1. Field of the Invention
This invention relates to an optical exposure apparatus for projecting a reduced pattern of an original mask pattern precisely onto a desired position of a workpiece to be exposed, and more particularly to an improvement of an alignment system of such an apparatus for precise alignment of the original mask pattern and the workpiece.
2. Description of the Prior Art
Semiconductor devices, such as transistors or integrated circuits, are usually fabricated by a series of diffusion steps. Each diffusion step involves applying a coat of photosensitive polymer, known as photo-resist, over a silicon dioxide layer on the surface of the semiconductor substrate. Then, the photo-resist is exposed to light through a mask having a predetermined pattern by means of an optical exposure apparatus. When the photo-resist is photographically developed, selected areas of the photo-resist are removed to expose the underlying silicon dioxide. The exposed silicon dioxide is then removed by an etching fluid which does not attack the photo-resist to expose the underlying semiconductor material. The photo-resist is then stripped from the silicon dioxide and impurities are diffused into the areas of the semiconductor material exposed by the openings in the silicon dioxide layer. A new silicon dioxide layer is either grown over the exposed portion of the semiconductor material during the diffusion process, or is subsequently deposited, and the above-mentioned steps are repeated as often as required for the formation of the desired circuitry.
As the above-mentioned optical exposure apparatus, a so called "step and repeat camera" is usually used. A schematic block diagram of the step and repeat camera of the conventional type is shown in FIG. 1.
Referring now to FIG. 1, the camera is principally composed of a light source 6, a condenser lens 7, a mask 1, a reduction lens 8, a moving table 4, and a table position control circuit 3. In actuality, the moving table 4 can be moved in the X and Y directions, but, for simplifcation, the table movement in the Y direction is not shown in FIG. 1.
In operating the camera, firstly, the mask 1, which has an original pattern 1a formed by an opaque material, is set on a stationary holder H.. And, a semiconductor wafer 2 is placed on the table 4. Then, the table position is adjusted so that the desired exposure position of the semiconductor wafer 2 is precisely located at the projection position of the reduced pattern of the original pattern. That is, the table position control circuit 3 controls the table position in response to a control signal S1 supplied from a control signal generator 9, so that a proper alignment is obtained. When this operation for controlling the table position has been finished, a signal S2 is delivered to an exposure control circuit 5, and then the light source 6 begins to operate in response to an exposure control signal S3 supplied from the circuit 5.
A dispersed light beam l.sub.1 from the light source 6 is formed into a parallel light beam l.sub.2 by the condenser lens 7, and then the mask 1 is irradiated by this parrallel light beam. A light beam l.sub.3 passing through the mask 1 is projected on the wafer 2 through the reduction lens 8, so that the photo-resist layer disposed on the wafer 2 is exposed by the reduced pattern of the original pattern.
In the above-mentioned conventional optical exposure apparatus, the accuracy of the alignment of the exposure position and projection position is decided approximately according to the accuracy of positioning the table 4. Therefore, in order to increase the accuracy of the fabrication of the semiconductor devices, it is necessary to provide a X-Y moving table having a very high accuracy. As a result, the moving table becomes very expensive and heavy in weight. Additionally, the moving speed of the table is restricted to small a value because of its heavy weight. In a result, the time period required for one time of exposure becomes long, and hence the efficiency of the fabrication decreses. Furtheremore, it is impossible to ignore the decrease of the accuracy of the table positioning under the influence of the changes of the circumferential conditions, such as temperature, during the exposure step.