JPH0727904B2 - Method of forming solder bumps - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a method for forming solder bumps, and is particularly used in the semiconductor industry.

(Prior Art) Semiconductor device bonding techniques are roughly classified into wire bonding techniques and wireless bonding techniques.

The former is to connect the electrode of the semiconductor chip and the lead terminal of the lead frame with a wire. Although this technology can sufficiently cope with the case where the number of connections is small, as the size of the electrode becomes 100 μm □ or less and the density becomes higher with the high integration of the element, there are many problems particularly in reliability. .

On the other hand, the latter method is to collectively bond the electrodes of the semiconductor chip and the lead terminals of the lead frame or the glass and the electrodes on the ceramic substrate,
Practical applications have been made to ensure reliability in response to high integration of devices.

As this wireless bonding technology, for example, a tape automated bonding method (TAB method),
Flip chip method, CCB method and the like are known. In these methods, bumps are usually formed on the electrodes of a semiconductor chip. For this bump, Pb which has been
-Sn solder is under consideration.

Conventionally, a bump made of Pb-Sn solder formed on an electrode of a semiconductor chip is as shown in FIG. In FIG. 7, an electrode 3 made of Al or an Al alloy or the like is patterned and formed on a silicon substrate 1 with an insulating film 2 such as a silicon oxide film, and a whole surface is covered with a passivation film 4 such as a silicon nitride film. After that, the passivation film 4 on the electrode 3 is selectively etched to expose the electrode 3. Cr, Ni, Mo, Cu, A on the exposed electrode 3
A base metal 5 made of u, Ag or the like is formed. Furthermore,
Solder bumps 6 are formed on the base metal 5.

The base metal 5 is provided to improve the bondability with solder. For this purpose, 1 to 3 metal layers are provided as the base metal 5, and various combinations have been studied.

By the way, the solder bumps 6 are usually formed by plating or vapor deposition, and various methods have been proposed, but these methods all have drawbacks as described below.

In the method by plating, for example, after the electrode drilling step is completed, the natural oxide film on the electrode is removed by reactive ion etching, the electrode part is covered with the plated resist, and only the underlying metal on the electrode is covered. Solder plating,
A step of etching unnecessary portions of the plating resist and the base metal is taken. However, although such a method does not have a problem in the case of forming gold bumps, when it is applied to the formation of solder bumps, the chemical resistance of the solder is not good, so when the base metal is etched, the solder also has an etching solution. Has the drawback of being eroded by. Therefore, Sn,
A method is used in which Pb is sequentially plated, and the base metal is etched and then heated to form an alloy.

Further, in the method by vapor deposition, for example, after the electrode drilling step is completed, the natural oxide film on the electrode is removed by reactive ion etching, and 1 to 3 layers of base metal are vapor deposited on the entire surface,
Patterning is performed, and after coating the resist in which only the underlying metal on the electrode is opened, solder is vapor-deposited, the resist is removed, and the unnecessary portion of the solder is removed. However, when using the vapor deposition method, Pb in the solder
Since the vapor pressure is different from that of Sn, it is difficult to form solder bumps having a eutectic composition.

In any case, the conventional method has a fundamental problem that leads to complication of the process, such as using a base metal and forming a mask so that solder is not plated or vapor-deposited on a portion other than the electrode portion. is there.

Therefore, a method of forming bumps on the Al electrode by soldering using ultrasonic waves has been proposed (Japanese Patent Laid-Open No. 53-
89368). However, the solder used in this method is S
Since n, Zn, Mo, Bi, and Sb are used as the reference components, they have the drawbacks that they adhere to portions other than the Al electrode and the bump height is low.

(Problems to be Solved by the Invention) The present invention has been made in order to solve the above-mentioned problems, and solder bumps can be directly formed without providing a base metal on electrodes, and the process is simplified. The purpose is to provide a possible method.

[Structure of the Invention] (Means and Actions for Solving Problems) In the method for forming a solder bump of the present invention, an electrode is exposed from an insulating film covering a substrate, and nitric acid, sulfuric acid, hydrochloric acid,
After contacting a solution containing at least one of phosphoric acid, oleic acid, palmitic acid, stearic acid, butyric acid, resin, sodium carbonate, zinc borofluoride, cadmium borofluoride, and hydrazine, at least contact the electrode with molten solder Then, ultrasonic waves are applied to the molten solder to destroy the natural oxide film on the electrode surface and to selectively attach the solder by alloying with the electrode.

The principle of the operation of the present invention is as follows. That is, when ultrasonic waves are applied to the molten solder that is in contact with the electrode portion, bubbles of metal vapor are generated in the molten solder due to ultrasonic energy. This bubble grows and disappears instantly, and becomes locally high temperature and high pressure. When the high-pressure bubbles are destroyed, a strong impact is given to the electrode surface, which destroys the natural oxide film on the electrode surface and selectively solders the exposed new surface of the electrode. In this way, solder bumps are formed on the electrodes.

However, when the above-mentioned means is used when the oxide film on the Al electrode surface is thick, the output of ultrasonic waves must be increased. However, increasing the output of ultrasonic waves in this way may cause holes in the passivation film.

Therefore, if the oxide film etching liquid is brought into contact with the electrode portion before the soldering operation to make the oxide film thin, it is not necessary to increase the output of ultrasonic waves during soldering, and it is possible to prevent the passivation film from being destroyed.

In the present invention, as the etching solution, nitric acid, sulfuric acid,
A solution based on hydrochloric acid or phosphoric acid, a solution based on oleic acid, palmitic acid, stearic acid, butyric acid, resin, sodium carbonate, zinc borofluoride, cadmium borofluoride, or hydrazine is used.

Specific examples include those having the following compositions. In the following, the numerical value after the substance name indicates% by weight, and the numerical value in parentheses for the inorganic acid indicates specific gravity or concentration. That is, cadmium borofluoride 10
-Ammonium borofluoride 8-triethanolamine 82
(No.1), Cadmium borofluoride 10-Zinc borofluoride 3
-Ammonium borofluoride 5-triethanolamine 82
(No.2), zinc borofluoride 10-ammonium borofluoride 8-triethanolamine 82 (No.3), hydrazine dihydrofluoride (No.4), triethanolamine 83-fluoroboric acid 10- Cadmium borofluoride 7 (No.5), phosphoric acid (1.71) 10-distilled water 90 (No.6), methyl alcohol 32-
Hydrochloric acid (1.19) 32-nitric acid (1.40) 32-hydrofluoric acid (40%) 4
(No. 7), nitric acid (1.84) 5-distilled water 95 (No. 8), oleic acid 90-palmitic acid or stearic acid 7-butyric acid 3 (N
o.9), oleic acid 90-butyric acid 5-resin, sodium carbonate 5 (No. 10) and the like. Although a flux containing a halide that may corrode the Al electrode is not mentioned here, a halide may be used in a field where the corrosion of the Al electrode does not pose a problem.

As a method of bringing the etching solution for the oxide film into contact with the electrode portion, a method of immersing the substrate in the etching solution, spraying the etching solution onto the substrate, or the like can be used. When immersing the substrate in the etching solution, the temperature and immersion time must be changed according to the etching solution, but usually at room temperature.
It only needs to be dipped for 10 seconds or less. Further, when increasing the etching rate, ultrasonic waves may be applied to the etching solution.

Next, as a specific means for carrying out soldering, the substrate is immersed in the molten solder in the solder bath, an ultrasonic transducer is inserted and ultrasonic waves are applied to the molten solder, or the molten solder is applied. The tank itself may be ultrasonically vibrated to apply ultrasonic waves to the molten solder, or a soldering iron capable of ultrasonic vibration may be used to bring the molten solder into contact with the electrodes and apply ultrasonic waves to the molten solder at the same time. Good.

The method of the present invention can be similarly applied whether the substrate is a semiconductor substrate or an insulating substrate such as glass or ceramics. When these substrates are dipped in the molten solder as described above, the entire substrate may be dipped or a part thereof may be dipped. Further, since solder does not adhere to an insulating film such as a passivation film other than the electrodes, it is not necessary to cover it with polyimide or the like, but it is necessary to protect it when metal other than the electrodes is exposed. When dipping a semiconductor wafer in molten solder, it may break along the dicing line and separate the semiconductor chips after performing blade dicing.To prevent this, the back surface of the wafer It is desirable to attach an adhesive tape for high temperature to, and further adhere to metal or glass, or to adsorb the back surface of the wafer with a vacuum chuck or the like. Further, blade dicing may be performed after forming the solder bumps.

Further, in a semiconductor element, Al or a material containing Al as a main component and Si or Cu added is often used as an electrode.
Any type of solder may be used for the electrodes made of these materials, but a Pb-Sn solder or a solder containing Ag or Cd is usually used. Also, since it may be difficult for solder to adhere to old electrodes, Zn
You may use the solder which added. Note that the surface of some electrodes may be covered with a hard oxide film,
In this case, ozone may be wiped as a pretreatment.

Further, it is desirable that the soldering operation is performed in a non-oxidizing atmosphere by flowing an inert gas. This is because when the atmospheric gas contains 5% or more of oxygen, Al oxidizes to deteriorate the wettability with the solder, and in the worst case, a defect in which the bumps are connected to each other occurs.

In the present invention, the frequency of the ultrasonic wave applied to the molten solder may be about 10 to 60 kHz, preferably 15 to 40 kHz.
This is because if the frequency is too low, the above-mentioned action is unlikely to occur, and if the frequency is too high, peeling of the electrodes or the like may occur. The output of ultrasonic waves may be about 2 to 500 W, but is preferably 10 to 300 W. On the other hand, the temperature of the molten solder is about 230 to 350 ° C, and the processing time is about 0.1 to 10 seconds. This is because when the temperature is high and the treatment time is long, the Al, which is a constituent element of the electrode, is dissolved, and conversely, when the temperature is low and the treatment time is short, the alloying of Al and solder is not performed. Preferably, the molten solder temperature is about 240 to 320 ° C. and the treatment time is about 0.5 to 5 seconds. Since a glass or ceramic substrate is less likely to be broken than a silicon or compound semiconductor substrate, the frequency of ultrasonic waves, output, temperature of molten solder, and the like can be increased. Moreover, since the temperature of the liquid becomes different depending on the solder component, it is preferable to select the temperature according to the solder component.

The substrate on which the solder bumps are formed as described above is mounted by the wireless bonding technique. For example, in the TAB method, a semiconductor substrate having solder bumps and a lead frame (tape) are aligned and thermocompression bonded. In this case, the conductor metals that make up the lead frame are Cu, Fe,
Al, Fe-Ni alloy, Au, Ag, Sn, etc. are used. What plated these metals may be used. However, flux is used when the lead frame side is Fe-Ni alloy. Further, in the case of Al, it is desirable to attach solder by the same method. In this case, when the space between the leads is narrow, it is effective to attach oil or a surfactant as in the case of the substrate. Further, in the flip chip method or the CCB method, the semiconductor chip on which the solder bumps are formed and the bumps on the glass or ceramic substrate on which the solder bumps are formed are thermally fused to each other.

As described above, according to the present invention, the solder bumps can be directly formed on the electrodes without using the base metal and without destroying the passivation film.
The process can be simplified and the time can be significantly shortened. Also,
The subsequent wireless bonding process can be easily performed.

(Example) Hereinafter, the Example of this invention is described with reference to drawings.

Example 1 After forming wirings / electrodes by a normal wafer process, a silicon wafer 11 was prepared in which a passivation film was deposited on the entire surface and openings for contact pads were formed. The wiring and electrodes are made of Al-2% Si-2% Cu having a film thickness of about 1 μm formed by a sputtering device,
A silicon nitride film is used as the passivation film. Each chip formed on this silicon wafer 11 is provided with 200 electrodes (contact pads) of 30 μm □. Blade dicing was not performed on this silicon wafer 11.

After placing this silicon wafer 11 in a blow box sealed with N ₂ gas, both immersion in an etching solution and ultrasonic soldering were performed therein as follows. This is to prevent oxidation of the Al electrode by blocking the wafer from air after etching.

First, this silicon wafer 11 was immersed in the above-mentioned No. 1 etching solution (cadmium borofluoride 10-ammonium borofluoride 8-triethanolamine 82).

Next, solder bumps were formed on the electrodes of the silicon wafer 11 by using an ultrasonic soldering apparatus as shown in FIG. In FIG. 1, a solder recirculation path 22 is formed in a solder bath 21 and a molten solder 23 is accommodated therein. The molten solder 23 flows through the inside of the reflux passage 22 by a stirring rod 24 rotated by a motor (not shown) and is jetted above the liquid surface to reflux. A double-sided adhesive tape for high temperature is attached to the back surface of the silicon wafer 11, and the silicon wafer 11 is vertically held in a state of being adhered to a glass plate (not shown) and immersed in the molten solder 23 jetting out. Then, the ultrasonic oscillator 25 is inserted into the molten solder 23 near the silicon wafer 11 to apply ultrasonic waves to the molten solder 23.

As the solder, 95Pb-Sn solder was used, and the solder bath temperature was maintained at 320 ° C. Further, ultrasonic waves having a frequency of 20 kHz and an output of 80 W were applied to the molten solder 23 by the ultrasonic vibrator 25, and the immersion time of the silicon wafer 11 was set to 1 second. During this soldering operation, nitrogen gas was caused to flow around at a flow rate of 10 / min to prevent oxidation of aluminum, which is a constituent element of the electrode, and tin in the solder.

By this operation, solder bumps were formed as shown in FIG. That is, before dipping, the electrode 33 is formed on the silicon substrate 31 via the oxide film 32, and the electrode 33 is exposed from the passivation film 34 covering the entire surface, but after dipping, solder bumps are formed on the electrode 33. 35 was directly joined and formed into a mountain shape. The bump height was 25 μm. An alloy layer of Al and Sn was thinly formed on the joint surface between the electrode 33 and the solder bump 35.

Then, the silicon wafer 11 was divided into individual chips by blade dicing. Separately from this, a TAB tape was prepared in which a copper lead frame was formed and the surface of which was gold plated. Then, the bumps of the chip and the lead terminals of the tape were aligned and heat-sealed at 270 ° C. No problem occurred in this inner lead bonding process.

Example 2 A silicon wafer 11 similar to that in Example 1 was prepared. In this case, Al-1% Si is used as the wiring / electrode, and 163 40 μm square electrodes (contact pads) are formed on each chip formed on the silicon wafer 11. It is expected that the silicon wafer 11 has been covered with a hard oxide film for a considerable period of time after the element formation. In addition, since the metal other than the electrode was partially exposed, the metal other than the electrode was masked with polyimide.

First, the surface of the silicon wafer 11 was sprayed with the above-mentioned No. 6 etching solution (phosphoric acid (1.71) 10-distilled water 90).

Next, solder bumps were formed on the electrodes of the silicon wafer 11 using an ultrasonic soldering device as shown in FIG. In FIG. 3, a solder return path 42 is formed in a solder bath 41, and molten solder 43 is accommodated therein. The molten solder 43 passes through the reflux path 42 by a stirring rod 44 rotated by a motor (not shown) and is jetted above the liquid surface in the central portion of the solder bath 41 to be refluxed. In the state where the back surface of the silicon wafer 11 is attracted by the vacuum chuck, the entire surface of the surface on which the electrode is formed is jetted out
Is immersed in the liquid surface. Then, the ultrasonic vibrator 45 is inserted from the bottom surface of the solder bath 41, and ultrasonic waves are applied to the molten solder 43 near the silicon wafer 11.

A Pb-Sn eutectic solder containing 2% of Ag was used as the solder, and the solder bath temperature was maintained at 260 ° C. Moreover, the ultrasonic transducer 45 causes the molten solder 43 to have a frequency of 30 kHz and an output of 5
The ultrasonic wave of 0 W is applied, and the immersion time of the silicon wafer 11 is 3
Seconds. During this soldering operation, Ar gas was
Flowed at a flow rate of / min.

By this operation, solder bumps having a bump height of 15 μm as shown in FIG. 2 were formed. Furthermore, the bump height is set to 25 μm by immersing it in a general solder bath that does not apply ultrasonic waves for 2 seconds.
And

Next, the silicon wafer 11 was blade-diced into individual chips, and inner lead bonding was performed between the silicon wafer 11 and a TAB tape in the same manner as in Example 1, but no problem occurred.

Example 3 A silicon wafer 11 similar to that in Example 1 was prepared. In this case, Al-1% Si is used for the wiring / electrode, and two 500 μm square electrodes (contact pads) are formed on each chip formed on the silicon wafer 11.

First, the above-mentioned No. 9 etching solution (oleic acid 90-palmitic acid 7-butyric acid 3) was dropped onto the electrodes of this silicon wafer 11. A trowel-type ultrasonic horn was inserted into the dropped liquid, and ultrasonic waves were applied at 20 kHz and 20 W for 5 seconds.

Next, using a soldering iron 51 capable of applying ultrasonic waves as shown in FIG. 4, while the molten solder 52 is dropped on the electrodes of the silicon wafer 11, ultrasonic waves are applied to form solder bumps. did.

A Pb-Sn eutectic solder was used as the solder, and the solder temperature was maintained at 240 ° C. Further, ultrasonic waves having a frequency of 40 kHz and an output of 30 W were applied to the molten solder 52, and the soldering time was 5 seconds. Further, in this case, since the soldering area was large, it was not necessary to flow an inert gas around.

As a result of this operation, the bump height is 100μ as shown in Fig. 2.
m solder bumps 35 are formed.

Next, the silicon wafer 11 was blade-diced into individual chips, and inner lead bonding was performed between the silicon wafer 11 and a TAB tape in the same manner as in Example 1, but no problem occurred.

The ultrasonic soldering device used in the present invention is not limited to the one used in Examples 1 to 3 above, but may be the one shown in FIG. 5, for example. In the ultrasonic soldering device shown in FIG. 5, the solder bath 61 itself vibrates ultrasonically and applies ultrasonic waves to the molten solder 62.

Further, in the above-mentioned first to third embodiments, the case where the present invention is applied to the wireless bonding of the TAB method has been described, but the present invention is not limited to this, and the present invention is not limited to the flip chip method or
It is also applicable to other wireless bonding such as CCB method. In this case, first, as shown in FIG. 6A, an electrode 33 is formed on a silicon substrate 31 via an insulating film, the entire surface is covered with a passivation film 34, and then an opening is provided on the electrode 33. 6 and the electrode 72 is formed on the glass or ceramic substrate 71 as shown in FIG. 6 (b), the whole surface is covered with the insulating film 73, and then the electrode 72 is provided with the opening portion. The solder bumps 35 are formed on the electrodes 33 and 72 by the method described in the first to third embodiments. Next, as shown in FIG. 6 (c), the solder bumps 35 of both are bonded by thermal fusion.

Furthermore, in the above-mentioned Examples 1 to 3, the present invention has been described for forming solder bumps on an electrode containing an additive such as Si and Cu with Al as a main component, but the present invention is that the electrode is a metal such as tungsten or molybdenum. Alternatively, the same applies to silicides of these metals.

[Effects of the Invention] As described in detail above, according to the method for forming a solder bump of the present invention, the solder bump can be directly formed on the electrode by an extremely simple process without causing damage to the passivation film. This is an industrially extremely advantageous effect in that the bonding technology can be easily introduced, and a semiconductor device with high bonding reliability can be manufactured in accordance with the miniaturization of elements.

[Brief description of drawings]

FIG. 1 is an explanatory view showing a method for forming solder bumps in Embodiment 1 of the present invention, FIG. 2 is a sectional view of a silicon wafer on which solder bumps are formed by the method of the present invention, and FIG. 3 is an embodiment of the present invention. 2 is an explanatory view showing a method for forming solder bumps in FIG. 2, FIG. 4 is an explanatory view showing a method for forming solder bumps in Embodiment 3 of the present invention, and FIG. 5 is a method for forming solder bumps in another embodiment of the present invention. Explanatory drawing showing No. 6,
FIGS. 7A to 7C are sectional views showing a process of wireless bonding in another embodiment of the present invention, and FIG. 7 is a sectional view of a conventional silicon wafer on which solder bumps are formed. 11 …… Silicon wafer, 21 …… Solder tank, 22 …… Reflux path, 23 …… Melting solder, 24 …… Stirring bar, 25 …… Ultrasonic vibrator, 31 …… Silicon substrate, 32 …… Insulating film, 33 …… electrode, 34 …… passivation film, 35 …… solder bump,
41 …… Solder bath, 42 …… Reflux path, 44 …… Stirring bar, 45 ……
Ultrasonic transducer, 51 …… Soldering iron, 52 …… Melting solder,
61 …… Solder tank, 62 …… Melting solder, 71 …… Glass or ceramic substrate, 72 …… Electrode, 73 …… Insulation film.

Claims (6)

[Claims] 1. An electrode is exposed from an insulating film covering a substrate, and nitric acid, sulfuric acid, hydrochloric acid, phosphoric acid, oleic acid,
After contacting a solution containing at least one of palmitic acid, stearic acid, butyric acid, resin, sodium carbonate, zinc borofluoride, cadmium borofluoride, and hydrazine, contact the molten solder with at least the electrode part to form the molten solder. A method for forming solder bumps, characterized in that ultrasonic waves are applied to destroy a natural oxide film on the surface of an electrode and to selectively attach solder by alloying with an electrode. 2. The method for forming solder bumps according to claim 1, wherein the solution is brought into contact with the electrode portion of the substrate and ultrasonic waves are applied to the solution. 3. The method for forming solder bumps according to claim 1, wherein the substrate is immersed in a molten solder bath. 4. The method for forming solder bumps according to claim 1, wherein the substrate is a semiconductor substrate or a glass or ceramic substrate. 5. The method for forming solder bumps according to claim 1, wherein the electrode contains aluminum as a main component and the solder contains Sn. 6. The method for forming solder bumps according to claim 1, wherein the treatment is performed in a non-oxidizing atmosphere.