JPH0343788B2 - - Google Patents

Description

[Detailed Description of the Invention] (1) Technical Field of the Invention The present invention provides a fuse-type semiconductor memory device that includes:
In particular, the present invention relates to a fuse-type semiconductor memory device using a polysilicon film as a fuse material.

(2) Technical background Fuse-type semiconductor memory devices (hereinafter referred to as fuse)
(abbreviated as ROM) is a process in which electrical connection circuits are formed in a matrix in a semiconductor device and the circuits are opened by melting a predetermined part using excessive current caused by pulses, or by opening the circuit.
Alternatively, "1" and "0" can be stored by keeping it closed without blowing it out. For this reason, fuse ROMs can be constructed very easily, including those that use only ordinary fuses, those that use diodes as fuses, and those that use the emitter, base, and collector circuits of transistors as fuses. There is.

Because this fuse ROM has a simple structure, it has the advantage that it can be made into a semiconductor memory device with the same number of bits in an area several times smaller than other P-ROMs. A typical example using this fuse ROM is shown in Figure 1. X ₁ to _{X 4} are each bit line, Y ₁ to _{Y 4} are each word line 2,
Bit lines and word lines form a matrix, and their intersections are connected by fuses indicated by 3, 4, and 5. This fuse is usually one containing a high concentration of impurities, such as a polysilicon film, or one made of a metal material, a diode, a transistor, or the like.

For example, when bit line X ₁ and word line Y ₁ are connected by fuse 3, and bit line X ₂ and word line Y ₂ are connected by fuse 4, such a state means that the information is '', _on the other hand, an excessive current is applied by a pulse or the _like between the bit line ” should be written.

In this way, the fuse ROM has a structure that is extremely simple in principle as it simply fuses and electrically disconnects each position in the matrix array, and by assembling a large number of matrix arrays, it is possible to create a storage device with a large number of bits. can be made.

(3) Problems with the conventional technology Apart from the diodes and transistors that are built into semiconductor devices as currently used fuse materials, fuse ROM is used as a wiring material.
is mainly an aluminum film or a polysilicon film, but the aluminum film, which is generally used widely for wiring in semiconductor devices, has too good electrical conductivity and has the disadvantage that it is required to melt an excessively high pulse current. Furthermore, in order to reduce this current, it is necessary to make the shape of the fuse at the blown part sufficiently thin, especially the width, which leads to problems in forming fine patterns and improving photoetching accuracy. Another way to reduce the fusing current is to reduce the thickness of the aluminum film, but the film thickness required when forming the fuse shape is different from the film thickness required to make contact with external wiring. It is not possible to satisfy both requirements with the same film thickness. Also, even if the film thickness is changed, the number of steps will increase,
Problems such as the etching control method for both methods also arise, making it even more difficult to find a common point.

(4) Purpose of the Invention The present invention eliminates the above-mentioned drawbacks.The purpose of the present invention is to utilize the difference in thermal conductivity caused by the difference in thickness on the insulating film to blow the fuse material at the stepped portion by flowing. This aims to reduce the fusing current and the area occupied by the fuse ROM in the circuit pattern.

(5) Structure of the Invention In other words, the fuse ROM of the present invention includes an insulating film provided with a stepped portion, the insulating film being thicker on the upper side of the stepped portion than on the lower side; and a thick fuse that is disposed above the stepped portion and is blown by melting above the stepped portion.

(6) Embodiment of the Invention The present embodiment will be explained below with reference to FIG. 2. First, an initial oxide film 7 of about 1 μm is formed on a P-type (100) 10Ωm silicon wafer 6 at 1000° C. in WetO ₂ .
Next, open the window and gate oxide film 8 by HCl oxidation.
Attach 300 to 1000Å. Depressurize polysilicon film 9
Windows are opened and patterned to a film thickness of 4000 Å using the CVD method. This polysilicon film 9 and the source and drain 10 are formed by ion implantation.
For example, a predetermined concentration and depth is formed using 5×10 ¹⁵ doses of As and 150 Kev. Next, the entire surface is covered with a PSG film 11 to open the sort and drain electrode windows.

Next, the aluminum film 12 is patterned as a wiring material, and finally the PSG film 14 is covered as a film to complete the process. The fuse portion 13 to be blown at this time is covered with the last PSG film and becomes a fuse ROM, and the element on the left side of FIG. 1 shows an example of a selection transistor.

The above polysilicon film was created using the low pressure CVD method.
When the width W of the fuse part that is attached to a film thickness of 4000 Å and is to be blown is 2.5 μm and the length is 10 μm,
At this time, by passing a pulse current of 100 mW for 10 μs through the fuse of the polysilicon film, it is easily blown out and programming at an arbitrary position becomes possible.

Furthermore, the present invention is characterized in that the part of the fuse formed on the insulating film to be blown is divided into an upper part and a lower part of the insulating film, and the upper part of the insulating film (mainly SiO ₂ ) is thinner. Since the polysilicon layer is thicker, its thermal conductivity is relatively lower than that of the lower layer, so a method is used in which the viscosity of the polysilicon film increases its fluidity due to heat, moves in a flowing manner from the stepped portion toward the lower layer, and is fused. Furthermore, in the present invention, since the transparent PSG glass 14 is covered as a film on the top, the melted polysilicon film does not scatter to the surroundings and have no adverse effect.
The melted part becomes cloudy and can be seen visually, providing convenient conditions for visual inspection.

Fuses illustrating one embodiment of the invention shown in FIG.
Although the fuse material of the ROM is limited to a polysilicon film, as described above, the present invention can also be applied when the fuse portion to be blown is formed using a material that flows from the upper stage to the lower stage, for example, a metal material such as an aluminum film. It is a place that can be reached.

(7) Effects of the Invention As explained above, when the initial oxide film is a thick field oxide film and the gate oxide film is a thin oxide film, if the thermal conductivity of both is used to fuse the stepped portion, the difference in viscosity The power consumption is about half that of conventional methods, and the fuse is cut vertically.
It has the advantage of reducing area in pattern layouts such as ROM.

[Brief explanation of drawings]

Fig. 1 is a configuration diagram showing a typical storage device of fuse ROM, and Fig. 2 is a cross-sectional view showing the selection transistor on the left side of the dashed line in this embodiment and the fuse ROM connected to it on the right side. be. 1: Bit line, 2: Word line, 3, 4, 5; Fuse, 6: Silicon wafer, 7: Initial oxide film, 8: Gate oxide film, 9: Polysilicon film, 1
0; Gate, drain region, 11, 14; PSG
Membrane, 13; Fuse blown part.

Claims (1)

[Scope of Claims] 1. An insulating film provided with a stepped portion, the insulating film being thicker on the upper side of the stepped portion than on the lower side; 1. A fuse-type semiconductor memory device comprising a thick fuse that is blown at the top.