Download Study of Ion Beam Sputtering using a Glow Discharge Ion Source

26
M.M. Abdelrahman
Study of Ion Beam Sputtering using a Glow Discharge Ion Source
M.M. Abdelrahman
Accelerators & Ion Sources Department, Nuclear Research Center,
Atomic Energy Authority, P.O. Box: 13759 Inchas, Cairo, Egypt.
(Received on 25 July, 2009)
In this work, sputtering yield in a glow discharge ion source system has been determined using the operating
parameters of the ion source. The sputtering yield is found to be varied between 0.4 to 1 atoms removed per
incident ion for nitrogen while for argon between 0.2 to 1.3 atoms removed per incident ion. The feature of this
ion source is high output ion beam current and small size. Operation of the ion source is quite simple since a
stable discharge can be obtained within a large range of main parameters such as, discharge voltage, discharge
current and gas pressure. Also, beam profile for argon ion beam produced from the glow discharge ion source
at Id = 2, 3 mA (discharge current) using argon gas with different gas pressures has been investigated.
Keywords: Glow discharge ion source, sputtering yield, argon gas pressure, plasma production.
INTRODUCTION
Sputtering is a process whereby atoms are ejected from a
solid target material due to bombardment of the target by
energetic ions. It is commonly used for thin-film deposition, etching and analytical techniques. Physical sputtering is
driven by momentum exchange between the ions and atoms
in the material, due to collisions [1]. The average number of
atoms ejected from the target per incident ion is called the
sputter yield and depends on the ion incident angle, the energy of the ion, the masses of the ion and target atoms, and
the surface binding energy of atoms in the target. The incident ions set off collision cascades in the target, when such
cascades recoil and reach the target surface with an energy
above the surface binding energy, an atom can be ejected.
When a target is bombarded with fast heavy particles, erosion of the target material occurs: this is termed sputtering
[2-7]. The glow discharge in sputtering is significantly dependent on the work function of the target material and pressure of the environmental gas [8]. Sputtered atoms ejected
into the gas phase are not in their thermodynamic equilibrium state, and tend to deposit on all surfaces in the vacuum
chamber. A substrate (such as a wafer) placed in the chamber
will be coated with a thin film. Sputtering usually uses argon
plasma. In the present work, we have studied the profile of
the argon beam produced from the glow discharge ion source
and determined the sputtering yield with the aid of the different operational parameters using argon and nitrogen gases at
the same gas pressure.
cm from the ion-exit aperture of the cathode and used to measure the output ion beam current. The working gas is admitted to the ion source through a hose fixed in a Perspex flange
at the upper side of the anode. A 10 kV power supply is used
for initiating the discharge (glow discharge) between the anode and the cathode.
FIG. 1: Schematic diagram of the ion source and its associated electrical circuit [9].
EXPERIMENTAL SETUP
A schematic diagram of the glow discharge ion source and
its associated electrical circuit is shown in Fig. 1. This glow
discharge ion source has been described before [9]. It consists of aluminium Pierce shape anode with small aperture to
confine the discharge and aluminium plane cathode. The anode has an internal diameter of 28 mm at the upper side and
10 mm diameter at the lower side and its length 17 mm. The
aluminium cathode has an aperture of diameter 1.5 mm and
length 5 mm. Both the Pierce anode and the plane cathode
are immersed in an insulator of Perspex material.
The collector (Faraday cup) is situated at a distance of 5
A complete vacuum system is used to evacuate the ion
source chamber. It consists of stainless steel silicon oil diffusion pump provided with electrical heater and backed by
rotary fore vacuum pump. The rotary pump is used to evacuate the system with ultimate pressure of 10−2 to 10−3 mmHg,
while the silicon oil diffusion pump is used to yield very low
pressure in the ion source vacuum chamber of the order of
10−5 mmHg. The silicon diffusion pump is surrounded by
water tubes for cooling during the operation. A liquid nitrogen trap is fixed between the ion source chamber and the silicon oil diffusion pump in order to prevent the silicon vapour
from entering the ion source chamber. The working gas is
27
Brazilian Journal of Physics, vol. 40, no. 1, March, 2010
transmitted to the ion source from a gas cylinder through a
needle valve to regulate the rate of gas flow.
240
chamber. The working gas is transmitted to the ion source from a gas cylinder
through
a needle valve1
-4
1 P = 4.0 x 10 mmHg
to regulate the rate of gas flow.
210
EXPERIMENTAL RESULTS
EXPERIMENTAL RESULTS
180
-4
2 P = 5.3 x 10 mmHg
-4
3 P = 6.8 x 10 mmHg
2
3
Ib (P A)
In this work, the distance between the anode and the cathode, dA−K is fixed at the optimum value [9] for stable discharge current
high output
ion beam
current.the
The
dis- and the cathode,
150
In and
this work,
the distance
between
anode
dA-K is fixed at the optimum value
charge characteristics of the glow discharge ion source such
as, discharge
voltage,
discharge
currentand
andhigh
output
ion beam
discharge
current
output
ion beam current. The discharge characteristics of the glow
[9] for stable
120
current are investigated using nitrogen and argon gases. Also
discharge
ion
source
such
as,
discharge
voltage,
discharge
current
and output ion beam current are investigated
the sputtering yield is determined using the experimental resultsusing
for nitrogen
and
argon
gases
on stainless
material
nitrogen
and
argon
gases.
Also thesteel
sputtering
yield has been90determined using the experimental results
(target).
for nitrogen and argon gases on Stainless steel material (target).
60
1- Ion Source Properties
1- Ion Source Properties
30
Ib (mA)
Id (mA)
Figure 2 shows the relation between the discharge cur2 showsvoltage,
the relation
current, Id, and the discharge voltage, Vd, at different
rent, Id , and Figure
the discharge
Vd , atbetween
differentthe
gasdischarge
pressures,
using nitrogen
gas.nitrogen
It is clear
increase
of increase of 0the discharge voltage was accompanied
It isan
clear
that an
gasP,pressures,
P, using
gas. that
0
0.4
0.8
1.2
1.6
2
the discharge voltage was accompanied by an increase of the
by ancurrent,
increase
of the
current, voltage;
where atthe
the same discharge voltage; the discharge
current is higher
discharge
where
at discharge
the same discharge
Id (mA)
discharge current is higher at high pressure than that at low
at high
pressure
thanthe
thatrelation
at lowbetween
pressure.the
Figure
shows the relation between the output ion beam current,
pressure.
Figure
3 shows
output3 ion
FIG. 3: Output ion beam current versus discharge current at differbeamIb,current,
and the discharge
Id , at different
niand theIb ,discharge
current,current,
Id, at different
gas pressures,
P, using nitrogen gas. It is obvious that, when the
ent pressures using nitrogen gas.
trogen gas pressures, P. It is obvious that, when the discharge
discharge
current
increases,
the output
beam and
current increases and high output ion beam current was
current
increases,
the output
ion beam
current ion
increases
highobtained
output ion
beam
current
was
obtained
at
low
pressures
at low pressures than that at high pressures.
was accompanied by an increase of the discharge current,
than that at high pressures.
where at the same discharge voltage; the discharge current is
higher
at high pressure than that at low pressure.
240
2.2
Figure
5 shows the
-4
-4 relation between the output ion beam
1 P = 4.0 x 10 mmHg
1 P = 4.0 x 10 mmHg
1
-4
-4 discharge current, I , at different gas
current,
I
,
and
the
b
d
mmHg
2
P
=
5.3
x
10
mmHg
2
P
=
5.3
x
10
2
210
-4
-4
3 P = 6.8 x 10 mmHg
3 PP,= 6.8
x 10argon
mmHg
3
2
1
pressures,
using
gas. It is obvious that, when the
1.8
discharge current increases, the output 2ion beam current in180
creases
and high output ion beam current was obtained at low
1.6
pressures than that at high pressures. 3
150
Fig. 6 shows beam profile for argon ion beam produced
1.4
from the glow discharge ion source at Id = 2 mA using ar1.2
120 gas. It is clear that the maximum value of the output
gon
ion beam is obtained at the center of the target at pressure
1
−4 mmHg and it decreases by increasing the
equal
90 to 8 × 10
0.8
pressure.
Fig. 7 shows beam profile for argon ion beam produced
60
0.6
from the glow discharge ion source at Id = 3 mA using argon gas. It is clear that the curves have the same behaviour
0.4
30
as shown in Fig. 6 but the maximum value of the output ion
0.2
beam increases by increasing the discharge current. Figs 6
and0 7 show the beam profile for argon gas at low voltage
0
0
0.8
1.2
1.6
2
(1.8 kV,
2.050.4
kV, respectively)
and high
pressure
(1.1 ×10−3
0
1
2
3
4
5
−4
mmHg and 9 ×10 ImmHg,
d (mA) respectively). It is clear that
Vd (KV)
, the beam profile is uniform distribution which is required
in many field of industrial applications as, cleaning of the
Fig.2: Discharge current versus discharge
Fig.3: Output ion beam current versus
materials, sputtering etc..(Broad Beam Surface). Figs 6 and
FIG. 2: Discharge current versus discharge voltage at different presvoltage at different pressures using
discharge current at different
sures using nitrogen gas.
7 show beam profile at high voltage (3 kV and 3.5 kV, renitrogen gas.
pressures using nitrogen gas.
spectively) and low pressure (8 ×10−4 mmHg). It was found
that, a peak appeared which is suitable for a thin focused ion
Figure 4 shows the relation between the discharge current,
beam and is required for ion beam applications of a narrow
Id , and the discharge voltage, Vd , at different argon gas presarea surface. The increase of the extraction voltage (accelersures P. It is clear that an increase of the discharge voltage
28
M.M. Abdelrahman
1807
1.8
-4
at I = 2 mA, V = 3 kV, P = 8x10 mmHg
d
-4
1.6
3
1.4
1405
1.2
120
1
clear that an increase of the discharge voltage was accompanied
0.8
nt, where at the same
discharge voltage; the discharge current is higher
Ib (µ A)
Id (mA)
een the discharge current, Id, and the discharge voltage, Vd, at different
d
4
1
at I = 2 mA, V = 2.05 kV,
d
d
2
-4
P = 9x10 mmHg
3
r
100
2
80
at I = 2 mA, V = 1.8 kV,
1
.
r
-4
1 P = 4.0 x 10 mmHg
-4
2 P = 5.3 x 10 mmHg
-4
3 P = 6.8 x 10 mmHg
1606
1
2
Ion beam current (arbitrary unit)
1 P = 4.0 x 10 mmHg
-4
2 P = 5.3 x 10 mmHg
-4
3 P = 6.8 x 10 mmHg
0.6
d
60
3
d
-3
P = 1.1x10 mmHg
r
0
-0.8
0.4
40
een the output ion beam current, Ib , and the discharge current, Id ,
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
Distance (mm)
0.2
20
n gas. It is obvious that, when the discharge current increases, the FIG. 6: Beam profile for argon ion beam produced from the ion
0
gh output ion beam
source 0at Id = 2 mA.
0
1
current
was
2
3
obtained
at
low
4
5
pressures
than 6that
at
0
10
Vd (KV)
5
voltage at different pressures using
argon gas.
-4
at
I160
d =
d
r
8
Fig.5: Output ion beam current versus
discharge current at different
6
pressures
gas.
at I using
= 3 mA, argon
V = 2.45 kV,
d
d
-4
P = 9x10 mmHg
4
r
Fig.6 shows beam profile for argon ion beam produced from the glow discharge ion source
180
1
1.2 1.4 1.6 1.8
Id kV,
(mA)
at I = 3 mA, V = 3.5
P = 8x10 mmHg
d
Ion beam current (arbitrary unit)
FIG. 4: Discharge current versus discharge voltage at different pressures
using argon
gas.
Fig.4:
Discharge
current versus discharge
1
-4
at nitrogen is more easily to ionize than argon, where nitrogen has two
.
0.2 0.4 0.6 0.8
1 P = 4.0 x 10 mmHg
-4
2 P = 5.3 x 10 mmHg
-4
3 P = 6.8 x 10 mmHg
2
2 mA using argon gas. It is clear that the maximum
value of the
output ion beam is obtained at
at I = 3 mA, v = 2.05 kV,
1
d
d
the140
center of the target at pressure equal to 8 x 10-4 mbar0and it decreases
by increasing
the pressure.
P = 1.1x10
mmHg
-3
r
-0.8
-0.6
-0.4
120
Ib (µ A)
0
0.2
0.4
0.6
0.8
Distance (mm)
2
100
-0.2
FIG. 7: Beam profile for argon ion beam produced from the ion
source at Id = 3 mA.
80
3
60
2 -Determination of the Sputtering Yield
40
The rate of sputtering process can be calculated from simple considerations of the volume removed per atom. In particular, the thickness removed per second by ion beam sputtering given by the expression [11]:
20
0
6
charge
es using
0
0.2 0.4 0.6 0.8
1
1.2 1.4 1.6 1.8
Id (mA)
FIG. 5: Output ion beam current versus discharge current at differFig.5:
Output
ent pressures
using argon
gas. ion beam current versus
discharge current at different
pressures using argon gas.
or argon ion
beam
produced
from
glow
discharge
ion source
ation
voltage)
decreases
the the
width
of the
ion beam [10].
ear that the maximum value of the output ion beam is obtained at
ual to 8 x 10-4 mbar and it decreases by increasing the pressure.
δ = 1.66 × 10−24 .
MS Ji
.
ρ
e
(1)
where M is the atomic mass of the target atoms, ρ is the density of the sputtered material, e is the charge of the electron,
ji is the ion beam current density at the target and S is the
sputtering yield (atoms removed per incident ion) given by
the relation:
S=
M1 lnW 1
M2 W cos θ
(2)
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Brazilian Journal of Physics, vol. 40, no. 1, March, 2010
where M1 is the mass of the incident ion, M2 is the mass of
the target particle and W is the energy of the incident ion and
θ is the angle between the incident beam and the target. By
substituting eq. 2 in eq. 1, and using the experimental results,
the rate of sputtering can be calculated, where in our case
θ = 0, so cos θ = 1. Figs. 8 and 9 show the relation between
the current density, J, in A/cm2 and the rate of sputtering, δ,
in µm/min. at constant pressure, P = 4 × 0−4 mmHg, using
pure nitrogen and argon gases. It is clear that at the same
current density, the rate of sputtering in case of argon gas is
higher than that for nitrogen gas at the same pressure.
1.1
rate of sputtering. N gas,
2
rate of sputtering, µmeter/minutes
1
-4
gas Pressure = 4x10 mmHg
0.9
0.8
0.7
0.6
0.5
0.4
0.3
5
10
15
20
25
30
35
40
45
2
J (A/cm )
FIG. 8: Relation between the current density, J ,in A/cm2 and the
rate of sputtering, δ, in µm/min. at p = 4×10−4 mmHg for nitrogen
gas on stainless steel material (target).
CONCLUSION
rate of sputtering, µmeter/minutes
1.4
1.2
Rate of sputtering, argon gas,
gas pressure = 4x10-4 mmHg
1
0.8
0.6
0.4
0.2
0
5
10
15
20
25
30
2
J (A/cm )
FIG. 9: Relation between the current density, J ,in A/cm2 and the
rate of sputtering, δ, in µm/min. at p = 4 × 10 − 4 mmHg for argon
gas on stainless steel material (target).
[1] R.Behrisch and W.Eckstein; Sputtering by Particle bombardment: Experiments and Computer Calculations from Threshold to Mev Energies. Springer, Berlin (eds.) (2007).
[2] F. Aumayr and HP. Winter, Phil. Trans. R. Soc. Lond. A 362,
77–102 (2004).
[3] J. F. Ziegler, J. Appl. Phys, 85, 1249-1272 (1999).
[4] H. Gnaser, Low-Energy Ion Irradiation of Solid Surfaces,
Springer- Verlag Berlin Heidelberg, Germany, 1999.
[5] Tatsuya Banno et al, Vacuum, 80, 667–670 (2006).
[6] S.Han et al, Vacuum, 78, 539–543 (2005).
[7] L. He et al, Nanotechnology, 19, 445610 (2008).
In this study, sputtering yield in a glow gas discharge ion
source system has been determined using the operating parameters of this ion source. The main features of this ion
source are high output ion beam current and small size. Operation of this ion source is quite simple since a stable discharge can be obtained within a large range of main parameters such as, discharge voltage, discharge current and gas
pressure. The sputtering yield is found to be varied between
0.4 to 1 atoms removed per incident ion for nitrogen while
for argon between 0.2 to 1.3 atoms removed per incident ion.
Also, beam profile for argon ion beam produced from the
glow discharge ion source at Id = 2, 3 mA (discharge current) using argon gas with different gas pressures has been
investigated. Indeed, it is found also that, at the same current
density, the rate of sputtering in case of argon gas is higher
than that for pure nitrogen gas. This ion source can be used
for etching, sputtering and micro-machining applications.
[8] K. Hine et al, The Second International Symposium on
Atomic Technology IOP Publishing, Journal of Physics: Conference Series 106,012019 (2008).
[9] M.M.Abdel Rahman, A.G.Helal, O.A.Mostafa and F.W.Abdel
Salam,, Journal of Nuclear and Radiation Physics, Vol. 3, No.
1, (2008).
[10] S.G. Zakhary, Rev. Sci. Instrum. 66, 12, December (1996).
[11] H.L.Garvin, Paper presented at KODAK Microelectronics
Seminar, San Diego, Calefornia, Dec.11-12 (1972).