Abstract:
A system for measuring the thickness of a wafer while it is being thinned this disclosed. The system and method provide integrating an optical reflectometer into a common wafer thinning apparatus. Using reflected optical signals from the top and bottom of the wafer, the thickness of the wafer is determined with time based calculations in real-time while thinning is occurring. Once the desired thickness has been reached, the thinning operation is halted. By performing the measurement in-situ, this system and a method prevent scrapping of wafers which are overthinned and the reloading of wafers which are too thick. Since an optical reflectometer is used, the measurement is contactless, and thus prevents possible damage to wafers during measurement.

Description:
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The present invention relates to an apparatus and method for monitoring the change in thickness of a substrate, and more particularly to monitoring the change in thickness of a substrate on a semiconductor workpiece, during a CMP operation. 
     II. Description of the Related Art 
     In the semiconductor industry, critical steps in the production of semiconductor products include the selective polishing and thinning of substrates. Types of substrates include wafers, glass or ceramic plates, etc. made form a variety of substances, and can be conductive or non-conductive. 
     Substrates are thinned and polished by any of several well-known methods, for example chemical-mechanical polishing (also known as CMP), reactive ion etching (RIE), wet etching, electrochemical etching, vapor etching, and spray etching. 
     It is extremely important with thinning of substrates to stop the process when the correct thickness has been achieved. With CMP, substrate material is selectively removed from a substrate by rotating the wafer against a polishing pad, or rotating the pad against the wafer, or both, with a controlled amount of pressure in the presence of a chemically reactive slurry. Removing too much or too little of the substrate, commonly referred to as overpolishing or underpolishing respectively, may result in improper die thickness and therefore result in scrapping of the wafer. Since many process steps have already taken place prior to a CMP step, scrapping a wafer during the thinning of a substrate can mean a significant financial loss. 
     Various methods have been employed to detect when the desired endpoint of the CMP process has been reached in order to stop the polishing operation. In the prior art, methods for CMP endpoint detection involve the following types of measurement: (1) mechanical measurement such as a dial indicator, (2) simple timing, (3) friction or motor current, (4) chemical analysis of the slurry, (5) capacitive, (6) non in-situ optical, (7) acoustical, and (8) conductive. These prior art methods each have inherent disadvantages such as inability for real-time monitoring, the need to remove the wafer from the polishing apparatus (not in-situ), or unreliability. 
     The mechanical measurement method is cheap and simple, but is relatively inaccurate, especially if the substrate is fixed to a backing plate, and requires removing the sample from the polishing fixture, which may result in damage to the substrate. The simple timing method gives large errors because it is affected by thickness variations of the substrate and polish rate variations caused by composition of the slurry, pressure of the wafer against the pad, type of pad, and relative rotational speeds. Monitoring the motor current change due to the change in friction produced between the wafer and the pad only provides a resultant value for the variations and provides indirect wafer monitoring at best, with average values for the wafer. Chemical analysis of the slurry requires transporting the slurry from the polishing pad to the analysis location, as well as the use of expensive instrumentation such as inductively coupled plasma (ICP) for atomic emission spectroscopy and does not provide true real time response. Capacitive measurements embed sensing elements in the polishing table below the polishing pad and thus do not provide a continuous and reliable measurement of the change during removal. Capacitive measurements are also especially ill suited for metal substrates on top of multiple levels of metal interconnections. A non in-situ optical method has also been used, but requires that the process be interrupted from time to time for measurement of the reflectivity or thickness change. Acoustical methods have also been proposed, however no encouraging data is available so far. Conductive methods monitor current flowing from electrodes embedded in either the polishing pad or the polishing table through the wafer. This type of method requires some kind of direct contact between the electrodes and the wafer surface as well as their exposure to the corrosive slurry and contact with the polishing pad, which can lead to contamination of the pad and possible scratching of the wafer. 
     Techniques for measuring the thickness of coatings on metal objects are also known. In U.S. Pat. No. 5,559,428, Li et al., an apparatus and method are disclosed which monitor the change in thickness of a semiconductor substrate by discriminatorily inducing a current in the substrate depending upon the conductivity of the substrate. If the substrate is conductive, eddy currents are induced in the substrate by generating an alternating electromagnetic field with a sensor which includes a capacitor and an inductor. However, this technique is dependent on the system accurately determining the material of the substrate. In addition, as this technique involves contacting the wafer, damage to the wafer may result. 
     What is needed is an in-situ real-time contactless monitoring of the change in thickness of a substrate which can be any material, including a conductive substrate. 
     SUMMARY OF THE INVENTION 
     The present invention provides a system for in-situ contactless measuring of the thickness of a workpiece while the workpiece is being thinned. Such a system prevents the need to re-load the workpiece into a thinning apparatus if the workpiece is determined to be too thick, or scrapping the workpiece if it has been overthinned or damaged due to contact by the measuring device. 
     According to one aspect of the invention, a common wafer inning apparatus is modified to include an optical reflectometer. The optical reflectometer constantly monitors the thickness of a wafer while the wafer is being thinned. Since the optical reflectometer uses light for measurement, it can perform a large range of measurements in a non-contact nature, with relatively high precision. In addition, executing the measurement in-situ rather than posteriori eliminates the repetitiveness of reloading the wafer to be thinned more. It also minimizes the occurrence of wafer scrap caused by out-of-spec thickness or damage to the wafer from the measurement process. 
     Multiple optical reflectometers may be used to obtain wafer thickness readings at more than one position on the wafer. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The foregoing and other advantages and features of the invention will become more apparent from the detailed description of preferred embodiments of the invention given below with reference to the accompanying drawings in which: 
     FIG. 1 is a side view of the thinning device constructed in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a cross sectional top view of the chuckface in accordance with a preferred embodiment of the present invention; 
     FIG. 3 is a cross sectional side view of the chuckface in accordance with a preferred embodiment of the present invention; 
     FIG. 4 is a side view of the optical signal source and vacuum source in accordance with a preferred embodiment of the present invention; 
     FIG. 5 is a cross sectional view of FIG.  4 . 
     FIG. 6 is an illustration of the path of the optical signals. 
     FIG. 7 shows a graph depicting the reflected signals at the top and bottom of the workpiece. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now to the drawings, where lie reference numerals designate like elements, there is shown in FIG. 1 a system  100  for thinning a semiconductor workpiece. The workpiece may be a semiconductor wafer, glass or ceramic substrate,) etc. The system  100  includes an outer shell fixture  700  with integrally connected components  125 ,  126 ,  127 , support  122  for the fixture  700 , chuckface  114 , and vacuum channel  110 . The fixture  700  and workpiece  120  are supported by a rotatable table  124 . The fixture is rotatable by means of drive  704  ad the rotatable table is movable by means of drive  702 . The vacuum channel  110  is a communication path/channel which is connected on its upper end to a source interface  200 , which provides an interface for a vacuum source  224  and an optical reflectometer  210  to connect to a hollowed center  112  of the vacuum channel  110 . The hollowed center  112  provides a communication path for the suction from the vacuum source  224  and light from the optical reflectometer  210  to reach the chuckface  114 . In order to facilitate the needs of the optical reflectometer  210 , the hollowed center  112  has an optical beam path running down its center. The optical reflectometer  210  is also connected to a PC  240 , which is used in the wafer thickness calculations. The vacuum channel  110  has a light collimator  230  at the top to direct the optical signal down the hollowed center to the chuckface. The chuckface  114  resides at the bottom of the system  100  and sits above the workpiece  120  during operation. A quartz optical window is cemented into a hole in the center of the chuckface to allow the optical signal to pass through, while maintaining vacuum in the hollowed center. Lastly, system  100  contains a grinding control system  400  with control lines  404 ,  402  to the thinning fixture  700  and control line  406  to the PC  240 . The PC  240  uses control line  406  to signal to the grinding control system  400  to stop the thinning process. The grinding control system  400  then in turn signals the thinning fixture  700  via control line  402  and the rotatable table  124  via control line  404  to halt operation. For purposes of the above exemplary description, we have assumed that both the thinning fixture  700  and rotatable table  124  are both rotating counter to each other. However, this invention will work if only one rotates relative to the other. 
     FIG. 2 illustrates a top cross sectional view of the chuckface  114 . FIG. 2 shows that the chuckface  114  includes an upper cavity  118 , several vacuum conduits  126 , and an optical duct  130 . The vacuum conduits  126  each have a communication path to the upper cavity  118 . These vacuum conduit  126  allow suction from the top of the chuckface to reach the bottom of the chuckface and hold a wafer in place during a CMP operation. 
     A side cross sectional view of the chuckface  114  is shown in FIG.  3 . The optical duct  130  is connected to an optical window  128 . The optical window is a quartz disk polished on both surfaces. 
     FIG. 4 shows the interface of the optical reflectometer  210  and vacuum source  224 . The vacuum source  224  is connected to a tube  204  via a connector  206 . The tube  204  is connected at its opposite end to a vacuum source inlet  500  on conversion tube  202 . The optical reflectometer  210 , which contains a light source, e.g. light emitting diode (“LED”), is connected to optical fiber  208 , which is terminated at the other end with a collimator and held in place by a x-y-z fiber positioner with pitch and yaw adjustment. The x-y-z translation stage fixture  212  also connects to an optical inlet  502  , which is a quartz window that allows the optical signal from the reflectometer  210  to pass into the vacuum channel  110  with no loss of vacuum. Conversion tube  202 , which communicates with the optical and vacuum resources  224 ,  210 , passes the vacuum and light to interface  200 , which connects to the hollowed center  112  of vacuum channel  110 . 
     As can be seen from FIG. 5, the x-y-z translation stage fixture  212  contains the x-y-z translation stage components  214  which provides output of an aligned light beam to an optical tube  218 . The x-y-z translation stage fixture  212  holds the x-y-z translation stage components  214  in proper orientation. The x-y-z translation stage components  214  receive the optical light from the optical reflectometer  210  via optical fiber  208  and properly align that beam of light to be in alignment with the optical tube  218  such that the optical signal may proceed down optical tube  218  to the vacuum channel  110 . The x-y-z translation stage is a mechanically adjusted device, where alignment is performed manually by adjusting the light&#39;s alignment so that a maximum reflectance peak is returned back to the reflectometer from the substrate which is being thinned. This indicates that the optical beam is at normal incidence to the substrate. Without such alignment the optical signal would never reach the chuckface  114 . The x-y-z translation stage components basically align the ray of light from optical fiber  208  until it is within proper coordinates to go straight down the optical tube  218 . 
     Operations begin with the vacuum source  224  providing suction and the optical reflectometer  210  emanating an optical signal. A wafer  120  to be thinned is placed under the chuckface  114 . The suction provided by the vacuum source is routed through connector  206 , tube  204 , through converter  202 , pass interface  200 , down the vacuum channel  110  through the hollowed center  112 , to the upper cavity of the chuckface  118  to the vacuum conduits  126 . This path of vacuum suction secures the wafer  120  to be thinned to the lower side of the chuckface  114 , during the thinning operation. At the same time an optical signal emanating from the optical reflectometer  210 , is routed through optical fiber  208 , and out of the collimator held by the x-y-z translation stage components  214 , through the conversion tube  202 , pass the interface  200 , down the hollowed center  112  of the vacuum channel  110  and through the chuckface  114  via optical window  128 . 
     During a thinning operation, the thing fixture  700  rotates in a direction counter to that of the rotating table  124 , while the vacuum channel  110  remains stationary. The friction created between the wafer  120  rotating, by means of the thinning fixture  700  and the rotatable table  124 , containing a chemical slurry, thins the wafer  120 . 
     Referring to FIG. 6, during thing the optical signal  400   a  is routed down the vacuum channel  110  and reflected back up the vacuum channel  110  when it reaches top  404  and bottom  406  surfaces of the wafer  120  respectively. A first reflected optical signal  400   b  occurs due to a reflection of the source optical signal  400   a  at the top surface of the wafer  404 . A second reflected optical signal  400   c  occurs due to the reflection of the source optical signal  400   a  at the lower surface of the wafer  406 . Other wafer structures between the top and bottom surfaces of the wafer  406  may result in additional reflected signals. A graphical representation of the two reflected signals  400   b  and  400   c  can be seen in FIG.  7 . Then, the optical reflectometer determines the optical thickness of the wafer being thinned by comparing reflected signals  400   b  and  400   c  to a reference optical signal, by calculating the difference between the time of arrival of the reflected signals  400   b  and  400   c , or by another suitable method. This information is then used by the PC 240 to calculate the physical thickness of the wafer. When the desired thickness is reached, the PC 240 signals to the grinding control unit  400  via control line  406  to stop the thinning function. The grinding control unit then in turn communicates to the thinning fixture  700  and rotating table  124  via control lines  402 ,  404 , respectively, to stop rotating thus stopping the thinning. 
     In another embodiment, one optical reflectometer may be multiplexed to make measurements on other wafers that are being polished on other polishing fixtures. 
     This present invention is more efficient than previously used methods because the thickness measurement is performed in-situ while preventing the scrapping or re-loading of workpiece ground to an incorrect thickness. 
     The scope of the present invention is not to be considered as limited by the specifics of the particular structures which have been described and illustrated, but is only limited by the scope of the appended claims.