Keysight (Agilent) 4156C Application Note

Introduction

Safe operation of next-generation
sub-micron devices requires that
power dissipation and signal levels be
scaled accordingly. To successfully
characterize these devices and semi­conductor processes, more precise
and noise free low current measure­ments are needed. The Agilent 4156C
precision semiconductor parameter
analyzer allows you to measure down
to the fA level. This application note
shows how to precisely evaluate sub­threshold characteristics of a MOSFET
device using the 4156C’s ultra low
current measurement capability.

Current Problems

Accurate low current measurements
on a wafer are difficult because of the
following reasons:

• Leakage and electric noise in the
measurement cables, and interface
between the measurement instru­ment and wafer prober

• Leakage and electric noise at the
interconnect wires and probe nee­dles due to insufficient guarding,
and the effects of light due to
incomplete shielding of the wafer

Due to these limitations, you have to
design special parametric test ele­ments. For example, in the measure­ment of oxide leakage current, you
may have to design test elements that
have larger areas to minimize the
effect of leakage and noise.

But these “scaled-up” test elements
waste valuable wafer layout area.
There is also the question of how
valid the data taken on large test
elements is for your actual “scaled­down” process. With the 4156C, you
can measure with confidence, the
actual MOSFET devices without
developing special higher current
test elements.

Measurements using the Agilent 4156C

The 4156C and Cascade Microtech
Summit series analytical wafer prober
together provide you with a solution
for fast measurement throughput at
these very low current levels.

• The 4156C’s A to D converter and
guard design of the measurement
system is newly designed, resulting
in revolutionary improvements in
resolution and accuracy. The 4156C
has 1 fA resolution and 20 fA offset.
Each of four SMUs has the same

resolution and accuracy. You are
free from worrying about cable
connections according to the pin
assignment of your devices.

• The measurement cables are also
re-designed by reviewing the mate­rials and forms. Kelvin Triaxial
Cable (Patent) minimizes electric
noise and leakage. Moreover, it
reduces the effect of electromotive
force (emf) in the cable.

• The Zero Offset function and Self
Calibration function reduce the
offset current and offset voltage in
the measurement system including
the measurement cables and probes.
Now you can perform ultra low
current test by eliminating errors
caused by thermal and emf effects.

• The Summit series prober was
designed to mate with the 4156C.
All measurement paths are fully
guarded and shielded, including
the wafer chuck (substrate) con­nection. You can measure surface
and bulk leakages to the full
potential of the 4156C.

Ultra Low Current dc Characterization of
MOSFETs at the Wafer Level

Application Note 4156-1

Agilent 4156C Precision Semiconductor Parameter Analyzer

2

Measurement Instruments and Connection

The 4156C’s kelvin-triaxial cables can
be connected directly to the optional
connector plate of the Summit prober.

You do not have to solder measurement
wires to the connector plate, so there
will be no flux and fingerprints that
increase leakage (Figure 1).

It is recommended to use 1.5 m kelvin
triaxial cables (option) to connect the
4156C and the prober. Shorter measure­ment cable has less leakage and noise.

Measurement of Threshold Characteristics of an n-channel MOSFET

The measurement block diagram and
measurement setup for the measure­ment of the threshold characteristics
of an n-channel MOSFET are shown
in Figure 2 and Figure 3. The sweep
step interval is kept small to reduce
charging currents caused by any
residual cable or probe capacitances.

To perform accurate ultra low current
measurements, use the measurement
conditions recommended in the MEA­SURE SETUP page shown in Figure 4.

In this page, the measurement range
should be set to Auto range or Limited
Auto 10 pA range. The 10 pA range is
the minimum range and has 1 fA reso­lution. Also the measurement integra­tion time is set to Medium or Long to
prevent the effect of electric noise.

On SWEEP SETUP page, you will
need to add a Hold Time at the begin­ning of non-zero voltage sweeps to
allow for initial charging time con­stants. Increase the Delay Time
between each point in the sweep as
needed to account for the effects of
dielectric absorption from residual
ground or guard capacitances.

The measurement environment
should be considered as well.

• Turn off any sources of mechani­cal vibration

Figure 1. Connector plate at the rear of the Summit 10500

Figure 2. Measurement block diagram

Figure 3. SWEEP SETUP page

Drain

Gate

Substrate

Source

SMU2

SMU4 SMU1

SMU3

Vg

Vsub

A

Id

Vd