Wafer scale manufacturing of high precision micro-optical components through X-ray lithography yielding 1800 Gray Levels in a fingertip sized chip

We present a novel x-ray lithography based micromanufacturing methodology that offers scalable manufacturing of high precision optical components. It is accomplished through simultaneous usage of multiple stencil masks made moveable with respect to one another through custom made micromotion stages. The range of spectral flux reaching the sample surface at the LiMiNT micro/nanomanufacturing facility of Singapore Synchrotron Light Source (SSLS) is about 2 keV to 10 keV, offering substantial photon energy to carry out deep x-ray lithography. In this energy range, x-rays penetrate through resist materials with only little scattering. The highly collimated rectangular beam architecture of the x-ray source enables a full 4″ wafer scale fabrication. Precise control of dose deposited offers determined chain scission in the polymer to required depth enabling 1800 discrete gray levels in a chip of area 20 mm$^{2}$ and with more than 2000 within our reach. Due to its parallel processing capability, our methodology serves as a promising candidate to fabricate micro/nano components of optical quality on a large scale to cater for industrial requirements. Usage of these fine components in analytical devices such as spectrometers and multispectral imagers transforms their architecture and shrinks their size to pocket dimension. It also reduces their complexity and increases affordability while also expanding their application areas. Consequently, equipment based on these devices is made available and affordable for consumers and businesses expanding the horizon of analytical applications. Mass manufacturing is especially vital when these devices are to be sold in large quantities especially as components for original equipment manufacturers (OEM), which has also been demonstrated through our work. Furthermore, we also substantially improve the quality of the micro-components fabricated, 3D architecture generated, throughput, capability and availability for industrial application. Manufacturing 1800 Gray levels or more through other competing techniques is either limited due to multiple process steps involved or due to unacceptably long time required owing to their pencil beam architecture. Our manufacturing technique presented here overcomes both these shortcomings in terms of the maximum number of gray levels that can be generated, and the time required to generate the same

THE FABRICATION OF HIGH-ASPECT RATIO GRATINGS FOR TALBOT INTERFEROMETER WITH MEDICAL IMAGING APPLICATION.

X-ray Phase contrast-based Talbot interferometer creates high contrast between weak and strong absorbing materials, which makes it effective in imaging soft tissues. However, its performance is bounded by the aspect-ratio, features and symmetry of its gratings. For 40 KeV energy X-rays, the analyzer grating thickness should be 100 µm or more to achieve \u3e 90% absorption in order to obtain high contrast images. Moreover, the smaller period in grating is desired for higher resolution. Therefore, researchers are exploring various fabrication techniques to achieve greater aspect-ratio gratings. Utilizing modern LIGA techniques, the aspect-ratio of gratings can be improved with a simplified and precise fabrication process. This thesis focuses on the fabrication of gratings with aspect ratio of 25; 100 µm tall and 8 µm period with 50 % duty cycle. X-ray lithography, electroplating and micro-machining were used during the fabrication of these gratings. Also, a silicon nitride based membrane X-ray mask with grating patterns was fabricated to perform X-ray exposure. Multiple approaches were implemented to optimize the processing conditions and parameters for gratings fabrication. The thesis experimentally compared the adhesion of PMMA resist acting as a mold, in which metal gratings were electroplated, to Copper oxide and titanium oxide. For each of two oxides, wafers were prepared separately, starting with depositing copper (Cu) and titanium (Ti) as seed layers and later oxidizing them. Later, both the wafers were bonded with 2.5 mm thick PMMA resist wafer. They are further flycut down to 100 µm and later is exposed at and XRLM-1 beamline at CAMD/LSU. The resist development results are compared and adhesion was analyzed for both copper oxide and titanium oxide. The adhesion in PMMA resist was better to copper oxide layer in comparison to the titanium oxide. However, titanium oxide is preferable because PMMA molds are damaged during copper oxide etching. Copper oxide, unlike titanium oxide, is not conductive which prevents electroplating of gratings; therefore, etching of copper oxide is required. Finally, the wafers were electroplated to nickel and later the resist is stripped. We have achieved gratings with an aspect ratio of ~21 with 4.64 µm period and 100 µm height

Colloidal inorganic nanocrystal based nanocomposites: Functional materials for micro and nanofabrication

The unique size- and shape-dependent electronic properties of nanocrystals (NCs) make them extremely attractive as novel structural building blocks for constructing a new generation of innovative materials and solid-state devices. Recent advances in material chemistry has allowed the synthesis of colloidal NCs with a wide range of compositions, with a precise control on size, shape and uniformity as well as specific surface chemistry. By incorporating such nanostructures in polymers, mesoscopic materials can be achieved and their properties engineered by choosing NCs differing in size and/or composition, properly tuning the interaction between NCs and surrounding environment. In this contribution, different approaches will be presented as effective opportunities for conveying colloidal NC properties to nanocomposite materials for micro and nanofabrication. Patterning of such nanocomposites either by conventional lithographic techniques and emerging patterning tools, such as ink jet printing and nanoimprint lithography, will be illustrated, pointing out their technological impact on developing new optoelectronic and sensing devices. © 2010 by the authors

Nanoimprint Lithography Technology and Applications

Nanoimprint Lithography (NIL) has been an interesting and growing field in recent years since its beginnings in the mid-1990s. During that time, nanoimprinting has undergone significant changes and developments and nowadays is a technology used in R&D labs and industrial production processes around the world. One of the exciting things about nanoimprinting process is its remarkable versatility and the broad range of applications. This reprint includes ten articles, which represent a small glimpse of the challenges and possibilities of this technology. Six contributions deal with nanoimprint processes aiming at specific applications, while the other four papers focus on more general aspects of nanoimprint processes or present novel materials. Several different types of nanoimprint processes are used: plate-to-plate, roll-to-plate, and roll-to-roll. Plate-to-plate NIL here also includes the use of soft and flexible stamps. The application fields in this reprint are broad and can be identified as plasmonics, superhydrophibicity, biomimetics, optics/datacom, and life sciences, showing the broad applicability of nanoimprinting. The sections on the nanoimprint process discuss filling and wetting aspects during nanoimprinting as well as materials for stamps and imprinting

Developing defect-tolerant demolding process in nanoimprint lithography

Demolding, the process to separate stamp from molded resist, is most critical to the success of ultraviolet nanoimprint lithography (UV-NIL). In the present study we first investigated adhesion and demolding force in UV-NIL for different compositions of a model UV-curable resist containing a base (either tripropyleneglycol diacrylate with shorter chain length or polypropyleneglycol diacrylate with longer chain length), a cross-linking agent (trimethylolpropane triacrylate) and a photoinitiator (Irgacure 651). The demolding force was measured using a tensile test machine after imprinting the UV resist on a silicon stamp. In general, the shorter monomer shows a larger demolding force. Decreasing the cross-linking agent content from 49 to 0 wt% results in a decreased adhesion force at the resist/stamp interface thereby facilitating the demolding. Demolding stress in general is mainly generated due to shrinkage of the resist in the UV curing step and also adhesion and friction at the stamp/resist interface in the subsequent demolding step. In the second part of this study the effect of resist compositions on the stress generation was studied by numerical simulation of the curing and demolding steps in UV-NIL. Input parameters required for the simulation were determined experimentally. As the cross-linking agent content increases the fracture strength of the resist also increases. At the same time, shrinkage stress due to cure and also adhesion at the stamp/resist interface both increase. By normalizing the overall maximum local stress by the fracture stress of the resist, we found that there is an optimum for the cross-linking agent content that leads to the most successful imprinting. In the third part of our study a simple method was developed to obtain the polymerization shrinkage stress exerted on the sidewalls of resist/stamp interface in UV NIL. This method is based on the measurements of demolding force which is the sum of adhesion and friction forces. The mean polymerization shrinkage stress on the sidewalls can readily be decoupled from overall demolding force by independently measuring the friction coefficient, adhesion force, and geometries of stamp structures. The polymerization shrinkage stress on the sidewalls is overall larger than adhesion and increases by adding more cross-linking agent to the resist composition

Soft Micro-Channels For Cell Culturing And Migration Studies

Various techniques and methods have been studied and developed to aid nerve regeneration and repairing nerve injuries. Among all, nerve grafting is the gold standard for bridging the gap between the injured nerve stumps. Despite the advantages of this technique, there are also various drawbacks that have encouraged the exploration of alternative, less invasive methods for promoting nerve regeneration. In this thesis, we have fabricated soft micro-channels for cell culturing and migration studies which could act as an interface capable of long-term, reliable, and high-resolution stimulation device for nerve regeneration. Micro-channels fabrication is performed using a ombination of photolithography technique and physical vapor deposition (PVD) methods. Initially, the surfaces of the micro-channels are treated with oxygen plasma to convert the surface of PDMS from hydrophobic to hydrophilic and to further provide an optimal environment for cells to adhere and grow. Next, in vitro studies were performed on the fabricated micro-channels to demonstrate feasibility of the platform to promote adherence and growth of PC12 cells (cell line derived from a pheochromocytomas of the rat adrenal medulla)

Traceable Standard for Sub - 100nm Metrology

As we approach the 65nm technological node, transistor gates with dimensions of the order of 40nm are being manufactured. As the device performance is directly related to the dimensions of the gate, critical dimension (CD) control becomes an important part of the fabrication process. Characterization of these small feature size, generally referred to as Metrology, is an indispensable ingredient of the semiconductor manufacturing processes. Metrology relies not only on the precision, but also the accuracy of commercially used metrology tools like the CD-SEM. To facilitate the magnification calibration of the CD-SEM, an easy access to standard reference artifact traceable to international specifications is an added advantage. Considerable literature is available for CD-SEM, which relies on in-house artifacts or general test objects. The absence of commercially available artifacts hinders evaluation of different CD-SEM. The objective of this abstract is to introduce the fabrication and characterization of artifacts for the sub-100nm metrology, which can be made available in wafer form at low cost. In this work, artifacts have been designed and fabricated for precise magnification calibration of the CD-SEM. The designing of the artifacts takes into account the proximity effect, a problem associated with the e-beam exposure, to produce dense grid type structure in the sub-100nm region. The structures are fabricated using the e-beam lithography tool, operated at 50KeV. The artifacts have been fabricated on a thin layer of negative resist HSQ spun on silicon substrate. Subsequent development in 0.26N TMAH gives a structure on silicon wafer, thereby eliminating contamination issues. Furthermore, characterization of the artifacts for line pitch determination is carried out using “Measure” (Spectel Corp.), which provides an absolute calibration of the image pixel size that can then be used to measure other features. The low values for the line edge roughness (LER) further facilitate precise linewidth metrology.\u3e/p\u3

FABRICATION OF MAGNETIC TWO-DIMENSIONAL AND THREE-DIMENSIONAL MICROSTRUCTURES FOR MICROFLUIDICS AND MICROROBOTICS APPLICATIONS

Micro-electro-mechanical systems (MEMS) technology has had an increasing impact on industry and our society. A wide range of MEMS devices are used in every aspects of our life, from microaccelerators and microgyroscopes to microscale drug-delivery systems. The increasing complexity of microsystems demands diverse microfabrication methods and actuation strategies to realize. Currently, it is challenging for existing microfabrication methods—particularly 3D microfabrication methods—to integrate multiple materials into the same component. This is a particular challenge for some applications, such as microrobotics and microfluidics, where integration of magnetically-responsive materials would be beneficial, because it enables contact-free actuation. In addition, most existing microfabrication methods can only fabricate flat, layered geometries; the few that can fabricate real 3D microstructures are not cost efficient and cannot realize mass production. This dissertation explores two solutions to these microfabrication problems: first, a method for integrating magnetically responsive regions into microstructures using photolithography, and second, a method for creating three-dimensional freestanding microstructures using a modified micromolding technique. The first method is a facile method of producing inexpensive freestanding photopatternable polymer micromagnets composed NdFeB microparticles dispersed in SU-8 photoresist. The microfabrication process is capable of fabricating polymer micromagnets with 3 µm feature resolution and greater than 10:1 aspect ratio. This method was used to demonstrate the creation of freestanding microrobots with an encapsulated magnetic core. A magnetic control system was developed and the magnetic microrobots were moved along a desired path at an average speed of 1.7 mm/s in a fluid environment under the presence of external magnetic field. A microfabrication process using aligned mask micromolding and soft lithography was also developed for creating freestanding microstructures with true 3D geometry. Characterization of this method and resolution limits were demonstrated. The combination of these two microfabrication methods has great potential for integrating several material types into one microstructure for a variety of applications

Fabrication of semiconductor nanowire multifunctional devices

Portable multi-functional devices can play a major role in the new age society embracing internet-of-things (IoT). Being able to perform primary functions such as sensing and secondary functions such as storing information is quite critical when out of connectivity. However, such bespoke devices are almost unheard of as it is very difficult to fabricate it due to several factors such as device architecture, dimension, scalability, and parasitic effects. This work describes the fabrication and characterization of a multi-functional device that acts an ultra-sensitive pressure sensor but is also capable of storing that information for a prolonged period. Both sensitivity and charge storage ability are attributed to the inclusion of one-dimensional (1-D) nanostructures. The alternating crystal phases in the as-grown gold (Au) catalyzed GaAs and self-induced AlGaAs/GaAs nanowires (NWs) were used in our case. This thesis discusses the fabrication, growth, characterization, integration and electrical testing involved to produce the multi-functional device. Bespoke nanowires were grown on a template prepared using a combination of nanosphere (NSL) and nanoimprint lithography (NIL) which provided a reproducible large-area periodic array of growth site at a relatively low cost. The inclusion of these NWs in the polymer helps enhance the relative permittivity of the host polymer by a factor of 40 making it an almost-perfect dielectric for a capacitive pressure sensor. NWs also acted as charge storage nodes allowing to extend the functionality. The technique consists of creating nanoholes in silicon dioxide (SiO2) to expose the silicon Si (111) beneath where self-induced NWs can nucleate, while nanodots deposited onto the Si (111) surface serve as catalyst seeds. For Au-catalysed NWs, a monolayer of self-assembled polystyrene nanospheres (PNS 300 nm) was created on a 2 inch Si wafer by spin coating and later etched for a short time before a very thin Au-catalyst layer was deposited. In turn, for self-induced, PNS monolayer was created onto a SiO2-Si substrate. A longer etch was required to reduce PNS diameter significantly to leave relatively larger spacing where chromium is blanket deposited. PNS were lifted off by sonicating the samples in toluene produce the periodic arrays of nanodots and nanoholes, respectively. The underlying SiO2 was etched further through the nanoholes to uncover the Si below. 200 nm holes and 30-70 nm dots were demonstrated through the bespoke methods. The patterned substrates served as master templates, subsequently copied using polydimethylsiloxane (PDMS) to produce a flexible stamp for nanoimprint lithography. A bi-layer resist lift off process was developed to print the replicated nanodots or nanoholes on large-area substrates onto which GaAs NWs were subsequently grown. GaAs NWs were extracted and mixed in PMMA to produce a composite dielectric which was sandwiched between electrodes to act as a capacitor. An order of magnitude increase in relative permittivity (ϵr) is observed after the addition of the NWs allowing a high signal to noise ratio output on the application of pressure. This is due to the addition of higher permittivity nano-filler in the matrix. Furthermore, it was demonstrated that encapsulated high aspect ratio NWs in a host (polymer in this case) can be integrated in devices to improve existing functionality. Devices were successfully fabricated for pressure sensing and memory using the above described low-cost high-volume process with high sensitivity and large memory window, respectively. This demonstration is one of the first steps in enabling low cost electronics without compromising on performance which is imperative for IoT

Quantum Transport Characterizations in Selective-Area Grown InGaAs Nanowire Networks

Selective-area-grown (also referred to as ‘templated’) semiconductor nanostructures have gen- erated considerable research interest in recent years as potential vehicles for the investigation of fundamental quantum phenomena as well as towards scalable networks for deployment in future quantum computers. Such structures made from III-V semiconductors, which can play host to effects such as strong spin-orbit interaction and Fermi-level pinning of the conduction band at the interface, are of interest due to the fast spin precession as well as being potential vehicles for realization of non-trivial topological states of matter. This nascent technology has relied on the bridging of growth techniques, nano-fabrication, low-temperature electronic transport studies, and compositional analysis during its development. The required expertise involved has resulted in fruitful collaborations between a number of re- search groups in different institutions, which has enabled steady progress. Devices have been fabricated on quasi-1D nanostructures comprised of InGaAs nanowires grown atop defect-free GaAs nanomembranes, first with bulk-doping of Si + donors in the InGaAs wires in order to demonstrate a proof-of-principle for the system. Later, devices utilizing modulation doping strategies in order to reduce impurity scattering - while still providing the necessary additional carriers for transport - have been realized, measured, and analysed. Devices with branched geometries have been produced and quantum intereference effects have been observed across the junctions. Important transport parameters such as mean free paths, coherence lengths and spin-orbit lengths have been extracted in order to characterize the structures and motivate the next steps along the evolution of the systems. Novel fabrication techniques have been developed and employed in order to investigate and control various aspects of the structures, including carrier density and electric fields across the wires