JPS63228738A - Formation of bump electrode - Google Patents

Abstract

PURPOSE:To prevent the short circuit of a bump electrode and an irregular shape, and to grow a bump layer in an excellent shape by previously removing a foundation layer except a bump electrode section and conforming the thickness of the foundation layer to a chemical etching rate. CONSTITUTION:A metallic thin-layer as a foundation layer 20 is applied onto an element substrate 10, and the thin-layer except a section to which a bump electrode 31 is formed is removed through chemical etching treatment. The thickness of a thin-film to which the composite foundation layer 20 is shaped is selected so as to acquire the relationship of the same magnitude in response to the relation-ship of the magnitude of chemical etching rates to each layer in a photo-etching process. A metallic layer 30 for the bump electrode is grown selectively onto the left layer 20. Consequently, even when the etching rates of each layer in the composite metallic thin-film as the foundation layer differ, the shapes of the end sections of all layers are arranged and all layers can be etched by one process. Accordingly, the short-circuit of the electrode is removed, and the electrode can be grown in an excellent shape.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for forming bump electrodes provided for connection with an external circuit in a semiconductor integrated circuit or the like called a flip chip, the method comprising: The present invention relates to a method of forming a bump electrode at a predetermined location on the top via a composite base layer made of a plurality of thin metal layers.

[Conventional technology]

The aforementioned bump electrodes are employed in various semiconductor devices for connection with external circuits. In particular, semiconductor integrated circuits usually have dozens of connection points with external circuits, so rather than connecting each connection point to the outside using wire bonding, it is better to configure it as a so-called flip chip in which bump electrodes are provided at each connection point in advance. However, if the electronic circuit device is connected to an external circuit in one step, the effort for assembling the electronic circuit device can be significantly reduced, and the area required for mounting the semiconductor element can also be significantly reduced. Figure 3 shows the main part of a conventional method for forming bump electrodes on a semiconductor element substrate. As shown in Figure (al), a wiring layer 12 made of aluminum or the like and an insulating layer 13 made of silicon nitride are layered on a semiconductor substrate 11 on which transistors and diodes are built, and a window made by photoetching is formed in this insulating layer 13. via separate wiring N12a
, 12b is provided with a bump layer 30 made of gold, m, solder, or the like so as to be in conductive contact with the bumps 30, 12b. However, as is well known, it is necessary to interpose a so-called base layer between the wiring layer 12 and the bump layer 30, so that the semiconductor substrate 11 degrees above the surface of the element substrate 10 comprising the wiring layer 12 and the insulating layer 13 is First, a base layer 20 is deposited by a vapor deposition method or a spackle method. This underlayer is usually a composite layer consisting of several thin metal layers,
Various metals are known as suitable materials for each metal thin layer, for example, TiN 11. PdJl12.
The Au layer 13 is sequentially deposited as a thin layer of 1<-> or less. Next, the entire upper surface of these metal thin layers is covered with a mask layer 51 made of photoresist, a window is formed by photoetching in the area where the bump layer is to be provided, and then the bump layer 30 is formed by electrolytic plating or the like so as to be in contact with the metal thin layer. grow selectively. This is the state shown in FIG. 3 (al).

Next, as shown in the figure), after removing the previous mask layer 51, a new mask layer 52 is applied using a photo-etching method so as to cover only the bump layer 30, and other areas are etched using a chemical etching process. The thin metal layers 11 to 13 are removed, and the mask layer 52 is also removed to obtain the state shown in FIG. If the bump layer 30 is made of gold or copper, it becomes a bump electrode as it is in the illustrated state, or if it is made of solder, it is further melted to form a bump electrode. In addition, in the above-mentioned etching process for composite metal thin layers, since the materials of each layer are different, different etching liquids are used to remove the thin layers of different metals, so it is customary to perform the etching process in three steps. .

[Problem that the invention seeks to solve]

By the way, unlike the thin metal layer for the base layer, the bump layer 30 described above needs to have a thickness of several tens of mm, so the thickness of the bump layer 30 shown in FIG.
It is easy for defects to occur in the photolithography process from the state shown in ) to the state shown in FIG. Bump layer 30 and composite metal thin layers 21 to 2 are coated with photoresist by spin coating.
The bump layer 3 is coated so as to cover the entire surface of the bump layer 3, and light is applied from above to expose the bump layer 3 in a predetermined pattern.
Since the thickness of the layer 52 is very large, it is difficult to uniformly expose the light during patterning. Therefore, even after the mask layer 52 on the thin composite metal layer is removed using a stripper liquid, the mask layer remains in the areas to be removed. In some cases, the mask layer may be removed in areas that should be left intact. If there is any residue left after removing the mask layer, the thin metal layer in that area will not be removed, resulting in a short circuit between the bump electrodes via the remaining thin metal layer, and conversely, if too much of the mask layer is removed, the thin metal layer will not be removed. If the bump layer is partially etched, reducing the size of the bump electrode, or if the shape of the mask layer around the base of the bump layer becomes irregular, the etching solution may be etched under the bump layer. Since it penetrates into the thin metal layer and reduces the adhesion strength of the bump electrode to the element substrate, problems such as falling off during subsequent assembly and poor contact during use are likely to occur. Furthermore, since the thickness of the mask layer tends to become thinner at the edge C of the bump layer shown in FIG. 3, the bump layer is also likely to be corroded by the etching solution in this case.

It is an object of the present invention to solve the problems of the prior art and to provide a method that can form bump electrodes reliably without fear of failure.

[Means for solving problems]

According to the present invention, the bump "I48i" is formed by a metal thin layer deposition process in which a plurality of metal thin layers are sequentially deposited in a predetermined order on an element substrate, and a composite metal thin layer deposited in this process. A photoetching process in which the remaining part of the gold r!I4 thin layer is removed by chemical etching, leaving only the base layer in the area where the bump electrode is to be formed in the layer plane, and the base masonry remaining after the photoetching process. The bump 1 scratching metal layer is selectively grown through a bump layer growth step, and the thickness of each thin metal layer constituting the composite base layer is determined according to the chemical etching rate for each layer in the photoetching step. This is achieved by selecting the same magnitude relationship accordingly.

[Effect]

The present invention focuses on the fact that the problem in the prior art is that a photolithography process is performed to remove unnecessary portions of the metal FI layer after forming a thick bump layer. The above-mentioned problem is basically solved by forming a bump layer after removing unnecessary portions of the bump layer. That is, in the metal thin layer deposition process mentioned above, after depositing a plurality of metal thin layers as base layers on the element substrate in a predetermined order, but before growing the bump layer as in the conventional case, In the next photo-etching step, unnecessary portions of the thin metal layer other than where the bump electrodes are provided are removed by chemical etching. Then, in the next bump layer growth step, a bump layer is selectively grown on the underlying layer left in the previous step, and the mask layer for selective growth at this time is also the same as the mask layer for selective etching in the previous step. Since all of them need only be provided on a surface with almost no difference in height, there is no risk of non-uniform exposure during the photolithography process as in the prior art, and accurate patterning can be achieved.

As described above, the method of the present invention basically overcomes the difficulties of the prior art by performing the step of growing a thick bump layer after removing the thin metal layer except for the underlying layer. As a result, it was found that there was another problem. This will be explained with reference to FIG. The structure of the element substrate 10 and the metal thin layers 111121 to 23 as base layers in FIG. 2 is the same as in FIG. As in the previous example, the gold V4FiI R as the base layer has a three-layer structure, but in this example, the bump layer will be grown in the subsequent bump layer growth process by electrolytic plating, so three layers will be used for the electrode at that time. The lowermost layer of TiFJ 21 was left unremoved in the previous photoetching process. The upper two layers removed in the previous process are the Pd layer 22 and the Au layer 23, and the etching solution used at that time is aqua regia, but as shown in the figure, the Pd layer 22 has a higher etching rate, so it is under-etched. Cut U has occurred. In the same figure (bl shows the state at the start of the bump layer growth process, as can be seen from the figure, the mask I?J41 was removed for removing the metal thin layer 22.23, and conversely, the metal thin layer 22.23 was removed.
Another mask layer 42 is provided covering the area where 3 was removed, ready for the growth of the bump layer 30 on top of the thin metal layer 22, 23 left as an underlying layer.

However, the eave portion of the Au layer 23 above the previous undercut U tends to bend or curl up as shown by B in the figure, and its tip may protrude above the surface of the mask layer 42. .

If such a protrusion exists, a false bump layer will grow as shown at 30a in the figure during electrolytic plating in the later bump layer growth process, which may cause the bump electrode to become irregular in shape and impair its function. be.

Although the depth of the upper undercut is 1μ or slightly more than that, if there is a dimensional difference between the Pd layer 22 and the Au layer 23 in the left-right direction in the figure, the thermal expansion coefficient of both layers will change. Since the thickness of the Au layer is different and the thickness of the Au layer is less than 1, bending B as shown in the figure is likely to occur when heated to around 100°C, such as during drying after cleaning.The cause of this problem is Pd1l122
Since there is a difference in etching rate between Au1W23 and Au1W23, both layers can be removed using different etching solutions with different selectivities. Selectivity is poor, and the etching rate for the Pd layer is inevitably higher.

It is also possible to change the combination of materials for the thin composite metal layer, but similar problems will occur even with other combinations of materials suitable for the underlayer.

Therefore, in the present invention, as a result of various experiments to solve this problem, the thickness of each thin metal layer constituting the composite underlayer is adjusted to have the same size relationship depending on the size relationship of the chemical etching rate for each layer in the photoetching process. In other words, in the case of the above example, the thickness of the Au11 layer, which has a smaller etching rate than the thickness of the Pd layer, which has a higher etching rate, is used in the present invention. The thickness of i is selected to be small. Experimental results show that when aqua regia is used as the etching solution, the ratio of the thickness of the Pd layer to the Au layer is between 10:1 and 40:1.

It is desirable to select a ratio of about 20:1. According to this ratio, the Au layer will be much thinner than the thickness conventionally used, but if the thickness is made too thin, it will not function as an Au layer. Since there may be cases where the number of people is lost, it is appropriate to set the value at the lower limit of 50 people.

Although such a means and the theoretical value for the above-mentioned desirable values are not necessarily certain, experiments and experience have shown that such a configuration can prevent the occurrence of undercuts U and bends B as shown in FIG. , and it becomes possible to simultaneously etch the two thin metal layers in one step using the same etching solution.

By combining this means with the above-described basic structure of the present invention, all of the above-mentioned intended problems can be solved.

〔Example〕

Hereinafter, the present invention will be explained in detail with reference to FIG.

The same parts as in FIGS. 2 and 3 before FIG.
It is assumed that the bump electrode has a three-layer structure including the layer 23, and that solder is used as the bump electrode material.

FIG. 1 (al) shows the state in which the base layer 20 has been deposited on the element substrate 1O by the metal thin layer deposition process, and the mask layer 41 has been further coated on the entire surface. The wiring layer 1 is made of aluminum containing a small amount of silicon on top of 11.
2, another wiring layer 12a is deposited in a predetermined pattern, and an insulating layer 13 made of silicon nitride or the like is deposited on the insulating layer 13 at a location where a bump electrode is to be provided.

It is assumed that a window leading to the upper surface of 12b is formed by photoetching. The Ti layer 21 , which is the lower two layers in the base layer 20 . The Pd layers 22 are sequentially deposited on the entire surface by, for example, a vapor deposition method to a thickness of about 0.5 mm, and then the top layer 41 layer 23 is deposited by a sputtering method to a thickness of approximately 1/20 of the Pd layer. . The mask layer 41 may be formed by spin-coding ordinary photoresist.

(in Figure 1) shows the state after the photo-etching process, and as shown in this example, the lowest rt layer in the underlayer is left without being etched away for use as an electric bridge during electrolytic plating later. After exposure by photolithography, the mask layer 41 applied after the previous step is removed with a stripper liquid, leaving only the portion where the bump electrodes will be provided, as shown in the figure. It is best to remove unnecessary parts of the Pd layer 22 and AuJi layer 23, which are the upper two layers of the underlayer, by using an aqua regia-based etching solution, for example, by dipping. In addition, it is better to use a mixed solution in which an appropriate organic acid is added to lower the etching rate and reduce the amount of side etching.This makes it easier to manage the etching time to about several tens of seconds, and also By using this type of mixed acid etching solution, the Ti layer can be left behind without removing much of it.

In addition, in order to prevent the mask layer 41 from being damaged and unnecessary etching of the base layer during this etching process, the mask layer is applied approximately 1.5 to 2 times thicker than the normal case. It is preferable to leave it at 0. This mask layer 4
1 is removed after the photo-etching process is completed, and is shown in the same figure (C1
Another mask 1142 is applied as shown in FIG.

Figure 1 (TC) shows the state after the bump layer growth process is completed.
The newly applied mask layer 42 is removed by photolithography only in the areas where the bump layer 30 is to be grown.
Using the Ti layer as one electrode, solder is plated to a thickness of about 40 nm as a bump JI30 by a normal electrolytic plating method. At this time, the problem Pd layer 2 surrounded by the small circle A in the figure
As shown in the enlarged figure (c1), the edges of both layers 2 and Au1i23 were etched in the previous process (they were etched together and there was no lifting or bending of Au1i3, The bump layer 30 is grown in the correct shape because it is sufficiently covered by the mask layer 42.

The main part of the method of the present invention ends with this bump layer growth step. After removing the mask layer 42 with a stripping liquid, the Ti layer 21 is selectively etched with weak hydrofluoric acid or the like having a high etching property against the Ti layer. All parts not covered by the upper two layers 22 and 23 are removed by etching. At this time, the Au layer 23
The Pd layer 22 serves as a mask layer, so to speak.
If the bump layer 30 is made of gold or copper, it can be used as it is as a hump electrode, but if it is made of solder as in this embodiment, it is melted by heating to form the state shown in FIG. 1 (dl).

Due to this melting, the bump layer 30 becomes a hump electrode 31 with a rounded tip as shown in the figure due to its surface tension.
At the same time, lower II! ! The three thin metal layers as a layer also partially dissolve into the solder and become integrated with the solder.

〔Effect of the invention〕

As is already clear from the above description, in the method of the present invention, before growing a thick bump layer, all of the thin composite metal layer as an underlayer is removed in the photo-etching process except for the areas where bump electrodes are to be provided. This eliminates the need to pattern photolithography on element substrates with highly protruding bump layers as in the past, and allows easy and practical photolithography patterning to accurately pattern unnecessary parts of the thin metal layer. can be removed. This eliminates problems such as mutual short circuits and irregular shapes of bump electrodes that have conventionally occurred. Furthermore, since the thickness of each thin metal layer constituting the composite base layer is selected to have the same size relationship according to the size relationship of the chemical etching L7-t for each layer in the photo-etching process, it is possible to Even if the etching rate is different for each layer of the composite metal ALL, it is possible to process and notch all the layers in one step using the same etching property and making the edge shapes uniform. As a result, the mask layer for the bump layer growth process is formed with a correct contour, so that the bump layer can be grown with a good shape.

[Brief explanation of drawings]

FIG. 1 is a partial cross-sectional view of a half-quad element showing each step of an embodiment of the method for forming a hump electrode according to the present invention, and FIG. 2 is a case in which the etching of a thin composite metal layer as a base layer for a bump electrode is irregular. FIG. 3 is a partial cross-sectional view of a semiconductor device showing the main steps of a conventional bump electrode forming method. In the figure, lO: element substrate, 11: semiconductor substrate, 12a, 12b
: wiring layer, 13: insulating layer, 20: base layer, 21: T
i layer, 22: Pd layer, 23: AuJ (5, 30; bump layer, 31: bump electrode, 41.42, 51, 52:
This is a mask layer. Figure 1

Claims (1)

[Scope of Claims] 1) A method for forming bump electrodes for connection with external circuits at predetermined locations on a semiconductor element substrate via a composite base layer made of a plurality of thin metal layers, the method comprising: A thin metal layer deposition process in which multiple thin metal layers are sequentially deposited in a predetermined order on top of the composite metal thin layer, and only the base layer is left in the area where the bump electrode is to be formed within the surface of the composite metal thin layer deposited in this process. A photo-etching step in which the remaining portion of the metal thin layer is removed by chemical etching treatment, and a bump layer growth step in which a bump electrode metal layer is selectively grown on the base layer left after the photo-etching step. Formation of a bump electrode characterized in that the thickness of each thin metal layer constituting the composite base layer is selected so as to have the same size relationship according to the size relationship of chemical etching rates for each layer in the photoetching process. Method. 2) In the method according to claim 1, the thin metal layer constituting the base layer includes a Pd layer and an Au layer deposited thereon, and the chemical etching process in the photoetching process is easy. A water-based etching solution is used, and Pd
A method for forming a bump electrode, characterized in that the ratio of the thickness of the layer to the Au layer is selected between 10:1 and 40:1. 3) A method for forming a bump electrode according to claim 2, characterized in that the ratio of the thicknesses of the Pd layer and the Au layer is approximately 20:1. 4) A method for forming a bump electrode according to claim 2, characterized in that the thickness of the Au layer is selected to be 50 Å or more. 5) A method for forming a bump electrode according to claim 2, wherein a mixed solution of nitric acid, hydrochloric acid, and an organic acid is used as the aqua regia-based etching solution.