Integrated Lithographic Molding for Microneedle-Based Devices

This paper presents a new fabrication method consisting of lithographically defining multiple layers of high aspect-ratio photoresist onto preprocessed silicon substrates and release of the polymer by the lost mold or sacrificial layer technique, coined by us as lithographic molding. The process methodology was demonstrated fabricating out-of-plane polymeric hollow microneedles. First, the fabrication of needle tips was demonstrated for polymeric microneedles with an outer diameter of 250 mum, through-hole capillaries of 75-mum diameter and a needle shaft length of 430 mum by lithographic processing of SU-8 onto simple v-grooves. Second, the technique was extended to gain more freedom in tip shape design, needle shaft length and use of filling materials. A novel combination of silicon dry and wet etching is introduced that allows highly accurate and repetitive lithographic molding of a complex shape. Both techniques consent to the lithographic integration of microfluidic back plates forming a patch-type device. These microneedle-integrated patches offer a feasible solution for medical applications that demand an easy to use point-of-care sample collector, for example, in blood diagnostics for lithium therapy. Although microchip capillary electrophoresis glass devices were addressed earlier, here, we show for the first time the complete diagnostic method based on microneedles made from SU-8

Nanogap Device: Fabrication and Applications

A nanogap device as a platform for nanoscale electronic devices is presented. Integrated nanostructures on the platform have been used to functionalize the nanogap for biosensor and molecular electronics. Nanogap devices have great potential as a tool for investigating physical phenomena at the nanoscale in nanotechnology. In this dissertation, a laterally self-aligned nanogap device is presented and its feasibility is demonstrated with a nano ZnO dot light emitting diode (LED) and the growth of a metallic sharp tip forming a subnanometer gap suitable for single molecule attachment. For realizing a nanoscale device, a resolution of patterning is critical, and many studies have been performed to overcome this limitation. The creation of a sub nanoscale device is still a challenge. To surmount the challenge, novel processes including double layer etch mask and crystallographic axis alignment have been developed. The processes provide an effective way for making a suspended nanogap device consisting of two self-aligned sharp tips with conventional lithography and 3-D micromachining using anisotropic wet chemical Si etching. As conventional lithography is employed, the nanogap device is fabricated in a wafer scale and the processes assure the productivity and the repeatability. The anisotropic Si etching determines a final size of the nanogap, which is independent of the critical dimension of the lithography used. A nanoscale light emitting device is investigated. A nano ZnO dot is directly integrated on a silicon nanogap device by Zn thermal oxidation followed by Ni and Zn blanket evaporation instead of complex and time consuming processes for integrating nanostructure. The electrical properties of the fabricated LED device are analyzed for its current-voltage characteristic and metal-semiconductor-metal model. Furthermore, the electroluminescence spectrum of the emitted light is measured with a monochromator implemented with a CCD camera to understand the optical properties. The atomically sharp metallic tips are grown by metal ion migration induced by high electric field across a nanogap. To investigate the growth mechanism, in-situ TEM is conducted and the growing is monitored. The grown dendrite nanostructures show less than 1nm curvature of radius. These nanostructures may be compatible for studying the electrical properties of single molecule

Fabrication of High-Performance Probes for Atomic Force Microscope (AFM)

Atomic force microscope (AFM) is widely used for topographical structure characterization. However, one serious issue with AFM imaging is the intrinsic artifact in the AFM image when mapping a none-flat surface (e.g. a deep and narrow hole/trench) where the tip cannot fully follow the sample surface. The natural solution to overcome this issue is by using thin and high aspect ratio (HAR) tips that can follow the sample surface more precisely. This thesis focuses on the fabrication of HAR AFM probes. The HAR tips are obtained by modifying regular AFM tips having a pyramid shape. The high aspect ratio structure in silicon, sitting on top of a pyramid base, is created by a dry plasma etching process, so the key is to form a hard metal dot right on top of the pyramid tip apex to act as the mask for silicon etching. Three approaches were developed to form the hard mask metal nano-dot on the tip apex. The first method (Chapter 3) employed metal deposition steps with the regular tip mounted on a tilted surface, and its etching back to leave behind metal only at the tip apex (the metal on the sidewall of the pyramid was etched away). Since both metal film deposition and its etching, as well as the subsequent dry plasma etching of silicon using the metal as mask to form the HAR structure, can be carried out on an entire wafer of regular AFM tips, this process is a low-cost and high throughput batch process. The second method (Chapter 6) utilized focused ion beam (FIB). FIB has been extensively used to fabricate HAR tips by milling away the silicon surrounding the tip axis, leaving behind a thin pillar or sharp cone of silicon at the pyramid axis. However, the FIB milling time for each tip is long, leading to high cost. Our method used FIB to mill away only a very thin layer of metal film to leave behind a metal dot at tip apex, thus the expensive FIB machine time is greatly reduced. The third method (Chapter 6) also utilized Ga-ion FIB, but instead of milling a metal dot mask pattern, the Ga ions were implanted to the tip apex area to act as a mask since Ga metal is resistant to fluorine-based plasma etching. For the above three approaches, silicon etching is very critical, so Chapter 5 covers our effort in developing silicon etching recipes using a non-switching pseudo-Bosch process with C4F8-SF6 gas, with a goal of obtaining vertical sidewall profile needed for HAR, high selectivity to mask, and high etching rate. As well, the etched silicon structures must be further sharpened to reduce its apex radius to below 10nm. So, Chapter 4 covers the process optimization of the oxidation sharpening process that involves thermal oxidation and subsequent oxide etching by HF. It was found that 950°C is a suitable oxidation temperature, and the oxidation sharpening can be carried out more than once to improve tip sharpness. Lastly, inspired by the first approach described above, we also developed the fabrication process for “edge probe”, for which the tip apex sits right at the end of the cantilever, and thus the tip location can be precisely determined in the view of the integrated optical microscope in an AFM system. Our method involves angle evaporation of a hard mask layer onto the AFM probe, followed by silicon dry etching that etches away the area not covered by the metal layer, i.e., the shadow area of the pyramid-shaped tip

Nanofabrication and its application in developing high aspect ratio (HAR) and edge Atomic Force Microscopy (AFM) probes

This thesis focuses on nanofabrication and its applications which are related to producing atomic force microscope (AFM) probes. This thesis is divided into four chapters. The first chapter brings a preliminary introduction to nanofabrication. The second chapter reviews the history of AFM and fabrication process of AFM probe. Equipping with the basic knowledge, Chapter 3, the main chapter, goes to our work on the batch fabrication of high aspect ratio (HAR) AFM tips. Last but not least, Chapter 4 focuses on another work, batch fabrication of edge probes. In order to obtain a more accurate image of surfaces, high aspect ratio tips are needed to reach the bottom of very deep and narrow trenches. However, currently all commercial HAR tips are produced in a slow, high-cost (~5-20x that of regular AFM tips) way. We have developed a new method to batch fabricate HAR tips. In this fabrication, two kinds of hard masks were deposited at a specific angle followed by two etching processes (dry etching and wet etching respectively). As a result, a small piece of hard mask was formed just on the apex of pyramid tip, which would be the protection layer in the following RIE step. The batch and lithography-free process makes it an efficient and low-cost method. The controllable profile, radius of curvature and aspect ratio of tips can be easily obtained by adjusting gas ratio and etching time in RIE. All the parameters and results were demonstrated clearly assisted by images and schematics. For edge probes, our method to batch fabricate tips on the edge is introduced step by step as well. The objective of every step is presented in detail assisted with schematics and tables

Effect of Low Temperature on The Fabrication of Microring Resonator by Wet Etching

Research related to semiconductor devices often relies on wafer fabrication. The fabrication of Silicon (Si) based devices by anisotropic wet etching can be affected by many etching parameters such as etching temperature, crystal orientation and percent of composition. Most of the anisotropic wet etchings by KOH solution done before were conducted at temperature over 70°C. We found that the temperatures are not suitable to fabricate ring waveguide as the waveguide wall will collapse at such high temperature. This study reports the etching characteristics of Si in KOH solution with 35% concentration at the temperature below 70°C. The etched wafer is targeted to be the basic structure for Microring Resonators (MRRs) based devices. This technique provides not only lower cost as compared to other etching technique, but also simple preparation. We found that low temperature manage to mold a good ring waveguide with low tendency to form rectangular structure due to crystal orientation. At 40°C, the best waveguide formation was obtained with a smooth waveguide surface, experiencing an etching rate of 0.066 μ min-1 and an appreciable ring waveguide structure. The effect of the low temperature on the fabrication of the MRRs devices has been investigated and studied

Fabrication of Hollow Silicon Microneedle Arrays for Transdermal Biological Fluid Extraction

This thesis presents the research in the field of microelectromechanical systems with the specific aim of investigating a microneedle based transdermal skin fluid extraction concept. This work presents an innovative double-side Deep Reactive Ion Etching (DRIE) approach for producing hollow silicon microneedle arrays for transdermal biological fluid extraction. The microneedles are fabricated from a double side polished wafer to a shank height of 200-300 μm with 300 μm center-to-center spacing. Moreover, the in vivo testing results are provided as well. In this thesis, several microfabrication techniques are investigated, developed and applied in the fabrication process. The first chapter brings an overview of nano-/microfabrication and MEMS for biomedical applications (drug delivery and biofluid extraction). Furthermore, the fundamental background of skin structure and interstitial fluid (ISF) is introduced as well. The second chapter clearly illustrates three key techniques specifically employed in the microneedle fabrication process which are photolithography, wet etching and dry etching. The third chapter presents a detailed literature review of microneedles in terms of its general concepts, structures, materials and integrated fluidic system. Eventually, Chapter 4 introduces the details of our method to fabricate hollow silicon microneedle arrays step by step. SEM images and in vivo testing results confirm that hollow silicon microneedle arrays are not only sharp enough to penetrate the stratum corneum but also robust enough to extract ISF out of skin. Ongoing work will focus on the optimization of the assemble extraction apparatus and the capillary filling of the holes

Elastocapillary folding using stop-programmable hinges fabricated by 3D micro-machining

We show elasto-capillary folding of silicon nitride objects with accurate folding angles between flaps of 70.6$\pm$0.1{\deg} and demonstrate the feasibility of such accurate micro-assembly with a final folding angle of 90{\deg}. The folding angle is defined by stop-programmable hinges that are fabricated starting from silicon molds employing accurate three-dimensional corner lithography. This nano-patterning method exploits the conformal deposition and the subsequent timed isotropic etching of a thin film in a 3D shaped silicon template. The technique leaves a residue of the thin film in sharp concave corners which can be used as an inversion mask in subsequent steps. Hinges designed to stop the folding at 70.6{\deg} were fabricated batchwise by machining the V-grooves obtained by KOH etching in (110) silicon wafers; 90{\deg} stop-programmable hinges were obtained starting from silicon molds obtained by dry etching on (100) wafers. The presented technique is applicable to any folding angle and opens a new route towards creating structures with increased complexity, which will ultimately lead to a novel method for device fabrication.Comment: Submitted to a peer reviewed journa

Exploring the surface-enhanced Raman scattering (SERS) activity of gold nanostructures embedded around nanogaps at wafer scale: Simulations and experiments

A unique way of converting free space light into a local electromagnetic field in small spaces is via metallic nanostructuring. In this work fabrication, experimental characterization and simulation of surface-enhanced Raman scattering (SERS) active specimens based on Au nanostructures are discussed. We used displacement Talbot lithography (DTL) to fabricate silicon nano-wedge substrates with Au nanostructures embedded around their apices. After the ion beam etching process, a nanogap is introduced between two Au nanostructures templated over nano-wedges, yielding specimens with SERS characteristics. The Au nanostructures and the nanogaps have symmetric and asymmetric configurations with respect to the wedges. With this nanofabrication method, various wafer-scale specimens were fabricated with highly controllable nanogaps with a size in the order of 6 nm for symmetric gaps and 8 nm for asymmetric gaps. SERS characteristics of these specimens were analyzed experimentally by calculating their analytical enhancement factor (AEF). According to finite-difference time-domain (FDTD) simulations, the Raman enhancement arises at the narrow gap due to plasmonic resonances, yielding a maximum AEF of 6.9 × 106. The results highlight the SERS activity of the nanostructures and ultimately comply with reliable substrates for practical applications