US 20050180216 A1
A memory may have access devices formed using a chalcogenide material. The access device does not induce a snapback voltage sufficient to cause read disturbs in the associated memory element being accessed. In the case of phase change memory elements, the snapback voltage may be less than the threshold voltage of the phase change memory element.
1. A method comprising:
forming a chalcogenide access device having a snapback voltage low enough to avoid disturbing data stored in a memory element while reading the memory element selected by the access device.
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This invention relates generally to memories utilized to store electronic information.
Generally, memories may include cells or bits arranged in rows and columns such that they may be individually accessed for reading, erasing, and programming. To this end, an access device, sometimes called a selection device, is provided to enable an individual bit on an individual row or column to be accessed. Thus, the row or column may include a number of cells or bits and one individual bit or cell along that column or row may be accessed by activating its access device.
Commonly, access devices are transistors or diodes. However, such access devices are formed in a semiconductor substrate. As a result, valuable real estate on the integrated circuit may be consumed for forming such access devices.
Thus, there is a need for better ways to access cells or bits in memories.
In accordance with one embodiment of the present invention, the access device 14 may use a chalcogenide material. In some embodiments, the memory element 16 may also use a phase change material, but the present invention is not so limited and other memory elements, including a wide variety of thin film memory elements, may be utilized including metal oxide memory elements and paraelectric memory elements, to mention two examples.
In order to select an element 16 on column CN and row RN, the access device 14 for the selected memory element 16 at that location may be operated. The access device 14 activation allows current to flow through the memory element 16 in one embodiment of the present invention.
After passing through the snapback region, in the on state, indicated at B, the device 14 voltage drop remains close to the holding voltage as the current passing through the device is increased up to a certain, relatively high, current level. Above that current level the device remains on but displays a finite differential resistance with the voltage drop increasing with increasing current. The device 14 may remain on until the current through the device 14 is dropped below a characteristic holding current value that is dependent on the size and the material utilized to form the device 14.
The snapback voltage is effectively the threshold voltage minus the holding voltage. By reducing the extent of the snapback voltage, read disturbs in the memory element 16 can be reduced. If the snapback voltage is sufficiently small, read disturbs can be avoided. In one embodiment of the present invention, the snapback voltage is reduced to that level that avoids read disturb problems in the associated memory element 16. In an embodiment where the memory element 16 also uses phase change material, the snapback voltage may be less than the threshold voltage of the memory element 16.
One advantage, in some embodiments, of using a chalcogenide material to form the access device 14 is that the access device 14 need not be formed in a semiconductor substrate. As a result, in some embodiments, stacks of memory arrays of the type shown in
In some embodiments of the present invention, the access device 14 does not change phase. It remains permanently amorphous and its current-voltage characteristics may remain the same throughout its operating life.
As an example, for a 0.5 micrometer diameter device 14 formed of TeAsGeSSe having respective atomic percents of 16/13/15/1/55, the holding current may be on the order of 0.1 to 100 micro-ohms in one embodiment. Below this holding current, the device 14 turns off and returns to the high resistance regime at low voltage, low field. The threshold current for the device 14 may generally be of the same order as the holding current. The holding current may be altered by changing process variables, such as the top and bottom electrode material and the chalcogenide material. The device 14 may provide high “on current” for a given area of device compared to conventional access devices such as metal oxide semiconductor field effect transistors or bipolar junction transistors.
In some embodiments, the higher current density of the device 14 in the on state allows for higher programming current available to the memory element 16. Where the memory element 16 is a phase change memory, this enables the use of larger programming current phase change memory devices, reducing the need for sub-lithographic feature structures and the commensurate process complexity, cost, process variation, and device parameter variation.
With this approach, there is no bias voltage between the unselected rows and unselected columns. This reduces background leakage current.
After biasing the array in this manner, the memory elements 16 may be programmed and read by whatever means is needed for the particular memory technology involved. A memory element 16 that uses a phase change material may be programmed by forcing the current needed for memory element phase change or the memory array can be read by forcing a lower current to determine the device 16 resistance.
In accordance with another embodiment of the present invention, shown in
With this approach, there is a voltage bias between unselected rows and unselected columns of plus and minus V/3. This may contribute to additional background leakage current relative to the embodiments shown in
After biasing the array in this manner, the memory element 16 can be programmed and read by whatever means is appropriate for the particular memory technology involved. A memory 16 using a phase change material may be programmed by forcing the current needed for the memory element to change phase or the memory array can be read by forcing a lower current to determine device resistance.
For the case of a phase change memory element 16, programming a given selected bit in an array can be as follows. Unselected rows and columns may be biased as shown in
Reading a phase change memory element 16 can be performed as follows. Unselected rows and columns may be biased as shown in
The above-described reading and programming protocols are merely examples of techniques that may be utilized. Other techniques may be utilized by those skilled in the art.
To avoid disturbing a set bit of memory element 16 that is a phase change memory, the peak current may equal the threshold voltage of the device 14 minus the holding voltage of the device 14 that quantity divided by the total series resistance including the resistance of the device 14, external resistance of device 16, plus the set resistance of device 16. This value may be less than the maximum programming current that will begin to reset a set bit for a short duration pulse.
A technique for manufacturing the devices 14 and 16, where both devices use chalcogenide material, that is particularly amenable to forming stacked arrays, is illustrated in
Over the layer 12, a layer of dielectric 20 may be formed which, in one embodiment, may be oxide. A number of bottom electrodes 22 may be defined in the dielectric layer 20. In some embodiments, the electrodes 22 may be titanium silicon nitride. In such case, the electrodes 22 may ultimately act as heaters for a phase change material to be added subsequently.
In one embodiment of the present invention, the structure that forms the memory element 16 may then be etched into a number of parallel lines and spaces (stripes) in the rowline direction, with the space between rowlines 28, shown in
In one embodiment, the phase change material 24 may be a phase change material suitable for non-volatile memory data storage. A phase change material may be a material having electrical properties (e.g., resistance) that may be changed through the application of energy such as, for example, heat, light, voltage potential, or electrical current.
Examples of phase change materials may include a chalcogenide material or an ovonic material. An ovonic material may be a material that undergoes electronic or structural changes and acts as a semiconductor once subjected to application of a voltage potential, electrical current, light, heat, etc. A chalcogenide material may be a material that includes at least one element from column VI of the periodic table or may be a material that includes one or more of the chalcogen elements, e.g., any of the elements of tellurium, sulfur, or selenium. Ovonic and chalcogenide materials may be non-volatile memory materials that may be used to store information.
In one embodiment, the memory material may be chalcogenide element composition from the class of tellurium-germanium-antimony (TexGeySbz) material or a GeSbTe alloy, although the scope of the present invention is not limited to just these materials.
In one embodiment, if the memory material is a non-volatile, phase change material, the memory material may be programmed into one of at least two memory states by applying an electrical signal to the memory material. An electrical signal may alter the phase of the memory material between a substantially crystalline state and a substantially amorphous state, wherein the electrical resistance of the memory material in the substantially amorphous state is greater than the resistance of the memory material in the substantially crystalline state. Accordingly, in this embodiment, the memory material may be adapted to be altered to one of at least two resistance values within a range of resistance values to provide single bit or multi-bit storage of information.
Programming of the memory material to alter the state or phase of the material may be accomplished by applying voltage potentials to the layers 12 and 26, thereby generating a voltage potential across the memory material 24. An electrical current may flow through a portion of the memory material 24 in response to the applied voltage potentials, and may result in heating of the memory material 24.
This heating and subsequent cooling may alter the memory state or phase of the memory material 24. Altering the phase or state of the memory material 24 may alter an electrical characteristic of the memory material 24. For example, resistance of the material 24 may be altered by altering the phase of the memory material 24. The memory material may also be referred to as a programmable resistive material or simply a programmable material.
In one embodiment, a voltage potential difference of about 0.5-1.5 volts may be applied across a portion of the memory material by applying about 0 volts to a lower layer 12 and about 0.5-1.5 volts to an upper layer 26. A current flowing through the memory material 24 in response to the applied voltage potentials may result in heating of the memory material. This heating and subsequent cooling may alter the memory state or phase of the material.
In a “reset” state, the memory material may be in an amorphous or semi-amorphous state and in a “set” state, the memory material may be in a crystalline or semi-crystalline state. The resistance of the memory material in the amorphous or semi-amorphous state may be greater than the resistance of the material in the crystalline or semi-crystalline state. The association of reset and set with amorphous and crystalline states, respectively, is a convention. Other conventions may be adopted.
Due to electrical current, the memory material may be heated to a relatively higher temperature to amorphize the memory material and “reset” memory material (e.g., program memory material to a logic “0” value). Heating the volume or memory material to a relatively lower crystallization temperature may crystallize memory material and “set” memory material (e.g., program memory material to a logic “1” value). Various resistances of memory material may be achieved to store information by varying the amount of current flow and duration through the volume of memory material.
The information stored in memory material 24 may be read by measuring the resistance of the memory material. As an example, a read current may be provided to the memory material using opposed layers 12, 26 and a resulting read voltage across the memory material 24 may be compared against a reference voltage using, for example, a sense amplifier (not shown). The read voltage may be proportional to the resistance exhibited by the memory storage element. Thus, a higher voltage may indicate that memory material is in a relatively higher resistance state, e.g., a “reset” state. A lower voltage may indicate that the memory material is in a relatively lower resistance state, e.g., a “set” state.
Under the top electrode 34 is the material 32 that may be a chalcogenide material. An example of such a material is SiTeAsGeX in an atomic percent ratio of 14/39/37/9/1 where X is indium or phosphorous. Another example is SiTeAsGeSSe in the atomic percent ratio of 5/34/28/11/21/1. Still another example is TeAsGeSSe in the atomic percent of 16/13/15/1/55.
The material 32 may be made relatively thin so that the holding voltage is close to the threshold voltage. The thickness of the material 32 may be in the range of about 20 to about 500 Angstroms in some embodiments and most advantageously about 200 Angstroms in one embodiment.
Then, referring to
Appropriate metal connections may enable each row 12 of each element 16 to be coupled to receive appropriate electrical potentials for biasing, reading, programming from an overlying conductive line (not shown) via the plugs 46. Thus, a large number of the plugs 46 may be provided.
Similarly, each column of devices 14 may include a plug 44 which, through patterning of the layer 36, may provide individual connections to each column of devices 14.
As a result of the configuration shown in
While an embodiment with a single access device 14 per cell or bit is illustrated, it is possible to use two or more access devices in series per bit or cell. This results in access devices 14 with effectively higher holding voltages and threshold voltages. For example, for a series combination of access devices, a combined threshold of about 3 volts and a combined holding voltage of about 2.8 volts may be achieved. More than two access devices may also be placed in series. Devices with different individual threshold voltages and holding voltages may be utilized to achieve a desirable targeted combination of holding and threshold voltages.
The system 500 may include a controller 510, an input/output (I/O) device 520 (e.g., a keypad display), a memory 530, and a wireless interface 540 coupled to each other via a bus 550. It should be noted that the scope of the present invention is not limited to embodiments having any or all of these components.
The controller 510 may comprises, for example, one or more microprocessors, digital signal processors, microcontrollers, or the like. The memory 530 may be used to store messages transmitted to or by the system. The memory 530 may also be optionally used to store instructions that are executed by the controller 510. During the operation of the system 500 it may be used to store user data. The memory 530 may be provided by one or more different types of memory. For example, a memory 530 may comprise a volatile memory (any type of random access memory), a non-volatile memory such as a flash memory, and/or phase change memory that includes a memory such as, for example, memory element 16 and access device 14.
The I/O device 520 may be utilized to generate a message. The system 500 may use the wireless interface 540 to transmit and receive messages to and from a wireless communication network with a wireless radio frequency (RF) signal. Examples of the wireless interface 540 may include an antenna or a wireless transceiver, such as a dipole antenna, although the scope of the present invention is not limited in this respect.
While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.