Safe and repeatable operation of next-generation deep-submicron devices requires reductions in power dissipation and signal levels accordingly. 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.
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, 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.
Multipin Probe-Card Designs
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 high-leakage 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.
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 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 µA 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 probe-card 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 picofarads. 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 probe-card 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 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 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 4156A 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 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 5 fA which does not adversely affect the parametric analyzer measurement. 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 high-speed 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.
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. Mr. Dangremond received a degree in business administration marketing at Portland State University. Cascade Microtech, S.W. 14255 Brigadoon Ct., Beaverton, OR 97005, (503) 626-8245.
Copyright 1997 Nelson Publishing Inc.
March 1997
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