Wafer probe station for low-current measurements

Abstract
A probe station includes a fully guarded chuck assembly and connector mechanism for increasing sensitivity to low-level currents while reducing settling times. The chuck assembly includes a wafer-supporting first chuck element surrounded by a second chuck element having a lower component, skirting component and upper component each with a surface portion extending opposite the first element for guarding thereof. The connector mechanism is so connected to the second chuck element as to enable, during low-level current measurements, the potential on each component to follow that on the first chuck element as measured relative to an outer shielding enclosure surrounding each element. Leakage current from the first chuck element is thus reduced to virtually zero, hence enabling increased current sensitivity, and the reduced capacitance thus provided by the second chuck element decreases charging periods, hence reducing settling times. With similar operation and effect, where any signal line element of the connector mechanism is arranged exterior of its corresponding guard line element, such as adjacent the chuck assembly or on the probe-holding assembly, a guard enclosure is provided to surround and fully guard such signal line element in interposed relationship between that element and the outer shielding enclosure.
Description




BACKGROUND OF THE INVENTION




The present invention is directed to probe stations adapted for making highly accurate low-current and low-voltage measurements of wafers and other electronic test devices. More particularly, the invention relates to such a probe station having a guarding system for preventing current leakage, a Kelvin connection system to eliminate voltage losses caused by line resistances, and an electromagnetic interference (EMI) shielding system.




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 are all well known and discussed extensively in the technical literature. See, for example, an article by William Knauer entitled “Fixturing for Low-Current/Low-Voltage Parametric Testing,” appearing in


Evaluation Engineering


, November, 1990, pages 150-153. See also Hewlett-Packard, “Application Note 356-HP 4142B Modular DC Source/Monitor Practical Application,” (1987) pages 1-4, and Hewlett-Packard, H-P Model 4284A Precision LCR Meter,


Operation Manual


(1991) pages 2-1, 6-9, and 6-15.




In guarding applications, a conductor surrounding or otherwise closely adjacent to a low-current line or circuit is maintained at the same potential as the line or circuit to reduce leakage currents therefrom, so that low-current measurements can be made accurately.




Kelvin connections compensate for voltage losses caused by line resistances which would otherwise cause errors in low-voltage measurements. This is accomplished by providing a source line and a measurement line (also referred to commonly as “force” and “sense” lines, respectively) to an interconnection point (the Kelvin connection) which is as close to the test device as possible. A high-impedance voltmeter is connected to this interconnection point through the measurement line to accurately detect the voltage without any significant flow of current or resultant voltage drop in the measurement line. This avoids the error which would otherwise occur if the voltmeter were to detect the voltage through the source line, due to the voltage drop that occurs in the source line.




Probe stations have previously been used for conducting tests with guarding, Kelvin connection, and EMI shielding techniques. However the custom set-up of such probe stations required for guarding and Kelvin connection procedures is time-consuming and, in some instances, limited as to effectiveness. For example, in an article by Yousuke Yamamoto, entitled “A Compact Self-Shielding Prober for Accurate Measurement of On-Wafer Electron Devices,” appearing in


IEEE Transactions on Instrumentation and Measurement


, Volume 38, No. 6, December, 1989, pages 1088-1093, a probe station is shown having a respective detachable triaxial connector mounted on the probe card and the chuck assembly which supports the test device. The intermediate connector element of a triaxial connector normally is utilized for guarding purposes. However the chuck assembly shown has only a chuck and a shield, with no separate guarding structure to which the intermediate connector element could be connected. Accordingly significant time-consuming alteration of such a station would be required to obtain both a guarded and shielded chuck assembly. The probes on the probe card, on the other hand, are both guarded and shielded; however there is no means of enabling each probe to be moved independently of the others in unison with its guard and shield to accommodate different contact patterns of test devices, thus sacrificing flexibility of the probe station. Also, there is no provision for Kelvin connections on the chuck assembly, which would require more than a single triaxial connector as shown.




Chuck assemblies are available which provide guarding and shielding components. For example, Temptronic Corporation of Newton, Mass. markets a thermal chuck assembly atop which is mounted an “add-on” supporting surface for the test device, with a copper guarding layer interposed between the add-on surface and the underlying chuck assembly and insulated from each by respective sheets of insulating material. This structure permits a signal line to be soldered to the add-on surface, a guard line to be soldered to the copper guarding layer, and a ground line to be soldered to the underlying chuck assembly which can then serve as a shield. However such wiring requires time-consuming set-up, particularly if Kelvin connections are also required. Moreover, the use of sheet insulation to insulate the copper guarding layer from the add-on surface and the underlying chuck assembly fails to provide as low a dielectric constant between the respective elements as is desirable to minimize leakage currents in view of the low level of current to be measured.




With respect to probe stations that are designed to accommodate the measurement of low levels of current, a sensitivity threshold is normally encountered below which further improvements in current sensitivity are difficult to reliably achieve. In most commercial probe stations that are of such design, this sensitivity threshold is typically reached at about 20-50 femtoamps. However, improvements in device fabrication and in the capabilities of commercially available test instrumentation make it desirable to reduce the sensitivity threshold to a level reliably within the single digit femtoamp range.




A particular difficulty encountered in low-level current measurements is the excessive time required for measurement voltages to stabilize with reference to the device under test after a shift in voltage has occurred at the electrical input to the probe station. This problem of excessive settling time, as it is referred to, increases as the level of current under measurement is reduced. That is, due to the residual capacitance existing between spaced apart conductors in the region surrounding the immediate test site, a certain amount of time is required for the conductors that are in direct connection with the test device to fully charge or discharge to their desired voltages, and the time required will increase as the current through the device decreases. If the residual capacitance and the degree of input voltage shift are moderately large and if the level of current being measured is moderately small, the probe station operator can encounter settling times that are upwards of two or three minutes. Clearly, then, it is desirable that settling times be generally reduced in order to reduce overall measurement time, particularly where the device under test is a wafer containing large numbers of discrete devices, each of which may individually require low-level current testing.




In addition to settling effects, measurements of low level currents are also susceptible to error due to electrical discharge effects which occur because of the acceptance and release of charge by nonconductors in the region surrounding the immediate test site. At very low currents, these discharge effects can significantly distort measurement values and hence contribute to unacceptable levels of measurement instability.




SUMMARY OF THE INVENTION




The present invention solves the foregoing drawbacks of the prior probe stations by providing a probe station having integrated and ready-to-use guarding, Kelvin connection and shielding systems, both for individually movable probes and for the chuck assembly.




In further preferred embodiments of the invention, an improved guarding system is provided for accurate and rapid measurement of very low-level currents.




The chuck assembly of the present invention may in preferred embodiments thereof comprise at least first, second and third chuck assembly elements electrically insulated from one another and positioned at progressively greater distances from the probe(s) along the axis of approach between them. At least one detachable electrical connector assembly is provided on the chuck assembly having respective connector elements connected matingly to the first and second chuck assembly elements, respectively, so as to provide a ready-to-use guarding system requiring only the easy detachable connection of a guarded cable to the connector assembly for immediate operability.




Preferably, a second such detachable electrical connector assembly is also provided having its corresponding connector elements connected, in parallel with those of the first connector assembly, to the first and second chuck assembly elements so as to provide a ready-to-use guarded Kelvin connection on the chuck assembly which becomes, immediately operable by the easy detachable connection of a second guarded cable to the second connector assembly. Thus one cable serves as a guarded source line and the other serves as a guarded measurement line.




Leakage currents in the chuck assembly are preferably minimized by the fact that the three chuck assembly elements are electrically insulated from one another by distributed patterns of dielectric spacers, rather than continuous dielectric sheets, so that large air gaps are provided between the respective chuck assembly elements to reduce the dielectric constant in the gaps between the elements.




In further preferred embodiments of the present invention, the second chuck assembly element is provided with respective upper, lower and skirting components to provide full guarding for the first chuck assembly element. In particular, respective surface portions on the upper, lower and skirting components extend opposite the upper, lower and peripheral surfaces, respectively, of the first chuck assembly element. Furthermore, a connector mechanism is provided that enables a nonzero potential to be established on the first chuck assembly element relative to ground, that is, relative to the outer shielding enclosure, and a substantially equal potential to be established on the second chuck assembly element.




In accordance, then, with a preferred method of use, the exemplary chuck assembly structure just described is energized via the connector mechanism so that the potential on the first element is effectively matched by a substantially equal potential on the second element whereby virtually no potential difference is developed in the region between the elements. As a result of this relationship and the arrangement of components of the second chuck assembly element, leakage current from the first chuck assembly element is reduced to virtually zero which enables low-level currents to be measured with increased sensitivity. Furthermore, with respect to low-level current measurements, settling times during startup and switchover phases of operation are reduced. That is, the second chuck assembly element, unlike the first, acquires or releases charge at a rate not limited by the large effective resistance presented by the device under test. Accordingly, the respective guarding components are able to achieve their full potential relatively quickly even though they are directly coupled capacitively to conductive surfaces of large area such as those on the outer shielding enclosure. The respective guarding components also serve as an effective barrier to stray radiation to the extent they are interposed between the element emitting such radiation and the first chuck assembly element. Therefore, relative even to the low levels of current being measured, the potential error or instability in each measurement is reduced to an insignificant level.




Individually movable probe holders are provided having not only ready-to-use guarded signal line cables and Kelvin connection cables, but also respective shields for the cables of each probe, the shields being movable independently in unison with each probe separately.




Where a line element of the connector mechanism that carries the signal is arranged exterior of its corresponding guard element, such as where it is separated out from the guard element for interconnection with another signal element, preferably a conductive guard enclosure is provided which surrounds the signal line element in interposed relationship between such element and the outer shielding enclosure. Furthermore, when a nonzero potential is established during low-level current measurement on the signal line element relative to ground, that is, relative to the outer shielding enclosure, preferably the connector mechanism is so connected to the guard enclosure as to enable a substantially equal potential to be established on the guard enclosure.




The signal line guarding system just described can thus be energized via the connector mechanism so that virtually no potential difference is developed between the signal line element and its surrounding guard enclosure. Hence, the level of leakage current flowing away from the signal line element is reduced to virtually zero which enables low-level currents in the system to be measured with increased sensitivity. Also, since there is a reduction in the combined area of the conductive surfaces to which the signal line element is capacitively coupled, less energy transfer and time is required for this line element to acquire its full potential, so that settling time is reduced. Moreover, if any transient shifts in electrical state should occur in relation to any nonconductor or conductor located outside the guard enclosure, this will have virtually no effect on the signal line element due to the effective barrier against radiation provided by the conductive guard enclosure, so that measurement instability is reduced.




The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.











BRIEF DESCRIPTION OF THE DRAWINGS





FIG. 1

is a partial front view of an exemplary embodiment of a wafer probe station constructed in accordance with the present invention.





FIG. 2

is a top view of the wafer probe station of FIG.


1


.





FIG. 2A

is a partial top view of the wafer probe station of

FIG. 1

with the enclosure door shown partially open.





FIG. 3

is a partially sectional and partially schematic front view of the probe station of FIG.


1


.





FIG. 3A

is an enlarged sectional view taken along line


3


A—


3


A of FIG.


3


.





FIG. 4

is a top view of the sealing assembly where the motorized positioning mechanism extends through the bottom of the enclosure.





FIG. 5A

is an enlarged top detail view taken along line


5


A—


5


A of FIG.


1


.





FIG. 5B

is an enlarged top sectional view taken along line


5


B—


5


B of FIG.


1


.





FIG. 6

is a partially schematic top detail view of the chuck assembly, taken along line


6





6


of FIG.


3


.





FIG. 7

is a partially sectional front view of the chuck assembly of FIG.


6


.





FIG. 8

is a partially sectional side view of a probe holder and probe.





FIG. 9

is a partially sectional bottom view taken along line


9





9


of FIG.


8


.





FIG. 10

is a partially sectional front view of an alternative exemplary embodiment of a wafer probe station constructed in accordance with the present invention.





FIG. 11

is a front detail view showing the lower elements of the chuck assembly of the wafer probe station of

FIG. 10

with hidden portions shown in cut-away view.





FIG. 12

is a partial top detail view showing the connector mechanism and the lower elements of the chuck assembly as viewed along line


12





12


of FIG.


10


.





FIG. 13

is a partial top view of the wafer probe station of

FIG. 10

with the outer enclosure door shown partially open.





FIG. 14

is a bottom view of an optional conductive panel in position on the upper guard component as viewed along line


14





14


in FIG.


10


.





FIG. 15

is a partially sectional side view of an alternative exemplary probe holder which is suitable for use in association with the wafer probe station of FIG.


10


.





FIG. 16

is a partially sectional bottom view taken along line


16





16


of

FIG. 15

with hidden portions shown in cut-away view.











DESCRIPTION OF THE PREFERRED EMBODIMENTS GENERAL ARRANGEMENT OF PROBE STATION




With reference to

FIGS. 1

,


2


and


3


, an exemplary embodiment of the probe station of the present invention comprises a base


10


(shown partially) which supports a platen


12


through a number of jacks


14




a


,


14




b


,


14


c,


14




d


which selectively raise and lower the platen vertically relative to the base by a small increment (approximately one-tenth of an inch) for purposes to be described hereafter. Also supported by the base


10


of the probe station is a motorized positioner


16


having a rectangular plunger


18


which supports a movable chuck assembly


20


for supporting a wafer or other test device. The chuck assembly


20


passes freely through a large aperture


22


in the platen


12


which permits the chuck assembly to be moved independently of the platen by the positioner


16


along X, Y and Z axes, i.e. horizontally along two mutually-perpendicular axes X and Y, and vertically along the Z axis. Likewise, the platen


12


, when moved vertically by the jacks


14


, moves independently of the chuck assembly


20


and the positioner


16


.




Mounted atop the platen


12


are multiple individual probe positioners such as


24


(only one of which is shown), each having an extending member


26


to which is mounted a probe holder


28


which in turn supports a respective probe


30


for contacting wafers and other test devices mounted atop the chuck assembly


20


. The probe positioner


24


has micrometer adjustments


34


,


36


and


38


for adjusting the position of the probe holder


28


, and thus the probe


30


, along the X, Y and Z axes respectively, relative to the chuck assembly


20


. The Z axis is exemplary of what is referred to herein loosely as the “axis of approach” between the probe holder


28


and the chuck assembly


20


, although directions of approach which are neither vertical nor linear, along which the probe tip and wafer or other test device are brought into contact with each other, are also intended to be included within the meaning of the term “axis of approach.” A further micrometer adjustment


40


adjustably tilts the probe holder


28


to adjust planarity of the probe with respect to the wafer or other test device supported by the chuck assembly


20


. As many as twelve individual probe positioners


24


, each supporting a respective probe, may be arranged on the platen


12


around the chuck assembly


20


so as to converge radially toward the chuck assembly similarly to the spokes of a wheel. With such an arrangement, each individual positioner


24


can independently adjust its respective probe in the X, Y and Z directions, while the jacks


14


can be actuated to raise or lower the platen


12


and thus all of the positioners


24


and their respective probes in unison.




An environment control outer enclosure is composed of an upper box portion


42


rigidly attached to the platen


12


, and a lower box portion


44


rigidly attached to the base


10


. Both portions are made of steel or other suitable electrically conductive material to provide EMI shielding. To accommodate the small vertical movement between the two box portions


42


and


44


when the jacks


14


are actuated to raise or lower the platen


12


, an electrically conductive resilient foam gasket


46


, preferably composed of silver or carbon-impregnated silicone, is interposed peripherally at their mating juncture at the front of the enclosure and between the lower portion


44


and the platen


12


so that an EMI, substantially hermetic, and light seal are all maintained despite relative vertical movement between the two box portions


42


and


44


. Even though the upper box portion


42


is rigidly attached to the platen


12


, a similar gasket


47


is preferably interposed between the portion


42


and the top of the platen to maximize sealing.




With reference to

FIGS. 5A and 5B

, the top of the upper box portion


42


comprises an octagonal steel box


48


having eight side panels such as


49




a


and


49




b


through which the extending members


26


of the respective probe positioners


24


can penetrate movably. Each panel comprises a hollow housing in which a respective sheet


50


of resilient foam, which may be similar to the above-identified gasket material, is placed. Slits such as


52


are partially cut vertically in the foam in alignment with slots


54


formed in the inner and outer surfaces of each panel housing, through which a respective extending member


26


of a respective probe positioner


24


can pass movably. The slitted foam permits X, Y and Z movement of the extending members


26


of each probe positioner, while maintaining the EMI, substantially hermetic, and light seal provided by the enclosure. In four of the panels, to enable a greater range of X and Y movement, the foam sheet


50


is sandwiched between a pair of steel plates


55


having slots


54


therein, such plates being slidable transversely within the panel housing through a range of movement encompassed by larger slots


56


in the inner and outer surfaces of the panel housing.




Atop the octagonal box


48


, a circular viewing aperture


58


is provided, having a recessed circular transparent sealing window


60


therein. A bracket


62


holds an apertured sliding shutter


64


to selectively permit or prevent the passage of light through the window. A stereoscope (not shown) connected to a CRT monitor can be placed above the window to provide a magnified display of the wafer or other test device and the probe tip for proper probe placement during set-up or operation. Alternatively, the window


60


can be removed and a microscope lens (not shown) surrounded by a foam gasket can be inserted through the viewing aperture


58


with the foam providing EMI, hermetic and light sealing.




The upper box portion


42


of the environment control enclosure also includes a hinged steel door


68


which pivots outwardly about the pivot axis of a hinge


70


as shown in FIG.


2


A. The hinge biases the door downwardly toward the top of the upper box portion


42


so that it forms a tight, overlapping, sliding peripheral seal


68




a


with the top of the upper box portion. When the door is open, and the chuck assembly


20


is moved by the positioner


16


beneath the door opening as shown in

FIG. 2A

, the chuck assembly is accessible for loading and unloading.




With reference to

FIGS. 3 and 4

, the sealing integrity of the enclosure is likewise maintained throughout positioning movements by the motorized positioner


16


due to the provision of a series of four sealing plates


72


,


74


,


76


and


78


stacked slidably atop one another. The sizes of the plates progress increasingly from the top to the bottom one, as do the respective sizes of the central apertures


72




a


,


74




a


,


76




a


and


78




a


formed in the respective plates


72


,


74


,


76


and


78


, and the aperture


79




a


formed in the bottom


44




a


of the lower box portion


44


. The central aperture


72




a


in the top plate


72


mates closely around the bearing housing


18




a


of the vertically-movable plunger


18


. The next plate in the downward progression, plate


74


, has an upwardly-projecting peripheral margin


74




b


which limits the extent to which the plate


72


can slide across the top of the plate


74


. The central aperture


74




a


in the plate


74


is of a size to permit the positioner


16


to move the plunger


18


and its bearing housing


18




a


transversely along the X and Y axes until the edge of the top plate


72


abuts against the margin


74




b


of the plate


74


. The size of the aperture


74




a


is, however, too small to be uncovered by the top plate


72


when such abutment occurs, and therefore a seal is maintained between the plates


72


and


74


regardless of the movement of the plunger


18


and its bearing housing along the X and Y axes. Further movement of the plunger


18


and bearing housing in the direction of abutment of the plate


72


with the margin


74




b


results in the sliding of the plate


74


toward the peripheral margin


76




b


of the next underlying plate


76


. Again, the central aperture


76




a


in the plate


76


is large enough to permit abutment of the plate


74


with the margin


76




b


, but small enough to prevent the plate


74


from uncovering the aperture


76




a


, thereby likewise maintaining the seal between the plates


74


and


76


. Still further movement of the plunger


18


and bearing housing in the same direction causes similar sliding of the plates


76


and


78


relative to their underlying plates into abutment with the margin


78




b


and the side of the box portion


44


, respectively, without the apertures


78




a


and


79




a


becoming uncovered. This combination of sliding plates and central apertures of progressively increasing size permits a full range of movement of the plunger


18


along the X and Y axes by the positioner


16


, while maintaining the enclosure in a sealed condition despite such positioning movement. The EMI sealing provided by this structure is effective even with respect to the electric motors of the positioner


16


, since they are located below the sliding plates.




Chuck Assembly




With particular reference to

FIGS. 3

,


6


and


7


, the chuck assembly


20


is of a unique modular construction usable either with or without an environment control enclosure. The plunger


18


supports an adjustment plate


79


which in turn supports first, second and third chuck assembly elements


80


,


81


and


83


, respectively, positioned at progressively greater distances from the probe(s) along the axis of approach. The lower chuck assembly element


83


is a conductive rectangular stage or shield


83


which detachably mounts conductive elements


80


and


81


of circular shape. In addition to having a lower surface


160


and a peripheral surface


162


, the upper chuck assembly element


80


has a planar upwardly-facing wafer-supporting or upper surface


82


having an array of vertical apertures


84


therein. These apertures communicate with respective chambers separated by O-rings


88


, the chambers in turn being connected separately to different vacuum lines


90




a


,


90




b


,


90




c


(

FIG. 6

) communicating through separately-controlled vacuum valves (not shown) with a source of vacuum. The respective vacuum lines selectively connect the respective chambers and their apertures to the source of vacuum to hold the wafer, or alternatively isolate the apertures from the source of vacuum to release the wafer, in a conventional manner. The separate operability of the respective chambers and their corresponding apertures enables the chuck to hold wafers of different diameters.




In addition to the circular elements


80


and


81


, auxiliary chucks such as


92


and


94


are detachably mounted on the corners of the element


83


by screws (not shown) independently of the elements


80


and


81


for the purpose of supporting contact substrates and calibration substrates while a wafer or other test device is simultaneously supported by the element


80


. Each auxiliary chuck


92


,


94


has its own separate upwardly-facing planar surface


100


,


102


respectively, in parallel relationship to the surface


82


of the element


80


. Vacuum apertures


104


protrude through the surfaces


100


and


102


from communication with respective chambers within the body of each auxiliary chuck. Each of these chambers in turn communicates through a separate vacuum line and a separate independently-actuated vacuum valve (not shown) with a source of vacuum, each such valve selectively connecting or isolating the respective sets of apertures


104


with respect to the source of vacuum independently of the operation of the apertures


84


of the element


80


, so as to selectively hold or release a contact substrate or calibration substrate located on the respective surfaces


100


and


102


independently of the wafer or other test device. An optional metal shield


106


may protrude upwardly from the edges of the element


83


to surround or skirt the other elements


80


,


81


and the auxiliary chucks


92


,


94


.




All of the chuck assembly elements


80


,


81


and


83


, as well as the additional chuck assembly element


79


, are electrically insulated from one another even though they are constructed of electrically conductive metal and interconnected detachably by metallic screws such as


96


. With reference to

FIGS. 3 and 3A

, the electrical insulation results from the fact that, in addition to the resilient dielectric O-rings


88


, dielectric spacers


85


and dielectric washers


86


are provided. These, coupled with the fact that the screws


96


pass through oversized apertures in the lower one of the two elements which each screw joins together thereby preventing electrical contact between the shank of the screw and the lower element, provide the desired insulation. As is apparent in

FIG. 3

, the dielectric spacers


85


extend over only minor portions of the opposing surface areas of the interconnected chuck assembly elements, thereby leaving air gaps between the opposing surfaces over major portions of their respective areas. Such air gaps minimize the dielectric constant in the spaces between the respective chuck assembly elements, thereby correspondingly minimizing the capacitance between them and the ability for electrical current to leak from one element to another. Preferably the spacers and washers


85


and


86


, respectively, are constructed of a material having the lowest possible dielectric constant consistent with high dimensional stability and high volume resistivity. A suitable material for the spacers and washers is glass epoxy, or acetal homopolymer marketed under the trademark Delrin by E.I. DuPont.




With reference to

FIGS. 6 and 7

, the chuck assembly


20


also includes a pair of detachable electrical connector assemblies designated generally as


108


and


110


, each having at least two conductive connector elements


108




a


,


108




b


and


110




a


,


110




b


, respectively, electrically insulated from each other, with the connector elements


108




b


and


110




b


preferably coaxially surrounding the connector elements


108




a


and


110




a


as guards therefor. If desired, the connector assemblies


108


and


110


can be triaxial in configuration so as to include respective outer shields


108




c


,


110




c


surrounding the respective connector elements


108




b


and


110




b


, as shown in FIG.


7


. The outer shields


108




c


and


110




c


may, if desired, be connected electrically through a shielding box


112


and a connector supporting bracket


113


to the chuck assembly element


83


, although such electrical connection is optional particularly in view of the surrounding EMI shielding enclosure


42


,


44


. In any case, the respective connector elements


108




a


and


110




a


are electrically connected in parallel to a connector plate


114


matingly and detachably connected along a curved contact surface


114




a


by screws


114




b


and


114




c


to the curved edge of the chuck assembly element


80


. Conversely, the connector elements


108




b


and


110




b


are connected in parallel to a connector plate


116


similarly matingly connected detachably to element


81


. The connector elements pass freely through a rectangular opening


112




a


in the box


112


, being electrically insulated from the box


112


and therefore from the element


83


, as well as being electrically insulated from each other. Set screws such as


118


detachably fasten the connector elements to the respective connector plates


114


and


116


.




Either coaxial or, as shown, triaxial cables


118


and


120


form portions of the respective detachable electrical connector assemblies


108


and


110


, as do their respective triaxial detachable connectors


122


and


124


which penetrate a wall of the lower portion


44


of the environment control enclosure so that the outer shields of the triaxial connectors


122


,


124


are electrically connected to the enclosure. Further triaxial cables


122




a


,


124




a


are detachably connectable to the connectors


122


and


124


from suitable test equipment such as a Hewlett-Packard 4142B modular DC source/monitor or a Hewlett-Packard 4284A precision LCR meter, depending upon the test application. If the cables


118


and


120


are merely coaxial cables or other types of cables having only two conductors, one conductor interconnects the inner (signal) connector element of a respective connector


122


or


124


with a respective connector element


108




a


or


110




a


, while the other conductor connects the intermediate (guard) connector element of a respective connector


122


or


124


with a respective connector element


108




b


,


110




b.






In any case, the detachable connector assemblies


108


,


110


, due to their interconnections with the two connector plates


114


,


116


, provide immediately ready-to-use signal and guard connections to the chuck assembly elements


80


and


81


, respectively, as well as ready-to-use guarded Kelvin connections thereto. For applications requiring only guarding of the chuck assembly, as for example the measurement of low-current leakage from a test device through the element


80


, it is necessary only that the operator connect a single guarded cable


122




a


from a test instrument such as a Hewlett-Packard 4142B modular DC source/monitor to the detachable connector


122


so that a signal line is provided to the chuck assembly element


80


through the connector element


108




a


and connector plate


114


, and a guard line is provided to the element


81


through the connector element


108




b


and connector plate


116


. Alternatively, if a Kelvin connection to the chuck assembly is desired for low-voltage measurements, such as those needed for measurements of low capacitance, the operator need merely attach a pair of cables


122




a


and


124




a


to the respective connectors


122


,


124


from a suitable test instrument such as a Hewlett-Packard 4284A precision LCR meter, thereby providing both source and measurement lines to the element


80


through the connector elements


108




a


and


110




a


and connector plate


114


, and guarding lines to the element


81


through the connector elements


108




b


and


110




b


and connector plate


116


.




Probe Assembly




With reference to

FIGS. 5B

,


8


and


9


, respective individually movable probes


30


comprising pairs of probe elements


30




a


are supported by respective probe holders


28


which in turn are supported by respective extending portions


26


of different probe positioners such as


24


. Atop each probe positioner


24


is a shield box


126


having a pair of triaxial connectors


128


,


130


mounted thereon with respective triaxial cables


132


entering each triaxial connector from a suitable test instrument as mentioned previously. Each triaxial connector includes a respective inner connector element


128




a


,


130




a


, an intermediate connector element


128




b


,


130




b


, and an outer connector element


128




c


,


130




c


in concentric arrangement. Each outer connector element


128




c


,


130




c


terminates by connection with the shield box


126


. Conversely, the inner connector elements


128




a


,


130




a


, and the intermediate connector elements


128




b


,


130




b


, are connected respectively to the inner and outer conductors of a pair of coaxial cables


134


,


136


which therefore are guarded cables. Each cable


134


,


136


terminates through a respective coaxial connector


138


,


140


with a respective probe element


30




a


having a center conductor


142


surrounded by a guard


144


. In order to provide adequate shielding for the coaxial cables


134


,


136


, especially in the region outside of the octagonal box


48


, an electrically-conductive shield tube


146


is provided around the cables


134


,


136


and electrically connected through the shield box


126


with the outer connector element


128




c


,


130




c


of the respective triaxial connectors


128


,


130


. The shield tube


146


passes through the same slit in the foam


50


as does the underlying extending member


26


of the probe positioner


24


. Thus, each individually movable probe


30


has not only its own separate individually movable probe holder


28


but also its own individually movable shield


146


for its guarded coaxial cables, which shield is movable in unison with the probe holder independently of the movement of any other probe holder by any other positioning mechanism


24


. This feature is particularly advantageous because such individually movable probes are normally not equipped for both shielded and guarded connections, which deficiency is solved by the described structure. Accordingly, the probes


30


are capable of being used with the same guarding and Kelvin connection techniques in a ready-to-use manner as is the chuck assembly


20


, consistently with full shielding despite the individual positioning capability of each probe


30


.




Preferred Alternative Embodiment of the Probe Station





FIG. 10

depicts a preferred alternative embodiment


220


of the wafer probe station which, like the basic embodiment depicted in

FIG. 3

, has the capability for providing guarded and Kelvin connections to the device under test but which also has additional features for facilitating extremely sensitive low-level current measurements. In particular, the alternative embodiment


220


includes a fully guarded movable chuck assembly


221


and a fully guarded probe-holding assembly


223


. These features are described below in further detail each under a separate subheading.




In the respective drawings of the alternative probe station


220


and the basic probe station, like reference numerals have been used to identify elements that are common to both systems. Thus, comparing

FIGS. 3 and 10

, it will be evident that the fully guarded movable chuck assembly


221


is carried on a rectangular plunger


18


for movement along X, Y and Z axes under the control of a motorized positioner


16


. As indicated by dashed lines in

FIG. 10

, the movable chuck assembly


221


has predetermined outer limits of horizontal movement


225


which, as previously described, are the result of interfering interaction between the upstanding margins which are on the bottom sealing plates


72


,


74


,


76


, and


78


.





FIG. 10

also shows a dashed line


227


signifying Z-axis or vertical movement of the chuck assembly


221


. The expansibility of resilient gasket


46


together with the limited vertical adjustability of the platen


12


provide a further mechanism, in addition to that of the motorized positioner, for shifting the chuck assembly


221


vertically relative to the upper half


42


of the environment control enclosure box. For the sake of convenience, the upper and lower halves


42


and


44


of the control enclosure will hereafter be collectively referred to as the outer shielding enclosure


229


to emphasize their importance in providing shielding for the chuck assembly against outside electromagnetic interference. At the same time, however, it will be recognized that the outer enclosure has several other significant functions including gas containment, light shielding and temperature control.




In certain respects, the connector mechanism


231


of the alternative probe station


220


resembles that of the basic probe station. For example, in order to enable low-voltage measurements to be made in relation to the chuck assembly


221


, the connector mechanism


231


includes both a source line and a measurement line to provide Kelvin-type connections to the chuck assembly. In particular, referring also to

FIG. 12

, the source and measurement lines each include an exterior connector


232


and


233


, a flexible connector assembly


235


and


237


, and an interior connector


239


or


241


, respectively. For purposes of low-level current measurement, either of these lines can be used, and thus the broader term signal line, as used hereinbelow, will be understood to refer to a line that is of either type.




In relation to the chuck assembly


221


, the exterior connectors


232


and


233


are mounted, as previously, on a vertical wall of the outer shielding enclosure


229


where they are accessible for detachable connection to an external signal line (e.g.,


243


or


245


) which is connected, in turn, to an external test instrument (not shown). The interior connectors


239


and


241


are mounted adjacent the chuck assembly


221


. Preferably, the flexible connector assemblies


235


and


237


each include an end connecting member by which such assembly is fastened detachably to its corresponding interior connector so that fuller access to the sides of the chuck assembly can be obtained, as needed, in order to facilitate replacement of particular chuck assembly elements. Each connector assembly


235


and


237


is flexible in order to accommodate relative movement between the chuck assembly


221


and the outer shielding enclosure


229


.




Preferably, the exterior connectors


232


and


233


, the connector assemblies


235


and


237


and the interior connectors


239


and


241


are each of triaxial configuration, that is, each includes a center (signal) conductor surrounded by an intermediate (guard) conductor which, in turn, is surrounded by an outer (shield) conductor. These elements, alternatively, can be of coaxial configuration if individual line shielding is not employed. The connector mechanism


231


as it relates to the chuck assembly


221


is further described under the subheading immediately below and, in particular, it is therein described how such mechanism differs from that of the basic probe station due to its fully guarded construction. That portion


231




a


of the connector mechanism relating to the probe-holding assembly


223


is described below under the separate subheading pertaining thereto.




Fully Guarded Chuck Assembly and Connector Mechanism




Referring to

FIG. 10

, as in the basic probe station, the chuck assembly


221


of the alternative probe station


220


includes a first or upper chuck assembly element


280


, a second or lower chuck assembly element


281


and a third chuck assembly element


283


which detachably mounts the first two elements. Referring also to

FIG. 11

, as in the basic system, the respective chuck assembly elements are electrically isolated from each other including by dielectric spacers


85


and O-rings


88


, and the first chuck assembly element has an upper surface


285


for horizontally supporting a test device, a lower surface


287


opposite the upper surface and a peripheral surface


289


vertically interconnecting the upper and lower surfaces.




However, in the alternative probe station


220


, the construction of the second chuck assembly element


281


is different than that previously described in certain important respects. In particular, in addition to having a lower component


291


, the second chuck assembly element further includes a skirting component


293


and an upper component


295


. These components, as explained in greater detail below, are electrically connected with each other and are arranged relative to each other so as to surround the first chuck assembly element


280


on all sides. More specifically, a surface portion


291




a


included on the lower component extends opposite the entire portion of the lower surface


287


on the first chuck assembly element, a surface portion


293




a


included on the skirting component extends opposite the entire portion of the peripheral surface


289


on the first chuck assembly element and a surface portion


295




a


included on the upper component extends opposite the entire portion of the upper surface


285


on the first chuck assembly element. Moreover, these relationships are maintained even when the chuck assembly


221


is brought to its predetermined outer limits of horizontal movement


225


. Thus, the surface portion


295




a


on the upper component is maintained opposite the entire portion of the upper surface


285


on the first chuck assembly element despite relative movement occurring therebetween.




Viewing this arrangement somewhat differently, it will be recognized that relative to any location on the respective surfaces


285


,


287


and


289


of the first chuck assembly element


280


, the second chuck assembly element


281


is considerably closer to such location than is the outer shielding enclosure


229


even along those angles of approach which do not lie perpendicular to such surfaces. Accordingly, electromagnetic interaction between the first chuck assembly element and its neighboring environment is only able to occur in relation to the second chuck assembly element. However, as fully described below, the connector mechanism


231


is so constructed as to enable the voltage potential on the second chuck assembly element to follow the potential which is on the first chuck assembly element. In accordance with this relationship, then, the first chuck assembly element is effectively isolated electrically from its neighboring environment.




In the preferred alternative probe station


220


depicted in

FIGS. 10-12

, the skirting component


293


is formed from a closed-sided strip of conductive material such as tin-plated steel. The strip is connected both mechanically and electrically to the lower component


291


by a plurality of threaded steel bolts


297


. Metal washers


299


which are seated on the bolts maintain the skirting component


293


in radially spaced-apart surrounding relationship to the first chuck assembly element


280


. In this manner, the surface portion


293




a


of the skirting component and the peripheral surface


289


of the first chuck assembly element are separated from each other by an open gap


301


so that the capacitance between these respective surfaces is minimized.




Referring to

FIG. 10

, the upper component


295


of the preferred alternative probe station


220


is formed from a sheet of conductive material such as tin-plated steel. The upper side of the sheet is attached to the top of the outer shielding enclosure


229


by several strips of insulative foam tape having double-sided adhesive as of a type sold commercially, for example, by the 3M Company based in St. Paul, Minn. In this manner, the upper component


295


is held in spaced relationship above the skirting component


293


so that each is separated from the other by an open gap.




The above form of construction is preferred over one in which no gap is provided between the skirting component


293


and the upper component


295


as may be achieved, for example, by fitting a resilient conductive gasket to the skirting component in such a manner that the gasket bridges the gap between the respective components. In this alternative but less desired form of construction, it is difficult to completely avoid abrasion of the upper component because the gasket or other bridging element will rub across the upper component when that component shifts horizontally relative to the outer shielding enclosure


227


. In this alternative construction, then, it is possible for small filings or other debris to be swept from the abraded surface of the upper component


295


into the central testing area causing possible damage to the device under test. In the preferred form of construction, on the other hand, the possibility of such damage has been avoided.




Centrally formed in the conductive sheet comprising the upper component


295


is a probing aperture


307


. As indicated in

FIG. 10

, the extreme end of each individual probe


30


can be inserted through this probing aperture in order to make contact with a wafer supported for test on the first chuck assembly element


280


. Referring also to

FIG. 14

, which shows the view looking toward the surface portion


295




a


of the upper component, the probing aperture


307


has an irregular diameter, that is, it is of a cross-like shape. As an option, a conductive panel


309


is preferably provided that selectively fits detachably over the probing aperture and that includes a central opening


311


, smaller in size than the probing aperture


307


, through which the extreme end of the electrical probe can be inserted, as shown. Because of its relatively smaller opening, the conductive panel


309


tends to reduce somewhat the range of horizontal movement of each electrical probe but, correspondingly, tends to increase the degree of electromagnetic isolation between the first chuck assembly element


280


and the outer shielding enclosure


229


since it extends the effective surface area of the surface portion


295




a


of the upper component. Hence, the conductive panel is particularly suited for use in those applications in which extremely sensitive current measurements are needed. Referring again to

FIG. 14

, the exemplary conductive panel


309


has a cross-like shape so that it covers the probing aperture


307


with only a small margin of overlap. Referring to

FIGS. 10 and 14

together, conductive pegs


313


project outwardly from the underside of the conductive panel. These pegs, as shown, are arranged into opposing pairs so that each pair can be wedged snugly between opposite corners of the probing aperture, thus preventing rotation of the conductive panel in its seated position on the upper component.




Referring to

FIG. 13

, the outer shielding enclosure


229


includes a loading aperture


315


,through which access to the chuck assembly


221


is obtained and a hinged door


68


for opening and closing the loading aperture. Along this portion of the outer shielding enclosure, the upper component


295


is divided into respective first and second sections


317


and


319


. The first section


317


is mounted inside the door for movement with the door as the door is being opened, and the second section


319


is mounted behind the surrounding portion


321


of the outer shielding enclosure. As previously described, insulated foam tape having double-sided adhesive is used to mount these sections so that each is electrically isolated from its respective mounting surface. As shown in

FIG. 13

, the outer edge


317




a


of the first section is slightly offset inwardly from the edge of the door


68


so that when the door is moved to its closed position in slight marginal overlap with the surrounding portion


321


, this brings the two sections


317


and


319


into physical contact with each other along an extended portion of their respective outer edges. To further ensure that there is good electrical contact between the first and second sections of the upper component, a conductive tab


323


is soldered to the underside or surface portion


295




a


of the first section so that when the door is closed such tab can establish oxide-removing wiping electrical contact with the underside or surface portion


295




a


of the second section.




In the preferred probe station


220


, not only is the chuck assembly


221


fully guarded but so too is the connector mechanism


231


. In particular, referring to

FIGS. 10 and 12

, the signal lines of the connector mechanism


231


by which the chuck assembly is energized are fully guarded by a first box-like inner guard enclosure


325


and a second box-like inner guard enclosure


327


. As is explained under the next subheading below, there is also a third box-like inner guard enclosure


329


(refer to

FIG. 15

) to provide guarding for that portion


231




a


of the connector mechanism associated with each probe-holding assembly


223


.




With respect to the ground connections established via the connector mechanism


231


, the outer conductor of each exterior connector


232


and


233


is electrically connected through the outer shell of such connector to the outer shielding enclosure


229


. Respective grounding straps


235




c


and


237




c


electrically interconnect the outer conductor of each connector assembly


235


and


237


, respectively, to the outer shielding enclosure. The outer conductor of each interior connector


239


and


241


is connected electrically through the outer shell of such connector to the third chuck assembly element


283


via a metal flange


331


that projects outwardly from the side of the third chuck assembly element. Accordingly, if detachable connection is made between either connector assembly


235


or


237


and the corresponding interior connector


239


or


241


, the third chuck assembly element


283


and the outer shielding enclosure


229


are then tied to the same potential, that is, to the ground potential of the system as maintained at either exterior connector


232


or


233


via the outer conductor of the external signal line (e.g.,


243


or


245


).




The inner and intermediate conductors of the interior connector


239


are separated out from their respective insulating members so as to form a signal (source) line element and a guard line element


239




a


and


239




b


, respectively. In relation to an inner or intermediate conductor, the term “line element” as used herein and in the claims is intended to refer to such conductor along any portion thereof where it is arranged exterior of its outside conductor(s), even if at some portion further back from its end the inner or intermediate conductor is surrounded by the outside conductor(s).




Referring also to

FIG. 11

, in similar manner, the inner and intermediate conductors of the interior connector


241


are separated out from their respective insulating members so as to form a signal (measurement) line element and a guard line element


241




a


and


241




b


, respectively. The respective signal line elements


239




a


and


241




a


are electrically tied together at the first chuck assembly element


280


thereby establishing a Kelvin connection with respect thereto. In particular, these signal line elements are inserted into respective holes


333


and


335


which are formed in the peripheral edge of the first chuck assembly element


280


where they are held detachably in place each by a respective set screw


337


or


339


that is adjusted by means of turning to its respective clamping position.




In order to provide full guarding in relation to each of the respective signal line elements


239




a


and


241




a


, a first box-like inner guard enclosure


325


is provided which is so arranged that it surrounds these elements in interposed relationship between them and the outer shielding enclosure


229


. In the preferred embodiment depicted, tin-plated steel panels are used to construct the first guard enclosure. In order to enable the leakage current flowing from either of the signal line elements


239




a


or


241




a


to be reduced to a negligible level, each of the guard line elements


239




b


and


241




b


is electrically connected, as by soldering, to the enclosure


325


, preferably on an inside wall thereof. Accordingly, by appropriate adjustment of the guard potential as carried by either guard line element


239




b


or


241




b


, the potential on the guard enclosure can be controlled so as to substantially follow the signal potential which is carried either by the signal (source) line element


239




a


or by the signal (measurement) line element


241




a


. Since leakage current from either signal line element


239




a


or


241




a


can thus be reduced to virtually zero, the measurement of very low-level currents can be made via either element. Moreover, to the extent that field disturbances occur in the region surrounding the first guard enclosure, such disturbances will be resolved at the first guard enclosure without affecting the stability at the signal as carried by either signal line element.




As indicated in

FIGS. 11 and 12

, the first guard enclosure


325


has a step


341


in its floor panel so that no part of the enclosure comes into either physical or electrical contact with the third chuck assembly element


83


. The first guard enclosure is electrically connected at its inside edges


345


to the skirting component


293


, as by soldering. Hence the guard potential as carried by either of the guard line elements


239




b


or


241




b


is conveyed to the lower and skirting components of the second chuck assembly element


281


via the first guard enclosure


325


, thereby enabling these components to provide guarding in relation to the first chuck assembly element


280


. The enclosure further forms a passage


347


that opens towards the first chuck assembly element


280


. In this manner, the respective signal line elements


239




a


and


241




a


are completely enclosed for full guarding by the first guard enclosure


325


as they extend through this passage for parallel electrical connection with the first chuck assembly element.




As previously mentioned, the various components of the second chuck assembly element


281


are electrically connected to each other, that is, the upper component


295


is electrically connected to the skirting component


293


as well as to the lower component


291


. In order to obtain this connection to the upper component, a coupling assembly


349


is provided. This coupling assembly is so constructed that the guard potential as carried by the intermediate (guard) conductor of either exterior connector


232


or


233


can be conveyed to the upper component via such coupling assembly in addition, for example, to being conveyed to the lower and skirting components via either of the guard line elements


239




b


or


241




b.






Referring to

FIG. 10

, the coupling assembly


379


preferably acquires the guard potential at a fixed connection point located adjacent the exterior connectors


232


and


233


. In preparation for this connection, the inner and intermediate conductors of the exterior connector


232


are separated out from their respective insulating members so as to form a signal (source) line element and a guard line element


232




a


and


232




b


, respectively. Similarly, the inner and intermediate conductors of the exterior connector


233


are separated out so as to form a signal (measurement) line element and a guard line element


233




a


and


233




b


, respectively. Opposite the exterior connector


232


, the inner and intermediate conductors of the connector assembly


235


are separated out to form a signal (source) line element and a guard line element


235




a


and


235




b


, respectively, while opposite the exterior connector


233


the inner and intermediate conductor of the connector assembly


237


are separated out to form a signal (measurement) line element and a guard line element


237




a


and


237




b


, respectively. As shown, the corresponding pairs of signal line elements are directly connected electrically by, for example, soldering signal line element


232




a


to


235




a


(to join the source line) and signal line element


233




a


to


237




a


(to join the measurement line).




In order to provide full guarding in relation to each of the corresponding pairs of signal line elements


232




a


and


235




a


or


233




a


and


237




a


, a second box-like inner guard enclosure


327


is provided which is so arranged that it surrounds these elements in interposed relationship between them and the outer shielding enclosure


229


. In the preferred embodiment depicted, tin-plated steel panels are used to construct the second guard enclosure. In order to enable the leakage current flowing from either of these pairs of signal line elements to be reduced to a negligible level, each of the guard line elements


232




b


,


233




b


,


235




b


and


237




b


is electrically connected, as by soldering, to the second guard enclosure


327


, preferably on an inside wall thereof. Hence, by appropriate adjustment of the guard potential as carried by either guard line element


232




b


or


233




b


, the potential on the guard enclosure can be controlled so as to substantially follow the signal potential that is carried either by the pair of signal line elements


232




a


and


235




a


or by the pair of signal line elements


233




a


and


237




a


. Since leakage current from either of the corresponding pairs of signal line elements


232




a


and


235




a


or


233




a


and


237




a


can thus be reduced to virtually zero, the measurement of very low-level currents can be made via either pair. Moreover, any field disturbances in the region surrounding the second guard enclosure will be resolved at such enclosure without affecting the stability of the signal as carried by either pair.




Referring to

FIGS. 10 and 12

together, the coupling assembly


349


includes a lower guard line element


351


, a pair of pass-through connectors


352


and


353


, a flexible connector assembly or cable


355


, and an upper guard line element


356


. To enable the coupling assembly to acquire the guard potential, one end of the lower guard line element


351


is electrically connected to the second guard enclosure


327


, as by soldering. Preferably, the pass-through connectors and the connector assembly are of coaxial configuration so that the center conductor of each is able to convey the guard potential from the lower guard line element to the upper guard line element. The upper guard line element


356


and the upper component


295


, in turn, are connected together electrically, as by soldering, so that the guard potential is conveyed to the upper component via the upper guard line element.




In an alternative construction, it is possible to run the lower guard line element


351


directly between the second guard enclosure


327


and the upper component


295


. However, such a construction would make it difficult to separate the upper and lower halves


42


and


44


of the outer shielding enclosure


229


should the operator wish to gain access to elements within the enclosure. In order to provide such access, in the preferred coupling assembly


349


shown, the connector assembly


355


has end connecting members


355




a


and


355




b


that connect detachably to each pass-through connector. Thus, upon detachment of either end connecting member, the two halves


42


and


44


of the outer shielding enclosure can be separated from each other to gain access to the interior of the enclosure.




In accordance with a preferred method of using the fully guarded chuck assembly


221


, test equipment suitable for guarded measurement of low-level currents is connected with a selected one of the exterior connectors


232


or


233


via an external line (e.g.,


243


or


245


). The first chuck assembly element


280


is then energized, that is, a current signal is established through a signal path which includes the probe


30


, the device-under-test (not shown), and that series of signal line elements


232




a


,


235




a


and


239




a


, or


233




a


,


237




a


and


241




a


which corresponds to the chosen connector


232


or


233


. A nonzero signal potential is thus developed on the first chuck assembly element


280


in relation to system ground, that is, in relation to the potential on the outer shielding enclosure


229


. As this occurs, a guard potential substantially equal to the signal potential is simultaneously conveyed to the upper component


295


via guard line elements


351


and


356


and to the lower and skirting components


291


and


293


via that series of guard line elements


232




b


,


235




b


and


239




b


or


233




b


,


237




b


and


241




b


which corresponds to the chosen connector. This guard potential is initially generated inside the test equipment by a feedback network of a design known to those of ordinary skill in the art. In accordance, then, with the foregoing procedure, the first chuck assembly element


280


is electrically guarded by the second chuck assembly element


281


.




Since, in accordance with the above method, almost no potential difference is developed between the first chuck assembly element


280


and the neighboring second chuck assembly element


281


, and since the geometry of the second chuck assembly element is such that it fully surrounds the first chuck assembly element, leakage current from the first chuck assembly element is reduced to negligible levels. A further reduction in leakage current is achieved by the first and second inner guard enclosures


325


and


327


which, being held at nearly the same potential as the signal line elements they respectively surround, reduce leakage currents from those elements. As a result, system sensitivity to low-level current is increased because the level of current that is allowed to escape detection by being diverted from the signal path is negligible.




In addition to increased current sensitivity, another major benefit of the fully guarded chuck assembly


221


is its capability for reducing settling time during low-level current measurements. During such measurements, the rate of charge transfer in relation to the first chuck assembly element


280


is limited by the amount of current that can flow through the device under test given the bias conditions imposed on that device, whereas the rate of charge transfer in relation to the second chuck assembly element


281


is under no such restriction. Accordingly, the second chuck assembly element


281


and also the first and second guard enclosures


325


and


327


are able to transfer sufficient charge so that each achieves its full potential relatively quickly, even though each is capacitively coupled to surrounding conductive surfaces of relatively large area such as those on the interior of the outer shielding enclosure


229


. Finally, in relation to the first chuck assembly element


280


and also to the signal line elements in the connector mechanism


231


, the second chuck assembly element


281


and each of the guard enclosures


325


and


327


act as barriers against stray electromagnetic radiation, thereby increasing signal stability.




The benefits provided by the fully guarded chuck assembly


221


in regard to low-level current measurements are achieved while, at the same time, preserving the capacity of the system for making low-level voltage measurements. As previously described, the connector mechanism


231


continues to provide separate source and measurement lines suitable for the establishment of Kelvin-type connections. Moreover, the first chuck assembly element


280


is movable relatively freely relative to each individual probe


30


without being encumbered by any of the elements that provide guarding. In particular, electrical connection is maintained between the upper component


295


and the skirting component


293


via the coupling assembly


349


despite horizontal or vertical movement occurring between these components. With respect to the first inner guard enclosure


325


and the second inner guard enclosure


327


, either vertical or horizontal movement is accommodated between these enclosures because of flexibility in the connector assemblies


239


and


241


.




Probe-Holding Assembly With Fully Guarded Connector Mechanism




The alternative probe station


220


preferably includes at least one fully guarded probe-holding assembly


223


. Referring to

FIGS. 15 and 16

, it will be recognized that from the standpoint of overall construction, each fully guarded probe-holding assembly


223


is generally similar to the probe-holding assembly of the basic probe station as depicted in

FIGS. 8-9

. As between

FIGS. 15-16

and

FIGS. 8-9

, like reference numerals have been used to identify elements common to both systems. It will be seen, in particular, that the portion


231




a


of the connector mechanism associated with the probe-holding assembly


223


preferably includes a pair of connectors


128


and


130


of triaxial configuration, each of which are mounted on an outer shielding enclosure or box


126


. These exterior connectors, then, are suitably configured to receive the respective source and measurement line cables


132


which arrive from the external test instrument (not shown) as needed to establish Kelvin-type connections in relation to the probe


30


.




The inner and intermediate conductors of the exterior connector


128


are separated out from their respective insulating members so as to form a signal (source) line element and a guard line element


128




a


and


128




b


, respectively. Similarly, the inner and intermediate conductors of the exterior connector


130


are separated out from their respective insulating members so as to form a signal (measurement) line element and a guard line element


130




a


and


130




b


, respectively. As in the basic system shown in

FIGS. 8 and 9

, each of the signal line elements


128




a


and


130




a


is electrically connected with the center conductor


142


of a respective probe element


30




a


via the center conductor of a corresponding coaxial connector


138


or


140


and the center conductor of a corresponding coaxial cable


134


or


136


. To provide a guarding capability in relation to each signal path, each guard line element


128




b


or


130




b


is electrically connected with the guard conductor


144


of its corresponding probe element


30




a


via the outside conductor of the corresponding coaxial connector


138


or


140


and the outside conductor of the corresponding coaxial cable


134


or


136


. Each exterior connector


128


or


130


further includes an outer shield element


128




c


or


130




c


both of which are electrically connected with the outer shielding box


126


. This box, in turn, is electrically connected with the shield tube


146


, so that when the shield tube is inserted into the octagonal steel box


48


, as previously described, the signal and guard lines will be fully shielded.




In order to provide full guarding in relation to each of the respective signal line elements


128




a


and


130




a


of the fully guarded probe-holding assembly


223


, the alternative probe station


220


includes a third box-like inner guard enclosure


329


. This guard enclosure is so arranged that it surrounds the respective signal line elements


128




a


and


130




a


in interposed relationship between them and the outer shielding enclosure or box


126


. In the preferred embodiment depicted in

FIGS. 15 and 16

, the third guard enclosure is constructed from tin-plated steel panels. The respective guard line elements


128




b


and


130




b


are both electrically connected, as by a respective wire


148


, to the enclosure


329


, preferably on an inside wall thereof.




During the measurement of low-level currents through the probe


30


, as previously described, the interconnections made between the connector mechanism portion


231




a


and the third guard enclosure


329


enable the potential on the guard enclosure


329


to be controlled so that such potential substantially follows the signal potential as carried by either signal line element


128




a


or


130




a


. In particular, the potential on the third guard enclosure is controlled either by adjustment of the guard potential on guard line element


128




b


or


130




b.






Since, in accordance with the above construction, the third guard enclosure


329


fully surrounds each signal line element


128




a


or


130




a


and will carry substantially the same potential as these elements, leakage current from either signal line element is reduced to virtually zero so that very low-level currents can be measured via either element. Moreover, any field disturbances in the region surrounding the third guard enclosure will be resolved at that enclosure without affecting the stability of the signal as carried by either signal line element.




Although a preferred alternative embodiment


220


of the probe station has been described, it will be recognized that alternative forms of the embodiment are possible within the broader principles of the,present invention. Thus, with respect to the fully guarded chuck assembly


221


, instead of having a closed-sided structure, either the skirting component


293


or the upper component


295


may have a mesh, open-slat or multilevel structure. Also, it is possible to position a dielectric sheet between the first chuck assembly element


280


and the skirting component


293


in order to form a sandwich-type structure. In yet a further possible modification, the first inner guard enclosure


325


can be integrated with the skirting component


293


so that, for example, the skirting component includes U-shaped side portions which serve as the first guard enclosure. Moreover, instead of having a box-like form, each guard enclosure can take the form of a cylinder or various other shapes.




The terms and expressions which have been employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims which follow.



Claims
  • 1. A probe station comprising:(a) a chuck assembly for supporting a test device; (b) a probe support for supporting a probe; (c) a positioning mechanism enabling movement of said chuck assembly relative to said probe support; (d) said chuck assembly including a conductive chuck assembly element having a laterally extending upper surface for supporting said test device; (e) a conductive component electrically insulated and extending laterally over said upper surface in opposed spaced relationship thereto so as to enable the establishment by a test instrument of a potential on said conductive component which variably follows that of said conductive chuck assembly element, said chuck assembly element being movable laterally by said positioning mechanism with respect to said conductive component, said conductive component forming at least one hole through which said probe is insertable so that an electrical connection can be made with said test device by said probe at different lateral positions of said chuck assembly element relative to said conductive component.
  • 2. The probe station of claim 1 wherein said conductive component has a more extensive lateral area than does said upper surface of said chuck assembly element.
  • 3. The probe station of claim 1 including a conductive enclosure for shielding said chuck assembly element and said conductive component against electromagnetic interference.
  • 4. The probe station of claim 1 including a further conductive component extending laterally beneath said chuck assembly element and insulated therefrom so as to enable the establishment of said potential on said further conductive component.
  • 5. The probe station of claim 4 including a conductive enclosure for shielding said chuck assembly element and said conductive components against electromagnetic interference, said enclosure including a conductive sidewall portion substantially surrounding said chuck assembly laterally, said chuck assembly being laterally movable by said positioning mechanism while said sidewall portion of said enclosure is kept motionless relative to said probe support, said further conductive component being electrically connected to a conductive skirting component arranged substantially in laterally surrounding relationship to said chuck assembly.
Parent Case Info

This is a continuation of U.S. patent application Ser. No. 08/855,735, filed on May 9, 1997, now U.S. Pat. No. 6,232,788, which is a continuation of U.S. patent application Ser. No. 08/508,325 filed on Jul. 27, 1995, now U.S. Pat. No. 5,663,653, which is a continuation of U.S. patent application Ser. No. 08/100,494 filed on Aug. 2, 1993, now U.S. Pat. No. 5,457,398, which is a continuation-in-part of U.S. patent application Ser. No. 07/896,853 filed on Jun. 11, 1992, now U.S. Pat. No. 5,345,170. U.S. patent application Ser. No. 08/508,325 filed on Jul. 27, 1995, now U.S. Pat. No. 5,663,653, is also a continuation-in-part of U.S. patent application Ser. No. 08/417,982 filed on Apr. 6, 1995, now U.S. Pat. No. 5,532,609, which, in turn, is a divisional of U.S. patent application Ser. No. 08/245,581 filed on May 18, 1994, now U.S. Pat. No. 5,434,512, which is a divisional of U.S. patent application Ser. No. 07/896,853 filed on Jun. 11, 1992, now U.S. Pat. No. 5,345,170.

US Referenced Citations (18)
Number Name Date Kind
3333274 Forcier Jul 1967 A
3710251 Hagge et al. Jan 1973 A
4115736 Tracy Sep 1978 A
4383178 Shibata et al. May 1983 A
4731577 Logan Mar 1988 A
4755746 Mallory et al. Jul 1988 A
4757255 Margozzi Jul 1988 A
4758785 Rath Jul 1988 A
4771234 Cook et al. Sep 1988 A
4845426 Nolan et al. Jul 1989 A
4856904 Akagawa Aug 1989 A
4884026 Hayakawa et al. Nov 1989 A
5077523 Blanz Dec 1991 A
5084671 Miyata et al. Jan 1992 A
5220277 Reitinger Jun 1993 A
5345170 Schwindt et al. Sep 1994 A
5434512 Schwindt et al. Jul 1995 A
5532609 Schwindt et al. Jul 1996 A
Foreign Referenced Citations (3)
Number Date Country
201205 Dec 1986 EP
1-209380 Aug 1989 JP
2-220453 Sep 1990 JP
Non-Patent Literature Citations (14)
Entry
Micromanipulator Company, Inc., “Test Station Accessories.” (month unavailable) (1983).
Micromanipulator Company, Inc., “Model 8000 Test Station.” (month unavailable) (1986).
“Model TPO3000 Series Thermochuk® Systems,” four-page product note, Temptronic Corporation, Newton, MA (May 1992 or earlier).
“Application Note 1 Controlled Environment Enclosure,” two-page application note, Temptronic Corporation, Newton, MA (May 1992 or earlier).
Micromanipulator Company, Inc. “Model 8000 Test Station.” (month unavailable) (1988).
Applebay, Harry F. Deposition transcript (pp. 61-67) with exhibits 581 A.B.C. describing Flexion AP-1 probe station sold in 1987 (May 1988).
“Cross Section Signatone S-1240,” one-page sketch prepared by Signatone counsel. Signatone, San Jose, CA (Feb. 1988 or earlier per Signatone).
“S-1240,” two-page product note, Signatone, San Jose, CA (Feb. 1988 or earlier per Signatone counsel).
Y. Yamamoto, “A Compact Self-Shielding Prober . . .” IEEE Trans., Inst. and Meas., vol. 38, pp. 1088-1093 (1989).
Temptronic's “Guarded” Chuck, one-page note describing guarding system of Temptronic Corporation of Newton, MA, dated Nov. 15, 1989.
Beck & Tomann, “Chip Tester,” IBM Technical Disclosure Bulletin, p. 4819 (Jan. 1985).
Article by William Knauer entitled “Fixturing for Low-Current/Low Voltage Parametric Testing,” appearing in Evaluation Engineering, (1990), pp. 150-153.
Hewlett-Packard, “Application Note 356-HP 4142B Modular DC Source/Monitor Practical Application,” (Nov. 1987), pp. 1-4.
Hewlett-Packard, H-P Model 4284A Precision LCR Meter, Operation Manual (Dec. 1991) pp. 2-1, 6-9 and 6-15.
Continuations (3)
Number Date Country
Parent 08/855735 May 1997 US
Child 09/784231 US
Parent 08/508325 Jul 1995 US
Child 08/855735 US
Parent 08/100494 Aug 1993 US
Child 08/508325 US
Continuation in Parts (2)
Number Date Country
Parent 07/896853 Jun 1992 US
Child 08/100494 US
Parent 08/417982 Apr 1995 US
Child 08/508325 Jul 1995 US