Abstract:
Provided are equipment and a method for measuring a transmittance of a photomask. The system includes an acoustic optical deflector (AOD) substrate interposed between a light source and the photomask. The AOD is adapted to deflect a laser beam to an oblique incidence angle with respect to a surface of the photomask. A radio frequency (RF) signal source is coupled with the AOD substrate. Varying the frequency of the signal applied to the AOD substrate acts to change the refractive degree of the substrate, thereby changing an angle of deflection of the incident laser beam. A photodetector is positioned to receive the laser beam passing through the photomask and is adapted to measure an intensity of the laser beam which has penetrated the photomask. As a result, a transmittance of the photomask can be measured under off axis illumination (OAI).

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
   This application claims the benefit of Korean Patent Application No. 10-2006-0120947, filed on Dec. 1, 2006, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to equipment and a method for fabricating a semiconductor device, and more particularly, to equipment and a method for measuring a transmittance of a photomask used in a photolithography process. 
   2. Description of the Related Art 
   Photomasks are used in microchip fabrication to form tiny circuit patterns on semiconductor substrates. Such photomasks typically include a light transmissive substrate such as quartz having deposited thereon a pattern of chrome defining a light blocking pattern. 
   In photolithography manufacturing, wafer circuit patterns are manufactured by projecting a pattern of light onto the photosensitive coating atop a wafer. The light pattern is formed by interposing the photomask between the light source and the wafer. Focused light from the source passes through the transmissive portions of the photomask but are blocked by the chrome pattern on the photomask. 
   Continuing manufacturing advances have decreased the size of these circuits and the critical dimensions (CD) of the circuit and photolithographic patterns. Several techniques have been developed to address problems with manufacturing semiconductor devices with small critical dimensions. 
   In a first technique, femto-second pulses from a laser are used to create correcting elements within the photomask that improve the uniformity of light transmission across the entire photomask. Improved uniformity is key to ensuring precise formation of patterns on the photomask and prevention of errors during the manufacturing process. 
   The correcting elements have varying characteristic depending upon the need. For example, the correcting elements may have a diameter of about 1 μm and a depth of about 30 μm. Furthermore, the correcting elements may be formed in a repeated structure and pitches of the correcting element may be adjusted to adjust the transmittance of the light transmissive substrate. Regions of the correcting elements with a small pitch have denser correcting elements, thereby lowering the transmittance compared to regions using correcting elements with a large pitch. A relationship between the pitches of the correcting elements and the transmittance of the light transmissive substrate can be observed using transmittance measuring equipment in exposure equipment for fabricating a photomask. 
   The second technique for reducing errors in small critical dimension photomasks is the use of off-axis illumination (OAI). Whereas conventional illumination through the photomask has been perpendicular to its surface, illumination through the photomask using off-axis light has been found to result in a higher resolution and a deeper depth-of-focus (DOF) margin. 
   Conventional techniques for OAI include either tilting the laser with respect to the photomask surface, or tilting the photomask itself. Each of these techniques, however, has a drawback in that the stage used to mount either the tilting laser or tilting photomask must be controlled with extremely tight tolerances. As a result, it has been extremely difficult to achieve accurate control of off-axis illumination through the photomask to measure the uniformity of its transmittance. 
   Accordingly, the need remains for methods and system which achieve more accurate control of OAI through a photomask. 
   SUMMARY OF THE INVENTION 
   The present invention provides equipment and a method for measuring a transmittance of a light transmissive substrate of a photomask, including ones having correcting elements for correcting the transmittance according to an incident angle of light. 
   According to an aspect of the present invention, there is provided a system for measuring the transmittance of a photomask. The system includes an acoustic optical deflector (AOD) substrate interposed between a light source and the photomask. The AOD is adapted to deflect a laser beam to an oblique incidence angle with respect to a surface of the photomask. A photodetector is positioned to receive the laser beam passing through the photomask and is adapted to measure an intensity of the laser beam which has penetrated the photomask. A radio frequency (RF) signal source is coupled with the AOD substrate. Varying the frequency of the signal applied to the AOD substrate acts to change the refractive degree of the substrate, thereby changing an angle of deflection of the incident laser beam. 
   According to another embodiment of the present invention, there is provided a method of measuring a transmittance of a photomask. The method includes deflecting a laser beam from a path normal to a surface to the photomask using an acoustic optical deflector (AOD) substrate. The deflected laser beam is then passed through the photomask. A photodetector is positioned under the photomask in a path adapted to receive and measure the intensity of the laser beam passing through the photomask. 
   Preferred embodiments of the invention additionally include varying a frequency of the RF signal applied to the AOD substrate to further adjust an angle of deflection of the laser beam through the AOD substrate. The photodetector is then moved in synchronicity with the change in source signal frequency by a degree mirroring the angle of deflection. The photodetector then measure the intensity of the laser beam passing through the photomask to determine its transmissivity. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
       FIG. 1  is a view illustrating a schematic structure of equipment for measuring a transmittance of a photomask according to an embodiment of the present invention; and 
       FIG. 2  is a flowchart of a method of measuring a transmittance of a photomask according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. 
     FIG. 1  is a view illustrating a schematic structure of equipment used for measuring a transmittance of a photomask according to an embodiment of the present invention. The equipment may be included with exposure equipment for fabricating the photomask. An acoustic optical deflector (AOD) substrate  20  is positioned between a light source  10  and a photomask  30 . The light source  10  may include an exposure light source such as an ArF laser or a KrF laser, similar or identical to the exposure light source for exposing a wafer. 
   The AOD substrate  20  may be formed of a crystal such as LiNbO 3 , BaTiO 3 , TeO 2 , or GaP and deflect an incident laser beam. A refractive index of the crystal of the AOD substrate  20  is varied at minute periods according to a radio frequency (RF) signal  22  applied from an external source to form an optical element. The optical element interferes with a laser beam  12  incident onto the AOD substrate  20  to refract the laser beam  12 . Also, the refracted degree of the laser beam  12  may be adjusted by a frequency of the RF signal  22  applied to an AOD driver. Thus, the frequency of the RF signal  22  applied to the AOD substrate  20  may be varied in order to vary a deflection angle of the laser beam  12  penetrating the AOD substrate  20 . As a result, the angle of the laser beam  12  incident onto the photomask  30  can be adjusted. Here, the AOD substrate  20  may be parallel with the photomask  30 . 
   An OAI method is used to improve a resolution and a depth-of-focus (DOF) margin during an exposure of a wafer. Here, a laser beam having passed a modified illumination system for an OAI may be incident at an angle between 2° and 12°, and preferably 4° and 11°, with respect to a vertical axis or an optical axis A of the photomask. A transmittance of a light transmissive substrate of a photomask in which correcting elements for correcting a transmittance are formed may be varied during OAI according to an incident angle of a laser beam. An AOD substrate may be used during a measurement of a transmittance of the photomask including the correcting elements to realize such an OAI. 
   The laser beam  12  deflected by the AOD substrate  20  penetrates a light transmissive substrate  31  and then reaches a photodetector  40  positioned under the photomask  30 . 
   The photomask  30  may include correcting elements  32  which are formed to predetermined pitches in the light transmissive substrate  31  to correct the transmittance of the light transmissive substrate  31 . As practiced in the prior art, high energy may be applied for a short time through an irradiation of a pulse laser having a femto-second lasting time onto a light transmissive substrate to change a property of the light transmissive substrate so as to form correcting elements. The correcting elements  32  may be formed in repeated solid structures, each having a diameter of about 1 μm and a depth of about 30 μm. Also, pitches of the correcting elements  32  can be adjusted to adjust the transmittance of the light transmissive substrate  31 . The photomask  30  may be a phase shift mask (PSM) or a binary mask (BM). 
   The photodetector  40  is a photoelectric converting device that converts light energy into electrical energy and generally includes a photodiode. The photodetector  40  measures an intensity of the laser beam  12  having penetrated the light transmissive substrate  31  and compares the intensity of the laser beam  12  with an intensity of a laser beam which has not penetrated the light transmissive substrate  31 . This measured ratio is equivalent to the transmittance of the light transmissive substrate  31 . Here, the photodetector  40  may use a beam splitter (not shown) positioned between the AOD substrate  20  and the photomask  30  to measure an intensity of light penetrating a transparent substrate and an intensity of light penetrating the photomask  30  so as to measure the transmittance in situ. 
   The photodetector  40  may move in a circular arc from a point of the AOD substrate  20  on which the laser beam  12  is incident, so as not to vary a path length of the laser beam  12 . The laser beam has a deflection angle varied by the AOD substrate  20  according to the RF signal  22 , penetrates the photomask  30 , and reaches the photodetector  40 . Here, the photodetector  40  may synchronize with the RF signal  22  to move with an adjustment of its angle from an optical axis A. In other words, the photodetector  40  may move from the optical axis A to cope with the deflection angle of the laser beam  12  caused by the AOD substrate  20 . Thus, differences in a transmittance of the laser beam  12  depending on incident angle of the laser beam  12  onto the photomask  30  may be compared with one another. 
   Also, the photodetector  40  can be positioned under the photomask  30  to move above a plane parallel with a plane of the photomask  30 . As a result, the photodetector  40  can measure the entire photomask  30 . Uniformity of a transmittance of the photomask  30  can be measured through the measurement of the transmittance of the entire photomask  30 . 
     FIG. 2  is a flowchart of a method of measuring a transmittance of a photomask according to an embodiment of the present invention. Referring to  FIGS. 1 and 2 , in operation S 10 , the AOD substrate  20  is provided between the light source  10  and the photomask  30  in the equipment for measuring the transmittance of the photomask. In operation S 20 , the photodetector  40  is provided under the photomask  30 , to measure an intensity of light penetrating the photomask  30 , i.e., an intensity of a laser beam. This can occur during, before, or after the AOD substrate  20  is provided. The AOD substrate  20  and the photodetector  40  may be included within the exposure equipment for fabricating the photomask, or may be provided as different equipment from the exposure equipment. 
   In operation S 30 , the light source  10  irradiates the laser beam  12  onto the AOD substrate  20 . In operation S 40 , the RF signal  22  is applied to the AOD substrate  20  to deflect the laser beam  12  from the AOD substrate  20  at a predetermined angle. As described above, an ArF laser or a KrF layer may be used as a light source used in wafer exposure equipment. The frequency of the RF signal  22  applied to the AOD substrate  20  can be adjusted to adjust the deflection angle of the laser beam  12 . The deflection angle may be within a range between 2° and 12°, preferably, between 4° and 11°. 
   In operation S 50 , the photodetector  40  moves to cope with the deflection angle of the laser beam  12  depending on the frequency of the RF signal  22  so as to measure the intensity of the laser beam  12  penetrating the photomask  30 . Here, the photodetector  40  may move in a circular arc at a predetermined distance from the point at which the laser beam  12  passes the AOD substrate  20 . Moving the detector along a circular path ensures that the detected laser beam  12  has a constant path length. The photodetector  40  may move above the plane parallel with the plane of the photomask  30  to measure the intensity of the laser beam  12  so as to measure the transmittance of the entire photomask  30 . Thus, the uniformity of the transmittance of the photomask  30  can be measured due to the measurement of the transmittance of the entire photomask  30 . 
   In particular, a relationship between pitches of the correcting elements and a transmittance of the light transmissive substrate may be measured under OAI and may be used to improve critical dimension uniformity within a pattern. 
   The measurement of the transmittance of the photomask under the OAI can be applied to a photomask and another material of which transmittance is to be measured, e.g., a light transmissive substrate. 
   As described above, in equipment and a method for measuring a transmittance of a photomask under OAI according to the present invention, a laser beam can pass an AOD substrate and be deflected at a predetermined angle to realize the OAI. Also, a photodetector can be moved to cope with the deflection angle of the laser beam to measure an intensity of the laser beam penetrating the photomask. As a result, the transmittance of the photomask can be measured under the OAI. 
   While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.