Etching a substrate: processes – Nongaseous phase etching of substrate – Substrate is multilayered
Reexamination Certificate
1999-11-01
2002-10-22
Powell, William A. (Department: 1765)
Etching a substrate: processes
Nongaseous phase etching of substrate
Substrate is multilayered
C216S013000, C216S020000, C216S100000, C216S102000, C216S105000, C438S754000
Reexamination Certificate
active
06468439
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the etching of metal layers in combination or electrical contact with other metal layers. The invention also relates to particular etchant solutions used to etch metal layers, and particularly to etch aluminum layers in Galvanic contact with other metal layers. Articles formed from etched multiple metal layers may be used in electronic devices, and more particularly in circuitry or circuit boards. The invention is especially advantageous when applied to manufacturing processes for thin circuit boards, particularly circuit boards with multiple layers of individually prepared circuits. These circuit boards provide physically durable structures, even when including fragile microstructure such as air bridges.
2. Background of the Art
Metals have been conveniently etched to form complex designs, including circuitry, for many years. A mask or resist layer in the shape of a pattern to be removed from a metal layer is placed over a surface of the metal layer, a solution that dissolves or removes the metal is conracted with the mask on the surface of the metal layer, and because the soltuion can contact only the exposed metal through the mask directly, a pattern of metal is removed from the metal layer that corresponds to the pattern in the mask. Although the underlying technology for this process appears fairly simple and direct, there are many complications in the actual performance of the process. For example, the resist must provide good resolution for the image, the etchant solution must have properties so that it etches the metal along vertical lines rapidly, yet doesn't undercut (etch along the side walls produced by the etching process), and with metal layers composites (elements having more than one metal layer, and layers of different metals) shoul be able to be etched one adjacent layer at-a-time, so that the solution does not etch completely through all of the layers. These are high performance standards that are not readily achieved,
A technique for forming a tri-layer metal structure is described in U.S. Pat. No. 5,428,250 to Ikeda et al. The tri-layer metal structure is formed on a glass substrate. The first layer is a Ta—M—N film, the second layer is a Ta film and the third (top) layer is a Ta—M—N film, where M is at least one atom selected from the group consisting of Molybdenum, Niobium, and Tungsten.
U.S. Pat. No. 5,153,754 to Whetten described a tri-layer metal structure formed on an LCD substrate where the first layer is a titanium (Ti) film, the second layer is a molybdenum (Mo) or aluminum (Al) film, and the third (top) layer is a titanium (Ti) film. In addition, colum 6, lines 56-70 describe a process to taper etch the tri-layer metal structure. When the second layer is a molybdenum film, the tri-layer structure is formed by wet etching the titanium first layer with fluoroboric acid (HBF
4
), wet etching the molybdenum second layer with PAWN (phosphoric acid, acetic acid, water and nitric acid), and dry etching the titanium third layer in a plasma barrel etcher with an atmosphere of CF
4
and O
2
(or SF
6
and O
2
). When the second layer is an aluminum film, the tri-layer structure is formed in a single etch step by an RIE (reactive ion etching) etch of BCl
3
, CCl
4
and O
2
.
U.S. Pat. No. 5,464,500 to Tsujimura et al. describes a tri-layer metal structure formed on a glass substrate. A silicon oxide layer is formed on the glass substrate. The first metal layer of Aluminum (Al) is formed on the silicon oxide layer. The second metal layer of aluminum oxide is formed on the first metal layer. The third metal layer of molybdenum is formed in the aluminum oxide layer. Beginning at column 3, line 60, a process for taper etching the tri-metal layer is described. As a result, the cross section of the first metal layer of aluminum is formed with a taper angle.
U.S. Pat. No. 4,824,803 to Us et al. describes a tri-layer metal structure formed on a glass layer wherein the first metal layer is a titanium (Ti) film, the second metal layer is an Aluminum (Al) film, and the third metal layer is a titanium film. As described beginning at column 2, line 45, the tri-metal structure is formed in a single RIE etch step of a chlorine based chemistry. As shown in
FIGS. 1
a
and
1
b
, the RIE etch step results in a non-tapered structure with vertical sidewalls.
U.S. Pat. No. 4,650,543 teaches a GaAs FET electrode wiring layer or bonding pad having a three-layered structure of Au/Pt/Ti or a two-layered structure of Al/Ti. The electrode wiring layer or the bonding pad is sometimes formed by a wet etching method but mainly by a lift-off method. A method of forming a bonding pad by wet etching is described. In this case, an insulating film is formed on a GaAs semiconductor substrate by CVD, and thereafter a contact hole is selectively formed in the insulating film. A metal film for forming a bonding pad is deposited on the overall surface of the substrate, and a resist pattern is formed thereon. Finally, the metal film is etched by wet etching using the resist pattern as a mask so as to form a bonding pad of the metal film on the hole of the insulating film. In this method, since the GaAs semiconductor layer is highly sensitive to chemical treatment, when the wet etching method is used, side etching occurs. For this reason, this method is inappropriate for forming a micropattern such as a gate electrode. Note that in a GaAs FET, a submicron micropattern must be formed. Therefore, a lift-off method was developed for micropatterning. This method is described in U.S. Pat. No. No. 3,994,758. However, the metal film formed by this method was formed by CVD (Chemical Vapor Deposition) at a low temperature because of a resist film. For this reason, bonding between a metal multilayer and a semiconductor substrate constituting an electrode pattern was inadequate. Therefore, the electrode pattern was easily removed during lifting off or wire bonding, thus degrading the yield in manufacturing of the GaAs FET. This Patent asserted an advance in the technology by the electrode pattern having a multilayer structure selected from the group consisting of Au/WN, Au/W/TiW and Au/Mo/TiW (elements on the left side are positioned uppermost with respect to the semiconductor substrate). In an ion milling technique used in that invention, etching is performed by bombarding a member to be etched with ions of an inert gas such as Ar or At+O
2
gas using a shower or beam type device. This technique is inert dry etching and is also called ion etching. This ion milling technique has been disclosed in, e.g., Solid State Tech. March 1983, Japanese Edition p. 51 to 62. In a reactive ion etching technique, by using a parallel-plate, microwave or ion-shower type device, dry etching is performed by reactive plasma using a reactive gas mixture such as CF
4
+O
2
or CF
4
+Cl while activating a member to be etched using an inert gas such as Ar gas.
U.S. Pat. No. 5,912,506 addresses perceived problems of
(a) thinning of additional metal layers crossing the edges of the multi-layer metal structure;
(b) shorts or pinholes formed in one or more insulator layers above multi-layer metal structure due to near vertical or undercut edges; and
(c) controlling the effective width of the multi-layer structure when using an extended non-directional over-etch.
These problems are variously addressed by the invention of that Patent. A multi-layer metal sandwich structure formed on a substrate includes a first metal layer formed on the substrate and a second metal layer formed on the first metal layer. The second metal layer has tapered side walls. The width of the first metal layer is different than the width of the second metal layer at the interface of the first metal layer and the second metal layer. The multi-layer metal sandwich may also include a third metal layer formed on the second metal layer. The second metal layer may also be substantially thicker than the first or third metal layers. A method for forming the multi-layer metal
Brenes Jose F.
Dufresne Michael J.
Finger Bruce A.
Ring Charles
Whitehurst Donald A.
BMC Industries, Inc.
Mark A. Littman & Assoc. P.A.
Powell William A.
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