Process for manufacturing an integrated transistor with thick oxide

Abstract: A process for manufacturing an integrated circuit provides for the formation of a matrix of floating-gate non-volatile memory cells having dual polysilicon levels, with the two polysilicon levels being isolated by a gate dielectric layer (4) and an interpoly dielectric layer (9) therebetween, and for the concurrent formation of one type of thick-oxide transistor (7) in peripheral zones to the matrix. The process of the invention provides for removal, during the step of defining the first-level polysilicon (5) in order to form the floating-gate structures of the memory cells, of the polysilicon (5) from the active area zone of the thick-oxide transistor (7), so that the gate oxide of the transistor (7) results from the superposition of said first (4) and second (9) dielectric layers.

Description: Field of the Invention This invention relates to a process for manufacturing integrated circuits which include non-volatile memory cells and peripheral transistors. The most up-to-date applications of integrated circuits concern systems which are supplied voltages in the 3-volt range. Such systems often utilize semiconductor storage facilities of the non-volatile type. In order to retain the characteristics of these devices at low supply voltages, it has been necessary to employ thin oxides (16 nm 160Å)-thick or less). However, it is well recognized that memories of the EPROM, EEPROM, FLASH types require high programming voltages (12V) which cannot be accommodated by such thin oxides. Accordingly, the requirement involves the concurrent provision of thick and thin oxides in one device, which burdens the process technology with at least one additional masking operation. As for the peripheral transistors, these are incorporated to circuits outside the array of memory cells, such as array control logics or circuits, and are frequently expected to withstand high voltages, on the order of 12 to 15 volts. In describing this invention, reference will be made specifically to transistors of the MOS type. Such transistors include an active dielectric, or gate dielectric, placed between the substrate and a gate made of an electrically conductive material, usually polysilicon. In general, the active dielectric consists of a layer of silicon oxide obtained by oxidation of the substrate at a high temperature. Background Art Processes for manufacturing integrated structures which include non-volatile memory cells, i.e. cell matrices set up into rows and columns, using dual levels of polysilicon with a gate dielectric layer and an isolating interpoly dielectric layer therebetween, have been known and extensively used in the industry. Such manufacturing processes may provide for a first masking and implanting step effective to define well regions having conductivity of a different type from that of the semiconductor, substrate, in order to provide N-channel and P-channel devices on one chip. Conventional processes essentially utilize, therefore, one mask to define the active areas of the individual integrated devices (transistors, floating-gate memory cells, etc.). The process normally provides for an additional masking step, in order to form field isolation implants, that is, to have the semiconductor implanted in those zones where a thick oxide layer will be later grown to define and isolate the individual active areas. The process may provide for an ATP (Anti-Punch Through) implantation of all those active areas that have the same type of conductivity. A relatively heavy threshold-raising implantation (EPM) is performed in the matrix area where the floating-gate memory cells are to be formed. A thin layer of gate oxide is then grown over the active areas. Subsequent to the depositing and doping of a first-level polysilicon layer (poly 1), a masking and etching step is usually carried out for a first definition of the first-level polysilicon that is to provide the floating gates for the memory cells. A thin dielectric layer is next grown and/or deposited over the entire structure to isolate the two polysilicon levels (interpoly). This thin dielectric isolation layer may be a multiple layer, typically comprised of a first oxidation layer, a thin second layer of deposited silicon nitride, and a third nitride oxidation layer (the so-called O.N.O. multi-layer) At this stage, a masking step is typically carried out in such processes using a mask commonly referred to as the "matrix mask", thereby to remove the interpoly dielectric isolation layer from all over the surface except the area being occupied by the memory cells (matrix). Thereafter, a second-level doped polysilicon layer is deposited which, in the formation area of the floating-gate memory cells, will thus be isolated from the interpoly dielectric layer by the first-level polysilicon portions previously defined. On the other hand, the second layer of doped polysilicon will directly overlie the existing first-level layer of polysilicon, in areas outside the area being occupied by the memory cell matrix, that is the areas where the transistors and other external circuitry devices are formed. Conventional processes may also require a masking step specifically to perform an implantation, known as the LVS (Light Voltage Shift) implant, for slightly adjusting the threshold of standard enhancement transistors in the circuitry, with the channel areas of the so-called "natural" (or low threshold) transistors screened off the LVS implant. By an additional masking (poly 2) and etching step, the second-level polysilicon is defined which, in the matrix area, is terminated at the interpoly dielectric layer and, in the circuitry area, also results in the underlying first-level polysilicon being defined, thereby defining the channel length of the transistors in the circuitry. A manufacturing process of the above-described kind is disclosed in prior Italian Patent No. IT1213249 and, inter alia, the corresponding patent publication GB-2,167,602, both in the name of this Applicant. Another manufacturing process of the above-described kind is disclosed in the European Patent Application EP-A-0542575 of FUJITSU. The underlying technical problem of this invention is to provide a process for forming non-volatile memory cells and thick-oxide peripheral transistors by a minimum number of steps, and accordingly, for lower production costs. This technical problem is solved by a process for manufacturing a thick-dielectric transistor, as indicated above and defined in Claims 1 to 4. The features of this invention will be more clearly apparent from the following detailed description of embodiments thereof, all shown in the accompanying drawings by way of non-limitative examples. Brief Description of the Drawings Figures 1 to 9 illustrate the most significant steps of a process according to the invention; Figure 10 is a plan view of the profiles of the masks used to define the thick-oxide transistor structure in a process according to the invention; Figure 11 is a sectional view of the thick-oxide transistor structure taken along line A-A' in Figure 10; Figure 12 is a sectional view of the thick-oxide transistor structure taken along line B-B' in Figure 10, orthogonally with the section view of Figure 11. Detailed Description A process for forming memory cells and thick-oxide transistors in accordance with the invention will now be described with reference, in particular, to a preferred embodiment thereof, illustrated by Figures 1-9. These figures are sectional views, not drawn to scale, showing schematically the sequential process steps of forming a non-volatile memory cell, a thick-oxide MOS transistor, and a direct contact. The partial constructions of the cell, the transistor and the direct contact are denoted by the reference numerals 20, 21 and 22, respectively. The basic construction of the memory cell 20 is that of all non-volatile memory cells, of either the EPROM, EEPROM or FLASH type, and comprised of a floating-gate MOS transistor. The thick-oxide transistor 21 is also a MOS transistor formed in accordance with the invention. The process specifically referred to in this description is a process of the MOS type, preferably as carried out with CMOS technology. The process of this invention may include conventional steps of forming, in a semiconductor substrate having conductivity of a first type, well regions with a second type of conductivity. The definition of active areas by a LOCOS (trademark of Philips, The Netherlands) method or PLANOX (Applicant's trademark) method, which may include an isolation implant for the areas where a thick layer of field oxide is to be grown, thereby to yield a structure like that shown in Figure 1, wherein reference 1 designates the monocrystalline semiconductor having a given type of conductivity on which the active areas are defined by isolating diffusions 2 and the field oxide 3. By an extensively used technique, a light implantation may be performed during this step at a high implanting energy, to provide an APT (Anti-Punch Through) implant, not shown to avoid overcrowded drawing figures. Also during this step, a relatively heavy implantation (EPM) is commonly performed, limited to the matrix area, by implanting a boron dosage of about 6.5 x 10 12 at 120keV, for example. Unlike the APT implant, the latter implant is shown in the drawing figures as EPM. Again in conducting a standard process sequence, a thin oxide layer 4 of a first type, having a thickness of 10 to 12 nm (100 to 120Å), is then formed over the active areas, as shown in Figure 2. The process is continued with the deposition of a first polysilicon layer 5 (poly 1), which is suitably doped with phosphorus (F), as shown in Figure 3. This is followed by a masking/etching step carried out on the poly 1 to define the floating gate 6 structures within the matrix area, and in accordance with the invention, remove the first-level polysilicon (poly 1) from the active area 7 of the transistor 21, as shown schematically in Figure 4. A thermal oxidation treatment is applied next which may also include intermediate steps of nitride deposition by a conventional technique to form a second type of oxide layer 9 serving as an interpoly isolation dielectric for the memory cell 20. This oxide layer 9 of the second type overlies the existing oxide layer 4 of the first type at the active area 7 of the transistor 21 and the contact area 8 farthest to the right in the figures. Thus, there will occur, in the active area zone 7 of the transistor 21, a superposition of two oxide layers, namely of the first-type oxide 4 and the second-type oxide 9. These oxide layers form, in accordance with the invention, a thick oxide whereon the transistor gate will then be formed. The thermal oxidation treatment also causes the dopant implanted during the EPM implantation step to be diffused, as marked symbolically'in Figure 5. According to the invention, and as shown in Figure 6, the so-called matrix mask is then formed to define a center or channel area of the transistor, as shown on the left in the sectional views which illustrate the formation of the thick-oxide transistor 21. Upon removal of the matrix mask, therefore, the channel area of the transistor 21 will be covered with a thick oxide layer of about 30 nm (300Å) resulting from the superposed oxide layers of the first and second types 4 and 9, respectively. A second layer 11 of poly 2 polycrystalline silicon is deposited next, as shown in Figure 7. This is followed by the formation of a mask 12 defining the second-level polycrystalline silicon or poly 2. By etching away the polysilicon through the poly 2 mask, the gate structure of the thick-oxide transistor 21 is defined as illustrated by the sectional view of Figure 8. Figure 9 is a sectional view showing the structure after the poly 2 mask is removed, and the source and drain regions of the integrated structures are implanted and diffused. The architecture of the thick-oxide transistor provided by the invention is fully illustrated in Figures 10, 11 and 12, which are respectively plan views of the related poly 1 and matrix and poly 2 active area masks, and two cross sections taken orthogonally through the transistor structure. The poly 2 layer 11 over the thick-oxide transistor area is short-circuited to the poly 1 layer 5 by direct superposition on the latter, over the field oxide 3, as shown by section B-B' in Figure 12. Thus, the invention does achieve its object. In practice, the invention allows thick-oxide transistors to be made through the utilization of classic masking operations in a dual polysilicon level process (of the DPCC type), thereby avoiding the need for an additional masking step, such as is required instead in conventional processes used to also form thick-oxide transistors. The process of this invention is uniquely simple, and no difficulties are encountered with its implementation at the production stage. It should be understood that many changes and modifications may be made unto the foregoing process for making non-volatile memory cells and thick-oxide transistors, without departing from the invention scope as defined in the appended claims.

Claims: A process of making a thick-dielectric transistor (21) having an active area (7) in an integrated circuit which also includes a matrix of floating-gate non-volatile memory cells, the process comprising the steps of: defining said active area (7) of the thick-dielectric transistor (21); defining an implant in the area of said matrix; simultaneously forming a gate dielectric layer (4) over said active area (7) of the thick-dielectric transistor (21) and said matrix area; forming a first-level polysilicon layer (5) over said matrix area on said gate dielectric layer (4) to form floating-gates for the memory cell within the matrix area; forming an interpoly dielectric layer (9) over said active area (7) of the thick-dielectric transistor (21) on said gate dielectric layer (4) and within said matrix area on said first-level polysilicon layer (5), said interpoly dielectric layer (9) and said gate dielectric layer (4) in the active area (7) of the thick-dielectric transistor (21) forming the gate dielectric thereof; forming and patterning a second-level polysilicon layer (11) on said interpoly dielectric layer (9) within said active area (7) of the thick-dielectric transistor (21) and within said matrix area, said patterned second-level polysilicon (11) over said active area (7) forming the gate of said thick-dielectric transistor (21). A process according to Claim 1, characterized in that the step of forming an interpoly dielectric layer (9) over said active area (7) of the thick-dielectric transistor (21) also includes the step of patterning said interpoly dielectric layer (9) over the active area (7) to define a channel area of said thick-dielectric transistor (21). A process according to Claim 2, characterized in that said gate dielectric layer (4) and said interpoly dielectric layer (9) overlap to form a single thick layer of oxide over the active area (7) of the thick-dielectric transistor (21). A process according to Claim 1, characterized in that it is a CMOS process.

Inventors: ROLANDI PAOLO, ROLANDI, PAOLO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: H01L21
IPC codes: H01L21, H01L27