WO2004101847A1

WO2004101847A1 - Method and apparatus for converting metal ion in solution to the metal state

Info

Publication number: WO2004101847A1
Application number: PCT/CA2004/000719
Authority: WO
Inventors: David Ericson
Original assignee: Century Circuits Inc.
Priority date: 2003-05-14
Filing date: 2004-05-14
Publication date: 2004-11-25
Also published as: US20040226407A1

Abstract

A metal ion conversion process, where the chelation component of a matrix of chemicals forms an electroless solution for converting metal ion to the metal state. The chelated metal complex is repeatedly circulated through a filter. The solution matrix is cycled back continuously through the filter and the metal ion is titrated back (colorimetrically) from various processes containing excess metal ion. The reactants required to complete said 'electroless' reaction are repleted continuously as said solution matrix cycles through said filter. The process is repeated continuously until the metal ion is exhausted or to the limit of the filter volume. Any residual metal ion present may then be polished down to acceptable limits by the same means and then resulting solution is discharged or used for further neutralization cycles.

Description

METHOD AND APPARATUS FOR CONVERTING METAL ION IN SOLUTION TO THE METAL STATE

FIELD OF THE INVENTION

The present invention relates to a process for the conversion of various forms of metal ion to the metal state, more specifically using an electroless metal solution matrix.

BACKGROUND OF THE INVENTION

The art describes the formulation of 'electroless' solutions, such as US Patent Serial No. 3,011 ,920. The purpose of such solutions is to impart a metal coating to a non conductive surface, for example, automotive plastic trim, thus the electron to convert the metal ion must be provided by chemical means instead of electrical. Said matrix is capable of providing an electron(s) to said metal ion for the conversion of said metal ion to the ground or metal state.

Although the discussion below focuses on the application of the invention to copper, it will be clear to a person skilled in the art how variations may be devised for other metals. Also, if more than a single type of metal ion is in solution, further steps may be employed to remove these other ions.

Electroless copper is a solution designed for the chemical deposition of copper metal on a non-metallic surface. Electroless copper metallization is used in a number of industries such as for plating on plastic and printed circuits. A fairly typical example is a double sided circuit board. On both sides of the board there is a copper foil, of varying thickness depending on the manufacturer and the particular function of the board. Some copper foil thickness are about one half ounce per square foot area of circuit board; others may be one ounce and greater. The board material is a combination typically of resin and glass fibres. Over the years resin technology has advanced significantly so the type of resin varies, depending on the dielectric constant desired. The overall structure of the assembly and type of glass fibres would affect drilling and other manufacturing aspects. A hole is drilled straight through this board (called an assembly or laminate) between the two sides. It then becomes desirable to metallize through this hole so the two sides (copper foils) are in electrical communication. There is no technique known whereby electrons may be applied using electrolytic means because the resin itself is not conductive. What is done is to immerse this assembly into a metal-ion containing solution where the metal will adhere to the non-metallic areas by an electrochemical process. This solution is called 'electroless' copper. Typically, the electroless process starts with a cleaning step. The surface to be coated may contain oils, soils, dirt and etc. The purpose of this step is to remove the surface contaminants and provide a nice clean surface. Typically, an acidic cleaner is used, but it might also be alkaline in some cases.

Then an important step, microetch, follows. Electroless copper is applied in the surface of the hole in the assembly as well as on the exposed copper foil surfaces of the board. The surface of a copper foil, depending on the particular foil, may be very smooth. The smoother it is, the less likely that the electroless copper will adhere to it, and it may actually peel away causing potential subsequent manufacturing problems. So it becomes necessary to roughen the surface of the foil before applying the electroless it up, that is to say to cause the surface topology to become pitted on a microscopic level. This involves a chemical process of removing a thin layer of copper from the copper surface of the foil, leaving a roughened surface topography. As a result, when electroless copper is applied, it will adhere tightly.

For this purpose, the assembly is immersed into a microetch solution containing an oxidizer which will take the copper from the ground metallic state and bring it to the Cu⁺² state, the copper becoming ionic and moving into solution. This microetch solution contain an oxidizer and Cu⁺² in an acidic medium. The question is how much Cu⁺² and how much acid, which depend on manufacturing characteristics individual to each manufacturer and job order. There is no way to predict with any degree of precision how much copper is oxidized to the +2 state. As well, the residual acidity that may be present in this solution may vary from moment to moment. As an example, between 20 to 30 grams per liter of copper in the ionic state may be present in an spent microetch solution.

After the assembly is removed from the microetch solution, the residual metal ions present on the workpiece must be removed in order not to contaminate subsequent processes with high levels of copper ion. This is done by attempting to capture much of the ion in a static or standing rinse known as a 'drag out'. The workpiece is briefly immersed in the drag out, then it proceeds to subsequent water rinses where the remaining metal ion is completely removed. The metal ion which diffuses into the water rinses must somehow be captured and converted into a manageable state; this highly concentrated metal ion solution may not be safely discharged.

This is dealt with historically in any one of several ways. First, being the oldest technique, is simple precipitation: the pH of the rinse is adjusted (raised) to form copper hydroxide (also known as sludge). Subsequently, there may be a possible coagulation step. Sludge, classified as a heavy metal, is typically collected in a filter press and then sent to a hazardous landfill site suitable for heavy metals, at great cost.

The second historical modality is ion exchange. What occurs are: collection, pH adjustment, then the ion exchange. The rinse effluent goes through an ion exchange column, then the upload is disposed (discharged to a sanitary sewer). Periodically the ion exchange resin will become charged with copper ion or metal ion and must be regenerated by typically an acid rinse. After this removal process the acid rinse itself needs to have the metal ion recovered so then a whole other, possibly sub-modality, is required, and that is typically electrowinning, which is electrolytic recovery of the metal ion.

Electrowinning depends upon many factors to operate efficiently. The first is high concentration of metal ion. When the concentration of the metal ion in question drops below a certain level, efficiency drops precipitously to a point of no return where one can literally do electrowinning for literally days or weeks without getting the ion down to the desired concentration. So the concentration parameter is a deficiency in that process.

The second deficiency is the acid itself. The acid has to be back neutralized so every time one conducts this operation to get the metal down to a low enough concentration to meet legal limits, one must do electrowinning for an extraordinarily long period of time, then back neutralize all this large volume of acid.

Third, electrowinning tends to be expensive: the parts rely on platinum electrodes which are very expensive, burn out easily, and are labour intensive. Changing the anodes and cathodes and regular maintenance of the electrowinning cell is labour intensive.

This therefore points to one embodiment of the present invention to remove copper ions from a low pH (acidic) rinse solution.

In some instances, a standing rinse also known as a 'drag out' is employed after microetch, the purpose of which is to capture excessive metal ion and diminish the ion concentration in the following rinses. The level of metal ion which is introduced into rinse water is a function of the ratio of the concentration of the copper ion in the microetch to that in the drag out. For example if the concentration ratio is 3:1 , two-thirds of the copper ion is captured by the drag out. The drag out is then pumped back into the microetch. A value to having this scheme comes when one possesses the capability to treat volumes of highly concentrated ion solutions, thus introducing the operation of one embodiment of the invention here. Typically, a running microetch solution will contain 20 to 30 grams per liter of copper ion; a drag out may contain levels of copper that are lower than that of the microetch, depending on the frequency and methodology of its management.

Following rinsing, the assembly will be treated by a catalyst. In these processes palladium is typically used as a catalyst, which is a standard industry catalyst. A thin coat at the ppm level is applied which adheres to the resin on the assembly; it also adheres to the surface of the metal (but primarily binds to the resin). There is further rinsing after that, just to rinse off residual palladium which may be present.

After catalysis the assembly goes into the copper electroless solution. An electroless solution comprises copper ions held preferably at high pH with a chelating agent. It is reacted with a source of hydroxide ion (for example, sodium hydroxide) and a reduction agent (for example, formaldehyde), in the following stoichiometrically balanced reaction: (also known as a Cannizzaro reaction)

Cu⁺² + 2HCHO + 4 0H^"→ Cu° + 2HCOO^" +2H₂ (1 )

The electrons are thus provided by chemical means as opposed to electrical means. Electroless copper has had a bad environmental reputation: large volumes of spent solution are generated, which are very difficult to neutralize and yet must be treated down to very low discharge limits.

As copper metal is deposited from a working electroless solution onto the assembly, the concentration of the reactants are decreased and the concentration of byproduct salts are increased. The components are pumped into the solution, which results in solution increase or 'growth'. The excess solution is withdrawn and treated through various treatment schemes, none of which are very efficient.

Electroless copper is chelated with EDTA, QUADROL or other like chelating or 'sequestering' agents. A variety of methods are employed to remove the metal ion including: complexation with various thio (sulphur) derivatives like DTC (sodium dimethyldithiocarbamate, or other carbamates) or other filtration methods employed together with various precipitation methods. Ion Exchange Resins have been used but are not highly reliable because the bond with EDTA can be stronger than that of the resin and thus the copper can bypass the resin treatment process under varying process conditions. Prior art describes the treatment of such 'electroless' solutions with ion exchange resins in US patent number 4,076,618, Zeblinsky et al. Further, when EDTA is to be combined with any waste stream, it necessitates the usage of strong precipitating agents like DTC, because if copper-EDTA is precipitated with traditional means, the newly freed EDTA is able to bond with other metal ions in the waste stream, which can then escape the treatment process. DTC actually forms a copper-sulfur bond, which is sufficiently strong as to resist the very high chelating power of EDTA with copper. However, this use of the DTC results in an insoluble compound (sludge).

All the copper that may contact the solution being neutralized must be precipitated with DTC and not just the copper chelated with EDTA. This results in a large amount of sludge, as well as the precipitation problems associated; also DTC is relatively expensive.

An example of an alternative approach to the neutralization of chelated copper may be found in Japanese patent no. JP54158059, which aims to decompose and remove organic contaminants contained in copper plating waste water at a high removal rate by adding palladium chloride to the waste water, which is kept at high pH, heating the waste water to form a precipitate, removing the precipitate, then subjecting the waste water to oxidizing treatment at low pH, followed by activated carbon. Copper plating waste water containing such contaminants as EDTA, formaldehyde, and copper is treated with an alkali agent to adjust the pH to above 12, followed by palladium chloride, and heated to 30-80 degrees C. After removal of the resulting precipitate, the waste water is treated with a mineral acid to pH 1-3, then treated with an oxidizer, again adding a mineral acid to adjust the pH to within 2-4, then treated with activated carbon, and subjected to solid-liquid separation to provide treated water. Among the numerous problems associated with this approach is that the material must be removed as a fine precipitate and is difficult to remove, dry, handle and dispose of.

SUMMARY OF THE INVENTION

This invention provides for a method for converting metal ions in solution to the metal state comprising the steps of: (a) providing an initial quantity of reaction solution containing a matrix of chemicals, the reaction solution comprising an electroless solution for said ions including a chelating agent for the ions; (b) providing a chamber for containing the reaction solution; (c) delivering the reaction solution to the chamber; (d) feeding the reaction solution in the chamber through a filter constructed of a polymer treated with a catalyst at a pre-determined filter exposure rate for reducing ions to the metal ground state; and (e) repeating steps (c) to (d) one or more times until ion concentration in the reaction solution is less than or equal to a minimal concentration limit.

In one aspect, the step of feeding the reaction solution through a filter above comprises the subsequent step of replenishing hydroxyl ions and reducer.

In a second aspect, the step of delivering the reaction solution to the chamber above comprises the further step of adding an aliquot of aqueous ions to the reaction solution if: the quantity of reaction solution is less than a pre-determined volume; and the ion concentration is greater than or equal to a pre-determined additive point.

In another aspect, the reaction solution is polished if the quantity of reaction solution is equal or greater than the pre-determined volume.

In a further aspect, the ion concentration is determined by an optical element. The optical element may comprise: a light emitter chosen from the group consisting of a light emitting diode and a laser; a wave guide for directing light generated by the light emitter into the reaction solution; a photo diode for receiving light from the wave guide through the reaction solution; and a computer with software to interpret light intensity reading from the photo diode as an indication of ion concentration.

Embodiments also include the variation where the ions are copper ions; and the electroless solution comprises EDTA as the chelating agent, and formaldehyde as a reducer.

In a variation, the initial quantity of electroless solution is derived by steps which include adding a blank solution to aqueous metal ions thereby chelating all metal ions in the electroless solution.

In another variation the method further comprises the step of rinsing the metallized copper to remove the residuals salts and EDTA carried forward.

In a further variation, the catalyst is palladium.

The filter may be constructed of reticulated foam in another variation.

In accordance with one aspect, the pre-determined filter exposure rate is above approximately 300 square feet per liter per minute.

In accordance with another aspect, the method further comprises the step of storing the metal depleted reaction solution for future use as a blank solution. In accordance with a further aspect, the reaction solution is at a temperature between about 90 degrees and 120 degrees Fahrenheit when fed through the filter.

Embodiments of the this invention also include an apparatus for converting copper ions in a reaction solution, the reaction solution comprising an electroless solution for said ions including a chelating agent for the ions, to the metal state comprising: a chamber for containing the reaction solution; a filter in fluid communication with the chamber providing a catalytic surface for metallic copper deposition from the reaction solution when fed through the filter, the filter surface being a polymer treated with a catalyst; and a heating element for monitoring and controlling the temperature of the reaction solution when fed through the filter; a pump element for circulating the reaction solution after flow through the filter to the chamber.

In accordance to one aspect, the apparatus further comprises elements for replenishing hydroxyl ions reducer, and adding more reaction solution.

In accordance to another aspect of the apparatus, the reaction solution is polished if the quantity of reaction solution is equal or greater than a pre-determined volume.

In accordance to a further aspect, apparatus comprises an optical element for determining the ion concentration in the reaction solution, the optical element comprising: a light emitter chosen from the group consisting of a light emitting diode and a laser; a wave guide for directing light generated by the light emitter into the reaction solution; a photo diode for receiving light from the wave guide through the reaction solution; and a computer with software to interpret light intensity reading from the photo diode as an indication of ion concentration.

In a variation of the apparatus, the ions are copper ions; and the electroless solution comprises EDTA as the chelating agent, and formaldehyde as a reducer.

In a second variation, the initial electroless solution is derived by steps which include adding a blank solution to aqueous metal ions thereby chelating all metal ions in the electroless solution.

In a another variation, the apparatus further comprises an element for rinsing the metallized copper to remove the residuals salts and EDTA carried forward.

In a further another variation to the apparatus, the catalyst is palladium.

In one aspect to the apparatus, the filter is constructed of reticulated foam. In another one aspect to the apparatus, the pre-determined filter exposure rate is above approximately 300 square feet per liter per minute.

In another one aspect to the apparatus, the heating element maintains the temperature of the reaction solution when fed through the filter at a temperature between about 90 degrees and 120 degrees Fahrenheit.

Embodiments of this invention also includes a method for converting metal ions in solution to the metal state comprising the steps of: (a) providing an initial quantity of reaction solution containing a matrix of chemicals, the reaction solution comprising an electroless solution for said ions including a chelating agent for the ions at a temperature between about 90 degrees and 120 degrees Fahrenheit; (b) providing a chamber for containing the reaction solution; (c) delivering the reaction solution to the chamber; (d) feeding the reaction solution in the chamber through a filter constructed of a polymer treated with a catalyst at a pre-determined filter exposure rate for reducing ions to the metal ground state; and (e) repeating steps (c) to (d) one or more times until ion concentration in the reaction solution is less than or equal to a minimal concentration limit.

The present invention provides a process for conversion of copper ion to the metal state, utilizing an electroless copper solution matrix, said process comprising the steps of:

(a) an aqueous solution containing a matrix of chemicals, containing the components necessary to form an 'electroless' solution for said copper ion. Said matrix is capable of providing electrons to said copper ion for the conversion of said copper ion to the metal state, while maintaining the temperature of said matrix between about 90 degrees and 120 degrees Fahrenheit.

(b) transporting and contacting said matrix from step (a) into a metal filter comprising of a large surface area polymer, such as polyether reticulated foam, having previously been treated with palladium chloride catalyst.

(c) repleting the reactants contained in said matrix to optimize and facilitate said 'electroless' reaction as said matrix circulates through the metal filter.

(d) maintaining the temperature in said filter between about 90 and 120 degrees Fehrenheit, so as to control the deposition rate of copper ions onto the filter material at a controlled rate but to prevent spontaneous deposition of copper metal onto non filter surface areas. The present invention also provides a process for the conversion of copper ions from non-chelated sources, such as plating wastes or ion exchange regenerant. As the copper ion concentration is decreased due to deposition as metal within the filter, the copper concentration is maintained by addition of concentrated copper ion, which is present from other processes; and continuing to replace the copper ion which is converted to metal state and to replete the reactants which are consumed in the electroless reaction until either the excess metal ion has been consumed or the volume limit of the filter is reached. When the physical limit of the filter is reached, or the excess metal ion has been consumed, the reactants are repleted, and the solution is transported until such time as the final desired metal ion concentration is achieved (polish).

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a reaction curve (time versus Copper ion concentration) of the overall reduction reaction.

DETAILED DESCRIPTION OF THE INVENTION

The process and machine, disclosed in the present invention, allow for the reduction of chelated copper, at high concentrations down to extremely low levels, and may be removed as a solid block of copper metal.

Typically, an electroless solution would run at a temperature between 90 and 120 degrees Fahrenheit in the presence of sufficient alkalinity and sufficient reductive power. Formaldehyde is volatile and will naturally come out of the solution over a very short period of time at elevated temperatures. The alkalinity may also drop due to the reaction itself. The process will stop when a given solution does not contain sufficient reactants to completely drive the reaction (1) through to the right (reducing all copper to the metallic ground state). Another possibility is that there may not be enough thermodynamic energy.

In order to conduct this reaction, a number of factors must be considered. First, enthalpy, ΔH: there must be a minimum energy to conduct the reaction. Heat has to be applied in order to carry on the reaction, preferably, between about 90 and 120 degrees Fahrenheit, though it could be more than that. However, the reaction will become vigorous at elevated temperatures. The reaction will conduct at a lower temperature than that described above but will be understandably (and sometimes unacceptably) slow.

Second, the reactants themselves must be present, including Cu⁺².

Third, a source of hydroxide. And fourth, a reducer must be present, which in this case is preferably formaldehyde. These are the essential components for the reaction.

A 5^th factor concerns the site at which the reaction will occur: it is undesirable to apply too much pressure to the previously mentioned elements such as to cause a spontaneous reaction forming copper metal, and resulting in sludge and particulate matter especially hard to deal with. The goal instead is to have the end product being a manageable product such as a block of copper metal. It is necessary thus to choose a catalytic material of a specific shape, size and density such as to cause this reaction to occur at the right rate.

The sixth factor is also to control the rate of the reaction: the rate at which the solution (factors 2, 3 and 4 above) at the correct temperature (factor 1 ) is impinged upon the reaction site (factor 5). The rate of impingement can also vary the reaction rate. For example, a pump may draw the electroless solution to and through the reaction site. Slowing or stopping the pump causes the overall reaction to slow or stop.

The invention seeks to exploit the very properties of the electroless copper reduction reaction. The microetch solution referred to earlier is an aqueous solution of Cu⁺² ion in an unchelated acidic matrix; and the spent electroless solution is a solution of Cu⁺² ion in an alkaline matrix which is chelated. At the outset those materials do not appear compatible, but this invention includes approaches to extract copper ions from both using the same core approach as will be described later. By controlling the temperature of reaction, the rate of reaction may be carefully managed. Also, the reactants (other than copper) tend to be decreased by side reactions and by volatility. Thus by replenishing the reactants, an optimum reaction 'pressure' may be maintained.

The invention uses thermodynamic properties, to push the chemical reaction of equation (1) above to near completion by maximizing the reaction sites available, manipulation of the temperature and concentration variables, and using Le Chatelier's

Principle. A new equilibrium is achieved with the amount of chelated copper held by

EDTA being very small.

While maintaining conditions conducive to electroless metal deposition, the solution is exposed to existing copper metal surface to initiate the reaction. This is done by rapid introduction and impingement of solution held initially in a chamber through a

'filter' of fairly large surface area and a specific surface area per unit of solution volume.

As an example, 20 liters of solution is put through a filter which contains 2 filter elements of 20 PPI at 12 inch diameter x 6 inch thickness per element. Calculations show that with a filter having a surface area of around 300 sq. ft., the solution is impinged around 3 - 5 US gallons per minute, which exposes the solution to approximately 300 to 1000 sq. ft. of autocatalytic filter area per litre per minute (the filter exposure rate). This high exposure rate per unit volume of solution is an important factor in achieving the high reaction rate that is required to efficiently reduce the metal ion concentration to acceptable levels within a short period of time. The preferred filter exposure rate is above approximately 300 sq. ft. of filter area per litre per minute.

This filter will serve to provide the surfaces upon which the reaction will initiate (the reaction site). This volume of solution should be impinged upon this filter at a given minimum rate in order to achieve a rapid reaction rate of metal ion conversion to the metal state. A minimum rate would apply for each set of circumstances and that rate would be the same rate that the copper ion is produced by their combined processes. If the copper ion is not reduced at this rate, then ionic copper would accumulate in drums and it would be necessary to have them removed by other means. Maintaining these conditions allows for a reduction in copper ion concentration to extremely low levels, 1 PPM and lower, which allows for direct discharge (provided other regulatory guidelines have been met) to the public waste stream in virtually any jurisdiction at the time of this document.

The metal filter itself is prepared from commercially available filtration media, such as reticulated foam material. The material used is selected for resistance to the reactants used. The pore size will govern the surface area in a given filter. Generally a pore size in the range of about 10 to 20 PPI is preferred, especially around 20 PPI (as indicated earlier). The filter media is prepared by treatment with a catalyst, typically of the standard palladium type. This renders the media surface active to initiate the electroless copper reaction.

Once copper metal atoms have been deposited onto the surface of the filter, the reaction will continue to likewise deposit metal ions on that same surface. A given filter can absorb many thousands of times its weight in copper metal atoms. In operation, the apparent bulk density of the block increases until it reaches a predetermined level, for example, 50 percent of that of solid copper metal. At that time the filter block is removed and replaced. (One could continue to increase that density until it approached that of the bulk density of copper itself, but then the flow rate would be significantly reduced and the rate of reaction would slow.) The block does not grow dimensionally to any significance, but rather the porosity decreases. The reaction rate is not significantly affected by this decrease in porosity, but clearly the filter must be replaced in practice before the flow rate is diminished to zero. When the media is new, that is after treatment with the catalyst, it typically weighs only a few grams in total per 'filter' unit; as copper begins to deposit on the reticulated matrix, the block weight will increase proportional to the amount of copper atom deposited and the 'apparent' density of the block increases.

The example dimensions of the filter referred to above are based on some practical limitations. That is to say a system must exist whereby a reasonable amount of material can be treated; i.e. one does not wish to create a system that would be incapable of handling the volume of material which would be generated by the user, or likewise so oversized that it would be impractical. The example diameter of the filter units allow for easy handling but most certainly could be varied. One does not want to create blocks that are so heavy they could not be easily lifted, or so small that they would be rapidly created and thus cause more labour than necessary in changing them. The porosity can be altered resulting in different surface area, and thus reaction rate. If it is decreased, that is to say less porous, the reaction rate would decrease, but the metal holding capacity would remain unchanged. Likewise an increase of porosity would have the inverse effect.

Other methods of catalysis are also possible. Such activated filters are available currently to treat electroless solutions, but without the additional reaction control are unable to effectively remove chelated metals to low enough levels to be useful.

In a preferred embodiment the filter is placed within the body of an apparatus.

Typically 2 or more filters are used, but these act as a single unit of filtration (known hereafter as just filter). The filter is treated previously with a catalyst, typically the same catalyst used for treating the production work for metallizing the nonmetallic surface, which is usually palladium. Electroless solution is sent through the filter, preferably by gravity and returned to the top of the filter in a cyclic mode of operation by pump. Byproduct ions of salts (HCOO^") will build up over time.

The filter typically remains submerged under solution. If the filter is allowed to be exposed to the atmosphere while running, it may oxidize, which will then result in rechelation and transfer back into the solution.

The reaction described earlier will conduct: the ionic copper plus the 4 hydroxide ions plus the 2 reduction molecules to produce copper in the metal state, which will deposit on the filter surface. The reaction will continue until any one of the three reaction components run out. During the reaction, it is necessary to replete the hydroxyl ion and the reducer in order to completely exhaust the ionic copper to the metal state

Thermal energy, sufficient heat to keep the reaction through to completion, must also be applied. When neutralizing acidic copper matrix, there is a negative change in enthalpy, which releases heat, so the reaction is promoted by this fact.

In a preferred embodiment of this invention, it is necessary to determine what point a particular solution has reached in the overall reaction. To do this, a preferred way is to sense the chemical concentration of copper ions in the solution; and this is preferably done optically. Chelated Cu⁺², being typically of a distinctive blue colour, has an absorbance in the red frequency range, so a light emitting diode (an emitter) and a photo diode (a receiver) are used that operate approximately in the 660 nanometer range (around 700 nm). A beam of light of the mentioned wavelength is transmitted by the light diode through the solution to a nearby photodiode. The atoms of chelated copper in the solution in the path of the light absorb some of the light, and the quantity of light which is detected corresponds inversely (approximately) to the actual concentration of Cu⁺². A computer maps the signal strength received from the photodiode to Cu⁺² concentration. The light emitter and receiver is typically immersed directly above the filter(s) so that a concentration reading can be obtained real-time and online.

A preferred embodiment uses glass tubes as waveguides extending through a holder with the ends of the tubes bend 90 degrees so that they face each other, with a gap between them as would be appropriate. Visually the tubes form the shape of a transposed letter 'J' placed beside another letter 'J'. So, when the light passes through the solution between the ends of the emitter and the receiver, some of the light in the desired band will be absorbed by the solution and the balance received by the photodiode (ignoring scattering and reflection). Both the emitter and the receiver are located at the opposite end of the glass tube waveguides.

Lasers of the correct frequency range may also be used as the emitter. When an incandescent light source is used without filtering, the signal produced is significantly reduced because photons other than the frequency absorbed by the copper-EDTA complex will also impinge upon the photodiode and create a false signal. This reduces the overall signal-to-noise ratio. The sensor side should preferably be a photodiode that does not discriminate between the frequency of light but rather and produces a current proportional to the amount of photons impinged upon it. A fibre optic waveguide may be used instead of glass. However, one issue with the use of fibre optics concerns the alignment of the fibre ends: since the preferred glass rod is much larger in diameter than fibre cable, it is easy to align these and to hold these in place without variation occurring. Chromatography is especially sensitive to positional alignment due to the fact that this has a large impact on the signal strength. Since the use of this invention is typically in a industrial (and thus hostile) environment, it is especially important that the design be rugged and robust.

Figure 1 shows a reaction curve (time versus Copper ion concentration). The reaction commences at a certain point with a certain electroless solution. This initial electroless solution may have two sources. For those who start out with electroless copper (e.g. after plating), this may be drawn from the excess electroless copper solution generated. For those instances which do not (e.g. spent microetch solution or rinse), this can be created simply by filling the apparatus with the unchelated copper solution and adding EDTA and other reactants if necessary. As time passes, the reduction process results in metallization of the copper ions and the copper ion concentration drops.

It is not necessary to know the volumes involved. Absorbance/transmittance are unitless measurements and the reaction time can be affected simply by increasing or decreasing the rate of feed of reactants (and temperature). The limiting factor is the combination discussed above, that is the rate of solution feed per unit of time per unit of surface area. In a preferred embodiment, a titrametric approach is used to solve the acid problem and the copper ion problem not needing to know what the initial concentration of the solution. That is to say, the precise amount of hydroxyls and reducers needed for the reaction may be determined titrametrically.

The microetch or other waste acid copper solution has unknown levels of both acid and copper ion. A spent electroless solution from a plating operation would have high pH and thus no hydrogen ions. These levels vary according to many factors between orders of magnitude, and it is virtually impossible or unworkable to predetermine these. It is highly desirable to construct a system that is able to solve those unknowns 'on the fly', offering great flexibility.

According to one aspect of this invention, a pre-determined absorbance point

(reflecting a certain cupper ion concentration) is selected. When this value is reached or just exceeded, a titration is performed where an aliquot of the original untreated electroless solution is added. Two conditions on the amount of aliquot should be observed. The first condition is that the H⁺ added is sufficiently smaller than the OH^" present in the system. The second condition is that the copper ion in the aliquot does not exceed the capability of the chelant present in the electroless solution to completely chelate it. In other words the total chelation available must not be completely occupied by the copper introduced; there must be an amount of chelant, for example EDTA, present in the free form. Otherwise the copper which is not bound with EDTA will precipitate or form a particulate matter which may foul the machinery. The aliquot of microetch introduced, therefore is sufficiently small so as to not change the foundational chemical parameters themselves, but operates within those parameters. The starting concentration values of OH^" and EDTA in the seed solution are not measured directly but are inferred from the behaviour of the system.

The amount of copper ions and acid are the 2 unknown variables in the aliquot. The addition of the aliquot results in an increase in the concentration of Cu⁺² in the reaction solution: effectively, pushing the reaction back from completion. The physical appearance of the reaction solution will get darker (bluer) as a result of the increase in chelated Cu⁺² concentration, and the amount of light being transmitted through to the photodiode will decrease, which is interpreted by the computer as a concentration increase.

Next, an assumption is made based on the increased absorbance of the solution when the aliquot is added as to how much reducer to add since the reducer that should be added is proportional 2 to 1 to the actual copper ion increase. Cu⁺² requires 2 electrons which are provided by the reducer. Alternatively, this amount of reducer can be inferred by observing the time that is taken to neutralize a given aliquot since the rate of reduction is predictable. This allows for approximate calculation of copper added in the aliquot and thus the amount of reducer required.

The other variable is the acid that may have been present in the aliquot that was added. A copper ion plus 4 hydroxyls plus the reducer goes to metal. Solve the acid/base component by the fact that this reaction conducts to metal. Since the reaction conducts by the addition of hydroxide ion, once the reaction proceeds, all residual acidity must have been neutralized. Each hydroxyl ion used to push back the reaction to the original pre-determined position would directly correspond to a necessary ion to reduce just the excessive amount of copper ion that had been added by the aliquot. That is observed optically as previously described. And once this optical difference has been observed the residual acidity has been neutralized by definition. Therefore, it is possible to iteratively solve that problem without ever knowing how much acid is present in the aliquot and return to the original pre-determined point. There are several reasons for solving this iteratively. One is essentially using Newton's method of approximation, continually looking to see if the desired value is getting closer or farther away. Based on this, an adjustment is made to the reactant feed on each iteration. This process is continuous and thus it is possible to move closer to the actual concentration value without ever actually knowing it. Secondly, one doesn't know if the concentration of the aliquot is actually changing between titrations, thus requiring the reactant feed recalculation. The reactants that are left in the solution between any given titration act as a buffer so that minor variations in the concentrations coming in the aliquot do not actually cause the reaction to halt. There is no need for electrowinning, to guess the concentration, to precipitate the solution, for pre-treatment, or pH adjustment. A further advantage of this approach is the reuse of EDTA where the initial copper in solution is unchelated (e.g. a microetch). A small amount of EDTA is added up front and chelates with copper ion. EDTA freed up during an initial part of the overall reduction reaction can then chelate with a residual copper ion for subsequent reduction of the latter into its metal state.

At some point the apparatus will be completely full of solution. At that point, this iterative process of the being added in a titrametic manner is stopped. The next step is called polishing, which comprises adding the reactants and thermodynamic energy to bring the system to the final desired copper concentration. Since the titration technique used to introduce the acid copper matrix previously discussed causes variations to occur in the concentration of the reactants, look for a second pre-determined optical absorbance point. That is to say, conduct a second titration where the reactants (hydroxide and reducer) are added in known quantities over known time. Once that second pre-determined point has been observed, the time required to achieve a repeatable result is known based on our prior knowledge of that system. At that second pre-determined point all factors are known: volume, concentration of metal ion, and that the concentration of reactants is contained in excess; therefore the time required to react the residual metal ion down to extremely low levels is now also known. Once that time has been observed, the solution is then discharged and the process is finished, (once this second pre-determined copper ion concentration point has been reached, there is a certain and predictable manner to finish the reaction (to reach the end point with zero copper ion concentration). There is a certain ΔH, certain amount of reducer and hydroxyl ion necessary to be added to reach that end point.

The zero concentration point is extremely hard to sense for a number of reasons: for example, the glass itself may become fouled. Rather, it is preferred to use the properties of the predictability of the reaction at this point to provide an environment to drive the reaction even farther along than the actual zero point. With this technique it is unnecessary to set zero.

In another preferred embodiment of this invention, the conversion of the rinse following the microetch replaces the drag out. Microetch may contain 20 to 30 grams of copper ion per litre when that highly concentrated solution is introduced into a water rinse by being carried over on the production. The thermodynamic situation becomes very

"undesirable" since entropy is increased.

So instead, this embodiment proposes to replace the 'drag out' with an electroless solution, which has been stripped of all of the copper ion, possibly using the methodology described above, providing what might be call a "blank" or 'production' solution. The blank solution contains the other components, especially EDTA, required for the metallization reaction. During this step, as microetch is dragged into that solution together with the production, the residual copper ion of the microetch will combine with the EDTA present in the blank solution forming a reaction solution for circulation through the apparatus embodiment described earlier. This would undergo the processing steps discussed earlier, one difference being that the aliquot introduced would be smaller, and thus this system would be much easier to control. So the copper ion will be metallized in one step avoiding those difficult treatment modalities discussed earlier. Standard water rinses of the metallized copper may follow this process to remove the residuals salts and

EDTA carried forward.

The advantages of the method discussed above are many: a given EDTA molecule may be being used multiple times, which substantially lower the cost related to that reactant. Additionally, the reactants that are not consumed during any given pass through the filter will be available for reaction on subsequent passes for different initial solution types, thus avoiding wastage of reactants. Also, the metal may be removed from the machine as a solid block, avoiding difficult and labor intensive filter changing procedures, as well as making disposal and recycling of the metal an easy procedure.

It will be appreciated that the above description relates to the preferred embodiments by way of example only, as copper would be in this case. Many variations on the apparatus and method for delivering the invention will be clear to those knowledgeable in the field, and such variations are within the scope of the invention as described and claimed, whether or not expressly described. It is clear to a person knowledgeable in the field that alternatives to these arrangements exist and these arrangements are included in this invention.

All documents and publications referred to in this paper are incorporated by reference in their entirety.

Claims

CLAIMSWhat is claimed is:

1. A method for converting metal ions in solution to the metal state comprising the steps of: (a) providing an initial quantity of reaction solution containing a matrix of chemicals, the reaction solution comprising an electroless solution for said ions including a chelating agent for the ions;

(b) providing a chamber for containing the reaction solution;

(c) delivering the reaction solution to the chamber; (d) feeding the reaction solution in the chamber through a filter constructed of a polymer treated with a catalyst at a pre-determined filter exposure rate for reducing ions to the metal ground state;

(e) repeating steps (c) to (d) one or more times until ion concentration in the reaction solution is less than or equal to a minimal concentration limit.

2. The method of claim 1 , wherein the step of feeding the reaction solution through a filter comprises the subsequent step of replenishing hydroxyl ions and reducer.

3. The method of claim 1 , wherein the step of delivering the reaction solution to the chamber comprises the further step of adding an aliquot of aqueous ions to the reaction solution if: the quantity of reaction solution is less than a pre-determined volume; and the ion concentration is greater than or equal to a pre-determined additive point.

4. The method of claim 3, wherein the reaction solution is polished if the quantity of reaction solution is equal or greater than the pre-determined volume.

5. The method of claim 1 , wherein the ion concentration is determined by an optical element.

6. The method of claim 5, wherein the optical element comprises: a light emitter chosen from the group consisting of a light emitting diode and a laser; a wave guide for directing light generated by the light emitter into the reaction solution; a photo diode for receiving light from the wave guide through the reaction solution; and a computer with software to interpret light intensity reading from the photo diode as an indication of ion concentration.

7. The method of claim 1, wherein the ions are copper ions; and the electroless solution comprises EDTA as the chelating agent, and formaldehyde as a reducer.

8. The method of claim 1 , wherein the initial quantity of electroless solution is derived by steps which include adding a blank solution to aqueous metal ions thereby chelating all metal ions in the electroless solution.

9. The method of claim 1 , further comprising the step of rinsing the metallized copper to remove the residuals salts and EDTA carried forward.

10. The method of claim 1 , wherein the catalyst is palladium.

11. The method of claim 1 , wherein the filter is constructed of reticulated foam.

12. The method of claim 1 , wherein the pre-determined filter exposure rate is above approximately 300 square feet per liter per minute.

13. The method of claim 1 , further comprising the step of storing the metal depleted reaction solution for future use as a blank solution.

14. The method of claim 1 , wherein the reaction solution is at a temperature between about 90 degrees and 120 degrees Fahrenheit when fed through the filter.

15. An apparatus for converting copper ions in a reaction solution, the reaction solution comprising an electroless solution for said ions including a chelating agent for the ions, to the metal state comprising: (a) a chamber for containing the reaction solution;

(b) a filter in fluid communication with the chamber providing a catalytic surface for metallic copper deposition from the reaction solution when fed through the filter, the filter surface being a polymer treated with a catalyst; (c) a heating element for monitoring and controlling the temperature of the reaction solution when fed through the filter; and

(d) a pump element for circulating the reaction solution after flow through the filter to the chamber.

16. The apparatus of claim 15, further comprising elements for replenishing hydroxyl ions reducer, and adding more reaction solution.

17. The apparatus of claim 16, wherein the reaction solution is polished if the quantity of reaction solution is equal or greater than a pre-determined volume.

18. The apparatus of claim 15, further comprising an optical element for determining the ion concentration in the reaction solution, the optical element comprising: a light emitter chosen from the group consisting of a light emitting diode and a laser; a wave guide for directing light generated by the light emitter into the reaction solution; a photo diode for receiving light from the wave guide through the reaction solution; and a computer with software to interpret light intensity reading from the photo diode as an indication of ion concentration.

19. The apparatus of claim 15, wherein the ions are copper ions; and the electroless solution comprises EDTA as the chelating agent, and formaldehyde as a reducer.

20. The apparatus of claim 15, wherein the initial electroless solution is derived by steps which include adding a blank solution to aqueous metal ions thereby chelating all metal ions in the electroless solution.

21. The apparatus of claim 15, further comprising an element for rinsing the metallized copper to remove the residuals salts and EDTA carried forward.

22. The apparatus of claim 15, wherein the catalyst is palladium.

23. The apparatus of claim 15, wherein the filter is constructed of reticulated foam.

24. The apparatus of claim 15, wherein the pre-determined filter exposure rate is above approximately 300 square feet per liter per minute.

25. The apparatus of claim 15, wherein the heating element maintains the temperature of the reaction solution when fed through the filter between about 90 degrees and 120 degrees Fahrenheit.

26. A method for converting metal ions in solution to the metal state comprising the steps of:

(a) providing an initial quantity of reaction solution containing a matrix of chemicals, the reaction solution comprising an electroless solution for said ions including a chelating agent for the ions at a temperature between about 90 degrees and 120 degrees Fahrenheit; (b) providing a chamber for containing the reaction solution;

(c) delivering the reaction solution to the chamber;

(d) feeding the reaction solution in the chamber through a filter constructed of a polymer treated with a catalyst at a pre-determined filter exposure rate for reducing ions to the metal ground state; (e) repeating steps (c) to (d) one or more times until ion concentration in the reaction solution is less than or equal to a minimal concentration limit.