Stock material or miscellaneous articles – Structurally defined web or sheet – Discontinuous or differential coating – impregnation or bond
Reexamination Certificate
2000-09-05
2003-02-18
Lam, Cathy (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Discontinuous or differential coating, impregnation or bond
C430S016000, C430S018000, C428S901000
Reexamination Certificate
active
06521328
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains generally to the etching of copper during the fabrication of microelectronic packages and, more particularly, to copper etching compositions and various components derived therefrom.
BACKGROUND OF THE INVENTION
At the present time, most chemical etching of copper on microelectronic substrates is performed with either copper or ferric chlorides, chromium salts, alkaline-ammonia, hydrogen peroxide-sulfuric acid or nitric acid compositions. Each of these compositions has certain limitations and disadvantages as described hereinbelow.
The metal etchants, in particular the chromium salts, create a deleterious environmental impact. It is also known that chromium salts are human carcinogens; therefore, their use and disposal are especially problematic.
Nitric acid, either alone or in combination with sulfuric acid or copper nitrate, has been reported by Battey (U.S. Pat. No. 4,695,348) to be useful for etching copper in circuit boards. However, nitrogous oxide gas is a byproduct of this process. Moreover, the process provides anisotropic etching only for certain orientations of copper (i.e., the top surface of the copper crystal structure must have a Miller index of (111) and (200) orientation). This requires first sputtering copper and then electrodepositing more copper. In lieu of sputtered copper, evaporative deposition or electroless deposition is also possible.
The alkaline-ammonia compositions are used commercially because they are relatively fast, have substantial copper-carrying capacity and are reasonably tolerant of some metal resists and some dry film resists. However, these same compositions have poor selectivity for copper versus other metals and alloys. Significant process control is required to achieve acceptable selectivity. It is also known that these compositions may not work well with fine line copper geometries. Furthermore, the dissolved copper is difficult to recover. Also, fumes from the ammonia composition present worker exposure concerns.
The hydrogen peroxide-sulfuric acid compositions used in copper etching processes are very clean to operate and can be recycled. However, these same compositions have relatively slow etching rates and require substantial cooling for stability control due to the autodecomposition reaction of the hydrogen peroxide. Additionally, both the performance of the process and the decomposition of the peroxide are very sensitive to trace impurities via homo- or hetero-catalysis. Stabilizers are necessary for peak performance but these are metal specific. Brasch (U.S. Pat. No. 4,378,270) teaches phenol-sulfonic acid for copper containing solutions. It is also known from Alderuccio, et al (U.S. Pat. No. 3,269,881) that these compositions are adversely affected by chloride or bromide ion at levels of 2 mg/liter, causing reduced etch rates. Elias (U.S. Pat. No. 4,130,455) teaches that the addition of sodium or potassium thiosulfate can counteract this effect.
But use of these additives does not address the basic problem of the catalytic decomposition of the peroxide discussed hereinabove. This decomposition has two important implications: firstly, the depletion of the peroxide in the etchant solution reduces the etchant rate; and secondly, there is potential for uncontrolled decomposition of large volumes of high temperature solutions, generating high concentrations of oxygen and increasing the safety risks therefrom. Because decomposition of the peroxide is accelerated at elevated temperatures, processing temperatures must be kept low. This adversely affects the rate of the etching process and exacerbates the already low copper-carrying capacity of the peroxide-sulfuric acid composition. Finally, the process generates voluminous quantities of copper sulfate, which are difficult to reclaim or dispose of. It is possible to electroplate out the copper from the copper sulfate. Typically, however, this copper does not deposit as an adherent homogeneous ingot deposit, but rather comes out as a powder which sluffs off and makes recovery difficult. Brasch (U.S. Pat. No. 4,378,270) describes the use of phosphoric acid to assist in producing smooth adherent copper deposits on a cathodic surface, but this is at the expense of a slower etchant rate.
The prior art etchants are used in processes to manufacture various types of microelectronic packages such as printed circuit boards having planar resistors and laminate chip carriers. For each of these packages, complex processes must be employed to circumvent the above-mentioned problems.
There are several problems in attempting to etch fine line (less than 0.004″ width) printed circuits when using classical processes and chemistry. These problems are defined as follows:
1) Fluid mechanics problems—With classical print and etch processes, a photoresist is applied to the printed circuit board. Dry film photoresist materials are generally available in thicknesses of 0.001″ and greater, and typically are 0.0012″ or greater. As etching progresses through the Cu, the trough defined by the photoresist and the etched Cu becomes progressively deeper and the depth to width aspect ratio becomes progressively greater. As the aspect ratio increases, the etch uniformity decreases due to the non-uniform nature of getting fresh etchant into this trough. One solution to this problem has been to use liquid resist materials that can be applied by spray or electrodeposition methods at a reduced thickness.
2) Galvanic etching—Many processors utilize a nickel/gold plated etch mask over the copper for etching. Due to the significant difference in electrode potentials between the Au and Cu metals, the copper is etched at an accelerated, almost uncontrollable rate in currently known chemistries, except possibly for ammoniacal etchants. Galvanic etch effects can be compensated for somewhat by increasing the width of the Ni/Au resist image, such that the etched cu image after galvanic etch effects is of the desired width. The down side to this technique is that it is not compatible with fine line/fine space etch requirements. This galvanic etch effect may also result in an undesirable line geometry after etching. Another potential solution would be to eliminate the Au, using only Ni as the etch mask. This solution would require an etchant that does not etch Ni, but does etch Cu. Until now, such an etchant has been unknown, except for chrome-sulfuric chemistries, which have their own problems of environmental impacts, waste treatment, and poor copper capacity.
3) Miscellaneous Au etch mask problems—In addition to the galvanic etch problems, Au masking suffers from very high material costs, and inability to use for innerlayer etching due to the inability to subsequently adhere epoxy to Au during composite lamination of the innerlayers.
Subtractive etching to produce fine line circuits is especially difficult for external circuit layers on plated through-hole (PTH) printed circuit boards. Prior art technology has been unable to provide uniform line widths along with high-yielding processes. This problem is primarily due to the thick photoresist that must be used to tent PTHs on printed circuit boards. Furthermore, the thickness of the photoresist layer is related to the width of area to be etched; viz., the greater the thickness of the photoresist, the greater the spacing must be between the protected circuit lines. This relationship therefore limits the density of the circuit lines if thick photoresists are necessary.
To fabricate a photoresist circuit pattern, prior art methods initially apply a thin copper foil to a substrate and then subsequently apply, image and develop a photoresist to create a circuit pattern. The exposed, thin copper foil in the through-holes is electroless and/or electrolytically plated with copper followed by a nickel-gold etch mask. The photoresist is then stripped away and the underlying thin copper foil is fast-etched while the etch mask theoretically protects the newly formed circuit lines and through-holes. This process is very difficu
Covert Kathleen L.
Hawley Kelly
Lauffer John M.
Moschak Peter A.
Fraley Lawrence R.
Hawley Kelly
Lam Cathy
Salzman & Levy
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