Combinatorial T hin F ilm Synthe sis of Cr2AlC; aComparison of Two Sputte ring M e tho ds

Department of Physics, Chemistry and Biology

Master’s Thesis

Combinatorial Thin Film Synthesis of Cr

AlC; a

Comparison of Two Sputtering Methods

Peter Carlsson

LiTH-IFM-EX--12/2573--SE

Department of Physics, Chemistry, and Biology Linköpings universitet

Master’s Thesis LiTH-IFM-EX--12/2573--SE

Combinatorial Thin Film Synthesis of Cr

AlC; a

Comparison of Two Sputtering Methods

Peter Carlsson

Supervisors: Per Eklund

IFM, Linköpings universitet

Marcela M. M. Bilek

The University of Sydney

David R. McKenzie

The University of Sydney

Examiner: Johanna Rosén

IFM, Linköpings universitet Linköping, 13 April, 2012

Avdelning, Institution

Division, Department Thin Film Physics Division

Department of Physics, Chemistry and Biology Linköpings universitet

SE-581 83 Linköping, Sweden

Datum Date 2012-04-13 Språk Language � Svenska/Swedish � Engelska/English � � Rapporttyp Report category � Licentiatavhandling � Examensarbete � C-uppsats � D-uppsats � Övrig rapport � �

URL för elektronisk version

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LiTH-IFM-EX--12/2573--SE

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Titel

Title Svensk titel_{Combinatorial Thin Film Synthesis of Cr2AlC; a Comparison of Two Sputtering} Methods

Författare

Author Peter Carlsson

Sammanfattning

Abstract

MAX phase materials have unique properties that combine metals and ceramics. These properties stems from the complex structure with extremely strong M-X bonds and weaker M-A bonds. Physical vapor deposition in form of magnetron sputtering is widely used for thin film syntheses. High Power Impulse Magnetron Sputtering (HIPIMS) was introduced in the mid-90s, today it is a state of the art thin film deposition method for researchers all over the world.

Combinatorial sputter deposition of Cr2AlC with HIPIMS and direct current

magnetron sputtering of the chromium target were compared. Aluminum and carbon were sputtered from individual targets with a shield in between of the three targets, making the combinatorial method possible.

X-ray diffraction showed that Cr2AlC was present at 550oC and scanning

elec-tron microscopy showed that there was not big difference in the morphology. Elec-trical measurements indicated that the film deposited with HIPIMS had a signif-icant higher electrical conductivity compared to the film deposited with direct current. x-ray photoelectron Spectroscopy gives an idea of the composition of the film, in one position there were oxygen through the film, the rest were dense films. By using a combinatorial approach Cr2Al was also found in the films. The two

films compared deposited with different methods, showed small differences but not the ones expected.

Nyckelord

Abstract

MAX phase materials have unique properties that combine metals and ceramics. These properties stems from the complex structure with extremely strong M-X bonds and weaker M-A bonds. Physical vapor deposition in form of magnetron sputtering is widely used for thin film syntheses. High Power Impulse Magnetron Sputtering (HIPIMS) was introduced in the mid-90s, today it is a state of the art thin film deposition method for researchers all over the world.

Combinatorial sputter deposition of Cr2AlC with HIPIMS and direct current

magnetron sputtering of the chromium target were compared. Aluminum and carbon were sputtered from individual targets with a shield in between of the three targets, making the combinatorial method possible.

X-ray diffraction showed that Cr2AlC was present at 550oC and scanning

elec-tron microscopy showed that there was not big difference in the morphology. Elec-trical measurements indicated that the film deposited with HIPIMS had a signif-icant higher electrical conductivity compared to the film deposited with direct current. x-ray photoelectron Spectroscopy gives an idea of the composition of the film, in one position there were oxygen through the film, the rest were dense films.

By using a combinatorial approach Cr2Al was also found in the films. The two

films compared deposited with different methods, showed small differences but not the ones expected.

Acknowledgments

I would like to thank the following people for making this thesis possible:

Dr. Johanna Rosén for giving me the opportunity to do this Master’s Thesis. Dr. Per Eklund for advising me on the writing of this Master’s Thesis.

Prof. Marcela Bielek and Prof. David McKenzie for giving me the

oppor-tunity to do this project at The University of Sydney.

Dr. Mark Tucker for guiding and discussing all experimental techniques and

thank you for the support during all the experimental work.

Alice Poppleton and Ben Treverrow for training me in the AJA sputtering

system.

Dr. Cenk Kocer for the help to setup the camera instruments for photography

of the samples.

Prof. Dougal Mcculloch for doing the XPS measurements. Matthew Field for analysing and summarize the XPS data.

And Jasmine Öberg for supporting me all the time during my work.

Contents

1 Introduction 3

1.1 Background . . . 3

1.2 Outline . . . 4

2 Theory 7 2.1 MAX Phase Materials . . . 7

2.1.1 Cr2AlC . . . 8

2.2 Thin Film Deposition . . . 8

2.3 Thin Film Deposition of MAX Phases . . . 10

2.4 Combinatorial Thin Film Deposition . . . 10

3 Experimental Details 11 4 Results and Discussion 15 4.1 Analyses of the Sample Regions . . . 15

4.1.1 Position 16,5 . . . 18

4.1.2 Position 11,-11 . . . 18

4.1.3 Position 0,0 . . . 19

4.1.4 Position -4,-19 . . . 22

4.1.5 Position -5,15 . . . 26

4.2 Lower Temperature Samples . . . 27

5 Summary and Conclusions 31 5.1 Recommendations . . . 32

Bibliography 33

Acronyms

HIPIMS High Power Impulse Magnetron Sputtering

DC Direct Current

DCMS Direct Current Magnetron Sputtering

PVD Physical Vapor Deposition

CVD Chemical Vapor Deposition

XRD X-ray Diffraction

SEM Scanning Electron Microscopy

XPS X-Ray Photoelectron Spectroscopy

FEG Field Emission Gun

RF Radio Frequency

Chapter 1

Introduction

This Master’s Thesis is the result of a collaboration between the University of Sydney, Linköping University and the Royal Melbourne Institute of Technology. The main project work has been done at the University of Sydney, where the depositions of the films, scanning electron microscopy and x-ray diffraction have been done. Colleagues at the Royal Melbourne Institute of Technology have done electrical measurements and x-ray photoelectron spectroscopy. The examination is at Linköping University.

1.1 Background

During the 60s and early 70s, Hans Nowotny’s group at Vienna University found over 100 of new solids. These solids were ternary compounds of either carbon or nitrogen combined with an early transition metal and a third element, usually

silicon or aluminum. Many of these phases were H-phases, Cr2AlC-structured

materials, and also Ti3SiC2-structured materials. Nowotny’s group were able to

identify the complex unit cell of these materials [1].

Nowotny’s discovery was remarkable, but it took until the mid-90s before these

materials were further investigated. Barsoum & El-Raghy fabricated Ti3SiC2 and

discovered some of the superb properties these materials have [2]. When Ti4AlN3

was discovered [3], it was realized that these three categories of phases have the

same basic structure and therefore also similar properties. The name Mn+1AXn

(MAX) phases (n=1, 2, or 3) was introduced.

The properties of MAX phases are a combination of ceramics and metals. They can be thermal shock resistant, have high stiffness and be corrosion resistant [4] and at the same time they can have high electrical and thermal conductivities and be machinable [5]. These unique properties give MAX phase materials potential to be used in applications like protective coatings, sensors, low friction surfaces, electrical contacts, high temperature devices and many more [6]. Because of this potential, research on MAX phase materials has grown in the last decade. To be able to commercialize MAX phases as coatings, low deposition temperature is needed and the market for low temperature coatings is big.

4 Introduction

Using Chemical Vapor Deposition (CVD), Nickl was the first to deposit a MAX

phase thin film in 1972, when a Ti3SiC2 thin film was synthesized [7]. The first

deposition of thin film MAX phases with Physical Vapor Deposition (PVD) was

made by Palmquist et al in 2002. Ti3SiC2 was synthesized by sputtering both

from a compound target and from three individual elemental targets [8]. Since 2002, there have been many reports on MAX phase syntheses by PVD.

Bulk syntheses of MAX phases typically occur close to thermodynamic equi-librium, while sputter deposition allow synthesis of phases that are not thermody-namically stable. It is known that sputtering can stabilize and grow metastable phases. Also bulk synthesis of MAX phases generally need higher temperature.

Palmquist et al were able to synthesize a new MAX phase Ti4SiC3 by using the

non thermodynamic equilibrium conditions that is possible with sputtering. Due

to these conditions, Palmquist et al discovered Ti5Si2C3 and Ti7Si2C5 with

prop-erties similar to MAX phases. Sputtering may be able to lower the deposition temperature and also grow new MAX phases, not possible to synthesize with bulk synthesis [9, 10].

This project will investigate thin film synthesis of the MAX Phase Cr2AlC,

using High Power Impulse Magnetron Sputtering (HIPIMS) and find the lowest possible deposition temperature, in comparison with Direct Current Magnetron Sputtering (DCMS) deposition. There are no published reports on the synthesis

of Cr2AlC using HIPIMS deposition to date. HIPIMS is known to provide a

more highly ionized deposition flux to the substrate compared to conventional DC sputtering [6]. Therefore it is of interest to investigate if HIPIMS can be used to synthesize MAX phase materials at lower temperatures than have been reported.

Deposition of Cr2AlC has been reported at the lowest temperature of any MAX

phase synthesis; it has been reported that Cr2AlC has been deposited using DC

sputtering at 550o_{C [11] and at 370}o_{C for 10 µm thick films [12]. Investigations}

will be done to study if HIPIMS synthesis can lower these temperatures further. Questions to be investigated in this project are listed below:

• How will the growth of Cr2AlC films be affected by the use of HIPIMS

deposition, compared to DC sputtering?

• Can Cr2AlC be synthesized at lower temperatures using HIPIMS than are

possible using DC sputtering? What is the lowest possible deposition

tem-perature for Cr2AlC when HIPIMS is used?

• What phases will appear on the sample when using a combinatorial ap-proach, and is there a difference in using DCMS compared to HIPIMS for the combinatorial approach and why/why not?

1.2 Outline

This chapter is an introduction to the thesis with the background to the subject. It continues with a theoretical chapter (chapter 2), where the theory of MAX phase materials, thin film deposition, thin film deposition of MAX phases, combinatorial

1.2 Outline 5

thin film deposition and Cr2AlC is discussed. In chapter 3, the experimental

details are described and there is a review of the equipment that was used. The following chapter (chapter 4) explains and discusses the results. The last chapter (chapter 5) is a summary of the project with some conclusions; there is also a suggestion of further work.

Chapter 2

Theory

2.1 MAX Phase Materials

There are three classes of Mn+1AXn (MAX) phases, for n=1,2 or 3. M is an

early transition metal, A is an A-group element and X is either C and/or N. The different stoichiometries are referred to as 211, 312 and 413. The elements for MAX phase are shown in Figure 2.1 [13, 14].

Figure 2.1: MAX phase groups [14].

MAX phase materials have a hexagonal structure, the unit cells for 211, 312 and 413 MAX phases are shown in Figure 2.2a. In the unit cell, there are octahedra

(M6X) with A layers in between. The A layers act as a mirror plane so a zigzag

structure arises, shown in Figure 2.2b. The three MAX phase structures have different stacking sequence. There are two M layers in 211 phases, three in 312 phases and four in 413 phases.

MAX phase materials have been reported to have unique properties, combining these of metals and ceramics. They can be easy machinable, thermally and electri-cally conductive, resistant to thermal shock and plastic at elevated temperatures. They can also be oxidation resistant, refractory, have a high specific stiffness and low density. The reason for these properties is the unique structure of the materi-als. The M-X bonds are an extremely strong mixture of metallic-covalent bonds,

8 Theory

(a) (b)

Figure 2.2: (a)The MAX phase unit cells, red is a M atom, blue is a A atom and

black is a X atom [6]. (b) A TEM image of the structure of Ti3SiC2, showing the

stacking sequence and the zigzag structure for MAX phases [14].

while the M-A bonds are relative weak. This combination of bonds in a layered structure is the explanation for MAX phase materials unique properties. The strong M-X bonds contribute to the damage tolerance, while the weaker M-A bonds contribute to the MAX phases machinability [6, 14].

2.1.1 Cr

AlC

The MAX phase Cr2AlC was discovered in the 1960s by Jeitschko et al, and

later studies have shown that Cr2AlC has excellent resistance to oxidation and

hot corrosion. The structure can be described as single layers of aluminum atoms

separating the chromium carbides into nanolaminates of Cr2AlC. At elevated

tem-peratures the aluminum atoms will diffuse to the surface and form a protective oxide [12, 15].

2.2 Thin Film Deposition

Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD) are widely used techniques for thin film deposition. CVD uses gases to chemically react to each other at a specific temperature. In PVD atoms are transferred from a solid target to a substrate where they form a film. It is possible to control the formation and the growth of the film. In evaporation, the target is heated to a certain degree where the material is evaporated and condensed on the substrate. In sputtering, an ionized gas is used to bombard the target so the target material is ejected to form a film on the substrate. [16]

In magnetron sputtering the target is a cathode and the substrate is an anode. An inert gas, generally argon, is ionized and accelerated to the negatively charged

2.2 Thin Film Deposition 9

Substrate

Target

Ar+

Figure 2.3: General sputtering

target. The ions will knock out atoms from the target and these atoms will then form a film on the substrate, see figure 2.3. Magnetrons are used to trap electrons close to the target and therefore the probability of ion collision is enhanced. A circular field of plasma over the target called the "race track” is formed. More ions are created and the sputter deposition rate is increased. The amount of electrons is also increased, therefore a lower working pressure can be used while the plasma is self-sustained. This leads to a higher mean free path for the sputtered atoms and thereby the deposition rate is enhanced [16, 17].

High Power Impulse Magnetron Sputtering (HIPIMS) is a sputtering technique where high power is applied to the cathode in pulses. The time-average target power is the same as in DCMS, but during a short pulse the power is very high. This results in a much higher plasma density and a higher ionization degree of the target material. The pulse-on time is short to avoid the target to melt. Thin films deposited with HIPIMS can generate a denser and smother film compared to DCMS. Therefore, HIPIMS can acquire unique morphologies for low temperature deposition. The energetic bombardment of HIPIMS increases the surface diffusion of the substrate and may cause a different preferred growth orientation compared to DCMS [17, 18].

Radio Frequency (RF) sputtering is mainly used for insulating material to avoid charge build up. An alternating potential in the MHz region and typically 5 to 30 MHz is used. That high frequency makes only the electrons mobile enough to follow the periodic change. The outcome is that the target is self-biased negatively by the electrons in the chamber and positive ions will be accelerated to the target so sputtering can occur.

Reactive sputtering is another method of sputtering. A gas (usually together with a inert gas) is introduced to the sputtering chamber during the deposition of a metallic target. The gas and the metallic compound will together form a film on the substrate. Reactive sputtering is good for creating e.g. oxides, nitrides and carbides[16].

10 Theory

2.3 Thin Film Deposition of MAX Phases

PVD is commonly used for thin film deposition of MAX phases, and DC sputtering is the method mostly used for MAX phase carbides. MAX phase nitrides have not been as much investigated, but reactive sputtering is the method used for the nitrides. Using reactive sputtering for MAX nitrides need an exact amount of nitrogen, else unwanted phases will be included in the film. Because of that, relatively little work has been done on reactive sputtering of MAX phases.

CVD has been used for thin film Ti3SiC2 MAX phase deposition. The

re-quired temperature for the process is between 1000-1300o_{C. It is possible to}

de-posit Ti3SiC2 with PVD at a lower temperature. With magnetron sputtering, it

is also easier to get a single phase of the MAX phase compared to CVD. When

using CVD extra phases occurs such as TiC, SiC, TiSi2 and Ti3Si2Cx [6].

2.4 Combinatorial Thin Film Deposition

If a large number of samples are produced and investigated in the same experiment, it is called combinatorial. Thereby a composition gradient is produced over the sample and a large number of compositions can be analyzed easily, without making a large number of different samples [10].

In the 1960’s co-evaporating and co-sputtering of two or more targets lead to a film composition continuum, thereby it was easier to investigate a large number of compositions in one sample and the possibility to find new materials increased [19].

An article by Mertens et al has been published on a combinatorial thin film

deposition of Cr2AlC. Four different phases were found in the sample, one pure

Cr2AlC phase. In the other three phases Cr23C and Cr2Al were mixed with Cr2AlC

[20].

Lofland et al used magnetron sputtering to synthesis combinatorial thin films

of the MAX phase (Ti1-xNbx)2AlC in 2009. They were able to identify a Nb-rich

Chapter 3

Experimental Details

A combinatorial sputtering approach was used to investigate a large field of com-positions. A shield was mounted between the three targets and the substrate, yielding a gradient of thickness for each material during sputtering. To be able to get the 211 stoichiometry in the middle of the sample the deposition parameters for each target needed to be calibrated. Chromium, aluminum and carbon films were deposited separately on three 50x50mm silicon substrate with parallel lines of ink at a 5mm interspaces. The ink and the film was removed by ultrasonication in isopropanol after the deposition. The thicknesses of the films were measured with a contact surface profiler at a grid of points with a 5mm distance across the surface of each sample, which resulted in 81 points. The density of the films were approximated to the bulk density, the expected error is not significant given the area of the composition space covered by the sample. By calculating the number of atoms deposited on the substrate per second for the 81 points, the power applied to the target could be determined to acquire the 211 stoichiometry in the center of the sample. Figure 3.1 shows the calculated incident composition space that the deposition should cover and the calculated thickness gradient.

Depositions were made with a AJA, Inc. magnetron sputtering system and six samples were synthesized with different temperatures and with the incident 211

stoichiometry in the middle of the sample (0,0). Polished Al2O3 (0001)-oriented

wafers with a diameter of 50 mm and a thickness of 0.5 mm were used. Figure 3.2 shows the coordinate system and where the targets are located in relation to the substrate. Aluminum was deposited with a DC power supply at 70 W, carbon with a RF power supply at 577 W and chromium with both DC at 87 W and HIPIMS. The HIPIMS deposition was made with a pulse of 70 µs at 150 Hz and a voltage of 750 V and a maximum current of 39A which is shown in figure 3.3. The parameters for the HIPIMS deposition of chromium were influenced by Hecimovic & Ehiasarian [22]. It was only possible to use the HIPIMS power supply on one target. To maximize the fraction of the deposited species produced by HIPIMS, the chromium target was chosen to be used as the HIPIMS target. RF sputtering of the carbon target was chosen as it produced the most reliable operation of the carbon target in the deposition system used. The different depositions are shown

12 Experimental Details

in table 3.1.

The chamber had a base pressure less than 2.5·10−6 _{Torr after the sample was}

heated. Argon gas was used during sputtering with a rate of 20 sccm (cm3_min−1

at STP). The pressure in the chamber during deposition was kept at 2.7 mTorr.

0 1 0.25 Carbon 0.5 0.75 0.75 1 Aluminium 0.5 0 0.25 0.25 0.5 Chromium 0.75 0 1 400 450 500 550 600 650 Thickness [nm]

Figure 3.1: The incident composition area and thickness of the deposited samples, shown in a ternary phase diagram.

y x (0,0) Al C Cr

Figure 3.2: The coordinate system of the samples in units of mm.

To be able to calibrate the deposition temperature of the samples, an infrared pyrometer was used. First the emissivity of an oxidised titanium metal sample was measured. Thereafter the surface temperature of the titanium sample was measured by using the Infrared pyrometer. Because the titanium sample had an

similar size, thickness and thermal conductivity as the Al2O3 wafer, the surface

13 -20 0 20 40 60 80 100 -200 0 200 400 600 800 1000 1200 V o lta g e ( V ) Time (µs) 0 20 40 C u rr e n t ( A )

Figure 3.3: Voltage (left, black) and current (right, blue) as a function of time during a single HIPIMS pulse on the chromium target.

Table 3.1: Deposited samples.

Sample no. Surface temp. (o_{C) Cr Power supply}

1 350 HIPIMS

2 450 HIPIMS

3 500 HIPIMS

4 550 HIPIMS

5 550 DC

X-Ray Diffraction (XRD) was used with a PANalytical X’pert MRD diffrac-tometer, equipped with a 6-axis sample stage. A Nickel filter and a 10 mm vertical slit was used between the source and the sample. Measurements were made in a

θ_{− 2θ geometry to find the different phases on the sample. Grazing incidence was}

used to increase the counts of the sample compared to the substrate.

Both frosted and mirror-like regions on the sample surface were visible to the naked eye. To be able to get images of this, a camera set up was made with a flash from the side, perpendicular to the camera. Scattered light from the frosty areas are lighter in the pictures and the mirror like areas are black.

Scanning Electron Microscopy (SEM) was used to investigate the surface

mor-phology of the samples, using a Zeiss EVO 50 SEM operating at 20kV with a LaB6

filament. Higher resolution SEM images were acquired by a Hitachi S-4500 Field Emission Gun (FEG) SEM, operating at 15kV.

Sample 4 and 5 were separated into pieces, five of these pieces contained the points of interest. This was made because further analysis needed smaller samples.

14 Experimental Details

Four-point probe electrical measurements were performed using a Signametrics PCI source meter, thermally evaporated NiCr contact pads and NiCr probes. A constant current I=1mA was applied between the outer contacts, the voltage V was measured between the inner contacts. The resistance R was calculated with equation (3.1).

R= V/I (3.1)

The thickness, length and width of the samples were measured with a FEG-SEM and are expected to be accurate to within 5%. The resistivity ρ was calculated with equation (3.2).

ρ=R· T · W

L (3.2)

Where R is the resistance, T the thickness, W the width and L the length. Re-sistance versus length and current-voltage measurements were also performed to confirm the low-resistance, ohmic properties of the NiCr contacts on the films.

X-Ray Photoelectron Spectroscopy (XPS) was performed on the films using a Thermo Scientific K-Alpha spectrometer with an Al anode x-ray source. A XPS depth profile was acquired by etching the surface for a total time of 750 seconds.

Chapter 4

Results and Discussion

After XRD measurements, it was decided to further analyze sample 4 and 5, to

compare HIPIMS and DCMS at 550oC.

The frosty regions on the samples seemed interesting, therefore further analyses took place at these positions. Figure 4.1 presents these regions and the five points of interest with the coordinates in millimeter from the center.

Figure 4.1: The samples synthesized at 550o_{C and the five analyzed areas with the}

coordinates in millimeter from the center. To the left synthesized with HIPIMS and to the right synthesized with DCMS.

4.1 Analyses of the Sample Regions

The five positions, 0,0; 16,5; 11,-11; -4,-19; -5,15 of the HIPIMS sample and the

DCMS sample, both synthesized at 550oC shown in figure 4.1, will be analyzed in

16 Results and Discussion 0 100 200 300 400 500 600 700 0 5 10 15 20 25 30 35 40 45 50 C o n ce n tr a tio n ( a t% ) Etch Time (s) Al C Cr O (a) HIPIMS 0 100 200 300 400 500 600 700 0 5 10 15 20 25 30 35 40 45 50 C o n ce n tr a tio n ( a t% ) Etch Time (s) Al C Cr O (b) DCMS Figure 4.2: XPS at position 0,0. 0 100 200 300 400 500 600 700 0 5 10 15 20 25 30 35 40 45 50 C o n ce n tr a tio n ( a t% ) Etch Time (s) Al C Cr O (a) HIPIMS 0 100 200 300 400 500 600 700 0 5 10 15 20 25 30 35 40 45 50 C o n ce n tr a tio n ( a t% ) Etch Time (s) Al C Cr O (b) DCMS Figure 4.3: XPS at position 16,5. this section.

The result from the depth profile XPS is shown in figure 4.2-4.6. The XPS was uncalibrated due to the absence of an Al/Cr/C/O standard. Therefore the actual percentage value is not to be considered, but it is of interest to look at the tendency of the curves to compare the films at different positions. In position 16,5, the HIPIMS film have a high amount of oxygen through the film, figure 4.3a. This could be due to that the film is porous in this location and thereby will oxygen get into the material and oxidize all through the film. In the other positions there is a clear drop of oxygen, this denotes that the film is denser in this positions. The XPS depth profile has not reached a steady state, a longer etching time would reach that state and a better result for the film composition would have been shown.

Table 4.1 shows the electrical measurements that was made on the samples. The conductivity is better for HIPIMS in three positions and it is the same as the DCMS film in one position.

4.1 Analyses of the Sample Regions 17 0 100 200 300 400 500 600 700 0 5 10 15 20 25 30 35 40 45 50 C o n ce n tr a tio n ( a t% ) Etch Time (s) Al C Cr O (a) HIPIMS 0 100 200 300 400 500 600 700 0 5 10 15 20 25 30 35 40 45 50 C o n ce n tr a tio n ( a t% ) Etch Time (s) Al C Cr O (b) DCMS Figure 4.4: XPS at position 11,-11. 0 100 200 300 400 500 600 700 0 5 10 15 20 25 30 35 40 45 50 55 C o n ce n tr a tio n ( a t% ) Etch Time (s) Al C Cr O (a) HIPIMS 0 100 200 300 400 500 600 700 0 5 10 15 20 25 30 35 40 45 50 55 C o n ce n tr a tio n ( a t% ) Etch Time (s) Al C Cr O (b) DCMS Figure 4.5: XPS at position -4,19. 0 100 200 300 400 500 600 700 0 5 10 15 20 25 30 35 40 45 C o n ce n tr a tio n ( a t% ) Etch Time (s) Al C Cr O (a) HIPIMS 0 100 200 300 400 500 600 700 0 5 10 15 20 25 30 35 40 45 C o n ce n tr a tio n ( a t% ) Etch Time (s) Al C Cr O (b) DCMS Figure 4.6: XPS at position -5,15.

18 Results and Discussion

Table 4.1: Electrical measurements, where R is the resistance, T is the thickness, L is the the length and W is the width.

Position R T L W Resistivity Conductivity

Units Ω nm mm mm Ω·m 1/(Ω·m) 0,0 HIPIMS 4.1 390 3.5 1.5 7.0·10−7 _1.4·106 16,5 HIPIMS 1.5 380 1.3 2.4 1.0·10−6 _9.6·105 16,5 DCMS 11 400 2.7 3.0 4.9·10−6 _2.0·105 11,-11 HIPIMS 8.2 370 7 2.1 9.3·10−7 _1.1·106 11,-11 DCMS 2.5 330 6 6 8.3·10−7 1.2·106 -4,-19 HIPIMS 1.4 400 2.5 2 4.5·10−7 _2.2·106 -4,-19 DCMS 5.2 420 2 1.8 2.0·10−6 _5.1·105 -5,15 HIPIMS 0.7 430 1.5 2.5 4.9·10−7 _2.0·106 -5,15 DCMS 4.5 410 2.2 2.3 1.9·10−6 _5.2·105

4.1.1 Position 16,5

The coordinate 16,5 from the center is a carbon rich area. The XRD grazing incidence is shown in figure 4.7 and the XRD θ − 2θ is shown in figure 4.8. The

diffraction pattern shows only Cr2AlC MAX phase peaks at this position, but the

peak to background ratio is very low and the 002 peak is weak. This indicates that the MAX phase crystals are only a small fraction of the films and the rest is probably amorphous. The film deposited with HIPIMS looks similar with some minor differences compared to the film deposited with DCMS. The 002 peak is bigger for DCMS and the 100 and 110 peaks is bigger for HIPIMS, this might denote that there is a difference in the preferred orientations between the two methods. On the contrary the intensity variation could just be an instrumental factor and the films could have the same orientation.

SEM images in figure 4.9 shows the morphology of the films deposited with HIPIMS and DCMS, the morphology looks similar for the both deposition meth-ods. Figure 4.10 is an image with higher resolution indicating the narrow crystal-lites that exist at this region.

4.1.2 Position 11,-11

11,-11 is the position analyzed with lowest amount of chromium. The XRD grazing incidence is shown in figure 4.11 and the XRD θ − 2θ is shown in figure 4.12. The

diffraction pattern shows that the Cr2AlC MAX phase is present in the film. There

are other small peaks indicating the presence of another phase, but there are not enough peaks to be sure of which phase. The grazing incidence and the θ − 2θ indicates a larger intensity of the 002 peak for DCMS, while the 100 peak is larger for HIPIMS.

The SEM images in figure 4.13 shows that there is no significant difference in the morphology of the two compared films. Figure 4.14 is a SEM image with higher

4.1 Analyses of the Sample Regions 19

10

20

30

40

50

60

70

80

90

2Tposition  (

₎

Intensity  (a.u.)

Cr

AlC  (002)

Cr

AlC  (100)

Cr

AlC  (1

10)

Cr

AlC  (1

16)

Cr

AlC  (103)

Cr

AlC  (106)

Cr

AlC  (109)

Figure 4.7: XRD grazing incidence at the position 16,5.

resolution of the HIPIMS film, showing a leaf like structure of the morphology.

4.1.3 Position 0,0

The position 0,0 is the center of the sample, where the incident composition was prepared to be the 211 stoichiometry for the MAX phase. The XRD grazing incidence is shown in figure 4.11 and the XRD θ − 2θ is shown in figure 4.12.

The diffraction pattern shows the Cr2AlC MAX phase peaks, but they are shifted

0.5◦_{, this could suggest the formation of (Cr,Al)}

2Cx with a=2.843 Å and c=4.285

Å [23].

The Cr2Al phase is also present in this region and the peaks are larger for

the HIPIMS film compared to the DCMS film, indicating that there is a higher

fraction of Cr2Al in the HIPIMS film. The Cr2Al peaks are not shifted.

The grazing incidence and the θ − 2θ shows a bigger 100 peak for the HIPIMS film. The MAX phase 002 peak is not present in the scans for the DCMS film, but there is a small 002 peak for the HIPIMS film in grazing incidence. There is

20 Results and Discussion

10

10

20

30

40

_{2Tposition  (}

50

60

₎

70

80

90

Intensity  (a.u.)

Substrate

Substrate

Substrate

Cr

AlC  (002)

Cr

AlC  (100)

Cr

AlC  (1

10)

Cr

AlC  (1

16)

Figure 4.8: XRD θ − 2θ at the position 16,5.

Figure 4.9: SEM images from the 16,5 position deposited with HIPIMS (left) and deposited with DCMS (right).

The SEM images in figure 4.17 shows that there is no significant difference in the morphology of the two compared films. Figure 4.18 shows the morphology

4.1 Analyses of the Sample Regions 21

Figure 4.10: SEM image from the 16,5 position.

10

20

30

40

50

60

70

80

90

2Tposition  (

₎

Intensity  (a.u.)

Cr

AlC  (002)

Cr

AlC  (100)

Cr

AlC  (1

10)

Cr

AlC  (1

16)

Cr

AlC  (103)

Cr

AlC  (106)

Cr

AlC  (109)

Cr

Al

 (300)

Figure 4.11: XRD grazing incidence at the position 11,-11.

from the HIPIMS film with higher resolution, showing a leaf like structure of the morphology, similar to the morphology at the position 11,-11.

22 Results and Discussion

10

20

30

40

50

60

70

80

90

2Tposition  (

₎

Intensity  (a.u.)

Substrate

Substrate

Substrate

Cr

AlC  (002)

Cr

AlC  (100)

Cr

AlC  (1

10)

Cr

AlC  (1

16)

Cr

AlC  (109)

Cr

AlC  (106)

Figure 4.12: XRD θ − 2θ at the position 11,-11.

Figure 4.13: SEM images from the 11,-11 position deposited with HIPIMS (left) and deposited with DCMS (right).

4.1.4 Position -4,-19

The position -4,-19 is a aluminum rich area. The grazing incident diffraction

4.1 Analyses of the Sample Regions 23

Figure 4.14: SEM image from the 11,-11 position.

10

20

30

40

50

60

70

80

90

2Tposition  (

₎

Intensity  (a.u.)

Cr

AlC  (100)

Cr

AlC  (1

10)

Cr

AlC  (1

16)

Cr

AlC  (103)

Cr

AlC  (106)

Cr

AlC  (109)

Cr

Al  (002)

0.5

_Shift

of Cr

AlC

Cr

Al  (103)

Figure 4.15: XRD grazing incidence at the position 0,0.

the DCMS sample shows no peaks at all. There is a shift in the Cr2AlC peaks

from the diffraction pattern in figure 4.19, that could suggest the formation of

24 Results and Discussion

10

20

30

40

50

60

70

80

90

2Tposition  (

₎

Intensity  (a.u.)

Substrate

Cr

Substrate

Substrate

AlC  (100)

Cr

AlC  (1

16)

65.341

Figure 4.16: XRD θ − 2θ at the position 0,0.

Figure 4.17: SEM images from the 0,0 position deposited with HIPIMS (left) and deposited with DCMS (right).

the DCMS sample, which have not been identified.

morphol-4.1 Analyses of the Sample Regions 25

Figure 4.18: SEM image from the 0,0 position. ogy.

10

20

30

40

50

60

70

80

90

2Tposition  (

₎

Intensity  (a.u.)

Cr

AlC  (100)

Cr

AlC  (1

10)

Cr

AlC  (1

16)

Cr

AlC  (103)

Cr

AlC  (106)

Cr

AlC  (109)

Cr

AlC  (002)

0.5

_Shift

of Cr

AlC

Cr

Al  (002)

Cr

Al  (103)

Cr

Al  (200/105)

26 Results and Discussion

10

20

30

40

50

60

70

80

90

2Tposition  (

₎

Intensity  (a.u.)

Substrate

Cr

Substrate

Substrate

AlC  (100)

Cr

AlC  (1

16)

Cr

AlC  (002)

Cr

AlC  (109)

65.332

Figure 4.20: XRD θ − 2θ at the position -4,-19.

Figure 4.21: SEM images from the -4,-19 position deposited with HIPIMS (left) and deposited with DCMS (right).

4.1.5 Position -5,15

The position -5,15 is a chromium rich area. Figure 4.22 shows the XRD grazing incident data and figure 4.23 shows the XRD θ − 2θ. The 002 peak is not present

4.2 Lower Temperature Samples 27

and the Cr2AlC peaks are shifted, therefore is it hard to tell if the MAX phase

is present or not. There are small peaks in the diffraction pattern that is not identified.

The SEM image in figure 4.24 shows the morphology of the films.

10

20

30

40

50

60

70

80

90

2Tposition  (

₎

Intensity  (a.u.)

Cr

AlC  (100)

Cr

AlC  (1

10)

Cr

AlC  (1

16)

Cr

AlC  (103)

Cr

AlC  (106)

Cr

AlC  (109)

Cr

AlC  (002)

0.3-0.6

_Shift

of Cr

AlC

Figure 4.22: XRD grazing incidence at the position -5,15.

4.2 Lower Temperature Samples

For the samples deposited at lower temperatures, the XRD diffraction pattern did not clearly show that the MAX phase was precent. Figure 4.25 shows the

XRD data for 350o_{C, 450}o_{C and 500}o_{C synthesized with HIPIMS. 350}o_{C clearly}

shows an amorphous structure of the sample. 450o_{C shows that a phase is weakly}

present. 500o_{C shows a phase that is similar to the MAX phase, but it could also}

be the (Cr,Al)₂Cx phase, previously reported for this temperature [23].

There is a big different between 450o_{C and 500}o_{C, so this is the temperature}

were crystallization starts to take place. At 450o_{C small peaks are visible, so}

28 Results and Discussion

10

20

30

40

50

60

70

80

90

2Tposition  (

₎

Intensity  (a.u.)

Substrate

Substrate

Cr

Substrate

AlC  (1

16)

65.419

Cr

AlC  (106)

75.909

Figure 4.23: XRD θ − 2θ at the position -5,15.

Figure 4.24: SEM images from the -5,15 position deposited with HIPIMS (left) and deposited with DCMS (right).

4.2 Lower Temperature Samples 29 Intensity  (a.u.) ₃₅₀o_C 450o_C 500o_C 50 60 10 20 30 40 70 80 90 2Tposition  (o₎

Figure 4.25: XRD for the samples deposited with HIPIMS at 350o_{C (blue), 450}o_C

Chapter 5

Summary and Conclusions

To easily investigate a large number of compositions, a combinatorial method was used.

In this project two films, deposited at 550o_{C and synthesized with HIPIMS}

and DCMS were compared. Analyses with Scanning Electron Microscopy showed that the films had similar morphology at the same positions. X-Ray Diffraction

indicated that the MAX phase Cr2AlC was synthesized. X-Ray Photoelectron

Spectroscopy verified that the films consists of chromium, aluminum, carbon and oxygen. Electrical measurements revealed that the HIPIMS film had better con-ductivity than the DCMS film in three of four positions.

Over the combinatorial area different structures and phases are present. In

position 0,0 and -4,-19 Cr2Al was synthesized. The DCMS film at position

-4,-19 shows some peaks that were not possible to identify. More analyses on this position would have been required to understand these phases. There are other peaks in the films that have not been identified, indicating that there are more phases present.

To be certain that the MAX phase is present, the XRD data need to contain the 002 peak and no peak shift can exist. Position 11,-11 is a position that satisfy these requirements. Position 16,5 is in the border zone for these conditions, but for the other three positions it is not possible to tell which phase is present with the data acquired in this project.

Synthesis of the MAX phase Cr2AlC by HIPIMS does not change the lowest

deposition temperature compared to DCMS. The lowest possible temperature for

depositing Cr2AlC is 550oC for both HIPIMS and DCMS. The XRD data from the

two methods have intensity variations for different orientations, this might stem from a different preferred orientation when using HIPIMS.

32 Summary and Conclusions

5.1 Recommendations

HIPIMS is only used on the chromium target in this project, that might not be enough to get an effect, if HIPIMS would be used on all three targets there could be a different result. It would be interesting to try different HIPIMS parameters, by using a higher power more energy could be transferred to the substrate to allow MAX phase formation at lower temperatures.

To be sure if the MAX phase was synthesized in all positions, transmission electron microscopy of each sample and position need to be done. In such a image there would be obvious if the zigzag structure from MAX phases is present or not.

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