Si Hardmask (Si-HM), EUV And Zero Defects

The multilayer system used in lithography consists of a planarizing carbon layer beneath a hardmask etch-transferring layer and capped with a standard photoresist coating. In the past, Brewer Science has discussed in-depth how the multilayer system helped to extend ArF (193 nm) immersion lithography to be able to print and transfer ever-shrinking features, ensuring enough process window especially on topographies in “Advanced Materials For High-Temperature Process Integration.” Brewer Science is striving to develop a line of Si-HM products to meet the new challenges expected in implementing EUV and multi-patterning for advanced nodes beyond 7 and 5 nm while implementing a “zero defect” policy.

Si Hardmask (Si-HM) metrology
The hardmask layer, also known as the middle layer, is a thin inorganic layer, typically with high silicon content, which provides the necessary etch selectivity to transfer a thin photoresist pattern into much deeper underlying structures. Even though this layer could be deposited using chemical vapor deposition (CVD), a spin-on Si hardmask (Si-HM) provides a much simpler and more flexible process. It allows the entire multilayer stack to be coated, exposed, and developed in an integrated track and scanner system, which reduces the complexity of transferring wafers between different modules. Compared to CVD, it normally requires a low-temperature (<250°C) process in ambient air conditions, which means no special chamber, vacuum or inert environment is needed.

Being a supplier of Si-HM for over 15 years, Brewer Science has seen this product continue its critical role in the development of future nodes beyond 7 and 5 nm. In the past 2-3 years, we witnessed the insertion of EUV into high-volume manufacturing (HVM). The current design rule puts EUV to work at its resolution limit on the very first node it sees. We are talking about a single pass of EUV exposure to print 20-nm contact holes (or pillars) and 14-nm half-pitch L/S with good CD uniformity (CDU), line-width roughness (LWR), enough process window (no collapse or scum), and of course defect-free. (See the image below as an example.) Considering the low photo-density in EUV, resulting in high shot noise, stochastics defects (random non-predictable defects and missing patterns) are becoming harder to control than ever before. The Si-HM acts as the very first etch-stop/masking layer right after the photoresist imaging and development. Every defect mode presented at the resist/Si-HM interface, within the Si-HM film, and generated during etch transfer will be passed on or even highlighted through subsequent etch steps and eventually into the device itself. The whole process benefits from having essentially a “defect-free” Si-HM at the beginning of each layer.

Fig. 1: 20-nm contact hole printing – CD uniformity (CDU) and defectivity challenges at extremely small scale.

Filtration
A part per billion defect rate can cause up to 10% yield loss, thus we strive for more than “zero defect.”  Each material along the supply chain must be in that quality grade for the final chip to be usable. Brewer Science invests heavily in research and development and collaborates with other industry leaders to elevate the standards on minimizing defects to perfect the manufacturing process. Over the years, we have implemented multi-path purification and filtration techniques to be able to push the metal ion contamination into part-per-trillion (ppt) level, reducing the presence of liquid particles and other gel formations to the minimum limit that state-of-the-art metrology can detect.

Solving a “sol-gel” problem – new chemistry design
Brewer Science is currently examining its Si-HM product line on the chemistry level to test and understand how each chemical bond is formed. One step of making Si-HM involves an intricate “sol-gel” chemistry process. The typical “sol-gel” process involves converting monomers into a colloidal solution (sol) that acts as the precursor for an integrated network (gel). However, the semiconductor process doesn’t need “sol” or “gel” – as they are the sources of defects. Instead, there is a high need for a defect-free, uniform 10- to 50-nm thin film across the whole 12-inch wafer. More importantly, the “sol-gel” process, if not controlled well, will render a product that is still under a dynamic chemical reaction equilibrium. It can pass all the specs defined by the customer, but when installed on the track, a defect can hit a customers’ line at the most unexpected moment. When a yield impact event is spotted, it is too late and extremely hard to trace the cause.

Stress testing
Brewer Science designed the OptiStack HM material series with this specific nature in mind. Our industry-recognized OptiStack HM800 and HM900 products feature room-temperature stability over 12 months, whereas most competitor’s similar offerings have to be stored and shipped under strictly controlled cold or freezing conditions. Using the latest defect inspection and review technologies, our effort involves stressing the system under long periods of elevated testing conditions. These tests push our product to the point of failure, which is more extreme than anything a customer fab would encounter. From this data, we understand the fundamental chemistry mechanism behind the failure modes and new versions of products can be designed to give the customer upgraded product experience.

Lower costs, better quality – supply chain management
By controlling the whole supply chain from Si monomer purification to the polymer synthesis and final product blending, Brewer Science is able to minimize defects, which sets us apart from our competitors who must rely on the quality their suppliers and sub-suppliers provide them. Not only is it hard and time-consuming to troubleshoot problems, but the cost accumulates at each level of the supply chain. This causes companies to either pass on the cost to the customer or cut corners on quality. By relying on a streamlined vertical integration scheme, Brewer Science can look at “zero defect” from each layer of the supply chain, examine how the defect modes evolve through each step and determine which step the purification will generate the most effect on final defect reduction. This leverage ensures the best quality on the final product, while also minimizing costs. The end result is a reduced cost of ownership (COO) and added engineering values to customers.

Fig. 2: Control the whole supply chain for quality and cost.

“Zero defect” is not just a slogan. Metrology, filtration, stress testing, first-principle understanding, new chemistry design and supply chain management are all factors we examine in our approach. At Brewer Science, we don’t just sit on our past success. We challenge ourselves on product design and improvement constantly, so we can share our future success with our customers.

Check out our Zero-Defect Program to how we approach our commitment to quality in every step of the process.