Abstract:
A method of measurement of topographic features on a surface of a substrate is presented, wherein a focused beam of particles falls onto the surface of the substrate, and backscattered particles are detected with a particle detector. An opaque material is interposed between the surface and the detector, and the position of the shadow of an edge of the opaque material on the detector is recorded. The relative position of the edge and the surface of the substrate is then determined, and the topography of the surface determined as the particle beam and the substrate are moved with respect to one another.

Description:
FIELD OF THE INVENTION 
     The field of the invention is the field of measurement of topological features on the surface of a substrate, principally but not limited to using focused electron beams and ion beams. 
     OBJECTS OF THE INVENTION 
     It is an object of the invention to produce a method of determining the topography of a surface of a substrate, particularly where the substrate has no sharp contrasts in material, crystallography, or angle. 
     It is an object of the invention to produce a method of determining the focusing conditions for an electron or an ion beam to focus the beam on a surface of a substrate, particularly where the substrate has no sharp contrasts in material, crystallography, or angle. 
     It is an object of the invention to produce a method of determining the depth of features in a generally flat, otherwise featureless surface. 
     It is an object of the invention to produce a method of determining features of sidewalls of a hole or trench in a substrate, particularly when the sidewall is sloped, vertical or undercut. 
     SUMMARY OF THE INVENTION 
     A focused particle bean, such as an electron, ion, atom, or molecular bean is directed on to the surface of a substrate. Scattered particles which travel in a straight line from the surface irradiated are collected in a particle detector. A particle blocking material having an edge is interposed between the surface and the particle detector, and the location of the shadow cast by the edge of the material is measured. The relative position of the surface and the edge casting the shadow can then be determined. Sweeping the particle beam can then be used to build up a topographic map of the surface. The depth and the sidewalls of holes and trenches are measured by appropriately changing the angle of incidence of the particle beam. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A shows a side view sketch of the most preferred embodiment of the invention. 
     FIG. 1B shows a graph of distance vs intensity for the embodiment sketched in FIG.  1 A. 
     FIG. 2A shows a perspective sketch of a preferred embodiment of the invention. 
     FIG. 2B shows a graph of distance vs time for the embodiment sketched in FIG.  2 A. 
     FIG. 3A shows a side view sketch of a preferred embodiment of the invention. 
     FIG. 3B shows a graph of distance vs time for the embodiment sketched in FIG.  3 A. 
     FIG. 4A shows a side view sketch of a preferred embodiment of the invention. 
     FIG. 4B shows a graph of distance vs time for the embodiment sketched in FIG.  4 A. 
     FIG. 5A shows a side view sketch of a preferred embodiment of the invention. 
     FIG. 5B shows a sketch of the detector illumination for the embodiment sketched in FIG.  5 A. 
     FIG. 6 shows a side view sketch of a preferred embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Measurement of the topography of a surface by scanning electron microscope (SEM) is very difficult if the surface has no sharp differences in material, crystal directions, or surface angles. Often, the operators searches for a dust particle or other feature on the surface just to focus the beam on to the surface. 
     The set up of the most preferred embodiment of the invention is shown in side view in FIG. 1A. A focused particle beam  10  is shown impinging on to a surface  11  of a substrate  12 . The surface  11  as shown is not flat, but has a deviation from the flat plane  13  which is drawn as an “average” surface. The particle beam  10  is shown impinging normal to the plane  13 , at an angle θ 1  of θ° to the normal  14  to the plane  13 . The particle beam  10  is most preferably a focused electron beam, but preferred embodiments of the invention use focused ion, atom, or molecular beams. Light beams (sometimes considered beams of quanta or particles) are specifically excluded as particles for the purposes of this specification. Particles  15  ejected from the surface  11  are shown flying in a straight line from the point of intersection of the particle beam  10  and the surface  11  to a particle detector  16 . The particle detector  16  is a point detector, a line detector, or most preferably an array detector such as an imaging electron detector CCD or CMOS array. Backscattered or low energy loss, electrons having low energy loss (LLE)&#39;s are the preferred particles  15  for the present invention. A body  17  opaque to particles  15  having an edge  18  is interposed between the point where the electron beam  10  impinges on the surface  11  and the detector  16 . The body  17  casts a “shadow” on the detector  16 , where the position of the shadow on the detector is determined by the relative positions of the intersection point and the edge  18 . FIG. 1A shows the electrons at the edge of the shadow forming an angle θ 2  to the normal  14 . Simple geometry shows that 
     
       
         tan θ 2   =d/h=s/y   
       
     
     and, if the distance h changes as the electron beam  10  is scanned, or alternatively, as the substrate  12  is translated perpendicular to the normal  14 , then 
     
       
         Δ h=d/s Δy   
       
     
     Since d/s may be made very small, very small deviations Δh lead to large values of Δy which are easily measured by a number of pixel lines on an imaging detector. 
     FIG. 1B shows a graph of the instantaneous intensity vs distance of the response of an imaging detector  16  to the set up depicted in FIG.  1 A. The intensity of backscattered electrons is proportional to sin 2  θ 2 , and is very small for electrons scattered nearly parallel to the surface. If the electron beam  10  is focused on the surface  11 , the solid line of FIG. 1B results. If the electron beam is focused above or below the surface, the dashed line provides a measure of the defocusing. Appropriate manipulation of the parameters of the electron beam  10 , or raising or lowering the substrate  12  in the electron beam chamber, is used to focus the electron beam  10  on any particular surface element of the surface  11  by making the “edge” of the curve in FIG. 1B as sharp as possible. 
     FIG. 2A shows a perspective sketch of a preferred embodiment of the invention. The electron beam  10  is shown scanning parallel to the edge  18  of the body  17  where the surface of the substrate  11  has a trench pattern with trenches  20  cut into the surface. The intensity of electrons on to the detector  16  is shown, as is the shadow line  22  from the edge  18  of the electron opaque material  17 , at an instantaneous instant of time. The distance y measured by the shadow line on the electron detector is shown as a function of time in FIG. 2B for the scanning embodiment sketched in FIG.  2 A. The depth of the trenches  20  is determined from the variations of y and the parameters h and d. 
     FIG. 3A shows a side view sketch of a preferred embodiment of the invention where the end elevation of a cut through the substrate  12  shows a trench  30  in the surface of the substrate. The focused electron beam  10  is shown impinging normal to the substrate, and sweeping at a constant rate perpendicular to the trench. The trench  30  has sloping sidewalls  32 . FIG. 3B shows a graph of distance vs time for the embodiment sketched in FIG. 3A, where the slope angle and depth of the trench are calculable from the measurements of y. 
     FIG. 4A shows a side view sketch of a preferred embodiment of the invention where the end elevation of a cut through the substrate  12  shows a trench  40  in the surface of the substrate. The focused electron beam  10  is shown impinging at an angle θ 1  to the substrate, and sweeping at a constant rate perpendicular to the trench. The trench  40  has vertical sidewalls  42 . FIG. 4B shows a graph of distance vs time for the embodiment sketched in FIG. 4A, where a first sidewall angle, sidewall topography, and depth of the trench are calculable from the measurements of y. The graph is now not symmetric, because the scattered electrons are cut off from reaching the bottom of the trench by the edge of the other sidewall. To measure the other sidewall, the angle θ 1  is reversed and the trench scanned again. 
     FIG. 5A shows a side view sketch of a preferred embodiment of the invention where the end elevation of a cut through the substrate  12  shows a trench  50  in the surface of the substrate. The focused electron beam  10  is shown impinging at an angle θ 1  to the substrate. The trench  50  has undercut sidewalls  52 . FIG. 5B shows an instantaneous sketch of the detector illumination for the embodiment sketched in FIG.  5 A. The detector illumination is shadowed at  22  by the edge  18  as noted above, and also by the edges of the trench at  54  and  56 . The shadows  54  and  56  also move as the electron beam sweeps across the trench. 
     FIG. 6 shows a side view sketch of a preferred embodiment of the invention, where the edge  63  of a trench  60  is used as an opaque block to the electrons to measure the topography of sidewall  62  and depth of the trench  60 . As noted above, measurements of the position of the electron beam  10  with respect to the edge  63  of the trench are used to determine the topography of the sidewall from measurements of the distance of the y of the shadow of the edge of the trench and from the known dimensions of the relative positions of the electron beam with respect to the edge  63  of the trench and the detector  16 . 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.