In this work, thin films of tin oxide doped with fluorine and of nominal surface resistance between 30 Ω and 40 Ω were deposited by RF sputtering in a plasma deposition system and their surface electrical resistances were evaluated for different conditions. To measure the resistivity of the fluorine-doped tin oxide film using an aluminum PAD with the same thickness and the same width as the film, a Minipa digital multimeter acting as an ammeter, a DC voltage source, and wires with banana-alligator connectors were used at six intervals (between the aluminum measuring tips and the PADs). Thus, a linear approximation and verification of the resistance value for each measured distance were undertaken. Resistances ranging from 44.10 Ω/ to 88.10 Ω/ for separations of 6 and 1 cm, respectively, were found. With the values obtained, the respective graphs were plotted for the six separations. The four-point method was employed to obtain four measurements (M1 to M4), and the curves of voltage as a function of the current were obtained. Values of resistance ranged from 11.4 Ω (M4) to 29.1 Ω (M3).

1. ISSN (online) 2583-455X
BOHR International Journal of Computer Science
2022, Vol. 2, No. 1, pp. 124–129
https://doi.org/10.54646/bijcs.018
www.bohrpub.com
Measurement of the Surface Electrical Resistance of SnO2:F
Thin Films
C. S. de Souza, J. R. R. Bortoleto, P. L. Sant’Ana∗ and S. F. Durrant
Technological Plasmas Laboratory, State University of Sao Paulo – UNESP, Sorocaba City,
Sao Paulo State, Brazil
∗Corresponding author: drsantanapl@gmail.com
Abstract. In this work, thin films of tin oxide doped with fluorine and of nominal surface resistance between 30 Ω
and 40 Ω were deposited by RF sputtering in a plasma deposition system and their surface electrical resistances
were evaluated for different conditions. To measure the resistivity of the fluorine-doped tin oxide film using an
aluminum PAD with the same thickness and the same width as the film, a Minipa digital multimeter acting as an
ammeter, a DC voltage source, and wires with banana-alligator connectors were used at six intervals (between the
aluminum measuring tips and the PADs). Thus, a linear approximation and verification of the resistance value for
each measured distance were undertaken. Resistances ranging from 44.10 Ω/ to 88.10 Ω/ for separations of 6 and
1 cm, respectively, were found. With the values obtained, the respective graphs were plotted for the six separations.
The four-point method was employed to obtain four measurements (M1 to M4), and the curves of voltage as a
function of the current were obtained. Values of resistance ranged from 11.4 Ω (M4) to 29.1 Ω (M3).
Keywords: RF sputtering, tin oxide thin films, surface electrical resistance.
INTRODUCTION
Sputtering began to be used in the early 1970s with the
use of radiofrequency sources. The magnetron sputtering
technique consists of the use of a magnet assembly, placed
strategically in relation to the cathode to generate a mag-
netic field with a closed path. The magnetron RF sputtering
technique presents advantages such as good adhesion of
the deposited films to the substrates; density of deposited
films equivalent to the substrate density; and deposition
of films from insulating, refractory, or multicomponent tar-
gets. The magnetic field functions as an electron trap near
the surface of the target in the plasma regime. Electrons
are confined in the magnetic field, causing the plasma
density to increase close to the target surface, thereby
increasing the concentration of ions of the inert gas and
providing a much more effective and localized bombard-
ment [1]. Since RF sputtering is very useful for deposition
of oxide thin films in plasma reactors, this kind of film
finds many applications in the energy and microelectronics
industries, such as the frontal contacts for solar cells, liquid
crystal dials, image sensors, OLEDs, and others. In fact,
at present, numerous applications, such as touch panels,
light-emitting diodes, and solar cells, require transparent,
conductive coatings [2–4]. Thus far, materials belonging to
the transparent conducting oxide (TCO) family have been
frequently used for this purpose. Most of the industry stan-
dard TCOs are n-type wide bandgap oxides (e.g., 3.1 eV),
such as In2O3, SnO2, and ZnO, whose conductivity can be
further tuned by aliovalent doping or the formation of oxy-
gen vacancies [5]. More specifically, tin oxide-doped thin
films are used in photo-converters, efficient electrodes for
the transport of free charges, in transparent thin film tran-
sistors, gas sensors, and the conductivity of SnO2 is highly
dependent on the pressure of some gaseous compounds
on its surface and in optical sensors [6]. In most cases,
electrical measurements represent the key to the analysis of
a wide range of semiconductor transport properties based
on oxide thin films. In fact, among these classes of films,
tin oxide thin films present a wide energy gap, ≈3.6 eV;
they have good electrical conductivity, good transparency
(average over 80% of the visible spectrum), reflect infrared
radiation, are chemically inert, and show good adhesion to
glass, and their more common dopants are Sb (antimony),
124

2. Electrical Surface Resistance SnO2:F Films 125
F (fluorine), Zn (zinc), or In (indium), which decrease the
resistivity of the films (from 10−3 to up to 10−4 Ω·cm) and
increase the reflection coefficient in the infrared region [7].
METHODS
Initially, glass substrates of 1.5 cm × 2.5 cm were ultrason-
icated three times for 20 min using pure water, industrial
detergent “Detlimp,” and isopropyl alcohol, respectively.
Then, the glass was positioned in the center of the upper
electrode in an RF sputtering deposition system. The exper-
imental setup consists of a stainless-steel vacuum chamber
(30 cm in height and 25 cm in diameter) with two horizon-
tal circular internal electrodes of equal dimension (11 cm
in diameter), and the system was evacuated by a rotary
pump (18 m3/h) down to 0.1 Pa. A tin target of 5 cm
in diameter was fixed inside the reactor. Needle valves
were employed to control the oxygen gas feed (of high
purity: up to 99.9995%) at 50 sccm. Glass substrates were
exposed directly to the plasma environment established
by the application of radiofrequency power (13.56 MHz)
at 100 W for 30 min. Figure 1 shows the principle of the
RF sputtering system used for tin oxide thin film deposi-
tion [1] with the permission of Scholar’s Press.
To measure the resistivity of the fluorine-doped tin oxide
film using an aluminum PAD with the same thickness and
width as the film, a Minipa digital multimeter was used
as an ammeter together with a DC voltage source and
wires with banana-alligator connectors. After the circuit of
Figure 2(a) was assembled, measurements were taken from
six different points on the film, using the distance between
the aluminum measuring tips and the PADs as a reference.
Then, the respective graph (V-I) was plotted for the six
distances of the PADs. Moreover, for the four parallel tips
method, two digital multimeters were used, one as an
ammeter and the other as a voltmeter simultaneously, a DC
voltage source, a perforated copper plate connected to the
wires, and the scheme of the measurements can be seen in
Figure 2(b).
Figure 1. Scheme of tin oxide thin film deposition method by
Magnetron Sputtering [1].
Figure 2. (a) Scheme for measuring surface resistance using PADs.
Rc indicates the resistance of the contacts (PAD, cables, and
multimeter), and surface resistance Rs = R W/L. (b) Four parallel
tips method: For a film of infinite width and length L with respect
to distances Sn, it is possible to calculate the resistivity ρ. Courtesy
of Dr. Jose Roberto R. Bortoleto: Technological Plasmas Laboratory,
State University of Sao Paulo, UNESP, Brazil.
RESULTS AND DISCUSSIONS
Table 1 presents the voltage and current values of the six
distances of the tips for the test with the fluorine-doped tin
oxide film. From the data of Table 1, the respective graphs
(V-I) were plotted for the six distances of the PADs as seen
in Figure 3.
Linear relationships were observed for all distances. As
the distance increases, the slope of the lines increases,
which indicates that for higher values of distance, the
surface resistance tends to decrease. From Figure 3, a linear
approximation can be obtained and the resistance value for
each measured distance can be evaluated. Table 2 presents
the parameters and surface resistance for each distance of
the PADs.
Table 1. Fluorine-doped tin oxide film voltage and current data
obtained from measurements using PADs.
V (V) I1 – 6.1 cm I2 – 5 cm I3 – 4 cm I4 – 3 cm I5 – 2 cm I6 – 1 cm
0.5 4.7 4.7 5.7 6.5 8.2 12.2
0.8 7.4 8.8 10.5 11.7 15 18.9
1 9.8 11.3 13.3 16.4 20.2 24.7
1.3 12.4 15.7 18 20.4 26.4 33.6
1.5 14.3 17.6 20.4 23.9 31.1 39.1
1.7 17.1 20.2 23.3 27.9 35.4 44.7
2 19.7 23.9 28.1 33.5 42 52.9
2.3 23.1 27.6 31.9 37.5 47.3 60.9
2.5 25.3 30.7 34.9 41.1 51.7 69
2.8 28.1 34 39.1 46.6 58.7 76.6
3 29.6 36.1 41.1 49 62.9 83.1
3.4 34.2 40.7 46.4 56.5 71.2 94.6
3.7 36.6 44.8 50.3 62 78.8 104.1
4 38.9 49.1 54.5 66.3 84 111.5
4.5 45 55.6 63.7 74.2 96.1 126.7
5 50.8 61.4 71.1 84.7 105.1 139.4
5.5 56.6 66.9 78.9 93.8 118.5 153.2
6 59.6 73.1 86 101.9 130.8 165.7

3. 126 C. S. de Souza et al.
Figure 3. Linear V vs. I curves of the six measured distances for
the tin oxide film:F.
Table 2. Values of the parameters of the linear approximations
of the curves in graph from Figure 3, as well as their respective
surface resistances.
Linear Approximation Results
Y = A + BX - V = A + B.I w = 2.5 cm
Error Error B Error RS
L A (V) A (V) B (Ω × 10−3) (Ω × 10−3) RS (Ω/) (Ω/)
1 cm 0.09 0.019 0.03 2.15 E-04 88.10 0.54
2 cm 0.09 0.019 0.05 2.87 E-04 57.43 0.36
3 cm 0.08 0.019 0.06 3.49 E-04 48.63 0.30
4 cm 0.08 0.025 0.07 5.70 E-04 43.55 0.35
5 cm 0.06 0.015 0.08 3.89 E-04 40.40 0.20
6 cm 0.05 0.024 0.10 7.44 E-04 41.09 0.31
The width of the film was about 2.5 cm, and, for the lin-
ear approximations, the linear coefficient A was neglected
owing to its low value and high error. In addition, it is
worth mentioning that to find the values of surface resis-
tances, as well as their respective errors, Equation (1) was
used, and this relation was deduced from the equation in
Figure 2(a) and elucidated as a function of the dimensions
W and L.
RS =
ρ
t
= R ·
W
L
(1)
Moreover, concerning the surface resistance results using
the four-point method in Figure 2(b), the data-reliable
results from four measurements were performed, one at
each point of the film with different distances between the
tips. The first and second measurements (M1) and (M2)
were made in the center of the film with the connectors on
the edge. The distances between connectors and edges of
the film are given in Table 3. The third measurement (M3)
was undertaken on top of the film edge, while the fourth
measurement (M4) was made on the left side of the film.
The distances between the connectors and the edges of the
film are also given in Table 3.
Table 3. Distances between the connectors and the edges of the
film for the four measurements (M1 to M4). Top and bottom
represents superior and inferior parts of the film, respectively.
Top Bottom Right Left
S1 (m) S2 (m) S3 (m) (m) (m) Side (m) Side (m)
M1 0.023 0.02 0.020 0.011 0.014 0 0
M2 0.010 0.01 0.007 0.011 0.014 0.02 0.02
M3 0.024 0.02 0.02 0 0.025 0 0
M4 0.007 0.007 0.01 0 0 0.06 0
Figure 4. Measurements of voltage V (mV) as function of current
I (mA) for the four attempts (M1 to M4) using the four-point
parallel method.
Table 4. Parameters of V and R of linear approximations of the
plots in the graphs of Figure 4 with their error.
Y = A + B.X - V = A + B.I
Measurements A (mV) Error A (mV) B (Ω) Error B (Ω)
M1 0.234 0.025 24.42 0.008
M2 0.7694 0.059 13.03 0.012
M3 0.1983 0.041 29.12 0.018
M4 0.49738 0.026 11.40 0.007
Then, it was possible to obtain the curves (V-I) for
the four measurements (M1 to M4) using the four-point
parallel method, as seen in the graph in Figure 4.
The plots are linear to about 1800 mV (M1) and to a
current of ∼95 mA (M2). The slopes of the lines were
different for each measurement, and condition M4 was
resulted from the lower values of S1, S2, and S3. For
greater values of S, the inclinations increase; this indicates
that for greater values of distance, the surface resistance
changes. From Figure 4, it was possible to calculate the
linear approximations of the curves of each measurement
and thus obtain the intercepts and slopes. Table 4 presents
the parameters of the linear approximations of the plots in
the graph in Figure 4.
With the values of B (V/I) of the linear approximations,
it is possible to determine the resistivity of the film for the

4. Electrical Surface Resistance SnO2:F Films 127
Table 5. Parameters of distances and its relationship between
these distances and the thin film measurements, the resistivities,
and its respective error for the four measurements.
S̄ (m) S/w L/S C RS (Ω/) Error RS (Ω/)
M1 0.021 0.820 3.000 1.214 29.65 0.009
M2 0.010 0.400 6.142 2.351 30.65 0.029
M3 0.023 0.911 2.700 1.089 31.73 0.019
M4 0.007 0.289 8.516 2.968 33.84 0.020
four measurements. Table 5 presents the mean values of
the distances between the tips, the relationships between
these distances and the thin film measurements, and the
resistivities and their respective errors for each of the
measurement procedures.
The surface resistance of the film ranged from 29.6 to 33.8
Ω/, which corresponds to a nominal surface resistance
of 30–40 Ω/. A small difference between the practical
and nominal values can be attributed to the imprecision in
measuring the distances. The length (L) and width (w) of
the thin film tested were 63 mm and 26 mm, respectively,
and the distances between the measuring tips approxi-
mated the average distance. The values of the geometric
correction factor, C, and resistivity were obtained through
Equations (2) and (3), respectively.
C =
π
(
πs
w
+ ln
1 − e− 4πs
w
− ln
1 − e− 2πs
w
+
e−
2π( L
s −2)s
w
1−e− 6πs
w
1−e− 2πs
w
1+e− 2πL
w
!# )
(2)
RS =
ρ
t
=
V
I
· C (3)
Making the distances Sn equal to S and taking into
account the geometric correction factor, for a rectangular
sample with length L, width w, and thickness t, we have
the following simplification (Equation (4)) to obtain the
electrical resistivity (Ω cm) of the film.
ρ =
V
I
· C · t (4)
TCOs are materials that display both high transparency
(80%) in the visible spectrum and low electrical resistivity
(10−3 Ω·cm). The increase in carrier concentration with
the amount of dopants results in a reduction in the resis-
tivity [8]. This is important when this film is applied to
resistive or capacitive touchscreen panels. An example of
such a panel is shown in Figure 5.
In addition, TCO serves as an electron pathway, which
connects the films with an external circuit and determines
the overall transmittance of the devices [10]. Moreover, the
TCO can be used for photocatalysis applications owing
to its advantageous electronic properties and conductivity
[11–14]. Typical TCO materials have been studied owing
Figure 5. Scheme of application of (a) resistive touchscreen panel
and (b) capacitive touchscreen panel. In both systems, a fluorinated
tin oxide film can be applied over a glass (rigid) substrate.
Touchscreens are prominently employed in smartphones, personal
computers, televisions, and car navigation due to their compact-
ness, reliability, and low power consumption. Reproduced from
Ref. [9] with permission from Caio Simons.
to their superior visible light transmittance (80%), low
resistivity (10−3 Ω cm), and relatively large band gap
(3 eV) [15, 16]. Fluorinated tin oxide was recently pro-
posed as a substitute material for indium tin oxide owing
to its low cost and excellent mechanical and chemical dura-
bility. To achieve high transmittance while maintaining the
electrical conductivity of tin oxide films, various methods
and processes have been suggested [17, 18] for doped SnO2
film fabrication. Consonni et al. [19] reported the electrical
properties of F-doped SnO2 films as a function of the film
thickness. Hence, the film thickness was not a determinant
factor to calculate the surface electrical resistance by the
methods used in this study, and thus, the resistivity of the
films was expressed as a function of t (film thickness), since
t was not evaluated. Even though all measurements made
here rely on a good approximation of Rs.
To understand how resistive or conductive a semicon-
ductor is, we must consider that the number of electrons
to be allocated to its bands is determined by its atomic
number Z and also by the geometry of the system. The
most important point to be highlighted here is that a
completely full band is “frozen,” that is, with or without
the application of an external electric field, the electrons in
this band do not contribute to the electric current. Only
an energy gain, greater than about 1 eV, could make an
electron jump from a full band to an empty band; this
then responds to the application of an electric field. In
contrast, electrons that are in a partially occupied band
respond easily to the application of external fields. These
have empty quantum states to which they can undergo
transitions produced by a small energy gain. These state
changes can also be caused by a thermal energy gain of the
order of kT, that is, by collisions with other particles [20].
This could explain why the temperature influences the sur-
face electrical resistance. However, this parameter was not
studied here because, in plasma deposition systems, the

5. 128 C. S. de Souza et al.
kinetics of charged particles is not well known nor easily
measured. When a sample is removed from the reactor, the
sample holder is cooled and the temperature of the glass
substrate is also influenced. Two processes, the increase
in the number of carriers and the increase in the number
of collisions (with increasing temperature), occur in any
material. The competition between them will determine
whether, in a given temperature range, the conductivity
increases or decreases with temperature. Both methods for
measurement of surface electrical resistance used here are
sufficient to determine R (Ω/) with a small standard
deviation (1 Ω/) using the aluminum PADs method
and (0.02 Ω/) using the four tips method.
CONCLUSION
Both methods (using aluminum PADs and four tips) were
consistent and useful to obtain the surface resistance of
a tin oxide thin film deposited by RF sputtering in a
plasma system. Applying the first method (PADs), values
of surface resistance changed as a function of the distance
between the aluminum measuring tips and the PADs.
Then, applying a linear approximation and checking the
resistance value for each measured distance, resistances
ranged from 44.10 Ω/ at 6 cm to 88.10 Ω/ at 1 cm of
separation. Applying the second method (four tips), the
values of surface electrical resistance reveal the nominal
value for the SnO2:F thin film, which ranged from ∼29.66
to 33.84 Ω/, and a minor error can be observed and
attributed to inaccuracy of the measurements of the dis-
tances. The calculations of the surface electrical resistance
are the key parameters for the requested applications of
these films, such as in the frontal contacts of solar cells,
liquid crystal dials, image sensors, and OLEDs, and are
essential for failure analysis and quality control of semi-
conductor materials and devices. For future studies, the
Van der Pauw method or concentric rings to measure the
surface resistivity of oxide thin films may be applied.
CONFLICT OF INTEREST
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
AUTHOR CONTRIBUTIONS
Sant’Ana P. L. contributed with experimental, plasma, and
the writing of the first draft. De Souza C.S. contributed
with surface measurements, and he is the owner of the
results. Bortoleto J.R.R. contributed as coordinator, concep-
tualization andreviewer, and with facilities. Durrant S.F.
contributed as reviewer of the text in all drafts.
FUNDING
This study was financed in part by the Coordenação de
Aperfeiçoamento de Pessoal de Nível Superior – Brasil
(CAPES) – Finance Code 001.
ACKNOWLEDGMENT
The author is very thankful to the Technological Plasmas
Laboratory from UNESP for all support and opportunities.
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