JP2003347487A - Semiconductor device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that a semiconductor pellet is exfoliated from a back electrode when operation is repeated in a semiconductor device wherein the semiconductor pellet is mounted on a hollow resin package by using a gold tin alloy. <P>SOLUTION: A semiconductor pellet 11 having a back electrode 11b wherein a titanium layer 12, a nickel layer 13 and a layer 14 composed of gold or silver are formed in order from a semiconductor substrate 11a side is mounted on a radiator 1 by using the gold tin alloy 6. <P>COPYRIGHT: (C)2004,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a semiconductor device,
In particular, the present invention relates to a semiconductor device in which a semiconductor pellet is fixed to a radiator via a low melting point alloy.

[0002]

2. Description of the Related Art In a power semiconductor device, in order to efficiently release heat generated by a semiconductor pellet to the outside, the semiconductor pellet is generally fixed to a radiator, and if necessary, the radiator is connected to an external radiator or the like. Connected to cooling device. An example of this type of semiconductor device is shown in FIGS. In the figure, 1
Is a plate-shaped radiator made of a material selected from metals, alloys, composite materials, and the like having good conductivity and thermal conductivity in consideration of thermal expansion due to high-temperature operation, and mounting through holes 1a on both sides;
1b is formed, and a plating layer 1c formed by laminating nickel 1d and gold 1e is formed at the center. 2a and 2b are plate-shaped leads orthogonally arranged on both sides of the intermediate position of the heat radiator 1,
Numeral 3 is a cylindrical resin frame portion, and the heat radiator 1 and the leads 2a, 2b are hermetically sealed and integrated at a crossing position of the heat radiator 1 and the leads 2a, 2b to form a resin package 4. Reference numeral 5 denotes a semiconductor pellet. A back electrode 5b is formed on the entire back surface of a semiconductor substrate 5a having a semiconductor element (not shown) formed therein, and the front surface is electrically connected to a main part of the internal semiconductor element. Electrodes 5c and 5d are formed. Reference numeral 6 denotes a low melting point alloy for bonding the semiconductor pellet 5 onto the heat radiator 1 of the resin package 4. Reference numerals 7a and 7b denote wires for electrically connecting the electrodes 5c and 5d on the semiconductor pellet 5 and the leads 2a and 2b. In order to reduce electric resistance and inductance, many of them are connected in parallel in the illustrated example. Reference numeral 8 denotes a lid for hermetically sealing the open end of the frame 3 of the resin package 4. The hollow resin package 4 supporting the radiator 1 and the leads 2 protects the main part such as the semiconductor pellet 5 from external force and corrosive gas, but separates the semiconductor pellet 5 and the wire 7 from the resin having a high dielectric constant by a certain distance. Therefore, it is used as a high-frequency semiconductor device package that operates in the GHz band.

[0003] A method of manufacturing this type of semiconductor device 9 will be described below. First, the resin package 4 is supplied into a high-temperature atmosphere and the radiator 1 is heated. Next, the low melting point alloy 6 is supplied onto the heated radiator 1 and melted. Then, the semiconductor pellets 5 are supplied onto the molten alloy 6, and the radiator 1 is cooled to fix the semiconductor pellets 5. Next, the resin package 4 is sent to a wire bonding step, and the electrodes 5 on the semiconductor pellet 5 are formed.
c, 5d and the leads 2a, 2b are electrically connected by wires 7a, 7b. When the work in the resin package 4 is completed in this way, the semiconductor device 9 completed through processes such as electrical characteristic inspection after sealing the opening end of the frame 3 with the lid 8 is shipped.

When the semiconductor device 9 is repeatedly operated and stopped, the temperature of the semiconductor pellet 5 rises and falls, and thermal expansion and contraction are repeated. However, the heat generated between the semiconductor pellet 5, the low melting point alloy 6 and the radiator 1 is reduced. If there is a large difference in the expansion coefficient, stress concentrates on the interface, and the stress concentration portion fatigues in a relatively short time, causing a fine crack. The fine cracks reduce the thermal conductivity between the semiconductor pellets 5 and the heat radiator 1, so that the heat generated in the semiconductor pellets 5 is prevented from being transferred to the outside, and the temperature of the semiconductor pellets 5 rises remarkably. It grows rapidly and destroys the semiconductor pellet 5 in a relatively short time. Therefore, the materials, shapes and dimensions of the heat radiator 1 and the low melting point alloy 6 are selected so that the thermal expansion coefficients of the heat radiator 1, the low melting point alloy 6 and the semiconductor pellet 5 do not greatly differ from each other.

On the other hand, the semiconductor device 9 shown in FIG.
When the semiconductor package 5 is heated and subjected to a thermal shock when it is fixed, fine cracks are generated in the hermetically sealed portion of the resin package 4. When these fine cracks grow and communicate with the inside and outside of the resin package 4, moisture is formed in the hollow portion. Infiltrate and accumulate further. This water vaporizes as the temperature rises and the vapor pressure rises, causing the electrodes 5c, 5d and wires 7a,
7b, wires 7a, 7b and leads 2a, 2
b infiltrates the connection interface, causing a local electrolysis reaction and corroding the connection interface. As the corrosion of the connection interface progresses, the electrical connection area decreases, the electrical resistance increases, and the characteristics of the electronic circuit device are changed. .

[0006] The resin used as the resin frame 3 is required to have heat resistance to withstand the melting temperature of the low melting point alloy 6.
If the heating time is prolonged, the resin will gradually carbonize even if it is below its heat-resistant temperature, and its electrical characteristics will deteriorate.
In the case of resin at 40 ° C., tin content 20
By using a gold-tin alloy having a melting point of 290 ° C. in weight%, consideration is given to completing the bonding operation in as short a time as possible so that the resin package 4 is not subjected to thermal shock during the manufacturing process.

[0007]

In a semiconductor device using such a hollow resin package, care must be taken not to cause cracks between the resin frame 3 and the radiator 1 and the lead 2 in an environment where temperature and humidity are controlled. Has been manufactured. As shown in FIG. 3, the back electrode 5b of the semiconductor pellet 5 has a multilayer structure in which a barrier layer 5e made of titanium, nickel or the like and a gold layer 5f having good wettability with respect to a gold-tin alloy are laminated. I have.

However, when the semiconductor pellets 5 in the hermetically sealed hollow portion are repeatedly heated and cooled, there is a problem that the semiconductor pellets 5 are separated from the back electrode 5b. As a cause of the separation of the semiconductor pellet 5 from the back surface electrode 5b, when the semiconductor pellet 5 is repeatedly heated and cooled, tin in the gold-tin alloy 6 permeates the barrier layer 5e and diffuses into the semiconductor substrate 5a. When water vapor contacts the water-soluble tin, the tin elutes from the back electrode 5b to the outside to form a porous region on the entire back surface of the semiconductor pellet 5 and reduce the adhesive force of the semiconductor pellet 5. it is conceivable that. Since the porous region from which tin has been removed has low thermal conductivity and high heat insulation, heat transfer from the semiconductor pellet 5 to the radiator 1 is impeded. Therefore, the semiconductor pellet 5 is overheated and becomes defective in a short time.

In order to solve such a problem, a low melting point alloy 6 containing no tin may be used. However, since the melting point is high, it is not suitable as an alloy for a resin package. To prevent the diffusion of tin in the gold-tin alloy, the thickness of the barrier layer 5e may be increased, but a few A in the forward direction is formed at the PN junction between the external lead 2b (drain electrode) and the radiator 1 (source electrode). The voltage Vf between the electrodes when a large current flows through the back electrode 5b
Changes not only due to the electrical resistance of the
Therefore, there is a limit in increasing the thickness of the barrier layer 5e.

[0010]

SUMMARY OF THE INVENTION The present invention has been proposed for the purpose of solving the above problems. In a semiconductor device in which a back electrode of a semiconductor pellet is fixed to a radiator through a low melting point alloy, the present invention provides The back electrode is provided by sequentially laminating a titanium layer, a nickel layer, and a gold or silver layer from the semiconductor substrate side, and provides a semiconductor device using a gold-tin alloy as a low melting point alloy.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a semiconductor device according to the present invention, a titanium layer, a nickel layer,
Since the semiconductor pellet having a back electrode in which gold or silver layers were sequentially laminated was connected to a radiator using a gold-tin alloy,
The penetration of tin in the low melting point alloy into the semiconductor substrate is prevented,
A semiconductor device which is electrically stable for a long time can be realized. The back electrode of this semiconductor device has a titanium layer with a thickness of 0.02 to 0.5 μm, a nickel layer with a thickness of 0.4 to 2 μm, and a gold or silver layer with a thickness of 0.01 to 2 μm.
By setting each to 2 μm, a semiconductor device having stable characteristics can be realized. Further, a nickel layer and a gold layer can be sequentially laminated on the surface of the radiator on which the low melting point alloy is to be adhered. The present invention is suitable for a semiconductor device having a structure in which semiconductor pellets are accommodated in a box-shaped resin package in which a part of a heat radiator is exposed at the bottom.

[0012]

FIG. 1 shows an embodiment of the present invention.
In the figure, the same parts as those in FIG. 2 are denoted by the same reference numerals, and redundant description will be omitted. In the semiconductor device 10, similarly to the semiconductor device 9 of FIG. The semiconductor pellet 11 is mounted, and the electrodes 11 c and 11 on the semiconductor pellet 11 are mounted.
d and the leads 2a and 2b are electrically connected to each other, and the opening end of the frame 3 is sealed with the lid 8. The titanium layer 12, the nickel layer 13, the gold or silver Tier 1
The semiconductor device 9 differs from the semiconductor device 9 in FIG. 2 in that a semiconductor pellet 11 having a back surface electrode 11 b in which layers 4 are sequentially stacked is fixed to the heat radiator 1 using a gold-tin alloy 6. The thickness of each layer of the back surface electrode 11b is 0.02 to 0.5 μm for the titanium layer 12, 0.4 to 2 μm for the nickel layer 13, and 0.1 μm for the gold or silver layer 14.
The thickness of the nickel layer is set to 0.4 to 2 μm. As in the semiconductor device 9 shown in FIG.

Comparative Examples 1 and 2
Tables 1 and 2 show the semiconductor device according to the present invention as Examples 1 and 2 in Tables 3 and 4, respectively.

[Table 1]

[Table 2]

[Table 3]

[Table 4]

As shown in Table 1, in Comparative Example 1, a laminate of a titanium layer (thickness 0.02 μm) and a gold layer (thickness 0.2 μm) (Comparative Example 1-No. 1) 0.02 μm) and a gold layer (2 μm in thickness) with different sintering conditions (Comparative Example 1-Nos. 2-3)
In each case, the initial Vf was good, but in Comparative Example 1-3, the compatibility with the low melting point alloy was inferior, and in Comparative Example 1-1 and 1-4 in the moisture resistance test (PCT) in a pressurized steam atmosphere. In the case of heating after mounting, Comparative Examples 1-2 and 1-4 are inferior, respectively, so that the overall evaluation is inferior and is not suitable for stable production.

In Comparative Example 2, as shown in Table 2, a nickel layer was disposed between a titanium layer (thickness 0.02 μm) and a gold layer (thickness 2 μm).
Comparative Examples 2-1 to μm, 0.05 μm, and 0.15 μm
It is classified into three types of 2-3. In each case, the sintering conditions were 250 ° C. for 20 minutes after mounting. In all the comparative examples, the conformability and PCT were good, but in the case of PCT which was mounted and then heated and performed, all the comparative examples failed, and further, in the comparative example 2-3, the initial Vf failed.

In Example 1, as in Comparative Example 2, a nickel layer was interposed between a titanium layer (0.02 μm in thickness) and a gold layer (2 μm in thickness), and the thickness of the nickel layer was 0.4 μm.
m and 0.5 μm, and the sintering conditions were 250 ° C. for 20 minutes after mounting, and 320 ° C.
It divided into two ways of 0 second heating, and compared about four combinations. In the case of the post-mounting heating shown in Examples 3-1 and 3-2, the initial Vf was slightly inferior, but the heating time at the time of mounting was extended, but the result was acceptable, and good results were obtained in Example 3 as a whole.

In Example 2, as shown in Table 4, a nickel layer (0.5 μm in thickness) was formed on a titanium layer (0.02 μm in thickness).
And a silver layer (thickness: 1 μm) was formed as an outer layer. The sintering conditions were as follows: the wafer was heated at 400 ° C. for 1 hour or mounted at 320 ° C. for 20 seconds. In this case, familiarity, initial Vf, PCT, P
All the test items of CT were passed, and it was found that a stable semiconductor device could be realized.

Although silver may cause migration in a high-humidity atmosphere, there is no practical problem by keeping the radiator 1 at a low potential such as by grounding. When a nickel layer is arranged between the titanium layer and the gold layer, the initial Vf increases. In order to improve this, wafer sintering may be performed.However, since the outermost gold layer disappears in a dimple shape and the compatibility with the low melting point alloy is deteriorated, wafer sintering cannot be performed, but the outermost layer is replaced with a silver layer. By doing so, wafer sintering becomes possible, and variations can be suppressed for each semiconductor pellet, and production can be easily performed. The thickness of the nickel layer is 0.4 μm
In the following, the barrier property is lowered, and if it exceeds 2 μm, a stable barrier property is obtained, but the cost is increased.
２2 μm.

The semiconductor device in which the semiconductor pellet 11 having the back electrode 11b shown in Examples 1 and 2 is mounted on the heat radiator 1 disposed in the hollow portion of the resin package 4 via the gold-tin alloy 6 is sealed. Even if the temperature inside the hollow portion rises and the pressure of water vapor rises, the semiconductor substrate 11 of tin in the gold-tin alloy
a is prevented by the nickel layer having a thickness of 0.4 μm or more, does not generate water-soluble tin, does not elute even in contact with high-pressure steam, can maintain the adhesive strength of the back electrode 11b, Peeling of the pellet can be prevented. Therefore, the heat generated by the semiconductor pellets can be efficiently transmitted to the heat radiator 1 and the temperature rise of the semiconductor pellets can be suppressed, so that a high power operation can be stably performed.

[0020]

As described above, according to the present invention, the problem that the semiconductor pellet is separated from the heat radiator in the semiconductor device in which the semiconductor pellet is mounted on the heat radiator by using a gold-tin alloy is solved, and high power is used. A semiconductor device that can operate stably can be realized.

[Brief description of the drawings]

FIG. 1 is a sectional side view of a main part of a semiconductor device according to the present invention;

FIGS. 2A and 2B show an example of a semiconductor device using a hollow resin package, wherein FIG. 2A is a partially cutaway plan view, and FIG.

3A is a side sectional view of the semiconductor device in FIG. 2, and FIG.
Shows an enlarged side sectional view of a main part.

[Explanation of symbols]

1 heat sink 2a, 2b lead 3 Frame 4 Resin package 10 Semiconductor device 11 Semiconductor pellet 11a Semiconductor substrate 11b Back electrode 11c, 11d electrode 12 Titanium layer 13 Nickel layer 14 Gold or silver layer

Claims (4)

[Claims] 1. A semiconductor device in which a back electrode of a semiconductor pellet is fixed to a heat radiator via a low melting point alloy, wherein the back electrode of the semiconductor pellet is formed from a semiconductor substrate side.
A semiconductor device comprising a titanium layer, a nickel layer, and a gold or silver layer sequentially laminated, and using a gold-tin alloy as a low melting point alloy. 2. The thickness of the titanium layer of the back electrode is 0.02 to 0.02.
0.5 μm, the thickness of the nickel layer to 0.4-2 μm,
2. The semiconductor device according to claim 1, wherein the thickness of the gold or silver layer is set to 0.01 to 2 [mu] m. 3. The semiconductor device according to claim 1, wherein a nickel layer and a gold layer are sequentially laminated on the surface of the radiator on which the low melting point alloy is to be adhered. 4. The semiconductor device according to claim 1, wherein the semiconductor pellet is housed in a box-shaped resin package having a part of the heat radiator exposed at the bottom.