Method of fabricating non-volatile memories, and non-volatile memory produced thereby

Abstract: A method comprising the steps of depositing a first (12) and second (17) polysilicon layer, separated by an oxide layer (13); selectively etching the second polysilicon layer (17) to form first gate regions (18); forming first substrate regions (26, 29) in the substrate (2) and laterally in relation to the first gate regions; selectively etching the first polysilicon layer (12) to form second gate regions of a length greater than the first gate regions (18); and forming in the substrate, laterally in relation to the second gate regions (12) and partially overlapping the first substrate regions (26, 29), second substrate regions (37, 38) of a higher doping level than the first substrate regions (26, 29).

Description: The present invention relates to a method of fabricating non-volatile memories, and to the non-volatile memory produced thereby. At present, non-volatile memories integrate, in the same substrate, memory cells and transistors fabricated simultaneously and hereinafter referred to in general as "devices". For this purpose, various methods are known and which, subsequent to defining the active areas, consist in implanting isolating regions; growing the field oxide; depositing a first polycrystalline silicon (polysilicon) layer; effecting a first shaping (definition) of the first polysilicon layer; depositing an intermediate dielectric layer (dielectric interpoly); removing the dielectric interpoly from the circuit areas not accommodating memory cells; depositing a second polysilicon layer; defining the gate regions in the transistors and memory cells; implanting source and drain; and, finally protecting and interconnecting the devices. In recent years, for integrated circuit transistors in general, a so-called ITLDD (Inverse T Low Doped Drain) structure has been devised, which has the advantage of withstanding the hot electron injection stress typical of submicrometer structures. For this reason, the ITLDD structure is among the most favoured for transistors with gate lengths below 0.5 µm, but presents serious difficulties in terms of production and industrialization, as outlined below. The first proposal for such a structure appeared in 1986 (cf. articles by HUANG T.Y. and others entitled "A novel submicron LDD transistor with inverse-T gate structure" IEEE-IEDM 1986, p. 742, and "A new LDD transistor with inverse-T gate structure" IEEE Electron Dev. Letters, 8, 1987, p. 151). These deal with a standard transistor in which the etching for defining the gate region is stopped at a certain level to give the desired structure, but not without obvious problems in terms of etching controllability and repeatability. So much so that, in the second of the above articles, alternative solutions are outlined for simplifying the etching stage, and, in a follow-up proposal, recourse is made by the same writer to two distinct gates, one wider and the other narrower, contacted by polysilicon spacers. An alternative to the above process is described, for example, in an article by IZAWA R. and others entitled "Impact of the Gate-Drain Overlapped Device (GOLD) for deep submicrometer VLSI" IEEE Transactions Elect. Dev. 35, 1988, p. 2088 and in DE-A-3 813 665. According to this process, which was later to become widely used, a first thin polysilicon layer is deposited; a very thin layer of native oxide (0.5-1 nm) is grown by exposure to air; a second polysilicon layer is deposited on the oxide layer; the second polysilicon layer is etched highly selectively, so as to stop at the thin oxide layer; the low-doped regions are implanted through the native oxide layer, the first polysilicon layer and the gate oxide layer; an LPCVD (Low Pressure Chemical Vapour Deposition) oxide layer is deposited and etched to define the lateral spacers; the first thin polysilicon layer is etched; partial lateral re-oxidation of the thin polysilicon layer is effected; and the high-doped regions are implanted. In this way, overlapping of the transistor gate region and the low-doped regions does not depend on the size of the spacers. The above known process is especially critical as regards selective etching of the second polysilicon layer, which requires extremely selective technology (100 to 1 is the figure mentioned) for selecting between the polysilicon and oxide, if the etching process is to be repeatable uniformly on a silicon wafer. What is more, no direct contact exists between the gate region parts separated by the native oxide layer, which, albeit thin, could impair electrical continuity. Finally, precise control of the lateral oxidation length of a doped polysilicon layer is not as straightforward as it would at first appear. To overcome the above drawbacks, further processes have been proposed, such as that described in an article by PFIESTER J. and others entitled "A self-aligned LDD/channel implanted ITLDD process with selectively-deposited poly gates for CMOS VLSI" IEEE-IEDM, 1989, p. 253). This consists in forming the gate regions inside openings in an appropriate oxide layer over the first polysilicon layer; removing the oxide layer; implanting the low-doped regions; forming oxide spacers; and implanting the high-doped regions. The process described in an article by MOON J.E. and others entitled "A new LDD structure: Total Overlap with polysilicon Spacer (TOPS)" IEEE Electron Dev. Letters, 11, 1990, p, 221, consists in forming appropriately thick polysilicon spacers for electrically contacting the top part of the gate region, formed from a thick polysilicon layer, and the bottom part of the gate region, formed from a thin polysilicon layer and separated from the first by an oxide layer. In this case, the low-doped regions are implanted through the oxide and thin polysilicon layers prior to forming the spacers, and the high-doped regions are implanted after they are formed. In the process described in an article by WEN D.S. and others entitled "A self-aligned inverse T fully overlapped LDD device for sub-half micron CMOS" IEEE-IEDM, 1989, p. 765, the problem of contacting the two parts of the gate region is solved by depositing a thin polysilicon layer after defining the thicker part of the gate region and before defining the oxide for the spacers, which obviously complicates the process by introducing additional deposits and etching. In this case also, a dummy oxide or TiN layer is used for arresting etching of the polysilicon in the thicker part of the gate region, thus resulting in selectivity problems, especially when overetching. Other processes proposed for fully overlapping the gate region and low-doped regions (cf. the article by HORI T. and others entitled "A new submicron MOSFET with LATID (Large-tilt angle implanted drain) structure" Int. Symp. VLSI Technology Dig. 1988, p. 15) consist in implanting the above regions by sharply tilting the beam and semiconductor wafer. Such a technique presents difficulties in controlling the junction profile, and, what is more, results in direct injection of the charge into the gate oxide. All the above processes (and others not mentioned) have met with only a limited amount of success, due to the manufacturing difficulties involved, and at any rate are limited in scope to integrated circuit transistors in general. Undoubtedly, an inverse-T profile as proposed is disadvantageous as regards capacitive coupling of the gate and drain regions, and detrimental to the speed of the device. Nevertheless, analysis of the structure has shown advantages are to be afforded by using such a profile for the gate region. Some of the above articles in fact show how optimizing dosage of the low-doped regions and overlap length may minimize the above drawback and so limit the theoretical coupling increase to 10%, the consequences of which are further reduced by virtue of a big improvement in the source and drain series resistance values. It is an object of the present invention to provide an improved method of fabricating non-volatile memories, based on the principle of partially overlapping the gate region on low-doped regions with an LDD type structure; which method provides for eliminating the drawbacks typically associated with known methods, for enabling troublefree, low-cost implementation, and for ensuring the production of high-performance devices. According to the present invention, there is provided a fabrication method as claimed in Claim 1. According to the present invention, there is also provided a non-volatile memory produced using the method according to the present invention and as claimed in Claim 5. A preferred non-limiting embodiment of the present invention, relative to the fabrication of a non-volatile memory, will be described with reference to the accompanying drawings, in which: Figs. 1 to 6 show cross sections of a wafer of semiconductor material at various stages in the method according to the present invention; Fig.7 shows a schematic view of the masks employed for the memory circuit transistors; Fig.8 shows a cross section of a circuit transistor, perpendicular to the Fig.1-6 section. The accompanying drawings show intermediate structures formed using the method according to the present invention, and relative to an EPROM memory and a pair of CMOS transistors, each integrated in a respective tub according to the twin tub process. The method commences with a silicon wafer 1 including a substrate 2 having twin tubs 4 and 5 of opposite conductivity, for housing respective circuit MOS transistors. The active areas are defined on wafer 1 using standard masking processes (the relative mask is shown by continuous line 3 in Fig.7, which shows, one on top of the other, the various masks employed in the method described herein for one of the circuit transistors), and isolation regions 6, 7, 8 of appropriate conductivity are implanted between the active areas. The field oxide is then grown (to form regions 10), and an N type implant made in the memory cell area (right half of Fig.1). Sacrificial oxidation is then effected, in known manner, on the active areas, and, following removal of the oxide layer so formed, gate oxide 11 (Fig.1) is grown thermally. On top of gate oxide 11, a layer of polycrystalline silicon (first poly 12), appropriately doped to improve conductivity, is grown and then shaped by means of standard masking and etching operations, so as to produce, in the memory cell area, polysilicon strips extending parallel to strip 12 in Fig.1 and in a plane perpendicular to that of the drawing. On top of first poly 12, a layer of dielectric material (dielectric interpoly 13) is formed, and a suitably thick second polysilicon layer (second poly 14) is deposited for protecting dielectric layer 13, and so producing the intermediate structure shown in Fig.1. An array mask is then deposited, so that a suitably shaped resist layer 16 covers the whole of polysilicon layer 14 in the memory cell area, and the active areas in the circuitry area. The design of the photolithographic mask at this stage differs as compared with that of standard processes, and, for a circuit transistor, is as shown by dotted line 15 in Fig.7. The portions of polysilicon and dielectric layers 14 and 13 not covered by resist layer 16 are then removed by plasma etching, to give the intermediate structure shown in Fig.2. Following removal of resist layer 16, a threshold adjusting implant is made in the circuit transistor area outside the memory cell array, and a third polycrystalline silicon layer 17 (Fig.3) is deposited. Layer 17 shortcircuits first poly layer 12 where second poly 14 and dielectric layer 13 have been removed (over field oxide regions 10 surrounding each circuit transistor, as shown also in the Fig.8 section), and elsewhere shortcircuits second poly layer 14 (over the active areas of the circuitry and in the memory cell area). Hereinafter, the layer resulting from overlapping layers 14 and 17 will be referred to as layer 18, since it does in fact act as a single layer. The top third layer 17 is then appropriately doped to improve conductivity. A 2500 Å (10 Å = 1 nm) thick silicide layer 19 (WSi 2 ) is then deposited and, over this, an LPCVD silicon oxide layer 20 (absent in the standard process, together with the relative etching operation). A suitably shaped photoresist layer is then deposited for defining the memory cell control gates and the circuit transistor gate regions, and so producing the structure shown in Fig.3, which also shows resist layer 21 after shaping. The relative mask is also shown by continuous line 22 in Fig.7. At this point, oxide layer 20, silicide layer 19, polysilicon layer 17 and layer 14 (if any) are plasma etched down to dielectric layer 13, which thus acts as a gate etching stop layer when defining the gate. Using the same mask, dielectric layer 13 is then etched by means of a further, polysilicon-selective, etching operation; a further resist mask 24 (Fig.4) is deposited over mask 21, so as to leave only the memory cells exposed; and N type doping ions (e.g. a 7E13 at/cm 2 dose of phosphorous at 80 KeV) are implanted through first polysilicon layer 12 and gate oxide layer 11, as shown by arrows 25 in Fig.4, so as to form, in the cell area of substrate 2, low-doped regions 26 aligned with the shaped stack formed by layers 13, 18, 19, 20, and so produce the intermediate structure shown in Fig.4. Following removal of the double resist layer 21, 24, a further mask (not shown in Fig.s 1-6 but the shape of which, for a single transistor, is shown by dot-and-dash line 28 in Fig.7) is deposited for implanting N type doping ions through first polysilicon layer 12 in the N channel transistor active areas of the circuitry. At this stage, a 6E13 at/cm 2 dose of phosphorous at 110 KeV is typically employed, for producing regions 29 in tubs 5 (Fig.5). Following removal of the above mask, a further mask (of the same shape as mask 28) is deposited for implanting P type doping ions through first polysilicon layer 12 in the P channel transistor active areas of the circuitry. At this stage, a 3.5E13 at/cm 2 dose of boron at 45 KeV is typically implanted, for producing regions 30 in tubs 4 (Fig.5). Following removal of the above mask, the overlapping layers in the stack are re-oxidized laterally in a controlled atmosphere to form a thin oxide layer 31 (Fig.5); and an LPCVD silicon oxide layer 32 is deposited, to produce the intermediate structure shown in Fig.5. Oxide layer 32 is plasma etched immediately to form lateral spacers 33 (Fig.6) to the side of stacked silicide and polysilicon layers 19 and 18, which stage is not provided for in standard processes. By means of further plasma etching, first polysilicon layer 12 is then shaped so that, as shown in the Fig.6 section, it is self-aligned with spacers 33, thus defining the floating gate of the memory cells (to the right in Fig.6) and the bottom part of the circuit transistor gate region (shortcircuited at the top part defined by layer 14 up- and downstream from the Fig.6 plane, as shown in Fig.8). Following minor protective re-oxidation (thin layers 34) of the lateral surface of layer 12 exposed by the above plasma etching operation, appropriate masks (similar to 28) are deposited for making an N type and P type implant self-aligned with the gate regions formed by the remaining portions of layer 12, and so forming regions 36, 37, 38 (the first a P type and the other two N types) in tubs 4, tubs 5 and memory cell substrate 2 respectively. The resulting intermediate structure is as shown in Fig.6. Transistors and cells similar in structure to known LDD (Light Doped Drain) devices are thus obtained, but in which, by virtue of the method employed, the bottom part of the gate region in the transistors and the floating gate in the cells partially overlap the low-doped regions. The method then continues with the usual steps including: depositing a dielectric layer in which the contacts are defined; depositing and shaping a metal layer; and, finally, depositing a final passivation layer for externally insulating the memory. The advantages of the method according to the present invention will be clear from the foregoing description. Firstly, by virtue of the ITLDD structure, it provides for producing transistors of improved performance, particularly as regards hot-carrier reliability, which is especially advantageous in the case of line decoding transistors subjected to high voltages. Secondly, a preferred embodiment provides the simultaneous production of memory cells and circuit transistors by exploiting the double shortcircuited poly concept, thus enabling, as far as circuit transistors are concerned, low-cost production of devices otherwise involving unusual or unfeasible manufacturing processes. To those skilled in the art it will be clear that changes may be made to the method and device as described and illustrated herein without, however, departing from the scope of the present invention.

Claims: A method of fabricating non-volatile memories including integrated devices in a substrate (2) of semiconductor material, said method comprising the steps of: depositing a gate oxide layer (11) on said substrate; depositing a first polycrystalline silicon layer (12) on said gate oxide layer; depositing a layer of insulating material (13) on said first polycrystalline silicon layer; depositing at least a second polycrystalline silicon layer (17) on said layer of insulating material; and selectively etching said second polycrystalline silicon layer (17) for forming upper gate regions (18) of a first length in a first direction, said upper gate regions defining control gate regions; selectively etching said layer of insulating material (13); forming, in said substrate and self-aligned with said upper regions (18), first substrate regions (26, 29) having a first doping level; then forming lateral spacers (33) to the side of said control regions (18); then selectively etching said first polycrystalline silicon layer (12) for forming lower gate regions below said upper gate regions, said lower gate regions having a second length, greater than said first length, in said first direction, said lower gate regions defining floating gate regions; and forming in said substrate, self-aligned with said lower gate regions (12), and partially overlapping said first substrate regions, second substrate regions (37, 38) having a second doping level, greater than said first doping level. A method as claimed in Claim 1, characterized in that said step of forming said first substrate regions (26, 29) is performed by doping ion implantation (25) through said first polycrystalline silicon layer (12). A method as claimed in Claim 1 or 2, characterized in that said step of forming said spacers (33) comprises the steps of depositing and selectively etching a second layer of insulating material (32). A method as claimed in one of the foregoing Claims for producing at least a memory cell and a circuit transistor, characterized in that said step of depositing said second polycrystalline silicon layer (17) is preceded by the step of shaping said layer of insulating material (13) for defining insulating portions of a predetermined width in a second direction perpendicular to said first direction; and in that, said second polycrystalline silicon layer contacts said first polycrystalline silicon layer (12) electrically at least at a portion of said second polycrystalline silicon layer extending from said insulating portions and in said second direction. A non-volatile memory comprising integrated devices in a substrate (2) of semiconductor material comprising a gate oxide region (11) arranged on said substrate; a polycrystalline silicon lower gate region (12) arranged on said gate oxide region and having a first length in a first direction, said lower gate region defining a floating gate; an insulating region (13) arranged on said lower gate region; a polycrystalline silicon upper gate region (18) on said insulating region , said upper gate region defining a control gate; lateral spacers (33) of insulating material extending at the side of said upper gate region (18) and said insulating region (13); and first substrate regions (26, 29) embedded in said substrate, laterally aligned with said upper gate regions (18), and having a first doping level; said upper gate region (18) presenting a second length, less than said first length, in said first direction; and in that said devices further comprising second substrate regions (37, 38) embedded in said substrate, laterally aligned with said first substrate regions, and beneath said floating gate regions (12); said second substrate regions having a second doping level greater than said first doping level. A memory as claimed in Claim 5, and including at least a memory cell and a circuit transistor; characterized in that said insulating region (13) is part of said circuit transistor and comprises an insulating portion of a predetermined width in a second direction perpendicular to said first direction; said lower (12) and upper (18) gate regions presenting a width, in said second direction, greater than said predetermined width, and are in direct electrical contact with each other at least at a portion of said upper gate region extending from said insulating portion and in said second direction.

Inventors: CLEMENTI CESARE, CRISENZA GIUSEPPE, CLEMENTI, CESARE, CRISENZA, GIUSEPPE
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1992
Main classification: H01L21
IPC codes: H01L27, H01L21, H01L29