TWI Industrial Member Report Summary 827/2005
By L J Ecclestone and J L Howlett
Background
Electronic devices and systems are used in many applications and are subjected to a wide range of environments. The ability of the components of such devices and systems to survive these environments is dependent on the system architecture and the materials used in their construction, as well as the particular nature of the environment to which they are subjected.
Operation at temperatures above 300°C (or high power operation at temperatures above 200°C) is too demanding for silicon integrated circuit based technologies. There are two candidate semiconductor device materials under consideration by the electronics industry for high temperature operations: silicon carbide (SiC) and gallium nitride (GaN). Of these two materials, SiC is currently commercially produced in single crystal wafer form of semiconductor grade, whilst GaN is epitaxially grown on substrates such as silicon carbide or sapphire. The greater commercial availability of SiC suggests this material is likely to be pre-eminent in the development of high temperature electronics, and so was selected as the semiconductor material for this project.
World-wide little work has been done on high temperature (>300°C) packaging technologies. Significant effort is required to establish which materials, metallisations, interconnection and protection techniques and qualification technologies are best suited to 300°C environments.
Without data on the long-term high temperature behaviour of the various potential first level interconnect material combinations, identification of the most effective architectures cannot be achieved. Whilst TWI has an extensive background over the past 40 years in electronics interconnect and packaging this does not extend to the SiC based systems. Therefore, in anticipation of high temperature SiC based devices being of interest and potential use by TWI Industrial Members this project has been undertaken.
This project had two broad aims. The first was to investigate the applicability of a range of evaluation methods for high temperature devices. The second aim was to generate some long term elevated temperature performance data on a model metallisation system.
For the purposes of this project tungsten was selected as the metallisation since it is currently used as an ohmic contact metal for SiC semiconductor devices. To reduce any reaction between the conducting tungsten layer and the SiC substrate use was made of a tungsten disilicide (WSi2) barrier layer. To minimise the likelihood of oxidation of the tungsten, a silicon nitride (Si3N4) passivation layer was used.
Objectives
- Assess an analytical approach to measure performance of a high temperature metallisation system.
- Determine the capability of tungsten coating systems on SiC substrates to provide stable operation at temperatures from 300°C to 400°C for high temperature electronic and sensor packaging applications.
- Determine the suitability of tungsten coating systems at higher temperatures up to 700°C.
- Assess improvements to tungsten coating systems through the use of passivating and barrier layers.