Characterization of the Plasma Properties of a Reverse Polarity Planar Magnetron Operated as an Ion Source

Full Paper

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Characterization of the Plasma Propertiesof a Reverse Polarity Planar MagnetronOperated as an Ion Source

Mukesh Ranjan,* Kishor K. Kalathiparambil, Naresh P. Vaghela,Subroto Mukherjee

In the present study the possibility of using a planarmagnetron configuration as an ion sourcehas been studied by inverting the polarity of a DC magnetron. The characterization of theplasma properties has been performed under various operating conditions using retardingpotential analyzer (RPA), Langmuir probe, and Faraday cup. The ion energy is 291 eV at 490 Vanode voltage and pressure of 5� 10�4 mbar measured using a RPA. Also the ion density is�1012 per cm3 as measured by Langmuir probe. Spread in energy is less (�3 eV) at lowerpressure and at higher anode voltage. This configuration behaves very well like an ion sourceand can be used for sputter cleaning of substrates and for ion beam-assisted deposition
applications.
Introduction

Ion sources have found various applications in the field

of surface cleaning, etching, sputtering, and ion beam-

assisted deposition.[1–3] Ion sources are also successfully

implemented in satellites in the form of plasma thrus-

ters.[4,5] Ions with a wide distribution of energies can be

produced using various ion sources. The broad energy

distribution and ease of operation make ion source

technology well suited to the surface modification of

materials. Ion sources are also used to produce nano-wires

and quantum dot on semiconductor surfaces by ion

sputtering.[6,7]

Today awide range of ion sources are available based on

different cathode and anode configurations, and various

types of power supplies (DC, RF, microwave) have been

used.[8] In most of the cases, magnets were also used to

increase the ionization efficiency. In the present study,

feasibility of using the DC planar magnetron concept as an

ion source has been investigated. It is simple to fabricate

and easy in operation. In a normal magnetron the target

(the electrode which is sputtered) is biased negative.

M. Ranjan, K. K. Kalathiparambil, N. P. Vaghela, S. MukherjeeFCIPT, Institute for Plasma Research, Sector-25 Gandhinagar,Gujarat, IndiaE-mail: [email protected]

Plasma Process. Polym. 2007, 4, S1030–S1035

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

However, in this investigation the target is biased positive

and hence called as anode. Paik and Kim,[9] Kim and

Lee,[10], and Oks[11] had also used the planar magnetron to

develop a negative ion source. They had used the

magnetron in the normal mode of operation, i.e., biasing

the target negative and not in reverse polarity as done in

the present investigation.

In the present investigation, the ion energy distribution,

current density, ion density, and the floating potential

confirm that planar magnetron biased inversely can also

be used as an ion source. The main advantage of this

system is that it can be used for sputter cleaning in one

mode and for thin film deposition in another mode.

Therefore, use of a different plasma source or supply for

sputter cleaning the surface can be avoided.

Experimental Part

Vacuum Chamber

The inverted polarity magnetron configuration was mounted on

the radial port of a 1 m diameter and 1 m length cylindrical

vacuum chamber (Figure 1). Using a 3 300 l � s�1 pumping

capability diffusion pump base pressure of 6�10�6 mbar could

be achieved. Retarding potential analyzer (RPA), Faraday cup, and

Langmuir probe could be simultaneouslymounted on the vacuum

DOI: 10.1002/ppap.200732323

Characterization of the Plasma Properties . . .

Figure 1. (a) Schematic diagram of the experimental chamber used. (b) Theion source in running condition.

chamber for diagnostics purposes. Argon gas was used for

experimental purposes.

Design of Ion Source

In the ion source a 76mm diameter Cu disk was used as an anode;

it was housed in a 120 mm diameter and 75 mm height hollow

grounded cathode with proper ceramic insulation between the

anode and cathode. The nearest gap between the anode and

cathode was kept fixed at 3.5 mm. Permanent ferrite magnets

with amagnetic field strength of 0.13 Twere set behind the anode.

A variable DC supply (1 kV/5 Amp) was connected between anode

and the grounded cathode. Argon gas was introduced through the

magnetron so as to have a system pressure of 0.2 mbar and to

create the discharge, bias in the range of 400–500 V was applied.

Initially, at this pressure a small glow was observed at the anode

center but at a lower pressure of 4�10�4 mbar this glow was

around 12 cm in length and covers the entire anode diameter. In

this configuration, discharge could be sustained at a pressure as

low as 2�10�4 mbar. Properties at various pressure and anode

voltages were studied.

Diagnostics Used

Retarding Potential Analyzer (RPA)

RPA was used for measuring the ion energy distribution. A design

mentioned in ref.[4,12,13] has been used with an extra electron-

retarding grid. The relationship between grid spacing y and the

potential difference between the grids V is given by[13]

y ¼ ldðeV=TeÞ3=4

Here ld is the Debye length and Te the electron temperature. The

Debye length to be used here is that corresponding to the plasma

density outside the electron repeller grid. Therefore, in order to

repel the electrons, V is generally few times Te and y� 4ld. For the

high-density plasma Debye length is short, and these conditions

become too difficult to satisfy. Therefore, very less transparency



grids are used to attenuate the plasma flux before it

encounters the repeller. It will increase ld. Transverse flow

of the ions is restricted by putting an entrance slit which

could allow ions to enter in a conical aperture (�208 to

þ208). The transparency of the analyzer after two meshes

was approximately 20%. The entrance aperture to the

device was at the apex of the grounded cone. The biased

ion retarding structure was a cylinder, closed on one end

except for a small hole, and closed on the other by a grid.

Ions which pass through this structure were collected by a

graphite collector.

Faraday Cup

Faraday cup was used for measuring the ion current

density (J) at a given location. It is basically a charge

collector plate, made of graphite. A guard ring was placed

around the collector to shield it from low energy ions

arriving from non-axial direction and also to reduce the

electrostatic edge effects. To minimize the edge effect around the

collector, a flat uniform sheathwas created by biasing the collector

and guard ring at the same potential.[4]

Langmuir Probe

A cylindrical Langmuir probe was used for measuring the floating

potential (Vf), plasma potential (Vp), electron temperature (Te), and

ion density (ni). Two similar probes were installed to scan the

properties axially as well as vertically from the anode surface

(Figure 1). Probe was made of tungsten wire of diameter 0.076 cm

and length 0.43 cm. Collisionless theory was used for the

calculations. The planar magnetron had a permanent magnet of

0.13 T behind the anode. Therefore, magnetic field intensity was

measured first in the region of probe measurement to avoid the

influence ofmagnetic field on probe. Themagnetic fieldwas found

to be 0.04 T at the anode surface (x¼ 0 for our vertical

measurement) and 5� 10�4 T at 5 cm distance from the anode.

Floating potential, plasma potential, and electron temperature

were calculated using I–V characteristics of the Langmuir probe. In

this study, the plasma potential was determined using the

derivative method. I–V characteristics goes through an inflection

at the plasma potential and the second derivative is equal to zero

at the plasma potential. The electron temperature was calculated

with the plot of semi-logarithmic electron value versus probe

potential by fitting a line in the electron retarding regime.

Results and Discussion

A very small probe diameter and probe axis perpendicular

to the field lines are recommended for use of Langmuir

probe in a weak magnetic field.[14–16] An extensive study

on the use of Langmuir probe in weak magnetic field

(0.05 T), especially in magnetron like device was done by

Passoth et al.[14] The following conclusions were made:

(i) For the cylindrical probe the influence of the magnetic

field is minimized when the probe axis is positioned

perpendicular to the field lines.

www.plasma-polymers.org S1031

M. Ranjan, K. K. Kalathiparambil, N. P. Vaghela, S. Mukherjee

Figure 3. Floating potential and plasma potential profile at a fixedanode voltage (525 V) and various pressure values, at 50 cmdistance from the anode.

Figure 2. Floating potential and plasma potential profile at var-ious anode voltages and fixed pressure of 5� 10�4 mbar, at 50 cmdistance from the anode.

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(ii) For small b<2 the reduction in electron current due to

the influence of

magnetic field does not exceed 20%. b is the ratio of

probe radius and mean gyroradius of the electron.

(iii) The ratio of magnetic field (B)/pressure (P) should not

be higher than several T mbar�1.

(iv) The influence of a weak magnetic field on the positive

ion current is negligible due to the bigger Larmor

radius of the positive ion.

As per point (i), Langmuir probe was kept perpendicular

to the field lines. Value of b turned out to be 0.29, much less

Figure 4. Floating potential (a) and electric field variation (b) versus distance fromthe anode at fixed anode voltage of 525 V and pressure of 5� 10�4 mbar.

than the recommended value. Also the B/P

ratio at 5 cm distance from the anode

turned out to be 1 and 0.05 for the pressures

of 5� 10�4 and 1� 10�2 mbar, respectively.

It is quite in agreement with point (iii) and

at a even higher distance of 5 cm this value

will be even lower. At the anode surface the

B/P ratio is a maximum (80) for 5�10�5 mbar pressure, since point (iv) states

about the negligible effect of the magnetic

field on the ions current. We have calcu-

lated the plasma density using the ion

saturation current. Therefore, under the

present experimental conditions the influ-

ence of the magnetic field on the probe can

be considered negligible.

Ion source was characterized keeping

two parameters as variables, namely anode

voltage and pressure. Floating potential (Vf)

and plasma potential (Vp) profiles at con-

stant pressure of 5� 10�4mbar and various

anode voltages are shown in Figure 2.

Plasma potential is �325 V at 400 V of



anode bias and becomes �365 V at anode bias of 600 V.

Figure 3 shows the decrease in floating as well as plasma

potential with increase in pressure from 5� 10�4 to 0.02

mbar at constant anode voltage of 525 V. Electron

temperature (Te) is found to be between 1.50 and

3.55 eV under the conditions mentioned in Figure 2

and 3. Most of the values of Vp–Vf also give the same Tevalue if calculated for the Ar ion. This information confirms

that electrons are following a Maxwellian distribution.

Floating potential reduces drastically with distance

from the anode [Figure 4(a)]. Floating potential is 430 V at 2

cm away from the anode and reduces to 380 V at 10 cm

DOI: 10.1002/ppap.200732323


Figure 5. Change in plasma density and electron temperature at various anode voltagesat a pressure of 5� 10�4 mbar, at a 50 cm distance from the anode.

distance. Floating potential becomes

constant at a distance of 60 cm from

the anode. Electric field variation is

shown in Figure 4(b) for the same

parameters. Assuming the electron

temperature does not vary appreciably

(�2 eV in the experiment as measured

by Langmuir probe) downstream of

device, the electric field then can be

estimated from the negative gradient of

the floating potential.[5] It can be

observed that the electric field in the

near field is fairly modest, but as

expected it decreases with increasing

down stream axial position.

In Figure 5 the electron temperature

and plasma densities are plotted for

various anode voltages. The ion density

is between 8� 1011 and 3� 1012 per cm3

and electron temperature between 1.50

and 3.55 eV.

Retarding potential analyzer results

are shown in Figure 6 and 7. Space

charge effect and secondary electron emission are the

issues which always remain in RPA design up to some

extent. These errors have been substantially minimized by

choosing a proper grid spacing and low transparency grids.

In RPA measurement, the entrance grounded grid remains

at the plasma potential. Therefore, the possibility of error

Figure 6. dn/dE at various anode voltages (a) and pressure values (b) at 50 cm distancefrom the anode, keeping pressure and anode voltage fixed.

in measurement due to plasma bound-

ary potential should also be considered.

There are a number of research papers

available which discuss about this

problem. Oksuz and Hershkowitz[17]

had experimentally verified the drop

in plasma potential in the plasma

boundary as predicted by Riemann[18]

in a theoretical model. They had con-

cluded that potential drop over the

presheath and transition sheath region

is given by Te/e and 2Te/3e, respec-

tively. Using these conclusions the

calculated drop in the present study

comes out to be 3.55 and 2.36 V for

3.55 eV electron temperature in pre-

sheath and transition sheath region. So,

percentage error in the energy mea-

surement is 0.8% or lesser. Such a small

error can be neglected in the case of

highly energetic ions as in our experi-

ment. But these effects of drop in

plasma potential near the plasma

boundary cannot be ignored when

ion energy is below 50 eV.



The ion energy distribution is shown for various anode

voltages and pressure values in Figure 6(a) and 6(b).

Figure 6(a) clearly indicates that the ion energy increases

with an increase in anode voltage and the distribution

becomes sharper for a given pressure. This means that the


M. Ranjan, K. K. Kalathiparambil, N. P. Vaghela, S. Mukherjee

Figure 7. FWHM and peak ion energy at various anode voltages (a) and various pressuresvalues (b), at 50 cm distance from the anode, keeping pressure and anode voltage fixed,respectively.

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energy spread is less at higher voltage. Peak ion energy

reduces at a higher pressure as shown in Figure 6(b).

Energy spread is also higher at higher pressure. FWHM

reduces with increase in anode potential and increases

with increase in pressure as shown in Figure 7(a) and 7(b).

Variation in peak ion energy is also plotted for various

Figure 8. Current density variation at various pressures (a) and anode voltages (b), at 50cm distance from the anode, as measured by Faraday cup.



pressure and anode voltages in

Figure 7(a) and 7(b), respectively. At

higher pressure, ions encountered more

collisions with neutrals and therefore a

decrease in ion energy peak can be seen

in Figure 7(b).

The ion current density was calcu-

lated using the Faraday cup. At 525 V

anode voltage and 5� 10�4 mbar pres-

sure, current density is found to be 0.26

mA � cm�2. Initially, ion saturation was

achieved and maximum current den-

sity is at the center (below anode), and

then reduces on both sides from 0.26 to

0.1 mA � cm�2 at a distance of 70 cm

away from anode. The current density is

plotted at various anode voltage and

pressures values in Figure 8(a) and 8(b),

respectively. Current density increases

substantially with increase in pressure

or anode voltage.

Conclusion

The functioning of the ion source can be understood like

this: electrons are accelerated to the anode and get

confined by the B field. The E�B drift is strong enough

to trap the electrons near to the anode surface and causes

more ionization. The working of the ion

source is based on the glow discharge

between an anode and the grounded

cathode. Below the anode surface

permanent magnets are located; in

front of the anode surface the magnetic

field creates a zone (‘‘magnetic trap’’)

where the electron loss rate is reduced

and the ionization probability is

enhanced. The ensuing ions, having

Larmor radii much larger than the

magnetic trap size, are accelerated by

the electric field away from the anode.

The process is strongly dependent on the

magnetic field and plasma parameters

such as ion flux, plasma potential,

electron temperature, and density.

Experimental observations reveal

that a planar magnetron configuration

can be used as an ion source by

inverting the polarity. It was found

that this ion source works over a large

pressure range of 2� 10�4–0.02 mbar

at 3.5 mm gap of anode to cathode. Best

DOI: 10.1002/ppap.200732323


operating conditions are found to be at high anode voltage

and lower pressure. Under such conditions, ion energy, ion

density increase and spread in energy are less. Maximum

ion energy was found to be 291 eV at 490 V anode voltage

and 5� 10�4 mbar operating pressure with Ar. Typical

plasma density was found to be of the order of 2� 1012

per cm3 at 500 V anode voltage. Current density is also

sufficiently high 0.26 mA � cm�2 at 50 cm away from the

anode at 525 V anode voltage and 5� 10�4 mbar pressure.

The results indicate that reverse polarity planar magne-

tron can be used as an ion source.

Received: September 10, 2006; Revised: November 9, 2006;Accepted: November 30, 2006; DOI: 10.1002/ppap.200732323

Keywords: DC discharge; density; distribution; Faraday Cup;IEDF; ion source; Langmuir probe; magnetron; RPA

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Characterization of the Plasma Properties of a Reverse Polarity Planar Magnetron Operated as an Ion Source