This application is related to United States patent application attorney docket number SC11858TP, entitled “Non-Volatile Memory Having A Reference Transistor And Method For Forming” filed concurrently herewith by Chindalore et al., and assigned to the assignee hereof.
This invention relates to integrated circuit memories, and more particularly to establishing a variable gate bias for a reference transistor in a non-volatile memory.
In a typical non-volatile memory having floating gate transistors, such as flash memory, a bit is programmed by changing a threshold voltage of a selected floating gate memory cell. An unprogrammed, or erased, cell will have a different threshold voltage than a programmed cell. A read operation is used to determine the programmed state of the floating gate memory cell and is performed by applying a predetermined voltage to a gate of the selected memory cell while a source terminal of the cell is coupled to a predetermined voltage, such as for example, ground. A resulting drain current is compared to a reference current in a sense amplifier to determine the programmed state. For example, if the cell current is greater than the reference current, then the cell may be considered to be in an erased state.
Likewise, if the cell current is less than the reference current, then the cell may be considered to be in a programmed state. The reference voltage, or current, is generally established to be about half way between a logic high voltage and a logic low voltage of the memory cells.
Several techniques have been used in the past to generate the reference voltage for memories that use voltage sensing. One technique used to generate the reference voltage depends on the use of “dummy cells”. A dummy cell is manufactured using the same process technology as the normal cells of a memory array in order to model the behavior of the normal cells as closely as possible. However, the dummy cell will be physically smaller to generate a reference voltage that is between a logic high voltage and a logic low voltage for the cell. The problem with this technique is that reducing geometries of the cells produces process problems in keeping the time-current ratio of the dummy cell to normal cell constant.
Also, a dummy cell that is the same size as a normal cell can be used. However, a voltage divider is used to provide a reduced gate voltage to the normal sized cell.
Another technique is to connect normal sized dummy cells in series or parallel combinations. One of the cells will be programmed to read a “zero” state and the other programmed to read a “one” logic state to produce the required reference voltage. However, this technique may create errors due to the non-linearity of the resistance with voltage.
Yet another technique involves the use of current mirrors to establish the reference voltage. However, current mirrors sometimes do not produce the desired current with an acceptable degree of accuracy.
A nanocrystal memory uses nanocrystals embedded in an insulator such as oxide as the charge storage medium. In a nanocrystal memory that employs hot carrier injection for programming and channel erase for erasing, it has been found that electrons tend to be trapped in the interface between the top oxide and the bottom tunnel oxide in areas between the nanocrystals. These electrons cannot be removed even with extended erase times and/or very high erase voltages. As a result of this trapping of electrons, the erased threshold voltage and the programmed threshold voltage both tend to increase with the cumulative number of program/erase operations. This creates a problem when choosing a reference voltage for a read operation. If the reference voltage is too high, the gate oxide may be damaged and read speed will be degraded. However, if the reference voltage is chosen to be too low, the read margins will suffer.
Therefore, there is a need for a circuit that can generate an accurate reference voltage in a nanocrystal memory cell that maintains speed and read margins throughout the life of the memory.
The foregoing and further and more specific objects and advantages of the instant invention will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment thereof taken in conjunction with the following drawings:
Generally, the present invention provides a non-volatile memory where the non-volatile memory cells comprise transistors having nanoclusters, or nanocrystals. The program and erase threshold voltages of the memory cell transistors increase as a function of the number of program/erase operations due to electron trapping. A data value stored in a selected memory cell is determined by comparing a cell current to a reference current. A reference transistor is used to provide the reference current. The reference transistor is made from a process similar to the one used to make the memory cell transistors, except that the reference transistor does not use nanoclusters. By using a similar process to make both the reference transistor and the memory cell transistors, a threshold voltage of the reference transistor will track the increasing memory cell transistor threshold voltage. A read control circuit is provided to bias the gate of the reference transistor. The read control circuit senses a drain current of the reference transistor and adjusts a gate bias, or voltage of the reference transistor to maintain the reference current at a substantially constant value relative to the cell current.
Reference circuit 42 provides a control signal 49 to row decoder 36, and provides a reference current to one input of sense amplifier 40. A second input of sense amplifier 40 is coupled to column decoder 34 for receiving a memory cell current from a selected memory cell. An output voltage representative of a logic state, or data value, stored in the selected memory cell is provided by sense amplifier 40 to output data buffer 44. Data buffer 44 then buffers and further amplifies the output voltage and provides a data signal labeled “OUTPUT”. Also included in memory 30 but not shown is an input data path for receiving data to be programmed into memory array 32. The input data path components are conventional for a non-volatile memory and a functional description of their operation is not necessary for purposes of describing the present invention.
Generally, memory array 32 includes a plurality of non-volatile memory cells that store information in response to changing a threshold voltage of the cell. Specifically, in the illustrated embodiment, memory array 32 includes a plurality of nanocrystal memory cells. Reference transistor 46 is fabricated to be the same as the memory cells of memory array 32, except that reference transistor 46 does not include the nanocrystals for storing information. Reference transistor 46 has a gate for receiving a reference voltage labeled “VREF”, a source terminal for receiving a source voltage labeled “VSR”, and a drain coupled to an input of sense amplifier 40. Memory cell 50 has a gate coupled to a drain/source terminal of row select transistor 52, a source terminal for receiving a source voltage labeled “VSC”, and a drain coupled to a drain/source terminal of column select transistor 54. The method for making memory array 32 and reference transistor 46 will be described in detail in the discussion of
In order for the threshold voltage of reference transistor 46 to track the threshold voltage of memory array 32, reference transistor 46 will be cycled each time memory array 32 is cycled. That is, reference transistor 46 will receive the same program or erase voltage as memory array 32, and for the same length of time, each time memory array 32 is programmed or erased. Because transistor 46 is made using the same process steps as a normal memory cell, as described below, charge will be trapped at the interface between a top oxide and a bottom tunnel oxide that cannot be removed. The threshold voltage will change as the threshold voltage of the memory cells change in response to the number of program/erase cycles. The actual program and erase operations implemented in memory 30 are conventional, and are not important for describing the present invention, and will therefore not be described further.
During a read operation, a predetermined memory cell is selected to provide a current to one input of sense amplifier 40. In response to an address, a row select signal labeled “RS” is provided to the gate of a coupling transistor 52 and a column select signal labeled “CS” is provided to the gate of a coupling transistor 54. In response to a control signal labeled “RE”, a reference voltage labeled “VREF” is provided to the gate of reference transistor 46 and a gate voltage “VCELL” is provided to the gate of a selected memory cell, for example, memory cell 50. Source voltages VSC and VSR are both at ground potential. The reference current iREF is provided to one input of sense amplifier 40, and the cell current iCELL is provided to the other input of sense amplifier 40. Sense amplifier 40 is a current sensing type of sense amplifier. Current sensing is typically used when the voltage difference between a logic high and a logic low cell voltage is relatively small. If cell current iCELL is greater than reference current iREF, then memory cell 50 has a low threshold voltage and is in the erased state. However, if cell current iCELL is lower than reference current iREF, then memory cell 50 has a high threshold voltage and is in the programmed state. In other non-volatile memories, a high threshold voltage may indicate an erased state and a low threshold voltage may indicate an erased state. Sense amplifier will provide an output voltage corresponding to the sensed state of the memory cell 50 to data output buffer 44. Data output buffer 44 will provide buffered output voltage OUTPUT to a circuit external to memory 30 (not shown).
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The use of a variable reference gate voltage in accordance with the present invention provides an accurate reference current in a nanocrystal memory cell that tracks the changing cell threshold voltages. This provides a nanocrystal memory that operates with optimum read margins throughout the life of the memory and maintains access speed.
In
Other circuitry 70 may be formed in other areas of the substrate. The other circuitry 70 may include the peripheral circuits of memory 30, such as decoders or sense amplifiers, or other circuits such as a microprocessor core or random logic that may, or may not, communicate with the memory 30. A first dielectric layer 72 is formed over the substrate. In the illustrated embodiment, the first dielectric layer is formed by thermally growing silicon dioxide on the surface of substrate 64 to a thickness of about 50 angstroms. A storage material layer 76 is formed over first dielectric layer 72. In the illustrated embodiment, storage material layer 76 comprises nanocrystals or nanoclusters. The nanocrystals or nanoclusters are very small silicon dots for storing charge in the memory cells of the array. In another embodiment, the dots may be formed from nitride or germanium. In addition, the dots may be formed from any metal such as Silver, Platinum, Gold, Tungsten or Tantalum. The dots are spaced apart at least on some regions so as to not be in direct contact with each other. A second dielectric layer 74 is formed overlying and surrounding the storage material layer 76. Second dielectric layer 74 is formed from high temperature oxide and deposited to a thickness of about 100 angstroms. A first barrier layer 78 is formed over the second dielectric layer 74. Barrier layer 78 is deposited nitride having a thickness of about 75 angstroms. Barrier layer 78 functions as a hardmask for subsequent processing steps and will be completely removed later. A layer of patterned photoresist 79 is deposited over first barrier layer 78 and patterned as shown in
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Various changes and modifications to the embodiments herein chosen for purposes of illustration will readily occur to those skilled in the art. For example, variations in the types of conductivities of transistors, the types of transistors, etc. may be readily made. To the extent that such modifications and variations do not depart from the scope of the invention, they are intended to be included within the scope thereof, which is assessed only by a fair interpretation of the following claims.