# Patent

## Title
Semiconductor device of a charge storage type

## Abstract

According to the present invention, a gettering layer is deposited both on the side surfaces and the bottom surface of a semiconductor chip. The semiconductor chip is then mounted on the board of a package so that a Schottky barrier is formed on the bottom surface. With this structure, metal ions that pass through the board of the package can be captured by the defect layer deposited on the side surfaces and/or the bottom surface of the semiconductor chip, and by the Schottky barrier.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device, and particularly to a thin-film semiconductor chip.
2. Related Background Art
Currently, as the functions provided for cellular phones, digital AV apparatuses and IC cards continue to be improved, there is a corresponding increase in the need for highly integrated, thin and compact chips that can be mounted in these devices. Thin chips are also used in the fabrication of an SiP (System in Package) or an MCP (Multi Chip Package), for which a plurality of semiconductor chips are mounted, at high density and in multiple layers, in a single package. Hence, to satisfy the demand for thin film semiconductors, the manufacture of thin chips is required.
An example technique whereby the bottom surface of a silicon substrate is ground to obtain a thin semiconductor chip is disclosed in Japanese Patent Laid-Open Publication No. Hei 7-302769.
The following manufacturing method is disclosed in this publication.
A diffusion layer and an electrode are formed on the top surface of a silicon substrate, and while the bottom surface is ground using a grindstone, a crystal defect layer is introduced. Titanium, Monel and silver are deposited, in the named order, on the bottom surface, to form the crystal defect layer and obtain an ohmic electrode. As the objective of this publication, an ohmic electrode is obtained by using a grindstone having a large grain size to intentionally introduce a crystal defect.
Generally, a semiconductor chip is sealed using a resin, and is attached, at its bottom surface, to an assembly board. According to the above described manufacturing method, since, a crystal defect layer is deposited on the bottom surface of the semiconductor chip, an ionized contaminant from the assembly board can be prevented from entering the semiconductor chip through its bottom surface. However, a contaminant that has passed through the resin, can not be prevented from entering the semiconductor chip at a side surface.
SUMMARY OF THE INVENTION
It is, therefore, one objective of the present invention to provide means for at least preventing a contaminant from entering a thin-film semiconductor at a side surface and, more preferably, for also preventing the contaminant from entering through the bottom surface of the chip.
For a semiconductor device according to the present invention, a defect layer for gettering a contaminant is formed on the side surfaces of a semiconductor chip.
More preferably, a defect layer for gettering a contaminant is formed on the bottom surface and the side surfaces of a semiconductor chip.
As described above, since the defect layer is at least deposited on the side surfaces of the semiconductor chip, a contaminant passed through an assembly apparatus or another member can be prevented from entering the semiconductor chip at its side surfaces, or at the bottom and the side surfaces, of the chip.
According to the present invention, as described above, a semiconductor chip can be protected from a contaminant, such as metal ions, during the assembly processing.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing a semiconductor chip according to a first embodiment of the present invention;
FIGS. 2A and 2B are diagrams showing a method for manufacturing a package that employs the semiconductor chip of the first embodiment;
FIG. 3 is a diagram showing the continuation of the manufacturing method shown in FIG. 2;
FIG. 4 is a diagram showing a modification of the first embodiment; and
FIG. 5 is a diagram showing a semiconductor device according to a second embodiment of the present invention.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The preferred embodiments of the present invention will now be described in detail while referring to the accompanying drawings, so as to more clearly define the above objective and other objectives, the features and the effects of the present invention.
FIG. 1 is a diagram showing a semiconductor chip according to a first embodiment of the present invention.
A crystal defect layer 2, for gettering a contaminant of metal ions, such as copper ions, is deposited on the side and the bottom surfaces of a semiconductor chip 1.
The presence of the crystal defect layer 2 prevents a contaminant from entering the semiconductor chip 1 through its side surfaces and its bottom surface.
FIGS. 2A, 2B and 3 are diagrams showing a method for manufacturing a semiconductor device according to the present invention. The manufacturing method will now be explained.
As shown in FIG. 2A, a diffusion layer, a source and gate electrodes and a circuit (none of them shown), which are components of a nonvolatile memory cell, are deposited on the top surface of an 800 μm thick semiconductor substrate 3 made, for example, of silicon. Then, a passivation film 4, made of SiN or SiON, for example, is deposited on the top surface of the resultant substrate 3. Since a fine nitride film is employed as the passivation film 4, a heavy element, such as a copper ion, can not pass through it. Next, the bottom surface of the semiconductor substrate 3 is mechanically ground, using a grindstone, until the thickness thereof is, for example, 110 to 120 μm. A grindstone should be selected that has a grain size large enough to form a crystal defect layer 2 on the bottom surface of the semiconductor substrate 3. The crystal defect layer 2 is a gettering layer for capturing a contaminant, such as a copper ion. Thereafter, the semiconductor substrate 3 is cut, using a blade cutter or a laser cutter, to separate the semiconductor chip 1, and the side surfaces of the chip are ground, using the grindstone, to obtain the crystal defect layer 2 for the side surfaces (see FIG. 2B). It should be noted that so long as a defect layer that can capture a contaminant has already been obtained for the side surfaces of the semiconductor chip 1 when the semiconductor substrate 3 is cut using the blade cutter, the mechanical grinding process may not be performed. Further, a blade used for grinding the bottom surface and a blade used for cutting out the semiconductor chip 1 may have the same sharpness, and a blade #2000, for example, is employed. In this case, the side and the bottom surfaces have about the same roughness. This processing is employed to manufacture the semiconductor chip 1.
As is shown in FIG. 3, the semiconductor chip 1 is mounted on the top surface of an assembly board 6 using mounting tape 11. Then, a pad 8 on the semiconductor chip 1 and a pad 9 on the assembly board 6 are electrically connected by a wire 10. And thereafter, a sealing resin 5 is applied to seal the semiconductor chip 1. Finally, soldering balls 7 are attached to the bottom surface of the assembly board 6. This processing is employed to obtain a BGA (Ball Grid Array) package.
The assembly board 6 is a printed board fabricated by gluing a copper foil to an insulating plate composed of a non-conductive material. The printed wiring for the assembly board 6 is then formed by patterning the copper foil. Thereafter a solder resist coating is applied to all portions of the top surface of the printed board 6 other than the printed wiring to which the wire 10 is soldered. Since copper is diffused in the solder resist, the copper ions in the solder resist are diffused during the assembly process shown in FIGS. 2 and 3. According to the invention, however, since the crystal defect layer 2 has been deposited on the bottom surface of the semiconductor chip 1, the copper ions are captured by the crystal defect layer 2 and are prevented from entering the semiconductor chip 1. And although the copper ions are diffused throughout the sealing resin 5 and reach the side surfaces of the semiconductor chip 1, they are absorbed by the defect layer deposited on the side surfaces of the semiconductor chip 1 and are prevented from entering the semiconductor chip 1. Further, since the passivation film 4 has been formed on the top surface of the semiconductor chip 1, the entry of copper ions through the top surface can also be prevented.
According to this embodiment, an explanation has been given for a non-volatile memory; however, the present invention is effective for other charge storage memories. That is, it is possible to prevent the occurrence of a charge storage failure caused by a contaminant, such as copper ions, that deteriorates the charge storage characteristics. An example charge storage memory can be a flash memory or a DRAM memory.
FIG. 4 is a diagram showing a modification of the first embodiment wherein semiconductor chips of the invention are laminated to constitute an MCP (a Multi Chip Package). This package is an assembly composed of semiconductor chips, such as a flash memory, an SRAM and a DRAM. A semiconductor chip 1, for example, is a flash memory, and a semiconductor chip 12 is a DRAM. It should be noted that mounting tape 11 is not shown in FIG. 4.
FIG. 5 is a diagram showing a semiconductor device according to a second embodiment of the present invention.
A semiconductor chip 1 is mounted on an assembly board 6 using a mounting member 12 that contains silver, so that the entire bottom surface of the semiconductor chip 1 is covered with the mounting member 12.
By employing the silver-containing mounting member 12, a Schottky barrier is formed between the entire bottom surface of a semiconductor device 3 and the mounting member 12. And since copper ions are very heavy, so long as the Schottky barrier is present, copper ions from the assembly board 6 can not pass the barrier and enter the semiconductor chip 1.
A manufacturing method used to produce this semiconductor device is as follows.
The semiconductor chip 1 is securely mounted on the assembly board 6 by using the silver-containing mounting member 12, so that the entire bottom surface of the chip 1 is covered with the mounting member 12. Since the other processes are the same as those in FIGS. 2 and 3, no further explanation for them will be given.
The present invention is not limited to these embodiments, and it is obvious that the embodiments can be variously modified within the scope of the technical idea of the invention. For example, a semiconductor device employing both the first and the second embodiments can also be provided. That is, a semiconductor device is also available wherein a crystal defect layer is deposited on the bottom surface of the semiconductor chip shown in FIG. 5, and the Schottky barrier is positioned between the defect layer and the assembly board. Furthermore, although in the second embodiment a mounting member containing silver is employed to form the Schottky barrier, so long as a Schottky barrier can be formed, another metal may be employed.

## Claims

1. A semiconductor device of a charge storage type comprising:
a semiconductor chip; and
a gettering layer formed on the side surface and the bottom surface of said semiconductor chip;
wherein said gettering layer is a crystal defect layer.

2. The semiconductor device according to claim 1, further comprising an assembly board which is fixed to the bottom surface of the semiconductor chip.

3. The semiconductor device according to claim 1, said device further comprising a passivation film covering the top surface of said semiconductor chip.

4. The semiconductor device according to claim 1, wherein said getting layer is adapted to capture a contaminant.

5. A semiconductor device comprising:
a semiconductor chip; and
an assembly board,
wherein the semiconductor chip is mounted on the assembly board so as to at least form a Schottky barrier across the entire bottom surface of the semiconductor chip.

6. A semiconductor device of a charge storage type comprising:
a semiconductor chip; and
a gettering layer formed on the side surface and the bottom surface of said semiconductor chip;
further comprising an assembly board which is fixed to the bottom surface of the semiconductor chip,
wherein said assembly board includes copper and said getting layer and a passivation film covering the top surface of said semiconductor chip are provided to prevent said copper of said assembly from diffusing to said semiconductor chip.

7. A semiconductor device comprising:
a semiconductor chip;
a gettering layer formed on the side surfaces and the bottom surface of the semiconductor chip;
a resin layer that covers the semiconductor chip; and
an assembly board provided for the bottom surface of the semiconductor chip by using a mounting member.

8. The semiconductor device according to claim 7, said device further comprising a passivation film covering the top surface of said semiconductor chip.

9. The semiconductor device according to claim 7, wherein said assembly board includes copper and said gettering layer and a passivation film covering the top surface of said semiconductor chip are provided to prevent said copper of said assembly from diffusing to said semiconductor chip through said resin layer.

10. The semiconductor device according to claim 7, said semiconductor device is a multi chip package.

11. The semiconductor device according to claim 7, said semiconductor device is a system in package.

12. The semiconductor device according to claim 7, said device further comprising a second semiconductor chip covered with said resin layer, said second semiconductor chip having gettering layers on the side and bottom surfaces thereof.

13. The semiconductor device according to claim 12, wherein said second semiconductor chip includes a DRAM.

14. The semiconductor device according to claim 12, wherein said second semiconductor chip includes a Flash memory.

15. The semiconductor device according to claim 12, said semiconductor device is a system in package.

16. The semiconductor device according to claim 12, said semiconductor device is a multi chip package.

17. The semiconductor device according to claim 16, further comprising a second passivation film covering the top surface of said second semiconductor chip.