JPH0442960A - Programmable element of semiconductor integrated circuit - Google Patents

Abstract

PURPOSE:To shorten a delay time without changing a writing voltage and to make it possible to form a high-performance programmable integrated circuit capable of performing a high-speed operation by a method wherein a projecting part is formed on the surface, which comes into contact to an insulating film, of an electrode which is used as a cathode among lower and upper electrodes. CONSTITUTION:A P-type silicon substrate 1 is isolated into a plurality of regions by a field oxide film 2 and an n<+> diffused layer 3 which is a lower electrode to be used as a cathode is formed in one region among the regions. A projecting part 4 is formed on the surface of the layer 3 and the upper part of the projecting part 4 is covered with an oxide film 5. An upper electrode 6 consisting of a polycrystalline silicon film is formed on the film 5. The film thickness of the film 5 is 30 nm or thereabouts, for example, but as the projecting part is formed by a breakdown strength t the time of application of a negative voltage to the lower electrode 3, the film thickness varies according to the shape of the projecting part or the like, but is reduced to 1/3 or thereabouts of 30 nm. Accordingly, a program can be written in a voltage of a degree identical with that to a film of a film thickness of 10 nm or thereabouts on a flat part.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to electrically programmable elements incorporated in semiconductor integrated circuits.

Among conventional semiconductor integrated circuits, so-called F ROM (PROM) can be electrically written with contents after the user purchases them.
rogrammable RO) [) is the RO with the desired content
M (Read Only Memory) is widely used because it can be obtained immediately.

Also, in the field of logic circuits, so-called P-type circuits that allow the user to electrically write the contents after purchasing the
L D (Programmable Logic
Device) is used for similar purposes. P
In order to configure an RCM or a PLD, it is necessary to use a programmable element that allows storage contents to be written electrically from the outside and that retains the storage contents even when the power is turned off.

Conventionally, a structure suitable for such a programmable element is as shown below, for example.

FIG. 2 is a sectional view showing the structure of a conventional programmable element.

As shown in the figure, a P-type semiconductor substrate 1 is separated by a field oxide film 12, and a highly impurity-concentrated diffusion layer 13 serving as a lower electrode is formed in one separated region.

An oxide layer 14 with a thickness of about 1 Onm and an upper electrode 15 made of polycrystalline silicon are sequentially laminated on the diffusion layer 13, and programming is performed by applying a voltage of about 20V between the upper electrode 15 and the diffusion layer 13. Apply voltage, oxide layer 1
This is done by breaking the insulation of 4 and making the upper electrode 15 and the diffusion layer 13 electrically conductive.

Problems to be Solved by the Invention In general, in conventional programmable elements such as those described above, in order to lower the voltage applied during programming, a thin insulating film is required, which increases the capacitance of the programmable element portion. As a result, a problem arises in that the delay time becomes longer and the operating speed becomes slower.

Means for Solving the Problems The programmable element of the present invention for solving the above-mentioned problems is formed in a semiconductor substrate of one conductivity type divided into a plurality of regions by a thick isolation oxide layer. The lower electrode includes a diffusion layer having a high impurity concentration and an opposite conductivity type, an insulating film formed on the lower electrode, and an upper electrode formed on the insulating film, and the lower electrode or the upper Among the electrodes, the electrode used as a cathode has a structure in which a convex portion is formed on the surface in contact with the insulating film.

Operation In the programmable element of the present invention, since the cathode side electrode in contact with the insulating film to be programmed has a convex portion, electric field concentration occurs when a voltage is applied between the upper and lower electrodes. Therefore, electrons are easily released from the cathode into the insulating film, and even a thick film can be programmed with a low voltage. In other words, under the same write voltage, the thickness of the insulating film can be increased, and as a result, the insulating capacitance can be reduced. Therefore, the delay time becomes shorter and the operating speed becomes faster.

Embodiment An embodiment of the programmable element of the present invention is shown in FIG.
This will be explained with reference to this.

As shown in FIG. 1, a P-type silicon substrate 1 is separated into a plurality of regions by a field oxide film 2, and an N° diffusion layer 3, which is a lower electrode used as a cathode, is formed in the − region. There is. A convex portion 4 is formed on the surface of the N-type scattering layer 3 by photoetching, and the upper portion thereof is covered with an oxide film 5. An upper electrode 6 made of polycrystalline/licon is formed on the oxide film 5. The thickness of the oxide film 5 is, for example, about 3 Onm, or the withstand voltage when a negative voltage is applied to the lower electrode 3 is about 1/3 due to the formation of the convex part, although it varies depending on the shape and form of the convex part. Therefore, programming can be performed with the same voltage as a film of about 1 On.m on a flat part. This protrusion has almost no relation to capacitance, and the capacitance of the programmable element is about 1/3 of the capacitance without the protrusion. In addition, withstand voltage in the reverse direction, concentration of electric field does not affect electron emission, so
The withstand voltage is about the same as when there is no convex part.

In the above embodiment, the N'' diffusion layer 3 was used as a cathode for convenience of explanation, but it is not necessary to follow the structure of the embodiment, and a convex portion may be provided at the bottom of the upper electrode 6 to serve as a cathode.

Effects of the Invention In the programmable element of the present invention, as a result of forming a convex portion on the surface of the cathode that is in contact with the insulating film to be programmed, the dielectric strength is maintained at the same level as when there is no convex portion, but the thickness of the insulating film is reduced. It is possible to make it thicker. Therefore, the insulating capacitance is correspondingly smaller than that without the convex portion, and the delay time can be shortened without changing the write voltage.

The result is therefore a high performance programmable integrated circuit with high speed operation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of a programmable element of a semiconductor integrated circuit according to the present invention, and FIG. 2 is a sectional view of a conventional programmable element. DESCRIPTION OF SYMBOLS 1...Substrate, 2...Field oxide film, 3...N° type scattering layer 4...Protrusion, 5
... Programming oxide film, 6 ... Upper electrode.

Claims (1)

[Claims] a lower electrode formed in a semiconductor substrate of one conductivity type divided into a plurality of regions by a thick isolation oxide layer, and comprising a diffusion layer having a higher impurity concentration than the substrate and of an opposite conductivity type; and a lower electrode formed on the lower electrode. and an upper electrode formed on the insulating film, and a convex portion is formed on a surface of the lower electrode or the upper electrode used as a cathode in contact with the insulating film. Characteristic programmable elements of semiconductor integrated circuits.