Download Multipin Solutions for On-Wafer Parametric Measurements

Multipin Solutions for On-Wafer
Parametric Measurements
by Larry Dangremond, Cascade Microtech
S
afe and repeatable operation of
devices requires reductions in power
waste valuable wafer layout area. Also, how valid is the data
taken on large test elements for the actual scaled-down process?
A solution to this problem is now available that allows the
characterization of MOSFET devices without developing special higher-current test elements.
dissipation and signal levels accord-
Multipin Probe-Card Designs
next-generation deep-submicron
ingly. To successfully characterize
semiconductor devices and processes, you need more precise
and noise-free low-current and low-voltage measurements.
Precision semiconductor parameter analyzers now measure
down to the femtoamp )fA) level, but this is of little value unless low-leakage connections are made to the device-under-test
(DUT). Accurate low-level measurements on a wafer are difficult because of:
Leakage and electric noise in the measurement cables and
interface between the measurement instrument and probe
card interface.
Leakage, electrical noise and capacitance at the interconnect lines and probe needles due to insufficient guarding.
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Typically, you do not expect to get as good a measurement
when using a multipin probe card as you would with individual manipulators and probe needles. There are several reasons why measurements are not as good when using a standard probe card.
A typical edge-connected probe card is made from a highleakage material such as glass-epoxy or FR-4. In this design,
the probe needles are connected directly to the board, the circuit is printed on the board, each probe needle contacts one run
on the board and the board uses a standard edge connector.
This type of design exhibits high capacitance and nanoamp
levels of leakage current between probe-card conductors that
are parallel for considerable distances. High leakage also exists
between the interface connection points at the edge of the card.
This unshielded type of system is also very susceptible to
electromagnetic currents generated by adjacent 50/60-Hz
power lines and sources. Leakage also exists at the connector
and in the cabling to the connector.
Leakage increases as the temperature increases, making this
a high-leakage board. This is the simplest type of probe card
and is the least expensive to make. Many improvements in the
design can produce a very low-leakage probe card.
Shield
Figure 1.
Keeping the capacitance low is important. A high capacitance
may cause throughput problems as the instrument’s settling time
increases, or mean that an extremely low measurement ramp
rate must be used when characterizing the DUT.
The only alternative to a complete low-leakage system is to
design elements that have larger areas to minimize the effect
of leakage and noise. These scaled-up test elements, however,
Figure 2.
What Can Be Done?
The first improvement is to remove the
wiring from the board
and use coaxial cabling wherever possible. Probe needles
usually are still connected to the FR-4
board but the parallel
runs on the board need
to be as short as possible Figure 3
to reduce leakage. This
design is still low-cost, but the performance is also quite low.
The coaxial cabling reduces pickup
from 50/60-Hz radiation but does not
take advantage of the guarding provided
by the parametric analyzer. There is still
considerable leakage in the high-capacitance coaxial cable and in the board
material itself, which increases with temperature. This all results in a lot of leakage, around 1 uA to 100 nA, and a lot
of capacitance, 200 to 500 pF.
A second improvement is the use of
triaxial cable, at least as far as the probecard edge connector. This type of cable
has the signal wire completely surrounded by a guard that is then completely surrounded by a shield (Figure
1). The guard shield is driven by the
parametric instrumentation guard/buffer
amplifier which eliminates leakage and
apparent capacitance.
Through the use of triaxial cabling,
leakage is 1 nA to 10 nA and the capacitance is reduced to tens of picofar-
ads. A good-quality low-noise triaxial
cable also helps to reduce noise currents
generated from vibration or thermo-electric effects in the cable. Unfortunately,
leakage still occurs at the edge-card connector and between runs of the probecard board, and both of these increase
with temperature.
More improvements can be made by
continuing the guard connection of the
triaxial cable onto the probe card itself.
By providing a run on both sides of the
signal line and connecting them to the
guard, the surface leakage between adjacent signals on the probe card is eliminated. This design brings the leakage
down to less than 1 nA. However, the
unguarded edge connector and internal
board leakage are still problems.
The best design in a conventional
needle-type probe card is the circular
card that uses spring pins as the electrical interface (Figure 2). This has an
upper ring with spring pins that carry
both the force wire and the associated
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Figure 4.
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guard wire from the parametric analyzer
to the mating circular probe card.
The force wire is surrounded by the
guard lines on the probe card to minimize leakage. This design eliminates the
leakage points at the edge connector and
provides good guarding from the test
instrument to the probe needle.
This probe card design drops leakage
to less than 1 pA and the effect of temperature change is considerably reduced.
Capacitance typically is less than 10 pF.
The leakage at temperature occurs because the guarding is only at the surface
The low-leakage probe card
is the missing piece of
the puzzle and critical
in evaluating next-generation
semiconductor devices
of the FR-4 or glass-epoxy board material so most of the leakage is internal to
the probe card itself.
Low-Leakage Probe-Card
Solutions
This level of performance was extremely good until Hewlett-Packard
recently introduced the HP E5250A
Low-Leakage Switch which allows
measurements to be made down to a few
femtoamps. In choosing a matching
probe-card system to complement the
new switching matrix, it is necessary to
select cabling, connectors and probes
that minimize the degradation of the signal to the measuring equipment. Also,
the probe card must be fast and easy to
change.
All of the probe cards described so far
were found lacking when used with an
HP 4 156A Precision Semiconductor Parameter Analyzer and the HP E5250A
Low-Leakage Switch Matrix. To address this problem, a new low-leakage
low-capacitance system was developed.
This new design uses a probe needle
mounted on a ceramic blade (Figure 3).
The signal line of the triax cable connects
directly to the ceramic blade to provide
high isolation.
A metal guard plated on the reverse
side of the blade provides crosstalk isolation from adjacent signals. This guard
connects to the triax
cable guard through
the circuit board.
The multiple triax
cables connect to a
multipin coax connector to allow
quick disconnect of
the probe card from
the probe station
while maintaining
signal integrity.
This type of
design eliminates figure 5.
the effects of leakage through the board material itself
because the blade sits on top of a driven guard. The probe
card exhibits low leakage up to a chuck temperature of
150°C (Figure 4). Since the measurement path does not
come in contact with the polyimide board materials, measurement-path leakage is greatly reduced.
The triaxial shield layer terminates at the metal shield
enclosure of the probe system. Up to 48 probes can be
mounted in a standard 4.5” rectangular-card-format and the
complete assembly is shielded by a metal box to further protect the integrity of the signal (Figure 5).
With this low-leakage probe-card system, measurements
are made with the same precision as with individual probe
needles. Leakage is less than 5fA which does not adversely
affect the parametric analyzer equipment. Probe-tip capacitance is ultra low for reduced instrument settling time and
accurate capacitance measurement. Typical measurements
such as the MOSFET transfer characteristics are observed
down to femtoamp levels.
Other Solutions
The quadrant probe card is a very flexible alternative when
fewer contacts are needed or when high-speed lines are
required (Figure 6). Quadrant probe cards are mounted in left
and right micropositioners and can be positioned as needed to
match the parametric probe pads. One quadrant probe card can
accommodate up to eight DC pins as well as up to three highspeed RF pins. To achieve low leakage, the individual ceramic probes are used and the signal line is fully guarded.
Separate triaxial cables and connectors complete the signal path back to the instrumentation. The performance is
equivalent to individual probes and individual positioners.
The high-speed RF pins are useful for AC applications such
as ring-oscillator measurements.
Figure 6.
Conclusions
The low-leakage probe card is the missing piece of the
puzzle and critical in evaluating next-generation semiconductor devices. Typical applications include MOS subthreshold current, oxide leakage, substrate current and junction
leakage. These measurements can be made accurately over
temperature with little degradation. Instead of sacrificing
valuable space on the wafer for scaled-up test structures,
devices now can be evaluated using the geometry used in the
final chip.
About the Author
Larry Dangremond has been the marketing manager of
the Probing Systems Business Unit at Cascade Microtech
since 1993. Before joining Cascade, he was a product marketing manager at Tektronix. Dangremond received a degree
in business administration marketing at Portland State
University. Cascade Microtech, S. W. 142.55 Brigadoon Ct.,
Beaverton, OR 97005, (503) 626-8245.
Reprinted from E-Evaluation Engineering, March 1997