# Patent

## Title
Phosphorescent nanotube memory device

## Abstract

An optical memory cell having a material layer associated with a pixel capable of emitting and receiving light. The material layer has phosphorescent material formed therein for storing data as light received from and emitted to the pixel.

## Background
FIELD OF THE INVENTION
The present invention relates to a memory cell, specifically a photon-based memory cell.
BACKGROUND OF THE INVENTION
Integrated circuit designers have always sought the ideal semiconductor memory: a device that is randomly accessible; can be written to or read from very quickly; is non-volatile, but indefinitely alterable; and consumes little power.
One common volatile memory is the DRAM in which information can be written to and read from as bits of data, e.g., a “1” or a “0,” where a “1” generally corresponds to one voltage state stored on a capacitor, and a “0” generally corresponds to another voltage state stored in the capacitor. The capacitor of the DRAM cell typically has an associated transistor that acts as a switch to allow the control circuitry on the memory chip to read from and write to the capacitor.
DRAM cells suffer from a number of shortcomings. First, the capacitor of a DRAM cell is extremely energy inefficient because capacitors of DRAM cells quickly lose their stored voltage, and need to be refreshed to prevent the cell from being discharged, resulting in high levels of energy consumption. Second, because DRAM cells are based on electrical signals, the speed of integrated chips are not only limited by the speed by which electrons travel through matter, but are also limited by the number of interconnections within the chip necessary to effect proper transfer and storage of the signals; these additional interconnections contribute to the problem of short circuits. Finally, the electrical signals used in conventional memory cells can interfere with each other, resulting in increased cross-talk, and decreased performance, which is undesirable.
Accordingly, there is a desire and need to construct a memory cell that improves upon the shortcomings of DRAM cells.
BRIEF SUMMARY OF THE INVENTION
Various exemplary embodiments of the invention provide a photon-based memory cell having a material layer having phosphorescent materials therein associated with a pixel capable of emitting and receiving light. The data is stored in the memory cell as radiation in the phosphorescent material.



BRIEF DESCRIPTION OF THE DRAWINGS
The above-described features and advantages of the invention will be more clearly understood from the following detailed description, which is provided with reference to the accompanying drawings in which:
FIG. 1 illustrates a cross-sectional view of a memory cell constructed in accordance with an exemplary embodiment of the invention;
FIG. 2 illustrates a partial cross-sectional view of a memory cell constructed in accordance with a second exemplary embodiment of the invention;
FIG. 3 illustrates a partial cross-sectional view of a memory cell constructed in accordance with a third exemplary embodiment of the invention;
FIG. 4 illustrates a partial top-down view of a memory cell array constructed in accordance with an exemplary embodiment of the invention; and
FIG. 5 is a block diagram of a processor system incorporating a memory device constructed in accordance with an exemplary embodiment of the invention.



DETAILED DESCRIPTION OF THE INVENTION
In the following detailed description, reference is made to specific exemplary embodiments of the invention. It is to be understood that other embodiments may be employed, and that structural and electrical changes may be made without departing from the spirit or scope of the present invention.
The term “semiconductor substrate” is to be understood to include any semiconductor-based structure that has an exposed semiconductor surface. The semiconductor structure should be understood to include silicon, silicon-on-insulator (SOI), silicon-on-sapphire (SOS), doped and undoped semiconductors, epitaxial layers of silicon supported by a base semiconductor foundation, and other semiconductor structures. The semiconductor substrate need not be silicon-based. The semiconductor could be silicon-germanium, germanium, or gallium arsenide. When reference is made to a semiconductor substrate in the following description, previous process steps may have been utilized to form regions or junctions in or over the base semiconductor or foundation. Also, the invention may be formed over non-semiconductor substrates.
The fabrication steps below are discussed as being performed in an exemplary order, however this order may be altered and still maintain the spirit and scope of the invention.
Referring now to the drawings, where like elements are designated by like reference numerals, FIG. 1 illustrates a partial cross-sectional view of a memory cell 100 constructed in accordance with an exemplary embodiment of the invention. The illustrated memory cell 100 includes a material layer 116 having phosphorescent material formed therein associated with a pixel 110 capable of emitting and receiving light. The illustrated pixel 110 has a light emitting element 120 and a light receiving element 130. The memory cell 100 can store and transmit bits of data, represented by 1's and 0's, similar to the conventional memory cells discussed above (e.g., a DRAM cell).
In operation of the memory cell 100, the light emitting element 120 (illustrated as a light emitting diode) is activated to emit radiation in the form of photons (depicted by the arrow) onto the material layer 116. The phosphorescent material in the material layer 116 over the pixel 110 absorbs the radiation emitted by the light emitting element 120 and reradiates the energy, also in the form of photons. The presence or absence of reradation of photons by the material layer 116 above the pixel 110 corresponds to a particular logic state potential, e.g., a 1 or 0, similar to the electrons stored on the capacitor of the DRAM, discussed above.
In reading the memory cell 100, the reradation (depicted by the arrow) by the phosphorescent material within the material layer 116 is read by a light receiving element 130 (illustrated as a phototransistor) when the light receiving element 130 is activated by electrical signals from external circuitry. Once activated, the reradation from the phosphorescent material layer 116 causes the light receiving element 130 to produce a detectable electrical signal, which is different from an electrical signal produced by light receiving element 130 when there is no light shone or reradiated from the overlying material layer 116. Thus, the output signal from the light receiving element 130 represents a logic “1” or “0” depending on whether the material layer 116 above the light receiving element is storing light or not.
Memory cell 100 does not suffer from the same drawbacks as conventional memory cells. Unlike the capacitors of conventional DRAM memory cells, memory cell 100 is much more energy efficient, and does not have to be refreshed every few milliseconds. Phosphorescent materials generally reradiate for up to several minutes, limited only by the half-life of the specific phosphorescent material used in the material layer 116. Therefore, memory cell 100 can be refreshed every minute or so, rather than every few milliseconds, resulting in a significant reduction in energy consumption. Refreshing is accomplished by reading the memory array containing a plurality of memory cells 100 and rewriting to those cells having light stored therein.
Memory cell 100 also has a faster read/write speed than conventional DRAM cells because the information is retained as photons, and not as an electrical potential; therefore, the information stored in the material layer 116 does not have to be transmitted through matter, as electronically stored information would. Additionally, because the information from the memory cell 100 is stored in the form of photons, a decreased number of electrical interconnections are necessary, reducing the number of fabrication short circuits compared with DRAM memory cells.
It should be noted that although the light emitting element 120 is illustrated as a light emitting diode, this is only exemplary. For example, the light emitting element 120 could include any device capable of emitting radiation, including visible light, x-rays, infrared radiation, or UV radiation. Similarly, although illustrated as a phototransistor, the light receiving element 130 could be any device capable of receiving radiation, and converting the radiation into an electrical signal. Moreover, a single integrated light emitting and light receiving structure can also be used for light emitting and light receiving elements 120, 130.
The material layer 116 could have a thickness in the range of about 100 Å to about 2000 Å, depending on the intended application. The material layer is formed by doping a material layer with phosphorescent materials. The phosphorescent materials used could be selected from the group consisting of copper activated polycrystalline zinc sulfides, rare earth doped alkaline earth sulfides, and organic dyes like bis(2-phenyl-benzoimiazole)acetylacetonato iridium. It should be noted that the materials listed are only exemplary, and should not be limiting in any way.
FIG. 1 also illustrates an optional light emitting device 132 in light communication with the memory cell 100. The light emitting device 132 may be used to perform a masked programming of an array of memory cells 100, similar to light programmable EEPROM arrays. The light emitting device 132 can also be used to globally erase a memory array by turning all memory cells on.
The light emitting device 132 could be integral to a memory die containing an array of memory cells for global erase programming. Alternatively, the light emitting device 132 could be located externally to the array containing memory cells. In an embodiment in which the light emitting device 132 is located externally to the array, the memory die in which the array is located would typically have a transparent element 180 allowing the light to enter the die to allow a light masked programming operation. The light emitting device 132 could comprise an epitaxial silicon layer having a plurality of light emitting diodes fabricated therein. The light emitting device 132 could also be constructed as a laser located externally to the memory die in which the array is contained, which can be programmable to light material layer 116 to set a programmed state of a memory array.
FIG. 2 illustrates a partial cross-sectional view of a memory cell 200 constructed in accordance with a second exemplary embodiment of the invention. Like the FIG. 1 memory cell 100, the FIG. 2 memory cell 200 has a material layer 216 associated with a pixel 110 capable of emitting and receiving light. The material layer 216 could have a thickness in the range from about 1000 Å to about 2000 Å. The material layer 216 of the illustrated memory cell 200 has a thickness of about 1500 Å.
The illustrated material layer 216 includes a plurality of nanotubes 216a that are substantially perpendicular to the pixel 110. The nanotubes 216a could have a length in the range from about 50 Å to about 150 Å, for example. The illustrated nanotubes may have a length of about 100 Å. Each nanotube 216a includes one or more of the phosphorescent materials discussed above with respect to FIG. 1. Because the nanotubes 216a are substantially perpendicular to the pixel 110, the amount of cross-talk between memory cells in an array (e.g., array 314 (FIG. 3)) can be further reduced.
The FIG. 2 memory cell 200 operates in substantially the same manner as the FIG. 1 memory cell 100, except that when the light emitting element 120 emits radiation onto the material layer 216, only those nanotubes 216a containing phosphorescent material localized above the pixel 110 will reradiate. Therefore, the reradiation is more localized, and the nanotubes reduce cross-talk among adjacent memory cells.
It should be noted that although the nanotubes 216a are illustrated as being at a 90° angle relative to the pixels 110, it is not intended to be limiting in any way. For example, the nanotubes 216a could be at any angle relative to the pixel 110. It should also be noted that not all of the nanotubes 216a are necessarily straight, and that the nanotubes 216a may contain bends and turns.
The material layer 216 could be formed by forming the nanotubes 216a in the material layer 216, and then doping the nanotubes 216a with one or more of the phosphorescent materials discussed above with respect to FIG. 1. Alternatively, the material layer 216 could be formed by implanting a thin silicon layer with boron and anodizing the silicon layer in a solution of hydrogen fluoride (HF) and isopropanol.
FIG. 3 illustrates a partial cross-sectional view of a memory cell 300 constructed in accordance with a third exemplary embodiment of the invention. Like the FIG. 1 and FIG. 2 memory cells 100, 200, the illustrated memory cell 300 has a material layer 316 having phosphorescent material contained therein associated with a pixel 310 capable of emitting and receiving light. The exemplary embodiment provides the material layer 316 within the pixel 310; between the light emitting element 120 and the light receiving element 130.
An array of memory cells including the illustrated memory cell 300 could be formed by providing an array of light receiving elements 130 in a silicon substrate and providing a material layer 316 over the light receiving element array. The material layer 316 could be doped with any of the phosphorescent materials discussed above with respect to FIG. 1. An epitaxial silicon substrate having an array of light emitting elements 120 could then be deposited over the material layer 316, forming an array of memory cells. It should be noted that the light emitting 120 and light receiving 130 elements should be substantially aligned with one another.
The FIG. 3 memory cell 300 operates in substantially the same manner as the FIG. 1 and FIG. 2 memory cells 100, 200, in that the light emitting element 120 writes data to the material layer 316, and the light emitting element 130 reads data from the material layer 316.
FIG. 4 illustrates a partial top-down view of a memory array 514 constructed in accordance with an exemplary embodiment of the invention. The memory cells 500 of the array 514 are arranged in rows and columns. Each illustrated memory cell 500 has a material layer 516 associated with a pixel 510. Phosphorescent regions 516a of the material layer 516 are provided in a pattern positioned above respective pixels 510 in the array 514. The separated phosphorescent regions 516a minimize cross-talk between adjacent memory cells 500.
The material layer 516 is selectively doped with phosphorescent material to create the phosphorescent regions 516a. The phosphorescent regions 516a are formed by forming a patterned photoresist layer over a material layer 516, such that the doping of the material layer 516 occurs through the pattern aligning phosphorescent regions 516a with the underlying pixels 510. Any phosphorescent material discussed above with respect to FIG. 1 could be used to dope the material layer 516, resulting in the phosphorescent regions 516a. 
FIG. 4 also illustrates a bitline 540 connected to column control circuitry and a wordline 550 connected to row control circuitry. Each memory cell 500 is written to or read from by activating the corresponding bitline and wordline, and further selecting either the light emitting element 120, or the light receiving element 130 for actuation. For example, to write to or read from a selected memory cell 500b, a corresponding bitline 540b and a corresponding wordline 550b are turned on, and the selected memory cell 500b can be either written to or read from, in accordance with a write or read signal which selects light emission and light reception as discussed above with respect to FIG. 1.
Although not illustrated, each memory cell 500 of the array 514 would have a write/read conductor, which could be positioned under each memory cell 500 of the array 514. In operation, the write/read conductor in one logic state activates the light emitting elements of the selected memory cells 500 to store bits of data. The write/read conductor in a second logic state activates the light receiving elements of selected memory cells 500 to read bits of data. It should be noted, that the write/read conductors could be connected to a transistors that are associated with the light emitting and light receiving elements of the memory cells 500. The transistors act as a switch that turn the light emitting and light receiving elements on and off.
It should be noted that the separated phosphorescent regions 516a shown in FIG. 4 could be incorporated into any of the memory cells discussed above with respect to FIGS. 1-3, i.e., memory cells 100, 200, 300. The material layer 116, 216, 316 over each pixel of each memory cell 100, 200, 300 could have separated phosphorescent regions formed therein, thereby reducing the amount of cross-talk among adjacent memory cells.
FIG. 5 illustrates a block diagram of a processor system 600 incorporating a memory device having a memory cell (e.g., memory cells 100, 200, 300) or a memory cell array (e.g., 514) constructed in accordance with exemplary embodiments of the invention. Examples of processor based systems include, without limitation, computer systems, camera systems, scanners, machine vision systems, vehicle navigation systems, video telephones, surveillance systems, auto focus systems, star tracker systems, motion detection systems, image stabilization systems, and others.
The exemplary system 600 includes a memory device 608 having one or more of the memory cells constructed in accordance with exemplary embodiments of the invention (e.g., memory cells 100, 200, 300). The memory device 608 could also incorporate the FIG. 4 memory cell array 514.
The system 600 generally comprises a central processing unit (CPU) 602, such as a microprocessor, that communicates with an input/output (I/O) device 606 over a bus 604. The memory device 608 also communicates with the CPU 602 over the bus 604. The system 600 also includes random access memory (RAM) 610, and can include removable memory 615, such as flash memory, which also communicates with CPU 602 over the bus 604. Imaging device 608 may be combined with a processor, such as a CPU, digital signal processor, or microprocessor, with or without memory storage on a single integrated circuit or on a different chip than the processor.
The above description and drawings illustrate preferred embodiments which achieve the objects, features, and advantages of the present invention. Although certain advantages and preferred embodiments have been described above, those skilled in the art will recognize that substitutions, additions, deletions, modifications and/or other changes may be made without departing from the spirit or scope of the invention. Accordingly, the invention is not limited by the foregoing description but is only limited by the scope of the appended claims.

## Claims

1. A memory die, comprising:
a semiconductor substrate supporting a memory cell array, the memory cell array comprising:
a plurality of memory cells, each memory cell further comprising:
a pixel capable of emitting and receiving light, and
a material localized over the pixel, the material having nanotubes formed therein, the nanotubes having phosphorescent material, wherein the phosphorescent material receives light from and emits light to the pixel; and

circuitry connecting the plurality of memory cells to the semiconductor substrate.

2. The memory die of claim 1, wherein each pixel includes a light emitting element and a light receiving element.

3. The memory die of claim 1, wherein the phosphorescent material is selected from the group consisting of copper activated polycrystalline zinc sulfides, rare earth doped alkaline earth sulfides, and organic dyes.

4. The memory die of claim 1, further comprising a light emitting layer over the material.

5. The memory die of claim 4, further comprising a transparent element between the light emitting layer and the material.

6. The memory die of claim 1, wherein the phosphorescent material is formed of a single phosphorescent material layer comprising a plurality of phosphorescent regions.

7. The memory die of claim 1, wherein the memory cells are arranged in rows and columns.

8. The memory die of claim 7, further comprising a wordline connecting each column to column circuitry and a bitline connecting each row to row circuitry.

9. The memory die of claim 1, further comprising read and write conductors associated with a light emitting element of each pixel.

10. A processor system, comprising:
a processor;
an array of memory cells provided on a semiconductor substrate, wherein at least one of the memory cells comprises:
a pixel capable of emitting and receiving light, and
nanotubes formed within a material associated with the pixel, the nanotubes being substantially perpendicular to the pixel, and containing a phosphorescent material; and

readout circuitry within the semiconductor substrate.

11. The processor system of claim 10, wherein the phosphorescent material consists of copper activated polycrystalline zinc sulfides.

12. The processor system of claim 10, wherein the phosphorescent material consists of rare earth doped alkaline earth sulfides.

13. The processor system of claim 10, wherein the phosphorescent material consists of organic dyes.

14. The processor system of claim 13, wherein the organic dyes consist of bis(2-phenyl-benzoimiazole)acetylacetonato iridium.

15. The processor system of claim 10, wherein the material has a thickness in the range of from about 1000 Å to about 2000 Å.

16. The processor system of claim 1, wherein the nanotubes have a length in the range from about 50 Å to about 150Å.