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Number 2241
Application Note Making Ultra-Low Current Measurements
Series with the Low-Noise Model 4200-SCS
Semiconductor Characterization System
Introduction
associated with connecting these grounds together. Sometimes,
however, the power line ground can be noisy. In other cases, a
Parametric characterization of semiconductor devices typically test fixture and probe station connected to the instrument may
requires making extremely low current measurements. For create a ground loop that generates additional noise. Due to these
MOSFET devices, the gate voltage controls the on/off of the concerns, ensuring low-level measurement accuracy demands
MOSFET, in other words, the drain current flow. The off state that the system grounding be thought out carefully.
current and the transition from the on state to the off state (the
subthreshold current) are both of critical importance in the per-
formance of a ULSI CMOS circuit. In modern, highly integrated GNDU
circuits, the off state current is typically on the order of just M 1
S
E
femtoamps. Furthermore, there are a number of device phenome- N
S
INSTRUMENT
CONNECTIONS
E
na that are of critical importance in device characterization, such SMU ONLY
SENSE LO
GUARD
as Gate Induced Drain Leakage (GIDL). Therefore, it is very F SENSE LO
PT 1 O
important to have a tightly integrated parametric characterization R
COMMON
C
system that is capable of making these ultra-low current E SMU AND GNDU
measurements. C SENSE
O
M
A typical semiconductor characterization system will M GUARD COMMON
O
include a DC characterization system such as the Keithley N
FORCE
4200-SCS, a switch matrix, a probe station, and cabling. Too LATION
ORY I
often, however, when a semiconductor characterization system is SLOT
configured, the system specifier will tend to concentrate on the 8
DC parametric instrumentation while neglecting the rest of the
system. In fact, the switch matrix and probe station chosen can
have a significant impact on the system's overall measurement
performance. In addition, even with a properly configured sys- Figure 1. Common vs. Earth Ground (Chassis)
tem, the system implementation itself can significantly affect
measurement integrity. System Grounding and Shielding
This application note will discuss several important aspects Technically, although grounding and shielding are closely
of making low current measurements with the Model 4200-SCS, related, they are actually two different issues. In a test fixture or
including grounding and shielding, noise in the measurement, probe station, the device and probing are typically enclosed in
and system settling time. soft metal shielding. This metal enclosure is used to eliminate
interference from power lines and high frequency radiation (RF
Grounding and Shielding Issues or microwave) and to reduce magnetic interference. Most semi-
conductor devices are also relatively light sensitive, so the
enclosure also prevents light from striking the device under test,
Instrument Common vs. Chassis Ground
which could produce a low level current flow that would inter-
Before discussing grounding in detail, it is important to distin- fere with making accurate low current measurements.
guish between the instrument common and the chassis ground.
The Model 4200-SCS and similar instruments have two grounds This metal enclosure is normally grounded for safety
available. One is called the common; the other is the power reasons. However, when an instrument is connected to the probe
ground. (See Figure 1.) When shipped from the factory, these station through triaxial cables, the point where ground connec-
two grounds are connected, but they are different. The common tions are made is very important. The configuration in Figure 2a
is the ground for the complete measurement circuit; it will affect illustrates a common grounding design error.
the system's low-level measurement performance. In contrast, Note that the instrument common and the chassis ground
the chassis ground is connected to the power line ground and is are connected. The probe station is also grounded to the power
mainly used for safety reasons. Usually, there are no problems line locally. Even more significantly, the measurement
OT
SMU
SL
1 OT
SMU
SL
1
OT
SL
2 OT
LO SL
E 2
NS LO
OT SE E
SL NS
3 OT SE
LO SL
E 3
TS NS E
SE NS LO
EN OT SE E
UM SL TS NS E
EN TR 4 EN SE NS
NS OT SE
UM INS LO UM SL
TR TIO E
E ENNS TR 4
INS EC LY LO
NS UM LO
DU SE NS INS
GN
NN ON SE D OT SE TR TIO E
E
CO U SEN AR SL INS EC LY LO
T E NS
GU LO 5 DU NN ON SE D SE NS
SM RC OT SE
SE LO FO GN CO U SEN AR
T SL E
Prober
E GU LO 5
SEN NS E SM RC
NS LO FO
S
SE
Prober
ON OT SE E
SL SE SEN NS E
MM E
6 NS
S
CO RC OT SE
E DU FO
ON
SL SE
GN MM E
N CO 6 RC
S D E E DU FO
AN OT NS N GN
E U SL SE D E
SE 7 E S AN NS
SM OT
GND
SEN RC E SE
FO U SE
SL E
SM 7
GND
ON SEN RC
F MM OT FO
O CO SL E ON
R 8 F MM OT
D RC
C AR E FO O CO SL E
GU RC R 8
E FO D
E
RC
C AR
RC
FO
E GU
FO
C
O
M C
M L O
TR M
O CN L
N PA M TR
O CN
N PA
L
LK TR
"Dark Box"
INT CN L
PA LK TR
"Dark Box"
INT CN
PA
L
TR
CN L
PA TR
IN CN
PA
GND
IN
OU
T
OU
T
GND
Model 4200-SCS Probe Model 4200-SCS Probe
Mainframe Station Mainframe Station
Figure 2a. Ground loops Figure 2b. Eliminating ground loops
instrument and the probe station are connected to different power it is amplified. When measuring extremely low currents, remote
outlets. The power line grounds of these two outlets may not be mounting of the PreAmp offers significant measurement accura-
of the same level all the time. Therefore, a fluctuating current cy advantages.
may flow between the instrument and the probe station. This
creates what is known as a ground loop. To avoid ground loops,
PreAmp
a single point ground must be used. Figure 2b illustrates a better
grounding scheme when a probe station is used.
42
00
CONTROL
-P
PREAMP
A-1
RE
Techniques for Minimizing
M OTE
PRE
AMP
OT
SMU
SL
1
SENSE
OT
SL
2
LO
E MA
NS DE
SE U.S IN
OT .A. 420
SL 0-M
3 AG
LO
FORCE
S E
NS E
NT SE NS
System Noise
ME SLOT SE
N RU 4
ME NS ST LO
RU TIO IN E
ST E
IN NNEC LYE LO
NS
DU SE NS
ON D OT SE
GN CO U SENS
T AR SL
5 E
SM GU LO RC
E LO FO
NS E
SE NS E
NS
S
ON OT SE
SL SE
MM E
CO 6 RC
E DU FO
N GN
S D E
AN OT NS