(193am) Tuning Pitch in Self-Assembled Block Copolymers through Homopolymer Addition: Effect of Homopolymer Molecular Weight on Lamellae Roughness

Moore’s Law, coined by Intel co-founder Gordon Moore in 1965, claims that the number of transistors per unit area on integrated circuits (IC) will double every year. Although IC development had consistently followed this trend from the 1970’s through the late 2000’s, the rate of increase has slowed within the past decade as the desired feature sizes have shrunk to the nanometer scale. The limiting step in further reduction of transistor size is lithography, which is the process of patterning and subsequently developing device features on a silicon wafer. It is well known in the lithographic community that there are significant challenges ahead in terms of developing lithographic techniques that can be used to pattern features with sub-10 nm spacing (pitch). Some technologies that are being investigated in the semiconductor industry are extreme ultraviolet lithography (EUV), nanoimprint lithography, electron-beam lithography, and directed self-assembled (DSA) block copolymers. Block copolymers (BCP) have the unique ability to microphase separate into a variety of morphologies, including lamellae, cylinders, spheres, and gyroids. These various microstructures can form entropically driven defective regions. To counteract this DSA methods such as graphoepitaxy and chemoepitaxy are employed that rely on an underlayer to guide the BCP into defect free states. Graphoepitaxy manipulates the topology of the underlayer to preferentially guide the BCP domains. On the other hand, chemoepitaxy is a method that uses energetically preferential pinning stripes in an underlayer to control the orientation and placement of the block copolymer’s phase separated features. These highly controlled DSA techniques drastically reduce the defectivity found in the BCP structure, and even more importantly, a desired BCP structure can be precisely reproduced over a large area. This precision and reproducibility presents DSA block copolymers as an intriguing method of patterning semiconductor features on the sub-10 nm scale.

In practice, chemoepitaxy is employed in the DSA of lamellar block copolymers by first coating a crosslinkable, slightly preferential, random copolymer and then etching away periodic stripes over this region. Then a homopolymer brush is synthesized and grafted to the removed regions forming block preferential pinning stripes. The density multiplication is defined as the frequency of pinning stripes in the underlayer. For example, a 1x density multiplication has one pinning stripe for every A-B domain, while a 2x density multiplication has one pinning stripe for every two A-B domains, etc. Previous studies have shown that a BCP film is less defective as the density multiplication decreases; however a density multiplication of 1x is impractical since if the underlayer could be patterned at this frequency then there would be no need for the BCPs. In this study, all BCP models are patterned atop a 2x density multiplication underlayer.

After the underlayer is patterned and the block copolymer film is coated atop the underlayer, the BCP will ideally coat the substrate’s surface and phase separate via DSA into the desired morphology with minimal defectivity. Upon removal, one of the BCP blocks will leave behind a mask that can be transferred into the substrate via etching.

Every BCP has a natural pitch (L₀)that is dependent upon factors including its molecular weight. This feature of block copolymers allows manufacturers to pattern a precise feature size at dimensions that beat the diffraction limited possibilities of other more conventional optical lithographic techniques. However, this also means that changing the desired feature size requires synthesizing a BCP of a different molecular weight (MW). The ability to modulate the natural pitch can reduce the necessity for low tolerances in batch-to-batch fluctuations of the block copolymer’s molecular weight. One approach to pitch modulation is blending homopolymer with the BCP and increasing L₀. Previous studies have shown that significant homopolymer concentrations increase lamellae line-edge roughness (LER) and line-width roughness (LWR). Prior results are limited to homopolymer of MW equal to ½ that of the BCP, but the effects of homopolymer MW on LER and LWR have not been well characterized. In this study, coarse-grained molecular dynamics simulations have been utilized to systematically study the influence of symmetric homopolymer loading of varying MW (â…›, ¼, and ½ of the BCP MW) on the behavior of a lamellae-forming BCP film with a density multiplication of 2x. The effective process windows for symmetric homopolymer loading will be presented, and the challenges of this method will be discussed.