James M. Fitz-Gerald

New approach to laser direct writing active and passive mesoscopic circuit elements

We have combined some of the major positive advantages of laser-induced forward transfer (LIFT) and matrix-assisted pulsed laser evaporation (MAPLE), to produce a novel excimer laser driven direct writing technique which has demonstrated the deposition in air and at room temperature and with sub-10 μm resolution of active and passive prototype circuit elements on planar and nonplanar substrates. We have termed this technique MAPLE DW (matrix-assisted pulsed laser evaporation direct write) and present its historical evolution from pulsed laser deposition. This paper describes the simplistic approach to carry out MAPLE DW, gives experimental conditions, and physical characterization results for the deposition of NiCr thin film resistors, Au conducting lines, and multilayer depositions of Au conductors and BaTiO3 dielectrics to produce prototype capacitors. In general, the electrical properties of the materials deposited (conductivity, dielectric constant, and loss tangent) are comparable or superior to those produced by other commonly used industrial processes such as screen printing. The mechanism of the MAPLE DW process, especially the novel aspects making it a powerful approach for direct writing all classes of materials (metals, oxide ceramics, polymers and composites), is also described.

Engineered Interfaces for Adherent Diamond Coatings on Large Thermal-Expansion Coefficient Mismatched Substrates

Adhesion of thin or thick films on substrates is a critical issue in systems where the thermal-expansion coefficients of the coating and bulk material are significantly different from each other. The large mismatch of the expansion coefficients results in the generation of very high stresses in the coating that may lead to delamination, cracking, or other deleterious effects. A method to increase the adherence of diamond coatings on tungsten-carbide and stainless steel substrates is reported based on a substrate-modification process that creates a three-dimensional thermally and compositionally graded interface. Scratch and indentation tests on diamond-coated steel and tungsten-carbide samples did not exhibit film fracture at the interface and concomitant catastrophic propagation of interfacial cracks.

Direct writing of electronic and sensor materials using a laser transfer technique

We present a laser-based direct write technique termed matrix-assisted pulsed-laserevaporation direct write (MAPLE DW). This technique utilizes a laser transparentfused silica disc coated on one side with a composite matrix consisting of the materialto be deposited mixed with a laser absorbing polymer. Absorption of laser radiationresults in the decomposition of the polymer, which aids in transferring the solute to anacceptor substrate placed parallel to the matrix surface. Using MAPLE DW, complexpatterns consisting of metal powders, ceramic powders, and polymer composites weretransferred onto the surfaces of various types of substrates with <10 micron resolutionat room temperature and at atmospheric pressure without the use of masks.Current trends for developing advanced electronic andsensor systems place great emphasis in achieving per-formance levels generally associated with integratedcircuits. This requires further miniaturization, while en-hancing the functionality and reliability of existing sys-tems. New strategies are needed in order to eliminate thelong lead times required for the fabrication of prototypesand evaluation of new materials and designs. The use ofrapid prototyping techniques such as direct write, whichdo not need photolithographic processing, provide a so-lution to the above requirements. Direct write technolo-gies do not compete with photolithography for size andscale but rather add a complementary tool for specificapplications requiring rapid turnaround and/or patterniteration, conformal patterning, or modeling difficult cir-cuits. Examples of direct write technologies for fabricat-ing or modifying metallic interconnects and/or otherelectronic passive elements include ink jet printing,

Laser direct writing of phosphor screens for high-definition displays

A laser-based forward transfer direct writing technique was used to deposit phosphor powder screens for high-resolution display applications. With this technique, called matrix-assisted pulsed-laser evaporation direct write, dense oxide phosphor powders of Y2O3:Eu (red) and Zn2SiO4:Mn (green) were deposited on alumina and polymer substrates. All processing was performed in air at room temperature. Cathodoluminescent measurements showed that the luminous efficiency of the phosphor powders was not degraded by the deposition process. A 6×6 red and green matrix with pixel sizes of 100 μm (250 lines per inch) with a 100 μm spot size is demonstrated; however, with smaller spot sizes this technique is easily scalable to pixel sizes 2500 lines per inch).

Laser-based processing of polymer nanocomposites for chemical sensing applications

Pulsed laser deposition (PLD) has been used to fabricate polymer/carbon nanocomposite thin films for use in chemical sensors (chemiresistors). Ethylene vinyl acetate copolymer (EVA) films (undoped and 20% carbon by weight) were deposited using an ArF excimer laser (193 nm) at fluences between 150 and 300 mJ/cm2. The structure and morphology of the deposited films were characterized using Fourier transform infrared spectroscopy (FTIR), Raman scattering, and transmission and scanning electron microscopy (TEM). An analysis of the FTIR spectra indicates that a film deposited using an undoped EVA target is primarily polyethylene, suggesting that the acetate group is photochemically or photothermally removed from the starting material. Gas phase measurements of the laser-evaporated material using a quadrupole time of flight mass spectrometer confirm the production of the acetyl radical on the target surface. Analysis of TEM of films deposited using C-doped targets shows that the carbon black particles (initiall...

Significant reduction of cathodoluminescent degradation in sulfide-based phosphors

The degradation of cathodoluminescent (CL) brightness under prolonged electron-beam excitation of phosphors has been identified as one of the outstanding critical issues for flat-panel field-emission displays. In this letter, we have demonstrated that a TaSi2 coating on Y2O2S:Eu3+ phosphors substantially inhibits the cathodoluminescent degradation characteristics without reducing its efficiency. The coating was deposited by pulsed laser deposition of TaSi2 targets onto a fluidized bed containing phosphor particles. Cathodoluminescent degradation experiments conducted at 2 keV and at 150 μA/cm2, showed that the CL brightness decreased by more than 50% after a Coulomb load of 15 C/cm2 on the uncoated material. In contrast, the TaSi2-coated phosphor powders showed much less degradation, with CL brightness only decreasing by approximately 12% after electron irradiation with the same dose.

Particle formation and growth from ozonolysis of α‐pinene

Observations of particle nucleation and growth during ozonolysis of α-pinene were carried out in Calspans 600 m3 environmental chamber utilizing relatively low concentrations of α-pinene (15 ppb) and ozone (100 ppb). Model simulations with a comprehensive sectional aerosol model which incorporated the relevant gas-phase chemistry show that the observed evolution of the size distribution could be simulated within the accuracy of the experiment by assuming only one condensable product produced with a molar yield of 5% to 6% and a saturation vapor pressure (SVP) of about 0.01 ppb or less. While only one component was required to simulate the data, more than one product may have been involved, in which case the one component must be viewed as a surrogate having an effective SVP of 0.01 ppb or less. Adding trace amounts of SO2 greatly increased the nucleation rate while having negligible effect on the overall aerosol yield. We are unable to explain the observed nucleation in the α-pinene/ozone system in terms of classical nucleation theory. The nucleation rate and, more importantly, the slope of the nucleation rate versus the vapor pressure of the nucleating species would suggest that the nucleation rate in the α-pinene/ozone system may be limited by the initial nucleation steps (i.e., dimer, trimer, or adduct formation).

Laser processing of polymer nanocomposite thin films

Current biotechnology and sensor research has enhanced the drive to establish viable methods for depositing high-quality polymer thin films. In this research, thin films of poly(methyl methacrylate) (PMMA) were prepared by matrix-assisted pulsed-laser evaporation (MAPLE). Up to 2wt% of carbon nanotubes were subsequently added to MAPLE target systems for deposition of polymer nanocomposite films. Targets were ablated using a 248nm (KrF) laser at fluences ranging from 0.045to0.75J∕cm2. In addition, polymer concentration in MAPLE targets was varied between 1 and 5wt% relative to the matrix solvent, in this case toluene. Films were deposited on Si substrates at room temperature in an Ar atmosphere. Molecular-dynamics simulations of MAPLE were utilized for interpretation of experimental observations. Particularly, the ejection of large clusters consisting of both PMMA and toluene molecules was studied and related to the observed morphology of the deposited films.

High-resolution three-dimensional reconstruction: A combined scanning electron microscope and focused ion-beam approach

The ability to obtain three-dimensional information has always been important to gain insight and understanding into material systems. Three-dimensional reconstruction often reveals information about the morphology and composition of a system that can otherwise be obscured or misinterpreted by two-dimensional images. In this article, we describe tomographic measurements with 10nm scale resolution, combining focused ion-beam processing with field-emission scanning electron microscopy to obtain a series of high-resolution two-dimensional cross-sectional images. The images were then concatenated in a computer and interpolated into three-dimensional space to assess and visualize the structure of the material. The results of this research demonstrate the use of tomographic reconstruction of Si–Si∕Ge and θ′ Al2Cu samples to reproduce the three-dimensional morphology with sub-10nm resolution.

Synthesis of spherical luminescent particulate coatings

In this communication, we show the feasibility of the pulsed laser ablation technique to make very thin, uniform luminescent coatings on particulate systems. To deposit continuous particulate coatings, the laser-induced plume from the target comes in contact with an agitated bed of core particles (size 1 μm). The pressure and nature of the background gas (inert or active) controls the cluster size of the particles in the laser plume. Experiments were conducted for laser deposition of phosphor nanoparticles on SiO2 core particles by pulsed excimer laser (wavelength=248 nm and pulse duration=25 ns) irradiation of a yttria:Eu (Y2O3:Eu3+) sputtering target.