Abstract:
A method and apparatus for controlling when a calibration cycle is started for a metrology tool. The method and apparatus exploits a correlation between a drift of a first parameter (e.g., film thickness measurement drift) and a drift of a second parameter (e.g., CD measurement drift). One embodiment of the method comprises measuring a film thickness on one or more reference substrates to determine when a drift component of these measurements exceeds a pre-determined range and thereafter calibrating the metrology tool when the drift component of the film thickness measurements exceeds the pre-determined range. Generally, the drift of the film thickness measurement will occur prior to substantial drift of the CD measurement occurring.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention generally relates to semiconductor substrate processing systems. More specifically, the present invention relates to a method and apparatus for controlling the calibration cycle of a metrology tool.  
         [0003]     2. Description of the Related Art  
         [0004]     Ultra large scale integrated (ULSI) circuits may include more than one million micro-electronic devices (e.g., transistors, capacitors, interconnecting lines, and the like) that are formed on a substrate (e.g., a silicon (Si) wafer) and which cooperate to perform various functions within the device. Fabrication of the electronic devices includes processes in which one or more layers of a film stack of such a device are deposited or etched, thereby forming one or more structures of the device being fabricated.  
         [0005]     During manufacturing processes, topographic dimensions of structures formed on the substrates are measured to verify that the substrate processing reactors are operating within desired ranges that facilitate high yield and productivity. One typical procedure comprises measuring the smallest widths of the structures, such as lines, columns, openings, spaces between the lines, and the like. Such widths are known as “critical dimensions”, or CDs, and are generally the most difficult elements of a structure to fabricate and measure. In advanced ULSI circuits, the critical dimensions generally are deep sub-micron dimensions having a nominal value of less than about 0.18 microns.  
         [0006]     Metrology tools for performing critical dimension (CD) measurements of topographic structures require periodic calibration to ensure the measurements remain accurate. Generally, metrology tools are calibrated based on a predetermined number of measurements performed since the most recent calibration or, alternatively, based on a time duration that has passed since a previous calibration. These methods of determining when the metrology tool should be calibrated are referred to as “timed” calibrations. Such a timed calibration does not assess actual performance of the metrology tool between calibration cycles such that inaccurate tool performance (e.g., invalid or inaccurate CD measurements) may occur before a calibration cycle is begun. Since CD measurements are used to control process parameters, processing a substrate based upon an incorrect CD measurement can destroy one or more substrates. Furthermore, such “timed” calibration may cause calibration of the tool when such calibration is unnecessary. Consequently, substrate processing throughput may be unnecessarily impacted.  
         [0007]     Therefore, there is a need in the art for a method and apparatus for anticipating when a metrology tool requires calibration such that the use of a calibration cycle can be accurately controlled.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention is a method and apparatus for controlling when a calibration cycle is started for a metrology tool. The method and apparatus exploits a correlation between a drift of a first parameter (e.g., film thickness measurement drift) and a drift of a second parameter (e.g., CD measurement drift). One embodiment of the method comprises measuring a film thickness on one or more reference substrates to determine when a drift of these thickness measurements exceeds a pre-determined range and thereafter calibrating the metrology tool. Generally, the drift of the film thickness measurement will occur prior to substantial drift of the CD measurement occurring. Thus, the risk of destroying substrates by using an inaccurate CD measurement is substantially reduced. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:  
         [0010]      FIG. 1  depicts a flow diagram of a method for anticipating when a metrology tool requires calibration in accordance with one embodiment of the present invention;  
         [0011]      FIG. 2  depicts a schematic, cross-sectional view of a reference substrate fabricated in accordance with the method of  FIG. 1 ;  
         [0012]      FIG. 3  depicts a series of exemplary timing diagrams showing the results of measurements performed on a reference substrate; and  
         [0013]      FIG. 4  depicts a schematic diagram of an exemplary integrated semiconductor substrate processing system of the kind used in performing portions of the inventive method. 
     
    
       [0014]     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.  
         [0015]     It is to be noted, however, that the appended drawings illustrate only exemplary embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
       DETAILED DESCRIPTION  
       [0016]     The present invention is a method for anticipating when a metrology tool requires calibration. The method monitors actual performance of a metrology tool to determine when the metrology tool requires a calibration. A metrology tool such as a NanoSpec 9000 series available from Nanometrics, Inc. of Milpitas, Calif. can measure both film thickness and the critical dimensions (CD) of structures on a substrate. In one embodiment of the invention, such a metrology tool is a component of an integrated substrate processing system used to fabricate topographic structures on substrates.  
         [0017]     The invention exploits the discovery that the drift of a first parameter (e.g., film thickness measurement drift of the metrology tool) anticipates the drift of a second parameter (e.g., CD measurement drift of the tool). Since film thickness measurement drift is less critical to substrate processing than CD measurement drift, the invention uses the thickness measurement drift to initiate a calibration cycle prior to the CD measurement drift becoming large enough to result in substrate damage.  
         [0018]      FIG. 1  depicts a flow diagram of one embodiment of the inventive method  100  for controlling calibration of a metrology tool.  
         [0019]     The method  100  starts at step  102  and proceeds to step  104 , wherein the metrology tool is calibrated. In one exemplary embodiment, the metrology tool is a NanoSpec 9000 series used within a TRANSFORMA™ system available from Applied Materials, Inc. of Santa Clara, Calif. The metrology tool may use one or more non-destructive optical measuring techniques, such as spectroscopy, interferometry, scatterometry, reflectometry, ellipsometry, and the like. The measured parameters include a thickness for patterned and blanket dielectric and/or conductive films, as well as topographic dimensions and profiles of structures fabricated using such films. As such, the metrology tool provides both film thickness and CD measurements.  
         [0020]     At step  106 , one or more reference substrates are provided to the metrology tool.  FIG. 2  depicts a schematic, cross-sectional view of an exemplary reference substrate  200 , e.g., silicon wafer. The illustration in  FIG. 2  is not depicted to scale and is simplified for illustrative purposes. The substrate  200  illustratively comprises a reference layer  204  formed on a material layer  202 . The reference layer  204  has a known thickness  214 . The reference layer  204  has a plurality of structures  210  and/or a plurality of features  212  formed therein. The widths of the structures and features are known. In one embodiment, dimensions for the reference layer thickness  214  and widths  206 ,  208  are selected to correspond approximately to the dimensions of the thickness and widths of the structures and/or features to be formed on non-reference wafers (e.g., product wafers) that are to be measured using the metrology tool.  
         [0021]     At step  108 , the metrology tool measures the thickness  214  of the reference layer  204  and, optionally, the width  206  of the structures  210  and/or features  212  on the reference substrate(s)  200 . A single reference substrate may be measured or multiple reference substrates measured and the measurement averaged. To anticipate when the metrology tool will require calibration again, the method  100  tracks the drift of the thickness measurement. To identify the drift, the reference substrate is periodically reintroduced to the metrology tool to facilitate periodic thickness measurements. The method determines the change in the newly measured thickness with respect to the originally measured thickness as thickness measurement drift (i.e., the current thickness is subtracted from the original thickness).  
         [0022]     At step  110 , the schedule for periodically reintroducing the reference substrate or substrates is established. Multiple reference substrates may be measured to establish an average thickness value. Such a schedule may be defined by time, e.g., a reference substrate is moved to the metrology tool every 10 minutes, or by substrate number, e.g., a reference substrate is moved to the metrology tool after every 20 wafers are processed. The schedule may or may not be periodic, e.g., reference substrates reintroduction may be spaced far apart just after a calibration and be spaced closely after a predefined time or number of product substrates.  
         [0023]     At step  112 , the method  100  queries whether the schedule indicates that it is time to reintroduce a reference substrate. If the query is negatively answered, at step  122 , a non-reference substrate (product substrate) is introduced into the metrology tool. At step  124 , the method measures the CD of the non-reference substrate. The method  100  then proceeds along path  126  to step  112  to query whether it is time for a reference substrate to be measured.  
         [0024]     If, at step  112 , the method  100  determines that a reference substrate is required, then, at step  114 , a reference substrate (or substrates) is provided to the metrology tool. At step  116 , the metrology tool measures the film thickness on the reference substrate. At step  118 , the thickness measurement drift is determined. The drift is the difference between the original thickness measurement (determined in step  108 ) and the present thickness measurement. At step  120 , the drift is compared to a threshold level to determine if the drift is excessive. An excessive drift in a thickness measurement is indicative that soon the CD measurement will have drifted to an unacceptable level. An inaccurate CD measurement can cause severe damage to one or more substrates that are processed in a manner that relies on the inaccurate CD measurement.  
         [0025]     If, at step  120 , an excessive thickness measurement drift is determined, the method  100  proceeds on path  123  to step  104  wherein the metrology tool is calibrated. On the other hand, if the drift is found to be within limits, (i.e., not excessive), the method  100  proceeds to step  112 .  
         [0026]      FIG. 3  depicts a graph that illustrates the results of periodic measurements performed on the reference substrate  200  ( FIG. 2 ). A first graph  310  depicts the thickness measurement drift results (y-axis  312 ) of the reference layer  204  versus time (x-axis  314 ). The drift values are the difference between the thickness measured for a currently used reference substrate and the thickness measured on the original reference substrate. A second graph  320  depicts the CD measurement drift results (y-axis  322 ) versus time (x-axis  324 ). The measurements of the thickness and CD are valid and useful when the results of such measurements remain within pre-determined ranges  316  (thickness) and  326  (CD) having lower limits  315  and  325  and upper limits  317  and  327 , respectively. The results of measuring the thickness and CD are illustratively depicted in  FIG. 3  as drifting over time in positive directions (i.e., beyond the limits  317 ,  327 ) of the respective axes  312  and  322 , however, such results may similarly drift in the opposite directions (i.e., beyond the limits  315 ,  325 ), as shown using broken lines.  
         [0027]     The results of measuring the thickness begin drifting beyond the pre-determined range  316  at a moment  330  preceding the moment  332  when the results of measuring the CD begin a statistically significant drift beyond the respective pre-determined range  326 . Therefore, the moment  330  is used to identify when the tool requires recalibration. As such, the recalibration cycle is begun before the CD measurements have attained a critical drift that could result in wafer damage.  
         [0028]     An example of an etch system that is integrated with an ex-situ metrology tool with the capability of measuring CDs and film thickness is Applied Materials&#39; TRANSFORMA™ system  400  ( FIG. 4 ). Detailed information describing Applied Materials&#39; TRANSFORMA™ system has been disclosed in a commonly assigned U.S. patent application Ser. No. 10/428,145, titled “Method and Apparatus for Controlling Etch Processes During Fabrication of Semiconductor Devices”, filed on May 1, 2003. The system comprises a controller  414 , a chamber or “mainframe”  401 , such as the CENTURA™ processing system for mounting a plurality of processing chambers  402 , e.g., conventional etch reactors, such as the DPSII™ silicon etch chambers, photoresist stripping chambers, such as the AXIOM® from Applied Materials, Inc., and one or more transfer chambers  403 , also called “load locks”. In one embodiment of the present invention, two etch processing chambers  402  and two photoresist stripping chambers  403  are mounted to the mainframe  401 . A robot  404  is provided within the mainframe  401  for transferring substrates between the processing chambers  402  and the transfer chambers  403 . The transfer chambers  403  are connected to a factory interface  405 , also known as a “mini environment” that maintains a controlled environment for the substrates.  
         [0029]     The factory interface  405  comprises a pair of robots  407  that move substrates from at least one tool buffer  408  (e.g., at least one front opening unified pod (FOUP)). The tool buffer  408  comprises a plurality of substrates. These substrates comprise one or more reference substrates  410  and non-reference substrates  412 . The robots  407  move the non-reference substrates  410  to/from the metrology tool  406  and the load locks  403 . The robot  404  moves the non-reference substrates from the load locks  403  to the process chambers  402  and  403  as well as amongst the process chambers  402  and  403 . In accordance with the invention, a reference substrate  410  is moved to the metrology tool  406  to determine metrology tool measurement drift and to perform metrology tool calibration.  
         [0030]     The metrology tool  406  is integrated in the factory interface and provides high-speed data collection and analysis for one or more substrates that enter the system  400 . In accordance with one embodiment of the present invention, the metrology tool  406  is capable of measuring both CD and film thickness. Such a tool is a NanoSpec 9000 series tool available from NanoMetrics, Inc. In other embodiments of the invention, two or more metrology tools may be used to measure film CD and thickness. The metrology tool  406  could also be placed at different location within the process system  400  or be located separate from the processing system.  
         [0031]     The controller  414  comprises a central processing unit (CPU)  416 , a memory  418 , and support circuits  420 . The CPU  416  may be any form of general-purpose computer processor that can be programmed to perform the method of the present invention. Control software  422  can be stored in memory  418 , such as random access memory, read only memory, removable storage, hard disk storage or any combination thereof. The support circuits  420  are conventionally coupled to the CPU  416  and may comprise cache, clock circuits, input/output subsystems, power supplies and the like. In operation, the CPU  416  executes the control software  422  which, in part, causes the TRANSFORMA™ system  400  to perform the method ( 100  in  FIG. 1 ) of the present invention.  
         [0032]     The invention may be practiced using other etch processes wherein parameters may be adjusted to achieve acceptable characteristics by those skilled in the arts by utilizing the teachings disclosed herein without departing from the spirit of the invention.  
         [0033]     The invention may be practiced using other semiconductor substrate processing systems wherein the processing parameters may be adjusted to achieve acceptable characteristics by those skilled in the arts by utilizing the teachings disclosed herein without departing from the spirit of the invention.  
         [0034]     While the foregoing is directed to the illustrative embodiment of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.