JP2899313B2 - Programmable element - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to electrically programmable devices incorporated in integrated circuits.

2. Description of the Related Art Among semiconductor integrated circuits, a so-called PROM (Programmable Programmable Read Only Memory) that allows a user to electrically write the contents after purchase.
Able ROM is widely used because a ROM (Read Only Memory) with desired contents can be obtained immediately.

Also, in the field of logic circuits, the contents can be electrically written after the user has purchased the so-called so-called logic circuit.
PLDs (Programmable Logic Devices) are used for similar purposes. In order to configure a PROM or PLD, it is necessary to use a programmable element that can electrically write stored contents from outside and retain the stored contents even when the power is turned off.

Conventionally, a structure suitable for such a programmable element has been described below, for example.

FIG. 2 is a sectional view showing the structure of a conventional programmable element, which will be described with reference to FIG.

As shown, a P-type semiconductor substrate 1 is separated by a field oxide film 2, and a diffusion layer 3 having a high impurity concentration serving as a lower electrode is formed in one separated region.

On the diffusion layer 3, an oxide layer 4 and an upper electrode 5 are sequentially laminated and formed. For programming, an appropriate voltage is applied between the upper electrode 5 and the diffusion layer 3, and This is done by breaking the insulation.

Problems to be Solved by the Invention In the above-mentioned conventional programmable element, in order to make the oxide layer 4 of high quality, that is, with few holes called pinholes and high withstand voltage, the diffusion layer 3 which is the base layer is required. Must be formed by thermal oxidation. Generally, when a diffusion layer containing a high-concentration impurity is thermally oxidized, the thickness of the oxide film formed varies depending on the impurity concentration, and a thicker oxide film is formed as the impurity concentration increases.

In the structure as shown in FIG. 2, the oxide layer 4 is in contact with the field oxide film 2. Generally, the diffusion layer 3 is formed using the field oxide film 2 as a mask. Therefore, at the end 6 of the field oxide film 2, a region 7 having a slightly lower impurity concentration is exposed due to lateral diffusion or receding due to etching of the field oxide film 2. doing. For the reasons described above, the thickness of the oxide layer 4 becomes thinner in this region than in other portions, and it is difficult to control the film thickness. For this reason, there is a problem that the withstand voltage of the programmable element is reduced and the variation thereof is increased, so that the writing characteristics are not stable.

Further, as shown in FIG.
In a structure that does not cover the entire area of the
There is also a problem that the oxide layer 4 exposed at the time of processing by etching and the diffusion layer 3 are damaged, and as a result, leakage current at the junction is likely to occur.

Means for Solving the Problems A programmable element according to the present invention for solving the above-mentioned problems has a lower half almost buried in a semiconductor substrate, and an end having inclined surfaces on both the upper surface and the lower surface. A lower isolation electrode formed of a high impurity concentration diffusion layer formed in the semiconductor substrate region in contact with the isolation oxide film; A program insulating film formed on a flat surface of the lower electrode separated from an end portion, and an oxide film formed by thermal oxidation on the lower electrode between the program insulating film and the isolation oxide film And an upper electrode formed so as to cover at least the entire surface of the program insulating film and drawn out onto the isolation oxide film via the oxide film, and an end portion of the diffusion layer serving as the lower electrode. Is from the end Both ends of the diffusion layer have a lower impurity concentration than the outside, and both ends of the diffusion layer are in contact with an inclined surface of a lower surface of an end portion of the isolation oxide film. The oxide film having a thickness greater than that of the film and having a breakdown voltage equal to or higher than a program voltage is formed so as to be continuous with the isolation oxide film.

In the programmable element of the present invention, the thickness of the insulating film to be programmed is substantially constant, so that the voltage required for dielectric breakdown is constant and stable writing characteristics can be obtained. Also, since there is no locally low withstand voltage region, the reliability of the unwritten element is high.

Embodiment An embodiment of the programmable element of the present invention is shown in FIG. 1 and will be described with reference to FIG.

As shown in the figure, a P-type silicon substrate 11 is separated into a plurality of regions by a field oxide film (hereinafter referred to as an isolation oxide film) 12 by selective oxidation, and an N ⁺ -type electrode serving as a lower electrode is formed in one of the separated regions. A diffusion layer 13 is formed.

The N ⁺ -type diffusion layer 13 has an oxide film 14 having an upper portion formed thermally.
Covered by The thickness of the oxide film 14 is the thickness of the isolation oxide film.
The thickness may be about 1/3 to 1/10 of the thickness of 12 and sufficiently withstand a voltage of 10 to 20 V used as a normal programming voltage, that is, 50 to 150 nm.

A part of the oxide film 14 is selectively removed, and a programming insulating film 15 is formed on the diffusion layer 13 in the removed region. The program insulating film 15 is connected to the oxide film 14 and further connected to the isolation oxide film 12. Therefore, the program insulating film 15 has a structure not in contact with the isolation oxide film 12. The insulating film 15 has a total thickness of 8 to 10 with the lower layer being a silicon oxide film and the upper layer being a silicon nitride film, for example.
If the thickness is 9 nm, the insulation can be broken in about 1 ms at a voltage of about 20 V.

Since the region having the insulating film 15 is not in contact with the isolation oxide film 12, and the impurity concentration of the diffusion layer 13 is substantially constant, the thickness of the insulating film 15 is also substantially constant. Thereby, stable writing characteristics can be obtained.

An upper electrode 16 is formed on the insulating film 15, and an end of the upper electrode 16 is on the oxide film 14. For this reason, the insulating film 15 and the diffusion layer 13 are not damaged even if a slight excessive etching is performed during the processing of the upper electrode 16.

In the above embodiment, the program insulating film 15 is a multilayer film for convenience of explanation, but it is not necessary to follow the configuration of the embodiment, and it may be a single layer film in which the diffusion layer 13 is thermally oxidized. .

Effect of the Invention In the programmable element according to the present invention, the thickness of the programmed insulating film is constant and is not locally thin, so that the writing characteristics and reliability are stable. As a result, a high-performance and high-reliability programmable integrated circuit is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view showing an embodiment of a programmable element of the present invention, and FIG. 2 is a sectional view showing a structure of a conventional programmable element. 11 ... substrate, 12 ... isolated oxide film, 13 ... N ⁺ type diffusion layer, 14
... oxide film, 15 ... insulating film for programming, 16 ... upper electrode.

Claims (1)

(57) [Claims] A semiconductor device according to claim 1, wherein the lower half is substantially buried in the semiconductor substrate, and
A thick isolation oxide film whose end portion has an inclined surface on both the upper surface and the lower surface and has a thinner thickness toward the tip portion; and a high impurity concentration formed in the semiconductor substrate region in contact with the isolation oxide film. A lower electrode formed of a diffusion layer, a program insulating film formed on a flat surface of the lower electrode separated from an end of the isolation oxide film, and a gap between the program insulating film and the isolation oxide film. An oxide film formed by thermal oxidation on the lower electrode, and an upper electrode formed so as to cover at least the entire surface of the program insulating film and drawn out on the isolation oxide film via the oxide film; An end portion of the diffusion layer serving as the lower electrode has a lower impurity concentration than the end portion, and both ends of the diffusion layer are in contact with an inclined surface of an end lower surface of the isolation oxide film. Low impurity concentration A programmable element, wherein the oxide film having a thickness larger than that of the program insulating film and having a breakdown voltage equal to or higher than a program voltage is formed on an end portion of the diffusion layer so as to be continuous with the isolation oxide film.