Patent Application
20050167284
1. Field of the Invention
The present invention relates to a method for wet processing, and more particularly to a method for removing photoresists by electrostripping. Specifically, the present invention relates to a method of mechanically or physically lifting the photoresist off a surface by the generation of gas bubbles.
2. Description of Related Art
Photoresists are widely used in electronics industry for pattern definition on a conductive surface before electrolytic plating. Photoresists are light-sensitive compounds, which upon irradiation by a suitable light or other radiation source change their chemical structure. Organic polymer resist layers are usually applied in the form of either a dry film or liquid. If the resist is of a “negative” photo defined type, the underlayer is exposed to a pattern defining artwork and the unexposed portion of the resist is developed off. In a “positive” photo defined type, the exposed areas are rendered soluble in a developing solution. As an alternative, the resist may be defined using a screen-printing process, or less commonly, using electron beam or laser ablation.
During the chemical process, such as plating, the photoresist is generally subject to structural and chemical modifications. Consequently, removal of the photoresist can present some problems. Also, the thermal stressing and ultraviolet hardening of the photoresist affects the ability to remove the photoresist. Furthermore, the chemical removal of photoresists continues to pose environmental concerns.
Applications for photoresists include second level (board) and first level (substrate) packaging as well as processing of semiconductor wafers and magnetic heads, among others. Stripping of photoresists may be performed by using wet chemical processing or by a dry process such as oxygen plasma etch. Wet chemical processes used for resist stripping may involve water based strippers such as sodium or potassium hydroxide, tetramethyl ammonium hydroxide (TMAH) or organic strippers such as N-methyl-pyrrolidone (NMP). The resist stripping involves a chemical attack of the stripper on the resist resulting in its surface modification, swelling, and at least some degree of dissolution in the stripper, which frequently leaves some residue. Stripping operations may also be aided by a spray or ultrasonic type agitation aimed at lifting the chemically attacked resist.
When resist stripping is performed by chemical means, the interface bond between the resist and the underlying metal surface is chemically broken. Generally, sodium hydroxide in water chemically attacks the resist. However, the chemical bond is normally weaker than the mechanical bond between the surfaces. It would be advantageous to employ a process that specifically targets the mechanical bonds. It is desirable to physically lift the photoresist from the underlying surface. It is also desirable to utilize the semiconductor wafer in-situ as part of the electrolytic process.
In U.S. Pat. No. 5,676,760 issued to Aoki, et al., on Oct. 14, 1997, entitled “METHOD FOR WET PROCESSING OF A SEMICONDUCTOR SUBSTRATE,” electrolyzed waters, including an anode water and a cathode water, are applied in a wet processing method for cleaning, etching, and rinsing processed semiconductor wafers. This process, however, is limited in that electrostripping and physical lifting of the resist are not performed.
In U.S. Pat. No. 4,968,398 issued to Ogasawara on Nov. 6, 1990, entitled “PROCESS FOR THE ELECTROLYTIC REMOVAL OF POLYIMIDE RESINS,” electrolysis is conducted using an exposed copper layer as a cathode on a polyimide resin and an insoluble anode to remove undesirable remaining polyimide resin from the substrate. Ogasawara, however, does not teach removing a photoresist, or replacing a typical chemical stripping processes. In the Ogasawara invention, photoresists are used to pattern a copper seed before copper electroplating, and stripped chemically from the copper surface. Moreover, photoresists are used to pattern the polyimide and copper surface before the polyimide etch step. Electrostripping of any photoresist is not taught or disclosed.
In U.S. Pat. No. 6,436,276 issued to Yakobson on Aug. 20, 2002, entitled “CATHODIC PHOTORESIST STRIPPING PROCESS,” a printed wiring board having photoresist on its surface is used as a cathode during electrolysis in an alkaline solution. However, Yakobson uses a solution that remains capable of chemically resist stripping. In contrast, the present invention uses a solution that is not capable of chemical resist stripping. Furthermore, Yakobson separates the cathode and insoluble anode compartments by an ion selective membrane. No requirement is imposed by the present invention; the electrolytic cell is a single compartment cell.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method for electrostripping photoresist in chemically inert solutions.
It is another object of the present invention to provide an electrolytic method for photoresist stripping by physically lifting the photoresist from a surface.
A further object of the invention is to provide an electrolytic method for photoresist stripping that replaces any chemical stripping process or chemical stripping solution.
It is yet another object of the present invention to provide an apparatus for photoresist stripping to physically lift the photoresist from a piecepart while utilizing the piecepart in the electrolytic process.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects, which will be apparent to those skilled in art, are achieved in the present invention, which is directed to a method of stripping a photoresist from a piecepart having a conductive layer under the photoresist, comprising: immersing the piecepart in a neutral solution; generating hydrogen gas bubbles in the neutral solution for a period of time to lift the photoresist from the piecepart; and removing the piecepart for rinsing and drying after the photoresist is lifted. The method includes applying a voltage potential across the conductive layer and a counter electrode, while both are in the neutral solution. The voltage potential is applied such that the conductive layer is a cathode and the counter electrode is an anode. The neutral solution may include sodium citrate, sodium orthophosphate, sodium sulfate, sodium nitrate, ammonium acetate, sodium acetate, or a water-based ionic conductive chemical.
In a second aspect, the present invention is directed to a method of stripping a photoresist from a piecepart having a conductive layer under the photoresist, comprising: immersing the piecepart in a neutral solution; immersing an inert counter electrode in the neutral solution; electrically applying a potential difference across the piecepart and the counter electrode such that the piecepart acts as a cathode and the counter electrode acts as an anode; generating hydrogen gas in the neutral solution to lift the photoresist off the piecepart; and removing the piecepart from the solution for rinsing and drying. The piecepart may comprise a circuit board, substrate, metal mask, semiconductor wafer, or magnetic head. The counter electrode may comprise conductive non-corrosive material, such as stainless steel meshes or platinum covered titanium. The step of generating hydrogen gas comprises inducing a current between the cathode and the anode. The neutral solution remains incapable by itself of stripping resist. The photoresist includes Riston 4840. The photoresist may have a thickness of 2 mils in one embodiment with an induced current density on the order of 50 mA/cm2, or a thickness of 4 mils in a second embodiment with an induced current on the order of 100 mA/cm2. In the first embodiment the piecepart may be immersed in the neutral solution at a concentration on the order of 50 g/l at room temperature. In the second embodiment, the piecepart may be immersed in the neutral solution at a concentration on the order of 150 g/l at 30° C. The method further includes maintaining the piecepart in the neutral solution with the voltage potential applied for approximately three to fifteen minutes.
In a third aspect, the present invention is directed to an apparatus for electrolytic stripping of a photoresist from a piecepart having a conductive layer under the photoresist, comprising: an electrolytic cell containing a neutral water-based ionic conductive solution, the piecepart, and an electrode; and a power supply having a negative terminal electrically attached to the conductive layer and a positive terminal electrically attached to the electrode, and generating a potential difference between the conductive layer and the electrode; such that hydrogen gas is generated in the solution when the potential difference is applied.
The features of the invention believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The figure is for illustration purposes only and is not drawn to scale. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawing, in which
In describing the preferred embodiment of the present invention, reference will be made herein to
The piecepart 12 may be a circuit board, substrate, metal mask, semiconductor wafer, magnetic bead, and the like. When immersed in the tank with a voltage potential applied, it acts as a cathode. The conductive metal on the piecepart is held at a negative potential. The insoluble counter electrode 20 may include such material as stainless steel mesh, platinum covered titanium, or the like, and is used as the anode. As discussed above, the solution is preferably, a neutral, inorganic salt, capable of carrying a current without decomposition or chemical attack on the piecepart. Importantly, this technique allows the solution to be non-toxic, and thus more environmentally friendly for disposal. The applied voltage potential causes a current to pass through the cell. It is the decomposition of water at the cathode that results in generation of hydrogen gas bubbles around the photoresist and at an interface between the photoresist and the conductive surface. The hydrogen bubbles mechanically lift the resist from the conductive surface. In addition, the pH of the electrolyte, in close vicinity of the conductive surface, becomes alkaline due to water decomposition/hydrogen gas evolution. This in-situ increase in the solution's pH aids in the resist stripping by attacking the interface between the resist and a conductive surface upon which the resist was deposited. As the resist stripping and hydrogen gas evolution takes place at the cathode, oxygen is generated at the insoluble anode. The stripped resist is not chemically degraded. It retains its original shape and pattern. When the stripping is completed, the resist floats to the top surface of the solution and is easily removed by filtering. The piecepart is then removed from the solution, rinsed and dried.
The following two embodiments illustrate the invention:
In a first preferred embodiment, a metal mask is patterned with a photoresist such as Riston 4840, generally on the order of 2 mils thick, and immersed in a sodium citrate solution at a concentration of 50 g/l at room temperature. A voltage potential is applied to induce a current between the mask and a stainless steel counter electrode (anode) at a current density of approximately 50 mA/cm2. At these conditions, complete lifting of the resist from the mask may be achieved in about three minutes.
In a second embodiment, a semiconductor wafer with a conductive copper layer is patterned with a photoresist, such as Riston 4840, generally on the order of 4 mils thick, and electroplated. Thereafter, the wafer is immersed in electrolytic cell containing 150 g/l of sodium sulfate solution at 30° C. A voltage potential is applied to induce a current between the semiconductor wafer and the counter electrode at a current density of approximately 100 mA/cm2. Current is passed through the cell for approximately seven minutes, resulting in a complete lifting of the resist from the wafer.
Typically, current densities may be adjusted to lift all of the resist from the piecepart in about three to fifteen minutes. Once the resist is lifted, the piecepart is removed from the solution, rinsed and dried.
