Electrical Characterization

BULK RESISTIVITY MEASUREMENT

• Bulk Resistivity Measurement of silicon wafers, ingots and blocks can

be realized with help of non-contact, non-destructive Eddy current
technique. When AC current flows in a coil, it' s magnetic field
induces circulating (Eddy) currents in the sample. The Eddy current
measurement is actually the measurement of the electrical loss in the
material. The measuring head moves above the sample in constant
height without touching of it. Because the measured signal depends
on sample resistivity, thickness, and distance between the probe and
sample, there is built-in distance sensor (based on capacitance
measurement), which measures in the same spot like Eddy sensor.
From the distance value and the Eddy signal, true resistivity values
can be obtained.
• The measured signal (Ueddy) always depends on Distance between the
coil and sample, additionally:
1.In case of Semi-infinite samples as blocks depends on Resistivity of
the material
2.In case of Samples with finite thickness as wafers depends
on Resistivity and thickness
3.In case of Thin layers depends on Sheet resistance
• The Eddy current is higher in good conducting material compared
to less conductive material.
BULK RESISTIVITY MEASUREMENT
The Four-Point Collinear Probe Method
• The most common way of measuring the resistivity of a
semiconductor material is by using a four-point collinear probe.
This technique involves bringing four equally spaced probes in
contact with a material of unknown resistance. The probe array
is placed in the center of the material,
The Four-Point Collinear Probe Method
Scanning Capacitance Microscopy (SCM)
• Scanning capacitance microscopy (SCM) is a powerful atomic force
microscopy (AFM) method for the characterization of semiconductor
devices due to its non-destructive operation as well as high spatial
resolution and electric sensitivity.
• Generally, SCM measures spatial capacitance variations on semiconductors
and thereby maps the carrier concentration and doping profiles in non-
uniformly doped samples. The application of SCM for failure analysis in the
semiconductor industry continues to grow since SCM maps localized
charges and electronic defects with a nanometer resolution. For example,
SCM is applied to characterize the gate oxide in metal-oxide-semiconductor
(MOS) devices: As the gate oxide gets thinner due to the reduction of
overall device dimensions, problems and failures can occur, caused by
charges trapped in the oxide layer during device operation. Presently, no
other characterization tools are able to locally resolve the location of the
failure and the electrical defects in two dimensions. In addition, SCM is
used in the field of ultrahigh density nonvolatile semiconductor memories.
Here, stored charges in the insulator layer of the metalinsulator-oxide-
semiconductor (MIOS) heterostructure of the non-volatile memory can be
imaged with a nanometer resolution by the SCM.
• SCM operates with a conductive materialcoated AFM tip and a highly
sensitive capacitance sensor in addition to regular AFM components. By
applying an AC voltage, the conductive tip in contact and the oxidized
semiconductor sample form a MOS capacitor with the tip and sample
acting as electrodes. This MOS capacitor consists of two capacitors in
series: one is the insulating oxide layer and the other is the active depletion
layer near the oxide/silicon interface as shown in figure 1. The total
capacitance is determined by the oxide thickness and the thickness of the
depletion layer, which depends on the carrier concentration in the silicon
substrate and the applied DC voltage (Vb ) between the tip and the
semiconductor. Figure 2 (next page) shows DC bias dependence of the
capacitance as high frequency C-V curves for p-type (in red) and n-type (in
blue) semiconductor samples. Samples with different dopant types display
opposite slope directions in their C-V curves. Figure 2 (b) shows DC bias
dependence of the differential capacitance derived as derivative from the
C-V curve in figure 2 (a).
• Scanning Capacitance Microscopy (SCM) is a powerful technique used to measure the dopant profiles
in semiconductor materials, particularly silicon wafers.
• 1. Principle of SCM:
• SCM operates by scanning a sharp conductive tip (similar to that in Scanning Tunneling Microscopy)
over the surface of a semiconductor. The capacitance between the tip and the sample changes based
on the local doping concentration, allowing for the measurement of the dopant profile.
• 2. Setup:
• Tip Preparation: A conductive tip is prepared, typically made of metal, and is carefully shaped to
achieve high resolution.
• Sample Configuration: The sample (e.g., silicon wafer) is placed on the stage of the microscope. It is
often biased with a voltage to enhance capacitance changes.
• 3. Capacitance Measurement:
• As the tip approaches the sample, it establishes a capacitive coupling. The capacitance depends on
the distance between the tip and the surface, the dielectric properties of the materials, and the local
doping concentration.
• The SCM setup measures the small capacitance variations as the tip scans across the surface.
• 4. Data Acquisition:
• The SCM tip scans in a raster pattern over the sample surface, measuring
the capacitance at each point. This results in a map of capacitance
variations, which correlates with doping levels.
• 5. Data Analysis:
• Calibration: To relate capacitance measurements to actual doping
concentrations, calibration is necessary. This can involve using known
dopant profiles or reference samples.
• Interpretation: The capacitance data is analyzed to extract information
about the doping profile. The relationship between capacitance and doping
concentration can be modeled using Poisson’s equation and other
semiconductor physics principles.
 Tunneling AFM (TUNA)

• Like Conductive AFM (C-AFM) , Tunneling AFM (TUNA) can be

used to localize electrical defects in semiconductor or data
storage devices, or to study conductive polymers, organics, or
other materials.
• TUNA works similarly to C-AFM but with higher current
sensitivity. TUNA characterizes ultra-low currents (<1pA)
through the thickness of thin films, and is of particular
importance when electrical characterization of such low-
conductivity samples is needed at high lateral resolution.
• To perform TUNA measurements on all Dimension series
microscopes, the user applies a selectable bias between the sample
and the conductive SPM tip, with the tip being on virtual ground. As
the tip is scanning the sample in contact mode and imaging the
topography, a linear amplifier with a range of 80fA to 120pA senses
the current passing through the sample. Thus, the sample’s
topography and current image are measured simultaneously, enabling
the direct correlation of a sample location with its electrical
properties.
• This technique is especially useful for the evaluation of dielectric films
with high resistivity and when local electrical properties strongly
affect the amount of current passing through the film. An important
application is the evaluation of gate oxides for transistors like silicon
oxide (SiO2). The current tunneling from the SPM tip through the
oxide strongly depends on film thickness, leakage paths (possibly
caused by defects) and charge traps. All of these may significantly
affect the properties and the integrity of the whole film, thus
compromising an entire device’s performance. With its high current
sensitivity and high lateral resolution, TUNA helps to identify and
characterize in homogeneities like those present in the sample
• TUNA is also very useful for the measurement of other higher
resistivity films, such as ferroelectric films and diamond-like-carbon
(DLC) films, which find widespread use in data storage. Further
examples of application include conductivity measurements on light-
emitting conductive polymers or nanotubes, to name a few. Figure
293.2a shows a rough schematic of the tunneling AFM setup.