Fabrication

A number of devices have been used throughout this thesis to conduct electrical measurements on grown material. The devices used were fabricated in-house using a number of techniques described in this section.

3.3.1 Optical Lithography

Optical lithography is a fabrication technique used to produce micron scale features for devices. The process involves transferring a polymer (resist) pattern onto a wafer through the using a mask and UV exposure. This patterned resist is then used as a stencil for deposition of etching processes used to fabricate micro devices.

Before lithography, the sample surface is rinsed with a primer to clean the surface and improve resist adhesion. The resist is dripped onto the sample, to completely cover the surface. Uniform spreading of the liquid resist is achieved by spinning the sample, the speed used also determines the thickness of the resist layer. This is then solidified by baking the sample on a hot plate. The desired pattern is transferred onto the resist, by placing a quartz and chrome mask containing the pattern over the sample and exposing the surface not covered by the chrome covered pattern to UV light. The UV light can either strengthen or weaken the polymer bonds depending on whether the process is negative or positive. Submerging the wafer in a ‘developer’ solution removes the weaker resist, to form the final patterned surface, ready for material deposition or etching. Additional flood exposures and baking may also be used for some resists for the formation of undercuts in the patterned resist. This is typically used to prevent deposition onto side walls which can cause problems in lift off. The general schematic for optical lithography is given in figure 3.8.

3.3.2 Deposition Techniques

Deposition of metals and oxides in this project were preformed by either sputtering, electron beam evaporation or thermal evaporation. Deposition typically requires low pressures (<10-6_{mbar) to prevent the incorporation of impurities into the evap-}

orated layer. The surface of the material receiving the evaporated metal must also be cleaned by either limited sputtering or submersion in a weak acid to remove sur- face impurities or native oxides. During deposition, a crystal monitor can be used to

Figure 3.8: A demonstration of a negative process typically used for metal contact deposition and positive lithography process used for defining a mesa.

measure the thickness of the deposited layer in-situ and achieve precise thicknesses.

Evaporation

In thermal evaporation, the target material is placed in a W or alumina crucible and the patterned substrate is mounted directly above. A shutter is positioned between the target material and the substrate. Once the chamber is at the appro- priate pressure, a current is passed through the crucible to induce Joule heating of the filament/crucible and the target material. Typically the current is ramped up

gradually until the desired deposition rate is reached to prevent flash evaporation of the target material. Typically, the shutter is held in place for a short amount of time once the desired deposition rate is reached, to allow the evaporation of any contaminants on the surface of the target material. Deposition onto the surface of the substrate is initiated and stopped by operating the shutter. Thermal evapora- tion is limited by the melting point of both the target material and the crucible. The thermal evaporator did not facilitate multiple sources and was not fitted with a crystal monitor, for this reason it was used to deposit single layer metallic contacts where thickness was not important.

Electron-beam evaporation follows a similar procedure, instead of Joule heat- ing, an electron gun is used to fire a beam of electrons and accelerate them onto the target material to produce heat. The e-beam evaporator contained multiple sources and had a minimum deposition rate of 0.1 ˚As−1. This technique was used for thin film deposition and multilayer depositions to minimise contamination between deposited layers. Both electron beam and thermal evaporation typically produce amorphous samples when evaporating oxides.

Sputter Deposition

The basic concept of sputter deposition is the acceleration of ions towards a target material in order to knock the target atoms free, to be deposited onto a substrate, this is illustrated in figure 3.9. In a sputterer, the target material is placed on a cathode, whilst the substrate is mounted opposite to the target on an anode. A small amount of inert gas is released in the vacuum chamber and ignited into a plasma. The ions in the plasma are accelerated towards the cathode and collide with the target. The transfer of kinetic energy ejects the neutral target atoms which proceed to travel in straight trajectories and deposit on the substrate. A magnetron sputterer uses magnets situated under the cathode to confine the free electrons in spiral like paths above the target to encourage the re-ionization of the plasma gas

Figure 3.9: A simplified diagram of DC sputtering deposition. The insert shows the ejection of a target atom after the impact of an ion.

and enhance sputtering rates.

The process described is defined as DC sputtering, which is not effective for insulating materials as a positive charge can build up on the target material and oppose the motion of sputtering ions, instead insulating films are deposited using radio frequency (RF) sputtering. RF sputtering alternates the potential (on the order of MHz) of the cathode such that the positive charge build-up can be dissipated on the reversal of the cathode.