JPH0310094A - Silver plating solution and silver plating method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to improvements in a silver plating solution used in the die bonding process in the semiconductor device manufacturing process, and a silver plating method using the silver plating solution. Regarding improvement.

In particular, it relates to improvements in silver plating solutions aimed at improving the smoothness of plated surfaces, and improvements to silver plating methods aimed at improving smoothness on plated surfaces.

[Conventional technology]

In the so-called Gui bonding process, in which a semiconductor chip is pasted onto a lead frame, etc. or other stems, etc., the stem, etc. is pasted by making the surface of the silver-plated surface good and smooth. In order to improve the wettability of adhesives used in silver plating, a method has conventionally been used in which a selenium compound or the like is added to a silver plating solution.

[Problem to be solved by the invention]

However, although this conventional method of adding selenium compounds, etc. to the silver plating solution is effective for improving the smoothness of the plating surface, the selenium compounds, etc. added to the silver plating solution may cause problems in plating. As a result, if the conditions for selenium to be able to diffuse into the semiconductor during the period after the semiconductor chip is bonded onto the stem are met, the above-mentioned selenium will become eutectoid in the semiconductor. Diffused within the chip, it may adversely affect the characteristics of the semiconductor, leading to a decrease in reliability of semiconductor devices manufactured using this semiconductor.

The aim of the invention is to eliminate this drawback.

The first object is to provide a silver plating solution that can smooth the plating surface without having the disadvantage of eutectoiding on the plating film and adversely affecting the properties of the semiconductor.

The second object is to provide a silver plating method that can further improve the smoothness of the plating surface compared to the case where this silver plating solution is simply used.

(Means for solving !ml) The first of the above two objectives is to add a small amount of imidazole CCs Ha Nz to an aqueous solution containing silver cyanide halide or silver nitrate, which is the main component of the silver plating solution.
) is achieved by adding silver plating solution. The amount of imidazole added is preferably in the range of 1 pp- to several 100 pp-.

Addition of several hundred pp- or more does not change the effect.

Further, the second purpose of the above two purposes is to use the silver plating solution according to claim [1] as the plating solution (3),
A first electrode (2) and a second electrode (1) serving as the object to be plated are used as electrodes, and a current is passed through the first electrode (2) as an anode, and then the second electrode ( This is achieved by repeatedly carrying out the process of 1) using a small amount of current as an anode.

The current used in the step of passing a current through the first electrode (2) as an anode and then passing a small current through the second electrode (1) as an anode may have a square wave shape or a triangular wave shape.

[Effect]

However, the present invention is based on the results of experiments, and its effects are not necessarily clear. However, it is possible to use a silver plating solution that does not contain selenium compounds, which eutectoidize with the plating film and adversely affect the characteristics of semiconductor chips. The surface of the silver plating carried out using this method has non-uniform crystal grain sizes and poor surface smoothness. It has been experimentally confirmed that the diameter is almost uniform and the surface has good smoothness. CA is a measure of surface smoothness.
M glossiness is commonly used, and CAM glossiness is 0.
Although it is considered that 2 or more is sufficient, imidazole (C, H) is used as an additive.

If you use a silver plating solution containing N8),
It has been experimentally confirmed that the CAM glossiness exceeds 0.2 and the smoothness is good. It has been experimentally confirmed that the CAM gloss level does not reach 0.2 when using a silver plating solution that does not contain the selenium compound mentioned above.
It seems clear that the silver plating solution to which imidazole (Cs Ha Nt ) is added as an additive contributes to improving smoothness.

Improving the smoothness of the silver plating surface is desirable in improving the wettability of the adhesive and increasing the effective bonding area when bonding a semiconductor chip to the silver plating surface with a conductive adhesive. It is known that
Rather, it is essential to improve the smoothness of the silver plating surface in order to ensure a conductive area. Further, the action of a current whose polarity is periodically reversed as an electrolytic current is estimated as follows. In other words, the silver atomic layer deposited on the plated surface by the positive polarity current in one cycle does not necessarily have a uniform thickness over the entire surface, and more silver atoms are deposited locally, forming protrusions. However, it is thought that a slight reverse scratching current following the positive polarity current performs a slight electrolytic etching on the plating surface, removing the above protrusions and improving the smoothness of the plating surface. .

[Example] Hereinafter, a silver plating solution and a silver plating method according to an example of the present invention will be further described with reference to the drawings.

This is an embodiment in which the silver plating solution described in claim [1] is used, but direct current is used as the current. Imidazole CC
s Ha N= ) to prepare a silver plating solution having the composition shown below.

Table 1 Composition of silver plating solution Bottom - Bean 1-1 - Northern cyanide potassium 120 g/l Potassium pyrophosphate 80 g/j2 dipotassium hydrogen phosphate 8 g/l Boric acid
35g/l imidazole
Potassium hydroxide (200 ppm) is added to this solution to adjust the hydrogen ion concentration so that the pH becomes 8.7.

42 material (an alloy containing 42% Ni and 5B% Fe) as a substrate was subjected to normal pretreatment (surface cleaning), and then copper strike plating was applied to a thickness of 0.2%.
n, and after performing replacement prevention processing,
An injection partial plating method was carried out using the above plating solution. The plating conditions in this plating method are as described below.

Table 2 Plating conditions 1-day pH 8,7 Bath temperature 65°C Anode Platinum-plated titanium nozzle power supply Single-phase full-wave rectified DC power supply Plating thickness Approx. 5
n Plating solution flow rate 5 m/sec Electrode path # 1 cm The plating film formed under the above conditions has a current density of 40~
The CAM glossiness is 0.2 or more in the range of 50A/dm'', the crystal grain size is uniform, and the smoothness is good.
As a result, there is no negative effect on semiconductor chips,
Die bonding was successfully performed.

In addition, the results of a plating method under the same conditions except that imidazole was not added (i.e., conventional matte plating method) showed that the CAM glossiness was 0.2 or less, and the crystal grain size was nonuniform. Yes, unevenness is observed on the surface, and the die
It was confirmed that during bonding, the adhesive did not spread sufficiently, causing adverse effects such as poor adhesion.

”” Unspoken This is an embodiment corresponding to claims 2] and [3].

This is an embodiment in which the silver plating solution described in claim [1] is used and an alternating current is used as the current.

First, as in the first example, imidazole (C3H4N,)
to prepare a silver plating solution having the same composition as above.

In the diagram shown in FIG. 1a, ■ is a lead frame on which silver plating is applied, 2 is a plating solution nozzle that also serves as the first electrode, and 3 is a silver plating whose composition is described in the first example. It is a jet of liquid. 4 is a plating current direction switching switch;
5 is a silver plating film.

First, the plating current direction switching switch 4 is connected so that the nozzle 2 becomes the positive electrode, and the plating solution 3 is injected from the nozzle 2 onto the surface of the lead frame 1. The plating current is passed from nozzle 2 to lead frame 1.
Through this process, the lead frame 1
A silver plating film 5 is formed on the surface.

Refer to Figure 1b Next, switch the plating current direction switch 4,
Connect so that lead frame 1 becomes the positive g1 girder,
By this process, the plating liquid 3 is injected from the nozzle 2 onto the surface of the lead frame 1, and the plating current is caused to flow from the lead frame 1 toward the nozzle 2 through the injected liquid jet 3. A part of the silver plating 1I15 formed on the surface of is removed. In this step, protrusions and the like formed on the silver plating 195 are removed.

Refer to FIGS. 1a and 1b again. Repeat the above two steps. As a result of repeating the plating process and the removal process, a plated surface with a substantially uniform crystal grain size and a highly smooth surface is realized.

In order to find the optimum conditions for the alternating current waveform, the current value and/or current ratio between the plating current and the removal current used in the repeating process of the plating process and the removal process described above, the following effect confirmation test was conducted. executed.

Refer to FIG. 1C, which is an example of using a pulsed square wave (corresponding to claim 3).
, Silver plating was carried out using a pulsed square wave alternating current having the waveform shown in FIG. 1c as an electrolytic current.

The current conditions and energization time conditions are as shown in Table 3.

(Leaving space below) Table 3 Effect confirmation test conditions icp iap missing nimu upper a)j''1 and 艷) -kuL^-7-5i''-m:,)
T:92p-("1!shi) 1:Jljl-("
! ”! -) 30 3 TO 1 40 4 10 4 10 10 4 10 10 5 10 10 10 60 6 10 10 70 7 10 8 to 1 TO 1 girl 2 ≦ (1m 1H4) TU -value 1 100 10 10 10 10 10 100 100 100 100 100 As a result of conducting a true number of experiments under the above conditions, t c
At a current density of p=40 to 60 A/dm, a higher CAM gloss than the first example was obtained.The crystal grain size of the plating surface was almost uniform, and the smoothness was extremely good.
The adhesive spread well on the plated surface, and the glue bonding was performed extremely well.

This is an embodiment corresponding to claim 4, see FIG. 1d. Silver plating was carried out using a triangular wave current shown in FIG. 1d as an electrolytic current. The current conditions and current application time conditions are as shown in Table 4.

Table 4 Effect confirmation test conditions icp
M) M) Picture) 30 4.3 4.3 140
4.3 5.8 150
4.3 7.2 160 4.3
8.7 170 4.3
10.2 180 4.3 11
．． 7 1 As a result of conducting a large number of experiments under the above conditions, at a current density of I c p = 50 to 70 A/dm', a high CAM gloss was obtained.

The crystal grain size on the surface is almost uniform, the smoothness is good, and die bonding can be carried out as well as in the case of the pulsed current shown in FIG. 1c. Furthermore, the plating film obtained in this example was subjected to a heat resistance test (350°cx3mtn
) and wire bonding pull test.
No practical problems were observed.

FIG. 2 shows the glossiness of the first example and the second example (Case 1 to Case 4) of the embodiment of the present invention, with the current density being plotted on the horizontal axis, where the glossiness value exceeds 0.2. When you do, you will be satisfied.

[Effects of the Invention] As explained above, since imidazole is used as an additive in the silver plating solution according to the present invention instead of a selenium compound, changes in semiconductor characteristics due to selenium diffusion into the semiconductor chip are prevented. It was experimentally confirmed that this does not occur. Furthermore, it was confirmed by an electron microscope [4] that the crystal grain size on the plating surface was almost uniform. In other words, the smoothness of this surface is improved, and the wettability of the adhesive on the plating surface is also improved, allowing the adhesive to spread sufficiently, ensuring an adhesive and conductive area, and ensuring good adhesive bonding. It was confirmed that it is easy to implement.

Further, as mentioned above, the silver plating method according to the present invention uses a silver plating solution containing imidazole as an additive, and furthermore,
Since an alternating current whose polarity is periodically reversed is used as the electrolytic current, a slight electrolytic etching due to the reverse positive current is achieved, and as a result, the smoothness of the plating surface is further improved, and the direct current of normal unidirectional current Compared to the case where the current is an electrolytic current, the CAM glossiness is significantly increased. That is, the smoothness of the plating surface is significantly improved, the bonding performance is further improved, and this can significantly contribute to improving the yield and reliability of semiconductor devices.

[Brief explanation of the drawing]

FIG. 1a is a diagram of a silver plating process (positive polarity current application) according to an embodiment of the present invention. FIG. 1b is a diagram of a silver plating process (reverse polarity current application) according to an embodiment of the present invention. FIG. 1c is a waveform diagram of a pulsed square wave current according to an embodiment of the present invention. FIG. 1d is a waveform diagram of a pulsed triangular current according to an embodiment of the present invention. FIG. 2 is a graph showing the glossiness (CAM) of the surface of the silver plating film obtained by the example of the present invention in comparison with that of the conventional method. 1 ・ ・ 2 ・ ・ 3 ・ ・ 4 ・ ・ 5 ・ ・ cp ap cp ap T ・ ・ Lead frame (object to be plated), plating solution nozzle, plating solution jet, plating current direction switching switch, silver plating film, ...Current value of pulsed square wave current, ...'r of pulsed triangular wave current! i current value, pulsed square wave positive polarity current conduction time, pulsed square wave reverse polarity current conduction time, increase time of pulsed triangular wave positive polarity current,
Decrease time of pulsed triangular wave positive polarity current, increase time of pulsed triangular wave reverse polarity current, decrease time of pulsed triangular wave reverse polarity current.

Claims (1)

[Scope of Claims] [1] A silver plating solution comprising a small amount of imidazole added to an aqueous solution containing silver halide cyanide or silver nitrate as a main component. [2] The silver plating solution according to claim [1] is used as the plating solution (3), and the first electrode (2) and the second electrode (1) as the object to be plated are used as the electrodes. A silver plating method characterized by repeatedly carrying out the steps of passing a current through the first electrode (2) as an anode, and then passing a slight current through the second electrode (1) as an anode. [3] A current is passed through the first electrode (2) as an anode,
3. The silver plating method according to claim 2, wherein the current used in the step of applying a small current to the second electrode (1) as an anode is a square wave. [4] A current is passed through the first electrode (2) as an anode,
3. The silver plating method according to claim 2, wherein the current used in the step of applying a small current to the second electrode (1) as an anode is a triangular wave.