JPH04318177A - Electroless copper plating bath - Google Patents

Abstract

PURPOSE: To prepare an electroless copper plating bath in such a manner that the nature of the layer of the copper plated on a substrate is greatly improved in terms of uniformity and strength on structure and further that the stability of a plating rate and the bath are made extremely uniform.

CONSTITUTION: The addition of a cyanide ion feedback of the predetermined amt. is executed at predetermined time intervals, by which the level of the cyanide ion stabilizer is maintained in the electroless copper plating bath 2 maintained at a predetermined specific temp. A computer system 16 programmable for the purpose of controlling the method is provided as well.

COPYRIGHT: (C)1992,JPO

Description

[Detailed description of the invention]

0001

TECHNICAL FIELD This invention relates to electroless copper plating baths and improved copper plating substrates, and more particularly to methods and programmable computer-controlled systems for electroless copper plating, and The present invention relates to substrates coated with copper using the system.

0002

BACKGROUND OF THE INVENTION Electroless copper plating of substrates, such as printed circuit boards, is a well-known process. Electroless copper plating bath is
Copper ions, along with reducing agents such as formaldehyde, complexing agents for the copper ions, typically ethylenediaminetetraacetic acid (EDTA), sodium hydroxide to control pH, and various additives such as surfactants and stabilizers. sources, generally containing copper sulfate. A commonly used stabilizer is a source of cyanide ions, such as sodium cyanide.

[0003] Underkofler et al. US Patent No. 3
No. 844799 maintains the cyanide ion concentration of the electroless copper plating bath at 0.0002-0.0004 mol and at the same time maintains the bath temperature at 70°C-80°C.
It teaches that the properties of plated copper layers can be improved. The range of cyanide ion concentrations required by this teaching is wide enough to allow up to 100% variation.

[0004] The above patents do not provide details on how to monitor the concentration of cyanide ions in order to make additions to keep the concentration within the required range. A common method for analyzing cyanide ions uses an electrode specific for cyanide ions and measures the rate at which silver metal is removed from a membrane that is in contact with a solution containing cyanide ions. The accuracy of this method is affected by the presence of ions that may be present in the bath as additives or impurities. For example, the presence of ions such as sulfides, halides, thiocyanates, or any ions that form insoluble complexes with silver, makes it difficult to accurately measure the actual cyanide concentration, especially at levels as low as 0.0002 molar. cause significant interference. Other methods of analysis include ion chromatography, polarography, colorimetry,
Including Kjeldahl and amperometric titration. In general, these methods lack the necessary precision and/or are slow, i.e., require considerable time to perform, thereby making it difficult to correct the deficiency of cyanide ions in the bath. causing noticeable delays.

[0005]

SUMMARY OF THE INVENTION The present invention provides that if the pH, copper ion and reducing agent concentrations are maintained at constant levels in accordance with the normal practice of those skilled in the art, the rate at which cyanide ions are consumed will be as low as the bath maintains. It is based on the discovery that it is directly proportional to the temperature at which The rate of consumption will vary for baths with different compositions, but for any given bath where the above parameters are kept constant, the rate of consumption of cyanide ions for a given temperature is directly proportional to that temperature. , and is reproducible, ie, the rate for a given temperature shows substantially the same pattern in all baths with the same chemical composition.

[0006] Based on the foregoing findings, it has therefore been found that for a given bath at a given temperature, how much cyanide ion can be added in any given elapsed time to restore the concentration of cyanide ions in the bath to the initial desired level. of cyanide ion must be added to the bath. Therefore,
Once the relationship between the rate of cyanide ion consumption at a chosen bath temperature has been determined, how much cyanide ion is added at any given time to maintain the desired cyanide ion concentration. It would be unnecessary to perform routine cyanide analyzes of that bath or any other baths of the same chemical composition during the operational life of the bath to determine whether the cyanide should be used.

Based on the above findings, the cyanide ion concentration in an electroless copper plating bath can be maintained substantially constant over extended operating times without resorting to repeated analyzes for cyanide ions in the bath. It has become possible to develop a system according to the present invention. As a result, the significant changes in cyanide ion content characteristic of conventional systems due to the time required to analyze each section as well as the delay introduced to correct the cyanide ion concentration after the defect has been determined are be excluded. Additionally, as described in detail below, the above discoveries have enabled the development of automated systems to maintain cyanide ion concentrations at desired levels in electroless copper plating baths.

A layer of copper plated on a substrate in an electroless plating bath operated in accordance with the present invention is less expensive than a layer of copper plated using the same bath but operated without the benefits of the present invention. It was also found that the material exhibited significantly improved physical properties. Similarly, bath stability and plating rate are very uniform, potentially resulting in long bath life and more consistent products.

The present invention, in its broadest aspect, provides a method for electroless plating of copper onto the surface of a substrate from an electroless copper plating bath comprising copper ions, a reducing agent, a complexing agent, and a stabilizer containing cyanide ions. (a) for a fixed value of copper ion concentration, the reducing agent concentration and the pH of the bath, and the characteristic consumption of cyanide ions in the bath and time for a fixed bath temperature; (b) maintaining the temperature of the bath substantially uniform at said fixed temperature; and (c) adjusting the copper ion concentration, reducing agent concentration, and pH of said bath at said fixed temperature. and (d) an amount sufficient to substantially replace cyanide ions consumed in the bath in accordance with the characteristic relationship established in (a). ions are added to the bath during plating of copper from the bath, the addition substantially bringing the cyanide ion concentration in the bath to a predetermined level during plating of copper from the bath. It consists of being carried out at intervals of sufficient time to maintain uniformity.

[0010] In certain embodiments, the present invention relates to a computer-controlled system for carrying out the method of the present invention, the system comprising a stabilizer comprising copper ions, a reducing agent, a complexing agent, and a cyanide ion; an electroless copper plating bath further comprising a thermal control means for regulating its temperature, a storage means comprising cyanide ion feedback, and coupled to the storage means for conveying a cyanide solution to the plating bath via a supply line. a combination of pump means, programmable computer means for monitoring the temperature of the plating bath and controlling the addition of a predetermined amount of cyanide ions at scheduled or calculated intervals;
(a) means for generating a base reference signal characteristic of a predetermined bath temperature; (b) sensing means for generating a signal characteristic of a temperature of said bath; and (c) said predetermined bath temperature. means for generating at scheduled time intervals a signal characteristic of a predetermined or calculated amount of cyanide ion feedback to be added to said bath at said predetermined time intervals; means for sensing the signal to activate the pump to deliver each of the scheduled or calculated amounts of the pump.

The present invention also provides electroless plating of copper on a substrate using the method and system described above, so that the properties of the plated copper layer and the bath stability and rate uniformity are improved. be done.

FIG. 1 shows a plot of cyanide ion loss versus temperature for a typical electroless copper plating bath.

FIG. 2 shows a schematic diagram of a representative computerized control system according to the present invention.

FIG. 3 schematically shows the components of the computer means shown in FIG.

The cyanide ion CN- has an exceptional ability to complex and sequester monovalent copper insofar as it removes chelating agents such as EDTA from monovalent copper/EDTA complexes. The resulting cuprous cyanide (CuC
N) has such stability that it behaves as an inert substrate in the plating bath. Cyanide ions react very quickly with monovalent copper, which often prevents other unstable reactions from occurring in electroless copper plating baths. However, cyanide ions can also react with divalent copper in any one of a number of ways, consuming the cyanide ions, but
It does not contribute to stabilizing the plating bath. The following is representative of these undesirable reactions with divalent copper, and cyanide is used in the form of potassium cyanide for illustration.

[0016]

[Chemical formula 1]

The above reactions and other related reactions are the main sources of cyanide consumption in electroless plating baths. The rate of consumption depends on monovalent and divalent copper, cyanide ion concentration, bath pH, reducing agent (formaldehyde) concentration,
and proportional to the temperature of the bath. If the concentration of copper ions and reducing agent and the pH of the bath are each maintained at constant levels during the operational life of the bath, as in normal industrial practice, the rate of consumption of cyanide ions depends only on the concentration of and the temperature of the bath. As mentioned above, it has been found that for a bath of a given composition, there is a direct proportional relationship between the rate of consumption and the temperature of the bath.

FIG. 1 shows a plot of cyanide ion loss body bath temperature over one hour for a typical electroless plating bath having the following composition: Cupric chloride 5.25 parts by weight, formaldehyde 2.40 parts by weight, EDTA 20.00 parts by weight, sodium cyanide 0.0017 parts by weight, sodium hydroxide 5.00 parts by weight, surfactant 0.10 parts by weight, Add water to make 1000.00 parts by weight.

Analysis of the cyanide content of the bath at various times and temperatures is carried out using an ion-specific electrode, by acidification of the sample, followed by steam distillation of hydrogen cyanide, collection of the distillate in a standard alkaline solution, and then a standard acid solution. This is done by measuring the amount of hydrogen cyanide liberated by back titration.

As mentioned above, the curve shown in FIG.
It is unique for a bath with said particular composition and is consistently exhibited by all baths with that particular composition. Similar but different plots would be obtained from baths with different compositions.

Based on the data shown in FIG. 1, a mathematical formula is derived from which the loss of cyanide ions at a given time interval can be calculated for any particular temperature of the bath at which the data shown in the figure is derived. . This formula then substantially maintains the cyanide ion concentration at the predetermined level without the need for further analysis of all the bath fluids to control the cyanide ion level.
It is used as the basis for determining the amount and frequency with which cyanide ions must be added to the bath.

The addition of cyanide ions at the required time intervals according to the invention can be carried out manually. However, in certain and preferred embodiments, the necessary additions are performed automatically using a computer control system.

Reference is now made to FIG. 2, which shows in schematic form a control system according to the invention. Electroless copper plating bath (
2) includes a supply line through which cyanide ion feedback is transmitted from the storage vessel (12) when the temperature controller (4), the temperature sensor probes (6) and (8) and the pump (14) are activated; (10) is provided. The temperature of the bath (2) is maintained at a predetermined level by a heater control system, shown generally as (18). Computer system (16
) are schematically illustrated in FIG. 3, as discussed further below. The bath temperature is controlled as follows. At scheduled intervals, a signal characteristic of the temperature of the bath (2) is detected by the sensor (6).
to the heater control, where comparator means compare the received signal with a base reference signal characteristic of the predetermined temperature at which it is desired to maintain the bath (2). If a difference between the signal received from the sensor (6) and the base reference signal is detected by the comparator, the latter generates a signal characteristic of the necessary adjustment that has to be made. These correction signals are used by the heater control system (18), which controls the temperature control means (4) to make the necessary corrections that have to be made to the bath temperature, either up or down depending on the case. operate. The temperature sensor (8) generates signals characteristic of bath temperature and these signals are used to monitor the heater control system (18) for proper operation. Although the above method of monitoring bath temperature is performed continuously at very short intervals, generally about 1 second, the frequency of monitoring can vary over a wide range and is not critical for successful operation of the system.

In addition to monitoring the temperature of the bath, the computer system (16) also monitors the loss of cyanide ions for a particular bath operating life at the operating temperature being used and monitored as described above. Cyanide ion additions are programmed to occur at scheduled time intervals based on a mathematical relationship of time. Timer means incorporated into the computer system cause a signal to be generated at a scheduled time, the signal being characteristic of the particular amount of cyanide ion to be added at any particular time. The signal so generated is fed to sensor means attached to the pump (14) for transmitting a predetermined or calculated amount of cyanide ions to the bath (2) via the supply line (10). Operate the pump for a sufficient period of time. pump (14) and bath (
A fluid flow sensor means (20) located in the supply line (10) between 2) is actuated by the movement of the fluid in the supply line and is supplied to the computer (16) and further actuates the pump (14). This serves to ensure that the cyanide ion feedstock is being transferred when The frequency with which the scheduled or calculated amount of cyanide ion is added can vary over a wide range. Advantageously, additions are programmed to occur at intervals on the order of 0.01-120 minutes and preferably on the order of about 1 minute to about 60 minutes.

Advantageously, the temperature at which the bath (2) is kept constant according to the invention is in the range from about 25°C to about 75°C and preferably from about 30°C to about 60°C. However, the bath temperature used in the method of the present invention is not critical;
And the above ranges are presented for illustrative purposes only. The optimum temperature for any given case can be determined by trial and
Determined by error method.

The concentration level at which cyanide ion (typically present in the potassium or sodium salt form) is maintained is advantageously from about 0.1 to about 100 ppm and preferably from about 0.5 to about 100 ppm. It is in the range of 50 ppm. However, these concentration ranges are given for illustrative purposes only and are not limiting, and the methods of the invention can be carried out at concentrations above or below these ranges.

The system of the invention has an additional advantageous feature in that the rate of loss of cyanide ions is directly proportional to the actual concentration of cyanide ions, ie, the higher the concentration, the faster the rate of consumption. The focus is on the remarkable and unexpected discovery that the If the amount of cyanide ion added at a given time is more or less than the amount actually needed at that particular time to maintain the desired level, the error will be accelerated due to the factors mentioned above. Corrected. Therefore, if the amount added is greater than the required amount, the increased level of cyanide causes the rate of consumption to increase accordingly, thereby quickly overcoming the error. Conversely, if the amount added is less than actually needed, the rate of consumption will be lower than at higher desired levels of cyanide ion and the error will also be rapidly corrected. This self-correcting effect is an added and unexpected advantage in the overall control achieved by the system of the present invention.

FIG. 3 schematically depicts the various components of the computer system designated generally as (16) in FIG. 2, the functionality of which is as previously described. As shown in FIG. 3, the microprocessor (22) serves to receive, process, and communicate appropriate signals characteristic of various functions. The timer means (24) provide a signal to the microprocessor (22) at preset intervals, which signal is characteristic of the time when one or other function of the system is to be activated. R.A.
The M/ROM component (26) contains the software for operation of the system. The temperature monitoring component is a cold junction compensated analog/digital conversion temperature control system, preferably with track and hold features, which receives signals from the temperature sensor (8) (see Figure 2) and converts these signals into Converting from analog to digital mode and transmitting it to the microprocessor (22) for calculation of the temperature database used to determine cyanide consumption. These signals are also compared to a base reference signal characteristic of the desired temperature level and the heater control system (
18) to monitor proper operation.

The computer (22) also controls the pump control component (30) at scheduled time intervals.
) and is characteristic of the time component, for which the pump (14) is actuated to transfer a quantity of cyanide ions from the storage vessel (12) to the bath (2). and the amount expected to be required by the bath at the time in question to maintain the desired cyanide level in the bath. Component (30) is provided with the necessary logic to send the appropriate signals to the operating mechanisms of the pump.

The unit (32) of the system allows the operator to adjust the program for the entire system and make all necessary changes to add to it;
and a standard terminal keyboard and video display monitor for accessing the system in order to monitor the actual operating parameters of the system during its operation. In another embodiment, the entire system is entirely preprogrammed and need only respond to alarm conditions such as pump failure, heater failure, empty feedstock reservoir, etc.

The electronic components necessary to perform the various functions described above for the systems shown and discussed with respect to FIGS. 2 and 3 are well known and commercially available. A detailed discussion of the circuitry involved in producing the above functions is therefore excluded. A description of commercially available components used in the computer system described above is provided below. It should be noted that these are shown for illustrative purposes and the invention is not limited to the use of these particular ones of the devices, either individually or in combination.

An example of a commercially available timer unit is SSAC
Inc. It is commercially available under the name DIGI-TIMER. An example of a microprocessor unit is an mch18 commercially available from Wintek Corporation, and a ROM/RAM component from Motorola Semiconductor.
mcm2814p commercially available from nductors
and mcm6206p unit. A typical display unit is manufactured by Digi-tal under the name VT420.
A typical terminal keyboard is available from Grayhill Inc. and is commercially available from Equipment Corporation. It is RS232/RS423 COM-PAC commercially available from . An example of a temperature control system component is manufactured by Protec Inc. as model d15d. An example of a pump control system component is a unit commercially available from Cr as a Series 6 module.
This unit is commercially available from Ydom Company.

Programmable computer control means for monitoring and adjusting copper ion, formaldehyde levels, pH and other parameters of electroless copper plating baths are well known and commonly used by those skilled in the art, and are described herein. It is not described in detail. These computer means generally include: means for generating a base reference signal characteristic of the predetermined level of the parameter to be monitored; sensing means for generating a signal characteristic of the actual level of each parameter in the plating bath; comparator means for generating a signal responsive to any difference between the reference signal and the base reference signal, and for indicating the need for any necessary adjustments to the actual levels of the various parameters in the bath; and means for sensing the comparison response. These means can be incorporated into the system of the invention, if desired, or kept separate therefrom.

The control method of the present invention, when used in electroless plating of copper on substrates, such as those used in the manufacture of printed circuit boards, provides a marked improvement compared to layers plated by conventional methods. This results in a layer of plated copper with properties. Illustratively, two identical substrates consisting of epoxy fiberglass reinforced sheets with copper foil laminated to one side were subjected to electroless copper plating. For both substrates, coating was carried out using a bath with the following composition: Copper sulfate pentahydrate 9.83 parts by weight, formaldehyde 1.90 parts by weight, EDTA 20.0
0 parts by weight, 0.002 parts by weight of sodium cyanide, 2.80 parts by weight of sodium hydroxide, 0.10 parts by weight of surfactant, 0.02 parts by weight of additives, and 1000.00 parts by weight of water.
Parts by weight. The bath was maintained at 60°C. One substrate (A
), plating was performed using the computer control system of the present invention. A 1.0 mil thick layer was plated. For other substrates (B), plating to the same thickness is done without the benefit of the control system of the present invention, cyanide levels are maintained in a conventional manner, and cyanide in the caustic soda component is maintained as required. Added to the bath. Plating times were on the order of 16 hours in both cases. The following properties were then determined for the copper layer on each substrate after dissolution in concentrated sulfuric acid and removal of the reinforcing layer. In tensile strength, Substrate A was 51,000 psi and Substrate B was 39,000 psi; in % elongation, Substrate A was 10 and Substrate B was 7.

The above description and examples are presented by way of illustration only and are not to be considered limiting. Various modifications readily apparent to those skilled in the art may be made without departing from the scope of the invention, which is defined solely by the claims.

[Brief explanation of the drawing]

FIG. 1 shows a plot of cyanide ion loss versus temperature for a typical electroless copper plating bath.

FIG. 2 shows a schematic diagram of an exemplary computerized control system according to the present invention.

FIG. 3 shows schematically the components of the computer means shown in FIG. 2;

[Explanation of symbols]

2... Electroless copper plating bath
4...Temperature controller 6...Temperature sensor probe 8
... Temperature sensor probe 10 ... Supply line
12...Storage container 14...Pump
16... Computer system 18... Heater control system 20... Fluid flow sensor means 22... Microprocessor 2
4. Timer 26.. ROM/RAM
28...Temperature system 30...Pump control
32...terminal and display

Claims (7)

[Claims] 1. A method for electroless plating of copper on the surface of a substrate from an electroless copper plating bath comprising copper ions, a reducing agent, a complexing agent, and a stabilizer containing cyanide ions, the improvement comprising: (a) For a fixed value of copper ion concentration,
establishing a characteristic relationship between consumption of cyanide ions in the bath and time for the reducing agent concentration and the pH of the bath and a fixed bath temperature; (b) at said fixed temperature;
maintaining the temperature of the bath substantially uniform; (c) maintaining the copper ion concentration, reducing agent concentration, and pH of the bath substantially uniform at these fixed values; and (d) Cyanide ions are added to the bath during plating of copper from the bath in an amount sufficient to substantially replace the cyanide ions consumed in the bath according to the characteristic relationship established in (a). In addition, the addition is made at intervals of time during plating of copper from the bath sufficient to maintain a substantially uniform cyanide ion concentration in the bath at a predetermined level. wherein the concentration of cyanide ions in the bath is maintained substantially uniformly at a predetermined level during plating of copper from the bath. 2. A system for maintaining a cyanide ion stabilizer at a predetermined level in an electroless copper plating bath at a constant temperature, the system comprising copper ions, a reducing agent, a complexing agent, and cyanide ions. an electroless copper plating bath comprising a stabilizer and also comprising thermal control means for regulating the temperature thereof; storage means comprising cyanide ion feedback; and a chamber for conveying the cyanide solution to the plating bath via a supply line comprising a combination of pump means associated with storage means, programmable computer means for monitoring the temperature of said plating bath and controlling the addition of a predetermined amount of cyanide ions at predetermined or calculated intervals; The computer means comprises: (a) means for generating a base reference signal characteristic of a predetermined bath temperature; (b) sensing means for generating a signal characteristic of a temperature of the bath; and (c) ) means for generating at scheduled time intervals a signal characteristic of a scheduled or calculated amount of cyanide ion feedback to be added to said bath at said scheduled time intervals; A system comprising means for sensing said signal to actuate said pump to deliver each of said predetermined or calculated amounts at each of said intervals. 3. Further comprising flow sensor means located in the supply line between the pump means and the plating tank, the flow means being configured to detect a rate of flow of cyanide ion feedback in the supply line. 3. The system of claim 2, wherein the system generates a signal. 4. The amount of cyanide feedback applied at any given time interval is the amount necessary at that particular time interval to restore the cyanide ion concentration in the bath to the level present therein. 3. The system of claim 2, wherein the amount is scheduled or calculated to be. 5. Further comprising programmable computer means for monitoring and adjusting the pH and concentration of copper ions and formaldehyde in said plating bath, said computer means characteristic for predetermined levels of each of said parameters. means for generating a base reference signal characteristic of the plating bath; sensing means for generating a signal characteristic of the actual level of the parameter in the plating bath; 3. The system of claim 2, including comparator means for generating a signal responsive to the difference, and means for sensing said comparison response to indicate the need for any necessary adjustments to the actual level of said parameter in said bath. . 6. An improved method for electroless plating of copper, the method comprising maintaining a predetermined level of cyanide ions at a predetermined temperature in an electroless copper plating bath used for the plating. In particular, the system comprises an electroless copper plating bath comprising copper ions, a reducing agent, a complexing agent, and a stabilizer comprising cyanide ions, and further comprising thermal control means for regulating the temperature thereof, cyanide ion feedback. and a pump means coupled to the storage means for conveying the cyanide solution to the plating bath via a supply line, monitoring the temperature of the plating bath and at scheduled or calculated intervals. comprising a combination of programmable computer means for controlling the addition of a predetermined amount of cyanide ions;
The computer means comprises: (a) means for generating a base reference signal characteristic of a predetermined bath temperature; (b) sensing means for generating a signal characteristic of a temperature of the bath; and (c) ) means for generating at scheduled time intervals a signal characteristic of a scheduled or calculated amount of cyanide ion feedback to be added to said bath at said scheduled time intervals; A method comprising means for sensing said signal to actuate said pump to deliver each of said predetermined or calculated amounts at each of said intervals. 7. A substrate electrolessly coated with a layer of copper using the method of claim 6.