Methods and apparatus for the in-process detection of workpieces with a physical contact probe

An apparatus for detecting the presence of extraneous material on a polishing pad during a chemical mechanical planarization (CMP) process uses a contact probe assembly. The contact probe assembly interrogates the surface of the polishing pad during processing of a workpiece and generates a control signal indicating the presence of extraneous material when the displacement of a contact stylus exceeds a threshold amount. The contact probe assembly produces a control signal in response to the detection of extraneous material and the control signal causes the CMP system to react in an appropriate manner to reduce damage to the workpieces being processed.

TECHNICAL FIELD 
The present invention relates, generally, to methods and apparatus for 
detecting the presence of extraneous objects on a processing element 
during a chemical mechanical planarization (CMP) process, and more 
particularly to an improved technique for detecting the in-situ, 
in-process loss or breakage of a semiconductor wafer using a probe that 
responds to physical contact with the wafer. 
BACKGROUND ART AND TECHNICAL PROBLEMS 
The production of integrated circuits begins with the creation of 
high-quality semiconductor wafers. During the wafer fabrication process, 
the wafers may undergo multiple masking, etching, and dielectric and 
conductor deposition processes. Because of the high-precision required in 
the production of these integrated circuits, an extremely flat surface is 
generally needed on at least one side of the semiconductor wafer to ensure 
proper accuracy and performance of the microelectronic structures being 
created on the wafer surface. As the size of integrated circuits continues 
to decrease and the number of microstructures per integrated circuit 
increases, the need for precise wafer surfaces becomes more important. 
Therefore, between each processing step, it is usually necessary to polish 
or planarize the surface of the wafer to obtain the flattest surface 
possible. 
Chemical mechanical planarization (CMP) processes and apparatus are well 
known to those skilled in the art, and need not be discussed in detail 
herein. Such processes generally involve attaching one side of the wafer 
to a flat surface of a wafer carrier or chuck and pressing the other side 
of the wafer against a flat polishing surface. In general, the polishing 
surface includes a polishing pad that has an exposed abrasive surface of, 
for example, cerium oxide, aluminum oxide, fumed/precipitated silica, or 
other particulate abrasives. Commercially available polishing pads can be 
formed of various materials known in the art. Typically, a polishing pad 
may be formed from a blown polyurethane, such as the IC and GS series of 
polishing pads available from Rodel Products Corporation in Scottsdale, 
Ariz. The hardness, density, color, reflectivity, and other 
characteristics of the polishing pad may vary from application to 
application, e.g., according to the material that is to be polished. 
During the polishing or planarization process, the workpiece or wafer is 
typically pressed against the polishing pad surface while the pad rotates 
about its vertical axis. In addition, to improve the polishing 
effectiveness, the wafer may also be rotated about its vertical axis and 
oscillated radially back and forth over the surface of the polishing pad. 
During the CMP process, workpieces occasionally become dislodged from the 
workpiece carrier, or they may break during polishing. If a dislodged 
workpiece, a part of a broken workpiece, or other extraneous material is 
allowed to remain on the polishing table, it could contact other 
workpieces and/or workpiece carriers on the same polishing table and 
thereby damage or destroy all of the workpieces on the table. Accordingly, 
it is desirable to detect the presence of a broken or dislodged workpiece 
immediately and to terminate processing until the situation can be 
rectified. Typically, this requires a thorough cleaning and/or replacement 
of the polishing pad, so that workpiece fragments and other debris can be 
removed so that they do not damage other intact workpieces. 
Systems for detecting the loss of workpieces or for detecting broken 
workpieces are currently known. For example, U.S. patent application Ser. 
No. 08/683,150, filed Jul. 18, 1996, and U.S. patent application Ser. No. 
08/781,132, filed Jan. 9, 1997, both assigned to SpeedFam Corporation of 
Chandler, Ariz., disclose two such systems. These patent applications are 
incorporated herein by reference. However, the presently known systems for 
detecting the loss of workpieces or for detecting broken workpieces may be 
unsatisfactory in several regards. For example, currently known systems 
that employ reflective optical signal processing may be limited to 
operation with a small number of similarly colored polishing pads. Such 
known systems may be ineffective for detecting wafer loss on a dark 
colored polishing pad or in an environment where the polishing pad may 
become discolored over time. Present systems may also be inadequate in CMP 
environments that employ a large amount of polishing slurry and/or 
polishing slurry having a variety of colors. Furthermore, the presence of 
slurry, deionized water, iodine (or other CMP compounds) on the pad and on 
the wafer itself tend to mask the reflected light signal, preventing the 
signal from being properly detected by the photodetector. Consequently, 
many presently known workpiece detection schemes often emit "false" 
readings whereupon machines are shut down and processing halted even 
though all disks remain intact within their respective carriers. 
Therefore, a reliable and robust technique and apparatus for detecting lost 
or dislodged workpieces on a CMP polishing pad is thus needed which 
overcomes the shortcomings of the prior art. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, methods and apparatus are 
provided which overcome many of the shortcomings of the prior art. 
In accordance with a preferred embodiment, a contact stylus is suitably 
mounted proximate the upper surface of a CMP polishing table, such that 
the contact stylus physically responds to the presence of a fragment of a 
wafer or a lost wafer upon the table. 
In accordance with a further aspect of the present invention, a processor 
produces an output indicative of the presence of extraneous material on 
the polishing pad in response to displacement of the contact stylus. 
In accordance with yet a further aspect of the present invention, if a 
dislodged or fragmented wafer is detected on the polishing pad, the 
processor will send a signal to a CMP machine controller, immediately 
stopping processing of the CMP machine, or at least those processes which 
could be adversely affected by the lost or fragmented workpiece. 
The above and other advantages of the present invention may be carried out 
by an apparatus for detecting the presence of extraneous material on a 
polishing pad during a planarization procedure. The apparatus includes a 
contact probe assembly for interrogating the surface of the polishing pad 
during the planarization procedure, where the contact probe assembly is 
capable of movement relative to the polishing pad. The apparatus also 
includes a detector for detecting displacement of the contact probe 
assembly when extraneous material physically contacts the contact probe 
assembly.

DETAILED DESCRIPTION OF A PREFERRED EXEMPLARY EMBODIMENT 
The subject invention relates to the in-process detection of a dislodged or 
fractured workpiece on a polishing pad using a probe that is responsive to 
physical contact with the dislodged or fractured workpiece. The preferred 
embodiment set forth herein relates to the detection of semiconductor 
wafers on a chemical mechanical planarization (CMP) polishing pad; it will 
be appreciated, however, that the principles of the present invention may 
be employed to detect workpieces or other materials in a variety of 
processing (e.g., polishing or planarization) environments such as, for 
example, hard disks and the like. 
Referring now to FIGS. 1 and 2, a wafer polishing apparatus 100 in the form 
of a CMP system is shown embodying the present invention. Wafer polishing 
apparatus 100 is suitably configured to accept wafers from a previous 
processing step, polish and rinse the wafers, and reload the wafers into 
wafer cassettes for subsequent processing. 
Discussing now the polishing apparatus 100 in more detail, apparatus 100 
includes an unload station 102, a wafer transition station 104, a 
polishing station 106, and a wafer rinse and load station 108. 
In accordance with a preferred embodiment of the present invention, one or 
more of cassettes 110, each holding a plurality of wafers, are placed into 
the machine at unload station 102. Next, a robotic wafer carrier arm 112 
removes the wafers from cassettes 110 and places them, one at a time, on a 
first wafer transfer arm 114. Wafer transfer arm 114 then individually 
lifts and moves each wafer into wafer transition section 104. That is, 
transfer arm 114 suitably sequentially places an individual wafer on each 
one of a plurality of wafer pick-up stations 116 which reside on a 
rotatable table 120 within wafer transition section 104. Rotatable table 
120 also suitably includes a plurality of wafer drop-off stations 118 
which alternate with pick-up stations 116. After a wafer is deposited on 
one of the plurality of pick-up stations 116, table 120 will rotate so 
that a new station 116 aligns with transfer arm 114. Transfer arm 114 then 
places the next wafer on the new empty pick-up station 116. This process 
continues until all pick-up stations 116 are filled with wafers. In the 
preferred embodiment of the invention, table 120 includes five pick-up 
stations 116 and five drop-off stations 118. 
Next, a wafer carrier apparatus 122, having individual wafer carrier 
elements 124, suitably aligns itself over table 120 so that respective 
carrier elements 124 are positioned directly above the wafers which reside 
in respective pick-up stations 116. The carrier apparatus 122 then drops 
down and picks up the wafers from their respective stations and moves the 
wafers laterally such that the wafers are positioned above polishing 
station 106. Once above polishing station 106, carrier apparatus 122 
suitably lowers the wafers, which are held by individual elements 124, 
into operative engagement with a polishing pad 126 disposed upon a lap 
wheel 128. During operation, lap wheel 128 causes polishing pad 126 to 
rotate about its vertical axis, for example, in a counter-clockwise 
direction as shown by arrow 134. At the same time, individual carrier 
elements 124 spin the wafers about their respective vertical axes and 
oscillate the wafers radially back and forth across pad 126 (substantially 
along arrows 133) as they press against the polishing pad. In this manner, 
the surface of the wafer will be polished or planarized. 
After an appropriate period of processing time, the wafers are removed from 
polishing pad 126, and carrier apparatus 122 transports the wafers back to 
transition station 104. Carrier apparatus 122 then lowers individual 
carrier elements 124 and deposits the wafers onto drop-off stations 118. 
The wafers are then removed from drop-off stations 118 by a second 
transfer arm 130. Transfer arm 130 suitably lifts each wafer out of 
transition station 104 and transfers them into wafer rinse and load 
station 108. In the load station 108, transfer arm 130 suitably holds each 
wafer while it is being rinsed. After a thorough rinsing, the wafers are 
reloaded into cassettes 132, which then transports the wafer to subsequent 
stations for further processing or packaging. 
Although CMP machine 100 is shown having five polishing stations, it will 
be appreciated that the present invention may be employed in the context 
of virtually any number of polishing stations. Moreover, the present 
invention may also be employed in circumstances where not all of the 
polishing stations are functioning at the same time. For example, many 
standard wafer cassettes carry twenty-four individual workpieces in a 
single cassette. Consequently, because there are often five workpiece 
chucks on a single CMP machine, the last four disks within a cassette may 
be polished at one time, leaving the fifth disk-holder empty. 
With continued reference to FIG. 2, a respective contact probe assembly 129 
is suitably configured on wafer carrier apparatus 122 near each carrier 
element 124. In accordance with a particularly preferred embodiment of the 
invention, each contact probe assembly 129 is suitably configured to 
interrogate at least a portion of the polishing pad surface proximate each 
carrier element 124 to detect the presence of extraneous material, such as 
a loose screw, a wafer, or a wafer fragment on the surface of polishing 
pad 126 as described in greater detail below. In the context of this 
description, polishing pad 126 is one example of a processing element with 
which the present invention may be configured to interact. 
Referring now to FIG. 3, a schematic representation of an exemplary contact 
probe assembly 129 in accordance with the present invention is illustrated 
in conjunction with a workpiece detection system 300. Although apparatus 
100 preferably includes a plurality of probe assemblies 129 for use with a 
number of carrier elements 124, only a single probe assembly 129 is 
illustrated in FIG. 3 for clarity. Detection system 300 preferably 
includes at least one probe assembly 129 for interrogating the upper 
surface 302 of polishing pad 126, a processor 304 for receiving and 
processing a control signal 306 generated by probe assembly 129, and a CMP 
controller 308 configured to control various components associated with 
polishing apparatus 100, e.g., carrier element 124 and lap wheel 128 with 
polishing pad 126. It should be appreciated that control signal 306 may be 
associated with any number of measurable electrical characteristics such 
as current or voltage and that control signal 306 may be a digital 
representation or an analog signal. 
As best seen in FIG. 3, an exemplary workpiece 310 is shown being held by 
carrier element 124 and polished by polishing pad 126 as described above 
in conjunction with FIGS. 1 and 2. For clarity, the other components of 
apparatus 100 are omitted from FIG. 3. Contact probe assembly 129 is 
suitably mounted proximate to and above polishing pad 126 such that, when 
in operation, a contact stylus 312 contacts (or nearly contacts) upper 
surface 302 of polishing pad 126. In this manner, and as described in more 
detail below, contact probe assembly 129 preferably monitors polishing pad 
126 for the presence of extraneous material during polishing of workpiece 
310. In the preferred embodiment, contact probe assembly 129 interrogates 
upper surface 302 in a substantially continuous manner during processing 
of workpiece 310. 
In an exemplary embodiment, contact probe assembly 129 may be mounted under 
the multi-head transport assembly (MHTA) which is part of carrier 
apparatus 122 in a position that will suitably allow contact probe 
assembly 129 to interrogate polishing pad 126 at an appropriate location. 
In accordance with the preferred embodiment, contact probe assembly 129 is 
mounted in a substantially stationary position relative to polishing pad 
126, and carrier element 124 is capable of rotational and translational 
movement relative to contact probe assembly 129. 
In accordance with the illustrated embodiment, at least one contact probe 
assembly 129 is suitably mounted proximate each carrier element 124 such 
that the associated contact stylus 312 is located directly in front of the 
respective carrier element 124. That is, if polishing pad 126 is rotating 
counter-clockwise, as shown by arrow 134 (see FIG. 2), contact stylus 312 
is positioned such that a wafer (or wafer fragment) will touch contact 
stylus 312 immediately upon becoming dislodged from carrier element 124. 
In this manner, contact probe assembly 129 will detect a dislodged wafer 
or wafer fragment as soon as possible to enable workpiece detection system 
300 to take the appropriate action, e.g., disable polishing apparatus 100 
before other wafers can be damaged by the broken/dislodged workpiece or 
other debris. Of course, if polishing pad 126 is rotating clockwise, then 
the associated contact stylus 312 should be positioned at the opposite 
side of each carrier element 124. 
It should be appreciated that contact probe assembly 129 may be angularly 
oriented relative to upper surface 302 rather than substantially 
perpendicular as shown in FIG. 3. Such an angular orientation may be 
desirable to enhance the detection sensitivity of contact probe assembly 
129 or to facilitate fine tuning for the specific application. 
In accordance with a particularly preferred embodiment of the present 
invention, and with continued reference to FIG. 3, contact probe assembly 
129 suitably generates control signal 306 in response to vertical 
displacement of contact stylus 312, relative to polishing pad 126, by at 
least a threshold amount. The predetermined threshold distance may be 
selected according to the specific application, the CMP environment, or 
the extraneous material intended to be detected. For example, a preferred 
embodiment configured to detect a semiconductor wafer may utilize a 
threshold amount within the range of 5 to 20 mils. This threshold amount 
is selected to ensure that workpiece detection system 300 reliably detects 
physical contact between a lost or fragmented wafer and contact stylus 
312. 
Control signal 306 may be an analog signal having a linear characteristic 
relative to the amount of displacement of contact stylus 312 (see FIG. 8). 
In the preferred embodiment, control signal 306 is characterized by a 
substantially linear current-to-displacement output function. Although not 
described in detail herein, workpiece detection system 300 may employ any 
number of conventional signal conditioning techniques, known to those 
skilled in the art, to suitably process control signal 306 before and/or 
after it is received by processor 304. It should be appreciated that 
contact probe assembly 129 may be additionally configured to produce a 
bias signal associated with the initial position of contact stylus 312, 
relative to upper surface 302 of polishing pad 126; control signal 306 may 
be associated with a linear increase (or decrease) in the bias signal that 
is suitably detected and processed by processor 304. As mentioned above, 
nothing limits the present invention to the use of analog control signals 
and control signal 306 may be alternately configured as a digital signal 
or a digital word associated with the displacement of contact stylus 312. 
Processor 304 is preferably configured to process control signal 306 and to 
produce an output signal 314 indicative of the presence of extraneous 
material proximate contact stylus 312. Processor 304 is preferably 
configured to indicate the presence of extraneous material on upper 
surface 302 when control signal 306 exceeds a predetermined threshold 
value. Consequently, workpiece detection system 300 refrains from 
indicating the presence of extraneous material on upper surface 302 when 
control signal 306 is less than the threshold value. The particular 
threshold value for control signal 306 (which is associated with the 
threshold displacement of contact stylus 312) may be selected according to 
the operating specifications of contact probe assembly 129 or other 
components of apparatus 100. It should be appreciated that, rather than 
employ a single threshold control signal value, workpiece detection system 
300 may be alternately configured to indicate the presence of a wafer on 
polishing pad 126 when control signal 306 falls within a predetermined 
voltage or current range. Such a configuration may be desirable to reduce 
false detections or to otherwise increase the reliability or robustness of 
workpiece detection system 300. 
According to a desired aspect of the present invention, if a wafer or wafer 
fragment is detected, processor 304 sends output signal 314 to CMP 
controller 308 which, in turn, immediately shuts down the machine. In a 
particularly preferred embodiment of the invention, CMP controller 308 may 
include a separate control device connected to the main 
processor/controller of polishing apparatus 100, or CMP controller 308 may 
be configured as part of the main unit. Further, CMP controller 308 may be 
configured as part of processor 304. As will be appreciated by one skilled 
in the art, CMP controller 308 may employ any type and configuration of 
controller capable of shutting down polishing apparatus 100. Output signal 
314 may alternatively, or additionally, trigger warning devices or control 
various other components of apparatus 100. Rather than disabling the 
machine, CMP controller 308 may produce an output indicative of the 
presence of extraneous material upon polishing pad 126 or suitably adjust 
an appropriate operating parameter of the system to reduce or eliminate 
the likelihood of damage. 
The operation and configuration of contact probe assembly 129 will now be 
described in conjunction with FIGS. 4-7. FIGS. 4 and 5 are phantom views 
of a probe actuator housing 400 and associated contact probe assemblies 
129; FIGS. 6 and 7 are schematic representations of contact stylus 312 in 
typical operating conditions. 
As described above, actuator housing 400 may be suitably mounted to 
polishing apparatus 100 in a position that enables contact probe 
assemblies 129 to effectively interrogate upper surface 302 of polishing 
pad 126 proximate wafer carriers 124 (see FIG. 2). In the exemplary 
embodiment shown in FIGS. 4 and 5, each actuator housing 400 has two 
contact probe assemblies 129 coupled thereto. The use of multiple contact 
probe assemblies 129 may be desirable to address space limitations of 
polishing apparatus 100 or to facilitate effective interrogation of 
differently sized polishing pads 126. Polishing apparatus 100 preferably 
employs at least one contact probe assembly 129 for each active wafer 
carrier 124, e.g., at least five in the exemplary embodiment shown in 
FIGS. 1 and 2. For purposes of this description, each contact probe 
assembly 129 is substantially similar in configuration and functionality. 
Consequently, only one contact probe assembly 129 will be described in 
detail below. 
Contact probe assembly 129 may be pivotally coupled to actuator housing 400 
such that it may be raised during inactive periods (see FIG. 5) and 
lowered to interrogate polishing pad 126 during processing of workpiece 
310 (see FIG. 4). Contact probe assembly 129 may be raised to provide 
clearance for efficient operation of polishing apparatus 100. In the 
preferred embodiment, contact probe assembly 129 is coupled to actuator 
housing 400 via an air cylinder 402, which, when appropriately activated, 
causes contact probe assembly 129 to pivot upward or downward. Although 
not shown, air cylinder 402 may be in communication with and controlled by 
CMP controller 308 or any other control system employed by polishing 
apparatus 100. Those skilled in the art should appreciate that any 
suitable device may be alternately utilized to engage contact probe 
assembly 129, e.g., an electromagnetic solenoid or a mechanical gear 
arrangement. 
In accordance with an exemplary embodiment of the present invention, 
contact probe assembly 129 generally includes contact stylus 312, a shaft 
404, and a displacement detector 406. As described above, contact stylus 
312 interrogates upper surface 302 of polishing pad 126 and deflects in 
response to physical contact with extraneous material present upon upper 
surface 302. Contact stylus 312 is coupled to one end of shaft 404, which 
is received within a sleeve bearing 408. Sleeve bearings and equivalent 
structures are well known in the art and any number of commercially 
available components may be utilized for sleeve bearing 408. Sleeve 
bearing 408 retains shaft 404 along a substantially constant axis and 
preferably limits movement of shaft 404 in a direction compatible with 
displacement detector 406 (described below). In the preferred embodiment, 
contact stylus 312 and shaft 404 are configured for translational movement 
in a substantially perpendicular direction relative to polishing pad 126 
when contact probe assembly 129 is in the lowered position (see FIG. 4). 
The end of shaft 404 opposing contact stylus 312 engages displacement 
detector 406 such that contact stylus 312 is physically coupled to 
displacement detector 406 via shaft 404. Displacement detector 406 may 
utilize any suitably configured actuator tip 410, e.g., a roller wheel or 
a ball bearing. Actuator tip 410 may be attached to shaft 404 or be 
configured to merely contact the end of shaft 404. When extraneous 
material causes contact stylus 312 to raise (see FIG. 7), shaft 404 
engages displacement detector 406, which produces an intermediate control 
signal (not shown). The intermediate control signal serves as an input to 
an amplifier 412, which suitably amplifies and/or conditions the 
intermediate control signal to produce control signal 306 (see FIG. 3). 
Those skilled in the art will appreciate that amplifier 412 may be 
configured in accordance with known technologies and that the specific 
operating parameters of amplifier 412 may be selected for compatibility 
with the detection sensitivity of displacement detector 406, the output 
range of displacement detector 406, the operating conditions of polishing 
apparatus 100, and/or any number of application-specific variables. 
In accordance with a preferred embodiment of the present invention, 
displacement detector 406 includes a linear displacement transducer. 
Although the present invention may utilize a displacement transformer, 
such transformers typically exhibit a relatively slow response time. To 
ensure that a broken or dislodged wafer is immediately detected, 
displacement detector 406 preferably has a response time within a range of 
1 to 20 ms. This preferred range may vary depending upon the rotational 
speed of polishing pad 126, the location of contact stylus 312 relative to 
wafer carrier 124, and other operating parameters. One linear displacement 
transducer suitable for purposes of the present invention is commercially 
available from Omron (part number D5M). Such commercially available linear 
contact displacement sensors may include a matched amplifier that may be 
suitable for use as amplifier 412. 
Contact probe assembly 129 may include a number of splash guards configured 
to prevent water, slurry, debris, and/or other processing materials from 
contaminating the components of contact probe assembly 129. In particular, 
a first splash guard 414 substantially surrounds shaft 404 and sleeve 
bearing 408. First splash guard 414 may cooperate with a second splash 
guard 416 to provide further protection for displacement detector 406. In 
the exemplary embodiment shown in FIGS. 4 and 5, first and second splash 
guards 414 and 416 are coupled together to form a telescoping arrangement. 
The telescoping arrangement enables first and second splash guards 414 and 
416 to move in conjunction with the displacement of contact stylus 312. In 
the preferred embodiment, first and second splash guards 414 and 416 are 
formed of TEFLON or a TEFLON coated metal. Alternatively, any water 
resistant and corrosion resistant material may be utilized for first and 
second splash guards 414 and 416. 
An alternate embodiment may employ a contamination rinse assembly instead 
of (or in addition to) first and second splash guards 414 and 416. Such a 
rinse assembly may dispense deionized water, or any other suitable rinse 
solution, to appropriately clean contact probe assembly 129. It should be 
appreciated that the rinse assembly may be suitably controlled to dispense 
the cleaning solution during processing of workpieces, between processing 
cycles, or at any other desired time. 
With reference to FIGS. 6 and 7, contact stylus 312 will be described in 
more detail. Contact stylus 312 is preferably formed of TEFLON, DELRIN, or 
an equivalent corrosion resistant material having a relatively low 
coefficient of friction. The particular material selected for contact 
stylus 312 may vary depending upon the downward pressure imparted by 
contact probe assembly 129, the composition of polishing pad 126, and/or 
other application-specific parameters. 
Contact stylus 312 has a substantially round perimeter, which enables 
consistent detection of material traveling in any direction relative to 
contact probe assembly 129. To facilitate effective displacement in 
response to physical contact with extraneous material, e.g., a wafer or a 
wafer fragment, the base of contact stylus 312 may be upwardly tapered 
from a central portion 600 to an outer portion 602 (see FIG. 7). Central 
portion 600 may be adapted to substantially reside upon upper surface 302 
when contact stylus 312 is in the bias position shown in FIG. 6. Contact 
stylus 312 may upwardly taper at an angle .theta. (FIG. 6) within the 
range of 10 to 20 degrees, and preferably at approximately 15 degrees, 
relative to upper surface 302. Angle .theta. may be selected according to 
the anticipated thickness of the extraneous material such that contact 
stylus 312 readily deflects upward to allow the extraneous material to 
pass between upper surface 302 and central portion 600. 
FIG. 9 is a schematic depiction of an alternate contact stylus 900 that may 
be employed by the present invention. Contact stylus 900 is preferably 
configured for angular movement about an axis of rotation 902 oriented 
substantially parallel to polishing pad 126. In other words, contact 
stylus 900 exhibits a pendulum motion in response to contact with a lost 
or dislodged wafer. The bias position of contact stylus 900 may be 
substantially perpendicular to upper surface 302, and any suitable 
detection device (not shown) may communicate with contact stylus 900 to 
monitor the rotation and/or the deflection of contact stylus 900 beyond 
the bias position. For example, displacement detector 406 (see FIGS. 4 and 
5) may be suitably modified to measure deflection of contact stylus 900. 
FIG. 10 is a schematic representation of an alternate displacement detector 
1000 that may utilize fiber optic sensor techniques. Displacement detector 
1000 preferably includes a plurality of optical fibers arranged in a fiber 
optic array 1002 and a trigger block 1004, which may be coupled to the end 
of shaft 404 (see FIGS. 4 and 5). Fiber optic array 1002 may be 
commercially available and configured in accordance with known 
methodologies such that each individual fiber emits a distinct detection 
beam 1006. Trigger block 1004, or any other suitable structure, is 
configured to interrupt at least one detection beam 1006 when the 
displacement of contact stylus 312 exceeds the threshold amount. In an 
alternate embodiment, displacement detector 1000 may employ a single 
optical fiber that functions as a binary switch. 
Processor 304 (see FIG. 3) may communicate with fiber optic array 1002 to 
determine how many detection beams 1006 are currently interrupted and/or 
unobscured. As shown in FIG. 10, the number of interrupted detection beams 
1006 is related to the displacement of contact stylus 312. If a 
predetermined number of detection beams 1006 are obscured by trigger block 
1004, then processor 304 generates output signal 314 for CMP controller 
308 (described above). The use of fiber optic array 1002 is desirable to 
enable displacement detector 1000 to establish a bias position associated 
with contact stylus 312. Thus, output signal 314 may be produced in 
response to the interruption of a differential number of detection beams 
1006, relative to the number of detection beams 1006 that are interrupted 
in the bias position. 
The sensitivity of displacement detector 1000 may be adjusted by 
appropriately configuring trigger block 1004. For example, and as shown in 
FIG. 11, trigger block 1004 may be formed with an angular face 1008 to 
adjust the number of detection beams 1006 that are interrupted by trigger 
block 1004 for a given displacement of contact stylus 312. Those skilled 
in the art should appreciate that it may be desirable to adjust the 
sensitivity of displacement detector 1000 to accommodate the operating 
parameters of associated conditioning circuits, amplifiers, processors, or 
other components of workpiece detection system 300. Furthermore, the 
specific manner in which the sensitivity of displacement detector 1000 is 
adjusted may vary from the embodiment shown in FIG. 11. 
In summary, the present invention provides a technique for the in-process 
detection of extraneous material upon a polishing pad; the technique 
employs a contact stylus mounted proximate the upper surface of the 
polishing pad. The contact stylus physically responds to the presence of a 
fragment of a wafer or a lost wafer upon the table. A processor produces 
an output indicative of the presence of extraneous material on the 
polishing pad in response to displacement of the contact stylus. 
Preferably, if a dislodged or fragmented wafer is detected on the 
polishing pad, the processor will send a signal to a CMP controller, which 
immediately disables the CMP machine, or at least those processes which 
could be adversely affected by the lost or fragmented workpiece. 
The present invention has been described above with reference to preferred 
embodiments. However, those skilled in the art will recognize that changes 
and modifications may be made to the preferred embodiment without 
departing from the scope of the present invention. For example, any 
suitable displacement sensor may be employed by the present invention in 
place of the linear displacement transducer described herein. In addition, 
the present invention is not limited to use with the specific polishing 
apparatus shown and described herein or to use with semiconductor wafers. 
These and other changes or modifications are intended to be included 
within the scope of the present invention, as expressed in the following 
claims.