Printing – Printing members
Reexamination Certificate
2002-07-16
2004-09-21
Colilla, Daniel J. (Department: 2854)
Printing
Printing members
C101S214000
Reexamination Certificate
active
06792856
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to printing, particularly to micro-contact printing, also called “soft lithography”, in which a flexible stamp transfers an “inked” pattern to a receiving surface by mechanical contact, the pattern often having very small features normally associated with optical lithography and other expensive methods. More generally, the invention relates to a precise and controlled way of bringing two surfaces into contact, and subsequently separating them.
BACKGROUND OF THE INVENTION
A number of printing techniques collectively known as “soft lithography” have been recently developed, spurred by the 1993 discovery of micro-contact printing, as described in A. Kumar and G. M. Whitesides, FEATURES ON GOLD HAVING MICROMETER TO CENTIMETER DIMENSIONS CAN BE FORMED THROUGH A COMBINATION OF STAMPING WITH AN ELASTOMERIC STAMP AND AN ALKANETHIOL INK FOLLOWED BY CHEMICAL ETCHING,
Appl. Phys. Lett
., 63, 2002 (1993), the disclosure of which is incorporated by reference herein in their entirety. Typically, in such a printing technique, a flexible, polymeric stamp, embossed with a pattern and coated with a chemical “ink”, is brought into contact with a receiving surface and then separated from it, thereby transferring the image to the receiving surface in the form of a molecular monolayer of the ink. A full review of the techniques of soft lithography has recently been given in Y. Xia and G. M. Whitesides, SOFT LITHOGRAPHY,
Angew. Chem. Int. Ed
., 37, 550 (1998) and in B. Michel, et al., PRINTING MEETS LITHOGRAPHY: SOFT APPROACHES TO HIGH RESOLUTION PATTERNING, to be published in
IBM Journal of Research and Development
(special issue on lithography), the disclosures of both of which are incorporated by reference herein in their entirety.
Soft lithography promises to deliver printing that is less costly than that available with conventional techniques, such as optical lithography, used routinely in semiconductor processing. Soft lithography's lower cost is possible because the per-print process is simpler than conventional techniques—there are fewer steps and fewer costly machines. Moreover, soft lithography can print large areas quickly, whereas traditional, optical techniques can print only small areas at a time, and must build up large areas by “stitching” (step and repeat), a slow process requiring an extremely precise and expensive machine known as a lithographic stepper.
To enable soft lithography, a printing method and apparatus are required to bring the stamp and the receiver into intimate contact, in a controlled and repeatable manner, such that the pattern on the stamp is transferred to the receiver with the greatest possible fidelity (i.e., with minimal distortion). To insure intimate contact everywhere, the printing method must prevent the trapping of gaseous bubbles (e.g., air bubbles) between the opposing surfaces of the stamp and the receiver. To insure repeatability, the printing apparatus must be automated. To achieve high fidelity, two requirements must be met. Firstly, the stamp itself should resist distortions in its own plane; such resistance is provided, for example, by the two-layer “hybrid stamps” described by B. Michel et. al., supra. Secondly, the printing apparatus must provide, when the stamp and the receiver come into contact, uniform contact pressure and uniform geometric conditions over the entire printed area, lest the stamp be non-uniformly strained and therefore the printed pattern distorted.
Several prior-art methods and machines attempt to provide the printing requirements needed for soft lithography. However, these prior-art methods are deficient in several respects. One such method is described in U.S. Pat. No. 5,669,303 entitled APPARATUS AND METHOD FOR STAMPING A SURFACE, issued Sep. 23, 1997. This apparatus brings a circular stamp, held at its edges, into gradual contact with a receiver. The stamp is treated as a membrane under variable pressure: the convex (lower-pressure) side of the curving stamp being gradually flattened against the receiver while the periphery of the stamp is held fixed. Although the gradual contact successfully eliminates the trapping of air bubbles, this method and apparatus clearly produces non-uniform strain in the stamp as the varying pressure stretches the membrane, thereby distorting the pattern. Acknowledging this distortion, various schemes were proposed to compensate it, but the manufacturing practicality of these schemes is doubtful, and it would clearly be preferable if the method did not engender the non-uniform strain in the first place.
Another prior-art apparatus and method are described by B. Michel et al., supra, as the “rocker cylinder printing tool”. In this method, the stamp is wrapped on a partial drum of radius R, and then “rocked” upon the receiving surface in a manner somewhat analogous to the motion of a rocking chair upon a floor. In other words, the method is like a printing press in which the receiver remains stationary while the axis of the rotating drum translates over it. The problems with this method are three-fold. Firstly, the embossed pattern on the stamp is stretched in the print direction due to the drum's curvature, introducing systematic distortion. Secondly, over the print cycle, the peak contact pressure between the stamp and the receiver is spatially non-uniform because it depends critically on the drum-to-receiver gap, which varies as the mechanism moves on account of unavoidable mechanical tolerances such as bearing runout and machining inaccuracies on the drum's surface. Attempting to minimize variations in peak contact pressure by introducing a compliant layer (known as a “soft pad”) behind the stamp simply trades peak-pressure non-uniformity for geometric non-uniformity; that is, as the soft pad compresses to accommodate gap changes, the local curvature of the stamp near the line of contact varies, and thus the tangential strain of the embossed pattern varies—this complex variation being superimposed on the systematic strain due to the drum's curvature. Thirdly, because the drum is both translating and rotating, the accuracy of printing depends critically on precisely matching the drum's translational speed &ngr; with its rotational speed &ohgr;; ideally, to roll without slipping and without straining the compliant stamp by frictional forces, the drum's velocity &ngr; should be exactly equal to &ohgr;R. However, this ideal matching is nearly impossible to accomplish to the tolerance (~1 ppm) required for high-accuracy, large-area applications—exactly the applications where soft lithography seeks to replace optical lithogtaphy. Thus the rocker-style printer is ill-suited to the task of soft lithography. In fact, a controlled experiment was performed in which feature-placement errors on two prints from the same stamp were measured—one print made with a well-engineered rocker printer, the other with an alternative scheme (such as the current invention), where the three problems discussed above are absent. The results demonstrate roughly a factor-of-three advantage in feature-placement accuracy for the latter method.
All three shortcomings of the rocker printer, of course, are shared by the “printing press” style of machine. In particular, the printing press shares the third shortcoming mentioned above (print accuracy dependent on precise matching of &ngr; to &ohgr;R): although the printing-press's drum rotates without translating, the receiver instead translates beneath it, so speed matching is still an issue. Although the printing press is, of course, suitable for images to be observed by the human eye, where feature-placement accuracy need not be better than about 10 to 20 &mgr;m, it appears to be unsuitable for the applications of soft lithography (e.g., printing patterns for electronic circuitry), where feature-placement accuracy on the order of 1 &mgr;m or better is required.
Accordingly, there is a need for an improved method and apparatus for transferring patterns from a stamp to a receiver with great fidel
Emmans Robert Scott
Fair Robert H.
Hall Shawn Anthony
Lovas Istvan
Nunes Ronald W.
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