# Patent

## Title
Method of forming a semiconductor device having bonding pad of the second chip thinner than bonding pad of the first chip

## Abstract

To provide a semiconductor device that enables high integration degree, and a manufacturing method therefor. A multi-chip module according to an embodiment of the present invention includes: a first semiconductor chip having a first bonding pad; a second semiconductor chip having a second bonding pad thinner than the first bonding pad; and a bonding wire connected with each of the first bonding pad and the second bonding pad, the first bonding pad being connected with a first bond side end portion of the bonding wire and the second bonding pad being connected with a second bond side end portion of the bonding wire.

## Background
The present Application is a Divisional Application of U.S. patent application Ser. No. 11/350,889, filed on Feb. 10, 2006 now U.S. Pat. No. 7,256,485.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method therefor. In particular, the invention relates to a semiconductor device having plural semiconductor chips and a manufacturing method therefor.
2. Description of Related Art
To keep up with recent tendencies to downsize an electronic device, there is an increasing demand to package semiconductor chips with high density or high integration degree. To that end, a multi-chip module prepared by integrating plural semiconductor chips into one package has bee developed (see Japanese Unexamined Patent Publication Nos. 2003-273314, 2000-68316, and 2000-114452, for example).
In general, the multi-chip module has plural semiconductor chips on a lead frame that are obtained by arbitrarily connecting transistors, resistors or such other elements to compose a circuit. FIG. 5 is a schematic diagram showing the structure of a conventional multi-chip module 10 where two semiconductor chips 12 are mounted on the same lead frame 11. As shown in FIG. 5, the semiconductor chips 12 are connected face up on the island of the lead frame 11 via the die mounting member 13. Formed on each of the semiconductor chips 12 are circuit wiring lines 14 and a bonding pad 15. The bonding pads 15 on the respective chips are connected with each other via a bonding wire 16.
Incidentally, in the aforementioned multi-chip module 10, the bonding pads 15 for bonding the semiconductor chips 12 through the bonding wire 16 are formed with the thickness enough to suppress damage on the semiconductor chips, for the purpose of minimizing damage on the semiconductor chips upon the bonding.
Along with recent high-density integration of semiconductor chips, there is a need to reduce a pitch of the circuit wiring lines 14, and to reduce the thickness of each circuit wiring line 14 for realizing a processing with high dimensional accuracy. However, the circuit wiring line 14 and the bonding pad 15 are formed at the same time, so if the bonding pad 15 is thick, the circuit wiring line 14 is accordingly thick. Thus, it is difficult to reduce a pitch of the circuit wiring lines 14, and to downsize the semiconductor chips 12. In addition, in the case of etching only the circuit wiring line 14, an additional step of increasing the thickness of the bonding pad 15 is required, which costs high.
SUMMARY OF THE INVENTION
A semiconductor device according to an aspect of the present invention includes: a plurality of semiconductor chips including: a first semiconductor chip having a first bonding pad; and a second semiconductor chip having a second bonding pad thinner than the first bonding pad; and a bonding wire connected with each of the first bonding pad and the second bonding pad, the first bonding pad being connected with a first bond side end portion of the bonding wire and the second bonding pad being connected with a second bond side end portion of the bonding wire. Hence, it is possible to reduce the thickness of a circuit wiring line, form circuit wiring lines at small pitches with high dimensional accuracy, and provide a semiconductor device of the high integration degree.
According to another aspect of the invention, a method of manufacturing a semiconductor device including a plurality of semiconductor chips, includes: arranging a first semiconductor chip having a first bonding pad; arranging a second semiconductor chip having a second bonding pad thinner than the first bonding pad; and bonding one end of the bonding wire to the first bonding pad and then bonding the other end of the bonding wire to the second bonding pad. Accordingly, the thickness of the circuit wiring line can be reduced without requiring an additional step of increasing the thickness of the bonding pad, and the circuit wiring lines can be formed at small pitches with high dimensional accuracy.



BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, advantages and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a schematic diagram showing the structure of a semiconductor device according to a first embodiment of the present invention;
FIG. 2A illustrates a method of manufacturing the semiconductor device according to the first embodiment of the present invention;
FIG. 2B illustrates a method of manufacturing the semiconductor device according to the first embodiment of the present invention;
FIG. 2C illustrates a method of manufacturing the semiconductor device according to the first embodiment of the present invention;
FIG. 2D illustrates a method of manufacturing the semiconductor device according to the first embodiment of the present invention;
FIG. 3A illustrates how to connect a first bonding pad with a bonding wire;
FIG. 3B illustrates how to connect a second bonding pad with a bonding wire;
FIG. 4 is a schematic diagram showing the structure of a semiconductor device according to a second embodiment of the present invention; and
FIG. 5 is a schematic diagram showing the structure of a conventional semiconductor device.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention will be now described herein with reference to illustrative embodiments. Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the invention is not limited to the embodiments illustrated for explanatory purposed.
First Embodiment
Referring to FIG. 1, a semiconductor device according to a first embodiment of the present invention is described. FIG. 1 is a schematic diagram showing the structure of a multi-chip module 100 as the semiconductor device of this embodiment. As shown in FIG. 1, the multi-chip module 100 includes a lead frame 101, a first semiconductor chip 102a, a second semiconductor chip 102b, and a die mounting member 103. From now on, description is given of a case where two semiconductor chips, the first semiconductor chip 102a and the second semiconductor chip 102b, are mounted on the same lead frame 101. A feature of this embodiment resides in a bonding pad 105 used for connecting between the first semiconductor chip 102a and the second semiconductor chip 102b. 
The lead frame 101 is a substrate supporting the first semiconductor chip 102a and the second semiconductor chip 102b. Formed on the lead frame 101 are islands (not shown) connected with the first semiconductor chip 102a and the second semiconductor chip 102b. The first semiconductor chip 102a and the second semiconductor chip 102b are connected face up on a corresponding island through the die mounting member 103.
The first semiconductor chip 102a and the second semiconductor chip 102b compose a circuit by arbitrarily connecting transistors, resistors, or other such elements in a previous step. Further, on the first semiconductor chip 102a, a circuit wiring line 104 and a first bonding pad 105a are formed. On the other hand, on the second semiconductor chip 102b, the circuit wiring line 104 and a second bonding pad 105b are formed. The circuit wiring line 104 and the bonding pad 105 are generally formed at the same time through the same process, for example, lithography or patterning. As a material for the bonding pad 105, a conductive material such as Al, AlSi, AlSiCu, or Cu can be used. The bonding pad 105 is described later in detail.
Further, the first bonding pad 105a is electrically connected with the second bonding pad 105b via a bonding wire 106. As a material for the bonding wire 106, Al-containing metal such as Al, AlSi, AlNi, or AlSiNi can be used. Further, the bonding pad 105 may be bonded to the bonding wire 106 based on a well-known wedge bonding method. The wedge bonding method implies a wire-bonding method using a wedge, which presses the bonding wire 106 against the bonding pad 105 with the edge of the ultrasonically vibrating wedge to bond the wire to the pad at the room temperature by means of ultrasonic vibrations under a load. Further, an Au wire may be used as the bonding wire 106 to carry out wedge bonding through low-temperature heating.
Referring now to FIGS. 2A to 2D, a method of manufacturing the semiconductor device of this embodiment is described. The bonding pad 105 is bonded to the bonding wire 106 through the above wedge bonding. For ease of explanation, the circuit wiring line 104 or the like is omitted from FIGS. 2A to 2D.
First of all, as shown in FIG. 2A, the bonding wire 106 is previously supplied from a wire spool (not shown) up to the edge of a wedge 107. Then, the wedge 107 is moved downward to bring the bonding wire 106 into contact with the first bonding pad 105a. Thereafter, the wedge 107 is further moved downward to press the bonding wire 106 against the first bonding pad 105a. 
As shown in FIG. 2B, the ultrasonic vibrations are transmitted from the wedge 107 while the bonding wire 106 is pressed against the first bonding pad 105a to bond the first bonding pad 105a to the bonding wire 106. Here, the junction between the first bonding pad 105a and the bonding wire 106 is referred to as a first bond. At the first bond, the tip end of the bonding wire 106 is not used for bonding with the bonding pad 105a but remains unused.
Next, as shown in FIG. 2C, the wedge 107 is moved upward up to a position just above the second bonding pad 105b while the bonding wire 106 is bonded to the first bonding pad 105a. After that, as shown in FIG. 2D, the wedge 107 is moved downward again to press the bonding wire 106 against the second bonding pad 105b. In this way, the ultrasonic vibrations are applied by the wedge 107 while the bonding wire 106 is pressed to bond the second bonding pad 105b to the bonding wire 106. The bonding wire 106 connects between the first bonding pad 105a and the second bonding pad 105b to form a loop as mentioned above. Here, a junction between the second bonding pad 105b and the bonding wire 106 is referred to as a second bond. In addition, the end portion on the first bond side of the loop of the bonding wire 106 connecting between the first bonding pad 105a and the second bonding pad 105b is referred to as a first bond side end portion, and an end portion thereof on the second bond side is referred to as a second bond side end portion.
After that, the bonding wire 106 is torn off and cut. Thus, at the second bond, there is a trace of the bonding wire 106 being torn off. As described above, the bonding wire form differs between the first bond and the second bond. Hence, whether the end portion of the loop of the bonding wire 106 is on the first bond side or the second bond side can be visually checked with ease through a microscope based on the form of the bonding wire 106. After the completion of the bonding for all portions, the lead frame 101, semiconductor chips 102, and the bonding wire 106 are wholly sealed to complete a multi-chip module 100.
Hereinbelow, the bonding pad 105 of this embodiment is described. As mentioned above, the bonding pad 105 is used for electrically connecting between two semiconductor chips, the first semiconductor chip 102a and the second semiconductor chip.
As shown in FIG. 3A, in the first bonding pad 105a that is first bonded to the bonding wire 106, the bonding wire 106 having the free end extends from the wedge 107 with a predetermined length and comes into contact with the first bonding pad 105a at an angle. The bonding wire 106 has no ball at its end, and its diameter is substantially uniform. In general, an angle between the first bonding pad 105a and the bonding wire 106 is 45°. Subsequently, the wedge 107 is further moved downward to press an extended portion of the bonding wire 106 against the first bonding pad 105a. When being pressed against the bonding wire 106, the first bonding pad 105a is considerably damaged. At this time, if the thickness of the bonding pad 105a is insufficient, the semiconductor chip 102a may be damaged.
On the other hand, the second bonding pad 105b is bonded to the bonding wire 106 after the first bonding pad 105a was bonded to the bonding wire 106. Accordingly, a middle portion of the bonding wire 106 comes into contact with the second bonding pad 105b. The bonding wire 106 connects the first bonding pad 105a with the second bonding pad 105b to form a loop, and thus is pulled upward. Hence, as shown in FIG. 3B, when coming into contact with the second bonding pad 105b, the bonding wire 106 is bent along the wedge 107 and extends almost in parallel to the second bonding pad 105b. Accordingly, a damage applied at the time when the wedge 107 moves downward to press the bonding wire 106 against the second bonding pad 105b is smaller than that of the first bonding pad 105a. 
In addition, not all energy of the ultrasonic vibrations upon the bonding is used for bonding between the bonding pad 105 and the bonding wire 106; there is an energy loss at the bonding wire 106. That is, upon the first bonding operation for bonding the first bonding pad 105a and the bonding wire 106, the tip end of the bonding wire 106 is the free end. Accordingly, the vibration energy generated during the bonding operation is wasted only on one side of the bonding wire 106, that is, the supplying side of the bonding wire 106.
On the other hand, the second bonding pad 105b is bonded to the bonding wire 106 after the first bonding pad 105a was bonded. At this time, the bonding wire 106 is being connected with the first bonding pad 105a. In this state, there is a loss of the vibration energy generated during the bonding operation on both sides of the bonding wire 106, that is, the supplying side and the first bond side. Accordingly, the vibration energy used for bonding the second bonding pad 105b to the bonding wire 106 is smaller than that for bonding the first bonding pad 105a. Hence, the second bonding pad 105b bonded in the second bonding operation is less damaged than the first bonding pad 105a bonded in the first bonding operation.
Thus, after bonding the first bonding pad 105a with the bonding wire 106, the second bonding pad 105b is bonded to the bonding wire 106, whereby the second bonding pad 105b can be made thinner than the first bonding pad 105a. 
On that account, if the second bonding pad 105b is thinner than the first bonding pad 105a that is first bonded, an internal circuit of the second semiconductor chip 102b is not so damaged as a result of bonding with the bonding wire 106. As described above, whether the first bonding pad 105a (or the second bonding pad 105b) is on the first bond side or the second bond side can be visually checked with ease through the microscope based on the form of the remaining portion of the bonding wire 106.
In this way, the bonding wire 106 is first bonded to the thick first bonding pad 105a and then bonded to the thin second bonding pad 105b. That is, the bonding wire 106 bonded to the first bonding pad 105a is bonded to the second bonding pad 105b thinner than the first bonding pad 105a. With such a structure, even if the second bonding pad 105b is thin, satisfactory bonding can be executed without damaging the semiconductor chips 102.
Further, the bonding pad 105 is generally formed concurrently with the circuit wiring line 104 through the same process. Thus, in the second semiconductor chip 102b, the circuit wiring line 104 to be formed together with the second bonding pad 105b can be formed with a smaller thickness. Accordingly, the circuit wiring lines 104 can be formed at small pitches with high dimensional accuracy to improve the integration degree of the second semiconductor chips 102b. Furthermore, the second semiconductor chip 102b can be downsized, so the size of the entire multi-chip module 100 can be reduced.
Further, in the second semiconductor chip 102b, it is unnecessary to form the thick second bonding pad 105b. Hence, a step of forming the second bonding pad 105b separately from the circuit wiring line 104 is unnecessary. Thus, it is possible to prevent an increase in the number of steps of the manufacturing process and to reduce a manufacturing cost.
An output chip supplies power and includes a power MOS transistor of a large current type. Therefore, the output chip is required to have thick circuit wiring lines for withstanding a large amount of current. Further, a control circuit chip is an LSI circuit chip where a logic circuit is configured. The control circuit chip is required to have thin circuit wiring lines for improving the integration degree. Accordingly, for example, the first semiconductor chip 102a having the thick first bonding pad 105a is used as the output chip that needs to have the thick circuit wiring line 104, while the second semiconductor chip 102b having the thin second bonding pad 105b is used as the control circuit chip with the high integration degree.
Second Embodiment
Referring to FIG. 4, a semiconductor device according to a second embodiment of the present invention is described. FIG. 4 is a schematic diagram showing the structure of the multi-chip module 100 as the semiconductor device of this embodiment. This embodiment differs from the first embodiment in that the second semiconductor chip 102b is mounted onto the first semiconductor chip 102a connected onto the lead frame 101. In FIG. 4, the same components as those of the first embodiment are denoted by like reference numerals and their description is omitted.
In the multi-chip module 100 according to this embodiment, an island (not shown) connected with the first semiconductor chip 102a is formed on the lead frame 101. The first semiconductor chip 102a is connected face up onto the island through the die mounting member 103.
Formed on the first semiconductor chip 102a are the circuit wiring line 104 and the first bonding pad 105a formed in the previous step. Further, an island 108 connected with the second semiconductor chip 102b is formed on the first semiconductor chip 102a. 
The second semiconductor chip 102b is connected face up onto the island 108 of the first semiconductor chip 102a through the die mounting member 103. The second semiconductor chip 102b has a chip size small enough not to cover the bonding pad 105a formed on the first semiconductor chip 102b. Formed on the second semiconductor chip 102b are the circuit wiring line 104 and the second bonding pad 105b. Further, the second bonding pad 105b is thinner than the first bonding pad 105a. 
The first bonding pad 105a is electrically connected with the second bonding pad 105b through the bonding wire 106. As mentioned above, after bonding the thick first bonding pad 105a with the bonding wire 106, the second bonding pad 105b is bonded to prevent the semiconductor chips 102 from being damaged.
Accordingly, the circuit wiring line 104 of the second semiconductor chip 102b can be made thin to improve the integration degree of the second semiconductor chip 102b. Accordingly, for example, the first semiconductor chip 102a can be used as an output chip that needs to have the thick circuit wiring line for withstanding a large amount of current. In addition, the second semiconductor chip 102b can be used as a control circuit chip that needs to have the thin circuit wiring line for improving the integration degree. Thus, a manufacturing cost can be reduced to improve the integration degree of the multi-chip module 100.
The present invention is applicable to a multi-chip module where the semiconductor chips 102 are stacked three-dimensionally.
Incidentally, this embodiment describes the example where the second bonding pad 105b of the second semiconductor chip 102b mounted on the first semiconductor chip 102a is made thicker than the first bonding pad 105a, but the present invention is not limited thereto. By changing the bonding order, the first bonding pad 105a of the first semiconductor chip 102a underlying the second semiconductor chip 102b may be thinner than the second bonding pad 105b. That is, in the multi-chip module where the chips are three-dimensionally stacked, whichever of the upper semiconductor chip and the lower semiconductor chip is selected, the bonding pad on the second bond side can be made thin.
Further, it is needless to say that the present invention is applicable to a package having plural islands.
As set forth above, in consideration of the bonding order upon the bonding between the semiconductor chips 102 with the bonding wire 106 of the multi-chip module 100, the bonding pad 105 connected with one of the semiconductor chips 102 can be formed with the smaller thickness at low costs. Accordingly, a line pitch of the circuit wiring lines 104 of the semiconductor chips 102 can be reduced, and thus the semiconductor chips 102 can be downsized with ease. This contributes to production of the semiconductor device of the high integration degree and small size.
It is apparent that the present invention is not limited to the above embodiment and it may be modified and changed without departing from the scope and spirit of the invention.

## Claims

1. A method of manufacturing a semiconductor device including a plurality of semiconductor chips, comprising:
arranging a first semiconductor chip having a first bonding pad;
arranging a second semiconductor chip having a second bonding pad thinner than the first bonding pad; and
bonding one end of the bonding wire to the first bonding pad and then bonding the other end of the bonding wire to the second bonding pad.

2. The method of manufacturing a semiconductor device according to claim 1, wherein each of the first bonding pad and the second bonding pad is bonded to the bonding wire through wedge bonding.

3. The method of manufacturing a semiconductor device according to claim 1, wherein the bonding wire is metal containing Al.

4. The method of manufacturing a semiconductor device according to claim 1, wherein the first semiconductor chip is an output chip, and the second semiconductor chip is a control circuit chip.

5. The method of manufacturing a semiconductor device according to claim 4, wherein the first semiconductor chip has a power MOS transistor.

6. The method of manufacturing a semiconductor device according to claim 1, wherein an external force applied to the second bonding pad upon bonding the second bonding pad to the bonding wire is smaller than an external force applied to the first bonding pad upon bonding the first bonding pad to the bonding wire.

7. The method of manufacturing a semiconductor device according to claim 1, further comprising:
preparing a lead frame;
mounting the first semiconductor chip on the lead frame;
mounting the second semiconductor chip on the lead frame.

8. The method of manufacturing a semiconductor device according to claim 1, further comprising:
preparing a lead frame;
mounting the first semiconductor chip on the lead frame;
mounting the second semiconductor chip on the first semiconductor chip.