The invention relates generally to a particle beam projection system for exposing a substrate through a mask. More particularly, the present invention relates to an alignment system for a particle beam projection system which is adapted for rapid and accurate registration of a mask and a substrate prior to exposure of the substrate to a beam of particles.
The tendency of integrated circuits in semiconductor technology is toward ever-decreasing structure dimensions in order to increase the density of the circuits and their switching speed. Photolithography, which is still used today in the majority of cases, is approaching the limits dictated by the physical resolution of optical systems. Structures having conductive lines in the submicron range of less than 1.mu. cannot be made with optical systems. The most promising methods for the production of such fine structures are particle beam processes. The following specification refers specifically to electron beam systems but the projection system described herein can also be applied analogously to other particle beam projection systems, such as ion beam.
For this kind of use, electron beam systems have a number of advantages: the resolution of the patterns made with them is not limited by diffraction effects; they can be made with high intensity and they are deflectable with relative ease and high precision.
The most progressive developments in this field operate in accordance with the raster or scanning principle. The electron beam is used as a very fine "pencil" with which the pattern to be exposed is directly written onto a semiconductor substrate coated with an electron beam sensitive layer. The pattern to be produced is provided in the storage of a computer controlling the deflection of the electron beam. The high flexibility of this type of pattern generation, however, involves a high amount of writing time. The throughput of exposed wafers in industrial production is therefore low.
In the production of circuits and circuit chips (chips) having a plurality of repeatedly appearing circuit elements, e.g. memory chips, the flexibility of the raster method is of secondary importance. On the other hand, costs can only be reduced by high chip throughput. Electron beam projection methods utilizing a mask and operating analogously to optical photolithographic methods offer this high throughput since their larger pattern areas are imaged on the substrate by means of the mask through electron radiation. Such systems are known as described in the publications by H. Koops et al., Optik 28, 5, 1968/1969, pp. 518-531; T. W. O'Keeffe, Solid State Electronics, Pergamon Press, 1969, Vol. 12, pp. 841-848; M. B. Heritage, "Electron Projection Micro Fabrication System", Journal of Vacuum Science Technology, Vol. 12, 1975, pp. 1135-1138 and U.S. Pat. No. 4,169,230 and copending U.S. patent application Ser. No. 70,453 filed Aug. 28, 1979, now U.S. Pat. No. 4,334,156 both assigned to the assignee of the present application.
The problem of increased production of highly integrated monolithic circuits, however, involves not only the possible resolution through the exposure method used but also the precision of the mutual alignment of mask and semiconductor substrate in each exposure step required during a manufacturing process. For achieving good overlapping (so-called "overlay"), registration has to be very precise and therefore requires a considerable amount of time. Registration is the detection of structures existing on the wafer prior to exposure, and the alignment of the pattern to be imaged (mask) relative to the existing structure.
Patterns for alignment (alignment marks) in electron beam processes are either markings of a material differing from that of the semiconductor substrate, and/or areas of a particular geometric design, e.g. edges. The impinging electron beam releases in these areas secondary electrons whose reflected or substrate-absorbed part can be measured and utilized as a signal.
For achieving a high signal to noise ratio the electron beam impinging at the alignment marks should have a high current density (see W. Stickel, "Method of optimizing registration signal for electron beam microfabrication" in Journal of Vacuum Science and Technology, Vol. 15, No. 3, May/June 78, pp. 901-905). This alignment method operates satisfactorily for raster processes. In mask exposure, however, the projection methods operate with an expanded electron beam of a relatively low current density. The secondary electrons released by this beam at the conventional alignment marks provide very low registration signals with a high noise factor. Prior art mask methods therefore had to adopt specific measures, which will be described below, to ensure satisfactory registration in spite of these obstacles.
One possible improvement consists in the multiple deflection of the beam over the mark with subsequent electronic integration for improving the signal to noise ratio, but this process is time-consuming.
Another suggestion involves switching the electron beam from one exposure mode (with low current density) to a registration mode with high current density (see the article by Heritage). However, the switching of the beam path is difficult to accomplish since the two beam paths cannot always be reproduced.
According to another suggestion (8th International Conference on Electron and Ion Beam Science and Technology, May 1978, page 984, Frosien et al.), switching is abandoned and mechanical diaphragms are pivoted into the beam path instead. This avoids the switching between the two modes of operation but the problems with the still insufficient current density remain. Furthermore, such mechanical adjustments are not realizable within very short periods.
Other important parameters in electron beam-lithographic methods include the method by which registration signals are detected and the geometric relations. The evaluation of the electrons reflected at alignment marks cannot always be used in shadow printing methods with electron beams since the distance between the mask and the semiconductor wafer is small (typically 0.5 mm) so that detector devices for electrons cannot be easily inserted. This differs from systems where there are electron-optical imaging means between the mask and the semiconductor wafer.
For automatic as well as manual registration, a method for quickly reaching a registered position should be provided. For this purpose the relative position of the objects at any moment and the direction and shift of the objects to reach the registered position must be known. In German Auslegeschrift 20 46 332, an alignment pattern is described for a photoelectric device, where the spacings between two respective mask openings are not constant. The spacing of the openings corresponds to an arithmetic series. For two-dimensional alignment, two rows of such mask openings arranged vertically to each other are provided. This arrangement has the disadvantage of a low transparency (ratio between the surface of the openings to the overall surface) so that if subjected to electron beam exposure systems the signal to noise ratio would be low. This arrangement also requires two detectors.
It is therefore the object of the present invention to provide a particle beam projection system which operates with mask projection and which permits a quick, precise and automatic registration. This object is achieved by the invention characterized in the main claim; embodiments of the invention are characterized in the subclaims.