JPH03135040A - Bonding fine wire for semiconductor use and manufacture thereof - Google Patents

Abstract

PURPOSE:To obtain the soft central part and a surface-layer part having a desired characteristic and to satisfy various characteristics required by a mounting body by a method wherein, when a bonding fine wire used to connect a semiconductor chip to a lead wire is manufactured, an alloy element whose concentration is changed continuously is contained from an outer circumference part to the central part of a conductor thin wire. CONSTITUTION:In order to enhance an overall characteristic of a bonding fine wire, an alloy concentration is changed continuously at a surface-layer part and in the central part. For example, in order to make a loop shape high, the fine wire must be soft; and therefore, the central-part side is made wide and highly pure or an element which does not increase a recrystallization temperature of the fine wire so much is added. In order to enhance reliability in a neck part, only recrystallized particles near the surface are made fine. In order to bond a ball part, the alloy concentration is made small as far as possible so that a ball can easily be deformed. When a characteristic at the surface-layer part and in the central part is especially important, a ratio of an average concentration within a range of 1/20 of a wire diameter from the surface to an average concentration of 1/2 of a wire diameter from the central part is set to 1.2 or higher.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a thin bonding wire that connects a semiconductor chip and leads, and particularly to a thin bonding wire containing an alloy element and a method for manufacturing the same.

(Prior art) Bonding thin wires are usually bonded to semiconductor chips and lead frames, and at this time, for example, strength, ductility,
Characteristics such as neck (heat affected zone) strength, loop height, and bondability are required. Conventionally, these properties have been improved by adjusting the composition with alloying elements. For example, Japanese Patent Publication No. 5735577, Japanese Patent Publication No. 57-3485
In the case of Au wire, Ca.

It is disclosed that adding Ge, Be, In, etc. is effective in improving the respective characteristics.

However, these alloying elements are added uniformly to the material by the melting method, and they are added in too large a quantity (several tens of parts per million).
If ω~several 100ppe+ or more) is added, it may not be possible to turn the ball into a true sphere when forming the ball by discharging with an electric torch, or the composition of the ball may become finer, making it difficult to bond to the chip. unable or
There was a problem in that it caused damage to the chip. Therefore,
Generally, the addition of alloying elements is from several pp+n to several tens of ppm.
Although it is limited to the range of heat affected zone near the ball part (
Semiconductor devices often fail due to insufficient strength of the neck (neck portion) and breakage of the neck portion, and countermeasures are desired.

Furthermore, if an attempt is made to increase the strength by adding alloying elements and making the structure of the heat-affected zone finer, the thin wire tends to become harder and the loop shape becomes lower.

If the loop shape is low, there is a high possibility that a short circuit will occur due to contact with a chip or the like, reducing the reliability of the semiconductor device.

In the case of Cu alloys, alloying is also effective in making the neck structure finer, but the problem arises that the hardness of the ball portion increases too much and the chip is damaged during bonding.

Furthermore, attempts have been made to improve the overall properties by adding multiple alloying elements in combination, but it is difficult to satisfy all properties, and conventional methods have limitations.

(Problems to be Solved by the Invention) The present invention achieves properties necessary for the thin wire, such as thin wire strength, pole neck strength, and The main purpose is to comprehensively improve joint reliability, loop shape, electrical conductivity, etc.

Another object of the present invention is to provide a bonding thin wire that has sufficient tensile strength to be made into a thin wire (ultra-fine wire) by wire drawing.

(Means for Solving the Problems) The gist of the present invention for achieving the above objects is as follows.

(1) A thin bonding wire for semiconductors characterized by containing an alloying element whose concentration changes continuously from the outer periphery to the center of the thin conductor wire.

(2) The above-mentioned (
1) The bonding thin wire for semiconductors as described above.

(3) The surface of the thin conductor wire is coated with an alloying element or a high-concentration alloy, and the thin conductor wire is subjected to a diffusion treatment in which the concentration of the alloying element changes continuously from the outer periphery to the center. A method for manufacturing a thin bonding wire for semiconductors.

(4) The surface of the thin conductor wire is coated with an alloying element or a high-concentration alloy, and the thin conductor wire is subjected to a diffusion treatment in which the concentration of the alloying element changes continuously from the outer periphery to the center, and then the wire is drawn. A method for producing a thin bonding wire for semiconductors, characterized in that:

The present invention will be explained in detail below.

As described above, in the present invention, in order to improve the overall characteristics of the bonding wire, the alloy concentration is continuously changed between the surface layer and the center of the wire.

Regarding each characteristic, for example, in order to increase the loop shape, the thin wire needs to be soft, and in this case, the center side of the thin wire must be wide and highly pure, or the thin wire must be recrystallized. It is sufficient to add elements that do not raise the temperature too much. Furthermore, in order to improve the reliability of the neck portion, it is effective to simply refine the recrystallized grains of the thin wire, particularly near the surface.

For joining the ball part, the alloy concentration should be kept as low as possible.
Make the ball easy to deform. Therefore, it is better to limit alloying to the vicinity of the surface.

From this point of view, in order to increase the loop and deformability of the ball, most of the center part should remain highly pure, and in order to increase the strength of the neck part, the surface layer should be made finer ( alloying)
Try to measure it. Therefore, in fine wire alloying, in order to satisfy both the characteristics that are important in the surface layer and the characteristics that are important in the center, it is preferable to change the concentration between the surface layer and the center. In practice, the ratio of the density on the back side (for example, the average density in a range of 1/20 of the wire diameter from the front surface) to the density in the center area (for example, the average density in a range of 1/3 of the wire diameter from the center) is 1.
It is preferably 2 or more, most preferably 1.5 or more.

When the ratio of the back surface concentration to the center concentration (Cs/Co) is 1.2 or less, no significant improvement is observed in the loop height or neck strength. An example is shown in which gold wire is alloyed with beryllium. After a copper-2% by weight beryllium alloy was uniformly deposited on the surface of the gold wire 0.5 φ by sputtering, the concentration ratio between the surface and center was adjusted by diffusion heat treatment at 400° C. or higher. After wire drawing to a wire diameter of 25 tlIaφ, finishing heat treatment was performed so that each wire had an elongation of approximately 4% until breakage.

Figure 1 shows the ratio of the concentration on the back surface and the concentration on the center of beryllium (C
Figure 2 shows the relationship between the loop height and the neck strength. The loop height is C
Loop height H when s/Co changes, and Cs/Co
When is 1.0 (alloy element concentration is uniform), the loop height (
HA) was expressed as the ratio HB/HA. Also 2IIIm
A pull test was conducted to examine the neck strength when ball bonding was performed with a span of . The ratio FB/FA of the strength (FB) when Cs/Co is 1.0 or more and the strength (F^) when Cs/Co is 1.0 was determined. Note that the concentration when beryllium was uniformly diffused was about 5 ppm.

As is clear from the figure, there was an effect when Cs/Co was 1.2 or more, and a remarkable effect was seen when Cs/Co was 1.5 or more.

In recent years, with the increase in the number of pins in ICs and LSIs, high-density bonding wiring has become possible, and as a result, wires with a diameter of 20 μm
The following ultra-fine wires are desired. However, when it comes to such ultra-fine wires, normal alloying results in insufficient wire strength, resulting in frequent wire breakage during wire drawing, and insufficient neck strength. For this reason, when the alloy is made to be highly alloyed, the formation of the ball during bonding becomes unstable and bonding failures often occur. As will be described later, the present invention provides wire drawing after diffusion treatment.
It is one of the features of the present invention that ultrafine wires of tm or less can be produced without trouble, and that such ultrafine wires of 20 or less can be obtained.

The method of forming a change in the concentration of alloying elements from the outer periphery to the center of the thin wire according to the present invention is carried out by coating the surface of the thin wire with a high concentration alloy by means such as vapor deposition or plating, and then performing a diffusion treatment. For the vapor deposition method, physical vapor deposition methods such as sputtering, ion blating, and vacuum deposition, and chemical vapor deposition methods such as plasma CVD are used, and for plating, commonly used immersion and electrolytic methods are used.

Diffusion of the coated alloy according to the present invention is carried out at a point where the wire has a large diameter, and then the wire is drawn to form a thin wire with a desired diameter. That is, in the present invention, when it is desired to obtain a fine wire with a diameter of 30 mm or less, for example, an intermediate material with a diameter of 0.2 mm or more is coated with an alloy, subjected to a diffusion treatment, and then drawn. By doing so, productivity can be extremely increased, and it is possible to prevent coarsening of grains, which occurs when a vapor deposition-diffusion treatment is performed after thinning.

Regarding electrical conductivity, it may be reduced depending on the alloy added, and if it is low, it may cause heat generation and lead to IC failure, but even if the amount of alloying elements added is the same, , the thin wire of the present invention is better than the conventional thin wire.

The metal thin wire used in the present invention is Au or Cu.

It is Al, and the alloying element coated on it is Au.

Cu, Al, Be, Ca, Ge, In, Sl.

Fe, Ga, Zn, Ba, Mg, Ni, Sn and La
, Eu, Ce, Nd, etc., and one or more of these are used depending on the purpose.

Cu thin wire with a wire diameter of 150mφ (Cu purity 99.999%)
Examples are shown below in which Au was vapor-deposited on the surface and then diffused at different temperatures.

Alloy (vapor deposition) element deposition film thickness Diffusion conditions (a)
Au 200 people 400℃, 4 hours (b)
A u 20G person 650℃, 4
Time (c) A u 200 people 90
Figure 3 (a) shows the diffusion status of elements after 4 hours of diffusion treatment at 0°C.
), (b).

Shown in (C). These figures show that the cross section of the thin wire is polished and subjected to X-ray analysis (EP) in the diametrical direction 2 of the thin wire 1 as shown in FIG.
MA line analysis) at 900°C.

In the heat treatment for 4 hours (e), although a decrease in the concentration of elements is observed in the center, the element is diffused almost uniformly throughout the wire, and there is no concentration gradient of the element. On the other hand, in (a).

Under the processing conditions of (b), there is a clear difference in density between the surface layer and the center of the line. Thereafter, when these 15 (Jimφ) Cu wires were drawn to 25 mmφ and subjected to the same X-ray analysis as described above, analysis results were obtained in which the element distribution was almost similar to that in the case of 150 mmφ.

Based on the above-mentioned phenomenon, the present inventors considered the diffusion of alloying elements as follows. In other words, when diffusing an alloying element into a metal, the diffusion distance (N) of the element is determined by the diffusion temperature (T
6K) and diffusion time (L). The relationship is approximately expressed as g-column 51. Here, DmD, exp T 22 and D is the diffusion coefficient, which can be found for each temperature if Do and Q are given.

Do and Q are constants specific to each element, for example, the "Metal Data Book" edited by the Japanese Society of Metals, or the CRC Handbook of chc+n+5tr published by CRC Press.
y anctPhysics” etc.

R is a gas constant and T is an absolute temperature.

In order to produce a thin wire in which the alloying element has a concentration gradient between the surface and the center, which is the gist of the present invention, it can be estimated by the following equation, where d is the wire diameter in the case of diffusion treatment.

d/2≧Kjl- Here, K is a constant and has a value approximately between 1 and 1o.

And the preferable range for diffusion conditions is d2:l■7≧d
1500, and the most preferable range is d/10≧Jb]−≧d150(1
becomes.

Applying the above calculation using the above-mentioned Au diffusion experiment into Cu thin wire as an example, we find that “CRC1landbookor
According to "Chc+++1stry and Physics", D -0.03cd/sac, Q = 42.f
A value of 3 kcal is given.

From the results of the diffusion experiment, it is thought that during heat treatment at 900°C or less for 4 hours or less, the alloy concentration changes between the outer periphery of the surface and the center, so when we calculate the relationship between d and KfI at that time, d≧44. Therefore, the value is about K-3 to 4,
It can be seen that it is within the above range.

Next, the types of elements to be added, the range of the steam thinning thickness, and the subsequent heat treatment conditions will be explained.

The above-mentioned elements may be used singly or in combination, or the concentration of only a specific element may be changed, and the other elements may be uniformly alloyed. Uniform alloying can be carried out in advance by a melting method or by subjecting the surface-coated alloy to a high-temperature, long-time diffusion treatment.

The vapor deposition, plating thickness, and diffusion treatment conditions for each element vary depending on the atomic size, solid solution amount, and diffusion coefficient of each element in the metal to be vapor deposited. Preferred elements to be deposited on Au and Cu wires and preferred thicknesses are illustrated in Tables 1 and 2. The wire diameter is the same, but when the wire diameter changes, it is necessary to change the evaporation rate in proportion to the wire diameter. Further, when selecting the coating thickness, the maximum and minimum values differ depending on the effect of each element on the characteristics. If the amount is too large, the balls formed during thin wire bonding will become too hard, causing problems such as damage to the semiconductor chip.

Table 1 Types of elements deposited on Au wire and coating thickness Element name Cu Be Ca Gc In Si Fe Minimum thickness (people) 50 10 7 +0 7
20 30W (person) 4000 2500 3
000 1000 700 2000 6o.

Element Name Minimum Thickness (person) M Ri S Mi (In) Ga Zn La Ba Mg Ni S
n8 7 B 14 27 5 7
800 700 800 +400 2800 50
0 700 Table 2 Types of elements deposited on Cu wire and coating thickness Element name Minimum thickness (8) Maximum thickness (person) A, ill Zn Fe Au Be
In8'3 5 10 12 380
0 600 1200 200 1200 6o.

(Example) Examples of the present invention will be shown below.

Example 1 An Au wire sample with a diameter of 1++ mφ and a length of 5 m was set in a rotary winding device having a delivery reel and a take-up reel, and this was placed in a vacuum chamber of an ion blating device. The wire rotates as it passes from the delivery reel through the deposition zone and onto the take-up reel. At this time, in the vapor deposition zone, alloying elements are uniformly vapor deposited on the surface of the Au wire. The conditions for vapor deposition are as follows.

Method: Ion-blating evaporation substance and thickness As shown in Table 3 (indicates the evaporation (alloy) element and film thickness for each sample) Vaporization atmosphere: 5 x 10-' Torr or less Evaporation method: Electron beam, 10kV, 1O~20On+A High frequency power: 0.1 kW Coil (wire) feeding speed: 30 to 100 cm/min After vapor deposition, the wire was subjected to diffusion treatment using two methods, A and B. The A treatment is a diffusion treatment in which the alloy concentration is made almost uniform from the wire surface to the center, and 7 in the above formula is
The temperature and time were set so that 6π- was greater than or equal to d/2 (d is the wire diameter). For cases where the value of the diffusion coefficient was not available as data, D was determined in advance by a diffusion experiment using a rod-shaped sample. The B treatment is a treatment method of the present invention in which the concentration of the alloying element is changed continuously between the wire surface and the center.
range) and the center (1/3 range from the center)
The temperature and time of the diffusion treatment were set so that the ratio of the average concentration of was 3 or more. That is, in the above formula, 761-
The selection was made under the condition that the ratio is approximately d/20. However, 5 was adopted for K.

For each sample with a diameter of 1 mm after each diffusion treatment, 25 φ
The ultra-fine wire was drawn. Then, each sample thin wire (N
α1-37) EPM-A analysis (see Figure 2),
Alternatively, the concentration ratio (Cs/CI) of the diffusion alloying element between the surface portion (Cs) and the center portion (Ci) was determined by chemical analysis. EP
For those with low sensitivity in MA analysis, chemical analysis was performed in which the surface and center portions of the thin wire samples were separated by acid dissolution.

As a result, the Cs/Ci of each sample treated with A was 1.2 or less, and the diffusion concentration was almost uniform, whereas the Cs/Ci of each sample treated with B was 3 or more, and the Cs/Ci was found at the surface and center. It became clear that the diffusion concentration of alloying elements with

Ball bonding and stitch bonding were performed on each of the above-mentioned samples, which had been subjected to a finishing heat treatment such that the wire drawing resistance was approximately 4%. The distance between the ball joint side and the stitch joint side was set to two. The maximum height of looping at that time is the average value of those obtained by processing A.

The average value of those obtained by B treatment is expressed as H[l, H, 3/HA
I asked for In addition, the hook breaking strength was measured by a hook test (pulling a hook across a loop portion formed by joining a ball and stitch). In this case, the average hook strength of the A treatment was designated as FA, and the B treatment was designated as F, and F a / F A was determined. Both loop height and hook strength are 80 δp.
The average value of 1 constant value was determined.

Table 4 shows the values of HB/HA and Fa/FA in each sample.

Table 4 Characteristic Values From the above results, it is clear that the B-treated material within the scope of the present invention is superior in both loop height and hook strength in each sample.

In addition, in order to investigate the electrical conductivity, we calculated the increase in electrical resistance due to alloying of each sample, that is, the residual resistance (ρI) by ilN.
Established. The residual resistance is determined from the ratio of the electrical resistance at room temperature (295K) and 4.2 (residual resistance ratio RRR). Then, when we calculated ρ8/ρ9 by using ρ4 for the A process and ρ□ for the B process (the present invention), the values were 1.2 or more in both cases, indicating that the present invention is superior. was clear.

Example 2 A 0.15 mm diameter Cu wire was subjected to ion blating treatment using the same apparatus and method as in Example 1.

Pure gold (99,999%) was used as the vapor deposition material, and the C
A uniform film was formed on 200 people on the U-ray. After that, diffusion treatment was performed under the conditions of 650°C x 4 hours (B treatment) and 950"C x 4 hours (B treatment, scope of the present invention), and the line was drawn.
A thin wire with a diameter of 5 m was used. FB/
When the FA was determined, all of them were 1.15 or more, and it became clear that the products according to the present invention exhibited excellent characteristics.

Example 3 The surface of a 0.15 mmφ Cu wire was coated with Au to a uniform film thickness of 500 people (using the same ion blating device and method as in Example 1), and then subjected to fire diffusion treatment. I did it.

■ 650℃, 4 hours (B treatment) ■ 950℃, 4 hours (A treatment) Cu was vapor-deposited on the surface of the 0.15 mmφ Au wire to a uniform film thickness of 7000 mm (using the same method as above). ), the following diffusion treatment was performed.

(2) 950° C., 4 hours (A treatment) The following two alloy wires (1) and (2) were produced by a melting method, rolled and drawn to a wire diameter of 0.15++usφ, and then intermediately annealed.

■ Alloy wire with the same concentration (0, O [1 lvt% Au) as ■ ■ Alloy wire with the same concentration (0.86 wt% Cu) as in ■ ■ All of the above ■ to ■ were each drawn into thin wires of 12 unφ. At this time, the number of wire breaks until 1000 mm was created in 12 mmφ was measured, and the results are shown in Table 5.

Table 5 Number of wire drawing process samples Magic wire breakage Alloy treatment method ■Cu wire OB treatment (inventive diffusion treatment) ■Cu wire 2 A treatment (vapor deposition - uniform diffusivity) ■Au wire
OA treatment (same as above) ■Cu wire 10 Melting method ■Au wire 2 Melting method Alloy addition amount Au RO, 061 wL% Cu O, 80〃 Au 0.08'' Cu 0.8G /1 ■ is the material targeted by the present invention. There was no disconnection, but ■ had the same amount of alloy, but due to uniform diffusion, disconnection was observed. ■: Same as ■, uniform diffusion, but there was no disconnection due to the high alloy concentration. However, when we conducted a bonding test with the chip, we found that the chip was damaged.
(2) is a melting method, and inclusions are observed, and wire breaks are observed in both cases. In particular, ■ has a low alloy concentration, compared to ■.
There were many disconnections. ■ is less than ■, but ■
Similarly, damage (scratches) caused by chip bonding was observed. These results show that the method of the present invention is superior.

(Effect of the invention) As explained above, the wire rod obtained by the present invention is
The addition of alloying elements eliminates the contamination of impurity elements that occur during melting methods, and the amount added can be freely selected.The combination of a soft core and a surface layer that provides the desired properties makes it easy to manufacture and bond. Furthermore, it is possible to obtain various characteristics required for a package as desired, and a reliable semiconductor device can be manufactured, so its industrial value is extremely large.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the ratio of the surface concentration and center concentration of a gold alloy and the loop height. Figure 2 is a diagram showing the relationship between the ratio of the surface concentration and center concentration of a gold alloy and the pull strength. FIG. 3 is a diagram showing the results of EPMA analysis of each line hod after diffusion treatment, and FIG. 4 is a diagram showing the EPME analysis position of the sample (line).

Claims (4)

[Claims] (1) A thin bonding wire for semiconductors characterized by containing an alloying element whose concentration changes continuously from the outer periphery to the center of the thin conductor wire. (2) The thin bonding wire for semiconductors according to claim 1, wherein the thin wire has a diameter of 20 μm or less. (3) The surface of the thin conductor wire is coated with an alloying element or a high-concentration alloy, and the thin conductor wire is subjected to a diffusion treatment in which the concentration of the alloying element changes continuously from the outer periphery to the center. A method for manufacturing a thin bonding wire for semiconductors. (4) The surface of the thin conductor wire is coated with an alloying element or a high-concentration alloy, and the thin conductor wire is subjected to a diffusion treatment in which the concentration of the alloying element changes continuously from the outer periphery to the center, and then the wire is drawn. A method for producing a thin bonding wire for semiconductors, characterized in that: