Metal-Etching Process

The field of the present invention is precise metal etching.

Thin gauge precision metal parts have been produced by photochemical fabrication processes. One such process consists of coating a clean metal sheet with a layer of photosensitive resistive material, then covering it with a photomask. The photomask defines covered areas and exposed areas to achieve a pattern of the final product. The material directly under the photomask remains solvent dissolvable after ultraviolet radiation is applied over the mask. The photoresist layer under the clear area of the photomask is hardened when the workpiece is exposed to ultraviolet radiation. The hardened photoresist covers the desired portion of the metal work piece and forms a chemically-resistant etch mask, leaving the unwanted portion to the attack of an etchant, such as a chromic acid solution or ferric chloride. After chemical etching, the hardened photoresist can be removed by a hot organic stripper. This method has its limitations. For example, it cannot produce a clearly defined, straight, sharp and narrow slit of 0.001 inches in a 0.01 inch thick metal sheet.

Another method, generally referred to as the bi-metal method is capable of greater accuracy. In this process, a temporary photoresist etch mask is replaced by a permanent etch mask such as nickel, gold or silver. In the aforementioned process, the photomask is a negative. In this process, the photomask is a positive of the product resulting from the final etch. The photoresist is covered by an appropriate mask and subjected to ultraviolet radiation. Once the exposed portions of the photoresist are hardened, a solvent is employed to develop the photoresist by washing away the unexposed portions covered by the mask during ultraviolet radiation. The photoresist thus covers the portions to be etched away in the final process. The exposed metal is then electroplated with the permanent mask. Once the permanent mask has been deposited, the hardened photoresist is washed away by hot organic strippers. A chemical etch is then used which does not etch the permanent mask but etches the now-exposed areas of the substrate previously covered by the hardened photoresist. In this process, electroplated nickel and beryllium-copper alloy is typically employed. The etchants used are ammonium hydroxide and cupric chloride or chromium trioxide and sulfuric acid. A difficulty with the bi-metal process is the expensive and limited types of materials which may be used for the process and to make up the substrate.

An improved metal etching process is the subject of this invention. The process consists of developing a photoresist on a substrate, plating the exposed substrate with an etchant resistant electroless nickel and etching the substrate not covered by the electroless nickel plating. One means by which electroless nickel may achieve etchant resistance is by employing a phosphorous rich nickel bath. Through such a practice, ferric chloride may be used which is capable of etching a variety of metals.

The process constitutes an improvement over the old step of electrolytically depositing a thick layer of nickel alloy onto a substrate. The improved method has the advantage of utilizing an inexpensive and highly corrosion resistant protective coating. The process enables less expensive material to be used as a substrate such as stainless steel and other iron alloys with or without magnetic properties. The process allows for a uniform deposition of material. In electroforming methods, the corners and edges of the substrate tend to receive heavier metal distribution due to nonuniform current densities. Also, the process allows for extremely fine and precise etching to be performed on the surface of the substrate.

Accordingly, it is an object of the present invention to provide an improved etching process. Other and further objects and advantages will appear hereinafter.

FIG. 1 is a side view of a photoresist coated metal substrate with a photomask prior to exposure to ultraviolet radiation;

FIG. 2 is a side view of the substrate after it has been exposed to ultraviolet radiation and developed;

FIG. 3 is a side view of the coated substrate after it has been developed and then had a nickel layer applied to it;

FIG. 4 is a side view of the substrate after the hardened photoresist is removed by hot organic stripper; and

FIG. 5 is a side view of the substrate after etching.

Turning in detail to the Figures, an improved process as one embodiment of the present invention is comprised in the following sequence of steps.

(1) A substrate, metal sheet 10, is scrubbed with pumice or other gritty cleaner. The metal 10 can be made of a material such as stainless steel 301, 302, 304, or 316; a steel alloy; or a nickel-iron alloy such as Kovar. Also, copper alloy such as brass or berylium-copper can be used as a metal substrate. Many sheet metal materials are acceptable for use as the substrate. After the metal substrate has been scrubbed, it is rinsed.

(2) A thin layer of electrolytic nickel 11 in a Wood's nickel chloride bath at 60 amperes per square foot is then applied or plated onto to the scrubbed metal surface. The application is to be for approximately two minutes followed by a rinse.

(3) The nickel plated and coated substrate is subsequently immersed in a 25% by volume solution of phosphoric acid at a 140.degree. Fahrenheit for three minutes.

(4) The nickel plated and coated substrate is then dried and a photoresist layer 12 applied. The photoresist layer may be "Waycoat 450" a trademarked product of Hunt Chemical. Other materials are available. It is preferable that the material is polymerized by ultraviolet light. The photoresist is normally applied as a liquid. The coating is then baked to solidify the photoresist on the surface of the substrate.

(5) A photomask 14 is then applied on top of the photoresist layer. The photomask 14 is a transparency, the darkened areas 16 thereof containing an emulsion and the clear areas 18 thereof not containing an emulsion. The surface of the substrate to be etched is beneath the clear area of the transparency while the darkened area masks ultraviolet radiation from the photoresist. See FIG. 1.

(6) With the photomask in place, the photoresist is exposed to ultraviolet radiation. After the coated substrate 10 has been exposed to ultraviolet light, the exposed, hardened photoresist 12 forms a temporary mask covering the unwanted portion of the substrate, the unwanted portion of the substrate being that portion that will be etched away. The desired portion of the substrate 10 is left under unpolymerized or unhardened photoresist 12 and subject to be dissolved by solvents.

(7) After exposure to ultraviolet radiation, the coated substrate 10 is then developed. Developing occurs in a solvent such as 1-l-l trichloroethane. Particular solvents are typically recommended for particular photoresists. See FIG. 2.

(8) After developing, the substrate 10 is then baked at 80.degree. Centigrade for ten minutes to add durability to the remaining developed photoresist for further processing.

(9) After the substrate is baked it is immersed in a solution of 50% hydrochloric acid for approximately 30 seconds. After immersion the coated substrate should then be rinsed.

(10) The substrate is further reactivated with a very dilute acidic ferric chloride solution. An appropriate solution consists of 5% hydrochloric acid and 4% ferric chloride. The coated substrate should then be rinsed.

(11) A thin layer 19 of electrolytic nickel is then applied to the nickel plated surface of the substrate which has been exposed by the selective removal of the photoresist coating in steps 7-10 above. This second thin layer 19 of electrolytic nickel is applied in a Wood's nickel chloride bath at 60 ampere per square foot for one minute. Some thin coating is preferred for proper adherence of the electroless nickel coating to follow. The Wood's nickel chloride bath is convenient as it may serve for both steps (2) and (11).

(12) The coated substrate 10 is then plated in an electroless nickel bath at 80.degree. to 90.degree. Centigrade to the desired thickness. The nickel coating 20 preferably consists of 88 to 90% nickel and 10 to 12% phosphorous, with a preferred minimum of 10% phosphorous. If substantially less than 10% phosphorous is used under the circumstances described above, the later use of ferric chloride will tend to etch the nickel. Phosphorous rich electroless nickel baths are commercially available. FIG. 3 is representative of the substrate 10 after this step.

(13) The hardened photoresist 12 should then be stripped using a commonly available photoresist stripper. The voids 22 left by the stripped photoresist are shown in FIG. 4.

(14) The substrate can then be etched in an acidic ferric chloride solution. The ferric chloride solution may be 45% by weight ferric chloride. The substrate can be etched in a commercially available etching machine. FIG. 5 shows that the exposed metal surface in FIG. 4 is etched away at 24 by the ferric chloride solution.

At the present time the bi-metal etched components (encoder disks, optical targets, etc.) available in the market consist of two components, i.e., electroplated nickel and beryllium-copper alloy. By the present invention, other and less expensive materials may be used. Thus, an improved etching process is disclosed for precise working of metals. It is intended that the above illustration of the invention is by way of example only and not intended to limit the invention in any way except in the spirit of the appended claims.