RESIST STRIPPING PROCESS

Abstract: A process for removing patterned resist from the surface of a substrate during manufacture of printed wiring boards is disclosed. The process includes the steps of contacting the patterned resist with a stripping solution containing an alkalinity source as well as a source of ammonium ions. The stripping solution is characterized in that it does not contain volatile organic compounds (VOCs).

Description: - 1 - RESIST STRIPPING PROCESS Field of the Invention The present invention is in the field of printed wiπng board fabπcation. More specifically, the present invention relates to a process for removing or stnppmg resist mateπal from the pπnted wiring board following plating or etching process steps. Background of the Invention The majority of pπnted wiring boards (PWB) that are currently manufactured are made utilizing a combination of subtractive (etching) and additive (plating) techniques. In order to define a conductive pattern on a surface of the PWB, an organic polymer resist layer, typically m the form of a dry film or a liquid, is applied to a copper-clad panel. In the case in which the resist is of a negative working photo-defined type, the resist- coated panel is exposed to electromagnetic energy, such as visible light or ultraviolet (UN) radiation, which is projected through a pattern. The pattern defines artwork corresponding to a positive or negative image of the metallic traces to be formed on the board. Exposed portions of the negative-type resist undergo a chemical change which allows them to remain on the board. The unexposed portions of the resist are developed off, (i.e., removed). In contrast, in a positive working system, the exposed areas of the resist are rendered soluble in the developing solution. Thus, the soluble areas are removed, leaving the patterned unexposed areas of resist on the board. Alternatively, the resist image may be defined or patterned using a screen printing process. Even less commonly, an electron beam or laser ablation can be used to define a pattern in the resist. After the resist image has been defined, the PWB surface will be exposed to plating or etching steps depending upon the specific board type and the particular manufacturing technique selected. Following this exposure, it is normally necessary to remove the photopolymer resist layer to allow further processing. The removal step is accomplished using a resist stπpping process. Strongly alkaline ("caustic") solutions are commonly used as resist removers. Among the most commonly used are sodium hydroxide and potassium hydroxide. The most significant drawbacks to the use of caustic solutions are their slow stπpping speed, short life and so-called "sheeting". This last effect results because caustic solutions tend to swell most resists, rather than break them up into small particles, and stπp them in large sheets. Such sheets are undesirable because they can re-deposit and stick to the copper surface of the board, creating - 2 - problems in subsequent etching steps. Also, sheets of stπpped resist can clog filters and nozzles in spray machines resulting in substantial maintenance problems. Caustic-based strippers are not commonly used for the "outerlayers" of PWB's where the resist is typically used to define the image for an electroplating step. This is due to the fact that it is virtually impossible to cleanly strip swelled resist between acid copper and tin/tin-lead overplated fine-line traces. In addition to that, caustic tends to attack tin or tin-lead etch resist. Thus, caustic strippers tend to be used for less demanding "innerlayers" or "print & etch" applications, where the resist is simply used to define the pattern for an etching step. Unfortunately, the problems of re-deposition and maintenance described above still remain. Another known type of stripping composition consists of a mixture of aliphatic or cyclic organic, (i.e., carbon containing), amines, organic quaternary ammonium hydroxides and organic solvents. Sometimes inorganic alkali metal hydroxides are also used as secondary alkalinity carriers. U.S. Patent No. 5,545,353 describes a photoresist stripper composition containing an organic polar solvent, an alkanolamine and a thiocarboxylic acid as a corrosion inhibitor. U.S. Patent No. 5,556,482 teaches a method of stripping photoresist with a composition comprising organic polar solvents, basic amines and an inhibitor. U.S. Patent No. 4,904,571 discloses a method for removing photoresist in solution comprising alcohols, ethers, ketones, chlorinated chlorocarbons, aromatic hydrocarbons, alkali metal hydroxides, carbonates, phosphates, pyrophosphates, borohydrides and organic amine borane compounds. In most known stripping compositions, the organic part of the stripper, containing amine and a solvent mixture, acts as the primary stripping agent. Such compositions are known to offer high performance, (i.e. high speed), long life and small stripped resist particle size. Unfortunately, the organic part of the stripper composition consists of volatile organic compounds (hereafter VOCs). Due to increasingly stringent government environmental laws and regulations, the printed wiring board industry currently is under heavy pressure to reduce VOC emissions. VOCs are typically defined as any volatile compound of carbon, (excluding methane, carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, ammonium carbonate and exempt compounds), which participates in atmospheric photochemical reactions. VOCs react photochemically in the troposphere with nitrogen oxides (NO x ) in the presence of ultraviolet radiation from sunlight and atmospheric oxygen to form ground-level ozone — the - -> - pnmary component of "smog", which is a mixture of NO x , peroxyacyl nitrate (CH 3 COONO 2 ), VOCs and ozone. As a result, VOCs are regulated as "ozone precursors" Summary of the Invention The present invention relates to processes for removing or stπpping resist mateπal from the pπnted wiπng board following plating or etching process steps More specifically, the present invention relates to a process for removing resist patterns from a substrate which compπses the steps of providing a substrate having a patterned or layered resist on a surface, providing a VOC-free resist stπpping solution which includes a source of an ammonium ion, and exposing the substrate to the stπpping solution for a time sufficient to remove all patterned or layered resist from the surface. Surpπsmgly, ammonium ions have been found to be very effective m stripping resist patterns or layers. Thus, the present invention provides a simple photoresist stπpping process that does not use VOCs, and as such, the invention eliminates a significant source of environmental contamination. Detailed Description One of the steps of the PWB manufactuπng process that is being targeted for VOC emission reduction, and eventual elimination, is resist stπppmg Surpπsmgly, it has been found that the organic mixture of amines, solvents and quaternary ammonium compounds commonly present in known resist stπpping compositions can be successfully replaced by an inorgamc non- VOC-contaming solution containing a source of ammonium ions, such as ammonia gas or ammonium hydroxide. Ammonia-based compounds are already used in the PWB industry, and ammonium hydroxide is used extensively in ammomacal etchants for vanous copper etching processes. As discussed above, the etching step is a step just preceding resist stπpping in mnerlayer production or subsequent to resist stπpping in outerlayer manufactuπng Ammonia is very soluble in aqueous solutions and once dissolved, depending on pH. it can react with hydrogen (hydromum) ions to form ammonium ions This reaction occurs if hydrogen ions are readily available, such as at low pH In alkaline solution, where hydrogen ions are not readily available, the ammonia remains in its gaseous form At a pH of greater than about 9 3 (at 25°C), 50% of ammonia is in the form of the ammonium ion, whereas at a pH of about 12, almost 100% of ammonia remains in gaseous form At elevated temperatures, the equihbnum is shifted further toward gas - 4 - Conventional resist stπppers are normally used under conditions at which the pH is greater than about 1 1 and the temperature is in the range of about 120-130°F (about 49-54°C) At these conditions, virtually all of the ammonia would stay in gaseous form and volatilize in a matter of hours As such, ammonia gas was never commercially utilized as a pnmary resist stnppmg agent Although U S Patent No, 3,980,587 discloses a method for stnppmg certain resists using a solution containing 10 to 30 lb per gallon (about 1.2-3.6 kg/1) of a concentrate containing ethylene glycol monobutyl ether as a pnmary resist stnppmg agent (50-55% by weight), EDTA salt, acetic acid and an alkaline stnpper component (19% by weight) containing a 30-1 mixture of potassium hydroxide and ammonium hydroxide, ammonium hydroxide at this negligible concentration would not contnbute appreciably to the stnppmg action of the organic stnppmg agent (ethylene glycol monobutyl ether) and the alkalinity earner (potassium hydroxide). Likewise, U.S Patent No 4,078,102 desenbes a process for stnppmg resist in a mixture of an aldehyde or a ketone as pnmary resist stripper agent and an activator, containing an alcoholic solution of ammonium, alkali and alkali earth metal hydroxides in proportion of 0.005 to 0.1 mole activator to 1 mole of aldehyde or ketone In a more preferred embodiments, the patent suggests a saturated isopropyl alcohol solution of sodium hydroxide (alkalinity earner) and cyclohexanone (pnmary resist stnpper) in proportion 0 006-0.01 to 1 Once again the stπpping action of this composition can be attributed to the organic part of the stnpper rather than to the inorganic alkalinity earner The concept behind current invention is to provide a fully aqueous VOC-free resist stnppmg process utilizing ammonia gas or other source of ammonium ions as a pnmary resist stnppmg agent without utilizing any volatile organic matenals The invention also relates to a method to maintain constant and sufficiently high ammonium ion concentration m alkaline solutions (pH>9) at elevated temperatures, (i.e , 80-140°F) Optional bath components include non-volatile inhibitors to prevent corrosion of metal surfaces exposed to the stnppmg solution, and antifoammg or defoammg agents to control foam generation if the solution is subjected to high agitation to enhance the stnppmg action In order to prolong the life of stnppmg solutions, most modern resist stnpper systems normally operate in so-called "feed and bleed" mode In this mode, a portion if the stnppmg bath is continuously removed and the bath is continuously replenished with fresh stnpper solution - 5 - Under these conditions ammonium ion concentration can be easily maintained at a constant level by utilizing any of the following methods: a) continuously feeding an ammonium hydroxide solution into the stripper solution; b) continuously feeding an alkaline ammonium salt (carbonate, carbamate, silicate, formate, phosphate, sulfite, etc.), which will convert into ammonium gas at pH>12 and 120- 130°F; c) continuously injecting compressed ammonia gas directly into the stripping solution; d) withdrawing and recovering ammonia gas from ammonia-rich ammoniacal etchant vapor and continuously injecting it into the stripping solution, thereby recycling it. The stripping solution can be applied to the developed photoresist using any of a wide variety of methods known in the art. For example, the board containing the developed photoresist may simply be immersed into a bath of the stripping solution. Alternatively, the stripping solution may be sprayed onto the board. It is noted that the methods described herein to apply the stripping solution to the developed photoresist are not intended to limit the invention in any way. It should further be recognized that the source of the ammonium ions is not intended to be limited to those species described above, but rather may be any ammonium ion source that can be used without introducing VOCs into the process. Likewise, it should be apparent that it is desirable to maintain the stripping solution at a caustic pH, both for the stripping ability of the caustic and for the ability of the caustic solution to convert ammonium ions into a source of ammonia gas. Although several compounds are described above for providing and/or maintaining the caustic nature of the stripping bath, it is to be understood that the invention is not intended to be limited strictly to those compounds. Rather, virtually any source of alkalinity can be used provided that it does not introduce VOCs into the process. Experimental Example 1 A copper-clad panel was hot-roll laminated using DuPont Riston 9020 dry film, exposed and developed in 1% sodium carbonate solution. A resist stripping solution containing 20 grams per liter of potassium hydroxide and 10 milliliters per liter of ammonium hydroxide (30% aqueous solution) was provided. The stripping solution was heated to about 125°F (about - 6 - 51.7°C) and the panel was immersed into the solution. In less than 60 seconds, all the resist was completely removed from the surface of the panel. Example 2 A copper-clad panel was hot-roll laminated using Dynachem Laminar GA dry film, exposed and developed in 1% sodium carbonate solution. A resist stripping solution containing 50 grams per liter of sodium hydroxide and 20 grams per liter of ammonium bicarbonate was provided. The stripping solution was heated to about 130°F (about 54.4°C) and the panel was immersed into the solution. In less than 60 seconds, all the resist was completely removed from the surface of the panel. Example 3 A copper-clad panel was hot-roll laminated using DuPont Riston 9015 dry film, exposed and developed in 1% sodium carbonate solution. A resist stripping solution containing 25 grams per liter of lithium hydroxide and 15 grams per liter of ammonium carbamate was provided. The stripping solution was heated to about 120°F (about 48.9°C) and the panel was immersed into the solution. In less than 60 seconds, all the resist was completely removed from the surface of the panel. Example 4 Three copper-clad panels were hot-roll laminated using DuPont Riston 9020 dry film, exposed and developed in 1% sodium carbonate solution. The stripping solution from Example 1 was heated to about 125°F (about 51.7°C) and the first panel was immersed into the solution. In less than 60 seconds, all the resist was completely removed from the surface of the panel. The solution was stirred at about 125°F (about 51.7°C) for 2 hours to volatilize ammonia. The second panel was immersed into the solution. After 60 seconds, the resist was not completely removed from the surface of the second panel. Ten milliliters of 30% ammonium hydroxide were added to the stripping solution, and the third panel was immersed into the solution. In less than 60 seconds, all the resist was completely removed from the surface of the third panel. Comparative Example 1 A copper-clad panel was hot-roll laminated using DuPont Riston 9020 dry film, exposed and developed in 1% sodium carbonate solution. A resist stripping solution containing 20 grams per liter of potassium hydroxide was provided. The stripping solution was heated to about 125°F (about 51.7°C) and the panel was immersed into the solution. After 60 seconds, the resist was not removed from the surface of the panel. - 7 - Comparative Example 2 A copper-clad panel was hot-roll laminated using DuPont Riston 9020 dry film, exposed and developed in 1% sodium carbonate solution. A resist stripping solution containing PC-4077 Resist Stripper (available from Alpha Metals PC Fab Division) 10% (by vol.) was provided. The stripping solution was heated to about 125°F (about 51.7°C) and the panel was immersed into the solution. After 60 seconds, the resist was completely removed from the surface of the panel. Equivalents From the foregoing detailed description of the specific embodiments of the invention, it should be apparent that a unique resist stripping process has been described. Although particular embodiments have been disclosed herein in detail, this has been done by way of example for purposes of illustration only, and is not intended to be limiting with respect to the scope of the appended claims which follow. In particular, it is contemplated by the inventor that various substitutions, alterations, and modifications may be made to the invention without departing from the spirit and scope of the invention as defined by the claims.

Claims: - 8 - CLAIMS 1. A process for removing patterned resist from a substrate which comprises the steps of: a) providing a substrate having a patterned resist on a surface; b) providing a VOC-free resist stripping solution which includes a source of an ammonium ion; and c) exposing the substrate to the stripping solution for a time sufficient to remove all patterned resist from the surface. 2. A process as in Claim 1, wherein the VOC-free resist stripping solution further includes a source of hydroxide ions. 3. A process as in Claim 2 wherein the source of hydroxide ions is selected from the group consisting of potassium hydroxide, sodium hydroxide, lithium hydroxide and combinations thereof. 4. A process as in Claim 1 wherein the source of the ammonium ion comprises ammonia gas. 5. A process as in Claim 1 wherein the source of the ammonium ion comprises ammonium hydroxide. 6. A process as in Claim 1 wherein the source of the ammonium ion comprises an ammonium salt. 7. A process as in Claim 1 wherein the ammonium salt is selected from the group consisting of ammonium carbonates, ammonium carbamates, ammonium sulfites, ammonium silicates, ammonium formates, ammonium phosphates, and combinations thereof. 8. A process as in Claim 1 wherein the VOC-free resist stripping solution further includes a non-volatile corrosion inhibitor. - 9 - 9 A process as in Claim 1 wherein the VOC-free resist stπpping solution further includes an antifoammg agent. 10 A process as in Claim 1 wherein the VOC-free resist stnppmg solution further includes a defoam g agent 11 A process as in Claim 1 wherein the exposing step compnses immersing the substrate in the stnppmg solution 12. A process as m Claim 1 wherein the exposing step compnses spraying the stπpping solution onto the substrate 13. A process as in Claim 1 wherein the source of the ammonium ion in the stnppmg solution is peπodically or continuously replenished. 14. A process as m Claim 13 wherein the replenishment compnses peπodically or continuously adding an ammonium hydroxide solution to the stnpper solution 15 A process as in Claim 13 wherein the replenishment compnses peπodically or continuously adding an alkaline ammonium salt to the stnpper solution 16. A process as in Claim 13 wherein the replenishment compπses peπodically or continuously adding ammonia gas to the stnpper solution. 17 A process as m Claim 1 wherein the pH of the stπpping solution is maintained at or above about 9.3 18 A process as in Claim 1 wherein the pH of the stπppmg solution is maintained at or above about 11.0 19 A process as in Claim 1 wherein the pH of the stπpping solution is maintained at or above about 12.0 - 10 - 20. A process as in Claim 1 wherein the stripping is carried out at a temperature in the range of about 80°F to about MOT.

Inventors: YAKOBSON ERIC, YAKOBSON, ERIC
Applicants: ALPHA METALS, ALPHA METALS, INC.
Application year: 1999
Main classification: G03F7
IPC codes: G03F7, H05K3