JPS63213678A - Electroless copper plating method - Google Patents

Abstract

PURPOSE:To accurately control the concn. of each component in a plating bath within a certain range, by calculating the amts. of a metal component and a reducing agent to be supplied from reaction rate constants related to the consumptions of the metal component and the reducing agent according to the progress of a plating reaction and by supplying them. CONSTITUTION:Reaction rate constants related to the consumptions of a metal component (copper) and a reducing agent (formaldehyde) according to the progress of a plating reaction during electroless copper plating are represented by KM and KR, respectively. When the constants KM, KR are used, a rate equation for the consumption of copper is represented by CM=Co-KM.t and that for the consumption of formaldehyde by CR=Co.exp(KR.t) (where Co is initial concn. and t is time). The amts. of the components consumed during the time from analysis to supply are estimated by reaction kinetics and the components are supplied by the estimated amts. added to the conventional amts. supplied. Thus, the concn. of each of the components is accurately controlled within a set range.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for electroless plating, and more particularly to a method for replenishing consumed bath components.

(Prior Art) Electroless copper plating is widely used as a surface treatment technique for connecting circuits on the front and back sides of a printed wiring board via through holes. Furthermore, in recent years, as circuits on printed wiring boards have become more compact, the diameter of through holes has tended to become smaller. Conventionally, such small diameter through holes were first formed by electroless plating to form a thin conductive layer, and then electroplated. A method of electrodepositing a copper film to a predetermined thickness using copper plating has been used. However, this method has disadvantages such as unnecessary electrodeposition of copper at the ends of the through holes and difficulty in electrodeposition in one direction. For this reason, it is convenient to form a conductive layer in small-diameter through holes using only electroless copper plating, which has good uniform electrodeposition properties, using copper sulfate as the copper source, EDTA as the complexing agent, and formalin as the reducing agent.
Thick electroless copper plating baths containing alkali hydroxide as a pl+ adjuster and additives such as dipyridyl and surfactants have been used. However, since the deposition rate of this plating bath is usually less than 2 .mu.m/Hr, operation for 15 to 20 hours is required to obtain the targeted 35 .mu.m coating. During this time, components such as copper, reducing agent, and alkali hydroxide are consumed as the plating reaction progresses, so these components are quantified every 15 to 20 minutes and the consumed amount is replenished to keep the concentration of each component within a certain range. Automatic control is performed to adjust the concentration, and the fixed range here is a range of 5% soil relative to the set concentration of each component. By always controlling the bath components mentioned above within a certain range, it is possible to not only keep the deposition rate of the resulting copper film constant, but also to influence the physical properties of the copper film itself. Management is extremely important.

(Problem to be Solved by the Invention) As mentioned above, the method of controlling the bath components within a certain range is to quantitatively analyze each component in the plating bath at fixed intervals, and calculate the amount consumed from the set value of each component. may be added to the bath using concentrated solutions of each component. However, in reality, the concentration of each component is determined in the following order: collection of the plating solution for analysis from the plating bath, quantitative analysis of each component, and replenishment of the consumed amount. ; There is a time difference (hereinafter referred to as a time lag) between the replenishment time of the contracted liquid and the replenishment time. In the plating bath, each component continues to be consumed during that time, so the amount of replenishment calculated as the shortfall from the set concentration of each component at the time of liquid collection will be less than the true amount of replenishment at the time of replenishment, so the analysis time The concentration of each component does not return to the set value even if the replenishment amount is replenished. Therefore, it is difficult to maintain the concentration of each component within a predetermined concentration range.

In order to solve the above-mentioned problems, the present invention provides a method for accurately controlling the concentration of bath components within a certain range by using replenishment amounts that take into account the consumption of bath components during the time lag. .

(Means for Solving the Problems) In order to achieve the above object, the present invention calculates the consumption of metal components and reducing agent from the respective reaction rate constants kW and kR related to the consumption of metal components and reducing agent as the plating reaction progresses. It is characterized by calculating and replenishing the amount of replenishment.

(Function) The reactions that occur in the electroless copper plating bath are mainly the copper reduction-precipitation reaction represented by the following formula (1) and the side reaction of the reducing agent represented by the following formula (2).

Cu"'+2HCHO+40tl--"Cu+2HC
OO-+ 211zO+Hz (1) 211CIIO
+OH−−4tlCOO−+ CHsOH(2)(11
, (As can be seen from Equation 21, consumption of formalin proceeds not only by the reduction of copper but also by the self-condensation reaction of formalin itself.

The replenishment method of the present invention calculates the reaction rate constants kHz and Ny from the above-described consumption reactions of copper and formalin, and adds the consumption amount of both components during the time lag to the replenishment amount at the analysis time to calculate the true value at the replenishment time. This is to calculate the supply amount. To find the reaction rate constants related to the consumption of copper and formalin, we utilize the fact that the consumption reactions of copper and formalin are zero-order and -order reactions, respectively.More quantitatively speaking, the reaction of copper and formalin is When the rate constants are kW and kR (M: metal, R: reducing agent), the consumption reaction rate equation for copper is C, = Co - k i t, and the consumption reaction rate equation for formalin is C, 1 = Co -exp(-
kR-t) (Co is the initial concentration, t is time).

In reality, since there is replenishment for the consumption of each component as the plating reaction progresses, the consumption reaction rate equation for the copper component and formalin is as follows.

First, regarding the copper component, the nth measurement value is Cn, n+1
The second measurement value is Cn. 3. Supply amount is Cs, measurement interval is T
Then, C11+i = Cn + Cs kH・T
Therefore, by substituting the actual measured value, amount of reinforcement, and time, k
Calculate the value of M. If the time lag time is T, the consumption amount of copper components during the time lag is kM - TI, so the consumption during the time lag ff1k, T is added to the replenishment amount determined by quantitative analysis, and the amount is transferred into the plating bath. Replenish.

On the other hand, for formalin, the nth measurement value is C'n, n
+If the first measurement value is C'g+1, the replenishment amount is C's, the measurement interval is T, and the replenishment time is t, then C',, +, -C'n-exp(-T =,) +
Since it is expressed as C'5exp(-(T-t) ·kR), the kRO value is calculated by substituting the actual measurement value, supply amount, and time into both equations. If the time lag time is T2, the consumption amount of formalin during the time lag is C′・exp(−T,・kR)
Therefore, the amount of reinforcement determined by quantitative analysis is added to the amount of reinforcement consumed during the time lag (-Tz.k+t) and then replenished into the plating tank.

(Example) An example of the present invention is shown below.

Copper sulfate 2.5 g/I! , EDTA 60g/l, formalin 2. -Og/l, dipyridyl 30 Akebono/1
2. Polyethylene glycol 0.5 g/I! Using an electroless copper plating bath with a bath temperature of 65°C and a bath load of 1
．． Tests were conducted at 5 dm2/l. In addition, the plating tank used was 100X80X80 (marked), and the liquid volume was 120
It is E.

In this example, the concentration of copper and formalin was measured at 20
The measurement was carried out every minute, and as means for measuring both components, an absorption photometry method in the visible region (740 nm) was used for copper, and a neutralization titration method using sodium sulfite was used for formalin.

Under the above conditions, copper replenishment solution (copper sulfate 50%
g/e, EDTA450 g/ff), 37% aqueous solution as formalin was used for continuous treatment, 5 mE of plating solution was collected every 20 minutes, and the alkali hydroxide concentration, formalin concentration, and copper concentration were measured in this order. went. As for copper and formalin, the measured values were processed by a computer using the method shown in the section on the effects above, and the amount of replenishment was added to the amount of replenishment at the time of analysis and the amount consumed up to the time of replenishment, and the amount was replenished into the plating tank. As a result, as shown in FIG. 1, both formalin concentration and copper concentration were within the controlled concentration range.

Furthermore, for comparison, when replenishing only the amount determined by analysis as in the past, the polymerin concentration and copper concentration did not fall within the control concentration range, as shown in Figure 2.

(Effects of the Invention) As explained above, according to the electroless copper plating method of the present invention, the amount of bath components consumed within the time difference between the analysis time and the replenishment time is predicted from reaction kinetics, and By adding the consumption amount to the conventionally used replenishment amount and replenishing, it is now possible to accurately control the metal component concentration and reducing agent concentration in the plating bath within each set concentration range. Further, it is clear that the properties of the resulting copper coating, especially the ductility, can be greatly improved.

[Brief explanation of the drawing]

FIG. 1 shows changes in the concentrations of copper and formalin in the plating bath when the replenishment method according to the present invention is used. FIG. 2 shows the changes in the concentration of copper and formalin in the plating bath when using the conventional replenishment method. 1... upper limit of the control concentration range, 2... lower limit of the control concentration range. Figure 1 (time) 1. η浬浓率ρi云L”-Eye 2, lower limit of the same Figure 2 (time) 1. Upper limit of the controlled concentration range 2, lower limit of the same

Claims (1)

[Claims] An electroless copper plating method characterized in that the amounts of metal components and reducing agents are calculated and replenished from reaction rate constants K_M and K_R relating to the consumption of metal components and reducing agents as the plating reaction progresses.