JP2003059958A - Forming method for micro-bump - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for uniformly forming the micro-bump of a height equal to or lower than 30 μm at low cost with high form accuracy from the metal paste of high viscocity. SOLUTION: A resist film 6 is formed on a wafer W equipped with a base electrode 3, a fine opening 7 is provided by selectively removing a part higher than the base electrode 3, metal paste P is supplied onto the resist film 6, pushed and applied into the fine opening 7 by a squeegee 8. Then, the wafer W is charged in a vacuum chamber 11 and defoamed under vacuum in the state of applying the metal paste P and continuously, excessive metal paste P on the resist film 6 is scraped away. Further, the metal paste P on the wafer W is prebaked and solidified and the resist film 6 is removed. Afterwards, the remaining metal paste P on the wafer W is baked, a micro-grain metal is integrated as a micro-bump M1 and this is fused and made into micro-bump M2 in the shape of micro-sphere.

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, and more particularly to a method of forming bumps, which are protruding electrodes, on a base electrode of a semiconductor chip such as an IC (integrated circuit device). Is.

[0002]

2. Description of the Related Art Miniaturized semiconductor chips have been mounted by the conventional DIP (dual in-line package) or QFP.
Instead of (quad flat package), mounting density,
CSP (chip size package), which is excellent in terms of mounting cost, and mounting by flip chip bonding are often used, and the connection between these semiconductor chips and the mounting board is high on the semiconductor chip side or the mounting board side. A bump having a height of about 300 μm and a height of about 60 μm for a small bump is formed.

However, in recent years, with the progress of higher frequency and higher speed of wireless communication systems represented by portable terminals such as mobile phones, controllers of information appliances, or in-vehicle terminals in highway toll collection systems, these terminals have been developed. There is a demand for smaller size and lighter weight, and it is desired to further increase the packaging density of semiconductor chips.

Conventionally, an electrolytic plating method or a wire bonding method has been adopted for forming the above bumps. However, in the electrolytic plating method, there is a fear that adjacent bumps are connected by a plating film, and pretreatment and posttreatment are performed. Including the process is extremely complicated and it is difficult to reduce the cost. Since the wire bonding method forms one bump at a time, it takes a long time to form a large number of bumps, and the shape of each bump tends to be different. Therefore, in Japanese Patent Laid-Open No. 9-64047, there is a wafer 2 provided with electrodes as shown in FIG.
1 is formed with a resist film 22 to form holes 2 at the bump formation locations.
3 is opened, and this is attached on the table 31 of the spin coater, and a low viscosity (8
The metal paste 31 of about 5 cps or less, that is, about 0.085 Pa · s or less) is dropped, the table 31 is rotated at a high speed by the drive motor 32, and the holes 23 of the resist film 22 are filled with the metal paste 31. A method is disclosed in which the metal paste 31 in the flat portion is removed by centrifugal force, the metal paste 31 in the hole 23 is pre-baked to remove the resist film 22, and then the metal paste 31 is baked to form bumps. Therefore, according to this method, the bumps have the same height as the thickness of the resist film 22 (10 in the embodiment).
It is said that the bumps can be formed easily and in a short time as compared with the conventional method.

A flip-chip bonding mounting method is known as C4 (controlled collapse chip connection) method, but a semiconductor chip having solder bumps is mounted together with flux on a mounting substrate coated with eutectic solder. A method widely used is that after placing, reflowing and cooling, filling an epoxy resin and an underfiller containing a filler as a main material between the mounting substrate and the semiconductor chip and thermosetting. The solder bump in the C4 method is formed by vapor-depositing on a masked semiconductor chip to locally form a solder film, and melting and spheroidizing the solder film. In addition, a method is used in which a hole is formed in a thick film resist formed on a semiconductor chip to evaporate solder, and the solder in the part other than the hole is lifted off together with the resist film to leave a solder bump. There is a method in which a paste is locally printed, transferred to a predetermined position of a semiconductor chip, and then melted and spheroidized to form a metal bump.

[0006]

The spin coating method of the above-mentioned Japanese Patent Laid-Open No. 9-64047 shown in FIG. 9 requires that the metal paste has low viscosity. The rate of removal by centrifugal force during spin coating is large, loss is likely to occur even if these are collected, and since the amount of solvent in the metal paste is large because of low viscosity, the metal paste was fired. Subsequent bumps may be too thin, or else small voids may easily form in the bumps.

In addition, it is difficult to form fine bumps by the method of masking and depositing on a semiconductor chip in the C4 method, the cost is high for vapor deposition, and solder with a composition other than the eutectic composition is desired. There is a problem that the evaporation rate cannot be controlled by the composition of the components. The method of selectively opening a thick film resist and vapor deposition is also the same, and it is difficult to form fine bumps. Further, in the method of locally printing and transferring the metal paste, since a part of the metal paste is easily attached to the mask to be removed and is removed, the bumps formed are likely to lack area uniformity.

The present invention has been made in view of the above problems, and it is possible to form bumps by using a highly viscous metal paste. Furthermore, for example, micro bumps having a height of 30 μm or less can be uniformly formed with high shape accuracy. It is an object of the present invention to provide a bump forming method that can be formed at low cost.

[0009]

The above-mentioned problems can be solved by the structure of claim 1, and the solution means will be described as follows.

A method of forming a micro bump according to claim 1 is
A method of forming a micro bump on an electrode on a surface of a semiconductor substrate, which comprises forming a resist film on a semiconductor substrate and selectively removing a portion on the electrode to provide a fine opening,
Supplying a metal paste consisting of ultrafine metal and an organic solvent on the resist film, applying a metal pesto with a squeegee or equivalent equipment, and vacuuming the semiconductor substrate with the metal paste applied to the resist film The forming method includes a step, a step of pre-baking the metal paste, a step of removing the resist film, and a step of baking the metal paste remaining on the semiconductor substrate.

The method of forming such a micro bump is
Since it is applied with a squeegee or equivalent equipment, it is possible to use a high-viscosity metal paste, and there is less loss than when spin-coating a low-viscosity metal paste, that is, a metal paste with a large amount of solvent. , Less likely to cause thinning and voids after baking applied metal paste. Further, since the semiconductor substrate to which the high-viscosity metal paste is applied is evacuated, it is difficult to fill the corners of the fine opening of the resist film with a missing portion.

A method of forming a micro bump according to a second aspect of the present invention is a method of forming a micro sphere by melting the micro bump formed by baking. Such a method of forming micro bumps increases contact pressure with the electrodes connected to the micro bumps to ensure the connection.
A method of forming a micro bump according to claim 3 subordinate to claim 1 includes a step of removing an excessive metal paste on the resist film between a step of vacuuming the semiconductor substrate and a step of pre-baking the metal paste. It is a forming method.
In such a micro bump forming method, since the excess metal paste is removed after vacuuming, excess metal paste is pre-supplied on the resist film to increase the filling degree of the metal paste into the fine openings of the resist film. Can be applied as.

A method of forming a micro bump according to a fourth aspect of the present invention, which is dependent on the first aspect, is a method of performing a step of baking a metal paste under vacuum. In such a micro bump forming method, micro air bubbles are not mixed into the formed micro bump. A method of forming a micro bump according to claim 1 which depends on claim 1 is a method of using a metal paste having a particle diameter of ultrafine metal particles of 0.001 μm or more and 0.1 μm or less. In such a micro bump forming method, it is possible to form a micro bump by integrating ultrafine metal particles by heating to a temperature of about 300 ° C.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION As described above, the method of forming a micro bump of the present invention is a method of forming a micro bump on an electrode on the surface of a semiconductor substrate. The step of selectively removing the portion to be provided with a fine opening, the step of supplying ultrafine metal paste on the resist film and pressing it into the fine opening with a squeegee or an equivalent device, and the metal paste was applied In this state, the semiconductor substrate is vacuumed, the metal paste is pre-baked, the resist film is removed, and the metal paste remaining on the semiconductor substrate is baked.

The resist film to be formed on the semiconductor substrate can be formed by using a commonly used resist as it is, and the type of resist is not limited. Also,
The resist is formed on the surface of the semiconductor substrate by a widely used spin coater. As will be described later, the height of the micro bump is determined by the thickness of the resist film. Then, a portion above the electrode of the semiconductor substrate is selectively removed to provide a fine opening, and a well-known photolithography technique is used for this selective removal.

The metal paste is an ultrafine particle metal, that is, gold (Au), silver (Ag), aluminum (Al), copper (C).
u) or other conductive metal and having an average particle size of 0.1 μm or less, for example, 0.01 μm to 0.08 μm
Those having a viscosity of about 2 Pa · s, which are dispersed in an organic solvent such as α-terpineol or toluene, are used. The composition of the metal paste is not specified unconditionally because it varies greatly depending on the spherical shape of the ultrafine particle metal used, that is, whether it is a true sphere or the degree of deviation from the true sphere, but, for example, if the metal is 10 to 10
40% by volume and 90-60% by volume of organic solvent are used. Then, a certain amount of metal paste containing the above ultrafine metal as a main component is supplied onto the resist film,
A squeegee, such as is used during screen printing, stretches and presses into fine openings. In this case, a machine tool having an equivalent function may be used instead of the squeegee, and a roller can also be used.

[0017] with the coated metal paste on the resist film, 10 ¹ accommodates a semiconductor substrate in a vacuum chamber
The degree of vacuum is maintained at about Pa to 10 ² Pa. As a result, the metal paste spreads to every corner of the fine opening formed in the resist film, and a part of the organic solvent is evaporated and removed. Then, the semiconductor substrate is taken out of the vacuum chamber, and the metal paste is heated at a temperature not higher than the boiling point of the organic solvent, for example, at a temperature of 100 ° C. to 150 ° C. for 10 minutes to 30 minutes to perform pre-baking to solidify the metal paste. That is, the amount of the organic solvent is reduced by this prebaking, and the ultrafine metal particles in the metal paste are pressed against each other with a large pressure and sintered. Of course, depending on the type of metal paste used, a heating device such as an infrared lamp may be installed in the vacuum chamber, and prebaking for heating in a vacuum atmosphere may be performed. In this pre-baking, the resist film 6 is not deteriorated by the heating until it cannot be removed, and the pre-baking is performed to such a strength that the solidified metal paste is not broken or partially chipped when the resist film 6 is removed. Need to have been.

Then, the resist film is removed by a well-known method such as removing with a stripping solution or by ashing, and then the solidified metal paste remaining on the semiconductor substrate is baked to integrate the ultrafine metal particles with each other. It's a bump. Then, the formed micro bumps can be heated to a higher temperature to be melted, and can be made into microspheres by the surface tension. In this way, the height is 30 μm
Hereinafter, for example, a micro bump having a height of about 10 μm can be formed.

[0019]

Next, the method of forming a micro bump of the present invention will be specifically described with reference to the drawings by way of examples.

(Embodiment) FIGS. 1 to 7 are views for explaining a method of forming a micro bump according to an embodiment. FIG. 1 shows a state in which a wafer W having a base electrode and an insulating film and having a resist film formed on the upper surface thereof is fixed to a rotary table 1 and a metal paste P is applied. That is, the wafer W
A resist film 6 having a predetermined thickness is formed on the upper surface of the wafer, and a portion corresponding to the base electrode of the wafer is selectively removed by a photolithography technique, and a plurality of fine openings 7 are patterned in the resist film 6. Has been formed.

The wafer W is positioned and fixed on the turntable 1. As shown in FIG. 2, which is an enlarged view of the left end portion of the turntable 1 in FIG. 1, the wafer W is placed on the turntable 1. A disc-shaped recess 2 is formed at a depth such that the upper surface of the wafer W slightly protrudes from the surface of the rotary table 1 when placed. The disk-shaped recess 2 is provided with a portion corresponding to the orientation flat portion of the wafer W so that the mounted wafer W can be positioned. The wafer W is fixed to the rotary table 1 by vacuum suction from the back surface side. Of course, the wafer W may be positioned and fixed by a method other than these.

Then, a certain amount of metal paste P is supplied onto the surface of the resist film 6 formed on the wafer W while gently rotating the rotation table 1. For the metal paste P, a mixed composition of Au having a particle diameter of 0.01 μm, for example, 30% by volume and α-terpineol of about 70% by volume, to which a small amount of a dispersant is added, is used. To be done. It is possible to supply a fixed amount of metal paste P in a syringe and use compressed air to push out a fixed amount, but it may be supplied by a method other than this. Then, as shown in FIG. 1, the squeegee 8 is used to push the metal paste P into the fine openings 7 of the resist film 6 with a pressure that does not damage or deform the resist film 6. The squeegee 8 may be made of polyurethane, which is used for screen printing.

Next, as shown in FIG. 3, the metal paste P
The coated wafer W is loaded into the vacuum chamber 11 provided with the vacuum pump 12 together with the rotary table 1 to perform vacuum defoaming. At this time, it is not always necessary to rotate the turntable 1. In addition, the wafer W coated with the metal paste P may be removed from the rotary table 1 and loaded into the vacuum chamber 11. By performing this vacuum defoaming, it is possible to prevent incomplete filling of the metal paste P due to the presence of minute bubbles in the fine openings 7 of the resist film 6. Also, the vacuum chamber 11
A mechanically operable squeegee or an equivalent pressing device may be installed therein to press and extend the metal paste P even during vacuum defoaming to further ensure the filling into the fine openings 7.

FIG. 4 shows the vacuum chamber 11 after vacuum degassing.
FIG. 6 is a diagram showing a state in which the wafer W is taken out from and the excess metal paste P existing on the resist film 6 is scraped off by the squeegee 8. By this operation, only the metal paste P filled in the fine openings 7 of the resist film 6 remains on the wafer W. Instead of scraping with a squeegee 8, you may wipe the part outside the fine opening 7 with a brush containing an organic solvent. The metal paste P remaining in the fine openings 7 is processed by the fine openings 7 after the steps described below.
Will be the micro bumps connected to the base electrode 3 of the wafer W below. Although the base electrode 3 of the wafer W is shown in FIG. 4, the wiring and the surrounding insulating film following it are omitted.

After scraping off the excess metal paste P with a squeegee 8, heating is performed at a temperature of about 100 ° C. for 10 to 15 minutes to perform pre-baking. The amount of the organic solvent remaining in the metal paste P is reduced by this prebaking, and the ultrafine metal particles are pressed by the surface tension of the organic solvent and sintered and solidified.

After prebaking, the resist film 6 is peeled off. For peeling off the resist film 6, a general method of immersing the wafer W in a peeling solvent of a resist peeling apparatus, for example, an organic solvent such as butyl cellosolve acetate to swell the resist film 6 and peel it off can be adopted. Of course, other than this, the resist film 6 may be removed by reacting it with a reactive gas by an ashing device which is widely used for peeling the resist film. FIG. 5 is a partial enlarged cross-sectional view showing the wafer W after the resist film 6 is removed and the solidified metal paste P together with the base electrode 3 and the insulating layer 4. On the surface of the wafer W, a base electrode 3, wirings 4 and an insulating layer 4 made of, for example, polyimide are formed. On the base electrode 3 exposed in the opening of the insulating layer 4, the solidified metal paste P after prebaking is formed. Indicates the remaining state.

In FIG. 6, the solidified metal paste P after prebaking in FIG. 5 is heated at a temperature of about 300 ° C. for about 15 minutes to evaporate the remaining organic solvent and integrate the ultrafine metal particles. FIG. 7 shows the cylindrical micro-bump M ₁ obtained by the above, and FIG. 7 shows the cylindrical micro-bump M ₁ of FIG. 6 which is further heated and melted, and is made into microspheres by the surface tension of the metal in the molten state. Shows a micro bump M ₂ obtained by cooling. Then, the wafer W on which the micro bumps M ₂ are formed is cut into a predetermined size to be a semiconductor chip. Also, FIG.
Is a process showing a micro bump forming process of the present invention.
8 is a flow chart, and since the contents of each step have been described in FIGS. 1 to 7, the description of FIG. 8 is omitted to avoid duplication.

Although the method of forming the micro-bumps of the present invention has been described with reference to the embodiments, the present invention is not limited to this, and various modifications can be made based on the technical idea of the present invention.

For example, in this embodiment, the case where the baking is performed in the atmosphere has been described, but the baking may be performed in a vacuum. It is possible to avoid the formation of minute voids when transitioning from the sintered state to the integrated state, and to prevent oxidation when the bump is a metal that is easily oxidized at high temperature. Further, when it is only necessary to prevent oxidation, the baking may be performed in an atmosphere of, for example, nitrogen gas instead of being performed in a vacuum.

Further, in this embodiment, the case where the metal paste P applied to the resist film 6 is applied and defoamed in vacuum, and the excess metal paste P present in the resist film 6 is scraped off by the squeegee 8 is shown. However, when it is possible to leave the excess metal paste P on the resist film 6 by scraping at the time of initial application, the scraping by the squeegee 8 after vacuum defoaming can be omitted. Further, although the case where the micro bumps are formed of Au, Ag, or an alloy thereof has been described in the present embodiment, the solder bumps may be formed by a paste of ultrafine metal particles made of a leaded or lead-free solder alloy.

[0031]

The method of forming a micro bump of the present invention is carried out in the form as described above, and has the following effects.

According to the method of forming a micro bump of the first aspect, since the high-viscosity metal paste is used, the loss is less than that in the case of spin-coating the low-viscosity metal paste with a large amount of solvent, and after baking, Does not cause thinning or voids. In addition, since the semiconductor substrate to which the high-viscosity metal paste is applied is evacuated, the fine openings of the resist film are filled in every corner and there are no missing parts, and uniform micro bumps with high shape accuracy can be formed at low cost.

According to the method of forming a micro bump of the second aspect, since the micro bump is made into a microsphere, the contact pressure at the contact point between the micro bump and the electrode to be connected is increased, and the reliability of the connection is improved. According to the method of forming a micro bump of claim 3, since the excess metal paste is removed after vacuuming, the metal paste is pre-excessively supplied onto the resist film to increase the filling degree of the resist film into the fine openings. It is possible to further improve the shape accuracy and uniformity of the formed micro bumps.

According to the method of forming a micro bump of claim 4, since the step of baking the metal paste is performed under vacuum, micro air bubbles are not mixed in the formed micro bump, and a highly reliable micro bump is obtained. Give a bump. According to the method for forming a micro bump of claim 5, the metal paste has a particle diameter of 0.001 μm or more and 0.1 or more.
Since the metal having a melting point of less than μm is used, even a metal having a melting point of around 1000 ° C. can be integrated to form a micro bump by heating at a temperature of about 300 ° C. Does not cause serious damage.

[Brief description of drawings]

FIG. 1 to FIG. 7 are views for explaining a method of forming a micro bump according to an embodiment. FIG. 1 shows a semiconductor substrate on which a patterned resist film is formed, which is fixed on a rotary table and metal. It is a figure which shows the state which is applying the paste.

FIG. 2 is a diagram showing a height relationship between a surface of a rotary table and a surface of a semiconductor substrate fixed to the surface.

FIG. 3 is a diagram showing a state in which a semiconductor substrate coated with a metal paste is vacuum degassed in a vacuum chamber.

FIG. 4 is a diagram showing a state in which excess metal paste is scraped off after vacuum degassing.

FIG. 5 is a partially enlarged cross-sectional view showing a semiconductor substrate, a metal paste after prebaking, a base electrode, and an insulating film after the resist film is removed.

FIG. 6 is a view showing a cylindrical micro bump formed by baking and integrating a metal paste after pre-baking.

FIG. 7 is a diagram showing a micro bump obtained by melting the micro bump of FIG. 6 to form a microsphere.

FIG. 8 is a process flow diagram showing a micro bump forming process.

FIG. 9 is a diagram showing a conventional bump forming method.

[Explanation of symbols]

1 ... rotary table, 2 ... dent, 3 ... base electrode,
3 '... Wiring, 4 ... Insulating layer, 6 ... Resist film, 7 ...
… Micro openings, 8… Squeegee, 11… Vacuum chamber, M… Micro bumps, P… Metal paste, W…
… Wafer.

Claims (5)

[Claims] 1. A method of forming a micro bump on an electrode on a surface of a semiconductor substrate, which comprises forming a resist film on the semiconductor substrate and selectively removing a portion on the electrode to form a fine opening. And a step of supplying a metal paste composed of an ultrafine metal and an organic solvent on the resist film, and applying the metal pesto by a squeegee or an equivalent tool, and the metal paste applied to the resist film in the state described above. A micro bump including a step of vacuuming the semiconductor substrate, a step of pre-baking the metal paste, a step of removing the resist film, and a step of baking the metal paste remaining on the semiconductor substrate. Forming method. 2. The method of forming a micro bump according to claim 1, wherein the micro bump formed by baking is melted to form a microsphere. 3. A step of removing the excess metal paste on the resist film is inserted between the step of vacuuming the semiconductor substrate and the step of prebaking the metal paste. 1. The method for forming a micro bump according to 1. 4. The method for forming a micro bump according to claim 1, wherein the step of baking the metal paste is performed under vacuum. 5. The method of forming a micro bump according to claim 1, wherein the metal paste has a particle diameter of the ultrafine metal particles of 0.001 μm or more and 0.1 μm or less.