Radiation imagery chemistry: process – composition – or product th – Radiation modifying product or process of making – Radiation mask
Reexamination Certificate
1999-08-31
2001-02-13
Rosasco, S. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Radiation modifying product or process of making
Radiation mask
C428S426000
Reexamination Certificate
active
06187484
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
BACKGROUND OF THE INVENTION
The present invention relates to irradiation of a workpiece through a patterned mask and, more particularly, to a laser ablation mask, its method of production, and its manner of use. Although the present invention is described in the context of laser ablation, it is contemplated by the present invention that the irradiation mask of the present invention is suitable for use in irradiation applications outside of the realm of laser ablation.
As will be appreciated by those of ordinary skill in the art, laser ablation has application in many diverse fields. Typically, laser ablation processes must be done at relatively high laser power and high accuracy with high throughput and a high degree of repeatability. Laser ablation masks have been employed to enhance the accuracy and repeatability of the process. The mask incorporates apertures that are transparent to the wavelength of the radiation output by the laser and is used to produce a similar pattern of apertures on each of a plurality of successive workpieces. However, high laser power and throughput typically have adverse effects on many laser ablation masks.
Typical laser power levels often exceed 1 watt/cm
2
. In ordinary metal masks fabricated from chromium, power levels above 1 watt/cm
2
cause separation of the metal from the underlying glass or quartz substrate because a substantial amount of laser energy is absorbed by the metal layer, even though a high percentage of the incident laser radiation is reflected. As a result, the metal of the mask itself, and not just the material of the workpiece, may be ablated by the laser. Accordingly, the useful life of a particular laser projection mask formed of metal is very limited at high power levels.
Due to the inability of metal masks to withstand the laser ablation process at desired laser power flux levels, masks composed of alternating dielectric films of silicon oxide and tantalum oxide of closely controlled thickness and differing refractive indices have been proposed and used in some applications. If the thicknesses of the layers are closely controlled with respect to the wavelength of the laser radiation and the respective refractive indices of the materials, a destructive interference pattern can be established to reflect a majority of the light incident on each dielectric layer pair. Desirable thicknesses and materials for these layers are on the order of 500 Angstroms for silicon oxide and 400 Angstroms for tantalum oxide. The transmitted radiation flux can be reduced to any arbitrary desired degree by increasing the number of dielectric layer pairs which are stacked together to form the mask. However, dielectric masks are difficult to manufacture and the materials proposed for use in the plurality of dielectric layer pairs are very difficult to pattern in order to form a mask. Accordingly, multi-layered dielectric masks have not yet provided a solution to the trade-off between mask cost and laser throughput requirements in laser ablation. As a result, there is a continuing need for an irradiation mask resists laser ablation and that represents a simplified and cost effective mask manufacturing process.
BRIEF SUMMARY OF THE INVENTION
This need is met by the present invention wherein an improved laser ablation mask, an improved method of mask production, and an improved process of workpiece irradiation are provided.
In accordance with one embodiment of the present invention, a method of producing a radiation reflective mask is provided comprising the steps of: (i) providing a substrate, wherein the substrate is transparent to radiation of a selected range of wavelengths; (ii) forming a metallic layer over an upper surface of the substrate, wherein the metallic layer is reflective of the selected wavelengths of radiation; (iii) forming at least one pair of dielectric layers over an upper surface of the metallic layer, wherein the pair of dielectric layers are arranged to reflect incident radiation at the selected wavelengths; and (iv) patterning the metallic layer and the pair of dielectric layers to form apertures therein, wherein the apertures render portions of the mask transparent to the selected wavelengths of radiation.
The metallic layer may be formed on the substrate. The dielectric layers may be formed on the metallic layer. The pair of dielectric layers are preferably arranged to establish a destructive interference pattern with respect to incident radiation at the selected wavelengths. The metallic layer is preferably patterned subsequent to its formation over the substrate. The dielectric layers are preferably patterned subsequent to their formation over the metallic layer.
In accordance with another embodiment of the present invention, a method of producing a radiation reflective mask is provided comprising the steps of: (i) providing a quartz substrate, wherein the substrate is transparent to laser radiation of a selected wavelength; (ii) forming an aluminum layer over an upper surface of the substrate, wherein the aluminum layer is reflective of the selected wavelength of laser radiation; (iii) forming a pair of dielectric layers over an upper surface of the metallic layer, wherein the pair of dielectric layers are arranged to establish a destructive interference pattern with respect to incident laser radiation at the selected wavelength, and wherein the pair of dielectric layers include a low index of refraction silicon dioxide layer formed over the upper surface of the metallic layer and a high index of refraction silicon nitride layer formed over the silicon dioxide layer; and (iv) patterning the pair of dielectric layers and the aluminum metallic layer to form a series of apertures in the mask, wherein each of the apertures extend through the pair of dielectric layers and the aluminum metallic layer, and wherein the apertures render portions of the mask transparent to the selected wavelength of laser radiation. The pair of dielectric layers may form an uppermost surface of the mask.
In accordance with yet another embodiment of the present invention, a method of producing a laser ablation mask is provided comprising the steps of: (i) providing a quartz substrate, wherein the substrate is transparent to laser radiation of a selected wavelength; (ii) forming an aluminum layer on an upper surface of the substrate, wherein the aluminum layer is reflective of the laser radiation, and wherein the aluminum layer is less than about 3 microns in thickness; (iii) forming a pair of dielectric layers on an upper surface of the aluminum layer to define an uppermost surface of the mask, wherein the pair of dielectric layers are arranged to establish a destructive interference pattern with respect to incident laser radiation at the selected wavelength, wherein the pair of dielectric layers include a low index of refraction silicon dioxide layer formed on the upper surface of the aluminum layer and a high index of refraction silicon nitride layer formed on the silicon dioxide layer, and wherein the thickness of the silicon dioxide layer is about 40 nm and the thickness of the silicon nitride layer is about 45 nm; (iv) forming a resist layer on the pair of dielectric layers; (v) patterning the resist layer to form a series of apertures therein; (vi) etching the pair of dielectric layers and the aluminum layer through the series of apertures in the resist layer to form a series of apertures in the mask, wherein each of the apertures extend through the pair of dielectric layers, and the aluminum layer, and wherein the apertures render portions of the mask transparent to the laser radiation; and (vii)removing the patterned resist layer.
In accordance with yet another embodiment of the present invention, a radiation reflective mask is provided comprising a substrate, a patterned metallic layer, and at least one pair of patterned dielectric layers. The substrate is transparent to radiation of a selected range of wavelengths. The patterned metallic layer is formed over an upper surface of the substrate an
Glass Thomas R.
Schofield Kevin H.
Killworth, Gottman Hagan & Schaeff, L.L.P.
Micro)n Technology, Inc.
Rosasco S.
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