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Controlling Variabilities When Integrating IC Fab Materials

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By Ed Korczynski, Senior Technical Editor, SemiMD/Solid State Technology

Semiconductor integrated circuit (IC) manufacturing has always relied upon the supply of critical materials from a global supply chain. Now that shrinks of IC feature sizes have begun to reach economic limits, future functionality improvements in ICs are increasingly derived from the use of new materials. The Critical Materials Conference 2016—to be held May 5-6 in Hillsboro, Oregon (cmcfabs.org)—will explore best practices in the integration of novel materials into manufacturing. Dr. David Thompson, Senior Director, Center of Excellence in Chemistry, Applied Materials will present on “Agony in New Material Introductions – minimizing and correlating variabilities,” which he was willing to discuss in advance with SemiMD.

Korczynski: With more and more materials being considered for use in high-volume manufacturing (HVM) of advanced ICs, how do you begin to selectively screen out materials that will not work for one reason or another to be able to reach the best new material for a target application?

Thompson: While there’s ‘no one size fits all’ solution to this, it typically starts with a review of what’s available and known about the current offerings. With respect to the talk at the CMC, we’ll review the challenges we run into after the materials system and chemistries are set and have been proven generally viable, but still require significant optimization in order to get acceptable yields for manufacturing. It’s a very long road from device proof of concept on a new materials system to a viable manufacturing process.

Korczynski: Since new materials are being considered for use on the atomic-scale in advanced devices, doesn’t all of this have to be done with control at the atomic scale?

Thompson: For the material on the chip, many mainstream analytical techniques are used to achieve atomic level control including TEMs and AFMs with atomic resolution during film development for many applications. Unfortunately, this resolution is not available for the chemicals we’re relying on to deposit these materials. For a typical precursor that weighs in the 200 Dalton range, a gram of precursor may have 5 × 1020 molecules. That’s a lot of molecules. Even with ppb (109) resolutions on analytical, you’re still dealing with invisible populations of >1010 molecules. It gets worse. While trace metals analysis can hit ppb levels, molecular analysis techniques are typically limited in the 0.1 to 0.01 percent resolutions for most semiconductor precursors and there may be impurities which are invisible to routine analytical techniques.

Ultimately, we rely on analytical techniques to control the gross parameters and disciplined process controls to verify suppliers produce the same compositions the same way, and to manage impurities. On the process and hardware side, it’s like threading the needle trying to get the right film at the right throughput, in a process space that’s as tolerant as possible to the inevitable variability in these chemistries.

Korczynski: With all of this investment in developing one specialty material supplier for advanced IC manufacturing, what is the cost to develop and qualify a second source?

Thompson: Generally, it’s not sustainable to release a product with dual specialty material sources. The problem with dual-sourcing is chemical suppliers protect their knowledge—not simple IP—but also their sub-supply-chains and proprietary methods of production, transport and delivery. However, given how trace elements in the formulation can change depending on conditions the molecules experience over time, the customer in many cases needs to develop two separate sub-recipes based on the specific vendor’s chemistry they are using. So, redundancy in the supply chain is prudent as is making sure the vendor can produce the material in different locations.

There are countless examples over the last 20 years of what I like to call ‘the agony of the supply-chain’ when a process got locked into using a material when the only supply was from a Ph.D. chemist making it in small batches in a lab. In most cases the initial batch of any new molecule is made at a scale that would fit in a coffee mug. Sometimes though scaling up the first industrial-scale batch can alter impurity factors that change yields on the wafer even with improved purification. So while a customer would like to keep using a small batch production, it’s not sustainable but trying to qualify a second vendor in this environment presents significant challenges.

Korczynski: Can you share an example with us of how your team brought a source of subtle variation under control?

Thompson: We had a process using a new metal film, and in the early development everything looked great. Eventually we observed a drift of process results that was more pronounced with some ampoules and less so with others. The root cause initially eluded us. Then, a bright Ph.D. on our team said it’s interesting that the supplier did not report a particular contaminant that would tend to be present as a byproduct of the reaction. The supplier confirmed it was present and variable at concentrations in the 100-300 ppm concentration in the blend. This contaminant was relatively more volatile than the main component due to vapor pressure differences and much more reactive with the substrate/wafer. It was found this variability in the chemistry induced the process variation on the wafer (as shown in Figure 1).

FIGURE 1. RESOLUTION OF SEQUENTIAL WAFER DRIFT VIA IMPURITY MANAGEMENT

Chasing impurities and understanding their impact requires rigor and a lot of data collection. There’s no Star Trek analyzer we can use to give us knowledge of all impurities present and the role of those impurities on the process. Many impurities are invisible to routine analytical techniques, so we work very closely with vendors to establish a chemistry analytical protocol for each precursor that may consist of 5-10 different techniques. For the impurities we can’t detect we rely on excellent manufacturing process control and sub-supply sourcing management.

Korczynski: Is the supply-chain for advanced precursors for deposition and etch supplying everything we need in early R&D?

Thompson: New precursor ideation—the science that leads to new classes of compounds with new reactivity that Roy Gordon, or more recently Chuck Winter, have  been doing in academia is critically important and while there are a few academics doing excellent work in this space, in general there’s not enough focus on this topic.While we see many IP protected molecules, too often they are obvious simple modifications to one skilled in the art, consisting of merely adding a functional group off of a ring, or mixing and matching known ligand systems. We don’t see a lot of disruptive chemistries. The industry is hunting for differentiated reactivity, and evolutionary precursor development approaches generally aren’t sufficiently disruptive. While this research is useful in terms of tuning a vapor pressure or thermal stability it only very rarely produces a differentiated reactivity.

Korczynski: Do we need new methodologies to more efficiently manage all of this?

Thompson: Applied has made significant investments over the last 5 years to help accelerate the readiness of new materials across the board. One of the best things about working at Applied is the rate at which we can learn and build an ecosystem around a new material. With our strength in chemistry, deposition, CMP, etch, metrology and a host of other technologies, we get a fast, strong feedback loop going to accelerate issue discovery, resolution and general learning around new materials.

On the chemical supply-chain front, the need is making sure that chemical vendors accelerate their analytical chemistry development on new materials. Correlating the variability of chemistry to process results and ultimately yield is the real battle. The more knowledge we have of a chemistry moving into development, the faster learning can occur. I explain to my team that we can’t be proactive and respond to things we didn’t anticipate. Situations where trying to develop the analytical technique to see the impurity responsible for causing (or resolving) a variability is to start out at a significant disadvantage. However, we’ve seen a good response from suppliers on new materials and significant improvement on the early learnings necessary to minimize the agony of new material introductions.



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