Jouko Vähäkangas

Chip cooling with integrated carbon nanotube microfin architectures

Efficient cooling of silicon chips using microfin structures made of aligned multiwalled carbon nanotube arrays is achieved. The tiny cooling elements mounted on the back side of the chips enable power dissipation from the heated chips on the level of modern electronics demands. The nanotube fins are mechanically superior compared to other materials being ten times lighter, flexible, and stiff at the same time. These properties accompanied with the relative simplicity of the fabrication makes the nanotube structures strong candidates for future on-chip thermal management applications.

Design aspects of microwave components with LTCC technique

Abstract Low temperature co-fired ceramic (LTCC) technology, widely used in the automotive industry, is now being employed in microwave applications. Several commercial materials with low dielectric losses at microwave frequencies and adequate thermomechanical properties have been introduced. Computer-aided design of three-dimensional circuits has also become available. These advances together with high-quality manufacturing technology have placed LTCCs at the forefront in the development of miniature microwave devices. The paper outlines LTCC technology placing emphasis on those essentials of the materials and processing technologies about which the microwave circuit designer needs to be aware. The discussion is illustrated by examples.The crucial issue of component reliability is also addressed. Although the integration of passive components into the structure improves reliability, the joints between the LTCC module and PCB remain as significant ‘weak link’. Therefore, thermomechanical and structural design is a key to reliable LTCC assemblies.Finally, some future trends the LTCC technology for microwave applications are outlined.

Thermal oxidation of porous silicon: Study on structure

The structural changes of porous silicon (PS) samples during oxidation are investigated and analyzed using various microscopy techniques and x-ray diffraction. It is found that the surface roughness of oxidized PS layers increases with the oxidation at 200–400°C and decreased at 600–800°C. At 800°C a partially fused surface is observed. The oxide formed on the wall of porous silicon skeleton is amorphous. The shifts of Si(400) peaks are observed in the x-ray diffraction patterns, which are correlated to the lattice deformation induced by thermal expansion coefficient mismatch between the grown SiO2 and the residual Si, and to the intrinsic stress caused by the Si–O bonds at the Si–SiO2 interface. These explanations are supported by thermomechanical modeling using three-dimensional finite element method.

Broadband BGA-Via Transitions for Reliable RF/Microwave LTCC-SiP Module Packaging

This letter presents two broadband ball grid array-via transition structures applicable in reliable radio frequency/microwave low-temperature co-fired ceramic system-in-package (LTCC-SiP) module packaging from dc up to the K-band. The first transition provides better electromagnetic shielding, while the second one exhibits 40% wider bandwidth by including an air-cavity in the LTCC module. To specifically address board-level reliability, novel plastic-core solder balls as large as 1100 mum were employed. The measured 1-dB cutoff frequencies of the transition structures, all the way from the printed circuit board up to the top surface of the module package, were 19 and 27 GHz. In addition, equivalent circuit models for the transitions were developed.

Room temperature chemical deposition of palladium nanoparticles in anodic aluminium oxide templates

Palladium nanoparticles are deposited in the pores of freestanding, mesoporous anodic aluminium oxide (AAO) templates via a chemical route at room temperature. In the precursor, Pd2+ ions are complexed with ammonia and reduced to Pd0 by adding an excess amount of formaldehyde. To enhance the infiltration of pores with the precursor, 2-propanol is added to the solution. After a few hours of plating at room temperature, crystalline Pd0 nanoparticles with an average size of ~4 nm form on the pore walls of the AAO templates. The effect of heat treatment on the structure of the AAO/Pd system is also studied. The specimens are investigated using FESEM, EFTEM/EDX, and electron and x-ray diffraction techniques. Preliminary catalyst activity analyses reveal good selectivity of ~90%, but low conversion of ~1.2% of carbon monoxide to methane in hydrogen flow at 330 °C on AAO/Pd. The oxidized form of the catalyst shows instantaneous oxidation with ~100% selectivity and conversion of carbon monoxide to carbon dioxide in oxygen flow at 140 °C.

An Ultra-Wideband BGA-Via Transition for High-Speed Digital and Millimeter-Wave Packaging Applications

This paper presents a high-performance BGA-via transition structure suitable for multilayer system-in-package (SiP) applications over a wide frequency range from DC up to the F-band. The main issues involved in designing and optimizing the entire vertical transition path, starting from a motherboard and ending at the top surface of a BGA module package are outlined. The module substrates were manufactured in a standard, multilayer low-temperature co-fired ceramic (LTCC) process. The ceramic modules with plastic-core solder balls were mounted on a motherboard using standard surface-mount assembly processes. The RF performance of the developed transition structure was validated with on-wafer scattering parameter measurements. The measured results correlated very well with full-wave electromagnetic (EM) simulations, exhibiting return and insertion loss values better than 22 dB and 0.6 dB, respectively, up to 50 GHz. Moreover, the EM simulations demonstrated that the 1-dB cut-off frequency of the complete BGA-via transition structure can be extended from 55 GHz up to nearly 70 GHz at the expense of poorer return loss.

Ultra-Wideband Shielded Vertical Via Transitions from DC up to the V-Band

This paper presents three shielded vertical via transition designs applicable in millimeter-wave module packaging from DC up to the F-band. The optimized transition structures were fabricated using a standard low-temperature co-fired ceramic (LTCC) process. The measured scattering parameter results of the back-to-back via transition structures showed an exceptionally wide bandwidth with return losses better than 18 dB up to 50 GHz. The extracted insertion loss values of the single transitions were less than about 0.4 dB at 50 GHz. Moreover, full-wave electromagnetic (EM) simulations demonstrated the high potential of two of these via transitions up to 70 GHz

Probe based manipulation and assembly of nanowires into organized mesostructures

A convenient approach to patterning inorganic and organic nanowires using a novel probe manipulator is presented. The system utilizes an electrochemically etched tungsten wire probe mounted onto a 3D actuator that is directed by a 3D controller. When it is engaged by the user, the movement of the probe and the forces experienced by the tip are simultaneously reported in real time. Platinum nanowires are manipulated into organized mesostructures on silicon chip substrates. In particular, individual nanowires are systematically removed from aggregates, transferred to a chosen location, and manipulated into complex structures in which selected wires occupy specific positions with defined orientations. Rapid prototyping of complex mesostructures, by pushing, rotating and bending conjugated polymer, i.e., polyfluorene, nanowires into various configurations, is also achieved. By exploiting the strong internal axial alignment of polymer chains within the polyfluorene nanowires, mesostructures tailored to exhibit distinctly anisotropic optical properties, such as birefringence and photoluminescence dichroism, are successfully assembled on fused silica substrates.

Low-Loss and Wideband Package Transitions for Microwave and Millimeter-Wave MCMs

This paper presents high-performance flip-chip, through-substrate via and board-level BGA transition designs applicable in microwave and millimeter-wave multichip module (MCM) assemblies. Full-wave electromagnetic analysis was performed to achieve optimized transition structures. Interconnection test structures were fabricated in ceramic substrates and their performance was validated using on-wafer scattering parameter measurements. Both the flip-chip transition and the through-substrate via transition with a height of 800 mum exhibited return losses better than 25 dB and 20 dB, respectively, with low transmission losses up to 50 GHz. Furthermore, the wideband board-level BGA transition with a standoff height of 500 showed low insertion loss (< 0.5 dB) over a wide frequency range, from direct current (dc) up to 32.5 GHz.

Interfacial reactions between Sn‐based solders and AgPt thick film metallizations on LTCC

Purpose – This paper aims to investigate the metallurgical reactions between two commercial AgPt thick films used as a solder land on a low temperature co‐fired ceramic (LTCC) module and solder materials (SnAgCu, SnInAgCu, and SnPbAg) in typical reflow conditions and to clarify the effect of excessive intermetallic compound (IMC) formation on the reliability of LTCC/printed wiring boards (PWB) assemblies.Design/methodology/approach – Metallurgical reactions between liquid solders and AgPt metallizations of LTCC modules were investigated by increasing the number of reflow cycles with different peak temperatures. The microstructures of AgPt metallization/solder interfaces were analyzed using SEM/EDS investigation. In addition, a test LTCC module/PWB assembly with an excess IMC layer within the joints was fabricated and exposed to a temperature cycling test in a −40 to 125°C temperature range. The characteristic lifetime of the test assembly was determined using DC resistance measurements. The failure mechan...