US3475242A - Process and apparatus for controlling metal etching operation - Google Patents

Description

K. J. RADIMER PROCESS AND APPARATUS FOR CONTROLLING METAL ETCHING OPERATION Filed Dec. 19, 1966 3 Sheets-Sheet 1 FIG. I.

'l .K' VEN TOR. KEN/V5 TH J: RAD/MEI A T TORNEV.

Oct. 28, 1969 K. J. RADIMER 3,

PROCESS AND APPARATUS FOR CONTROLLING METAL ETCHING OPERATION Filed Dec. 19. 1966 3 Sheets-Sheet 2;

INVENTOR. KENNE TH J. RA DIMER A 7' TORNE' X Oct. 28, 1969 K. J. RADIMER I 3,475,242

PROCESS AND APPARATUS FOR CONTROLLING METAL ETCHING OPERATION Filed D90. 19. 1966- v 3 Sheets-Sheet 3 FIGS United States Patent 3,475,242 PROCESS AND APPARATUS FOR CONTROLLING METAL ETCHING OPERATION Kenneth J. Radimer, Little Falls, N.J., assignor to FMC Corporation, New York, N.Y., a corporation of Delaware Continuation-impart of application Ser. No. 522,694, Jan. 24, 1966. This application Dec. 19, 1966, Ser.

Int. Cl. C23f 1/02 US. Cl. 156-49 15 Claims ABSTRACT OF THE DISCLOSURE This application is a :continuation-in-part of my application, Serial No. 522,694, filed Jan. 24, 1966.

This invention is directed to an improved method for conducting an etching process at a predetermined or preselected etch rate and, more particularly, it is concerned with the etching of copper foil mounted on a backing for the purpose of preparing so-called printed circuits for use in electronic apparatus and to apparatus useful in said method.

My patent application Serial No. 522,694, discloses a process and apparatus for determining the rate at which metal is dissolved in commercial metal etching operations by measuring the rate at which a metal test article is dissolved in the metal etching bath. The rate of metal dissolution thus determined is utilized to control the addition of active etchant to the metal etching bath, to control a process variable, or to do both. Certain of the etching solutions widely used commercially contain two or more active components. Metal etching baths normally should contain an optimum concentration of active components. During operation, these active components are consumed at different rates with the result that replenishment of the difierent bath components is required at different and, usually, unpredictable rates.

Etching systems, such as the solution used in chemical milling, which contains ferric chloride and thiourea or a related compound, and such metal etching solutions as those containing mercuric chloride and ammonium persulfate, disclosed in U.S. Patent 2,978,301, issued Apr. 4, 1961, illustrate such complex systems.

The mercuric chloride-persulfate system is also complicated by the hydrolysis of the persulfate to degradation products, some of which are active etchants. Each of these has a different etch rate at the same normality. The etch rates in such systems are also affected by the hydrogen ion concentration and the concentration of catalysts which may purposely be added, or which may be brought into the process during operation. It is advantageous separately to control the addition of each of the active chemicals which affects the etching process. It is also preferable to be able to control the process variables separately.

It is an object of the present invention to provide a method for controlling the addition of one or more active components to a metal etching solution.

It is a further object of the present invention to provide a method for controlling the rate at which a metal dissolves in a metal etching bath by separately controlling the concentration, rate of addition of various components of the bath, and the process variables.

It is also an object of this invention to provide apparatus operative to develop a basis for controlling the process variables.

Other objects and advantages of this invention will in part be obvious and will in part become apparent from the disclosure and drawings herein.

In its broadest aspects, the present invention provides a method for controlling the etch rate of a metal etching bath by means of controlled addition of at least one active agent in a metal etching bath containing two or more components that affect the etch rate, by first determining the difference in the rate of dissolution of a metal article in a standard metal etching solution being used for etching, and a second identical metal article which is etched with the same solution containing an additional amount of one of the active agents being controlled, or which has first been treated in a concentrated solution of that active agent being controlled, before being dissolved in the same etching solution as was used on the first article. If the second metal article dissolves at a different rate from the first, a need is indicated for addition of the active agent being controlled. An increase in this difference, other factors being constant, indicates a need for a larger addition of the active agent to the metal etching solution being tested.

In more specific terms, reference should first be had to the following equations identifying a persulfate etching bath:

These equations indicate first the usual hydrolysis of perdisulfate to the bisulfate and permonosulfate ion.

The second equation indicates ionization or hydrolysis of this second ion to a bisulfate and hydrogen peroxide form.

The third equation indicates the decomposition of the hydrogen peroxide to water and oxygen.

Interestingly, each of the peroxygen components of this very complicated mixture is an etchant and no one of them is of the same degree of effectiveness as the next under a given set of conditions. The complexity of attempting to control etch rate of such baths on the basis of measuring the concentration of one or another component is such as to be not only discouraging, but susceptible to such complications as to be undesirable. The problem of control of a solution or control of an etch rate in a solution of persulfate is best related to the overall rate of etch without reference to the concentration of any particular one of these ions.

Frequently, mercuric chloride is used as an accelerator of the persulfate etchant. It can be added to the persulfate solution, but the amount needed is very critical to maintain optimum etch rate in the bath. That is, optimum etch rate may be with a mercuric chloride concentration of the order of 3-5 parts per million, However, addition to a concentration beyond 5 parts per million will not necessarily produce a proportional increase in the rate of solution. Hence, optimum control is best achieved in the overall etch by maintaining the concentration of ingredients at a level on the low side of the maximum so that the addition of the mercuric chloride can produce a significant increase on the etching rate. Similarly, presence of too much ammonium persulfate in the etchant can decrease etch rates. Hence, for control purposes, it is best to operate the etch bath at a rate of 510% below a maximum rate of etch so that the bath will be immediately sensitive to additives.

In accordance with this invention, therefore, it is the purpose of the method and of the technique to develop an overall etch rate conrtoller based upon measuring the point of actual physical disappearance of a sample of the material being etched at a given level or depth in the solution by physically inspecting the material being etched at the desired level and scanning it over a predetermined distance as it approaches that level, in order to develop a range within which control can be achieved. My method, therefore, consists of simultaneously feeding at least two tapes carrying copper, or other metal to be etched, the said tapes being identical to each other and of the same metal as that being etched, into an etching bath, such as a persulfate bath, which is catalyzed with mercuric chloride to a concentration which is an appropriate optimum level, but on the low side of optimum. Since the two tapes are physically identical, they should etch at the same rate and the disappearance of copper should be detectable at a given point in the solution corresponding to the level reached by the tape at that point, based upon the rate of consumption of copper in the etch bath. One tape is fed directly into the solution, and the second tape is first fed through a mercuric chloride solution, so as to develop on its surface an effective small incremental concentration of mercury, so as to cause the tape to be etched more rapidly when it enters the etching solution with the first tape. Under these circumstances, the two tapes travelling at the same rate will have different levels of copper disappearance as they travel through the solution, and the difference in levels can be used as a measure of a need for mercuric chloride in the etching solution, the measurement being used as a signal to effect an addition of mercuric chloride to the solution and, under the circumstances where the disappearance of the copper is substantially at the same level, at a deep point in the solution, it can be used as a measure of the need for the addition of persulfate. The actual rates of etching of the tapes may not be identical with etching rates in the bath being controlled because of differences in the mode of application of etchant to the copper tape and to the printed circuit board in the etching bath, so that the method is actually one for controlling a bath to produce a pre-selected etch rate in the etching bath rather than an effort simply to measure the rate of etch on any particular surface of copper in the solution. It is of course obvious that the device described can also be used to measure etch rates on the copper tapes.

The invention, accordingly, is based upon this method of developing an overall control of etch of a copper foil in a given etching bath at a preselected rate, by comparing a copper foil sample etched in the etchant being controlled with a sample having an incrementally larger mercury activity, and utilizing the difference as a measure of the need for additional mercuric chloride catalyst,

The apparatus described herein in connection with the accompanying drawings provides a means for practicing the method of this invention, in which FIGURE 1 is a phantom view of the path of the two tapes through the apparatus;

FIGURE 2 is a right side elevation of the apparatus, partially in section; and

FIGURE 3 is a front elevation of FIGURE 2, also partially in section.

The application of the basic process of the invention is illustrated in the control of a metal etching bath using a tape in which a metal foil is laminated to a flexible nonmetallic tape. A first such metal tape is treated in a concentrated solution of the first active agent and then etched in a solution taken from the commercial metal etching bath being controlled. The test metal etching baths should preferably be maintained at the temperature used in the etching machine associated with the control device. The time necessary to dissolve the pre-dipped metal foil on the first tape is determined. Similarly, a second standard metal tape is treated with the same metal etching solution but without having been pre-dipped and the time necessary to dissolve the metal foil on the second tape is determined. The additional increment of time necessary to dissolve the second strip when compared with the time necessary to dissolve the first test strip is determined. During operation, an increase in this time increment is indicative of a need for addition of the first active agent.

The following specific example of the process illustrates this invention.

A copper foil (one ounce per square foot being 0.0014 inch thick) laminated to a transparent polyester tape backing, hereinafter referred to as the copper-tape, was predipped by passing it through a solution of mercuric chloride containing 25 parts per million of mercury at a rate such that its residence time was one minute and then etched in a one molar ammonium persulfate standard solution. The copper dissolved in three minutes and 13 seconds when the standard solution contained 5 parts per million of dissolved mercury added as mercuric chloride; three minutes and 25 seconds when the standard solution contained three parts per million of mercury; and four minutes and 37 seconds when the standard solution did not contain any dissolved mercury. Standard copper-tape which had not been pre-dipped in the concentrated mercuric chloride solution was also dissolved in the same standard etching baths. The bath containing 5 parts per million of dissolved mercury required three minutes and 45 seconds to dissolve the copper; the bath containing three parts per million of dissolved mercury required four minutes and ten seconds to dissolve the copper; and the bath which did not contain dissolved mercury required 11 minutes and 15 seconds to dissolve the copper. The foregoing information is utilized in a system for controlling the concentration of mercuric chloride at a level of 5 parts per million in a commercial metal etching bath containing about 1 molar ammonium persulfate solution.

Two test copper-tapes, side by side, are moved into a portion of 1 molar ammonium persulfate metal etching bath being tested, preferably at a constant speed. Control of the dissolved mercury concentration at 5 parts per million was desired. One of the test copper-tapes was first passed through a mercuric chloride solution containing 25 parts per million of mercury for a period of 1 minute. While immersed in the test solution, each of the coppertapes is monitored by a sensor which moves along the copper-tape, preferably in the direction in which the copper-tape is moving, and determines the time necessary to dissolve the subject copper-tape. The sensor for the predipped copper-tape had been adjusted to lag behind the sensor for the untreated copper-tape by a distance corresponding to an etching time difference of 32 seconds. When the bath being tested contains 5 parts per million of dissolved mercury, both sensors are actuated at the same instant. When the concentration in the test bath drops below 5 parts per million of dissolved mercury, the pre-dipped copper-tapes sensor is actuated before the other sensor. Thus, when the bath contained three parts per million of mercury, the additional increment required in etching time would be 13 seconds. If the bath contained no dissolved mercury, the additional increment would be 6 minutes and 6 seconds. Where the sensors determine a larger time increment than that determined from the standard solutions, the information is utilized to add mercuric chloride to the etching bath tested. This may be accomplished by using the action of the predipped tapes sensor to open a solenoid valve and the action of the other sensor to close it. The valve may be controlled to introduce a constant small flow rate of mercuric chloride solution into the commercial metal etching bath. This may be repeated in each sensing cycle with the result that the concentration of mercuric chloride would be adjusted upward until the desired concentration was attained. Additions of mercuric chloride during each of a number of consecutive cycles might be required.

More complex control systems utilize the determination of the difference of time necessary to dissolve the untreated tape when compared with the time necessary to dissolve the pre-dipped tape, to determine the actual concentration of dissolved mercury in the bath. This could then control addition of the necessary quantity of mercuric chloride to attain the optimum production concentration.

The mercury concentration is adjusted as already described. Therefore, an optimum concentration of mercuric chloride catalyst will be maintained in the etching bath. With such an optimum mercuric chloride catalyst concentration present, it is possible to know of to determine the maximum etching rate which one can obtain with any given etching equipment being used with persulfate etchant. The apparatus which is the subject of this invention can be used to add persulfate as required to produce the desired etch rate which, most desirably, is selected to be only slightly less than the maximum etch rate obtainable with catalyzed persulfate in the given etching system. One of the two copper bodies being etched in this device is monitored at a predetermined level for the presence of copper at a level below the free-surface of the etchant or beyond the point at which the etchant is first applied to the body being etched. The predetermined level is calculated by the rate of transport of the tape multiplied by the thickness of the copper being etched away, divided by the rate of etching at the desired catalyst concentration. The predetermined level may also be determined by experimentation with the test device. If copper on a tape has been etched away at this pre-selected level, no additional persulfate is required. If the copper is still present at that level, additional persulfate can be injected into the etchant.

The method of the present invention for controlling the etch rate of a metal etching bath may be simplified in operation by deter-mining conditions under which the metal tape which first passes through the concentrated solution dissolves at almost or exactly the same rate as the metal tape which passes through the metal etching bath controlled at the desired concentration and rate of etching. This may be accomplished by adjusting the concentrated solution so that it results in an etching rate the same as, or only slightly greater than, that of the etching rate desired in the metal etching bath. In operation, slower dissolution of the metal tape etched in the metal etching bath being controlled is directly indicative of the need for addition of etching agent. In eifect, this variant of the invention utilizes a zero predetermined difference in the metal etching rate of the desired metal etching bath, and the more concentrated solution.

In another variant of the method, the control of the etch rate may be simplified by predetermining a rate of addition of the etching agent being controlled, to maintain the etching rate in the metal etching bath at, or just below, the desired metal etching rate. During operation, slower dissolution of the metal tape in the metal etching bath when compared to the tape first passed through the concentrated solution of etching agent is indicative of the need for more rapid addition of the etching agent. :In this variant of the invention, the decline in etching rate that would be expected because of any predicted continuous loss of etching agent from the bath is largely, but not completely, eliminated by adding etching agent at a rate almost sufficient to compensate for the expected loss. The addition of the necessary remaining etching agent to control the system, is accomplished by the apparatus and process of this invention.

The method of the invention is directly applicable to spray etch techniques. The point of entry of the tape into the spray can be treated in the same fashion as the point of entry of the tape through the free surface of an etch solution.

A suitable apparatus for practicing the process of the present invention is illustrated in the drawings herein.

The apparatus illustrated is particularly adapted for the control of one or two active etchant reagents in a complex system, e.g., the control of mercuric chloride and if desired, the control of persulfate, in the system described hereinbefore. The path of the two copper- tapes 10 and 11 through the apparatus is depicted in FIGURE 1. Each of the copper-tapes is identical, being a thin copper foil fixed on a flexible strip of transparent polyester backing.

Copper-tape 10 is passed from supply reel 12 directly into reservoir tank 16 which contains etching solution. Copper-tape 11 is taken from supply reel 13 and passed into a pre-dip section comprising tank 14 which contains a solution of the active etchant being monitored, e.g., mercuric chloride, and is then rinsed in tank 15. Tape 11 is then passed into and through reservoir tank 16.

The path of copper- tapes 10 and 11 through reservoir tank 16 are identical. The etching conditions are identical. The thickness of copper on the copper-tape, and the distance travelled through the etching solution are correlated with the rate of travel of the tapes so that all the copper be dissolved before the tape reaches guide rods 17.

The point at which the copper is dissolved from each of said copper-tapes is determined by the sensing apparatus which carries a light source-photocell combination for scanning each of the copper-tapes. Copper-tape 10 is scanned by light source 23 and photocell 24. Copper-tape 11 is scanned by light source 25 and photocell 26. These light source-photocell combinations ascend and descend at a rate faster than the descending copper-tapes. The light source-photocell combination are activated only while descending. The respective light source-photocell combinations determine the point at which the copper is dissolved from the copper-tape which it is scanning.

The tapes after dissolution of the copper, are passed around guide rods 17 and then around guide rods 27. The now upwardly ascending tapes 10 and 11, are pulled by driven take-up reels 28 and 29. After passage through reservoir tank 16, they are passed between capstans 30 and spring loaded guide rollers 31, and then taken up on reels 28 and 29.

The preferred apparatus is illustrated in detail in FIG- URES 2 and 3. As depicted therein, copper-tape 10 is passed directly to reservoir tank 16 from tape supply reel 12. Copper-tape 11 is taken from tape supply reel 13, passed over guide rod 18, and through the mercuric chloride solution in tank 14 around guide roller 19. Coppertape 11 is then passed over guide rod 32, through rinse tank 15 around guide roller 33, and then positioned by guide rods 34 and passed into reservoir tank 16.

The portions of the apparatus which are in contact with the corrosive etching solution, and particularly those immersed in reservoir tank 16, are constructed of corrosion resistant material such as polymethyl methacrylate, and preferably utilize stainless steel fasteners when the etchant is ammonium persulfate.

'Each of the light source-photocell combinations is supported on a panel which is controlled to ascend and descend in reservoir tank 16 in the space between the descending and ascending paths of tapes 10 and 11, Photocell 24 is supported on panel 40. Light source 23 is connected to panel 40 by link 41. In a similar fashion, photocell 26 is supported on panel 42. Light source 25 is connected to panel 42 by link 43. Panels 40 and 42 are secured to each other by adjusting screw 44. In certain operations, particularly to avoid corrosive attack, the light-source photocell combinations may be mounted outside of the tank. The light sources would be on one side of the tank, positioned to illuminate light paths through front transparent windows positioned in line with the paths of the tapes being scanned and then through transparent rear windows to the externally mounted aligned photocells.

Panel 40 is supported in its ascending and descending traverse by tubular arm 45. Panel 42 is similarly supported by tubular arm 46. Both of these tubular arms are aflixed to connecting link 47. Electrical connections to the light sources and from the photocells on the panels, are passed through the tubulars arms 45 and 46, and then to outside electrical connections through wires 48.

Tubular arms 45 and 46 are controlled in their ascending descending movement by attachment of link 47 to chain 50 which rides on lower sprocket 51 and upper sprocket 52. Control of the sensing apparatus so that it is activated only when descending is achieved by contact of rollers 53 with contact switch 54.

Power for the movement of the tape take-up reels 28 and 29, and also for the movement of chain 50 is supplied through shaft 55, which is supported by bearings 56. Shaft 55 drives sprocket 51. It also drives the tape take-up reels 28 and 29 through slipping elastic drives 57 and 58 respectively. Shaft 55 is driven from sprocket 59 by means of chain drive 60, which in turn is powered by a conventional motor and reduction gear to provide the desired speed.

Additional units may be used for etchant components being controlled in addition to the two components which may be controlled by the aforedescribed apparatus, and for process variables being controlled. Such additional units could be constructed in tandem operating from the same motor shaft.

In another variant of the method, the pre-dip section may be avoided. Instead, each of the copper-tapes are etched in separate reservoir tanks under identical conditions. The solution in one tank will be a portion of the metal etching bath being controlled. The solution in the other tank will be the same solution, to which has been added an incremental amount of one active etching agent.

In the preferred method of this invention, with the test metal strip in the form of a continuous strand, e.g., a wire and preferably a foil, measurement of the rate of dissolution of the metal utilizes transmitted light and optical sensors which move along the strip as aforedescribed. Other means for measuring the rate of dissolution may be utilized, such as ahe thickness of the strip, the ability to absorb other radiations, such as X-rays, gamma rays, etc., electrical resistivity, detecting system-s depending on capacitance of a conducting plate in proximity with the strip and other light radiation systems depending upon reflectance of impinged light rather than transmission of such light.

The preferred sensing device of the present invention utilizes a light source and a light pickup travelling along a tape and determining the point at which the tape is entirely dissolved. Equivalent systems may be used which do not involve a moving sensor, or measure a point of metal dissolution less than complete dissolution. Thus, a plurality of photocells positioned along the tape could be utilized to determine the time necessary to dissolve the tape. The photocells are connected to a commutator. A brush riding on the commutator could sense the time necessary to dissolve the metal. Using two such devices, one for the pre-dipped tape, and the other for the untreated tape, the addition of the requisite active agent into the etching bath could be controlled. Another system utilizes a wholly electrical system rather than a light system in which an AC potential is impressed between the tape and a number of sensing electrodes positioned along the tape. Where such electrodes are parts of small conductivity cells, the conductance of the cell consisting of the copper on the tape and each small electrode will be comparatively large if copper on the tape remains unetched near a given small electrode, while the conductance of the cell consisting of an electrode located near that portion of the tape from which the copper has been etched will be comparatively small. A suitable circuit can utilize this information in a manner similar to that described hereinbefore.

What is claimed is:

1. The method for controlling the etching rate of a metal etching bath containing at least two active etching agents by the controlled addition of at least a first etching comprising comparing the rate at which a first metal article dissolves in the metal etching bath being controlled, with the rate that an identical second metal article dissolves in an etching solution that differs from said metal etching bath by containing a higher concentraction of said first etching agent at the surface of said second metal article, whereby when said first metal article dissolves at a rate different from that of said second metal article, an addition of said first etching agent is made to change the etching rate.

2. The method of claim 1 comprising (i) treating a first metal article in a concentrated solution containing an amount of a first etching agent in excess of the amount desired in the metal etching bath, and then contacting said first article with a metal etching bath containing the desired concentration of the bath components, and measuring the rate of dissolution of said first metal article;

(ii) contacting a second metal article with said metal etching bath and measuring the rate of dissolution of said second metal article;

(iii) determining the difference in the rates of dissolution of said two metal articles;

(iv) treating a first test metal article with said concentrated soltuion, and then contacting said first test metal article with the metal etching bath being controlled and measuring the rate of dissolution of said first test metal articles;

(v) contacting a second test metal article with said metal etching bath being controlled and measuring the rate of dissolution of said second test metal article and (vi) determining the difference in the rates of dissolution of said two test metal articles, and then comparing said difference with that determined in step No. (iii) hereof, whereby when this second difference is larger than said first difference, an addition of said first etching agent is required to increase the etching rate.

3. The method of claim 2 wherein said metal articles are metal tapes.

4. The method of claim 3 wherein the difference in rates at which the metal tapes dissolve is determined by measuring the time required to completely dissolve the metal of each of said tapes.

5. The method of claim 4 wherein each of said tapes is a copper foil laminated to a flexible plastic film backmg.

6. The method of claim 5 wherein said metal etching bath is an aqueous persulfate-mercuric chloride etching bath.

7. The method of claim 6 wherein said method is used to control the etching rate by control of the amount of mercuric chloride etchant added to said metal etching bath.

8. The method of claim 7 wherein the addition of persulfate etchant to said metal etching bath is controlled by determining whether the copper has been completely dissolved from the tape after the elapse of a predetermined amount of time.

9. Apparatus for controlling the etching rate of a metal etching bath by controlling the concentration of an etching agent in said metal etching bath comprising a container having at least two compartments, one for contacting a solution containing said etching agent with a metal strip, and a second for contacting a portion of the metal etching 'bath being controlled with at least two metal strips,

means for contacting a metal strip with said solution and then contacting said metal strip with said metal etching bath,

means for contacting a second metal strip with said metal etching bath under the same conditions as those for contacting said first metal strip with said bath, and

means for measuring the rate that metal is dissolved from each of said metal strips as a result of contact with said metal etching bath. 10. The apparatus of claim 9 containing means for comparing the difference in the rates at which metal is dissolved from each of said strips and controlling the addition of said etching agent to said metal etching bath in response to the difference in said rates.

11. The apparatus of claim 9 containing means for passing a metal tape on a plastic hacking into a tank containing said metal etching bath;

means for passing an identical meta-l tape on a plastic backing through a tank containing a concentrated solution of said etching agent and then passing said tape into a tank containing the metal etching bath;

means for passing said two metal tapes in side by side relationship through said metal etching bath, and

means for determining the point along the path of travel of each of said tapes through said metal etching bath at which the metal is dissolved from said plastic backing.

12. The apparatus of claim 11 wherein said means for determining each of said points at which said metal is dissolved, is the combination of a light source directed across the path of said tape and at least one photoelectric cell positioned at the opposite side of said tape.

- metal etching bath.

References Cited UNITED STATES PATENTS 2,762,036 9/1956 Triman 1567 2,933,094 4/1960 Cunniff et a1. 134-57 X 2,978,301 4/1961 Margulies et al. 15618 X 3,013,569 12/1961 Sterczala 15618 3,032,753 5/1962 Knapp et al. 156345 X 3,300,362 1/1967 Crosfield 156345 HAROLD ANSH-ER, Primary Examiner JOSEPH C. GIL, Assistant Examiner US. Cl. X.R.

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