Process for fabricating tunnel-oxide nonvolatile memory devices

Abstract: A process including the steps of forming a gate oxide layer (50) on a semiconductor substrate (10); masking the gate oxide layer with a nitride mask (52); forming openings (57) in the gate oxide layer (50) using the nitride mask; and forming, at the openings, tunnel oxide regions (19) of a thickness smaller than the thickness of the gate oxide layer. The nitride mask (52) presents a thickness smaller than the width (l) of the openings (57) to improve etching of the gate oxide layer (50) and subsequent washing, protects the covered layers (51) when etching the gate oxide and growing the tunnel oxide regions (19), and is removed easily without damaging the exposed layers.

Description: The present invention relates to a process of fabricating tunnel-oxide nonvolatile memory devices, in particular EEPROM memories. As is known, EEPROM memories are programmed by storing electric charges in a floating gate region and tunneling current through a thin oxide region (Fowler-Nordheim tunneling); the thin oxide region (tunnel oxide) is surrounded by a thicker oxide region (gate oxide, used in the sensing transistor of the cell for reading the cell). The gate oxide region located between the floating gate region and the substrate provides for: a) increasing the coupling coefficient between the floating gate and overlying control gate (which also depends, among other things, on parasitic capacitances, such as that between the floating gate and the substrate, and hence on the thickness of the gate oxide); b) reducing any undesired parasitic phenomena, such as band-to-band tunneling (i.e. the presence of parasitic currents due to charges passing directly from the valence band to the conduction band of the silicon); and c) preventing electron entrapment in the source region of the cell, which would result in asymmetric aging curves (i.e. an asymmetric shift of the threshold curves of erased and written cells, which must be avoided to achieve a good degree of read reliability). EEPROM memories also require an implanted region (normally phosphorous for N-channel cells) beneath and to the side of the tunnel oxide region and partially superimposed on the drain region of the cell to ensure electric continuity between the substrate region beneath the tunnel oxide (tunnel area) and the drain region of the cell. In EEPROM memories of the type described, it is desirable to minimize the tunnel oxide area, which seriously affects the overall area of the cell and hence of the memory itself, and so enable the fabrication of memories with smaller, i.e. a greater number of, cells. Moreover, reducing the tunnel area also provides for enhanced coupling of the floating gate and control gate regions, which therefore explains the effort currently being made to reduce the size of the tunnel oxide region to the minimum permitted by the lithographic process by which it is formed. On the other hand, current technology presents a series of problems preventing this minimum area from being achieved, as explained below with reference to Figures 1-3 showing a known EEPROM cell, and Figures 4-7 showing the known process by which the cell is fabricated. Figures 2 and 3 show a portion of a known memory device 1 comprising a cell 2 and a selection transistor 3. Number 10 in Figures 2 and 3 indicates the P-type substrate of device 1, which is formed, using or not the twin-tub technique, inside a wafer of monocrystalline semiconductor material (silicon), and which houses an N-type source region 11 of the cell, an N-type implanted electric continuity region 12, an N-type drain/source region 13 defining the drain region of cell 2 and the source region of selection transistor 3, and an N-type drain region 14 for selection transistor 3. Regions 11-14 all face the surface 15 of substrate 10. Stacked on surface 15 are the following regions forming cell 2 and the relative sensing transistor: a gate oxide region 18 of cell 2, incorporating a thin tunnel oxide region 19; a floating gate region 20 of polycrystalline silicon; an interpoly dielectric region 21; and a control gate region 22 of polycrystalline silicon. Again on surface 15 and to the right in Figure 2 are shown a gate oxide region 25 and a gate region 26 of polycrystalline silicon, both pertaining to selection transistor 3. An intermediate dielectric layer 30 covers the structure and insulates the various layers. As shown in the Figure 3 section, floating gate region 20 is insulated and surrounded at the top and sides by interpoly dielectric layer 21 (preferably an ONO layer comprising superimposed silicon Oxide-silicon Nitride-silicon Oxide) and at the bottom by gate oxide 18 and tunnel oxide 19 of the cell. Portion 15' of surface 15 beneath tunnel oxide region 19 forms the tunnel area; the distance between floating gate region 20 and surface 15 is minimum (about 7-10 nm (70-100 Å)) at portion 15', and is greater (about 20 nm (200 Å)) at gate oxide region 18; and portion 31 of the substrate between continuity region 12 and source region 11 forms the channel of cell 2. As shown in Figure 3, electric continuity region 12 and channel region 31 (not shown in the Figure 3 section) are defined laterally, widthwise, by a thick field oxide layer 32. Device 1 is formed using the masks illustrated in Figure 1, in which 4 indicates the tunnel mask; 5 (dotted line) the mask for defining the floating gate region widthwise; 6 a self-align etching mask (explained below); 7 the active area mask (also corresponding to the source/drain implant of cell 2 and selection transistor 3); and 8 a mask for forming drain contact D. Device 1 is fabricated using a typical CMOS (twin- or single-tub) process wherein, commencing with P-type substrate 10 and using the nitride active area mask 7 covering the active area of cell 2 and transistor 3, a field oxide layer 32 ( Figure 3) is grown to define the active areas of device 1; after growing a sacrificial oxide layer and a photolithographic masking step, capacitors are implanted to form continuity region 12; after removing the sacrificial oxide layer, a pre-gate oxide layer 42 (eventually forming part of gate region 18 of cell 2 and gate oxide region 25 of selection transistor 3) is deposited; tunnel mask 5 is deposited, comprising a layer of photosensitive material 43 covering the whole of layer 42 except for a window 41 where tunnel region 19 is to be formed (Figure 4); and dedicated etching is performed to clean surface 15, remove the exposed part of layer 42, and so obtain the intermediate structure shown in Figure 5. At this point, tunnel mask 5 is removed; tunnel oxide region 19 is grown; and, at the same time, the thickness of pre-gate oxide layer 42 also increases to form a gate oxide layer 44 of a thickness equal to the desired final thickness of gate oxide region 18 (and region 25), thus resulting in the intermediate structure shown in Figure 6. A layer of polycrystalline silicon (poly1) 45 is then deposited and doped to give the intermediate structure shown in Figure 7. As of this point, the process may proceed in various ways, depending on whether memories with one or two polycrystalline silicon levels are being formed, on whether or not the two polycrystalline silicon layers are shorted at the circuit transistors, etc.. A process currently employed by the Applicant, for example, comprises the steps of: masking poly1 layer 45 to define the floating gate regions 20 of the cells widthwise (horizontal direction in Figures 1 and 3); depositing the composite ONO layer; etching ONO layer 21 at the selection transistors and at the circuit portion of device 1; depositing and doping a second polycrystalline silicon layer (poly2); depositing a tungsten silicide layer (not shown in Figures 2 and 3 for the sake of simplicity); etching the poly2 layer without removing the mask; masking and self-align etching the ONO layer and etching the poly1 layer using mask 6 to define floating gate and control gate regions 20 and 22 lengthwise (vertical direction in Figure 1); removing mask 6; source/drain implanting to form regions 11, 13, 14; and finish steps including forming intermediate dielectric layer 30, forming the contacts and interconnecting layer/s, and depositing the passivation layer. In the above fabrication process, the step of forming tunnel oxide region 19 is especially critical for various reasons: 1. The quality of the tunnel oxide depends to a critical extent on how thoroughly pre-gate oxide 42 is removed and surface 15 is cleaned. 2. The span of tunnel window 41 is about 0.5-0.7 µm or less, that is, much smaller than the thickness of resist layer 43 (1-1.5 µm). Consequently, when wet removing pre-gate oxide 42, thorough wetting of the whole of window 41 by the etching solution and uniform etching of layer 42 are difficult to ensure. 3. When plasma etching to remove the exposed portion of layer 42, surface 15 of the substrate is normally damaged, thus impairing the quality of the tunnel oxide. 4. When plasma etching, residue is easily formed on account of the high, closely spaced resist walls, thus resulting in contamination of surface 15 of the substrate and tunnel oxide layer 19. No satisfactory solutions to the above problems have as yet been devised, the standard practice substantially consisting in increasing the area of the tunnel oxide region, by using windows larger than the lithographic minimum or by devising ways of self-aligning the masks, or in using etching solutions also comprising a surface-active agent to improve the wettability of the surface etching step. Nevertheless, the problem still remains. Increasing the area of the tunnel oxide region results in larger cells and very often in more complex configurations; whereas alterations to the etching solution must be optimized in each individual case, and do not always allow reproducible results. Patent Abstracts of Japan, vol 13, no. 58 (E-714), 9 February 1989 & JP-A-63 246875 disclose a process for manufacturing EEPROM memories. This known process is disadvantageous since it is susceptible to stress on the surface of gate oxide layer, due to the different coefficient of expansion of gate oxide layer and nitride layer. Moreover, any defects (pinholes) in the nitride may result in etching of gate oxide layer when forming the openings of the tunnel oxide regions, and hence in impaired quality of the gate oxide. It is an object of the present invention to provide a fabrication process enabling the formation of a good quality tunnel oxide region unaffected by the problems typically associated with known solutions. According to the present invention, there is provided a process of fabricating tunnel-oxide nonvolatile memory devices, in particular EEPROM memories, as claimed in Claim 1. A preferred, non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a plan view of a number of masks used to fabricate a known EEPROM memory device; Figure 2 shows a cross section along line II-II in Figure 1 of the known memory device; Figure 3 shows a cross section along line III-III in Figure 1 of the known memory device; Figures 4-7 show cross sections, as in Figure 2, of the memory cell of the known device at various steps in the fabrication of the cell; Figures 8-12 show cross sections, as in Figures 4-7, of an embodiment of the fabrication process according to the present invention. In the embodiment shown in Figures 8-12, in which the regions common to the known solution in Figures 4-7 are indicated using the same numbering system, a gate oxide layer 50 is grown on a substrate 10 of semiconductor material, typically monocrystalline silicon doped to present P-type conductivity, and housing an N-type continuity region 12. Unlike the known process described above, layer 50 already presents the desired final thickness of gate oxide region 18 (Figures 2 and 3). A thin layer (20-50 nm) of polycrystalline or amorphous silicon 51 is then deposited, preferably doped in situ, e.g. by adding phosphine or arsine in known manner to the silane in the deposition furnace to improve its electric characteristics; and a thin layer (20-50 nm) of silicon nitride 52 is deposited to obtain the intermediate structure shown in Figure 8. At this point, a photolithographic masking step is performed to form a resist mask 54 presenting a window 55 at the portion in which the tunnel oxide region is to be formed (Figure 9); and nitride layer 52 and polycrystalline or amorphous silicon layer 51 are plasma etched to give the intermediate structure shown in Figure 9, wherein layers 51 and 52 define a tunnel window 56 of the same area as window 55. Mask 54 is then removed; a wash step is performed; using the mask formed by layers 51 and 52, the exposed portion of gate oxide layer 50 is wet etched, e.g. using a diluted or buffered hydrofluoric acid solution, to give the Figure 10 structure in which gate oxide layer 50 presents an opening 57 of width l ; and tunnel oxide region 19 is grown thermally (e.g. at T=800°C in an environment containing dry oxygen or in the presence of traces of steam) to give the Figure 11 structure. Since gate oxide layer 50 is masked by layers 51 and 52, tunnel oxide region 19 is only grown at window 56. Then, nitride layer 52 is wet removed, e.g. using a hot phosphoric acid solution at 150°C; the first polycrystalline silicon layer 45 is deposited to give the intermediate structure in Figure 12; and the usual steps for forming intermediate dielectric 21, depositing the second polycrystalline silicon layer, patterning and implanting, etc. are performed as described above. As stated, in the above process, opening 57 in gate oxide layer 50 is formed of width l (about 0.5-0.7 µm) corresponding to one side of tunnel oxide region 19 (square in shape when viewed from above), so that the thickness of the mask (equal to the total thickness of layers 51 and 52, i.e. 40-100 nm) is much smaller than the width of opening 57. Reducing the thickness of mask 51, 52 used when etching gate oxide layer 50 provides for improving the wettability of the etching window, for more uniform etching, and for more thorough washing and residue removal as compared with the known process. Moreover, after removing mask 54, nitride layer 52 permits much more thorough washing of the gate oxide surface than that permitted in the known process by the presence of exposed pre-gate oxide. For example, washing may be performed using oxide etching solutions (in particular, diluted or buffered hydrofluoric acid solutions). Finally, the process described enables the use of a wider range of solutions for etching layers 52, 51 and forming tunnel window 56, with no danger of detaching the resist layer forming mask 54. In the above process, the protective polycrystalline or amorphous silicon layer 51 is not removed, and subsequently contacts first polycrystalline silicon layer 45 with which it forms the floating gate region. The presence of layer 51 therefore in no way interferes with the formation of cell 2. The advantages of the processes according to the present invention will be clear from the foregoing description. In particular, using nitride as the masking material when etching oxide layer 50 is especially advantageous on account of nitride being depositable to a thickness equal to a fraction of the width of the tunnel oxide region of the cells (0.5-0.7 µm) while at the same time maintaining the required resistance to etching of the oxide. Moreover, it is removable without damaging the existing layers, i.e. tunnel oxide 19 and polycrystalline or amorphous silicon layer 51 in the described embodiment. Clearly, changes may be made to the process as described and illustrated herein without, however, departing from the scope of the present invention as defined in the appended claims. For example, though nitride is currently preferred as the masking layer material for removing portions of gate or pre-gate oxide where the tunnel oxide is to be formed, other materials with the thickness and selective oxide etching characteristics indicated may also be used.

Claims: A process of fabricating tunnel-oxide nonvolatile memory devices, in particular EEPROM memories, the process comprising the steps of: forming, on a semiconductor substrate (10), a layer of insulating material (50) of a first thickness; depositing a protective layer (51) of semiconductor material on top of said layer of insulating material (50); forming, on top of said protective layer (51) a masking layer (52) of a material with chemical-physical resistance characteristics to etching conditions of said layer of insulating material (50); forming, in said masking layer, said protective layer and said layer of insulating material, openings (57) having a width (1) greater than the thickness of said masking layer (52); and forming, within said openings, a tunnel oxide (19) of a second thickness smaller than said first thickness. A process as claimed in Claim 1, characterized in that said material of said masking layer (52) presents thermal resistance characteristics comparable with those of ceramic materials, and is impervious to oxidation. A process as claimed in Claim 1 or 2, characterized in that said masking layer (52) presents a thickness of less than a quarter of said width (1) of said opening (57). A process as claimed in any one of the foregoing Claims, characterized in that said masking layer (52) is made of nitride. A process as claimed in any one of the foregoing Claims, characterized in that said step of forming a masking layer comprises the steps of depositing said masking layer (52); forming a mask (54) of photosensitive material having first windows (55) at said openings (57) to be formed; forming second windows (56) in said masking layer, within said first windows; and removing said mask of photosensitive material. A process as claimed in Claim 5, characterized in that said first thickness of said layer of insulating material (50) is equal to a predetermined desired final thickness; and said step of forming a tunnel oxide comprises the step of growing said tunnel oxide (19) within said second windows (56) in said masking layer (52). A process as claimed in Claim 6, characterized in that said step of growing said tunnel oxide (19) is followed by the steps of removing said masking layer (52) and depositing a semiconductor gate layer (45). A process as claimed in Claim 7, characterized in that said step of forming second windows (56) also comprises the step of removing exposed portions of said protective layer (51). A process as claimed in Claim 8, characterized in that said semiconductor gate layer (45) is deposited over said protective layer. A process as claimed in Claim 8 or 9, characterized in that said protective layer (51) is of polycrystalline or amorphous semiconductor material. A process as claimed in Claim 10, characterized in that said protective layer (51) is a doped N-type layer.

Inventors: BALDI LIVIO, BALDI, LIVIO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: H01L21
IPC codes: H01L21