MESA+ Institute for Nanotechnology

The trend in information technology is toward large-screen, high-resolution, low-power, low-cost and flexible displays.

Flat panel displays have a large area of applications and it has been predicted that the display market is going to increase in the next years. Amorphous-silicon transistors, while widely used nowadays as switching devices in displays, have too low carrier mobility and poor stability for integrating the circuitry on glass or plastic substrate. Therefore, transistors with good characteristics need to be realized at low temperature, compatible with the cheep glass or polymer substrate.

The gate dielectric is fundamental for the good functioning of the transistor, however until recently the quality of the dielectric was degraded considerably with decreasing the temperature.

The challenge of this PhD research is to develop a technology for realizing a high quality dielectric in terms of interface properties and breakdown field at near room temperature.

A low-temperature deposition method called electron cyclotronic resonance (ECR) PECVD has been used for depositing silicon oxide and silicon nitride films. Optimising the film properties has proved to be difficult due to the opposite dependencies of electrical properties upon the deposition parameters. For low pressure and high microwave power, the ECR plasma was very energetic and the ions bombard efficiently the substrate. Therefore, for these conditions, dense layers with low hydrogen content and excellent dielectric strength can be produced. The ECR plasma deposited SiO2 obtained at near room temperature exhibits resistivity, breakdown field and charge-to-breakdown similar to the ones of thermally grown oxide. However, very high microwave power and extremely low pressure have deleterious effects upon the interface due to ion bombardment. Therefore the film optimisation required studying in detail the effects of various deposition parameters upon the film properties.

The hydrogen incorporation at high pressure and low microwave power proved to be responsible for the increased leakage current, high trapping behaviour and poor reliability. Stoichiometric Si/O ratios were obtained by highly diluting the SiH4 precursor in helium.

Silicon nitride layers deposited at near room temperature exhibited also excellent bulk properties and under 1 at.% hydrogen content. The oxygen contamination was minimised at low pressures for an increase dissociation degree of N2. Oxygen atoms have a beneficial role upon the films interface with silicon, without degrading the dielectric strength; therefore silicon oxynitride can be a solution for obtaining layers at near room temperature with both good interface and bulk properties.

In order to improve furthermore the film properties, Jet Vapour Deposition was explored. A high-speed jet of helium transporting the SiH4 to the wafer was produced with a convergent-divergent nozzle. The shape and dimensions of the nozzle were obtained by simulating the gas flow, according too the fluid dynamics. Experiments have shown that increasing the pressure at the entrance of the nozzle can eliminate the deleterious effect that high pressure has upon the bulk properties. The extra kinetic energy brought by the supersonic jet densified the deposited films. Annealing processes did not modify the dielectric strength of the layers, showing that ECR –JVD plasma is suitable for low temperature deposition.

Metal–oxide –semiconductor capacitors were used mainly for investigating the dielectric properties. The ECR and JVD films were better than LPCVD and rf-PECVD dielectrics. Transistors made with low temperature dielectrics confirmed the high quality of the films in terms of low threshold voltage and high mobility.