# Patent

## Title
Dual non-volatile memory cell comprising an erase transistor

## Abstract

The present disclosure relates to a non-volatile memory cell on a semiconductor substrate, comprising a first transistor comprising a control gate, a floating gate and a drain region, a second transistor comprising a control gate, a floating gate and a drain region, in which the floating gates of the first and second transistors are electrically coupled, and the second transistor comprises a conducting region electrically coupled to its drain region and extending opposite its floating gate through a tunnel dielectric layer.

## Background
BACKGROUND
Technical Field
The present disclosure relates to non-volatile memories and in particular to a memory array and memory cell structure of the type described in the US application 2013/0228846, which is hereby incorporated by reference in its entirety.
Description of the Related Art
As a reminder, FIG. 1 represents such a memory array structure MA0 and shows memory cells Mi,j, M1−1,j, Mi,j−1, Mi−1,j+1 of the above-mentioned type, belonging here to two adjacent physical pages Pi, Pi−1 of the memory array, of respective ranks “i” and “i−1”. The memory cells Mi,j, Mi−1,j, Mi,j+1, Mi−1,j+1 are read- and write-accessible through bit lines BLj, BLj+1, a word line WLi−1,i and control gate lines CGLi, CGLi−1. Each memory cell comprises a floating-gate transistor, respectively Ti,j, Ti−1,j, Ti,j+1, Ti−1,j+1. The drain terminals D of the transistors Ti,j, Ti−1,j are connected to the bit line BLj and the drain terminals of the transistors Ti,j+1, Ti−1,j+1 are connected to the bit line BLj+1. The control gates CG of the transistors Ti,j, Ti,j+1 are connected to the control gate line CGLi and the control gates CG of the floating-gate transistors Ti−1,j, Ti−1,j+1 are connected to the control gate line CGLi−1.
Furthermore, each floating-gate transistor Ti,j, Ti−1,j, Ti,j+1, Ti−1,j+1 has its source terminal coupled to a source line SL through a select transistor ST. The select transistors ST of the memory cells Mi,j and Mi−1,j share the same control gate CSG and the two memory cells are thus referred to as “twins”. Similarly, memory cells Mi,j+1 and Mi−1,j+1 are twin memory cells and their select transistors ST have a common control gate CSG. Each common control gate is preferentially a vertical gate embedded in a substrate receiving the memory array MA0, the source line SL also being an embedded line. These common control gates CSG, or twin memory cell select gates, are connected to the word line WLi−1,i.
Such memory cells are channel-erased or programmed, i.e., by putting the substrate to a positive erase voltage or negative programming voltage causing electric charges to be extracted from their floating gates or electric charges to be injected into their floating gates, by Fowler Nordheim effect.
More particularly, a memory cell is erased by combining the positive voltage applied to the substrate with a negative voltage applied to the control gate of its floating-gate transistor, while the control gate of the floating-gate transistor of the twin memory cell receives a positive erase-inhibit voltage preventing it from being simultaneously erased (FIG. 11 of the above-mentioned application).
Similarly, a memory cell is programmed by combining a negative voltage applied to the bit line of the memory cell and to the substrate, with a positive voltage applied to the control gate of its floating-gate transistor, while the control gate of the floating-gate transistor of the twin memory cell receives a negative program-inhibit voltage preventing it from being simultaneously programmed (FIG. 12 of the above-mentioned application).
Finally, a memory cell is read by applying a positive voltage to the control gate of its floating-gate transistor, as well as a positive voltage to the corresponding bit line, while the twin memory cell, which is connected to the same bit line, receives on its control gate a negative read-inhibit voltage preventing it from being simultaneously read (FIG. 9 of the above-mentioned application).
This memory array structure having twin memory cells comprising a shared vertical select gate embedded in the substrate, offers the advantage of having a small footprint. The channel erase method used with this memory array structure is well suited to the production of a page-erasable memory array but less so to the production of a word-erasable memory array. This emerges by comparing the word-erasable memory array represented in FIG. 24 of the above-mentioned application, with the page-erasable memory array represented in FIG. 23 of this application, the former being more complex than the latter. Therefore, for the memory array to be word-erasable, each control gate line CGL, instead of being connected to all the memory cells of a page, must be divided into a plurality of control gate lines with one control gate line per word. This causes a noticeable complexification of the word line and column decoders, and requires providing various voltage switches to control, within each page, the control gate lines of the different words.
It can thus be desired to provide an improvement of this memory array and memory cell structure that is more appropriate for the implementation of a word-erasable memory, and does not complexify the control units of the memory array.
This conventional memory array and memory cell structure also employ a word line decoder capable of applying a positive read voltage to a memory cell that must be read, while applying a negative read-inhibit voltage to its twin memory cell, as explained above.
It can thus also be desired to provide another improvement of this memory array and memory cell structure that enables a memory cell to be read without applying any negative voltage to the twin memory cell.
BRIEF SUMMARY
Some embodiments of the present disclosure relate to a non-volatile memory cell on a semiconductor substrate, comprising a first floating-gate transistor comprising a control gate, a floating gate and a drain region, a second floating-gate transistor comprising a control gate, a floating gate and a drain region, wherein the floating gates of the first and second floating-gate transistors are electrically coupled, and the second floating-gate transistor comprises a conducting region electrically coupled to its drain region and extending opposite its floating gate through a tunnel dielectric layer.
According to one embodiment, the floating gates of the first and second floating-gate transistors are formed by a same layer of a conducting material.
According to one embodiment, the conducting region is a doped region of the substrate.
According to one embodiment, the memory cell comprises at least one select transistor coupling a source region of the first floating-gate transistor to a source line.
According to one embodiment, the select transistor comprises a vertical control gate embedded in the substrate.
Some embodiments of the present disclosure also relate to a non-volatile memory on a semiconductor substrate, comprising at least one memory cell according to the present disclosure, a first bit line electrically coupled to the drain region of the first floating-gate transistor, and a second bit line electrically coupled to the drain region of the second floating-gate transistor.
According to one embodiment, the memory comprises an erasing circuit that erases the memory cell by Fowler Nordheim effect, configured for extracting negative electric charges from the floating gate of the second floating-gate transistor through the conducting region.
According to one embodiment, the memory comprises a programming circuit that programs the memory cell by Fowler Nordheim effect, configured for injecting negative electric charges into the floating gate of the second floating-gate transistor through the conducting region.
According to one embodiment, the memory comprises programming circuit that programs the memory cell by hot-electron injection, configured for injecting negative electric charges into the floating gate of the first floating-gate transistor by means of a current circulating in the transistor.
According to one embodiment, the memory comprises a reading circuit that reads the memory cell through the first floating-gate transistor.
According to one embodiment, the memory comprises a first memory cell according to the present disclosure and a second memory cell of the same structure as the first memory cell, having a select transistor having the same control gate as the select transistor of the first memory cell.
According to one embodiment, the memory comprises a bit line electrically coupled to the drain region of the first floating-gate transistor of the first memory cell, another bit line electrically coupled to the drain region of the first floating-gate transistor of the second memory cell, and yet another bit line, electrically coupled both to the drain region of the second floating-gate transistor of the first memory cell and to the drain region of the second floating-gate transistor of the second memory cell.
Some embodiments of the present disclosure also relate to a method for manufacturing on a semiconductor substrate a memory cell according to the present disclosure, comprising the steps of forming in the substrate isolating trenches delimiting at least a first and a second strip of substrate, doping the second strip of substrate to render it conducting, forming on the substrate a floating gate arranged transversally to the two strips of substrate, with interposition of a first dielectric layer, forming a control gate on the floating gate with interposition of a second dielectric layer, to obtain a gate stack, and doping the two strips of substrate on either side of the gate stack, to generate drain and source regions of the first floating-gate transistor and at least one drain region of the second floating-gate transistor, the conducting region opposite the floating gate of the second floating-gate transistor being formed by a region of the second strip of substrate doped before forming the gate stack.
According to one embodiment, the method comprises a step of forming in the substrate a conducting trench arranged transversally to the strips of substrate and forming, after doping the two strips of substrate, an embedded vertical gate of a select transistor of the memory cell.
Some embodiments of the present disclosure also relate to a method for erasing a non-volatile memory cell on a semiconductor substrate, the memory cell comprising a first floating-gate transistor comprising a control gate, a floating gate, a tunnel dielectric layer between the floating gate and the substrate, a drain region and a source region, the method comprising the steps of: providing a second floating-gate transistor comprising a control gate, a floating gate and a drain region, coupling the floating gates of the first and second floating-gate transistors, providing in the second floating-gate transistor a conducting region electrically coupled to its drain region and extending opposite its floating gate through a tunnel dielectric layer, and applying a negative electric potential difference between the control gate and the drain region of the second floating-gate transistor, so as to extract negative electric charges from the floating gate of the second floating-gate transistor through the conducting region.
According to one embodiment, the method comprises the steps of providing a first bit line electrically coupled to the drain region of the first floating-gate transistor, providing a second bit line electrically coupled to the drain region of the second floating-gate transistor, providing a control gate line electrically coupled to the control gates of the first and second floating-gate transistors, and applying the negative electric potential difference between the control gate and the drain region of the second floating-gate transistor through the control gate line and the second bit line.



BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
Some embodiments of and methods for manufacturing a memory array and memory cell structure according to the present disclosure, and methods for reading and writing memory cells according to the present disclosure, will be described below in relation with, but not limited to, the accompanying figures, in which:
FIG. 1 described above is the wiring diagram of a conventional memory array and memory cell structure,
FIG. 2 is the wiring diagram of a first embodiment of a memory array and memory cell structure according to a first improvement according to the present disclosure,
FIG. 3 is a cross-section of a memory cell in FIG. 2,
FIG. 4 is another cross-section of the memory cell,
FIG. 5 shows voltages applied to the memory array in FIG. 2 to erase a memory cell,
FIG. 6 is a cross-section of a memory cell in FIG. 5, and shows voltages applied to the memory cell,
FIG. 7 is another cross-section of the memory cell, and shows the voltages applied to the memory cell,
FIG. 8 shows voltages applied to the memory array in FIG. 2 to program a memory cell by Fowler Nordheim effect,
FIG. 9 is a cross-section of a memory cell in FIG. 8 and shows voltages applied to the memory cell,
FIG. 10 is another cross-section of the memory cell and shows the voltages applied to the memory cell,
FIG. 11 shows voltages applied to the memory array in FIG. 2 to program a memory cell by hot-electron injection,
FIG. 12 is a cross-section of a memory cell in FIG. 11 and shows voltages applied to the memory cell,
FIG. 13 shows voltages applied to the memory array in FIG. 2 to read a memory cell,
FIG. 14 is a cross-section of a memory cell in FIG. 13 and shows voltages applied to the memory cell,
FIGS. 15 to 24 show steps of a method for manufacturing a memory cell shown on FIGS. 2 to 4,
FIG. 25 is the wiring diagram of a memory comprising the memory array in FIG. 2,
FIG. 26 is the wiring diagram of a second embodiment of a memory array and memory cell structure according to the first improvement,
FIG. 27 is the wiring diagram of a first embodiment of a memory array and memory cell structure according to a second improvement according to the present disclosure,
FIGS. 28 to 32 show steps of a method for manufacturing a memory cell shown on FIG. 27,
FIG. 33 shows voltages applied to the memory array in FIG. 27 to read a memory cell,
FIG. 34 is the wiring diagram of a memory comprising the memory array in FIG. 27,
FIG. 35 is the wiring diagram of a second embodiment of a memory array and memory cell structure according to the second improvement.



DETAILED DESCRIPTION
FIG. 2 is the wiring diagram of one embodiment of two memory cells Ci,j, Ci−1,j and of a memory array MA1 according to a first improvement according to the present disclosure of the memory array and memory cell structure in FIG. 1.
The memory cells are read- and write-accessible via a first bit line RBLj, a second bit line EBLj, a word line WLi−1,i, and two control gate lines CGLi, CGLi−1. The memory cell Ci,j belongs to a physical page Pi of the memory array and the memory cell Ci−1,j belongs to an adjacent page Pi−1. The pages Pi, Pi−1 may comprise various other memory cells and the memory array MA1 may comprise various other pages.
The memory cell Ci,j comprises two floating-gate transistors TRi,j, TEi,j the floating gates FGr, FGe of which are interconnected, the floating-gate transistor TRi,j being dedicated to reading the transistor memory cell and the floating-gate transistor TEi,j being dedicated to erasing the memory cell. According to one embodiment, the floating gates FGr, FGe are interconnected by manufacturing the two floating gates from a same conducting element CFG.
The transistor TRi,j has a control gate CGr connected to the control gate line CGLi, a drain terminal D connected to the bit line RBLj and a source terminal S connected to the drain terminal D of a select transistor ST the source terminal S of which is coupled to a source line SL. The transistor TEi,j has a control gate CGe connected to the control gate line CGLi, a drain terminal D connected to the bit line EBLj and a source terminal S connected to the drain terminal D of a select transistor ST the source terminal S of which is coupled to a source line SL.
The memory cell Ci−1,j has the same structure as the memory cell Ci,j and comprises two floating-gate transistors TRi−1,j, TEi−1,j the floating gates FGr, FGe of which are interconnected and/or formed by a same conducting element CFG. The transistor TRi−1,j has a control gate CGr connected to the control gate line CGLi−1, a drain terminal D connected to the bit line RBLj and a source terminal S connected to the drain terminal D of a select transistor ST the source terminal S of which is coupled to a source line SL. The floating-gate transistor TEi−1,j has a control gate CGe connected to the control gate line CGLi−1, a drain terminal D connected to the bit line EBLj and a source terminal S connected to the drain terminal D of a select transistor ST the source terminal S of which is coupled to a source line SL.
The select transistors ST associated with the floating-gate transistors TRi−j, TRi−1,j have a common control gate CSG connected to the word line WLi−1,i, that is preferentially produced in the form of a vertical gate embedded in a substrate receiving the memory array MA0. Similarly, the select transistors ST associated with the floating-gate transistors TEi,j, TEi−1,j have a common embedded vertical control gate CSG that is connected to the word line WLi−1,i. The memory cell Ci,j is thus, structurally, the equivalent of the combination of the memory cells Mi,j, Mi,j+1 shown on FIG. 1, the floating gates of which have been electrically coupled. Similarly, the memory cell Ci−1,j is, structurally, the equivalent of the combination of the memory cells Mi−1,j, Mi−1,j+1 shown on FIG. 1 the floating gates of which have been electrically coupled. The two memory cells may thus be referred to as “dual” memory cells. As their select transistors ST have the same control gate in pairs, these memory cells may also be referred to as “twin” memory cells.
However, the floating-gate transistor TEi,j differs from the floating-gate transistor Ti,j+1 of the memory cell Mi,j+1 in that it comprises a conducting region IS that extends opposite its floating gate FGe with interposition of a tunnel dielectric layer, and which is electrically coupled to its drain terminal D. Similarly, the floating-gate transistor TEi−1,j differs from the floating-gate transistor Ti−1,j+1 of the memory cell Mi−1,j+1 in that it comprises a conducting region IS that extends opposite its floating gate FGe through a tunnel dielectric layer, and which is electrically coupled to its drain terminal D.
FIG. 3 is a cross-section of one embodiment of the transistors TEi,j, TEi−1,j and of their respective select transistors ST. The floating-gate transistors are produced on a substrate PW, for example made of silicon, forming the upper region of a semiconductor chip WF. The chip WF is initially a wafer on which several integrated circuits are produced, which is then cut into individual chips.
The source lines SL of the select transistors ST are formed by a deep doped region n0 that here forms a collective source plane for the entire memory array. The common control gate CSG of the select transistors ST is formed with a conducting material, for example polysilicon (polycrystalline silicon), deposited in a trench made in the substrate, and isolated from the latter by a dielectric layer D0. This “conducting trench” also forms the word line WLi−1,i according to an axis perpendicular to the plane of the figure.
The floating gates FGe of the transistors TEi,j, TEi−1,j are arranged on either side of the trench CSG, and are supported by the substrate PW through a tunnel dielectric layer D1. They are formed here by a conducting part, for example made of polysilicon, which extends up to the transistors TRi,j, TRi−1,j (not shown in this section plane, cf. FIG. 4) to also form the floating gates of these transistors.
The control gates CGe of the transistors TEi,j, TEi−1,j extend above the floating gates through a dielectric layer D2. They are formed here by conducting strips of polysilicon that also form the control gate lines CGLi, CGLi−1 according to an axis perpendicular to the plane of the figure.
Doped regions n2 and n3 implanted on either side of the gate stack FG/CGe respectively form the drain (D) and source (S) regions of the transistors TEi,j, TEi−1,j, the regions n3 also forming the drain regions (D) of the select transistors ST. The source regions (S) of the select transistors ST are here formed by the layer n0, the common vertical gate CSG of the select transistors extending here up to the region n0. In one alternative embodiment, the lower end of the conducting trench CSG does not reach the region n0 and a deep doped pocket is implanted between the trench and the layer n0 to form the source region of the select transistors ST.
The gate stacks FG/CGe of the transistors TEi,j, TEi−1,j are covered with a dielectric layer D3 over which the bit line EBLj extends. Contacts C1 pass through the layer D3 to electrically couple the bit line EBLj to the drain regions n2 (D) of the transistors TEi,j, TEi−1,j. The conducting regions IS of the transistors TEi,j, TEi−1,j are here doped regions n1 of the substrate which extend beneath the floating gates FG, between the drain n2 and source n3 regions of the transistors TEi,j, TEi−1,jand are thus covered with the tunnel dielectric layer D1. The dielectric layers D0, D1, D2 and D3 are for example made of silicon dioxide SiO2.
FIG. 4 is a cross-section of the floating-gate transistors TRi,j, TRi−1,j of the memory cells Ci,j, Ci−1,j and of their respective select transistors ST. The structure of these transistors is substantially identical to that of the transistors TEi,j, TEi−1,j and will not be described again. The control gates CGe of these transistors are formed by the polysilicon parts forming the control gate lines CGLi, CGLi−1, and their floating gates FG are formed by the same polysilicon parts as those which form the floating gates of the transistors TEi,j, TEi−1,j (FIG. 3). Their drain regions n2 (D) are coupled by contacts C1 to the bit line RBLj which extends over the dielectric layer D3.
The transistors TRi,j, TRi−1,j differ from the transistors TEi,j, TEi−1,j in that they do not have the conducting region IS extending beneath the floating gate FG. Thus, when these transistors are biased by an appropriate gate voltage, a conducting channel CH1 or CH1′ can form between the drain region n2 (D) and the source region n3 (S). A vertical conducting channel CH2 or CH2′ can also form between the drain n3 (D) and source (S) regions of the select transistor ST if the common vertical gate CSG of these transistors simultaneously receives a bias voltage. More particularly, the vertical channel region CH2 of the select transistor ST of the memory cell Ci,j extends opposite a first face of the embedded vertical control gate CSG, and the vertical channel region CH2′ of the select transistor ST of the memory cell Ci−1,j extends opposite a second face of the embedded vertical control gate, and thus opposite the channel region CH2 of the select transistor of the memory cell Ci,j.
The transistors TRi,j, TRi−1,j may also differ from the transistors TEi,j, TEi−1,j by the thickness of their tunnel dielectric layer D1, which can be different from the one that extends beneath the floating gates of the transistors TEi,j, TEi−1,j, this choice being offered to those skilled in the art depending on the method for programming the memory cells selected, i.e., by Fowler Nordheim effect by means of the transistors TEi,j, TEi−1,j or by hot-electron injection by means of the transistors these two options being described below.
Unlike the transistors TRi,j, TRi−1,j, the transistors TEi,j, TE,i−1,j cannot have any conducting channel CH1 controlled by the voltage applied to them, due to the fact that the region extending between their drain n2 and source n3 regions is short-circuited by the doped region n1 (FIG. 3). These transistors are thus always on whatever their gate voltage, and cannot be used to read the memory cells Ci,j, Ci<1,j. However, they can be used to erase the memory cells in conditions where no drain-source current passes through them, i.e., by Fowler Nordheim effect. They may also be used to program the memory cells in conditions where no drain-source current passes through them, i.e., also by Fowler Nordheim effect, as we will see it below.
As a result, the select transistors ST associated with the transistors TEi,j, TEi−1,j are not used and are present here only to streamline the manufacturing of the memory cells, in accordance with one embodiment of a manufacturing method described below. It may indeed be simpler to produce a useless transistor within a set of transistors used, when the non-production of the useless transistor would involve additional masking and photolithography steps. As, firstly, these select transistors are on when the word line to which they are connected receives a positive voltage, and as, secondly, the floating-gate transistors TEi,j, TEi−1,j are always on due to their region IS, one should ensure that the corresponding bit line EBLj cannot simultaneously receive a voltage different from zero.
In short, the transistor TEi,j can be used as transistor for erasing the memory cell Ci,j by Fowler Nordheim effect, which involves a static programming without any programming current, whereas the transistor TRi,j can be used as transistor for reading the memory cell. Similarly, the transistor TEi−1,j can be used as transistor for erasing the memory cell C1−1,j by Fowler Nordheim effect and the transistor TRi−1,j can be used as transistor for reading the memory cell. The bit line RBLj can be used as bit line for reading and the bit line EBLj as bit line for erasing the memory cell Ci,j or the memory cell Ci−1,j.
As regards the programming of the memory cells Ci,j, Ci−1,j, some embodiments of the present disclosure provide two methods, between which those skilled in the art may choose, i.e., a programming method by Fowler Nordheim effect by means of the erase transistor TEi,j or TEi−1,j, or a programming method by hot-electron injection by means of the read transistor TRi,j, TRi−1,j.
Methods for erasing, programming and reading cells of the memory array MA1 will be described below, assuming as an example that it is desirable to erase, program and read the memory cell Ci,j.
Erasing of a Memory Cell by Fowler Nordheim Effect via the Eerase Transistor TEi,j 
A method for erasing the memory cell Ci,j without erasing the memory cell Ci−1,j, via the erase transistor TEi,j, is described in Table 1 in the Annex, which is an integral part of the description.
FIGS. 5 and 6 show this erasing method. FIG. 5 is the wiring diagram in FIG. 2 in which the voltage values contained in Table 1 have been entered. FIG. 6 is a cross-section of the transistors TEi,j, TEi−1,j. identical to that of FIG. 3, in which the voltage values contained in Table 1 have been entered.
The conducting region IS of the transistor TEi,J is taken to the positive voltage EBLV applied to the bit line EBLj, here 6V, through the contact C1 and the drain region n1 (D) of the transistor. As the control gate CGe of the transistor TEi,j is taken to the negative voltage CGVi, here −8V, a voltage difference dV equal to −14V (FIG. 6) appears between this control gate and the conducting region IS which causes electrons to be extracted from the floating gate FGe by Fowler Nordheim effect, which puts the transistor TEi,j to the erased state. As the bit line RBLj connected to the floating-gate transistor TRi,j is at high impedance, this transistor does not play any role in the process of erasing the memory cell. As the floating gate of the transistor TRi,j is however electrically coupled to that of the transistor TEi,j, the transfer of electric charges also causes the erasing of the transistor TRi,j, the entire memory cell Ci,j being thus erased through the transistor TEi,j.
The conducting region IS of the transistor TEi−1,j of the twin memory cell Ci−1,j is also taken to the positive voltage EBLV applied to the bit line EBLj here 6V, through the contact C1 and the drain region n1 (D) of the transistor. As the control gate CGe of the transistor TEi−1,j, is taken to the positive voltage CGVi−1, here 3V, a voltage difference dV equal to −3V appears between this control gate and the conducting region IS, which is insufficient to extract electrons from the floating gate of the transistor. The twin memory cell Ci−1,j, is thus not erased.
This memory array and memory cell structure thus enables individual erasing of each memory cell, i.e., erasing by bit. This possibility allows a memory erasable by bit, by word or by page to be produced indifferently without changing the general structure of the memory array or of its control units.
FIG. 7 is a cross-section of two transistors TEi,j+i, TEi−1,j−1 of two memory cells Ci,j+1, Ci−1,j+1 (not represented on FIG. 2 or 5) that are contiguous to the memory cells Ci,j, Ci−1,j. The memory cells Ci,j+1, Ci−1,j+1 are connected to the same word line WLi−1,i as the memory cells Ci,j, Ci−1,j but are connected to a different bit line EBLj+1 that receives the default voltage EBLV*, here 0V. The transistors TEi,j−1, TEi−1j+1 have their control gates CGe connected to the same control gate lines CGLi, CGLi−1 as the transistors TEi,j, TE1−1,j and thus receive the same voltages CGVi, here −8V, and CGV1−1,1, here 3V. Thus, the voltage difference dV between the control gate CGe of the transistor TEi,j+1 and its conducting region IS is equal to −8V and this transistor undergoes an erasing stress, i.e., a low-intensity spurious erasing that could, if the transistor were in the programmed state, and after many cycles of erasing other memory cells connected to the control gate lines CGLi, CGLi−1, substantially alter its threshold voltage and thus cause a corruption of its state, and thus a corruption of the data bit associated with the programmed state.
Furthermore, the voltage difference dV between the control gate CGe of the transistor TEi−1,j+1 and its conducting region IS is 3V and this transistor does not undergo any erasing stress, the control gate line CGLi−1 being taken to only 3V. Similarly, default voltages applied to the memory cells connected to other word lines WL (not represented on the figures) do not cause any erasing stress in these memory cells.
Finally, the erasing method according to the present disclosure enables not only each memory cell to be individually erased, but also limits the appearance of an erasing stress on the memory cells connected to the same control gate line, whereas various other known erasing methods, enabling only erasing by word, or otherwise by page, also cause an erasing stress on the memory cells connected to other word lines. The management of the erasing stress, by methods for refreshing the memory cells known per se, is thus simplified, given the smaller number of memory cells to be refreshed. It is possible, for example, to decide to initiate a sequence for refreshing the memory cells of a word line after N programming cycles of memory cells of this word line, by providing an erase cycle counter associated with the word line.
Programming of a Memory Cell by Fowler Nordheim Effect via the Erase Transistor TEi,j 
A method for programming the memory cell Ci,j without programming the memory cell Ci−1,j, via the erase transistor TEi,j, is described by Table 2 in the Annex.
FIGS. 8 and 9 show this programming method. FIG. 8 is the wiring diagram in FIG. 2 in which the voltage values contained in Table 2 have been entered. FIG. 9 is a cross-section of the transistors TEi,j, TEi−1,j identical to that of FIG. 3, in which the voltage values contained in Table 2 have been entered.
The conducting region IS of the transistor TEi,j is taken to the voltage EBLV applied to the bit line EBLj, here 0V, through the contact C1 and the drain region n1 (D) of the transistor. As the control gate CGe of the transistor TEi,j is taken to the positive voltage CGVi here 14V, a positive voltage difference dV equal to 14V (FIG. 9) appears between this control gate and the conducting region IS which causes electrons to be injected into the floating gate FGe by Fowler Nordheim effect, which puts the transistor TEi,j to the programmed state. As the bit line RBLj connected to the floating-gate transistor TRi,j is at high impedance, this transistor does not play any role in the process of programming the memory cell. As the floating gate of the transistor TRi,j is however electrically coupled to that of the transistor TEi,j, the transfer of electric charges also causes the programming of the transistor TRi,j, the entire memory cell Ci,j being thus programmed through the transistor TEi,j.
The conducting region IS of the transistor TEi−1,j of the twin memory cell Ci−1,j is taken to the voltage EBLV applied to the bit line EBLj, here 0V, through the contact C1 and the drain region n1 (D) of the transistor. As the control gate CGe of the transistor TEi−1,j is taken to the positive voltage CGVi−1, here 3V, a voltage difference dV equal to 3V appears between this control gate and the conducting region IS, which is insufficient to inject electrons into the floating gate of the transistor. The twin memory cell is thus not programmed.
FIG. 10 is a cross-section of the transistors TEi,j+1, TEi−1,+1 of the neighboring memory cells Ci,j+1, Ci−1j−1 already described in connection with FIG. 7, connected to the same word line WLi−1,i as the memory cells Ci,j, C1−1,j but connected to the neighboring bit line EBLj+1 that receives the default voltage EBLV*, here 6V. The transistors TEi,j−1, TE,—i j+i have their control gates CGe connected to the same control gate lines CGL, as the transistors TEi,j, TE1−1,j and thus receive the same voltages CGVi, here 14V, and CGV1−1, here 3V. Thus, the voltage difference dV between the control gate CGe of the transistor TEi,j+1 and its conducting region IS is 8V and this transistor undergoes a programming stress, i.e., a low-intensity spurious programming.
Furthermore, the voltage difference dV between the control gate CGe of the transistor TE1−1,j+1 and its conducting region IS is 3V and this transistor does not undergo any programming stress, the control gate line CGLi−1 being taken to only 3V. Similarly, default voltages applied to the memory cells connected to other word lines WL (not represented on the figures) do not cause any programming stress in these memory cells.
Finally, this programming method, like the erasing method previously described, only causes an electrical stress to the memory cells connected to the same control gate line, the effects of which can be cancelled out by a refreshing method of the above-mentioned type.
Programming of a Memory Cell by Hot-Electron Injection via the Read Transistor TRi,j 
A method for programming the memory cell Ci,j without programming the memory cell Ci−1,j, via the read transistor TRi,j, is described by Table 3 in the Annex.
FIGS. 11 and 12 show this programming method. FIG. 11 is the wiring diagram in FIG. 2 in which the voltage values contained in Table 3 have been entered. FIG. 12 is a cross-section of the transistors TRi,j, TRi−1,j identical to that of FIG. 4, in which the voltage values contained in Table 3 have been entered.
The transistor TRi,j receives the positive voltage CGVi, here 10V, on its control gate and is in the on state, the conducting channel CH1 appearing in the substrate PW beneath the gate stack FG/CGr. The select transistor ST associated with the transistor TRi,j receives the positive select voltage SV, here 1 to 2V, on its embedded vertical gate CSG, and is in the on state, the vertical conducting channel CH2 appearing opposite the gate CSG. As the bit line RBLj is taken to the positive voltage RBLV, here 4V, and the source line SL is coupled to the ground (0V), a current circulates from the bit line to the source line through the transistor Ti,j and the corresponding select transistor ST. This current corresponds to a flow of electrons HE shown on FIG. 12, in the opposite direction to the current. This flow of electrons contains high kinetic energy electrons (hot electrons) that bypass the doped region n3 (“cold” region). Some of these electrons are injected into the floating gate at an injection point HI, causing the programming of the transistor TRi,j, and the programming of the erase transistor TEi,j that does not play any role in the programming process here. However, the transistor TRi−1,j of the twin cell receives the voltage CGVi−1 that is equal to 0V, such that it does not undergo any spurious programming process, nor any of the other read transistors of the memory array, which receive only zero voltages.
Reading of a Memory Cell via the Read Transistor TRi,j 
A method for reading the memory cell Ci,j via the read transistor TRi,j, is described by Table 4 in the Annex.
FIGS. 13 and 14 show this reading method. FIG. 13 is the wiring diagram in FIG. 2 in which the voltage values contained in Table 4 have been entered. FIG. 14 is a cross-section of the transistors TRi,j, TRi−1,j identical to that of FIG. 4, in which the voltage values contained in Table 4 have been entered.
The transistor TRi,j receives the positive voltage CGVi here 2 to 3V, that is lower than the threshold voltage of the programmed transistor but greater than the threshold voltage of the erased transistor. If the transistor TRi,j is in the erased state, i.e., if it has a threshold voltage Vt lower than the voltage CGV the conducting channel CH1 appears in the substrate PW, beneath the gate stack FG/CGr. The select transistor ST associated with the transistor TRi,j receives the positive select voltage SV, here 3V, on its embedded vertical gate CSG, and is in the on state, the vertical conducting channel CH2 appearing opposite the embedded gate CSG. As the bit line RBLj is taken to the positive voltage RBLV, here 1V, and the source line SL is coupled to the ground (0V), the transistor TRi,j is passed through by a read current Ir that circulates from the bit line to the source line. However, this current Ir is zero if the transistor TRi,j is in the programmed state, i.e., if it has a threshold voltage greater than the voltage CGVi. A current amplifier (not represented) connected to the bit line RBLj enables the presence or absence of the current Ir to be detected, and the erased or programmed state of the transistor TRi,j can thus be deduced, to which a logical value, 0 or 1, is allocated by convention.
The transistor TRi−1,j of the twin memory cell receives the negative voltage CGVi−1, here −2V. This transistor, if it is in the erased state, can have a threshold voltage close to zero. The application of a negative gate control voltage ensures that it remains in the off state. Indeed, as this transistor is connected to the same bit line RBLj as the transistor TRi,j, being read, rendering it conducting could corrupt the reading of the transistor TRi,j.
FIGS. 15 to 22 show steps of a method for manufacturing the memory cells Ci,j, Ci−1,j, described above. FIG. 15 shows a preliminary step of forming in the substrate PW three isolating trenches STI0, ST1, ST2 of STI type (“Shallow Trench Isolation”) that delimit two strips of substrate S1, S2 in which the memory cells will be produced. This step is preceded by a step of implanting in the substrate the embedded layer n0 forming a source plane (not shown on the figure) or of implanting several source lines. A source plane is generally preferred to multiple source lines if it is provided to erase the memory cells by hot-electron injection.
During a step shown on FIG. 16, a conductive trench is formed transversally to the strips S1, S2, by etching the substrate, depositing the dielectric layer D0 (not shown) and depositing a polysilicon layer P0 and etching the latter. The trench is intended to form both the word line WLi−1,i and the embedded vertical control gate of the select transistors ST of the memory cells.
During a step shown on FIG. 17, the strip of substrate S2 is doped by implanting N-type dopants, the strip of substrate S1 being masked during this operation. This step makes it possible to produce the conducting region IS that will extend beneath the floating gate of the erase transistors TEi,j, TEi−1,j,
During a step shown on FIG. 18, the tunnel dielectric layer D1 described above is deposited on the substrate PW, then a strip of polysilicon P1, intended to form floating gates, is deposited on the strips of substrate S1 and S2.
During a step shown on FIG. 19, the dielectric layer D2 is deposited on the substrate PW, then a polysilicon layer P2 is deposited on the layer D2. The layer P2 is then etched with the layer D2 as well as with the layer P1 to form the control gate lines CGLi, CGLi−1, and, beneath the latter, the common floating gates CFG, resulting from the simultaneous etching of the layer P1.
During a step shown on FIG. 20, the strips of substrate S1, S2 are doped by implanting dopants self-aligned with the control gate lines CGLi, CGLi−1 and with the word line WLi−1,i the strip of substrate S2 thus being doped a second time. This step generates the source S and drain D regions of the transistors TRi,j, TEi−1,j, TEi,j, TEi−1,j, and the drain regions of the select transistors ST.
During a step shown on FIG. 21, the dielectric layer D3 is deposited on the substrate and orifices are made in the layer D3, then are metallized to form the contacts C1. Contacts C1 extend above the drain regions D of the transistors TRi,j, TRi−1,j and others above the drain regions D of the transistors TEi,j, TEi−1,j.
During a step shown on FIG. 22, a metal layer M1 (“metal 1”) is deposited on the substrate, then is etched to obtain two conducting strips that form the bit lines RBLj and EBLj, the former being arranged on the contacts C1 produced above the drain regions D of the transistors TRi,j, TRi−1,j and the latter arranged on the contacts produced above the drain regions D of the transistors TEi,j, TEi−1,j.
FIG. 23 is identical to FIG. 22 and shows the surface area occupied in width W and in length L by each memory cell Ci,j, Ci−1,j, the assembly forming a “basic brick” 20 of the memory array, containing two twin dual cells, the repetition of which enables a memory array MA1 of variable size chosen according to the intended application to be designed. The contacts C1 of the memory cell Ci,j are in this case shared with a memory cell adjacent to this memory cell (not represented), and the same is true concerning the contacts C1 of the memory cell C1.1j. Thus, only half of the surface area occupied by each contact C1 is considered to be part of the “basic brick” 20.
Although these memory cells Ci,j, Ci−1,j have a surface area that is twice those shown on FIG. 1, those skilled in the art will note that the semiconductor surface area they occupy barely differs from that occupied by conventional memory cells having planar-type select transistors that are not shared, due to the fact that the embedded vertical select gates considerably reduce their surface area and even more to the fact that they are shared.
Furthermore, according to one embodiment of the disclosure, a memory array according to the present disclosure may comprise a first memory area produced from memory cells as described on FIG. 1, forming a page-erasable mass memory, and a second memory area produced from memory cells according to the present disclosure, forming a bit- or word-erasable data memory, offering a finer erasing granularity than the mass memory and more suited to certain applications.
FIG. 24 is a cross-section of the memory cell Ci,j according to a section plane AA′ shown on FIG. 23 and perpendicular to the section plane of FIGS. 3 and 4, showing together the transistors TRi,j, TEi,j and their common floating gate FGT. This figure also shows that it is possible to further reduce the width W of the memory cell by reducing the width of the central isolating trench STI0 that separates the transistors TRi,j, TEi,j, this isolating trench not needing to have the isolation width usually chosen for trenches STI1, STI2 that separate neighboring memory cells, since the transistors TRi,j, TEi,j are electrically coupled.
FIG. 25 is the wiring diagram of a memory MEM1 comprising the memory array MA1 according to the present disclosure, only the cells Ci,j, Ci−1,j being represented. The memory comprises a control circuit CCT1, a word line decoder RD1, a column decoder CD1, as many sense amplifiers SA as the number of bits of a word DTR to be read in the memory, for example an eight-bit word B0-B7, and programming latches BLT1 to apply voltages to the bit lines RBLj or EBLj according to a word DTW to be written in the memory, for example an eight-bit word B0-B7.
The word line decoder RD1 controls the voltages applied to the control gate lines CGLi, CGLi−1 and to the word line WLi−1,j according to a most significant address A(n-1)-A(x) of a word, or line address. The decoder CD1, in combination with the latches BLT1, controls the voltages applied to the bit lines RBLj, EBLj according to a least significant address A(x-1)-A(0) of the word, or column address, the line and column addresses forming together the address A(n-1)-A0 of a word to be read or to be written in the memory array. In read mode, the decoder CD1 couples the sense amplifiers SA to the bit lines RBLj coupled to the memory cells to be read, and the sense amplifiers supply the word DTR.
The circuit CCT1 comprises for example a central unit CPU, a voltage generator VGEN, and address and data registers. It executes read or write commands, controls the decoders, supplies the voltages for the read and write operations (erasing-programming), provides the decoders with the most significant and least significant addresses, and if necessary executes a program for refreshing the memory cells.
Although the improvement that has just been described was initially designed to be applied to a memory cell structure of the type represented in FIG. 1, it will be understood by those skilled in the art that some embodiments of this improvement may be applied to other types of memory cells.
As an example, FIG. 26 represents one embodiment of this improvement applied to memory cells without any select transistor. The memory array MA1′ represented comprises memory cells Ci,j, Ci−1,j each comprising a floating-gate transistor TRi,j, respectively TRi−1,j and a floating-gate transistor TEi,j, respectively TEi−1,j, of the same structure as those described above. The transistor TRi,j has a drain terminal connected to the bit line RBLj, a control gate CGr connected to the word line WLi, and a source terminal directly connected to the source line SL. The transistor TEi,j comprises a drain terminal connected to the bit line EBLj, a control gate CGr connected to the word line WLi, and a non-connected source terminal. As above, the floating gate FGr of the transistor TRi,j is electrically coupled to the floating gate FGe of the transistor TEi,j and the latter comprises the conducting region IS opposite its floating gate, enabling the memory cell to be erased. The memory cell Ci−1,j has an identical structure and the description above applies by replacing the index i with the index i−1.
Other alternatives could be provided, for example by removing the source terminal of the transistors TEi,j, TE1−1,j in the embodiment in FIG. 26 or in the embodiment in FIG. 2, or by removing the select transistors ST associated with the transistors TEi,j, TEi−1,j in the embodiment in FIG. 2.
FIG. 27 is the wiring diagram of a memory array MA2 comprising two memory cells Ci,j, Ci,j+1 according to a second improvement of the memory array and memory cell structure in FIG. 1, this improvement being implemented here in combination with the improvement previously described in connection with FIG. 2.
The memory cells are read- and write-accessible via a first bit line RBLj, a second bit line RBLj+1, a third bit line EBLj,j+1, a word line WLi and two control gate lines CGL1i, CGL2i. The memory cell Ci,j belongs to a physical page Pi of the memory array and the memory cell Ci−1,j belongs to an adjacent physical page P1−1. The pages Pi, Pi−1 may comprise various other memory cells and the memory array MA1 may comprise various other pages.
In accordance with the previous improvement, the memory cell Ci,j comprises two floating-gate transistors TRi,j, TEi,j the floating gates FGr, FGe of which are interconnected, the floating-gate transistor TRi,j being dedicated to reading the transistor memory cell and the floating-gate transistor TEj being dedicated to erasing the memory cell. As above, the floating gates FGr, FGe may be formed by a same conducting element CFG and each transistor TRi,j, TEi,j comprises a conducting region IS that extends opposite its floating gate through a tunnel oxide layer.
The transistor TRi,j has a control gate CGr connected to the control gate line CGL1i, a drain terminal D connected to the bit line RBLj and a source terminal S connected to the drain terminal D of a select transistor ST the source terminal S of which is coupled to a source line SL. The floating-gate transistor TEi,j has a control gate CGe connected to the control gate line CGL1i, a drain terminal D connected to the bit line EBLj,j+1, and a source terminal S connected to the drain terminal D of a select transistor ST the source terminal S of which is coupled to a source line SL. The memory cell Ci,j+, has the same structure as the memory cell Ci,j and comprises two floating-gate transistors TRi,j+1, TEi,j+1, the floating gates FGr, FGe of which are interconnected or formed by a same conducting element CFG. The transistor TRi,j+1 has a control gate CGr connected to the control gate line CGL2i, a drain terminal D connected to the bit line RBLj+1 and a source terminal S connected to the drain terminal D of a select transistor ST the source terminal S of which is coupled to a source line SL. The floating-gate transistor TEj+1 has a control gate CGe connected to the control gate line CGL1i, a drain terminal D connected to the bit line EBLj,j+1, and a source terminal S connected to the drain terminal D of a select transistor ST the source terminal S of which is coupled to a source line SL.
The select transistors ST associated with the floating-gate transistors TRi,j, TRi,j+1 have a common embedded vertical control gate CSG. Similarly, the select transistors ST associated with the floating-gate transistors TEi,j, TEi,j+1 have a common embedded vertical control gate CSG.
The memory array MA2 thus differs from the memory array MA1 in FIG. 2 in that the transistors TRi,j, TRi,j+1 of the twin memory cells Ci,j, Ci,j+1 are connected to different bit lines RBLj, RBLj+1. As will be seen below, these two bit lines enable a method for reading the memory cells to be implemented that does not require, when reading a memory cell, a negative voltage to be applied to the control gate line CGL1, or CGL2i of the twin memory cell.
It will first be noted that adding an extra bit line in a structure of two twin memory cells as represented in FIG. 1, involves increasing the surface area of the pair of memory cells, considered as “basic brick” of a memory array, because the latter does not enable two bit lines to be received. On the other hand, a structure of two twin dual memory cells of the type represented in FIG. 2 enables the two bit lines RBLj, RBLj+1 dedicated to reading the memory cells to be provided in addition to the bit line EBLj,j+1 dedicated to erasing the memory cells, without increasing their surface area. This will be shown in connection with FIGS. 28 to 32, that show an alternative of the manufacturing method previously described for manufacturing a pair of memory cells forming a “basic brick” of the memory array, this alternative making it possible to produce the two bit lines RBLj, RBLj+1 instead of the bit line RBLj without increasing the surface area of the pair of memory cells.
The initial steps of the manufacturing method, described above in connection with FIGS. 15 to 21, are not modified. The step in FIG. 22 of manufacturing the bit lines RBLj, EBLj, is replaced with the step shown on FIG. 28.
During the step in FIG. 28, a metal layer M1 (“metal 1”) is deposited on the substrate, then etched to obtain firstly a conducting strip that forms here the bit line EBLj,j+1 (previously referred to as EBLj) and secondly two conducting rectangles parallel to the bit line EBLj,j+1, that form two bit line sections P1(RBLj), P1(RBLj+1). These two bit line sections extend above the contacts C1 that have been produced above the drain regions D of the transistors TRi,j, TRi,j+1 during the step shown on FIG. 21.
During a step shown on FIG. 29, a dielectric layer D4 is deposited on the substrate, and orifices are made in the layer D4 above the bit line sections P1(RBLj), P1(RBLj+1,), and are then metallized to form the contacts C2.
During a step shown on FIG. 30, a metal layer M2 (“metal 2”) is deposited on the substrate, then etched to obtain two other conducting rectangles forming two bit line sections P2(RBLj), P2(RBLj+1) perpendicular to the sections P1(RBLj), P1(RBLj+1) and which extend above the contacts C2.
During a step shown on FIG. 31, a dielectric layer D5 is deposited on the substrate, and orifices are made in the layer D5 above the bit line sections P2(RBLj), P2(RBLj+1,), and are then metallized to form the contacts C3.
During a step shown on FIG. 32, a metal layer M3 (“metal 3”) is deposited on the substrate, then etched to obtain two conducting strips that form the bit lines RBLj and RBLj+1, the former being arranged on the contacts C3 coupled to the drain region D of the transistor TRi,j, and the latter arranged on the contacts C3 coupled to the drain region D of the transistor TRi,j+1. As shown in FIG. 32, the bit lines RBLj and RBLj+1 are parallel to the bit line EBLj,j+1, that is situated two metal levels below them, and the assembly fits into the template of the cells Ci,j, Cj+1 without requiring any increase in their surface area.
The memory cells are programmed and erased as described above. They are, however, read through the bit lines RBLj, RBLj+1 in a manner that is going to be described.
Readin of a Memory Cell Via a Bit Line RBLj or RBLj+1 
A method for reading the memory cell Ci,j in FIG. 27 via the read transistor TRi,j and the bit line RBLj is described by Table 5 in the Annex.
FIG. 33 is a wiring diagram identical to that in FIG. 27 in which the voltage values contained in Table 5 have been entered. The transistor TRi,j receives on its control gate the positive voltage CGV1i, here 2 to 3V, and becomes on if it is in the erased state. The select transistor ST associated with the transistor TRi,j receives the positive select voltage SV on its control gate, here 3V, and is in the on state. As the bit line RBLj is taken to the positive voltage RBLVj, here 1V, and the source line SL is coupled to the ground (0V), the transistor TRi,j is passed through by a read current Ir that circulates from the bit line to the source line. The transistor TRi,j+1 of the twin memory cell receives the zero voltage CGV2i. This transistor, if it is in the erased state, can have a threshold voltage close to zero, or even negative, and switch to the on state. Indeed, the bit line RBLj+1 is not connected to any sense amplifier during the reading of the transistor TRi,j and the transistor TRj+1 is not read. Thus, even in the absence of a negative voltage on the control gate line CGL21, the transistor TRi,j+1 cannot corrupt the reading of the transistor TRi,j, and vice-versa when reading the transistor TRi,j+1.
FIG. 34 is the wiring diagram of a memory MEM2 comprising a memory array MA2 according to the present disclosure, only the cells Ci,j, Ci,j+1 being represented. The memory comprises a control circuit CCT2, a word line decoder RD2, a column decoder CD2, sense amplifiers SA and programming latches BLT2.
The word line decoder RD2 controls the voltages applied to the control gate lines CGL1i, CGL2i and to the word line WL, according to a most significant address A(n-1)-A(x) of a word (line address). The decoder CD2, in combination with the latches BLT1, controls the voltages applied to the bit lines RBLj, RBLj+1, EBLj,j+1 according to the column address A(x-1)-A(0) of the word. In read mode, the decoder CD2 couples the sense amplifiers SA to the bit lines RBLj connected to the memory cells that must be read, and the sense amplifiers supply a word DTR read in the memory, for example of 8 bits B0-B7. The circuit CCT2 comprises, like the circuit CCT1 previously described, a central unit CPU, a voltage generator VGEN, and address and data registers. It executes read or write commands, controls the decoders, supplies the voltages for the read and write operations (erasing-programming), provides the most significant and least significant addresses, and if necessary executes a program for refreshing the memory cells.
According to one embodiment, the word line decoder RD2 is configured to be able to distinctly control the voltages applied to the twin control gate lines CGL1i, CGL2which here have the same most significant address A(n-1)-A(x). This distinct control of the voltages can be reserved for the erasing operations, to apply a positive voltage to these memory cells situated on a twin page of the one containing the memory cell(s) being erased (Cf. FIGS. 6 and 7). However, in read mode, the decoder may apply the same voltage to the twin control gate lines CGL1i, CGL2i or even to all the control gate lines of the memory array to limit the logic gate switches and thus limit the current consumption of the memory, because the memory cells are read-selected by means of the word lines WL.
In such an embodiment, the decoder RD2 receives, in addition to the most significant address A(n-1)-A(x) of a word, the least significant bit A(0) of the least significant address A(x-1)-A(0) of the word. The decoder RD2 also receives from the circuit CCT2 an information signal EPR that indicates to it whether the address decoding to be done occurs in the context of reading, erasing or programming memory cells. If the decoding occurs in the context of erasing, the decoder RD2 differentiates the two control gate lines CGL1i, CGL2i according to the bit A(0). In other words, the decoder RD2 selects the control gate line CGL1i if the bit line RBLj is designated by the full address received by the memory, or selects the control gate line CGL2i if the bit line RBLj+1 is designated by the full address received by the memory. In one equivalent alternative, the decoder may receive a signal from the column decoder CD2, indicating to it which of the two control gate lines must be selected. Those skilled in the art may naturally provide other embodiments of the decoder, aiming for example to distinctly control the voltages applied to the twin control gate lines CGL1i, CGL2i in reading, programming and erasing mode.
Although the second improvement that has just been described was initially designed to be applied to a memory cell structure according to the first improvement, as shown on FIG. 2, it will be understood by those skilled in the art that some embodiments of this second improvement may be applied to other types of memory cells.
As an example, FIG. 35 shows an example of application of the second improvement to the memory cell structure represented in FIG. 1, and shows a memory array MA2′ comprising four memory cells Di,j, Di,j+1, Di,j+1, Di,j+3 of the same structure and the same arrangement as the cells Mi,j, Mi−1,j, Mi,j+1, Mi−1,j+1 in FIG. 1, but considered as being part of the same logic page, i.e., having the same most significant address. The memory cells Di,j, Di,j+1 are thus twin memory cells and share the same select gate CSG. The memory cells Di,j+2, Di,j+3 are twin memory cells and share the same select gate CSG. The control gates of the floating-gate transistors Ti,j, Ti,j+2 of the memory cells Di,j, D,i,j+2 are connected to the control gate line CGL1i and the control gates of the floating-gate transistors Ti,j+1, Ti,j+3 of the memory cells Di,j−1, Di,j+3 are connected to the control gate line CGL2i. The drain terminals of the twin memory cells Di,j, Di,j+1, instead of being connected to the same bit line, are connected to different bit lines BLj, BLj+1. Similarly, the drain terminals of the twin memory cells Di,j+2, Di,j+3, instead of being connected to the same bit line, are connected to different bit lines BLj+2, BLj+3.
ANNEX







TABLE 1







Fowler Nordheim erasing of Ci, j via TEi, j, FIGS. 5 and 6










Ref.
Description
Sign
Example














CGVi
Erase voltage applied to the control gate CGLi of
negative
−8
V



the transistors TRi, j, TEi, j of the memory cell Ci, j



(erase-selected memory cell) via the control gate



line CGLi


CGVi−1
Erase-inhibit voltage applied to the control gate
positive
3
V



CGLi−1 of the transistors TRi−1, j, TEi−1, j of the



memory cell Ci−1, j (twin memory cell not erase-



selected) via the control gate line CGLi


EBLV
Erase voltage applied to the bit line EBLj
positive
6
V










RBLV
Voltage applied to the bit line RBLj
—
HZ*











SV
Select voltage applied to the word line WLi−1, j
—
0
V



common to the twin memory cells Ci−1, j, Ci, j


SPV
Source line voltage applied to all the source lines
—
0
V



SL (or to the source plane)


VB
Electric potential of the substrate PW
—
0
V


CGV*
Default voltage applied to all the other control gate
positive
3
V



lines CGL


EBLV*
Default voltage applied to the non-selected bit lines
—
0
V



EBL


RBLV*
Default voltage applied to a non-selected bit line
—
0
V



RBL (for example “RBLj+1”)


SV*
Voltage applied to the non-selected word lines WL
—
0
V





*High impedance, i.e., line disconnected from the rest of the circuit













TABLE 2







Fowler Nordheim programming of Ci, j via TEi, j, FIGS. 8 and 9










Ref.
Description
Sign
Example














CGVi
Voltage applied to the control gate CGLi of the
positive
14
V



transistors TRi, j, TEi, j of the memory cell Ci, j



(program-selected memory cell) via the control gate



line CGLi


CGVi−1
Voltage applied to the control gate CGLi−1 of the
positive
3
V



transistors TRi−1, j, TEi−1, j of the memory cell Ci−1, j



(twin memory cell not program-selected) via the



control gate line CGLi


EBLV
Voltage applied to the bit line EBLj
—
0
V










RBLV
Voltage applied to the bit line RBLj
—
HZ











SV
Select voltage applied to the word line WLi−1, i
—
0
V



common to the twin memory cells Ci−1, j, Ci, j


SPV
Source line voltage applied to all the source lines
—
0
V



SL (or to the source plane)


VB
Electric potential of the substrate PW
—
0
V


CGV*
Voltage applied to all the other control gate lines
positive
3
V



CGL


EBLV*
Voltage applied to the non-selected bit lines EBL
—
6
V










RBLV*
Voltage applied to the non-selected bit lines RBL
—
HZ











SV*
Voltage applied to the non-selected word lines WL
—
0
V
















TABLE 3







Programming of Ci, j by injection, via TRi, j, FIGS. 11 and 12










Ref.
Description
Sign
Example














CGVi
Voltage applied to the control gate CGLi of the
positive
10
V



transistors TRi, j, TEi, j of the memory cell Ci, j



(program-selected memory cell) via the control gate



line CGLi


CGVi−1
Voltage applied to the control gate CGLi−1 of the
—
0
V



transistors TRi−1, j, TEi−1, j of the memory cell Ci−1, j



(twin memory cell not program-selected) via the



control gate line CGLi










EBLV
Voltage applied to the bit line EBLj
—
0 V or





more











RBLV
Voltage applied to the bit line RBLj
positive
4
V


SV
Select voltage applied to the word line WLi−1, i
positive
1-2
V



common to the twin memory cells Ci−1, j, Ci, j


SPV
Source line voltage applied to all the source lines
—
0
V



SL (or to the source plane)


VB
Electric potential of the substrate PW
—
0
V


CGV*
Voltage applied to all the other control gate lines
—
0
V



CGL


EBLV*
Voltage applied to the non-selected bit lines EBL
—
0
V


RBLV*
Voltage applied to the non-selected bit lines RBL
—
0
V


SV*
Voltage applied to the non-selected word lines WL
—
0
V
















TABLE 4







Reading of Ci, j via TRi, j, FIG. 13 and 14










Ref.
Description
Sign
Example














CGVi
Voltage applied to the control gate CGLi of the
positive
2-3
V



transistors TRi, j, TEi, j of the memory cell Ci, j (read-



selected memory cell) via the control gate line CGLi


CGVi−1
Voltage applied to the control gate CGLi−1 of the
negative
−2
V



transistors TRi−1, j, TEi−1, j of the memory cell Ci−1, j



(twin memory cell not read-selected) via the control



gate line CGLi


EBLV
Voltage applied to the bit line EBLj
—
0
V


RBLV
Voltage applied to the bit line RBLj
positive
1
V


SV
Select voltage applied to the word line WLi−1, i
positive
3
V



common to the twin memory cells Ci−1, j, Ci, j


SPV
Source line voltage applied to all the source lines SL
—
0
V



(or to the source plane)


VB
Electric potential of the substrate PW
—
0
V


CGV*
Voltage applied to all the other control gate lines
—
0
V



CGL


EBLV*
Voltage applied to the non-selected bit lines EBL
—
0
V


RBLV*
Voltage applied to the non-selected bit lines RBL
—
0
V


SV*
Voltage applied to the non-selected word lines WL
—
0
V
















TABLE 5







Reading of Ci, j via TRi, j and RBLj, FIG. 33










Ref.
Description
Sign
Example














CGV1i
Voltage applied to the control gate CGL1i of the
positive
2-3
V



transistors TRi, j, TEi, j of the memory cell Ci, j (read-



selected memory cell) via the control gate line



CGL1i


CGV2i
Voltage applied to the control gate CGL2i of the

0
V



transistors TRi, j+1, TEi, j+1 of the memory cell Ci, j+1



(twin memory cell not read-selected) via the



control gate line CGL2i


EBLV
Voltage applied to the bit line EBLj, j+1
—
0
V


RBLVj
Voltage applied to the selected bit line RBLj of the
positive
1
V



pair of memory cells


RBLVj+1
Voltage applied to the non-selected bit line RBLj+1

0
V



of the pair of memory cells


SV
Select voltage applied to the word line WLi
positive
3
V



common to the twin memory cells Ci, j+1, Ci, j


SPV
Source line voltage applied to all the source lines
—
0
V



SL (or to the source plane)


VB
Electric potential of the substrate PW
—
0
V


CGV*
Voltage applied to all the other control gate lines
—
0
V



CGL


EBLV*
Voltage applied to the non-selected bit lines EBL
—
0
V


RBLV*
Voltage applied to the non-selected bit lines RBL
—
0
V


SV*
Voltage applied to the non-selected word lines WL
—
0
V









The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.

## Claims

1. A method, comprising:
manufacturing on a semiconductor substrate a memory cell that includes:
a first floating-gate transistor having a control gate, a floating gate, and a drain region;
a second floating-gate transistor having a control gate, a floating gate, and a drain region, wherein:
the floating gates of the first and second floating-gate transistors are electrically coupled to each other, and the second floating-gate transistor comprises:
a tunnel dielectric layer extending along the floating gate of the second floating-gate transistor; and
a permanently conductive region electrically coupled to the drain region of the second floating-gate transistor and extending along an opposite side of the tunnel dielectric layer with respect to the floating gate of the second floating-gate transistor, the method comprising:


forming in the substrate isolating trenches delimiting first and second strips of the substrate,
making the second strip permanently conductive by doping the second strip,
forming a first dielectric layer on the substrate and forming the floating gate of the first floating gate transistor on the first dielectric layer, the floating gate being arranged transversally to the two strips of substrate,
forming a second dielectric layer on the floating gate of the first floating gate transistor and forming the control gate of the first floating gate transistor on the floating gate with of the first floating gate transistor, to obtain a gate stack, and
doping the first and second strips of substrate on opposite sides of the gate stack, thereby generating drain and source regions of the first floating-gate transistor and a drain region of the second floating-gate transistor, the permanently conducting region opposite the floating gate of the second floating-gate transistor being formed by a region of the second strip, which is doped before forming the gate stack.

2. The manufacturing method according to claim 1, comprising forming in the substrate a conductive trench arranged transversally to the first and second strips and forming, after doping the first and second strips, an embedded vertical gate of a select transistor of the memory cell.

3. A method, comprising:
erasing a non-volatile memory cell on a semiconductor substrate, the memory cell including a first floating-gate transistor comprising a control gate, a floating gate, a tunnel dielectric layer between the floating gate and the substrate, a drain region and a source region, the erasing including:
applying a negative electric potential difference between a control gate and a drain region of a second floating-gate transistor that also includes a floating gate, a tunnel dielectric layer, and a permanently conductive region electrically coupled to the drain region of the second floating-gate transistor and extending on an opposite side of the tunnel dielectric layer with respect to the floating gate of the second floating-gate transistor, the applying including extracting negative electric charges from the floating gate of the second floating-gate transistor through the permanently conductive region.

4. The method according to claim 3, wherein:
the drain region of the first floating-gate transistor electrically coupled to a first bit line,
the drain region of the second floating-gate transistor is electrically coupled to a second bit line,
the control gates of the first and second floating-gate transistors are electrically coupled to a control gate line, and
applying the negative electric potential difference between the control gate and the drain region of the second floating-gate transistor includes applying the negative electric potential difference through the control gate line and the second bit line.

5. A method, comprising:
forming first and second floating-gate transistors of first and second memory cells, respectively, the forming of the first and second floating-gate transistors including:
forming a plurality of isolation trenches in a substrate, first and second portions of the substrate separating the plurality of isolation trenches from each other;
forming a conductive trench in the plurality of isolation trenches and the first and second portions of the substrate;
forming first and second conductive regions on opposite sides of the conductive trench by doping the first portion of the substrate;
forming a first dielectric layer on the first and second conductive regions;
forming first and second floating gates on the first dielectric layer; and
forming source and drain regions by doping the first and second portions of the substrate.

6. The method of claim 5, wherein the isolation trenches are extending in a first direction, and the conductive trench is extending in a second direction that is substantially perpendicular to the first direction.

7. The method of claim 5, further comprising:
forming a third floating-gate transistor of the first memory cell, the third floating-gate transistor having a third floating gate; and
forming a fourth floating-gate transistor of the second memory cell, the fourth floating-gate transistor having a fourth floating gate.

8. The method of claim 7, further comprising electrically coupling the third and fourth floating gates to the first and second floating-gates, respectively.

9. The method of claim 5, further comprising:
forming a second dielectric layer on the first and second floating gates; and
forming control gate lines on the second dielectric layer.

10. The method of claim 9, wherein the forming of the control gate lines includes:
forming a conductive layer on the second dielectric layer; and
removing portions of the conductive layer and the second dielectric layer.

11. The method of claim 9, further comprising:
forming a third dielectric layer on the substrate and the control gate lines; and
forming conductive vias electrically coupled to the source and drain regions in the third dielectric layer.

12. The method of claim 11, further comprising forming bit lines electrically coupled to the conductive vias on the third dielectric layer.