ath

eden

owiummplms. Attlet/mthe

cal Vae. However, besides the lmrate coradio

sfully tr and Clasmat powmperatf lms

and even higher rates, exceeding 2.5 m/min were achieved for the

dc and pulsed dc magnetron sputtering depositions [9] or of

Plasma Jet (HCPJ

diameter of 2.5 mm) from chromium powered by pulsed dc generator

Surface & Coatings Technology 205 (2011) 41694176

Contents lists available at ScienceDirect

Surface & Coatin

l sCrN lm depositions in N2 plasmas. Even though the hollow cathodeshave already been used indirectly to enhance deposition of Cr and CrNlms using the Activated Reactive Evaporation [4,5], as to ourknowledge there have been no other attempts yet to use the Crhollow cathodes directly as a target either for non-reactive or forreactive PVD of Cr based lms.

A motivation for the present work was to test the pulsed dc

was used for PVD of Cr and CrN lms. A schematic diagram of theexperimental setup is shown in Fig. 1. The generator (ENI, RPG-50)was operated in a constant power mode in the interval between 150and 350 W at 250 kHz with 500 ns pulses. The CrN lms weredeposited in pure nitrogen and the Cr lms in pure argon, both at gaspressures of 67 Pa (0.5 Torr) in the chamber. The target-to-substratedistance was 10 mm. The lms were deposited on unheated silicondeposition in the same hollow cathode systempower might be considered as a technically mto the rf power. Several successful attemptshave already been reported in depositions of

Corresponding author.E-mail address: hana.barankova@angstrom.uu.se (H

0257-8972/$ see front matter 2011 Elsevier B.V. Aldoi:10.1016/j.surfcoat.2011.03.013es. The rates up to aboutosition in the Ar plasma A cylindrical hollow cathode (outer diameter of 6 mm, internalcathode on unheated silicon or steel substrat1 m/min were obtained for the Cr lm deprepresents the most prospective onquality, a sufciently high depositionplating is a challenge. Recently thecathodes made from Cr were succesmodes for high-rate depositions of Cexperiments the Hollow Cathode Ppowered by a 13.56 MHz generator. Acathode outlet was heated up to the teresulted in very high deposition rates omparable with electro-frequency (rf) hollowested in the diffuse arcrN lms [13]. In thoseJet (HCPJ) system wasers above 200 W the Crures over 1300 C whichin the jet axis below the

this paper we compare the pulsed dc power generation with the rfgeneration in the HCPJ system at the same average power delivered tothe plasma and discuss similarities and differences in the PVDprocesses as well as comparison of the lm properties in both thenon-reactive depositions of the Cr lms in argon plasma and thereactive deposition in pure nitrogen plasma.

2. Experimental arrangement, because the pulsed dcore feasible alternativewith the pulsed powerCrN lms, e.g. the pulsed

substrates placethermocouple tesubstrate heateddid not exceed 3about 50 C loweat comparable pwas measured bments Co).. Barnkov).

l rights reserved.) depositions of Cr and CrN were published in [11]. Inreplacements of chromium electroplamethods, among which the Physichnology by alternativepor Deposition (PVD)

superimposing the pulsed signal over the dc or rf power [10], havebeen presented, too. The rst results on pulsed dc Hollow CathodeThe more stringent environmental legislation initiated large-scaleting teComparison of pulsed dc and rf hollow c

H. Barnkov , L. BrdosUppsala University, Angstrom Laboratory, Plasma Group, Box 534, SE-751 21 Uppsala, Sw

a b s t r a c ta r t i c l e i n f o

Article history:Received 8 November 2010Accepted in revised form 3 March 2011Available online 12 March 2011

Keywords:Diffuse arc depositionPulsed dc hollow cathodeRadio frequency hollow cathodeChromium nitride (CrN)Chromium (Cr)

A cylindrical chromium hollwas used for PVD of chromplasmas, respectively. A codepositions of Cr and CrN deposition rate of Cr lmsreached with the cathode ouand CrN lms exceeded 1 mstudied and compared with

1. Introduction

j ourna l homepage: www.eode depositions of Cr and CrN lms

cathode powered by a pulsed dc generator working in a constant power modeand chromium nitride lms on silicon substrates in argon and nitrogen

arison of the pulsed dc process with the radio frequency hollow cathodeat identical power levels shows considerable differences particularly in thethe pulsed power above 250 W the hot cathode/diffuse arc regimes weretemperature as high as 1300 C and the maximum deposition rates of both Crin. The resulting lm properties, e.g. themicrostructure andmorphology werelms obtained by the rf hollow cathode PVD.

2011 Elsevier B.V. All rights reserved.

dc magnetron sputtering [6], pulsed laser ablation [7], or the highpower impulse magnetron sputtering [8]. Comparative studies, e.g. of

gs Technology

ev ie r.com/ locate /sur fcoatd on a thermally insulated sample holder with amperature sensor. The maximum temperature at theby the plasma itself during the deposition process50 C. Note that the pulsed dc plasma regimes led tor substrate temperatures than those in the rf plasmaower [1]. The temperature of the cathode outlet wally an infrared thermometer M90H (Mikron Instru-

the

4170 H. Barnkov, L. Brdos / Surface & Coatings Technology 205 (2011) 41694176The thickness of lmswasmeasured by a VeecoDektakV 200-stylusproler and/or determined by scanning electron microscopy (SEM) onfracture cross-sections. The morphology and microcrystalline structureof the Cr and CrN lms deposited at different powers were studied indetails on SEM images taken from both the lm surfaces and fracturecross-sections, as well as by the X-Ray Diffraction (XRD). The lm

Fig. 1. Experimental arrangement with the cylindrical Cr hollow cathode powered from(HCPJ) discharge at 300 W.nanohardness was measured by a XP Nanoindenter with Berkovichtip at loads between 3 and 5 mN.

0

500

1000

1500

0 50 100 150 200 250 300 350 400

Cath

ode

tem

pera

ture

[C]

Power [W]

ArAr

N2

N2

Pulsed DCRF

arcing

Fig. 2. Comparison of the Cr hollow cathode wall temperatures 7 mm above the HCPJoutlet in pulsed dc and rf power regimes in argon and nitrogen plasmas.3. Results and discussion

3.1. Film deposition rates

Fig. 2 shows the Cr hollow cathode temperatures measured 7 mmabove the cathode outlet versus power. Themeasurements are taken for

pulsed dc generator. The photograph is taken from the Ar Hollow Cathode Plasma Jetboth pulsed dc and rf regimes, in Ar and N2 plasmas. The highest valuesare observed for the Ar plasma in the pulsed dc mode. The voltages arehigh (several hundred V) in the pulsed dc regime and especially incombination with a high secondary electron emission coefcient for Ar,the ion bombardment leads to very high temperatures. At 150W thehollow cathode plasma had a tendency of arcing and the temperature

0

500

1000

1500

2000

2500

3000

0 50 100 150 200 250 300 350 400

Pulsed DC

Film

gro

wth

rate

[nm/

min]

Power [W]

Cr CrN CrN

Cr

RFarc

glow

Fig. 3. The maximum PVD rates of Cr and CrN lms versus power in the pulsed dc and rfHollow Cathode Plasma Jets (HCPJ).

Fig. 4. SEM micrographs of the Cr lms deposited on Si at different pulsed dc powers. Data are shown on the lm thickness and the PVD time for each layer.

Fig. 5. SEMmicrographs of the fracture cross-sections of the Cr lms deposited on Si at different pulsed dc powers. The scale bars and the time of the PVD process are given for each lm.

4171H. Barnkov, L. Brdos / Surface & Coatings Technology 205 (2011) 41694176

almost approached the melting point for Cr (1857 C). We have noexplanation yet of these anomalous regimes. We speculate that theeffect might be connected with nitrogen adsorbed at the cathode walls(either from atmospheric air or from the nitrogen experiments) andwith its strongly exothermic reactions with Ar metastables, see [12].Also, in this regime, pulsed dc mode in Ar, the distributions oftemperature values from the tip of the Cr cathode along the cathodelength towards the electrode holder indicate that the temperature ishighest in the distance of about 10 mm from the outlet. For the rf hollowcathodes and also for pulsed dc regimes in the nitrogen plasma theregion of the maximum temperature is several mm closer to the outlet[11].

In the rf regime the space charge sheaths are formed inside thehollow cathode; the self-bias voltage across the space-charge sheathand the space-charge sheath thicknessdependon thegas. Themeasuredvoltages and electric elds are higher for N2 than in Ar (see e.g. [13]),which leads to more efcient ion bombardment and higher tempera-tures at the cathode in the N2 plasma.

Themaximumdeposition rates (in the axis of the hollow cathode) ofthe Cr and CrN lms as functions of the power in the pulsed dc and rfregimes, respectively, are shown in Fig. 3. It is seen that in the pulsed dcregime the deposition rates of Cr lms are higher than the depositionrates of the CrN lms at powers below 300W, i.e. approximately in thesame power regionwhere the Cr cathode temperature in the Ar plasmaexceeds the cathode temperature in the N2 plasma and where therelease of Cr by controlled evaporation is more efcient. For the rfgeneration, the CrN deposition rate was always higher than the Crdeposition rate, following the evolution of cathode temperatures [11].

It is also seen that at the same power the rf deposition rates of CrNlms are higher than those in the pulsed dc regime, while the Cr lmsare deposited substantially faster in the pulsed dc regime. Thedifference in Cr (in Ar) deposition rates can be attributed to thetemperature difference at the Cr cathode, while for CrN (in N2) thetemperatures are almost the same. It is assumed that in the pulsedregime (0.5 s on, 3.5 s off) even though the release of Cr is high, theionization proceeds only in on part of the pulse and the reactionsbetween the neutral Cr and N2 are slower that the reactions betweenthe ionized species.

The results indicate that the rf powered hollow cathode could bebetter for the reactive deposition of CrN lms while the pulsed dcpower can provide higher rates in the non-reactive Cr depositions.The increased metal content in the alloy lms was reported for thepulsed dc plasma as compared to the rf regime in [10].

The deposition rates of CrN lms always exceeded 1 m/min in thediffuse arc regimes where the temperature of the hollow cathodeoutlet surpassed 1300 C. This was usually the case for the averagepower300 W. An exceptionally high rate, up to almost 3 m/min incertain cases, has been detected in pulsed dc deposition of Cr lms at150 W (see Fig. 3).

3.2. Microstructure and morphology of Cr and CrN lms

Cr and CrN lms deposited on Si substrates in the pulsed dc regimeswere studied by SEM and compared with the lms deposited in the rfregimes. SEM images of the Cr lms deposited at different pulsed dcpowers are shown in Fig. 4. A brous crystalline microstructure of the

4172 H. Barnkov, L. Brdos / Surface & Coatings Technology 205 (2011) 41694176Fig. 6. Comparison of Cr lms deposited in the pulsed dc plasma and the rf plasma regimes at 150 W and 300 W.

pulsed dc deposited Cr lms is seen in images of the cross-sectionalmicrographs in Fig. 5. At highest powers the Cr bers have submicronwidths. The arc based regimes in the argon pulsed dc hollow cathodeplasma led to thick lms (300 m/80 min) with a characteristicfeature of the increasing grain widths with the growing lm thickness.

It is interesting to note that featureless Cr lms grown in the rfhollow cathode plasma in Ar have a very dense crystalline structurecomparable to the monocrystalline lms. However, their growth ratesare much lower than in the pulsed dc plasma (see Fig. 3). Acomparison of the Cr lms grown at 150 W and 300 W in the pulseddc plasma and the rf plasma respectively is shown in Fig. 6. Theseresults show that the generation mode considerably affects both thegrowth rate and the morphology of Cr lms.

The SEM micrographs of the CrN lms deposited by the reactivePVD in the nitrogen hollow cathode plasma at different pulsed dcpowers are shown in Fig. 7. The thinner lms (below 10 m) are ne-

grained and smooth. This is similar to the growth of CrN lms in the rfhollow cathode plasma. For comparison the surface of a CrN lmgrown in the rf plasma at 250 W is shown in Fig. 7, too. The lmmorphology at different pulsed dc powers can be seen on SEM imagesof the fracture cross-sections in Fig. 8. The last image in the set in Fig. 8is the cross-section of the CrN lm grown in the rf plasma. The CrNlms grown in the reactive regimes can also be described as brous upto 250 W and columnar above this power.

3.3. Crystalline structure of Cr and CrN lms

Similarly as for the Cr and CrN lms deposited by the rf hollowcathode plasma PVD [13] the XRD analyses conrmed that all Cr andCrN lms deposited by the hollow cathode plasma PVD in the pulseddc regime are crystalline. However, both the main orientation of

e l

4173H. Barnkov, L. Brdos / Surface & Coatings Technology 205 (2011) 41694176Fig. 7. SEMmicrographs of the CrN lms deposited on Si at different pulsed dc powers. Th

CrN lm grown at 250 W is given for the comparison.m thickness is shown in parenthesis for each individual surface. The rf plasma deposited

4174 H. Barnkov, L. Brdos / Surface & Coatings Technology 205 (2011) 41694176crystals and the texture coefcients of lms are dependent on thepower level as well as on the power regime.

A comparison of the XRD diagrams acquired from Cr lmsdeposited at two different powers in the pulsed dc plasma and therf plasma, respectively, is shown in Fig. 9. The main feature in all lmsis Cr (110). The Cr lms deposited in the low power (150 W and200 W) pulsed dc regimes also exhibit peak (211) at about 81.5 (notshown). This reection disappears at higher power deposited lms.

A comparison of the XRD diagrams from CrN lms deposited in thepulsed dc and rf plasmas is shown in Fig. 10. CrN deposited at the highpower (350 W) in the pulsed dc regime is characterized bypreferential orientation (111). With decreasing power, at 250 W,CrN (200) becomes more prominent. For the rf HCPJ, CrN (111) isprominent for both, 300 and 250 W. This is connected with thedeposition rate versus power curves. In the rf regime both 300 and

Fig. 8. SEM images of the fracture cross-sections of the CrN lms deposited on Si at differenplasma deposited CrN lm grown at 250 W is given for the comparison.250 W correspond to very high deposition rates, while in the pulseddc regime the deposition rate at 250 W is much lower. Thus, the highdeposition rates are linked to (111) preferential orientation, see alsoour previous work on the rf hollow cathode plasma [3].

3.4. Mechanical properties of Cr and CrN lms

A comparison of nanoindentation hardness and Young's modulus ofCr and CrN lms deposited at different powers in pulsed dc plasma andin the rf plasma is summarized in Table 1. Typical values reported in theliterature, shown in Table 1 relate to themicrohardness of lms. For arcevaporated Cr lms values between 8 and 16 GPa were reported [14].For dc pulse sputtered CrN lms [6], arc evaporated CrN lms [15] andarc ion-plated CrN lms [16] the reported microhardness is in theinterval between 16 GPa and 31 GPa. The nanoindentation hardness of

t pulsed dc powers. The time of the PVD process is given for each individual lm. The rf

CrN

(111)

CrN

(200)

CrN

(220)

350W

250W

1 104

2 104

3 104

4 104

5 104

Inte

nsity

[Cou

nts]

0

Pulsed DC

4175H. Barnkov, L. Brdos / Surface & Coatings Technology 205 (2011) 416941760

1 104

2 104

3 104

4 104

5 104

40 45 50 55 60 65 70

Inte

nsity

[Cou

nts]

2 theta []

Cr (1

10)

300W

150W

Cr (2

00)

300W

150W

Pulsed DC

RF

1 104

2 104

3 104

4 104

5 104

Inte

nsity

[Cou

nts]

0CrN lms produced by themagnetron andHIPIMS sputterings ts to thesame interval as well [17]. Note that the pulsed dc plasma deposited Crlms exhibit somewhat lower nanoindentation hardness as well aslower Young's modulus than the rf plasma deposited lms. This mightbe connected with their brous microstructures. The CrN lmsdeposited in the dc pulsed and rf plasma regimes exhibit comparableproperties.

4. Conclusions

The pulsed dc power has been proven to be an interesting andprospective alternative to the rf plasma in the hollow cathode plasmaPVD of Cr and CrN lms. However, considerable differences have beenfound between the pulsed dc and rf hollow cathode plasma regimes inboth the PVD rates and in properties of the deposited lms. Thesedifferences can be very important for selection of power generator forrequired lm applications.

The lm deposition rates always exceed 1 m/min in the diffusearc regimes where the temperature of the hollow cathode outletsurpasses 1300 C. An exceptionally high rate, up to almost 3 m/minin certain cases, has been detected in the pulsed dc deposition of Crlms in argon plasma at 150 W. This was accompanied by anenhanced temperature of the cathode outlet and a cavity-shapederosion inside the cathode. The reason for this effect is not understoodyet.

Heating of the substrate by the pulsed dc plasma is lower than bythe rf plasma, resulting in about 50 C lower temperature. This fact isimportant for PVD of lms on temperature sensitive substrates.

The pulsed dc plasma enhances the PVD of Cr lms and causes abrous crystalline microstructure, while the rf plasma enhances thereactive PVD of CrN lms.

Fig. 9. Comparison of the XRD diagrams acquired from Cr lms grown on silicon in thepulsed dc HCPJ and in the rf HCPJ. The full line curves represent 300 W, the dotted linecurves represent 150 W.250W

300W

CrN

(200)

CrN

(220)

040 45 50 55 60 6535

2 theta []

1 104

2 104

3 104

4 104

5 104

Inte

nsity

[Cou

nts]RFCr lms deposited both in the pulsed dc and in the rf regimesexhibit dominant (110) orientation. CrN lms deposited at very highdeposition rates (above 1 m/min) exhibit (111) preferential orien-tation, both for the pulsed dc and rf hollow cathode plasmas. Lowerdeposition rates lead to CrN (200) being more prominent.

The Cr lms deposited in the pulsed dc plasma exhibit somewhatlower nanoindentation hardness as well as lower Young's modulusthan the Cr lms deposited by the rf plasma. The CrN lms depositedin both kinds of power regimes exhibit comparable properties.

It is evident that due to exceptionally high PVD rates of Cr and CrNlms achieved in the hollow cathode plasma systems in the diffuse arcmodes for both power regimes, as well as due to superior properties ofthe lms, the hollow cathode plasma technology represents a verypromising system for the industrial replacement of conventionalelectroplating.

Fig. 10. Comparison of the XRD diagrams of CrN lms grown on silicon in the pulsed dcHCPJ and in the rf HCPJ. The full line curves represent 350 (300) W, the dotted linecurves represent 250 W.

Table 1Comparison of typical values of the nanoindentation hardness and the Young's modulusmeasured for Cr and CrN lms deposited in the pulsed dc HCPJ and in the rf HCPJ atdifferent powers delivered into the plasma.

Film and PVDpower regime

Power[W]

Depositiontime [min]

Filmthickness(max) [m]

Nanoindent.hardness[GPa]

Young'smodulus[GPa]

Cr pulsed dc 300 10 11 45 120140CrN pulsed dc 150 100 4.2 1923 180230CrN pulsed dc 250 30 4 1722 200290Cr rf 300 10 2.9 1315 249CrN rf 250 20 35 23 278Typical values reported: Cr 816

CrN 1631

Acknowledgements

Financial supports from the Foundation for Strategic Environmen-tal Research (MISTRA) and from the Johan Gustaf Richerts ScienticFoundation in Sweden are gratefully acknowledged. The authors arethankful to Dr. Lars-Erik Gustavsson from the Plasma Group for theSEM images and for XRD measurements of lms.

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[3] H. Barnkov, L. Brdos, L.-E. Gustavsson, J. Vac. Sci. Technol. A23 (2005) 959.[4] T. Sato, M. Tada, C. Huang, H. Takei, Thin Solid Films 54 (1978) 61.[5] A.J. Novinrooz, H. Seyedi, J. Achiev. Mater. Manufact. Eng. 17 (2006) 189.

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of Vacuum Coaters (SVC), Louisville, AprilMay 2007, p. 63.[12] H. Barnkov, L. Brdos, S. Berg, Appl. Phys. Lett. 67 (1995) 1521.[13] H. Barnkov, L. Brdos, Surf. Coat. Technol. 120121 (1999) 704.[14] D.A. Dudko, A.E. Barg, Y.V. Milman, R.K. Ivashchenko, N.P. Zakharova, Powder

Metall. Met. Ceram. 32 (1994) 717.[15] O. Knotek, F. Lofer, H.-J. Scholl, Surf. Coat. Technol. 45 (1991) 53.[16] A. Kondo, T. Oogami, K. Sato, Y. Tanaka, Surf. Coat. Technol. 177178 (2004) 238.[17] G. Greczynski, J. Jensen, L. Hultman, IEEE Trans. Plasma Sci. 38 (11) (2010) 3046.

4176 H. Barnkov, L. Brdos / Surface & Coatings Technology 205 (2011) 41694176

Comparison of pulsed dc and rf hollow cathode depositions of Cr and CrN filmsIntroductionExperimental arrangementResults and discussionFilm deposition ratesMicrostructure and morphology of Cr and CrN filmsCrystalline structure of Cr and CrN filmsMechanical properties of Cr and CrN films

ConclusionsAcknowledgementsReferences