Sourabh K. Saha

A surface diffusion model for Dip Pen Nanolithography line writing

Dip Pen Nanolithography is a direct write process that creates nanoscale dots and lines. Models typically predict dot and line size via assumption of constant ink flow rate from tip to substrate. This is appropriate for dot writing. It is however well-known, though models rarely reflect, that the ink flow rate depends upon writing speed during line writing. Herein, we explain the physical phenomenon that governs line writing and use this to model tip-substrate diffusion in line writing. We accurately predict (i) the increase in flow rate with writing speed and (ii) line width within 12.5%.

Characterization of the Dip Pen Nanolithography Process for Nanomanufacturing

Dip pen nanolithography (DPN) is a flexible nanofabrication process for creating 2-D nanoscale features on a surface using an “inked” tip. Although a variety of ink-surface combinations can be used for creating 2-D nanofeatures using DPN, the process has not yet been characterized for high throughput and high quality manufacturing. Therefore, at present it is not possible to (i) predict whether fabricating a part is feasible within the constraints of the desired rate and quality and (ii) select/design equipment appropriate for the desired manufacturing goals. Herein, we have quantified the processing rate, tool life, and feature quality for DPN line writing by linking these manufacturing metrics to the process/system parameters. Based on this characterization, we found that (i) due to theoretical and practical constraints of current technology, the processing rate cannot be increased beyond about 20 times the typical rate of ∼1 μm2 /min, (ii) tool life for accurate line writing is limited to 1–5 min, and (iii) sensitivity of line width to process parameters decreases with an increase in the writing speed. Thus, we conclude that for a high throughput and high quality system, we need (i) parallelization or process modification to improve throughput and (ii) accurate fixtures for rapid tool change. We also conclude that process control at high speed writing is less stringent than at low speed writing, thereby suggesting that DPN has a niche in high speed writing of narrow lines.

Design of a Compact Biaxial Tensile Stage for Fabrication and Tuning of Complex Micro- and Nano-scale Wrinkle Patterns

Wrinkling of thin films is a strain-driven process that enables scalable and low-cost fabrication of periodic micro- and nano-scale patterns. In the past, single-period sinusoidal wrinkles have been applied for thin-film metrology and microfluidics applications. However, real-world adoption of this process beyond these specific applications is limited by the inability to predictively fabricate a variety of complex functional patterns. This is primarily due to the inability of current tools and techniques to provide the means for applying large, accurate, and nonequal biaxial strains. For example, the existing biaxial tensile stages are inappropriate because they are too large to fit within the vacuum chambers that are required for thin-film deposition/growth during wrinkling. Herein, we have designed a compact biaxial tensile stage that enables (i) applying large and accurate strains to elastomeric films and (ii) in situ visualization of wrinkle formation. This stage enables one to stretch a 37.5 mm long film by 33.5% with a strain resolution of 0.027% and maintains a registration accuracy of 7 μm over repeated registrations of the stage to a custom-assembled vision system. Herein, we also demonstrate the utility of the stage in (i) studying the wrinkling process and (ii) fabricating complex wrinkled patterns that are inaccessible via other techniques. Specifically, we demonstrate that (i) spatial nonuniformity in the patterns is limited to 6.5%, (ii) one-dimensional (1D) single-period wrinkles of nominal period 2.3 μm transition into the period-doubled mode when the compressive strain due to prestretch release of plasma-oxidized polydimethylsiloxane (PDMS) film exceeds ∼18%, and (iii) asymmetric two-dimensional (2D) wrinkles can be fabricated by tuning the strain state and/or the actuation path, i.e., the strain history. Thus, this tensile stage opens up the design space for fabricating and tuning complex wrinkled patterns and enables extracting empirical process knowledge via in situ visualization of wrinkle formation.

Predicting the Quality of One-Dimensional Periodic Micro and Nano Structures Fabricated via Wrinkling

Wrinkling of thin films due to buckling-based surface instabilities is a fast and inexpensive technique for template-free fabrication of periodic micro/nano scale structures. Although one-dimensional (1-D) periodic micro and nano structures have been fabricated via wrinkling in the past, wrinkling is not yet appropriate for a manufacturing environment. This is because it is currently not possible to predict and control the quality of the fabricated patterns. Pattern quality is quantified in terms of the uniformity of the pattern, i.e., defect density within the patterned area. Herein, we (i) identify the process parameters that affect pattern quality, (ii) model the effect of these parameters on wrinkling quality and (iii) quantify the feasible operating region for a target pattern quality. During wrinkling, dislocation defects are observed due to local geometric imperfections such as voids or variations in the material properties. We have developed a finite element model of the wrinkling process that accounts for voids in the material. The wavelength and amplitude predictions of this model were found to be within ∼13% of the experimental observations. Also, it was found that below a threshold void size, the non-uniformity in the pattern due to voids decays with an increase in the applied compressive strain. This provides a practical means to minimize the non-uniformity in 1-D wrinkled patterns by increasing the compression. However, the defect density due to surface cracks increases with an increase in the compressive strains. Our analysis enables one to identify and predict the feasible operating region within which uniform 1-D patterns can be obtained, thereby improving manufacturability via wrinkling.Copyright

An Automated Stage for Scalable Imprinting of DNA Nanowires Based on a Self-Aligning Technique

Molecular combing is an established technique for aligning DNA nanowires on a surface. When performed on micro-patterned surfaces, this technique can be used to reliably align and stretch DNA nanowires across micro pillars. Imprinting of these aligned DNA nanowires is an affordable technique for fabrication of arrays of nano-scale channels across micro-scale reservoirs. In the past, DNA combing and imprinting (DCI) have been performed manually to fabricate polymer chips that are used in biomedical applications such as gene therapy and drug delivery studies. Automation of the DCI process is necessary to improve the yield and to scale-up production for these applications. However, existing automated techniques are not appropriate for DNA nanowire imprinting because these techniques cannot handle (i) fragile stamps and (ii) individual chip scale stamps of size ∼1 cm2. Herein, we present the design, fabrication and performance evaluation of an imprinting stage that enables (i) handling fragile stamps via low-cost equipment and (ii) production scale-up via simultaneous handling of multiple stamps. The stage is based on a self-aligning imprinting technique that passively aligns a stamp parallel to the substrate thereby enabling simultaneous imprinting of multiple stamps via a single stage. This self-alignment technique minimizes nanowire breakage by ensuring (i) minimal in-plane stamp motion during imprinting and (ii) that the contact forces do not exceed the weight of the stamp. Based on this technique we have designed/fabricated a stage that can simultaneously handle three stamps and is capable of further scale-up. The stage consists of a movable platform that is mounted on linear bearings and is actuated via a stepper motor. Stamps are loaded onto a holder that is mounted on the movable platform via kinematic couplings. This allows one to rapidly attach and detach the holder from the stage and also makes it possible to handle fragile stamps during loading/unloading. Imprinting of DNA nanowires with a manual stage has demonstrated the feasibility of the self-alignment scheme. Experiments that were performed to test the alignment capability of the stage verify that conformal stamp contact can be achieved across all three stamps even in the presence of an angular misalignment of 5° between the stamp and the glass slide. This ability to simultaneously align multiple stamps is a critical step in being able to scale-up and fully automate the DCI process.Copyright

Metric mapping: a new method to aid in the design of nanomanufacturing systems

The creation of new systems for nanomanufacturing can be catalysed by a knowledge of manufacturing systems. A method for comparing macro-scale manufacturing metrics and nanoscale manufacturing processes allows a design engineer to objectively evaluate similarities between macro-scale and nano-scale processes. The result of mapping manufacturing metrics is that a design engineer will have a better understanding of which currently-available manufacturing technology can be easily modified to enable a given nano-scale process. Furthermore, a design engineer will be able to clearly identify areas where new technology must be developed in order to satisfy the performance requirements for a nanomanufacturing process not met by existing technology. A metric mapping method for comparing manufacturing process metrics to nanomanufacturing processes has been developed, which can be used to aid in the design of systems for manufacturing nano-scale products; metric mapping has been utilised to help create a machine for enabling high-throughput dip pen nanolithography.