Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – Having insulated gate
Reexamination Certificate
2001-06-14
2003-03-18
Chaudhuri, Olik (Department: 2813)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
Having insulated gate
C438S763000
Reexamination Certificate
active
06534368
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to memories and, more particularly, to memory cells having chalcogenide memory elements.
2. Background of the Related Art
In the field of integrated circuit memory devices, there is a continuing trend toward memories that have store more information, consume less power, operate faster, take up less space, and cost less to make. While these are often competing interests, memory manufactures strive to make advances in these areas to remain competitive. Thus, the ability to manufacture small memory cells efficiently is crucial in maximizing the performance and cost-efficiency of a memory device.
Popular memories today include dynamic random access memories (DRAMs), static random access memories (SRAMs), read only memories (ROMs), and flash memories. Certain basic characteristics are shared by these memories. For example, these memories typically include one or more memory arrays, where each array has a plurality of memory cells arranged in rows and columns. Other than these basic characteristics, however, these memories possess many different attributes. By way of a general comparison, ROMs and flash memories do not exhibit true random access as do DRAMs and SRAMs. Also, DRAMs are volatile memories, i.e., they require constant power to retain the contents of the memory, while SRAMs, ROMs, and flash memories are non-volatile memories. Furthermore, DRAMs typically require less area on a die than the other memories, but DRAMs generally do not exhibit the fastest access times. Thus, as can be appreciated due to the many trade-offs between these different memory configurations, the type of memory used greatly depends upon the requirements of the system in which it is used.
One reason for these differences may be understood by referring to the memory cells used by these various memories. Although the memory cells of these different memories store data in the form of an electrical charge, the memory cells take different forms. The form of a memory cell may dictate many of a memory's characteristics. For instance, the memory cell of a typical dynamic random access memory (DRAM) generally includes a memory element and an access device. The memory element is typically a small capacitor, which stores data as the presence or absence of a charge on the capacitor. The access device, typically referred to as an access transistor, coupled to the small capacitor to control the charging and discharging of the capacitor.
DRAMs possess many desirable features, such as large storage capacity, high storage density, and ease of manufacture. However, due to the type of memory cell used, DRAMs also require periodic refreshing, i.e., the capacitors need to be periodically recharged, to maintain the stored information. Although the memory cells of SRAMs, ROMs, and flash memories do not require refreshing, they suffer from disadvantages, such as lower storage densities, larger size, and greater cost to manufacture.
Instead of using memory cells that store information in the form of an electrical charge, memory cells may be manufactured of a material that is capable of storing information. Chalcogenides are a class of materials that may be used to store information in an integrated circuit memory. Chalcogenide material may be electrically stimulated to change states, from an amorphous state to increasingly crystalline states. In the amorphous state, chalcogenide material exhibits a high electrical resistivity. As chalcogenide material progresses into an increasingly crystalline state, its electrical resistivity generally decreases. Because chalcogenide material retains its programmed state even after removal of the electrical stimulus, chalcogenide-based memories are non-volatile. As an added benefit, chalcogenide elements may be programmed into two or more states. Thus, chalcogenide-based memories may operate as traditional binary memories or as higher-based memories.
In chalcogenide-based memories, the memory cells are typically formed by disposing chalcogenide material between two electrodes. The size of the contact area between the electrode and the chalcogenide material appears to be related to the speed, power requirements, and capacity of a chalcogenide-based memory. It has been observed that the chalcogenide memory element has an active area which may be less than the area of the entire chalcogenide memory element. The size of this active area can be controlled by controlling the size of the contact area between the electrode and the chalcogenide element. The size of the active area appears to be related to the programming current and/or time required to achieve the desired state change. Thus, to increase programming rates and optimize programming current, it is desirable to minimize the dimension of the electrode contacting the chalcogenide element.
Techniques for reducing the contact area have focused on forming smaller sized electrodes. One conventional method for forming the electrode includes forming a receiving cavity in a dielectric layer and then depositing a conductive material in the cavity. Photolithography or etching has been used to establish the initial cavity through the insulator layer. However, these methods suffer from technological constraints upon the hole size. Other conventional techniques for forming the cavity in the insulator layer have included the application of a high current pulse to open a hole having a diameter on the order of 0.1-0.2 microns. The size of the cavity, and therefore the size of the electrodes, achieved through this method is generally unpredictable. Hence, the need remains for a method for repeatably and accurately establishing a small contact area between an electrode and a chalcogenide memory element.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, there is provided a method of forming a memory cell. A first electrode is formed. A memory element is electrically coupled to the first electrode. The memory element is formed using a photolithographic technique having a resolution limit. The memory element has a contact portion that has an area smaller than the resolution limit. A second electrode is formed and electrically coupled to the contact portion of the memory element.
In accordance with another aspect of the present invention, a method of forming a memory cell is provided. A first electrode is formed. A memory element is formed and electrically coupled to the first electrode. The memory element has a contact portion with an area of less than 0.04 microns
2
. A second electrode is formed and electrically coupled to the contact portion of the memory element.
In accordance with a further aspect of the present invention, there is provided a method for manufacturing a memory cell for a memory device. A first electrode is formed. A memory element is formed and electrically coupled to the first electrode. A pattern is formed over a portion of the memory element. At least a portion of the memory element is removed to form a protruding portion of the memory element generally underneath the pattern. A second electrode is formed and electrically coupled to the protruding portion.
In accordance with yet another aspect of the present invention, there is provided a method for manufacturing a memory cell. A first electrode layer of conductive material is formed. A chalcogenide layer is formed and electrically coupled to at least a portion of the first electrode layer. A pattern is formed over a desired contact location on the chalcogenide layer. At least a portion of chalcogenide layer is etched away to form a protruding portion of chalcogenide generally underneath the pattern. A dielectric layer is formed on the chalcogenide layer. A portion of the dielectric layer is removed to expose at least a portion of the protruding portion. A second electrode layer is formed and electrically coupled to the exposed
Blum David S
Chaudhuri Olik
Fletcher Yoder & Van Someren
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