Abstract:
Purging of a light beam path in an effective manner that minimizes the affect of the purging requirement on system throughput. In one embodiment, the invention is incorporated into a birefringence measurement system that has several components for directing light through a sample optical element and thereafter detecting and analyzing the light. The segment of the beam path through the sample is isolated to reduce the volume that requires continual purging.

Description:
TECHNICAL FIELD 
   This invention relates to purging of contaminants from the path of a light beam, such as a short-wavelength light beam that can be used for photolithography or for measuring the properties of an optical element. 
   BACKGROUND OF THE INVENTION 
   The optical lithography industry is currently transitioning to the use of very short exposure wavelengths for the purpose of reducing line weights (conductors, etc.) in integrated circuits, thereby to enhance performance of those circuits. In this regard, the next generation of optical lithography systems will use laser light having a wavelength of about 157 nanometers, which wavelength is often referred to as deep ultraviolet or DUV. 
   It is important to precisely determine the optical characteristics of the optical elements that are used in systems that employ DUV light. Such an element may be, for example, a calcium fluoride (CaF2) lens of a scanner or stepper. Birefringence is one such characteristic of the optical element. 
   Birefringence is an intrinsic property of many optical materials, and may also be induced by external forces. The terms retardation or retardance represent the integrated effect of birefringence acting along the path of a light beam traversing a sample optical element. If the incident light beam is linearly polarized with the direction of polarization different from the fast axis of the sample, the two orthogonal components of the polarized light will exit the sample with a phase difference, called the retardance. The unit of retardance can be length, such as nanometers (nm). It is also frequently expressed in units of phase angle (waves, radians, or degrees), which angle is directly proportional to the retardance (nm) divided by the wavelength of the light (nm). A path “average” birefringence for a sample is sometimes computed by dividing the measured retardation magnitude by the thickness of the sample. Oftentimes, the term “birefringence” is interchangeably used with and carries the same meaning as the term “retardance.” Thus, unless stated otherwise, those terms are also interchangeably used below. 
   Since the retardance of an optical element is a characteristic of both the optical material and the wavelength of the light that penetrates the material, a system for measuring retardance properties (hereafter usually referred to as a birefringence measurement system) of an optical element employed in a DUV optical setup must also operate with a DUV light source and associated components in order to precisely detect and process the associated light signals. 
   There are several problems associated with the use of DUV light in applications such as birefringence measurement or photolithography. One problem concerns absorption of DUV light by oxygen present in the system environment, and in the light beam path in particular. In this regard, the oxygen molecules (as well as other contaminants such as water vapor or trace amounts of hydrocarbons) absorb the DUV light, thus attenuating the light and reducing the signal necessary to make accurate birefringence measurements of the sample. 
   One way of eliminating the oxygen (as well as other contaminants) in the system environment is to purge the system with nitrogen (N 2 ). Purging, and the maintenance of a purged system, however, will often require reductions in throughput or large, expensive purging systems, especially in instances where a large number of optical components are involved, or the equipment incorporating the optical elements is large. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to effective purging of a light beam path, while minimizing the affect of the purging on system throughput. 
   In one embodiment, the invention is incorporated into a birefringence measurement system that has several components for directing DUV light through a sample and thereafter detecting and analyzing the light. A segment of the beam path, including the path through the sample, is isolated in a chamber to reduce the volume in the equipment that requires continual purging. Moreover, purging gas is directed in a localized or focused manner that maintains a low-oxygen, purged beam path. The chamber is designed to quickly reestablish a purged environment when the chamber is occasionally exposed to ambient oxygen, such as during the loading and unloading of a sample. 
   Other advantages and features of the present invention will become clear upon study of the following portion of this specification and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram of one preferred system that incorporates the light-beam-path purging of the present invention. 
       FIG. 2  is a somewhat schematized side view of the system diagrammed in  FIG. 1 . 
       FIG. 3  is a three-part diagram showing an embodiment of the invention wherein a sample optical element that is held in an inclined or tilted orientation is moved relative to the beam path. 
       FIG. 4  is an enlarged detail diagram of the embodiment of  FIG. 3  illustrating movement of purging gas delivery tubes relative to a movable, tilted sample. 
   

   DETAILED DESCRIPTION 
   A system that incorporates the light-beam-path purging of the present invention is depicted in  FIG. 1 . Such a system may be a birefringence measurement system that, as mentioned above, operates with a deep ultraviolet (DUV) light source and associated components for detecting and processing the light signals that are transmitted through a sample optical element. 
   It is noteworthy here that the birefringence measurement system discussed next is only one of many possible embodiments for the beam path purging system of the present invention, and it is to be kept in mind that the particular components of the birefringence measurement system are described here for illustrating the underlying invention. Also, although the invention is often characterized as beam path purging, it is understood that the environment of the system in an isolated chamber enclosing the sample is also purged. 
   The birefringence measurement system  100  under consideration is arranged in three primary compartments or modules. An upper module  102 , a lower module  104  and a sample module  106 . 
   The upper module  102  includes a DUV light source  120  that can be, for example, a deuterium lamp combined with a monochromator. The lamp irradiates a wide range of wavelengths. The monochromator is set to select the wavelength that is desired for the particular birefringence measurement application (such as 157 nm+/−10 nm). It is contemplated that other lamps, such as mercury lamps and xenon lamps, can be used for birefringence measurements in different spectral regions. In this regard, it is noted that the hereafter described beam path purging features of the present invention are not limited to a beam having a particular wavelength, such as 157 nm, although the importance of effective purging is increased at wavelengths in the DUV portion of the spectrum. 
   It is also contemplated that a light beam may be generated by a laser source, such as an excimer- or fluorine-type laser. 
   The light beam  121  emanating from the source  120  is directed through a polarizer  124  and then though a first photoelastic modulator (PEM 1 )  126  that modulates the polarization of the beam before the beam is directed into the sample module  106  for passage through the sample optical element  136  (hereafter, “sample”). The sample may be, for example, a lens blank for which a retardance characteristic is to be determined. The sample  136  is mounted to a sample holder  134  in a manner that permits passage of the beam  121  through the sample. The holder is a X/Y stage type that is controllable for moving the sample in a translational sense along orthogonal (X and Y) axes so that birefringence data can be collected for a plurality of locations across the surface of the sample. After passing through the sample the beam passes into the lower module  104 . The particulars of the lower module  104  are discussed next before returning to the description of the sample module  106  and its contents. 
   The lower module  104  includes a second photoelastic modulator (PEM 2 )  128  that modulates the polarization of the beam  121  that emanates from the sample  136 . The beam  121  then passes through an analyzer  130  before reaching an adjacent detector  132 . 
   As one aspect of the present invention, the system is constructed and arranged so as to minimize the volume (through which the light beam passes) that must be continually purged of oxygen. The modular arrangement of the system components is convenient for accomplishing this. In particular, the components of the upper module  102  discussed above are sealed in a casing, as depicted at  140  in the  FIG. 1  diagram. The casing  140  may be any suitable material, such as metal sheeting, that is constructed to enclose all of the upper module optical components from ambient atmosphere. Control cabling (such as shown at  122 ) and other connectors passing through the walls of the casing  140  are sealed at the casing wall in a conventional way so that there remains a sealed volume inside the casing. 
   The upper module  102  may include a cabinet that houses the upper module casing  140  and any related equipment, such as a controller  123  for controlling the light source  120  and the photoelastic modulator  126 . 
   The upper module casing  140  can be a single enclosure or a stack of separate enclosures of each component. The casing(s)  140  is purged of oxygen and sealed from ambient air. Alternatively, the casing  140  may be evacuated. The casing  140  is mounted in airtight fashion to the top wall  105  of the sample module  106  ( FIG. 1 ). The aperture of the photoelastic modulator  126  aligns with an aperture in that wall  105  so that the beam  121  enters the sample module  106  to pass through the sample  136 . 
   Similarly, the components of the lower module  104  discussed above are sealed in a casing, as depicted at  142  in  FIG. 1 . The casing  142  may be any suitable material, such as metal sheeting, that is constructed to enclose all of the lower module components from ambient atmosphere. Control cabling (such as shown at  133 ) and other connectors passing through the walls of the casing  142  are sealed at the casing wall in a conventional way. 
   The lower module  104  includes a cabinet that houses the lower module casing  142  and any related equipment, such as a computer  135  for processing the signal data collected by the detector  132 . 
   The lower module casing  142  can be, like the upper module casing, a single enclosure or a stack of separate enclosures of each component. The casing  142  is purged and sealed from ambient air, and mounted in airtight fashion to the bottom wall  107  of the sample module  106 . The aperture of the second photoelastic modulator  128  in the lower module  104  aligns with an aperture in that wall  107  so that the beam  121  propagating through the sample  136  passes into the set of optical components of the lower module  104 . 
   Although the light beam  121  passes through all three modules, the isolated internal chamber defined by the sample module  106  is hermetically sealed from the other two modules of the system. The sample module  106  encloses the sample holder  134  and the sample  136  and can be exposed to oxygen because of the need to provide access to that module (as explained more below) for changing or rearranging a sample. That is, the necessary, occasional access to the interior of the sample module exposes that chamber to oxygen in the atmosphere surrounding the system. Thus, this module  106  receives the local or focused purging mentioned above. One can appreciate that by isolating the sample module  106 , only a relatively small volume of the overall optical system (that is, the system including all of the optical elements through which the light beam passes) will require continual purging. 
   One preferred mechanism for purging includes a supply of nitrogen  150 , which can be stored in liquid form. The liquid nitrogen is expanded to a gaseous state and heated to ambient temperature via a temperature controlled heat exchanger  152  that is interconnected between the supply  150  and the sample module  106 . Other gases, such as helium, could be used instead of nitrogen. The heat exchanger  152  is housed in a compartment that is thermally isolated from the sample module  106 . 
   The focused application of the purging gas includes a pair of gas delivery tubes  154 ,  156  that are pressurized with purging gas that is provided by bifurcated tubing  158  that conducts the gas from the heat exchanger  152  to the interior of each tube  154 ,  156 . Gas flow to the tubing  158  is controlled by an electronic valve  155  that, under control of the computer  135 , can be closed whenever the beam is not in use. 
   The upper tube  156  is mounted to the wall  105  of the sample module  106  in a manner such that it is axially aligned with the light beam  121 . Thus, after the beam  121  exits the sealed, upper-module casing  140  it propagates through the nitrogen-saturated interior of the tube  156  to the sample  136 . 
   It is noteworthy here that the light source  120 , which generates a significant amount of heat during use, is thermally isolated from the sample chamber  106 , including the tubing that delivers purging gas within the sample chamber. The compartmented temperature-controlled heat exchanger  152  is also thermally isolated from the light source as well as the sample chamber. As a result of this thermal isolation, the temperature of the purging gas delivered to the sample remains stable so that no temperature gradients are induced in sample, which gradients could vary the optical characteristics of the sample. 
   Preferably, the upper tube  156  has a telescopic extension  160 . The extension  160  can be retracted to provide clearance for easily replacing the sample  136 , and extended once the sample is in place on the holder  134  so that the open end of the tube extension  160  is adjacent to the upper surface of the sample. The gas pressure supplied to the tubes is selected so that the purging gas exiting the tube extension  160  provides a positive pressure in the gap that resides between the tube and the sample surface, thereby preventing the entry of oxygen into this otherwise exposed segment of the beam path. 
   The lower purge-gas delivery tube  154  is mounted to the bottom wall  107  of the sample module  106  so that it is axially aligned with the beam  121 . Thus, after the beam  121  propagates through the sample  136 , it passes through the interior of the tube  154  and to the lower module casing  142  as described above. Accordingly, the beam  121  propagates through the nitrogen-saturated interior of the tube  154  in the path to the lower module  104 . 
   Preferably, the open end of the lower tube  154  extends to be adjacent to the undersurface of the sample holder  134  (hence, adjacent to the sample). The pressure of the purging gas is selected so that the gas exiting the tube  154  provides a positive pressure in the gap between the tube and the sample undersurface, thereby preventing the entry of oxygen into this segment of the beam path. 
   It is contemplated that a single, focused gas delivery tube will suffice, with or without a telescopic extension. Also, in some birefringence measurement systems, the beam is completely or partly reflected from the sample toward a separate set of detection components. It will be clear to one of ordinary skill that the present invention can be used with such a system by sealingly enclosing those other components (such as by a casing similar to the ones  140 ,  142  discussed above) and arranging another purging gas delivery tube in axial alignment with the reflected light beam. 
   It is also contemplated that the gas purging tubes could be sized and arranged with axes perpendicular to that of the beam, thereby to direct the purging gas across the beam path in instances where it might not be desirable or possible to have the light beam pass through the interior of the tubes. 
   Although the purging gas is focused where its presence is most important, along the path of the light beam, it is noted that the entire interior chamber of the sample module  106  is purged of oxygen. To this end, one preferred embodiment of the present invention includes additional or secondary purging gas delivery, as by a separate port  159  fed from the heated source of nitrogen and controlled by a computer controlled electronic valve  157 . The provision of the secondary purging permits a dual or staged approach to purging. For example, the secondary purging (via port  159 ) may be applied substantially continuously during operation of the system  100 , thereby maintaining a very low concentration of oxygen in the supply chamber. For efficient use of the purging gas, the primary or focused gas purging (via delivery tubes  154 ,  156 ) may be used only at times when the light beam is directed through the sample chamber. 
   It is noted that even though only a single port  159  and valve  157  are depicted, it will be understood that several such ports and valves could be used for effective secondary purging described here. 
   An exhaust vent  164  is provided, with or without an associated fan, with a one-way flap or check valve to prevent exhausted gas from returning from the vent to the chamber of the sample module  106 . 
     FIG. 2  shows how, in a preferred embodiment, the sample module  106  may be accessed by an operator through hinged doors  170 ,  172  that, when closed, seal the chamber of that module. As mentioned, such access to the module may introduce unwanted oxygen into the sample module  106 . In any event, since the volume of the sample module is relatively small, owing to its isolation from the remaining modules of the optical system, an adequately purged chamber within the sealed sample module can be rapidly reestablished after the module doors  170 ,  172  are again closed. The beam path through the sample module  106  is immediately purged owing to the effect of the purging gas delivery (tubes  154 ,  156  and port  159 ) discussed above. 
   It is contemplated that the secondary purging provided via the above-discussed port  159  (or any similar secondary purging-gas delivery system) may be optional. That is, in instances where the interior chamber of the sample module  106  is configured with as small a volume as practical, and where that chamber is carefully isolated (including well-sealed access doors), the focused, delivery of gas, such as via tubes  154 ,  156 , will suffice for maintaining a tolerably low oxygen concentration in the sample chamber. 
   It is also pointed out that because the sample module  106  is equipped with a minimum amount of components, contamination sources from those components (such as by material disintegration or out-gassing) are similarly minimized. 
   In some imaging optical systems, including optical lithographic step-and-scan systems, calcium fluoride lenses are typically used to bend the light rays in order to achieve certain numerical apertures. Therefore, it is important to study the angular dependence of birefringence in calcium fluoride samples relative to the incident light beam. One way to analyze this angular dependence is to orient the surface of the optical sample at an inclined or tilted orientation to establish the desired angle of incidence for the beam. 
   The diagram of  FIG. 3  schematically illustrates an example of how a tilted optical element sample  236  may be traversed (here in a linear, “Y,” direction) across the fixed path of a light beam  221  in a system that is otherwise comparable to the birefringence measurement system  100  discussed above (isolated, purged sample chamber, etc.). The sample  136  is incrementally traversed by the X/Y stage sample holder  234  so that birefringence data can be collected over a plurality of locations across the surface of the sample. 
   It will be appreciated that as a result of the movement of the tilted sample  136  relative to the light beam  221  there will occur a change in the distance between the sample surface and the ends of purging gas delivery tubes that surround the beam. As an aspect of the present invention, there is provided a purging gas delivery system that is adjustable to account for the movement of a tilted sample relative to the light beam and that also maintains the gas purging tubing such that the tubing opens adjacent to the surface of the sample. This embodiment is illustrated in  FIGS. 3 and 4  and discussed next. 
     FIG. 3(   a ) illustrates one position of the tilted sample  236  relative to the light beam  221  where a relatively low leading edge  237  of the sample crosses the path of the beam  221 . In this embodiment, a telescopic, upper purging-gas delivery tube  256  is provided as shown schematically in  FIG. 3  and in  FIG. 4 . Beneath the sample, there is a similar telescopic, lower purging-as delivery tube  254 . 
   In the sequence of drawings  FIGS. 3(   a )– 3 ( c ) it is shown how the upper gas deliver tube  256  is retracted and the lower gas delivery tube  254  extended as the sample  236  is traversed from left to right in the figure. From that figure, it can be appreciated that the ends of the purging gas tubes are maintained in close proximity to the surfaces of the sample, thereby ensuring that the gap that resides between the tube and the sample remains under positive pressure from the purging gas that flows from the tubes. 
   With reference to  FIG. 4 , the adjustable purging-gas tubes  256  and  254  can be configured in any of a number of ways. In one embodiment, the telescopic upper tube  256  is mounted to the top wall  105  of the sample module and supplied with purging gas through one branch of by bifurcated tubing  258  that corresponds to the tubing  158  described above. 
   An extension part  260  of the upper tube  256  is connected to a linear actuator  262  that is mounted adjacent to the tube  256 . The actuator  262 , under the control of the computer  135 , is operable to extend and retract the connected extension part  260  in the opposing directions shown by arrow  264  in  FIG. 4 . The lower, telescopic tube  254  is similarly extended and retraced by a computer controlled linear actuator  263 . 
   The sample holder  234  may be constructed to hold the sample  236  in a particular angle relative to the incident light beam  221 . In such an instance, the computer  135  is programmed with information that correlates the Y-position of the sample holder with the position of the sample surfaces so that the linear actuators may be controlled to maintain the ends of the tubes  256 ,  254  in close proximity with the respective surfaces of the sample as the sample is traversed. For example, with reference to  FIG. 3 , the linear actuators are controlled to incrementally retract the upper tube  254  and extend the lower tube  256  as the sample is traversed from left to right in that figure. 
   The tubes are retractable by an amount sufficient to ensure (in this embodiment) there is no contact between the lower tube and the leading end  237  of the sample, or between the upper tube  254  and the trailing end  239  of the sample. 
   It is also contemplated that the sample holder  234  may be designed to rotate the sample to facilitate, for example, analysis of the sample&#39;s birefringence properties at a number of different angles of incidence to the light beam. For example, the holder  234  shown in  FIG. 4  secures the sample  236  about aligned pivot posts  240   241 . A servomotor  235  is connected to one post or shaft  241  and operable by the computer for rotating the sample to the desired angle for analysis. In one embodiment, the servomotor is provided with an encoder that provides shaft  241  position information to the computer. The position information is processed to determine the relative positioning of the sample surfaces, and the lengths of the purging gas tubes  254 ,  256  are timely adjusted (via the linear actuators  262 ,  263 ) to maintain the respective tube ends in close proximity with the sample surfaces, as discussed above. 
   For example, the servo motor  235  can be driven to rotate the sample  236  from the angled orientation shown in solid lines in  FIG. 4  to a horizontal position as shown by dashed lines  243 . The upper tube is thus extended in the direction shown by arrow  245  until the end of that tube is adjacent the sample surface. The lower tube  254  is correspondingly retracted. 
   Although preferred and alternative embodiments of the present invention have been described, it will be appreciated that the spirit and scope of the invention is not limited to those embodiments, but extend to the various modifications and equivalents as defined in the appended claims.