Static information storage and retrieval – Read/write circuit – Differential sensing
Reexamination Certificate
2002-06-13
2003-12-09
Nguyen, Tan T. (Department: 2818)
Static information storage and retrieval
Read/write circuit
Differential sensing
C365S185280
Reexamination Certificate
active
06661724
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of memory devices. Specifically, embodiments of the present invention relate to methods and systems for programming a memory device.
BACKGROUND ART
Presently, electronic memories come in a variety of forms and serve a variety of purposes. For example, one type of memory is flash memory. Generally, flash memories are used for non-volatile, fast information storage in devices such as digital cameras and home video consoles. Flash memory may be considered a solid state storage device.
In general, flash memory is a type of electrically erasable programmable read-only memory (EEPROM). It has a grid of columns and rows with a cell that has one of two transistors at each intersection. One of the transistors has a floating gate. When the floating gate has a neutral or positive net charge stored it conducts and the cell has a value of one. To change the value to a zero a process called Fowler-Nordheim (FN) tunneling is utilized.
FN tunneling is used to alter the placement of electrons in the floating gate. For example, a voltage is applied to the control gate above the floating gate and drain to the ground. This electric field causes the floating-gate transistor to act similar to an electron gun. That is, the electrons are pushed through and trapped on the other side of the TOX layer, giving it a negative charge. These negatively charged electrons act as a barrier between the control gate and the floating gate. A cell sensor then monitors the level of the charge passing through the floating gate. If the flow through the gate is greater than 50 percent of the charge, then it has a value of one. However, when the charge passing through the gate drops below the 50 percent threshold, the value changes to zero. Normally, a blank EEPROM has all of the gates fully open, giving each cell a value of one.
The electrons in the cells of a flash-memory can be returned to a normal state (e.g., one) by the application of an electric field (e.g., a higher voltage). Furthermore, flash memory utilizes in-circuit wiring to apply the electric field either to the entire chip or to predetermined sections known as blocks. This electrical field erases the target area of the chip, which can then be rewritten. Therefore, flash memory works much faster than traditional EEPROMs because instead of erasing one byte at a time, it erases a block or the entire chip. In addition, flash memory is non-volatile and as such it will maintain its data without an external source of power. Thus, it is extremely useful with removable memory media such as digital cameras, digital music players, video consoles, computers, and the like.
Generally speaking, EEPROMs generate high voltages to induce electrons to move on and off of a floating gate in a metal oxide semiconductor field effect transistor (MOSFET) which charges the transistors threshold voltage. By changing the voltage that effects the threshold of the device one can effect the current that will flow through it when it is powered. Logic one or zero can be determined by sensing the amount of current that can flow through the transistor at a given control gate voltage.
The electrons are free to move with no actual physical connection to the gate. So what is needed is an induction of the electrons to get them to move on or off of the poly-silicon electrode. This is done by applying high voltage to nodes that are near the floating gate. For example, a high voltage may be applied to the drain, source, or in some cases an extra gate electrode located nearby. Electrons move either because they are ballistic (e.g., high energy) or they tunnel due to quantum effect. In both cases, they move on and off the floating electrode. In flash technology, quantum tunneling is used. However, the electrons may be moved by ultraviolet light (UV) or X-ray.
The key is to get the right number of electrons on or off the electrode. However, since there is no way to touch the electrode, there is no way to measure the exact number of electrons thereon. Thus, the number of electrons thereon must be inferred based on the variations of the current flow through the transistor of a programmed device. If too many electrons are on or off the device, early failure of the device may occur. In addition, there may be degradation in the current flow. Thus, to maximize reliability, better control of the number of electrons actually placed on the device is needed.
Currently, the voltages are generated by a voltage pump that applies an electric field for a certain period of time during the memory programming process. Thus, by controlling both voltage and time, a basic statistical inference can be made with regard to the amount of electrons that will be moved. Then, the device can be tested for the correct current flow parameters. For example, a statistical process may be used to define a number of volts and a time period for applying the voltage that will result in correct programming.
Thus, for a specific manufacturing process, a publication by the manufacturer may state a voltage of 10 volts needs to be applied for 10 milliseconds to ensure proper operation or device programming. However, there are many statistical variations within such a method, and therefore deleterious effects may result within the device depending on the environment in which programming occurs. Moreover, in order to resolve the reliability of the published voltages and times, a further trial and error process is typically done to ensure that the correct current is indeed passing through the device. Such excessive testing may be both monetarily inhibiting and time consuming.
SUMMARY OF INVENTION
Thus, a need exists for an improved method for programming a flash or other non-volatile memory device. A further need exists for a method for programming a flash memory device, which can apply the most efficient programming voltage and programming time for a specific memory device. A further need exists for a method for programming a memory device which can proficiently utilize the memory device temperature when calculating the most efficient programming voltage and programming time for a specific memory device. Still another need exists for a method for programming a memory device which is compatible with existing memory device programming processes.
A method for programming a memory device is disclosed. In one method embodiment, the present invention receives a measurement from a temperature sensor located near a memory to be programmed. Next, a pre-existing transformation is electronically accessed. Then, the measurement from the temperature sensor is processed in conjunction with the transformation which provides a programming time for a memory device as a function of a programming voltage and the temperature. The programming voltage is then applied to the memory device for the length of time specified by the programming time in order to properly program the memory device. In one embodiment, the memory device is flash memory.
In one exemplary system, a host computer system downloads data to a programmable device to be programmed. The programmable device contains a programmable memory array, a voltage pump, a pulse width generator, a temperature sensor, a controller, and a transformation as described above. The controller accesses the transformation based on the current temperature reading from the sensor. Programming voltage and pulse width data supplied from the transformation are provided to the charge pump and pulse width generator. Memory programming then commences using techniques known in the art.
In one embodiment, the present invention is implemented in a microcontroller included on a single substrate. The microcontroller includes a microprocessor, a plurality of internal peripherals, an interconnecting component, an external coupling port, and a memory for storing instructions. The microprocessor processes information. The plurality of internal peripherals are programmably configurable to perform a variety of functions associated with the microcontroller. The interconnect
Rouse Mark
Snyder Warren
Cypress Semiconductor Corporation
Nguyen Tan T.
Wagner , Murabito & Hao LLP
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