1. Field of the Invention
The present invention generally relates to a method of fabricating a semiconductor device and, more particularly, to a method of fabricating a semiconductor device having a Lead On Chip (LOC) construction.
Recently, in association with the high performance and high density of semiconductor devices, the chip size of a semiconductor device is increasing. Therefore, a significantly high level of fabrication technology is required in order to meet a demand for a small package size.
In this background, a semiconductor device having a Lead On Chip (LOC) construction is proposed. In a LOC semiconductor device, leads are provided on a semiconductor chip. Since such a construction allows the leads to overlap the semiconductor device in a top view, the size of a semiconductor device can be reduced.
In the semiconductor device having the LOC construction, the semiconductor chip is fixed on the leads using an adhesive. Thus, it is necessary to apply the adhesive to the leads with precision.
A second aspect of the semiconductor device having the LOC construction is that, as semiconductor devices are built with increasingly high density of constituting elements, increasingly minute formation of a circuit on the semiconductor chip is required. Accordingly, that area (hereinafter, referred to as a circuit area) on the semiconductor chip on which a circuit is formed is exposed to effects such as an external stress.
this reason, a protective film is formed on a circuit area of the semiconductor chip. For proper protection of the circuit area, a high-quality protective film forming technology is required.
2. Description of the Related Art
Conventionally, the fabrication method described below has been employed to manufacture a semiconductor device having the LOC construction.
First, a lead frame having leads is produced by blanking such that a flat base formed of a material suitable for a lead frame is cut appropriately. Subsequently, a non-conductive adhesive which also serves as a protective film is provided on leads formed in the lead frame. The semiconductor chip is secured on the leads using the non-conductive adhesive. Wiring is then done according to a predetermined design, a package is produced and the semiconductor chip is sealed. This completes a process for fabricating a semiconductor device.
Two methods described below have generally been used to provide the non-conductive adhesive on the leads.
One of the methods is film pasting. FIG. 1A shows a sheet of adhesive film 1 (protective film) formed of a non-conductive adhesive. As shown in FIG. 1B, the adhesive film 1 is die-cut to a predetermined size that corresponds to an area of a lead frame.
In a process separate from the process of cutting the adhesive film 1, a lead frame 2 having leads 3 is produced by blanking such that a base is cut appropriately. As shown in FIG. 2, the cut adhesive film 1 is fitted adhesively on the leads 3 of the lead frame 2. As a result of the process described above, the adhesive film 1 is properly provided on the leads 3.
The other of the methods that have generally been used to provide the non-conductive adhesive on the leads is printing. In the printing of the non-conductive adhesive, the lead frame 2 having the leads 3 is first produced by blanking such that a base is cut to an appropriate size. Subsequently, a mask 4 is provided on the lead frame 2 having the leads 3, whereupon, as shown in FIG. 3, a non-conductive adhesive 6 (hereinafter, simply referred to as an adhesive) which also serves as a protective film is printed on the lead frame 2 using a squeegee 5.
As shown in FIG. 4, openings 7 are formed in the mask 4 at positions that correspond to the positions of the leads 3. Accordingly, as shown in FIG. 5, by printing the adhesive 6 using the mask 4, the adhesive 6 is properly provided on the leads 3.
While FIGS. 1 through 5 show the non-conductive adhesive 6 provided on the leads 3, it is also possible to provide the non-conductive adhesive (protective film) 6 on the semiconductor chip.
FIGS. 6 and 7 show how a protective film 6b is provided on a semiconductor chip 8; and FIG. 8 shows a semiconductor device 9 fabricated by the method of FIGS. 6 and 7. In FIGS. 6 through 8, those components that correspond to the components of FIGS. 1 through 5 are designated by the same reference numerals.
The printing method as described with reference to FIG. 3 is employed in order to provide the protective film 6b on the semiconductor chip 8. In this printing method, the mask 4 is provided on the semiconductor chip 8 (which may be in a wafer state). Subsequently, as shown in FIG. 6, a protective film material 6a which forms the protective film 6b is printed on the semiconductor chip 8 using the squeegee 5.
The openings 7 are formed on the mask 4 at positions that correspond to a circuit area formed on the upper surface of the semiconductor chip 8. Therefore, as shown in FIG. 7, by printing the protective film material 6a using the mask 4 and then causing the protective film material 6a to be hardened, the protective film 6b is formed on the circuit area on the semiconductor chip 8.
As shown in FIG. 8, the semiconductor device 9 fabricated by the method of FIGS. 6 and 7 is constructed such that the semiconductor chip 8 is encapsulated in a resin package 9a. Generally, a filler is mixed in the resin package 9a.
If the protective film 6b is not formed on the semiconductor chip 8, the filler collides against the circuit area on the semiconductor chip 8, when the package is formed, thus causing a damage on the circuit formed by the microfabrication technology. Provision of the protective film 6b ensures that the circuit area on the semiconductor chip 8 is protected and prevents an unfavorable effect caused by the filler from occurring.
However, in the film pasting described above, a plurality of dies for die-cutting the adhesive film 1 are required because the adhesive films 1 having different shapes should be prepared for different lead patterns formed in the lead frame. Therefore, the film pasting method has a drawback in that it is not a general-purpose method and the cost of dies is relatively high.
When the adhesive film 1 is die-cut, waste adhesive films are produced however efficient the die-cutting process may be. The adhesive film 1 is formed of a polyimide and is relatively expensive. Therefore, wasting of the adhesive film 1 causes the cost of the product to increase.
In the printing method, the mask 4 is required for each of the lead patterns. Further, the printing method has a drawback in that, in case the lead pattern is complex or has a small pitch, bridging of the adhesive 6 or invasion thereof into the back of a lead may occur, thus preventing a subsequent wiring process and a package forming process from being performed properly.
Still another problem with the conventional printing method is that it is necessary to apply a pressure on the squeegee 5 while it is being moved on the mask 4. A pressure from the squeegee 5 is transmitted to the semiconductor chip 8. Therefore, the circuit area on the semiconductor chip 8 is scraped by the squeegee 5 via the mask 4, thus causing a stress to be built up in the circuit area. As a result, the circuit surface may be damaged even before the non-conductive adhesive 6 or the protective film 6b is formed.
Another problem with the semiconductor device having the LOC construction fabricated by the conventional method is that, since the leads 3 are provided above the circuit area, the leads 3 may come in contact with the circuit area when the leads 3 are joined with the semiconductor chip 8, thus causing a damage on the circuit area.