Jeffrey R. Strahan

Polymeric Cross-Linked Surface Treatments for Controlling Block Copolymer Orientation in Thin Films

The orientation of cylinder-forming poly(styrene-block-methyl methacrylate) [P(S-b-MMA)] was investigated on two sets of polymeric surface treatments: 10 para-substituted polystyrene derivatives with <10 mol % poly(4-vinylbenzyl azide) and a series of poly(styrene-random-4-vinylbenzyl azide) [P(S-r-VBzAz)] copolymers with 5-100 mol % poly(4-vinylbenzyl azide). The copolymers were spin-coated to form thin films and then cross-linked by heating. The resulting films exhibited a range of surface tensions from 21 to 45 dyn/cm. Perpendicular orientation of P(S-b-MMA) cylinders was achieved with poly(p-bromostyrene) and all the [P(S-r-VBzAz)] copolymer surface treatments, most notably the homopolymer of poly(4-vinylbenzyl azide). Films made from these simple copolymers are as effective as random terpolymer alignment layers commonly made from both block monomers and a cross-linkable monomer.

EUV Resist Requirements: Absorbance and Acid Yield

The challenge in obtaining good resist performance in terms of resolution, line width roughness and sensitivity at EUV wavelength forces to make more efficient use of photons that reach the wafer plane than has been the case for traditional optical lithography. Theory demonstrates that the current absorbance levels of EUV resists are quite far from optimal and absorbance should be increased. The most attractive pathway to achieve this is by increasing the fluorine content of EUV resists. The viability of this approach has been demonstrated using non-chemically amplified PMMA as model resist and comparing its photospeed with a fluorinated analogue. It has been demonstrated that the photospeed increases due to improved resist absorbance by ~1.5X, which is close to 1.7X that is predicted by the difference in absorbance. Further modeling studies support the experimental results and indicate an optimum for total film absorbance of ~0.20- 0.25. Compared to current platforms this would correspond to an increase in photospeed by ~1.7X which is accompanied with an improvement in LWR of ~1.14X. Combining this approach with the trends in EUV resists to increase PAG loading and include sensitizer in order to improve photospeed will likely provide a path for EUV resists that will meet the specifications that are required for the 32nm and 22nm node.

Fluorinated polymethacrylates as highly sensitive nonchemically amplified e-beam resists

In an effort to improve on the sensitivity of commercial nonchemically amplified e-beam resists, four polyacrylates functionalized with -CF3 and/or CH2CF3 alkoxy substituents were studied. The -CF3 substituent is known to increase backbone-scission efficiency while simultaneously eliminating acidic outgassing and cross-linking known to occur in -halogen substituted polyacrylates. Contrast curves for the polymeric -CF3 acrylates, generated through e-beam exposure, showed that the resists required an order of magnitude less dose than the current industry standards, poly(methyl methacrylate) (PMMA) and ZEP. The fundamental sensitivity of these materials to backbone scissioning was determined via 60Co -ray irradiation. The chain scissioning, G(s), and cross-linking, G(x), values calculated from the resulting change in molecular weight demonstrated that all fluorinated resists possess higher G(s) values than either PMMA or ZEP and have no detectable G(x) values. Utilizing e-beam and EUV interference lithographies, the photospeed of poly(methyl -trifluoromethacrylate) (PMTFMA) was found to be 2.8× and 4.0× faster, respectively, than PMMA.

Self-aligned patterning on a flexible substrate using a dual-tone, thermally activated photoresist

The fabrication of electronic devices on flexible substrates represents an opportunity for the development of flexible display technologies, large area devices, and roll-to-roll manufacturing processes. Traditional photolithography encounters alignment and overlay limitations when applied to flexible substrates. One solution to the overlay challenges is imaging of two device layers in a single lithographic exposure. To enable the simultaneous patterning of two device layers, a new photoresist system was developed. Prior work on dual-tone photoresists introduced formulations capable of storing two independent images, but the reported systems are incompatible with the reactive ion etch processes commonly used today. This paper describes a dual-tone photoresist system that maintains the ability to store two independent latent images, distinguished by the incident exposure light wavelength, simultaneously remaining compatible with reactive ion etch image transfer processes.

Extension of 248 nm Monte Carlo, mesoscale models to 193 nm platforms

Current minimum feature sizes in the microelectronics industry dictate that molecular interactions affect process fidelity and produce stochastic excursions like line edge roughness (LER). The composition of future resists is still unknown at this point, and so simulation of various resist platforms should provide useful information about resist design that minimizes LER. In the past, researchers developed a mesoscale model for exploring representative 248 nm resist systems through dynamic Monte Carlo methods and adaptation of critical ionization theory. This molecular modeling uses fundamental interaction energies combined with a Metropolis algorithm to model the full lithographic process (spin coat, PAB, exposure, PEB, and development). Application of this model to 193 nm platforms allows for comparison between 248 and 193 nm resist systems based on molecular interactions. This paper discusses the fundamental modifications involved in adapting the mesoscale model to a 193 nm platform and investigates how this new model predicts well-understood lithographic phenomena including the relationship between LER and aerial image, the relationship between LER and resist components, and the impact of non-uniform PAG distribution in the resist film. Limited comparisons between the 193 nm system and an analogous 248 nm platform will be discussed.