Orven F. Swenson

Laser-Enabled Advanced Packaging of Ultrathin Bare Dice in Flexible Substrates

Embedding ultrathin semiconductor dice in flexible substrates provides unique capabilities for product designers and makes products such as smart bank cards and radio-frequency identification banknotes possible. Most of the current work in this area is directed toward handling, embedding, and interconnecting the ultrathin chips. Relatively little attention is paid to another critical process step-placing the flexible and very fragile ultrathin die onto the flexible substrate reliably and in a cost-efficient manner, suitable for high throughput assembly. The presented laser-enabled technology for embedding ultrathin dice in a flexible substrate was developed at the Center for Nanoscale Science and Engineering, North Dakota State University, Fargo, ND, to address this problem. The technology has been successfully demonstrated and proven for the fabrication of an RFID tag.

Solution-Based Synthesis of Crystalline Silicon from Liquid Silane through Laser and Chemical Annealing

We report a solution process for the synthesis of crystalline silicon from the liquid silane precursor cyclohexasilane (Si(6)H(12)). Polysilane films were crystallized through thermal and laser annealing, with plasma hydrogenation at atmospheric pressure generating further structural changes in the films. The evolution from amorphous to microcrystalline is characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy and impedance spectroscopy. A four-decade enhancement in the electrical conductivity is attributed to a disorder-order transition in a bonded Si network. Our results demonstrate a potentially attractive approach that employs a solution process coupled with ambient postprocessing to produce crystalline silicon thin films.

Laser sintering of direct write silver nano-ink conductors for microelectronic applications

Direct Write Technologies are reaching the goal of entirely printable microelectronic devices on flexible polymeric substrates. In the present work, nanoparticle inks deposited on low temperature polyimide substrates using Maskless Mesoscale Material Deposition (M3D®) technology were laser sintered using a continuous wave 1.06 micron Nd:YAG laser. In-situ measurements were made during sintering to capture the dynamic change in bulk resistivity as a function of deposited energy per volume of sintered material. It was shown that less than 0.05-μJ/μm3 started the sintering and 0.34-μJ/μm3 was enough for sintering the deposited samples regardless of initial resistance.

Noncontact Selective Laser-Assisted Placement of Thinned Semiconductor Dice

New laser-induced forward transfer (LIFT) techniques promise to be a disruptive technology by enabling high-volume placement of ultrathin bare dice. Limitations of current die-attach techniques such as pick-and-place are presented and discussed which inspired the development of this new placement method. The thermo-mechanical selective laser-assisted die transfer (tmSLADT) process is introduced as an application of the unique blistering behavior of a dynamic releasing layer when irradiated by low-energy-focused UV laser pulses. The potential for tmSLADT to be the next generation LIFT technique is demonstrated by the “touchless” transfer of 65-μm-thick silicon tiles between two substrates spaced 195 μm apart. Additionally, the advantages of an enclosed blister actuator mechanism over previously studied ablative and thermal releasing techniques are discussed. Finally, experimental results indicate that this nonoptimized die transfer process compares with, and may exceed, the placement precision of current assembly techniques.

Collimated Aerosol Beam Deposition: Sub-5-

Materials deposition based upon directed aerosol flow has the potential of finding application in the field of flexible electronics where a low-temperature route to printed transistors with high mobilities remains elusive. NDSU has been actively engaged in addressing this opportunity from the following two perspectives: 1) developing an appreciation of the basic physics that dominate aerosol beam deposition toward engineering a robust method that allows the realization of deposited features with sub-5 μm resolution; and, 2) developing an understanding of the mechanistic transformations of silane-based precursor inks toward the formation of electronic materials at atmospheric-pressure. In this paper, we will briefly discuss the genesis of a new materials deposition method termed collimated aerosol beam direct-write (CAB-DW) where precision linewidth control has been realized using a combined theoretical/experimental approach. Next, we will discuss progress using Si6H12 (cyclohexasilane-a liquid silane) as a precursor for solution-processed diodes and transistors. Finally, we demonstrate the ability to CAB-DW Si6H12-based precursor inks for printing Si-based semiconductors.

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We report that when silicon is evaporated onto a tungsten field emitter at room temperature and the tip is then annealed at higher temperatures in order to carry out surface migration and thermal desorption studies, part of the silicon diffuses into the interior. When the tip is flashed at 2500 °K to produce a clean surface, all the silicon is not removed and it can be brought out again by annealing between 1000 and 1500 °K. This is demonstrated by a silicon buildup on the surface with an activation energy of 9.3 ± 0.5 kcal/mole. The rate of buildup is enhanced by the application of high voltage.

m Resolution of Printed Actives and Passives

Ion mobility spectrometry with a photoemissive electron source is a promising approach for monitoring vapors of highly electronegative species such as chlorinated solvents and explosives. Electrons are generated over well-defined intervals by ultraviolet irradiation of a metal plate or metal-coated window by either a flashlamp or a pulsed laser beam, so no gating of the drift process is required. The negative ion mobility spectrum of air exhibits features predominately due to clusters of oxygen anion with water molecules. These species readily transfer electrons to chlorinated aliphatic compounds that undergo dissociative electron attachment to generate chloride ions. The ion mobility spectra change in a predictable fashion, permitting real-time detection of chlorinated species at low ppmV concentration. In this presentation we shall describe our methodology, display the response characteristics of our instrument, and summarize our investigations of the relevant ion-molecule reactions.

DIFFUSION OF SILICON INTO TUNGSTEN IN FIELD-EMISSION MICROSCOPE.

Ultrathin flip-chip semiconductor die packaging on paper substrates is an enabling technology for a variety of extremely low-cost electronic devices with huge market potential such as RFID smart forms, smart labels, smart tickets, banknotes, security documents, etc. Highly flexible and imperceptible dice are possible only at a thickness of less than 50 μm, preferably down to 10-20 μm or less. Several cents per die cost is achievable only if the die size is ≤ 500 μm/side. Such ultrathin, ultra-small dice provide the flexibility and low cost required, but no conventional technology today can package such die onto a flexible substrate at low cost and high rate. The laser-enabled advanced packaging (LEAP) technology has been developed at the Center for Nanoscale Science and Engineering, North Dakota State University in Fargo, North Dakota, to accomplish this objective. Presented are results using LEAP to assemble dice with various thicknesses, including 350 μm/side dice as thin as 20 μm and less. To the best of our knowledge, this is the first report of using a laser to package conventional silicon dice with such small size and thickness. LEAP-packaged RFID-enabled paper for financial and security applications is also demonstrated. The cost of packaging using LEAP is lower compared to the conventional pick-and-place methods while the rate of packaging is much higher and independent of the die size.

Real-time monitoring of chlorinated aliphatic compounds in air using ion mobility spectrometry with photoemissive electron sources

Gas dynamic cold spray was first discovered in the 1980s and has since been used as a surface coating process for depositing metals, metal-ceramic composites, metal-carbon nanotube composites and other composite materials onto both flexible and rigid substrates. We recently developed a focused cold spray material deposition tool termed Micro Cold Spray (MCS). MCS is a direct-write tool applicable for printed electronics and has been used to print conductive trace patterns as thin as 50 μm wide using copper, aluminum and tin micro powders. Unlike conventional aerosol processing at 10–100 m/s, aerosol particles in the MCS process are accelerated to speeds greater than 500 m/s. In this paper the possibility to accelerate, focus, collimate, and deposit aerosol particles is theoretically explored using a finite difference approximation method to simulate the flow of Helium through a symmetric converging-diverging nozzle of throat diameter 200 μm. A Lagrangian particle tracking algorithm is used to calculate the particle trajectories and corresponding velocities. This paper presents a comparison of the effect of Stoke’s drag force and Saffman’s lift force on the trajectory and velocity of copper particles 3 μm in diameter.Copyright

Laser-assisted ultrathin bare die packaging: a route to a new class of microelectronic devices

Materials deposition based upon directed aerosol flow has the potential of finding application in the field of flexible electronics where a low-temperature route to printed transistors with high mobilities remains elusive. NDSU has been actively engaged in addressing this opportunity from the following two perspectives: (1) developing an appreciation of the basic physics that dominate aerosol beam deposition toward engineering a robust method that allows the realization of deposited features with sub-5 mum resolution; and, (2) developing an understanding of the mechanistic transformations of silane - based precursor inks toward the formation of electronic materials at atmospheric-pressatmospheric-pressureure. In this paper, we will briefly discuss the genesis of a new a materials deposition method termed collimated aerosol beam direct-write (CAB- DW) where precision linewidth control has been realized using a combined theoretical/experimental approach. Next, we will discuss progress using Si6H12 (cyclohexasilane - a liquid silane) as a precursor for solution-processed diodes and transistors. Finally, we demonstrate the ability to CAB- DW Si6H12-based precursor inks for printing Si-based semiconductors.