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1. Atomistic Investigation of Material Deformation Behavior of Polystyrene in Nanoimprint Lithography

   https://doi.org/10.3390/surfaces3040043  

   Odujole, Jahlani; Desai, Salil (December 2020, Surfaces)

   null (Ed.)

   This research investigates deformation behavior of polystyrene (PS) as a thermoplastic resist material for the thermal nanoimprint lithography (T-NIL) process. Molecular dynamics modeling was conducted on a PS substrate with dimensions 58 × 65 × 61 Å that was imprinted with a rigid, spherical indenter. The effect of indenter size, force, and imprinting duration were evaluated in terms of indentation depth, penetration depth, recovery depth, and recovery percentage of the polymer. The results show that the largest indenter, regardless of force, has the most significant impact on deformation behavior. The 40 Å indenter with a 1 µN of force caused the surface molecules to descend to the lowest point compared to the other indenters. An increase in indenter size resulted in higher penetration depth, recovery depth, and recovery percentage. Higher durations of imprint cycle (400 fs) resulted in plastic deformation of the PS material with minimal recovery (4 Å). The results of this research lay the foundation for explaining the effect of several T-NIL process parameters on virgin PS thermoplastic resist material. 

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2. Molecular Dynamics Simulation of Poly Acrylic Acid as a Resist Material for Thermal Nanoimprint Lithography Processes

   Odujole J., Desai S. (October 2020, Proceedings of the Industrial Engineers Research Conference 2020)

   Nanoimprinting of polymers lays the foundation for several electronic and biomedical devices. Process parameter optimization have been conducted using thermal nanoimprint (T-NIL) experimentation. However, the underlying deformation mechanism of specific polymers under varying process condition needs further exploration. This research investigates the deformation behavior of poly acrylic acid (PAA) as a thermoplastic resist material for the T-NIL process. Molecular dynamics modeling was conducted on a PAA substrate imprinted with a rigid, spherical indenter. The effect of indenter size, force, and imprinting duration on the indentation depth, penetration depth, recovery depth, and recovery percentage of the polymer was evaluated. The results show that the largest indenter, regardless of force has the most significant impact on deformation behavior. The results of this research lay foundation for explaining the effect of several T-NIL process parameters on virgin PAA thermoplastic resist material. 

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3. Molecular Dynamics Investigation of the Deformation Mechanism of Gold with Variations in Mold Profiles during Nanoimprinting

   https://doi.org/10.3390/ma14102548  

   Gaikwad, Abhaysinh; Desai, Salil (May 2021, Materials)

   null (Ed.)

   Understanding the deformation behavior during nanoimprint lithography is crucial for high resolution patterning. Molecular dynamics modeling was implemented to investigate the effect of different mold profiles (cylindrical, rectangular, and spherical) on the von Mises stress, lattice dislocations, and material deformation. Relatively higher von Mises stress (1.08 × 107 Pa) was observed for the spherical mold profile compared to the rectangular and cylindrical profiles due to the larger surface area of contact during the mold penetration stage of NIL. Substantial increases in the von Mises stress were observed for all the mold geometries during the mold penetration stage. The von Mises stresses had a reduction in the relaxation and mold retrieval stages based on the rearrangement of the gold atoms. The lattice dislocation during the deformation process revealed the formation of the BCC structure which further reverted to the FCC structure after the mold retrieval. The polyhedral template matching (PTM) method was used to explain the retention of the FCC structure and subsequent ductile behavior of the substrate. The cylindrical mold had the lowest percentage spring back in both of the orthogonal directions and thus replicated the mold profile with high-fidelity as compared to the spherical and rectangular molds. The findings of this research can aid the design of molds for several applications. 

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4. Molecular Dynamics Study of the Quenching Effect on Direct Nanoimprint of Gold

   Abhaysinh Gaikwad, Jahlani Clarke (May 2019, Proceedings of the 2019 IISE Annual Conference)

   Nanoimprinting of gold has the potential to make significant impact in the fabrication of miniaturized devices including optical devices, photonic crystals and biochemical sensors. The deformation behavior and recrystallization of gold structures has profound impact on the accurate replication of the imprinted pattern. An important step in the nanoimprint lithography (NIL) process is the cooling of the mold and resist assembly to room temperatures. The rate of cooling, commonly termed as quenching determines the final shape of the NIL features. Molecular dynamics simulations were implemented to study the nanoimprinting of gold substrates using a silicon mold. Simulations were conducted at room temperature (298 K), gold recrystallization (473K) and melting point (1000K) temperatures. The effect of different cooling rates (quenching) on the deformation behavior and spring back of gold was investigated. These include 0.01ns, 0.05ns, and 0.2ns time steps of quenching at 1fs per time step. The modified embedded atom method (MEAM) potential was used for the system molecules. Fast cooling at 298 K and 473 K whereas, intermediate cooling at 1000 K resulted in good pattern transfer. High fidelity nanoscale features were achieved with a gradient cooling proportional to the initial heating temperatures. The findings of this research provide insight on the effect of quenching in the material deformation and spring back during direct metal nanoimprinting. 

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5. Multi-cycling nanoindentation in additively manufactured Inconel 625 before and after laser peening

   https://doi.org/10.1088/2051-672X/ac7736  

   Tajyar, Ali; Brooks, Nicholas; Vaseghi, Majid; Hackel, Lloyd; Momeni, Kasra; Davami, Keivan (June 2022, Surface Topography: Metrology and Properties)

   Abstract In this research, a room temperature multicycle nanoindentation technique was implemented to evaluate the effects of the laser peening (LP) process on the surface mechanical behavior of additively manufactured (AM) Inconel 625. Repetitive deformation was introduced by loading-unloading during an instrumented nanoindentation test on the as-built (No LP), 1-layer, and 4-layer laser peened (1LP and 4LP) conditions. It was observed that laser-peened specimens had a significantly higher resistance to penetration of the indenter and lower permanent deformation. This is attributed to the pre-existing dislocation density induced by LP in the material which affects the dislocation interactions during the cyclic indentation. Moreover, high levels of compressive stresses, which are greater in the 4LP specimen than the 1LP specimen, lead to more effective improvement of surface fatigue properties. The transition of the material response from elastic-plastic to almost purely elastic in 4LP specimens was initiated much earlier than it did in the No LP, and 1LP specimens. In addition to the surface fatigue properties, hardness and elastic modulus were also evaluated and compared. 

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