Radiation imagery chemistry: process – composition – or product th – Effecting frontal radiation modification during exposure,...
Reexamination Certificate
1999-01-04
2001-05-29
McPherson, John A. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Effecting frontal radiation modification during exposure,...
C430S322000, C355S046000, C355S053000, C355S077000
Reexamination Certificate
active
06238852
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
None
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
REFERENCE TO MICROFICHE APPENDIX
None
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a maskless lithography system that provides large-area, seamless patterning using a reflective spatial light modulator such as a Digital Micromirror Device (DMD), which consists of an array of micromirrors, each being settable to an “on” reflective angle as directly addressed by a control system, so as to reflect a pattern via a projection subassembly onto a photosensitive panel of a substrate;
and more particularly relates to a throughput doubling configuration which, simultaneously with patterning onto a first substrate panel from the DMD array of “on” pixels, provides a duplicate pattern, which is the complement of the pixel pattern on the first panel, of “off” pixels, via a second projection subassembly onto a photosensitive panel of a second substrate, thus using the normally-rejected non-pattern “off” pixel radiation reflections of the DMD, simultaneously, to provide a second pattern on the second substrate panel.
2. Description of Related Art
Importance of Maskless Lithography Technology
High-throughput lithography systems are critical elements in the commercial fabrication of microelectronic components. They are used for high-volume production of small-area packages such as integrated circuits as well as large-area patterns such as printed circuit boards and flat-panel displays. High-throughput is important because the amount of time required to manufacture a microelectronic device has a significant effect on its cost in the consumer market. Optical lithography is the most widely used technology used for high-volume production because it can achieve high throughput via the parallel nature of its pattern generation, in which a large number of features are simultaneously printed onto a substrate during a single exposure This parallel processing using optical lithography is achieved by illuminating a mask in order to transfer its features onto the substrate. Since the features of a printed circuit are created by using a mask comprising a replica of the circuit's features, the complexity and size of a circuit directly influence the complexity and size of the corresponding mask which must be used to print it. However, regardless of the size and complexity of any circuit pattern, once the appropriate mask is on hand, optical lithography can precisely and efficiently generate that circuit.
Even with the substantial benefits of parallel processing, there exists a variety of factors which can significantly reduce the speed and efficiency at which optical lithography can proceed. For example, the intensity of the illumination and the sensitivity of the resist largely determine how quickly a single exposure can take place; the exposure speed can be improved by increasing the illumination intensity and/or the resist sensitivity. For circuits (such as those consisting of many layers of features) requiring many individual exposures, the switching of masks between exposures can represent a significant amount of overhead time. In order to print a new layer, a mask must be placed in position and accurately aligned before an exposure can take place. This positioning of the mask reduces the throughput. Additionally, a microelectronic fabrication facility must have an in-house stock of masks in order to generate the numerous circuits which it supplies to the consumer market. Purchasing and maintaining these masks results in large overhead costs.
Other issues associated with the use of a mask in optical lithography can also lead to lower productivity. For example, when developing a prototype circuit there is generally a need to obtain masks rapidly so that the prototype can be quickly manufactured and tested. However, since masks are typically manufactured by outside vendors with specialized equipment, it could take many weeks to procure a prototype mask, leading to delays in product development. Additionally, for prototype work the mask is often used only temporarily and then must be discarded when the prototype is refined, leading to large overhead expenses. These expenses can become quite significant for prototype circuits requiring multiple masks.
Another notable property of the mask is that its cost goes up with its size. Thus a mask used to print micron-sized features for flat panel displays—which are on the order of 1 ft.
2
—could be significantly more expensive than a mask used to print similar-sized features over a much smaller area. Prototype development requiring the use of numerous large-area masks for each prototype generation can be enormously expensive.
The use of a mask thus enables lithography to achieve high volume throughput with parallel processing, but it can represent a significant burden. The use of a mask affects both the manufacturing process (with the overhead time required to switch masks and the cost of maintaining an in-house stock of masks) and the prototype development process (with the long mask-acquisition time and the expense of constructing and discarding numerous masks for prototype development). The use of a mask also affects the printing of large-area circuits, which require large-area masks. It would be of great benefit if the parallel processing power of a mask were maintained, while eliminating the need to switch masks during circuit production, reducing overhead manufacturing time. Additionally, overhead manufacturing costs could be reduced if it were not necessary to maintain a stock of production masks. Prototype development could proceed rapidly and economically if masks were eliminated, while maintaining the benefits of parallel exposure. Large-area circuit patterns could be printed more economically if large-area masks were not needed to achieve high-throughput. Thus, a technology which provides parallel exposure without the use of masks could significantly improve the productivity of microelectronic fabrication.
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 is briefly described.
Contact and Proximity Printing Systems
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 light to transfer the mask image to the substrate. A proximity system maintains a uniform gap 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, but feature size variation becomes a limitation. 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)
{fraction (1/2+L )}, and therefore any nonuniformity in the gap d results in feature size variation. This limitation becomes more severe for larger panels as it becomes more difficult to maintain a uniform gap d between the mask and the substrate. It should be emphasized that since contact and proximity tools use masks to generate circuit patterns, they are both subject to the limitations noted in the previous secti
Anvik Corporation
Kling Carl C.
McPherson John A.
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