Patent Document

CLAIM FOR PRIORITY 
     This application claims the benefit of priority to German Application No. 102 61 323.0, filed in the German language on Dec. 27, 2002, the contents of which are hereby incorporated by reference. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to an apparatus and a method for determining physical properties of a mask blank, and in particular to an apparatus and a method for determining mechanical stresses and/or flexures of a mask blank. 
     BACKGROUND OF THE INVENTION 
     Mask blanks are used in machining tools for manufacturing integrated circuits in order to serve as an exposure mask for example in a stepper device. Usually, a mask blank comprises a carrier device, preferably made of glass having a thickness of about 6 mm, to which is applied a molybdenum-silicon nitride layer preferably having a thickness of 100 nm and, over the latter, a chromium layer likewise having a thickness of 100 nm as hard mask. Arranged above that is a photoresist layer having a thickness of 500 nm, for example. 
     In order to be able to serve as a patterned mask, the mask blank or the layers deposited on the glass must be patterned in various machining steps. This patterning is effected for example in pattern generators. For this purpose, the mask blank is quite generally clamped in a fixing or clamping device. Customary clamping devices clamp a mask blank by means of mechanical spring elements. This often leads to mechanical stresses or flexures of the mask blank and thus to a reduced quality of the patterned mask. 
     Hitherto, such dislocations and strains have only been detected or identified at the end of the process for producing the mask blank or the mask in the form of registration fluctuations, accuracy or position fluctuations and other faults. Mask blanks which lie outside a tolerance range have to be separated out by sorting. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus and a method for determining physical properties of a mask blank, whereby the quality assurance and/or inspection of the mask blanks is simplified and in situ determination is made possible. 
     The present invention essentially includes generating a tomogram of a mask blank by introducing a predetermined light and detecting the light which has been scattered in the mask blank, thereby localizing scattering sources for example as a result of flexures, mechanical stresses caused by the absorber layer or as a result of clamping the mask blank or on account of inclusions or impurity particles. 
     In the present invention, the problem mentioned in the introduction is solved, in one embodiment, by providing an apparatus for determining physical properties of a mask blank having an illumination device for radiating a predetermined light laterally into the mask blank; a detection device opposite the illumination device for detecting the light which has been scattered and/or runs through the mask blank; and an evaluation device for determining physical properties of the mask blank from the light which has been scattered and/or has run through the mask blank and has been detected in the detection device. 
     In this way, a tomogram of the mask blank is generated in the evaluation device, preferably a computer, thereby characterizing predetermined properties or the state of the mask blank. 
     In accordance with one preferred embodiment, the illumination device has a laser device or laser diodes. 
     In accordance with a further preferred embodiment, the illumination device, the mask blank and the detection device are accommodated in a mask production tool. 
     In accordance with a further preferred embodiment, the illumination device and the detection device are arranged such that they are diametrically opposite and rotatable by at least 180° in the plane of the mask blank around the mask blank situated inbetween. 
     In accordance with a further preferred embodiment, the illumination device and the detection device are arranged pointing inward in a recess, in which the mask blank is arranged. 
     In accordance with a further preferred embodiment, laser diodes as illumination device and laser detectors as detection device are arranged alternately next to one another, in which case, preferably, the laser diodes are connected in series and the light which runs through the mask blank can be detected simultaneously by the laser detectors. 
     In accordance with a further preferred embodiment, a data analysis of the light detected by the detection device on the basis of a Fourier transformation for the purpose of creating a characteristic of the mask blank can be provided in the evaluation device. 
     In accordance with a further preferred embodiment, the mask blank is fixed in a fixing device with an adjustable clamping force. 
     In accordance with a further preferred embodiment, the fixing of the mask blank in the fixing device is provided by piezoelements, preferably tube piezoelements. 
     In accordance with a further preferred embodiment, the clamping force of the fixing device of the mask blank is adjustable depending on the evaluated predetermined properties of the mask blank. 
     In accordance with a further preferred embodiment, the fixing device of the mask blank has at least three separately drivable clamping elements, preferably piezoelements. 
     In accordance with a further preferred embodiment, scattering sources, such as, for example, inclusions or mechanical stresses and/or flexures in the mask blank are calculated and localized in the evaluation device on the basis of the received data of the detection device. 
     In accordance with a further preferred embodiment, on the basis of the data of localized scattering source brought about by a mechanical flexure, an adjustable fixing device, which clamps the mask blank, is driven in such a way that the flexure is reduced. 
     In accordance with a further preferred embodiment, the detection of the light which runs through the mask blank in the detection device is effected by utilizing the Kerr effect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention are illustrated in the drawings and explained in more detail in the description below. 
       In the figures: 
         FIG. 1  shows a diagrammatic oblique view of an apparatus for determining physical properties of a mask blank for elucidating a first embodiment of the present invention. 
         FIG. 2  shows a diagrammatic oblique view of an apparatus for determining physical properties of a mask blank for elucidating a second embodiment of the present invention. 
         FIG. 3  shows a diagrammatic oblique view of a mask blank tomogram for elucidating the functioning of one embodiment of the present invention. 
         FIG. 4  shows a diagrammatic oblique view of a dynamic clamping device for a mask blank for elucidating a detail of one embodiment of the present invention. 
     
    
    
     In the figures, identical reference symbols designate identical or functionally identical constituent parts. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a mask blank  10 , which essentially comprises a carrier device  11 , e.g. a glass body having a thickness of about 6 mm with a rectangular base area, and a layer sequence  12  deposited thereon. The layer sequence  12  on the carrier device  11  has, by way of example, a molybdenum-silicon nitride layer, at the periphery a chromium layer and in the vertical direction a terminating photoresist layer. 
     An illumination device  14  oriented toward a side area  13  of the mask blank  10 , preferably a laser, radiates a predetermined light  15  into the mask blank  10  essentially perpendicularly to the side wall  13  of the mask blank  10 . The light  15 , running essentially parallel to the layer sequence  12 , runs through the mask blank  10  and is if appropriate scattered at dislocations, strains, other deformations such as mechanical stresses or flexures or at defects or impurity particles in the mask blank  10 . Light  16  which runs through the mask blank  10  or is scattered therein is received in a detection device  17 . The detection device  17  is diametrically opposite the illumination device  14 , the mask blank  10  being arranged such that it lies inbetween. The illumination device  14 , the detection device  17  and the mask blank  10  lie in one plane. 
     In order now to be able to create a tomogram or a map of the blank interior the illumination device  14  coupled with the detection device  17  is moved at least by an angle α of at least 180° around the mask blank  10 , so that in an evaluation device  18 , e.g. a computer, scattering sources present in the blank material and the position of said scattering sources can be computationally deduced from the combination of the emitted and detected light beams  15 ,  16 . 
     The laser light  16  scattered in the mask blank  10  is stored in a location-dependent manner in the computer  18 . The blank map thus generated may then be used as a reference for any further tool (pattern generator, metrology, etc.) or as a basis for corrections. A map generated in this way is likewise suitable for identifying the mask blank quality, in which case this determination of the predetermined properties such as e.g. mask blank flexures and included particles in the mask blank  10  may likewise be carried out during the machining process of a mask blank  10  in a blank machining tool. 
     An evaluation of the data of the detection device  17  in the evaluation device  18  is preferably effected on the basis of a Fourier transformation. The evaluation device  18  not only processes and analyzes the data of the detection device  17  but preferably likewise controls the illumination device  14  and thus the incident light  15 . 
     In a second embodiment in accordance with  FIG. 2 , the mask blank  10  is identical to the mask blank described with reference to  FIG. 1 . In accordance with the second embodiment, an illumination device  14  comprises a plurality of light sources  19 , e.g. laser diodes. A detection device  15  likewise comprises a plurality of detectors  20 , preferably laser detectors, which, like the laser diodes  19 , are arranged in a manner distributed around the mask blank  10  in one plane with the mask blank  10 . The illumination device  14  and the detection device  17  are preferably arranged internally in a recess  21  for receiving the mask blank  10 , both the illumination device  14  and the detection device  17  being positioned essentially perpendicularly to the side areas  13  of the mask blank  10 . 
     The laser diodes  19  and the laser detectors  20  are preferably arranged alternately next to one another. The laser diodes  19  are connected in series, and the detectors  20  simultaneously receive the laser scattering waves which have been scattered in the mask blank  10 . The data supplied by the detectors  20  are preferably detected or evaluated by utilizing the Kerr effect. In this case, too, as described with reference to  FIG. 1 , a data analysis of the output data of the detectors  20  is carried out in an evaluation device  18 , preferably a computer, e.g. with the aid of the Fourier transformation, a blank characteristic, tomogram or a map of the mask blank  10  being created from the items of information generated. Such a tomogram may thereupon be used, in each step for producing the mask blank  10 , as a reference for measurement data or blank alterations in the individual process steps. 
     If the mask blank  10  is fixed in a fixing device (not illustrated) with an adjustable clamping force, then a flexure of the mask blank  10  possibly brought about by the clamping may be identified and corrected if the evaluation device  18  is connected to the adjustable clamp or fixing device. 
       FIG. 3  illustrates a diagrammatic oblique view of a blank map  10 ′, distortions, mechanical stresses in the mask blank  10  or flexures  22  of the mask blank  10  being illustrated by the flexed grid arrangement. In order to ensure a high quality standard of the mask blank  10 , it is necessary to avoid distortions or flexures  22  of the mask blank  10 , which can be identified in a blank map  10 ′. 
       FIG. 4  illustrates a detail, namely a fixing device  23  in the form of adjustable clamping elements, preferably piezoelements such as tube piezoelements, for example. The mask blank  10  is preferably fixed by means of three piezo devices  23 , which can each be driven variably by means of a separate DC voltage  24 . If an apparatus according to the invention, for example according to the embodiment as shown in  FIG. 1  or  2 , has localized a mechanical strain  22  of the mask blank  10  with the aid of a blank map  10 ′ in accordance with  FIG. 3 , then the clamping elements  23 , which can be driven independently of one another, can act on the mask blank  10  in such a way that the mechanical strain or flexure  22  is corrected or reduced. Application of a voltage  24  to the tube piezos used as clamping or fixing device  23  causes them to expand. A tube piezoelement has a tube structure in this case, the adjustable DC voltage  24  being connected to the outer wall, i.e. the tube exterior, by one pole and to the inner wall, i.e. the tube interior, by one pole. The regulation of the DC voltage  24  may be provided externally for example with the aid of a potentiometer (not illustrated). In this way, a variable clamping force and thus the mechanical stress in a mask blank  10  can be controlled actively and, consequently, detected flexures of the mask blank  10  can be reduced. 
     Although the present invention has been described above on the basis of preferred exemplary embodiments, it is not restricted thereto, but rather can be modified in diverse ways. 
     Thus, the illumination device is not restricted to laser-based devices, but rather may also be provided for example by an illumination device with long coherent wavetrains such as a mercury vapor lamp, for example. Furthermore, geometries other than rectangular arrangements and thus also geometries of the illumination device or of the detection device such as a circular configuration, for example, are also conceivable, in principle.

Technology Category: g