Abstract:
A method of correcting physically conditioned errors in the measurement of an object detects an image of the object to be measured, measures the imaged object, determines a measurement error caused by structural surroundings of the object, and corrects the measurement result in dependence on the measurement error.

Description:
FIELD OF THE INVENTION 
     The present invention refers to a method of correcting physically conditioned errors in the measurement of an object, and especially to a method of correcting physically conditioned errors in the measurement of the width of microscopic objects whose dimensions are of the same order of magnitude as the wavelength of the illumination source. 
     BACKGROUND OF THE INVENTION 
     The further development in the field of semiconductor technology leads to ever finer chip structures and this entails increasing circuit complexity. In addition to problems in the production of such structures, problems also arise in the examination of such structures, viz. when the structures produced are imaged for checking them, e.g. by measuring the width thereof. An example for a conventional method is the measurement of structure widths making use of optical microscopes, the object to be measured being illuminated in transmitted light or incident light and the resultant intensity image being subsequently observed and measured. 
     These methods are disadvantageous insofar as the intensity image is composed of superpositions originating from the structure to be measured as well as from structures which are arranged in the vicinity of the structure to be measured. It follows that the intensity image of the structure to be measured and therefore the measurement result are directly influenced and corrupted by the surroundings of the object to be measured. This phenomenon is known under the name of “proximity effect”. Due to the corruption of the intensity images, it is impossible to make any reliable statements with regard to the shape and the dimensions of the structures produced and therefore the reliability of the production process, when the width of the object is measured. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide a method of correcting physical errors in the measurement of an object so that measurement errors originating from influences of neighbouring structures are avoided. 
     A method of correcting physically conditioned errors in the measurement of an object, includes 
     (a) detecting an image of the object to be measured; 
     (b) measuring the object imaged in the detected image; 
     (c) determining a measurement error caused by structures which are arranged in the vicinity of the object to be measured; and 
     (d) correcting the measurement of step (b) in dependence on the measurement error determined. 
     The present invention is based on the finding that the corruption of the intensity image and the resultant effect on the measurement result—the corruption being produced by the surroundings of an object to be measured when this object is being irradiated or illuminated—can be corrected by correcting the image of the object to be measured with due regard to the error contributions from the object surroundings. Starting from the image produced in this way, the influence of the structural surroundings or object surroundings on the measurement result is eliminated. The measurement result corrected in this way will then correspond to the actual physical dimensions of the object to be measured. 
     According to one aspect a correction method is provided in the case of which, starting from the image produced, an intensity image of the object to be measured and of its surroundings is detected, whereupon a correction value is determined from the global image especially for the structure to be measured, the correction value depending on structures which are arranged in the vicinity of the object to be measured. Finally, the intensity image is measured and corrected in dependence on the value determined. The result of this measurement is then always a measurement value which has undergone correction and which therefore approaches the actual magnitude of the object to be measured more closely. 
     According to a further aspect, the detected image or intensity image contains the object to be measured and the structural surroundings thereof, so that, when the correction value or correction values is/are being determined, the measurement errors are determined and corrected in dependence on the structural surroundings contained in the intensity image detected. 
     According to a further aspect, the detected image or intensity image contains the object to be measured and the relevant surroundings thereof, which have an influence on the correction value to be calculated. Furthermore, a layout description of the object to be measured and imaged, respectively, and of the structural surroundings thereof is provided, and the correction factor for the measurement result is determined only in dependence on the layout description provided. 
     According to a further aspect, the microscopic object imaged as an image or intensity image is measured and the actual intensity image is approximated to the ideal conditions by means of a correction function. 
     In addition to a light source, also other radiation sources, such as an electron beam source or an X-ray source, are suitable to be used as an irradiation source. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, preferred embodiments of the present invention are described in detail making reference to the drawings enclosed, in which: 
     FIG. 1 shows a flow chart representing a first embodiment of the method according to the present invention; and 
     FIG. 2 shows a flow chart representing a second embodiment of the method according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the present invention, a method is provided, which permits a correction of physically-conditioned errors in the measurement of an object; in the description of the preferred embodiments following hereinbelow, the method for optical microscopy is described as an exemplary method; to a person skilled in the art it will, however, be obvious that the method according to the present invention can be used for all image-forming measurement methods. 
     Quite generally speaking, an image of the object to be measured is produced according to the present invention, the imaged object is measured, and the measurement result is corrected with due regard to the object surroundings so that the corrected measurement result then corresponds to the actual physical dimensions of the object to be measured and can be used reliably in the case of further processing. 
     FIG. 1 shows a first flow chart representing a first embodiment of the method according to the present invention. In a first step  100 , the object to be measured is illuminated by means of light, and in step  102  an image of the object to be measured is produced. Following this, an intensity image is produced in step  104  from the image produed in step  102 , and the imaged object is measured in step  106 . Subsequently, correction calculation in dependence on a measurement error determined takes place in step  108  and the corrected measurement result is outputted in step  110 . According to the embodiment shown in FIG. 1, the possible measurement error is determined in step  112  in that the measurement error or the possible measurement error caused by the structural surroundings of the object is calculated in dependence on the image produced in step  102  and is then supplied for correction calculation in step  108 . 
     In the case of optical microscopy, the transfer/transmission characteristics of the measurement microscope are calculated on the basis of the characteristic data of the microscope, viz. the wavelength and the spectrum of the illumination source, the type of illumination (incident light or transmitted light), the magnification factor, the objective values (numerical aperture and sigma). 
     Taking these calculated transfer characteristics as a basis, the result of the measurement carried out in step  106  is corrected in step  108  in such a way that the effects of the optical “proximity effect” on the measurement result are eliminated so that the measurement value determined corresponds to the actual physical dimensions. 
     With regard to the present invention, reference is made to the fact that this invention is used especially for objects to be measured having dimensions in the case of which the imaging processes used and the structural density cause the above-mentioned measurement errors, e.g. line broadening and the like. These objects have dimensions in the microscopic range, which are of the same order of magnitude as the wavelength of the illumination source, e.g. 0.5 æm to 10 æm. 
     Making reference to FIG. 2, a second embodiment of the method according to the present invention is described, the blocks and steps, respectively, which have already been described on the basis of FIG. 1 being designated by the same reference numerals. The present embodiment differs from the embodiment described in FIG. 1 insofar as the calculation of possible measurement errors is carried out independently of the image produced in step  102 . According to the present embodiment, the data required for determining the measurement error are received in step  200  from an external source independently of the actually produced image. These external data include e.g. the original structure, for example in the form of a layout description, which represents the expected value of the object to be measured including its structural surroundings. On the basis of these data, the possible measurement error is calculated and the correction which has already been described making reference to FIG. 1 is carried out. 
     In the embodiment shown in FIG. 2, the image can, according to one implementation, be produced in step  102  in such a way that only the object to be imaged is detected, without detecting the structure surrounding this object. The correction of the measurement errors caused by the surroundings is effected in the manner described hereinbefore on the basis of a calculation of a possible measurement error in step  200 . 
     In the case of both methods, correction can take place in step  108  in dependence on a specific correction function which approximates to the ideal conditions. 
     It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.