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(1) Field of the Invention
This invention relates to projection lithography systems for imaging on curved substrates, and more particularly relates to a large-area lithography system featuring a curved mask that is identical in size and shape to the curved substrate, for the purpose of achieving a constant optical path length for conjugate image points in order to maintain the substrate surface within the depth-of-focus of the projection optics, thereby providing an effective depth-of-focus significantly greater than the depth-of-focus of the projection optics. Additionally, the system has provisions for compensating for magnification errors that arise as a result of imaging a curved mask onto a curved substrate. The system performs patterning on curved surfaces by means of small-field seamless scanning techniques to achieve high resolution over an entire large-area curved substrate. This invention also relates to a technique for fabricating the appropriate curved projection masks, using a planar contact printing technique to replicate a planar pattern onto a flexible film which is stretched over a curved mask blank and secured by a frame, with the option of using the system itself to replicate the original mask onto an indeterminate number of curved fused-silica mask blanks, by projection printing.
(2) Description of Related Art
Although microlithographic patterning has traditionally been performed on planar substrates, primarily for the fabrication of computer chips and microcircuits, there exist many applications which are based on microcircuits or circuit-like elements fabricated on curved surfaces. For example, CCD (charge coupled device) arrays constructed on spherical silicon substrates, rather than flat substrates, could significantly enhance the performance of digital imaging systems. As with any conventional lens system, an increased collection efficiency and larger fields-of-view could be achieved by utilizing a curved image plane, in this case a curved CCD array, matched to the inherent curvature of focus of the optical system (the Petzval curvature). Although the individual pixels for CCD arrays are typically xcx9c25 xcexcm in size, the electrical interconnects are of the order of 1 xcexcm in width. We note that microcircuits having features 1 xcexcm and below generally are fabricated using projection lithography, and thus projection techniques would be useful for patterning curved CCD""s. Another area of potential widespread application is in the fabrication of frequency selective surfaces (FSS), which are used as electromagnetic windows usually in the millimeter-wave and microwave portions of the electromagnetic spectrum. FSS consist of a repeating array of identical elements, typically on a flexible dielectric material such as Kapton. The individual elements are often in a tripole or quadrupole configuration, with the sizes of the elements of the same scale as the wavelength of selectivity. In order to optimize the bandwidth and polarization responses of FSS arrays, and to improve the uniformity of the responses with respect to angle of incidence, the FSS elements often are fabricated as closed contours with the contour linewidth measuring a fraction of the design wavelength, of the order of tens or hundreds of microns. Active FSS, which have electronic devices, such as diodes, integrated into their periodic structures, are based on semiconductor device technology and are therefore fabricated using traditional semiconductor-processing materials and techniques. The active elements must be printed with high resolution, of the order of microns in scale. Thus the fabrication of both passive and active FSS requires the use of high-resolution patterning technology, with resolution down to the micron scale for active FSS.
Many techniques have been investigated for patterning high-resolution features on curved surfaces, with a number of methods based on new materials-based processes and applications. For example, there exist a variety of imprinting techniques based on soft-lithography methods, using stamps or molds to replicate patterns on curved surfaces. (Y. Xia and G. Whitesides, Angew Chem. Int. Ed., 37, 550-575, (1998)). A somewhat different fabrication method utilizes conventional photolithography processes, by which a flat substrate is first patterned using planar fabrication techniques; the planar substrate is subsequently mechanically deformed into the desired curved shape, so that after deformation the features reside on a surface having the desired curvature (see, for example, Z. Suo, Appl. Phy. Lett, 74, 1177-1179 (1999)). Several variations of the deformation approach have been utilized for FSS fabrication on curved dielectric substrates. For example, planar substrates may be patterned and subsequently deformed into the desired FSS shape either by heat-forming or by assembling patterned strips directly onto the curved substrate. (T. Wu, Frequency Selective Surface and Grid Array, John Wiley and Sons, 1995) Contact printing directly onto FSS substrates has also been investigated, using a mask that conforms to the curved substrate. See, for example, U.S. Pat. No. 5,395,718, Jensen et al., CONFORMAL PHOTOLITHOGRAPHIC METHOD AND MASK FOR MANUFACTURING PARTS WITH PATTERNED CURVED SURFACES, Mar. 7, 1995. See also, U.S. Pat. No. 5,552,249, Jensen et al., METHOD FOR MAKING A MASK USEFUL IN THE CONFORMAL PHOTOLITHOGRAPHIC MANUFACTURE OF PATTERNED CURVED SURFACES, Sep. 3, 1996.
As described by Jensen, et al., the curved masks used in this process have been fabricated by laser direct-writing, utilizing a three-axis stage to control the position of the mask during the writing process. Masks fabricated in this manner have been successfully used to pattern images on curved substrates by contact printing. However, as with planar contact printing methods, throughput is limited by the vacuum pulldown time, and cost efficiency is reduced by the need to replace masks, as they are subject to degradation resulting from their intimate contact with the curved substrates.
Other methods for patterning onto curved surfaces, such as direct-writing, could be successfully used, having the substrate situated on an x, y, z-stage, with z-adjustments for maintaining the substrate in the focus of the writing beam. However, direct-writing is slow and would therefore be unsuitable for high-throughput fabrication. For example, for patterning on planar substrates, projection lithography is the preferred method because of the much higher throughputs that can be achieved, compared to direct writing. Although traditional projection lithography techniques using planar masks can be used for very high throughput lithography on planar substrates, there are currently no very high throughput techniques described in the literature for projection imaging onto curved surfaces, even for relatively large features, tens or hundreds of microns in size. This is primarily due to the limited depth-of-focus (DOF) of projection imaging systems. For example, for conventional steppers and scanners achieving a resolution of the order of a micron, the DOF is of the order of 10 microns, which is far too small for imaging a planar mask onto curved CCD or FSS substrates having height variations of typically several centimeters and several tens or hundreds of centimeters, respectively. Even for projection systems having a resolution of only 100 microns, which would be sufficient for certain passive FSS, the DOF would be of the order of only several millimeters, which is not sufficient for imaging a planar mask onto typical FSS structures, which have height variations significantly greater than several millimeters. We note that it would be possible to perform projection imaging onto curved substrates, using planar masks, by modifying conventional steppers or scanning systems such that the z-position of the substrate could be adjusted during patterning to maintain the substrate within the DOF of the imaging system. However, to operate in this mode, it would be necessary to restrict the size of the image field such that the height of the substrate does not vary by more than the DOF of the imaging system, over any substrate region equal in size to the image field. For practical curved-substrate imaging applications, such as CCD fabrication, the maximum-allowable image field would consequently not be much larger than many microns. Using such a technique, throughputs would be higher than for contact printing but significantly lower than for projection printing onto planar substrates. Thus the use of conventional projection lithography systems operating with planar masks would not be suitable for very high throughput lithographic patterning onto curved surfaces such as CCD arrays and FSS. However, it should be noted that a method for achieving improved throughputs by projection printing using planar masks has been described in copending U.S. patent application Ser. No. 09/176,920 now U.S. Pat. No. 6,304,316, MICROLITHOGRAPHY SYSTEM FOR HIGH-RESOLUTION, LARGE-AREA PATTERNING OF CURVED SURFACES, Jain, Farmiga and Dunn, filed Oct. 22, 1998. See also U.S. Pat. No. 5,285,236, LARGE-AREA, HIGH-THROUGHPUT, HIGH-RESOLUTION PROJECTION IMAGING SYSTEM, Jain, Feb. 8, 1994. The copending Jain et al. application describes the Anvik system for seamless hexagonal complementary projection scanning using planar masks, for projection printing onto curved substrates using a deformable mirror in the imaging system to enhance the DOF, and z-actuators to maintain the substrate within the enhanced DOF region. The Anvik system is effective, but its wide range of capability does not always provide the optimum economics in mass production, and there may be limits to the degree of enhanced performance that can be achieved by using the deformable mirror. There remains a need for a more effective, economical system for mass-production projection imaging of microelectronics features on curved substrates.
There are a number of existing techniques for imaging onto a curved surface, in addition to Jensen""s method, which was noted earlier. See, for example, U.S. Pat. No. 3,694,080, Malsky, METHOD AND APPARATUS FOR PHOTOPRINTING CONDUCTOR PATTERNS ON THE CURVED SURFACES OF TRANSDUCERS, Sep. 26, 1972, which describes a technique using a cylindrical substrate with the cylinder revolving past a small scanning station for projection patterning via a curved mask, keeping the focus effectively unchanged at the scanning station while the cylinder revolves. Malsky is limited to rotational presentation of the substrate in a step and repeat imaging system.
Another technique, a contact method which permits the curved surface to be a compound curve such as a radome for an aircraft, uses a contact photo mask which is adhered to a metallic layer on the inside of the radome. After exposure, an etching step leaves a pattern in the metallic lawyer. See, for example, U.S. Pat. No. 4,388,388, Kornbau et al, METHOD OF FORMING METALLIC PATTERNS ON CURVED SURFACES, Jun. 14, 1983. See also U.S. Pat. No. 5,344,729, Akins et al, CONFORMAL PHOTOMASK FOR THREE-DIMENSIONAL PRINTED CIRCUIT BOARD TECHNOLOGY, Sep. 6, 1994.
The prior art suffers from the problems of contact printing, including wear and damage due to contact, or does not have the resolution required for present-day needs. There is a continuing need for high-resolution non-contact projection imaging for patterning intricate details on curved surfaces.
This invention provides to a large-area lithography system the capability of patterning onto a curved substrate, using a curved mask in order to achieve a fixed track length for conjugate object and image points, thereby maintaining the curved substrate surface within the depth-of-focus (DOF) of the projection optics; this invention also provides the capability for controlling magnification errors arising as a result of the curved patterning technique. This invention provides these capabilities while retaining the option of small-field seamless scanning techniques to achieve high resolution over the entire large-area curved substrate.
The object of the invention is to make possible a high-resolution projection imaging operation on a curved substrate with height variations significantly greater than the depth-of-focus of the imaging optics.
Another object of the invention is to permit scanning projection imaging, by minimizing and compensating for destructive magnification error.
A feature of the invention is a curved mask having a curvature that is identical to, i.e., having the same size and shape, as the curved substrate.
Another feature of the invention is the inverted orientation of the curved substrate with respect to the curved mask when the mask and substrate are mounted on a common scanning platform, thus keeping the track length constant for conjugate image points, with motion provided to the projection lens to compensate for magnification error.
An advantage of the invention is that it makes it possible to perform high-resolution projection imaging on curved surfaces having topography variations greater than the depth-of-focus of the imaging system.
Other objects, features and advantages of the invention will be apparent to those skilled in the art, in view of the drawings and written description.