The present invention relates to a method of and apparatus for correcting defects on a plane surface of a phase shift mask, and more particularly to a method of and apparatus for repairing defects by means of removing a bump defect which partially projects from a region to be flat or plane.
When an etching is performed with respect to the bump defect for a repair in order to remove the bump defect on the phase shift mask, it is necessary to stop the etching at a stage where the etching to a level as the same as a reference plane is performed. The reason why the etching is stopped is that, if the etching is continued over the necessity, there is generated a new region in which a phase changes, thereby resulting that the new region becomes a new defect. Accordingly, it is necessary to properly monitor an etching amount with respect to the bump defect, namely, a depth of the etching, and at the same time, it is necessary to properly monitor exact quantities of the etching to the reference plane corresponding to a reference phase. Therefore, the followings (A)-(D) have been proposed as main method for measuring a fine height and depth:
(A) Probe method;
The method is that a fine probe traces a bump and divot defect on a surface of a sample by moving with a contact to the sample surface with extremely small contact pressure, for example, several milligram (mg) or several microgram (.mu.g). Both of an STM (Scanning Tunneling Microscope) and an AFM (Atomic Force Microscope) measure a bump and divot defect by the basically same principle as the probe method.
(B) Stereoscopy method;
The method is that a sample is investigated from two different angles to obtain a two-dimensional electronic image, a characteristic point is extracted from the image, and a parameter denoting the bump and divot defect such as a height or incline angle of an edge is calculated on the basis of a discrepancy amount of the image by using the characteristic point as the standard. This method is widely used in a measurement of the shape processed in a semiconductor field, and disclosed in "M.Miyoshi, F.Komatsu, N.Matsuda and K.Okumura, Measurement of the geometrical dimensions in the VLSI using the scanning electron microscope, Proc. XIth Int.Cong. on Electron Microscopy, Kyoto, 1986, pp.657-658", for example.
(C) Double detector method;
The method is to calculate a slant angle .PHI. which is an angle between the sample surface and a virtual vertical axis, a height h and a section profile on the basis of strength signals a and b detected from two detectors A and B which are disposed at positions each symmetrical to an optical axis in accordance with the following equations (1): EQU tan .PHI.=k(a.sup.2 -b.sup.2)/(a.sub.N +b.sub.N).sup.2 ( 1)
Where k is a coefficient, and a.sub.N and b.sub.N are a signal strength of the detectors A and B at .PHI.=0.degree., respectively. A range of .PHI. is less than 75 degrees to establish the above equation (1). This is described in "Extended Abstract of the 32-th Spring Meeting of JSAP (1985), p.333(29p-k-9), 'A section measurement by SEM' written by Suganuma".
(D) Contamination method;
The method is a simple method for measuring a three-dimensional shape, in which a measured object is scanned along a straight line by a vertical incidence having a slant angle a=0.degree. against the sample to form a contamination line on a surface of the sample, then the sample is inclined, and parameters such as a slant angle .PHI. and a height h for representing the three-dimensional shape are obtained from a change amount of the contaminating line caused by an inclination of the sample. This content is disclosed in "R.A.Hoover, Measuring surface variations with the scanning electron microscope using deposited contamination lines, J. Phys. E. Sci. In strum 4, pp. 747-749, 1971".
It is desired to apply a so-called in situ measurement which measures a depth of the sample as it is without taking-out in a field of a sputtering (an ion milling) using a focused ion beam (IFB) generation apparatus and a beam assist etching using a charge beam such as an ion and an electron beam.
In the case where it is necessary to process by a focused ion beam in order to correct a defect of a phase shift mask, the in situ measurement is the only method for the measurement in order to increase a processing precision.
However, since it is necessary for the contact needle method or scanning probe method described in the above paragraph (A) to measure the sample in a different position by one taking out the sample, the method can not be called as the in situ measurement.
Furthermore, the stereoscopy method of the item (B) is easy to apply to the in situ measurement in view of the etching by the ion beam or the electron beam as a premise. However, even though he method is suitable for a measurement of the etching depth and an elimination of a defect or a deposit layer having the known height, the method is unsuitable for the etching to the defect and layer having an unknown height, namely, a requirement that the etching must be stopped at the point coinciding with the reference plane.
Still furthermore, the double detectors method of the item (C) is not practical because an application range of the measurable inclined angle .PHI. is limited less than 75.degree., and this method is also unsuitable for the etching without the occasion that the height of the defect has been previously known in the same manner of the stereoscopy method.
Moreover, the contamination method of the item (D) results a pollution of the sample because the contamination is positively added to the sample, thereby not applying in the semiconductor field.