Selective Resistive Sintering: A Novel Additive Manufacturing Process

Selective laser sintering (SLS) is one of the most popular 3D printing methods that uses a laser to pattern energy and selectively sinter powder particles to build 3D geometries. However, this printing method is plagued by slow printing speeds, high power consumption, difficulty to scale, and high overhead expense. In this research, a new 3D printing method is proposed to overcome these limitations of SLS. Instead of using a laser to pattern energy, this new method, termed selective resistive sintering (SRS), uses an array of microheaters to pattern heat for selectively sintering materials. Using microheaters offers significant power savings, significantly reduced overhead cost, and increased printing speed scalability. The objective of this thesis is to obtain a proof of concept of this new method. To achieve this objective, we first designed a microheater to operate at temperatures of 600⁰C, with a thermal response time of ~1 ms, and even heat distribution. A packaging device with electrical interconnects was also designed, fabricated, and assembled with necessary electrical components. Finally, a z-stage was designed to control the airgap between the printhead and the powder particles. The whole system was tested using two different scenarios. Simulations were also conducted to determine the feasibility of the printing method. We were able to successfully operate the fabricated microheater array at a power consumption of 1.1W providing significant power savings over lasers. Experimental proof of concept was unsuccessful due to the lack of precise control of the experimental conditions, but simulation results suggested that selectivity sintering nanoparticles with the microheater array was a viable process. Based on our current results that the microheater can be operated at ~1ms timescale to sinter powder particles, it is believed this new process can potentially be significantly quicker than selective laser sintering by increasing the number of microheater elements in the array. The low cost of a microheater array printhead will also make this new process affordable. This thesis presented a pioneering study on the feasibility of the proposed SRS process, which could potentially enable the development of a much more affordable and efficient alternative to SLS

Integrated movable micromechanical structures for sensors and actuators

Movable pin-joints, gears, springs, cranks, and slider structures with dimensions measured in micrometers have been fabricated using silicon microfabrication technology. These micromechanical structures, which have important transducer applications, are batch-fabricated with an IC-compatible process. The movable mechanical elements are built on layers that are later removed so that they are freed for translation and rotation. An undercut-and-refill technique, which makes use of the high surface mobility of silicon atoms undergoing chemical vapor deposition, is used to refill undercut regions in order to form restraining flanges. Typical element sizes and masses are measured in micrometers and nanograms. The process provides the tiny structures in an assembled form avoiding the nearly impossible challenge of handling such small elements individually

Cantilever beam microactuators with electrothermal and electrostatic drive

Microfabrication provides a powerful tool for batch processing and miniaturization of mechanical systems into dimensional domain not accessible easily by conventional machining. CMOS IC process compatible design is definitely a big plus because of tremendous know-how in IC technologies, commercially available standard IC processes for a reasonable price, and future integration of microma-chined mechanical systems and integrated circuits. Magnetically, electrostatically and thermally driven microactuators have been reported previously. These actuators have applications in many fields from optics to robotics and biomedical engineering. At NJIT cleanroom, mono or multimorph microactuators have been fabricated using CMOS compatible process. In design and fabrication of these microactuators, internal stress due to thermal expansion coefficient mismatch and residual stress have been considered, and the microactuators are driven with electro-thermal power combined with electrostatical excitation. They can provide large force, and in- or out-of-plane actuation. In this work, an analytical model is proposed to describe the thermal actuation of in-plane (inchworm) actuators. Stress gradient throughout the thickness of monomorph layers is modeled as linearly temperature dependent Δσ. The nonlinear behaviour of out-of-plane actuators under electrothermal and electrostatic excitations is investigated. The analytical results are compared with the numerical results based on Finite Element Analysis. ANSYS, a general purpose FEM package, and IntelliCAD, a FEA CAD tool specifically designed for MEMS have been used extensively. The experimental results accompany each analytical and numerical work. Micromechanical world is three dimensional and 2D world of IC processes sets a limit to it. A new micromachining technology, reshaping, has been introduced to realize 3D structures and actuators. This new 3D fabrication technology makes use of the advantages of IC fabrication technologies and combines them with the third dimension of the mechanical world. Polycrystalline silicon microactuators have been reshaped by Joule heating. The first systematic investigation of reshaping has been presented. A micromirror utilizing two reshaped actuators have been designed, fabricated and characterized

Mechanical properties of thin polysilicon films by means of probe microscopy

A new method for tensile testing of thin films is being developed. An electrostatic grip apparatus was designed and implemented to measure the elastic and ultimate tensile properties (Young's modulus, Poisson's ratio and tensile strength) of surface micromachined polysilicon specimens. The tensile specimens are 'dog-bone' shaped ending in a large 'paddle' for electrostatic gripping. The test section of the specimens is 400 micrometers long and with 2 micrometer X 50 micrometer cross section. The method employs Atomic Force Microscope (AFM) or Scanning Tunneling Microscope (STM) acquired surface topologies of deforming specimens to determine (fields of) strains. By way of the method of Digital Image Correlation (DIC), the natural surface roughness features are used as distributed markers. The effect of markers artificially deposited on the surface is examined computationally. Also the significance of other parameters on property measurements, such as surface roughness, has been examined computationally. Initial results obtained using the tensile test apparatus are presented

Doctor of Philosophy

dissertationNew hydrogel-based micropressure sensor arrays for use in the fields of chemical sensing, physiological monitoring, and medical diagnostics are developed and demonstrated. This sensor technology provides reliable, linear, and accurate measurements of hydrogel swelling pressures, a function of ambient chemical concentrations. For the first time, perforations were implemented into the pressure sensors piezoresistive diaphragms, used to simultaneously increase sensor sensitivity and permit diffusion of analytes into the hydrogel cavity. It was shown through analytical and numerical (finite element) methods that pore shape, location, and size can be used to modify the diaphragm mechanics and concentrate stress within the piezoresistors, thus improving electrical output (sensitivity). An optimized pore pattern was chosen based on these numerical calculations. Fabrication was performed using a 14-step semiconductor fabrication process implementing a combination of potassium hydroxide (KOH) and deep reactive ion etching (DRIE) to create perforations. The sensor arrays (2×2) measure approximately 3 × 5 mm2 and used to measure full scale pressures of 50, 25, and 5 kPa, respectively. These specifications were defined by the various swelling pressures of ionic strength, pH and glucose specific hydrogels that were targeted in this work. Initial characterization of the sensor arrays was performed using a custom built bulge testing apparatus that simultaneously measured deflection (optical profilometry), pressure, and electrical output. The new perforated diaphragm sensors were found to be fully functional with sensitivities ranging from 23 to 252 μV/V-kPa with full scale output (FSO) ranging from 5 to 80 mV. To demonstrate proof of concept, hydrogels sensitive to changes in ionic strength were synthesized using hydroxypropyl-methacrylate (HPMA), N,N-dimethylaminoethyl-methacrylate (DMA) and a tetra-ethyleneglycol-dimethacrylate (TEGDMA) crosslinker. This hydrogel quickly and reversibly swells when placed environments of physiological buffer solutions (PBS) with ionic strengths ranging from 0.025 to 0.15 M. Chemical testing showed sensors with perforated diaphragms have higher sensitivity than those with solid diaphragms, and sensitivities ranging from 53.3±6.5 to 271.47±27.53 mV/V-M, depending on diaphragm size. Additionally, recent experiments show sensors utilizing Ultra Violet (UV) polymerized glucose sensitive hydrogels respond reversibly to physiologically relevant glucose concentrations from 0 to 20 mM

Low pressure chemical vapor deposition of silicon nitride films from tridimethylaminosilane

In this study amorphous stoichiometric silicon nitride films were synthesized by low pressure chemical vapor deposition (LPCVD) using tri(dimethylamino) silane (TDMAS) and ammonia (NH3). The growth kinetics were determined as a function of temperature in the range of 650 - 900 °C, total pressure in the range of 0.15 - 0.60 Torr, and NH3/TDMAS flow ratio in the range of 0 - 10. At constant condition of pressure (0.5 Torr), TDMSA flow rate (10 sccm) and NH3 flow rate (100 sccm), the deposition rate of as-deposited silicon nitride films was found to follow an Arrehnius behavior in the temperature range of 650 - 800 °C with an activation energy of 41 ± 3 kcal mol-1. The film characterizations including compositional, structural, physical, optical and mechanical properties were determined by using XPS, RBS, X-ray diffraction, Nanospec interferometry, Ellipsometer, FTIR, UV Visible, as well as other techniques. The results demonstrated the feasibility of using TDMAS in the synthesis of high quality silicon nitride films by LPCVD

Low pressure chemical vapor deposition of boron nitride thin films from triethylamine borane complex and ammonia

Boron nitride thin films were synthesized on Silicon and quartz substrates by low pressure chemical vapor deposition using triethylamine-borane complex and ammonia as precursors. The films were processed at 550°C, 575°C and 600°C at a constant pressure of 0.05 Torr at different precursor flow rates and flow ratios. Several analytical methods such as Fourier transform infrared spectroscopy, x- ray photo-electron spectroscopy, ultra-violet/visible spectrophotornetry, ellipsometry, surface profilometry and scanning electron microscopy were used to study the deposited films. The films deposited were uniform, amorphous and the composition of the films varied with deposition temperature and precursor flow ratios. The stresses in the film were either mildly tensile or compressive. Dielectric constant characterization of LPCVD boron nitride was made using metal-insulator-semiconductor (MIS) and metal-insulator-metal (M IM) structures. The boron nitride films were stable and showed dielectric constant values between 3.8 and 4.7. The limitation of attaining lower values could be due to the presence of carbon as an impurity in the film and the presence of mobile charge carriers in the films as well as at the substrate-film interface

Load deflection analysis for determining mechanical properties of thin films with tensile and compressive residual stresses

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1995.Includes bibliographical references (leaves 24-25).by Mayank T. Bulsara.M.S

Silicon carbide process development for microengine applications : residual stress control and microfabrication

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004.Includes bibliographical references.The high power densities expected for the MIT microengine (silicon MEMS-based micro-gas turbine generator) require the turbine and compressor spool to rotate at a very high speed at elevated temperatures (1300 to 1700 K). However, the thermal softening of silicon (Si) at temperatures above 900 K limits the highest achievable operating temperatures, which in turn significantly compromises the engine efficiency. Silicon carbide (SiC) offers great potential for improved microengine efficiency due to its high stiffness, strength, and resistance to oxidation at elevated temperatures. However, techniques for microfabricating SiC to the high level of precision needed for the microengine are not currently available. Given the limitations imposed by the SiC microfabrication difficulties, this thesis proposed Si-SiC hybrid turbine structures, explores key process steps, identified, and resolved critical problems in each of the processes along with a thorough characterization of the microstructures, mechanical properties, and composition of CVD SiC. Three key process steps for the Si-SiC hybrid structures are CVD SiC deposition on silicon wafers, wafer-level SiC planarization, and Si-to-SiC wafer bonding. Residual stress control in SiC coatings is of the most critical importance to the CVD process itself as well as to the subsequent wafer planarization, and bonding processes since residual stress-induced wafer bow increases the likelihood of wafer cracking significantly. Based on CVD parametric studies performed to determine the relationship between residual stresses in SiC and H2/MTS ratio, deposition temperature, and HCl/MTS ratio, very low residual stress (less than several tens of MPa) in thick CVD SiC coatings (up to -50 pm) was achieved.(cont.) In the course of the residual stress study, a general method for stress quantification was developed to isolate the intrinsic stress from the thermal stress. In addition, qualitative explanations for the residual stress generation are also offered, which are in good agreement with experimental results. In the post-CVD processes, the feasibility of SiC wafer planarization and Si-to-SiC wafer bonding processes have successfully been demonstrated, where CVD oxide was used as an interlayer bonding material to overcome the roughness of SiC surface. Finally, the bonding interface of the Si-SiC hybrid structures with oxide interlayer was verified to retain its integrity at high temperatures through four-point flexural tests.by Dongwon Choi.Ph.D

MEMS-Based Silicon Nitride Thin Film Materials and Devices at Cryogenic Temperatures for Space Applications

Microshutter arrays, scheduled to be launched in 2011 as part of NASA's James Webb Space Telescope (JWST), will be the first micro-scale optical devices in outer space using MEMS technology. As the microshutter arrays consist of electrical and mechanical components and must operate in a cryogenic environment reliably over a 10 year mission lifetime, a fundamental challenge for the development of this device is to understand the mechanical behaviors of the micro-scale materials used and the possible failure mechanisms at 30 K. This thesis investigates the mechanical properties and reliability of low-stress LPCVD silicon nitride thin films, the structural materials of the microshutter arrays, at cryogenic temperatures. A helium-cooled cryogenic measurement setup installed inside a focused-ion-beam system is designed, implemented, and characterized to obtain a cryogenic environment down to 20 K. Resonating T-shaped cantilevers with different "milling masses" are used to measure the Young's modulus of silicon nitride thin films, while the fracture strength is characterized by bending tests of these beams. A passive high-sensitivity microgauge sensor based on displacement amplification is introduced to measure residual stress and coefficients of thermal expansion, which are critical for the device performance. To achieve accelerated fatigue study of the microshutter arrays, a novel mechanical-amplifier actuator is designed, fabricated, and tested to emulate their torsional operating stress. Furthermore, nano-scale tensile fatigue tests are demonstrated using similar mechanical-amplifier actuators. The research results of this thesis provide important thin film material parameters for the design, fabrication, and characterization of the microshutter arrays. Moreover, the presented test devices and experimental techniques are not limited for space applications only but can be extended for characterization of other thin film materials used in MEMS and microsystems