KR20010041221A - Resist stripping process - Google Patents

Abstract

The present invention is directed to a method of removing patterned resist from a substrate surface in manufacturing a printed wiring substrate. The method includes contacting the patterned resist with a stripping solution containing an alkali source and ammonium ions. The stripping solution is characterized in that it does not contain a volatile organic compound (VOC).

Description

How to remove resist {RESIST STRIPPING PROCESS}

Most printed wiring boards (PWBs) currently manufactured are a combination of subtractive (etching) and additive (plating) techniques. In order to form a conductive pattern on the surface of the PWB, an organic polymer resist layer, usually present in dry film or liquid form, is applied to a copper-clad panel.

When the resist is a negative functional photo-forming type, when the resist coating panel is exposed to electromagnetic energy, such as visible or ultraviolet light, it is projected through the pattern. The pattern forms artwork corresponding to a positive or negative image of a metal trace formed on the substrate. Exposed areas of negative resists cause chemical changes that leave these areas on the board. Unexposed areas of the resist are developed and removed. In contrast, in a positive action system, the exposed sites in the resist become soluble in the developing solution, so removing the soluble sites leaves the patterned unexposed sites in the resist on the board. Alternatively, a screen printing process can be used to form or pattern the resist image. Although not commonly seen, electron beams or laser ablation may be used to form patterns in the resist.

After the resist image is formed, the PWB surface is exposed to a plating or etching step, depending on the particular board type and the particular fabrication technique chosen. After this exposure, it is necessary to remove the photopolymer resist layer to perform further processing. The removal step is performed using a resist stripping process.

Strong alkaline ("caustic") solutions are commonly used as resist removers. The most commonly used removers are sodium hydroxide and potassium hydroxide. The biggest problem with using caustic solutions is their slow removal rate, short life and so-called "sitting". Sitting here refers to the tendency of the caustic solution to expand most of the resist, rather than breaking them into smaller particles and stripping them into larger sheets. Such sheets are undesirable in that they are deposited again in the subsequent etching step and adhere to the copper surface of the board. In addition, the removed resist sheet clogs the filters and nozzles on the spraying machine, causing substantial maintenance problems.

Caustic removers are not commonly used in "outer layer of PWB". Here, resist is generally used to form an image of the electroplating step. This makes it practically impossible to clean strip the expanded strip between fine line traces overplated with acid copper and tin / tin-lead. In addition, caustic tends to attack tin or tin-lead etch resists. Therefore, caustic strippers are used in applications requiring less "inner layer" or "print & etch", where the resist simply forms a pattern of etching steps. Unfortunately, however, the above-mentioned problems of re-deposition and maintenance still remain.

Another type known as stripping composition consists of mixtures with aliphatic or cyclic organic (ie carbon containing), amines, organic quaternary ammonium hydroxides and organic solvents. Sometimes inorganic alkali metal hydroxides are used as secondary alkali carriers. U.S. Patent 5,543,353 discloses photoresist stripper compositions containing organic polar solvents, alkanol amines and thiocarboxylic acids as corrosion inhibitors. US Pat. No. 5,556,482 describes a method for removing photoresist with a composition containing an organic polar solvent, a basic amine and an inhibitor. U.S. Patent 4,904,571 discloses the removal of photoresist in solutions containing alcohols, ethers, ketones, chlorinated chlorocarbons, aromatic hydrocarbons, alkali metal hydroxides, carbonates, phosphates, pyrophosphates, borohydrides, and organic amine borane compounds. It describes how.

In most known stripping compositions, the stripper organic component containing the amine and the organic solvent serves as the main component of the stripping. Such compositions are known to provide high performance (i.e. high speed), long life and small resist particle size removed. Unfortunately, however, the organic component of the stripper composition consists of volatile organic compounds (hereinafter referred to as "VOC"). Thus, due to increasingly stringent government environmental laws and regulations, the printed wiring board industry is currently under severe pressure to reduce VOC emissions.

VOC is generally defined as any carbon volatile compound (except methane, carbon monoxide, carbon dioxide, carbonate, metal carbide or carbonate, ammonium carbonate and excluded compounds). These VOC compounds participate in the photochemical reaction of the atmosphere. VOCS Photochemically reacts with nitrogen oxides (NOx) in the troposphere in the presence of ultraviolet light from sunlight and atmospheric oxygen to form ground-level ozone--the main component of "smog", these include NOx peroxyacylnitrites. Rate (CH ₃ COONO ₂ ), VOCs and ozone. As a result, VOCs are regulated as "ozone precursors".

The present invention relates to a method of removing or stripping resist material from a printed wiring board after a plating or etching step. More specifically, the present invention relates to a method of removing a resist pattern from a substrate, the method comprising providing a substrate having a patterned or layered resist on a surface, the method of removing a resist free of VOC Providing (the solution comprises an ammonium ion source), exposing the substrate to the stripping solution for a time sufficient to remove all of the patterned layer or layered resist from the substrate. Surprisingly, ammonium ions have been shown to be very effective in stripping resist patterns or layers. Thus, the present invention provides a simple photoresist removal process that does not use VOCs, and as the process itself, the present invention removes substantial resources that are environmental pollution.

The present invention relates to the field of printed wiring board fabrication. More specifically, the present invention relates to a process for removing or stripping resist material from a printed wiring board after a plating or etching process step.

One step in the PWB manufacturing process aimed at reducing VOC emissions and ultimately complete removal is resist stripping. Surprisingly, the amines, solvents and quaternary ammonium compounds typically present in known resist stripping compositions can be successfully replaced with inorganic non-VOC containing solutions containing ammonium ion sources such as ammonia gas or ammonium hydroxide. Ammonia-based compounds have already been used in the PWB industry, and ammonium hydroxide has been widely used as ammonia corrosive in various copper etching processes. As described above, the etching step is a step after the resist stripping in the outer layer manufacturing process or just before the resist stripping in the inner layer manufacturing.

Since ammonia has a very high solubility in aqueous solution, once dissolved according to pH, it reacts with hydrogen (hydronium) ions to form ammonium ions. If hydrogen ions are readily available, this reaction occurs at low pH. In alkaline solutions, if hydrogen ions are not readily available, ammonia remains in gaseous form. Under pH greater than about 9.3 (at 25 ° C.), 50% of ammonia is present in ammonium ion form, while at a pH of about 12 almost 100% ammonia is present in gaseous form. Under elevated temperature, the equilibrium is further shifted towards the gas.

Conventional resist strippers are used when the pH is greater than about 11 and the temperature is in the range of about 120-130 ° F. (about 49 ° C. to 54 ° C.). Under these conditions, all ammonia stays in gaseous form with the naked eye and becomes volatile over time. Ammonia gas, by itself, has never been used as the main component of a resist stripping agent. U.S. Patent No. 3,980,587 discloses an alkali stripper component (19 wt%) containing ethylene glycol monobutyl ether, EDTA salt, acetic acid, and a 30: 1 mixture of potassium hydroxide and ammonium hydroxide as the main component of the resist stripping agent (50-55 wt%). Although it is disclosed a method for stripping a constant resist using a solution containing 10 to 30 lb (about 1.2 to 3.6 kg / l) per gallon, such a negligible concentration of ammonium hydroxide There is no noticeable contribution to the stripping action of the organic stripping agent (ethylene glycol monobutyl ether) and the organic carrier (potassium hydroxide).

Similarly, U.S. Patent No. 4,078,102 discloses a method of stripping resist in a mixture of aldehydes or ketones as a resist stripper active ingredient and activator, wherein the mixture provides an alcohol solution of ammonium, alkali and alkaline earth metal hydroxides. It contains about 0.005 to 0.1 moles of activator to 1 mole of aldehyde or ketone. In a more preferred embodiment, the patent proposes a saturated isopropyl alcohol solution consisting of sodium hydroxide (alkaline carrier) and cyclohexanone (resist stripper main component) in a ratio of 0.006 to 0.01 to 1. The stripping action of this composition is due to the organic component of the stripper and not the inorganic alkali carrier.

Yet another concept of the present invention is to provide a complete aqueous VOC-free resist stripping process using other raw materials of ammonium ions or ammonia gas as the main component of the resist stripping agent without using any volatile organic material. The present invention also relates to a method for maintaining a constant and sufficiently high ammonium ion concentration in an alkaline solution (pH> 9) at elevated temperatures (ie 80-140 ° F.). Optional bath components include non-volatile inhibitors that prevent corrosion of the metal surface exposed to the stripping solution and foam inhibitors or defoamers to control foam formation when vigorously stirring the solution to enhance stripping action.

In order to extend the life of the stripping solution, most modern resist stripper systems operate in a so-called "feed and discharge" mode. In this mode, the stripping bath is removed continuously and the bath is continuously replenished with fresh stripper solution.

Under these conditions, the ammonium ion concentration can be easily maintained at a constant level using any of the following methods:

a) a method of continuously feeding the ammonium hydroxide solution into the stripper solution;

b) continuously feeding alkaline ammonium salts (carbonates, carbamates, silicates, formates, phosphates, sulfites, etc.), which are converted to ammonium gas at pH> 12 and 120 ° F-130 ° F;

c) continuously injecting compressed ammonia gas directly into the stripping solution;

d) A method for removing and recovering ammonia gas from the ammonia corrosive vapor of ammonia concentration and then recycling it by continuous injection into the stripping solution.

Stripping solutions can be applied to developed photoresists using a wide variety of methods known in the art.

For example, a substrate containing developed photoresist may simply be immersed into a stripping solution bath. Alternatively, the stripping solution can be sprayed onto the substrate. The present invention is not limited only to the methods presented herein for applying stripping solutions to developed photoresists.

The ammonium ion material is not limited to the above, but any ammonium ion material can be used as long as it can be used without introducing VOCs into the process. Alternatively, it is possible to maintain the stripping solution at a caustic pH, which refers to the caustic stripping capacity and the caustic solution's ability to convert ammonium ions into ammonia gas. Although some of the compounds described above may be used to maintain and / or provide the caustic properties of the stripping bath, the present invention is not particularly limited to these compounds. Any raw material can be used as long as it is alkaline provided that VOCs are not introduced into the process.

Experiment

Example 1

The copper clad panel was laminated with a hot roll using a DuPont Riston 9020 dry film and then exposed and developed in 1% sodium carbonate solution. A resist stripping solution was provided containing 20 milliliters per liter of potassium hydroxide and 10 milliliters per liter of ammonium hydroxide (30% aqueous solution). The stripping solution was heated at about 125 ° F. (about 51.7 ° C.) and then the panel was immersed in this solution. In less than 60 seconds all resist was removed completely from the panel surface.

Example 2

Using a Dynachem Laminar GA dry film, the copper clad panel was laminated to a hot roll and then exposed and developed in 1% sodium carbonate solution. A resist stripping solution was provided containing 50 g per liter of sodium hydroxide and 20 g per liter of ammonium bicarbonate. This stripping solution was heated at about 130 ° F. (about 54.4 ° C.) and then the panel was immersed in the solution. In less than 60 seconds, all resist was completely removed from the panel surface.

Example 3

Using a DuPont Riston 9015 dry film, a copper clad panel was laminated to a hot roll and then exposed and developed in 1% sodium carbonate solution. A resist stripping solution was provided containing 25 g per liter of lithium hydroxide and 15 g per liter of ammonium carbamate. This stripping solution was heated at about 120 ° F. (about 48.9 ° C.) and the panel was immersed in the solution. In less than 60 seconds, all resist was completely removed from the panel surface.

Example 4

Using a DuPont Riston 9020 dry film, a copper clad panel was laminated to a hot roll and then exposed and developed in 1% sodium carbonate solution. The stripping solution obtained in Example 1 was heated to about 125 ° F. (about 51.7 ° C.) and the first panel was immersed in the solution. In less than 60 seconds all of 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 make ammonia volatile. The second panel was immersed in the solution. After 60 seconds, the resist was completely removed from the second surface. 10 ml of 30% ammonium hydroxide was added to the stripping solution and the third panel was immersed in the solution. In less than 60 seconds, all resist was completely removed from the third panel surface.

Comparative Example 1

The copper clad panel was wound up on a hot roll and laminated using a DuPont Riston 9020 dry film, then exposed and developed in 1% sodium carbonate solution. A resist stripping solution containing 20 g per 1 liter of potassium hydroxide was provided. The stripping solution was heated at about 125 ° F. (about 51.7 ° C.) and then the panels were immersed in this solution. After 60 seconds, the resist was not removed from the surface of the panel.

Comparative Example 2

The copper clad panel was wound up on a hot roll and laminated using a DuPont Riston 9020 dry film, then exposed and developed in 1% sodium carbonate solution. A resist stripping solution was provided containing 10% (by vol) PC-4077 resist stripper (Alpha Metals PC Fab Division). The stripping solution was heated to about 125 ° F. (about 51.7 ° C.) and then the panel was immersed into the solution. After 60 seconds, the resist was completely removed from the surface of the panel.

It will be appreciated from the specific embodiments of the present invention that a characteristic stripping process has been proposed. Although specific embodiments have been described, this is for the purpose of specification only and is not intended to limit the claims set forth below. In particular, the inventors understand that various substitutions, changes, and modifications are possible without departing from the scope and spirit of the invention as defined in the appended claims.

Claims (20)

a) providing a substrate having a patterned resist on the surface b) providing a VOC free resist stripping solution comprising an ammonium ion source; c) exposing the substrate to the stripping solution for a time sufficient to remove all patterned resist from the surface. The method of claim 1 wherein the resist stripping solution free of VOCs further comprises a hydroxide ion source. The method of claim 2, wherein the hydroxide ion source is selected from the group consisting of potassium hydroxide, sodium hydroxide, lithium hydroxide, and combinations thereof. The method of claim 1 wherein the ammonium ion source comprises ammonia gas. The method of claim 1 wherein the ammonium ion source comprises ammonium hydroxide ions. The method of claim 1 wherein the ammonium ion source comprises an ammonium salt. The method of claim 1 wherein the ammonium salt is selected from the group consisting of ammonium carbonate, ammonium carbamate, ammonium sulfite, ammonium silicate, ammonium formate, ammonium phosphate and combinations thereof. The method of claim 1 wherein the VOC free resist stripping solution further comprises a nonvolatile corrosion inhibitor. The method of claim 1 wherein the resist stripping solution free of VOCs further comprises a foam inhibitor. The method of claim 1 wherein the resist stripping solution free of VOCs further comprises a defoamer. The method of claim 1 wherein the exposing step comprises immersing the substrate into the stripping solution. The method of claim 1 wherein the exposing step comprises spraying the stripping solution onto the substrate. The method of claim 1 wherein the ammonium ion source in the stripping solution is fed periodically or continuously. The method of claim 13, wherein said supply comprises adding ammonium hydroxide solution periodically or continuously to the stripper solution. 14. The method of claim 13, wherein said supply comprises adding alkaline ammonium salts periodically or continuously to the stripper solution. The method of claim 13, wherein the supply comprises adding ammonia gas periodically or continuously to the stripper solution. The method of claim 1, wherein the pH of the stripping solution is maintained at about 9.3 or greater. The method of claim 1, wherein the pH of the stripping solution is maintained at about 11.0 or higher. The method of claim 1 wherein the pH of the stripping solution is maintained at 12.0 or higher. The method of claim 1, wherein stripping is performed at a temperature range of about 80 ° F. to about 140 ° F. 3.