Process for the manufacture of floating-gate non-volatile memories

Abstract: The process described requires the formation of floating-gate non-volatile memory cells entirely similar in structure to those produced by known processes, and comprises an annealing treatment at relatively low temperature (430°C) to repair damage due to plasma treatments. To obtain threshold voltage values for the cells close to the theoretical values, especially for cells with particularly extended interconnections, the cells are subjected to ultraviolet radiation before the annealing treatment, in order to neutralize any electrical charges present in the floating-gate electrodes of the cells.

Description: The present invention relates to the manufacture of semiconductor electronic devices and, more particularly, to a process for the manufacture of a floating-gate non-volatile memory on a monocrystalline silicon substrate by the operations specified in the precharacterizing clause of claim 1. It is known that in the usual manufacturing processes certain treatments of the silicon wafers in which the electronic devices are formed generate electrical potentials on the wafer which may damage the active dielectrics of the electronic components being manufactured, in particular the thin layers of silicon dioxide which form the gate dielectrics of MOS field-effect transistors and the gate dielectrics of cells of programmable non-volatile memories, for example EPROM, EEPROM and "flash" memories. This may result in the production of electronic components whose electrical characteristics are degraded with respect to those theoretically obtainable. The treatments most responsible for this damage are those which require the use of plasmas for dry etching or for certain operations of chemical deposition from the vapour phase (plasma enhanced CVD). To repair the damage caused in this way, there is a known method of using high-temperature annealing treatments. However, it is not always possible to use sufficiently high temperatures to ensure the restoration of the properties of the dielectrics. Indeed, in practically all processes it is necessary to form on the substrate at least one layer of material which sets a maximum limit to the temperatures which can be used in the subsequent operations. A case in point is that of the layers of aluminium required to form interconnecting paths on one or more levels; these impose a temperature limit of approximately 450°C. It is also known that certain annealing treatments are sufficiently effective even at relatively low temperatures. For example, certain defects of the dielectrics of MOS devices at the interface with the substrate can be largely eliminated by heating to 400-450°C in a hydrogen-containing atmosphere. The defects in question are those caused by silicon atoms which have a free bond at the interface with the dielectric and therefore tend to trap electrons. This causes a change in the theoretical electrical characteristics of the devices, for example an increase in the threshold voltage of MOS transistors with respect to the design value. With the aforesaid treatment, within the stated temperature range, the hydrogen reacts with the silicon atoms with a free bond to saturate the states at the interface between dielectric and silicon and impede the trapping of electrons. It has been found that this annealing treatment, although effective in MOS transistors, enabling threshold voltage values very close to the theoretical values to be obtained, does not have the same effect on floating-gate non-volatile memory cells, although these are structurally and functionally similar to MOS transistors, particularly in cells whose control gate electrodes are connected to particularly long connecting paths. Indeed, if the threshold voltage of a memory cell is measured after the annealing treatment described above and after an appropriate erasure treatment by ultraviolet (UV) radiation, which has the purpose of rebalancing any excess charges in the floating-gate electrode caused by the manufacturing process, values higher than those expected according to the design parameters are found. These results have been documented, for example, in the article by H. Fang et al., in International Electronic Device Meeting, 1994, page 467. US-A-5 279 981 discloses a manufacturing process substantially according to the preamble of claim 1. Patent Abstracts of Japan, Vol. 17, No 19, describes a method of removing charge stored on a floating gate by irradiation with UV rays during an etching process. Similar treatment by UV rays is known from Patent Abstracts of Japan, Vol. 12, No. 457, and Vol. 9, No. 43. The object of the present invention is to improve the manufacturing process of a floating-gate non-volatile memory with memory cells having threshold voltage values equal, or at least very close, to the theoretical values. This object is achieved by means of the process specified and characterized in a general way in Claim 1. The invention will be more easily understood from the following description of one of its embodiments, provided by way of non-limiting example in relation to the attached drawing, in which Figures 1 and 2 show, in a plan and sectional view respectively, a memory cell which may be made by the process according to the invention. A known process for the manufacture of floating-gate non-volatile memory cells in an integrated circuit requires the formation of pairs of planar regions doped with n-type impurities, designed to form the source and drain regions of the memory cells, on a monocrystalline silicon substrate 12 doped with p-type impurities. Figures 1 and 2 show a single "flash" memory cell, but a cell of any floating-gate non-volatile memory could be shown in its place. A thin layer of silicon dioxide 17, acting as the gate dielectric, a floating-gate electrode 16 made of polycrystalline silicon, and a control gate electrode 18 separated from the floating-gate electrode by a relatively thick layer of dielectric 19 are formed, by known photolithographic techniques, between the source and drain regions, numbered 10 and 11 respectively in the drawing, and precisely above the channel 13 which they delimit. The control gate electrode 18 is formed from a layer of aluminium, suitably deposited and treated by known photolithographic techniques, from which are also formed any necessary interconnecting paths between the components of the integrated circuit in which the memory is formed. The figures show only contact strips, indicated by the numbers 20, 21 and 22, for connection, respectively, to the control gate electrode 18 and to the source and drain regions 10 and 11. The known process may also require the formation of further metallic paths, which are not shown, by formation from other metallic layers deposited on different levels insulated from each other. The operations required to obtain the structure illustrated and described in a summary way above comprise treatments, for example plasma etching for the formation of the interconnecting metallic paths, which cause alterations in the dielectrics of the devices. In order to repair, among other things, the damage caused by these treatments, the known process requires annealing in a hydrogen-containing atmosphere at approximately 430°C after the formation of the final level of deposited metal. The process continues with the formation of a layer 23 of dielectric material for the final insulation and passivation of the integrated circuit, the opening of windows in this layer and the formation of superficial contacts and metallic areas (pads) for the connection of the electronic devices of the integrated circuit to the outside. Finally, the structure is subjected to ultraviolet radiation for the "erasure" of the memory cells. According to the invention, the process described above is modified by the introduction of an ultraviolet radiation treatment after the last treatment which may cause alterations in the dielectrics and before the annealing in a hydrogen-containing atmosphere. The intensity and duration of the radiation depend on the thicknesses and geometries of the structure and are selected in such a way as to neutralize as far as possible any negative charges present in the floating-gate electrodes, and are in practice identical to those used for the erasure of the memory in the finished device. Measurements made at the end of the process according to the invention demonstrated that the threshold voltage values of the memory cells were substantially equal to the theoretical design values. A hypothetical explanation of the physical phenomena responsible for the improved performance, at least in respect of the threshold voltage, is as follows. In the known process, during the annealing treatment the diffusion of H+ hydrogen ions towards the oxide/silicon interface is vigorously opposed by the strong electrical field produced by the electrons trapped in the floating gate as a result of the plasma treatments, and therefore these ions do not contribute to the passivation process at the interface between the silicon substrate and gate oxide, which is therefore incomplete. With the preliminary erasure treatment according to the invention, however, the electrons trapped in the floating-gate electrodes are neutralized, so that the subsequent annealing treatment is able to have its full effect, since there is no longer an electrical field opposing the diffusion of hydrogen ions towards the oxide/silicon gate interface.

Claims: Process for the manufacture of a floating-gate non-volatile memory on a monocrystalline silicon substrate (12), comprising the operations of: forming in the substrate (12) pairs of planar regions (10, 11) doped with impurities, separated by a channel (13) and designed to form source and drain regions of cells of the memory, forming on the substrate a gate dielectric (17) and a floating-gate electrode (16) made of polycrystalline silicon for each cell, and forming at least one layer (21, 22, 18) of a material which sets a maximum limit (450°C) on the temperatures which can be used in the subsequent operations of the process, subjecting the substrate (12), after the formation of the said layer (21, 22, 18), to at least one treatment which causes alterations in the gate dielectrics and to a subsequent annealing treatment at a temperature (430°C) below the said maximum limit to repair the alterations in the gate dielectrics caused by said at least one treatment, characterized in that , after the treatment which causes alterations and before the annealing treatment, the substrate is subjected to a controlled dose of ultraviolet radiation whose intensity and duration are such that any negative charges present in the floating-gate electrodes (16) of the cells are neutralized, and in that the annealing treatment is carried out in a hydrogen-containing atmosphere. Process according to claim 1, in which said intensity and duration of the ultraviolet radiation depend on the thicknesses and geometries of the structure. Process according to claim 2, in which said intensity and duration of the ultraviolet radiation are substantially identical to those used for the erasure of the memory in the finished device. Process according to Claim 1, 2 or 3 in which the treatment which causes alterations is a plasma treatment. Process according to Claim 1, 2, 3 or 4, in which the formation of at least one layer (21, 22, 18) which sets a maximum limit on the temperatures comprises the deposition of a layer of aluminium. Process according to any of the preceding claims, in which the annealing treatment is carried out at a temperature of approximately 430°C.

Inventors: ALBA SIMONE, COLOGNESE ANDREA, GHIO EMILIO, ALBA, SIMONE, COLOGNESE, ANDREA, GHIO, EMILIO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: H01L21
IPC codes: H01L21