharpen their competitive edge in this highly competitive, quickly shifting industry should consider investigating new UHP valve technology. For example, new valves such as the recently released UHP ALD20 valve from Swagelok have shown potential to grant semiconductor manufacturers the high-quality, precise depositions they need without sacrificing process efficiency. Therefore, these valves can open new opportunities for innovation in ALD processes.
Advancing the semiconductor market is a continuous journey of iterative improvement and meeting new performance demands. While current demand has largely been driven by the proliferation of smartphones, cloud computing, and the Internet of Things, even more demanding applications are just around the corner. Artificial intelligence, 5G, and autonomous driving are just a few examples.
To enable the success of these technologies, it is imperative that semiconductor tool OEMs and fabs equip themselves with the right components, technology, and expertise that drive ongoing innovation. Adjusting processes and system designs to make the best use of advanced components like new ALD valves is one way to do just that. Another way is working with the right suppliers and partners who can help make these technologies the most effective.
About the author
Matt Ferraro is Product Manager, Semiconductor, Swagelok Company. He can be reached at matthew.ferraro@swagelok.com.