7 February 2012

Cambridge’s new form of hafnium oxide boosts dielectric constant from 20 over 30

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.

Developed by Dr Andrew Flewitt’s research team in the Department of Engineering’s Electronic Devices & Materials Group, the new form of hafnium oxide provides higher dielectric constant compared with existing forms of hafnium oxide, which is already a key material in the electronics industry.

Normally, metal oxides are produced on substrates by sputtering (where some of the atoms of an electrode are ejected as a result of bombardment by heavy positive ions). However, one of the problems when attempting to make high-quality electronic materials through sputtering is the difficulty in precisely controlling the energetics of the deposition process, and hence the material properties such as defect density.

To enable much greater control of the material properties, Flewitt’s team began using HiTUS (High Target Utilisation Sputtering) deposition technology to promote plasma sputtering. Hafnium oxide was one of the first materials that the team looked at.

Hafnium oxide is an electrical insulator that is used in optical coatings, capacitors and transistors, among other applications. In particular, many firms are using it to replace silicon dioxide in transistors, due to its high dielectric constant (ratio of electric displacement in a medium to the intensity of the electric field producing it). The higher the dielectric constant of a material, the higher its capacitance (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 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 about 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,” he adds. “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 not only higher effective resistivity but also 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 also makes the material ideal for optical coatings and more efficient solar cells, it is reckoned.

Cambridge Enterprise Ltd, the university’s commercialization group, is currently seeking commercial partners for collaborative development and licensing of the new material.

Tags: Hafnium oxide

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