Optical: systems and elements – Holographic system or element – Using a hologram as an optical element
Reexamination Certificate
1997-11-14
2003-09-09
Juba, Jr., John (Department: 2872)
Optical: systems and elements
Holographic system or element
Using a hologram as an optical element
C359S033000, C359S900000, C430S005000, C355S053000, C219S121600, C219S121700, C219S121770
Reexamination Certificate
active
06618174
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to methods and apparatus for microlithography, photopatterning, machining and materials processing and, more particularly, to high-efficiency in-line holograms that combine the functions of a lens and a standard amplitude mask in one device.
BACKGROUND OF THE INVENTION
There are many industrial applications and processes that require precise patterning of a workpiece, two such applications being, for example, fabricating microcircuits, and forming circuit board interconnections. For instance, the demand for compact electronics packaging has seen the means for forming interconnections among microcircuits evolve from the use of peripheral interconnections (i.e., connections around the edge of the package) to the use of flexible ball grid arrays (BGA) on the surface of the package. This newer BGA packaging and thin, flexible interconnection method requires the creation of an array of hundreds of vias (i.e., holes) on the order of 25 &mgr;m diameter in a thin multi-layer laminate insulating layer, such as polyimide (for example, KAPTON polyimide, sold under this trademark by DuPont).
Traditional means for accomplishing precise patterning of a workpiece by micromachining include mechanical drilling, chemical etching, contact printing, and projection photolithography. In recent years, however, lasers have been shown to be a valuable and often preferred means for performing high-precision micromachining because of their directionality, coherence, high intensity and high photon energy.
The specific interaction between the laser beam and the workpiece depends on the laser wavelength and the material comprising the workpiece. For instance, infra-red wavelength and visible wavelength laser beams focused to a small spot on the workpiece provide intense localized heating which vaporizes most workpiece materials. However, such localized heating can have the undesirable side-effect of thermally damaging the workpiece. On the other hand, ultraviolet (UV) wavelength lasers (such as excimer lasers) provide photons with sufficient energy to excite the electrons that form the molecular bonds of certain workpiece materials such as polyimide. Sufficient excitation of the bonding electrons with a tightly focused beam results in the localized disassociation of the material with little or no heating of the workpiece. This process is referred to as “ablation.”
In a typical laser-based micromachining application, a laser is used to irradiate the surface of a workpiece in order to form a desired pattern thereon or therein. One method of laser-based micromachining involves a mask-based step-and-repeat operation, wherein the mask is illuminated with a laser beam, and a projection lens images the mask onto the workpiece. While this method is capable of forming small well-defined spots and is well-suited for forming arbitrary shapes or figures, the method is inefficient with its use of available light because the mask blocks a portion of the beam in order to form the pattern. Also, the step-and-repeat method is time-consuming, particularly when hundreds or thousands of spots need to be patterned on each of a multitude of workpieces.
Another method of laser-based micromachining involves scanning a laser beam over the workpiece with a flying-spot scanning apparatus. However, this apparatus is fairly complex and expensive, and is generally not well-suited for forming arbitrary shapes and figures, and it has limited processing capacity or “thruput” (up to about 1000 holes/second) because of its serial mechanical nature.
To increase “thruput” (the number of workpieces that can be processed in a given time interval) and to simplify the apparatus for step-and-repeat laser micromachining, there have been recent efforts to develop laser micromachining methods and apparatus that employ various types of multiple-focusing means for simultaneous drilling multiple holes (i.e., forming holes in “parallel” rather than serially). Such means include conventional lenses, fresnel zone plates (FZP's), computer-generated holograms (CGHs), diffractive optical elements, and binary phase gratings.
Because there is some confusion in the patent literature regarding the definition of the above multiple-focusing means, the following definitions are used herein.
A FZP is a plate with concentric transparent and opaque annular rings or ring sections that transmit and block alternating Fresnel zones on a wavefront thereby allowing the transmitted light to positively interfere and come to a focus. An FZP can also be made with refractive zones instead of opaque zones, so that the phase of the light is changed to be in phase with the other zones, rather than simply being blocked. For FZP's used to create an image other than a single focus spot, the zone pattern is calculated and then produced by digital means and lithography, as is referred to as a “kinoform.”
A holographic optical element (HOE) is an optical component used to modify light rays by diffraction, and is produced by recording an interference pattern of two laser beams and can be used in place of lenses or prisms where diffraction rather than refraction is desired.
A hologram is a continuous diffracting region created by two or more interfering beams in which the phase information of the wavefronts in the object is converted to intensity or phase variations. The continuous diffracting region can also be computer-generated. Each point on the hologram contains information about the entire object, and thus any portion of the hologram can, in principle, reproduce the entire three-dimensional image of the object via wavefront reconstruction.
Diffractive optical elements (DOEs) have zones of refraction, phase shift, or amplitude modulation with a scale that allows for the directional control of diffraction effects. A DOE can have a focusing effect as in an FZP, or it can have more complicated effects such as chromatic correction or aspherical distortion correction. Diffracting optical elements are made using computation to describe the zones of diffraction, and then producing these zones in a suitable substrate surface by means of diamond turning or by lithographic processes common to semiconductor manufacturing or injection molding.
A binary optical element is a diffracting optical element having a binary or “flat-top” zone profile.
In addition, the phrase “in-line” as used herein denotes a geometry in which is coaxial, i.e., disposed along a common axis.
Laser micromachining methods and apparatus employing the above multiple-focusing means are generally faster and more efficient than step-and-repeat micromachining, contact printing, and projection photolithography. However, these multiple-focusing apparatus and methods also have their own shortcomings and limitations.
U.S. Pat. No. 5,233,693 to Zumoto et al. discloses an in-line optical projection micromachining apparatus. The apparatus comprises a mask having apertures and reflective parts in between, and a hemispherical reflective member for returning the light reflected off the reflective parts of the mask back toward the open areas of the mask. A projection lens is used to image the mask onto a workpiece. While this system operates in-line, it is fairly complex because the projection lens for most applications would not be a single lens element, but a multi-element well-corrected lens system capable of imaging very small features. In addition, when the mask features to be patterned are small relative to the total area of the mask, the amount of light transmitted by the mask will be relatively low, even with the hemispherical reflective member present.
U.S. Pat. No. 5,481,407 (the '407 patent) to MacDonald et al. discloses a laser-based method and apparatus for creating small holes having a desired shape (e.g., circular, square, oval, etc.) by laser ablation. The focusing means is a segmented array of FZPs, wherein the form of the individual FZPs comprising the segmented array determines the shape of the holes. While this technique allows for a multitude of holes t
Parker Julie W.
Parker William P.
Diffraction, LTD
Downs Rachlin & Martin PLLC
Jr. John Juba
Leas James Marc
Neiman Thomas N.
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