A novel material developed by researchers at the University of Cambridge is opening up new
possibilities for next generation electronic and optoelectronic devices, and
paving the way for further component miniaturization.
The material, a new form of hafnium oxide, was developed by Andrew
Flewitt’s, PhD, research group in the Department of Engineering. The material
provides exceptionally high dielectric constant compared with currently
existing forms of hafnium oxide, which is already a key material in the
electronics industry.
Metal oxides are used in a wide variety of applications.
Normally, they are produced on substrates by sputtering, a process by which
some of the atoms of an electrode are ejected as a result of bombardment by
heavy positive ions. One of the problems when attempting to make high-quality
electronic materials through sputtering, however, is the difficulty in
precisely controlling the energetics of the deposition process, and hence the
material properties such as defect density.
In order to enable much greater control of the material
properties, Flewitt and his team began using a novel deposition technology to
promote plasma sputtering. The technology, known as HiTUS (High Target Utilization
Sputtering), was developed by a U.K.-based company, Plasma Quest Ltd. One of
the first materials that the Cambridge
team looked at using HiTUS was hafnium oxide.
Hafnium oxide is an electrical insulator which is used in
optical coatings, capacitors, and transistors, among other applications. Many
companies are currently using hafnium oxide to replace silicon dioxide in
transistors, due to its high ratio of electric displacement in a medium to the
intensity of the electric field producing it, known as a dielectric constant.
The higher the dielectric constant of a material, the higher its capacitance—the
ability to store an electric charge.
Hafnium oxide forms in different crystalline and
polycrystalline structures: monoclinic, cubic, and orthorhombic. However, an
amorphous form is preferable to polycrystalline forms due to the absence of
grain boundaries, the point at which two crystals in a polycrystalline material
meet. Grain boundaries act as conduction paths through thin films of the
material. They not only reduce the resistivity, but lead to a non-uniformity in
conductivity over a large area, which itself leads to spatial non-uniformity in
device performance However, until now amorphous hafnium oxide has had a relatively
low dielectric constant of around 20.
The form of hafnium oxide developed by Flewitt has a
dielectric constant higher than 30.
“Most people thought that all amorphous hafnium oxide had to
exist in the monoclinic-like phase,” says Flewitt. “What we’ve shown is that it
can exist and does exist in a cubic-like phase. This is similar to amorphous
carbon, where you can get diamond-like properties out of amorphous carbon
material.”
Amorphous dielectrics are more homogenous than other forms,
allowing improved uniformity from one device to another, and the absence of
grain boundaries results in higher effective resistivity, as well as less
optical scatter.
The material is produced using a room-temperature,
high-deposition rate process, making it particularly suitable for plastic
electronics and high-volume semiconductor manufacturing. The absence of grain
boundaries makes the material ideal for optical coatings and more efficient
solar cells.
Cambridge Enterprise, the University’s commercialization
group, is currently seeking commercial partners for collaborative development
and licensing of this material.
SOURCE – University of Cambridge
