Langmuir probe and optical emission spectroscopy ...

Langmuir probe and optical emission spectroscopy studies in magnetron sputtering plasmas for Al-doped ZnO film deposition B. B. Sahu, Jeon G. Han, Masaru Hori, and Keigo Takeda Citation: Journal of Applied Physics 117, 023301 (2015); doi: 10.1063/1.4905541 View online: http://dx.doi.org/10.1063/1.4905541 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/117/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Correlations between 1/f noise and thermal treatment of Al-doped ZnO thin films deposited by direct current sputtering J. Appl. Phys. 115, 204502 (2014); 10.1063/1.4879095 Deposition and composition-control of Mn-doped ZnO thin films by combinatorial pulsed laser deposition using two delayed plasma plumes J. Appl. Phys. 112, 044904 (2012); 10.1063/1.4747935 Compositional study of vacuum annealed Al doped ZnO thin films obtained by RF magnetron sputtering J. Vac. Sci. Technol. A 29, 051514 (2011); 10.1116/1.3624787 Transparent conducting Al-doped ZnO thin films prepared by magnetron sputtering with dc and rf powers applied in combination J. Vac. Sci. Technol. A 25, 1172 (2007); 10.1116/1.2748809 Al-doped zinc oxide films deposited by simultaneous rf and dc excitation of a magnetron plasma: Relationships between plasma parameters and structural and electrical film properties J. Appl. Phys. 83, 1087 (1998); 10.1063/1.366798

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JOURNAL OF APPLIED PHYSICS 117, 023301 (2015)

Langmuir probe and optical emission spectroscopy studies in magnetron sputtering plasmas for Al-doped ZnO film deposition B. B. Sahu,1,a) Jeon G. Han,1 Masaru Hori,2 and Keigo Takeda2 1

NU-SKKU Joint Institute Plasma-Nano Materials (IPNM), Center for Advance Plasma Surface Technology (CAPST), School of Materials Science and Engineering, Sungkyunkwan University, Suwon, South Korea 2 Plama Nanotechnology Research Center, Nagoya University, Nagoya, Japan

(Received 9 September 2014; accepted 24 December 2014; published online 8 January 2015) This work reports investigation of the Al-doped ZnO (AZO) film deposition process, at different working pressures, in a conventional magnetron sputtering system. The primary goal of this study is to investigate the plasma formation and deposition process using various diagnostic tools, by utilizing low-temperature deposition process. In addition, this paper also presents a systematic Langmuir probe (LP) analysis procedure to determine the maximum information about plasma parameters. For the present study, we have extensively used LP method to characterize the deposition process for the control of plasma parameters. Along with the LP method, we have also used optical emission spectroscopy diagnostic to examine the favorable deposition condition for the fabrication of conductive AZO film. Utilizing diagnostics, this also reports measurements of ion current density, substrate temperature, and deposition rates to fabricate low resistivity films of 3 mX cm. C 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4905541] V I. INTRODUCTION

Transparent conductive oxide (TCO) thin films have attracted considerable attention in many thin film devices such as gas sensors, transparent electrodes, and solar cells.1,2 In particular, the most capable TCO materials is the indium tin oxide (ITO) films. However, scarcity and high price are some disadvantages of the ITO thin films.3 Recently, for this reason, zinc oxide (ZnO) and aluminum doped zinc oxide (AZO) thin films have attracted much more attention than ITO films. ZnO is an important semiconducting material, because of its wide bandgap (3.37 eV) and high excitation binding energy 60 meV at the room temperature. It has the richest group of nanostructures among all materials.4 There are different techniques for the synthesis and characterization of ZnO and doped ZnO depending upon their application prospects in the development of material’s area. Owing to their excellent optical and electrical properties, AZO films have the high potential for TCO applications.5,6 ZnO has high transparency in the near-ultraviolet and visible spectral regions, wide conductivity range, and conductivity can change under oxidation/photo-reduction condition.6 However, pure ZnO films lack the stability in terms of thermal edging in air and/or corrosive environments.7 Therefore, group II and group III metal ions, such as aluminum (Al), indium (In), gallium (Ga), cadmium (Cd), and copper (Cu), have been doped with the polycrystalline ZnO films to enhance their electrical, structural, and optical properties.8 Doping is desirable to get high transparency, stability, and high conductivity. Aluminum doping is particularly suitable for this purpose. In view of the suitability of application, AZO thin films with high transmittance in the visible region and low optical bandgap and resistivity may be controlled by using Al doping amount.9 Existing literature shows different physical and chemical methods for the aluminum incorporation in ZnO. These a)

Corresponding author: [email protected]

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include dc and radio frequency magnetron sputtering (rf MS),1,5 pulsed laser ablation,10 chemical beam deposition,11 chemical vapor deposition,12 sol-gel,13 electrolysis technique,14 sprays pyrolysis15 among others. At the present time, magnetron sputtering systems become the promising sources for the deposition of thin films with unique physical and chemical characteristics of a variety of industrial branches, in microelectronics, cutting tools, or reflecting layers manufacture, etc., due to its simplicity in structure and high deposition rate. Conventional MS (CMS) and facing targets sputtering (FTS) are familiar to be the promising processes for low-temperature film synthesis, because of the significant reduction of damage in the film structure, by suppression of impinging high-energy particles on the growing film.16,17 Furthermore, in low-temperature deposition process, defect annealing by thermal activation is not predictable, and defect creation needs the suppression during the deposition process.18,19 In particular, during the deposition process of TCO films like ITO and AZO, the substrates are exposed to high-energy recoiled Ar atoms and oxygen ions (O), which are accelerated through the sheath, and obtain high energy. Moreover, to obtain quality TCO film with low resistivity, the energy of particles impinging on the substrate should be adjusted. Also, in the low-temperature process, high ionization rate, high electron temperature, and high plasma density are desirable to obtain high-quality films.18 Thus, one requires suitable and systematic diagnostics to control plasma parameters and understanding of the sputtering process under different operation conditions. Thus, it is necessary to investigate the plasma formation and favorable deposition conditions for making conductive or low resistivity films. Further, it is recognized that MS systems are useful for sputter etching and thin film deposition. Such devices use a magnetic field, to trap electrons above a cathode,20 which allow operation at low neutral pressures than similar unmagnetized plasmas. These confined electrons are responsible for

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C 2015 AIP Publishing LLC V

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ionization in the plasma or creation of ions that are accelerated into the target cathode by the plasma sheath, causing the necessary sputter etching of target atoms. Furthermore, it is necessary that there should be equal flux of electrons to the anode to compensate the flux of ions to the cathode target. However, electrons must escape from the magnetic trap to reach the anode. Thus, basic plasma diagnostics are crucial to recognize the fundamental plasma surface interactions in this process. We investigate the CMS processes with different plasma processing conditions by varying working pressure. Thus, the investigations described in this paper focus predominantly on the plasma diagnostics. Moreover, this work also presents a precise procedure for LP analysis in detail, which enables one to get more possible information about the plasma parameters and their role in the process plasmas. Along with LP and optical emission spectroscopy (OES) diagnostics, we have also diagnosed the deposition conditions by measuring substrate temperature and substrate ion current density to fabricate low resistivity films. The paper has the following sections: Section II briefly explains about the experimental setup and operation parameters for this work; Sec. III illustrates the LP technique in detail and explains briefly about the OES method. Section IV presents salient results and discussion using various diagnostics to investigate the sputtering process in CMS, and Sec. V is the concluding section. II. EXPERIMENTAL SYSTEM AND OPERATION PARAMETERS

Figure 1 shows the schematic of the CMS experimental system, which include vacuum system, plasma chamber, magnetron source, DC power source, substrate, and diagnostic systems. The magnetron sputtering reactor is about 25 cm in diameter and 45 cm long. The detail of the experimental system is present in the earlier work.21,22 We maintain a base

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pressure of (1–3)  105 Torr in the plasma chamber for several hours, prior to the commencement of the experiments. The figure also shows various diagnostic ports together with two quartz windows as the vacuum viewport and for OES diagnostic. The figure also depicts the presence of a magnetic null region in the mid-plane, where we have taken some LP measurements. The substrate position is at a distance of 6 cm from target and 9 cm below the mid-plane. The source is a circular planar DC magnetron with water cooling target ZnO:Al (2%) of about 4 in. The operating pressure variation is 3–20 mTorr at a fixed power density 3.8 W/cm3 (300 W of DC power). III. PLASMA DIAGNOSTICS

The main plasma characterization in the present studies is using the LP system (M/s. Hiden Analytical Ltd., England). The LP system uses window-based graphics ESPsoft application software, which allows acquisition and storage of the LP I-V characteristics. All the spectral data are acquired through optical fibre (M/s. Ocean optics) with Acton Spectra pro 500i spectrometer. The spectrometer, which has a resolution 0.05 nm, 10 lm wide entrance slit, and 1200 grooves mm1 grating, is used in conjunction with a PIMAX Princeton instrument CCD camera connected to a PC. The software for the spectra acquisition is WinSpec32TM. We have also measured the ion current density using a copper plate on the substrate and substrate temperature by an oscilloscope and thermocouple, respectively. The electrical resistivity of the film is measured by the 4-point probe method. As discussed earlier, to investigate the favorable deposition condition, it is necessary to study the plasma formation by measuring the plasma parameters and their role in the process plasmas. Even though there are numerous works,23–27 reporting high electron energy (warm electrons) and low energy (bulk electrons) population in different plasma conditions, it is also necessary to make a systematic analysis of plasma parameters, simultaneously. The following discussion presents the systematic diagnostic procedure for the LP data analysis for this work and brief information about the OES method. Langmuir probe (LP): For the LP, the length of the cylindrical probes tip of tungsten wire is 2 mm and the diameter 0.5 mm. In this work, we use LP data for obtaining the bulk plasma density (n0), the bulk electron temperature (Te), the warm electron temperature (Tw), warm plasma density (nw), the plasma potential (Vp), electron energy distribution function (EEDF), etc. A. Plasma density and electron temperatures

FIG. 1. Schematic of the experimental CMS system used for the present study.

A typical I-V characteristic curve for LP measurement in the present experiments is shown in the Fig. 2(a). I-V curve is obtained by measuring the collected current (I) by the probe for each bias potential V. To interpret the curve, one can consider an idealized non-equilibrium collisionless, Maxwellian, and unmagnetized plasma. Thus, the mean free paths (k) of collision of all particles are larger than the characteristic lengths rp (probe radius) and kd (Debye length). Let Te, Ti, and Ta are the temperature (in K) of bulk

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electrons, ions, and neutral atoms. Then, the electron temperature (Te) is much higher than those of ions (Ti) and neutral atoms (Ta). This means Te  Ti  Ta. In accordance with the bias potential V, the current from the plasma, I ¼ Ii þ Ie, is the sum of an ion Ii and electron Ie currents. For very large negative bias voltages V  Vp (the plasma potential), the electrons are repelled, while ions are attracted by the probe. The collected ion current from the bulk plasma is limited by the electrostatic shielding of the probe, and I decreases slowly (for very negative V  Vp) with increasing bias voltage V.28 In this region, the current, I ¼ ion saturation current (Isat). Usually, the collected current by the probe due to electrons, Ie, is considered positive, while the current due to positive ions, Ii, is taken to be negative. In the region of ion saturation, I ¼ Isat, while in the region of electron saturation, V > Vp and I ¼ electron saturation current (Ie, sat). In these regions, a positive and negative space charge sheath forms between the plasma and the probe surface. If the sheath thickness is much smaller than the probe dimension, the charge collection surface of the probe will have approximately the same area as the active surface of the probe. The most interesting part of the probe I-V characteristic is the transient region (represented by the purple lines in Fig. 2(a)), when between the probe surface and plasma there is a positive charge sheath that rejects partially the electrons coming from plasma. This indication corresponds to the end of the ion saturation region or break point, and the corresponding current and the probe bias are, respectively, called as is called as Isat and ion saturation voltage (Vsat). Indeed, in this region, V is negative with respect to the plasma potential (Vp), and the probe collects all the positive ions entering the probe sheath and the only electrons that are fast enough to reach the probe surface. By inspection, we fit this region by a linear fit using the software in MATLAB platform. Beyond this region, the curve tends to bend upward, and the current becomes more and more positive. In this region, the electron current has an exponential dependence on V. The electron current Ie can be obtained by subtracting the ion saturation current Isat from the total probe current I. We consider that the plasmas produced by DC magnetron discharges have multi-temperature electron populations. For the two-temperature electron population, these temperatures appear as two linear portions in the ln (Ie) versus V curve as seen in Fig. 2(b). If Ie, b and Ie, w are the two electron population currents corresponding to the bulk (Te) and warm electron temperature (Tw), respectively, and Tw > Te then the total electron current Ie ¼ Ie; b þ Ie; w :

(1)

At large negative probe bias (V), i.e., for V  Vp, the plasma potential (space potential), it is reasonable to assume that Ie  Ie, w, since I2 corresponds to Tw. Assuming   eðV–Vp Þ ; (2) Ie; w ¼ I0 exp Tw

FIG. 2. Figure shows a typical LP data and LP analysis technique. Operating condition and its location are given in the text. (a) Typical I-V characteristic; (b) Plot of ln (Ie) versus V for the plot in (a) after subtracting the ion saturation current. Figure (b) shows the two linear regions corresponding to two electron populations with temperatures Tw  15.3 eV and Te  2.5 eV; (c) C(s) versus s plot (d) second derivative of I-V curve for obtaining the plasma potential. The other plasma parameters are n0 1.38  1010 cm3, nw  3.8  108 cm3, Vp  4.7 V.

one obtains [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 115.145.194.90 On: Fri, 09 Jan 2015 00:14:51

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lnðIe; w Þ ¼ lnðCÞ þ

eV ; Tw

(3) 1

w Þ2 where e is the electron charge, and I0 ¼ Ie0; w ¼ A enw ð2 Tpm e is the thermal current of warm electrons [A ¼ 2prl, A is the probe area, l is the length of probe tip, and r is the radius of the wire], which means the value of Ie, w at V ¼ Vp and   eVp : (4) C ¼ I0 exp – Tw

It is obvious that Tw can be obtained directly from the inverse slope of ln (Ie, w) versus V curve (for large probe voltages) and C from the intercept of this curve on V axis. Thus, knowing Tw and C, one can obtain Ie, w at any probe voltage and, therefore, Ie, b from Eq. (1). From the inverse slope of ln (Ie, b) versus V curve, one can compute Te. For determining the plasma density, there can be two approaches. One of the methods is using the electron retarding current region (Fig. 2(b)). Note that the value of Ie0, w may be calculated from an arbitrary probe voltage value V* in the linear ln (Ie, w) section, say, for example, V*  30 V or 25 V in Fig. 2(b). Thus, one can calculate the density of warm electrons (nw) using the value of Tw as Ie0; w ¼

Ie; w ðV  Þ h  i: e V –V exp ð Tw p Þ

(5)

Furthermore, knowing Tw and Ie0, w, one can obtain Ie, w at any probe voltage and, therefore, Ie, b from Eq. (1). Analogous to Ie0, w as above, one can calculate Ie0, b, to estimate the density n0 of bulk electrons. However, it may be noted that the choice of different probe voltage V* can produce different values of Ie0, w, which in turn give a wide variation of nw and n0. Note that in a similar approach analogous to Eq. (5), Sheridan et al.29 has used to evaluate the plasma parameters like Tw, nw, Tb, and n0, from the LP data. In their work, they have first estimated the Vp from the 1st derivative of the probe current, using the parabolic approximation. Based on the approximate Vp, they have modelled the probe current to be a combination of ion and electron currents. The ion current term is used to model the orbit-limited Is, which is considered to remain constant and that does not take into account the variation of Is with V. Also, in the measurements of LP data, they have not considered the ion saturation current. Moreover, the flat ion and electron saturation currents and a sharp knee at the plasma potential in the I-V characteristic are the ideal probe features that are rarely seen in practice. For high-density plasmas, there is also the possibility of a very high electron current; one may not track the electron saturation region and hence, Vp. In addition, the probe with high negative bias at the ion-attracting mode, can operate without its thermal damage in the plasmas about few tens times denser than at electron-collecting mode. Thus, it is also necessary to use an alternative approach for the estimation of the bulk plasma density from ion-collecting region and Vp. Note that the ion current is evaluated by the electron temperature (Te) when Te  Ti is counterintuitive and needs

some explanation.30 The physical reason for the dependence Is  (kBTe/mi)1/2 has to do with the creation of a sheath around a negatively biased probe. Another approach for measuring the plasma density is using Lam’s theory30,31 using the ion saturation current (Fig. 2(a)). Lam’s theory provides a convenient and accurate procedure for determining the plasma density n0. Moreover, it takes into account the variation of the ion saturation current, Is, with probe voltage. For cylindrical probes at moderately negative probe potential, Lam’s theory gives –

V  VP ðeIi rp Þ2=3

 ¼

2mi e

1=3 CðsÞ;

(6)

with s¼

Ii ; IB

Ii ¼

Is ; ðe:lÞ

 IB ¼ 1:9rp n0

2kB Te mi

1=2 ;

(7)

where kB is the Boltzmann constant, Te is the bulk electron temperature (in K), and Ii the ion particle current per unit length of the probe. Note that Ii now depends on V, since Is is also a function of V. One can express the functional dependence of the parameter C(s) on s as31     1=2 d dG 1 s ¼ s ds ds G 2

where G ¼ s3=2 CðsÞ;

(8)

with the boundary condition that lims!1 GðsÞ ¼

 3=2 4 3 ðs  1Þ3 : 2

(9)

We solve Eq. (8) numerically to generate the requisite data points for calculation. Figure 2(c) shows the dependence of C(s) on s. For a given probe voltage V, the ion particle current Ii can be obtained from the corresponding Is and using Eq. (6) one can determine the value of C(s). The resulting value of s is obtained from the C(s) versus s plot (Fig. 2(c)) which in turn determines IB, and thus, plasma density n0 from Eq. (7). Note that the plasma density n0 in the present experiment is usually of the order 1010 cm3 and the electron temperature is Te  2 eV. This yields a sheath width 0.1 mm for which rp/kd  2.5. Thus, for lower n0 values and small LPs, the sheath expansion could contribute an increase in the collected current because the effective area of the probe is the sheath area and not the geometric probe surface area. Moreover, LP I-V characteristics (in Fig. 2(a)) have rounded knees (even though knee of electron saturation is not clearly visible, but trend is rounded) and ion saturation currents that increase gradually with increasing voltage at high negative bias.28 One can expect here that the sheath expansion occurs for both the ion and electron currents. The sheath expansion effect may be modeled and incorporated in the ideal probe characteristic as a linear function of the bias voltage for the ions and electrons.28,32 The lack of saturation is related to the fact that the sheath is formed around the probe, and this

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sheath expands with increasing probe bias voltage. Sheaths form around any electrode in a plasma if the bias voltage differs from plasma potential (Vp). In the limiting case (n0  1010 cm3 and Te  2 eV) one can approximate the Isat as a function of applied bias voltage and not as a constant value Ii ¼ Isat. However, from the point of view of LP analysis, the presented approach, with the moderate rp/kd  2.5 values, can also provide approximate information about the plasma parameter. For plasmas with low plasma density, one can also estimate the n0 value using the procedure described below by Eqs. (10) and (11). If n0 1010 cm3 and Te  2 eV, then kd will be

0.1 mm. Thus, the expansion of the sheath will contribute negligibly to the current as the probe bias is increased. In the case of high-density plasmas also this analysis method would be helpful. In the present experiment, near the magnetron, the plasma density is expected to be high. Moreover, when the plasma contains an appreciable fraction of high energy (few tens of eV) ionizing primary electrons along with the secondary electrons resulting from ionization, it is necessary to subtract the Ii from the total current (I) to get accurate values of Ie. In addition, Lam’s theory is strictly valid for collisionless unmagnetized plasma with Maxwellian electrons. Also, in the present experiment, we have taken the LP measurements in a similar environment. However, it can be applied even to magnetized plasma provided the electrons are in thermal equilibrium (at least locally), and the ion Larmor radius (rLi) is much larger than probe radius (rp) and rp  Debye length (kd). In this limiting case, the ion saturation current is not affected much. B. Plasma potential

In principle, one can use the electron saturation part of the LP characteristics to determine the plasma potential Vp.30,32 However, for high-density plasmas, it is not easy to obtain the electron saturation region. In addition, determination of the "electron saturation knee" involves uncertainties. This procedure is, therefore, quite unsatisfactory. Another method, commonly used to determine Vp, is to measure the floating potential Vf of an emissive33 probe or use the emissive probe characteristics.34,35 The emissive probe modeling by Fruchtman and his coworkers showed that the emitted current by the emissive probe is limited by space charge, and the error of the plasma potential is in the order of 1.5Te.36 Note that, this theory of emissive probe, which takes into account finite collection area of the probe with respect to the sheath, is done for unmagnetized electrons. In the present experiments, the LP measurements (Fig. 1) are carried out in the magnetic null region. However, for magnetized plasmas, such as used in magnetrons (as in the present study), it may be a bit more complicated. Emissive probes utilize electron emission to measure Vp and the details of the emissive sheaths are crucial to its operation. The analysis of emissive probe data is based on the separation of the cold and hot IV traces that takes place near the plasma potential. This electron emission process is insensitive to the plasma flow because it only depends on the local plasma potential (Vp), rather than the electron kinetic energy. Moreover, a

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departure from Maxwellian energy distribution functions of electrons could additionally complicate the situation with the sheath voltage of the emitting probe. In addition, emissive probes do not provide needful data on temperature and plasma density as collecting probes. Although these probes have been investigated for a long time, there still remain many issues regarding the space charge effects and the emitted electron current.37,38 Detailed discussion on the analysis of this approach is beyond the scope of this paper. Numerous works reporting the emissive probe techniques along with kinetic approach of plasma sheaths surrounding the electron emitting surfaces are present in the literatures.36–39 In the present experiments, the plasma potential (Vp) is measured from the maximum in the first derivative of the I-V characteristic. This method is also is equivalent to the zero value of second derivative (Fig. 2(d)). Note that, in noisy plasma where the latter method is more prone to noise, Vp may be estimated by extrapolating the current from the electron saturation and retarding region. The determination of the plasma potential in the present magnetron sputtering experiments was mainly for the study plasma production and the effect of plasma in the deposition process condition. Note that Vf may also be determined fairly accurately from LP curve, since it is the probe potential corresponding to zero probe current. Also, Vp may be determined along with the other plasma parameters (like n0, Te etc.) and hence, use of a single LP for all measurements is very convenient. The other method to deduce the Vp using the Vf is as follows. Note that positive ions continue to be collected by the probe until the bias voltage, V reaches plasma potential (Vp), at which point ions begin to be repelled by the probe. For V  Vp, all positive ions will be repelled, and the ion current to the LP will be negligible, Ii  0. For a Maxwellian ion distribution (at temperature Ti), the dependence of the ion current taken to be the negative current on V is given by30,32 Ii ðVÞ ¼ Isat exp ½eðVp  VÞ=Ti  ¼ Isat

for

for

V Vp ;

V < Vp ;

(10)

where e is the electronic charge. When Ti is comparable to Te, Isat is given by40 1 e n0 vi;th A; (11) 4 qffiffiffiffiffiffiffi where n0 is the ion density and vi;th ¼ p8 mTii is the ion thermal speed. When Te Ti, the Isat is not evaluated by the ion thermal speed, but rather is decided by ion sound speed, cs ¼ (Te/mi)1/2 to yield the Bohm current, Isat  (Te/mi)1/2.41 One can estimate the plasma density n0 from Eq. (11). For V  Vp, the probe collects electron saturation current, Ie,sat. For V < Vp, the electrons will be partially repelled by the probe. For a Maxwellian electron velocity distribution, the electron current decreases exponentially with decreasing V. The electron current as a function of V can be expressed as follow: Isat ¼

Ie ðVÞ ¼ Ie;sat exp ½eðVp  VÞ=Te  ¼ Ie;sat

for

V > Vp :

for

V Vp ; (12)

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Note that, the floating potential (Vf), in the region V < Vp, is the bias voltage at which Ii(Vf) þ Ie(Vf) ¼ 0. Thus, using Eqs. (10) and (12), one can write Ie;sat exp ½eðVp  Vf Þ=Te  ¼ Isat :

(13)

Using Eq. (11) in Eq. (13), the expression containing Vp will be produced as     eðVf –Vp Þ 1 2pme –1 ; (14) ¼ ln Te mi 2 where me is the electron mass. Thus, Vp can be determined by known Te and Vf. Note that the estimated ratio of rp /kd is nearly 2.5. Note that Chen and Arnush41 have calculated that the ratio of the difference between the Vp and Vf to the Te derived from Eq. (14) is the value of 5.18 for the argon plasma. In the present case, for the cylindrical probe with rp/kd  2.5 this ratio is the order of 4.1. Note also that the present experiments are conducted in the DC environment. First, LP measurement does not require any RF compensation, and there is no concern of distortion in the I-V characteristic which can affect the jVfj. This can be due to the other possibilities. Second, the presence of high-energy or non-Maxwellian electron tails would also increase jVfj. Finally, ion collisions in the sheath could also influence this change, but electron collisions do not affect the present results since electrons are considered to be Maxwellian. The later cases are expected to affect a change in the jVfj and hence, the change in the value from 5.18 to 4.1. Note that plasmas in the processing applications can compose of ions of different masses. There can also be the possibility of both positive and negative ions. One can determine the ion masses by the mass spectrum analysis using a quadrupole mass spectrometer (QMS). All positive ions are accelerated in the sheath, subject to the same electric fields. As mentioned above, ions will enter the sheath with the velocity, cs, which will be different for lighter and heavier ions. Moreover, the left-hand side of Eq. (14) corresponds to the sheath voltage (Vsh). When multiple ions are present, mi is calculated by summing the abundance weighted ion masses as the ion reduced mass, which can be estimated from the mass spectrum analysis using a QMS. In multicomponent plasmas (such as SF6 gasPplasmas), mi is calculated by the effective mass “1=mi ¼ j Ij =mi j ,” where Ij is the ratio of spectrum intensity of the “jth” ion. The negative ion reduced mass can also be calculated in the same way.42,43 In this case, one needs to estimate the mass by taking ratios of the ion saturation and electron saturation current ratio in electronegative-gas mixture plasma to that in reference noble gas plasma. In RF produced plasmas, the plasma potential fluctuates with time. In this case, the ratio of ion sheath transit time (iion) to the RF period determines the effect on the sheath voltage.44 The ions enter the sheath at various phases of the RF cycle; they acquire a different energy as they cross the sheath. However, in such case, one requires RF compensation to minimize this effect.41 In the present experiment, the experimental gas is only Ar that precludes

the possibility of negative ion formation. Also, in the DC plasma environment, there is no distortion in the sheath potential due to RF. It can be seen from Eq. (4) that knowing the plasma potential the density of warm electron population (nw) can be determined. Equation (4) may be rewritten as to find the value I0 as follow:   eVp : (15) I0 ¼ C exp Tw It may be noted that at V ¼ Vp no sheath develops around the probe and the charges reach its surface because of their thermal motion. Thus, the probe collects the thermal flux of electrons (Ce) and ions (Ci), called as random flux. The flux of electrons and ions is given by  1  1 1 8 kB Te 2 1 2Te 2 ¼ na ; Ca ¼ na pma pma 4 2

(16)

where the subscript “a” represents electron (e) and ion (i) and kB is Boltzmann constant. Their corresponding current densities are Ja, thermal ¼ qa Ca (qa charge). Moreover, ion mass mi  me and kBTe  kBTi. Thus,  1 Ji;thermal 2 Jnet ¼ Je;thermal þ Ji;thermal ¼ Je;thermal 1 þ Je;thermal  Je;thermal :

(17)

In consequence, the probe biased at the plasma potential drives an electric current (see Eq. (2)) from the plasma even in the absence of potential difference (V - Vp ¼ 0) between the conductor and the surrounding plasmas. I0 divided by the probe area A is the random flux of the warm electrons Je, thermal arriving at the surface of a retarding probe. Here, Je, thermal may be defined as30 Je; thermal

 1 1 2Tw 2 ¼ nw : pme 2

(18)

Thus, the warm electron density, nw may be obtained using this relation. C. EEDF

In the present study, the EEDF is estimated from the second derivative of the electron current to the Langmuir probe at bias voltages below the plasma potential30,45   rffiffiffiffiffiffi f ð E Þ d 2 Ie 2me 2e pffiffiffi ¼

; (19) 2 2A m d V e E e where E ¼ Vp – V, f(E) is the EEDF and the distribution of the form f(E)/冑E is also familiar as the EEPF. EEDF measurements using the LP require dedicated electronics to reduce the noise level to provide a proper condition for the signal process to get the second derivative of probe current data with respect to the bias voltage. Moreover, with highspeed data acquisition and data storage size of the computer,

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LP techniques can analyze the EEDF directly from the probe data. Then, the primary apprehension for developing the analysis algorithm becomes how to minimize the noise level during the data processing. Particularly, when the warm electron population increases in practical plasmas, the EEDF becomes bi-Maxwellian, so the algorithm needs to handle the tail of the EEDF. Most algorithms for EEDF analyzes are adapted to the Druyvesteyn formula, which uses the second derivative of the probe current with respect to the bias voltage (V) of the electrical probe.30,45 Branner et al.46 used a sophisticated automatic plotting device to evaluate the second derivative of the LP data by applying the second harmonics of the sinusoidal signal superimposed on the DC probe voltage. Godyak et al.47,48 used an EEDF circuitry that uses a filter-differentiator to eliminate high-frequency data and a waveform analyzer to suppress the noise. However, those methods require experience and knowledge of handling electronics to tune and to control the gain of the electronic circuit. Moreover, as the speed of computer-aided data acquisition system has improved, digitized probe data can be analyzed through an analysis algorithm, accelerating the performance of probe diagnostics. In the present study, we have used interactive graphics-based software in Matlab platform for the computation of plasma parameters. The EEDF analysis procedure is shown in Fig. 3. The software generates a mirror-image data set of the raw probe Ie-V data. These data are conjugated at the endpoint of the electron saturation of the original probe signal. Normally, the conjugated datum is located far from the plasma potential so that the regenerated data may not influence the accuracy of the analysis. Later, the process probe data with an axis-symmetric new data set are regenerated. The axis-symmetry reconstructed data set acquire a bellshaped profile. Noise filtering process is carried out in the next step using bi-orthogonal wavelet transform (BWT), which is composed of the wavelet and scaling functions. Using the wavelet function, the overall structure of the signal is obtained as the low-pass filter, and the scaling function is applied to obtain the informative signal data as the high-pass filter. Note that each wavelet transform process is based on decomposition and recombination processes, which can reduce the noises efficiently with less loss of important information. During the process of the decomposition and the recombination, the threshold of the noise level to be filtered is calculated from the statistics by using the hard threshold method reported by Donoho.49 The BWT has bell-shaped wavelet functions, so it is simple to filter the bell-shaped data signals. Vp is obtained from the first derivative of the probe data processed by using the BWT. To acquire the second derivative of the probe data, a continuous wavelet transform (CWT) is utilized. Note that CWT is based on the convolution process of a mother wavelet function, the differentiation, and the smoothing processes can be carried out simultaneously to produce differentiation data with suppression of noise. It may also be noted that the details of the differentiation procedure using CWT are present in the literature.50 The conjugated part is then eliminated from the CWT processed data and is restored as the size of original data set. Corresponding to the zero value of the fitted second

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FIG. 3. Flow chart to obtain the EEDF from the LP data.

derivative data, Vp is estimated. Then, Vp provides a reference bias voltage (V) to obtain the EEDF using Eq. (19). D. OES

The OES technique is non-invasive, easy to implement, and measurements are fast. OES method is passive and based on recording light emitted from the plasma. Plasma particles can be excited to higher electronic states through collisions with electrons. Note that relaxation of excited particles, which are present in the process chamber, to lower energy states is the origin of emitted photons in the form of light. Energy of released photon corresponds to the energy difference between excited and lower energy levels and corresponding with the wavelength of spectral line described by relation

k¼

hc ; EU  EL

(20)

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where EU and EL are, respectively, upper and lower energy states, c is the speed of light, and h is Planck’s constant. Since the energy of the transition during the excitation is a characteristic feature of the particle species, the analysis of photon energy can reveal the constituents of the plasma. Note that even if spectral lines of some particles are not detected by OES, those particles could be still present in discharge plasma, however, would not be excited. Basic information on relevant plasma spectroscopy that often used population models such as the corona model (CM) and the collision radiative model (CRM), is present in the literature.51–53 It is well known53 that the expression for the intensity of spontaneous emission by an excited level L* of density [L*] (cm3) created in the plasma (power emitted per unit of volume and solid angle, W m3 sr1) and detected by an optical system (lenses, optical fibre, monochromator, and detector) is IP ¼

1 t D

½L  ht Aij ¼ K v ½L ; 4p detection

(21)

where Kt is a constant relevant to each emitted line of frequency t (s1) which includes the detection system response (Dtdetection ) and Einstein’s coefficient for spontaneous emission Aij (s1) for emission from the upper level i to lower level j. According to Pantel et al.,54 the loss frequency of the first excited state of argon by electronic collision is of the order of 106–107 s1, for an electronic density below 1012 cm3. We assume for any excited levels (see Table I) with spontaneous emission frequency Aij  107 s1 between states i and j that the losses by spontaneous photon emission are the dominant process compared with the electronic impact loss. Table I present the collection53 of more relevant data for clarity [In the table, the levels are given in general coupling notation (for Ar I in Paschen notation), from upper (first) to lower. Here, k, EU, f, and Aij are, respectively, the wavelength of the transition (nm), energy of the upper level (cm1 (eV)), oscillator strength (dimensionless), and spontaneous emission probability (s1)]. However, for nonradiative species, the losses by electronic impact may be taken into account. The collision rate coefficients ki (cm3 s1) may be defined by hri vei, where ri is the cross section (cm2) of the collision i and ve is the electronic velocity (cm s1). The collision rate coefficients depend on the pressure through the electronic energy distribution function. Table I also illustrates that the spectral line of the transition of Ar in TABLE I. Spectroscopy data showing various Ar emissions and high energy electronic transitions.

Species Ar I

Ar II

k

Transition

696.54 750.39 763.51 811.53 443.02 391.48

3p54p(2p2)- 3p54s(1S5) 3p54p(2p1)- 3p54s(1S2) 3p54p(2p6)- 3p54s(1S5) 3p54p(2p9)- 3p54s(1S5) 3p44p(4D0)- 3p44s(4P) 3p44p(4D0)- 3p43d(4D)

EU (eV) Lower level 13.34 13.48 13.17 13.08 19.61 19.61

f

Aij (108)

Metastable 0.029 0.067 Radiative 0.13 0.472 Metastable 0.24 0.274 Metastable 0.51 0.366 Radiative 0.31 0.53 Metastable 0.007 0.032

the plasma corresponding to the energy 13–19 eV should be detected by LP measurements. IV. RESULTS AND DISCUSSION

Figure 2 is a typical LP data at Ar pressure of 3 mTorr, showing the various plasma parameters. The LP position is in the mid-plane (Fig. 1) and at a distance of 3 cm away from the target surface. The figure also shows the presence of both warm and bulk electron populations. For measuring n0, the ion current region of the probe characteristic is favorable for diagnostics as the electron drain and consequently the plasma perturbation is minimal. Even though there is the possibility of perturbation in the plasma due to high applied probe bias, there is no distortion in the IV characteristics (see Fig. 2(a)) even at high ve bias voltage range of 140 to 100 V range. Calculation of n0 in the present study is on the basis of Lam’s. Note that Lam’s theory pertains to the treatment of the attraction of positive ions by an electric probe in the collisionless plasma given by Bernstein and Rabinowitz55 for spherical and cylindrical probes (monoenergetic ions and Maxwellian electrons). This method was extended by Laframboise56 to a fully self-consistent theory considering Maxwellian ions and Maxwellian electrons. The other theory assumes that cold ions (Ti/Te ¼ 0) move radially toward the probe, which suggests that all ions entering the probe sheath reach the probe tip. Allen et al.57 showed that this is valid for spherical probes but not for cylindrical ones as suggested by Chen.58 Nevertheless, Chen used this assumption also for cylindrical probes. Measurements with such probe geometry may be interpreted with more accuracy by Chen’s theory than by the approach of Laframboise, due to destruction of the orbital motion of the ions by collisions. This theory of radial motion is valid for a cylindrical probe as long as the number of collisions across the sheath is small (typically less than one), but not zero. Currently accepted theories, used in low-pressure laboratory plasmas, usually assume an infinitely long probe and a collisionless sheath. Following this approach is the general orbital motion (OM) theory of Laframboise,53 which reduces to the orbital motion limited (OML) regime57 for probe radius to electron Debye distance ratio rp/kd 3. The plasma density n0 in the present experiment is usually of the order 1010 cm3, for which rp/kd  2.5 (for Te ¼ 2 eV). This discussion suggests that the use of Lam’s theory may also be appropriate for the present studies. Figure 4 present the EEDFs using the procedure in Fig. 3, corresponding to the situation reported in Fig. 2. It gives a comparison of the EEDF determined experimentally with standard distributions as the Maxwellian and the Druyvesteyn30,45 (evaluated with Te  2.5 eV). As expected, the Druyvesteyn distribution drops faster than the Maxwellian distribution for energies above 20 eV. However, the Maxwellian EEDF lies well above both distributions beyond 20 eV, indicating clearly the presence of higher energy electrons than predicted by either the experimental or the Druyvesteyn. The nature of EEDF for experimental or the Druyvesteyn distributions are similar. This

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FIG. 4. EEDF profiles are corresponding to the LP data in Figure 2. It also shows a comparison with Maxwellian and Druyvesteyn distribution.

result supports our earlier assertion that plasma production within the system is primarily by impact ionization of warm electrons. Note that a very common problem in LP diagnostics is probe contamination. Contamination of the probe tip, in the process plasmas, with the low-conductivity layers can introduce additional resistance into the probe circuit that leads to severe distortion at the peak in the measured second derivative of the probe characteristic.48 In the present experiments, we use a thin glass capillary tube that supports and covers the connection between the probe tip (tungsten wire) and the ss tube of 1.5 mm outer diameter. The LP tip exposed to the plasma is about 2 mm. Also, we frequently change the LP tip which reduces the concern of contamination. Another possibility of distortion from the LP measurements can occur from the probe circuit resistance.59 One can expect here that the Druyvesteynization of EEDF in Fig. 4 may result from measurement errors and/or filtering of the noise signal. In this context, one can anticipate the electron-neutral collisions to be negligible in the area of probe perturbation that include the probe sheath and pre-sheath. For the DC magnetron plasmas, in the present measurements, there is also the absence of the RF compensated filter circuit, which can contribute additional circuit resistance. One can estimate the Debye length, kd  0.1 mm [for n0  1010 cm3 and Te  2 eV]. For a probe tip radius (rp) of 0.025 mm and length (l) of 2.0 mm, it can be seen that it will easily satisfy the small probe assumptions59   pl rp ln ; kd  mean free path of electrons; 4 rp probe current ð few mAÞ  discharge current ðhundreds of mAÞ:

probe with another stray capacitance. It can be possible that the EEDF distortion (in Fig. 4) is due to the processing of the raw data during the wavelet transform process as described earlier. Further, we investigate the sputtering process using the emission spectra, which are recorded over a wide wavelength range of 200–900 nm for the same operating condition of Figures 2 and 4. OES result in Figure 5 shows strong Ar I emissions (see Table I for clarity) of 3p54p ! 3p54s transitions in the range 750–811 nm. Figure 5 clearly show Ar metastable radicals of 750.39 nm, 763.51 nm, and 811.53 nm. It is obvious that the intensity of the spectral line corresponding to 811 nm (2p9 ! 1S5) is about 2–3 times more than the lines of 763.51 nm (2p6 ! 1S5) and 750 nm (2p1 ! 1S2). Thus, OES data show the signature of high energetic Ar radicals corresponding to Ar-811.53 line. Note that LP analysis (Fig. 2) also signals the presence of 15.3 eV electron population that favors well with the OES data in Fig. 5. Also, EEDF measurement in Fig. 4 shows the clear signature of high electron energy tail. The ions of the plasma move towards the cathode and cause the sputtering of target atoms. Figure 5 also shows the clear evidence of sputtered Zn atoms by the presence of strong emission line corresponding to 307.6 nm (4s4p ! 4S2). However, the lines corresponding to wavelengths 472.2 nm (4s5s ! 4S4p) and 481.05 nm (4s5s ! 4S4p) exhibit low intensity, which indicates lower excited Zn atoms (corresponding to these lines) or low-sputtering rate at low operating pressure 3 mTorr. Also, there is no signature of Al atom for the line 396.15 nm (3s23p 2 P3/20–3s24s 2S1/2), possibly due to its low (2%) content in AZO target. This result suggests that one has to change the operating pressure and investigate the condition, where the line intensities of all lines should be appreciable, for higher sputtering rate by using diagnostics. In order to investigate the sputtering process and favorable deposition condition, the operating pressure is varied from 3 to 20 mTorr. Figure 6 presents the corresponding LP data, showing various plasma parameters, at the same applied DC power of Fig. 2. Note that near the target and in the midplane (Figure 1), the magnetic field is expected to be

(22)

Thus, the plasma perturbation near the collecting probe tip is believed to be small. Moreover, we use a commercial ESPION LP system, which utilizes sweeping voltage power supply. The hardware employs a floating analog-to-digital converter to record the voltage across R. The probe voltage Vp is measured on the ground side of R so as not to load the

FIG. 5. OES data, at 3 mTorr with 300 W DC power, showing emission spectral lines corresponding to Ar and sputtered Zn atoms.

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very low (magnetic null region). In this study, the conventional magnetron’s magnetic field is non-uniform as one move from the midplane to upward and downward direction. For the present measurements, the location of the LP is in the same location from target surface as in Fig. 2. Figures 6(a) and 6(b) clearly illustrate the presence of two-electron populations. n0 (in Fig. 6(a)) decreasing smoothly from 1.38  1010 cm3 to 2  109 cm3. Comparison between Figs. 6(a) and 6(b) indicates that the profile for the n0 is tracking the profiles of Tw, which suggests that the bulk plasma (n0) formation is due to the high-energy population of energy Tw. Figure 6(c) shows that Vp does not change much for all pressure, varying only marginally between 4.5 and 5 V. Corresponding to the variation of Vp, the overall variation of Tw is between 15 and 5 eV with the change in pressure from 3 to 20 mTorr. The lower value of Vp indicates that warm electrons with energy 5–15 eV are not potential trapped. It is obvious from the LP (Figs. 2, 4, and 6) and the OES (Fig. 5) data that there is the signature of the warm populations in the discharge. It is also evident that there is no electron cyclotron resonance (ECR) like discharges in the present work. As discussed above, ionization in the plasmas is mainly dependent on the electron energy or temperature or the presence of the high energy tail on the EEDF. Note that such an energetic tail can be produced by Landau damping of waves in the plasma.60 It may also be noted that plasma is a highly nonlinear medium which can result in nonlinear features even in the DC power coupled plasma sources. The richness of the plasma medium has resulted in numerous studies being carried out in both unmagnetized61,62 as well as magnetized geometries.63,64 In addition, probing the plasmas through LPs or current probes have shown the oscillatory nature riding on a DC level, which carry signatures of instabilities in the plasmas. Some of the experiments65,66 has reported that the secondary electrons emitted from the cathode of a glow discharge can drive beam-plasma-type instabilities. However, detailed investigation of the mechanism of the formation of warm electrons is beyond the scope of this

paper. In addition, due to enhanced collisions with neutrals at high pressure, the effect of Landau damping will be less, which causes a decrease in Tw value or depletion of the high energy tail (to be discussed later in Fig. 10). (According to the electron-neutral collision cross section data of Ar,64 the mean free paths are, respectively, 14.5 cm, 7.5 cm, 5 cm, 3.5 cm, and 3.1 cm for corresponding values of Tw in Fig. 6(b) at different pressures.) Moreover, thermal equilibrium is clear for the bulk electrons for which the temperature Te only varies between 2.5 and 2 eV for the variation from low to high pressure. It is obvious that, between pressure 3 to 15 mTorr, nw [in Figure 6(a)] increases smoothly from 3.8 to 4.8  108 cm3. Since Vp increases (slowly from 4.7 to 5.1 V) and Tw decreases (from 15.3 to 7.1 eV) in this region, the nw variation may be attributed partly to plasma pressure (p ¼ nw Tw) balance and partly to the Boltzmann factor (exp (eVp/Tw)). Between 15 to 20 mTorr, nw decreases gently from 4.8 to 4.0  108 cm3, and this variation seems to be as per the Boltzmann factor, while Tw drops from 7.1 to 5.3 eV in this operating condition. Figure 7 shows the collision rate coefficient in units of second1 (s1) using collision cross section (r) data67 corresponding to the data of Tw in Figure 6(b) as discussed above in Sec. III. We note that the warm electron populations with Tw (15–11 eV) are near the threshold for electron impact ionization for argon (15.76 eV) for pressure range of 3–6 mTorr. The impact ionization cross-section (IICS) data67 for electrons on argon atoms show that the IICS increases quickly between 10 – 20 eV and saturates nearly 40 eV. Thus, the plasma density, n0 is higher at this pressure range. As discussed above, Tw acquires low value for higher pressures due to high collision rates. The overall trend of nw and Tw indicates that the impact collision frequency will be proportional to the product nw ve, where ve {¼冑(8Tw/pme)} is the average speed of the warm electrons. An examination of the radial n0 profile (not shown) can show that it tracks essentially the nw冑Tw profile. It is apparent that the Fig. 7 has very similar profiles of n0 in Fig. 6(a). It shows that bulk plasma production throughout the system is due to the warm electrons.

FIG. 6. Figure shows profiles of plasma parameters, at the same location and operating conditions in Fig. 2 at different working pressures.

FIG. 7. Impact collision frequency (hr vei) showing the connection with the profile of n0 in Figure 6.

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min before coming to saturation. However, at higher pressure 15–20 mTorr, temperature saturates at an earlier time 2 min than the low-pressure range. The small mean free paths (3 cm) for electron-neutral collisions at high pressure can help in thermalization giving rise to lower substrate temperatures at high pressure. Figure 9(c) presents the measurement of resistivity showing the electrical properties of films, for the operating conditions in Figure 9(a). The figure also shows the deposition rate, which increases with pressure and tend to be insensitive near 20 mTorr. Also, as discussed earlier, the film with lowest resistivity can be obtained in the pressure range of 15 mTorr. In order to look into the plasma conditions just above (1 cm) the substrate, we present LP data in Fig. 10, which show the normalized EEDFs corresponding to the operating FIG. 8. OES diagnostic showing the dominant lines in Figure 5 at various pressures.

The emission spectra in the range of 300–900 nm from the plasma are shown in Fig. 8 for the same operating conditions in Fig. 6. For clarity, Fig. 8 only presents the sensitive emission spectral lines given in Fig. 5 for the comparative study. Data show strong Ar I emission of 3p54p ! 3p54s transitions of 750.39 nm, 763.51 nm, and 811.53 nm wavelengths for all pressure range indicating presence of higher energetic electrons or excited radicals in the discharge. The figure clearly shows that the spectral line intensities of Ar do not vary significantly with increasing pressure. However, emission intensities (of lines 307.6 nm, 472.2 nm, and 481.05 nm) corresponding to sputtered Zn atoms increases sharply as the pressure varies from 3 to 15 mTorr; beyond 15 mTorr the intensities do not change much, which indicates the saturation in the sputtering rate of Zn atoms at high pressure in the range of 15–20 mTorr. Also, there is a small trace (low intensity) of Al atom for the line 396.15 nm at high pressures. With the help of the above diagnostics (Figs. 6 and 8), AZO films are deposited on high quality Corning EAGLE XG AMLCD glass (color lines) substrates with different operating conditions. To investigate the favorable deposition condition, we measure the substrate current density, substrate temperature, deposition rate, and film resistivity. The prepared film thickness is 200 nm. Note that the substrate current density here refers to the ion bombardment on the substrate. Figure 9(a) shows that the ion current density rapidly falls as the pressure has increased from 3 to 15 mTorr, before coming to saturation between 15 and 20 mTorr. Comparison with the LP data in Fig. 6, it may be anticipated that because of enhanced collision with neutrals at high pressure Tw decreases quickly by thermalization. Also, as per the earlier discussion, the collisional mean free paths reduce quickly from 14.5 cm to 3 cm as the pressure increases from 3 to 20 mTorr. These data suggest that the operating pressure range 15–20 mTorr will be a favorable condition for the deposition process. Figure 9(b) presents the corresponding increase in substrate temperature from the room temperature with time in second (s). For the pressure range of 3–15 mTorr, the temperature gradually increases up to 3

FIG. 9. Figure shows (a) Ion current density on the substrate, (b) substrate temperature, and (c) deposition rate and resistivity of the films, at various pressures.

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conditions in Figures 6, 8, and 9. The figure also specifies the respective bulk electron temperatures (Te) depending on the operating conditions. As expected, the normalized EEDF data show a broader peak at low temperature, which is in accordance with the LP measurements at the mid-plane (in Figure 6). Probably, due to lack of confinement because of very low magnetic field near the substrate, there is no signature of high energy tails. Note that there is electrostatic confinement (Te < eVp) of bulk electrons even in the very low magnetic field region near the substrate. Also, n0 values (not shown) do not change much, which is within the range 1.5–2.6  109 cm3. These data also suggest that high pressure with low electron temperature is the favorable condition for the AZO sputtering process. Note that, in the present studies, we have varied the operation pressure for the fixed target power of 300 W. Plasma diagnostics and film resistivity data show that the pressure range of 15 mTorr would be the favorable condition for preparation of conductive AZO film. Note that a similar work68 also investigates the effects of deposition pressure and power on the electrical, optical, and structural properties of AZO films deposited by the RF magnetron sputtering. In this work, Zhang et al.68 achieved a very low resistivity of 3.6  104X cm. It was also seen that the film properties of AZO films are dependent on operating pressure and discharge power. We wish to note that the present study describes and investigates the deposition process for the AZO films in the DC magnetron sputtering. The detailed characterization of chemical composition and morphology of the films are beyond the scope of this paper. In addition, there are also other possibilities, which will be undertaken, in the future work, to investigate the AZO film deposition with the lowest possible resistivity. Accordingly, one can also utilize higher target powers, FTS system, RF power, nitrogen/hydrogen gas for further investigation to optimize highly conductive and quality AZO film using dedicated film analysis tools like X-ray diffraction (XRD), XPS, FESEM, UV-visible, etc. Thus, investigations present in this work essentially involve plasma diagnostics using LP and OES methods to understand

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the plasma formation and control plasma parameters, and study the sputtered deposition processes. LP can straightforwardly give information about many plasma parameters, which can be combined with OES to explain more insight of the sputtering plasmas, as in the present case. It may be noteworthy that a key feature of the LP analysis procedure used is that the results obtained are fairly robust. Thus, minor changes in the ion saturation current or its subtraction procedure alter the bulk electron temperature and density only marginally. The robustness of the results about the warm population may be described as follows. For instance, when the LP analysis indicates the absence of the warm population for a given data set, moderate changes in the ion saturation current or its subtraction procedure does not alter this conclusion for that data set. On the other hand, when the LP analysis signals the presence of such a population, changes in the ion current or subtraction procedure, though they can alter the warm temperature and its density by a fair amount, cannot reverse this conclusion or even change the character of the population significantly. Also, EEDF measurement gives the confidence about our analysis technique. It may be kept in mind that, in the present experiments, it is not so much the exact values of the warm electron temperature and density that are critical, but the presence of the warm electron population itself. In this sense, the LP analysis procedure used for this work is fairly robust and satisfactory. V. CONCLUSIONS

This work reports systematically experimental studies of plasma parameters and the diagnostics of plasmas in the CMS system for the fabrication of conductive Al-doped ZnO film. For the straightforward evaluation of the plasma parameters, this study also focuses on the development of the methodical LP analysis procedure for its use in low-temperature and low-pressure laboratory plasmas. Along with LP and OES diagnostics, we have also diagnosed the deposition environment by measuring substrate temperature, ion flux, and deposition rate to investigate the favorable condition for fabricating AZO film, with low resistivity of mX, at different operating pressures. ACKNOWLEDGMENTS

This work was supported by Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (MSIP) (20110031643) and Korea Institute for the Advancement of Technology (KIAT) funded by the Ministry of Trade, Industry and Energy (MOTIE) (N0000590). 1

FIG. 10. Normalized EEDFs near the substrate location at the operating conditions in Figures 8 and 9.

M. W. Park, K. Y. Chun, J. S. Lee, D. J. Kwak, Y. M. Sung, and Y. T. Hyun, Electron. Mater. Lett. 5, 109 (2009). 2 J. G. Han, J. Phys. D: Appl. Phys. 42, 043001 (2009). 3 Y. S. Lim, S. G. Seo, B. B. Kim, H. S. Choi, W. S. Seo, Y. S. Cho, and H. H. Park, Electron. Mater. Lett. 8, 375 (2012). 4 C. Jagadish and S. J. Pearton, Zinc Oxide Bulk Thin Films and Nanostructures (Elsevier, New York, 2006).

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