Islam A. Salama

Laser synthesis of carbon-rich SiC nanoribbons

A nanosecond pulsed laser direct-write and doping (LDWD) technique is used for the fabrication of carbon-rich silicon carbide nanoribbons heterostructure in a single crystal 4H–SiC wafer. Characterization by high-resolution transmission electron microscope and selected area electron diffraction pattern revealed the presence of nanosize crystalline ribbons with hexagonal graphite structure in the heat-affected zone below the decomposition temperature isotherm in the SiC epilayer. The nanoribbons exist in three layers each being approximately 50–60 nm thick, containing 15–17 individual sheets. The layers are self-aligned on the (0001) plane of the SiC epilayer with their c axis at 87° to the incident laser beam. The LDWD technique permits synthesis of heterostructured nanoribbons in a single step without additional material or catalyst, and effectively eliminates the need for nanostructure handling and transferring processes.

Modelling of microvia drilling with a Nd : YAG laser

Drilling of microvias with Nd : YAG laser of wavelength 1.06 µm is studied for polymer materials used in high density electronic packaging applications. Generally thermal ablation is the dominant mechanism for such polymer materials in the nanosecond and microsecond laser pulse regimes. A transient thermal model is developed to describe the drilling process. Since a fraction of the laser energy is absorbed inside the polymer layer, volumetric heating occurs and an overheated small metastable region can form inside the layer. Thus the drilling mechanism involves thermal phase change, chemical decomposition and possibly explosion due to rapid thermal expansion and vaporization inside the polymer material. The volumetric heating can cause large thermal stresses inside the material, resulting in convex forms at the free surface of the substrate. Experimental results show the formation of such convex surfaces during laser irradiation. These surfaces eventually rupture and the material is removed explosively due to high internal pressures.

Laser direct writing and doping of diamond-like carbon, polycrystalline diamond, and single crystal silicon carbide

Nanosecond pulsed Nd:yttrium–aluminum–garnet laser treatment is applied to alter the electric properties of free standing polycrystalline diamond substrates, diamond-like carbon (DLC), and 4H–SiC single crystal wafers. In the case of DLC samples, the laser irradiation, nitrogen doping, as well as the incorporation of cobalt into the DLC layer reduces its electric resistance. Laser fabrication of Schottky barrier diode at the DLC–cobalt contact is demonstrated, and its nonlinear 1/C2 vs V curve indicates a nonuniform dopant distribution. The nitrogen dopant profile in the laser-doped SiC wafer is obtained by secondary ion mass spectroscopy and the corresponding nitrogen diffusion coefficient under laser processing parameters is calculated. Laser doping enhances the nitrogen diffusivity in SiC and allows in situ fabrication of metal contacts with no additional materials. Scanning electron microscopy, wavelength dispersive spectroscopy, and x-ray photoelectron spectroscopy are used to study the surface compo...

Laser microprocessing of wide-bandgap materials

Laser direct-write and doping technique (LDWD) is used to introduce variations in electric properties of wide band gap materials such as SiC and diamond. Conductive, p-type doped, n-type doped and insulative tracks are created on different diamond and SiC substrates using this method. The effects of various processing parameters such as laser-matter interaction time, number of repeated exposures, and type of irradiation environment are investigated. SEM, SIMS, XPS and Raman spectroscopy are used to study the effect of laser irradiation on the microstructure, chemical binding and to obtain dopant depth profile in the substrates, respectively. LDWD technique proved to enhance the dopant (nitrogen) diffusivity into SiC resulted in a diffusion coefficient (available in paper)that is four orders of magnitudes faster than the reported value (5 x 10-12 cm2s-1). Process modeling is conducted to study the atomistic of laser-doping process and to utilize laser irradiation to increase both dopant penetration and concentration. Laser doping of nitrogen alters the Raman spectrum of the 4H-SiC suggesting that Raman spectroscopy can be used as a non-contact method to characterize the laser-doped SiC.

Embedded Multi-die Interconnect Bridge (EMIB) -- A High Density, High Bandwidth Packaging Interconnect

The EMIB dense MCP technology is a new packaging paradigm that provides localized high density interconnects between two or more die on an organic package substrate, opening up new opportunities for heterogeneous on-package integration. This paper provides an overview of EMIB architecture and package capabilities. First, EMIB is compared with other approaches for high density interconnects. Some of the inherent advantages of the technology, such as the ability to cost effectively implement high density interconnects without requiring TSVs, and the ability to support the integration of many large die in an area much greater than the typical reticle size limit are highlighted. Next, the overall EMIB architecture envelope is discussed along with its constituent building blocks, the package construction with the embedded bridge, die to package interconnect features. Next, the EMIB assembly process is described at a high level. Finally, high bandwidth signaling between the die is discussed and the link bandwidth envelope is quantified.

One-dimensional transient analysis of volumetric heating for laser drilling

Generally laser energy is considered to interact only with the substrate surface, as in metals, where the laser beam does not propagate into the substrate beyond a very small absorption depth. There are, however, many instances, particularly for ceramics and polymers, where the laser beam can penetrate into the substrate to substantial depths depending on the laser wavelength and laser-material interaction characteristics. Specifically there are polymeric dielectrics used as multilayer electronic substrates in which a laser beam of wavelength 9.3μm can penetrate into the substrate. The laser energy interacts at the substrate surface as well as inside the substrate. This particular aspect of laser-material interactions is important in laser drilling of blind microvias in polymeric multilayer electronic substrates. A one-dimensional transient heat conduction model including vaporization parameters is constructed to analyze this behavior. The absorption coefficient of the dielectric is also considered in thi...

Laser direct write doping of wide-bandgap semiconductor materials

Gas immersion laser doping (GILD) and molten precursor (predeposition and drive-in diffusion) laser doping are demonstrated in SiC. Trimethylaluminum (TMAI) and nitrogen are the precursors used to produce p-type and n-type SiC, respectively. Nd:YAG and excimer laser nitrogen doping in SiC epilayer and single crystal substrates increases the dopant concentration by two orders of magnitude and produces both deep (500-600 nm) and shallow (50 nm) junctions, respectively. Laser-assisted effusion/diffusion is introduced and utilized to dope Al in SiC wafers. Using this technique, a 150 nm p-type doped junction is fabricated in semi-insulating 6H-SiC and n-doped 4H-SiC wafers. This technique is used to p-type dope Mg in single crystal GaN to concentrations up to 10/sup 21/ cm/sup -3/ and 5/spl mu/m penetration., respectively, are achieved using various laser doping techniques. Laser direct write (LDW) conversion provides metallization to process Ohmic and rectifying contacts in these doped semiconductors, without the addition of metal, in order to conduct junction and device testing.

Effect of large deflection angle on the laser intensity profile produced by acousto-optic deflector scanners in high precision manufacturing

Laser beam scanners have found wide applications in a variety of laser-assisted advanced microprocessing technologies such as printing, patterning, and doping. Traditional galvo-scanners affect the accuracy of beam positioning and repeatability in high precision manufacturing due to mechanical motion of the mirrors and backlash errors. The purpose of this study is to analyze an acousto-optic deflector (AOD) to achieve high diffraction efficiency and high deflection angle. Conventional AODs are operated with one-dimensional refractive index variation induced by modulating only the acoustic wave frequency. The effect of two-dimensional refractive index variations, which can be achieved by modulating both the phase and frequency of the acoustic waves, is analyzed. This study, therefore, advances the current AOD technology from just acoustic wave frequency modulation to a new class of AODs involving both phase and frequency modulations. This new type of AOD is designed by developing an analytic model based on...

Dynamic two-dimensional refractive index modulation for high performance acousto-optic deflector.

The performance of an acousto-optic deflector is studied for two-dimensional refractive index that varies as periodic and sinc functions in the transverse and longitudinal directions, respectively, with respect to the direction of light propagation. Phased array piezoelectric transducers can be operated at different phase shifts to produce a two-dimensionally inhomogeneous domain of phase grating in the acousto-optic media. Also this domain can be steered at different angles by selecting the phase shift appropriately. This mechanism of dynamically tilting the refractive index-modulated domain enables adjusting the incident angle of light on the phase grating plane without moving the light source. So the Bragg angle of incidence can be always achieved at any acoustic frequency, and consequently, the deflector can operate under the Bragg diffraction condition at the optimum diffraction efficiency. Analytic solutions are obtained for the Bragg diffraction of plane waves based on the second order coupled mode theory, and the diffraction efficiency is found to be unity for optimal index modulations at certain acoustic parameters.

Two-dimensional analytic modeling of acoustic diffraction for ultrasonic beam steering by phased array transducers

HighlightsFourier model for analytic expression of displacement vector due to acoustic waves.Filon model for quasi‐analytic expression of the displacement vector.Composite wavefront due to interference and diffraction of acoustic waves.Ultrasonic beam steering by phased array transducers.Beam steering can deflect laser beams at large angles in acousto‐optic deflectors. Abstract Phased array ultrasonic transducers enable modulating the focal position of the acoustic waves, and this capability is utilized in many applications, such as medical imaging and non‐destructive testing. This type of transducers also provides a mechanism to generate tilted wavefronts in acousto‐optic deflectors to deflect laser beams for high precision advanced laser material processing. In this paper, a theoretical model is presented for the diffraction of ultrasonic waves emitted by several phased array transducers into an acousto‐optic medium such as TeO2 crystal. A simple analytic expression is obtained for the distribution of the ultrasonic displacement field in the crystal. The model prediction is found to be in good agreement with the results of a numerical model that is based on a non‐paraxial multi‐Gaussian beam (NMGB) model.