Abstract:
An improved wafer polishing technique. The apparatus as configured prevents the destruction of a thrown wafer and facilitates quick restart of the polishing process on a previously thrown wafer without shutting down or reinitializing the apparatus. When a thrown wafer is detected, a part is moved quickly, e.g., by using a supplemental part raising element.

Description:
BACKGROUND 
     The present invention pertains to surface machining of semiconductor wafers, and in particular to chemical/mechanical polishing (CMP) of silicon and other types of semiconductor wafers. 
     In the commercial production of semiconductor wafers, a semiconductor wafer undergoes successive operations in which relatively thin layers of conductive, semiconductive and dielectric materials are formed on one of the wafer&#39;s major surfaces by metalization, sputtering, ion implantation and other conventional techniques. Although the thickness of such layers is measured in terms of microns or micro inches, the exposed surfaces must be polished flat, in preparation for successive layering operations. 
     A variety of equipment is commercially available for planarizing and otherwise preparing wafer surfaces using a variety of techniques, including chemical/mechanical polishing (CMP) processes. Typically, the layered surface (device side) of the wafer is placed face down on a polish pad carried on a rotating table. A chemically active media, which may also have abrasive particles, is introduced onto the polish table from a dispenser. This media migrates between the wafer and the polish pad. 
     A carrier and compressible backing pad apply a downforce to the back side of the wafer, pressing the device side of the wafer against the polishing pad surface. The polishing pad is supported in place by a polishing tub. An inner sidewall of this tub provides a fence around the circumference of the pad. A guard arm extends laterally over the moving polish pad surface and functions to condition this surface as the wafer is polished. 
     The carrier applying downforce to the wafer is rotatably driven about a vertical axis so as to rotate the wafer with respect to the moving polish pad surface, thereby increasing the relative motion between the wafer and the polish pad. The carrier and hence the wafer is also reciprocated back and forth along an arc, usually intersecting a radial line originating at the center of the polish pad. An optical measuring device, such as that manufactured by NOVASCAN, is coupled to the polishing system. This device optically measures the thickness of the wafer and sends a signal to the polishing system when the proper thickness is obtained. 
     Wafers are polished in batches. At the beginning of a polishing process a technician is required to program a number of parameters into the system&#39;s computer. Such parameters include defining (i) the polishing interval, (ii) the rotatably-driven speed of the carrier, (iii) the number of wafers in the batch, and (iv) other related specifications. 
     In order to maintain the wafer underneath the carrier despite sideways or lateral dislodging forces, the carrier exerts a suction force to the backside of the wafer. This force is often insufficient to retain the wafer, thereby causing it to become dislodged. It has been observed in commercial wafer polishing operations that, despite precautions to the contrary, a dislodged wafer is thrown with destructive force against the fence of the polishing tub. In certain cases, a thrown wafer was observed to break after coming to rest on the polish pad, when it then crashes against either the laterally extending carrier or guard arm. 
     A sensor mounted on the carrier detects a thrown wafer condition. An audible alarm is then immediately sounded. A technician typically shuts down the system completely to remove the broken wafer. At this time, the optical measuring device is reset and the system reinitialized. This can consume time and cost. 
     SUMMARY OF THE INVENTION 
     The present disclosure is directed to an improved wafer polishing apparatus and method of operating such apparatus. A method of facilitating quick restart of a polishing process on a previously thrown non-broken wafer, is disclosed that carries out processing a wafer, identifying a thrown wafer condition, transmitting a signal to the main controller indicative indicating the thrown wafer condition, and causing, an element to be raised to prevent contact with a thrown wafer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a CMP apparatus for polishing a semiconductor wafer arranged in such manner as to prevent a thrown wafer from becoming damaged. 
     FIG. 2 is a high level block diagram of a CMP apparatus constructed in accordance with the present invention. 
     FIG. 3 is a block diagram of the second pneumatic controller in FIG.  2 . 
     FIG. 4 is a partial perspective view of a portion of the polish pad and of the padded fence surrounding an outer periphery of the polish pad portion. 
     FIG. 5 is an operational flow diagram of the main controller shown in FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a typical CMP apparatus for polishing a semiconductor wafer. The polishing apparatus shown generally as  10  includes a polishing wheel assembly shown generally as  11 . The polishing wheel assembly includes a rotatable table  12  to which is attached a polish pad  13 . The polish pad  13  includes a polish pad surface  13 ′ facing upward. A conventional pad is a Rodel IC1000 polyurethane pad. The rotatable table  12  is rotated by shaft  14  in the direction indicated by arrow  15 . The polish pad  13  is typically polyurethane foam having open pores and is about 22 inch in diameter and 0.050 inch thick. 
     A wafer carrying assembly shown generally as  17  includes a wafer carrier  18 , shown holding wafer  16 . A pressure plate  19  is secured to the wafer carrier  18  for applying pressure to the wafer carrier and wafer. In the embodiment shown, a hollow spindle  20  is coupled to the pressure plate and driven by an externally coupled device that moves the wafer carrier assembly  17  in the directions shown by the arrows  21 ,  22  and  23 . As shown by the arrow  31 , pressure can be applied to the spindle  20  by a weight load, and/or a pressurized fluid such as compressed air can be used to exert pressure on the upper surface of wafer carrier  18  by supplying the pressurized fluid to space  24  of the wafer carrier assembly. The force is essentially uniform over the surface of the wafer carrier and wafer. The wafer carrier assembly in a preferred embodiment moves over one-half of the pad surface  13 . A guard arm is shown positioned on the other half of the pad surface  13 . The guard arm operates in conventional manner to condition the polishing pad surface  13 ′ which is rotated by the rotatable table  12  upon which it is carried. 
     The inventor has found that a thrown wafer has a generally consistent trajectory path. In accordance with the preferred embodiment, it has been determined that by selectively positioning the guard arm  32  relative to the carrier assembly, it is possible to prevent a thrown wafer from crashing into the guard arm  32 , immediately upon its becoming dislodged. Of course, the optimum position of the guard arm  32  is configuration specific and can only be determined by simple experimentation. 
     During polishing, a slurry is applied by a sprayer means to the surface of the pad  13 . The slurry is forced into the open pores of the polish pad  13  and also forms a layer of slurry on the pad surface which flows between the wafer  16  carried by the wafer carrier assembly  17  and the polishing pad  13  of polishing wheel assembly  11 . Any suitable slurry may be used. Silica based slurries such as Cabot SC112 are preferred. 
     As previously explained, the rubbing friction between a wafer and the polish pad surface  13 ′ could cause the wafer  16  to slide laterally away from the wafer carrier  18 , until it is eventually fully freed and released with great force toward the periphery of the polish pad  13 . A sensor  33  mounted on the carrier detects a thrown wafer condition. In response thereto, an alarm sounds, and the polishing operation is terminated. 
     A block diagram of the essential operating components constituting the CMP apparatus  10  is shown in FIG. 2 . The main controller  50  provides overall system control. The main controller  50  is shown generally connected to a first pneumatic controller  51 , a second pneumatic controller  52  and a third pneumatic controller  53 . Each of the first, second and third pneumatic controllers  51 - 53  receive electronic signals from the main controller  50  at appropriate times to control the driving speeds and/or raising and lowering action of each of the wafer carrier assembly  17 , and guard arm  32 , and the polishing wheel assembly  11 , respectively. 
     A display  55  and keyboard  56  are coupled to the main controller to allow a technician to monitor a polishing operation as well as to enter batch specific polishing specifications. 
     The basic operational flow of the main controller  50  is shown in FIG. 3 . The main controller continually scans for a signal from sensor  33  indicative of a thrown wafer condition (step  100 ). Upon receipt of such signal (step  110 ), main controller  50  sounds an alarm  54  alerting a technician (step  120 ). Simultaneously with the sounding of the alarm  54 , main controller  50  generates appropriate signals to each of the first, second and third pneumatic assemblies  51 - 53  (steps  130 ,  140 ,  150 ). In response, first pneumatic assembly  51  powers a motor in the wafer carrier assembly  17  to cause the vertical raising of the wafer carrier  18 . Similarly, pneumatic assembly  52  powers a motor coupled to the guard arm to immediately raise the guard arm. In similar fashion, the third pneumatic assembly  53  is powered to significantly slow down the rotating speed of the rotatable table  12 . 
     The wafer carrier  18  and the guard arm  32  are raised to prevent a dislodged wafer from crashing against either component. The slowing of the rotatable table speed provides an improved window of opportunity within which to raise either the carrier  18  or guard arm  32 . A thrown wafer typically bounces of a surrounding wall around the periphery of the polish pad  13 . Eventually, the thrown wafer comes to rest at or near the edge of the polish pad  13  where it is carried under rotational action circumferentially thereon. Unless the guard arm  32  and/or carrier are raised in time, the rotationally-carried wafer will crash. By slowing the traveling speed of the wafer (i.e., reducing the speed of the rotating table  12 ), the wafer must travel a similar distance in greater time to reach either the guard arm  32  and carrier  18 . 
     FIG. 4 is a partial perspective view of a portion of the polish pad and of a padded fence surrounding an outer periphery of the polish pad portion. The inventors have found that by also padding the surrounding wall  35  around the periphery of the polish pad  13 , the likelihood of a wafer breakage at initial impact is significantly improved. In the illustrative embodiment, this is accomplished by providing a removable, padded fence  36  about the polish pad. The padded fence  36  is preferably comprised of a rubber polyurethane stripping with adhesive backings so the padded fence can be periodically removed as debris and other particles become trapped on the surface thereof. 
     The combination of padding the surrounding periphery and the lifting of the guard arm and/or carrier out of the moving path of a thrown wafer, together with the slowing of the rotating table, reduces the possibility of thrown wafer being broken. 
     In accordance with the embodiment, a thrown (non-broken) wafer is re-mounted by a technician onto the rotatable wafer carrier  18  and the polishing process restarted without substantial technician intervention and without fully shutting down the system. 
     A suitable CMP apparatus for practicing the present invention are polishing devices manufactured by WESTECH, such as model 372M. Current versions of such polishing devices are provided without a padded fence as proposed herein. Consequently, a large number of thrown wafers are broken in large part due to the initial impact of a thrown wager against an unpadded surrounding wall. In addition, the WESTECH guard arm is not automatically lifted in response to a thrown wafer condition thus causing thrown and yet-non-broken wafers to crash thereon and break. 
     It should appreciated that a device such as the WESTECH device can be readily adapted to include the new feature of a padded fence, as well as the added functionality of raising the guard arm, raising the carrier, and reducing the speed of the rotating table, under the now timed-control of the existing main controller. 
     Current versions of existing devices, such as those manufactured by WESTECH, were not designed to fast lift the guard arm or carrier. Fast lift, in accordance with the preferred embodiment, is achieved by use of a second pneumatic controller  52  configured substantially as shown in FIG.  2 . When a thrown wafer condition is detected, the main controller  50  transmits a signal to the second pneumatic controller  52 . A signal discriminator  70  analyzes the signal to determine whether the signal from the main controller  50  is for normal operation of the guard arm  32  or in response to a thrown wafer condition. In the former case, the discriminator  70  fires up a primary solenoid  71  to slowly raise the guard arm  32 . In a thrown wafer situation, the discriminator  70  fires up both the primary solenoid  71  as well as a boost solenoid  72  to more quickly raise the guard arm  32 . The firing of the boost solenoid  72  only when a wafer is thrown significantly cuts down on the wafer of the guard arm during normal operation. 
     Because a thrown wafer is generally thrown away from the laterally extending position of the carrier  18 , a boost solenoid is generally not necessary on many configurations. The lift speed of the carrier is fast enough to ensure its being moved before possible contact with a thrown wafer. Of course, a boost solenoid could be employed if a given CMP configuration would render it necessary to reduce contact with a thrown wafer. 
     As previously explained, the padded fence  36  is preferably comprised of rubber polyurethane stripping with adhesive backings. However, any material capable of absorbing the impact of a thrown wafer to prevent it from breaking is considered a suitable equivalent. It is envisioned that the padded fence  36  include any suitable adjusting means to facilitate sizing of the padded fence for use with different CMP devices. 
     Although only a few embodiments have been described in detail above, other modifications are possible. For example, other CMP machines, besides the ones specifically described herein, can be used.