Pattern forming apparatus
This invention relates to an apparatus for forming a pattern of a large scale integration semiconductor element by utilizing a fine pattern. 
     Of the processes utilized in fabrication of semiconductors, the lithography process for the formation of a fine pattern on a substrate is the most important and photolithography using light plays the leading role today in the lithography process. 
     In the lithography process, as larger integration of the semiconductor elements prevails, it is required to improve resolution of the fine pattern and pattern alignment necessary for the formation of the fine pattern at a desired position. 
     With the photolithography, however, the wavelength of light for exposure constrains the resolution to a limit in the formation of a fine pattern having a line width of about 1 μm. 
     In order to assure the pattern alignment, it is also required to detect the position of the pattern to be formed on the substrate with high precision. 
    
    
     Description will be made hereinbelow in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a fragmentary sectional view of a substrate useful in explaining disadvantages encountered in a prior art pattern forming apparatus; 
     FIG. 2 is a schematic diagram showing a fundamental construction of a pattern forming apparatus embodying the invention; 
     FIG. 3 is a schematic diagram of another embodiment of the invention; and 
     FIG. 4 is a graph useful in explaining effects brought about by the pattern forming apparatus of the invention. 
    
    
     Referring to FIG. 1, a pattern 10 formed on a substrate 1 and to be detected for aligning the substrate 1 is covered with a photoresist layer 2 with an uneven surface. Therefore, refraction of light for pattern detection becomes irregular, thus impairing precise position detection. 
     This invention contemplates the elimination of the above drawbacks and has for its object to provide a novel pattern forming apparatus effective to improve resolution of a fine pattern and precision of the pattern alignment. 
     According to one aspect of the present invention, there is provided a pattern forming apparatus for projecting a pattern, which is formed in advance on a reticle, upon a workpiece at a certain reduction ratio, comprising an illumination system for illuminating the pattern for forming an optical pattern image, reduction lenses for projecting the optical pattern image at a certain reduction ratio upon a photoresist layer formed on a substrate to expose the photoresist layer, and liquid sustaining means for filling a gap between at least a portion of the reduction lenses and the photoresist layer with an optically transparent liquid having a refractive index of more than 1 (one). 
     Now, description will be made in detail, by way of example, of preferred embodiments of this invention. 
     The fundamental principle of the present invention will first be described. In general, the resolution limit R (μm) of a pattern projection optics used for the pattern formation for semiconductor elements is expressed as, 
     
         R=0.61×λ/(n×sin θ) 
    
     where 
     λ: wavelength (μm) of light in a vacuum used for exposure, 
     n: refractive index of ambient atmosphere in an exposure system, 
     sin θ: fixed value of reduction lenses 
     n×sin θ: numerical aperture. 
     For larger integration of semiconductor elements, the pattern for the formation of the semiconductor element should be finely drawn and the resolution limit R of the pattern projection optics should be improved. 
     Accordingly, it has hitherto been contrived to minimize the exposure light wavelength or increase the fixed value sin θ of the reduction lenses. However, the physical condition constrains the variation of these quantities to limits. It has been found that the conventional pattern projection and exposure has assumed to carry out the exposure in air or in a vacuum in which n=1 always stands. The present invention makes it possible to drastically improve the resolution limit R by using an optically transparent liquid having a refractive index n of more than 1 so as to thereby vary the numerical aperture, the resolution limit being inversely proportional to the numerical aperture. 
     Specifically, a pattern forming apparatus 20 of the invention has a fundamental construction as shown in FIG. 2. The apparatus 20 comprises an illumination system 6 comprised of a lamp housing unit and condenser lenses (not shown), a reticle 5 provided with a pattern, reduction lenses 4, a semiconductor substrate 1 carried on a wafer stage (not shown) and having a surface on which a photoresist layer 2 is formed, a liquid layer 3 filling a gap between the photoresist layer 2 and at least a portion of the reduction lenses 4, and a pattern detector 7 for positioning the reticle 5 and the substrate 1. 
     In operation, a positioning pattern formed on the reticle 5 and a positioning pattern 10 formed on the substrate 1 are first brought into alignment by means of the pattern detector 7 through the reduction lenses 4. An optical path 11 in this alignment process is shown by solid lines. Then, the reticle 5 provided with a predetermined magnified pattern is illuminated by the illumination system 6 to form an optical pattern image of the magnified pattern. This optical pattern image is reduced and projected by the reduction lenses 4 upon the photoresist layer 2 coated on the semiconductor substrate 1 to expose the photoresist layer 2. An optical path 12 in this process is shown by dotted lines. After the first exposure process, the wafer stage carrying the semiconductor substrate 1 is moved (stepped) and a next exposure field is exposed (i.e. exposure is repeated) in the same manner as the first exposure process. By repeating the exposure process, a predetermined pattern is formed over the entire area of the substrate 1. The pattern forming apparatus 20 of the type as described above is usually termed a reduction projection aligner, and its general construction is referred to as STEP &amp; REPEAT CAMERA, for the detail of which reference may be made to U.S. Pat. No. 4,057,347. 
     The pattern forming apparatus 20, according to an embodiment of the present invention is featured by the use of the liquid layer 3 formed and sustained between the reduction lenses 4 and the photoresist layer 2 as has already been described above. The present embodiment defines the pattern forming apparatus 20 by the construction wherein the semiconductor substrate 1 provided with the photoresist layer 2 and a portion or the entirety of the reduction lenses 4 are immersed in the liquid layer 3. This liquid layer 3 contains a liquid which can transmit the optical pattern image, that is, optically transparent and has a refractive index of more the 1 (one). Needless to say, the liquid should not be reactive with materials of the reduction lenses 4, photoresist layer 2 and semiconductor substrate 1, i.e. should have chemically stable characteristics. Examplified as the liquid of this nature are trichlorotrifluoroethane (C 2  Cl 3  F 3 ), chlorobenzene (C 6  H 5  Cl), and water. 
     In order to form and sustain the liquid layer 3, the liquid is filled in a pool 19 and the reduction lenses 4 and the semiconductor substrate 1 are disposed in the pool 19. Alternatively, in accordance with another embodiment 20&#39; of the invention as shown in FIG. 3, a member 9 may be provided which surrounds the reduction lenses 4 and forms and sustains the liquid layer 3, and the sustaining member 9 may be equipped with a nozzle 13. Liquid supplied to the nozzle 13 in a direction of an arrow 8 may be drawn out toward the semiconductor substrate 1. With this modification also, the reduction lenses 4 and the photoresist layer 2 are put in immersion in the liquid layer 3. 
     According to the foregoing embodiments, when the refractive index n of atmosphere ambient of the exposure system is varied to be more than 1 (one) under the condition that a high resolution reduction lenses 4 usable and available for this invention has a fixed value as defined by sin θ=0.28 at a wavelength λ=0.436 μm, the resolution limit R is improved as shown in FIG. 4. 
     Specifically, while the conventional exposure in air achieves a resolvable line width or resolution limit R of 0.95 μm, the use of a liquid of, for example, n=1.36 or n=1.53 such as trichlorotrifluoroethane or chlorobenzene permits a drastic improvement of the resolution limit R to 0.69 μm or 0.62 μm. 
     When the refractive indices of the photoresist layer 2 formed on the substrate 1 and of the liquid layer 3 are made equal or almost equal, there occurs no or almost no such error in position detection, which is ascribed to the unevenness of the photoresist layer and has occurred in the conventional system upon detection of the pattern 10 by the position detector 7, even with a photoresist layer 2 having an uneven surface as shown in FIG. 2. In other words, the optical affect of the unevenness of the photoresist layer 2 can be eliminated essentially by melting the optical interface at the photoresist surface in an optically uniform medium, thereby ensuring a stable and highly precise pattern detection. 
     The present invention is also advantageous in that since the liquid used in the invention can be kept clean by, for example, distillation, it is expected that failure due to the deposition of dust or particles contained in air to the surface of the photoresist layer 2, which would occur in the conventional apparatus, can be prevented. Especially, when forming a fine exposure field of less than 1 μm line width with a reduction projection aligner, difficulties have been encountered in removing fine particles contained in air which may otherwise be deposited to the fine pattern. But, such difficulties can advantageously be overcome by the present invention. 
     Further, since the liquid layer 3 has a larger heat capacity than air, the substrate 1 can be less affected by temperature variations in the course of exposure and possible alignment error due to in-plane distortion of the substrate can be prevented in advance. 
     While the foregoing embodiments have been described by way of the reduction projecting aligner, the present invention may also be applied advantageously to a 1:1 projection aligner for forming a pattern on a substrate, and a checker or a measuring instrument for a fine pattern on a substrate.