Abstract:
A system comprising a radiation source, a holder operable to securely hold at least one mask oriented to receive radiation emitted from the radiation source, a projection system operable to direct radiation passing through the at least one mask, and an image capture system operable to receive radiation directed by the projection system and capture a projected image of the at least one mask.

Description:
BACKGROUND  
       [0001]     Optical proximity correction (OPC) is a term used to refer to a collection of techniques used to correct distortions in sub-wavelength photolithography. These distortions include line width variations, shortening of lines, and rounding of corners that are dependent on pattern density and other proximity factors. OPC is used to alter the photomask geometries to anticipate and compensate for these proximity effects. Typically, a photolithography mask is designed according to circuit geometries and layout, and then its geometries are modified in an OPC step. The OPC-modified photomask pattern comprises “serifs” added to line corners, “jogs” or extensions added to line-ends, and other OPC features that generally increase the non-linearity of the mask pattern.  
         [0002]     Although OPC has been an important technique to compensate for optical proximity effects, it also has introduced difficulties in another area of photomask production. The successful manufacture of these masks requires the detection, measurement and evaluation of defects on the photomasks. However, the OPC features added to the photomask pattern has made the mask verification process very difficult. Conventional metrology tools are designed to measure the distance between line edges. If the edges are irregular curves, making edge-to-edge measurement becomes a challenge task. For example, some metrology tools require an edge used in measurements to be a straight edge of at least 2 μm long. Because of the added OPC features, this requirement is difficult to satisfy for the number of measurements needed for mask verification.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0003]     Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.  
         [0004]      FIG. 1  is a simplified schematic diagram of an embodiment of a system for photolithography mask critical dimension measurement; and  
         [0005]      FIG. 2  is a simplified flowchart of an embodiment of a method for photolithography mask critical dimension measurement. 
     
    
     DETAILED DESCRIPTION  
       [0006]      FIG. 1  is a simplified schematic diagram of an embodiment of a system  10  for photolithography mask critical dimension measurement. System  10  comprises a radiation source  12  that may emit visible light, ultraviolet (UV), deep ultraviolet (DUV), extreme ultraviolet (EUV), X-ray, or radiation in other suitable spectra. A mask  14  that has been modified after optical proximity correction or OPC is placed on a mask holding stage or held by a mask holder  16 . Mask holder  16  is operable to hold one or more masks. The mask  14  is positioned over a projection system  18  that may include one or more lenses, for example. The projection system  18  is operable to direct radiation passing through the mask  14 . An image capture system  20  is positioned proximately to projection system  18 . The image capture system  20  is operable to receive radiation directed by the projection system  18  and capture a projected image of the mask  14 . Image capture system  20  may comprise an array of sensors such as CCD (charge-coupled device) image sensors, CMOS (complimentary metal-oxide semiconductor) image sensors, and other suitable image sensors. Alternately, image capture system  20  may be a photoresist that is operable to receive a transfer of the pattern from OPC mask  14 .  
         [0007]     The radiation source  12 , mask holder  16 , and projection system  18 , as well as the general setup and distances between the components shown in  FIG. 1  should be identical or very similar to that used during photolithography using the OPC mask to transfer the pattern thereon to a wafer or the photoresist on a wafer during integrated circuit manufacture. Alternatively, the same tool or system for photolithography during integrated circuit fabrication may be used for critical dimension metrology as described herein.  
         [0008]     A tool such as a critical dimension scanning electron microscope (CD SEM)  22  or another suitable tool may be used to make critical dimension measurements of projected image at a predetermined number of points on mask  14 . These predetermined number of points are preferably previously identified by given X- and Y-coordinates, for example. CD measurement tool  22  is coupled to a comparison tool or system  24 . The predetermined number of OPC mask critical dimension measurements are compared, by system  24 , with the same critical dimension measurements of the mask prior to the OPC step stored in a database  26 , for example. System  24  may comprise a microprocessor, a computer or another suitable processing device.  
         [0009]      FIG. 2  is a simplified flowchart of an embodiment of a method for photolithography mask critical dimension measurement. In step  30 , one or more masks  14  are put into position either on a stage or held in a holder  16  ( FIG. 1 ). In step  32 , light or radiation is directed through OPC mask  14  and further directed by projecting system  18  onto image capture system  20 . In step  34 , image capture system  20  receives the projected or aerial image of the OPC-modified patterns of the mask and captures as electrical signals. This projected image is used by a measurement tool such as a CD SEM  22  to make a predetermined number of critical dimension measurements at predetermined locations of the projected image in step  36 . Because the aerial image (either captured by sensors or by photoresist) of the OPC mask lacks the serifs and jogs, customary methods of taking critical dimension measurements may be used. The OPC mask projected image critical dimension measurements are then provided to a comparison system  24 , which compares the OPC mask measurements with the critical dimension measurements of a pre-OPC mask stored in a database  26  in step  38 . The database  26  comprises Graphic Design System (GDS) data. The database  26  comprises an image file of GDS data. The OPC mask should generate an projected image that is very similar or identical to the mask prior to the addition of OPC features. Therefore, any critical dimension measurement difference between the OPC mask projected image and the pre-OPC mask may be used to verify and correct the mask.  
         [0010]     It may be seen that, instead of making critical dimension measurements of an OPC mask, which includes OPC features such as serifs and jogs, the measurements are instead made on a projected image of the OPC mask. Such projected image lacks the serifs, jogs and other non-linear OPC features. Therefore, the critical dimension measurement and verification tasks become possible for semiconductor wafer manufacturing.  
         [0011]     Although embodiments of the present disclosure have been described in detail, those skilled in the art should understand that they may make various changes, substitutions and alterations herein without departing from the spirit and scope of the present disclosure. Accordingly, all such changes, substitutions and alterations are intended to be included within the scope of the present disclosure as defined in the following claims.