The present invention is related to a semiconductor programmable device, and more specifically to a field programmable ROM (read only memory) or logic device.
There are several kinds of memory cells used for programmable read only memory (PROM) devices, such as fuse type, diode type and so on. For example, Japanese Patent 52-40540 (1977) by T. Wada et al. discloses a capacitor and diode type memory cell, which basically discriminates between a written or unwritten state by determining whether a shorted or isolated state of a thin dielectric film exists.
FIG. 1(a) shows a cross sectional structure of the capacitor and diode type memory cell. In the figure, 10 is a semiconductor substrate, 12 is a diffusion region, 14 is a dielectric layer, 16 is an electrode formed on it, 18 is contact region for the substrate and 20 is an electrode which contacts to region 18. The electrode 16, the dielectric layer 14 and the diffusion region 12 form a capacitor, and a diode is formed by the diffusion region 12 and the substrate 10. The equivalent circuit of such memory cell, therefore, is as shown in FIG. 1(b).
If a positive high voltage is applied to the electrode 20, and a negative high voltage is applied to the electrode 16, the dielectric layer beneath the electrode 16 is electrically broken down, and the memory cell is in a written state. So long as such voltage is not applied, the memory cell keeps the state of FIG. 1(b), corresponding to an unwritten state. The written and unwritten states correspond to data "1" and "0", and hence the memory cells functions as a programmable memory cell.
FIG. 2 is an example of a fuse and diode type memory cell disclosed in Japanese Patent 53-26462 (Priority in U.S. 1971, Ser. No. 181503) by S. A. Appas et al. In FIG. 2(a), which shows a cross sectional structure of a memory cell, 10 is an n-type semiconductor substrate, 12a and 12b are a p-type and n.sup.+ -type diffusion layers respectively, 14 is an insulation layer, and 22 and 24 are conductor layers. The thickness of the insulation layer 14 over the diffusion layer 12b is controlled to as to be thin. If a high voltage is applied a current flows in a diode formed by the diffusion layers 12a and 12b, the metal of the conductor layer 22 diffuses through a thin portion of the insulator layer 14, and forms a conductive filament 22a. An equivalent circuit of the memory cell of FIG. 2(a) is shown in FIG. 2(b). The conductive filament 22a becomes a fuse element, the conductor layer 24 becomes a word line, and the diffusion layer 12a becomes the bit line of the memory cell. The programming (writing) in the memory cell is done by applying a high voltage between the word line and bit line so that a high current flows through the conductive filament to burn it out.
FIG. 3 is a capacitor and diode type memory matrix disclosed in Japanese laid open Patent 53-132281 (1978) by K. Okada. FIG. 3(a) is a cross sectional view. In the figure, 10 is a p-type semiconductor substrate, 10a is an n-type epitaxial layer, 10b is an n.sup.+ -type buried layer, 10c is a p.sup.+ -type isolation layer, 12 is a p.sup.+ -type diffusion layer, 14a is an isolation layer and 16 is an electrode. In this configuration, the electrode 16, the isolation film 14a and the p.sup.+ -type diffusion layer 12 form a capacitor, and the diffusion layer 12 and the n-type epitaxial layer 10a form a diode. The capacitor and the diode compose a memory cell similar to that of FIG. 1. FIG. 3(b) is a memory matrix formed of such memory cells. The writing (programming) of the memory is done by applying a voltage between a bit line and a word line in a direction so that a current flows in the diode and burns out the insulation film 14a of the capacitor.
As has been described above, the capacitor and diode type PROM is programmed by short circuiting the insulation film of the capacitor, and a short circuit corresponds to "1" data. The reliability of the memory, therefore, depends on the stability of the broken down insulation films. FIG. 4 is an enlarged cross sectional view of an electrically broken down part of the insulation film. In the figure, 14 is the insulation layer (usually it is a silicon dioxide layer), and 26 and 28 are the electrodes. When a voltage V is applied between the electrodes, current I does not flow unless the voltage exceeds the breakdown voltage, and the resistance between the electrodes is substantially infinity.
But when a dielectric breakdown occurs, a large current I flows. FIG. 4 shows a state after a breakdown has occurred. The insulation layer 14 is bridged by a diffused region 14b of the electrode material which is surrounded by an oxidized region 14c, and the electrodes 26 and 28 are short circuited to each other. The oxidized region 14c is formed by an oxidation of the electrode metal by oxygen supplied from the insulation layer (usually it is an oxide layer). In such device, when the ambient temperature is increased, the oxidized region grows further and it eats away little by little the diffused region 14b of the electrode material. The resistance between the electrodes 26 and 28 increases gradually, and at last the electrodes are disconnected from each other. This is equivalent to the unwritten state.
FIG. 5 shows the variation of V-I characteristics of a memory cell. When the memory cell is programmed, or written, the resistance between the electrodes are small, and so the V-I curve of the memory cell is a straight line as shown the solid line. But, as time passes, the slope of the V-I curve shifts in the direction of the arrow and gradually becomes small as shown by the broken lines. This means an increase of the resistance of the memory cell or a decrease of the read out current and operation speed of the memory cell.
This defect may be avoided by selecting the material of the electrode which is not oxidized for the oxide insulation layer, or not nitrided for a nitride insulation layer. But in the present-state-of-the-art the material applicable to the electrode is limited to a small number of materials such as tungsten or molybdenum. The material is limited by the fabrication process of the semiconductor devices, such that it must withstand a temperature applied during the fabrication, it must have a good affinity to the insulation layer or to a wire bonding, and its patterning must be easy and so on.
Considering the affinity to the oxide insulation layer, for example, the electrode material must have a low free energy against oxidation. This means the material diffused into the insulation layer is oxidized, and the resistance of the diffused region is apt to vary its value, and hence, the reliability of the memory is not very good.