39

3. Filtered Cathodic Vacuum Arcs

3.1. Introduction

‘Vacuum arc’ is a term used to describe a direct current (DC) glow discharge

involving the explosive emission of plasma from the surface of a conductive

electrode. The electrode material itself is used to sustain the discharge without the

need for a background gas and the process can therefore occur under vacuum

conditions. When the plasma material is emitted from the cathode surface the term

‘cathodic vacuum arc’ is used. Emission from the anode surface is also possible under

certain conditions, termed ‘anodic vacuum arc’.

3.1.1. Historical review

A.W.Wright performed the first documented experiments on the vacuum arc as a

deposition technique in the 19th century, investigating the deposition of metal films

on the walls of glass discharge tubes [1]. Thomas Edison filed a patent in 1884

claiming “the process of plating with electrical conducting material by forming an

electric arc in vacuum”[2]. Edison utilised the technique for depositing a conducting

layer onto phonographic wax cylinders to produce an inverse impression of the

original recording for use as a template. Intermittent research was conducted on

vacuum arcs until the 1960’s when Kikuchi et al. published work on “structures of

metal films produced by vacuum-arc evaporation method”[3]. Since this work a

renaissance of research on vacuum arc technology has occurred. A major stimulus for

40

this renewed interest was the research undertaken for the development of vacuum

interrupters [4].

3.1.2. Arc thin film deposition

A significant drawback in the utilisation of vacuum arcs for thin film deposition is

contamination of the plasma by molten droplets of cathode material. Although being

on average of the order of a few micrometres in diameter, these droplets are termed

macroparticles. Development of the macroparticle filter has been instrumental in the

adoption of vacuum arcs as thin film deposition plasma sources in industry and

research. Pioneering work on filter design and efficiency was undertaken by Aksenov

and co-workers from the Kharkov group in the former USSR. This work produced the

most popular design of macroparticle filter employed today; the magnetic solenoid

toroidal duct system [5].

Deposition of ceramic materials can be achieved by the introduction of a reactive gas

into the deposition chamber. Ionisation of the gas is facilitated by collisions with the

cathodic arc plasma and ceramic compounds are deposited by chemical reactions

between the gas ions and the metal ions. Titanium nitride is commonly produced by

this method. It was this material, with its attractive gold colour, which caught the eye

of J. Filner, a New York based precious metals dealer, who subsequently bought

soviet vacuum arc technology to the West around 1980.

Condensation of pure carbon plasmas from graphite cathodic arcs produces a

diamond-like carbon (DLC) film with a high proportion of sp3 (diamond) bonds.

41

Cathodic arcs are unusual in their ability to produce hydrogen free DLC (often termed

tetrahedral amorphous carbon or ta-c) and a significant portion of cathodic arc

research has been focused on the development of the technology for this purpose.

3.1.3. Arc Ion Source

In contrast to most conventional plasma sources, a very large proportion of the plasma

produced by a cathodic arc is ionised. This allows the plasma to be manipulated

electro-magnetically. It is for this reason that the magnetic solenoid can be employed

as a macroparticle filter. Another way in which this feature is exploited to advantage

is through the extraction of ions from the plasma by the use of charged extraction

grids to create a metal ion source. Extensive development of this technology has been

undertaken by Ian G. Brown and the plasma applications group at Lawrence Berkeley

Laboratory, California [6]. The ion extractors and cathodic arc source are collectively

trade named “metal vapour vacuum arcs” (MEVVA).

Recent interest in plasma based ion implantation has seen cathodic arcs utilised as a

source of metal ions for MePIIID. This thesis is concerned primarily with MePIIID

using cathodic arcs for modifying the surface properties of polymers. Deposition of

ultra thin films on insulators by cathodic arc is a secondary, but equally interesting,

concern of this thesis.

42

3.2. General Considerations

3.2.1. Cathodic arc components

Modern practical cathodic vacuum arcs consist of several essential components; a

conductive cathode from which the plasma is derived; an anode, which is essentially

an electron-collecting electrode; a trigger to initiate the discharge; a power supply;

and a vacuum chamber (figure 3.1). Additional components may include magnetic

confinement coils and a macroparticle filter.

Figure 3.1: Schematic showing the essential components of a cathodic vacuum arc.

Choice of a cathode material is limited only by its ability to conduct a current. All

pure metallic species as well as conductive alloys, graphitic carbon and doped

semiconductors are potential cathodes. Un-doped semiconductors can also be heated

to increase the carrier electron concentration. It is the cathode material that determines

the composition of the plasma. Cathode designs are almost invariably a solid disc with

one end connected to the power supply and the other circular surface being the region

Power supply Vacuum pump

Anode

Cathode

Trigger

Plasma

43

of arcing and plasma production. A large range of cathode diameters exist, ranging

from thin wires to tens of centimetres.

The anode must be immersed in the plasma plume that originates from the cathode

surface. The location of the anode must be such that it does not impede the flow of the

majority of the plasma, whilst being close enough to the region of plasma production

to collect enough electrons to sustain the discharge. Common anode designs include a

cylinder around the cathode through which the majority of plasma can flow, or a flat

collecting plate with a central hole that allows the majority of plasma to pass through.

3.2.2. The Arc Discharge

Both electrodes are housed in a vacuum chamber and the vessel evacuated. Before an

arc is initiated a potential is established between the electrodes. This pre-discharge

potential is of the order of many tens of volts. The arc is initiated by creating a small

amount of plasma to provide current continuity between the electrodes. This can be

achieved by a number of different methods. Physical contact with the cathode by a

mechanical trigger electrode held at anode potential is a common method. Non-

contact methods such as high-voltage flashover from a trigger electrode or laser

ablation of the cathode material are also employed.

Once an electrical connection is made between the primary electrodes the arc is self-

sustaining. Unlike more conventional discharges that require an ionised gas as a

conductive medium, it is the cathode material itself that acts as the ‘switch’ between

the electrodes. So long as the power supply can maintain a potential difference

44

between the electrodes, and the anode can effectively collect electrons from the

cathode, the arc will run until all the cathode material is ablated. The burn voltage is

the potential difference sustained between the cathode and the anode during the

discharge, as distinguished from the pre-discharge potential applied between the

electrodes. Since the power supply is generally of low impedance, the electrical

resistance of the plasma is the primary determinant of the burn voltage.

3.2.3. Pulsed vs continuous

Cathodic vacuum arc plasma sources can be grouped into two classes: continuous (or

DC) and pulsed. Essentially all cathodic arc discharges are DC discharges. The

distinction between DC and pulsed discharges comes about due to the short burn

times of the pulsed arc and not due to any oscillatory nature of the arc. DC arcs are

generally operated at much lower currents than their pulsed counterparts and

consequently exhibit markedly different current-voltage and plasma characteristics. In

general, DC arcs draw currents in the range of 20 to 200 A with burn voltages

between 10 and 100V. It is interesting to note that at very low operating currents

(<10A), DC arcs exhibit high-frequency oscillatory fluctuations. Pulsed arcs generally

draw currents from a few hundred amps up to tens of kiloamps with burn voltages

similar to that of their DC counterparts.

DC and pulsed arcs require different power supplies. DC arcs generally utilise a

continuous current source similar to a welding power supply. Pulsed arcs require high

instantaneous currents and often utilise a capacitor bank as a power reservoir. One of

the limiting factors of the pulsed arc repetition rate is the charging time of the

45

capacitors. A more severe limitation is the resistive heating load of the electrical

components. The cooling capability requirement is generally the major limitation in

total power usage for both classes of arc. Since the erosion rate of the cathode is

closely linked with the power dissipated in the cathode, the plasma production

capability of cathodic arcs is in part limited by the ability to cool the electrical

components.

3.3. Cathode Spots

Unlike conventional glow discharges, in a vacuum arc the current continuity at the

cathode cannot be provided by the charged particles from the plasma column. At the

cathode surface the arc current is channelled through micrometer diameter bright

spots called cathode spots. These spots have an extremely high current density that

leads to enhanced ionisation and energy transfer to the electrode. Energetic ions and

electrons are emitted from the spot and provide the metal vapour necessary to sustain

the discharge in vacuum environment. In addition to production of the plasma species,

macroparticles are produced by local heating and explosive emission of molten

droplets from the spot region. Theoretical models of physical processes at the arc

spots vary widely, and are far from conclusive. For a review of the current state of the

theory see [7]. A number of experimentally observed parameters underlie the

theoretical models, some of which will be discussed here. Table 3.1 gives a summary

of characteristic values of the more common measurements.

46

Table 3.1: Cathodic arc characteristic parameters.

Burning voltage (V)

Current per spot (A)

Current density (A.m-2)

Spot velocities (m.s-1)

Ion velocities (km.s-1)

15-25 ref [8]

0.5 – 200 ref [9]

109-1012 ref [8]

0.1-100 ref. [7]

5-20 ref. [8]

3.3.1. Current per spot

The current per spot is fairly constant for a given cathode material in vacuum. If the

arc current is varied, spots are observed to split or extinguish to maintain this

parameter. There is a large variation in the average current per spot for different

materials, from around half an amp for solid mercury to a few hundred amps for

carbon and tungsten [9]. The burning voltage also varies for different materials but is

confined to a much smaller range, from 16-25V [8]. Surface contaminants reduce the

burning voltage by about 3-5V. Anders [10] convincingly relates the burning voltage

to the cohesive energy of the cathode material.

3.3.2. Current density

The current density in an arc spot is extremely high. Measurement of the current per

spot can be performed quite accurately. In contrast, estimates of the active area of the

spot are difficult and subject to large uncertainties. Visual observations of the spots by

high speed camera is subject to errors arising from accurate determination of the

active area, as distinct from the luminous area which may be in part ascribed to

conductive heating. Additionally, the expanding plasma emits an intense optical

signal that can distort the estimates of the spot diameter.

47

Estimates of the spot dimensions by post-arc observations of the arc craters by

electron microscopy are also prone to errors. Whilst it is relatively straightforward to

measure the area of damage on the cathode surface, exactly how much of the

damaged area was the conduit for the current is difficult to determine due to the

explosive ejection of molten material from the spot during the arc. Because of these

uncertainties, experimental estimates of the arc current densities range from 109 to

1012 A.m-2. Theoretical models suggest that the density may be an order of magnitude

higher [11]. Beilis et al. developed a self-consistent model for copper cathodes based

on experimental observations [12]. According to their model the current density is 1 x

1010 A.m-2, 20µs after spot ignition for a 15V burning voltage and 40A arc current. At

the same point the plasma density at the spot is 1.5 x 1026 m-3.

3.3.3. Ion velocities

As a result of the high currents and plasma densities, a feature of vacuum arcs is the

relatively high kinetic energy imparted to the plasma ions. The very high plasma

density creates strong pressure gradients which, combined with high local electric

fields, cause the acceleration of ions to supersonic drift speeds [13]. Ion velocities

range from 0.5 - 2 x 104 ms-2, almost independent of ion mass and charge state [8].

This provides a unique condition for depositing thin films, affecting the film stress

and hardness by imparting energy to the growing film through ion impacts. Additional

energy can be given to the ions by applying a potential to the substrate, making use of

the high degree of ionisation in the cathodic arc plasma, for example by plasma

immersion ion implantation.

48

3.3.4. Ion Charge States

During the discharge the majority of the plasma atoms are ionised and the ion-to-

electron ratio is usually limited to around 0.1 at distances greater than a few

millimetres from the cathode [14]. Due to the extremely high current densities the

material ejected from the spot region contains energetic ions with charge states up to

six [15]. Most metal species, at low arc currents, have an average ion charge state of

between 1 and 3. Brown [6] gives a detailed table of the ion charge states and their

relative proportions in vacuum arc plasmas for a large range of cathode materials,

produced at arc voltages around 100V.

It has been observed that external magnetic fields affect the distributions of charge

states [15]. High arc currents (>1kA) have also been shown to increase the average

charge state [15]. These two effects can be related by considering that an increased

arc current results in the production of multiple arc spots. Being a conduit for a large

current, each arc spot is the source of a large magnetic field that acts on surrounding

spots in the same way as an externally applied field, thereby affecting the charge state

proportions. In addition, the average ion charge state during the discharge decreases

from an initial maximum to a steady state after around 100µs [16]. This is likely to be

due to surface contaminants and adsorbed gases affecting the electronic properties of

the cathode surface.

49

3.3.5. Spot types

Changes in the work function of the cathode by surface contaminants and adsorbed

gases has been proposed as a mechanism by which to explain the observation of

cathode spots with dramatically different properties on the same cathode material

[17]. In general, spots can be classified into two groups, unimaginatively named type

1 and type 2 spots. Beilis [7] summarised the results of a number of investigations

into cathode spot classification. Type 1 spots are associated with high-speed motion

(10-100m/s) and short lifetime (<10µs) with a comparatively small current per spot

(<10A). Type 2 spots exhibit lower speeds (~0.1m/s), longer lifetimes (~100µs), and

spot currents greater than 10A. These values are quoted for copper cathodes. Type 1

spots are attributed to the presence of surface contaminants. This will be discussed

further in the chapter 5.

3.3.6. Retrograde motion

An unresolved problem in the theory of cathode spot motion is the inability to

comprehensively explain the observed motion of cathode spots, especially in a

magnetic field. Single cathode spots in the absence of externally applied fields exhibit

a random motion across the cathode surface. Spot behaviour in the presence of an

externally applied magnetic field is somewhat more complicated. Electromagnetic

theory predicts that an electric current density, J, in the presence of a magnetic flux

density, B, is subject to a force, J x B. In contrast, when observed in an external

magnetic field parallel to the surface of the cathode (transverse magnetic field),

cathode spots move in a direction opposite to that predicted by the theory. This

50

phenomenon is termed “retrograde motion” and numerous attempts to explain it have

proved either inconclusive or contradictory [18]. Retrograde motion has been shown

to be influenced by the gas pressure in the vacuum chamber [19]. As the background

pressure increases, retrograde motion slows, and then reverses at a critical pressure.

Juttner and Kleberg [20] used high-speed microscopy to investigate the motion and

structure of cathode spots in transverse magnetic fields. They found evidence of

plasma jets emitted from the cathode spots in the retrograde direction (figure 3.2).

Cathode spot motion was observed to follow the direction of these jets, moving in

jumps over distances of 50-300µm. The hypothesis put forward, originally proposed

by Drouet [21], was that the plasma in the spot vicinity is confined at the retrograde

side by the magnetic field. The inability of the plasma to expand under the forces

exerted from the cathode spot below causes the confinement to become unstable and

jets of plasma are emitted toward the retrograde side at velocities of 5km.s-1 on

average.

Figure 3.2: Images of plasma jets bursting toward the retrograde side of the arc spots. Image from reference [18].

51

High-current arcs exhibit simultaneous multiple arc spots on the cathode surface.

Each spot carries a current in the direction normal to the cathode surface and

consequently generates a transverse magnetic field. Other cathode spots subsequently

exhibit retrograde motion and the total field from all the spots appears as a repulsive

force between the spots. As such, high-current arc spots move outward from the

ignition point with a velocity proportional to the total current in the arc and the

distance from the ignition point. This will be shown in more detail in chapter 5.

3.4. DC arc applications

DC vacuum arcs have been studied and utilised within the School of Physics at the

University of Sydney for more than 15 years [19]. Numerous additions and

modifications have been included over the years. At present there are two DC arcs,

one primarily used for metals and the other for carbon, both utilised on a regular basis

for deposition and implantation. Recently, a pulsed vacuum arc has been constructed,

a description of which will form chapter 5 of this thesis. Development of the pulsed

version was in part based on the design of the DC versions, warranting a review of the

state of the art of the DC machines. A number of brief investigations and applications

of the DC arcs, performed by the author, are also presented here. DC arcs were also

used to perform metal ion implantation into polymers, which forms the bulk of the

next chapter of this thesis.

52

3.4.1. DC Arc Design

The basic design of the present incarnation of DC arcs is shown in figure 3.3. The

system shown is for the carbon arc, the metal arc version being essentially identical

save a few minor differences. The system can essentially be divided into three distinct

but coupled components; the vacuum arc plasma source, the magnetic macroparticle

filter, and the deposition chamber.

Figure 3.3: Schematic of the DC cathodic arcs showing the location of the anode, cathode, substrate holder, filter coils, gas ports and water-cooling. (Image courtesy of Dr Richard Tarrant, with permission).

An oil diffusion pump evacuates the entire vacuum chamber to a base pressure of

around 1 x 10-6 Torr. To stabilise the arc argon is often introduced, and the pressure

B3

Substrate holder

Anode

C a t h o d e

W a t e r c o o l i n g

G a s i n l e t B

G a s i n l e t A

S c a l e : 200 mm

B2

B1

B4

Port A

Port B

53

raised to around 1 x 10-4 Torr. Ionisation of the gas near the cathode provides a more

consistent conductive medium that limits arc instabilities, reducing the occurrence of

arc extinction. At these pressures the mean free path of the ions is many times larger

than the vacuum vessel and plasma transport efficiency is not compromised. At higher

pressures, collisions with background gas molecules reduce the efficiency of the

plasma transport.

The plasma source is powered by a commercial arc welding power supply, which

provides a potential difference between a 76 mm diameter, water-cooled cathode and

a 125 mm diameter, tubular, water-cooled anode. The latter is located, on average,

50mm from the front surface of the cathode. A mechanical trigger, maintained at

anode potential, is brought into contact with the cathode, initiating a plasma that acts

as the current conducting medium between the anode and cathode. Currents as low as

40A and as high as 200A can be maintained between the electrodes. Large volumes of

plasma and macroparticles expand outward from the cathode surface, through the

anode, and into the macroparticle filter duct. A significant proportion of cathode

material is deposited on the anode and vessel walls surrounding the cathode. Magnetic

containment coils are used to limit the outward expansion of the ionised plasma (B1,

B2 and B3 in figure 3.3), coupling the plasma into the filter duct and reducing losses

to the walls. Electron heating of the vacuum chamber around the cathode limits the

continuous operation of the arc to around 10 minutes. Water pipes attached to the

chamber wall near the cathode assist in cooling.

The macroparticle filter consists of a quarter-turn toroidal duct, along which the

ionised plasma is magnetically guided by means of an externally applied magnetic

54

field (figure 3.3). The field is produced by hundreds of turns of copper wire wound

around the exterior of the duct (B4 in figure 3.3), through which currents of around

ten amps are passed. This produces magnetic fields inside the duct of the order of tens

of millitesla parallel to the duct axis. These field strengths are not sufficient to

magnetically deflect the ions around the ninety-degree bend. Rather, the electrons

gyrate at the larmour radius and follow the field lines around the duct bend. The ions

are constrained by the electric field established by the electrons and are also deflected

around the bend. Unionised particles and large macroparticles are not deflected and

impact the duct wall. Whilst not present in the current version, baffles are often used

to impede the passage of macroparticles.

3.4.2. Plasma properties

3.4.2.1. Ion densities

The material entering the deposition chamber consists of purely ionised plasma with a

high directed velocity. Experimental determination of the plasma properties is

important since it allows predictions of sheath dynamics and dose estimates in PIII.

Various plasma diagnostic techniques have been utilised to determine the plasma

properties in the deposition chamber. Langmuir probes have been used to determine

the plasma density, electron temperature and floating potential of plasmas under

different arc and filter conditions. In this example, the characteristics of a carbon

plasma were measured by an undergraduate physics student, Paul Thompson, under

supervision from the author [22]. The probe measured the plasma density, electron

temperature and floating potential across the plasma flow at the location of the

55

substrate holder (shown in figure 3.3). The plasma density was adjusted by varying

the arc current and the confinement and filter coil currents. Table 3.2 shows the arc

and filter coil currents used to produce the different conditions, designated as low-,

medium- and high-density.

Table3.2: Values of arc current and coil currents (see figure 3.3) used to produce three different carbon plasma densities. The measured plasma densities are shown in figure 3.4.

Currents (A)

Setting Arc B1 B2 B3 B4

Low 75 0.8 0.2 2 5

Medium 75 0.8 0.2 3 9

High 93 0.8 0.2 5 14

The floating potential varied from –19 V to –28 V across the beam, showing no trend

with either the location or the plasma density. For all three settings the electron

temperature remained fairly constant across the plasma path, at values around 5 eV.

Figure 3.4 shows the plasma density as a function of position across the plasma

stream. Positive values of distance correspond to the direction of the outer radius of

the duct. The plasma density shows a marked increase toward the outer radius of the

duct for the high-density settings. This is due to the ion momentum dragging the

plasma toward the outer radius. Ion density for the low- and medium-density cases

shows a slight increase toward the outer radius of the duct, to a maximum of 4 and 9 x

1015 ions.m-3, respectively. The high-density case peaks at a significantly higher

maximum value of 3.2 x 1016 ions.m-3.

56

Figure 3.4: Plasma density at the substrate as a function of location relative to the centre of the duct for three settings of plasma density.

3.4.2.2. Ion energies

One of the distinctions of vacuum arc plasmas compared to other metal ion sources is

the high intrinsic energy of the ions. A HIDEN mass-energy analyser was used to

determine the relative concentrations of titanium and nitrogen ions and their energies

during conditions used for deposition of titanium nitride thin films [23]. Ion energies

were found to be as large as 80 eV when measured with respect to the grounded

vessel walls. These energies are extremely large when compared with the native

energies of ions produced by other physical vapour deposition techniques, such as

magnetron sputtering.

57

It is this high ion energy that is responsible for creating the thermodynamic conditions

that result in the formation of tetrahedral amorphous carbon (ta-C) when carbon is

deposited by cathodic arc [24]. The relative proportion of sp2 (graphitic) and sp3

(diamond-like) carbon bonds in the deposited film is a function of the average energy

of the depositing ions. A maximum sp3 fraction is obtained at deposition energies

between 20-100eV, depending on the deposition rate, as this affects the growth

temperature, which in turn affects the sp3 fraction [25]. This is exactly the range of

the native energies of cathodic arc produced carbon ions. ta-C grown by filtered

cathodic arc plasma condensation exhibits extremely high hardness and Young’s

modulus and also extremely high intrinsic stress. This last property results in very

poor adhesion to the substrate by deposited ta-C films. Adhesion can be improved by

growing films with a pulse-biased substrate (PIII), alternately removing and applying

the bias for extended periods during the deposition. This process results in the

formation of a multilayered carbon film with alternating layers containing high and

low sp3 content. In this way the intrinsic stress is alleviated and adhesion enhanced

without significant loss of hardness [26].

3.4.3. Film adhesion

Combining cathodic arc deposition with PIII results in energetic deposition with

implantation, which can enhance the adhesion of thin films. A simple demonstration

of this is shown in figure 3.5. An 80nm copper film was first deposited on a

polycarbonate substrate by DC cathodic vacuum arc. The arc current was 80A with a

background argon gas pressure of 0.3mT, resulting in a deposition rate of 0.13nm.s-1.

A circular metallic aperture, electrically connected to the PIII power supply, was then

58

placed on the film and 20µs high voltage pulses were applied at 250Hz whilst a

further 40nm of copper deposited. The experiment was performed at voltages of 1, 5

and 10kV.

Figure 3.5: Adhesion enhancement of copper films to polycarbonate substrates by PIII. Implantation energies of 1, 5, and 10kV were used for images 1, 2 and 3 respectively. The green semicircle indicates the implanted region.

Adhesion of the film was improved for higher implantation voltages when subjected

to the “sticky-tape test”, as observed in figure 3.5. The semi-circular area exposed to

the ion implantation is outlined in green for clarity. A plausible explanation for the

observed adhesion improvement is that ions were implanted through the pre-deposited

film, forming an intermixed layer with the substrate. An alternative explanation could

be that heating and densification of the deposited film by ion implantation increases

the contact area between film and substrate. TRIM calculations suggest that 10kV

copper ions should not penetrate the initial 80nm copper film, a result that favours the

latter explanation.

59

3.4.4. Ceramic films

If a reactive gas is introduced into the chamber during deposition, the gas is ionised

by the metal plasma. This can result in incorporation of the gas in the growing film to

produce ceramics. A commercially developed application of this is the production of

hard titanium nitride films for tool coatings [27]. A further example of DC arc

deposition in combination with PIII is the use of PIII to implant reactive ions into

metal to form different compounds. Early investigations of PIII used nitrogen plasmas

to harden tool steel in a process known as iron nitriding [28].

In the example presented here, titanium nitride films were grown on silicon substrates

by introducing nitrogen into the deposition chamber during titanium plasma

condensation. A pure titanium arc was run at 75A in a background of 0.4mTorr argon.

High purity nitrogen was introduced 1 minute after the beginning of the deposition at

a flow rate of 23.4sccm. The nitrogen reacted with the depositing titanium, leaving a

residual nitrogen gas pressure of 0.4mTorr.

60

Figure 3.6: Titanium nitride colour changes caused by ion implantation by PIII. Samples 1 is cathodic arc deposited TiN. Samples 2-4 are nitrogen, oxygen and hydrogen implanted TiN respectively.

Gold coloured films indicated that approximately stoichiometric TiN films had been

produced. Post-deposition PIII of 20 kV hydrogen, nitrogen and oxygen ions was then

performed. The films were observed to change colour after implantation, suggesting a

change in the chemical structure of at least the surface regions of the film (figure 3.6).

Considering the large interest in coatings technology for decorative applications this

technique may hold commercial promise.

3.4.5. Polymer PIII

As was mentioned in the previous chapter, charge accumulation results in electric

field enhancement across the sheath and subsequent breakdown, observable as bright

arcs that damage the substrate surface. The next chapter describes how a thin

conductive film can be used to circumvent the charging problem. Prior to that work, a

number of experiments were performed to investigate the feasibility of the technique.

61

In one such experiment, a 12.5nm aluminium film (thickness measured by

profilometer) was deposited on polycarbonate using a JAVAC magnetron sputter

coater. The argon pressure was 3.3mTorr and the sputtering current was 150mA. The

coated polycarbonate was then placed on a copper substrate holder and a 50mm

diameter circular aluminium aperture, electrically contacted to the PI3 power supply,

was placed on top. Good electrical contact was made between the aperture and the

aluminium film. The substrate holder was then placed in the deposition chamber of

the carbon cathodic arc with the substrate surface parallel to the direction of plasma

flow (figure 3.3).

A carbon arc was initiated in vacuum (<10-6 Torr) with a current of 80A. Magnetic

field currents of 2, 0.4, 0.4 and 12A were used for coils B1-B4, respectively,

corresponding to a density regime midway between the medium- and high-density

settings in table 3.2. 20µs, 20kV pulses were applied to the substrate at 1000Hz, and

the arc was run for 10 minutes. Carbon ions were drawn from the plasma, implanting

through the aluminium film and into the polymer substrate. Upon removal from the

deposition chamber the substrate was lightly wiped with a dry tissue. The metal film

easily came away from areas that had been exposed to the plasma. The underlying

polymer showed signs of darkening due to carbonisation, which is attributed to cross-

linking.

62

Figure 3.7: Optical micrograph of a scratch made in carbon implanted polycarbonate. The implanted region on the left shows a higher density of cracks indicating embrittlement of the surface when compared with the unimplanted region on the right.

Figure 3.7 shows an optical micrograph of a scratch made in the surface of the

polymer by a scalpel blade. The region that has been implanted shows a higher

density of cracks around the scratch than the unimplanted region, suggesting an

embrittlement of the surface due to ion implantation. This qualitatively demonstrates

the effectiveness of PIII for surface modification of polymers. Further evidence and

quantitative measurements shall be presented in the next chapter.

63

3.5. References

1. Wright, A.W., on the production of transparent metallic films by the electrical

discharge in exhausted tubes. Am. J. Sci. Arts, 1877. 13: p. 49-55.

2. Edison, T.A., Manuscript patent application, in Edison papers project,

unpublished. 1884.

3. M. Kikuchi, S.N., H. Ohmura, S. Oketani, Structures of metal films produced

by vacuum-arc evaporation method. J. Appl. Phys., 1965. 4: p. 940.

4. Greenwood A, Vacuum switching of high current and high voltage at power

frequencies,, in Handbook of vacuum arc science and technology, P.M.a.D.S.

RL Boxman, Editor, Noyes: New Jersey. p. 590-624.

5. Aksenov, I.I., et al., Soviet Journal of Plasma Physics, 1978. 4: p. 425.

6. Brown, I.G., Vacuum arc ion sources. Rev. Sci. Instrum., 1994. 65(10): p.

3061.

7. Beilis, I.I., State of the theory of Vacuum Arcs. IEEE transactions on Plasma

Science, 2001. 29(5): p. 657-670.

8. Davis, W.D. and H.C. Miller, Analysis of the electrode products emitted by

DC arcs in a vacuum ambient. Journal of Applied Physics, 1969. 40(5): p.

2212-2221.

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