Abstract:
A system uses reflectance spectrophotometry to characterize a sample having any number of structures. The system uses toroidal mirrors that are shaped in such a way that the angle of reflectance off of the target is small. The small angle of reflectance may allow for simplification of calculations and can result in a faster processing time. In addition, a more accurate measurement can be achieved when the reflected beam is close to normal.

Description:
FIELD 
   The invention relates to optical measurement systems, and, more particularly, to reflectance spectrophotometry. 
   BACKGROUND 
   In many industries such as semiconductor manufacturing the characterization of surface structures comprise an important step in verifying the integrity of the manufacturing process. These structures include critical dimensions (CD&#39;s), depth, profile, etc. One method of characterizing structures is to use reflectance spectrophotometry. 
   Reflectance spectrophotometry is a technique where a beam of light is directed toward a target. The light reflects off of the target and is collected in a spectrophotometer. When structures are arranged in a repeating pattern, even if the structures are non-symmetrical, evidence of the structure pattern shows up in the reflected light. By analyzing the properties of the collected light and comparing them to the properties of the original light source, properties of the structures, such as those used in diffraction gratings for example, can be determined. 
     FIG. 1  shows an example of the disclosure of U.S. Pat. No. 5,991,022 by Buermann, Forouhi, and Mandella, which describes a spectrophotometric apparatus with toroidal mirrors with the desired characteristics stated above. A light source  102  produces a beam of light  104 . The beam  104  strikes mirrors  106  A, B in route to illuminating the substrate  108 . Light reflected off of the substrate  108  is directed by mirrors  106  C, D into the photodetector  110 , which is a spectrophotometer. Data from the photodetector  110  is sent to a computer  112  for processing. The angle of incidence and angle of reflection are shown with θ i  and θ r  respectively. 
     FIG. 2  shows the path of travel for light beam for the system shown in  FIG. 1 . This is a side view. The light beam  104  comes in from the left side of the page. The light beam  104  strikes the mirror  106 B, and is reflected toward the substrate  108 . For illustrative purposes only, the substrate  108  and the cross-sectional areas  202 ,  204  of the light beam  104  are shown in a slight isometric configuration. The cross-sectional area  202  of the beam  104  is not reduced as the beam  104  reflects off of the mirror  106 B. Likewise, the cross-sectional area  204  of the beam  104  as the beam  104  strikes the substrate  108  is not reduced. Thus, in this prior art example, the beam  104  travels in its entirety from the light source  102  to the substrate  108 . 
     FIG. 3  shows the path of travel for a light beam for the system shown in  FIG. 1 . This is a front view, which looks at the system from the left side of  FIG. 2 . Again, for illustrative purposes only, the cross sectional areas  202 ,  204  and the substrate  108  are shown in a slight isometric configuration. In this view, the beam  104  approaches the mirror  106 B from above the page. The beam  104  reflects off of the mirror  106 B toward the substrate  108 . The beam  104  is not reduced as it travels from the light source  102  to the substrate  108 . 
   It should also be apparent from the prior art system shown in  FIG. 1  that the beam  104  that reflects off of the substrate  108  is not reduced in its cross-sectional area before it reaches the photodetector  110 . 
   In an apparatus used to characterize structures using reflectance spectrophotometry, it is desirable that light reflected from the material is directed into a spectrophotometer by an optical relay that has a minimum of aberrations. First, it is desirable to eliminate the chromatic aberrations to achieve an accurate measurement. However, lenses and mirrors have other, nonchromatic aberrations as well. These aberrations include spherical aberration, coma, astigmatism, curvature of field, and distortion. All lenses and mirrors suffer from these aberrations to some extent, even if they are perfectly machined. The existence of these aberrations represents a fundamental limitation on the nature of a lens or mirror—a limitation that is generally neglected in the paraxial approximation of introductory texts. Since the structures of interest often are patterned structures, such as integrated circuits, diffraction gratings, or contact holes, the structures usually are small and the areas that they are comprised of are small. Consequently, the measurement area is desirably small enough to fit within the entire pattern, yet large enough so that there are repeating structures in the measurement area. Thus, it is desirable that a reflectance spectrophotometric apparatus be able to image a small area, on the order of 50 microns in diameter, of the area of interest to a spectrophotometer with as little aberration as possible. It is also desirable that the apparatus include hardware for translating the target with respect to the imaging optics so that different regions of the target may be characterized. 
   One disadvantage for systems that use larger angles of incidence is that they do not correctly measure trenches with high aspect ratios (i.e. deep and narrow). With these systems it is possible that the incoming light will not reach the bottom of the trench before striking a wall. This effect is sometimes referred to as “shadowing.” In order to obtain an accurate measurement, is it desirable that the beam strikes the bottom of the trench and reflect out of the trench without hitting the side walls of the trench. 
   Another disadvantage for systems that use larger angles of incidence is that they take more time to determine structure geometries when the trenches of the geometry in question are parallel to the plane of the angle of incidence. In order to speed the calculation time, smaller angles of incidence can be used. 
   Thus, it is desirable for a reflective spectrophotometric device to have an angle of incidence that is small. 
   SUMMARY 
   This document describes a system that characterizes structures of a sample. The system includes a light source and mirrors for directing and collecting light. The angle of incidence for the collected light is small. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  shows a prior art measurement system. 
       FIG. 2  shows a prior art path of travel for a light beam. 
       FIG. 3  shows a prior art path of travel for a light beam. 
       FIG. 4  shows an example of a reflective spectrophotometric system. 
       FIG. 5  shows an example of the path of travel for a light beam. 
       FIG. 6  shows an example of the path of travel for a light beam. 
       FIG. 7  shows an example of the plane of the angle of incidence. 
       FIG. 8  shows an example of the plane of the angle of incidence relative to features that are being measured. 
       FIG. 9  shows an example of a diffraction grating. 
   

   DESCRIPTION 
   The measurement system described in this application has several advantages over the prior art. First of all, by using a small angle of incidence for the beam of light, the calculations required to determine the properties of the sample are greatly diminished for diffraction gratings. Fewer calculations mean a faster processing time. It has been said in the past that time is money, and, in a manufacturing or evaluation situation, this axiom is particularly true. Another advantage to the small angle of incidence system is that the accuracy goes up as the angle of incidence diminishes. In addition, the “shadowing” effect (mentioned above) can be reduced or eliminated with small angles. Where some prior art systems might have measured a sample from a variety of angles of incidence in order to improve the overall accuracy of the system, the disclosed measurement system has proven to be at least as accurate, and in most cases more accurate, than the prior art while using only one angle of incidence for measurement purposes. The combination of only needing one measurement angle and a faster processing time for that one angle leads to dramatic increases in productivity. 
     FIG. 4  shows an example of a reflective spectrophotometric system. In this example, which is a side view, a light source  402  emits a beam  404  of light. The beam  404  is directed to the substrate  408  with two mirrors  406 A, B. The beam  404  is directed to the photodetector  410 , which is a spectrophotometer, with two mirrors  406 C, D. In this side view, the beam  404  appears to travel either parallel with or perpendicular to the surface of the substrate  408 . This is in contrast to the prior art shown in  FIG. 1 , where the beam  104  travels at a non-perpendicular angle toward the substrate  108  and away from the substrate  108 . 
     FIG. 5  shows a front view of some of the elements shown in  FIG. 4 . This view shows what might be seen from the left side of the page from  FIG. 4 . The beam  404  reflects off of the mirror  406 B toward the substrate  408 . The cross sectional area of the beam  404  comprises two parts: the reflected portion  512  and the discarded portion  510 . The discarded portion  510  is marked with crosshatching. The discarded portion  510 , in this example, strikes the backside of the mirror  406 C, and is discarded. The reflected portion  512  continues through the system. The central axes of the incident beam  502  and the central axis of the reflected beam  504  are shown. The angle of incidence θ i  is measured from the normal to the substrate  408  to the central axis  502 . Likewise, the angle of reflectance θ r  is measured from the normal of the substrate  408  to the central axis  504 . Also shown are the conical beam sections which includes the incident conical beam section  506  and the reflective conical beam section  508 . These sections  506 ,  508  are not truly conical in this case, due to the discarded portion  510 . Arrows are shown to indicate the path of travel of the beam  404 . 
   One will note that the prior art example shown in  FIG. 3  shows the cross sectional area of the beam  104  fully intact. Furthermore, the beam  104  in  FIG. 3  approaches the substrate  108  perpendicular to the substrate  108  in that front view. In contrast,  FIG. 5  shows the beam  404  with an angle of incidence and angle of reflectance in the front view. 
   The mirrors  406  B, C are desirably of a size and distance from the substrate  408  in order to achieve a proper illumination spot size, while achieving a small cone angle. The trade off between spot size and cone angle left for the engineer to determine, depending on the requirements of the system. 
   The angle of incidence is also balanced with discarding a portion of the beam. The smaller the angle of incidence, the larger the discarded portion  510 . Thus, there is a balancing act between the amount of light propagated through the system versus the angle of incidence relative to the substrate  408 . The more light, the better the signal and the better the measurement. But, as is discussed above, the smaller the angle of incidence, the faster and more accurate the analysis (for certain types of structures). A good compromise has been found to be an angle of incidence of about 3.5° or less. When central axes  502 ,  504  have an angle of about 3.5°, the weighted average angle of incidence of any particular path of travel is roughly 4°. This makes it possible for the system to perform nearly as well as normal incidence beam system. 
   In this application, further discussion of angles of incidence or reflectance refer to the angle between the central axis of the beam  404  relative to the normal of the substrate  408  (or sample). 
     FIG. 6  shows an example of components laid out in the system. In this example, the light source  402  emits the beam  404  at an angle that is about the same as the angle of incidence relative to the substrate  408 . Likewise, the photo detector  410  is configured to receive the beam  404  at an angle that is about the same as the angle of reflectance relative to the substrate  408 . Thus, the mirrors  406 A, B, C, D need only translate the beam  404  relative to the plane of the page of  FIG. 6 . 
   The mirrors shown in  FIGS. 4-6  may be toroidal mirrors or off-axis parabolic mirrors. A mirror pair works as an optical relay. Toroidal pairs have the advantage of virtually eliminating chromatic aberrations and minimizing non-chromatic aberrations. It may be possible to build a system where the light source  402  emits the beam  404  in such a manner that only one mirror  406 B is required to direct the beam  404  toward the substrate  408 . Another possibility might be to build a system where the photodetector  410  can receive the beam  404  directly after it reflects off of the mirror  406 C. In order to obtain the properly shaped mirrors  406 B, C, one would normally cut oversized toroidal mirrors to the appropriate shape. 
   Off-axis parabolic mirrors offer certain advantages compared to toroidal mirrors. An off-axis parabolic mirror can exactly collimate light from an off axis point, whereas a toroidal mirror cannot precisely collimate the light. A pair of toroidal mirrors cancels most of the axial aberration present. However, a pair of toroidal mirrors may not correct the aberrations of off-axis points. Off-axis parabolic mirrors that share a common axis generally have low axial and off axis aberrations, with the foci generally being on the same axis. If off-axis parabolic mirrors can be purchased “off the shelf,” then there can be significant cost savings. 
   Off-axis parabolic mirrors have certain disadvantages compared to toroidal mirrors. The addition of flat folding mirrors is usually needed to fit into the optical system. The additional mirrors reduce the signal throughout the system. Off-axis parabolic mirrors with offset axes generally have low axial but very large off-axis aberrations. 
   It has been geometrically shown that the angle of incidence can go as low as 7° (for a cone angle of 14° and a spot size of 50 μm, for example) without having the cross sectional area of the beam  404  reduced. For angles of 6° or less, the mirrors  406 B, C would normally interfere with each other. The discarded portion  510  comprises 2%, 3.5%, 5.3%, 7.2%, 9.5%, and 12% of the cross sectional area of the beam  404  when the angle of incidence is 4.5°, 4.25°, 4.0°, 3.75°, 3.5°, and 3.25° respectively. 
   Measurements of orthogonal geometries, such as gratings, can vary with the angle of incidence. When the trenches of the geometry in question are parallel to the plane of the angle of incidence, smaller angles of incidence translate to faster calculations. The plane of the angle of incidence is defined by the angle of incidence and the normal of the surface. It has been observed that when these angles are 5° or less that speed gains are observed. When the trenches of the geometry in question are perpendicular to the plane of the angle of incidence, there is not an appreciable difference in calculation time with respect to angles of incidence. 
     FIG. 7  shows an example of the plane of the angle of incidence. The plane  702  is defined by the axis of incident light  502  and the normal to the substrate  704 . 
     FIG. 8  shows an example of the plane of the angle of incidence relative to features that are being measured. In this example there are two protrusions  802 ,  804 . The first protrusion  802  is parallel to the plane  702 . The second protrusion  804  is perpendicular to the plane  702 . When the angle of incidence is small, calculations regarding features that are parallel to the plane  702  (protrusion  802  for example) are easier and faster than when the angle of incidence is large. 
     FIG. 9  shows an example of a diffraction grating. The diffraction grating  902  has several trenches  904 . From the point of view of the bottom of the trenches  904 , there are several protrusions  906 . Thus, if a length of either a trench  904  or a protrusion  906  is parallel, or roughly parallel, to the plane of the angle of incidence  702  and the angle of incidence is small, then the critical dimension calculation time is faster. 
   It is also possible to use a beam splitter to direct the light to and from the sample. The disadvantage of using a beam splitter is that 75% of the light is lost by either for flexion or passthrough in the undesired direction. The advantage is that the angle of incidence is 0°. Thus, using a beam splitter allows one to achieve the fastest possible calculation times for certain types of structures and allow more accurate measurement for structures with high aspect ratios (depth to opening). 
   It will be apparent to one skilled in the art that the described embodiments may be altered in many ways without departing from the spirit and scope of the invention. Accordingly, the scope of the invention should be determined by the following claims and their equivalents.