Declan Casey

High efficiency Si integrated micro-transformers using stacked copper windings for power conversion applications

This paper details the design, fabrication, and characterization of silicon integrated micro-transformers. Two types of race-track shaped micro-transformers, single copper winding or single layer metal (SLM) and double copper winding or double layer metal (DLM) were designed and fabricated using standard CMOS processing. The DLM devices have higher inductance density than SLM devices realized within similar footprint area. The design study showed that the efficiency of micro-transformers increased from 37% for SLM designs to over 75% for DLM transformers at 20 MHz.

Energy Storage: Battery Materials and Architectures at the Nanoscale

Enterprise Ireland (Project CFTD07325). European Commission (EU Framework 7 project Nanofunction, (Beyond CMOS Nanodevices for Adding Functionalities to CMOS) www.Nanofunction.eu EU ICT Network of Excellence, Grant No.257375)

High frequency DC-DC converter with co-packaged planar inductor and power IC

The paper introduces the trend of integration and miniaturization of power converters with potential for enhanced efficiency, form factor reduction and cost reduction. To demonstrate the concept of highly integrated switched mode power supply with integrated magnetic, a system-in-package DC-DC converter using a stacked co-packaging approach is developed. A system approach was taken to the design, and functional integration, using 3-D packaging for realizing a power supply in package solution (PwrSiP). The target integrated converter is capable of handling an input voltage of 5V and frequencies up to 40MHz. A DC-DC converter IC on a 0.35μm CMOS process was designed to meet this goal. In parallel with the IC design, technology development for on-silicon integrated micro-inductors was completed to achieve small-form factor and extremely low profile. A maximum measured efficiency of 83% and 78% was achieved on the stacked converter operating at 20MHz and 40MHz, respectively. The stacked approach showed a 30% area reduction compared to side-by-side implementation with external discrete inductor.

Electroless nickel/gold Ohmic contacts to p-type GaN

A solution based approach to forming Ohmic contacts to p-type GaN is described. Electroless plated Ni∕Au contacts are shown to compare favorably with traditional evaporated contacts, with contact resistivities ρc in the region of 10−2Ωcm2. These values are readily achieved after a rapid thermal annealing in an O2 atmosphere. The tunneling nature of the contact is confirmed via temperature dependant measurements. X-ray diffraction measurements confirm the similarity between evaporated and plated contacts. Current-photocurrent (I-L) and current-voltage (I-V) measurements from light emitting diodes formed using an electroless p-type contact are shown. Electroless deposition of the contact metals allows for a reduction in processing time and cost.

High-aspect-ratio photoresist processing for fabrication of high resolution and thick micro-windings

DC winding losses remain a major roadblock in realizing high efficiency micro-magnetic components (inductors/transformers). This paper reports an optimized photoresist process using negative tone and acrylic based THB-151N (from JSR Micro), to achieve one of the highest aspect ratio (17:1) and resolution (~5 µm) resist patterns for fabrication of thick (~80 µm) micro-winding using UV lithography. The process was optimized to achieve photoresist widths from 5 µm to 20 µm with resist thickness of ~85 µm in a single spin step. Unlike SU-8, this resist can be readily removed and shows a near-vertical (~91°) electroplated Cu side-wall profile. Moreover, the high resolution compared to available resist processes enables a further reduction in the footprint area and can potentially increase the number of winding thereby increasing the inductance density for micro-magnetic components. Resistance measurements of electroplated copper winding of air-core micro-inductors within the standard 0402 size (0.45 mm2 footprint area) suggested a 42% decrease in resistance (273 mΩ–159 mΩ) with the increase in electroplated Cu thickness (from 50 µm to 80 µm). Reduction of the spacings (from 10 µm to 5 µm) enabled further miniaturisation of the device footprint area (from 0.60 mm2 to 0.45 mm2) without significant increase in resistance.

Invited) Integrated Microinductors on Semiconductor Substrates for Power Supply on Chip

Enterprise Ireland (commercialisation fund technology development project CFTD/2008/331); Higher Education Authority (“INSPIRE” Higher Education Authority via PRTLI4)

Electrochemical Properties of Cu Deposition from Methanesulphonate Electrolytes for ULSI and MEMS Applications

Methanesulfonic acid (MSA) is an alternative electrolyte system which could replace sulfuric acid in practical applications. MSA is a natural product, is readily biodegradable and has a low toxicity [1, 2]. This results in easily treatable effluents that are less hazardous compared to all other commercial baths such as sulfate, chloride, fluoride, etc. Nevertheless, MSA is a strong electrolyte and its conductivity in water is similar to other strong acids such as sulfuric or hydrochloric and higher than that of other organic acids [1]. However, little information exists in the literature regarding the fundamental properties of such electrolytes.

Electroless metal deposition for IC and TSV applications

Ultrathin film electroless deposition of Cu and Ni is shown for IC and TSV barrier layer / interconnect applications as an alternative to vacuum based deposition techniques. Cu films of approximately 20 nm were achieved while coherent electroless Ni can be deposited to single digit nm levels. The use of self-assembled monolayers facilitates electroless deposition in high aspect ratio structures. This activation process in combination with ultrathin film barrier/seed layer deposition by electroless processing enables scaling for both IC and TSV interconnect applications.

Metal contacts to p-type GaN by electroless deposition

Initial results are presented on the electroless deposition of metal contacts to p-type gallium nitride (GaN). Deposition procedures were developed for the deposition of both nickel and tungsten-cobalt (W-Co) contacts onto p-type GaN. Attempts to deposit platinum on p-type GaN failed, despite the fact that electroless platinum deposition was successfully achieved on other substrate types. Nickel contacts were overlaid with gold and annealed in oxygen ambient to form ohmic contacts with specific contact resistivity values down to 2x10-2 &OHgr;cm2. Measurements at elevated temperatures up to 140 degrees C showed that the specific contact resistivity was almost independent of temperature. The tungsten-cobalt contacts showed rectifying behaviour even after annealing at 650 degrees C. This makes this contact type a possible candidate for Schottky contacts in high temperature applications.

Packaging technology for high power blue-green LEDs

High brightness LEDs (HBLEDs) have been fabricated on GaN semiconductor material grown on sapphire substrate. These devices provide an optical output power in excess of 50 mW at a driving current of 1 amp. For this high current application, large size (1.8 mm × 0.6 mm) GaN LEDs are flip-chip mounted onto a heat sink to provide a low thermal resistance path from the junction to the ambient. For the flip-chip mounting, a Au/Sn/Au solder and a Au/Au thermal compression bonding process have been optimized. The bond strength of the Au/Sn solder joints and the Au-Au bonds is measured through shear testing. Good bond strength results of 224 g/f for the Au/Sn/Au solder and 288 g/f for the solid Au bonds have been achieved. The thermal modeling of the assembly is done with a finite element analysis and the optimum design has been adopted for this high current application. At present these assemblies are under lifetime test and so far nearly 6000 hours of continuous operation has been achieved.