Microfabrication of plasma nanotorch tips for localized etching and deposition

Journal ArticleAbstract-We present the microfabrication and initial testing of an AFM-tip like device, or nanotorch, that is capable of generating a very localized microplasma at its tip. The submicron region near its tip provides a unique manufacturing environment where new methods for controlled direct-write micro and nanofabrication can be tested. The device has been fabricated using both surface and bulk micromaching techniques. We demonstrated both localized submicrometer oxidazion patterning and imaging with the same device. Preliminary experiments have also been carried out demonstrating localized plasma etching of a polymer surface at atmospheric conditions with an AC voltage of 1000V

Microfluidics and Nanofluidics: Science, Fabrication Technology (From Cleanrooms to 3D Printing) and Their Application to Chemical Analysis by Battery-Operated Microplasmas-On-Chips

The science and phenomena that become important when fluid-flow is confined in microfluidic channels are initially discussed. Then, technologies for channel fabrication (ranging from photolithography and chemical etching, to imprinting, and to 3D-printing) are reviewed. The reference list is extensive and (within each topic) it is arranged chronologically. Examples (with emphasis on those from the authors’ laboratory) are highlighted. Among them, they involve plasma miniaturization via microplasma formation inside micro-fluidic (and in some cases millifluidic) channels fabricated on 2D and 3D-chips. Questions addressed include: How small plasmas can be made? What defines their fundamental size-limit? How small analytical plasmas should be made? And what is their ignition voltage? The discussion then continues with the science, technology and applications of nanofluidics. The conclusions include predictions on potential future development of portable instruments employing either micro or nanofluidic channels. Such portable (or mobile) instruments are expected to be controlled by a smartphone; to have (some) energy autonomy; to employ Artificial Intelligence and Deep Learning, and to have wireless connectivity for their inclusion in the Internet-of-Things (IoT). In essence, those that can be used for chemical analysis in the field for “bringing part of the lab to the sample” types of applications

Fabrication of localized plasma gold-tip nanoprobes with integrated microchannels for direct-write nanomanufacturing

pre-printWe present the microfabrication and characterization of an AFM-tip like device with integrated gas delivery microchannel for the generation of localized microplasmas. The device plasma is generated within a submicron region around its tip for direct-write micro and nanofabrication. The device is fabricated by forming a tall, sharp micromolded gold tip in a KOH etched inverted pyramid followed by thermo-compression bonding and consecutive tip transfer, microfluidic channel patterning and formation of supporting cantilever beam. The tall tip overcomes the height problems of previous designs. Preliminary experiments have been carried out demonstrating the generation of localized microplasma at atmospheric conditions with 1,000V AC stimulation. By mounting the device to a commercialized AFM station and operated in tapping mode, imaging with the same device has also been demonstrated

GaN/AlGaN Avalanche Photodiode Detectors for High Performance Ultraviolet Sensing Applications

The shorter wavelengths of the ultraviolet (UV) band enable detectors to operate with increased spatial resolution, variable pixel sizes, and large format arrays, benefitting a variety of NASA, defense, and commercial applications. AlxGa1-xN semiconductor alloys, which have attracted much interest for detection in the UV spectral region, have been shown to enable high optical gains, high sensitivities with the potential for single photon detection, and low dark current performance in ultraviolet avalanche photodiodes (UV-APDs). We are developing GaN/AlGaN UV-APDs with large pixel sizes that demonstrate consistent and uniform device performance and operation. These UV-APDs are fabricated through high quality metal organic chemical vapor deposition (MOCVD) growth on lattice-matched, low dislocation density GaN substrates with optimized material growth and doping parameters. The use of these low defect density substrates is a critical element to realizing highly sensitive UV-APDs and arrays with suppressed dark current under high electric fields.Optical gains greater than 5X10 (exp 6) with enhanced quantum efficiencies over the 350-400 nm spectral range have been demonstrated, enabled by a strong avalanche multiplication process. Furthermore, we are developing 6X6 arrays of devices to test high gain UV-APD array performance at ~355 nm. These variable-area GaN/AlGaN UV-APD detectors and arrays enable advanced sensing performance over UV bands of interest with high resolution detection for NASA Earth Science applications

Doctor of Philosophy

dissertationMicroplasmas are currently used in displays, two-terminal breakdown switches, light sources, and medical instruments. They can also be used in miniaturized particle accelerators, micro-X-ray generators, UV and extreme UV sources, gas sensors, and in micropropulsion thrusters. They are also excellent candidates for applications in harsh environments that usually lead to the breakdown of silicon electronics. Here we develop their unique applications in X-band microwave analog and digital devices and circuits. To enable these applications, we identified a breakdown region, called sub-Paschen regime that enables generation of atmospheric plasmas at low voltages. The sub-Paschen regime, involves devices with a breakdown gap below 10 m in 1 atmosphere in air. This newly discovered operation regime enabled us to design plasma devices with relatively low operation voltages of 50-100 V. We developed microplasma devices similar to metal oxide semiconductor field effect transistors (MOSFETs) with drain, source, and gate regions that used plasma channels for switching or amplification. The gate field effect was successfully tested under both direct current (dc) and alternating current (ac) excitations. A drain current modulation frequency up to 7 GHz was obtained. Additionally, we implemented logic gates with microplasma devices to realize simple Boolean logic operations including OR, AND, NOT, and XOR. The gates were then combined to obtain a 1-bit half-adder circuit. The MOPFET developed in this work achieved 3x reduction in the breakdown (device turn-on) voltage by operating in the sub-Paschen regime. In addition to the scaling in breakdown voltage, the microplasma field effect transistors (MOPFETs) are at least 50x smaller compared to plasma transistors reported in the past. The smallest MOPFET used in this work had a source-drain gap of 1 μm and showed unprecedented functionalities derived from plasmas at a microscale

Microplasma chemical reaction enhancement by laser modification of dielectric surface topography

A newly developed etching technique, infrared laser ablation, for microplasma device fabrication is introduced. The ablated surface provides a topography that is distinct from the surface created by conventional techniques. Chemical, optical, and electrical experiments have been conducted to observe the difference in performance between devices fabricated with the conventional and with the new technique. Carbon dioxide (CO2) dissociation and ozone (O3) generation have been observed to verify the difference. Using the new laser ablation technique, CO2 dissociation energy efficiency has been increased from 13.5 ± 3.1% to 17.4 ± 5.7%. Furthermore, introducing 10% H2 (by vol.) into CO2 with the laser-ablated device has increased the energy efficiency to 23.5 ± 4.6%. In short, total energy efficiency increase of ~75% has been achieved by combining microplasmas with the new ablation technique and the H2 mixing. Optical emission spectroscopy observation shows that the CO2+ Fox-Duffendack-Barker system is dominant at low H2 flow rates, but the CO Angstrom bands start to dominate as H2 in the reactant mixture composition is increased. Using the residual gas analyzer (RGA), mass 30 (ethane or formaldehyde) and mass 46 (ethanol or formic acid) have been observed when H2 is mixed into the CO2 microplasmas. Calculated from the RGA signal, the maximum amount of ethanol generated is ~0.4 sccm when 100 sccm (80% CO2 and 20% H2 by vol.) is flowed into a single microplasma device (“chip”). Using the laser-ablated device, the efficiency of generating O3 has been increased by 7-11% depending on the flow rate. Microcavities within the microchannel generated by laser ablation have been observed, and the average cavity diameter has been calculated to be ~33 μm, with cavity density of ~300 mm-2. Intensified charge coupled device (ICCD) images of these cavities indicate that they discharge at lower applied voltage, while the observed optical emission intensity has been measured at ~2 times higher than typical microplasma regions at any given voltage. Furthermore, the laser-ablated device that contains cavities has higher electrical conductivities. Stark broadening has been measured, and the electron density has been calculated to be 1.2×10^16 ± 0.8×10^15 cm-3 and 1.1×10^16 ± 0.8×10^15 cm-3 for the laser-ablated and powder-ablated chips, respectively. Current-voltage (i.e. I-V) characteristics of laser-ablated chips show ~6% lower breakdown voltage. Also, higher current, compared to that of powder-ablated chips, at any given voltage has been observed for the laser-ablated chips. Owing to the higher surface area of laser-ablated chips, these electrical observations agree with increased field emission effect. As the dimensions of individual cavities will play an important role in further optimizing the plasma-surface interaction, production of uniform cavities has been attempted. Uniform truncated upside-down conical shapes with bottom and top diameters of ~150 μm and ~300 μm, respectively, have been fabricated. The laser ablation technique also has shown procedural advantages over the conventional technique. From a new microchannel design to a complete microplasma chip, micropowder ablation takes ~150 hours, whereas laser ablation technique requires only ~27 hours. Furthermore, no need for consumable chemicals, such as photoresist or silicone molds, makes the laser ablation technique a safer and more economical option as a surface ablation tool for microplasma production

The Journal of Microelectronic Research 2008

https://scholarworks.rit.edu/meec_archive/1016/thumbnail.jp

The study and characterisation of plasma microfluidic devices

Controlling the behaviour of atmospheric pressure plasmas and their interaction with polymeric materials is of major interest for surface modification applications across multidisciplinary fields intersecting biomedical engineering, bio-nanoengineering, clinical/medical science, material science and microelectronics. The aim of the present work is to investigate the behaviour of atmospheric pressure dielectric barrier discharges in closed systems (microfluidic devices) and open systems (glass capillary devices) and their polymer-surface interactions. Atmospheric pressure microplasma jets operating in helium gas have been used to modify locally the surface energy of polystyrene (PS) and to interact directly with the surface of analytes using a novel plasma assisted desorption ionisation (PADI) method causing desorption and ionization to occur. Although atmospheric pressure micro-jets are now widely studied for the treatment of materials there is still a lack of understanding of the fundamental plasma-surface processes. A number of recent studies using plasma micro-jets for the surface modification of polymerics have used systems in which the emerging plume impinges directly the substrate head-on. Here, by placing the micro-jet side-on to the substrate we can observe how different flow regions of the jet affect the sample, allowing individual effects to be seen. In addition, this configuration may prove an efficient way of treating samples with reduced or no surface damage. These conclusions are considered to be an important contribution to the study of complex mechanisms underpinning the behaviour of radicals and reactive species in surface modification processes of polymeric materials. The study of the behavioural mechanism involved in the plasma was done using various diagnostic techniques such as electrical measurements, optical emission spectroscopy (OES), Time-averaged and time-resolved ICCD Optical Imaging and Schlieren Photography. The filamentary discharge mode was observed in bonded microchannels using metallic and liquid-patterned electrodes. The treated surfaces were characterised using various techniques such as X-ray photoelectron spectroscopy (XPS), Atomic Force Microscopy (AFM), Optical profiling measurements and Water Contact Angle (WCA) measurements. Schlieren photography has been used to indentify regions of laminar (pre-onset of visual instability) and turbulent flows (post-onset of visual instability) in the exiting gas stream and the nature of their interaction with the substrate surface. The length of both regions varies depending on operating parameters such as frequency, applied voltage and flow rate. WCA results from treated polystyrene (PS) samples exposed directly facing the microjet reveals a change from hydrophobic (high contact angle) to a hydrophilic (low contact angle) surface with substantial reductions in WCA (~ 50 to 60 °) occurring in downstream regions where the turbulent gas mixed with air impinges the substrate surface. In contrast, only small changes in WCA (~ 10 to 20 °) occur in regions where the gas flow is laminar. AFM imaging of treated PS samples reveal holes and ripple like effect with a much larger area than that of the capillary seen on treated samples positioned “head-on” and directly facing the sample but this was not seen using the side-on configuration. The results indicate that excited air species (either mixed or entrained in the He gas flow) which exist only in regions of turbulence are the main agents causing surface covalent bond breaking leading to surface modification. This thesis reports on atmospheric pressure microdischarges and their applications, a brief summary of work done so far including major results, using new and existing technologies including those under development in terms of design, properties and working conditions

Confinement of microplasma in silicon trenches with widths as small as 2.5 μm

In plasma science, the ability to confine plasma in cavities with characteristic dimensions less than 1 μm would represent a major milestone. In this thesis, realization of microplasma inside channel devices with the characteristic dimensions of 5 μm and 2.5 μm is discussed. Data collected during the characterization of 5 μm devices are consistent with Paschen’s curve. Operating devices with a characteristic dimension of 2.5 μm are also introduced in this thesis. Finally, a few potential strategies for creating “nanoplasma” devices are discussed