The invention pertains to a device for projection-copying of a mask onto a workpiece, in particular a semiconductor substrate for the production of integrated circuits, in which case the pattern of the mask is projected via a projection lens onto a photosensitive layer of the workpiece after in order to align the mask and workpiece relative to one another alignment patterns of the mask and adjustment areas on the workpiece have been projected onto each other by means of an adjustment light with a bandwidth of at least 3 nm through the projection lens.
Projection lenses for the lithographic production of integrated circuits are charcterized by a large picture field with a diameter which typically lies between 10 and 30 mn and a large numerical aperture when there is a great resolution capability which is limited with respect to diffraction. Because of the necessity of projecting different pictures one on top of the other in such a way that they precisely cover one another, the distortion in the entire field must not exceed 0.2 .mu.m and the picture field must be completely level, i.e., any convexity must not exceed 0.3 .mu.m.
At the present state of the art, lenses which meet such stringent requirements can be corrected up to the diffraction limit only for a very narrow wavelength range, which is to be understood as a bandwidth of a few nm.
This narrow correction area is selected in such a way that within it the sensitivity of the photosensitive resist is as high as possible and, on the other hand, an appropriate source of illumination is available. Typical correction ranges are 406 nm.+-.4 nm or 436 nm.+-.4 nm, corresponding to the most intense spectral lines of mercury discharge lamps.
As mentioned above, it is vital for the projection of the pattern of the mask to take place not only with good picture quality, but also with complete precision of positioning. The precision with respect to the lateral coordinates (X, Y, .theta.) is necessary in this case in order for the successive patterns to be correctly allocated, but in addition it is also necessary to precisely focus an entire picture plane since the depth of focus of the above-described lenses is very slight.
The alignment of the mask and workpiece is preferably carried out through the lens itself, and in this process the adjustment areas for lateral adjustment are defined by marks of the most widely varying structure. Per se, the unaltered reflective surface of the workpiece itself is sufficient for focusing.
Focusing as such does not necessarily require that the workpiece be observed through the lens. For instance, it is a known process to determine the distance between the lens and the workpiece by means of capacitive sensors with the aid of the flow resistance which the annular gap between the lens and the workpiece presents to a discharging gas, or from the eigen-frequency of an air resonance section between the workpiece and the lens. A prerequisite in this case, however, is that there be a very short distance between the lens and the workpiece, but such a short distance is precisely what is avoided through the use of projection illumination procedures, in contrast to the obsolete contact procedures. When optical focusing is carried out without the use of the projection lens, a tightly packed beam of light is obliquely directed onto the center of the field to be illuminated and the point where the reflected light beam strikes is observed; the location of this point is a measure of the location of the illumination field itself. The light used is frequently laser light (a HeNe laser). A disadvantage in this case is the fact that it is impossible to distinguish whether the change in the position of the light point on the receiver is caused by a change in the angle of incidence or the Z position. In practice, it is assumed that the angle of incidence is constant, i.e., that the reflective wafer surface is always vertical to the optical axis. This condition is never rigorously met; sometimes there are even large deviations from the ideal position. When laser light is used, another problem arises due to diffraction effects at the wafer surface (speckle), in particular if the surface has already been structured (higher manufacturing stages).
Since, when adjustment is done through the projection lens, the system-related disadvantages of the above-described procedures do not arise, there is great interest in solving the sub-problems which still exist with this type of adjustment. When adjustment is done through the projection lens, problems arise when the adjustment light is not identical to the illumination light for which the projection lens is corrected. Initially we think here of the case where the wavelength range of the adjustment light lies outside of the range of the spectral sensitivity of the photosensitive resist to avoid having the marks on the workpiece be destroyed by the adjustment process. The differences with respect to focal length and magnification which the lens shows depending on the type of light used can be compensated for by bending the adjustment beam through a pair of mirrors or by lengthening the beam by means of intermediate glasses or by shifting the location of the protector for adjustment. Since tests can readily determine the extent to which the focal length and magnification of the lens differ at the corresponding illumination wavelength and adjustment wavelength, overall the consequences of this difference can be easily handled; with respect to the position where the device is optimally aligned with the adjustment light, prior to illumination being carried out a shift is simply made which takes the differing behavior of the lens in the two cases into account.
Since, with an illumination wavelength which deviates from the adjustment wavelength, the picture defects of the lens can be corrected only when the bandwidth of the adjustment light is relatively small, it is assumed that the adjustment light used should, in principle, be narrowband. The frequenlty-made suggestion that laser light be used for adjustment purposes has not proven out in practice since, due to the coherence of this light, diffraction effects (speckle) occur which distort the measurement results. Generally mercury discharge lamp light, the natural line width of which is approximately 3 nm, is thus used for adjustment.
Surprisingly enough, it has been found that, when the adjustment is carried out with mercury light through the projection lens in the way described in the introduction, a nonsystematic error arises, the cause of which was found to be the fact that the reflection capacity of the workpiece is dependent on the processing stage of the workpiece and, in addition, that this capacity varies on the surface of the workpiece. Not only does the color of the reflected adjustment light deviate slightly from the color of the input adjustment light, but differences in the color of the reflected adjustment light appear from workpiece to workpiece and from mark to mark on the same workpiece. If it is assumed that, in addition to the spectral line itself, the adjacent area of the radiation background in a total width of, for example, 10 nm is also typically passed by the narrowband interference filter in front of the adjustment light sources, it is still amazing that the differences of 1-2 nm which arise overall in the wavelength of the reflected adjustment light still have an effect on the precision of the adjustment. Due to the heavy frequency dependency of the lens used, on the one hand, and the extreme demands imposed on adjustment precision, on the other, however, this is indeed the case.
The invention thus is based on the recognition that it is not sufficient, as was previously the assumption, to take into account the differences which arise, regarding the focal length and magnification of the lens, in the illumination wavelength on the one hand and, on the other, and adjustment wavelength which is assumed to be constant, but rather it is necessary to take into account the effect of the difference in the reflection behavior of the workpieces, which difference cannot be known in advance, and which leads to a change in the spectral composition of the input adjustment light.