Publications

2018
X. Xu, Kwon, H., Gawlik, B., Estakhri, N. M., Alu, A., Sreenivasan, S. V., and Dodabalapur, A., “Enhanced Photoresponse in Metasurface-Integrated Organic Photodetectors,” Nano Lett., vol. 18, no. 6, pp. 3362-3367, 2018. DOIAbstract
In this work, we experimentally demonstrate metasurface-enhanced photoresponse in organic photodetectors. We have designed and integrated a metasurface with broadband functionality into an organic photodetector, with the goal of significantly increasing the absorption of light and generated photocurrent from 560 up to 690 nm. We discuss how the metasurface can be integrated with the fabrication of an organic photodiode. Our results show large gains in responsivity from 1.5x to 2X between 560 and 690 nm.
A. Cherala and Sreenivasan, S. V., “Molecular dynamics modeling framework for overcoming nanoshape retention limits of imprint lithography,” Microsystems & Nanoengineering, vol. 4, no. 3, 2018. Publisher's VersionAbstract
Complex nanoshaped structures (nanoshape structures here are defined as shapes enabled by sharp corners with radius of curvature <5 nm) have been shown to enable emerging nanoscale applications in energy, electronics, optics, and medicine. This nanoshaped fabrication at high throughput is well beyond the capabilities of advanced optical lithography. While the highest-resolution e-beam processes (Gaussian beam tools with non-chemically amplified resists) can achieve <5 nm resolution, this is only available at very low throughputs. Large-area e-beam processes, needed for photomasks and imprint templates, are limited to similar to 18 nm half-pitch lines and spaces and similar to 20 nm half-pitch hole patterns. Using nanoimprint lithography, we have previously demonstrated the ability to fabricate precise diamond-like nanoshapes with similar to 3 nm radius corners over large areas. An exemplary shaped silicon nanowire ultracapacitor device was fabricated with these nanoshaped structures, wherein the half-pitch was 100 nm. The device significantly exceeded standard nanowire capacitor performance (by 90%) due to relative increase in surface area per unit projected area, enabled by the nanoshape. Going beyond the previous work, in this paper we explore the scaling of these nanoshaped structures to 10 nm half-pitch and below. At these scales a new "shape retention" resolution limit is observed due to polymer relaxation in imprint resists, which cannot be predicted with a linear elastic continuum model. An all-atom molecular dynamics model of the nanoshape structure was developed here to study this shape retention phenomenon and accurately predict the polymer relaxation. The atomistic framework is an essential modeling and design tool to extend the capability of imprint lithography to sub-10 nm nanoshapes. This framework has been used here to propose process refinements that maximize shape retention, and design template assist features (design for nanoshape retention) to achieve targeted nanoshapes.
2017
N. Khusnatdinov, Resnick, D. J., Singhal, S., Grigas, M. M., and Sreenivasan, S. V., “Development Of An Inkjet-enabled Adaptive Planarization Process,” SPIE Photomask Technology Conference, vol. 10451. SPIE, Monterey CA, 2017. Publisher's VersionAbstract
Nanoimprint lithography manufacturing utilizes a patterning technology that involves the field-by-field deposition and exposure of a low viscosity resist deposited by jetting technology onto the substrate. The patterned mask is lowered into the fluid which then quickly flows into the relief patterns in the mask by capillary action. Following this filling step, the resist is crosslinked under UV radiation, and then the mask is removed, leaving a patterned resist on the substrate. The technology faithfully reproduces patterns with a higher resolution and greater uniformity compared to those produced by photolithography equipment. Throughputs of 80 wph have been demonstrated, and mix and match overlay of 3.7nm 3 sigma has been achieved. The technology has already been successfully applied as a demonstration to the fabrication of advanced NAND Flash memory devices. A similar approach can also be applied however to remove topography on an existing wafer, thereby creating a planar surface on which to pattern. In this paper, a novel adaptive planarization process is presented that addresses the problems associated with planarization of varying pattern densities, even in the presence of pre-existing substrate topography. The process is called Inkjet-enabled Adaptive Planarization (IAP). The IAP process uses an inverse optimization scheme, built around a validated fluid mechanics-based forward model that takes the pre-existing substrate topography and pattern layout as inputs. It then generates an inkjet drop pattern with a material distribution that is correlated with the desired planarization film profile. This allows a contiguous film to be formed with the desired thickness variation to cater to the topography and any parasitic signatures caused by the pattern layout. In this work, it was demonstrated that planarization efficiencies of up to 99.5% could be achieved, thereby reducing an initial similar to 100nm wafer topography down to as little as 0.6nm.
P. Joseph, Singhal, S., Abed, O., and Sreenivasan, S. V., “Fabrication of self-aligned multilevel nanostructures,” Microelectronic Engineering, vol. 169, pp. 49-61, 2017. DOIAbstract
Multilevel three-dimensional nanostructures are essential in many integrated nanoelectronic and nanophotonic applications. With the continued shrinking of critical device dimensions, extremely precise nanoscale overlay is required between multiple individual levels of these integrated devices. Multilevel nanoimprint lithography has been proposed in the literature as a potential solution to this overlay problem. In this context, self-aligned (perfectly aligned) multilevel templates (SAMT) for multilevel nanoimprint lithdgraphy are proposed in this article. By combining nanolithography, atomic layer deposition, and highly selective reactive ion etch, SAMTs can enable the fabrication of sophisticated integrated devices. Four specific self-aligned multilevel fabrication techniques have been demonstrated that result in symmetric multilevel structures, bilaterally symmetric multilevel structures, tubular structures, and asymmetric multilevel structures, all in the sub-100 nm scale. When used in conjunction with a nanoimprint lithography process, the SAMTs can enable high-throughput patterning of various nanoelectronic and nanophotonic devices using a single patterning step with perfect alignment and overlay. SAMTs further enable large area patterning, such as wafer-scale patterning and roll-to-roll patterning on flexible substrates, without compromising perfect overlay. (C) 2016 Elsevier B.V. All rights reserved.
S. V. Sreenivasan, “Nanoimprint lithography steppers for volume fabrication of leading-edge semiconductor integrated circuits,” Microsystems & Nanoengineering, vol. 2017, no. 3, 2017. Publisher's VersionAbstract
This article discusses the transition of a form of nanoimprint lithography technology, known as Jet and Flash Imprint Lithography (J-FIL), from research to a commercial fabrication infrastructure for leading-edge semiconductor integrated circuits (ICs). Leading-edge semiconductor lithography has some of the most aggressive technology requirements, and has been a key driver in the 50-year history of semiconductor scaling. Introducing a new, disruptive capability into this arena is therefore a case study in a "highrisk-high-reward" opportunity. This article first discusses relevant literature in nanopatterning including advanced lithography options that have been explored by the IC fabrication industry, novel research ideas being explored, and literature in nanoimprint lithography. The article then focuses on the J-FIL process, and the interdisciplinary nature of risk, involving nanoscale precision systems, mechanics, materials, material delivery systems, contamination control, and process engineering. Next, the article discusses the strategic decisions that were made in the early phases of the project including: (i) choosing a step and repeat process approach; (ii) identifying the first target IC market for J-FIL; (iii) defining the product scope and the appropriate collaborations to share the risk-reward landscape; and (iv) properly leveraging existing infrastructure, including minimizing disruption to the widely accepted practices in photolithography. Finally, the paper discusses the commercial J-FIL stepper system and associated infrastructure, and the resulting advances in the key lithographic process metrics such as critical dimension control, overlay, throughput, process defects, and electrical yield over the past 5 years. This article concludes with the current state of the art in J-FIL technology for IC fabrication, including description of the high volume manufacturing stepper tools created for advanced memory manufacturing.
P. Jurney, Agarwal, R., Singh, V., Choi, D., Roy, K., Sreenivasan, S. V., and Shi, L., “Unique size and shape-dependent uptake behaviors of non-spherical nanoparticles by endothelial cells due to a shearing flow,” Journal of Controlled Release, vol. 245, pp. 170-176, 2017. DOIAbstract
The size and shape of nanoparticle (NP) drug carriers can potentially be manipulated to increase the drug delivery efficacy because of their effects on particle margination and interactions with various cells in vivo. It is found in this work that the presence of a physiologically relevant shearing flow rate results in very different size and shape-dependent uptake behavior of negatively charged, non-spherical polyethylene glycol (PEG) hydrogel NPs by endothelial cells (ECs) cultured in a microchannel compared to uptake of either identical NPs in static culture or spherical particles in a shear flow. In particular, larger rod- and disk-shaped PEG NPs show more uptake than smaller ones, opposite to the size effect observed for spherical particles in a flow. Moreover, the trend observed in this dynamic uptake experiment also differs from that reported for uptake of similar PEG NPs by ECs in a static culture, where the smaller disks were found to be uptaken the most. These differences suggest that the increasing rotational and tumbling motions of larger-size non-spherical NPs in the flow play a dominant role in NP margination and cell interaction, compared to Brownian motion, gravity, and cell membrane deformation energy. These findings suggest that the coupling between NP geometry and shear flow is an important factor that needs to be accounted for in the design of the size and shape of nanocarriers.
2016
S. Singhal, Grigas, M. A., and Sreenivasan, S. V., “Mechanics-Based Approach for Detection and Measurement of Particle Contamination in Proximity Nanofabrication Processes,” Journal of Micro and Nano-Manufacturing, vol. 4 (3), no. 031004, 2016. Publisher's VersionAbstract
In spite of the great progress made toward addressing the challenge of particle contamination in nanomanufacturing, its deleterious effect on yield is still not negligible. This is particularly true for nanofabrication processes that involve close proximity or contact between two or more surfaces. One such process is Jet-and-Flash Imprint Lithography (J-FIL (TM)), which involves the formation of a nanoscale liquid film between a patterned template and a substrate. In this process, the presence of any frontside particle taller than the liquid film thickness, which is typically sub-25 nm, can not only disrupt the continuity of this liquid film but also damage the expensive template upon contact. The detection of these particles has typically relied on the use of subwavelength optical techniques such as scatterometry that can suffer from low throughput for nanoscale particles. In this paper, a novel mechanics-based method has been proposed as an alternative to these techniques. It can provide a nearly 1000 x amplification of the particle size, thereby allowing for optical microscopy based detection. This technique has been supported by an experimentally validated multiphysics model which also allows for estimation of the loss in yield and potential contact-related template damage because of the particle encounter. Also, finer inspection of template damage needs to be carried out over a much smaller area, thereby increasing throughput of the overall process. This technique also has the potential for inline integration, thereby circumventing the need for separate tooling for subwavelength optical inspection of substrates.
A. Cherala, Chopra, M., Yin, B., Mallavarapu, A., Singhal, S., Abed, O., Bonnecaze, R., and Sreenivasan, S. V., “Nanoshape Imprint Lithography for Fabrication of Nanowire Ultracapacitors,” IEEE Transactions on Nanotechnology, vol. 15, no. 3, 2016. Publisher's VersionAbstract
Emerging nanoscale applications in energy, electronics, and medicine require high throughput patterning with complex shape control at the nanoscale that is beyond the capabilities of optical lithography and block copolymer-based self-assembly. We introduce a technique for creating precise two-dimensional nanoshapes with sharp corners by imprint lithography, and apply it to enable shaped silicon nanowire capacitors that significantly exceed standard nanowire capacitor performance due to relative increase in surface area per unit projected area. The patterning technique employs atomic layer deposition to fabricate a template with diamond-like shapes consisting of corners with 2 nm radii of curvature. Template materials and chemical staining of the imprint polymer enable precise imaging of the template and replicated resist. Continuum mechanics appears no longer applicable at the length scale of similar to 3 nm. A systematic increase in the radius of the imprinted corner is observed contrary to predictions by a linear elastic continuum model shedding new light on shape relaxation of polymers, and on the limits of nanoshape replicability by imprint lithography. Novel diamond-shaped silicon nanowires (100-nm half-pitch) have been fabricated using nanoshape imprint followed by metal-assisted chemical etching, and have been incorporated into shaped nanowire capacitors that exceed standard nanowire capacitors performance by similar to 90%. The 3-nm resolution limit does not degrade the performance of the shaped capacitor. This increased capacitance validates the ability to preserve nanoshape cross section during patterning and deep etching over large areas. Lithographic scaling to 10-nm half-pitch has the potential to further increase capacitance by a factor of 10.
S. Singhal and Sreenivasan, S. V., “Influence of discrete drop locations on film thickness uniformity in UV-nanoimprint lithography,” Journal of Micro and Nano-Manufacturing, vol. 164, pp. 139-144, 2016. DOIAbstract
Multi-nozzle inkjetting is the material deposition approach for a variant of UV-nanoimprint lithography called Jet-and-Flash Imprint Lithography (J-FIL). J-FIL has several advantages with respect to addressing pattern density variations in the template as well as achieving low material waste when compared with spin-coating. In this work, the influence of discrete drop placement in J-FIL on residual layer thickness uniformity has been investigated with the help of a novel nonlinear lubrication theory model, which takes into account the compliant nature of the template in the presence of a moving contact line. This work is directed at thin residual layers (sub-20 nm), where a correlation exists between minimum drop resolution, inkjet nozzle pitch and residual layer thickness uniformity. Numerical simulations reveal that the non-uniformity resulting from discrete drop placement can exceed desired process tolerances when low film thicknesses are desired with relatively larger drop volumes. The results have been verified experimentally to reveal the same trend. This work can be useful in determining the correlation of inkjet drop resolution with inkjet nozzle pitch to meet process tolerances in the J-FIL process. (C) 2016 Elsevier B.V. All rights reserved.
P. Ajay, Cherala, A., Yin, B. A., Moon, E. E., Pease, F. R., and Sreenivasan, S. V., “Multifield sub-5nm overlay in imprint lithography,” Journal of Vacuum Science & Technology B, vol. 34, no. 061605, 2016. Publisher's VersionAbstract
One significant advantage of imprint lithography (IL) over photolithography is that the field size is not limited optically, and so, throughput can be very high (>100 cm(2)/s). But, in applications requiring precise (sub-5nm) overlay, the field size is limited by the distortion (between template and wafer) to about the same field size (26 x 33mm) as employed in current photolithographic tools for semiconductor integrated circuits. This reduces the throughput of current IL tools to less than that of current photolithographic tools. Here, the authors have, for the first time, created a multifield (dual and quad fields) nanoscale overlay capability by optimally combining (1) Precision mechanical actuators around the periphery of the fields which can correct for magnification and shear over the whole field, and (2) high resolution intrafield isotropic expansion and contraction using an array of local temperature control units. The authors have developed control algorithms for sub-5 nm overlay precision over up to four fields using thermomechanical simulations, and the authors have experimentally validated the approach. This research has the potential to significantly improve IL throughput without compromising nanoscale overlay. (C) 2016 American Vacuum Society.
R. Zhu, Brueck, S. R. J., Dawson, N., Busani, T., Joseph, P., Singhal, S., and Sreenivasan, S. V., “Scatterometry for nanoimprint lithography,” Journal of Vacuum Science & Technology B, vol. 34, no. 06K503, 2016. Publisher's VersionAbstract
Angular scatterometry is used to characterize the nanostructure parameters of two samples: a high dielectric contrast similar to 100-nm period Al wire-grid polarizer (WGP), and a low dielectric contrast similar to 130-nm period photoresist grating on a flexible polycarbonate substrate; both fabricated by nanoimprint lithography. The zero-order diffraction (reflection) is monitored for a large incident angle range from 8 degrees to 80 degrees. For the WGP, four wavelengths (244-, 405-, 633-, and 982-nm) are used to study the dependence of the scatterometry parametric determination as a function of the sample pitch to wavelength ratio (p/lambda: 0.41-0.1). A 4-nm thick native Al2O3 layer was added to the scatterometry simulation and dramatically improved the cross-correlations between results at the different wavelengths. For the photoresist samples, the scatterometry results at 405 nm are compared with atomic force microscopy measurements and the master grating structure. The scatterometry results are sensitive to inhomogeneity of the sample and show a capability for classifying different types of macroscopic defects. (C) 2016 American Vacuum Society.
2015
R. Agarwal, Jurney, P., Raythatha, M., Singh, V., Sreenivasan, S. V., Shi, L., and Roy, K., “Effect of Shape, Size, and Aspect Ratio on Nanoparticle Penetration and Distribution inside Solid Tissues Using 3D Spheroid Models,” Advanced Healthcare Materials, vol. 4, no. 15, pp. 2269-2280, 2015. Publisher's VersionAbstract
Efficient penetration and uniform distribution of nanoparticles (NPs) inside solid tissues and tumors is paramount to their therapeutic and diagnostic success. While many studies have reported the effect of NP size and charge on intratissue distribution, role of shape, and aspect ratio on NP transport inside solid tissues remain unclear. Here experimental and theoretical studies are reported on how nanoscale geometry of Jet and Flash Imprint Lithography-fabricated, polyethylene-glycol-based anionic nanohydrogels affect their penetration and distribution inside 3D spheroids, a model representing the intervascular region of solid, tumor-like tissues. Unexpectedly, low aspect ratio cylindrical NPs (H/D approximate to 0.3; disk-like particles, 100 nm height, and 325 nm diameter) show maximal intratissue delivery (>50% increase in total cargo delivered) and more uniform penetration compared to nanorods or smaller NPs of the same shape. This is in contrast to spherical NPs where smaller NP size resulted in deeper, more uniform penetration. Our results provide fundamental new knowledge on NP transport inside solid tissues and further establish shape and aspect ratio as important design parameters in developing more efficient, better penetrating, nanocarriers for drug, or contrast-agent delivery.
R. Zhu, Munoz, A., Brueck, S. J. R., Singhal, S., and Sreenivasan, S. V., “Metrology of a 50 nm HP wire-grid polarizer: a SEM-scatterometry comparison,” Metrology, Inspection, and Process Control for Microlithography XXIX. Proceedings of SPIE, San Jose, CA USA, 2015. Publisher's VersionAbstract
The capabilities and limitations of angular scatterometry for a structure pitch much less than the optical wavelength are experimentally investigated using a 100-nm pitch Al-wire grid polarizer on a SiO2 substrate. Three CW laser sources of wavelengths (244 nm, 405 nm and 633 nm) are used to measure the 0-order diffraction (reflection) across an incident angle range of 8 degrees to 80 degrees. The grating profile is defined by seven parameters (pitch, bottom linewidth, top linewidth, fused silica undercut, Al thickness, horizontal and vertical extent of top rounding). Rigorous coupled wave analysis (RCWA) simulations show that the reflectivity versus angle results are sensitive to changes in all of these parameters. The simulations act as a baseline library for the scatterometry measurements. Fitting the experimental curves with the corresponding simulation parameters results in a determination of the grating profile. As expected the shorter wavelength measurements provide the most sensitivity, but good precision is obtained at all three wavelengths. The measurements are in good agreement with destructive cross section scanning electron microscopy measurements.
A. Cherala, Shumaker, P., Mokaberi, B., Selinidis, K., Choi, B. J., Meissl, M. J., Khusnatdinov, N. N., LaBrake, D., and Sreenivasan, S. V., “Nanoscale Magnification and Shape Control System for Precision Overlay in Jet and Flash Imprint Lithography,” IEEE-ASME Transactions on Mechatronics, vol. 20, no. 1, 2015. Publisher's VersionAbstract
Jet and flash imprint lithography steppers have demonstrated unprecedented capability for patterning of sub-25-nm features for semiconductor manufacturing. A critical requirement for such patterning is the ability to overlay one layer of a device to a previously printed layer. In this paper, the design and development of a nanoprecision mask magnification/shape control system (MSCS) for the unique requirements of imprint-based overlay is presented. Imprint specific topics such as in-liquid overlay and distortions, and on-tool overlay metrology are discussed. The MSCS presented here has demonstrated 10-nm mix and match overlay (mean + 3 sigma) capability that approaches performance of state-of-the-art photolithography tools.
V. Singh, Agarwal, R., Jurney, P., Marshall, K., Roy, K., Shi, L., and Sreenivasan, S. V., “Scalable Fabrication of Low Elastic Modulus Polymeric Nanocarriers With Controlled Shapes for Diagnostics and Drug Delivery,” Journal of Micro and Nano-Manufacturing, vol. 3(1), no. 011002, 2015. Publisher's VersionAbstract
A new process, decoupled functional imprint lithography (D-FIL), is presented for fabricating low elastic modulus polymeric nanocarriers possessing Young's modulus of bulk material as low as sub-1 MPa. This method is employed to fabricate sub-50 nm diameter cylinders with >3: 1 aspect ratio and other challenging shapes from low elastic modulus polymers such as N-isopropylacrylamide (NIPAM) and poly(ethylene glycol) di-acrylate (PEGDA), possessing Young's modulus of bulk material <10 MPa which is cannot otherwise be imprinted in similar size and pitch using existing imprint techniques. Standard imprint lithography polymers have Young's modulus >1 GPa, and so these polymers used in nanocarrier fabrication in comparison have very low elastic modulus. Monodispersed, shape-and size-specific nanocarriers composed of NIPAM with material elastic modulus of <1MPa have been fabricated and show thermal responsive behavior at the lower critical solubility temperature (LCST) of similar to 32 degrees C. In addition, re-entrant shaped nanocarriers composed of PEGDA with elastic modulus <10 MPa are also fabricated. Nanocarriers fabricated from PEGDA are shown with model imaging agent and anticancer drug (Doxorubicin) encapsulated in as small as 50 nm cylindrical nanocarriers.
2014
A. Cherala, Choi, B. J., Lu, X., and Sreenivasan, S. V., “Active wafer shape modulation using a multi-actuator chucking system,” Precision Engineering, vol. 38, no. 4, pp. 783-790, 2014. Publisher's VersionAbstract
Jet and Flash Imprint Lithography has proven to be a viable alternative to optical lithography for fabrication of sub 30 nm nanostructures for large volume semiconductor manufacturing. Machine throughput, overlay and process defectivity that meet and exceed the International Technology Roadmap for Semiconductors (ITRS) are essential for commercial viability of any new lithography technology. Jet and Flash. Imprint Lithography uses an inkjet head to dispense a grid of liquid drops on the wafer surface to match the volume requirements of the pattern being imprinted. Wafer shape modulation has been shown to increase imprinting speed significantly by reducing air bubble trapping in the drop interstitial sites. A wafer shape modulation chuck that can address arbitrary field locations and sizes on a wafer with a novel actuation scheme that minimizes the number of actuators while increasing imprinting speed and reducing process defects significantly is presented. (C) 2014 Elsevier Inc. All rights reserved.
Y. Zhai, Mathew, L., Rao, R., Sreenivasan, S. V., Willson, G. C., and Banerjee, S. K., “Vertical finFET with Salicide Contact for Potential Power Applications,” ECS Journal of Solid State Science and Technology, vol. 3, no. 10, pp. 203-206, 2014. Publisher's VersionAbstract
We report a vertically diffused finFET with high-kappa gate dielectric and metal gate for potential power applications. Electron beam lithography and deep silicon reactive ion etching (RIE) were utilized to form the fins, followed by atomic layer deposition of Al2O3 high-k dielectric and TiN to form the metal gate. Self-aligned silicide (salicide) was formed on the top of the fins to lower the contact resistance. The devices exhibit excellent performance with steep sub-threshold swing (SS), low drain-induced barrier lowering (DIBL) and a high I-ON/I-OFF ratio. The breakdown voltage test (BVDSS) was also characterized for potential power applications. (C) 2014 The Electrochemical Society. All rights reserved.