Influence of Ar/Kr ratio and pulse parameters in a Cr-N high power pulse magnetronsputtering process on plasma and coating propertiesKirsten Bobzin, Nazlim Bagcivan, Sebastian Theiß, Jan Trieschmann, Ricardo Henrique Brugnara, Sven

Preissing, and Ante Hecimovic

Citation: Journal of Vacuum Science & Technology A 32, 021513 (2014); doi: 10.1116/1.4865917 View online: http://dx.doi.org/10.1116/1.4865917 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/32/2?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in CrNx films prepared using feedback-controlled high power impulse magnetron sputter deposition J. Vac. Sci. Technol. A 32, 02B115 (2014); 10.1116/1.4862147 Structure of multilayered Cr(Al)N/SiO x nanocomposite coatings fabricated by differential pumping co-sputtering Appl. Phys. Lett. 103, 201913 (2013); 10.1063/1.4831736 Thermal stability and tribological properties of CrZr–Si–N films synthesized by closed field unbalancedmagnetron sputtering J. Vac. Sci. Technol. A 27, 867 (2009); 10.1116/1.3116589 Synthesis and mechanical properties of Cr Mo C x N 1 x coatings deposited by a hybrid coating system J. Vac. Sci. Technol. A 26, 146 (2008); 10.1116/1.2821736 Investigation of the nanostructure and wear properties of physical vapor deposited CrCuN nanocompositecoatings J. Vac. Sci. Technol. A 23, 423 (2005); 10.1116/1.1875212

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Influence of Ar/Kr ratio and pulse parameters in a Cr-N high power pulsemagnetron sputtering process on plasma and coating properties

Kirsten Bobzin, Nazlim Bagcivan, Sebastian Theiß, Jan Trieschmann, andRicardo Henrique Brugnaraa)

Surface Engineering Institute, RWTH Aachen University, D-52072 Aachen, Germany

Sven Preissing and Ante HecimovicInstitute of Experimental Physics II, Research Department Plasmas with Complex Interactions,Ruhr-University Bochum, D- 44780 Bochum, Germany

(Received 4 October 2013; accepted 4 February 2014; published 24 February 2014)

Krypton is sometimes used in physical vapor deposition processes due to its greater atomic mass

and size compared to argon, which leads to a lower gas incorporation and may have beneficial

effects on kinetics of the coating growth. In this paper, the authors investigate the plasma

composition and properties of deposited high power pulse magnetron sputtering Cr-N coatings for

discharges with various Ar/Kr ratios and for various pulse lengths of 40 ls, 80 ls, and 200 ls,

keeping the average discharge power constant. The results show that an addition of Kr influences the

discharge process by altering the ignition and peak values of the discharge current. This influences

the metal ion generation and growth conditions on the substrate by reducing the nucleation site

densities, leading to a predominantly columnar grow. However, the deposition rate is highest for an

Ar/Kr ratio of 120/80. The integral of the metal ion and atom emission exhibits the same trend,

having a maximum for Ar/Kr ratio of 120/80. By decreasing the pulse length, the deposition rate of

coatings decreases, while the hardness increases. VC 2014 American Vacuum Society.

[http://dx.doi.org/10.1116/1.4865917]

I. INTRODUCTION

High power pulse magnetron sputtering (HPPMS) is a

physical vapor deposition technology, where high power is

applied to the magnetron in short pulses, with a duty cycle

below 10%. Longer pulses, where the discharge is sustained

for a several milliseconds, have been achieved when using

the modulated pulsed power (MPP) technique. During MPP

discharge, the duty cycle can be as high as 30%.1 First investi-

gations of the HPPMS plasma using optical emission spectros-

copy (OES) during sputtering of Cr and Ti targets have shown

an increase in the metal ion-to-neutral ratio with increasing

power compared to direct current magnetron sputtering.2 This

results in formation of dense plasmas with a high fraction of

ionized species. It offers the possibility to influence the proper-

ties of deposited coatings by variation of the process parame-

ters in terms of, e.g., density or mechanical properties.3 The

analysis of Cr-N coatings shows changes of the density and

the surface roughness as the peak target current increases,

while the deposition rate decreases drastically.4 Greczynski

et al. reported the effect of process parameters such as pulsing

frequency, energy delivered to the target in each pulse, and

pulsed negative substrate bias on the growth of HPPMS Cr-N

coatings.5 The investigations show that a finer grain structure

may be obtained at a higher frequency. Furthermore, the high

bias voltage is necessary to produce column-free coatings.

In the magnetron sputtering process, argon is the most

commonly used inert gas due to its large atomic mass, inert

chemistry, and relatively low cost. The choice of the process

gas may significantly affect the production and transport of a

range of energetic species to the substrate and subsequently

the growth and properties of deposited material.6 Krypton is

sometimes used due to its greater atomic mass and size com-

pared to argon, which leads to lower gas incorporation and

may have beneficial effects on the kinetics of the coating

growth.7 In our previous work, we have investigated the influ-

ence of the addition of Kr and the variation of HPPMS pulse

parameters in CrAlN process regarding the coating properties.8

Until now, there has not been an extensive study to inves-

tigate influence of the Kr partial pressure on the plasma com-

position and the properties of HPPMS Cr-N coatings. The

aim of this contribution is to investigate both properties of

the deposited Cr-N coatings and the plasma composition for

a discharge where the Ar/Kr ratio is varied between 200/0

and 60/140, in order to understand the influence of the Kr

gas on the deposition process in an industrial scale unit.

Additionally, various pulse lengths of 40 ls, 80 ls, and

200 ls have been applied with an Ar/Kr ratio of 120/80 in

order to investigate the influence on the plasma composition

and the properties of the deposited coatings.

The plasma composition has been investigated using a

fast intensified charge-coupled device (ICCD) camera and

recording the plasma emission parallel to the target surface.

Early results using OES applied to an HPPMS discharge

have shown an abundance of the ion emission lines9 and a

very dynamic plasma development strongly dependent on

the discharge current waveform.10–13 Spatial resolution was

obtained using a fast ICCD camera mounted parallel to the

target surface allowing measurement of the plasma emission

from surface up to 8 cm away from the target surface. The

emission of metal and Ar atoms and ions was observed using

band interference filters with full width at half maximuma)Electronic mail: brugnara@iot.rwth-aachen.de

021513-1 J. Vac. Sci. Technol. A 32(2), Mar/Apr 2014 0734-2101/2014/32(2)/021513/9/$30.00 VC 2014 American Vacuum Society 021513-1

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(FWHM) of 5 nm, allowing a transmission of emission from

only a single species. In addition to OES, we have performed

measurements of the ion saturation current at the substrate

using an oscilloscope in order to further understand the dep-

osition process and to link it with properties of the deposited

coatings.

II. EXPERIMENT

A. Coating deposition

For the coating deposition, an industrial scale coating unit

CC800/9 Custom by CemeCon AG, W€urselen, Germany,

was used. The unit is built up to allow a plasma characteriza-

tion in an industrial scale coating unit. Therefore, it is

equipped with the different flanges offering the possibility to

reach the plasma from every side of the unit. Furthermore,

the unit was equipped with a HPPMS power supply made by

CemeCon AG, W€urselen, Germany. The used HPPMS

power supply generates a rectangular pulse shape. The rec-

tangular cathode, 88 mm � 500 mm, was equipped with a

chromium target (purity: 99.9%). The deposition parameters

are listed in Table I. The substrates made of cemented car-

bide were positioned in front of the target without any rota-

tion to determine the deposition rate and correlate it to the

plasma diagnostics. The pressure, the mean cathode power,

and the frequency were kept constant at 0.45 Pa, 5 kW, and

500 Hz, respectively. Two sets of variations were carried

out. In the first variation, the pulse length was kept constant

at 200 ls and the Ar/Kr ratio was changed. The flow of both

Ar and Kr was kept constant at 200 sccm while the ratio of

Ar to Kr gas was varied to obtain the following ratios of the

flows: Ar/Kr: 200/0, 160/40, 120/80, and 60/140. The dis-

charge waveforms are presented in Fig. 1(a). The nitrogen

flow was pressure controlled. Therefore, the nitrogen flow

varied in the range of 36 sccm and 42 sccm depending on

the gas composition. In a second variation, the gas composi-

tion was kept constant (Ar/Kr 120/80) while the pulse length

was set to 40 ls, 80 ls, and 200 ls. The corresponding dis-

charge waveforms are presented in Fig. 1(b).

B. Plasma characterization

The light emission of metallic atoms and ions was

recorded as an indication of the spatial distribution of metal

vapor sputtered from the target using a fast ICCD camera.

The camera was triggered with acquisition times of 1 ls in

steps of 5 ls in order to obtain a temporal evolution. The

band pass interference filters with transmittance having

FWHM of 5 nm were used to distinguish the light emission

of the plasma constituents. The lines and filters are chosen

so the light transmitted through one filter can originate from

a single species. The observed emission lines were 312.0 nm

(Chromium ion), 427.5 nm (Chromium atom), 487.9 nm

(Argon ion), and 763.5 nm (Argon atom). Appropriate filters

to adequately isolate the emission from the Kr lines were not

available in the time of the experiment; therefore, we are not

able to report on the Kr emission. The fast ICCD camera

from Princeton Instruments 1024i was mounted on top of the

deposition chamber recording the plasma emission from the

surface of the rectangular target up to a distance of 8 cm

away from the target. The distance was limited by the win-

dow flange diameter.

In Fig. 2, a typical plasma emission distribution for a dis-

charge with 200 ls pulse length and Ar/Kr ratio 120/80 is

shown with two maxima at the position of the racetracks.

Since the view is along the longer axis of the rectangular tar-

get, the emission along the racetrack dominates, resulting in

an image with two maxima. Two emission maxima appear to

FIG. 1. (Color online) Waveforms of the discharge current and voltage in a

discharge with (a) variation of the Ar/Kr flow ratio and (b) the pulse length.

TABLE I. Deposition parameters.

Process parameter Unit Value

Substrate cemented carbide

Deposition time t min 60

Temperature at the heaters Th�C 560

Argon flow F(Ar) sccm 200/160/120/60

Krypton flow F(Kr) sccm 0/40/80/140

Nitrogen flow F(N2) for various

Ar/Kr ratios

sccm 36/39/40/42 (pressure

controlled)

Pressure p mPa 450

DC bias voltage VB V �100

Mean cathode power kW 5

Pulse frequency f Hz 500

Pulse length ls 200/80/40

Nitrogen flow F(N2) for various

pulse lengths

sccm 40/39/37 (pressure

controlled)

Substrate rotation None

021513-2 Bobzin et al.: Influence of Ar/Kr ratio and pulse parameters in a Cr-N HPPMS process 021513-2

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be asymmetric. Since the bottom emission maximum in Fig.

2 is positioned close to the edge of the flange and the upper

emission maximum is positioned in the center of the flange,

the emission from upper maximum was used for the analysis.

The analysis consisted of extracting the intensity over the

straight line spanning perpendicular from the target surface

up to 8 cm distance limited by the edge of the flange (marked

with the white dashed line). The resolution of the camera’s

CCD array is 1024 � 1024 pixels. The line over which we

took the intensity spans over approximately 434 pixels.

Therefore, the resolution of technique described here is

approximately 5 pixel/mm. Each image was taken with a dif-

ferent delay. The intensity was then plotted as a function of

distance and time; one example is shown in Fig. 3, providing

information on spatial and temporal evolution of the species

emission. Another possibility for an analysis of images taken

by fast ICCD camera is the Abel inversion;27 however, it

requires a cylindrical symmetry that does not hold for the

rectangular targets.

At this point, it is very important to stress out the limita-

tion of this diagnostic method. Assuming that an HPPMS

discharge is a corona discharge, the recorded intensity Ia can

be regarded as a function of the observed species density na,

electron density ne, and reaction rate coefficient f(e)14

Ia ¼ nanef eð Þ: (1)

This implies that, not knowing the spatial distribution of

the electron density, it is not possible to deduct any quantita-

tive conclusions regarding the density of the emitting spe-

cies. Therefore, at this stage, all conclusions are qualitative,

comparing different species within the same pulse or com-

paring the integral of the emission intensity for the same dis-

charge power.

C. Coating characterization

In order to evaluate the morphology and the thickness of

coatings, scanning electron microscope (SEM) micrographs

of fractured cross sections were taken (ZEISS DSM 982

Gemini) using secondary electrons mode. Hardness and

Young’s modulus were determined by means of nanoinden-

tation, using Nanoindenter XP, by MTS Nano Instruments.

The indentation depth did not exceed 1/10 of the coating

thickness. The evaluation of the measured results was based

on the equations according to Oliver and Pharr.15 A constant

Poisson’s ratio of v¼ 0.25 was assumed. The phase analysis

was carried out via x-ray diffractometry with a grazing inci-

dence x-ray diffractometer (XRD) 3003, General Electric.

All measurements were performed using Cu-Ka (wavelength

k¼ 0.15406 nm) radiation operated at 40 kV and 40 mA.

III. RESULTS AND DISCUSSION

A. Difference in discharge current waveforms fordifferent Ar/Kr ratios

From Fig. 1(a), it is evident that the current rise, the peak

value, and time until the peak current is reached differ for

discharges with different Ar/Kr ratio. To quantify the influ-

ence of the Ar/Kr ratio, we fitted a straight line on the rising

current, from the ignition to the first 20 ls, shown in Fig.

4(a), and extracted the value of the slope, representing the

current rise time. From Fig. 4(b), it can be seen that the cur-

rent rise time is nearly linearly proportional to the Ar/Kr ra-

tio with higher current rise time for discharges with smaller

contents of Kr gas. The observed dependence can be

explained by the effect working gas has on the sputtering

process, and consequently the ionization process. Secondary

electrons, the electrons generated from the target surface and

accelerated through the cathode sheath, are essential for the

discharge,16 having abundance of kinetic energy to generate

ions. The secondary electron yield is 0.108 for Ar ions and

0.079 for Kr ions impinging on Cr targets with an electron

work function of 4.5 eV.7 A higher secondary electron yield

results in a higher density of energetic electrons and

enhanced ionization resulting in a faster current rise.

In addition to the higher secondary electron yield, a sput-

ter yield of target material is also higher for Ar compared to

Kr. It is 1.72 for Ar ions compared to 0.89 for Kr ions,FIG. 3. (Color online) Temporal and spatial distribution of Crþ emission for

a discharge with 200 ls pulse length and Ar/Kr ratio 120/80.

FIG. 2. (Color online) Emission of Crþ ion at 30 ls from the discharge igni-

tion (200 ls pulse length and Ar/Kr ratio 120/80).

021513-3 Bobzin et al.: Influence of Ar/Kr ratio and pulse parameters in a Cr-N HPPMS process 021513-3

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calculated using SRIM (Ref. 26) with kinetic energy of

700 eV. The higher sputter yield will sputter more target ma-

terial, Cr atoms with ionization energy of 6.76 eV, which has

a twofold effect. The lower ionization energy and thus

higher probability of ionization together with a higher ioni-

zation cross section, 8�10�20 m2 for Cr (Ref. 17) compared

to around 4�10�20 m2 for Ar and Kr,18 additionally enhance

the current rise. A higher current leads to an enhanced sput-

tering and elevated densities in the target vicinity resulting

in a gas rarefaction effect19 where the Ar gas is pushed away

from the target surface. Consequently, the sputtering process

will change from a gas dominated to a metal dominated sput-

tering. Since the singly charged metal ions do not generate

secondary electrons, the current will start to decrease. A

high current rise and a faster transition to the metal domi-

nated sputtering result in higher peak values and shorter

times to reach the peak current.

B. Influence of gas composition on coating properties

For the determination of thickness and morphology, cross

section fractures of the coatings were analyzed using SEM.

Figure 5 shows SEM images of the coatings deposited at var-

ious Ar/Kr ratios. The deposition rate was nearly constant.

The highest deposition rate was achieved at Ar/Kr ratio of

120/80. However, an increase of Kr flow leads to a predomi-

nantly columnar growth.

Table II summarizes the results for the gas variation

including the mechanical properties. The H3/E2 ratio can be

used as a parameter to evaluate the resistance of materials

due to plastic deformation. The resistance to plastic defor-

mation is reduced with high H3/E2 ratios.20 The coating

FIG. 5. SEM cross section fractures for determination of thickness and morphology of the coatings deposited at various Ar/Kr flow ratios.

FIG. 4. (Color online) (a) Straight lines fitted to the first 20 ls of the current

rise for various Ar/Kr ratios and (b) the corresponding current rise times as a

function of Ar/Kr ratio.

021513-4 Bobzin et al.: Influence of Ar/Kr ratio and pulse parameters in a Cr-N HPPMS process 021513-4

J. Vac. Sci. Technol. A, Vol. 32, No. 2, Mar/Apr 2014

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deposited without Kr shows a higher hardness and H3/E2 ra-

tio compared to the processes with Kr. An increase of Kr

flux leads to a decrease of hardness. These effects can be

explained by the fact that a lower energy impact leads to a

formation of a columnar coating microstructure, lower hard-

ness, and H3/E2 ratio. The phase composition of the Cr-N

coatings was analyzed using XRD. Figure 6 shows the XRD

patterns of the coatings deposited at various Ar/Kr fluxes.

All coatings indicate the formation of a crystalline Cr2N hex-

agonal structure. The diffraction peaks of the crystallo-

graphic planes (110), (111), (112), (300), (113), (302), and

(222) corresponding to hexagonal Cr2N are identified.

Furthermore, a preferential growth of (111) grains is

observed in all samples except for the coating deposited in a

discharge with Ar/Kr ratio of 60/140. Chromium nitride gen-

erally forms in face-centered-cubic (fcc) CrN or hexagonal

Cr2N crystalline structures. In the magnetron sputtering, the

formation of CrN and Cr2N depends on the nitrogen to inert

gas, e.g., argon, concentrations. In this work, the nitrogen

concentration was pressure controlled and varied between

18% and 21%. The occurrence of a hexagonal Cr2N phase in

the HPPMS process with nitrogen content below 30% is in

agreement with Ref. 5.

C. Influence of pulse length on coating properties

In the second set of measurements, the pulse length was

varied between 40 ls, 80 ls, and 200 ls. Figure 7 shows the

SEM images of the coatings deposited at various pulse

lengths. By leaving the mean cathode power constant

(5 kW), it can be seen that the peak current is strongly influ-

enced by the length of the pulse. At 40 ls, the power supply

is switched off in the current maximum. By decreasing the

pulse length, the ionization of metal atoms is increased. This

leads to a formation of a dense and fine-grained coating

microstructure (cf. Fig. 7). However, the deposition rate is

significantly lower compared to a pulse length of 200 ls.

Table III shows the results of mechanical properties for

various pulse lengths. The coating deposited at 40 ls shows

the highest hardness (27.3 6 2.5 GPa) and H3/E2 ratio. The

higher ionization of the metal atoms with 40 ls leads to a

higher energy impact on the growing coating contributing to

an enhanced hardness compared to the coatings deposited at

80 ls and 200 ls.

Figure 8 shows the XRD patterns of the coatings depos-

ited at various pulse lengths. As in Fig. 6, the coatings indi-

cate formation of hexagonal Cr2N crystal structure. A

preferential growth of (111) grains is observed for all coat-

ings. However, the diffraction peak (111) of the coating

deposited at 40 ls is widest, which could be explained by a

decrease in a grain size. The decrease in the grain size,

beside the dense morphology of the coating, probably con-

tributes to an increase in the coating hardness according to

the Hall–Petch effect.21

D. Plasma diagnostic of deposition process by fastICCD camera

The light emission of different species plotted as a func-

tion of distance and time is shown in Fig. 9 for a discharge

with 200 ls pulse length and Ar/Kr ratio of 120/80. For all

species, the highest emission, spatially, is localized close to

the target. Since the intensity is among other a product of

the electron density, as shown in Eq. (1), the intensity will

be localized close to the target, due to an abundance of

energetic electrons confined within the closed magnetic

field lines. Temporarily, the highest emission differs for dif-

ferent species. The peak of intensity for all species is visi-

ble at around 40 ls corresponding in time to the peak

discharge current, which is in accordance to previously

published results.3,4 The Ar0 emission has an unusual light

pattern stretching 6 cm into the discharge in the ignition

phase. Our interpretation is that electrons from the previous

discharge are accelerated by the collapsing cathode sheath.

The accelerated electrons are exciting Ar atoms resulting in

the Ar emission from the target towards the substrate. This

phenomenon will be discussed in detail in a future

publication.

In order to observe the temporal evolution of each species

in detail, we have plotted values of the intensity from dis-

tance of 5 mm, shown in Fig. 10. The Ar0 emission raises

FIG. 6. (Color online) XRD phase analysis of the coatings deposited at vari-

ous Ar/Kr flux ratios.

TABLE II. Properties of the coatings deposited at various Ar/Kr flux ratios.

Sample Gases Ar/Kr Pulse length (ls) Deposition rate (lm/h) Hardness (GPa) Young’s modulus (GPa) H3/E2 (GPa)

1 200/0 200 5.9 25.2 6 2.3 438 6 40 0.083

2 160/40 200 6.2 24.2 6 2.7 422 6 36 0.079

3 120/80 200 6.4 24.4 6 2.3 428 6 35 0.079

4 60/140 200 5.9 20.3 6 2.7 414 6 49 0.048

021513-5 Bobzin et al.: Influence of Ar/Kr ratio and pulse parameters in a Cr-N HPPMS process 021513-5

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first, excited by the electrons left from the previous pulse

and accelerated in the collapsing cathode sheath. As the dis-

charge current rises the sputtering process begins, resulting

in an increase of the Arþ emission, Arþ being the ion im-

pinging onto the target, and increase in the Cr0 emission, Cr

atoms being product of the sputtering process. Several ls

later the Crþ emission onsets as the sputtered material is ion-

ized. The transition to metal dominated sputtering, due to

gas rarefaction, is observed as the emission of Ar0 atoms

decrease around 40 ls. Due to limitations of the power sup-

ply, the target voltage decreases in time. The voltage

decrease in conjunction with a less efficient metal dominated

sputtering process6 results in the current decrease. Figure 10

shows that the emission of Arþ and Crþ ions and Cr0 atoms

decreases simultaneously with the discharge current

decrease. The decrease of Arþ and Crþ ions is understand-

able, since the current is proportional to the number of ions

impinging on the target. Less ions impinging on the target

will result in a reduced sputtering and therefore decrease of

Cr ion and atom emission intensity. The change of the slope

of the Ar0 emission, around 40 ls, indicates the transition

from a metal to Ar dominated discharge, visible in faster

decay of Crþ emission compared to Arþ emission. These

results show the importance of understanding that the gener-

ation and ionization of the sputtered particles are closely

related to the current profile.

Assuming that the electron density profile is comparable

for similar discharge current shapes, it follows that the emis-

sion intensity, from Eq. (1), is proportional to the density of

the emitting species. The integral of the Cr0 and Crþ emis-

sion is compared to the deposition rate of coatings in

Fig. 11. The comparison is made with the Cr0 and the Crþ

emissions as these are the constituents of the deposited Cr-N

coating.

Figure 11(a) shows results for a discharge with variation

of the gas composition. The results show that the integral of

the Crþ emission does not change significantly for different

Ar/Kr ratios, while the integral of Cr0 emission peaks at

Ar/Kr ratio of 120/80. The sum of two integrals also exhibits

a peak at Ar/Kr ratio of 120/80. It corresponds well with the

deposition rate that also exhibits a peak for Ar/Kr ratio of

120/80.

Figure 11(b) shows the results for discharge with varia-

tion of the pulse length. The integral of Cr0 emission does

not significantly change when varying the pulse length. The

integral of Crþ emission is highest for the discharge with

40 ls; it decreases for discharge with 80 ls and does not

change significantly for the discharge with 200 ls. The sum

of two integrals therefore increases for longer pulses. The

same trend is observed for the deposition rate, an increase

with longer pulses. However, it exhibits close to a linear

increase while the sum of the integrals exhibits a sharp

increase by increasing the pulse length from 40 ls to 80 ls,

followed by a small increase when increasing the pulse

length to 200 ls. An explanation for the discrepancy

between the sum of integrals and the deposition rate could

be found in the peak current difference reached for dis-

charges with different pulse lengths. Since the average

power was kept constant, the discharges with a shorter pulse

reached higher peak currents. Higher discharge currents pro-

duce higher plasma densities22 and consequently increase

the fraction of ionized material and the ionization states.

Doubly charged metal atoms have been abundantly

observed in HPPMS discharges.23 Applying a bias onto the

substrate, the average energy of singly charged ions is

increased by value of the bias; however, the average energy

of doubly charged ions is increased twice. The aim of apply-

ing the bias is to increase the energy of incoming ions that

transfer their kinetic energy to ad-atoms at the surface

increasing the ad-atom mobility and improving the density

of the coatings. However, when the ion energy exceeds a

certain value, the ions start to sputter the coating. In that

FIG. 7. SEM cross section fractures for determination of thickness and morphology of the coatings deposited at various pulse lengths.

TABLE III. Properties of the coatings deposited at various pulse lengths.

Sample Gases Ar/Kr Pulse length (ls) Deposition rate (lm/h) Hardness (GPa) Young’s modulus (GPa) H3/E2 (GPa)

3 120/80 200 6.4 24.4 6 2.3 428 6 35 0.079

5 120/80 80 3.5 24.9 6 2,2 439 6 37 0.080

6 120/80 40 2.1 27.3 6 2.5 476 6 45 0.089

021513-6 Bobzin et al.: Influence of Ar/Kr ratio and pulse parameters in a Cr-N HPPMS process 021513-6

J. Vac. Sci. Technol. A, Vol. 32, No. 2, Mar/Apr 2014

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sense, the abundance of doubly charged ions will negatively

affect the coating growth and result in thinner coatings.

Another aspect that can contribute to a decrease of the depo-

sition rate with increasing peak current is the self-sputtering

effect close to the cathode, due to the elevated density of

Crþ and Cr2þ.11 Since the emission of Cr2þ is in the near

UV part of the spectrum and the wavelength range of our

ICCD camera is not sensitive enough in that part of the

spectrum, no information about the Cr2þ emission could be

measured.

E. Ion saturation current at the substrate

The evolving crystalline microstructure is connected to

number of displacements per atom (DPA) in the growing

film. Prenzel et al. have recently shown that the DPA can be

expressed as a product of displacements per ion (DPI), the

ion flux and duty cycle of the bias, divided by the growth flu-

ence.24 For a DC bias used in this study, the duty cycle is

equal to 1, the deposition rate does not change significantly;

therefore, we can assume that the growth fluence does not

FIG. 9. (Color online) Normalized light intensity as a function of distance and time of (a) Ar0, (b) Arþ, (c) Cr0,and (d) Crþ for a discharge with 200 ls pulse

length and Ar/Kr ratio 120/80.

FIG. 8. (Color online) XRD phase analysis of the coatings deposited at vari-

ous pulse lengths.

FIG. 10. (Color online) Temporal evolution of the plasma species intensity

and the discharge current extracted from Fig. 9 at a distance of 5 mm for a

discharge with 200 ls pulse length and Ar/Kr ratio 120/80.

021513-7 Bobzin et al.: Influence of Ar/Kr ratio and pulse parameters in a Cr-N HPPMS process 021513-7

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change significantly and we can safely assume it is constant.

Therefore, the DPA can be regarded as a product of the DPI

and the ion flux.

In Fig. 5, the SEM images show a dense and fine-grained

microstructure of the coating deposited by a discharge with

pure Ar, shifting toward a columnar microstructure of the

coating deposited by a discharge with Ar/Kr ratio of 60/140.

The transition from a dense and fine-grained microstructure

toward a predominantly columnar microstructure can be

explained by a reduction of the DPA and consequently a

reduction of the nucleation site density. The reduction of the

nucleation site density promotes a columnar growth while

the higher nucleation site density promotes a denser coating

growth.25 Using the SRIM code,26 it can be calculated that the

DPI for Ar ion impinging onto the Cr2N coating with an

energy equivalent to the bias voltage of �100 V, equals 1.7,

while for Kr ion with the same energy the DPI equals 1.5.

Furthermore, the ion saturation current measured on a

negatively biased flat probe placed at the exact position

where the samples have been deposited is shown in Fig.

12(a). The results show the highest ion current for discharge

with Ar/Kr ratio of 200/0, the ion current is reducing when

Ar/Kr ratio is increasing. The trend and relative amplitude of

the ion saturation current change is very similar to the trend

and relative amplitude of the discharge current. The

influence of the Ar/Kr ratio on the discharge current shape

has been discussed in Sec. III A. The results indicate that a

higher DPI for Ar impinging ions and higher ion saturation cur-

rent at the substrate result in highest DPA. Introducing Kr in the

discharge, both DPI and the ion saturation current at the sub-

strate are reduced leading to a reduction of the nucleation site

density resulting in a predominantly columnar microstructure.

The SEM images in Fig. 5 show a dense and fine-grained

microstructure for the coating deposited by a discharge at

40 ls pulse length and a more columnar microstructure de-

posited by a discharge at 80 ls and 200 ls pulse length. The

analogous argument implies that for this set of measurement,

with Ar/Kr ratio constant, the DPA is proportional to the ion

saturation current arriving at the substrate. The ion saturation

current measured for discharges with different pulse length,

presented in Fig. 12(b), shows almost two times higher ion

currents for 40 ls discharge compared to 80 ls and 200 ls

discharges. Consequently, the DPA for the discharge with

40 ls pulse length will be elevated, resulting in an increased

nucleation site density and a denser microstructure.

IV. SUMMARY

In this contribution, we have investigated both plasma

composition and properties of HPPMS Cr2N coatings depos-

ited with various Ar/Kr ratios and at various pulse lengths of

40 ls, 80 ls, and 200 ls, keeping the average discharge

FIG. 11. (Color online) Comparison between the integral of the Cr0 and the

Crþ emission, and the deposition rate for (a) gas composition variation and

(b) pulse length variation.

FIG. 12. (Color online) Ion saturation current as a function of time for (a)

gas composition variation and (b) pulse length variation.

021513-8 Bobzin et al.: Influence of Ar/Kr ratio and pulse parameters in a Cr-N HPPMS process 021513-8

J. Vac. Sci. Technol. A, Vol. 32, No. 2, Mar/Apr 2014

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power constant. An addition of Kr resulted in a change of

both the plasma dynamics and the properties of the deposited

coating. It has been observed that for a constant discharge

power the current rise is impeded by an introduction of Kr

that has been explained by a reduced efficiency of the Kr gas

to sputter the target material and generate secondary elec-

trons. Consequently, the achieved peak discharge currents

were reduced. The lower peak discharge currents resulted in

lower ion currents reaching the substrate, which combined

with lower DPI for Kr compared to Ar resulted in a reduction

of the nucleation site densities, dominant columnar growth,

and a decrease of the coating hardness. The highest deposi-

tion rate has been observed for Ar/Kr ratio of 120/80 and for

pulse length of 200 ls. The integrals of light emitted by Cr0

atoms and Crþ ions above the racetrack and over the pulse

length have been compared to the deposition rate. A qualita-

tive comparison shows similar trends, an increase with lon-

ger pulses and a variation in the deposition rate when

changing the Ar/Kr ratio, with highest values obtained for

Ar/Kr ratio of 120/80. Regarding the mechanical properties,

a maximum of hardness (27.3 6 2.5 GPa) was reached for

Cr2N coating deposited at 40 ls and Ar/Kr 120/80.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support

of the German Research Foundation (DFG) within the trans-

regional collaborative research center TRR87/1 “Pulsed high

power plasmas for the synthesis of nanostructured functional

layers” (SFB-TR 87) subproject A5, C6, and Research

Department Plasmas with Complex Interactions.

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