Mercury probe and method

The disclosed mercury probe has a mercury reservoir and one or more passages extending from the reservoir to an aperture plate for engaging a test wafer. Dross that tends to form at the wafer-contact end of the mercury passage(s) is removed by returning all of the mercury in the passage(s) into the reservoir, where the dross is captured. Means is provided to introduce air into the passage(s) between the reservoir and the aperture(s) of the aperture plate, for disconnecting the mercury-probe segment(s) of the mercury from the reservoir and, where there are plural mercury-probe segments, for disconnecting them from each other.

BACKGROUND 
The present invention relates to mercury-probe test apparatus and a method 
of its operation. Such test apparatus is widely used for testing 
semi-conductor wafers, being exemplified in U.S. Pat. Nos. 3,794,912 
issued Feb. 26, 1974 to P. J. W. Severin et al, and 4,101,830 issued July 
18, 1978 to John H. Greig. 
A wafer to be tested is positioned against an aperture plate in such test 
apparatus and mercury is displaced along a passage into contact with the 
wafer. In the course of repeated test cycles, bits of dirt tend to 
accumulate at the contact end of the mercury in the passage, degrading the 
tests. In order to clear the mercury in the passage of dross, it has been 
customary to expel some of the mercury, either at intervals or before each 
test. This involves a separate preparatory operation. Moreover, special 
care is needed in handling the expelled mercury to avoid the hazard of 
scattered mercury. 
A variety of tests can be performed with mercury probes, each requiring an 
appropriate number of contacts. Where the tests involve capacitance 
measurements, the stray capacitance of the test apparatus must be taken 
into account. In bridge circuits, small values of stray capacitance can be 
"zeroed out" without appreciably affecting the test accuracy. A large 
amount of stray capacitance tends to interfere with the test accuracy. The 
reservoir represents a large contributor to the stray capacitance in 
apparatus where it remains connected to the wafer-engaging mercury, for 
example the apparatus shown in the Greig patent mentioned above. The 
problem is compounded where there are plural mercury-probe contacts each 
connected to its own mercury reservoir. There, the stray capacitance 
between reservoirs tends to be especially large. 
SUMMARY OF THE INVENTION 
The illustrative embodiments of the invention shown in the accompanying 
drawings and described in detail below incorporate a number of novel 
features which will be clear from that detailed description below. In one 
novel aspect, a reservoir of mercury is used in each test cycle to provide 
the mercury that contacts the test wafer. That mercury is returned to the 
reservoir after the test. Two ways to perform this step are described in 
detail below. Dross that might have been introduced at the mercury probe 
apertures tends to accumulate at the surface of the mercury in the 
reservoir, where it is captured. Mercury having a fresh surface enters the 
passage(s) to the contact aperture(s) in each new test cycle. 
Mercury-probe test apparatus of the kind just described may have a 
mercury-filled passage extending from the reservoir to the test 
aperture(s) in the aperture plate when the mercury makes contact with the 
test wafer. In such apparatus the reservoir is electrically connected to 
what may be called the "mercury-probe segment", that is, the mercury in 
the passage extending to the test aperture. Moreover, with mercury filling 
the passage, if the passage were divided to form plural mercury-probe 
contacts, the mercury in the passage would connect those mercury-probe 
contacts to each other. 
In the apparatus detailed below, the mercury in the mercury-passage is 
parted so that the (each) mercury-probe segment becomes insulated from the 
reservoir. In the described apparatus air is introduced into the mercury 
passage for separating the (each) mercury probe segment from the rest of 
the mercury in the system. Where a common reservoir is used for plural 
mercury-probe contacts, a common air inlet is used in the illustrative 
apparatus not only for disconnecting the mercury-probe segments from the 
reservoir but also for disconnecting those segments from each other. 
In one sense, the present apparatus represents an improvement over the 
mercury probe in the Greig patent, supra. The Greig apparatus shifts 
mercury from the reservoir to contact the test wafer by developing vacuum 
in the interface between the aperture plate and the test wafer. In the 
novel illustrative apparatus where vacuum is used in this way and where 
the reservoir is below the level of that interface, mere opening of an 
intermediate point in the mercury passage(s) to the atmosphere introduces 
air and effects disconnection of the (each) mercury probe segment from the 
mercury filling the passage to the reservoir. If Greig's vacuum were not 
used, and if air pressure were applied to the mercury in the reservoir to 
shift the mercury along the passage(s) to the mercury-probe segments, air 
that is introduced under pressure partway along the mercury passage could 
be provided for disconnecting the mercury-probe segment(s) from the 
reservoir and from each other. However, use of air under pressure for 
displacing the mercury can lead to hazardous scattering of mercury, so 
that use of vacuum as in Greig and described below is distinctly superior. 
The nature of the invention in its various aspects including the foregoing 
and other objects, advantages and novel features, will be better 
appreciated on the basis of the accompanying drawings and the detailed 
description below of several illustrative embodiments.

THE ILLUSTRATIVE EMBODIMENTS 
In FIG. 1, a wafer W rests on support 10. For some tests, the material of 
support 10 (or a large contact area on support 10) is nickel or aluminum 
or other electrical conductor. Plate 12 of electrical insulation rests on 
the wafer. A tube 14 provides a passage from reservoir 20 to aperture 17. 
Tube 14 includes segment 14a that extends into plate 12. Where segment 14a 
is of insulation, circuit connection 16 of the mercury is made by a wire 
as of nickel or stainless steel extending through the wall of the tube 
segment. In a preferred construction, tube segment 14a is of metal such as 
stainless steel that extends only partway through plate 12 (FIG. 1A) and 
is aligned with aperture 17. The metal tubing provides the circuit 
connection to the segment of mercury extending to test aperture 17. 
A "T" fitting connects vent line 18 to tube 14. In this way tube 14 is 
divided into segment 14a and another segment 14b that extends into mercury 
reservoir 20. Valve 19 is interposed in vent line 18. A vacuum line 22 
equipped with valve 24 extends through the sealed cover 20a of the 
reservoir. Of course, all of the tubing 14a, 14b and 18 that at times 
contains mercury should be of a material such as plastics, stainless steel 
or other material that does not form an amalgum. Such materials cause the 
mercury to form a convex surface, i.e. to bulge into the tubing, when an 
end of the tubing is immersed in mercury (FIG. 2A). 
A vacuum line 26 equipped with valve 28 extends through plate 12 to a 
circular groove 30 in plate 12 that surrounds aperture 17. 
In use of the apparatus, valves 19, 24 and 26 are closed initially. Valve 
26 is opened, developing vacuum across the surface of the wafer within 
groove 30 and drawing mercury from the reservoir into tube 14 and into 
contact with the surface of wafer W. The opposed surfaces of wafer W and 
aperture plate 12 are flat and smooth, but vacuum develops rapidly at 
aperture 17 by virtue of microscopic scratches in the wafer-engaging face 
of plate 12. 
Next, vent valve 19 is opened. The mercury between the "T" fitting and the 
reservoir and any mercury present in vent line 18 drops back into the 
reservoir by siphon action, leaving only the limited amount of mercury in 
tube segment 14a extending into contact with wafer W. This is the 
"mercury-probe segment" mentioned above. The vacuum in line 26 is 
maintained, so that atmospheric pressure behind the mercury tends to 
flatten the meniscus of the mercury at the surface of the wafer. The 
mercury-probe segment is connected into the test circuit by lead 16. 
Notably, the large mass of mercury in the reservoir and the mercury 
previously in tube segment 14b become disconnected from lead 16. In this 
way, the test circuit is freed of the stray capacitance that would develop 
but for the disconnection. It is understood that tube 14b is of 
insulation, where such disconnection is to occur. 
After the tests are completed and before plate 12 is lifted to enable the 
test wafer to be removed, vent valve 19 is closed, vacuum valve 26 is 
closed (so that aperture 17 shifts to atmospheric pressure) and vacuum 
valve 24 is opened. The vacuum that develops in reservoir 20 withdraws the 
air in segment 14b and then transfers the mercury in tube segment 14a into 
the reservoir. Obviously--and in ordinary practice--tube 14 is not of such 
gross diameter as to enable air from the wafer surface to bubble past the 
mercury that is to be lifted toward the reservoir. All of the mercury in 
tubing 14 is returned to the reservoir and dross that may have entered 
aperture 17 is drawn into the reservoir where it rises to the surface. In 
the ensuing test operation, fresh mercury enters tubing 14, thus tending 
to provide a clean mercury probe contact automatically. 
After all the mercury in tubing 14 has been returned to the reservoir, the 
vacuum should be shut off. Otherwise, air from tubing 14 bubbles through 
the mercury in the reservoir, producing turbulence. This effect is 
minimized by using a weak vacuum in line 22. Also, at the cost of added 
complexity, the returning mercury can be directed to the reservoir via a 
different line through cover 20a that would terminate above the surface of 
the mercury. 
Experience shows that, for consistent results in tests generally, and for 
the very success of certain tests, a fresh surface of mercury should 
engage wafer W in each new test cycle. However, in usual mercury-contact 
test equipment, dross tends to accumulate at the contact end of the 
mercury in the tube. 
A self-cleaning mercury contact is realized in the apparatus of FIG. 1 by 
virtue of the vacuum developed in the reservoir that extracts all the 
mercury from the passage to the test apparatus 17. Any dross that may have 
entered at aperture 17 tends to float and accumulate in reservoir 20. In 
each test cycle, mercury stripped of dross is drawn from reservoir 20 and 
shifted to aperture 17. 
For certain tests, it is not important to disconnect the mercury in tubing 
segment 14a from the rest of the mercury. However, in some tests, it is 
important to eliminate prominent values of stray capacitance. Even if only 
one mercury contact is involved, it may be important to minimize stray 
capacitance by segregating the mercury-probe segment 14a of the passage. 
In Schottke diode tests, the wafer has a thin oxide layer on its surface 
engaged by the mercury probe, and measurements of capacitance are made. 
Some stray capacitance can be "zeroed out" in the test circuit. This is 
readily done with respect to the stray capacitance between the mercury in 
tube segment 14a and the large-area contact 10. However, a much larger 
amount of stray capacitance is developed between the large-area contact 
and the reservoir whenever the reservoir remains connected to the mercury 
probe segment of tube 14a. That larger stray capacitance is more difficult 
to "zero out", and it tends to degrade the accuracy of the measurement. 
Opening of valve 19 after mercury fills tubing segment 14a disconnects the 
reservoir electrically and eliminates its stray capacitance. 
In some tests it may be unnecessary to part the mercury in segment 14a from 
that in the reservoir. In that case, the vent provision 18, 19 and the 
vacuum provision 22, 24 could be omitted or it could be disregarded. At 
the end of the tests, vacuum at groove 30 is interrupted by shutting 
vacuum valve 26. When that occurs, air can reach aperture 17, allowing the 
mercury in tubing 14 to return to the reservoir by siphon action. Even if 
the vacuum line 22 were omitted or not used, the mercury can be 
self-cleaning, simply by limiting the immersion of tube 14b in the mercury 
to a depth h discussed below in connection with FIGS. 2 and 2A. 
FIG. 2 illustrates mercury probe apparatus that provides a single reservoir 
serving four mercury contacts. Such four-point-probe contacts enable 
resistivity measurements to be made by passing current through the wafer 
between the outermost two mercury-probe contacts and making voltage 
measurements across the inner two mercury contacts. In other tests, three 
and even two mercury-probe contacts are used, served by the common 
reservoir. Indeed, where two mercury-probe contacts are used, as in making 
Schottke diode tests and whenever stray capacitance is of concern, the 
apparatus of FIG. 2 has the further advantages discussed above of 
disconnecting the reservoir and its associated stray capacitance from the 
mercury that is in the test circuit. 
In FIG. 2, wafer W rests on support 32, and is held firmly against the 
support when vacuum is developed between the wafer and support 32. This 
occurs when valve 34 in vacuum line 36 is opened, developing vacuum in the 
circular groove 38. The vacuum develops across the interface beweeen the 
wafer and plate 32 by virtue of microscopic scratches in the surface of 
plate 32. Groove 38 surrounds apertures 42 in plate 32. 
Four segments of tubing 40a (as in FIG. 1A) extend to apertures 42, to 
provide four mercury-probe contacts to the wafer. A common length of 
tubing 40b extends into the mercury reservoir 44. Manifold 40c connects 
probe segments 40a to each other and to a vent line 46, which is 
controlled by vent valve 48. Portions 40a, 40b and 40c are collectively 
identified as tubing 40. Manifold 40c is of electrical insulation. 
With valve 48 closed, opening of vacuum valve 34 develops vacuum across the 
interface of the wafer and plate 32, and vacuum also develops in tubing 40 
and vent line 46. Tubing 40 fills with mercury. When tests are to be made, 
vent valve 48 is opened, allowing the mercury in portions 40b and 40c of 
the system to drop back into the reservoir. The vacuum is maintained in 
the interface between the wafer and its support. Therefore the mercury is 
retained at apertures 42 and in tube segments 40a. There is no problem of 
the mercury dropping out of apertures 42 and tube segments 40a inasmuch as 
their diameters are not unduly large. 
The desired tests are conducted with vacuum valve 34 open. Opening of vent 
valve 48 causes discharge of the mercury in manifold 40c, so that tubing 
segments 40a are electrically disconnected from reservoir 44 and from each 
other. Suitable tests connections 50 are made to tube segments 40a. 
When the tests are completed, vacuum valve 34 is closed, the interface 
between wafer W and plate 32 returns to atmospheric pressure, and the 
mercury in probe apertures 42 and tube segments 40a drops into the 
reservoir. 
Dross any enter test apertures 42 and would interfere with subsequent 
tests. The dross can be eliminated automatically in this apparatus by 
limiting the depth of immersion h of tube 40b in the mercury of the 
reservoir. While the test apparatus of FIG. 2 includes four probes 40a, 
42, this feature is also effective for one or more probes. 
The lower end of tubing 40b should be immersed far enough so that, when 
mercury is withdrawn from the reservoir to fill tubing 40, the mercury in 
the reservoir does not fall below the opening in the lower end of tube 
40b. This is easily arranged. The bore of tubing 40 is made small, so that 
only a small volume of mercury is needed to fill tubing 40, and the 
diameter of the reservoir is made appropriately large so that the level of 
the mercury in the reservoir does not fall excessively in filling the 
tubing with mercury. 
With a limited depth of immersion h (FIG. 2A), surface tension across the 
tube opening depresses the mercury as shown. During a test, mercury fills 
tube segments 40a. Following the test, all of that mercury drops into the 
reservoir. The sudden transfer of the mercury from the tubing into the 
reservoir tends to carry with it any dross introduced at the contact 
apertures 42. Once in the reservoir, the dross floats and is captured, as 
in the apparatus of FIG. 1. 
For self-cleaning of the mercury, the depth h of immersion of tube 40b into 
the mercury in the reservoir before mercury is drawn into tubing 40 should 
be no greater than about: 
##EQU1## 
where .rho.=density 
where 
S=surface tension 
g=acceleration of gravity 
r=radius of the tube. 
For mercury, .rho.=13.5 and S=487 dynes/cm; and g=980 cm/sec.sup.2. The 
above expression for h is derived by noting that the surface tension of 
the mercury that resists entry of the mercury into the tube is in 
equilibrium with the buoyant force tending to elevate the mercury into the 
tube. At equilibrium: 
EQU S.multidot.2.pi.r=.-+..multidot..pi.r.sup.2 hg 
For mercury, S=487 dynes/cm, and .rho.=13.5. 
Accordingly, 
##EQU2## 
In an example, where the bore diameter of the tube immersed in the mercury 
of the reservoir is 0.043 cm or 0.0165 inch, h=1.75 cm=0.7 inch or 11/16 
inch. 
This is the theoretical maximum immersion of the tube for dependable 
self-clearing of dross from the mercury in the apparatus of FIG. 2. The 
depth h can be determined empirically for any bore diameter simply by 
using a sample of the tubing and determining the maximum immersion that 
occurs before mercury rises appreciably into the tube. However, since the 
transfer of mercury into the reservoir in practice tends to be sudden, 
this depth may be exceeded slightly. 
In FIG. 3 as in FIG. 1, contact to the wafer is made from above, and FIG. 3 
like FIG. 2 provides multiple mercury-probe contacts that are disconnected 
from each other and from the reservoir during tests. In FIG. 3, wafer W 
rests on support plate 52 that may be of insulation or of metal, depending 
on the tests to be performed. 
Plate 54 of electrical insulation has a circular groove 56 connected to 
vacuum line 58 and vacuum valve 60. Within the circle of groove 56, plate 
54 has multiple probe apertures 62 aligned with tube segments 64a of the 
form in FIG. 1A. A return tube 64b provides a mercury passage from 
reservoir 66 to tube segments 64a via manifold 64c. Portions 64a, 64b and 
64c are collectively identified as tubing 64. Tubing 64c is of electrical 
insulation. Tube segment 64b extends through a seal in sealed cover 66a of 
the reservoir. A vacuum line 68 and a vacuum valve 70 (when open) develop 
vacuum in reservoir 66. A vent line 72 equipped with vent valve 74 extends 
from manifold 64c. 
The sequence of operations in FIG. 3 parallels that of FIG. 1. However, in 
FIG. 3, the plural mercury-probe segments identified with tubes 64a are 
disconnected both from the reservoir and from each other when vent valve 
74 is opened. At this time, vacuum valve 60 is open and vacuum valve 70 is 
closed. The apparatus of FIG. 3 thus eliminates the stray capacitance of 
the reservoir from each and all the mercury-probe contacts, and it 
operates in a manner that automatically removes dross that may enter the 
mercury-probe apertures. 
In each of FIGS. 1, 2 and 3, the vent valve is said to be either open or 
closed, at different times in the sequence of operations. However, 
experience shows that if these valves are nearly closed or if they leak 
ever so slightly, then there would be no need to manipulate them. In fact, 
they can be replaced by a length of vent line that has a pin-hole in its 
end remote from the mercury-probe segments 14a, 40a and 64a. In operation, 
when vacuum develops in the mercury-probe segments, the mercury-passage 
fills quickly with mercury. Once mercury fills apertures 17, 42 and 62, 
the vacuum is blocked from the vent line. Air entering the pin-hole--or 
the leaky valve 19, 48 and 74--of the vent line can then allow the mercury 
in the tubing (except that in the mercury-probe segments) to return to the 
reservoir by siphon action. This slight air leak does not interfere with 
subsequent operation of each apparatus to withdraw the mercury in 
mercury-probe segments 14a, 40a and 64a due to gravity or reservoir 
vacuum. 
The illustrative embodiments of the invention shown in the accompanying 
drawings and described in detail above are subject to various 
modifications and the novel features may be variously applied by those 
skilled in the art. Consequently, the invention should be construed 
broadly in accordance with its full spirit and scope.