Reactive sputtering

The sputtering deposition technique was developed several decades ago as an alternative to other metallization techniques. Ever since it has been gaining popu- larity and it has been modified to deposit various materials. The advantages of the sputtering technique are:

• Versatility, as many different materials may be deposited.

• Good adhesion of the deposited film as the species reach the sample with an energy up to several eV .

• Good control of the deposition rate.

In this dissertation a sputtering reactor for the deposition of AlN films has been used. A pulsed DC magnetron reactive sputtering deposition equipment has been used.

Sputtering principle

The sputtering deposition technique is based on the bombardment of a target material with the ionized species from a cold plasma. The impinging ions with the target material produces the material ablation and the deposition on the exposed surfaces. The process follows the following steps:

1. Plasma formation of an inert gas (typically Ar is used) 2. Positive ion acceleration and target bombardment 3. Energy transfer

4. Material ablation from the target

The target acts as the negative electrode for the plasma discharge. Therefore the heavy positive ions are accelerated across the stealth toward the target. The impact of the ion on the target produces secondary electrons that are accelerated toward the plasma where new ions are generated by the collisions of the electrons. This is the mechanism that balances the ion loss and production for achieving a self maintained plasma discharge.

The plasma may be generated using any high atomic mass inert gas. The high atomic mass is required to enhance the bombardment of the target and increase the material ablation. Typically Ar (m = 40) is used due to availability and cost reasons.

Magnetron effect

If a magnetic field is added in the stealth region the electron trajectory will the modified. The electrons will describe helical trajectories increasing the impact ratio and therefore enhancing the ionization cross section. Thus the discharge current is

increased and the deposition rate is higher. The most common configuration is to use a planar magnet so the field lines are parallel to the target surface closing at the center. This configuration modifies the trajectory of the secondary electrons and confines them to the target proximity. The effect is that the plasma has a higher ionization but is magnetically confined to the proximity of the target. Hence the substrate is separated from the plasma and the damage by high energy radiation from the plasma is largely reduced.

Pulsed DC signal

Another improvement to the technique is the used of a pulsed DC signal rather than a continuous voltage. It has the advantage over an RF signal that high sputtering duty cycles are maintained while improves the DC sputtering by reducing the electric arc formation in the chamber. The pulsed DC principle is illustrated in figure 2.4.

The deposition of dielectric material produces accumulation of insulating mate- rial on the chamber walls. During the discharge, this material becomes charged and uncontrolled electric arcs may be formed. The electric arcs produce undesired reac- tions that lead to contamination of the deposited film. Moreover, discharge arcs may eventually damage the deposition equipment.

Pulsing the signal prevents the arc formation and solves this problem. The DC signal is maintained during a time τon and the target is negatively polarized with

hundreds of volts. After this time the voltage is reverted and the target is positively polarized, typically in the range of 20 V . This period is called the reverse period, τrev and is maintained to allow the controlled discharging of the chamber surfaces.

Usually a 90% duty cycle signal is sufficient to prevent the arc formation. More details may be found in [85].

Reactive sputtering

The reactive sputtering technique consists in adding a reactive gas to the plasma used to sputter the metallic target. It has to be taken into account that the presence of a reactive gas in the plasma changes the discharge conditions and the plasma interac- tions. The reactive species get also accelerated toward the target. The metallic atoms reaching the substrate surface act as a sink for the reactive species. Although the capture speed depends on the number of metallic atoms reaching the substrate, the composition, the structure and the temperature of the process, the most important parameter is the reactive gas proportion in the chamber.

If the reactive gas flux is small compared to the inert gas flux, it gets completely captured by the growing film on the substrate. However if stoichiometry in the layer is overcome, the trapping of the gas turns inefficient and the concentration of reactive gas in the plasma increases significantly. If this increase takes place the reactive gas reacts on the target surface contaminating it. Getting back to the efficient trapping regime requires reducing the reactive gas partial pressure far below stoichiometry, in order to allow the contaminated layer on the target to be sputtered away.

Deposition setup used in this thesis

The deposition chamber is shown on figure 2.5. It is a custom equipment that has been built and optimized at the ISOM facilities by Dr. G. F. Iriarte. Here a general description of the equipment will be included, for an in depth description of the setup and calibration please refer to [86, 87].

The system is formed by the following elements:

• A dual chamber with a precharge chamber. Two chambers are used in order to maintain a good vacuum in the main deposition chamber.

• Heated susceptor. The sample can be heated up to 900◦_{C. However, this feature}

was not used in this thesis and all the depositions were performed at room temperature.

• Dual pumping system. A rotary and turbomolecular pumping system is used to ensure base pressures of 10−4 _{mtorr which is sufficiently low for high quality}

Figure 2.5: Reactive sputtering equipment used in this thesis for the AlN film deposition. • Gas system. A gas control system is used for controlling the fluxes of Ar and

N2 injected in the chamber as well as the working pressure.

• Target. A 99.9995% aluminum target is used for the sputtering. The target holder allows adjusting the target-substrate distance. The target holder also has a planar magnetron setup.

• Pulsed DC 500 W power source.

Deposition process

Although all the substrates are prepared for the deposition by undergoing a clean- ing procedure and the precharge chamber was used, a chamber preparation process was applied prior to every deposition. After the samples are loaded and the base pressure is achieved the target is sputtered only with Ar for cleaning it from con- taminants. During this process the sample is placed under a shutter in order to prevent the metal deposition. After a determined cleaning time, N2 is insufflated in

the chamber and the sputtering is initiated at the preconfigured parameters. The shutter covering the sample is only opened after the reactive sputtering conditions have been stabilized.