Optical Analysis of RF Sputtering Plasma Through Colour Characterization

Article Optical Analysis of RF Sputtering Plasma through Colour Characterization

Ali Salimian *, Roohollah Haghpanahan, Abul Hasnath and Hari Upadhyaya * Centre for Advanced Materials, School of Engineering, London South Bank University, 103 Borough Road, London SE1 0AA, UK; [email protected] (R.H.); [email protected] (A.H.) * Correspondence: [email protected] (A.S.); [email protected] (H.U.)

Received: 24 February 2019; Accepted: 6 May 2019; Published: 10 May 2019

Abstract: The photometric properties of an radio frequency (RF)-based sputtering plasma source were monitored through optical spectroscopy. The colour of the plasma source was deduced based on conventional chromaticity index analysis and it was compared to the direct spectral data plots of the emission peaks to investigate the possibility of characterising the plasma based on its specific colour and exploring the potential of defining a new method by which the plasma sputtering process can be addressed based on the plasma colour parameters. The intention of this investigation is to evaluate the possibility of simplifying the monitoring and assessment of the sputtering process for applied scientists operating plasma sputter deposition systems. We demonstrate a viable potential for this technique in terms of providing information regarding the stability of the plasma, chamber pressure, and plasma power; however, further work is underway to verify and assess a relationship between the quality of the thin film coating and the colour characteristics of the deposition plasma. Here, we only focus on the feasibility of such an approach and demonstrate interesting observations. We observed a linear relationship between the colour functions and the plasma power, while the stability of the sputtering plasma can be assessed based on the plasma colour functions. The colour functions also follow a unique pattern when the working gas pressure is increased.

Keywords: sputtering; colour functions; plasma; TCO

1. Introduction Transparent conductive oxides (TCO) are materials that have attracted a significant amount of interests due to their vast application areas. They are an essential part of various optoelectronic devices, solar cell modules, light emitting diodes, and flexible electronics that involve optical elements and the physical vapor deposition (PVD) viz. sputtering under vacuum is one of the methods of depositing a range of materials as a thin film coating, mobile phone touch screens, perovskite solar cells, and OLEDs are examples of technologies that rely on TCOs [1]. When sputter deposition of complex materials, such as metal oxides belonging to the TCO family of materials, is carried out, one can imagine how vital the sputtering conditions are to the way that the atoms are deposited and laid on the substrate surface, which will ultimately define the work function, electron affinity, band-gap, and all of the relevant functional parameters of the as-deposited material. However, plasma sputter deposition technique does not behave like a standard chemical reaction or a process with defined steps. For example, researchers who apply this technique for TCO preparation usually report their findings by stating the condition of the sputtering process, such as chamber pressure, plasma power and the gas composition of the chamber during deposition. However, from one sputter machine to another, depending on the dimensions of the chamber, the size of the targets, and a few other design related issues, TCO coatings with different properties are obtained, even if the sputtering conditions are maintained according to an earlier report. The authors have constantly experienced this ambiguity. We believe there is another way

Coatings 2019, 9, 315; doi:10.3390/coatings9050315 www.mdpi.com/journal/coatings Coatings 2019, 9, 315 2 of 19 of defining the sputtering conditions. Surely, the plasma in the sputtering procedure is the core of the reaction and is fundamental to the deposition of the thin film coatings. By evaluating the plasma, we can define a set standard for deposition procedures, regardless of the machine being used. We explore this concept by considering the sputtering target surface and the plasma as a light source. However, characterising plasma through optical spectroscopy is nothing new. Currently, plasma physicists study and investigate plasma by means of Emission spectroscopic techniques [2–8]. The diagnosis of laboratory plasma is usually carried out by optical emission spectroscopy (OES), through which numerous analytical techniques are established to determine the plasma properties, such as electron density, plasma temperature, elements recognition, and quantification of elements that are present in the plasma. The radiative behavior of the atomic constituents of any plasma can only be predicted if the expected populations of possible states are known and the atoms obey the Boltzmann distribution for every possible state and radiation energy density. Subsequently, it can be assumed that a thermal equilibrium condition is present [4–9]. However, creating such a condition in the laboratory would be almost impossible, and hence the concept of local thermal equilibrium (LTE) condition is considered. LTE can be described as a state where the Boltzmann and Saha equation, which governs the distribution of energy level excitation and ionization temperature, are equal to the Maxwell–Boltzmann distribution of free electron velocities [8–10]. Achieving the LTE condition depends on defining the plasma by a common temperature T and the existence of sufficient large electron density. McWhirter proposed a criterion where there is a need for a critical amount of electron density for LTE conditions to exist. This criterion is demonstrated by Equation (1), where Ne is the electron density, T is the plasma temperature, and ∆E is the energy gap [11–14].

12 1 3 3 Ne 1.6 10 T 2 (∆E) cm (1) ≥ × The temperatures of ions and electrons are directly proportional to their random average kinetic energy, and the distribution of velocities for each particle is governed by the Maxwell distribution when thermal equilibrium conditions apply. Under the LTE conditions, the same temperature is assumed for electrons, ions, and atoms in the plasma and the plasma temperature is referred to as the temperature of the electron [4]. Various methods of plasma temperature determination exist, however, the simplest method by far is the Ratio method, which utilises the intensity of two spectral lines for calculating the temperature of the plasma. The intensity of a spectral line associated with a transition is expressed by Equation (2). In this equation, Iij is the intensity of transition from state i to state j, Aij is the transition probability between the two states, c is the speed of light, N is the number density of electrons, h is the plank constant, gj is the statistical weight of the upper level (j) and λij is the wavelength corresponding to the transition, Ej is the energy of level (j), T is the excitation temperature and k is the Boltzmann constant, and the U(T) is referred to as the partition coeﬃcient, which accounts for the probability and degeneracy of the two states (i and j).

hcAij gjN Ej Iij = e− kT (2) λijU(T)

From the spectral data obtained, two spectral lines can be chosen from the same species and their ionization stages, e.g., Ar I, where there is a large diﬀerence in the upper energy level. By taking a ratio (I1)/(I2) of the intensities of the two selected lines, the constants cancel each other out and we will be left with Equation (3), where I1 and I2 are the intensity of comparative peaks due to their respective ij transition [15]. E E I1 g1 A1 λ2 [ ( 1− 2 )] = e − kT (3) I2 g2 A2 λ1 Coatings 2019, 9, 315 3 of 19

Solving Equation (3) will give the value of T in electron volts. This method is the simplest method of calculating the temperature of the plasma, and its accuracy is conditional upon using two lines with a maximum diﬀerence in their upper energy states. As an alternative to the ratio method discussed, a method that is referred to as the Boltzmann method can be applied to calculate the temperature of the electron. This method also utilises Equation (2); however, with a rearrangement. The equation is rearranged to give Equation (4). Taking the natural logarithm of both sides of Equation (4) would give us Equation (5) [4,12–15]. ! λijIij N Ej = e− kT (4) hcAij gij U(T) ! ! λijIij 1 N ln = Ej + ln (5) hcAij gij −kT U(T)

By plotting the first term of Equation (5) against Ej, the slope of the plot can give us the value of 1/kT, from which value of T can be extracted. The electron density can be calculated by using various methods, for example, by applying the Stark broadening relationships. The Stark broadening is caused by the electric field of electrons and ions interfering, which results in the broadening of the spectral line. The interference of the mentioned electric fields causes fluctuations in the field of plasma as the radiating atoms are surrounded by the interfering electrons and ions. The electric field of the electrons or ions causes a perturbation of energy levels that are close to the continuum, while simultaneously affecting the externally applied electric field, which ultimately causes the observed spectral broadening. The magnitude of this broadening in terms of its full width half maximum (FWHM) ∆λ1/2 is given by Equation (6).

1 Ne Ne 4 1 Ne ∆λ1/2 = 2ω + 3.5A (1 1.2ND− 3 )ω( ) (6) 1016 1016 − 1016

In this equation, ω is the electron impact parameter, A is ionic impact broadening parameter, Ne is the electron density, and ND is the Debye shielding parameter. The first term in this equation is associated with broadening that is caused by electrons and the second term is associated with the broadening that is caused by ions [16]. As the emissions from argon species is considered for the measurements, we can hence eliminate the second term of the equation that relates to the other ions to obtain Equation (7) [15–17]. Hence, rearranging the equation and obtaining the ∆λ1/2 value can calculate the Ne. Ne ∆λ1/2 = 2ω( ) (7) 1016 All of the methods that we have mentioned so far are some of the OES methods of measuring the electron density and electron temperature in a plasma. These methods are familiar among plasma physicists; however, when it comes to the application of sputtering plasma and thin film analysis, using the above-mentioned techniques will require the necessary expertise and precision spectroscopy and data interpretation. Many scientists or operators who use plasma deposition for the preparation of thin film coatings would ultimately require a machine that would be easy to use and produce consistent thin film coatings. Our objective is to simplify the operation of the sputtering technique for the users and possibly propose a simpler monitoring diagnostic methodology for the plasma deposition process. We are reporting a novel strategy of the in-situ characterization of plasma in a sputtering process by translating the emission spectra into a colour indexing (chromaticity) pattern that is able to quickly monitor any changes in the plasma character with a change in the operating parameter conditions during the course of the process, which in turn provides a better handle to reflect the instabilities of the tool and its optimizing parameters. This is a radical conceptual proposition that can potentially simplify the evaluation of the sputtering plasma for applied scientists by theorising a unique characterisation parameter for particular plasma deposition conditions, just like a fingerprint. As an analogy, to clarify Coatings 2019, 9, 315 4 of 19 our objective, we can divert our attention to the human genome that defines each individual person based on the composition and arrangement of billions of complex genetic coding embedded within each person’s DNA, complex laboratory procedures can be used to sequence and encode the genetic makeup of individuals; however, a simpler approach to distinguish people from one another is to check their fingerprint or iris patterns, rather than conducting the complex genetic complexity that defines the individual. Indeed, that is the objective of this communication. By considering the plasma as a light source, we will focus on its most apparent character; its colour. We will be focusing on the colour of the sputtering plasma light and identifying patterns that may relate its colour to certain fundamental parameters that are involved in generating the plasma, such as the driving power, the working gas, and its pressure. Plans are underway to further investigate the relationship between the thin film coatings that are deposited via this technique and the plasma colour with reports in due time. The colour of any light source can be mathematically described through what is known as colour spaces. A colour space is a completely-specified scheme for describing the colour of light, ordinarily using three numerical values (called coordinates). An important colour space, defined by the International Commission on Illumination (CIE, the initials of its French name), is the CIE XYZ colour space [18]. However, it is wholly defined in terms of human’s perception of light through their eyes. If two instances of light appear to a viewer to be the same colour, they are the same colour. Colour is usually recognized by the viewer as having two aspects:

Luminance, as an indication of the “brightness” of the light. • Chromaticity, the property that distinguishes colours. • Colour is not a primary physical property, like the temperature or pressure of a gas. Colour is related to the energy of photons that make up the light. Plotting the intensity of the photon energies (power) that are associated with the relevant wavelength will produce a spectral plot. The intensity and distribution of the emitted wavelengths that ultimately shape the plot determine the chromaticity of the light; its overall “vertical scale” determines its luminance [18]. In fact, there can be several instances of light with different spectrums, which nevertheless have the same colour. This situation is called metamerism. Through mathematics, the XYZ colour spaces are used to construct the x,y colour function, which is presented in Figure1A, where the specified area under the defined emission spectrums are used to calculate the coordinates in Figure1B. A luminance-chromaticity space (Figure1B) is then constructed by defining two values; x and y, as follows: X x = (8) Z + Y + X Y y = (9) Z + Y + X In these equations, as described in Figure1B, the X, Y, and Z represent the area under emission peaks in the regions. By plotting the x against y, the CIE x, y chromaticity diagram is constructed and is used to give a specific sense to a particular colour (Figure1B). Another photometric parameter is the radiance and spectral radiance of the plasma light. The radiance indicates how much radiant flux is emitted by a surface when it is received by an optical system looking at that surface. The radiance value is calculated by measuring the Radiant flux of an emission source, which is the radiant energy emitted per unit time. Spectral radiance expresses radiance as a function of frequency or wavelength and the spectral plot of the emission is based on the radiance value as a magnitude of intensity [18]. Within a sputtering plasma, a significant number of energy transitions are occurring, leading to a complex emission spectrum. These emissions ultimately define and construct the spectral plot of the plasma as a light source that is observed in plasma. The efficiency and characteristics of the sputtering process depend on the characteristics of the plasma, which in turn depend on the density of the particles and energies within the plasma [19]. Through complex procedures, some of which Coatings 2019, 9, x FOR PEER REVIEW 4 of 18

will be focusing on the colour of the sputtering plasma light and identifying patterns that may relate its colour to certain fundamental parameters that are involved in generating the plasma, such as the driving power, the working gas, and its pressure. Plans are underway to further investigate the Coatingsrelationship2019, 9, 315between the thin film coatings that are deposited via this technique and the plasma5 of 19 colour with reports in due time. The colour of any light source can be mathematically described wethrough already what discussed, is known the electronas colour temperature spaces. A andcolour electron space density, is a completely-specified meta-stable atoms, and scheme ions canfor bedescribing deduced the and colour estimated of light, from theordinarily spectral using plot [20three–23 ].numerical If the colour values parameters (called cancoordinates). be identified An andimportant linked colour to the existingspace, defined plasma by analysis the Internationa techniques,l Commission it can benefit on applied Illumination scientists (CIE, to performthe initials and of its French name), is the CIE XYZ colour space [18]. However, it is wholly defined in terms of human’s conduct thin film depositions and can have an easier method of assessing the specifics of the plasma perception of light through their eyes. If two instances of light appear to a viewer to be the same being used. colour, they are the same colour. Colour is usually recognized by the viewer as having two aspects: Due to the extensive optoelectronic applications, high interest in transparent conductive oxides, such• asLuminance, ITO, AZO, as and an IZO,indication and their of the deposition “brightness” characteristics of the light. via plasma sputtering exists and there is• plentyChromaticity, of literature the available property from that various distinguishes research colours. groups [ 24,25]. These materials are of high interest to many photovoltaic researchers, and hence we have focused on evaluating the light from a sputtering magnetronColour plasma is not sourcea primary that physical was fitted property, with an IZOlike the target temperature and the objective or pressure here isof toa gas. take Colour a radical is diversionrelated to from the energy the complexity of photons of thethat conventional make up the plasma light. Plotting characterisation the intensity techniques of the andphoton observe energies the colour(power) of that the plasmaare associated and explore with the feasibilityrelevant wavelength of exploiting will the produce colour characteristicsa spectral plot. of The the intensity plasma asand an distribution indicator of of the the coating emitted process. wavelengths The authors that ultimately realise that,shape still, the aplot significant determine number the chromaticity of energy transitionsof the light; occur its overall in the UV “vertical and IR scale” region determine of the spectrums its luminance that are not [18]. accounted In fact, for there when can constructing be several ainstances conventional of light chromaticity with different diagram; spectrums, however, which this worknevertheless is an initial have focus the same on clarifying colour. This the feasibility situation ofis suchcalled a radicalmetamerism. approach, Through after which mathematics, further e fftheorts XYZ can becolour invested spaces on are to modelling used to construct a plasma the colour x,y analysiscolour function, systemthat which can is provide presented a shortcut in Figure into 1A, the where characterisation the specified of area a sputtering under the plasma.defined emission spectrums are used to calculate the coordinates in Figure 1B.

Figure 1. (A) The x,y International Commission on Illumination (CIE) chromaticity index. (B) The CIE Figure 1. (A) The x,y International Commission on Illumination (CIE) chromaticity index. (B) The CIE XYZ colour function diagram where the curves are the CIE standard colour matching functions, the XYZ colour function diagram where the curves are the CIE standard colour matching functions, the values for x and y are obtained from the spectral plot of the emitting light, by calculating integral of the values for x and y are obtained from the spectral plot of the emitting light, by calculating integral of peaks X, Y, and Z and applying Equations (8) and (9). These peaks represent the emission wavelengths the peaks X, Y, and Z and applying Equations (8) and (9). These peaks represent the emission from the light source at the specific wavelengths, for example, Z, represents the area under emission wavelengths from the light source at the specific wavelengths, for example, Z, represents the area peak from 380 to 550 nm. under emission peak from 380 to 550 nm. 2. Experimental A luminance-chromaticity space (Figure 1B) is then constructed by defining two values; x and y, as follows:The sputtering machine used for these experiments was a V6000 unit that was manufactured by scientific vacuum systems limited (SVS Ltd., Wokingham, UK) with a vacuum chamber of 𝑋 ~40 cm 40 cm 40 cm. The magnetron𝑥= of the V6000 unit was fitted with a six inch 99.99%(8) pure × × 𝑍+𝑌+𝑋 Indium Zinc Oxide (IZO) target material with copper indium back bond for efficient thermal dissipation. Spectral data from the plasma was obtained by𝑌 placing an in-vacuum collimator optic probe that was 𝑦= (9) made by Plasus GmbH (Mering, Germany).𝑍+𝑌+𝑋 The probe was installed on the magnetron, so that it horizontally collected light from ~1.5 cm away from the surface of the target and at a distance of 4 cm from the edge of the target (Figure2A). The unique feature of the optic collimator is the honeycomb structure of the photon inlet, which traps the sputtering particle and prevents gradual coating of the collimators quartz window. This is highly important, as any coating of the inlet quartz window will Coatings 2019, 9, 315 6 of 19 undermine the reliability of the data obtained. We tested the quartz lens for any possible coating by running a continuous 5 h deposition experiment followed by dismantling the collimators (Plasus GmbH, Mering, Germany) and using UV/VIS spectroscopy (Bibby Scientific Ltd, Staffs, UK) to check the inlet quartz window for any coatings formed to ensure the data were not undermined (Figure3A). The collected light was then guided to two different spectrometers; a Jeti Specbos 1201 spectrometer (Jena, Germany) that calculates the chromaticity index of the light, the Jeti was programmed, so that, Coatings 2019, 9, x FOR PEER REVIEW 6 of 18 for each measurement, it took 20 readings and inputted the average. A second spectrometer; Plasus spectrometer;Emicon Spectrometer Plasus Emicon (Mering, Spectrometer Germany) that (Mering, generates Germany) a detail that spectral generates plot of a the detail emission spectral was plot used of thefor detailedemission spectral was used analysis. for detailed spectral analysis.

(A)

(B)

Figure 2. (A) In-vacuumIn-vacuum collimatorcollimator optics optics was was placed placed adjacent adjacent to theto the magnetron magnetron and and the emissionthe emission data datawere were transferred transferred to the to spectrometers the spectrometers via quartz via quar ﬁber,tz fiber, the distance the distance between between the collimator the collimator and target and targetedge was edge 4 cm.was (4B cm.) The (B Collimator.) The Collimator.

Using the IZO target ﬁtted onto the magnetron, a series of tests were carried out by igniting and running the plasma under various conditions. The working gas, working gas pressure, and radio frequency (RF) plasma power were varied and data were collected. Two types of working gases were used, pure Argon (Gas Ar) and 95% Argon: 5% Hydrogen (Gas ArH). Table1 deﬁnes the experimental regime undertaken.

(A)

(B)

Figure 3. (A) The quartz lens of the collimator was tested using UV/VIS transmission spectroscopy (320 to 900 nm) to ensure the lens was not coated during the experiments. (B) The unique feature of the optic collimator is the honeycomb structure of the photon inlet which traps the sputtering particle and prevents gradual coating of the collimators quartz window.

Coatings 2019, 9, x FOR PEER REVIEW 6 of 18 spectrometer; Plasus Emicon Spectrometer (Mering, Germany) that generates a detail spectral plot of the emission was used for detailed spectral analysis.

(A)

(B)

Figure 2. (A) In-vacuum collimator optics was placed adjacent to the magnetron and the emission Coatingsdata2019 were, 9, 315 transferred to the spectrometers via quartz fiber, the distance between the collimator and7 of 19 target edge was 4 cm. (B) The Collimator.

(A)

(B)

Figure 3. (A) The quartz lens of the collimatorcollimator was testedtested using UVUV/VIS/VIS transmission spectroscopy (320 toto 900900 nm)nm) toto ensure ensure the the lens lens was was not not coated coated during during the the experiments. experiments. (B )(B The) The unique unique feature feature of the of opticthe optic collimator collimator is the is honeycombthe honeycomb structure structure of the of photonthe photon inlet inlet which which traps traps the sputteringthe sputtering particle particle and preventsand prevents gradual gradual coating coating of the of collimators the collimators quartz quartz window. window.

Table 1. The experimental regime conducted to obtain the spectral results from the plasma light source under various conditions in 3 separate test groups. Test 1: where the stability of the plasma was monitored at 100 W under 1.9 10 3 mbar of working gas pressure, under Ar and ArH gas; Test 2: The × − spectral emission of the plasma was monitored while increasing the power of stabilized plasma from 100 to 300 W under 1.9 10 3 mbar of Ar; Test 3: The spectral emission of an stabilized plasma was × − monitored at 100 W plasma power under various Ar and ArH gas pressures. (Note: separate spectral analysis of the plasma power was carried out up to 350 W).

Test Regime Variable Parameter Power (RF) Working Gas Pressure (mbar) Test 1 Time 100 W Ar, ArH 1.9 10 3 × − 100 W 300 W Test 2 Plasma power → Ar 1.9 10 3 @ 100 W per min × − 1.2 10 3 7 10 3 Test 3 Gas pressure 100 W Ar, ArH × − → × − @5 10 4 per min × −

3. Results

3.1. Chromaticity of the Plasma Emissions The emission characteristics in terms of the whole spectral plot and the chromaticity index were obtained for each test group. Test 1, from the moment that the plasma is ignited, the spectral data can be obtained from it. However, by plotting the chromaticity x, y values of the plasma light from a cold start, these values will keep varying for up to 2 h. After 2 h, the variance in the x and y values that were obtained will signiﬁcantly reduce; this is the point where we can consider the plasma to be stabilized or more stable. Figure4 presents this observation. Coatings 2019, 9, x FOR PEER REVIEW 7 of 18

Using the IZO target fitted onto the magnetron, a series of tests were carried out by igniting and running the plasma under various conditions. The working gas, working gas pressure, and radio frequency (RF) plasma power were varied and data were collected. Two types of working gases were used, pure Argon (Gas Ar) and 95% Argon: 5% Hydrogen (Gas ArH). Table 1 defines the experimental regime undertaken.

Table 1. The experimental regime conducted to obtain the spectral results from the plasma light source under various conditions in 3 separate test groups. Test 1: where the stability of the plasma was monitored at 100 W under 1.9 × 10−3 mbar of working gas pressure, under Ar and ArH gas; Test 2: The spectral emission of the plasma was monitored while increasing the power of stabilized plasma from 100 to 300 W under 1.9 × 10−3 mbar of Ar; Test 3: The spectral emission of an stabilized plasma was monitored at 100 W plasma power under various Ar and ArH gas pressures. (Note: separate spectral analysis of the plasma power was carried out up to 350 W).

Test Regime Variable Parameter Power (RF) Working Gas Pressure (mbar) Test 1 Time 100 W Ar, ArH 1.9 × 10−3 100 W → 300 W Test 2 Plasma power Ar 1.9 × 10−3 @ 100 W per min 1.2 × 10−3 → 7 × 10−3 Test 3 Gas pressure 100 W Ar, ArH @5 × 10−4 per min

3. Results

3.1. Chromaticity of the Plasma Emissions The emission characteristics in terms of the whole spectral plot and the chromaticity index were obtained for each test group. Test 1, from the moment that the plasma is ignited, the spectral data can be obtained from it. However, by plotting the chromaticity x, y values of the plasma light from a cold start, these values will keep varying for up to 2 h. After 2 h, the variance in the x and y values that were obtained will Coatingssignificantly2019, 9, reduce; 315 this is the point where we can consider the plasma to be stabilized or more stable.8 of 19 Figure 4 presents this observation.

(A) (B)

Figure 4. The x and y data obtained from the plasma during th thee first first hour of ignition at 100 W for a magnetron fittedfitted with Indium Zinc Oxide (IZO) target under gas Ar (A,B) and gas ArH (C,D) along with data obtained 2 h afterafter ignition.ignition. During this 1 h interval the plots remain relatively stable as presented in the right side figures.figures. The stabilized data were gathered over an additional thirty minutes period. As it can be seen (A,C) prior to stabilisation the x,y coordinates are scattered across the plot area until the plasma is stabilized atat whichwhich pointpoint thethe plotsplots fallfall withinwithin closeclose proximityproximity ((BB,,DD).).

To demonstrate the complexity of relying on the conventional methods, such as the peak intensity ratio method, to monitor the progress of the plasma toward stability, the spectral plot of the 100 W Argon plasma over a period of one hundred and thirty minutes post-plasma ignition are presented in Figure5. As an example, particular transitions that were previously evaluated by other authors for measuring electron temperature and density were chosen to measure these data [26]. The ratio plot that is presented in Figure5A is related to the intensity ratio of 404.44 nm (Ar-I) transition 3s23p5(2P◦1/2)5p 2[3/2] 2 to 3s23p5(2P◦3/2)4s 2[3/2]◦ 1 and 426.62 nm (Ar-I) transition 3s23p5(2P◦3/2)5p 2[3/2] 2 to 3s23p5(2P◦3/2)4s 2[3/2]◦ 1. The ratio plot presented in Figure5B is related to the intensity ratio of 480.60 nm (Ar-II) transition 3s23p4(3P)4p 4P◦ [5/2] to 3s23p4(3P)4s 4P [5/2] and the 470.23 nm (Ar-I) transition 3s23p5(2P◦3/2)5p 2[1/2] 1 to 3s23p5(2P◦1/2)4s 2[1/2]◦ 1. It can be seen that it will be challenging to apply the peak intensity method as an instant real time monitoring technique. This highlights the novelty of using colour function as a mean to monitor the plasma stability as the chromaticity plot of the x and y values clearly demonstrates a linear path toward the stable x, y chromaticity values. Test 2, the RF power that was used to ignite and run the plasma is the source of energy for ionising the working gas and driving the sputtering process. Higher energy will result in more excited and metastable atoms to be created, and consequently will increase the bombardment of the target surface. During this part of our investigation, after stabilising the plasma at 100 W, the power was increased up to 300 W and data was collected at ~9 s intervals under argon. A clear linear relationship can be observed between the chromaticity value and the plasma power, as presented in Figure6. The values of x and y decrease with increasing the RF power and a shift toward blue in a linear manner is also observed. Coatings 2019, 9, x FOR PEER REVIEW 8 of 18

To demonstrate the complexity of relying on the conventional methods, such as the peak intensity ratio method, to monitor the progress of the plasma toward stability, the spectral plot of the 100 W Argon plasma over a period of one hundred and thirty minutes post-plasma ignition are presented in Figure 5. As an example, particular transitions that were previously evaluated by other authors for measuring electron temperature and density were chosen to measure these data [26]. The ratio plot that is presented in Figure 5A is related to the intensity ratio of 404.44 nm (Ar-I) transition 3s23p5(2P°1/2)5p 2[3/2] 2 to 3s23p5(2P°3/2)4s 2[3/2]° 1 and 426.62 nm (Ar-I) transition 3s23p5(2P°3/2)5p 2[3/2] 2 to 3s23p5(2P°3/2)4s 2[3/2]° 1. The ratio plot presented in Figure 5B is related to the intensity Coatings 2019, 9, x FOR PEER REVIEW 8 of 18 ratio of 480.60 nm (Ar-II) transition 3s23p4(3P)4p 4P° [5/2] to 3s23p4(3P)4s 4P [5/2] and the 470.23 nm (Ar-I)To transition demonstrate 3s23p 5(2P°3/2)5the complexityp 2[1/2] of 1 relyingto 3s23 pon5(2P°1/2)4 the conventionals 2[1/2]° 1. Itmethod can bes, seensuch that as itthe will peak be challengingintensity ratio to method,apply the to peakmonitor intensity the progress method of as the an plasma instant toward real time stability, monitoring the spectral technique. plot of This the highlights100 W Argon the plasmanovelty overof using a period colour of functionone hundred as a meanand thirty to monitor minutes the post-plasma plasma stability ignition as theare chromaticitypresented in Figureplot of 5. the As xan and exam y valuesple, particular clearly transitionsdemonstrates that a werelinear previously path toward evaluated the stable by other x, y chromaticityauthors for measuring values. electron temperature and density were chosen to measure these data [26]. The ratio plot that is presented in Figure 5A is related to the intensity ratio of 404.44 nm (Ar-I) transition 3s23p5(2P°1/2)5p 2[3/2] 2 to 3s23p5(2P°3/2)4s 2[3/2]° 1 and 426.62 nm (Ar-I) transition 3s23p5(2P°3/2)5p 2[3/2] 2 to 3s23p5(2P°3/2)4s 2[3/2]° 1. The ratio plot presented in Figure 5B is related to the intensity ratio of 480.60 nm (Ar-II) transition 3s23p4(3P)4p 4P° [5/2] to 3s23p4(3P)4s 4P [5/2] and the 470.23 nm (Ar-I) transition 3s23p5(2P°3/2)5p 2[1/2] 1 to 3s23p5(2P°1/2)4s 2[1/2]° 1. It can be seen that it will be challenging to apply the peak intensity method as an instant real time monitoring technique. This highlights the novelty of using colour function as a mean to monitor the plasma stability as the chromaticity plot of the x and y values clearly demonstrates a linear path toward the stable x, y Coatings 2019, 9, 315 9 of 19 chromaticity values.

(A) (B)

Figure 5. The intensity ration of four peaks are presented over 130 min. (A) The intensity ratio of the (Ar I) 404.44 nm and the (Ar I) 426.62 nm emission peaks. (B) The intensity ratio of the (Ar I) 470.4 nm and the 480.60 nm emission peaks. It can be seen that through time the increase or decrease of intensity ratios is not uniform across all of the selected peaks, hence why it will be challenging to use a method, such as peak intensity method for identifying stable plasma state during the operation of apparatus.

Test 2, the RF power that was used to ignite and run the plasma is the source of energy for ionising the working gas and driving the sputtering process. Higher energy will result in more excited and metastable atoms to be created, and consequently will increase the bombardment of the target surface. During this part of our investigation, after stabilising the plasma at 100 W, the power was increased up to 300 W and data was collected at ~9 s intervals under argon. A clear linear relationship can be observed between the chromaticity value and the plasma power, as presented in Figure 6. The values of x and y decrease with increasing the RF power and a shift toward blue in a Figure 6. The relationship between the power of plasma (radio frequency (RF)) and the x and y values. linearFigure manner 6. The is alsorelationship observed. between the power of plasma (radio frequency (RF)) and the x and y values. The plasma power was raised from 100 to 300 W at a rate of 100 W per minutes and the x and y values The plasma power was raised from 100 to 300 W at a rate of 100 W per minutes and the x and y values were obtained every ~9 s. It can be seen that the x and y values are reduced and the plot demonstrates a were obtained every ~9 s. It can be seen that the x and y values are reduced and the plot demonstrates linear trend when the sputtering power is increased. These plots indicate that the colour of the plasma a linear trend when the sputtering power is increased. These plots indicate that the colour of the is shifting toward deep blue with increasing the power. plasma is shifting toward deep blue with increasing the power. In Test 3, an interesting pattern is observed by increasing the pressure of the chamber under each Gas (Ar or ArH), after the plasma has been stabilized, which is highlighted in Figures7 and8. The test was started with the lowest pressure achievable based on the mass ﬂow controller of the machine (1.2 10 3 mbar) and then the pressure was increased. With increasing the pressure, the x and y values × − startFigure to increase, 6. The but,relationship at a certain between point, the apower loop of is plasma made (a(radio U-turn frequency shape) (RF)) and and it starts the x and to decrease y values. with higherThe pressure. plasma power was raised from 100 to 300 W at a rate of 100 W per minutes and the x and y values were obtained every ~9 s. It can be seen that the x and y values are reduced and the plot demonstrates a linear trend when the sputtering power is increased. These plots indicate that the colour of the plasma is shifting toward deep blue with increasing the power.

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In Test 3, an interesting pattern is observed by increasing the pressure of the chamber under each Gas (Ar or ArH), after the plasma has been stabilized, which is highlighted in Figures 7 and 8. The test was started with the lowest pressure achievable based on the mass flow controller of the machine (1.2 × 10−3 mbar) and then the pressure was increased. With increasing the pressure, the x Coatingsand y values2019, 9, 315start to increase, but, at a certain point, a loop is made (a U-turn shape) and it starts10 of 19to decrease with higher pressure.

(A) (B)

(C)

FigureFigure 7.7.The Thex andx andy data y data obtained obtained by Jeti by spectrometer Jeti spectrometer from the from plasma the at plasma 100 W 2at h after100 W stabilization 2 h after forstabilization a magnetron for ﬁtteda magnetron with IZO fitted target with under IZO gas target Ar atunder various gas pressuresAr at various (A). Newlypressures added, (A). pleaseNewly conﬁrm.added, please As the confirm. Argon pressureAs the Argon is increased, pressure the is increased,x and y values the xincrease and y values until increase about 2.5 until10 about3 mbar 2.5 × − and× 10 then−3 mbar start and to decrease then start (B ).to The decreasex and y (plotB). The demonstrates x and y plot a U-turn demonstrates formation, a indicatingU-turn formation, that the colourindicating is initially that the shifting colour toward is initially white shifting and then toward reverting white towardand then blue reverting (C). toward blue (C).

(A) (B)

(C)

FigureFigure 8.8.The Thex andx andy data y data obtained obtained By Jeti By spectrometer Jeti spectrometer from the from plasma the at plasma 100 W 2at h after100 W stabilization 2 h after forstabilization a magnetron for ﬁtted a magnetron with IZO fitted target with under IZO gas target ArH atunder various gas pressures ArH at various (A). As pressures the Argon ( HydrogenA). As the mixArgon pressure Hydrogen is increased, mix pressure the x andis increased,y values the increase x and until y values about increase 2.5 10 until3 mbar about and 2.5 then × 10 start−3 mbar to × − decreaseand then ( Bstart). The to decreasex and y plot (B). demonstrates The x and y plot a U-turn demonstrates formation, a U-turn indicating formation, that the indicating colour is initiallythat the shiftingcolour is toward initially white shifting and toward then reverting white and toward then bluereverting (C). toward blue (C).

The spectral plot of the plasma emission under various pressures clearly shows an increase in the intensity of some of the transition lines with increasing the pressure, which ultimately aﬀects the x and y reading (for example, the data obtained from IZO target and Argon gas at various pressures is presented in Figure9). Coatings 2019, 9, x FOR PEER REVIEW 10 of 18

The spectral plot of the plasma emission under various pressures clearly shows an increase in the intensity of some of the transition lines with increasing the pressure, which ultimately affects the Coatingsx and2019 y, 9reading, 315 (for example, the data obtained from IZO target and Argon gas at various pressures11 of 19 is presented in Figure 9).

(A)

(B)

FigureFigure 9. The 9. The effect effect of working of working gas gas pressure pressure observed observed on on the the spectral spectral plot plot (200 (200 to to 600 600 nm) nm) of of the the plasma withplasma IZO target with andIZO Argontarget and gas Argon only atgas various only at vari workingous working gas pressures. gas pressures. When When compared compared to theto thex and y plotsx in and Figures y plots7 andin Figures8, it can 7 and be seen 8, it thatcan be all seen we observethat all we from observe the spectra from the is spectra an increase is an inincrease the intensity in of certainthe intensity peaks which of certain makes peaks the assessmentwhich makes of thethe dataassessment complex of withthe data further complex evaluation. with further All of these evaluation. All of these increases in intensity can be summarised as presented in Figures 7 and 8 by increases in intensity can be summarised as presented in Figures7 and8 by evaluating the colour of evaluating the colour of the plasma light. The inset in the second section of the plot demonstrates the the plasma light. The inset in the second section of the plot demonstrates the complexity of analysing complexity of analysing every single peak for the amount of slight increase in one of the peaks. (A): everyspectral single plot peak 200 for to the 400 amount nm; (B): ofspectral slight plot increase from in400 one to 600 of thenm. peaks. (A): spectral plot 200 to 400 nm; (B): spectral plot from 400 to 600 nm. The rise in the intensity of all the peaks is simply due to the larger number of excited atoms and wellThe known, rise in thehowever, intensity the ofexact all mechanism the peaks isby simply which duethe x to, y thevalues larger make number such a ofU-turn excited pattern atoms is and wellpossibly known, related however, to complex the exact switching mechanism of the byelectronic which transitions, the x, y values and hence make it suchis related a U-turn to the patternarea is possiblyunder related emission to complexpeaks of certain switching transitions of the across electronic the spectrum transitions, and andthese hence peak areas it is relatedare related to the to area underthe emission microenvironment peaks of certainof the plasma transitions and the across interact theion spectrum of the plasma and theseconstituents, peakareas distribution are related of to the microenvironmentenergy, electron temperature, of the plasma and charge and thedensities interaction involved, of but the the plasma exact mechanism constituents, needs distribution further of energy,attention electron and temperature, research, this suggests and charge that there densities is a possibility involved, of butcorrelating the exact the mechanism colour to the needsplasma’s further physical state. attention and research, this suggests that there is a possibility of correlating the colour to the plasma’s This is currently being investigated by the authors through the monitoring of the above- physical state. mentioned factors affecting the plasma constituents and will be reported separately. Here, the core ofThis the is current currently communication being investigated is a focus by on the the authorscurrently through observed the data monitoring in terms of of colour the above-mentioned indications. factors affFinally,ecting another the plasma particular constituents observation and was will made; be reported once the separately. IZO target material Here, the is exposed core of theto gas current communicationArH and under is a a focus stable on plasma, the currently switching observed from ArH data to Ar in termsdoes not of lead colour into indications. a direct swift stable Finally, another particular observation was made; once the IZO target material is exposed to gas ArH and under a stable plasma, switching from ArH to Ar does not lead into a direct swift stable reading, rather for a period up to ~1 h, the values of x and y fluctuate. The fluctuation is only observed when going from gas ArH to Ar, and not from Ar to ArH under the same protocol, which indicates the possible incorporation of hydrogen into the target material similar to the target poisoning process, however this cannot be verified prior to close investigation of the target material. Figure 10 presents this observation. Coatings 2019, 9, x FOR PEER REVIEW 11 of 18 Coatings 2019, 9, x FOR PEER REVIEW 11 of 18 reading, rather for a period up to ~1 h, the values of x and y fluctuate. The fluctuation is only observed reading, rather for a period up to ~1 h, the values of x and y fluctuate. The fluctuation is only observed when going from gas ArH to Ar, and not from Ar to ArH under the same protocol, which indicates when going from gas ArH to Ar, and not from Ar to ArH under the same protocol, which indicates the possible incorporation of hydrogen into the target material similar to the target poisoning process, the possible incorporation of hydrogen into the target material similar to the target poisoning process, however this cannot be verified prior to close investigation of the target material. Figure 10 presents however this cannot be verified prior to close investigation of the target material. Figure 10 presents Coatingsthis observation.2019, 9, 315 12 of 19 this observation.

Figure 10. Switching back and forth from Ar gas to ArH gas. (A) Switching from Ar to ArH occurs Figure 10. Switching back and forth from Ar gas to ArH gas.gas. ( A) Switching from Ar to ArHArH occursoccurs swiftly. (B) Switching from ArH to Ar, the plasma colour demonstrates a colour change as the x,y swiftly. ( B)) Switching from from ArH to Ar, the plasmaplasma colourcolour demonstrates a colour change as the x,y values change over the course of ~1 h prior to stabilisation in a linear fashion after the target has been values change over the course of ~1 h prior to stabili stabilisationsation in a linear fashion after the target has been exposed to Argon hydrogen mixture. This observation can be interpreted as either the rate by which exposed to Argon hydrogen mixture. This observatio observationn can be interpreted as either the rate by which hydrogen is disappearing from the chamber and/or possible formation of target hydrogen hydrogen isis disappearing disappearing from from the chamberthe chamber and/or an possibled/or possible formation formation of target hydrogenof target compounds hydrogen compounds at the surface of the target (target poisoning by hydrogen) and the time that is required atcompounds the surface at of the the surface target (targetof the poisoningtarget (target by hydrogen)poisoning andby hydrogen) the time that and is the required time that for the is required target to for the target to revert back its original state while switching from Ar to ArH does not demonstrate revertfor the back target its to original revert stateback whileits original switching state from while Ar switching to ArH does from not Ar demonstrate to ArH does any not pattern, demonstrate rather any pattern, rather a swift instant jump is observed. aany swift pattern, instant rather jump a isswift observed. instant jump is observed. 3.2. Optical Optical Emission Spectros Spectroscopycopy of the Plasma Emissions 3.2. Optical Emission Spectroscopy of the Plasma Emissions In the previous section, we focused on the chroma chromaticityticity (colour) of the emission, parallel to these In the previous section, we focused on the chromaticity (colour) of the emission, parallel to these studies we obtained the optical emission spectrosco spectroscopypy of the plasma, and here we shall demonstrate studies we obtained the optical emission spectroscopy of the plasma, and here we shall demonstrate the results that are associated with the experimentsexperiments involving only the Argon gas as a comparativecomparative the results that are associated with the experiments involving only the Argon gas as a comparative guide to enable to discuss the observations seen through chromaticitychromaticity analysis.analysis. guide to enable to discuss the observations seen through chromaticity analysis. During TestTest 1,1, the the stability stability of of the the plasma plasma was was monitored monitored through through the colourthe colo functions,ur functions, at the at same the During Test 1, the stability of the plasma was monitored through the colour functions, at the sametime, wetime, obtained we obtained the optical the optical emission emission data of data the spectra of the spectra and divided and divided the spectrum the spectrum to seven to sections: seven same time, we obtained the optical emission data of the spectra and divided the spectrum to seven sections:a, b, c, d, e,a, f,b, and c, d, the e, UV.f, and Figure the UV. 11 presents Figure 11 these presents sections: these the sections: UV covered the UV emissions covered from emissions300–400 from nm, sections: a, b, c, d, e, f, and the UV. Figure 11 presents these sections: the UV covered emissions from 300–400a: 400–430, nm, b:a: 431–449,400–430, b: c: 431–449, 450–500, c: d: 450–500, 500–600, d: e: 500–600, 600–700, e: and600–700, f: 700–800 and f: 700–800 nm. Figures nm. Figures12 and 1213 300–400 nm, a: 400–430, b: 431–449, c: 450–500, d: 500–600, e: 600–700, and f: 700–800 nm. Figures 12 andrepresent 13 represent the possible the possible emission emission transitions transitions in these regions.in these Theregions. area underThe area each under region each was region calculated was and 13 represent the possible emission transitions in these regions. The area under each region was calculatedand monitored. and monitored. The area via The integral area undervia integral each peakunder region each waspeak then region used was for furtherthen used analysis for further of the calculated and monitored. The area via integral under each peak region was then used for further analysisdata to monitor of the data the variations to monitor of the areavariations as a dependent of the area of variousas a dependent experimental of various parameters experimental that was analysis of the data to monitor the variations of the area as a dependent of various experimental parametersalready discussed: that was duration already of emissiondiscussed: stability, duration RF of power emission applied stability, to the magnetron, RF power andapplied the various to the parameters that was already discussed: duration of emission stability, RF power applied to the magnetron,operating Argon and the pressures. various operating Argon pressures. magnetron, and the various operating Argon pressures.

Figure 11. The emission spectra of the plasma were segmented into seven sections: a, b, c, d, e, f and Figure 11. The emission spectra of the plasma were segmented into seven sections: a, b, c, d, e, f and UV. Where thethe UVUV coveredcovered emissionsemissions fromfrom 300–400300–400 nm.nm. a:a: 400–430, b: 431–449, c: 450–500, d: 500–600, 500–600, UV. Where the UV covered emissions from 300–400 nm. a: 400–430, b: 431–449, c: 450–500, d: 500–600, e: 600–700, 600–700, and and f: 700–800 700–800 nm. nm. The The area area under under each each region region was was calculated calculated and and monitored. monitored. e: 600–700, and f: 700–800 nm. The area under each region was calculated and monitored.

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(A) (A)

(B) (B) Figure 12. The emission spectrum and the associated Argon, Indium, Zinc and Oxygen states within FigurefarFigure 12. infra-redThe 12. The emission (A emission) and the spectrum spectrumUV section and and (B the )the of associated associatedthe spectrum Ar Argon, gon,obtained Indium, Indium, from Zinc the Zinc andsputtering and Oxygen Oxygen plasma. states states within These within far infra-redregionsfar infra-red are (A) not and(A) andaccounted the the UV UV section for section when (B ()B of)the of the thechromatici spectrumspectrumty obtained obtainedindices are from from calculated the the sputtering sputtering through plasma. the plasma. colour These These regionsfunctions.regions are not are accounted not accounted for when for when the chromaticity the chromatici indicesty indices are calculated are calculated through through the colour the colour functions. functions.

(A) (A)

(B) (B) Figure 13. Cont.

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(C)

Figure 13. The emission spectrum and the associated Argon, Indium, Zinc, and Oxygen states within the 400–700 nm section of the spectrum obtained from the sputtering plasma. These are the areas (including the unmarked peaks) used for calculating the colour coordinates by measuring the area under the peaks as discusses earlier and elaborated in Figure 1. Spectral range, (A): 400–500 nm, ( B) 500–600 nm, (C): 600–700 nm. (C)

Figure 13. The emission spectrum and the associated Argon, Indium, Zinc, and Oxygen states within FigureThe area 13. underThe emission all of the spectrum regions and was the summed, associated and Argon, then Indium, the ratio Zinc, of andthe Oxygenarea under states each within section the 400–700the 400–700 nm nm section section of of the the spectrum spectrum obtained obtained fromfrom the the sputtering sputtering plasma. plasma. These These are the are areas the areas of the study was calculated as a percentage of the total area under the peaks to visualise how the (including(including the unmarkedthe unmarked peaks) peaks) used used for for calculating calculating the the colour colour coordinates coordinates by bymeasuring measuring the area the area emissionsunder from the thepeaks associated as discusses earliertransition and elab changedorated in Figureduring 1. Spectralplasma range, stabilisation (A): 400–500 nm,and ( Bwhen) the under the peaks as discusses earlier and elaborated in Figure1. Spectral range, ( A): 400–500 nm, parameters,500–600 i.e., nm,power (C): 600–700and Argon nm. pressure, were changed. In Figure 14, the stability of the plasma (B) 500–600 nm, (C): 600–700 nm. over time is monitored over a total period of 5000 s at intervals 1000 s, e.g., T1: 0 to 1000 s, T2: 1000– 2000 s, etc.The area under all of the regions was summed, and then the ratio of the area under each section Theof the area study under was allcalculated of the regions as a percentage was summed, of the total and thenarea under the ratio the ofpeaks the to area visualise under how each the section It can be seen that some of these regions demonstrate a progressive increase in their emission of theemissions study was from calculated the associated as a percentage transition changed of the total during area plasma under thestabilisation peaks to and visualise when howthe the emissionsratio parameters,when from compared the i.e., associated power to the and overall transition Argon ratio, pressure, changed while were some during changed. demonstrate plasma In Figure stabilisation a 14,reduced the stability and ratio when ofof emissionthe plasma parameters, to the i.e.,whole powerover emission time and is Argonmonitored intensity. pressure, over Ultimately, a total were period changed. the of area 5000 In unders Figure at intervals the 14, thepeak 1000 stability s,readings e.g., T1: of 0 thetend to 1000 plasma to s, move T2: over 1000– toward time is monitoredstabilisation,2000 s, overetc. and a is total this period very much of 5000 in sagreement at intervals with 1000 our s, e.g.,observation T1: 0 to 1000of the s, chromaticity T2: 1000–2000 data. s, etc. It can be seen that some of these regions demonstrate a progressive increase in their emission ratio when compared to the overall ratio, while some demonstrate a reduced ratio of emission to the whole emission intensity. Ultimately, the area under the peak readings tend to move toward stabilisation, and is this very much in agreement with our observation of the chromaticity data.

Figure 14. These data demonstrate the ratio to the total spectral cover from 300 to 800 nm of the area under thethe peakpeak inin each each of of the the segments segments during during the the first first 5000 5000 s from s from the ignitionthe ignition of the ofplasma. the plasma. The timeThe series is divided into five sections of 1000 s. As can be seen, the plasma emission is changing through time Figureseries 14.is divided These data into demonstrate five sections the ratioof 1000 to the s. totalAs can spectral be seen, cover the from plasma 300 to 800emission nm of theis changingarea the time, in terms of the total area being covered under the peaks in each segment. Regions (a,f) throughunder the the time, peak in in terms each ofof thethe se totalgments area during being the covered first 5000 under s from the the peaks ignition in each of the segment. plasma. TheRegions demonstrate(a,f) demonstratetime series an is increase dividedan increase ininto peak infive peak area sections whilearea of while (1000b–e ) (s.b are –Ase) gradually canare begradually seen, reducing. the reducing. plasma emission is changing through the time, in terms of the total area being covered under the peaks in each segment. Regions It can be seen that some of these regions demonstrate a progressive increase in their emission ratio Figure(a,f )15 demonstrate presents anthe increase effect in of peak the area RF while power (b– eapplied) are gradually to the reducing. magnetron on the emission peak whenareas of compared the spectral to the segments, overall ratio, where while it is someclearly demonstrate noticeable athat reduced an increase ratio of in emission the emission to the ratio whole is Figure 15 presents the effect of the RF power applied to the magnetron on the emission peak emissionobserved intensity.in sections Ultimately, a, b, and c, thewhile area d, undere, and thef demonstrate peak readings a reduction. tend to move There toward seems to stabilisation, be a linear areas of the spectral segments, where it is clearly noticeable that an increase in the emission ratio is andtrend is in this both very cases. much This in agreement is interesting with and our it observation matches our of theobservation chromaticity of the data. chromaticity indices, observed in sections a, b, and c, while d, e, and f demonstrate a reduction. There seems to be a linear whichFigure also demonstrated15 presents the a e linearffect of relationship the RF power with applied plasma to thegenerating magnetron power. on the emission peak areas of thetrend spectral in both segments, cases. This where is interesting it is clearly and noticeable it matches that our an observation increase in of the the emission chromaticity ratio indices, is observed which also demonstrated a linear relationship with plasma generating power. in sections a, b, and c, while d, e, and f demonstrate a reduction. There seems to be a linear trend in both cases. This is interesting and it matches our observation of the chromaticity indices, which also demonstrated a linear relationship with plasma generating power. Coatings 2019, 9, 315 15 of 19 Coatings 2019, 9, x FOR PEER REVIEW 14 of 18

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Figure 15. These data demonstrate the ratio (to the total spectral cover from 300 to 800 nm) of the area underFigure thethe 15. peakpeak These inin each eachdata of demonstrateof the the segments segments the in ratio in response respon (to these tototal to the the spectral RF RF plasma plasma cover power frompower applied300 applied to 800 for nm) for maintaining ofmaintaining the area the under the peak in each of the segments in response to the RF plasma power applied for maintaining plasma.the plasma. The The peak peak area area associated associated with segmentwith segment (a–c) tend(a–c) to tend be increasing to be increasing with increasing with increasing the applied the the plasma. The peak area associated with segment (a–c) tend to be increasing with increasing the power,applied while power, emissions while emissions associated associated with sections with (sectionsd–f) are ( reducing.d–f) are reducing. applied power, while emissions associated with sections (d–f) are reducing. Figure 1616 presentspresents thethe e ffeffectect of of the the chamber chamber pressure pressure on on the the regions regions of theof the emission emission spectrum spectrum and the ratioFigure of the 16 peak presents areas the to effect the overall of the areachamber covered. pressure Just on like the the regions previous of the cases, emission we canspectrum see that andand the the ratio ratio of of the the peak peak areas areas to to the the overall overall areaarea covered. JustJust likelike the the previous previous cases, cases, we we can can see see that that the different sections of the spectrum demonstrate different trends in terms of increase or decrease thethe different different sections sections of of the the spectrum spectrum demonstrate demonstrate different trendstrends in in terms terms of of increase increase or or decrease decrease of of of the peak areas. Once again, each section is demonstrating either an increasing or decreasing ratio thethe peak peak areas. areas. Once Once again, again, each each section section is is demonstdemonstrating eithereither an an increasing increasing or or decreasing decreasing ratio ratio of of of emission areas to the overall spectral emission area under the peaks. Although the UV emission emissionemission areas areas to to the the overall overall spectral spectral emissionemission area under thethe peaks.peaks. Although Although the the UV UV emission emission region (300–400 nm) and the f section of the spectrum (700 to 800 nm) are not used for calculating regionregion (300–400 (300–400 nm) nm) and and the the f fsection section of of the the spectrumspectrum (700 toto 800800 nm) nm) are are not not used used for for calculating calculating the the thechromaticitychromaticity chromaticity indicates, indicates, indicates, the the thedata data data were were were presentedpresented presented he herere to to demonstratedemonstrate demonstrate the the the spectral spectral spectral property property property of of ofthe the plasmaplasma at at those those regions. regions. The The results results for for thethe UVUV sectionsection of the the spectrum spectrum are are separately separately presented presented in in FigureFigure 1717. .17. Where Where the the UV UV emissions emissions peak peak area area demonst demonstratedemonstrate anan increasingincreasing trend trend over over the the 5000 5000 s period s period andand demonstratedemonstrate demonstrate a a linearly alinearly linearly increasing increasing increasing relationship relationshiprelationship with with the plasma thethe plasmaplasma power, power,power, the higher the the higher chamberhigher chamber chamber pressure seemspressurepressure to reduceseems seems to the to reduce reduce ratio ofthe the the ratio ratio UV of emissionsof the the UV UV emisemis to thesions overall to thethe emission overalloverall emission withinemission the within within spectrum. the the spectrum. spectrum.

FigureFigure 16. 16.These These data data demonstrate demonstrate the the ratioratio (to(to the total spectral cover cover from from 300 300 to to 800 800 nm) nm) of ofthe the area area underFigureunder the16. the peakThese peak in data in each each demonstrate of of the the segments segments the ratio inin response resp(to theonse total to varying spectral the the cover chamber chamber from pressure.300 pressure. to 800 Sections nm) Sections of the(a, (ea ,areaf,e) ,f) demonstrateunderdemonstrate the peak and andin increase each increase of whilethe while segments (b– d()b demonstrate–d )in demonstrate response a decreaseto avarying decrease of intensitythe of chamber intensity when pressure. increasingwhen increasing Sections the chamber (thea,e ,f) pressurechamber to pressure 5.5 10 to3 mbar. 5.5 × 10−3 mbar. demonstrate and× increase− while (b–d) demonstrate a decrease of intensity when increasing the chamber pressure to 5.5 × 10−3 mbar.

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(a) (b)

(c)

Figure 17. TheseThese data data demonstrate demonstrate the ratio ratio (to the the total total spectral cover from 300 to 800 nm) of the area under the peak in the UV section (300–400 nm) of thethe spectrum.spectrum. The UV emissions demonstrate an increasing trend over the 5000 s pe period,riod, followed by a slig slightht decline and they demonstrate a linearly increasing relationshiprelationship withwith the the plasma plasma power. power. The Th highere higher chamber chamber pressure pressure seems seems to beto reducingbe reducing the theratio ratio of the of UVthe emissionsUV emissi toons the to overall the overall emission emission within within the spectrum. the spectrum. (a): Plot (a): tsability Plot tsability test over test 5000 over s 5000period, s period, (b): Eff (ectb): ofEffect plasma of plasma power power on UV on emission UV emission ratio, (ratio,c): Eff (ectc): Effect of pressure of pressure on UV on emission UV emission ratio. ratio. 4. Discussion 4. DiscussionHere, we have demonstrated how the complex analysis of the emission spectrum can be summarisedHere, we by have applying demonstrated the colour how coordinates the complex of the an plasmaalysis light.of the The emission calculation spectrum of the can colour be summarisedfunctions is extractedby applying through the colour the area coordinates that is covered of the under plasma the light. emission The peakscalculation of the of plasma the colour light functionsthat are aff isected extracted by various through parameters. the area that Higher is covered plasma under power the simply emission provides peaks more of the energy plasma toward light thatthe ionizationare affected of by the various working parameters. gas atoms. Higher A higher plasma density power of simply ionized provides atoms, in more turn, energy give a toward higher theflux ionization of emission of the and, working as such, gas increase atoms. theA higher intensity density of the of spectra.ionized atoms, By increasing in turn, give the powera higher of flux the ofplasma, emission the and, number as such, and increase momentum the intensity of the argon of the ions spectra. and sputteringBy increasing particles the power are both of the increased, plasma, whichthe number results and in higher momentum bombardment of the argon rate of ions the targetand sputtering surface and particles higher mobilityare both ofincreased, the atoms which at the resultsdeposited in higher thin film bombardment surface. The rate higher of the mobility target of surface the atoms and willhigher reduce mobility the stress of the at atoms the thin at filmthe depositedsurface, and thin hence film facilitates surface. The further higher crystallisation mobility of[ 27the]. atoms will reduce the stress at the thin film surface,The and discharge hence facilitates current linearly further increasescrystallisation with [27]. increasing the plasma power, irrespective of the chamberThe pressure.discharge Thecurrent energy linearly of the increases sputtered with atoms incr arrivingeasing atthe the plasma surface power, of the substrateirrespective increases of the withchamber increasing pressure. the The plasma energy power. of the sputtered Parallel to atoms this, thearriving effect at of the high surface energy of the bombardment substrate increases of the withsubstrate increasing surface the is alsoplasma increased, power. providingParallel to thermal this, the energy effect atof thehigh surface energy atoms bombardment to become of more the substratemobile. Overall, surface theis also higher increas plasmaed, providing power should thermal lead energy to more at crystallinethe surface depositionatoms to become of thin more films. mobile.However, Overall, excessive the powerhigher can plasma have power an adverse should effect lead by to causing more crystalline degradation deposition of the thin of film thin surface, films. However,resulting inexcessive high defect power density can have [28]. an We adverse have demonstratedeffect by causing that degradation the colour of functions the thin demonstratefilm surface, resultinga linear relationship in high defect with density the plasma [28]. We power, have hencedemonstrated giving this that technique the colour the functions potential demonstrate of defining a linearrelationship relationship between with the the film plasma properties power, and hence the x and givingy colour this technique coordinates, the we potential will be exploringof defining this a relationshipfurther in future between work. the film properties and the x and y colour coordinates, we will be exploring this furtherIf allin offuture the peakswork. would increase at the same rate with increasing the power, the chromaticity parametersIf all ofshould the peaks not would change, increase but in Figure at the6 same we observed rate with a increasing linear trend the in power, how the thex chromaticityand y values parametersare plotted should against not each change, other. but This in indicatesFigure 6 we that observed as the plasma a linear power trend isin increasedhow the x andand diy valuesfferent areenergy plotted bands against tend toeach respond other. toThis the indicates higher magnitude that as the of plasma the power. power This is increased is currently and under different our energyinvestigation bands andtend the to reasonrespond that to wethe have higher the magnit sectionude 400 of to the 500 power. nm section This of is the currently spectrum under divided our investigation and the reason that we have the section 400 to 500 nm section of the spectrum divided into three sections is part of our efforts on understanding this observation; however, as yet, our

Coatings 2019, 9, 315 17 of 19 into three sections is part of our efforts on understanding this observation; however, as yet, our findings are not conclusive and hence no further explanation or claims can be made at present. However, from an applied scientist or an operator of a plasma deposition system, the relationship observed in figure six is a simple observational method if in depth an operator does not require theoretical knowledge. The broadening of the spectral lines, as discussed in the introduction, are ultimately related to the electron density via the stark broadening process. The colour functions are calculated from the area under the emission peaks, hence this broadening is taken into consideration during the colour coordinate measurements and present a simplified form of data for the operator, who may be only interested in basic monitoring protocols and not necessarily wish to indulge in to the complex statistical mechanics of the plasma. A similar argument can be applied for the emission line shapes which are strongly influenced by the interaction of the radiating atoms or ions with surrounding particles. All of the spectral data that were obtained during these experiments were carried out using the same distance between the collimator head and the plasma. It is important to acknowledge that this distance is highly important and it can influence the spectra of the plasma and it is part of our ongoing research objectives to investigate various distances of the collimator and the plasma. Through the data presented in Figure5, we also demonstrated that, when trying to identify the stability of the plasma through the line ratio method, there may be confusion as certain peaks may increase or decrease, irrespective of their initial magnitude and ratio, while our proposed colour index values can provide a simple root to follow. Figure 13 presents some of the possible transitions that need assessment when using the conventional line intensity techniques, while, through colour coordinate calculations, the overall area of the peaks is considered, hence simplifying the monitoring process for a user with limited need of extensive plasma physic knowledge. However, the colour coordinates do not take into consideration the emissions in the UV and far-red part of the spectrum. Hence, an alternative colour function for this application can be envisaged. If this method is matured in the future, then it can be beneficial for applied scientists and those who utilise plasma sputtering for thin film coatings and depositions. The colour of the plasma is derived by the quantity and state of various meta-stable atoms and ions that constitute it. In an analogical overview, a simplified shortcut can be established to characterise the plasma by being able to identify the relationship between the colour of the plasma and physical events within the plasma, just as the fingerprint pattern is used to characterise people rather than their individual genome sequence. Higher chamber pressure lowers the voltage at which the plasma can be ignited and maintained. This is because higher chamber pressure increases the probability of the ionisation process of argon. Higher chamber pressure on the other hand means that more argon atoms are available to be energised to bombard the target surface; however, a higher concentration of argon atoms means that there will more collision between the sputtered atoms or ions and argon atoms. These collisions can lead to lesser energetic atoms arriving at the surface of the substrate [29]. A correlation can clearly be seen between the x and y coordinates of the chromaticity index of the plasma light and certain parameters, such as plasma stability, the working gas and the associated chamber pressure and the plasma power that affect the x and y values. This highlights a clear potential of pursuing and investigating the proposed concept and, if successful for a given target material to be sputtered, regardless of the manufacturer of the sputtering deposition machine, matching the colour parameters can indicate exact similar plasma operating conditions. Meanwhile, theoretically, there is a possibility of operating a plasma under two different conditions and getting similar colour values, we have partially investigated this, where, by altering the chamber pressure and plasma power, similar x or y values can be mimicked, but having an identical x and y values at the same time so far has not been observed, this itself will be a future experimental protocol. The x and y coordinates can give an excellent, easy indication of the point at which the plasma is stable when compared to more complex calculation methods. The stabilisation process itself is an interesting concept to consider, slight variation in parameters that are required for driving plasma can create unstable conditions, and the colour factor can be an excellent indicator of this. Coatings 2019, 9, 315 18 of 19

We have demonstrated that, under unique chamber pressure and plasma power, we can have a distinct x and y value that can be utilised by the operators of the machine to create a library of the thin film property relevant to particular x and y coordinate values, making the process of monitoring the thin film quality significantly easy. However, what has been reported here is just the beginning of a concept that bears potential. The colour coordinates that are driven from the plasma here do not take into consideration the UV and IR emissions. Additionally, in future efforts, the relationship between the coating properties and the plasma photometric need to be established. Future efforts on further investigation of this concept require establishing the relationship between the chromaticity parameters and the plasma’s inner parameters, such as electron temperature and charge density, to enable us to generate a reliable relationship between the mentioned factors. Thereby, the first step in any future development of this concept is to prepare and create an alternative colour coding that will take the UV and IR emissions into account and will create a colour index beyond our vision.

Author Contributions: Conceptualization, A.S. and H.U.; Methodology, A.S.; Software, R.H.; Validation, A.S. and H.U.; Formal Analysis, A.S. and R.H.; Investigation, A.S.; Resources, H.U.; Data Curation, A.S., R.H., and A.H.; Writing—Original Draft Preparation, A.S.; Writing—Review & Editing, R.H. and H.U.; Visualisation, A.S.; Supervision, H.U. and A.S.; Project Administration, H.U.; Funding Acquisition, H.U. Funding: This research was funded by Grand Challenge Research Fund (GCRF) toward the SUNRISE program, No. EP/P032591/1. Acknowledgments: We would like to thank Scientific Vaccum Systems UK and Brunel University London for their support on this project. Conflicts of Interest: The authors declare no conflict of interest.

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