Patent Number: 
Section: description

Referring to FIG. 1, a typical projection electron beam lithography system is generally shown at 10. Lithography system 10 comprises an exposure column unit 12 and a control unit 14. The exposure column unit 12 includes an electron beam generator 16 having a cathode 18, a grid 20 and an anode 22. A slit 24 is provided in the exposure column unit 12 for rectangular shaping of the electron beam. A lens 26 is provided for converging this shaped beam. A slit deflector 28 is provided for deflecting a position of the shaped beam to a mask 30 based on a deflection signal. The mask 30 is mounted movably in a horizontal direction between two opposing lenses 32 and 34. Deflectors 36-39 are provided to deflect the beam between lenses 32 and 34 based on position information to select an aperture in the mask 30. The exposure column unit 12 further includes a blanking aperture electrode 40 for cutting off or passing the beam in response to a blanking signal, a lens 42 for converging the beam, an aperture 44, a refocus coil 46 and a lens 48. Also, a dynamic focus coil 50, a set of dynamic stigmator coils 52, an objective lens 54 projecting the beam onto a wafer 55 are provided. A main deflector 56 and a sub deflector 58 position the beam on the wafer in response to exposure position signals. A stage 60 for carrying the wafer and moving it in X-Y directions and alignment coils 62-65 are provided. Such is merely exemplary of a typical exposure control unit. The control unit 14 includes a processor 72 and associated memory having stored design data. Control management for each of the aforementioned components is provided by control unit 14, as is well known. Details of the control management are well known and are not the subject of the present invention. Such lithography systems are well known and the above is provided for illustration purposes only, it is not in any way intended to limit the present invention which is applicable to lithography systems/tools in general. In the prior art, pattern resolution in the system was maximized using a test pattern on a mask. This test pattern would have geometries (e.g., lines or elements) which are smaller (or finer) than the lines or elements of a production pattern on a production mask. In the present invention, the system maximizes pattern resolution using a test mask having test pattern geometries that are the same size as (and even larger than that of) the geometries of a production pattern of a production mask. However, unlike in the prior art, the test wafer is exposed to the test mask image for a period of time shorter than a product pattern exposure. It has been found that image quality differences are more easily detected at these lower exposure dose levels, just as they are also more easily detected with smaller (finer) geometries in the prior art. Adjustments to the system during setup are made in the same manner as in the prior art, such being well known, e.g., current changes to lenses and correction coils, deflection positioning and stage positioning. In this way, smaller geometries for the pattern of the test mask are not required. The ease in detection of image quality differences is evidenced with reference to FIGS. 2-4, where a five-by-five test patten of a x+ symbol is shown with the in-axis stigmator varied by row and the off-axis stigmator varied by column. In FIG. 2 the test pattern is exposed at 1.0xc3x97nominal dose and the x+ symbols all appear to be of similarly good image quality. However, in FIG. 3 the test pattern is exposed at 0.9xc3x97nominal dose and the x+ symbols at the perimeter are clearly of a lesser quality than the x+ symbols at the center. One can see that the rectangles that make up the x+ symbols are no longer of uniform width and quality. This is even more prominent in FIG. 4 where the test pattern is exposed at 0.8xc3x97nominal dose. At this level information is lost at the perimeter and the apparent image quality degrades the further away from the center these features are located. A review of these FIGS. 2-4 clearly shows that evaluating the image quality at a below nominal dose level would enhance the sensitivity to stigmator adjustment errors. This enables an operator to make more accurate stigmator correction adjustments and thereby improves the resolution for product patterns exposed at the nominal dose levels. In other words, adjusting the system at the 0.8xc3x97nominal dose (FIG. 4) for optimal image quality would provide better ultimate pattern resolution than adjusting the system at 1.0xc3x97nominal dose (FIG. 2), since small errors are more readily apparent. The exposure dose of these symbols (features) at 10%-20% below that required for normal resolution, is such that the bottoms of the processed features are just on the verge of clearing to the substrate interface after normal development. In this state small variations in the system image quality have a dramatic effect on the perceived quality of the developed feature (symbol). Testing of this process on an electron beam projection lithography tool with a particular resist process has shown that by dosing a stigmator varied pattern with 200 nm features at xcx9c85% of the nominal dose, the clearing of the features is so marginal that one can easily discern the best image quality parameters using an optical microscope for evaluation. Scanning electron microscope evaluation of 80 nm lines and spaces printed after image quality adjustment confirmed these results. For other resist processes the optimum test pattern dose would have to be determined by experimentation, but should be in the range of 75-90% of the nominal dose. Referring to FIG. 5, a simulation of developed trenches is shown at a nominal dose and at 0.75xc3x97nominal dose at focus settings from xe2x88x9210 microns to +10 microns. At the nominal dose the developed trenches for all focus settings in the range xe2x88x926 microns through +6 microns from the nominal image plane would look similar based on top down optical microscope evaluation. At 0.75xc3x97nominal dose for the resist conditions simulated here a noticeable difference in the trenches occurs at focus settings of xe2x88x924 microns and +4 microns from the nominal image plane, further evidencing the advantages of the present invention. While the above exemplary embodiment is direct towards lithography using a mask with an electron beam such is equally applicable to lithography using a mask with an ultraviolet light (beam) and direct write lithography with an electron beam. In each of these lithography systems, the system can be adjusted to maximize pattern resolution using the method (process) of the present invention. In direct write lithography a test pattern is written at a reduced exposure dose, with such information being used for setup of the system. Also in developing a pattern on a mask (a photolithographic mask) using an electron beam, a test pattern can be projected at a reduced exposure dose, with such information being used for setup of the system. While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.