Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Non-single crystal – or recrystallized – active junction...
Reexamination Certificate
1997-05-30
2001-09-18
Lee, Eddie (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Non-single crystal, or recrystallized, active junction...
C257S209000
Reexamination Certificate
active
06291836
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a programmable, non-volatile memory device (PROM), comprising a system of programmable non-volatile memory cells arranged in a matrix of rows and columns and provided with a first set of selection lines parallel to the columns and with a second set of selection lines parallel to the rows, a memory cell being associated with each point of intersection between the selection lines. The invention also relates to a method of manufacturing such a device.
Programmable semiconductor memories or PROMs are known in various shapes. One of the earliest PROM types made use of fuses, where programming of a selected cell implies that the connection between a word line and a bit lines is broken in that the fuse is melted. These memories may be readily manufactured by generally known i.c. techniques, but they have the disadvantage that information once written cannot be erased any more. This means that a new chip is to be used for writing new data. In addition, the cells themselves cannot be tested during production and it is necessary to provide extra test cells on the chip which cannot be utilized for the memory. Another type of programmable memories is known under names such as EEPROM, EPROM, Flash EPROM. Each memory cell here comprises an MOS transistor with floating gate. The information is written in the form of electric charge at the floating gate and thus determines the threshold voltage of the transistor. These memories are erasable in principle, which means that separate test cells are unnecessary. A disadvantage is that the memory cells are comparatively large, which renders it difficult to manufacture memories with a very large number of bits. A third type of programmable memories, also erasable, is based on a resistance change in materials upon the transition between the crystalline and the non-crystalline state. Such memories are known inter alia under the abbreviated designation MIM (Metal-Insulator-Metal). These memories require for each cell besides the MIM element also a selection element such as a transistor or a diode. In addition, a chalcogenide material which does not form part of standard silicon processes is often used for the switchable element.
SUMMARY OF THE INVENTION
The invention has for its object inter alia to provide a programmable semiconductor memory which is erasable and which has a very high density. The invention further has for its object to provide such a memory which can be manufactured by silicon techniques which are known per se.
A programmable non-volatile memory device of the kind described in the opening paragraph, according to the invention, is characterized in that each memory cell is exclusively formed by a diode whose anode and cathode are each conductively connected to a selection line, at least one of the anode and cathode regions comprising a layer of hydrogenated, silicon-containing amorphous semiconductor material. Experiments have shown that it is possible, for example in a rectifying junction in hydrogenated amorphous silicon, for the current to be changed in the forward direction in that a large current is passed across the junction during a short time. The current in the forward direction is found to be very strongly reduced then compared with a non-stressed diode. It is not clear at this moment what the physical background of this effect is. Probably degradation occurs in the material owing to the generation of additional states within the forbidden band. These states can be eliminated again through heating. Each diode in a matrix of diodes may or may not be programmed by means of current, depending on the information to be written, corresponding to a “1” or a “0”. In contrast to a fuse memory, no separate selection element is necessary now for each cell. In addition, each diode can be returned to its original state again through a heating step in which the degradation in the semiconductor material is eliminated. This renders it possible to test each cell itself after production, and separate test cells are unnecessary.
An important embodiment of a device according to the invention with which a memory of maximum density can be obtained is characterized in that the layer of semiconductor material forms a stack with the selection lines at the area of the intersection between the selection lines and is connected at the upper side to one of the selection lines and at the lower side to the other selection line which crosses the former selection line.
In a simple embodiment, the diode is formed by a p-i-n diode, the letter i denoting “intrinsic” here, which means in practice a semiconductor material which is not purposely n-type or p-type doped. Materials may be used for the conductor tracks here which form an ohmic connection with the n-type and p-type zones of the p-i-n diodes.
A further embodiment of a device according to the invention is characterized in that at least one of the selection lines is formed by a metal track, and in that the diode is a Schottky diode arranged between this metal track and the layer of semiconductor material. It was found in practice that a Schottky diode has the advantage that mainly the forward characteristic of the diode changes, whereas the current in the reverse direction does not change, or only slightly, which has advantages in reading. It is possible to use for the metal track, for example, a metal from the group: Mo, W, TiW, Pt, and Cr., which form good rectifying junctions with intrinsic &agr;Si:H.
Various hydrogenated Si compounds may be taken for the amorphous semiconductor material, such as SiGe, SiC, or SiN. A simple embodiment is characterized in that the layer of semiconductor material is formed by a layer of hydrogenated amorphous silicon.
The properties of the amorphous semiconductor material, and thus of the diode to be formed, depend strongly on the circumstances under which the material is formed, in particular on the degree to which the dangling bonds are bound to hydrogen. According to the invention, a method of manufacturing a device of the kind described above is characterized in that the layer of amorphous semiconductor material is formed by means of PECVD (Plasma Enhanced CVD) at a temperature of at most 400° C., and preferably at a temperature of at most approximately 250° C. It was found that a suitable semiconductor material with a high concentration of hydrogen atoms bound to dangling bonds can be manufactured in this manner.
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“Resistless High Resolution Optical Lithography on Silicon”, by N. Kramer et al, 1995 American Institute of Physics, Appl. Phys. Lett. 67 (20), Nov. 13, 1995, pp. 2989-2991.
Kramer Niels
Lodders Wilhelmus H. M.
Niesten Maarten J. H.
Oversluizen Gerrit
Baumeister Bradley WM.
Biren Steven R.
Lee Eddie
U. S. Philips Corporation
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