WO2007001595A2

WO2007001595A2 - Non-volatile electrically alterable memory cell for storing multiple data and an array thereof

Info

Publication number: WO2007001595A2
Application number: PCT/US2006/014151
Authority: WO
Inventors: Andy Yu; Ying Go
Original assignee: Andy Yu; Ying Go
Priority date: 2005-06-21
Filing date: 2006-04-13
Publication date: 2007-01-04
Also published as: CN101506981A; TW200707760A; WO2007001595A3

Abstract

A memory cell that includes a control gate disposed laterally between two floating gates where each floating gate is capable of holding data. Each floating gate in a memory cell may be erased and programmed by applying a combination of voltages to diffusion regions, the control gate, and a well. A plurality of memory cells creates a memory string, and a memory array is formed from a plurality of memory strings arranged in rows and columns.

Description

NON-VOLATILE ELECTRICALLY ALTERABLE MEMORY CELL FOR STORING MULTIPLE DATA AND AN ARRAY THEREOF

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention

[0002] The present invention relates to logic gate structures, and more particularly, to an electrically erasable and programmable read-only memory (EEPROM) and to Flash EEPROMs employing metal-oxide-semiconductor (MOS) floating gate structures. [0003] 2. Description of the Related Art

[0004] Electrically erasable and programmable non-volatile semiconductor devices, such Flash EEPROMs are well known in the art. One type of Flash EEPROM employs metal-oxide-semiconductor (MOS) floating gate devices. Typically, electrical charge is transferred into an electrically isolated (floating) gate to represent one binary state while an uncharged gate represents the other binary state. The floating gate is generally placed above and between two regions (source and drain) spaced-apart from each other and separated from those regions by a thin insulating layer, such as a thin oxide layer. An overlying gate is disposed above the floating gate provides capacitive coupling to the floating gate, allowing an electric field to be established across the thin insulating layer. "Carriers" from a channel region under the floating gate are tunneled through the thin insulating layer into the floating gate to charge the floating gate. The presence of the charge in the floating gate indicates the logic state of the floating gate, i.e., 0 or 1. [0005] Several methods can be employed to erase the charge in a floating gate. One method applies ground potential to two regions and a high positive voltage to the overlying gate. The high positive voltage induces charge carriers, through the Fowler-Nordheim tunneling mechanism, on the floating gate to tunnel through an insulating layer that separates the overlying gate and the floating gate into the overlying gate. Another method applies a positive high voltage to a source region and grounds the overlying gate. The electric field across the layer that separates the source region and the floating gate is sufficient to cause the tunneling of electrons from the floating gate into the source region. [0006] Typically, one control gate and one floating gate form a memory cell and store only one piece of data. Accordingly, to store a large number of data, a large number of memory cells are needed. Another problem faced with traditional memory cells is miniaturization. Shrinking the scale of transistors has made it more difficult to program the floating gate devices, and reduces the ability of the floating gate devices to hold a charge. When the overlaying gate cannot induce enough voltage onto the floating gate, the floating gate cannot retain enough charge for a meaningful read-out. Therefore, the traditional transistor layout is reaching a limitation in miniaturization.

SUMMARY OF THE INVENTION

[0007] In one aspect, the invention is an electrically alterable memory device. The memory device includes a semiconductor substrate and a semiconductor well. The semiconductor substrate is doped with a first dopant in a first concentration, and a semiconductor well, adjacent the semiconductor substrate, is doped with a second dopant that has an opposite electrical characteristic than the first dopant. The semiconductor well having a top side on which two spaced-apart diffusion regions are embedded. Each diffusion region is doped with the first dopant in a second concentration greater than the first concentration. The two diffusion regions includes a first diffusion region and a second diffusion region, and a first channel region is defined between the first diffusion region and the second diffusion region. The memory device also includes a first floating gate, a second floating gate, and a control gate. The first floating gate is disposed adjacent the first diffusion region and above the first channel region and separated therefrom by a first insulator region. The first floating gate has a first height and is made from a conductive material and capable of storing electrical charge. The second floating gate is disposed adjacent the second diffusion region and above the first channel region and separated therefrom by a second insulator region. The second floating gate has a second height and is made from a conductive material and capable of storing electrical charge. The control gate is disposed laterally between the first floating gate and the second floating gate. The control gate is separated from the first floating gate by a first vertical insulator layer and separated from the second floating gate by a second vertical insulator layer. The control gate is disposed above the first channel region and separated therefrom by a third insulator region. The control gate has a third height and is made from a conductive material. [0008] In another aspect, the invention is an electrically alterable memory string. The memory string includes a plurality of memory devices, each memory device having a control transistor capable of storing a plurality of data. The plurality of memory devices has a fast end and a second end. A first select transistor connected to the first end, a second select transistor connected to the second end, and a connector connecting the first select transistor to a bit line.

[0009] In another aspect, the invention is an electrically alterable nonvolatile memory array. The memory array includes a plurality of memory strings, each memory string having a fast connector connected to a drain of a first select transistor in the memory string, a second connector connected to a gate of the first select transistor, a third connector connected to a gate of a memory cell transistor in the memory string, and a fourth connector connected to a gate of a second select transistor in the memory string. The plurality of memory strings are arranged in such way that the drain of the first select transistor in a fast memory string is connected to a source of the second select transistor in an adjacent second memory string. The memory array also includes a plurality of bit lines, wherein each bit line being connected to the first connector of every memory string, a plurality of first select lines, wherein each first select line being connected to the second connector of every memory string, a plurality of control lines, where each control line being connected to the third connector of every memory string, and a plurality of second select lines, wherein each second select line being connected to the fourth connector of every memory string.

[0010] Other advantages and features of the present invention will become apparent after review of the hereinafter set forth Brief Description of the Drawings, Detailed Description of the Invention, and the Claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a top plan view of a plurality of memory strings according to one embodiment of the invention. [0012] FIG. 2A is a cross sectional view of the memory string taken along line 2-2 of FIG. 1.

[0013] FIG. 2B is a cross sectional view of an alternative embodiment of the memory string taken along line 2-2 of FIG. 1.

[0014] FIG. 3 is a cross sectional view of the memory string taken along line 3-3 of FIG. 1.

[0015] FIG. 4 is a cross sectional view of the memory string taken along line 4-4 of FIG. 1.

[0016] FIG. 5 is a top plan view of a plurality of memory strings according to one alternative embodiment of the invention.

[0017] FIG. 6A is a cross sectional view of yet another alternative embodiment of the memory string taken along line 6-6 of FIG. 5.

[0018] FIG. 6B is a cross sectional view of yet another alternative embodiment of the memory string taken along line 6-6 of FIG. 5.

[0019] FIG. 6C is a cross sectional view of yet another alternative embodiment of the memory string taken along line 6-6 of FIG. 5.

[0020] FIG. 7 is a schematic of a top plan of a plurality of memory cells according to one embodiment of the invention.

[0021] FIG. 8 is a schematic of a top plan of a plurality of memory ceils according to one alternative embodiment of the invention.

[0022] FIG. 9 lists several combinations of operational voltages according to one embodiment of the invention.

[0023] FIG. 10 is a cross sectional view of a unitary embodiment of a memory cell according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0024] Three electrically programmable and erasable non-volatile memory strings are shown in FIG 1. Each memory string 100 includes an active region 106 running vertically and a plurality of control gates 102 running horizontally across multiple memory strings. The active region is heavily doped with a first dopant. The control gate is formed by polysilicon or other suitable material. A plurality of floating gates 104 are disposed adjacent to the control gate 102 and over the active region 106. Each control gate 102 is surrounded by two floating gates 104 on two sides. [0025] The combination of two floating gates 104 surrounding one control gate 102 over one area of the active region 106 forms a memory cell 103. Each memory cell 103 stores two data, one on each floating gate 104. Each memory string 100 may have many memory cells 103. The memory ceils 103 on a memory string 100 are delimited by a first select gate 116 and a second select gate 120. The first select gate 116 and the second select gate 120 run horizontally over all memory strings 100 and over the active region 106. The area of the active region 106 not covered by the floating gates 104, the control gates 102, and the select gates 114, 116, 118, 120 are doped diffusion regions. A vertical connector 121 connects the active region 106 to a bit line 110 that runs vertically through multiple memory strings 100. [0026] Each memory string 100 is connected to an adjacent memory string 100 through the active region 106. The separation of memory cells 103 in one memory string 100 from memory cells 103 of an adjacent memory string 100 may be accomplished through an isolation layer 122, such as localized oxidation (LOCOS), recessed LOCOS, shallow trench isolation (STI), or full oxide isolation. A plurality of memory strings 100 may form a high density memory array.

[0027] FIG. 2A is a cross section view 200 of a memory cell 103 taken along line 2-2 in FIG. 1. The memory cell 103 includes a semiconductor substrate 202 and a well 204 on the top of the substrate 202. The substrate is doped with a first dopant, which can be either N type or P type. The well 204 is a semiconductor doped with a second dopant with an electrical characteristic that is opposite of the first dopant. Two spaced-apart diffusion regions 106a and 106b, which are part of the active region 106, are placed on the top side of the well 204. The diffusion regions 106a, 106b are doped with the same dopant used for doping the substrate 202 but doped with a concentration that is higher than that of the substrate 202. A channel region 234 is defined between two diffusion regions 106a, 106b. An insulating layer 230 is placed on the top of the well 204 and the diffusion regions 106a, 106b. The insulating layer 230 may be formed by an insulating oxide material or other suitable insulating materials. Though FIG. 2A illustrates the diffusion regions 106a, 106b implemented in a single well, it is understood that other implementations, such as twin wells, triple wells, or oxide isolation well may also be used. The separation of active devices may be accomplished through localized oxidation (LOCOS), recessed LOCOS, shallow trench isolation (STI), or full oxide isolation. A first floating gate 104a of polysilicon material is placed above the channel region 234 and adjacent diffusion region 106a. The first floating gate 104a may overlap slightly with the diffusion region 106a; however, excessive overlapping may reduce the length of the channel region 234. The first floating gate 104a is separated from the channel region 234 by a tunnel channel 214a (also known as tunnel oxide) of the insulating layer 230. The thickness of the tunnel channel 214a should be thin enough to allow removal of electrons from the first floating gate 104a under the Fowler- Nordheim tunneling mechanism, but thick enough to prevent the occurrence of a leakage current between the first floating gate 104a and the well 204. The length of the tunnel channel 214a under the first floating gate 104a can be smaller than one lambda, where the lambda is defined by the technology used. For example, if the technology uses 0.18μm, then one lambda is defined as 0.18μm.

[0028] A second floating gate 104b of polysilicon material is placed above the channel region 234 and adjacent diffusion region 106b. The second floating gate 104b may overlap slightly with the diffusion region 106b; however, excessive overlapping may reduce the length of the channel region 234. The second floating gate 104b is separated from the channel region 234 by a tunnel channel 214b (also known as tunnel oxide) of the insulating layer 230. The thickness of the tunnel channel 214b should be thin enough to allow removal of electrons from the first floating gate 104b under the Fowler- Nordheim tunneling mechanism, but thick enough to prevent the occurrence of a leakage current between the second floating gate 104b and the well 204. [0029] A control gate 102 is placed above the channel region 234, laterally between the first floating gate 104a and the second floating gate 104b. The control gate 102 is separated from the first floating gate 104a by a first vertical insulating layer 212a and from the second floating gate 104b by a second vertical insulating layer 212b. The insulating layers 212a, 212b can be oxide- nitride-oxide or other suitable material. The control gate 102 is separated from the channel region 234 by a separation channel 216 (also known as separation oxide) of the insulating layer 230. The thickness of the separation channel 216 should be thick enough to sustain the stress from the control gate's 102 voltage variation. The voltage at the control gate 102 may vary during operation of the memory cell 103 and cause stress on the separation channel 216, thus leading to the deterioration of the separation channel 216. The control gate 102 may be formed by a polysilicon grown at a different stage as the floating gates 104a, 104b. The control gate 102 is connected to control gates in other memory cells in different memory strings. The control gate 102 is surrounded by two floating gates 104a, 104b. [0030] The fast floating gate 104a has a first height measured from its bottom edge to its top edge and the second floating gate 104b has second height also measured from its bottom edge to its top edge. The control gate 102 has a third height measured from its bottom edge to its top edge. The first height, the second height, and the third height may be identical or may be different. The first height and the second height may be taller or shorter than the third height.

[0031] The cross section view of one alternative embodiments of the memory cell 103 taken along line 2-2 in FIG. 1 is shown in FIG. 2B. FIG. 2B illustrates a cross section view 300, where each floating gate 104a and 104b surrounds the control gate 102 on more than one lateral side. Because of greater exposure of the surface of a floating gate 104a, 104b to the control gate 102, greater the coupling ratio between the control gate's voltage and the floating gate voltage can be achieved.

[0032] Referring back to FIG. 2A, when a voltage is applied to the control gate 102, through a coupling effect, a voltage is induced on the floating gates 104a, 104b. The voltage induced depends on a coupling ratio between the control gate 102 and the floating gate 104a. The coupling ratio is defined as the capacitance ratio between the capacitance between the control gate 102 and the floating gate 104a and the capacitance between the floating gate 104a and the substrate 204.

C(CG/FG) = capacitance between the control gate and the floating gate C(FG/Substrate) = capacitance between the floating gate and the substrate Gamma = coupling ratio

JCG Gamma =

FG ^

C

[FG) { β; ubstrate)

When a V_CG is applied to the control gate, the voltage at the floating gate is:

VFG = VcG x Gamma

[0033] The coupling effect depends on the thickness of the layers 212a, 212b separating the control gate 102 from the floating gates 104a, 104b and the area on each floating gate 104a, 104b exposed to the coupling effect. The coupling effect can be easily increased by increasing the area of the floating gate 210 exposed to the control gate 212, and the area of the floating gate 210 exposed to the control gate 212 may be increased by increasing the height 234 of the control gate 212 and the height 232 of the floating gate 210. A capacitor is formed between the control gate 102 and each floating gate 104a, 104b. When a floating gate 104a, 104b is surrounded by a control gate 102 in more than one lateral side, the coupling effect is increased and the capacitance between the floating gate 104a, 104b and the control gate 102 is increased. If the layer 212a, 212b separating the control gate 102 and the floating gate 104a, 104b is too thin, a leakage current may occur between the floating gate 104a, 104b and the control gate 102 when the floating gate 104a, 104b is charged with electrons. If the layer 212a, 212b is too thick, the coupling ratio may be low, resulting in a low voltage in the floating gate. One workable coupling ratio is between 50%-80%, i.e., 10 V applied to the control gate 102 results in 5 V to 8 V induced in the floating gate 104a, 104b. The combination of the control gate 102, the floating gates 104a, 104b, and the diffusion regions 106a, 106b forms a control transistor. The control transistor is capable of holding two data independently, one in each floating gate 104a, 104b. Each floating gate 104a, 104b may be independently programmed. [0034] The induction of voltage on the floating gate 104a, 104b is important when erasing or programming a memory cell 103. When programming the floating gate 104b of a memory cell 103 of N-type diffusion, a positive high voltage (Vpp) between 4V and 1 IV is applied to the control gate 102, and the diffusion region 106a and the well 204 are left at 0 V. A positive high voltage between 4V and 11V is also applied to the diffusion region 106b. The positive high voltage depends on the technology used. A voltage is induced to the floating gates 104a, 104b by the Vpp at the control gate 102 through the coupling effect. When the control gate 102 is at the Vpp and inducing voltages onto the floating gates 104a, 104b, the channel 234 between the diffusion regions 106a and 106b are conductive. With the channel 234 being conductive and the diffusion region 106b at the Vpp, electrons flow between the diffusion regions 106a, 106b, and the phenomenon of impact ionization (several occurrences) occurs near the diffusion region 106b. The impact ionization occurs when charge carriers moving toward the diffusion region 106b generate electron-hole pairs from the lattice near the drain junction (diffusion region 106b). The generated carriers look for high positive voltage and are injected into the floating gate 104b. The carriers emitted from the source region 106a experience lateral electrical field between the diffusion regions 106a and 106b. The average carder energy is higher near the drain junction of the diffusion region 106b. The impact ionization tends to occur near the diffusion region 106b. Of free electrons, only lucky few will be injected into the floating gate 104b, and this is known as Lucky electron model. The amount of electrons injected into the floating gate 104b depends on the positive high voltage applied to the control gate 102 and the duration of this positive high voltage. To program the floating gate 104a, the similar process may be used but the voltages at the diffusion regions 106a and 106b are reversed, i.e., a positive high voltage is applied to the diffusion region 106a while the diffusion region 106b and the well 204 are at zero volt. [0035] A ramping positive high voltage (Vppr) may be applied to the control gate 102 to program a floating gate 104b in a memory cell of P-type diffusion. A positive high voltage between 4V and 11V is initially applied to the control gate 102, and this positive high voltage is gradually ramped down to OV and then ramped up back to 4V-1 IV. A positive high voltage is applied to the diffusion region 106a and OV is applied to the diffusion region 106b. When the control gate 102 is at the positive high voltage of 4V-1 IV, a voltage is induced onto the floating gate 104b and the channel 234 between the diffusion regions 106a and 106b is turned off. Although the floating gate 104b is at a positive voltage level, no electrons are injected into the floating gate 104b because the channel 234 is off and there is no flow of electrons between the diffusion regions 106a and 106b. As the voltage at the control gate 102 ramps down, the potential difference between the control gate 102 and the well 204 turns on the channel between the diffusion regions 106a and 106b, and electrons start to flow in the channel 234. The voltage at the floating gate 104b also drops as the voltage at the control gate 102 ramps down, but the voltage at the floating gate 104b is still sufficient to cause some high energy electrons (also known as hot electrons) to be injected into the floating gate 104b. When the control gate 102 reaches zero voltage, the channel 234 is turned on, but no electrons are injected into the floating gate 104b because the floating gate 104b is also at zero voltage. When the voltage at the control gate 102 starts to ramp up back to 4V-11V, the voltage at the floating gate 104b also ramps up, and high energy electrons from the channel 234 start to be injected into the floating gate 104b again. When the control gate 102 is at positive high voltage of 4V- 1 IV, the channel 234 is turned off, electrons stop flowing, and no more electrons are injected into the floating gate 104b. The number of electrons injected into the floating gate 104b depends on the duration of the ramp down/up process and the concentration of dopants in the channel region. This voltage ramping process may be repeated for the floating gate to retain enough charge to represent a logic state properly. Once charges of electrons are inside of the floating gate 104b, the floating gate 104b may hold the charges for years. The voltage ramping may also be used to program memory cells of N-type diffusion.

[0036] The amount of charge injected into a floating gate 104a, determines the threshold voltage for the control transistor formed by the control gate 102, the floating gates 104a, 104b, and the diffusion regions 106a, 106b. The floating gate 104a may hold different amount of charges, thus having different threshold voltages. In one embodiment of the invention, through repeating the voltage ramping process, the floating gate 104a may have four different levels of threshold voltages and capable of representing four logic states. The four logic states may be read and distinguished by measuring the current flowing between the diffusion regions 106a, 106b.

[0037] A P-type diffusion memory cell may also be programmed with a different mechanism. Applying a negative voltage between -1V and -10V to diffusion region 106b, a positive high voltage to the control gate 102, and a voltage between OV and Vcc to the well 204, charges can be programmed into floating gate 104b. The high positive voltage of the control gate 102 induces a voltage into the floating gate 104b. The difference of potential between the well 204 and the diffusion region 106b causes a soft avalanche breakdown between the diffusion region 106b (P-type) and the well 204 (N- type). Some of the electrons from this soft breakdown are injected into the floating gate 104b because the floating gate 104b is at higher voltage. [0038] A negative voltage is applied between -4.5V and -10V to the control gate 102, a positive high voltage is applied to the well 204 when it is desired to erase charges in the memory cell 103 of N-type diffusion. The negative voltage at the control gate 103 is induced to the floating gates 104a, 104b. The combination of an induced negative voltage at the floating gates 104a, 104b and positive high voltages at the well 204 forces electrons out of the floating gates 104a. 104b and into the well 204, thus removing the electrons from the floating gates 104a, 104b. The electrons are removed through the Fowler-Nordheim tunneling mechanism.

[0039] A negative voltage is applied between -4.5V and -10V to the control gate 102, a positive high voltage is applied to the well 204 and the diffusion regions 106a, 106b when it is desired to erase charges in the memory cell 103 of P-type diffusion. The mechanism to remove the electrons is similar to what has been described above for the N-type diffusion except that the positive high voltage is needed at the diffusion regions 106a and 106b because otherwise the channel 234 may be floating at an unknown voltage and impeding the exit of electrons from the floating gates 104a, 104b. [0040] When it is desired to read the content from a floating gate 104a of a memory cell 103, a voltage between OV and Vcc is applied to the control gate 102, a voltage between 0 V to Vcc is applied to diffusion region 106b, and OV to -2V is applied to two select gates (not shown in FIG. 2A) in the memory string, one at each end of the memory string. The voltage at the control gate 102 turns on the portion of the channel 234 under it. The threshold voltage (Vt) for the floating gate 104b is lowered because of drain-induced barrier lowering (DIBL) and a depletion region is created under the floating gate 104b. If the floating gate 104a has charge, the portion of the channel 234 under it will be on and a current flows from diffusion region 106b to diffusion region 106a. The channel 234 under the floating gate 104a and the control gate 102 is on, the current passes under the floating gate 104a and the control gate 102, and then enter the depletion region under the floating gate 104b. The current will continue to flow through the depletion region under the floating gate 104a toward the diffusion region 106a. Because the select gates are at OV to -2V, the current resulting from the electron flow is sensed by a bit line and a sense- amplifier connected to the bit line. The data stored in the floating gate 104a comes out from the drain of the control transistor. When the floating gate 104a is programmed to store different levels of charge and thus with different levels of threshold voltage, the intensity of the current flowing between diffusion region 106a and diffusion region 106b depends on the threshold voltage of the floating gate 104a. The intensity of this current can be sensed by the sense-amplifier, thus the logic level of the floating gate 104a determined. [0041] If the floating gate 104a is without charge, then the portion of the channel 234 under the floating gate 104a will not be turned on and there will be no current or small leakage current flowing between diffusion region 106b and diffusion region 106a. The leakage current should be different from the current flowing when the floating gate 104a is charged. If the floating gate 104a is not charged, then no channel is established between the diffusion regions 106a and 106b and the sense-amplifier will not be able to detect any current. The absence of a current between the diffusion regions 106a and 106b indicates the floating gate 104a is without electrons. A floating gate 104a with electrons is assigned to a first logic state while a floating gate 104a without electrons or with too few electrons is assigned to an opposite second logic state. Other operations not described here can be easily understood based on voltages listed in FIG. 9 and operations described above by those skilled in the art. When reading the content of a floating gate 104a in an N- type diffusion device, OV is applied to 106a, and IV to 2.5V is applied to 106b. The voltage in the diffusion region 106b will lower the threshold voltage of the floating gate 104b because of DIBL effect.

[0042] FIG. 3 is a cross section view 400 taken along line 3-3 in FIG. 1. FIG. 3 illustrates a cross section view of a memory string 100. The memory string 100 includes a substrate 202 doped with a first dopant, which can be either N type or P type, and a well 204 doped with a second dopant with an electrical characteristic that is opposite of the first dopant. A plurality of diffusion regions 106, which are part of the active region 106 in FIG. 1, are placed on the top side of the well 204. The diffusion regions 106 are doped with the same dopant used for doping the substrate 202 but doped with a concentration that is higher than that of the substrate 202. A plurality of control transistors are placed adjacent each other. Each control transistor includes a control gate 102, two floating gates 104, and two diffusion regions 106, one diffusion region being the drain of the control transistor while the other diffusion region is the source. Two adjacent control transistors share one common diffusion region 106. Each control transistor is a memory cell. There is one select transistor 402 at one end of this "string" of memory ceils and another select transistor 404 at the other end of the string of memory cells. There is a vertical contact 406 connecting one diffusion region 106 at the end of the memory string to a bit line 108 in FIG. 1. A diffusion region 106 from one memory string 100 is connected to a diffusion region 106 of an adjacent memory string 100 (shown in FIG. 1). FIG. 4 is a cross section view 500 taken along line 4-4 in FIG. 1. It is shown that memory strings represented by a floating gate 104 are separated from each other by isolation layers 122.

[0043] FIG. 5 is a top plan view of an alternative embodiment of the invention. In this embodiment the memory ceils 603 in one memory string 600 are between two diffusion regions 106a, 106b. Each control gate 102 runs horizontally in FIG. 5 and across different memory strings 600. Each memory string 600 is delimited by two select gate transistors SGOa, SG1a in a manner similar as that depicted in FIG. 1. One STI 612 separates one diffusion region of one memory string 600 from a diffusion region for an adjacent memory string. The diffusion region 106a is connected to a bit line 602 through a buried contact and the diffusion region 106b is connected to a bit line 604 also through a buried contact. When it is desirable to read a data from a floating gate 104b of a memory cell 603, the control gate 102 for the selected memory cell is turned on. A bit line 604 is connected to a source voltage. The voltage at the bit line 604 is propagated through the diffusion region 106b to the memory cell 603. The read operation at the memory cell 603 is similar to that described for FIG. 2. The data is read from the drain of the control gate transistor, bit line 602. The program and ease operations for the embodiment of FIG. 5 are same as those previously described for FIG. 2. For the embodiment of FIG. 5, there is no need to turn on the control transistors of unselected memory cells. As matter of fact, the control transistors of unselected memory ceils are turned off to prevent short between diffusion regions 106a and 106b.

[0044] FIG. 6A is a cross section view 700 taken along line 6-6 in FIG. 5. The floating gates 104a, 104b are surrounded by the control gate 102 from top and two lateral sides. FIG. 6B is a cross section view 800 of an alternative embodiment taken along line 6-6 in FIG. 5. FIG. 6C is a cross section view 900 of yet another alternative embodiment taken along line 6-6 in FIG. 5. It is also illustrated in FIG. 6C two oxidation sections 612a, 612b. Each oxidation section 612a disposed on the top of a diffusion region 106a. The oxidation section 612 does not divide a diffusion region 106 into two, but it does lessen the capacitance of the diffusion region 106. When the capacitance of a diffusion region 106 is smaller, the fast is the speed a data can be read out from a memory cell 603.

[0045] FIG. 7 is a schematic representation of part of a memory array 1000 made from the memory strings 100. Memory cells 1002 in one memory string (running vertically in FIG. 7) are interconnected. The drain of one memory cell is connected to the source of an adjacent memory cell. Each memory string includes two select transistors 1003, 1005 one at each end of the memory string. One end of the memory string is connected to a bit line 1018 and also connected to an adjacent memory string. The memory strings are disposed parallel to each other and the resulting memory array are organized in rows and columns. The select transistors 1003 of odd columns are controlled by one select line 1004, while the select transistors 1003 of even columns are controlled by another select line 1006. Similarly, the select transistors 1005 of odd columns are controlled by one select line 1016, while the select transistors 1005 of even columns are controlled by another select line 1014. The control transistors in one memory row are interconnected together and controlled by a control line. Data operations to one floating of a memory cell in one memory string is controlled by activating proper control line, select lines, and bit lines as described above for FIG. 2A. The activation of control lines and select lines depends on an X-address decoder (not shown) and the activation of bit lines depends on a Y-address decoder (not shown). Each bit line may be connected to a charging transistor and a discharging transistor (not shown) that are also controlled by the Y-address decoder. [0046J FIG. 8 is a schematic presentation of a memory array 1100 made from memory strings 600. One side of a control transistor of the memory cell 1112 is connected to a bit line 1102, other side of the control transistor is connected to another bit line 1104, and the gate of the control transistor is connected to a control gate line 1106. Other select logics for enabling and selecting each memory cell are not shown in FIG. 8 but are easily understood by those skilled in the art.

[0047] The thickness of each gate (control gate, and floating gate) depends on the manufacturing process; currently most common thickness is about 3000 Angstroms or 0.3 micron. The thickness of the tunnel channels 214a, 214b depend also on manufacturing process. However, a preferred thickness for the tunnel channels 214a, 214b is between 70 Angstroms and 110 Angstroms. Similarly, the thickness of the insulating layer separating the control gate 102 from the well 204 is between 150 Angstroms and 250 Angstroms. The materials and measurements mentioned heretofore are for illustration purposes and not intended to limit the scope of the present invention. It is recognized that as technology evolves, other suitable materials and manufacturing processes may be employed to realize the present invention. It is also understood that the structures disclosed heretofore can be easily implemented by any of existing semiconductor manufacturing processes known to those skilled in the art. It is also understood that the voltages illustrated in FiG. 9 is for illustration purposes, and other voltage combinations may be used. For example, voltages may be reduced for embodiments that have a large coupling ratio, and small voltages make manufacturing easier and enhance reliability.

[0048] FIG. 10 illustrates a unitary flash memory cell according to the invention. The unitary flash memory cell includes a semiconductor substrate 1210 and a well 1208 on the top of the substrate 1210. The substrate is doped with a first dopant, which can be either N type or P type. The well 1208 is a semiconductor doped with a second dopant with an electrical characteristic that is opposite of the first dopant. One diffusion region 1212 is placed on the top side of the well 1208. The diffusion region 1212 is doped with the same dopant used for doping the substrate 1208 but doped with a concentration that is higher than that of the substrate 1210. A floating gate 1202 is placed adjacent to the diffusion region 1212, but separated by a tunnel channel (also known as tunnel oxide or insulator tunnel) 1214. The floating gate 1202 can be made of conductive or semiconductive material. The most adequate material is polysilicon with N-tyep dopant. The floating gate 1202 may overlap slightly with the diffusion region 1212. The thickness of the tunnel channel 1214 should be thin enough to allow removal of electrons from the floating gate 1202 under the Fowler-Nordheim tunneling mechanism, but thick enough to prevent the occurrence of a leakage current between the floating gate 1202 and the well 1208.

[0049] A control gate 1204 is placed parallel to the floating gate 1202 and separated thereof by an insulating layer 1206. The insulating layer 1206 can be oxide-nitride-oxide or other suitable material. The control gate 1204 is separated from the well 1208 by an insulator 1216, i.e. oxide. The thickness of the insulator 1216 should be thick enough to sustain the stress from the control gate's 1204 voltage variation. The insulator 1216 under the control gate 1204 is thicker than tunnel oxide 1214 to sustain the stress and have better reliability, if the insulator 1216 is made of oxide. The voltage across the insulator 1216 under the control gate 1204 being higher than the voltage across the tunnel oxide 1214. One can discharge the electrons from floating gate 1202 by applying a negative voltage to the control gate 1204 and applying a positive voltage to the diffusion region 1212. The electrons are discharged through the tunnel oxide 1214.

[0050] The electrons can be injected into the floating gate 1202 from a depletion region between the diffusion region 1212 and well 1208 by applying a positive voltage to the control gate 1202 and a negative voltage to the diffusion region 1212. The negative voltage preferably is between -1V to - 10V. The well 1208 is biased at OV or a small positive voltage. The electrons will be generated from valence to conduction band when the junction between the diffusion region 1212 and well 1208 is reversed biased. Some of these electrons will be injected into the floating gate 1202 through the tunnel oxide 1214 by overcoming the potential barrier between the well 1208 and the tunnel oxide 1214.

[0051] Although, the present application is described for Flash EEPROMs, it is understood that the invention is equally applicable for one-time- programmable (OTP) memories, multiple-time-programmable (MTP) memories, and other non-volatile memories.

[0052] While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the present invention as set forth in the following claims. Furthermore, although elements of the invention may be described or claimed in the singular, the plural is contemplated unless limitation to the singular is explicitly stated.

Claims

CLAIMSWhat is claimed is:

1. An electrically alterable memory device, comprising: a first semiconductor layer doped with a first dopant in a first concentration; a second semiconductor layer, adjacent the first semiconductor layer, doped with a second dopant that has an opposite electrical characteristic than the first dopant, the second semiconductor layer having a top side; two spaced-apart diffusion regions embedded in the top side of the second semiconductor layer, each diffusion region doped with the first dopant in a second concentration greater than the first concentration, the two diffusion regions including a first diffusion region and a second diffusion region, and a first channel region defined between the first diffusion region and the second diffusion region; a first floating gate having a first height and comprised of a conductive material, the first floating gate disposed adjacent the first diffusion region and above the first channel region and separated therefrom by a first insulator region, the first floating gate capable of storing electrical charge; a second floating gate having a second height and comprised of a conductive material, the second floating gate disposed adjacent the second diffusion region and above the first channel region and separated therefrom by a second insulator region, the second floating gate capable of storing electrical charge; and a control gate having a third height higher than the first height and the second height and comprised of a conductive material, the control gate disposed laterally between the first floating gate and the second floating gate, the control gate separated from the first floating gate by a first vertical insulator layer and separated from the second floating gate by a second vertical insulator layer, the control gate acting as a word select line, the control gate further being above the first channel region without overlapping the two spaced-apart diffusion regions and separated therefrom by a third insulator region.

2. The memory device of claim 1 , wherein the first dopant having a P-type characteristic and the second dopant having an N-type characteristic.

3. The memory device of claim 1 , wherein the first dopant having an N- type characteristic and the second dopant having a P-type characteristic.

4. The memory device of claim 1 , wherein the first insulator region having a thickness that allows tunneling of charge between the first floating gate and the first channel region.

5. The memory device of claim 4, wherein the thickness of the first insulator region is between 70 Angstroms and 110 Angstroms.

6. The memory device of claim 1, wherein the second insulator region having a thickness that allows tunneling of charge between the second floating gate and the first channel region.

7. The memory device of claim 6, wherein the thickness of the second insulator region is between 70 Angstroms and 110 Angstroms.

8. The memory device of claim 1 , wherein the first vertical insulator is made from a silicon dioxide having a thickness that provides capacitance between the first floating gate and the control gate, and the first vertical insulator preventing leakage between the first floating gate and the control gate.

9. The memory device of claim 1 , wherein the first vertical insulator is made from an oxide nitride oxide having a thickness that provides capacitance between the first floating gate and the control gate, and the first vertical insulator prevents leakage between the first floating gate and the control gate.

10. The memory device of claim 1, wherein the second vertical insulator is made from a silicon dioxide having a thickness that provides capacitance between the second floating gate and the control gate, and the second vertical insulator preventing leakage between the second floating gate and the control gate.

11. The memory device of claim 1 , wherein the second vertical insulator is made from an oxide nitride oxide having a thickness that provides capacitance between the second floating gate and the control gate, and the second vertical insulator preventing leakage between the second floating gate and the control gate.

12. The memory device of claim 1 , wherein the first height of the first floating gate is taller than the third height.

13. The memory device of claim 1 , wherein the first height of the first floating gate is shorter than the third height.

14. The memory device of claim 1 , wherein the first height of the first floating gate is same as the third height.

15. The memory device of claim 1 , wherein the first floating gate and the second floating gate each being capable of storing multiple levels of charge.

16. The memory device of claim 1 , wherein the first floating gate and the second floating gate each being capable of storing four levels of charge.

17. The memory device of claim 1 , wherein a charge is transported from the first channel region to the second floating gate when a first combination of voltages is applied to the first diffusion region, the second diffusion region, the control gate, and the second semiconductor layer.

18. The memory device of claim 17 wherein the first combination of voltages comprises: applying a zero voltage to the second semiconductor layer; applying a positive high voltage to the first diffusion region; applying a zero voltage to the second diffusion region; and applying a positive high voltage to the control gate.

19. The memory device of claim 17, wherein the first combination of voltages comprises: applying a positive high ramp down voltage followed by a ramp up voltage to the control gate; applying a positive high voltage to the first diffusion region; applying a zero voltage to the second diffusion region; and applying a positive high voltage to the second semiconductor layer.

20. The memory device of claim 17 wherein the first combination of voltages comprises: applying a Vcc voltage to the second semiconductor layer; applying a negative voltage to the second diffusion region; and applying a positive high voltage to the control gate.

21. The memory device of claim 17 wherein the first combination of voltages comprises: applying a OV voltage to the second semiconductor layer; applying a negative voltage to the second diffusion region; and applying a positive high voltage to the control gate.

22. The memory device of claim 17 wherein the first combination of voltages comprises: applying a positive voltage to the second semiconductor layer; applying a negative voltage to the second diffusion region; and applying a positive high voltage to the control gate.

23. The memory device of claim 1 , wherein charge inside the second floating gate can be determined when a second combination of voltages is applied to the first diffusion region, the second diffusion region, the control gate, and the second semiconductor layer.

24. The memory device of claim 23, wherein the second combination of voltages comprises: applying a zero voltage to the second semiconductor layer; applying a voltage between OV and Vcc voltage to the first diffusion region; and applying a voltage between OV and Vet to the control gate.

25. The memory device of claim 23, wherein the second combination of voltages comprises: applying a Vet voltage to the second semiconductor layer; applying a voltage between Ov and Vcc to the first diffusion region; applying a Vet voltage to the second diffusion region; and applying a voltage between OV and Vcc voltage to the control gate.

26. The memory device of claim 1, wherein charge is removed from the second floating gate when a third combination of voltages is applied to the first diffusion region, the second diffusion region, the control gate, and the second semiconductor layer.

27. The memory device of claim 26, wherein the third combination of voltages comprises: applying a negative voltage to the control gate; and applying a positive high voltage to the second semiconductor layer.

28. An electrically alterable non-volatile memory string comprising: a plurality of memory devices, each memory device having a control transistor capable of storing a plurality of data, the plurality of memory devices having a first end and a second end; a first select transistor connected to the first end; a second select transistor connected to the second end; and a connector connecting the first select transistor to a bit line.

29. The memory string of claim 28, wherein the first select transistor being further connected to the second select transistor of an adjacent memory string.

30. The memory string of claim 28, wherein the control transistor being capable of storing data representing four logic states.

31. The memory string of claim 28, wherein the control transistor being capable of storing data representing multiple logic states.

32. The memory string of claim 28, one memory device being connected to an adjacent memory device, whereby a drain of one memory device being connected to a source of an adjacent device.

33. An electrically alterable non-volatile memory string comprising: a plurality of memory devices, each memory device having a control transistor capable of storing a plurality of data, each memory device having a first end and a second end; a first diffusion region connected to the first end of each memory device; a second diffusion region, spaced-apart from the first diffusion region, connected to the second end of each memory device; and a plurality of connectors connecting the control transistor of each memory device to a bit line.

34. The memory string of claim 33, wherein the control transistor being capable of storing data representing four logic states.

35. The memory string of claim 33, wherein the control transistor being capable of storing data representing multiple logic states

36. The memory string of claim 33, wherein the memory devices are connected in parallel to each other between the first diffusion region and the second diffusion region.

37. An electrically alterable, non-volatile memory array, comprising: a plurality of memory strings, each memory string having a first connector connected to a drain of a first select transistor in the memory string, a second connector connected to a gate of the first select transistor, a third connector connected to a gate of a memory cell transistor in the memory string, and a fourth connector connected to a gate of a second select transistor in the memory string, wherein the plurality of memory strings are arranged in such way that the drain of the first select transistor in a first memory string is connected to a source of the second select transistor in an adjacent second memory string; a plurality of bit lines, wherein each bit line being connected to the first connector of every memory string; a plurality of first select lines, wherein each first select line being connected to the second connector of every memory string; a plurality of control lines, where each control line being connected to the third connector of every memory string; and a plurality of second select lines, wherein each second select line being connected to the fourth connector of every memory string.

38. The memory array of claim 37, wherein the memory cell transistor of one memory string is separated from a memory cell transistor of adjacent memory string by an isolation layer.

39. The memory array of claim 38, wherein the isolation layer is a shallow trench isolation.

40. An electrically alterable memory device, comprising: a first semiconductor layer doped with a first dopant in a first concentration; a second semiconductor layer, adjacent the first semiconductor layer, doped with a second dopant that has an opposite electrical characteristic than the first dopant, the second semiconductor layer having a top side; two spaced-apart diffusion regions embedded in the top side of the second semiconductor layer, each diffusion region doped with the first dopant in a second concentration greater than the first concentration, the two diffusion regions including a first diffusion region and a second diffusion region, and a first channel region defined between the first diffusion region and the second diffusion region; a first floating gate having a left side, and a right side and comprised of a conductive material, the first floating gate disposed adjacent the first diffusion region and above the first channel region and separated therefrom by a first insulator region, the first floating gate capable of storing electrical charge; a second floating gate having a left side, and a right side and comprised of a conductive material, the second floating gate disposed adjacent the second diffusion region and above the first channel region and separated therefrom by a second insulator region, the second floating gate capable of storing electrical charge; and a control gate comprised of a conductive material, the control gate disposed laterally between the first floating gate and the second floating gate, the control gate separated from the first floating gate by a third insulator layer and separated from the second floating gate by a fourth insulator layer, the control gate covering the first floating gate on at least right side and left side, the control gate further covering the second floating gate on at least right side and left side, the control gate further being above the first channel region and separated therefrom by a third insulator region.

41. The memory device of claim 40, wherein the first dopant having a P- type characteristic and the second dopant having an N-type characteristic.

42. The memory device of claim 40, wherein the first dopant having an N- type characteristic and the second dopant having a P-type characteristic.

43. The memory device of claim 40, wherein the first insulator region having a thickness that allows tunneling of charge between the first floating gate and the first channel region.

44. The memory device of claim 43, wherein the thickness of the first insulator region is between 70 Angstroms and 110 Angstroms.

45. The memory device of claim 40, wherein the second insulator region having a thickness that allows tunneling of charge between the second floating gate and the first channel region.

46. The memory device of claim 45, wherein the thickness of the second insulator region is between 70 Angstroms and 110 Angstroms.

47. The memory device of claim 40, wherein the third insulator is made from a silicon dioxide having a thickness that provides capacitance between the first floating gate and the control gate, and the third insulator preventing leakage between the first floating gate and the control gate.

48. The memory device of claim 40, wherein the third insulator is made from an oxide nitride oxide having a thickness that provides capacitance between the first floating gate and the control gate, and the third insulator prevents leakage between the first floating gate and the control gate.

49. The memory device of claim 40, wherein the fourth insulator is made from a silicon dioxide having a thickness that provides capacitance between the second floating gate and the control gate, and the fourth insulator preventing leakage between the second floating gate and the control gate.

50. The memory device of claim 40, wherein the fourth insulator is made from an oxide nitride oxide having a thickness that provides capacitance between the second floating gate and the control gate, and the fourth insulator preventing leakage between the second floating gate and the control gate.

51. The memory device of claim 40, wherein the first floating gate and the second floating gate each being capable of storing multiple levels of charge.

52. The memory device of claim 40, wherein the first floating gate and the second floating gate each being capable of storing four levels of charge.

53. An electrically alterable memory device, comprising: a first semiconductor layer doped with a first dopant in a first concentration; a second semiconductor layer, adjacent the first semiconductor layer, doped with a second dopant that has an opposite electrical characteristic than the first dopant, the second semiconductor layer having a top side; two spaced-apart diffusion regions embedded in the top side of the second semiconductor layer, each diffusion region doped with the first dopant in a second concentration greater than the first concentration, the two diffusion regions including a first diffusion region and a second diffusion region, and a first channel region defined between the first diffusion region and the second diffusion region; a first floating gate comprised of a conductive material, the first floating gate disposed adjacent the first diffusion region and above the first channel region and separated therefrom by a first insulator region, the first floating gate capable of storing electrical charge; a second floating gate comprised of a conductive material, the second floating gate disposed adjacent the second diffusion region and above the first channel region and separated therefrom by a second insulator region, the second floating gate capable of storing electrical charge; and a control gate having at least two lateral sides and comprised of a conductive material, the control gate disposed laterally between the first floating gate and the second floating gate, the control gate separated from the first floating gate by a first vertical insulator layer and separated from the second floating gate by a second vertical insulator layer, the control gate being covered by the first floating gate on more than one lateral side and covered by the second floating gate on more than one lateral side, the control gate being separated from the first channel region by a third insulator region.

54. The memory device of claim 53, wherein the first dopant having a P- type characteristic and the second dopant having an N-type characteristic.

55. The memory device of claim 53, wherein the first dopant having an N- type characteristic and the second dopant having a P-type characteristic.

56. The memory device of claim 53, wherein the first insulator region having a thickness that allows tunneling of charge between the first floating gate and the first channel region.

57. The memory device of claim 56, wherein the thickness of the first insulator region is between 70 Angstroms and 110 Angstroms.

58. The memory device of claim 53, wherein the second insulator region having a thickness that allows tunneling of charge between the second floating gate and the first channel region.

59. The memory device of claim 53, wherein the thickness of the second insulator region is between 70 Angstroms and 110 Angstroms.

60. The memory device of claim 53, wherein the first vertical insulator is made from a silicon dioxide having a thickness that provides capacitance between the first floating gate and the control gate, and the first vertical insulator preventing leakage between the first floating gate and the control gate.

61. The memory device of claim 53, wherein the first vertical insulator is made from an oxide nitride oxide having a thickness that provides capacitance between the first floating gate and the control gate, and the first vertical insulator prevents leakage between the first floating gate and the control gate.

62. The memory device of claim 53, wherein the second vertical insulator is made from a silicon dioxide having a thickness that provides capacitance between the second floating gate and the control gate, and the second vertical insulator preventing leakage between the second floating gate and the control gate.

63. The memory device of claim 53, wherein the second vertical insulator is made from an oxide nitride oxide having a thickness that provides capacitance between the second floating gate and the control gate, and the second vertical insulator preventing leakage between the second floating gate and the control gate.

64. The memory device of claim 53, wherein the first floating gate and the second floating gate each being capable of storing multiple levels of charge.

65. The memory device of claim 53, wherein the first floating gate and the second floating gate each being capable of storing four levels of charge.

66. An electrically alterable memory device, comprising: a semiconductor substrate doped with a first dopant in a first concentration; a semiconductor well, adjacent the semiconductor substrate, doped with a second dopant that has an opposite electrical characteristic than the first dopant, the semiconductor well having a top side; a diffusion region embedded in the top side of the semiconductor well, the diffusion region doped with the first dopant in a second concentration greater than the first concentration; a floating gate having a first height, the floating gate disposed adjacent the diffusion region and above and separated therefrom by an insulator tunnel, the floating gate capable of storing electrical charges; and a control gate having a second height and comprised of a conductive material, the control gate disposed laterally adjacent the floating gate, the control gate separated from the floating gate by a vertical insulator layer, the control gate further being separated from the semiconductor well by an insulator region.

67. The memory device of claim 66, wherein the vertical insulator is made from a silicon dioxide having a thickness that provides capacitance between the floating gate and the control gate, and the vertical insulator preventing leakage between the floating gate and the control gate.

68. The memory device of claim 66, wherein the vertical insulator is made from an oxide nitride oxide having a thickness that provides capacitance between the floating gate and the control gate, and the vertical insulator prevents leakage between the first floating gate and the control gate.

69. The memory device of claim 66, wherein the first height of the floating gate is taller than the second height.

70. The memory device of claim 66, wherein the first height of the floating gate is shorter than the second height.

71. The memory device of claim 66, wherein the first height of the floating gate is same as the second height.

72. The memory device of claim 66, wherein the floating gate being capable of storing multiple levels of charge.

73. The memory device of claim 66, wherein the floating gate being capable of storing four levels of charge.

74. The memory device of claim 66, wherein the electrical charges are discharged from the floating gate through the insulator tunnel when a negative voltage is applied to the control gate and a positive voltage is applied to the diffusion region.

75. The memory device of claim 66, wherein the electrical charges are injected into the floating gate from a junction between the diffusion and the semiconductor well when the junction is reversely biased and a positive voltage is applied to the control gate.