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Preliminary observations of the plasma etching system involved characterization of the system and development of a safe working protocol. As part of this the electrical layout was observed and the breakdown voltage was calculated. Residence time was also calculated and spectroscopic measurements were conducted. Determining these initial parameters enabled the continued investigation into the etching system.

Post deposition treatment methods are routinely utilized for removal of unwanted material or to modify a material structure. Chemical etching is commonly employed, which involves the risk of contamination from the hazardous working chemicals. Specialist processing conditions may require elevated temperatures and/or vacuum environment for operation. Combination of the environment and working conditions results in an expensive and potential harmful process (to the operator and external environment).

This research focused on a plasma etching system that utilizes the afterglow from an atmospheric pressure glow discharge. This was a low temperature process, where by activated species were ejected from the glow towards the sample surface. These active species interacted with the sample and etched the surface. The plasma was established by using a dielectric barrier discharge (DBD) arrangement with nitrogen as the main feedgas for discharge. High voltage and frequency discharge permitted the efficient breakdown of nitrogen at atmospheric pressure. Critical for effective etching was the uniformity of the process. This was accomplished by the application of the DBD configuration and glow discharge, which allowed stable uniform discharges for etching.

In this design the discharge was parallel to the sample, to avoid any direct interaction. Gas flow was directed through the plasma region towards the sample. A schematic of the system configuration is given in Figure 63.

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Figure 63 -Dielectric Barrier Discharge Plasma Nozzle System

Dielectrics are a poor conductor but are effective at permitting electrostatic fields. They are employed into the discharge design to localise the plasma without establishing an electrical short. Electrodes are situated on either side of the dielectrics and when the electric field was applied, breakdown of the gas occurred within this region. Nitrogen was the bulk feed gas mixture for the system, and was effectively broken down by the electrostatic field. Excited species generated in the plume were ejected from the plasma due to the gas flow direction. Etching of the sample occurred from interaction of these excited species. Waste by-products were removed by localised extraction in the housing.

Generation of the electrostatic field was from an AC supply that was tuned to first order resonance frequency. Application of resonant frequency enabled maximum transmission of electrical energy into the plasma. This reduced losses in the electrical components through heat and other sources.

Page | 158 To give a brief overview of the system regarding the etching process, the following flow diagram (Figure 64) gives a typical process route taken during experimentation. Additional steps may be taken for specific processing steps for example the HCl introduction and techniques to monitor the process conditions. Note not all steps of the process are explicitly listed.

Figure 64 – Flow diagram of plasma etching process

Having conceived a work flow it was important to begin characterisation of the plasma system. The next section of work observed the route taken to refine system parameters and configuration. Within this body of work, the etching investigation was split into two sections, tin oxide and zinc oxide etching. A distinction was noted between these film compositions requiring tailoring of the etching parameters. Small quantities of hydrochloric acid (HCl) were introduced into the nitrogen flow for F: SnO2 modification. Zinc oxide alternatively established a genuine etching effect operating with nitrogen as the bulk feed gas. The preceding sections focused on the modifications to the plasma system for improved etching followed by tin oxide and zinc oxide modification.

Page | 159 Establishing the plasma required a series of specific electronic components, enabling the discharge, manipulation of process parameters and measurement of the input values. Depicted in Figure 65 is the configuration required to develop and sustain a discharge. Multiple improvements have been made to several parts of the configuration, but the essential layout remained unchanged. Development of this system was conducted in house at the University of Salford by Dr Hodgkinson, and further details of this system are documented in the paper (333).

Page | 160 Calculation of the power input involved the standard equations of power in relation to current and voltage. Due to the configuration of the system, measurements of the voltage only assumed half the input voltage (point of measurement takes one half of the signal generator). Power and breakdown voltage can be calculated from equation (6. 1) and (5. 3) respectively.

(6. 1)

(6. 2)

- Power

– Voltage Input Primary Side – Current

Also to note the transformer possesses a turn‟s ratio of 1:85. The breakdown voltage generated was sufficient to breakdown nitrogen and oxygen at atmospheric pressure. However these idealised calculations did not account for losses due to heat and other sources in the system.

Characterization of the plasma configuration began by uniformity assessment and establishing the residence time experienced by the sample as it travels under the nozzle. Several trial samples etched under the nozzle were analysed by AFM over the entire area. As the plasma width between the electrodes was greater than the sample size and sufficient gas distribution system the samples were uniformly etched. Minimal variation was noted in surface structure and roughness in the AFM data confirming the etching uniformity.

The width of the nozzle was noted as the separation of the dielectrics within the housing as shown in Figure 66. Activated species should be present in this area; this was established as the active etching region. Residence time was therefore depicted as the time taken for the sample to pass through the active region. Residual etching effects were omitted from this

Page | 161 calculation, as their contribution to accuracy was not determinable. The purpose of this measurement was to give an indication of time rendered in the active region per pass. Table 17 shows the measured times of multiple passes under the active region and equation (5. 3)

gives the calculated residence time.

Figure 66 – Side View of Plasma Nozzle

Pass Time (s) Interval (s)

1 21.67 21.67 2 43.56 21.89 3 65.55 21.99 4 87.72 22.17 5 109.36 21.64

Page | 162 Length of measured translation – 276mm

Seconds‟ average pass

(6. 3)

(6. 4)

Etching of F: SnO2 compared to ZnO required a small quantity of hydrochloric acid to be activated in the plasma. Exploration into the plasma discharge composition may indicate the active species responsible for etching. Assessment of the plasma was conducted by spectroscopic analysis. Spectroscopic measurements were taken of the plasma from a line of sight, pointing directly into the discharge, as shown in Figure 67. These measurements were performed by Dr J Cowpe and Dr R Pilkington through the University of Salford.

Page | 163 Optimum collection of incident light found the nozzle angled at 45o degrees, with the optical fibre positioned within distance of the plume to fully saturate its 12o catchment angle. Quartz was introduced into the measurement field to prevent any adverse effects on the optical fibre whilst retaining optimal light transmission. Established was a standard discharge of 3.4 kHz where by the spectroscopic measurements were taken.

Analysis of the plasma from narrow slit spectroscopy confirmed the plasma was non-thermal. Observed in the spectral analysis is the high presence of nitrogen, oxygen and traces of aluminium. Interestingly the high traces of oxygen indicated several sources possibly stemming from the nitrogen cylinder, atmosphere or the alumina electrodes. Variation in the intensity of the peaks residing in the higher wavelengths varied with time during the discharge. This may have been due to small developments of filamentary discharges or excitation of oxygen sites upon the alumina plates. Despite the source of the gasses, assuming the high etching rates observed originated from interaction with oxygen and nitrogen in the discharge. Preliminary testing of added oxygen noted an increase in the etching effect, with a saturation point suppressing the effects upon reaching a critical flow level. To note the spectral analysis (Figure 68) shows the suggested species in the discharge and cannot be completely confirmed without further in-depth analysis.

Assessment of the plasma etching system has developed an operational work flow and initial system characterization. Spectroscopic measurements have shown some of the possible etchant species available in the discharge. Consideration of the available species will be critical for effective etching of films. Further work introduces improvements to the waste handling system and develops an upgraded etching environment.

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