Marcin Mysliwiec

Silver micropowders as SiC die attach material for high temperature applications

This work is devoted attaching technology between SiC structures and DBC substrates for creating SiC devices able to work at temperature up to 350°C. Our current work was concentrated on finding so called “pressure sintering” procedure in air using Ag micro particles. A special test samples with a size corresponding to the dimension of the SiC structures were assembled to DBC substrates with different surface finishing by Ag micro powder sintering. In the first series of experiments DBC substrates with Cu electroplated by Ni (3÷5 μm) and Au (above 1 μm) were used. It was found that by modifying application procedure of Ag micro powder onto DBC substrate with Cu/Ni/Au metallization, it is possible to obtain good adhesion between attached samples. The sintering is performed in air at temperature of 400°C for 40 min and pressure of 10 MPa. In the second series of experiments the SiC structures with Ni/Au metallization were assembled to DBC substrate with Cu/Ni/Au metallization. The adhesion higher than 10 MPa was obtained for such prepared samples.

TiAl-based ohmic contacts on p-type SiC

Titanium-aluminum alloys were successfully used to form low resistance ohmic contacts to p-type SiC. This work concerns two Al-Ti alloy compositions. Contacts were prepared by magnetron sputtering of bilayer Al-Ti and trilayer Ti-Al-Ti thin films and rapid thermal annealing at temperatures range 900°C ÷ 1000°C. Using scanning electron microscopy and profiler, an investigation of surface morphology of annealed contacts was conducted. The best resistivity of 5.8·10−5 Ωcm2 was attained on 100nm/26nm Al/Ti metallization of 80% at. Al alloy composition annealed at 1000°C for 2 min. Relatively low roughness of 30 nm was observed on trilayer Ti/Al/Ti metallization.

Die attach by diffusion Sn-Ag-Sn soldering in high temperature electronics applications

The aim of this work was assessment of application diffusion soldering in die attachment for the electronic devices operating at temperatures above 300 °C. Assembly process was based on the intermetallic compound formation in Sn, Cu and Ag systems at temperatures above Sn melting point. New-created compounds should be thermally stable in high temperatures. The influence of the following parameters: temperature, pressure, process time and application of flux on mechanical strength (shear strength) was investigated. It was concluded that mechanical strength of joints is good and influence of some of the process parameters is stronger than others. The best soldering results were achieved for process were flux was applied, temperature at 395 °C, pressure higher than 10 MPa, and process time longer than 10 minutes. In the next series of experiment the soldering temperature was decreased to the range 270 °C ÷ 300 °C and still good mechanical strength of joint was possible. In diffusion soldering it is necessary to apply no-clean flux. It makes soldering process easier and better mechanical properties were achieved.

High-temperature properties of Schottky diodes made of silicon carbide

This paper refers to properties of silicon carbide Schottky diodes at high values of their internal temperature. The investigated diode was elaborated at Warsaw University of Technology. Characteristics of this diode were measured at different cooling conditions in wide range of dissipated power. At each operating point the value of the internal temperature of the investigated diode was measured with the use of the impulse electrical method. On the basis of the obtained results of measurements thermal properties of the considered diode were discussed, as well as the possibility of using such a construction of the diode at very high values of its internal temperature (up to 500μC) was considered.

Thermal properties of Ag sintered layer used as interconnect material in microelectronics packaging

The paper deals with the thermal properties of Ag sintered layer used as interconnect material for SiC die assembly to DBC substrate. In our investigation an IR camera were used for measurement of thermal properties of Ag sintered layers. The changes of thermal conductivity were measured in temperature range from 69°C up to 221°C. The thermal conductivity of Ag sintered layers thickness 15 μm, made from micro particles, a flake shape, was in the range 7-10 W/mK and is stable in this temperature range.

Aspects of applying flip-chip technology for SiC power devices assembly

The aim of our paper is to consider the possibility of applying flip-chip technology for assembly of SiC Schottky diode into a ceramic package. Ag micropowder was used for assembly SiC structure to DBC interposer of the ceramic package. Ag or Au balls were used as flip-chip connection material. The parameters of I-V characteristics were used as a quality factor to determine the Schottky diode after hermetization into ceramic package in comparison with bare SiC diode.

Aspects of SiC diode assembly using Ag technology

The aim of our paper is to consider the possibility of applying pure Ag technology for assembly of SiC Schottky diode into a ceramic package able to work at temperatures up to 350°C. Ag micropowder was used for assembly SiC structure to DBC interposer of the ceramic package. Ag wire bonds as well as flip-chip technology using Ag balls were used as material for interconnection systems. The parameters of I-V characteristics were used as a quality factor to determine the Schottky diode after hermetization into ceramic package as well as after ageing in air at 350°C in comparison with characteristics of bare SiC diode.

Fluxless pressure Ag sintering in creation of Au-Ag connection systems

This work presents the results of manufacturing of connections based on Au-Ag metallization scheme. This type of connection is made without application of flux, thus no flux cleaning step is required. Such Au — Ag metallization scheme was chosen to simulate similar conditions as in assembly of SiC or GaN devices. Experiment was made to test various conditions of process and pick most satisfying values of time and temperature. Mechanical properties of joints were tested. The highest shear strength was achieved for GaN assembled to DBC substrate with 1 μm Sn layer by Ag sintering at 280 °C and 30 min. Additionally high current tests were performed on Cu ribbon (Ag plated) connections. No degradation was observed for current as high as 80 A/mm2.

Combination of solid-liquid interdiffusion and sintering bonding for GaN devices

In this paper the combination of solid-state interdiffusion with silver sintering is presented as a supplementary technique for assembly of high temperature devices. The design of the experiments technique is used to study the influence of process temperature, time and thickness of used Sn layers on the assembly process. Identical pressure for assembly and flux application was used in every experiment. In the study GaN chips (Ammono) with Au bottom metallization and DBC (alumina ceramic, double sided covered Cu) were used as test materials. Shear test was performed to test adhesion of the chip to the substrate. Results of statistical analysis of adhesion are presented and the best process parameters for GaN assembly to DBC substrate are identified.

Solder joints for flexible connection working at low temperatures

This paper presents flexible electrical connections operating at low temperatures. This kind of connections is used in packages where two devices operate at different temperatures - in this case, the IR detector and electronic control circuit. The IR detector operates at a temperature as low as 200 K, while the control circuit is operating in the temperature range of 290 K-300 K. The challenge is to create an elastic electrical connection between those devices. These connections must have low resistivity and at the same time should transfer as little heat as possible from the control circuit to the IR detector. 35 mm epoxy-phenolic foil with 5 mm Ni film was used as a flexible connection. The In-Sn solder was applied to create electrical connections between the Ni film and Au pads on both IR detector and I-Tera MT RF laminate. Electrical and mechanical properties of the InSn solder joints in temperatures from -40 °C to +30 °C were investigated. Two types of solder joints were taken into consideration: the flip chip type and the lap type. It was found that the InSn flip chip connections are able to work properly at DC mode in low temperatures. Further investigation will be conducted in AC mode for the lap solder connections.