Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
1998-10-22
2001-10-16
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S047000, C355S055000, C355S086000
Reexamination Certificate
active
06304316
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to microlithography, and more particularly relates to a projection microlithography system for high-resolution patterning on a large-area, photo-sensitive substrate having planarity deviations that exceed in magnitude the depth-of-field parameter of the projection imaging optics of the lithography system.
BACKGROUND OF THE INVENTION
Importance of Lithography Technology for Patterning on Curved Surfaces
Large-area electronic, mechanical and electromechanical structures on curved surfaces with integrated functional capability are rapidly growing in significance in a number of applications. In the military environment, this is evidenced by the desire for faster, lighter, and greater-functionality hardware, requiring active electronic circuitry and/or large-area micromechanical structures such as sensors and actuators. In the commercial sector, the availability of active microelectronic devices and micromechanical structures on large surfaces, especially those printed conformably on curved surfaces, holds enormous promise for applications in communications, integrated sensors and controls, and medical sectors. Technologies currently do not exist to produce such devices and structures.
As the level of functionality in a microelectronic or micromechanical device or structure increases, it becomes more important to develop manufacturing technologies and systems that will enable large-area printing on curved substrates at a reasonable cost. The curvature of the substrate may be predetermined and well defined, e.g., a spherical shell of known radius, or it may be random and unknown, e.g., a random, two-dimensional topography variation combining concave and convex regions. Existing approaches are unattractive because they provide very low throughputs (e.g., electron-beam or other focused-beam direct writing), or involve expensive additional process steps (e.g., chemical-mechanical polishing), or have a short depth of focus, and they cannot print conformably (e.g., all existing optical projection lithography techniques). This invention discloses a new lithography technology that enables conformable patterning with high resolution and high throughput on large-area substrates which may have curvature in one or more directions.
In the manufacturing of electronic devices and modules, the production equipment represents a major cost element, accounting for approximately two-thirds of the total facility cost. Of the many process steps involved in the fabrication of such devices, the most critical are those required for successively patterning various layers of photoresist and other materials. The capabilities and cost-effectiveness of the lithography technology impact the performance and cost of the device, and ultimately determine the size and cost at the end-system level. This makes patterning tools the largest and most critical component of the total production equipment investment. Typical costs of individual tools range between $2 and 5 million. A high-volume production facility would have multiple lithography tools. In addition, operating expenses add several hundred thousand dollars per year to the net cost of running such tools. Thus, in order to make significant progress toward broad realization of greater-functionality electronic and mechanical systems, there is a need to develop and implement new high-resolution, high-throughput lithography equipment for conformable patterning on curved, large-area surfaces.
Current Lithography Technologies
Currently available exposure systems can be classified into four general categories: (a) contact and proximity printing systems, (b) various types of projection systems (conventional, step-and-repeat, and scanning), (c) focused-beam direct-writing systems, and (d) holographic imaging systems. Each of these will be briefly described below. A contact printer consists of a fixture to align and hold the substrate in hard contact with the mask which is then illuminated with high-intensity collimated light to transfer the mask image to the substrate. In a proximity system a uniform gap is maintained between the mask and the substrate. Most contact printers use one or two mercury arc lamps, with powers in the range 2-8 kW. The radiation used for exposure is in the UV region from ~250 nm to ~430 nm, the rest of the light in the visible and the infrared being filtered away to minimize heating. The useful UV power represents less than 1% of the total wattage, indicating the extremely poor utilization efficiency of such lamp sources. A major limitation of contact printing is generation of defects on the substrate due to repeated contacting of the resist-coated surface, which results in lower yields. Frequent mask-substrate contact also degrades the mask life, which leads to higher overall costs. These problems are somewhat less severe in proximity printing. However, since the resolution R in a proximity system using radiation of wavelength &lgr; depends on the mask-substrate gap d, as given by R=(&lgr;d/2)
½
, any nonuniformity in the gap results in feature size variation. This limitation becomes more severe for larger panels as it becomes more difficult to maintain a uniform gap between the mask and the substrate. Finally, neither contact nor proximity lithography tools can print on curved surfaces.
A variety of projection imaging systems are used in fabrication of microelectronic modules.
Single-field, or conventional, projection tools are those in which the image field of the lens is sufficient to accommodate the entire substrate. Typically, a projection lens with a 1:1 magnification is used. For different design resolutions, the maximum image field size of the projection lens is different: whereas a 1 mil resolution can be obtained over a 4 inch square field, the imageable area for 1 micron resolution must be limited to a field diameter no larger than 2-3 cm. Thus, conventional projection printing systems are limited by the fundamental trade-off between the desired resolution and the largest substrate they can image.
In a step-and-repeat type of projection system the total substrate area to be patterned is broken up into several segments which are then imaged one at a time by stepping the substrate under the lens from one segment to the next Due to the increased overhead from the stepping, settling and aligning steps for each segment, steppers deliver low throughputs. Most step-and-repeat systems use reduction imaging, typically with a 2:1, 5:1 or 10:1 ratio. Generally, systems with larger reduction ratios provide higher resolution, but also lower throughput. Steppers also have a performance shortcoming due to stitching errors between adjacent exposure segments. When a stepper is used for semiconductor chip lithography, the individual chip sites are separated by nonimaged areas (‘streets’) through which the chips are diced. Since these streets contain no patterns, there is no requirement to precisely stitch adjacent segments together. However, in a large panel, as for a circuit board or a display, the entire substrate is often one pattern; there are no un-imaged areas between segments. The segments, therefore, must be butted next to each other with great precision. Poor lithography due to stitching errors is one of the most significant yield detractor in the production of large-area products. Recognizing the throughput limitations of steppers, many types of scanning projection tools have been developed over the last two decades. The most well known of these use a reflective ring-field imaging system. Exposures are made by scanning the mask (and the substrate) across an illumination beam in the shape of an annulus, which is necessitated by the geometry of the zone of good image correction. The scanning ring-field imaging concept requires primary imaging mirrors that are approximately three times larger than the size of the substrate. As a result, such scanners, although capable of good resolution, are extremely expensive and incapable of handling most large panel sizes. Their throughputs are al
Dunn Thomas J.
Framiga Nestor O.
Jain Kanti
Adams Russell
Anvik Corporation
Fuller Rodney
Kling Carl C.
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