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Surface Review and Letters, Vol. 15, No. 5 (2008) 653–659c© World Scientific Publishing Company

MACRODROPLET REDUCTION AND GROWTHMECHANISMS IN CATHODIC ARC PHYSICAL

VAPOR DEPOSITION OF TiN FILMS

MUBARAK ALIDepartment of Physics,

COMSATS Institute of Information Technology (CIIT),Park Road, Islamabad, Pakistan

mubarak74@comsats.edu.pk

ESAH HAMZAHFaculty of Mechanical Engineering, Universiti Teknologi Malaysia,

81310, Skudai, Johor, Malaysiaesah@fkm.utm.my

TAHIR ABBASDepartment of Physics, Bahauddin Zakariya University,

Multan, 60800, Pakistantahir abbas@yahoo.com

MOHD RADZI HJ. MOHD TOFFAdvanced Materials Research Center (AMREC),

SIRIM Berhad, Kulim Hi-Tech Park, 09000 Kulim, Kedah, Malaysiamradzit@sirim.my

ISHTIAQ A. QAZINational University of Sciences and Technology,

Tamiz-ud-din Road, Rawalpindi, Pakistanishtiaq qazi@iese.edu.pk

Received 12 February 2008

Cathodic arc physical vapor deposition (CAPVD) a technique used for the deposition of hardcoatings for tooling applications has many advantages. The main drawback of this techniqueis the formation of macrodroplets (MDs) during deposition resulting in films with roughermorphology. The MDs contamination and growth mechanisms was investigated in TiN coatingsover high-speed steel, as a function of metal ion etching, substrate bias, and nitrogen gas flowrate; it was observed that the latter is the most important factor in controlling the size andnumber of the macrodroplets.

Keywords: Cathodic arc; PVD; TiN; macrodroplets; morphology; surface roughness.

1. Introduction

Cathodic arc physical vapor deposition (CAPVD)technique is widely used as a hard coating depositiontechnique because of its many advantages such as,high deposition rate and strong adhesion between the

resulting film and substrate. Emission of macroparti-cles ejected from the cathode during process is a seri-ous problem since the quality of coating is degraded.The problem has been investigated by a number ofresearch groups and quite a few innovative ideas have

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been put forward to overcome the problem such ascooling the cathode, decreasing the arc current, andaccelerating the movement of arc spots.1 Tai et al.2

have reported that the number of macro droplets(MDs) over a fixed film area varied directly with arccurrent, deposition time, and target thickness andinversely with the substrate distance, substrate bias,and nitrogen gas pressure. Attempts have thereforebeen made to separate the arc plasma and the MDsby means of different filtering systems.3–6

The Arc bond sputter (ABS) PVD technique wasdeveloped by combining the features of the cathodicarc and magnetron deposition processes.7 This tech-nique yields coatings with high levels of adhesion,dense structure, and provides a smooth, MD freesurface. The resulting system is complex and henceexpensive.

In view of the above, the authors decided torevisit the problem and this paper describes ourattempts to deposit MD free films by varying thefilm deposition conditions.

2. Experimental Details

Titanium nitride (TiN) films were deposited on high-speed steel (HSS) discs (diameter: 10mm; thick-ness: 6mm) using the Hauzer Techno Coating (HTC)625/2 ARC coating system. The substrate wascleaned in an ultrasonic bath using a mild solvent-free detergent for 30min. This was followed by rins-ing with running water and air-drying. Just beforeloading the substrate into the coating chamber, itwas blow-dried with high-pressure nitrogen gas toremove any dust or contamination on the surface.

The film deposition was done with the substratebiased with a DC power source in order to induceproper ion bombardment on the growing surface toassist in the formation of desirable structure, grainsize, and film density. The chamber was evacuatedto a pressure of approximately 4 × 10−6 m bar andthen back filled with nitrogen gas in the range of∼ 10−3 m bar. The complete deposition procedurehas been described earlier.8

The average distance between the high purity(> 99.99 at %) titanium arcs and the substrate holderwas about 18 cm. The turntable substrate holder wasrotated continuously by its axis to provide a uniformcoating and to form the desired phase; the rotationspeed was set to 60% of the total, i.e. 100%. The

substrates were pre-biased at a −1000V potentialand subjected to metal ion etching for 5min usingtwo titanium arc sources operating at arc currents of100 A. A summary of the deposition parameters canbe seen in Table 1. The chemical composition mea-sured by EDX is given in Table 2. A qualitative chem-ical composition analysis of HSS can be seen in Fig. 1.

The surface morphology of the coatings depositedunder different substrate biases, nitrogen gas flowrates and with/without metal ion etching wasobserved using a field emission scanning electronmicroscope (FE-SEM, model LEO-1525). The topog-raphy of the surface samples was examined using thescanning probe microscope (model SPM-9500J2, Shi-madzu Corporation), with an AFM that was con-trolled via the Microsoft Window 95 OS. AFM withcontact mode has been used to investigate the mod-ifications of TiN-coated HSS. AFM was also usedto study growth mechanisms by using the lateralapproach. Average surface roughness of uncoatedand coated samples was measured using surfaceroughness tester (Model-Surftest SJ-301, Mitutoyo).

3. Results and Discussion

3.1. Reduction in macrodroplets viasubstrate bias

For comparison, Figs. 2(a) and 2(b) show electronmicrographs of TiN coating, on HSS substrate at0 V and −150V bias, respectively. In our experimen-tal work, we observed a gradual reduction in thesize and number of the MDs on the substrate, asthe bias voltage was increased from zero to upwards.The improvement of the film quality due to highernegative bias can be explained as follows:

The macroparticles evaporated from the sourcebecome negatively charged by interacting with theelectrons, as the latter are more mobile than theplasma ions. The repulsive force between the chargedmacroparticles and the negatively biased substrateresults in increased adatom mobility at the sur-face and a uniform deposition. Again, because ofthe high negative bias the Ar/N plasma ions strikethe substrate surface with higher energy raising itstemperature and smoothing out the film surface.Additionally, the re-sputtering from the non-uniformareas of the coating becomes a significant factor inelimination of asperities, voids, and surface rough-ness. In particular, the paper revealed that the

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Macrodroplet Reduction and Growth Mechanisms in CAPVD of TiN Films 655

Table 1. List of deposition parameters and average surface roughness.

Titanium Bias Substrate N2 gas Argon gas Cathode Surface roughnession-etching voltage temperature flow rate flow rate current Ra (ISO, 97)

(min) (−V) (◦C) (sccm) (sccm) (A) (µm)

5 0 300 200 50 100 0.435 25 300 200 50 100 0.425 50 300 200 50 100 0.355 75 300 200 50 100 0.275 100 300 200 50 100 0.215 150 300 200 50 100 0.185 50 300 100 50 100 0.495 50 300 200 50 100 0.255 50 300 300 50 100 0.330 50 300 200 0 100 0.148 50 300 200 50 100 0.45

Table 2. Chemical com-position of substratematerial (HSS).

Element Weight %

C 1.0Cr 4.0V 2.0Mo 5.0W 6.0Fe Balance

Fig. 1. Chemical composition analysis of HSS substrateusing EDX.

elimination in MDs mechanism is primarily relatedwith charged particle movement in plasma sheath. Arecent study9 shows that coarse particles on the TiNfilm surface are significantly decreased both in size

(a)

(b)

Fig. 2. SEM micrographs of TiN-coated HSS at sub-strate bias of (a) zero and (b) −150 V.

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656 M. Ali et al.

and in amount when pulsed bias arc deposition usedwith bias voltage of −300V.

3.2. Reduction in macrodroplets vianitrogen gas flow rate

The presence of nitrogen is essential for the film for-mation process and, up to a certain level, increas-ing the flow rate helps in improving the film quality.Figures 3(a) and 3(b) show that by increasing the N2

gas flow rate from 100 sccm to 200 sccm a significantdecrease both in size and number of MDs resulted.

Increasing the partial pressure of the reactivegas during deposition can decrease the number ofmacroparticles in reactively cathodic arc evaporation

(a)

(b)

Fig. 3. SEM micrographs of TiN-coated HSS at(a) 100 sccm and (b) 200 sccm.

(CAE) coatings.10–12 In TiN deposition, the com-pound layer of TiN, which has been shown to formon the surface of the Ti cathodes, has a significantlyhigher melting point (2930◦C) compared with the Ticathode (1660◦C).13 This has in turn been shown toreduce the number of Ti macroparticles deposited insubsequent TiN coatings.13,14

By increasing the flow rate of the N2 gas at a con-stant pumping speed, the pressure inside the cham-ber increased. Therefore, the species responsible forre-sputtering face hinder to reach the substrate sur-face. Lower macroparticle generation rate combinedwith higher collision rate lead to optimum around200 sccm. By increasing the nitrogen gas flow ratethus from 200 sccm to 300 sccm an increase in growthdefects and surface roughness was observed. Meansurface roughness measured via surface roughnesstester at N2 gas flow rate of 300 sccm and 200 sccmwere 0.33 µm and 0.25 µm, respectively. As expected,the rise in surface roughness at 300 sccm is caused bythe loss of ion bombardment energy due to the highpressures of nitrogen used. In addition, the increasein surface roughness at 100 sccm is because of anincrease both in size and number of MDs. Again, thenumber of MDs as well as the rate of growth defectshad mutually settled at 200 sccm and hence helpedin decreasing the surface roughness; when compareto 100 and 200 sccm. The physical mechanism behindthis has already been discussed by Karr et al.15

The macroparticle content of titanium filmsdeposited using a steered vacuum cathodic arc hasbeen investigated by Swift.16 Swift16 found that thenumber and mass of macroparticles arising from acathode spot are proportional to the cathode spotvolume in steered cathodic arc evaporation PVDtechnique. In our experimental work, random arc forcathode evaporation at constant rate was used. Thenumber density and average sizes of droplets andcraters were lower in the steered-arc sample than inthe random arc sample.17

It has been found that the macroparticle num-ber decreases with increasing steering magnetic fieldstrength, and experimental results are found to beconsistent with the model in which the number andmass of macroparticles arising from a cathode spotare proportional to the cathode spot volume.

The thickness of the deposited coatings at N2 gasflow rate of 100 sccm and 200 sccm was 4.8µm and

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Macrodroplet Reduction and Growth Mechanisms in CAPVD of TiN Films 657

4.3µm, respectively, whereas the deposition time wasthe same.

3.3. Reduction in macrodroplets viapretreatment/etching

Ion bombardment is most often used prior to thinlayer deposition as substrate surface cleaning andpossibly heating it up to the deposition tempera-ture. The aim is to reduce the amount of impuritieson the substrate surface by the bombardment of ionparticles.

(a)

(b)

Fig. 4. AFM 3D topography images at (a) without and(b) with 5min ion etching.

It is quite difficult to justify via SEM (whichis not considered suitable technique to study thenucleation and growth mechanisms) that MDs pro-duced during etching stage, whereas growth defectsduring coating stage. Therefore, we deliberately cre-ated some minor scratches on polished HSS surfacejust after standard metallography and studies havebeen carried out via atomic force microscopy (AFM).Three-dimensional (3D) topography image taken viaAFM can be seen in Fig. 4(b), which clearly indicat-ing that MDs produced during etching stage.18

Macrodroplets were observed for sample at ionetching for 16min and they were deposited in theform of domes on the surface of the substrate, asshown in Figs. 5(a) and 5(b). Figures 5(a) and 5(b)

(a)

(b)

Fig. 5. Interface micrographs showing growth of MDsat ion etching for 16min.

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658 M. Ali et al.

(a)

(b)

Fig. 6. Qualitative chemical composition of spectrum 1and 2 (Fig. 5).

show that the MDs growth initiated at the substratesurface. This is due to ion etching effect. Figures 5(a)and 5(b) show same sample micrographs but differ-ent locations for EDX analysis, i.e. spectrum 1 and2. Qualitative analyses confirm this observation asshown in Fig. 6 with spectrum 1 and 2. Spectrum 1shows the presence of TiN coating, whereas spec-trum 2 shows the presence of substrate elements (Fe,O, Cr, Si, V) with the coating elements.

As it stands that with ion bombardment thatif not properly controlled a surface with inade-quately removed impurities and degraded surfacemorphology due to the induced surface layer peel-ing may develop. In our case, we observed thation etching led to increased MD concentration.Figures 7(a) and 7(b) clearly demonstrate the coat-ing microstructure without ion etching and with

(a)

(b)

Fig. 7. SEM micrographs of TiN-coated HSS at(a) without and (b) with 8 min Ti ion etching.

16min ion etching. Further investigations of the rawsubstrate and the ion-bombarded substrate by AFMrevealed [Figs. 4(a) and 4(b)] that these microstruc-tures are different from the film with MDs in thatthese were non-spherical. With their origin in thesubstrate, they protruded out of the subsequentlydeposited film and appeared as macrodroplets! Inany case, they were harmful to the film and degradedthe film quality. Ion etching, therefore, was not help-ful in our case.

4. Conclusions

It was observed that HSS surface peeling occurredduring the ion etching stage and the resulting spikesprotruded through the TiN film, resulting in films

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Macrodroplet Reduction and Growth Mechanisms in CAPVD of TiN Films 659

with deteriorated surface features. Increasing N2 gasflow rate up to 200 sccm helped to reduce the MDsize and number; at higher rates, of say 300 sccm,the behavior was reversed. A negative bias ofaround −150V was found to be very effective in tack-ling the macrodroplet problem.

Acknowledgments

Thanks are due to the Government of Malaysia forthe scholarship for Dr. Mubarak Ali, the MalaysianMinistry of Science, Technology and Innovationfor the IRPA funding Project No: 03-02-06-0067PR0006/06-02 and to AMREC SIRIM Berhad forthe facilities.

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