Abstract:
A metrology system includes a sample holder and a backside reflective element. The backside reflective element causes light that is transmitted through the sample to be reflected and transmitted a second time, in the opposite direction, through the sample. A variable collection range can be adjusted to place the sample, the reflective element or both within the collection range. The collection range is the range of focused light that will be detected. The system can be controlled to move one or both of the sample and the reflective element in or out of the collection range or to alter the optics to adjust the collection range so that one or both of the sample and reflective element are in the collection range. Thus, the metrology system can be configured to operate in reflectance mode, transmittance mode or a mixed reflectance/transmittance mode.

Description:
FIELD OF THE INVENTION 
   The present invention relates to optical sampling of thin films and transparent substrates, and in particular to optical sampling using reflectance, transmittance and mixed modes. 
   BACKGROUND 
   Optical sampling of thin films is conventionally performed using either specular reflectance or transmittance. Conventionally, reflectance measurements and transmittance measurements are made separately. Specular reflectance measurements are based on the fraction of light intensity that is reflected from a sample surface.  FIG. 1A  shows a simplified schematic diagram of a conventional reflectance measurement of a sample  10 . It should be understood that sample  10  typically includes one or more thin film layers (not shown). As shown in  FIG. 1A , the sample  10  is exposed to a light beam  12  having an intensity Io. Part of the light beam  12  is reflected off sample  10  as light beam  14 , which has an intensity Ir. The light beam  14  is collected with a detector (not shown). The reflectance, R, can then be measured as the ratio of the intensities of the reflected and incident light beams, as follows: 
             R   =       Ir   Io     .             eq   .           ⁢   1             
 
The reflected intensity Ir of light beam  14  is less than the incident intensity Io of light beam  12  so that R&lt;1.
 
   Transmittance measurements are based on the fraction of light intensity that is lost as a beam passes through a sample.  FIG. 1B  shows a simplified schematic diagram of a conventional transmittance measurement of a sample  20 . Again, it should be understood that sample  20  typically includes one or more thin film layers (not shown) that are to be measured. As shown in  FIG. 1B , the sample  20  is exposed to a light beam  22  having an intensity Io. Part of the light beam  22  is transmitted through sample  20  as light beam  24 , which has an intensity It. The transmitted light beam  24  is collected by a detector (not shown). The transmittance, T, can then be measured as the ratio of the intensities of the transmitted and incident light beams, as follows: 
             T   =       It   Io     .             eq   .           ⁢   2             
 
As with reflectance, light intensity is lost upon transmittance so that the transmitted intensity It of light beam  24  is less than the incident intensity Io of light beam  22  so that T&lt;1.
 
   On example of transmittance measurement is found in a technique known as Mirror Backed Infrared Reflection Absorption Spectroscopy (MBIRRAS) that prescribes placing a mirror a fixed distance behind an absorbing film sample and which is described in “Reflectance FT-IR for monitoring chemical reactions in chemically amplified photoresist for 0.25 μm X-ray lithography”, Christopher Gamsky, Ph.D. Dissertation, University of Wisconsin-Madison, 1995. The mirror and sample are spaced by a Teflon ring and held fixed by pressure from front and back plates. Incident light passes through the sample, reflects off the mirror and passes back through the sample. Measurements are explicitly performed at an oblique angle of incidence, e.g., 40° from normal, in order to avoid collecting light reflected from the surface of the sample. The air gap, which is the width of the Teflon ring, is selected to minimize interference fringes caused by collecting both the sample reflected and mirror reflected beams. 
   Typically, reflectance and transmittance measurements are preformed over some continuous range of wavelengths such as the mid-IR (400 cm −1  to 4000 cm −1 ). These spectra will normally contain features that become more or less pronounced as material properties, e.g., concentrations, change. It is therefore possible to correlate feature strength with these material properties and thereby measure these material properties. Typically, there is some range of the feature strength over which either reflectance or transmittance is usable and a single sample may contain some features within the range of a transmittance measurement and some features within the ranges of a reflectance measurement. In addition, some samples may have features that cannot be measured well by either reflectance or transmittance. In these cases it becomes necessary to modify the sample, such as the film thickness, for monitoring purposes—a modification that would be costly and require additional correlation to the originally unmeasurable sample. 
   Accordingly, what is needed is a metrology device that is easily configurable to operate in reflectance mode, transmittance mode or a mix of reflectance and transmittance mode. 
   SUMMARY 
   A metrology system, in accordance with the present invention, can be configured to operate in reflectance mode, transmittance mode or a mixed reflectance/transmittance mode. The metrology system includes a sample holder and a backside reflective element. The backside reflective element causes light that is transmitted through the sample to be reflected and transmitted a second time, in the opposite direction, through the sample. A variable collection range can be adjusted to place the sample, the reflective element or both within the collection range. The collection range is the range of focused light that will be detected. The system can be controlled to move one or both of the sample and the reflective element in or out of the collection range or to alter the optics to adjust the collection range so that one or both of the sample and reflective element are in the collection range. 
   Thus, in one aspect of the present invention, an apparatus for optically measuring characteristics of a sample includes a light source that produces a light beam along an optical path; a sample support for holding a sample within the optical path; and a reflective element within the optical path and downstream of the sample support. The apparatus also includes a means for positioning a sample held on the sample support, the reflective element, or both the sample held on the sample support and the reflective element within a collection range. The means for positioning may be, e.g., at least one actuator coupled to at least one of the sample support and the reflective element to move at least one of the sample support and the reflective element into and out of the collection range. Alternatively, the means for positioning may be, e.g., the optical elements that can adjust the collection range to include at least one of the sample and the reflective element in or out of the collection range. The apparatus also includes a light detector in the optical path, wherein the light detector receives light reflected from within the collection range. 
   In another aspect of the present invention, a method of measuring a characteristic of a sample includes producing a light beam to be incident on a sample; reflecting a portion of the light beam off the sample to form a reflected light beam; transmitting another portion of the light beam through the sample in a first general direction to form a transmitted light beam; reflecting the transmitted light beam back toward the sample; transmitting the transmitted light beam through the sample in a second general direction to form a second transmitted light beam, the second general direction being opposite the first general direction; configuring a collection range, the collection range being a range within which light is reflected; and detecting light reflected within the collection range. Configuring the collection range includes, e.g., moving at least one of the sample and a reflective element to a desired position in or out of the collection range, where the reflective element reflects the transmitted light beam back toward the sample. Alternatively, configuring the collection range includes, e.g., adjusting at least one optical element to alter the focus of the light beam between the sample and a reflective element that reflects the transmitted light beam back toward the sample; and adjusting at least one optical element to alter the focus of the light that is detected. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  shows a simplified schematic diagram of a conventional reflectance measurement of a sample. 
       FIG. 1B  shows a simplified schematic diagram of a conventional transmittance measurement of a sample. 
       FIG. 2  shows a simplified schematic diagram of measuring a sample using reflection and transmission of light in accordance with an embodiment of the present invention. 
       FIG. 3  shows a simplified schematic diagram of metrology device configured to operate in reflectance mode in accordance with an embodiment of the present invention. 
       FIG. 4  shows a simplified schematic diagram of metrology device configured to operate in transmittance mode in accordance with an embodiment of the present invention. 
       FIG. 5  shows a simplified schematic diagram of metrology device configured to operate in a combination of reflectance mode and transmittance mode in accordance with an embodiment of the present invention. 
       FIGS. 6A ,  6 B and  6 C show simplified schematic diagrams of a metrology device with a sample mounted on a sample holder over a reflective element, where the metrology device is operating in reflectance mode, transmittance mode, and mixed mode, respectively. 
       FIGS. 7A and 7B  show simplified schematic diagrams of a metrology device in accordance with an embodiment of the present invention in which a beam splitter is not used and the light beam is obliquely incident on the sample. 
       FIG. 8  shows another embodiment of the optical path of a system that may be used in accordance with the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 2  shows a simplified schematic diagram of measuring a sample  100  using reflection and transmission of light in accordance with an embodiment of the present invention. It should be understood that sample  100  can include one or more film layers, which are not shown in  FIG. 2 , and that sample  100  may be, for example, a silicon wafer or flat panel display or other such similar device. 
   As shown in  FIG. 2 , a light beam  102  having an intensity Io is incident on sample. Light beam  102  may be, for example, a beam of light from a FTIR spectrometer, such as the type manufactured by Midac Corp., in Irvine Calif. A portion of light beam  102  is reflected off sample  100 , e.g., at the surface of sample  100  or at the interfaces of various overlying layers. The reflected light beam  104  has an intensity Ir. 
   Another portion of light beam  102  is transmitted through sample  100  resulting in transmitted light beam  106  having an intensity It. A mirror  108  is positioned beneath sample  100  such that the transmitted light beam  106  is reflected back towards sample  100  in the form of reflected transmitted light beam  107 , as shown in FIG.  2 . The mirror  108  should be highly reflective to the particular wavelengths of transmitted light beam  106 . Thus, for example, if light beam  102  is an infrared light beam, mirror  108  should be coated with gold, which is highly reflective to infrared light. The reflected transmitted light beam  107  passes through sample  100  a second time, resulting in transmitted light beam  110  having an intensity of It′. Thus, light passes through sample  100  twice, thereby increasing sensitivity to properties associated with bulk light absorption. 
   It should be understood that while  FIG. 2  shows light beams  102 ,  104 ,  106 ,  107 , and  110  separated, these light beams may overlap, for example, if light beam  102  is normally incident on sample  100 . Moreover, if light beam  102  is incident on sample  100  at an oblique angle, light beams  102 ,  104 ,  106 ,  107 , and  110  will not overlap but reflected light beams, i.e., light beam  104  and light beam  107  will be reflected from the point of incidence on sample  100  (or the interface of any overlying layers) and mirror  108 , respectively. Thus, the arrows in  FIGS. 1 and 2  do not represent actual light rays, but rather the general direction of energy flow. 
   The light collected is the sum of the reflected light beam  104  with intensity Ir and the transmitted light beam  110  with intensity It′. The collected light can be written as a sum of ratios relative to the intensity of the incident light beam  102  resulting in a quantity S, as follows: 
             S   =       Ir   Io     +       It   ′     Io               eq   .           ⁢   3             
 
where the first term, Ir/Io, is the conventional reflectance and the second term, It′/Io, is the transmittance for a beam passing through sample  100 , reflectance off mirror  108  and passing through sample  100  a second time. Thus, the quantity S contains both the reflectance and transmittance type information.
 
   The combination of reflectance and transmittance type information in quantity S is advantageous because it permits the simultaneous measurement of sample properties that primarily affect measured reflectance and sample properties that primarily affect measured transmittance. In addition, the transmittance type measurement, It′/Io, is twice as sensitive to properties of the sample that affect transmittance, consequently, permitting higher precision measurements of samples with weaker absorption, such as those with lower concentrations, thin films and weak absorption cross-sections. 
   In an embodiment of the present invention, the quantity S can be modified as follows: 
             S   =       a   ×     Ir   Io       +     b   ×       It   ′     Io                 eq   .           ⁢   4             
 
where “a” and “b” are changeable constants with 0&lt;a&lt;1 and 0&lt;b&lt;1. Thus, any particular measurement of the quantity S can be tailored to the particular sample of interest. For example, a sample with a low concentration of weakly absorbing species might require that the constant “a” is approximately 0 and the constant “b” is approximately 1 to emphasize the absorption term, while a sample with very strongly absorbing species might require working in reflectance such that “a” is approximately 1 and “b” is approximately 0. A sample between these two extremes might be most sensitively measured with both “a” and “b” in the middle of their ranges or by combining a sequence of reflectance/transmittance measurements in one measurement.
 
     FIG. 3  shows a simplified schematic diagram of a metrology device  150  with sampling optics, shown as beam splitter  152  and objective lens  154  that measures the quantity S and that is configured to operate in reflectance mode, i.e., a≅1, b≅0, in accordance with an embodiment of the present invention. It should be understood that the beam splitter  152  and objective lens  154  are shown in  FIG. 3  for exemplary purposes, and that beam splitter  152  and objective lens  154  may actually include any desired number of components and lenses. Moreover, the beam path of light shown in  FIG. 3  is also exemplary and that any desired beam path may be followed. 
   As shown  FIG. 3 , metrology device  150  includes a light source  151  that produces a light beam  151   a  along an optical path. The light beam  151   a  is transmitted through beam splitter  152  and objective lens  154  and is incident on sample  156 . A reflective element, e.g., mirror  160 , is in the optical path downstream of the sample  156 . Metrology device  150  also includes a collection lens  162  within the optical path and a detector  164 . An aperture stop  165  is also disposed in the beam path. A collection range of the optics, which is indicated by broken lines  158 , is a range in which any light reflected back toward detector  164  will be received. Light reflected back toward detector outside collection range  158  will not be detected. Thus, as shown in  FIG. 3 , light that is transmitted through sample  156  and reflected off mirror  160  will not be focused onto the light detector  164 , causing the constant “b” in equation 4 to be approximately 0. However, light that is incident on sample  156  is within the collection range  158  and is therefore properly focused by objective lens  154  and collection lens  162  to be received by the light detector  164 . Consequently, the constant “a” in equation 4 is approximately 1. The size of the collection range  158  can be adjusted by adjusting the numerical aperture of objective lens  154  and collection lens  162  through adjustment of the size of the aperture stop  165 . 
     FIG. 4  shows a simplified schematic diagram of metrology device  150  adjusted so that the mirror  160  falls within the collection range  158  of the optics. Consequently, metrology device  150  is operating in transmittance mode, i.e., a≅0, b≅1, in accordance with an embodiment of the present invention. As shown  FIG. 4 , because sample  156  is outside the collection range  158 , light that is reflected from the surface of sample  156  will not be collected by the light detector, causing the constant “a” of equation 4 to be approximately 0. The light that is transmitted through sample  156 , however, is reflected off mirror  160 , which is within the collection range  158  of the optics. Thus, the light that is reflected off mirror  160  passes through sample  156  a second time, focused by objective lens  154  and is received by the light detector, causing the constant “b” of equation 4 to be approximately 1. 
     FIG. 5  shows a simplified schematic diagram of metrology device  150  adjusted so that both the mirror  160  and sample  156  fall within the collection range  158 . Consequently, metrology device  150  is operating in a mixed mode, i.e., neither “a” nor “b” are approximately zero, in accordance with an embodiment of the present invention. Thus, light that is reflected from the sample  156  and the mirror  160  will be focused by objective lens  154  and collection lens  162  and will be received by the light detector  164 . Thus, the constants “a” and “b” from equation 4 will both fall somewhere within their full ranges, i.e., between 0 and 1. 
   In one embodiment of the present invention, different modes of operation may be achieved by varying the spacing between the mirror and the sample.  FIGS. 6A ,  6 B and  6 C show simplified schematic diagrams of a device  170  with a sample  172  mounted on a sample holder  174  over a mirror  176  where the device  170  is configurable to operate in reflectance (FIG.  6 A), transmittance mode ( FIG. 6B ) and in mixed mode (FIG.  6 C), in accordance with an embodiment of the present invention. Sample holder  174  and mirror  176  are coupled to at least one actuator  178  via arms  180  and  182 , respectively. Actuator  178 , which may be two separate actuators, moves mirror  176  and sample  172  via sample holder  174  to place mirror  176 , sample  172 , or both mirror  176  and sample  172  within the collection range illustrated by broken lines  184 . Of course, if desired, actuator  178  may control only mirror  176  or sample holder  174 . Actuator  178  may be any appropriate mechanism that can accurately position sample  172  and/or mirror  176 , as is well known in the art. 
   In another embodiment of the present invention, rather than adjusting the physical location of the sample and/or mirror, the collection range may be altered, e.g., by appropriately adjusting objective lens  154 , collection lens  162  and aperture stop  165 , shown in  FIGS. 3 ,  4 , and  5 . Thus, by properly adjusting objective lens  154 , collection lens  162  and aperture stop  165  to increase the collection range  158 , as is well understood by those skilled in the art, both mirror  160  and sample  156  will be located within the collection range, and metrology device  150  will operate in mixed mode. By decreasing the collection range  158  only mirror  160  or sample  156  will be within the collection range and thus metrology device  150  will operate in transmittance mode or reflectance mode. If desired, both the collection range  158  may be adjusted along with moving the sample and/or mirror as described in reference to  FIGS. 6A ,  6 B, and  6 C. 
   The selectivity of the reflectance mode may be improved by eliminating effects from the mirror  160 . Thus, for example, the mirror  160  may be shuttered with a non-reflecting material or with a 45 degree mirror. Alternatively, the mirror  160  may itself be tilted. The selectivity of the transmittance mode may likewise be improved by tilting the sample  156 . 
     FIGS. 7A and 7B  show simplified schematic diagrams of a system  200  in accordance with an embodiment of the present invention in which a beam splitter is not used and the optical path is obliquely incident on sample  204 . As shown in  FIGS. 7A and 7B , the light beam  202 , which has an intensity Io, is focused by mirror  203  to be incident on sample  204  ( FIG. 7A ) or mirror  206  ( FIG. 7B ) at an oblique angle. The reflected light beam  208 , which has an intensity Is, is received by mirror  209  and reflected to a light detector. Thus, system  200  may be used in reflectance mode as shown in  FIG. 7A  or in transmittance mode as shown in FIG.  7 B. 
     FIG. 8  shows another embodiment of the optical path of a system  220  that may be used in accordance with the present invention.  FIG. 8  is the optical path, e.g., of an FTIR, in which the beam splitter and objective lens are replaced with two parabolic mirrors  222  and  224 . In the transmittance mode or the mixed mode, the light beam  223  from parabolic mirror  222  intersects the sample at a slightly different point than the light beam  225  to parabolic mirror  224 . As with the system  200  described in reference to  FIGS. 7A and 7B , with the emphasis on marginal rays, system  220  will be more binary in its selection between reflectance and transmittance modes. By moving the parabolic mirrors  222  and  224  closer or farther apart and/or masking parts of the surfaces of the parabolic mirrors  222  and  224 , the control of the binary selection of system  220 , as well as system  200 , may be altered. 
   Although the present invention is illustrated in connection with specific embodiments for instructional purposes, the present invention is not limited thereto. Various adaptations and modifications may be made without departing from the scope of the invention.