Self-aligned split-gate flash memory cell and its contactless flash memory arrays

[0002] The present invention relates to a split-gate flash memory cell and its flash memory array and, more particularly, to a self-aligned split-gate flash memory cell having a single-side dish-shaped floating-gate structure and its contactless flash memory arrays.

[0003] A flash memory cell structure can be basically divided into two categories: a stack-gate structure and a split-gate structure, in which the gate length of a stack-gate structure can be defined by using a minimum-feature-size (F) of technology used and, therefore, the stack-gate structure is often used in existing high-density flash memory system for mass storage applications. The stack-gate flash memory cells can be configured into an array of a matrix form according to the basic logic function, such as NOR-type, NAND-type or AND-type. In general, a stack-gate flash memory cell in a NOR-type or AND-type array is programmed by the channel hot-electron injection (CHEI), however the programming efficiency is low and the programming power is large. Moreover, the gate length of a stack-gate flash memory cell is difficult to be scaled down due to the punch-through effect if the channel hot-electron injection is used as a programming method. For a NAND-type array, the stack-gate flash memory cells are interconnected in series with common-source/drain diffusion regions, the density is high but the read speed is relatively slow as compared to that of a NOR-type array. Moreover, the programming speed of a NAND-type array is relatively slow due to the Fowler-Nordheim tunneling across the thin tunneling-oxide layer between the source/drain diffusion region and the floating gate being used as a programming method.

[0004] The split-gate structure having a select-gate region and a stack-gate region offers in general a larger cell size as compared to that of a stack-gate structure and is usually configured to be a NOR-type array. Two typical split-gate flash memory cell structures are shown in FIG. 1A and FIG. 1B. FIG. 1A shows a split-gate flash memory device having the floating-gate layer 111 formed by a local-oxidation of silicon (LOCOS) technique, in which the floating-gate length is defined in general to be larger than a minimum-feature-size of technology used due to the bird's beak formation; the control-gate 115 is formed over a LOCOS-oxide layer 112 and a thicker select-gate oxide layer 114; a poly-oxide layer 113 is formed over a sidewall of the floating-gate layer 111; a source diffusion region 116 and a drain diffusion region 117 are formed in a semiconductor substrate 100 in a self-aligned manner; and a thin gate-oxide layer 110 is formed under the floating-gate layer 111. The split-gate structure shown in FIG. 1A is programmed by mid-channel hot-electron injection, the programming efficiency is high and the programming power is low as compared to the channel hot-electron injection used by the stack-gate structure. Moreover, the over-erase problem of the split-gate structure can be prevented due to a high threshold-voltage for the select-gate region, so the control logic circuits for erasing and verification can be simplified. However, there are several drawbacks for FIG. 1A: the cell size is larger due to the non self-aligned control-gate structure; the gate length can't be easily scaled down due to the misalignment of the control-gate with respect to the floating-gate; the coupling ratio is low and higher applied control-gate voltage is required for back erase; and the field-emission tip of the floating-gate layer is difficult to be controlled due to the weak masking ability of the bird-beak oxide.

[0005] FIG. 1B shows another split-gate structure, in which the floating-gate layer 121 is defined by a minimum-feature-size (F) of technology used; a thin tunneling-oxide layer 120 is formed under the floating-gate layer 121; a select-gate dielectric layer 122 is formed over the select-gate region and the exposed floating-gate layer 121; a control-gate layer 123 is formed over the select-gate dielectric layer 122; and a source diffusion region 124 and a double-diffused drain region 125, 126 are formed in a semiconductor substrate 100. From FIG. 1B, it is clearly visualized that similar drawbacks as listed for FIG. 1A are appeared except that the erasing site is located at the thin tunneling-oxide layer 120 between the floating-gate layer 121 and the double-diffused drain region 125,126.

[0006] It is therefore an objective of the present invention to provide a self-aligned split-gate flash memory cell having a cell size being smaller than 4F².

[0007] It is another objective of the present invention to provide a higher coupling ratio for a self-aligned split-gate flash memory cell.

[0008] It is a further objective of the present invention to provide a reproducible tip-cathode structure for the self-aligned split-gate flash memory cell with a higher field-emission efficiency.

[0009] It is yet another objective of the present invention to provide two contactless architectures for self-aligned split-gate flash memory arrays.

[0010] Other objectives and advantages of the present invention will be more apparent from the following description.

[0011] A self-aligned split-gate flash memory cell of the present invention is formed on a semiconductor substrate of a first conductivity type having an active region isolated by two parallel shallow-trench-isolation (STI) regions, wherein each of the two parallel STI regions is filled with a first raised field-oxide layer. A cell region can be divided into three regions: a common-source region, a gate region, and a common-drain region, wherein the gate region is located between the common-source region and the common-drain region. The common-source region comprises a first sidewall dielectric spacer being formed over a sidewall of the gate region and on a portion of a first flat bed being formed by a common-source diffusion region of a second conductivity type in the active region and two etched first raised field-oxide layers in the two parallel STI regions. The common-drain region comprises a second sidewall dielectric spacer being formed over another sidewall of the gate region and on a portion of a second flat bed being alternately formed by a common-drain diffusion region of the second conductivity type in the active region and two etched second raised field-oxide layers in the two parallel STI regions. The gate region comprises a single-side dish-shaped floating-gate structure being formed on a first gate-dielectric layer with a first intergate-dielectric layer being formed on its top portion and a second intergate-dielectric layer being formed on its inner sidewall and tip portion; and a planarized control/select-gate conductive layer being at least formed over a second gate-dielectric layer and the first/second intergate-dielectric layers in the active region and a portion of the first/second raised field-oxide layers in the two parallel STI regions, wherein the single-side dish-shaped floating-gate structure is preferably formed by an isotropic dry or wet etching process.

[0012] The self-aligned split-gate flash memory cell of the present invention as described is used to implement two contactless array architectures: a NOR-type flash memory array and a parallel common-source/drain conductive bit-lines flash memory array. The contactless NOR-type flash memory array comprises a plurality of common-source conductive bus lines being formed alternately in a first direction; a plurality of common-drain conductive islands being at least formed over the plurality of active regions along each of the common-drain regions between the plurality of common-source conductive bus lines; a plurality of self-aligned split-gate flash memory cells being formed between each of the plurality of common-source conductive bus lines and its nearby common-drain conductive islands with the elongated planarized control/select-gate conductive layer being acted as a word line in the first direction; and a plurality of bit-lines integrated with the plurality of common-drain conductive islands being simultaneously patterned and etched in a second direction being perpendicular to the first direction.

[0013] The contactless parallel common-source/drain conductive bit-lines flash memory array of the present invention comprises a plurality of common-source conductive bit lines and a plurality of common-drain conductive bit lines being formed alternately in a first direction; a plurality of self-aligned split-gate flash memory cells being formed between each of the plurality of common-source conductive bit lines and each of the plurality of common-drain conductive bit lines; and a plurality of word lines integrated with a plurality of planarized control/select-gate conductive islands being simultaneously patterned and etched in a second direction being perpendicular to the first direction.

[0014] FIG. 1 shows the schematic diagrams of the prior art, in which FIG. 1A shows a cross-sectional view of a split-gate flash memory cell with a tip-cathode structure for erasing; FIG. 1B shows a cross-sectional view of a split-gate flash memory cell with a source-side erase structure.

[0015] FIG. 2 shows the schematic diagrams of the present invention, in which FIG. 2A(a) shows a cross-sectional view of a pair of self-aligned split-gate flash memory cells in a contactless NOR-type flash memory array; FIG. 2A(b) shows a cross-sectional view of a pair of self-aligned flash memory cells in a contactless parallel common-source/drain conductive bit-lines flash memory array; FIG. 2B(a) shows a top plan view of a contactless NOR-type self-aligned split-gate flash memory array; FIG. 2B(b) shows a top plan view of a contactless parallel common-source/drain conductive bit-lines self-aligned split-gate flash memory array; FIG. 2C(a) shows a schematic circuit diagram of a NOR-type self-aligned split-gate flash memory array; and FIG. 2C(b) shows a schematic circuit diagram of a contactless parallel common-source/drain conductive bit-lines self-aligned split-gate flash memory array.

[0016] FIG. 3A through FIG. 3F show the process steps and their cross-sectional views of forming a shallow-trench-isolation structure for a self-aligned split-gate flash memory cell and its contactless flash memory arrays of the present invention.

[0017] FIG. 4A through FIG. 4O show the process steps and their cross-sectional views of forming a self-aligned split-gate flash memory cell and its contactless flash memory arrays of the present invention.

[0018] FIG. 5A through FIG. 5D show various cross-sectional views of a contactless NOR-type self-aligned split-gate flash memory array of the present invention as indicated in FIG. 4O(a).

[0019] FIG. 6A through FIG. 6D show various cross-sectional views of a contactless parallel common-source/drain conductive bit-lines self-aligned flash memory array of the present invention as indicated in FIG. 4O(b).

[0020] Referring now to FIG. 2A(a) and FIG. 2A(b), there are shown the cross-sectional views of a pair of self-aligned split-gate flash memory cells of the present invention being configured into two array architectures shown in FIG. 2B(a) and FIG. 2B(b), respectively, in which FIG. 2A(a) shows a cross-sectional view along the A-A′ line for a contactless NOR-type flash memory array shown in FIG. 2B(a) and FIG. 2A(b) shows a cross-sectional view along the A-A′ line for a parallel common-source/drain conductive bit-lines flash memory array shown in FIG. 2B(b).

[0021] From FIG. 2A(a), the common-source region comprises a pair of first sidewall dielectric spacers 316abeing formed over each sidewall of the nearby gate regions and on a portion of a first flat bed being formed alternately by an etched first raised field-oxide layer 304c(not shown in FIG. 2A(a), best seen in FIGS. 5D and 6D) and a shallow heavily-doped source diffusion region 315bof a second conductivity type formed within a common-source diffusion region 315a; a common-source conductive bus line 317bbeing formed over the first flat bed between the pair of first sidewall dielectric spacers 316a, and a first planarized thickoxide layer 318abeing formed over the common-source conductive bus line 317b. The gate region comprises an elongated planarized control/select-gate conductive layer 307c, 313chaving a portion formed at least on a second gate-dielectric layer 311bin a select-gate region and another portion formed at least over the first intergate-dielectric layer 306band the second intergate-dielectric layer 312a, wherein the first intergate-dielectric layer 306bis formed over a single-side dish-shaped floating-gate structure 302c; a second intergate-dielectric layer 312ais formed over an inner sidewall and a top portion of the single-side dish-shaped floating-gate structure 302c; the single-side dish-shaped floating-gate structure 302eis formed over a first gate-dielectric layer 301c; and an implanted region 310bof a first conductivity type having a shallow implant region as indicated by the dash line for threshold-voltage adjustment and a deep implant region as indicated by the cross symbols for forming a punchthrough stop is formed under the second gate-dielectric layer 311b. For the common-drain region, a second sidewall dielectric spacer 321ais formed over another sidewall of the gate region and on a portion of a second flat bed being alternately formed by an etched second raised field-oxide layer 304eand a shallow heavily-doped drain diffusion region 320bof a second conductivity type formed within a common-drain diffusion region 320a; and a planarized common-drain conductive island 322bis formed over the second flat bed outside of the second sidewall dielectric spacer 321a. As shown in FIG. 2A(a), a fourth sidewall dielectric spacer 314ais formed over a portion of the elongated planarized control/select-gate conductive layer 307c,313c; a fifth sidewall dielectric spacer 319ais formed over another portion of the elongated planarized control/select-gate conductive layer 307c, 313c; and a first interconnect-metal layer 325aintegrated with the planarized common-drain conductive islands 322bare simultaneously patterned and etched by a hard masking layer including a masking dielectric layer 326aand its two sidewall dielectric spacers 327a(not shown in FIG. 2A(a), best seen in FIGS. 5A-5D and 6A-6D) to form a bit line transversely to the common-source conductive bus line 317b.

[0022] From FIG. 2A(b), the common-source region comprises a pair of first sidewall dielectric spacer 316bbeing formed over each sidewall of the nearby gate regions and on a portion of a first flat bed being formed alternately by an etched first raised field-oxide layer 304cand a shallow heavily-doped source diffusion region 315bof a second conductivity type formed within a common-source diffusion region 315a; a common-source conductive bus line 317bbeing formed over the first flat bed between the pair of first sidewall dielectric spacers 316b; and a first planarized thick-oxide layer 318bbeing formed over the common-source conductive bus line 317b. The gate region comprises a planarized control/select-gate conductive island 324b, 313d, 307dhaving a portion formed at least on a second gate-dielectric layer 311bin a select-gate region and another portion formed at least over the first intergate-dielectric layer 306band the second intergate-dielectric layer 312a, wherein the first intergate-dielectric layer 306bis formed over a single-side dish-shaped floating-gate structure 302c; a second intergate-dielectric layer 312ais formed over an inner sidewall and a tip portion of the single-side dish-shaped floating-gate structure 302c; the single-side dish-shaped floating-gate structure 302cis formed over a first gate-dielectric layer 301c; and an implanted region 310bof a first conductivity type having a shallow implant region as indicated by the dash line for threshold-voltage adjustment and a deep implant region as indicated by the cross-symbols for forming a punch-through stop is formed under the second gate-dielectric layer 311b.

[0023] For the common-drain region shown in FIG. 2A(b), a second sidewall dielectric spacer 321bis formed over another sidewall of the gate region and on a portion of a second flat bed alternately formed by an etched second raised field-oxide layer 304eand a shallow heavily-doped drain diffusion region 320bof a second conductivity type formed within a common-drain diffusion region 320a; a common-drain conductive bus line 322bis formed over the second flat bed outside of the second sidewall dielectric spacer 321b; and a second planarized thick-oxide layer 323bis formed over the common-drain conductive bus line 322b. As shown in FIG. 2A(b), a first interconnect-metal layer 325aintegrated with the planarized control/select-gate conductive islands 324b, 307d, 313dare simultaneously patterned and etched by a hard masking layer including a masking dielectric layer 326aand its two sidewall dielectric spacers 327ato form a word line transversely to the common-source/drain conductive bus lines 317b, 322b.

[0024] FIG. 2B(a) shows a top plan view of a contactless NOR-type flash memory array of the present invention, in which the cross-sectional view along the A-A′ line is shown in FIG. 2A(a). As shown in FIG. 2B(a), a plurality of active regions (AA's) and a plurality of parallel STI regions (STI lines) are formed alternately on a semiconductor substrate 300 of a first conductivity type, wherein a first raised field-oxide layer 304bis formed in each of the plurality of parallel STI regions; a plurality of common-source conductive bus lines 317b(CS0,CS1) are formed alternately and transversely to the plurality of parallel STI regions, wherein each of the plurality of common-source conductive bus lines 317bis formed over a first flat bed between a pair of first sidewall dielectric spacers 316band the first flat bed is formed alternately by an etched first raised field-oxide layer 304cand a shallow heavily-doped source diffusion region 315bof a second conductivity type formed within a common-source diffusion region 315a; a plurality of planarized common-drain conductive islands 322bare formed alternately between the plurality of common-source conductive bus lines 317band over the second flat beds between the pair of second sidewall dielectric spacers 321a, wherein each of the second flat beds is alternately formed by an etched second raised field-oxide layer 304eand a shallow heavily-doped drain diffusion region 320bformed within a common-drain diffusion region 320a; a plurality of self-aligned split-gate flash memory cells are located between each of the plurality of common-source conductive bus lines 317band the plurality of planarized common-drain conductive islands 322b; a plurality of bit lines 325a(BL0-BL2) integrated with the plurality of common-drain conductive islands 322bare simultaneously patterned and etched to be perpendicular to the plurality of common-source conductive bus lines 317bby using a plurality of hard masking layers, wherein each of the plurality of hard masking layers includes a masking dielectric layer 326abeing aligned above the active region (AA) and two sidewall dielectric spacers 327abeing formed over each sidewall of the masking dielectric layer 326a. The various cross-sectional views as indicated by the B-B′ line, the C-C′ line, the D-D′ line, and the E-E′ line as shown in FIG. 2B(a) are shown in FIG. 5A through FIG. 5D, respectively. The schematic circuit diagram of FIG. 2B(a) is shown in FIG. 2C(a).

[0025] FIG. 2B(b) shows a contactless parallel common-source/drain conductive bit-lines flash memory array of the present invention, in which a plurality of active regions (AA's) and a plurality of parallel STI regions (STI lines) are alternately formed over a semiconductor substrate 300 of a first conductivity type with each of the plurality of parallel STI regions being filled with a first raised field-oxide layer 304b(not shown in FIG. 2B(b), best seen in FIGS. 5C and 6C); a plurality of common-source conductive bus lines 317b(CS0, CS1) and a plurality of common-drain conductive bus lines 322b(CD0, CD1) are formed alternately and transversely to the plurality of parallel STI regions; a plurality of self-aligned split-gate flash memory cells are formed between each of the plurality of common-source conductive bus lines 317band its nearby common-drain conductive bus lines 322b; and a plurality of word lines (WL0-WL2) integrated with a plurality of planarized control/selectgate conductive islands 324b, 313d, 307dare simultaneously patterned and etched to be perpendicular to the plurality of common-source/drain conductive bus lines 317b, 322bby using a plurality of hard masking layers with each of the plurality of hard masking layers comprising a masking dielectric layer 326abeing aligned above the active region and two sidewall dielectric spacers 327abeing formed over each sidewall of the masking dielectric layer 326a. Similarly, each of the plurality of commonsource conductive bus lines 317bis formed over a first flat bed between the pair of first sidewall dielectric spacers 316band the first flat bed is alternately formed by an etched first raised field-oxide layer 304cand a shallow heavily-doped source diffusion region 315bof a second conductivity type formed within a common-source diffusion region 315a; each of the plurality of common-drain conductive bus lines 322bis Formed over a second flat bed between a pair of second sidewall dielectric spacers 321band the second flat bed is alternately formed by an etched second raised field-oxide layer 304e(not shown in FIG. 2B(b), best seen in FIGS. 5A and 6A) and a shallow heavily-doped drain diffusion region 320bof a second conductivity type formed within a common-drain diffusion region 320a. The various cross-sectional views as indicated by the B-B′ line, the C-C′ line, the D-D′ line, and the E-E′ line shown in FIG. 2B(b) are shown in FIG. 6A through FIG. 6D, respectively The schematic circuit diagram of FIG. 2B(b) is shown in FIG. 2C(b).

[0026] From FIG. 2B(a) and FIG. 2B(b), the width and the space of a shallow-trench-isolation (STI) region can be defined to be a minimum-feature-size(F) of technology used; a common-source region and a common-drain region can be defined to be X₁F and X₃F, respectively; and a control/select-gate region of a self-aligned split-gate flash memory cell can be defined to be X₂F. It is clearly seen that X₁, X₂and X₃are the scaling factors and can be defined to be equal to unity, then the cell size as marked by a dash square in FIG. 2B(a) and FIG. 2B(b) is equal to 4F². Therefore, the cell size of the present invention can be fabricated to be smaller than 4F²by controlling the scaling factors X₁, X₂or X₃. It should be emphasized that the self-aligned split-gate flash memory cell of the present invention can be erased by the tip cathode formed by the single-side dish-shaped floating-gate structure with a higher field-emission efficiency due to a higher coupling ratio of the self-aligned structure used in the present invention. Similarly, the self-aligned split-gate flash memory cell can be erased by tunneling the stored electrons from the floating-gate layer to the common-source diffusion region if the tip-cathode is covered by a thicker second intergate-dielectric layer.

[0027] Accordingly, the major advantages and features of the present invention are summarized below:

[0028] (a) The self-aligned split-gate flash memory cell of the present invention can be made to have a cell size smaller than 4F².

[0029] (b) The self-aligned split-gate flash memory cell of the present invention can be erased with a higher field-emission efficiency through the single-side dish-shaped floating-gate structure.

[0030] (c) The two contactless flash memory arrays of the present invention offer a highly common-source conductive bus line over the common-source diffusion regions with smaller bus-line resistance and smaller bus-line capacitance with respect to the semiconductor substrate as compared to those of the buried diffusion layer of the prior art.

[0031] (d) The contactless NOR-type flash memory array of the present invention offers a highly conductive bit line being integrated with common-drain conductive islands with lower bit-line resistance.

[0032] (e) The contactless parallel common-source/drain conductive bit-lines flash memory array of the present invention offers a highly conductive layer for both common-source conductive bus line and common-drain conductive bus line with smaller bus-line resistance and smaller bus-line capacitance with respect to the semiconductor substrate.

[0033] (f) The contactless parallel common-source/drain conductive bit-lines flash memory array of the present invention offers a highly conductive word line being integrated with planarized control/select-gate conductive islands with lower word-line resistance.

[0034] Referring now to FIG. 3A through FIG. 3F, there are shown the process steps and their cross-sectional views of fabricating a shallow-trench-isolation (STI) structure for a self-aligned split-gate flash memory cell having a single-side dish-shaped floating-gate structure and its contactless flash memory arrays. FIG. 3A shows that a first gate-dielectric layer 301 is formed on a semiconductor substrate 300 of a first conductivity type; a first conductive layer 302 is formed on the first gate-dielectric layer 301; a first masking dielectric layer 303 is formed on the first conductive layer 302; and a plurality of masking photoresist PR1 are formed over the first masking dielectric layer 303 to define a plurality of active regions (AA's) (under PR1) and a plurality of parallel shallow-trench-isolation regions (STI lines) (between PR1). The first gate-dielectric layer 301 is preferably a thermal-oxide layer or a nitrided thermal-oxide layer having a thickness between 80 Angstroms and 120 Angstroms. The first conductive layer 302 is preferably made of doped polycrystalline-silicon or doped amorphous-silicon as deposited by low-pressure chemical-vapor-deposition (LPCVD) and its thickness is preferably between 1000 Angstroms and 3000 Angstroms. The first masking dielectric layer 303 is preferably made of silicon-nitrides as deposited by LPCVD and its thickness is preferably between 500 Angstroms and 3000 Angstroms. It should be noted that the width and the space of each of the plurality of masking photoresist PR1 can be defined to be a minimum-feature-size (F) of technology used, as indicated in FIG. 3A.

[0035] FIG. 3B shows that the first masking dielectric layer 303, the first conductive layer 302, and the first gate-dielectric layer 301 outside of the plurality of masking photoresist PR1 are sequentially removed and the semiconductor substrate 300 is then anisotropically etched to form shallow trenches; and subsequently, the plurality of masking photoresist PR1 are stripped. The depth of shallow trenches in the semiconductor substrate 300 is preferably between 3000 Angstroms and 8000 Angstroms.

[0036] FIG. 3C shows that the planarized field-oxide layers 304aare formed to fill up the gaps as shown in FIG. 3B. The planarized field-oxide layer 304ais preferably made of silicon-oxides, phosphosilicate glass (PSG), or boro-phosphosilicate glass (BPSG), as deposited by high-density plasma (HDP) CVD or CVD and is formed by first depositing a thick silicon-oxide film 304 to fill up the gaps and then planarizing the deposited thick silicon-oxide film 304 using chemical-mechanical polishing (CMP) with the first masking dielectric layer 303aas a polishing stop.

[0037] FIG. 3D shows that the planarized field-oxide layers 304aare etched back to a depth equal to a thickness of the first masking dielectric layer 303ato form the first raised field-oxide layers 304b.

[0038] FIG. 3E shows that the first masking dielectric layers 303aare removed by hot-phosphoric acid or anisotropic dry etching to form a flat surface being formed alternately by the first raised field-oxide layer 304band the first conductive layer 302a. The flat surface is favorable for fine-line lithography later on.

[0039] FIG. 3F shows that a second masking dielectric layer 305 is formed over the flat surface shown in FIG. 3E. The second masking dielectric layer 305 is preferably made of silicon-nitrides as deposited by LPCVD and its thickness is preferably between 4000 Angstroms and 12000 Angstroms. The cross-sectional view of an active region as marked by the F-F′ line in FIG. 3F is shown in FIG. 4A.

[0040] Referring now to FIG. 4A through FIG. 4O, there are shown the process steps and their cross-sectional views of fabricating a self-aligned split-gate flash memory cell having a single-side dish-shaped floating-gate structure and its contactless flash memory arrays. FIG. 4A shows a small portion of a memory array, in which a plurality of masking photoresist PR2 are formed over the second masking dielectric layer 305 to define a plurality of first virtual-gate regions VG1 (between PR2) and a plurality of second virtual-gate regions VG2 (under PR2). Each of the plurality of first virtual-gate regions VG1 includes a common-drain region and a pair of first control-gate regions. Each of the plurality of second virtual-gate regions VG2 includes a common-source region and a pair of second control-gate regions. It should be noted that both the first virtual-gate region VG1 and the second virtual-gate region VG2 are scalable.

[0041] FIG. 4B shows that the second masking dielectric layer 305 outside of the plurality of masking photoresist PR2 is removed and then the plurality of masking photoresist PR2 are stripped; and the exposed first conductive layer 302ais etched isotropically by using either a dry etching or a wet etching.

[0042] FIG. 4C shows that the first intergate-dielectric layers 306aare formed over the shaped first conductive layer 302aas shown in FIG. 4B. It should be emphasized that the first intergate-dielectric layer 306ais preferably a thermal poly-oxide or nitrided thermal poly-oxide layer.

[0043] FIG. 4D shows that a planarized second conductive layer 307ais formed over the first intergate-dielectric layer 306ain each of the plurality of first virtual-gate regions VG1 and is then etched back to form an etched-back second conductive layer 307b. The planarized second conductive layer 307ais preferably made of doped polycrystalline-silicon as deposited by LPCVD and is formed by first depositing a thick second conductive layer 307 to fill up the gaps and then planarizing the deposited thick second conductive layer 307 using CMP with the second masking dielectric layer 305aas a polishing stop. It should be noted that a metal-silicide layer can be formed over 307bby using either a self-aligned silicidation process or a deposition technique as described for an etched-back second conductive layer 307band is preferably a tungsten-silicide (WSi₂) layer. The etched-back second conductive layer 307bcan be a metal layer such as a tungsten (W) layer.

[0044] FIG. 4E shows that a planarized dielectric layer 308ais formed over the gap in each of the first virtual-gate regions by first depositing a thick silicon-oxide film 308 to fill up the gaps and then planarizing the deposited thick silicon-oxide film 308 using CMP with the second masking dielectric layer 305aas a polishing stop.

[0045] FIG. 4F shows that the second masking dielectric layers 305ain each of the second virtual-gate regions are removed by hot-phosphoric acid or anisotropic dry etching and a third sidewall dielectric spacer 309ais formed over each sidewall formed by the removed second masking dielectric layer 305ain each of the second virtual-gate regions; and subsequently, the first conductive layer 302abetween the pair of third sidewall dielectric spacers 309ain each of the second virtual-gate regions is removed by using anisotropic dry etching. An ion-implantation is then performed in a self-aligned manner to form an implanted region 310aof a first conductivity type in the semiconductor substrate 300 of the active regions, in which the implanted region 310amay include a shallow implant region as indicted by a dash line for threshold-voltage adjustment and a deep implant region as indicated by the cross symbols for forming a punch-through stop. The third sidewall dielectric spacer 309ais preferably made of silicon-nitrides as deposited by LPCVD and its spacer width is mainly used to create a buffer region for sidewall oxidation of the first conductive layer 302blater on. It is clearly seen that the third sidewall dielectric spacer 309aoffers two purposes: one is to prevent the tip-cathode from damages due to anisotropic dry etching; the other is to offer a buffer region for sidewall oxidation without smearing out the sharp shape of the tip cathode. As a consequence, a reproducible tip cathode with higher field-emission probability can be easily obtained by the present invention.

[0046] FIG. 4G shows that the first gate-dielectric layers 301aoutside of the third sidewall dielectric spacers 309aare removed by dipping in a dilute hydrofluoric-acid solution and a thermal oxidation process is then performed to form a second gate-dielectric layer 311aover the exposed semiconductor substrate 300 and a second intergate-dielectric layer 312aover each sidewall of the exposed second conductive layer 302b.

[0047] FIG. 4H shows that the third sidewall dielectric spacers 309aare removed by using hot-phosphoric acid. It is clearly seen that the second intergate-dielectric layer 312abeing formed over the sidewall is thicker than that on the tip cathode and a higher field-emission efficiency of the tip-cathode of the present invention is expected. Moreover, a thermal nitridation process can be performed in a N₂O or NH₃ambient to nitride the second intergate-dielectric layer 312aand the second gate-dielectric layer 311abefore removing the third sidewall dielectric spacers 309afor increasing the endurance of the second intergate-dielectric layer 312aover the tip-cathode. FIG. 4H also shows that a planarized third conductive layer 313ais formed over the gap in each of the second virtual-gate regions and is formed by first depositing a thick third conductive layer 313 over the formed structure and then planarizing the deposited thick third conductive layer 313 using CMP or etch-back.

[0048] FIG. 4I shows that the planarized third conductive layer 313ais etched back to a thickness of the planarized dielectric layer 308a. The third conductive layer 313bis preferably made of doped polycrystalline-silicon and is further implanted with a high-dose of doping impurities. Similarly, the third conductive layer 313bcan be a composite conductive layer such as a tungsten-silicide layer over a doped polycrystalline-silicon layer as described for the second conductive layer 307b. FIG. 4I also shows that a fourth sidewall dielectric spacer 314ais formed over each sidewall of the planarized dielectric layer 308ain each of the second virtual-gate regions; a plurality of masking photoresist PR3 are formed over the planarized dielectric layer 308aand a portion of the fourth sidewall dielectric spacers 314a; the third conductive layer 313bis etched back first to a top surface of the first raised field-oxide layers 304bin the parallel STI regions, then the first raised field-oxide layers 304bare then anisotropically etched back to a top surface of the second gate-dielectric layer 311a; and subsequently, the remained third conductive layers 313bare anisotropically removed. An ion-implantation is performed in a self-aligned manner by implanting doping impurities across the second gate-dielectric layer 311ainto the semiconductor substrate 300 of the active regions to form the common-source diffusion regions 315aof a second conductivity type. The common-source diffusion regions 315acan be lightly doped, moderately doped, or heavily doped.

[0049] FIG. 4J shows that the second gate-dielectric layers 311ain each of the common-source regions are removed by dipping in a dilute hydrofluoric-acid solution or an anisotropic dry etching and the etched raised field-oxide layer in each of the parallel STI regions is simultaneously etched to form a first flat bed along each of the common-source regions; and subsequently, the plurality of masking photoresist PR3 are then stripped. The first flat bed is formed alternately by a common-source diffusion region 315aand an etched first raised field-oxide layer 304c. FIG. 4J also shows that a pair of first sidewall dielectric spacers 316aare formed over each sidewall of the common-source region and on a portion of the first flat bed; a common-source conductive layer 317bis formed over the first flat bed between the pair of first sidewall dielectric spacers 316ain each of the common-source regions; and a first planarized thick-oxide layer 318ais formed over the common-source conductive layer 317band the pair of first sidewall dielectric spacers 316a. The first sidewall dielectric spacer 316ais preferably made of silicon-nitrides or silicon-oxides as deposited by LPCVD. The common-source conductive layer 317bis preferably made of doped polycrystalline-silicon as deposited by LPCVD and is formed by first depositing a thick fourth conductive layer 317 over the gaps between the pair of first sidewall dielectric spacers 316aand then planarizing the deposited thick fourth conductive layer 317 using CMP with the fourth sidewall dielectric spacers 314aas a polishing stop and subsequently etching back the planarized fourth conductive layer 317a. The common-source conductive layer 317bcan be a composite conductive layer having a tungsten-silicide (WSi₂) layer formed over a doped polycrystalline-silicon layer, in which the tungsten-silicide layer is formed by a well-known self-aligned silicidation process or a deposition technique similar to that of the common-source conductive layer 317bbeing made of doped polycrystalline-silicon. It should be noted that the doped polycrystalline-silicon layer 317bcan be further implanted with a high-dose of doping impurities to act as a dopant diffusion source for forming a shallow heavily-doped source diffusion region 315bof the second conductivity type within each of the common-source diffusion regions 315a. It should be emphasized that the common-source conductive layer 317bcan be a tungsten-metal layer over a barrier-metal layer and then over a silicided doped polycrystalline-silicon layer. The first planarized thick-oxide layer 318ais preferably made of silicon-oxides, phosphosilicate-glass (P-glass) or boro-phosphosilicate glass (BP-glass), as deposited by high-density plasma (HDP) CVD or CVD. Similarly, the first planarized thick-oxide layer 318ais formed by first depositing a thick-oxide film 318 over the formed structure and then planarizing the doposited thick-oxide film 318 using CMP with the fourth sidewall dielectric spacers 314aas a polishing stop.

[0050] FIG. 4K shows that a plurality of masking photoresist PR4(a) is formed over each of the common-source regions and a portion of the fourth sidewall dielectric spacers 314aas shown in FIG. 4J and the planarized dielectric layers 308aare selectively removed by using a buffered hydrofluoric-acid solution, then the plurality of masking photoresist PR4(a) are stripped. Subsequently, a fifth sidewall dielectric spacer 319ais formed over each sidewall of the fourth sidewall dielectric spacers 314ain each of the plurality of first virtual-gate regions VG1 and on the etched-back second conductive layer 307b.

[0051] FIG. 4L shows that a plurality of masking photoresist PR4(b) are again formed over the same positions; the etched-back second conductive layer 307band the first intergate-dielectric layer 306aoutside of the fifth sidewall dielectric spacers 319aare sequentially removed and the first raised field-oxide layers 304bare then etched back to a level equal to the top surface of the first gate-dielectric layer 301b, and subsequently, the first conductive layers 302bare removed in a self-aligned manner; and an ion-implantation is performed in a self-aligned manner to form a plurality of common-drain diffusion regions 320aof a second conductivity type in the semiconductor substrate 300 of the active regions along each of the common-drain regions. The common-drain diffusion region 320acan be lightly doped, moderately doped, or heavily doped.

[0052] FIG. 4M shows that the first gate-dielectric layer 301bbetween a pair of fifth sidewall dielectric spacers 319aare removed by dipping in a dilute hydrofluoric acid solution or anisotropic dry etching and the etched second raised field-oxide layers in each of the common-drain regions are simultaneously etched to form a second flat bed; and subsequently, the plurality of masking photoresist PR4(b) are stripped. The second flat bed is alternately formed by an etched second raised field-oxide layer 304eand a common-drain diffusion region 320a. FIG. 4M also shows that a second sidewall dielectric spacer 321ais formed over each sidewall of the common-drain region and on a portion of the second flat bed being alternately formed by an etched second raised field-oxide layer 304eand a common-drain diffusion region 320a. The second sidewall dielectric spacer 321ais preferably made of silicon-nitrides or silicon-oxides as deposited by LPCVD.

[0053] FIG. 4N(a) shows that a planarized fifth conductive layer 322ais formed over the second flat bed between the pair of second sidewall dielectric spacers 321ain each of the common-drain regions. The planarized fifth conductive layer 322ais preferably a planarized doped polycrystalline-silicon layer or a planarized tungsten layer over a barrier-metal layer and over a silicide layer. The planarized fifth conductive layer 322acan also be formed by first depositing a thin doped polycrystalline-silicon layer and then implanting heavily with a high dose of doping impurities to act as a dopant diffusion source for forming a shallow heavily-doped drain diffusion region 320bof a second conductivity type within each of the common-drain diffusion regions 320aand then depositing a thick tungsten-silicide or tungsten film to fill up the gaps; and subsequently, a planarization process is performed by using CMP or etching-back. It should be noted that a barrier-metal layer such as a titanium-nitride(TiN) or tantalum-nitride(TaN) layer can be formed over the thin doped polycrystalline-silicon layer before depositing a thick tungsten-silicide or tungsten film.

[0054] FIG. 4N(b) shows that a fifth conductive layer 322bis formed over the second flat bed between the pair of second sidewall dielectric spacers 321ain each of the common-drain regions and a second planarized thick-oxide layer 323ais formed over the fifth conductive layer 322band the pair of second sidewall dielectric spacers 321ain each of the common-drain regions. Similarly, the fifth conducive layer 322bcan be made by using the same process technique as discussed for the fourth conductive layer 317b. The second planarized thick-oxide layer 323ais preferably made of silicon-oxides, phosphosilicate glass (p-glass), or boro-phosphosilicate glass (BPSG) as deposited by HDPCVD or CVD.

[0055] FIG. 4O(a) shows that a first interconnect-metal layer 325 being formed over the formed structure shown in FIG. 4N(a) together with the planarized fifth conductive layers 322aare simultaneously patterned and etched to form a plurality of bit lines 325aand a plurality of the planarized fifth conductive islands 322bby using a plurality of hard masking layers, wherein each of the plurality of hard masking layers includes a masking dielectric layer 326abeing aligned above the active region and two sidewall dielectric spacers 327a(shown in FIG. 5) being formed over each sidewall of the masking dielectric layer 326a. The various cross-sectional views shown in FIG. 4O(a) are shown in FIG. 5A through FIG. 5D, respectively.

[0056] FIG. 4O(b) shows that the curve portions of the first/second planarized thick-oxide layers 318a/323aand the first/second sidewall dielectric spacers 316a/321abeing made of silicon-oxides are etched back first; the fourth/fifth sidewall dielectric spacers 314a/319abeing made of silicon-nitrides are then removed by using hot-phosphoric acid or anisotropic dry etching; subsequently, a planarized sixth conductive layer 324ais formed over the second/third conductive layers 313c/307c; and a first interconnect-metal layer 325 being formed over a flat surface together with the planarized sixth conductive layers 324a, the second conductive layers 307c, and the third conductive layers 313care simultaneously patterned and etched to form a plurality of word lines 325aand a plurality of planarized control/select-gate conductive islands 324b, 313d, 307dby using a plurality of hard masking layers, wherein each of the plurality of hard masking layers includes a masking dielectric layer 326abeing aligned above the active region and two sidewall dielectric spacers 327a(shown in FIG. 6) being formed over each sidewall of the masking dielectric layer 326a. The planarized sixth conductive layer 324ais preferably made of tungsten-silicide (WSi₂) or tungsten (W). It should be noted that the first/second planarized thick-oxide layers 318a/323aand the first/second sidewall dielectric spacers 316a/321acan be etched back to a top level of the second/third conductive layer 307c/313cand the fourth/fifth sidewall dielectric spacers 314a/319aare removed; subsequently, a first interconnect-metal layer 325 is formed and patterned. It is clearly seen that the planarized sixth conductive layer 324ais not required for the later structure (not shown) with sacrificing the thickness of the first/second planarized thick-oxide layers 318b/323b. The various cross-sectional views as indicated in FIG. 4O(b) are shown in FIG. 6A through FIG. 6D, respectively.

[0057] Referring now to FIG. 5A through FIG. 5D, there are shown various cross-sectional views as indicated in FIG. 4O(a). FIG. 5A shows a cross-sectional view along each of the common-drain regions as indicated by the B-B′ line shown in FIG. 4O(a), in which a plurality of bit-lines 325atogether with a plurality of planarized fifth conductive islands 322bbeing patterned and etched by a plurality of hard masking layers are formed over a second flat bed being alternately formed by an etched second raised field-oxide layer 304eand a shallow heavily-doped drain diffusion region 320bof a second conductivity type being formed within a common-drain diffusion region 320a; and each of the plurality of hard masking layers includes a masking dielectric layer 326abeing aligned above the active region and two sidewall dielectric spacers 327abeing formed over each sidewall of the masking dielectric layer 326ato eliminate misalignment.

[0058] FIG. 5B shows a cross-sectional view along the floating-gate region as indicated by the C-C′ line shown in FIG. 4O(a), in which a first intergate-dielectric layer 306bis formed over the floating-gate layer 302c; a planarized control/select-gate conductive layer 307cis alternately formed over the second raised field-oxide layer 304dand the first intergate-dielectric layer 306b; a fifth sidewall dielectric spacer 319ais formed over the planarized control/select-gate conductive layer 307c; and a plurality of bit lines 325aare patterned and etched by a plurality of hard masking layers as described.

[0059] FIG. 5C shows a cross-sectional view along each of the select-gate regions as indicated by the D-D′ line shown in FIG. 4O(a), in which the planarized control/select-gate conductive layer 313cis alternately formed over a first raised field-oxide layer 304band a second gate-dielectric layer 311b; the fourth sidewall dielectric spacer 314ais formed over the planarized control/select-gate conductive layer 313c; and a plurality of bit lines 325abeing formed over the fourth sidewall dielectric spacer 314aare patterned and etched by a plurality of hard masking layers as described.

[0060] The implanted region 310bof a first conductivity type being formed under the second gate-dielectric layer 311bincludes a shallow implant region as indicated by the dash line for threshold-voltage adjustment and a deep implant region as indicated by the cross symbols for forming a punch-through stop.

[0061] FIG. 5D shows a cross-sectional view along the common-source region as indicated by the E-E′ line shown in FIG. 4O(a), in which the fourth conductive layer 317bis formed over a first flat bed being alternately formed by an etched first raised field-oxide layer 304cand a shallow heavily-doped source diffusion region 315bof a second conductivity type being formed within a common-source diffusion region 315a; a first planarized thick-oxide layer 318ais formed over the fourth conductive layer 317b, and a plurality of bit lines 325abeing patterned and etched by a plurality of hard masking layers as described are formed over the first planarized thick-oxide layer 318a.

[0062] Referring now to FIG. 6A through FIG. 6D, there are shown various cross-sectional views shown in FIG. 4O(b). FIG. 6A shows a cross-sectional view along each of the common-drain regions as indicated by the B-B′ line shown in FIG. 4O(b), in which a fifth conductive layer 322bis formed over a second flat bed being alternately formed by an etched second raised field-oxide layer 304eand a shallow heavily-doped drain diffusion region 320bof a second conductivity type being formed within a common-drain diffusion region 320a; a second planarized thick-oxide layer 323bis formed over the fifth conductive layer 322b; and a plurality of word lines 325abeing formed over the second planarized thick-oxide layer 323bare patterned and etched by a plurality of hard masking layers as described in FIG. 5A.

[0063] FIG. 6B shows a cross-sectional view along the floating-gate region as indicated by the C-C′ line shown in FIG. 4O(b), in which a plurality of word lines 325atogether with a plurality of planarized control/select-gate conductive islands 324b, 307dbeing simultaneously patterned and etched by a plurality of hard masking layers as described are formed on the first intergate-dielectric layers 306band a portion of nearby second raised field-oxide layers 304d; the first intergate-dielectric layer 306bis formed on the floating-gate layer 302c; and the floating-gate layer 302cis formed on a first gate-dielectric layer 301c.

[0064] FIG. 6C shows a cross-sectional view along each of the select-gate regions as indicated by the D-D′ line shown in FIG. 4O(b), in which a plurality of word lines 325atogether with a plurality of planarized control/select-gate conductive islands 324b, 313dare patterned and etched by a plurality of hard masking layers as described, and each of the planarized control/select-gate conductive islands 324b, 313dis formed over the second gate-dielectric layer 311band a portion of nearby first raised field-oxide layers 304b. An implanted region 310bof a first conductivity type is formed under the second gate-dielectric layer 311bbetween the nearby first raised field-oxide layers 304b, which includes a shallow implant region as indicated by the dash line for threshold-voltage adjustment and a deep implant region as indicated by the cross symbols for forming a punch-through stop.

[0065] FIG. 6D shows a cross-sectional view along each of the common-source regions as indicated by the E-E′ line shown in FIG. 4O(b), in which a fourth conductive layer 317bis formed over a first flat bed being alternately formed by an etched first raised field-oxide layer 304cand a shallow heavily-doped source diffusion region 315bof a second conductivity type being formed within a common-source diffusion region 315a; a first planarized thick-oxide layer 318bis formed over the fourth conductive layer 317b; and a plurality of word lines 325abeing formed over the first planarized thick-oxide layer 318bare patterned and etched by a plurality of hard masking layers as described.

[0066] While the present invention has been particularly shown and described with a reference to the present examples and embodiments as considered as illustrative and not restrictive. Moreover, the present invention is not to be limited to the details given herein, it will be understood by those skilled in the art that various changes in form and details may be made without departure from the true spirit and scope of the present invention.