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. 
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.