Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – Having insulated gate
Reexamination Certificate
2000-06-19
2001-07-17
Nelms, David (Department: 2818)
Semiconductor device manufacturing: process
Making field effect device having pair of active regions...
Having insulated gate
C438S265000
Reexamination Certificate
active
06261904
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to semiconductor memory devices.
BACKGROUND OF THE INVENTION
Non-volatile memory devices are currently in widespread use in electronic components that require the retention of information when electrical power is terminated. Non-volatile memory devices include read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), and electrically erasable programmable read only memory (EEPROM) devices. EEPROM devices differ from other non-volatile memory devices in that they can be electrically programmed and erased. Flash EEPROM devices are similar to EEPROM devices in that memory cells can be programmed and erased electrically. However, Flash EEPROM devices enable the erasing of all memory cells in the device using a single electrical current pulse.
Typically, an EEPROM device includes a floating-gate electrode upon which electrical charge is stored. The floating-gate electrode overlies a channel region residing between source and drain regions in a semiconductor substrate. The floating-gate electrode together with the source and drain regions forms an enhancement transistor. By storing electrical charge on the floating-gate electrode, the threshold voltage of the enhancement transistor is brought to a relatively high value. Correspondingly, when charge is removed from the floating-gate electrode, the threshold voltage of the enhancement transistor is brought to a relatively low value.
The threshold level of the enhancement transistor controls current flow through the transistor by application of appropriate voltages to the gate and drain. When the threshold voltage is high, no current will flow through the transistor, which is defined as a logic 0 state. Correspondingly, when the threshold voltage is low, current will flow through the transistor, which is defined as a logic 1 state. This feature is identical to FET operation, except the floating gate in an EEPROM FET alters the threshold voltage dependent upon the presence of charge within the floating gate.
One type of EEPROM device utilizes a polycrystalline silicon or metal layer for the floating-gate electrode. Electrons are transferred to the floating-gate electrode through a dielectric layer overlying the channel region of the enhancement transistor. The electron transfer is initiated by either hot electron injection, or by Fowler-Nordheim tunneling. In either electron transfer mechanism, a voltage potential is applied to the floating-gate by an overlying control-gate electrode.
The EEPROM device is programmed by applying a high positive voltage to the control-gate electrode, and a lower positive voltage to the drain region which transfers electrons from the channel region to the floating-gate electrode. The EEPROM device is erased by grounding the control-gate electrode and applying a high positive voltage through either the source or drain region of the enhancement transistor. Under erase voltage conditions, electrons are removed from the floating-gate electrode and transferred into either the source or drain regions in the semiconductor substrate.
Another type of EEPROM device utilizes an oxide-nitride-oxide (ONO) layer for the fabrication of the floating-gate electrode. During programming, electrical charge is transferred from the substrate to the silicon nitride layer in the ONO structure. Voltages are applied to the gate and drain creating vertical and lateral electric fields, which accelerate the electrons along the length of the channel. As the electrons move along the channel, some of them gain sufficient energy to jump over the potential barrier of the bottom silicon dioxide layer and become trapped in the silicon nitride layer.
Electrons are trapped near the drain region because the electric fields are the strongest near the drain. Reversing the potentials applied to the source and drain will cause electrons to travel along the channel in the opposite direction and be injected into the silicon nitride layer near the source region. Because silicon nitride is not electrically conductive, the charge introduced into the silicon nitride layer tends to remain localized. Accordingly, depending upon the application of voltage potentials, electrical charge can be stored in discrete regions within a single continuous silicon nitride layer.
Non-volatile memory designers have taken advantage of the localized nature of electron storage within a silicon nitride layer and have designed memory circuits that utilize two regions of stored charge within the ONO layer. This type of non-volatile memory device is known as a two-bit EEPROM.
The two-bit EEPROM is capable of storing twice as much information as a conventional EEPROM in a memory array of equal size. A left and right bit is stored in physically different areas of the silicon nitride layer, near left and right regions of each memory cell. Programming methods are then used that enable two-bits to be programmed and read simultaneously. The two-bits of the memory cell can be individually erased by applying suitable erase voltages to the gate and to either the source or drain regions. The structure and operation of this type of memory device is described in a PCT application having the International Publication Number of WO/07000 entitled “TWO BIT NON-VOLATILE ELECTRICALLY ERASABLE AND PROGRAMMABLE SEMICONDUCTOR MEMORY CELL UTILIZING ASYMMETRICAL CHARGE TRAPPING”, the contents of which are fully incorporated herein by reference.
The present invention is directed to the type of memory cell described above that utilize ONO to achieve two-bit operation. Programming of such cells generates significant electrical fields that influence programming of adjacent cells. This can alter the programmed state of a cell adjacent to the one being programmed, or can draw charge to the center of the ONO layer between the opposing bit storage regions where it is extremely difficult, if not impossible, to remove. Therefore, a desirable two-bit ONO memory device would have improved isolation between bits stored on the same device.
SUMMARY OF THE INVENTION
A process for making an array of two-bit floating gate transistors for a semiconductor memory device, comprising:
forming the array of two-bit floating gate transistors having floating gates partially overlying parallel rows of bit-line oxide; and,
forming isolation spacers between the parallel rows of bit-line oxide that divide the floating gates.
According to an aspect of the invention, a process is provided for making an array of two-bit floating gate transistors for a semiconductor memory device, comprising:
providing a silicon wafer substrate having parallel rows of bit-line oxide and a floating gate ONO layer on said silicon wafer substrate and said bit-line oxide, said floating gate ONO layer comprising a first silicon dioxide layer, a silicon nitride layer on said first silicon dioxide layer, and a second silicon dioxide layer on said silicon nitride layer;
depositing a hard mask layer on said floating gate ONO layer;
creating a multitude of openings in said hard mask layer centered upon said rows of bit-line oxide and centered between said rows of bit-line oxide;
depositing a spacer layer on said hard mask layer;
reducing said spacer layer to spacers within said openings by anisotropically etching said spacer layer;
etching through said second silicon dioxide layer and said silicon nitride layer within said openings, thereby exposing said first silicon dioxide layer;
at least partially filling said openings with third silicon dioxide; and,
removing said hard mask layer and said spacers and leaving at least a portion of said third silicon dioxide at least partially filling said openings.
According to a further aspect of the invention, a process is provided for making a semiconductor device having an array of two-bit floating gate transistors, comprising:
providing a silicon wafer substrate having parallel rows of bit-line oxide and a floating gate ONO layer overlying said silicon wafer substrate and said bit-line oxide, said ONO layer compris
Pham Tuan
Rangarajan Bharath
Templeton Mike
Advanced Micro Devices , Inc.
Amin & Turocy LLP
Nelms David
Vu David
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