Study of RF/DC Co-Sputtered TiAlN Plasma Using Langmuir Probe for Prospective Industrial Applications

Study of RF/DC Co-Sputtered TiAlN Plasma Using Langmuir Probe for Prospective Industrial Applications

Ashwini Kumar Singh*, Neelam Kumari and P. K. Barhai

Department of Applied Physics, Birla Institute of Technology, Mesra, Ranchi-835215, INDIA

Abstract

Titanium Aluminium nitride thin films prepare by magnetron RF/DC co-sputtering system is commercially used for production of hard coatings for the industrial uses. Plasma parameter like plasma potential, floating potential, electron temperature, electron density and ion density plays vital role in defining the properties of the thin films. To understand these physical phenomenons, we have studied the plasma behavior with the help of Langmuir probe constructed by tungsten wire of small diameter. Several studies shows different type of relationship between these parameters with the plasma, but till date no study shows how these phenomenon are in particular responsible for the better properties and at what conditions best result should be obtained for industrial applications.

Keywords: Titanium aluminium nitride, Magnetron sputtering, Plasma Diagnostics, Langmuir Probe

Nomenclature

A = Langmuir probe area, m2

ne = electron number density, m-3 e = electron charge, C

k = Boltzmann constant

Te = electron temperature, eV Ti = ion temperature, eV

Ie = electron current, A

Vp = plasma potential, V

Ies = electron saturation current, A Ve = electron Velocity, ms-1

Iis = ion saturation current, A Vi= ion Velocity, ms-1

ni = ion number density, m-3 VB= Probe voltage (V)

fe() = Electron energy distribution function (EEDF)

Vf = floating potential, V

1. Introduction

   Reactive magnetron co-sputtering is widely used technique for the metal oxide and metal nitride surface coatings. In these types of devices magnetic and electrical fields are used in combination to achieve the specific properties due to the surface treatment/coating. Plasma species in the discharge are governed by the reaction of gases and metals in particular ratio due to the crossed magnetic and electrical fields, cathode (material used for sputtering), pressure, discharge power density and sputter gas used [1]. The nature of such plasma discharge is complex in nature and can be understood by monitoring the plasma parameters like density, temperature, energy etc. [2-4]. Magnetron sputtering process of thin film growth

   is widely used for the applications like antireflection coatings, decorative/protective coatings, metallic interconnects in microelectronics, magnetic recording heads, thin film solar cells, etc. This plasma assisted process enhances the efficiency of deposition rate and is devoid of any chemical reactants so it is eco- friendly [5-8].

   TiAlN thin films had found its use in the field of hard coating in last decades due to its high hardness (30-35GPa), oxidation resistance in the range (850-9000C) and higher corrosion resistance. TiAlN is superior to TiN, properties of the coating is governed by the Al content and the phase structure of the coatings. Due to this reason TiAlN coating is used in the dry and high speed machining tools with incremented tool life and reduced machine downtime [9-15]. These coatings can also be used as high-density complementary metal-oxide-semiconductor (CMOS) memory devices [16], temperature controller, solar collectors [17-20] and bio-medical implants [21]. TiAlN thin films are in general economically prepared by the magnetron sputtering method for the industrial use. Intensive study of the plasma properties of thin film is required for the growth phenomenon and various properties of the coatings. This paper is one of such studies that specifically emphasize the corresponding plasma

   behavior with the coating conditions.

   Plasma parameter governs the feature of PVD coatings formed on the test/field substrates. To understand the exact behavior of plasma these parameter needs to be studied under continuous observations. Langmuir probe is one of the simplest probes giving most accurate result of

   Present work emphasizes the nature of TiAlN plasma inside the magnetron co-sputtering system and elaborates the plasma information required for the better coating conditions of field samples. Plasma energy and gas ratio based study in relation with the plasma pressure carried out in the form of I-V curve extracted by using small diameter Langmuir probe. From these studies I-V characteristics are monitored and using respective mathematical relations different parameters are calculated for plasma diagnostics [37-40].

2. Experimental setup

   In the Plasma Laboratory at Birla Institute of Technology, Mesra, Ranchi RF/DC magnetron co- sputtering deposition system (Vacuum technique pvt. Ltd., Banglore) was used for the study of TiAlN plasma. It consists of dual magnetron in which pure (99.99%) Ti target is connected to DC (1kW) power supply and Al target to RF (300W) power supply, MKS type 247 mass flow controllers, sample holder and view port. Targets (50 mm diameter, 5 mm thick) were mechanically clamped to the magnetron cathode of the sputtering system. Langmuir probe of small diameter (0.5mm) and 5 mm length covered with glass sleeve inserted in the chamber from the front port as shown in the schematic diagram (Fig. 1).

   RF Power

   DC Power

   RF Power

   DC Power

   Cathode

   Langmuir Probe

   Cathode

   plasma parameters. It is in use since its inception and is used for the plasma diagnostics in most of the research labs around the world. It is a tool for the direct measurement of plasma potential and

   I

   current inside the deposition chambers and the

   current voltage (I-V) curve so obtained is used for V

   the measurement of ion density, electron density,

   Plasma

   Ions

   Sheath

   Secondary Electrons

   Anode

   electron temperature and electron energy distribution function(EEDF). From the I-V curve

   DC Biasing

   EEDF is calculated using Druyvesteyn formula in terms of second derivative of the I-V data [1, 22- 28]. Experimental and theoretic methods were employed by various authors for the measurement of plasma properties. Langmuir probe [29-32], laser induced fluorescence [33, 34] and optical emission spectroscopy (OES) [35, 36] are few of the methods used in research laboratories for plasma diagnostics.

   Figure 1. Schematic Diagram of RF/DC Magnetron Co- Sputtering System with Langmuir probe.

   Probe is placed inside the chamber in such position that it penetrated the plasma and does not have any effect on the thin films deposited during the plasma studies. Si substrate is placed in the chamber to verify the effect of plasma parameters on the film properties. Substrates were ultrasonically cleaned for 30 minutes using

   isopropyl alcohol and dried at room temperature prior to deposition. Substrates were mounted on the sample holder and the chamber is evacuated to the

   Electron velocity:

   base pressure of ~10-6 mbar. In presence of Ar

   target surface and the substrates were sputter etched at 100Watt power for 20min inside the deposition chamber to avoid the contamination before deposition. After sputter cleaning, Ar: N2 gas flow rate was adjusted to 4:6 ratios and working pressure of ~ 3.5 x 10-2 mbar was maintained during deposition. Plasma parameters and films were obtained at different RF/DC power ratios; the deposition conditions are summarized in our earlier communication [15]. I-V characteristics

   curve recorded at the given power after the discharge was stabilized. Probe current measure in respect to the biasing voltage of -50 to +50Volts. Plasma parameters are calculated with the help of I-V characteristic curve and the Graphical User

   1

   e

   e

   V = 8 kTe 2 (4)

   me

   Ion velocity:

   1

   V = 8 kTe 2 (5)

   i

   i

   mi

   3

   Current(mA)

   Current(mA)

   2

   1

   Original graph

   Normalized graph

   Interface (GUI).

   -50

   0

   0 50

3. Measurements

   1. Plasma parameters

      Langmuir probe theory is used for the study of plasma parameters. It states that when probe biasing voltage (VB) is sufficiently negative than the plasma potential (VP) probe collects the ion saturation current (Iis). Probe continuous to collect positive ion till VB equals VP after that probe starts repelling the ions. For VB » VP, current through probe is electron saturation current (Ies); VB < VP, probe starts partially repelling the electrons and when VB « VP, probe repels most of the electrons hence electron current reduces to zero (Ie =0). In case of Maxwellian electron velocity distribution, electron current decreases exponentially with decrease in biasing voltage. Using these equations we can calculate the plasma parameters [1, 26, 27,

      32, 41]:

      The electron current:

      Probe voltage(V)

      Figure 2. Langmuir probe I-V characteristics for different RF/DC power.

      I-V plot was obtained after calculating true electron current by subtracting ion saturation current from total probe current. From the Langmuir I-V plot plasma potential (VP) and floating potential (Vf) are determined (Fig. 2). Floating potential (Vf) is the potential at zero probe current and plasma potential (VP) determined by intersection of the tangent drawn at electron saturation and electron retardation region of the In (Ie)-V plot, Te can also be determined from the In (Ie)-V plot (Fig. 3). Energy of the bombarding particles is given by the difference of plasma potential and floating potential (VP-Vf). Plasma potential can also be determined by zero crossing of the second derivative of I-V curve (Fig. 4).

      2×100

      1×100

      Ie VB =

      Ies

      exp e VP VB , V

      B

      B

      kTe

      VP

      (1)

      0

      ln(I)current

      ln(I)current

      -1×100

      Ies , VB > VP

      The electron saturation current:

      1

      I = en A kTe 2

      es e

      es e

      2me

      The electron temperature:

      (2)

      -2×100

      -3×100

      -4×100

      -5×100

      -6×100

      -7×100

      -8×100

      0 10 20 30 40 50

      e

      e

      T = VB

      InI e

      (3)

      Probe Voltage(V)

      Figure 3. Variation of ln (Ie) with probe voltage.

      Electron velocity is found to be nearly 103 times that of ion velocity. Ion velocity is in the

      average electron energy (< >) and effective electron temperature (<Teff>) [1, 26, 27, 42].

      2 (

      2 (

      H

      2m 2e£ d2I

      range of 103 ms1 while that of electron is 106 ms1 inside the chamber. When electron temperature is

      fe() =

      e )2

      e A me

      dV2 (8)

      considerably higher than ion temperature (Te >>Ti)

      n = max f d

      (9)

      the ion saturation current is determined by Bohm

      Sheath Criterion instead of ion thermal speed.

      e 0 e

      < >= 1 max fe d

      (10)

      0.4 ni 0

      0.3

      Vp=6.09V

      <T >= 2 < > (11)

      d2I/dV2

      d2I/dV2

      0.2

      0.1

      D2=0

      eff 3

      0.0

      -40 -20 0 20 40

      -0.1

      -0.2

      -0.3

      Probe Voltage(V)

      1.20E+011

      1.00E+011

      8.00E+010

      Figure 4. Variation of second derivatives of Langmuir probe trace for RF/ DC power.

      The ion saturation current (Iis) is given as:

      1

      Iis = IBohm = 0.6eAni kTe 2 (6)

      m

      m

      i

      When Te >>Ti plasma is isotropic and the

      6.00E+010

      4.00E+010

      2.00E+010

      0.00E+000

      0 10 20 30 40 50

      Electron energy(eV)

      probe sheath is non-collisional and thick. The area is Orbital Motion Limited (OML) region and the probe theory is OML probe theory. The ion current in the OML regime is [42]:

      i i

      i i

      I = An e e(VB VP ) (7)

      8mi

      20

      12

      11 18

      Plasma potential(V)

      Plasma potential(V)

      Floating potential(V)

      Floating potential(V)

      10

      16

      9

      8 14

      7 12

      6

      10

      5

      4 8

      S1 S2 S3 S4 S5 S6

      Substrate

      Figure 5. Variation of Plasma potential (Vp) and (Vf) with RD/DC power.

      Druyvesteyn formula is used for the calculation of Electron Energy Distribution Function (EEDF), where probe current is double differentiated with respect to biasing voltage. On integration EEDF gives electron density (ne),

      Figure 6. Normalized EEDF as a function of power ratio of 1.6(S4).

      Normalized EEDFs at different RF/DC power were used to evaluate the electron density (ne EEDF), average electron energy and effective electron temperature Teff .Fig. 6 shows the normalized EEDF as a function of energy at RF/DC power. Plasma parameters (Fig. 7), i.e., Te, Teff , Vp (slope), Vp (EEDF), Vf (slope) and particle density (Fig. 8), i.e., ne (expt.), ni (OML) and ni (Bohm) are calculated from different method and at different power. The error bars in the measurements done by Langmuir probe are generally in the range of 1217%.

   2. Thin film characterization:

      Crystallographic analyses of the deposited films were performed by GIXRD (Model: Bruker D8 Advance X-ray diffractometer). Surface morphological analysis was done by Atomic Force Microscope (AFM NT-MDT, Solver Pro 47). We have published these results in earlier communication [15]. Surface hardness of the deposited thin film is done by Nano-indenter (CETR, Bruker, DFH-5)

      20

      Te

      18 Teff

      V (slope)

      16 p

      Plasma parameters

      Plasma parameters

      Vf(slope)

      14 Vp(EEDF)

      12

      10

      8

      6

      4

      2

      S1 S2 S3 S4 S5 S6

      Substrate

      Figure7. Plasma parameters as a function of RF/DC power.

      1. GIXRD analysis

         The x-ray diffraction patterns obtained from the film surface (Fig. 9) shows that the film formed is polycrystalline in nature. Consisting of cubical and hexagonal TiAlN crystals in the coating matrix. It has cubic TiAlN (100) peak at (21.79) JCPDS Card no: 37-1140 and hexagonal TiAlN

         (114) peak placed at (68.92) JCPDS Card no: 18- 0070. XRD spectra reveals that the coatings with low Al content show small hexagonal (Ti, Al) N phase and major cubic (Ti, Al) N Phase. This might have resulted due to the incorporation of cubic TiN in place of hexagonal AlN phase [15, 43].

      2. Hardness Analysis

The variation of hardness and Youngs modulus of the deposited films are shown in Fig.

10. It shows that the hardness varies from 10.4 GPa to 37.4 GPa and Youngs modulus from

180.01 GPa to 550.82 GPa with increase in power ratio [Fig. 10]. The variation of hardness & Youngs modulus of the deposited films shows that the film reached maximum hardness of 37.4 GPa. It can be inferred that when the electron temperature was higher the Ti and Al species reacted with N and the film so formed had better grain distribution as well as have maximum hardness. Atomic Force microscopy image of the sample shows even nano grains on the surface with better smoothness (Fig. 11) [15].

1. Results and discussion

   The nature of the current-voltage (I-V) characteristics for different RF/DC power as shown in Fig. 2 depends not only on the plasma behavior, geometry of the Langmuir probe but also on the compensation circuit used for the RF&DC plasma investigation. System pressure is kept such that the mean free path for charge particle-neutral collision is in centimeter range whereas charged particles were collected collisio free. Second derivative of the I-V curve at zero crossing was used for calculating plasma potential (Fig. 4). Constant discharge was maintained by keeping the system pressure constant by Ar+N2 gas flow controlled through MFC controller. The Vp & Vf as shown in Fig. 5 gives energy of the sputtered particles bombarding the substrate. Increase in RF/DC power resulted in increased current to the cathodes, in result supplying more number of electrons in the plasma for ionization. This phenomenon is frequently observed in the magnetron sputtering system. Negative biasing of the cathode increased with increase in electron concentration in plasma causing more target material to be ejected into the plasma from the target surface. Electrons with high energy leaving the target surface follow helical motion in large gyro radius.

   The electron energy distribution function f(), determined to check the plasma electrons Maxwellian nature using Druyvesteyn relation (Eq. 8). EEDF plot (Fig. 6) shows it is measured as a function of the probe potential VB (taken as electron energy ) for the plasma study. It shows that there are two peaks one at lower energy range 3.25eV and another at higher energy range 7.50eV which confirms the presence of two groups of electrons having different temperature. This type of plasma behavior is also noticed by other authors while plasma characterization and analysis. Existence of dual nature suggests that the Ar+N2 plasma is non- Maxwillian. High energy electrons group arises as a result of large number of ionizing collisions between accelerated ions and neutral gas atoms producing secondary electrons due to strong electric field in vicinity of cathode. When high energy group electrons repelled by cathode returned to the plasma by virtual anode, in elastically collides with neutral gas atoms losing most of the energy and contribute as low energy electron group. The thermalization time required between the two

   groups of electrons is not enough for electrons to leave the cathode fall region and hence the electrons not able to redistribute as one Maxwellian group.

   Sputter rate of a RF/DC magnetron is defined by the sputter-ejected atoms as combination of;

   (1) the fast moving atoms ejected from target surface and (2) movement of thermalized atoms arising due to diffusion process. By Increasing RF/DC power the number of electrons getting confined within the plasma increases; these electrons lose their energy to a greater extent during drift ionizing collisions, increasing particles density. This leads to the decrease in the electron temperature value from 4.8 to 3.2 eV (Fig. 7). Theoretically for plasma electrons to be Maxwellilan, effective electron temperature calculated from the normalized EEDF <Teff> should be equal the electron temperature. Here it is observed that the effective electron is slightly higher than the electron temperature [24]. The generated plasma electrons are thus non- Maxwellian having high energy tail in EEDF that strongly supports our observations.

   fairly uniform plasma with acceptable uncertainty/errors in the measurement in the processing chamber. Other researchers have also stated in their respective works that ion-ion and ion-atom collisions within the space charge sheath surrounding the cylindrical probe influences the ion orbital motion [44].

   Diffraction profile obtained from the film surface (Fig. 9) confirms the formation of TiAlN phase which is polycrystalline in nature. It is supported by the presence of the cubic TiAlN

   (100) peak appearing at (21.79) and hexagonal TiAlN (114) peak placed at (68.92). In the spectra it is clear that (100) phase increase with increased power and the other phase (114) reduces with the power. Spectra reveals that the small hexagonal (Ti, Al)N phase and major cubic (Ti, Al)N Phase in the coating is due to variation of Al content in the film. This phenomenon might have been resulted due to the incorporation of cubic TiN in place of hexagonal AlN phase [15, 43].

   TiAlN(110)

   Particle density(exp) (1016)

   Particle density(exp) (1016)

   Intensity (a.u)

   Intensity (a.u)

   100 TiAlN(114)

   S5

   10 S4

   1

   0.1

   ne(exp) ni(OML)

   ni(Bohm)

   S1 S2 S3 S4 S5 S6

   Substrate

   S2

   S1

   10 20 30 40 50 60 70 80 90

   Angle (2)

   Figure 8. Particle density (ne (expt.), ni (OML) and ni (Bohm) ) as a function of RF/DC power.

   The ne values calculated from different methods is observed in the range 1.05×1016 to 1.84×1017 cm-3 with variation of applied power [Fig. 8]. In magnetron sputtering system electrons

   are confined to the cathode, increase in applied power results in increased sputter rate. Magnetic and electric field (E × B) around the cathode present increases lifetime and the electron path. Motion of electron increases the collision probability between electron and atom causing increased ionization rate. This results in increased plasma density at increased power due to enhanced ionization with lower diffusion observed in our experimental and theoretical calculation shown in Fig. 8. Observations show

   Figure 9. GIXRD diffractograms of deposited films prepared at different RF/DC power.

   Nano indenter analysis shows that hardness varies from 10.4 GPa to 37.4 GPa and Youngs modulus from 180.01 GPa to 550.82 GPa with increase in power ratio [Fig. 10]. Hardness & Youngs modulus of the deposited films shows that the film reached maximum hardness of 37.4GPa (S4). Increase and decrease in hardness value is incorporated with the crystal nature and the Al concentration in the film. These values are maintained at predefined ratio by applying appropriate system pressure and the power/current to the cathodes. It is evident that higher concentration of (100) phase than (114) phase makes the film surface harder in comparison to lower concentration ratios. It can be inferred that

   when the electron temperature was higher the Ti and Al species reacted with N and the film had better grain distribution as well as have maximum hardness.

   region characterized by different electron density, plasma potential and temperature in relation with the applied power. It had been observed that in high plasma density region it is approximately 1016 – 1017 ions/cm-3. Plasma density measured by

   40

   35

   30

   Hardness (GPa)

   Hardness (GPa)

   25

   20

   15

   10

   5

   0

   S1 S2 S3 S4 S5 S6

   Test Samples

   600

   550

   500

   450

   400

   350

   300

   250

   200

   150

   Bohm Sheath Criterion and Orbital Motion Limited (OML) theory too confirms the same.

   Young's Modulus(GPa)

   Young's Modulus(GPa)

   The deposited films crystallographic analysis shows that TiAlN nitride film with 100 and 114 orientation. Hardness analysis confirmed that when Ti, Al and N ions are in proper compositional ratio, the films have best hardness. The surface roughness and the film thickness can be improved by increasing the deposition time if more thickly and smooth films are required. In general TiAlN coating are mainly used for the hard and protective coatings and as per the nano- hardness analysis, it can be inferred that better

   Figure 10. Variations of (a) Hardness and (b) Youngs modulus of deposited films Prepared with different RF/DC power.

   Figure 11. AFM image showing nano sized grains of Substrate (S4).

   Atomic Force microscopy image of the sample shows even distribution of nano grains on the surface (Fig. 11). It is observed that the nano grains with less roughness forms smooth surface having improved surface properties.

2. Conclusion

   In the present work Plasma parameter are investigated for two cathodes (RF&DC) in comparison with the TiAlN thin film properties by means of Langmuir probe. Probe parameters measurement shows existence of weak plasma

   properties are achieved at 8:5(S4) power ratio. For industrial application of reactive magnetron sputering system, best coating can be achieved at RF/DC ratio of 1.6. Atomic Force Microscopy image confirms the surface morphology consisting of nano-grains of TiAlN that is responsible for the smoothness and hardness of the thin film.

   Acknowledgement

   The authors are grateful to DST, GOI and MHRD, GOI for financial support under FIST and TEQIP respectively. The authors are also grateful to Birla Institute of Technology, Mesra, Ranchi, for providing the necessary facility and support for the present work. The authors want to acknowledge the cooperation of CIF, BIT, Mesra, Ranchi and UGC DAE consortium for Scientific Research, Indore for the characterization works. The authors want to acknowledge Dr. Mukul Gupta and Mr. Akhil Tayal, UGC DAE consortium for Scientific Research, Indore for the help in XRD study of the films reported in this paper.

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