For the lithographic generation of patterns with elements in the micron and submicron range, such as integrated semiconductor circuits, conventional photolithographic methods reach their physical limits that are set by the light wave length. If corpuscular beams, such as electron or ion beams, are used for exposure, then the limits are extended far into the submicron range; on the other hand, at such small dimensions the problems connected with mask accuracy and purity and errors caused by the lithographic system itself during exposure are aggravated.
Of the previously tested corpuscular beam lithographic processes and means, so-called proximity printers, wherein a transmission mask is imaged by a wide diameter electron beam at a 1:1 scale on a substrate covered with an electron-sensitive layer, are of particular interest. Such arrangements afford a high throughput and thus an inexpensive production process. Details of the related processes are described, for instance, in the article "Electron-beam proximity printing--a new high-speed lithography method for submicron structures" by H. Bohlen et al. in IBM J. Res. Devel., Vol. 26, No. 5, September 1982, p. 568 et seq. That method provides for the substrate to be shifted in steps, generating in each case a mask shadow projection image corresponding, for example, to a complete semiconductor chip. Mask errors, impairing the operation of the semiconductor circuit, may lead to the entire exposed substrate becoming unusable.
Therefore, such processes necessitate that the mask and the imaging characteristics of the entire lithographic system be tested to ensure that they are error-free, focusing in particular on the following errors:
erroneous mask patterns, PA1 mask distortions, PA1 impurity particles on the mask, PA1 errors in the imaging system.
Mask distortions may occur, for example, if the mask is unduly heated during its manufacture or by the incident beam. Errors of the imaging system affect, for instance, in an uncontrollable manner the inclination of the parallel beam incident on the mask, thus leading to displacements in the shadow image. Causes of such errors may be insufficiently homogeneous magnetic fields or changes in the magnetic field by induced eddy currents occurring when a relatively thin electron beam is made to rapidly raster-scan the mask for its full illumination.
For compensating for geometrical mask errors, the inclination of the electron beam is locally altered during the above-described raster illumination, so that the shadow image is displaced accordingly. This method necessitates, however, that the mask errors be known with great precision. With steadily smaller structures, such measurement is extremely elaborate and expensive, the main difficulty being that the apparatus required for measuring is not readily controllable with regard to error sources. Moreover, distortions of the shadow projection image, that are caused by inaccuracies of the remaining lithographic system rather than by mask errors, cannot be controlled in that way.
For accurately measuring masks with submicron structures, the probes used must be extremely fine, in order to reach the desired resolution. With such fine probes, for example, a strongly focused electron beam, the local thermal load on the tested mask increases, or the measuring signal available is extremely low and subject to much noise.
The transmission masks necessary for shadow projection corpuscular beam lithgraphy represent the mask pattern as physical apertures (hole pattern), as no material is known that is completely transparent to corpuscular beams. Such masks are particularly sensitive to impurity particles which even in hyperpure rooms settle on the mask, thus drastically reducing the yield of very large scale integrated circuits. Protecting a mask against impurity particles by protective foils, as is done in optical photolithography, is, therefore, ineffective, with corpuscular beams.