U.S. patent application number 11/975477 was filed with the patent office on 2008-02-21 for probe station.
This patent application is currently assigned to Cascade Microtech, Inc.. Invention is credited to Gavin Fisher, Brad Froemke, Rod Jones, Anthony Lord, Pete McCann, Peter Navratil, Scott Runbaugh, Jeff Spencer, Craig Stewart.
Application Number | 20080042675 11/975477 |
Document ID | / |
Family ID | 27616495 |
Filed Date | 2008-02-21 |
United States Patent
Application |
20080042675 |
Kind Code |
A1 |
Navratil; Peter ; et
al. |
February 21, 2008 |
Probe station
Abstract
A probe station.
Inventors: |
Navratil; Peter; (Tualatin,
OR) ; Froemke; Brad; (Portland, OR) ; Stewart;
Craig; (Bloxham, GB) ; Lord; Anthony;
(Banbury, GB) ; Spencer; Jeff; (Vernonia, OR)
; Runbaugh; Scott; (Tigard, OR) ; Fisher;
Gavin; (Chinnor, GB) ; McCann; Pete;
(Beaverton, OR) ; Jones; Rod; (Gaston,
OR) |
Correspondence
Address: |
CHERNOFF, VILHAUER, MCCLUNG & STENZEL
1600 ODS TOWER
601 SW SECOND AVENUE
PORTLAND
OR
97204-3157
US
|
Assignee: |
Cascade Microtech, Inc.
|
Family ID: |
27616495 |
Appl. No.: |
11/975477 |
Filed: |
October 19, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10759481 |
Jan 16, 2004 |
|
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|
11975477 |
Oct 19, 2007 |
|
|
|
10285135 |
Oct 30, 2002 |
6777964 |
|
|
10759481 |
Jan 16, 2004 |
|
|
|
60351844 |
Jan 25, 2002 |
|
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Current U.S.
Class: |
324/750.22 ;
324/754.03; 324/754.23; 324/756.07 |
Current CPC
Class: |
G01R 31/2831 20130101;
G01R 31/2886 20130101; G01R 31/2887 20130101; G01R 1/18 20130101;
G01R 31/311 20130101 |
Class at
Publication: |
324/755 |
International
Class: |
G01R 31/02 20060101
G01R031/02 |
Claims
1. A method of probing a device under test comprising: (a)
supporting said device under test on a first support; (b)
supporting an electrical probe on a second support; (c) supporting
an optical probe on a third support; and (d) moving said electrical
probe relative to said device under test in a first direction
relative to the surface of said device under test to sense
electrical characteristics of said device under test while said
optical probe remains substantially aligned with the edge of said
device under test to sense optical characteristics of said device
under test.
2. The method of claim 1 wherein said first support is a chuck.
3. The method of claim 2 wherein said chuck includes a
substantially planar upper surface.
4. The method of claim 1 wherein said second support is a platen
positioned above said device under test.
5. The method of claim 4 wherein said platen defines an opening
therein though which a portion of said electrical probe
extends.
6. The method of claim 1 wherein said first support is located
between said second support and said third support.
7. The method of claim 1 wherein said second support and said third
support remain stationary with respect to each other while said
first support moves relative to said device under test while said
moving.
8. The method of claim 1 wherein said first direction is a z-axis
direction.
9. The method of claim 1 wherein said electrical probe may be moved
independently of said optical probe.
10. A probe station for testing a device under test comprising: (a)
a first platen supporting an electrical probe; (b) a chuck
supporting said device under test; (c) a second platen supporting
an optical probe; (d) said first platen and said second platen
positioned at different elevations from one another and different
elevations from said chuck; (e) said second platen movable relative
to said chuck in a perpendicular direction, said first platen
movable relative to said chuck in a perpendicular direction; (e) at
least 70% of the top surface of said second platen terminating in
free space when said optical probe is not supported thereon.
11. The probe station of claim 10 wherein said first platen and
said second platen are movable in a perpendicular direction
relative to one another.
12. The probe station of claim 10 wherein at least 80% of the top
surface of said second platen terminating in free space when said
optical probe is not supported thereon.
13. The probe station of claim 10 wherein at least 90% of the top
surface of said second platen terminating in free space when said
optical probe is not supported thereon.
14. The probe station of claim 10 wherein at least 95% of the top
surface of said second platen terminating in free space when said
optical probe is not supported thereon.
15. The probe station of claim 10 wherein said second platen has a
greater top surface area than said first platen.
16. The probe station of claim 10 wherein said second platen has a
smaller top surface area than said first platen.
17. The probe station of claim 10 wherein said first platen is
maintained in position with respect to said second platen by
gravity such that if said probe station were turned upside down
said first platen would freely fall away from said second
platen.
18. A probe station for testing a device under test comprising: (a)
a first platen supporting an electrical probe; (b) a chuck
supporting said device under test; (c) said first platen is
positioned at a different elevation than said device under test;
(d) a movement mechanism laterally displacing said first platen in
a controlled manner such that at least 20% of the surface area of
said first platen is laterally displaced to a spatial region not
previously occupied by said first platen prior to said lateral
displacement.
19. The probe station of claim 18 wherein said controlled manner is
at least 30% of said surface area.
20. The probe station of claim 18 wherein said controlled manner is
at least 40% of said surface area.
21. The probe station of claim 18 wherein said controlled manner is
at least 50% of said surface area.
22. A probe station for testing a device under test comprising: (a)
a first platen supporting an electrical probe; (b) a chuck
supporting said device under test; (c) said first platen is
positioned at a different elevation than said device under test;
(d) a movement mechanism angularly displacing said first platen in
a controlled manner such that said first platen is tilted at an
angle of at least 5 degrees with respect to the angle of said first
platen when probing said device under test.
23. The probe station of claim 22 wherein said controlled manner is
at least 10 degrees.
24. The probe station of claim 22 wherein said controlled manner is
at least 45 degrees.
25. The probe station of claim 22 wherein said controlled manner is
at least 75 degrees.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/759,481, filed Jan. 16, 2004, which is a
continuation of U.S. patent application Ser. No. 10/285,135, filed
Oct. 30, 2002, now U.S. Pat. No. 6,777,964, which claims the
benefit of U.S. Provisional App. No. 60/351,844, filed Jan. 25,
2000.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a probe station.
BRIEF SUMMARY OF THE INVENTION
[0003] Probe stations are designed to measure the characteristics
of electrical devices such as silicon wafers. Probe stations
typically include a chuck that supports the electrical device while
it is being probed by needles or contacts on a membrane situated
above the chuck. In order to provide a controlled environment to
probe the electrical device, many of today's probe stations
surround the chuck with an environmental enclosure so that
temperature, humidity, etc. may be held within predetermined limits
during testing. Environmental enclosures protect the device from
spurious air currents that would otherwise affect measurements, and
also facilitate thermal testing of electrical devices at
other-than-ambient environmental conditions. Environmental
conditions within the enclosure are principally controlled by a dry
air ventilation system as well as a temperature element, usually
located below the chuck, that heats or cools the electrical device
being tested through thermal conduction.
[0004] Many probe stations also incorporate guarding and
electromagnetic interference (EMI) shielding structures within or
around the environmental enclosures in order to provide an
electrically quiet environment, often essential during high
frequency testing where electrical noise from external
electromagnetic sources can hinder accurate measurement of the
electrical device's characteristics. Guarding and EMI shielding
structures are well known and discussed extensively in 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.
[0005] Probe stations incorporating EMI shielding structures will
usually at least partially surround the test signal with a guard
signal that closely approximates the test signal, thus inhibiting
electromagnetic current leakage from the test signal path to its
immediately surrounding environment. Similarly, EMI shielding
structures may provide a shield signal to the environmental
enclosure surrounding much of the perimeter of the probe station.
The environmental enclosure is typically connected to earth ground,
instrumentation ground, or some other desired potential.
[0006] To provide guarding and shielding for systems of the type
just described, existing probe stations may include a multistage
chuck upon which the electrical device rests when being tested. The
top stage of the chuck, which supports the electrical device,
typically comprises a solid, electrically conductive metal plate
through which the test signal may be routed. A middle stage and a
bottom stage of the chuck similarly comprise solid electrically
conductive plates through which a guard signal and a shield signal
may be routed, respectively. In this fashion, an electrical device
resting on such a multistage chuck may be both guarded and shielded
from below.
[0007] FIG. 1 shows a generalized schematic of a probe station 10.
The probe station 10 includes the chuck 12 that supports the
electrical device 14 to be probed by the probe apparatus 16 that
extends through an opening in the platen 18. An outer shield box 24
provides sufficient space for the chuck 12 to be moved laterally by
a positioner 22. Because the chuck 12 may freely move within the
outer shield box 24, a suspended member 26 electrically
interconnected to a guard potential may be readily positioned above
the chuck 12. The suspended guard member 26 defines an opening that
is aligned with the opening defined by the platen 18 so that the
probe apparatus 16 may extend through the guard member 26 to probe
the electrical device 14. When connected to a guard signal
substantially identical to the test signal provided to the probe
apparatus 16, the suspended guard member 26 provides additional
guarding for low noise tests. Such a design is exemplified by EP 0
505 981 B1, incorporated by reference herein.
[0008] To provide a substantially closed environment, the outer
shield box 24 includes a sliding plate assembly 28 that defines a
portion of the lower perimeter of the shield box 24. The sliding
plate assembly 28 comprises a number of overlapping plate members.
Each plate member defines a central opening 30 through which the
positioner 22 may extend. Each successively higher plate member is
smaller in size and also defines a smaller opening 30 through which
the positioner 22 extends. The sliding plate assembly 28 is
included to permit lateral movement of the positioner 22, and hence
the chuck 12, while maintaining a substantially closed lower
perimeter for the shield box 24.
[0009] Referring to FIG. 2, in many cases the semiconductor wafers
that are tested within such a probe station are edge coupled
photonics devices. Edge coupled photonics devices are normally
arranged within each semiconductor wafer in orthogonal arrays of
devices. Typically, the wafers are sliced in thin strips of a
plurality individual optical devices, as illustrated in FIG. 3.
Edge coupled photonics devices may include, for example, lasers,
semiconductor optical amplifiers, optical modulators (e.g.,
Machzhender, electro-absorption), edge coupled photo-diodes, and
passive devices. Referring to FIG. 4, many such photonics devices
provide light output through one side of the device. Normally, the
photonics devices receive light through the opposing side of the
device from the light output. On another side of the device one or
more electrical contacts are provided. In typical operation, the
light provided by the device may be modulated or otherwise modified
by changing the input light and/or the electrical signal to the
device, or the electrical output may be modulated or otherwise
modified by changing the input light. Similarly, other photonics
devices are surface coupled where the electrical contact and the
light output (or light input) are both on the same face of the
device, as illustrated in FIG. 5. On such surface coupled photonics
device is a VCSEL laser.
[0010] Referring to FIG. 6, a typical arrangement to test such
photonics devices within a probe station is shown. A set of
electrical probe positioners 50 are arranged on the platen to
provide electrical signals to and from the device under test, as
needed. In addition, one or more optical probe positioners 60 are
positioned on the platen to sense the light output from the device
under test or provide light to the device under test. As it may be
observed, when testing devices that include both optical and
electrical attributes the number of positioners may be significant
thereby potentially resulting in insufficient space on the platen
to effectively accommodate all the necessary positioners. In
addition, the opening provided by the platen is normally relatively
small so that the space available for extending the probes through
the platen is limited. This limited space significantly increases
the difficulty in positioning the electrical and optical probes.
Similarly, the end of the optical probes typically need to be
positioned within 0.10 microns in x/y/z directions which is
somewhat awkward from a position on the platen above the chuck.
Moreover, the angular orientation of the end portion of the optical
probe likewise needs to be very accurate to couple light between
the optical probe and the device under test which is similarly
difficult. In many applications extreme positional and angular
accuracy is needed to couple the optical waveguide or free space
optical path (i.e., optical probe) to a photonics device or another
optical waveguide. Moreover, during the testing of wafers the
optical probes frequently tend to be out of alignment requiring
manual alignment for each photonics device while probing each of
the devices.
[0011] What is desired, then, is a probe station that facilitates
accurate alignment of electrical and optical probes.
[0012] 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 SEVERAL VIEWS OF THE DRAWINGS
[0013] FIG. 1 shows a cross sectional view of an existing probe
station.
[0014] FIG. 2 illustrates a wafer with photonics devices
thereon.
[0015] FIG. 3 illustrates a strip of photonics devices.
[0016] FIG. 4 illustrates an edge coupled photonics device.
[0017] FIG. 5 illustrates an upper surface coupled photonics
device.
[0018] FIG. 6 shows a cross sectional view of the probe station of
FIG. 1 with electrical and optical probes.
[0019] FIG. 7 shows a pictorial view of a modified probe
station.
[0020] FIG. 8 shows a pictorial view of another modified probe
station.
[0021] FIG. 9 shows a pictorial view of yet another modified probe
station.
[0022] FIG. 10 shows a pictorial view of the support assembly for
the probe station of FIG. 7.
[0023] FIG. 11 shows a pictorial view of a further modified probe
station.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
[0024] During testing, the end of the optical probes are typically
aligned with the edge of the device under test while the electrical
probes are typically aligned with the contacts on the upper surface
of the device under test, with both the electrical probes and the
optical probes being supported by the platen. In many cases, the
entire platen is moved in the z-axis direction for selectively
contacting the electrical probes on the device under test.
Alternatively, the chuck is moved in a z-axis direction. The z-axis
movement of the platen permits consistent simultaneous relative
movement of all the electrical and optical probes. Each component
of the device under test is successively moved in x and/or y
lateral directions relative to the electrical probes using a chuck
or other support to a location under the electrical probes.
[0025] The present inventors considered the z-axis movement of the
platen or chuck to perform simultaneous probing and came to the
realization that normal z-axis movement of the platen typically
brings the probes into contact with the device under test with
sufficient additional z-axis movement to result in lateral
scrubbing of the contact surfaces to provide a good contact. This
additional z-axis movement for the electrical probes, which may
vary depending on the particular circuit being probed, different
electronic components, the planarity of the devices, and
differences in the height of the different contacts between
devices, may result in inaccurate alignment of the optical probes
which are likewise being moved in the z-axis direction together
with the platen or chuck. The alignment of the optical inputs and
outputs of the devices tends not to vary in the same manner as the
contacts, if they vary significantly at all. In summary, the
appropriate z-axis movement of the electrical probes varies
depending on the particular device being tested; while the
appropriate z-axis movement of the optical probes tends to be at a
substantially fixed location with respect to the device under test,
which may not be consistent with the z-axis movement provided for
the electrical probes. Moreover, the relatively long optical device
tends to expand and contract with temperature variations of the
environment resulting in additional difficulty properly positioning
the optical probe.
[0026] In light of the foregoing realizations the present inventors
determined that the traditional probe station should be modified in
some manner to facilitate at least partial independent movement or
otherwise separation of the optical probes and electrical probes.
Referring to FIG. 7, a modified probe station 100 includes a chuck
102 that supports a device under test 104. The device under test
104 in many instances is one or more photonics devices. An upper
platen 106 defines an opening 108 therein and is positioned above
the chuck 102. The opening 108 may be, for example, completely
encircled by the upper platen 106 or a cutout of a portion of the
upper platen 106. Electrical probes 110 are supported by the upper
platen 106. The platen 106 is supported by a plurality of supports
112a, 112b, 112c, and 112d. Positioned below the supports 112a-112d
is a lower platen 114. The optical probes 116 are supported by the
lower platen 114. A microscope, not shown, may be used to position
the device under test 104 relative to the probes 110 and 116.
During probing the upper platen 106 is moved in a z-axis direction
to make contact between the electrical probes 110 and the device
under test 104. The x and/or y position of the chuck 102 (hence the
device under test 104) relative to the electrical probes 110 is
modified, and thereafter the upper platen 106 is moved in a
z-direction to make contact between the electrical probes 110 and
the device under test 104. During testing the optical probes 116
are aligned with the edge of the device under test 104.
[0027] In the case that the device under test is moved in a
direction perpendicular to the edge of the device under test 104
being tested, it may be observed that the optical probes 116 may
not need to be repositioned for each device being tested. If
realignment of the optical probes 116 are necessary, there is a
good likelihood that minimal adjustment is necessary. In
particular, there is a high likelihood that the elevation of the
optical probe 116 is accurate (or nearly so) because the chuck 102
is moving within a horizontal plane for testing the device under
test 104. It may be observed that optical probes 116 are
effectively decoupled from the z-axis motion of the upper platen
106. Moreover movement of the upper platen 106 for bringing the
electrical probes 110 into contact with the device under test 104
does not result in movement of the optical probe 116 with respect
to the device under test 104. Similarly, it may be observed that
movement of the optical probes 116 does not result in movement of
the electrical probes 110.
[0028] As illustrated in FIG. 7, it may be observed that there is
substantial open space on the lower platen 114 to position the
optical probes 116. Further, the open space permits operators to
access the optical probes 116 to make adjustments, as necessary.
For example, the lower platen 114 may include at least 70% of its
surface area free of other components and structures, such as the
chuck and supports, available for the positioning of optical
components thereon. More preferably, at least 80%, 85%, 90%, and
95% of the surface area of the lower platen 114 is free of other
components and structures. Moreover, from a region defined by the
perimeter of the supports, the lower platen 114 has preferably 70%,
80%, 85%, 90%, or 95% of the surface area of the upper platen free
from other components and structures thereon in any outward
direction, such as +x, -x, +y, or -y directions. This free space
more readily permits the attachment of free space optics thereon,
which frequently require substantial space and flexibility to set
up. The size of the upper platen 106 may have less surface area,
the same surface area, or greater surface area than the lower
platen 114. For example, the lower platen 114 (e.g., optical
platen) may have a surface area that is 25%, 35%, or 50% or more
greater than the upper platen 106 (e.g., non-optical platen). This
increased surface area of the lower platen 114 relative to the
upper platen 106 permits more open access to the lower platen 114
to locate optical components thereon without limitations resulting
from the proximity upper platen 106. Preferably the lower platen
114 is a single integral member or otherwise a rigidly
interconnected set of members. It is of course to be understood
that the system may include more than two platens, as desired. In
addition, the electrical components may be located on the lower
platen, as desired. Also, the optical components may be located on
the upper platen, as desired, which may include holes therein for
an optical breadboard if desired. Furthermore, with the upper
platen being maintained in position principally by gravity, such
that it would become detached from the supports if the probe
station were turned up side down, a set of different upper platens
may be provided, each of which is designed to be particularly
suitable for a particular test. For example, some upper platens may
be small, large, oval, rectangular, thin, thick, etc.
[0029] Another feature that may be included is the capability of
removing or otherwise moving the upper platen out of the way for in
a controlled manner. The movement of the upper platen facilitates
the adjustment and installation of the optical components
thereunder. For example, a mechanical support mechanism may be
included that supports the upper platen while the platen is moved
with respect to the remainder of the probe station, and in
particular the lower platen. For example, the upper platen may be
displaced such that at least 20% (or at least 30% or at least 40%
or at least 50%) of its surface area is laterally displaced beyond
its original position on the supports. Alternatively, the upper
platen may be tilted upwardly. For example, the upper platen may be
tilted such that it is at least 5 degrees (or at least 10 degrees
or at least 20 degrees or at least 45 degrees or at least 75
degrees) of its surface area is tiled with respect to its position
when probing, such as horizontal.
[0030] Referring to FIG. 8, a modified probe station 200 includes
an upper platen 206 supported by a set of upper supports 212a-212d.
The upper supports 212a-212d extend through respective openings
220a-220d in a lower platen 214 and are supported by a base 222.
The lower platen 214 is supported by a set of supports 224a-224d
which is supported by the base 222. The supports 224a-224d and the
supports 212a-212d are preferably adjustable in height. The chuck
202 extends through an opening 226 in the lower platen 214 and is
supported by the base 222. With this structure, one or more optical
probes 216 supported by the lower platen 214 may be simultaneously
moved in the z-axis direction with respect to a device under test
204 supported by the chuck 202. Also, one or more electrical probes
210 may be simultaneously moved in the z-axis direction with
respect to a device under test 204 supported by the chuck 202.
Furthermore, one or more electrical probes 210 may be
simultaneously moved in the z-axis direction with respect to the
optical probes 216, or vise versa, both of which may be moved
relative to the device under test 204. This permits effective
realignment of one or more optical probes 216 with respect to the
edge of the device under test 204. In this manner, at least a
portion of the alignment of the optical probes 216 may be performed
by the probe station, as opposed to the individual positioners
attached to the optical probes 116. It is to be understood that the
lower platen 214 is preferably positioned at a location below the
device under test 204 while the upper platen 206 is positioned
above the device under test 204. Also, it is to be understood that
the lower platen 214 may be positioned at a location above the
device under test 204 while the upper platen 206 is likewise
positioned above the device under test 204. Also, it is to be
understood that the lower platen 214 may be positioned at a
location below the device under test 204 while the upper platen 206
is likewise positioned below the device under test 204. Moreover,
the range of movement of the supports may permit the upper platen
206 and/or the lower platen 214 to be moved from a position above
the device under test 214 to a position below the device under test
214, or from a position below the device under test 214 to a
position above the device under test 214.
[0031] Referring to FIG. 9, a modified probe station 300 includes
the chuck 202 being supported by the lower platen 214. In this
manner, the chuck 202 and the lower platen 214 will move together
in the z-axis. This is beneficial, at least in part, to assist in
maintaining the relative alignment between the optical probes and
the device under test.
[0032] Referring to FIGS. 7-9, the lower platen (or the upper
platen) may include a set of openings 170 defined therein suitable
for engaging an optical device. Typically the openings 170 are
arranged in an orthogonal array. The openings 170 provide a
convenient mechanism for interconnection between the lower platen
and the optical probes.
[0033] The probe station facilitates the testing of a photonics
device that includes an optical test path, which is optimized based
upon optical characteristics. In addition, the probe station
facilitates the testing of a photonics device that includes an
electrical test path, which is similarly optimized based upon
electrical characteristics. Typically multiple electrical probes
are supported and simultaneously brought into contact with the
device under test. In this manner, the probe station includes a
structure that brings together optimized electrical test paths and
optimized optical test paths together on the device under test.
[0034] Referring to FIG. 10, the upper platen 106 (or other
platens) is supported by a plurality of supports 350a-350d.
Preferably the platen 106 is supported by a set of contacts
352a-352d. The contacts 352a-352d are preferably not fixedly
interconnected with the upper platen 106, but rather maintained in
contact by the force of gravity free from a fixed interconnection,
such as a screw or bolt. Accordingly, the upper platen 106 may be
removed from the supports 350a-350d by merely lifting the upper
platen 106. A set of interconnecting members 354, 356, and 358 may
be included to provide increased rigidity to the supports
350a-350d. In addition, the length of the interconnecting members
354, 356, 358 may be adjustable, such as extending through the
supports 350a-350d or otherwise including a length adjustment
mechanism for the interconnecting members themselves. In this
manner the upper platen 106 may be lifted from the supports
350a-350d, the position of the supports 350a-350d and relative
spacing thereof modified, and the upper platen 106 repositioned on
the supports 350a-350d. In addition, a mechanical lift mechanism
358 may be included to raise and lower the upper platen 106. Also,
the supports 350a-350d may include internal height adjustment for
z-axis movement. Further, computer controlled lift control
mechanisms may likewise be used. Moreover, it may be observed that
the upper platen 106 may be moved in the z-axis direction, and in
the x and/or y direction by simply moving the upper platen 106. In
an alternative embodiment, the supports 350a-350d may include
horizontal movement structures to move the upper platen 106 in the
x and/or y directions. As one example, the horizontal movement
structures may be a set of rollers that permit the selective
lateral movement of the upper platen 106.
[0035] Referring to FIG. 11, a substantially enclosed environment
400 may be provided around the device under test. The environment
may be electrically connected to an earth ground potential, an
instrument ground potential, a guard potential, a shield potential,
or otherwise remains floating. An optical box 402 may be provided
within the lower region of the probe station to provide a
substantially light tight environment around the device under test,
which may be useful for many applications. The optical box 402
preferably includes a plurality of sealable openings to permit
access to the optical probes. An electrical box 404 may be provided
within the upper region of the probe station to provide a
substantially noise controlled environment around the electrical
probes, which may be useful for many applications. The electrical
box 404 may be electrically connected to an earth ground potential,
an instrument ground potential, a guard potential, a shield
potential, or otherwise remains floating.
[0036] 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.
* * * * *