Advancements in nanotechnology have created the need for efficient means of communication of electrical signals to nanostructures, which can be addressed using low resistance contacts. In order to study and estimate the resistance of such contacts or the resistance posed by the interface(s) in such contacts, accurate test structures and evaluation techniques need to be used. The resistance posed by an interface is quantified using its specific contact resistivity (SCR), and although multiple techniques have been utilized, inaccuracies of such techniques in measuring values of SCR lesser than ( &lt; 10-8 Ω ldr cm2 ) have been reported. In this letter, an approach for estimating very low values of SCR (lower than the previously limiting ( &lt; 10-8 Ω ldr cm2 )) using a cross Kelvin resistor test structure is demonstrated using aluminum to titanium silicide ohmic contacts, with a minimum estimated SCR value of 6.0 times 10-10 Ω ldr cm2

Bhaskaran, M

Holland, A

Sriram, S

RMIT Research Repository

IEEE ELECTRON DEVICE LETTERS, VOL. 29, NO. 3, MARCH 2008 259
Accurate Estimation of Low (< 10−8 Ω · cm2) Values
of Specific Contact Resistivity
Madhu Bhaskaran, Student Member, IEEE, Sharath Sriram, Student Member, IEEE, and Anthony S. Holland
Abstract—Advancements in nanotechnology have created the
need for efficient means of communication of electrical signals
to nanostructures, which can be addressed using low resistance
contacts. In order to study and estimate the resistance of such con-
tacts or the resistance posed by the interface(s) in such contacts,
accurate test structures and evaluation techniques need to be used.
The resistance posed by an interface is quantified using its specific
contact resistivity (SCR), and although multiple techniques have
been utilized, inaccuracies of such techniques in measuring values
of SCR lesser than 10−8 Ω · cm2 have been reported. In this
letter, an approach for estimating very low values of SCR (lower
than the previously limiting 10−8 Ω · cm2) using a cross Kelvin
resistor test structure is demonstrated using aluminum to titanium
silicide ohmic contacts, with a minimum estimated SCR value of
6.0 × 10−10 Ω · cm2.
Index Terms—Contact resistance, cross Kelvin resistor (CKR),
specific contact resistivity (SCR).
I. INTRODUCTION
ADVANCEMENTS in nanotechnology have created theneed for efficient means of communication of electrical
signals to nanostructures [1]. Electrical contacts made to such
nanodevices need to pose minimum possible contact resistance.
In order to study and estimate the resistance of such contacts
or the resistance posed by the interface(s) in such contacts,
accurate test structures and evaluation techniques need to be
used. These will pave the way to the identification of new
materials and/or contact architectures to develop nanoscale low
resistance contacts.
The resistance posed by an interface is quantified using its
specific contact resistivity (SCR), which is denoted using ρc (in
ohm square centimeters) [2], and multiple techniques have been
utilized in measuring SCR values. Cross Kelvin resistor (CKR)
test structures were shown to be suitable for the measurement of
low values of SCR, but the use of cumbersome error correction
curves to estimate the value of SCR and inherent inaccuracies
in the technique served as deterrents from the widespread use
of this estimation technique [3]–[6]. Using a combination of
analytical calculations and finite element modeling, a simplified
approach to this problem of SCR estimation using the CKR test
structures with varying contact sizes has been developed by the
authors of this letter [7]. This letter demonstrates the accuracy
Manuscript received October 29, 2007; revised December 4, 2007. The
review of this letter was arranged by Editor K. De Meyer.
The authors are with the Microelectronics and Materials Technology
Centre, School of Electrical and Computer Engineering, RMIT University,
Melbourne, Vic. 3001, Australia (e-mail: madhu.bhaskaran@gmail.com).
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LED.2007.915378
Fig. 1. Schematic of a CKR test structure. Notations used in the manuscript
are denoted.
of this technique in estimating SCR lower than the previously
limiting values of 10−8 to 10−7 Ω · cm2.
The accurate evaluation of low values of SCR (related to low
values of contact resistance) will be demonstrated using two-
layer ohmic contacts with aluminum and titanium silicide thin
films. Although these ohmic contacts have been characterized
before [8], they remain one of the very few ohmic contacts for
which very low (< 10−8 Ω · cm2) values of SCR have been
reported.
II. CKR TEST STRUCTURE
The CKR test structure [9]–[11], as shown in Fig. 1, consists
of two “L”-shaped regions (of width w), consisting of the two
materials of interest, which, in this case, are aluminum (upper
“L”) and titanium silicide (lower “L”). For contact resistance
estimation, the CKR test structures using contacts of defined
areas (circular or square contacts with diameter or side d,
respectively) are used; achieved by defining the contact in an
intermediate insulator layer, as shown in Fig. 1.
The value of resistance measured from the two voltage
taps of the CKR is the Kelvin resistance RK , which is a
combination of the contact resistance Rc and other parasitic
resistances (due to sheet resistance of surrounding material).
Rc is inversely proportional to the area from the standard
resistance–area relationship. This implies that as the area de-
creases, by decreasing the diameter d, Rc increases, and beyond
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260 IEEE ELECTRON DEVICE LETTERS, VOL. 29, NO. 3, MARCH 2008
Fig. 2. Comparison of the measured SCR values before, after 30 min, and
after 300 min of annealing at 450 ◦C for the CKR test structures with
silicide arm width of 9 µm. The curves drawn for each set of data points are
representative and are only to assist the reader in extrapolation of the data.
a point will dominate the parasitic effects. An infinitesimally
small contact will exhibit maximum Rc, and the RK measured
will be the closest to the actual Rc value. Hence, by obtaining
a number of resistance values, which include parasitic errors,
the true SCR value (ρc, which is different from the measured
SCR ρ′c) can be estimated by extrapolating to an ideal point
contact.
III. EXPERIMENTAL DETAILS
Silicon samples coated with 100-nm titanium were patterned
(using photolithography) to form the titanium silicide (TiSi2)
arms of the CKR test structures. Formation conditions of C54
titanium silicide are described in [12]. After the formation of
TiSi2, 100 nm of silicon dioxide (SiO2) was deposited on the
samples and patterned by etching in a buffered hydrofluoric
acid solution to form circular contacts of a variety of diameters.
A 600-nm aluminum (Al) layer, which was deposited by elec-
tron beam evaporation, was patterned to form the upper arms of
the CKR test structures.
These steps defined the CKR structures of varying contact
sizes d and three different CKR arm widths w. These Al−TiSi2
contacts were annealed, in an atmosphere of 3% hydrogen in
nitrogen, at 450 ◦C or 500 ◦C in steps of 30 min, to convert them
from Schottky to ohmic contacts. The SCR measurements were
carried out using a micromanipulator probe station after each
annealing step in order to study their variations with annealing
time (up to 300 min) and temperature (450 ◦C or 500 ◦C).
Annealing was not carried out at temperatures beyond 500 ◦C
due to the diffusion of aluminum into silicon through the
titanium silicide thin film [13], [14].
IV. RESULTS AND DISCUSSION
The SCR measurements were interpreted using the technique
described in [7]. The graphs shown in Fig. 2 indicate the
measured SCR values ρ′c for Al−TiSi2 ohmic contacts annealed
at 450 ◦C; these measured values include parasitic resistance
TABLE I
MEASURED AND ESTIMATED VALUES OF SCR FOR Al−TiSi2 OHMIC
CONTACTS ANNEALED AT 450 ◦C FOR DIFFERENT ANNEALING
DURATIONS (CKR ARM WIDTH OF 9 µm)
TABLE II
MEASURED AND ESTIMATED VALUES OF SCR FOR Al−TiSi2 OHMIC
CONTACTS ANNEALED AT 500 ◦C FOR DIFFERENT ANNEALING
DURATIONS (CKR ARM WIDTH OF 9 µm)
contribution, and the true SCR value ρc is extracted from a
collection of these values. Fig. 2 shows the measured SCR
values for the CKR structures with varying contact diameters
and arm width of 9 µm. The graph clearly indicates the steep
fall in the SCR values on annealing the contacts for 30 min.
Subsequent annealing further reduces the values. Table I sum-
marizes the results obtained. The lowest measured ρ′c value
(8.6 × 10−9 Ω · cm2) can be extrapolated to yield a true SCR
value of 2.0 × 10−9 Ω · cm2.
Similar measurements were also carried out by annealing
at 500 ◦C, and the results are summarized in Table II. It can
be observed that the increased annealing temperature promotes
faster decrease in the measured SCR values. Fig. 3 illustrates
the SCR values measured after 180 min of annealing for
the different CKR linewidths. The measured SCR values are
higher for widths of 21 and 32 µm due to larger parasitic
resistances contributed by the silicide sheet resistance, which
increases as w increases [7]. The lowest measured ρ′c value
(2.0 × 10−9 Ω · cm2) can be extrapolated to yield an SCR value
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BHASKARAN et al.: ACCURATE ESTIMATION OF LOW (< 10−8 Ω · cm2) VALUES OF SCR 261
Fig. 3. Comparison of the measured SCR values after 180 min of annealing
at 500 ◦C for the CKR test structures with silicide arm widths of 11, 21, and
32 µm. The curves drawn for each set of data points are representative and are
only to assist the reader in extrapolation of the data.
of 6.0 × 10−10 Ω · cm2. Further annealing beyond 180 min did
not reduce the SCR values. This SCR value of 6.0 × 10−10 Ω ·
cm2 is the lowest reported for any two-layer contact.
The error in the measured SCR values is very small, which
can be observed from the limited scatter in the data presented.
The true SCR values estimated from the measured set of
values has an error of about 25%, which is highly acceptable,
considering the range of the SCR values under consideration
(10−10 to 10−8 Ω · cm2). These experimental results were also
verified using finite element modeling.
V. CONCLUSION
The results discussed in this letter highlight the ability of the
proposed technique, using the CKR test structures, in accurately
estimating very low values of SCR, with the lowest estimated
value of 6.0 × 10−10 Ω · cm2 for the aluminum to titanium
silicide contacts. The use of this simplified approach will be
beneficial in evaluating submicrometer contact structures [15]
and developing electrical contacts to nanodevices.
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Accurate estimation of low [< (10^-8) ohm . cm²] values of specific contact resistivity

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Accurate estimation of low [&lt; (10^-8) ohm . cm²] values of specific contact resistivity

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Accurate estimation of low [< (10^-8) ohm . cm²] values of specific contact resistivity