Val R. Marinov

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.

Built-up edge analysis of orthogonal cutting by the visco-plastic finite-element method

Abstract The behavior of the work materials at the chip-tool interface under extremely high strain rates and temperatures is more like that of viscous liquids than that of normal solid metals. Under these circumstances, the principles of fluid mechanics can be invoked to describe the metal flow in the neighborhood of the cutting edge. In the present paper, an Eulerian finite-element model is presented that simulates metal flow in the vicinity of the cutting edge when machining a low carbon steel with a carbide cutting tool. The work material is assumed to obey the visco-plastic (Bingham solid) constitutive law and the Von Mises criterion. Heat generation is included in the model, assuming adiabatic conditions within each element. The mechanical and thermal properties of the work material are accepted to vary with the temperature. The model is based on the virtual work-stream function formulation. Emphasis is given to analyzing the formation of the stagnant metal zone ahead of the cutting edge. The model predicts flow field characteristics such as material velocity, effective stress and strain-rate distributions as well as the built-up layer configuration.

Volumetric Display Based on Two-Photon Absorption in Quantum Dot Dispersions

A volumetric display technology based on two-photon absorption in quantum dots was investigated. We derive and validate a closed form expression for the luminance produced by dispersed quantum dots at the focal point of an infrared pulsed laser beam and we demonstrate using theory and experiments that voxels can be efficiently produced with commercially available solid-state pulsed lasers in off-the-shelf core-shell CdSe-ZnS quantum dots dispersed in toluene.

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.

A microstrip patch antenna manufactured with flexible graphene-based conducting material

In this paper, a unique process for fabricating microstrip patch antennas with flexible graphene-based conductors is presented. In particular, this manufacturing process uses a commercially available micro-cutter to cut the outline of the patch antenna from the flexible graphene-based conductors, and then this piece is attached to a grounded FR4 substrate using adhesive to create a unique printed antenna. The design was modeled using a commercial simulator, and a prototype was fabricated and measured. Overall, it was shown that the S-parameter simulations agreed fairly well with measurements, and that this manufacturing process has the potential to develop more complicated designs, such as meander-line dipoles for example, that are difficult to cut-out manually.

A low cost flexible passive UHF RFID tag for sensing moisture based on antenna polarization

Cost optimization and performance are the continuous improvement areas in any technology. In this paper a flexible dipole antenna on a passive UHF RFID tag is designed on an inexpensive paper substrate which can sense the moisture based on the polarization of the antenna. An inexpensive paper substrate and copper (Cu) layer are used for the flexibility and the optimization of manufacturing cost. The dipole antenna is matched with the complex conjugate of the power harvesting circuit of the Higgs2 Integrated Circuit (13.88 -j143.6 at the frequency of 915 MHz). The key characteristic of this design is the sensitivity of the antenna polarization on the passive RFID tag to the moisture content in the paper substrate. In simulations, the antenna is circularly polarized when the substrate is dry (relative permittivity, ϵr= 2.38) and the antenna is linearly polarized when the substrate is wet (relative permittivity, ϵ r= 35.35). The read ranges of the fabricated dipole antenna (i.e., prototype RFID moisture sensor) are measured and the desired performance is achieved with overall tag dimensions of 0.144λo × 0.133λo where λo is the free space wavelength at 915 MHz.

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

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.

Optically transparent and structurally sound silica aerogels: insights from a process study

Optically transparent and structurally sound silica aerogels: insights from a process study Aerogels are internally nanostructured materials characterized with a plethora of unique properties. Monoliths with high optical transparency made of silica aerogels were some of the first and still one of the most important classes of aerogels. Experiments and theory indicate that optical transparency and structural integrity of silica aerogels are negatively correlated. Other than optimal combination of processing conditions during aerogel fabrication can result in either highly transparent but cracked or in crack-free but less transparent and even opaque aerogels monoliths. Results are presented from the study of the relationship between the properties of single-step tetramethoxysilane (TMOS) base-catalyzed silica aerogels and the processing conditions, both at the alcogel preparation step and during the supercritical liquid CO2 drying process. Crack-free aerogel monoliths with good optical transparency were obtained with TMOS:methanol (MeOH) molar ratios of 1:16 and TMOS:ammonia (NH4OH) molar ratios of 1:0.05, CO2-MeOH exchange rates of about 1.25 ml/min, and autoclave decompression rates of 70 KPa/min. Adding glycerol in the sol-gel stage had a positive effect on the aerogel monolithicity but, even without glycerol, crack-free silica aerogels can be obtained by reducing the depressurization rate of the autoclave. A strict control and careful selection of the aerogels processing conditions within the set of parameters identified will enable the fabrication of structurally sound silica aerogels with good optical properties essential for a number of applications.

Simple, inexpensive, and reliable, high density interconnect technology for flexible electronics applications

The ultra-small form factor of the electronic components along with a reliable and inexpensive high-density interconnect technology are the key factors in bringing flexible electronics to the next level of miniaturization without compromising performance and substantially increasing production cost. The common thick-film methods used today to fabricate conductive trace patterns on flexible boards are ineffective for resolutions below 50 µm. Thin-film technologies can fabricate high resolution patterns; however, they are prohibitively expensive for the low-cost flexible electronics applications. A number of direct-write methods were developed to address the mesoscale (i.e., from 1 µm to 100 µm) range; however, these sequential material additive technologies are characterized with a low throughput and high production cost.

Modelling of spectral down-converter based on cadmium-free quantum dots for photovoltaics

In this paper we analyze the applicability of cadmium-free luminescent quantum dots to spectral down-conversion process, study its influence on short-circuit current change in several types of solar cells using Monte-Carlo simulation.