DSA and EUV should be envisioned as complementary, not competing, techniques that will eventually become mainstream for fine-pitch lithography.
BY DOUGLAS J. GUERRERO, Ph.D., Brewer Science, Rolla, MO
Advances in lithography have always been critical in the drive toward each subsequent semiconductor node. Anticipating limitations in the scaling ability of immersion lithography, the industry has been pursuing next-generation lithography techniques. Several techniques have been proposed, including extreme ultraviolet (EUV) lithography, multibeam electron-beam lithography, nanoimprint lithography and directed self-assembly (DSA) of block copolymers.
DSA attracted a great deal of interest from major semiconductor manufacturers for several years, following its initial development in the early 2000s. However, it has since fallen out of favor to some extent, in part because of advances in EUV lithography as a result of focused investment in that technology. Recent developments in DSA materials and processing promise to overcome concerns that have delayed its implementation.
Choosing an appropriate lithography technique does not need to be an either-or proposition. The greatest opportunity may lie in leveraging both EUV lithography and DSA. Although these two technologies are sometimes seen as competing, it makes more sense to envision them as complementary. This article explains how lithography may benefit by taking advantage of both EUV and DSA, and why previously existing roadblocks may no longer pose obstacles.
The material defines the pattern
Unlike most lithography techniques, where the mask defines the pattern, in DSA the pattern exists in the material itself. The original block copolymers (BCPs) for DSA combine polystyrene (PS) and poly(methyl methacrylate) (PMMA), two polymers that naturally segregate themselves into separate phases. Adjusting the relative proportions of PS and PMMA in the PS-b-PMMA material changes the morphology from spherical to cylindrical to lamellar (FIGURE 1). The product of the Flory interaction parameter, χ, and the segment length determine the spacing of the ordered structure. The higher the value of χ, the finer the pitch of the resulting structure.
Standard PS-b-PMMA materials have relatively low χ, which limits the pitch to 20nm or larger. Some materials manufacturers are considering chemistries other than PS-b-PMMA to produce high-χ BCPs, replacing the PMMA component with polydimethylsiloxane or polyhydroxystyrene. Modifying PS-b-PMMA is another approach to increase χ. In this manner, it is possible to tune χ, the molecular weight and the glass transition temperature to achieve lamellar spacing between 14nm and 40nm under various annealing conditions.
The process flow for BCP deposition is straightforward. A neutral layer spin-coated onto the substrate allows for the BCP to separate into its individual domains during the thermal annealing process. The neutral layer allows for domain separation because it does not have affinity for either of the polymer chains in the BCP. Polymer domain separation is responsible for pattern formation.
The DSA deposition process uses one of two basic approaches (FIGURES 2 and 3). Graphoepitaxy leverages topography to align the BCPs, depositing them into relatively deep trenches. Guide patterns define the trenches, confining the BCPs into configurations in which they align in a preferred direction. Chemical epitaxy, or chemoepitaxy, is based on a chemical pattern on a flat substrate, on top of which the BCPs self-align.
The semiconductor industry is pursuing both graphoepitaxy and chemoepitaxy approaches, favoring the former for producing fine-pitch vias and the latter for creating arrays of parallel lines.
Annealing temperatures are in the range of 250°C to 275°C, making them compatible with standard semiconductor processing. The annealing step can be lengthy—up to two hours to create structures with sufficiently low defect rates—adding cost to the process.
PS-b-PMMA BCPs are being manufactured in high- volume quantities. Worldwide, 1.1 million tons of the material are currently in use for a variety of applica- tions. This quantity is greater than the needs of the entire semiconductor industry. Therefore, although no commercially produced DSA materials are currently targeted for semiconductor applications, the infra- structure is in place to scale up production of suitable materials when the industry is ready.
Why DSA is attractive now
DSA was added to the ITRS roadmap in 2007. Major semiconductor industry players originally believed DSA would enter commercial production anywhere between the 14nm and 7nm logic nodes, and even sooner for DRAM; but so far that has not come to pass. A survey at the 2016 DSA Symposium suggested that the technology is still not ready for the mainstream and won’t be for several years. But some IDMs would like to accelerate the process, and there are reasons to believe this is not only possible, but desirable.
Decreasing the wavelength to 193nm immersion lithography has enabled line width and spacing down to 80nm. Techniques such as self-aligned quadruple patterning (SAQP) can create even smaller features through multiple lithography/etch iterations, but at the expense of adding lithography steps, each requiring a custom mask.
Immersion lithography is reaching its limits, providing an opportunity for next-generation lithographic techniques. Designs with critical dimensions (CD) in the range of 10nm to 30nm create a sweet spot for these state-of-the-art techniques.
Advances in EUV lithography are one factor that has led the industry to favor it over DSA. Today’s EUV materials have greater sensitivity compared with older- generation products, therefore requiring lower UV doses; and line roughness has improved as well. EUV lithography can create vias with 30nm or 40nm spacing that are not feasible with immersion lithography.
DSA enables even finer resolution than the semicon- ductor industry currently demands. Feature sizes are just now approaching a level where DSA can be especially effective. If these trends continue, the technique is poised to be widely adopted before the end of this decade.
DSA and EUV: Better together?
The most effective solution may lie in leveraging EUV and DSA technologies to take advantage of the strengths of each. Both methods can achieve resolution levels that are compatible with the N7 and N5 logic nodes. EUV lithography is well-suited to patterning designs with multiple different pitches, down to line width and spacing around 30nm. For such fine pitches, however, the number of mask steps required may make the technique prohibitively expensive. Local CD uniformity (LCDU) can also be a concern, especially at high throughput rates.
The initial hard-mask lithography process is the same for both EUV and DSA, but they diverge during pattern processing. Once the BCPs are deposited, DSA can achieve 30nm feature size without requiring additional masks. Annealing naturally separates the two phases into the correct morphology. The DSA process, however, is best suited to designs with a single pitch.
EUV can be used to pattern lower-resolution features on a chip, plus create spacers for subsequent DSA deposition. This combination provides the greatest design flexibility while streamlining the fabrication process, eliminating processing steps and reducing mask costs. LCDU is also better than with EUV alone.
DSA is best suited for devices with multiple repeating, regular fine-pitch features. Therefore, it likely will first be implemented in DRAM storage, later migrating to use in via layers on logic devices. Graphoepitaxy, especially using EUV to deposit the spacers, can enable more complex designs using DSA, where different regions of the chip require different pitches. This will presumably be the approach of choice for logic chips.
Despite the promise that leveraging both DSA and EUV offers, the semiconductor industry will only migrate to this approach once suppliers can convince IDMs that the materials have overcome their technical limita- tions. DSA has suffered from several challenges that have delayed its adoption: Primary issues are defectivity, pattern placement accuracy, ease of integration into manufacturing flows, and cost. But there is reason to be optimistic, as advances in chemistry and processing methods are improving all these metrics.
Overcoming technical challenges
The 2016 DSA Symposium survey identified defec- tivity as the greatest technical challenge. Defectivity and cost are related, in that the lowest defect levels are seen with the longest annealing times. While annealing for as little as five minutes causes the two phases to separate, the resulting material contains far too many defects to be suitable for commercial use.
Wafers are typically annealed one at a time, which can make the cost of annealing prohibitive. However, recent research using batch annealing in a vertical furnace showed great promise for reducing cost. By annealing 150 wafers in parallel for 30 minutes, researchers were able to demonstrate sufficiently low defect levels at a cost lower than that of SAQP.
Using both DSA and EUV has the potential to alleviate the problem of pattern placement errors. For example, EUV lithography can create prepatterned holes for doublet vias. The two vias may merge during the EUV process but will then automatically separate during DSA. Without DSA, an additional lithography step may be required to avoid merged vias.
This approach of leveraging EUV and DSA for fine- pitch vias is most reliable when the via shape is optimized. Studies have shown that a peanut shape, rather than an elliptical one, is ideal for creating doublet vias with minimal risk for pattern placement errors, even at the challenging N5 node.
Collaborating to advance DSA adoption
The semiconductor industry has extensive experience with lithography, but DSA requires a shift in mindset. BCP materials are not something that the industry is used to, and revolutionary rather than evolutionary changes in materials and processes can face resistance. DSA needs to be demonstrated on real devices before it can achieve traction in the semiconductor market.
Collaborative efforts between semiconductor industry materials suppliers and chemical companies with deep experience in BCPs are one route to bridge this gap. One such collaboration is currently underway. Brewer Science has teamed up with Arkema, a company with two decades of experience producing BCPs, but little leverage with the semiconductor industry. The partnership, begun in 2015, has led to pilot production of DSA materials, paving the way for the technique to move out of the laboratory and into commercial semiconductor products.
DSA and EUV should be envisioned as complementary, not competing, techniques that will eventually become mainstream for fine-pitch lithography at the N7 node and beyond. Partnerships between materials and chemical companies are poised to enable this transition, unlike previous efforts by single organizations.