Abstract:
An x-ray reflectometry system for measuring thin film samples. The system includes an adjustable x-ray source, such that characteristics of an x-ray probe beam output by the x-ray source can be adjusted to improve the resolution of the measurement system. The x-ray probe beam can also be modified to increase the speed of evaluating the thin film sample, for situations where some degree of resolution can be sacrificed. In addition, or alternatively, the system can also provide an adjustable detector position device which allows the position of the detector to be adjusted to increase the resolution of the system, or to reduce the time it takes to evaluate the thin film material.

Description:
RELATED APPLICATION  
       [0001]    The present application claims the benefit of U.S. Provisional Application Serial No. 60/261,154, filed Jan. 11, 2001, titled X-RAY REFLECTANCE MEASUREMENT SYSTEM WITH ADJUSTABLE RESOLUTION. 
     
    
     
       TECHNICAL FIELD  
         [0002]    X-ray reflectometry (XRR) is a technique for measuring the thickness of thin films in semiconductor manufacturing and other applications. The present invention relates to such a measurement system and provides for making adjustments to components of the system to improve the operation of the system.  
         BACKGROUND OF THE INVENTION  
         [0003]    There has been significant interest in developing x-ray reflectance techniques for analyzing thin films, and particularly thin metal films. Thin metal films are not easily analyzed using conventional optical metrology techniques that rely on visible or UV wavelengths since metal films are opaque at those wavelengths. X-rays are of interest since they can penetrate metals.  
           [0004]    The basic concepts behind measuring thin metal films on a substrate using an x-ray reflectance technique are described in U.S. Pat. No. 5,619,548, issued Apr. 8, 1997, and incorporated herein by reference. As described therein, a beam of x-rays is focused to strike the thin metal sample over a range of angles from near grazing incidence to a few degrees. A photodetector array detects the reflected x-rays over a range of angles of incidence. In this configuration, interference effects are created between the x-rays which reflect off the upper surface of the sample and at the interface between the thin film layer and the substrate. These interference effects vary as a function of angle of incidence. A plot of the change in intensity of the x-rays detected at the photodetector as a function of angle of incidence reveals periodic fringes, the spacing of which is a function of film thickness. Additional film properties, such as density and surface roughness, can be inferred from other characteristics of the reflectivity profile, such as the fringe amplitude or the location of the critical angle (onset of total external reflection).  
           [0005]    As with many systems, there are many trade-offs involved in the design parameters of an XRR system. For example, as the thickness of the films being measured increases, the spacing (as a function of angle of incidence) between the fringes becomes smaller. In order to be able to analyze such closely spaced fringes, it is desirable to maximize the resolution of the system. In particular, the spread of angles of incidence detected by any one pixel in the detector array should be as small as possible.  
           [0006]    One drawback associated with increasing the resolution of the system is that the flux or amount of energy received by each pixel is typically reduced. Reduced flux results in a less favorable signal to noise performance which in turn increases the time needed for successful measurements. While the trade-off may be required to measure thicker films, this increase in time would be an undesirable, and unnecessary, penalty when measuring thinner films.  
           [0007]    Typically systems are designed to balance the need to make measurements of thicker films which require higher resolution, with the need to make measurements of thinner films quickly and efficiently. The goal is to balance these competing factors so that the resulting measurement instrument will have a good balance between resolution and signal to noise performance. However, such systems do not allow physical characteristics of the measurement instrument to be adjusted to optimize them for measuring different films with a range of different thickness.  
           [0008]    It was recognized by the inventors herein that an improved system would allow the operator the freedom to adjust the resolution to best suit the measurement of a particular sample. For example, when measuring very thin films, the fringe spacing is quite large and high resolution is less important. In such a case, it would be helpful if the user could adjust the system to increase the flux thereby improving signal to noise performance and measurement speed. One design approach for increasing the flux is to move the detector array closer to the sample. Another approach is to tilt the X-Ray source such that apparent width of the source, as imaged on the sample, is increased.  
           [0009]    When measuring thicker films, the spacing between fringes is reduced. In such a case, having a high-resolution system is critical in being able to obtain accurate measurements. Therefore, it would be helpful if the operator could maximize resolution even if it meant that measurement time would be increased, since without sufficient resolution, information about the layer could not be derived at all. One design approach for increasing resolution is to move the detector array farther away from the sample. Another approach is to tilt the X-Ray source such that apparent width of the source, as imaged on the sample, is reduced.  
         SUMMARY  
         [0010]    In order to achieve these goals, the inventors herein propose an XRR system that includes one or more mechanisms that would permit the operator to adjust the resolution of the system for a particular measurement. In one embodiment, the operator is able to adjust the distance of the photodetector array from the sample. As this distance is increased, the resolution will be increased.  
           [0011]    In another embodiment, the user is able to control the effective width of the x-ray probe beam imaged on the sample. The smaller the effective width of the x-ray probe beam, the higher the resolution. The effective width is controlled by adjusting the angle of the x-ray emission material.  
           [0012]    Such a system can be implemented in a simultaneous multiple angle of incidence XRR system of the type described in the above-cited U.S. patent. Further details of an embodiment of an XRR system developed by the assignee herein can be found in PCT Publication WO 01/71325 A2 published Sep. 27, 2001, and incorporated herein by reference (referred to herein as the &#39;325 application). 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a simplified view of the of the measurement system disclosed in the &#39;325 application.  
         [0014]    [0014]FIG. 2 is a graph showing the high-resolution fringe profile for a thin film sample.  
         [0015]    [0015]FIG. 3 is a graph showing the fringe profile for a thin film sample.  
         [0016]    [0016]FIG. 4 is a graph showing a low-resolution fringe profile for a thin film sample.  
         [0017]    [0017]FIG. 5 illustrates the effect of pixel width and apparent x-ray source width on measurement resolution.  
         [0018]    FIGS.  6 ( a - b ) show an adjustable x-ray source where the take off angle of the x-ray emission material can be varied.  
         [0019]    [0019]FIG. 7 shows an embodiment of an XRR measurement system with an adjustable x-ray source and an adjustable detector positioner.  
         [0020]    [0020]FIG. 8 shows the effect of varying take off angles on fringe resolution for a thin film sample.  
         [0021]    [0021]FIG. 9 shows the relationship between tube voltage and output flux as it relates to take off angle. 
     
    
     DETAILED DESCRIPTION  
       [0022]    As discussed above, x-ray reflectometry (XRR) is a technique whereby the reflectivity of a sample is measured at x-ray wavelength (Angstrom range) over a spread of angles. These angles typically range from 0° (grazing incidence along the surface of the sample) to as large as a few degrees. From the behavior of the reflectivity one can infer properties of the sample such as material composition or thickness.  
         [0023]    A view of the XRR system disclosed in &#39;325 application for simultaneous measurements of the reflectivity over a range of angles is shown in FIG. 1. As shown in FIG. 1 the source  100  generates an x-ray beam  101  that is incident upon an x-ray reflector  102 , which is typically a monochromator. X-rays are then focused upon the sample being evaluated  106  which is positioned on a supporting stage  104 . X-rays incident upon the sample are then reflected and detected with a position-sensitive detector  108  (such as a photodiode array).  
         [0024]    Reflected x-rays  110  are captured in the top half of the detector  108 , while the incident beam  112  can be measured by lowering the stage and reading the bottom half of the detector. By properly normalizing the two profiles (as described in the &#39;325 application) one can determine the reflectivity as a function of angle. One of the key features of such a profile is the appearance of fringes whose spacing is inversely related to the thickness of the film under study.  
         [0025]    [0025]FIG. 2 shows the reflectivity of a perfect, 1500 Å copper (Cu) film on top of 250 Å of tantalum (Ta) on top of a Si substrate. The fine fringes (e.g.  300 ,  302 ,  304 ,  306  in FIG. 2) arise from interference in the thick Cu layer; the broad envelope fringes (e.g.  308 ,  310 ,  312 ) arise from the interference in the thin Ta layer. The plot shown in FIG. 2, however, is idealized, and not what a real system would measure. In practice, the ability of the XRR system to resolve angle is limited. For instance, the finite-width of the individual detecting elements (pixels) of the photodetector leads to an averaging of a finite range of angles. In one embodiment a 2° angular range is covered by 500 pixels which means that the angular resolution is no better than 4 m° (i.e. 2 degrees divided by 500 pixels). The impact of this effect is illustrated in the graph of FIG. 3 which was generated assuming a 4 m° system resolution. Clearly the fringe contrast has been reduced as compared to FIG. 2. For a 10 m° system as shown in FIG. 4, the fringe contrast generated by the interference with the thick copper layer is attenuated to the point where, depending on the noise characteristics of the measurement, the fringes might not even be distinguishable. In contrast the fringes generated by the interference in the thinner layer of Ta are still visible ( 308 ,  310 ,  312 ).  
         [0026]    Although the size of the detecting elements of the detector plays a role in the resolution of the system, it is not the only factor to consider. In real systems the x-ray source has a finite size which means the x-ray beam at focus (sample surface) will also have a finite size. This causes a smearing of angles which is illustrated in FIG. 5.  
         [0027]    [0027]FIG. 5 shows the interplay between the source and a single pixel of the detector (the widths are grossly exaggerated for this illustration). The total angular range of data collected by this pixel is the difference between the angles of the two extreme rays  501 , and  502  shown. As the source width is increased (i.e. the area of x-ray emission material is increased) the angular range collected by each pixel of the detector also increases. In fact, should the width of the source, at focus on the sample exceed that of the pixels it becomes the dominant effect in determining the system resolution. Alternatively, it can be appreciated that if the detector is moved farther away from the sample, the angular range would be reduced.  
         [0028]    One way to improve the resolution is to use a smaller source. However, the size of the x-ray emission material  700 , shown in FIG. 6( a - b ), is limited by the one&#39;s ability to focus the electron beam  702  (which is used to stimulate the x-ray emission material) and by thermal constraints (the smaller the x-ray emission material the harder it is to wick away the heat). In practice, the lower limit of length of the x-ray emission material  700  is in the range of 100 μm-500 μm. If an x-ray probe beam generated by the x-ray emission material were imaged onto the sample surface, with a width in this range, the degradation of the system resolution, due to the width of the probe beam, would far exceed that caused by the pixel width (which tends to be 25 μm in practice). (The actual size of the source imaged by the monochromator depends on the monochrometer acceptance angle and aberrations.) To minimize this degradation in resolution, the x-ray emission material  700  can be rotated relative to the optical system, such as monochromator, (which is used to focus the x-ray probe beam) so that the apparent width of source, as seen by the optical system, through an aperture  704  disposed in the housing  706  is reduced; thereby reducing the effective width of the x-ray probe beam. This is illustrated in FIG. 6( a - b ), where the effective width of the x-ray beam projected to the optical system, is related to the sine of the angle of the x-ray emission material  700  relative to the aperture  704  in the housing  706 . The relationship is such that the effective x-ray probe beam  710  for an angle of approximately 80 degrees is significantly wider than the effective x-ray probe beam  708  for an angle of 5 degrees. Thus, by adjusting the angle of the x-ray emission material the effective width of the probe can be reduced to as little as approximately 5 μm. This angle of the x-ray emission material relative to the aperture is referred to as the take-off angle.  
         [0029]    While reducing the take-off angle does limit the apparent source size, it comes at a price. One typically wants to run these tubes (i.e. the x-ray source) at high voltages to increase the x-ray flux of the line of interest (e.g., kα line). Theoretically, the boost in flux varies as the voltage to the 3/2 power. At higher voltages, however, the x-rays tend to get generated deeper in the target material. When this x-ray emission material is then tilted the x-rays have to tunnel through a substantial amount of material to make their way out of the x-ray source. This causes a substantial loss in flux. At some point, as the take-off angle is reduced, further increases in tube voltage return no increase in x-ray flux at all. This is illustrated in FIG. 9 where curve  1010  corresponds to the flux/voltage relationship for a very low take off angle, and curves  1020  correspond to the flux/voltage relationship at a higher take-off angles, such that an increase in voltage corresponds to an increase in flux over a wider range than for the low take take-off angle of  1010 .  
         [0030]    As is clear from the above there are a number of inter-related factors which lead to trade-offs in system performance. At small take-off angles a narrow x-ray probe beam is generated and potentially high resolution results; at large take-off angles a wider probe beam and a stronger signal results, but the resolution reduced.  
         [0031]    [0031]FIG. 7 shows a system that provides an adjustable x-ray source  800  and an adjustable detector positioning device  830 . As discussed above in connection with FIG. 6, an x-ray source  800  operates by projecting electrons into the x-ray emission material  804 , which responds by projecting x-rays.  
         [0032]    The x-ray emission material  804  is contained in a housing  810  and mounted to an adjustable mounting  812 . In one embodiment the adjustable mounting  812  is rotatable, which allows for x-ray emission material  804  to be rotated relative to an the aperture  814  in the housing  810 . This adjustable mounting  812  could be a rotating stage that is pivoted about an axis that runs horizontally through the x-ray emission material. The x-rays which are emitted through the aperture  814  form an x-ray probe beam that is reflected by the optical system  840 , which focuses the x-ray probe beam on the sample  824 . This optical system as discussed in the &#39;548 patent can include a monochromator. As discussed above the effective width of the x-ray probe beam  806  is a function the take off angle of the x-ray emission material  804  relative to the aperture  814 . In one embodiment the position of the adjustable mounting  812  can be controlled by a processor  816 . The processor  816  sends signals  820  to a motor  818  which is coupled with the adjustable mounting  812  and causes it to rotate, and thereby change the take off angle of the x-ray emission material  804 . In some circumstances it may be beneficial to provide a manual adjustor which is coupled to the adjustable mounting  812  so that an user can manually adjust the take off angle.  
         [0033]    The processor  816  can include a number of separate processors and controllers or it could be a single processor. The term “processor” as used herein refers to processing elements used to process information and control elements of the measurement system. The processor  816  receives signals  822  from the detector  802 , and based on the signals  822  determines the amplitude of x-rays relative to their angle of incidence on the sample  824 . The processor can then uses this information to generate a display  842  of information, such as fringe information, as shown in FIGS.  2 - 4 . Where the processor  816  determines that the features of the fringes are poorly defined due to poor resolution, as in FIG. 4, the processor can send a signal  820  to the motor  818  causing the rotation of the adjustable mounting  812  to reduce the take off angle. Alternatively, if the processor  816 , detects that system is operating at a higher resolution than necessary, as in FIG. 2, then process can send a signal to the motor  818  causing motor to adjust the adjustable mounting  812  to increase the take off angle, thereby reducing the resolution, but increasing the speed of the measurement.  
         [0034]    Alternatively, or in addition, the system can also allow the user to input information  826  regarding the thin film sample  824  to be measured. The processor can then access stored information  828  regarding the optimum settings for the adjustable x-ray source  800  for the particular characteristics of the sample  824  as input by the user, and adjust the position of the adjustable mounting  812  accordingly.  
         [0035]    In a similar fashion the processor  816  can also control the positioning of the detector  802  relative to the sample that is being measured. In one embodiment the detector is mounted to a carriage  832  that is engaged with a positioning track  834 , which allows the detector  802  to be moved either closer to, or further away from, the sample  824  being measured. It should be recognized that alternatively the system could allow for the sample, the x-ray source, and the optical system to move relative to the detector which could be fixed in single position. As shown in FIG. 7, the position of the detector  802  relative to the sample can be adjusted using a motor  836  that is controlled by signals  838  from the processor  816 . For situations where it is difficult to resolve features of the fringe, as in FIG. 4, the processor  816  can send signals to the motor  836  causing the position the detector  802  to be move further from the sample  824 , thereby increasing the resolution of the system. In other situations, where the resolution is greater than needed, the processor can send signals to the motor  836  causing the detector to be moved closer to the sample, which decreases the resolution but increases the speed with which measurements can be made. In some circumstances it may be beneficial to provide a manual adjustor which is coupled to adjustable detector positioning device to allow the user to manually adjust the position of the detector.  
         [0036]    It should be recognized that while the system shown in FIG. 7 includes both a means for adjusting the apparent width of the source, and a means for adjusting the position of the detector, it may be preferable to implement a system which includes either the one or the other. As is apparent from the discussion above the resolution of the system can be adjusted by changing the position of the detector relative to the sample, or by changing the apparent width of the source. Thus, it is not necessary for a system to include the ability to adjust both the apparent width of the source and the position of the detector relative to the sample.  
         [0037]    [0037]FIG. 8 shows the resolution and output powers for several different take-off angles. In FIG. 8 the four curves  902 ,  904 ,  906  and  908  have been spatially separated for clarity. The top curve  902  represents the smallest take-off angle; five degrees. The three curves  904 ,  906  and  908  below the top curve represent take-off angles of ten, fifteen and nineteen degrees respectively. As can be seen, as the take-off angle is reduced (so that the effective width of the x-ray probe beam is reduced), the peaks become more pronounced and can be more easily analyzed.  
         [0038]    While the method and apparatus of the present invention has been described in terms of its presently preferred and alternate embodiments, those skilled in the art will recognize that the present invention may be practiced with modification and alteration within the spirit and scope of the appended claims. The specifications and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. Further, even though only certain embodiments have been described in detail, those having ordinary skill in the art will certainly understand that many modifications are possible without departing from the teachings thereof. All such modifications are intended to be encompassed within the following claims.