Abstract:
The invention is a method and apparatus for determining characteristics of a sample. The system and method provide for detecting a monitor beam reflected off a mirror, where the monitor beam corresponds to the intensity of light incident upon the sample. The system and method also provide for detecting a measurement beam, where the measurement beam has been reflected off the sample being characterized. Both the monitor beam and the measurement beam are transmitted through the same transmission path, and detected by the same detector. Thus, potential sources of variations between the monitor beam and the measurement beam which are not due to the characteristics of the sample are minimized. Reflectivity information for the sample can be determined by comparing data corresponding to the measurement beam relative to data corresponding the monitor beam.

Description:
RELATED APPLICATION  
       [0001]    The present application claims the benefit of U.S. Provisional Application SerIAL No. 60/337,678, filed Nov. 9, 2001, titled ACCURATE SMALL-SPOT SPECTROMETRY INSTRUMENT which is incorporated herein by reference in its entirety. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to metrology instruments, and more particularly to spectrometry instruments that measure the reflectivity of a sample.  
         BACKGROUND  
         [0003]    In semiconductor manufacturing and other fields it is desirable to make quantitative measurements of a sample&#39;s reflectivity properties over very small, selectable areas and over as broad a band of wavelengths as possible. Instruments for making these measurements typically incorporate microscope-like objective lenses for focusing light on the sample. One type of illumination source is a Xenon arc lamp. In order to scan to various positions on the sample of interest a portion of the optics can be moved with respect to a stationary portion of the optics, or the sample can be moved with respect to at least some portion of the optics, or both.  
           [0004]    A common issue with such instruments is guaranteeing the stability of the light which is incident upon the sample, or at least knowing the spectral intensity of the incident light, so that the detected light reflected from the sample can be evaluated relative to the intensity of the light incident on the sample. Since reflectivity is defined as the ratio of the intensity of light reflected off the sample relative to the intensity of light incident upon the sample, accurate reflectivity measurements depend knowing the incident light intensity.  
           [0005]    There are several factors that can make it difficult to determine the intensity of the light upon the sample. One factor is that the characteristics of most sources of light change with time, and thus the intensity of the incident light can vary with time. Another factor is that where there is relative motion of the illumination source and the rest of the optics, either via (nearly) collimated light paths or optical fibers, there can be changes in the transmission efficiency of illuminating light as a function the scan position, or scan state. Here scan state includes the history of previous scan positions. This is important, for example, with some architectures using a fiber to transmit light from the light source. One prior system is shown in US patent application, publication US 2002/0021441 A1 (SMALL SPOT SPECTROMETRY INSTRUMENT WITH REDUCED POLARIZATION) and in PCT application, international publication number WO 00/57127 (METHOD AND APPARATUS FOR WAFER METROLOGY); both of these references are incorporated herein by reference in their entirety.  
           [0006]    [0006]FIG. 1 shows another type of prior system  100 . The system  100  includes a light source  102  and a transmission means  104  for light generated by the light source  102 . The light transmitted through the transmission means  104  is then transmitted through a collimating lens  108 , and leaves the collimating lens as light beam  106 . The light beam is then incident upon a beam splitter  110 . A first beam  140  is transmitted from the beam splitter through a lens  144  and then through a plate  146  having pinhole to receive the first beam  140 . The first beam  140  is then transmitted through a transmission means  148  and received by a detector  150 . In response to the light received, the detector  150  generates a monitor signal corresponding to the received light. This monitor signal from the detector  150  is received by a processor  160  which analyzes the monitor signal and uses it relative to a signal generated by detector  130 .  
           [0007]    In addition to the beam  140  being transmitted through the beam splitter  110 , beam  112  is also reflected from the beam splitter  110  through an objective lens  114  and onto a spot  118  on a sample  116  being analyzed. Some portion  113  of the light  112  is reflected off the sample and back through the objective lens  114 . This light  113  is further transmitted through the beam splitter  110  and off a turn mirror  122  and through a lens  124 . The resulting light beam is then transmitted through a pinhole in a plate  126  and into a transmission means  128 . The light transmitted through the transmission means  128  is received by the detector  130 . In response to receiving this light the detector  130  generates a sample signal which corresponds to the received light. This sample signal is received by the processor  160  where it is analyzed relative to the monitor signal received from the detector  150 .  
           [0008]    The fact that prior systems provide for transmitting a monitor beam  140  and a measurement beam  113  through different transmission paths and provide for using different detectors ( 150  and  130 ) for detecting the monitor light beam and the measurement light beam introduces a number of potential sources which could generate variations in the monitor signal relative to the measurement signal which are not related to the reflective properties of the sample. What is needed is a system which reduces possible sources of extrinsic variations in the monitor beam relative to the measurement beam.  
         SUMMARY OF THE INVENTION  
         [0009]    One embodiment herein provides a system for measuring characteristics of a sample. This system includes a light source for generating a beam of light which is directed toward a sample, and a mirror which can be moved between a first position and a second position, wherein in the first position the mirror is positioned between the light source and the sample such that light generated by the light source is reflected off the mirror and transmitted through a first path. The first path consists of a reflection path. In the second position the mirror is positioned such that it is not between the light source and the sample, and light generated by the light source is reflected off the sample and transmitted through the reflection path. This system also provides a detector coupled to the reflection path which generates a monitor signal in response to receiving light reflected from the mirror, and generates a measurement signal in response to light reflected from the sample.  
           [0010]    Another embodiment includes a method for determining characteristics of a sample in a system having a light source, and a movable mirror. The method includes generating a light beam and directing the light beam toward the sample, and positioning the movable mirror such that it is in a first position, where it reflects the light beam along a first path, wherein the first path consists of a reflection path. The method also includes generating a monitor signal which corresponds to the light reflected from the mirror, and positioning the movable mirror such that it is in a second position, where it does not reflect the light beam which is directed toward the sample, wherein the light beam which is directed toward the sample is reflected off the sample along the reflection path. The method of this embodiment also includes generating a measurement signal which corresponds to the light reflected from the sample, and analyzing the measurement signal relative to the monitor signal to determine properties of the sample.  
           [0011]    Another embodiment includes a system for measuring characteristics of samples. The system includes a light source for generating a beam of light, and beam splitter for directing the beam of light toward a sample. The system also includes a lens disposed between the beam splitter and the sample for focusing the beam of light on the sample, such that a measurement beam of light is reflected off the sample, wherein after the measurement beam is reflected off the sample, it is transmitted through a reflection path. The system includes a detector positioned to receive light transmitted through the reflection path wherein in response to receiving light transmitted through the reflection path the detector generates a signal corresponding to the light transmitted through the reflection path, and a mirror which can be moved between a first position and a second position, wherein in the first position the mirror is positioned between the beam splitter and the sample, such that light directed by the beam splitter toward the sample is incident upon the mirror and reflected through a first path, wherein the first path consists of the reflection path, wherein in the second position the mirror is positioned such that light directed by the beam splitter toward the sample is reflected off the sample along the reflection path. The system further includes a processor coupled to the detector which uses a first signal generated by the detector in response to receiving light reflected from the mirror and a second signal generated by the detector in response to light reflected from the sample, to determine characteristics of the sample. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a schematic view illustrating a prior art spectrometer system.  
         [0013]    [0013]FIG. 2 is a schematic view of an embodiment of the present invention in a first state.  
         [0014]    [0014]FIG. 3 is a schematic view of an embodiment of the present invention in a second state.  
         [0015]    [0015]FIG. 4 is top view of an embodiment of a movable mirror and its mounting.  
         [0016]    [0016]FIG. 5 is a side view of an embodiment of a movable mirror and components which enable its motion. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    [0017]FIG. 2 illustrates one embodiment of a multiplexed spectrometer system  200  of the present invention in a first state. The system  200  includes a light source  102  coupled to a transmission means  104 , such as an optic fiber. The light transmitted through the transmission means  104  is then transmitted through a collimating lens  108  which transmits a beam of light  106 . The beam of light  106  is then incident upon a beam splitter  110 . An absorber  202  absorbs light passing through beam splitter  110 . Illumination beam  206  reflects from the beam splitter  110  toward a sample  116 . A movable mirror  204  is shown in a monitor position. In the first position, the mirror  204  is positioned between the beam splitter  110  and the sample  116 , such that it reflects beam  208  through a reflection path. As part of this reflection path, the reflected beam  208  passes through the beam splitter  110 , reflects off an optional turn mirror  122 , and passes through focusing optics  224 , such as a lens, onto a pinhole in a plate  226 . Reflected beam  208  then passes through a transmission means  228 , such as an optic fiber and is then received by a detector  230 . Typically, the detector  230  will be a spectrometer, which detects the intensity of different wavelengths of light. One type of spectrometer includes an optical element for angularly dispersing a light beam as a function of wavelength. This dispersed light is then measured by an array of detector elements. With system  200  in the first state, detector  230  generates a monitor signal corresponding to the intensities at different wavelengths of light reflected from movable mirror  204 , and not from sample  116 . The processor  160  then uses the monitor signal as an indication of intensity in the illuminating light, based on the assumption that mirror  204  does not change with time, i.e., between calibrations, as discussed below.  
         [0018]    [0018]FIG. 3 shows the system  200  in a second state where the movable mirror  204  is in second position. As shown in the second position, the mirror  204  is not between the beam splitter  110  and the sample  116 . As in FIG. 2, the light beam  106  is incident upon the beam splitter  110 , and the absorber  202  absorbs light passing through the beam splitter  110 . Again illumination beam  206  reflects from beam splitter toward the sample  116 . In this second state, beam  206  passes through an objective lens  114  and is incident upon at least a small spot  118  on the sample  116 . Reflected beam  208  is then transmitted through the reflection path, as described above, and received by detector  230 . The detector  230  then generates a sample signal indicative of the reflectance of spot  118 .  
         [0019]    Apparatus  200  is generally for measuring the reflectivity of a sample. Reflectance is defined as the ratio of intensities incident upon and reflected from the sample. System  200  must be calibrated in order to measure reflectance. Calibration finds the relationship between measured signals and samples with known calibration reflectivities. Calibration may include the steps of estimating the calibration reflectivities. Calibration preferably allows for changes to system  200  over relatively long time scales, e.g., months. In the discussion that follows, all signals from spectrometer  230  are preferably dark corrected by subtracting signals collected with no light, e.g., from transmission means  104 , as is known in the art. Further, the signals may alternatively be additionally corrected for scattering of light within spectrometer  230 , as is also known in the art. A sample signal is collected with mirror  204  retracted so that sample  116  reflects reflected beam  208 , as in FIG. 3, and a monitor signal collected with mirror  204  reflecting reflected beam  208 , as shown in FIG. 2.  
         [0020]    In an embodiment of system  200 , and in another embodiment as described in PCT application no. PCT/US00/07709 entitled APPARATUS FOR WAFER METROLOGY (which is hereby incorporated by reference in its entirety) the system  200 , at any desired time when system is in use, locate spot  118  on a reference sample whose reflectivity does not change with time, and collect a reference measurement signal. The reference measurement signal has a corresponding reference monitor signal. In some embodiments, the reference measurement and monitor signals are collected at a time as close as practical to the time for collecting the sample measurement and monitor, e.g., with a time difference of less than a minute.  
         [0021]    The estimated value of the reflectance is given by 
           R ( λ,r ) ˜=S (1,  λ,r ) S (2,  λ, r   0 )/ [F 1( λ,r ) S (2,  λ,r ) S (1,  λ, r   0 )]− F 0( λ,r )/ F 1( λ,r )  Eq 1 
         [0022]    where S(         , λ,r) is the measurement signal from the sample at location r when mirror  204  is in its second state, S(         , λ, r) is the corresponding monitor signal at the same location (e.g. with the optics in the same position as when the measurement signal for location r is generated) when mirror  204  is in its first state. S(         , λ, r 0 ) is the measurement signal from the reference reflector at r 0  acquired at a proximate time, S(         , λ, r 0 ) is the corresponding reference monitor signal, F0(λ,r) and F1(λ,r) are first and second calibration functions, λ is wavelength, r specifies the position of spot  118  relative to sample  116 , and r 0  is the relative position of the reference sample. The calibration functions are the result of minimizing  
               ∑     n   =   1     N            (           S        (     n   ,   1   ,   λ   ,   r     )            S        (     n   ,   2   ,   λ   ,     r   0       )             S        (     n   ,   2   ,   λ   ,   r     )            S        (     n   ,   1   ,   λ   ,     r   0       )           -     F0        (     λ   ,   r     )       -       F1        (     λ   ,   r     )              R   c          (     n   ,   λ   ,   r     )           )     2             Eq  2                               
 
         [0023]    with respect to F0(λ,r) and F1(λ,r). R c (n,λ,r) are reflectance of calibration samples. The signals S and reflectances R c  have an additional integer index n=1,2, . . . ,N that labels the calibration samples. N is the number of calibration samples. In the preferred implementation, N=2, the reflectance of the calibration samples are known, and the expression in Eq 2 is minimized with respect to F0(λ,r) and F1(λ,r) separately for each wavelength λ and position r.  
         [0024]    In an alternative embodiment, parameters of one or more calibration samples, such as the thicknesses of films, are unknown. In this case, the following expression is minimized  
               ∑   λ            ∑     n   =   1     N            (           S        (     n   ,   1   ,   λ   ,   r     )            S        (     n   ,   2   ,   λ   ,     r   0       )             S        (     n   ,   2   ,   λ   ,   r     )            S        (     n   ,   1   ,   λ   ,     r   0       )           -     F0        (     λ   ,   r     )       -       F1        (     λ   ,   r     )              R   c          (     n   ,   λ   ,   r     )           )     2               Eq  3                               
 
         [0025]    with respect to the unknown parameters of the calibration samples and F0(λ,r) and F1(λ,r) for all wavelengths simultaneously. The minimization is repeated for each position r.  
         [0026]    In alternative embodiments, Eq 2 may include terms with additional calibration functions multiplied by powers of R c . Eqs 1 and 2 thus apply to the linear, or first order calibration, and higher order calibrations are possible. In yet alternative embodiments, the reference reflector and measurements associated with it may be left out. In such cases, movable mirror  204  serves as the reference reflector. Alternative version of Eqs. 1 and 2 would yield measured reflectivity and calibration functions.  
         [0027]    As seen above, reflectance is generally related to the ratio between the sample and monitor signals. Because both sample measurement signal and monitor signal come from light in the reflection path, in different states of the instrument  200 , changes in this ratio are due to the reflectivity changes of the sample  116  and mirror  204  only, and not potential variations caused by utilizing different reflection paths and/or different detectors for the monitor and measurement signals. The reflection path is such that the light reflected off the mirror to the detector includes only elements which the light reflected from the sample to the detector will travel through. Thus, there are no elements in the reflection path from the mirror to the detector which are not included in the reflection path from the sample to the detector.  
         [0028]    Thus, in prior systems there is a much higher likelihood that the ratio between the monitor signal and the measurement signals depends not only on the reflectivity characteristics of the sample, which are presumed unknown, but also on the effects of the different reflection or transmission paths, and different characteristics of different detectors. For example, referring to FIG. 1, a spec of dust on lens  144  would affect the ratio measurement/monitor ratio. However, referring to FIGS. 2 and 3, a spec of dust on lens  224  will affect the measurement and monitor signals equally so that their ratio will remain constant. Similarly, different changes in temperature of detectors  130  and  150  in FIG. 1 are likely to change their efficiencies differently, and thus affect the measurement/monitor ratio. However, in the present invention there is only one detector, so the ratio will not change as long as the temperature and efficiency of the detector do not change over the short time required to sample the two signals. Thus, the present invention can provide for enhanced measurement accuracy of the reflection characteristics of the sample.  
         [0029]    Movable mirror  204  may be implemented as shown in FIGS. 4 and 5. Viewed from above, as in FIG. 4, mirror  302  is held with bracket  304  which is allowed to rotate about axel  306  to positions  305   a  and  305   b  by bearings  308 . Support  310  holds the bearings in a fixed relation to the rest of the optics in system  200 , beam splitter  110 , as shown in FIGS. 2, 3, and  5 . Motor  312  turns axel  306 , and consequently bracket  304 . Motor  312  preferably has hard stops associated with locations  305   a  and  305   b , to allow these positions to be highly reproducible. Two spaced bearings  308  are preferred to constrain motion of bracket  304  to be in a plane so that mirror  302  is always perpendicular to the optical axis of associated with path  206 . Many alternative embodiments are possible. For example, mirror  302  may be allowed to rotate 360° about axel  306 . Mirror  302  may rotate continuously, with the precise acquisitions times for measurement and monitor signals synchronized with the rotation. In yet alternative embodiments, both the sample and monitor signals may be sums of over alternating sample and monitor signal portions. This allows for, e.g., variations in lamp intensity at a faster rate.  
         [0030]    While the present invention has been described in terms of the embodiments discussed above, those skilled in the art will recognize that the present invention may be practiced with modification to the above described embodiment and still be and within the spirit and scope of the appended claims. For example, one alternative embodiment could provide for positioning the movable mirror between the objective lens and the sample. Thus, the specifications and figures herein are 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 other modifications are possible without departing from the teachings herein. All such modifications are intended to be encompassed within the following claims.