Floating gate memory fabrication methods comprising a field dielectric etch with a horizontal etch component

[0002] The present invention relates to floating gate nonvolatile memories.

[0003] A floating gate nonvolatile memory cell stores information by storing an electrical charge on its floating gate. The floating gate is capacitively coupled to the control gate. In order to write the cell, a potential difference is created between the control gate and some other region, for example, the source, drain or channel region of the cell. The voltage on the control gate is capacitively coupled to the floating gate, so a potential difference appears between the floating gate and the source, drain or channel region. This potential difference is used to change the charge on the floating gate.

[0004] In order to reduce the potential difference that has to be provided between the control gate and the source, drain or channel region, it is desirable to increase the capacitance between the control and floating gates relative to the capacitance between the floating gate and the source, drain or channel region. More particularly, it is desirable to increase the “gate coupling ratio” GCR defined as CCG/(CCG+CSDC) where CCG is the capacitance between the control and floating gates and CSDC is the capacitance between the floating gate and the source, drain or channel region. One method for increasing this ratio is to form spacers on the floating gate. See U.S. Pat. No. 6,200,856 issued Mar. 13, 2001 to Chen, entitled “Method of Fabricating Self-Aligned Stacked Gate Flash Memory Cell”, incorporated herein by reference. In that patent, the memory is fabricated as follows. Silicon substrate 104 (FIG. 1) is oxidized to form a pad oxide layer 110. Silicon nitride 120 is formed on oxide 110 and patterned to define isolation trenches 130. Oxide 110 and substrate 104 are etched, and the trenches are formed. Dielectric 210 (FIG. 2), for example, borophosphosilicate glass, is deposited over the structure to fill the trenches, and is planarized by chemical mechanical polishing (CMP). The top surface of dielectric 210 becomes even with the top surface of nitride 120. Then nitride 120 is removed (FIG. 3). Oxide 110 is also removed, and gate oxide 310 is thermally grown on substrate 104 between the isolation trenches. Doped polysilicon layer 410.1 (FIG. 4) is deposited over the structure to fill the recessed areas between the isolation regions 210. Layer 410.1 is polished by chemical mechanical polishing so that the top surface of layer 410.1 becomes even with the top surface of dielectric 210.

[0005] Dielectric 210 is etched to partially expose the “edges” of polysilicon layer 410.1 (FIG. 5). Then doped polysilicon 410.2 (FIG. 6) is deposited and etched anisotropically to form spacers on the edges of polysilicon 410.1. Layers 410.1, 410.2 provide the floating gates.

[0006] As shown in FIG. 7, dielectric 710 (oxide/nitride/oxide) is formed on polysilicon 410.1, 410.2. Doped polysilicon layer 720 is deposited on dielectric 710 and patterned to provide the control gates.

[0007] Spacers 410.2 increase the capacitance between the floating and control gates by more than the capacitance between the floating gates and substrate 104, so the gate coupling ratio is increased.

[0008] This section is a brief summary of some features of the invention. The invention is defined by the appended claims which are incorporated into this section by reference.

[0009] In some embodiments of the present invention, before the floating gate polysilicon is deposited, the trench dielectric 210 is subjected to an etch which includes a horizontal etch component. For example, a wet etch can be used. Consequently, the sidewalls of dielectric 210 become recessed away from the active areas (see FIG. 13 for example). Therefore, the floating gate polysilicon 410 is wider at the top (FIG. 14). The gate coupling ratio is therefore increased.

[0010] The invention is not limited to polysilicon, silicon oxide, or other particular materials, or to particular dimensions, memory structures, or fabrication processes. Other features are described below.

[0011] FIGS. 1-7 show cross sections of prior art nonvolatile memory structures in the process of fabrication.

[0012] FIGS. 8-16, 17A, 17B show cross sections of nonvolatile memory structures in the process of fabrication according to the present invention.

[0013] This section describes some embodiments to illustrate the invention. The invention is not limited to these embodiments. The materials, conductivity types, dimensions, and other details are given for illustration and are not limiting.

[0014] In some embodiments, the memory array fabrication starts with substrate isolation. The pertinent fabrication processes can be identical or similar to the processes of FIGS. 1-2. FIGS. 8-10 illustrate one variation commonly practiced in DRAM technology. A doped region of type P- is formed in monocrystalline semiconductor substrate 104 as described, for example, in U.S. Pat. No. 6,355,524 issued Mar. 12, 2002 to H. T. Tuan et al. and incorporated herein by reference. This region is isolated by P-N junctions (not shown).

[0015] Field dielectric regions are fabricated by shallow trench isolation (“STI”) technology using a process like that of FIGS. 1-2. More particularly, silicon dioxide 110 (pad oxide) is formed on substrate 104 by thermal oxidation or some other technique (FIG. 8). Silicon nitride 120 is deposited on oxide 110. Nitride 120 is patterned photolithographically, using a photoresist mask (not shown), to define substrate isolation trenches 130. Oxide 110 and substrate 104 are etched through the openings in nitride 120. Trenches 130 (“STI trenches”) are formed in the substrate as a result. An exemplary depth of trenches 130 is 0.25 μm measured from the top surface of the substrate 104. Other depths are possible. Trenches 130 will be filled with dielectric to provide isolation between active areas 132 of substrate 104.

[0016] In FIG. 8, the trenches have sloping sidewalls, and the trenches are wider at the top than at the bottom. In some embodiments, the trenches have vertical sidewalls, or the trenches are wider at the bottom. The invention is not limited by any shape of the trenches.

[0017] Nitride 120 is subjected to a wet etch to recess the vertical edges of the nitride layer away from trenches 130. See FIG. 9. This step reduces the aspect ratio of the holes that will be filled with dielectric 210 (these holes are formed by the openings in nitride 120 and oxide 10 and by the trenches 130). The lower aspect ratio facilitates filling these holes.

[0018] A thin layer 210.1 of silicon dioxide is thermally grown on the exposed silicon surfaces to round the edges of trenches 130. Silicon dioxide 210.2 (FIG. 10) is deposited by a high density plasma process. Oxide 210.2 fills the trenches and initially covers the nitride 120. Oxide 210.2 is polished by a CMP process that stops on nitride 120. A planar top surface is provided.

[0019] In the subsequent figures, the layers 210.1, 210.2 are shown as a single layer 210. This dielectric 210 will be referred to as STI dielectric or, more generally, field dielectric.

[0020] Nitride 120 is removed selectively to oxide 210 (FIG. 11). This can be done by a wet etch (e.g. with phosphoric acid).

[0021] Then oxide 210 is etched (FIG. 12). This etch includes a horizontal component that causes the sidewalls of dielectric 210 to be laterally recessed away from active areas 132. This etch can also remove the oxide 110. The etch can be an isotropic wet etch selective to silicon nitride. A buffered oxide etch or a dilute HF (DHF) etch is used in some embodiments.

[0022] In one embodiment, the top surface of dielectric 210 is laterally offset from the top surface of active areas 132 by an amount X=300 Å at the end of this etch. Some of dielectric 210 may be etched out of the trenches 130 near the active areas 132, and the sidewalls of trenches 130 may become exposed at the top, but this is not necessary. In some embodiments, the trench sidewalls are exposed to a depth Y=300 Å. These dimensions are exemplary and not limiting.

[0023] As shown in FIG. 13, silicon dioxide 310 (tunnel oxide) is thermally grown on the exposed areas of substrate 104. An exemplary thickness of oxide 310 is 95 Å.

[0024] As shown in FIG. 14, conductive polysilicon layer 410 (floating gate polysilicon) is formed over the structure. Polysilicon 410 fills the areas between oxide regions 210 and initially covers the oxide 210. Polysilicon 410 is polished by a CMP process that stops on oxide 210. The structure has a planar top surface at this step, with the top surface of polysilicon 410 being even with the top surface of oxide 210. Polysilicon 410 can be doped in-situ or after the deposition. Polysilicon 410 projects over the isolation trenches 130.

[0025] An optional etch of oxide 210 lowers the top surface of oxide 210 to a level below the top surface of polysilicon 410. See FIG. 15. A timed wet etch can be used for this purpose.

[0026] Then dielectric 710 (FIG. 16) is formed over the structure. In one embodiment, dielectric 710 is ONO (oxide/nitride/oxide), i.e. a combination of a silicon dioxide layer, a silicon nitride layer, and another silicon dioxide layer. Control gate polysilicon 720 is deposited on dielectric 710 and patterned. Polysilicon 720 is made conductive by doping. Layers 710, 410 can be patterned after the patterning of layer 720 as appropriate. Due to the etch of oxide 210 described above in connection with FIG. 15, the polysilicon 720 extends down along the sidewalls of polysilicon layer 410. The capacitive coupling between the layers 410, 720 is consequently increased. Note S. Aritome et al., “A 0.67 um²SELF-ALIGNED SHALLOW TRENCH ISOLATION CELL (SA-STI CELL) FOR 3V-only 256 Mbit NAND EEPROMs”, IEDM 1994, pages 61-64, incorporated herein by reference. See also the aforementioned U.S. Pat. No. 6,355,524.

[0027] A wide range of floating gate memories can be fabricated using the teachings of the present invention. Stacked gate memories, split gate memories, and other types of memories, including flash and non-flash EEPROMs, can be fabricated. An example split gate flash memory array is illustrated in FIGS. 17A, 17B. This memory is a modification of the memory disclosed in the aforementioned U.S. Pat. No. 6,355,524. FIG. 17A shows a cross section of the array in the same cross sectional plane as FIG. 16. FIG. 17B shows a cross section in the plane that is marked B—B in FIG. 17A. This plane passes along a column of the array between the trenches 130.

[0028] The memory fabrication proceeds as in FIGS. 8-16, and then as in U.S. Pat. No. 6,355,524. Briefly, a silicon nitride layer 1710 is deposited on polysilicon 720. Layers 1710, 720, 710, 410 are etched to form a number of stacks 1720 (FIG. 17B). Each stack 1720 extends along one row of the array. Polysilicon 720 provides a control gate line for each row. Polysilicon 410 provides the floating gales. The individual floating gates in each row are separated from each other due to the CMP of polysilicon 410 described above in connection with FIG. 14.

[0029] Dielectric spacers 1730 (FIG. 17B) are formed on the sidewalls of each stack 1720 from a combination of silicon dioxide and silicon nitride layers. Gate dielectric 1724 is formed on substrate 104. A polysilicon layer 1740 is deposited and etched anisotropically to form conductive spacers over the sidewalls of each stack. Layer 1740 is removed from one sidewall of each stack. The remaining spacers 1740 provide wordlines for the corresponding memory rows.

[0030] N+ doped source/drain regions 1750, 1760 are formed in substrate 104. Regions 1750 (“bitline regions”) are adjacent to the wordlines 1740. Regions 1750 are connected to the bitlines (not shown). Regions 1760 (“source line regions”) of each row are merged together into a continuous source line. Each row shares its source line with an adjacent row.

[0031] The invention is not limited to the embodiments described above. For example, pad oxide 110 (FIG. 8) can be omitted. The X dimension (FIG. 12) can be zero. X can also be negative, i.e. the oxide 210 can encroach on the active areas 132. The Y dimension can also be zero or negative. The invention is not limited to any particular materials or memory structures. The invention is not limited to STI technology. Field dielectric regions 210 can be formed by another technology, known or to be invented. The invention is defined by the appended claims.