JPH0817835A - Bump forming method - Google Patents

Abstract

PURPOSE:To avoid non-adhesion caused by the effect of an oxide film by a method wherein a process in which the oxide film thinly formed on the surface of a pad is mechanically removed, is conducted before formation of a bump. CONSTITUTION:A tool 1 dedicated for removal of an oxide film is different from a bonding tool 7, and diamond or sapphire or ruby and the like in the width of an electrode pad is fixed to the tip 2 of the tool 1. The tip 2 of the tool 1 is brought into contact with the surface of a pad 3, and after a part of a natural oxide film 4 on the surface of the pad 3 has been removed by scratching the surface by shifting the tip 2 in a horizontal direction, a ball bump 9 is formed on the pad 3 where the oxide film is removed. Accordingly, as a pad surface keeps an aluminum film exposed, a strong adhesive strength can be formed between metal and metal, and a bump, having an excellent adhesive reliability, can be formed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bump forming method,
In particular, in a method of forming a metal bump on an LSI (high-density integrated circuit) chip, a wafer, or a circuit board, a method of mechanically removing a natural oxide film on the pad surface and then bonding the metal bump to each pad surface to form a bump .

[0002]

2. Description of the Related Art An outline of a conventional bump forming method is as follows.
First, a method of forming a metal bump on a pad of an LSI element will be described. As this method, a method in which metal bumps are collectively formed on each pad in a semiconductor wafer state, and a method in which individual LSI chips are separated from a semiconductor wafer by dimming,
The method is divided into a method of forming bumps after forming individual LSI chips. As the former method, for example, “Microelectronics packaging handbook (Microelectronics pack
aging Handbook) 2nd edition issued by Nikkei BP
90 pages, 291 pages, 300 pages and 301 pages ",
Has been stated. That is, the aluminum electrode formed on the surface of the semiconductor wafer is subjected to surface cleaning by DC sputtering to remove oxides, photoresist, etc. on the surface. BLM (Ball Limit)
A ball control metal layer, called a metallizing metal, is formed on the aluminum electrode by a vapor deposition method.
As an example of BLM, chromium-copper-gold (Cr-Cu-A
The three-layer metal of u) is typical. A lead-tin (Pb-Sn) solder is deposited on the BLM. Next, the Pb-Sn pad is melted by reflowing at 350 ° C. in a hydrogen (H ₂ ) atmosphere furnace, and a spherical bump due to surface tension is formed on the aluminum pad via the BLM. As another method of collective formation, there is a method by plating. Also in this case, it is necessary to form another plating base metal on aluminum as a pretreatment for plating.

A typical example of the latter method is a ball bump method in which a ball is formed with a metal wire, and the ball is bonded to an aluminum pad of an LSI chip to form a bump. Normal wire bonding is
After ball bonding on the aluminum pad, stretch this wire in a loop without cutting the wire,
Wedge bonding to pads on the substrate. On the other hand, in the ball bump method, after bonding a ball to an aluminum pad, a wire is cut to form a bump. However, the mechanism of bonding to the aluminum pad is common to ordinary wire bonding. References include pages 312 and 313 of the aforementioned “Microelectronics Packaging Handbook”.
Page and the like. As described herein, the bonding of the gold wire to the pad requires ultrasonic vibration or high temperature and pressure of 300-400 ° C., or both, to the aluminum pad via a ball. By adding, remove the aluminum surface oxide,
We have a metal welded joint between gold and aluminum.

This mechanism is described in “HYBRIDS”, 1987, Vol.
3, International Hybrid Microelectronics Association, Japan Headquarters ", pages 6 and 7. That is, when the gold wire ball is heated to 300 ° C. to 350 ° C. and pressed on the aluminum pad with a pressure of 70 g to 100 g, the gold ball plastically deforms on the aluminum thin film. Due to the plastic deformation of the gold ball, the irregularities of the slip line generated on the ball surface are stamped on the aluminum, and fine wrinkle-like irregularities are formed at the joint interface, and in accordance with this, irregularities also occur on the aluminum surface and the aluminum surface The oxide film is destroyed and some clean surfaces appear. Then, diffusion and reaction of Au and Al occur on this surface to form a bond.

The ultrasonic bonding method utilizes a mechanism of applying ultrasonic vibration to a gold ball to plastically deform the ball and breaking the aluminum oxide film by friction and sliding of the gold ball on the surface of an aluminum pad. . This joining method enables joining at room temperature.

As another method for removing the oxide film from the aluminum pad, there is a method utilizing plasma etching, chemical etching, or the like (Japanese Patent Laid-Open No. 63-289,941). According to this method, plasma etching or chemical etching such as hydrofluoric acid is used as a pretreatment for performing Au wire bonding on an aluminum pad in order to remove a natural oxide film and dirt. Thereafter, it is described that by performing ordinary wire bonding, bonding properties are improved and reliability is improved.

Next, a method of forming a metal bump on a pad on a circuit board will be described. The method of forming a metal bump on a pad of a circuit board is also broadly classified into a method of forming a bump on a pad on a board at once and a method of forming a bump one point for each pad.

As a batch bump formation method, there is a method in which solder bumps, metal bumps and the like are formed by plating or vapor deposition sputtering as in the case of an LSI wafer. In this case, the oxide film on the pad surface is removed by a chemical treatment as a pretreatment for plating, or the surface oxide film is removed by plasma etching or reverse sputtering as a pretreatment for vapor deposition or sputtering. As another example of the batch bump forming method, there is a printing reflow method using a solder paste. A solder paste is printed on a predetermined pad position, and then a solder bump is formed by reflow. In this case, the surface oxide film on the pad is removed by a chemical reaction of the flux contained in the solder paste.

As a method of forming bumps one by one on each pad on a circuit board, there are few reports on the prior art, but it is considered that the formation method on LSI pads can be basically used.

[0010]

The method of forming bumps on a pad of an LSI element or a pad of a circuit board in a semiconductor wafer state has the following problems. When a metal bump is formed by plating, it is impossible to form a plating directly on the metal oxide film, and therefore, it is necessary to remove this oxide film by chemical etching. In particular, in the case of an aluminum pad of an LSI element, since it is difficult to stably plate another metal directly on the aluminum metal, after forming the other metal by vapor deposition, sputtering, or the like,
A plating technique is required. Even when a metal bump is formed by vapor deposition or sputtering, pretreatment such as plasma etching or reverse sputtering is essential. Further, a mask is required to form vapor-deposited or sputtered metal only on the pad.

In any case, in the batch processing method, a pre-processing apparatus such as a chemical etching apparatus (which requires waste water treatment), a plasma etching apparatus, or a sputtering apparatus is required as processing before bump formation. For an aluminum pad, a plating base metal layer and a pre-deposition metal layer are required, and there has been a problem that the configuration is complicated.

Next, the method of forming metal bumps on individual pads of an LSI element or individual pads on a circuit board has the following problems. To form a bump on an aluminum pad of an LSI by a ball bonding method using a wire bonding method, an ultrasonic method, a thermocompression bonding method,
And a thermosonic method using both of them. By these methods, basically, the surface oxide film of aluminum can be broken, and a bond between the Au hole and Al can be obtained. However, the size of the bump that can be formed by the ball bonding method is almost determined by the diameter of the wire. For example, the lower limit of the diameter of a practical 25 μmφ wire is approximately 80 μm. That is, the conventional ball bonding method is not suitable for forming minute bumps having a diameter of 80 μmφ or less. This is because, by applying an ultrasonic wave, a load, heat, or the like, the plastic deformation of the ball is caused, thereby breaking the oxide film of the pad metal, so that the ball diameter becomes large during the plastic deformation. . Furthermore, when the ball diameter is reduced, if ultrasonic energy, pressure, or thermal energy to such an extent that the oxide film is broken, pad peeling, under-pad cracks, etc. occur. If these energies are reduced, poor bonding with low bonding strength occurs. In order to avoid this, if plasma etching, chemical etching, or the like is put in the process, an expensive device or an extra process is required, resulting in high cost.

The problem of the method of forming metal bumps one by one on the pads on the circuit board is also common with the pads on the LSI chip. In particular, when the circuit board is a general printed circuit board, the application of the ball bump technology by the conventional wire bonding is very difficult because the adhesive force of the pad is greatly deteriorated by heat.

In order to solve the above conventional problems,
The present invention has the following problems. (1) It is not necessary to chemically etch the oxide film on the surface before forming the metal bump. (2) Pretreatment such as plasma etching or reverse sputtering is not required. (3) No mask is required. (4) Even a minute bump having a diameter of 80 μmφ or less can be easily formed. (5) Prevent peeling of the pad and cracks under the pad. (6) It is not necessary to prepare expensive equipment or to add additional steps. (7) Make sure that the adhesion of the pad does not decrease. (8) The manufacturing method is simple, reliability is improved, and cost is not increased.

(9) It is not necessary to add a wastewater treatment device.

[0016]

The structure of the present invention is a bump forming method for bonding a metal bump to an electrode pad formed on a main surface of a semiconductor element or a circuit board, wherein an oxide film on the main surface of the electrode pad is formed. The method further comprises a first step of partially removing it by mechanical means, and a second step of joining the metal bump to the main surface of the electrode pad from which the oxide film has been partially removed in the first step. And the first and second
Is processed in an inert atmosphere or a reducing atmosphere.

[0017]

1A to 1F are sectional views or perspective views showing a bump forming method according to a first embodiment of the present invention in the order of steps.

1 (a) to 1 (f), in this embodiment, the removal of the oxide film 4 on the main surface of the pad 3 and the bonding are performed in one step.
₂ , Ar, etc.) or in a reducing (H ₂ ) atmosphere, and not in a natural (atmospheric) atmosphere or an oxidizing atmosphere.

In this embodiment, first, as shown in FIG. 1A, an oxide film removing tool 1 is prepared as a dedicated tool different from a bonding tool, and a tip 2 has a contact surface with a pad 3. Diamond, sapphire, ruby, or the like, which is flat and about the width of an electrode pad, is fixed. The removal tool 1 has sufficient strength so that the tool itself does not deform when the tip 2 is pressed against the pad 3, and there is no problem even if it is large.

Next, as shown in FIG. 1B, a semiconductor element such as a semiconductor chip 5 is placed on a bonding stage (not shown), and the tip 2 of the removal tool 1 is placed on the main surface of the semiconductor chip 5. One electrode (bonding) pad 3
Then, the tip 2 is lightly contacted with the surface of the pad 3, and the tool 1 is moved in a direction parallel to the main surface of the semiconductor chip 5 and in the left or right uniaxial direction 40.

As shown in FIG. 1C, the tip 2 moves while removing the main surface of the pad 3 while scratching it, and the metal oxide film 4 on the main surface is partially removed.

Here, the thickness of the aluminum oxide film 4 on the pad 3 made of aluminum is about 5 nm to 10 n.
m, and the tool 1 is pressed against the surface at a constant pressure and moved in the horizontal direction, thereby causing a plastic deformation of a shift on a crystal plane of the aluminum surface and exposing a clean aluminum surface. When the exposure is completed, the tip of the removal tool 2 is pulled up from the pad 3. In the next step, the bonding tool is moved to the position of the pad, and the metal bump is formed by the ball bump method as follows.

First, in FIG. 1D showing the step of forming a metal ball in the ball bump method, the capillary 7 is arranged above the pad 3 made of aluminum on the main surface of the semiconductor chip 5. The capillary 7 has an opening at the center, and the tip of the metal wire 6 penetrating the opening is formed into a spherical shape by sparking with a heater (not shown), and a metal ball 8 is formed.

Thereafter, as shown in FIG. 1E, the metal ball 8 is pressed against the electrode pad 3 of the semiconductor chip 5 by a capillary 7 on a stage (not shown) to deform the ball 8, The ball 8 is bonded to the electrode pad 3.
Finally, the metal wire 6 is broken by moving the capillary 7, and a metal projection 9 is formed on the electrode pad 3 as shown in FIG. Examples of the metal wire 6 include gold, gold alloy, Pd,
Al, Cu or the like can be used. Since the oxide film has already been removed from the surface of the pad 3 and the clean metal aluminum is exposed, a high adhesive bond between the metal of the ball 9 and the aluminum pad 3 can be obtained. This embodiment can be performed even when the semiconductor chip 5 is in the state of a semiconductor wafer. When the powder of the removed oxide film is generated, it is preferable to appropriately remove the powder by spraying and suctioning. still,
The natural oxide film 4 formed on the surface of the electrode pad 3
A residual oxidizing gas may be formed thin not only in the air or an oxidizing atmosphere but also in an inert atmosphere.

2 (a) to 2 (f) show a second embodiment of the present invention.
6A and 6B are cross-sectional views or perspective views showing the bump forming method of the embodiment in the order of steps.

[0026] In FIGS. 2 (a) to (f), in this embodiment, the entire process of the oxide film removal of the semiconductor chip to bonding, inert gas (N _2, Ar, etc.) in the atmosphere,
Alternatively, it is performed in a reducing (H ₂ ) atmosphere.

In this embodiment, first, as shown in FIG. 2 (a), a tool 21 having a tip 22 is prepared, and this tool 21 is a dedicated tool for removing an oxide film, and is subjected to ultrasonic vibration. It has a sufficient strength so as not to be absorbed, and a diamond, sapphire, ruby or the like having a width of the electrode pad is fixed to the tip 22 thereof. Here, the contact surface of the tool tip 22 has fine irregularities. The diameter of the tool tip 22 is 80μ
mφ, and the diameter of the indentations with fine irregularities is 1 μm to 1
The range is preferably 0 μmφ. 2 (b),
As shown in (c), the tip 22 of the removal tool 21 is brought into contact with the main surface of the bonding pad 23 of the semiconductor chip 25. Here, as shown in FIG. 2C, the removal tool 2
1 to the metal oxide film 2 on the main surface of the pad 23 by applying an ultrasonic wave.
A part of 4 is removed by ultrasonic friction. This ultrasonic vibration preferably reciprocates in the left and right direction 41 with respect to the main surface of the semiconductor chip 25. 2 (b) and 2 (c)
23 in which the oxide film on the pad surface has been removed in the step of
Is sent to the bump formation process in the above atmosphere, and the oxide film 2
A metal tape is punched by the punching method described below to form bumps on the electrode pad 23 in which 4 is removed and the metal aluminum film is exposed.

This punching method will be described with reference to FIGS. In FIG. 2D, a die 28 having an opening into which a pressing tool (punch) 26 is inserted is prepared on the bonding pad 23 on the main back surface of the semiconductor element 25, and a metal tape 27 is placed on the die 28.
Is placed. First, the positions of the holes of the dice 28 are aligned on the bonding pads 23 of the semiconductor chip 25. The metal tape 27 is placed on the die 28. Tape 2
Both ends of the tape 7 are tensioned by a jig (not shown), and the tape 27 is so tight that it does not move when punched. Next, as shown in FIG. 2E, the pressing tool 26 is lowered from above, the metal tape 27 is punched out between the die 28 and the metal tape 27 is pressed against the bonding pad 23 on the semiconductor element 25 as it is. The diameter of the pressure tool 26 is made slightly smaller than the diameter of the hole of the die 28 to facilitate punching of the metal tape 27. In this embodiment, the size of the hole of the die 28 is 50 μmφ, and the diameter of the pressing tool 26 is 45 μmφ. In this case, the thickness of the metal tape is preferably 30 μm. The punched metal tape 28 ′ is pressed into a bump shape on the aluminum pad 23. Since the metal oxide film has been removed from the surface of the pad 23 in the above-described process, and the metal tape 28 'is also a metal such as Au or Pd which is difficult to form an oxide film, the bump 29 is formed as shown in FIG. The pad 23 and the pad 23 can obtain a strong adhesive force only by being pressed against each other, and can obtain a highly reliable joint. In this embodiment, as the metal tape 27, Au, P
Although the case of d has been described, since at least the surface in contact with the pad 23 may be Au or Pd, a multilayer film formed by plating the lower surface of Cu, Ni, or solder with Au or Pd may be used. With this punching method, the bump 29
The bump diameter is determined by the punch and die diameters when formed, and the pressing force can be appropriately controlled, so that a minute bump having a diameter of 80 μmφ or less can be easily formed.

Incidentally, in the first embodiment, as shown in FIG.
Instead of the steps of (a) to (c), the steps of FIGS. 2 (a) to 2 (c) showing the second embodiment may be used, or FIGS. 1 (d) to 1 (f). 2 instead of the process
The steps (d) to (f) may be used.

In the second embodiment, the steps of FIGS. 1A to 1C may be used instead of the steps of FIGS. 2A to 2C, and FIG. 2D. Instead of the steps of (f) to (f), the steps of (d) to (f) of FIG. 1 may be used.

FIGS. 3A and 3B are a sectional view and a perspective view, respectively, showing a part of the bump forming method of the third embodiment of the present invention. All the steps of this embodiment are also carried out in the same atmosphere as the first and second embodiments.

3 (a) and 3 (b), in this embodiment, a tool 31 is a dedicated tool for removing an oxide film and has a strength sufficient to cope with a rotational movement with respect to an axis. Further, a diamond, sapphire, ruby, or the like having a width of about the electrode pad is fixed to the tip 32 thereof. First, the tip 32 of the removal tool is attached to the circuit board 3.
5 and is brought into contact with the main surface of the electrode pad 33. Next, the tool tip 32 is rotated with respect to the axis as shown by the arrow direction 42, and a part of the metal oxide film 34 on the main surface of the pad 33 is removed by rotational friction. Here, the metal of the pad 33 is Cu plating,
Solder plating, Sn plating, Pd plating or the like is preferable, and the thickness of the oxide film on the surface is considerably different due to the difference in the metal composition and the thermal history before the formation of the bump. The optimum conditions for the number of rotations and the contact time of the tool 31 should be determined by the composition and thickness of the oxide film. For example, for solder plating, at a rotation speed of 10 rpm to 100 rpm,
A contact time of 0.1 second to 1 second is sufficient.

Incidentally, the pad 3 of the tip 32 of the third embodiment is
The contact surface of the tool 3 is flat, and the rotation direction of the tool 31 may be opposite to the arrow direction 42.

Immediately after the oxide film 34 on the surface of the pad 33 is removed, a bump is formed on this pad by a ball bonding method or a punching method. When this ball bonding method is adopted, it is the same step as in FIGS. 1D to 1F shown in the first embodiment, and when the punching method is adopted, it is the same as in the second embodiment. Since the steps are common to those shown in FIGS. 2D to 2F, description and illustration thereof will be omitted.

Conventionally, when the circuit board is a printed circuit board, ultrasonic waves and heat are applied when the bumps are formed by the ball bump method, and pressure and heat are applied when the bumps are formed by the punching method. There is a problem that the adhesive strength is deteriorated and it cannot be practically applied. However, if the first, second, and third embodiments are used, the oxide film on the main surface of the pad is mechanically removed beforehand, so that the metal bump and the pad are brought into contact with each other with a weak pressure to form a strong bond. You can get it. That is, as shown in the third embodiment, the bumps can be formed on the pads on the printed board by the ball bump method or the punching method.

In the third embodiment, the method of removing the oxide film from the pad of the circuit board by the rotational frictional force is described. However, the scratching method of the first embodiment and the second embodiment are described. It can also be applied to ultrasonic methods (although weak ultrasonic waves are required for printed boards).

As described above, the bump forming method according to the embodiment of the present invention is performed in an inert atmosphere or a reducing atmosphere, and the natural oxide film on the pad surface is removed by various methods immediately before bonding. A dedicated tool different from the bonding tool is used, and the steps from oxide film removal to bonding are performed in the same atmosphere as a series of operations.

[0038]

As described above, according to the present invention, a series of operations from the oxide film removal to the bump formation are performed in an inert atmosphere or a reducing atmosphere to eliminate the influence of the oxide film. As a result, highly reliable bonding can be performed, so that the above-mentioned problems can be achieved.

Further, according to the present invention, ultrasonic waves during ball bonding can be eliminated or extremely weakened by the ball bump method, and heat and pressure can be reduced, so that damage to pads can be reduced. There is an effect that the amount of plastic deformation of the pole bump to be formed can be reduced, and a minute bump can be formed.

Further, in the present invention, the bump can be formed even by the punching method, and there is also an effect that a minute bump can be formed. In particular, since the metal oxide film is removed by mechanical means, the apparatus can be simplified. Therefore, there is also an effect that a highly reliable bonding can be obtained at low cost.

[Brief description of drawings]

FIG. 1 (a) and (b), (c), (d) to (f)
3A to 3C are a cross-sectional view, a perspective view, and a cross-sectional view, respectively, illustrating a bump forming method according to a first embodiment of the present invention in the order of steps.

2 (a) and (b), (c), (d) to (f)
FIG. 5A is a sectional view, a perspective view, and a sectional view showing a bump forming method of a second embodiment of the present invention in the order of steps.

3A and 3B are a cross-sectional view and a perspective view, respectively, showing a part of a bump forming method according to a third embodiment of the present invention in the order of steps.

[Explanation of symbols]

1,21,31 Oxide film removal tool 2,22,32 Diamond or sapphire
Tip of ruby etc. 3,23,33 Bonding pad or electrode pad 4,24,34 Oxide film on pad surface 5,25 LSI chip 35 Circuit board 6 Wire 7 Cavary 8 Metal ball 9 Bump 26 Pressing tool (punch) 27 Metal Tape 28 Dice

Claims (5)

[Claims] 1. A bump forming method for bonding a metal bump to an electrode pad formed on a main surface of a semiconductor element or a circuit board, wherein an oxide film on the main surface of the electrode pad is partially removed by mechanical means. 1) and a second step of bonding the metal bump to the main surface of the electrode pad from which the oxide film has been partially removed in the first step, and the first and second steps. A bump forming method, wherein the step is performed in an inert atmosphere or a reducing atmosphere. 2. The bump forming method according to claim 1, wherein a dedicated tool different from the joining means used in the second step is used as the mechanical means used in the first step. 3. A means for removing an oxide film by ultrasonic vibration is used as a mechanical means for the first step.
The method for forming bumps as described above. 4. The bump forming method according to claim 1, wherein as the mechanical means of the first step, a means having a tip for rotating the electrode pad in contact with the main surface of the electrode pad is used. 5. The bump forming method according to claim 1, wherein the second step is a step of punching a metal tape into a predetermined size and pressing the punched portion as it is against the main surface of the electrode pad to form a metal bump. .