Triaxial probe assembly

Details Triaxial probe assembly Triaxial probe assembly

2001-03-23

2004-03-02

Karlsen, Ernest (Department: 2829)

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

Of individual circuit component or element

C324S765010

Reexamination Certificate

active

06700397

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an apparatus and method of manufacture for wafer probe station systems and the use of guarding and shielding systems for limiting electrical leakage currents and noise. More particularly, the invention relates to approaches for providing a chuck apparatus system and a probe assembly which facilitate guarding and shielding techniques for improving the accuracy of low current and low voltage measurements of a device-under-test (DUT), typically a wafer containing one or more integrated circuits.
Modern wafer probe stations have been developed for making accurate low voltage and low current measurements of semiconductor integrated circuit wafers and other electronic component applications. Wafer probe stations having a guarding system have been developed for reducing current leakage, with Kelvin connection systems and the like to eliminate voltage losses associated with conductive line resistances, and electromagnetic interference (EMI) shielding elements for minimizing the effects of parasitic capacitance and noise in the test environment. The technique of guarding to minimize current leakage during low current measurements, the use of Kelvin connections for low voltage measurements, and the provision of EMI shielding portions are well known and discussed extensively in the technical literature. In guarding applications, in particular, an isolated conductor surrounding or otherwise positioned closely adjacent to low current circuitry, and maintained at the same or nearly the same potential provided as the low current circuit conductors, reduces leakage currents such that the low current measurements may be made accurately. In shielding applications, conductive material connected to ground potential reduce the effects of EMI from external and probe station electronics and other noise on test measurements.
The need to observe device behavior with very low level current and voltage measurements is being driven by the ongoing reduction in the integrated circuit semiconductor device geometry in order to increase circuit density, facilitate higher speeds, and reduce power consumption. Decreasing the scale of the circuit can provide the aforementioned improvements, however, tradeoffs in performance may also occur. A number of factors can adversely affect low level voltage and current measurements, including, impedances in which an impedance or current path unintentionally shares a noise source or other instrumentation, the transfer of a noise voltage through usually coupled incidental inductances, magnetically coupled noise, incidental capacitive coupling, charge transfer due to the proximity of charge bodies to the test circuitry, and the like. These mechanisms often perturb measurements taken in integrated circuit devices requiring very low level measurements. The measurement of current values in the high attoampere and the low femtoampere regime is particularly difficult in the presence of interfering sources that may be capable of generating current flow of electrons which, though minuscule, may be substantial relative to the very low voltage and low currents being measured.
In one known approach to providing a guarded and shielded chuck assembly, the assembly includes multiple conductive chuck elements spaced vertically and electrically insulated from each other. The upper chuck element supports the test wafer, and a conductive ring mechanically attached to one of the lower chuck elements surrounds the outer periphery of the chuck assembly to serve as a guard element. In such known assembly, an annular air gap between the chuck assembly elements and the surrounding guard ring serves as a dielectric to isolate the guard ring from the conductive wafer support element. A dielectric material may also be present in the annular gap. The size of the annular space provided in such a design directly affects its dielectric properties and capacitance, and in turn the degree of isolation from the support surface on which testing occurs. However, maintaining the desired registration between the chuck elements and the guard ring in such a design may be difficult. Even slight offsets in the associated mechanical connections between the various elements or in the shape of the guard ring can affect the registration and detrimentally alter the performance of the chuck.
Another known approach involves use of a chuck assembly in which the wafer support layer is a first conductive material sputtered on the upper surface of an insulator element, which in turn rests atop a second conductive chuck element. An electrically isolated dish has a bottom portion which extends laterally below the second conductive element, and an annular side wall which extends around the outer periphery of the chuck assembly and terminates vertically opposite the insulator element. The dish may be connected as a shield and the second conductive element as a guard. Such an approach may be suitable in certain applications, but does not provide significant guarding around the side periphery of the conductive support surface and the location of testing. In addition, with the annular side wall of the shield opposing the metal sputtered insulator element, parasitic and parallel capacitance may occur between the shield and the conductive test surface and distort test measurements.
Probe stations commonly include at least one manipulator that sits on the probe station platen and supports a probe holder, which is typically a metal shaft, either straight or bent, that holds the probe tip on one end and is held by the manipulator on the other. The probe tip is the part of the unit that actually touches the device under test. Both probe holders having built in tips and others using changeable or disposable tips have been developed. Several coaxial and triaxial probe assemblies are available for making low voltage or low current measurements. In a triaxial set up, the probe tip is connected to the center conductor of the triaxial cable, a middle conductor extending along the probe holder is driven as a guard and an outer shield conductor is referenced to ground. Such probe assemblies have been used for applications such as measuring device voltage and current, characterization of bi-polar and FET devices, and characterization of high speed devices.
One known triaxial probe assembly uses a conductive needle tip that is removably attached to the forward end of a horizontally extending probe holder for positioning the needle to engage the DUT. The needle projects at an angle to the longitudinal axis as it extends through an angled passageway in the holding portion. The tip is held in position via a set screw inserted into an internally threaded bore that opens to the forward end of the holding portion for pushing the needle against the passageway wall and clamping it against sliding movement.
One problem with the above-described arrangement is that there are competing considerations between using a set screw that is large enough to avoid stripping the screw threads while keeping the size of the holding portion including the set screw to a minimum for fitting the holding portion under a microscope so as not to obscure the line-of-sight to the area between the tip of the needle and DUT and for providing sufficient room to manipulate the probe tip in the area around the DUT, particularly where other probes are simultaneously being used on the same DUT. In practice, the holding portion is larger than desired and the set screw is still fairly small so that manipulation thereof has been found to be difficult.
Another problem is that clamping the very thin needle can create undue stresses on the needle shaft such as where the screw may cause small indentations or surface irregularities to form. It is these points where stress concentrations can occur leading to needle failure and requiring a time-consuming and tedious needle change-out operation, not to mention the loss of the cost of the broken tip.
Because of the precision placements of the tip that are required, it is essential that the

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