Moore And More: The Increasing Importance Of Materials
Had there been an electronics technology roadmap at the start of the 1940s, it may well have foreseen the coming of the “silicon age" with the evolution from the thermionic valve to the semiconductor transistor.
Such a roadmap may even have forecast the transition from Bardeen, Brattain and Shockley’s first germanium based transistor to the thousands of integrated silicon based transistors from which the earliest microprocessor chips were formed.
It was in 1965 that Gordon Moore of Intel observed that the complexity of integrated circuits increased exponentially over time, roughly “doubling" every two years.
“Moore’s Law" has continued to hold true for the last four decades, during which time the Semiconductor Industry Association (SIA) has established a formal industry roadmap to predict and forecast technology trends aimed at ensuring continued development in the field of semiconductors.
This roadmap is known as the International Technology Roadmap for Semiconductors (ITRS).
For most of the interceding years since the ITRS was established, the focus of attention has been on device scaling, with ever decreasing feature dimensions being the key to achieving greater levels of integration and improved transistor performance.
In recent years, it has become apparent that physical limitations and spiraling fab costs means that continued reductions in feature size cannot continue indefinitely.
The ITRS fully recognizes the limitations of continued scalability and recent roadmap updates clearly indicate that the way forward for future technological evolution is expected to be based on the development of novel materials based technologies which are compatible with the pre-existing manufacturing infrastructure
The increasing demand for radio frequency (RF) wireless connectivity, high processing speeds and portability with its associated requirement for low power consumption, has already led to the widespread adoption of materials based solutions in the form of gallium arsenide (GaAs) devices.
Similarly, the adoption of materials manipulation techniques such as high-k dielectrics, strained silicon and silicon on insulator signal a trend towards the industry entering a new materials era.
In addition to high speed and low power consumption, next generation applications are likely to demand integration of photonic and CMOS functionality within a single chip, a challenge that can only be addressed from a materials perspective.
Although silicon has long been established as the de-facto standard semiconductor material, germanium, the material from which the first transistor was made, offers substantially higher electron and hole mobility and consequently can achieve far higher operating speeds, for a given device dimension.
Traditionally, silicon has become the material of choice because of its relative abundance and lower cost as well as its mechanical strength and its excellent native oxide SiO2 that forms an ideal insulating interface with silicon.
Germanium by contrast is a brittle material with poor native oxide properties and, being a less common commodity is comparatively expensive. However, the recent introduction of deposited high-k gate dielectrics to replace the traditional silicon dioxide, now affords the superior electronic properties of Germanium a new lease of life within mainstream CMOS manufacturing.
Beyond the 22nm device node, the ITRS roadmap has predicted the development of ‘new materials to replace silicon as a alternate device channel to increase the saturation velocity and maximize drain currents in MOSFETs, while minimizing leakage currents and power dissipation for technologies scaled to 16nm and beyond’
In order to address such stringent requirements, engineers at IQE’s manufacturing plant in Cardiff, UK have developed a new range of engineered substrates including germanium on insulator (GeOI).
Engineered GeOI substrates are produced using a unique layer transfer process from a ‘proprietary lattice matched substrate’ to produce a material with extremely low defectivity levels and excellent across wafer thickness uniformity. The GeOI subtrate is manufactured using conventional epitaxial growth techniques, which eliminates the use of bulk Ge wafers, and offers a cost effective solution to future CMOS requirements.
Removal of co-transferred material is achieved using highly selective etch methods resulting in smooth Ge layers with excellent across wafer thickness uniformity and extremely high crystal quality. Typical Ge layer thicknesses are of the order 10-100nm, with an across wafer thickness uniformity of ~3% and a surface roughness of 0.5nm. The thickness of both the final Ge device layer, and the buried oxide layer, can be tailored to suit the specific application.
The hybrid approach provides a virtual germanium substrate on top of a silicon substrate which means that the enhanced mobility performance that can be achieved in partially or fully depleted germanium devices can be produced using established, “CMOS safe" production processes and can employ the same range of dopants used in standard silicon processes.
The engineered substrates therefore allow device designers to look beyond the performance constraints imposed by existing silicon technologies to push the boundaries of future CMOS devices for generations to come
Additionally, the inherent photonic properties of GeOI also provides a potential platform for advanced, multi-junction photovoltaic devices for high efficiency solar cells.