Threshold and memory switching devices containing a body of chalcogenide material were invented by S. R. Ovshinsky at Energy Conversion Devices (ECD) and used since the 1960's. Such switching devices typically included a pair of spacedly disposed electrodes with an active chalcogenide material therebetween. The type of switching device thereby formed (i.e., threshold or memory) depended upon the particular composition of chalcogenide material used.
The basic structural and operational characteristics for threshold and memory devices are set forth hereinafter. Each type of device is similar to the others in most structural aspects, however, their respective operational characteristics differ greatly.
In regards to structure, each device is basically a body of material which is reversibly electrically switchable from one detectable ohmic state to a different detectable ohmic state. This body of material is typically connected in an electrical circuit by means of a pair of electrodes placed on either extremity of the body of material.
Structurally, each device differs from the other in only a few key aspects, such as diameter and composition of the body of material. In the threshold switch, the typical diameter of the body of material ranges from about 10 microns to about 400 microns. An exemplary composition of the body of material for a threshold switch is As.sub.34 Te.sub.28 S.sub.21 Ge.sub.16 Se.sub.1. For the memory device, the dimensions of the body of material are typically much smaller than those of the threshold device. The diameter ranges from about 0.25 microns to about 2 microns. A commonly used composition for the body of material in memory devices is Ge.sub.15 Te.sub.81 Sb.sub.2 S.sub.2.
Operationally. threshold and memory switches respond to input electrical signals very differently. The threshold switch starts out in a high ohmic state and stays in this state until a signal voltage greater than the switch's "threshold" voltage is received. When this threshold voltage is reached, the switch will drop to a lower ohmic state and stay in this state until the current through the switch drops to a minimum holding value. In contrast thereto, when a memory switch is "set" to a low ohmic state, the switch remains in that state until another electrical signal "resets" the switch to its high ohmic state.
These operation differences stem from the compositional differences of the body of material forming the switch. Each of the materials for a threshold and memory switch has an amorphous atomic structure in its high ohmic state. However. in its low ohmic state, the memory switch's material is crystalline/polycrystalline, while the threshold switch's material does not crystallize. Therefore, once the current flowing through the switch drops. the threshold switch reverts automatically to its high ohmic state, while the memory switch remains in its low ohmic (crystalline) form. To revert the memory switch to its high ohmic (amorphous) state, another electrical signal of higher voltage is required.
Many different combinations of atomic elements, when combined in the proper proportions and fabricated in the prescribed manner, have been shown to inherently produce a chalcogenide material having the aforementioned threshold or memory switching characteristics. Examples of such chalcogenide materials and switching devices fabricated therewith, are found in the following list of U.S. patents, all of which are assigned to the assignee of the present invention, and all of the disclosures of which are hereby incorporated by reference:
______________________________________ LIST OF PATENTS TO BE INCORPORATED BY REFERENCE ______________________________________ 3,271,591 3,611,063 3,343,034 3,619,732 3,571,669 3,656,032 3,571,670 3,846,767 3,571,671 3,875,566 3,571,672 3,886,577 3,588,638 3,980,505 ______________________________________
Also incorporated herein by reference is the disclosure of commonly assigned U.S. patent application Ser. No. 07/642,984, filed on Jan. 18, 1991, entitled "Electrically Erasable Phase Change Memory".
While the aforementioned list of patents and application are representative of some known chalcogenide switching materials and device configurations, they are, by no means, inclusive of all possible materials and device structures. When used herein, the term "chalcogenide material" should not be limited to those switching materials disclosed and/or incorporated by reference, but is intended to refer to any thin-film chalcogenide alloy material capable of being used as switching devices when ohmic contacts are formed thereacross.
After extensive investigation and failure analysis of prior art device structures and fabrication processes, it has been determined that each prior art device/process contains inherent design weaknesses which reduce the yield of usable devices and repeatability of the electrical switching characteristics of these devices. These weaknesses can be grouped into three categories: a) poor step coverage of the oxide material when non-planarized structures are used; b) unreliable switching due to edge conductivity; and c) contamination of the active body of thin-film chalcogenide material.
Poor step coverage or thinning of the active body of chalcogenide material over the deposited layers of insulating material results in variability of the electrical switching characteristics of the device. This variability in switching characteristics is due to the requirement that the body of active chalcogenide material must cover a layer of insulating material that is thick enough not to break down when the voltage is applied thereacross. The step coverage problem must be solved while bearing in mind the fact that the amorphous chalcogenide materials are incompatible with most conventional semiconductor processing techniques heretofore used to alleviate such a problem. An example of such a chalcogenide switching structure which suffers from poor step coverage is shown in commonly assigned U.S. Pat. No. 4,809,044 to Pryor, et al., the disclosure of which is hereby incorporated by reference.
Switching of chalcogenide material due to edge conduction is caused by filamentary conduction paths which are not restricted from the perimeter of the pore in which the chalcogenide material is contained. Note that in such a structure the opposite surfaces of the body of chalcogenide material are contacted by first and second electrodes, respectively. The electrical current supplied by a source travels along a path of least resistance defined as that path from the first electrode along the periphery of the body of chalcogenide material to the edge of the second electrode. Such edge conduction results, by definition. in current shunting at the structure's perimeter thereby short-circuiting the device. In the extreme, the electrically conductive material at the edge of the second electrode (such as molybdenum) can be driven into the body of chalcogenide material, thereby forming a permanent conduction path. An example of a device structure which can suffer from edge conduction is shown in FIG. 1, which structure will be described in detail hereinafter.
In another problem initiated by prior art fabrication techniques, the upper surface of the active chalcogenide material can become contaminated by the etching procedure in which the opening (also synonymously referred to hereinafter as a "pore" or "via") in the lower layer of insulator material through which the upper electrode or external contact electrically communicates with the body of chalcogenide material is produced. This contaminated portion of chalcogenide material can result in variability of the electrical switching characteristics of a device fabricated therefrom and/or breakdown of that device due to changes in the composition of the chalcogenide switching material. Efforts to remove this contaminated portion typically results in electrical variation due to stoichiometry and thickness changes of the body of said active chalcogenide material.
Accordingly, one of the principal objects of the present invention is to fabricate an improved structural arrangement of the plurality of thin-film layers of the chalcogenide electrical switching devices of the instant invention which overcomes the aforementioned design weaknesses.