Abstract:
An apparatus and associated method for reducing thermal damage on a specimen during an inspection which includes a radiation source for supplying a beam of radiation, and a means for adjusting a first energy level of the beam of radiation to a second energy level as the beam of radiation is variably positioned from a first location on the surface of the wafer to a second location on the surface of the wafer.

Description:
BACKGROUND 
   The subject matter described herein relates to surface inspection, and more particularly to illumination energy management in surface inspection. 
   As design rules and process windows continue to shrink, integrated circuit (IC) manufacturers face challenges in achieving and maintaining yields and profitability while moving to new processes. The challenges have become more difficult because inspection systems are required to capture a wider range of physical defects on wafer surfaces. One such inspection system includes the use of lasers, which provide high sensitivity to detect small defects, and a relatively high throughput. 
   Lasers can cause surface damage to a semiconductor wafer, e.g., from thermal shock from the laser during a surface inspection process. In some inspection systems the wafer rotates about a central axis during the inspection process. Hence, the wafer surface near the central axis moves at a slower velocity than the wafer surface near the outer edge of the wafer. Accordingly, damage tends to occur near radial inner portions of a wafer surface because relatively more energy/mm 2  is imparted to the inner surface. 
   SUMMARY 
   Described herein are systems and accompanying methods for managing the amount of laser power applied to the surface of a semiconductor wafer during a surface inspection process. 
   In one aspect, the laser power can be adjusted as a continuous or discrete function of the radial distance of the laser beam spot from the center of the wafer. 
   In another aspect, a filter may be interposed between the laser origin and the wafer, such that the filter attenuates a portion of the laser power that varies as a function of the radial distance of the laser beam spot from the center of the wafer. 
   In yet another aspect, laser power can be managed by varying the spot size of the radiation beam incident on the surface of the wafer as a function of radial distance of the laser beam spot from the center of the wafer or by varying the speed of rotation of the wafer as a function of the radial distance of the laser beam spot from the center of the wafer. 
   Additional aspects are set forth in part in the detailed description which follows. It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The detailed description is described with reference to the accompanying FIGS. In the FIGS, the left-most digit(s) of a reference number identifies the FIG in which the reference number first appears. The use of the same reference numbers in different FIGS indicates similar or identical items. 
       FIG. 1  is a simplified schematic illustration of one embodiment of a surface inspection system. 
       FIG. 2  is a simplified schematic illustration of components of one embodiment of a radiation management apparatus. 
       FIG. 3  is a flowchart representing one embodiment of a method for managing radiation during an inspection process. 
   

   DETAILED DESCRIPTION 
   Described herein are exemplary systems and methods for illumination energy management in surface inspection. In the following description, numerous specific details are set forth in order to provide a thorough understanding of various embodiments. However, it will be understood by those skilled in the art that the various embodiments may be practiced without the specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to obscure the particular embodiments. 
   Various methods described herein may be embodied as logic instructions on a computer-readable medium. When executed on a processor the logic instructions cause a processor to be programmed as a special-purpose machine that implements the described methods. The processor, when configured by the logic instructions to execute the methods described herein, constitutes structure for performing the described methods. 
     FIG. 1  is a simplified schematic view of a typical surface inspection system  100 . To simplify  FIG. 1 , some of the optical components of the system have been omitted, such as components directing the illumination beams to the wafer. A wafer  102  is illuminated by a normal incidence beam  104  and/or an oblique incidence beam  106 . Wafer  102  is supported on a chuck  108  which is rotated by means of a motor  110  and translated in a direction by gear  112  so that beams  104  and/or  106  illuminate an area or spot  102   a  which is caused to move and trace a spiral path on the surface of wafer  102  to inspect the surface of wafer  102 . Motor  110  and gear  112  are controlled by controller  114  in a manner known to those skilled in the art. 
   The area or spot  102   a  illuminated by either one or both beams  104 ,  106  on wafer  102  scatters radiation from the beam(s). The radiation scattered by area  102   a  along directions close to a line  116  perpendicular to the surface of the wafer and passing through the area  102   a  is collected and focused by lens collector  118  and directed to a photo-multiplier tube (PMT)  120 . Since lens  118  collects the scattered radiation along directions close to the normal direction, such collection channel is referred to herein as the narrow channel and PMT  120  as the dark field narrow PMT. When desired, one or more polarizers  122  may be placed in the path of the collected radiation in the narrow channel. 
   Radiation scattered by spot  102   a  of wafer  102 , illuminated by either one or both beams  104 ,  106 , along directions away from the normal direction  116  is collected by an ellipsoidal collector  124  and focused through an aperture  126  and optional polarizers  128  to dark field PMT  130 . Since the ellipsoidal collector  124  collects scattered radiation along directions at wider angles from the normal direction  116  than lens  118 , such collection channel is referred to as the wide channel. The outputs of detectors  120 ,  130  are supplied to a computer  132  for processing the signals and determining the presence of anomalies and their characteristics. 
   Various aspects of surface inspection system  100  are described in U.S. Pat. No. 6,271,916 and U.S. Pat. No. 6,201,601, both of which are incorporated herein by reference. An exemplary surface inspection system is available from KLA-Tencor Corporation of San Jose, Calif., the assignee of the present application. 
     FIG. 2  is a simplified schematic illustration of optical components  200  of surface inspection system  100  in accordance with an embodiment. It should be understood that optical components  200  can be included and integrated into surface inspection system  100 , with only the modifications necessary to encompass embodiments described herein. For clarity, some components of the surface inspection system have been omitted from  FIG. 2 . 
   Optical components  200  of surface inspection system  100  direct illumination beam(s)  104 ,  106  to wafer  102 . Accordingly, optical components  200  include at least one radiation light source, such as a laser  202 , and a filter or attenuator  204  that controls the energy level of incidence beam(s)  104 ,  106  that are delivered to wafer  102 . As discussed in more detail below, in one embodiment, motion controller  114  controls the variable positioning of attenuator  204  to set the energy level of the laser power in system  100 . Motor  110  and gear  112  are also controlled by motion controller  114  to rotate and translate wafer  102  as appropriate to achieve the proper scanning motion. 
   Although  FIG. 2  depicts a single laser  202 , a greater number of lasers may be included among optical components  200 , as appropriate for a particular application. Further, alternative radiation light sources, such as, for example, xenon lamps, light-emitting-diodes and the like, may be used instead of laser  202 . 
   Attenuator  204  may be, for example, an addressable array of selected neutral fixed-density filters; a continuously variable neutral density filter; a plurality of polarizers that includes at least one rotatable polarizer; a rotating polarization retarder placed in front of a polarizer and the like, all of which are known in the art. Beam  106  passes through attenuator  204 , which produces an attenuated, collimated beam with a desired power level. 
   Referring now to  FIG. 2  in one operational embodiment, the dosage or energy level D of laser  202  is automatically adjusted as a function of the radial scan distance of the laser beam spot or scan spot from the center of wafer  102 . As illustrated in Equation 1, with a given laser power P, wafer rotational speed ω, and wafer translational speed V x , dosage D (light power density×exposure time) is approximately proportional to:
 
 P /(ω× r+V   x )  (1)
 
where r is equal to the current scan radius of the scan spot from the center of wafer  102 . The effective dosage D, therefore, increases as the radius of the scan radius decreases, thus reaching a maximum at r=0.
 
   In accordance with one embodiment, as the scan radius r approaches 0, the laser power P is simultaneously ramped down. In operation, laser controller  206  of surface inspection system  100  can be made to drive laser  202  in a laser power feedback loop, thus ramping a laser power profile as a function of scan radius r. Alternatively, a calibration table can be provided from which a correction factor for discrete scan radius can be determined. The calibration table can be system specific. 
   The loss of signal and thus the potential loss of signal/noise ratio (S/N) is compensated by a simultaneously adjustable noise filter and amplitude correction. The S/N can be determined as set forth in Equation 2, where R t  is equal to the tangential spot size and R r  is equal to the radial spot size. 
   
     
       
         
           
             
               
                 
                   Signal 
                   Noise 
                 
                 ∝ 
                 
                   
                     
                       P 
                       
                         
                           R 
                           t 
                         
                         × 
                         ω 
                         × 
                         r 
                       
                     
                   
                   × 
                   
                     1 
                     
                       R 
                       r 
                     
                   
                 
               
             
             
               
                 ( 
                 2 
                 ) 
               
             
           
         
       
     
   
   In this embodiment, a constant S/N, that is, a constant energy level, can be maintained while ramping down laser power P in approximate proportion to scan radius r, if R r , R t , ω are maintained as a constant. That is:
 
P∝r  (3)
 
It has been shown that the maximum dosage D may be reduced by a factor of up to 10 as the scan radius r approaches 0 without negatively affecting the sensitivity of surface inspection system  100 .
 
   In another embodiment, dosage D can be adjusted by varying the rotational speed ω of the wafer as a function of the radial distance of the beam spot from the center of the wafer. As shown in Equation 4, if P, R r , R t  are maintained constant, then rotational speed ω is approximately proportional to the scan radius from the center of wafer  102  as follows: 
   
     
       
         
           
             
               
                 ω 
                 ∝ 
                 
                   1 
                   r 
                 
               
             
             
               
                 ( 
                 4 
                 ) 
               
             
           
         
       
     
   
   Accordingly, in operation, controller  114  can cause the speed of motor  110  to vary the rotational speed of wafer  102 , while simultaneously translating wafer  102  under incidence beam(s)  104 ,  106 . In this manner, as r approaches 0, the rotational speed of wafer  102  increases to reduce the energy/mm 2  imparted to the inner surface of wafer  102 . 
   In another embodiment, dosage D can be adjusted by varying the spot size as a function of the radial distance of the beam spot from the center of wafer  102 . As shown in Equation 5, if P, R r , ω are maintained constant, then spot size R t  is approximately proportional to the scan radius from the center of wafer  102  as follows: 
   
     
       
         
           
             
               
                 
                   R 
                   t 
                 
                 ∝ 
                 
                   1 
                   r 
                 
               
             
             
               
                 ( 
                 5 
                 ) 
               
             
           
         
       
     
   
   Accordingly, in operation, laser controller  206  can operate to continuously refocus laser beam(s)  104 ,  106  using a focusing device, such as a lens assembly and the like, to cause the spot size of the beam to vary. In this manner, as r approaches 0, the spot size can be increased to reduce the energy/mm 2  imparted to the inner surface of wafer  102 . 
   In some embodiments, dosage D is adjusted by interposing a filter or attenuator  204  between the laser origin and wafer  102 , where the filter attenuates a portion of the laser power that varies as a function of the radial distance of the beam spot from the center of wafer  102 . 
   In one operational embodiment, attenuator  204  of surface inspection system  100  can be configured for the selection of or conditioning of filters, polarizers, and the like, to pass or reject specific wavelengths to set appropriate attenuation levels. In this embodiment, scan motion can be synchronized with attenuation value and proper amplitude correction, such that dosage D is automatically adjusted as a discrete function of the radial distance r of the scan spot from the center of wafer  102 . 
   In another operational embodiment, attenuator  204  is motorized such that it can be variably positioned by motion controller  114  between laser  202  and wafer  102 . In this manner, incidence beam(s)  104 ,  106  travels through attenuator  204  as the beam is delivered to wafer  102 . By manipulating and thus varying the distance of attenuator  204  from the origin or source of the radiation from laser  202 , the power level of beam(s)  104 ,  106  is also made variable. In this embodiment, scan motion can be synchronized with attenuation value and proper amplitude correction, such that as the scan radius r approaches 0, the proper attenuation value is provided by the adjustment of the position of attenuator  204  relative to laser  202 . 
     FIG. 3  is a flowchart which provides a general method for managing the radiation or laser power by employing the various embodiments described herein. Referring to  FIG. 3 , at operation  310  a surface scan is initiated. In various embodiments initiating a surface scan includes causing a surface of a wafer to be impinged by a beam of radiation at a first location such that the beam of radiation imparts a first level of energy to the surface. At operation  315  the radiation energy level imparted to the surface is varied as a function of the radial distance from the center of the wafer as the radiation beam moves from the first location on the surface of the wafer to a second location on the surface of the wafer. 
   In one embodiment, varying the energy level includes varying the power level of the radiation beam as a function of radial distance of the radiation beam from the center of the wafer. 
   Alternatively, varying the energy level may include varying the speed of rotation of the wafer as a function of the radial distance of the radiation beam from the center of the wafer, or varying a spot size of the radiation beam as a function of radial distance of the radiation beam from the center of the wafer. 
   In another alternative embodiment, adjusting the first energy level to the second energy level includes varying the position of a filter relative to the origin of the radiation beam as a function of radial distance of the radiation beam from the center of the wafer. 
   Reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least an implementation. The appearances of the phrase “in one embodiment” in various places in the specification may or may not be all referring to the same embodiment. 
   Thus, although embodiments have been described in language specific to structural features and/or methodological acts, it is to be understood that claimed subject matter may not be limited to the specific features or acts described. Rather, the specific features and acts are disclosed as sample forms of implementing the claimed subject matter.