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By Jeff Hochberg, Zyvex Instruments
Download a printable copy here.
It is difficult to electrically characterize
very small circuit elements in current-generation semiconductors.
Most modern prober systems have unacceptably large metal pad
requirements in order to effect such measurements, and unwieldy
manual mechanical probe stations with optical microscopes
cannot resolve fine features. With standard gate dimensions
of less than 90 nm and space budgets shrinking continuously,
the smallest probe pad dimensions required for most prober
systems remain fixed at about 50 microns. This limitation
is largely the result of the inaccuracy of probe movements
and the size of the probe tips. The Zyvex Test System is designed
to overcome these limitations by offering 5 nm movement precision
with probe tip diameters less than 20 nm, and current-measuring
capability down to 1 pA.
For general purpose resistance measurements
and I-V curve generation,
2-point electrical measurements are normally used. However,
when the resistance to be measured is relatively low, or the
resistance of the probes or the contacts is relatively high,
a 4-point probe will yield more accurate results. The Zyvex
Test System employs a Keithley Model 4200 Semiconductor Characterization
System, with a measurement resolution of 100 fA and a Zyvex
S100 Precision Manipulator System. With a combined system
noise floor below 55 fARMS, current measurements
of 1pA are readily achievable.
In order to determine a resistance, Ohm’s
law is used: R = V/I.
A known current is sourced and flows through the
unknown resistance. The voltage that develops across the resistance
is measured and resistance is determined by dividing the measured
voltage by the sourced current. A problem that occurs when
using a 2-wire setup is that the voltage is measured not only
across the resistance in question, but includes the resistance
of the leads and contacts as well (Figure 1).
When using an ohmmeter to measure resistances above a few
ohms this added resistance is usually not a problem. However,
when measuring low resistances or when contact resistance
may be high, obtaining accurate results with a two wire measurement
may be a problem.
Figure 1. 2-point measurement
A solution to the problem of 2-point measurements
in which the lead and contact resistance is measured along
with that of the device under test (DUT) is the 4-wire or
“Kelvin” measurement. Because a second set of
probes is used for sensing and since negligible current flows
in these probes, only the voltage drop across the device under
test is measured (Figure 2). As a result,
resistance measurement or I-V curve generation is more accurate.
Figure 2. 4-point measurement
The Zyvex Test System is a positioning and testing
tool for micro- and nano-scale research and development applications.
It accommodates up to four positioners (three-dimensional
stages) which grasp, move, test, and optimally position micro-and
nano-scale samples in Scanning Electron Microscopes (SEMs)
and Focused Ion Beam Systems (FIBS).
The Zyvex Test System employs a Keithley Model 4200 Semiconductor
Characterization System (SCS) which is an automated instrument
designed to provide IV and CV characterization of semiconductor
devices and test structures. Its advanced digital sweep parameter
analyzer combines speed and accuracy for deep sub-micron characterization.
Tests are easily and quickly configured and run from the Keithley
Interactive Test Environment (KITE). KITE is an application
program designed and developed specifically for characterizing
semiconductor devices and materials. Source and measurement
functions for a test are provided by up to eight Source-Measure
Units (electronic instruments that source and measure DC voltages
and currents). Test capabilities are extended by support of
a variety of external components. For less precise measurement
and testing applications, other Keithley instruments may be
employed to configure the system more closely to the requirement.
To implement a 4-point measurement (Figure
3), all 4 positioners of the test system are used.
Each positioner controls a single probe. The probes are positioned
in a line on the DUT. The outer and inner probes that are
closest to each other on the DUT connect to the same Source-Measure
Unit (SMU). The outer probe connects to the force terminal
on the SMUs and the inner probe connects to the sense terminal
on the SMU.
Figure 3. Schematic Diagram of Zyvex S-100 test head with
Keithley 4200
Because the Zyvex test head employs coaxial
cables and the 4200 uses triaxial cables, an adapter is required.
The signal lines of the coax and triax cables should be tied
together and the shields should be tied to each other as well.
The guard of the triax is left open. Keithley’s #237-BNC-TRX
adapters can be used. Caution should be used with other adapters
because they may not interface between the coax and triax
cables in the same way. Note that the connections between
the 4200 and S100 can be made at the patch panel rack or on
the BNC breakouts (Figure 4).
Figure 4. S100 Block Diagram
The Keithley Interactive Test Environment
(KITE) program on the 4200 is used to run the test. The “4-point”
interactive test module which is found in the KITE folder
titled 2 wire-resistor can be used to generate results for
the I-V curve. This is a pre-configured routine that implements
4 point measurements using 4 cables and 2 SMUs as described
above.
The Zyvex Test System is an effective measurement
tool for IV characterization of nanoscale components. With
4-point probe capability, 1pA accuracy, and 5 nanometer precision
movement, the system offers a unique combination of features
that are ideal for semiconductor characterizations.
1. Cable/Harness
Testing Made Easy™
2. Zyvex S100 System Manual
3. Keithley Low Level Measurements 5th Edition
4. Keithley 4200-SCS Semiconductor Characterization System
Reference Manual
5. Keithley 4200-SCS Semiconductor Characterization System
User’s Manual
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