PROCESS FOR MANUFACTURING CMOS INTEGRATED DEVICES WITH REDUCED GATE LENGTHS

Abstract: A process for manufacturing CMOS integrated devices with gate lengths of less than one micron and high supply voltage is described. In order to improve the resistance of CMOS devices to breakdown and punch-through phenomena without cost increases with respect to conventional CMOS processes and limiting as much as possible the introduction of resistances in series to the transistors, less doped source and drain regions (31) being provided in only one of the two MOS transistors, e.g. in the N-channel transistor, to increase the breakdown voltage, an oppositely doped region (13′), e.g. with P-type doping, being provided around the source and drain regions (31) of this first transistor to protect this first transistor against punch-through, and doped wells (14) being provided around the source and drain regions (24) of the complementary transistor, which is e.g. a P-channel transistor; said doped wells (14) being oppositely doped with respect to said source and drain regions (24) but having a lower doping level than the region of the body (1) of semiconductor material which accommodates the complementary transistor, in order to increase the breakdown voltage of the P-channel complementary transistor.

Description: The present invention relates to a process for manufacturing CMOS integrated devices with a reduced gate length according to claim 1. In particular, the process relates to high voltage fed CMOS integrated devices with gate lengths of less than one micron. It is known that the punch-through and breakdown of the junction are important limiting factors in reducing the dimensions of devices, since the high concentrations of doping in the channel required to avoid punch-through, can lead to an early breakdown of the junction, due to the increase of the electric field at the junction. Manufacturing methods are currently known, such as the Lightly Doped Drain (LDD) method, which have the purpose of controlling and reducing the maximum electric field peak. According to these methods, a light implanting of ion species adapted to generate regions with the same type of conductivity as the source and drain regions of the transistors to be manufactured is first performed by means of two successive separate masks. Silicon oxide spacer structures are then generally formed at the sides of the gate regions of the transistors to mask a heavy ion implanting of said source and drain regions. The introduction of these LDD regions on N-channel transistors and on P-channel transistors, however, is entailing some disadvantages, since in any case it produces a worsening of the electric characteristics of the device, especially of its speed, due to the resistances which are introduced in series to the source and drain regions of the transistors and thus reducing the value of the current flowing through the transistor. This known LDD process furthermore entails a hardly negligible cost increase with respect to conventional CMOS processes, due to the two masks required to produce the individual LDD regions in the two transistors. Given this situation, the aim of the present invention is to provide a process for manufacturing CMOS integrated devices with reduced gate lengths, capable of obviating the disadvantages of the known art, and in particular of providing a protection against breakdown and punch-through phenomena with no substantial increase in the number of manufacturing steps of conventional CMOS processes, and more precisely without increasing the number of required masking steps. Within the scope of this aim, a particular object of the present invention is to provide a manufacturing process which is capable of limiting as much as possible the introduction of resistances in series to the transistors, so as to limit the worsening of the electric characteristics of the CMOS devices with respect to conventional processes. Not least object of the present invention is to provide a process which comprises per se known and used process steps, so as to allow the use of machines which are commonly employed in the electronics industry, with thoroughly controllable procedures and results. This aim, the objects mentioned and others which will become apparent hereinafter are achieved by a process for manufacturing CMOS integrated devices with reduced gate lengths as described in the appended claims. The characteristics and advantages of the invention will become apparent from the description of a preferred but not exclusive embodiment, illustrated only by way of non-limitative example in the accompanying drawings, wherein: figures 1 to 6 are transverse sectional views taken through a silicon wafer, illustrating different successive steps of the process according to the invention. The process according to the invention initially comprises conventional steps to obtain oppositely doped regions in a body or substrate of semiconductor material 1 to provide the complementary transistors. In the illustrated embodiment, wells 2 with P-type conductivity and wells 3 with N-type conductivity are diffused in the monocrystalline silicon substrate 1 by means of conventional methods. Then a step of active area definition is carried out, leading to the forming of insulating oxide regions 5 which separate the individual active areas; in figure 1 the reference numeral 6 indicates the active area intended to accommodate the N-channel transistor, while 7 indicates the active area intended to accommodate the P-channel transistor. Then a gate oxide layer, indicated by the reference numeral 8 in figure 1, is grown by conventional methods on the surface of the wafer of semiconductor material. Consequently the gate regions of the individual transistors is deposited, by conventional methods, again by depositing, doping and shaping a layer of polycrystalline silicon. The structure illustrated in figure 2 is thus obtained; in this figure, the reference numeral 10 indicates the gate region of the N-channel transistor which is to be accommodated in the P-well 2, and the reference numeral 11 indicates the gate region of the complementary P-channel transistor which is to be accommodated in the N-well 3. According to the invention, in order to obtain wells for protecting the individual transistors of CMOS devices against punch-through, a light implanting with a P-type ion species is then performed on the entire substrate. This implanting is indicated in the figure by the arrows 12, and is leading to the accumulation of the implanted ions in the not shielded substrate regions. The shielded regions include the insulations 5 and the gate regions 10 and 11. Said ions are consequently diffused by means of an appropriate thermal treatment. Regions 13 consequently form in the P-type well 2, said regions having also P-type conductivity but being more heavily doped; regions 14 form in the N-type well 3, and the concentration of the doping agent of said well 3 is partially compensated in these regions. Regions 14 with N⁻ type conductivity thus form inside the well 3. Further, according to the invention, an N implanting is performed on the entire substrate to form the LDD regions. This step is shown in figure 4, in which the arrows 18 indicate the N implanting leading to the forming of the N-type surface regions 19 inside the wells 13, and to the forming of the regions 20, also of the N type, inside the regions 14. Then the P⁺ junctions are implanted; said junctions being intended to form the source and drain regions of the P-channel transistor provided in the N well. This step is shown in figure 5, which illustrates the resist mask 22 covering the regions which must not be implanted, such as e.g. the areas of the substrate which are to accommodate the N-channel transistors. In the figure, the arrows 23 indicate the P-type implanting (performed e.g. with boron) leading to the forming of the source and drain regions 24 of the P-channel transistor. It should be noticed that in this figure the N-type regions 20 have no longer been drawn since the high P⁺ dose implanted to form the junctions leads to a complete compensation of the N ions implanted earlier. In this figure, the reference numeral 14' furthermore indicates the remaining portion of the region 14. This portion 14' thus comprises in practice a well which completely surrounds the source and drain regions 24 of the P-channel transistor; in said well the concentration of the doping agent of the N well 3 is partially compensated, thus reducing the intensity of the maximum electric field peak in the P-channel transistor and raising the value of its breakdown voltage. Then, according to the invention, after removing the mask 22, silicon oxide spacer structures are provided in a conventional manner to the sides of the gate regions, as indicated by the reference numeral 27 for the N-channel transistor and by the reference numeral 28 for the P-channel transistor and as illustrated in figure 6. Consequently the surface of the substrate is again masked to implant the N⁺ junctions. A resist mask, indicated by the reference numeral 29 in figure 6, is then deposited in a conventional manner and covers the regions to be shielded (from implantation), and in particular the regions comprising the P-channel transistors. Then the N⁺ implanting is performed, as indicated in figure 6 by the arrows 30. This implanting leads to the production of N⁺-type regions 31 to the sides of the spacer structures 27. Said regions 31, are comprising the source and drain regions of the N-channel transistor, extend completely inside the region 13, which thus comprises a well, indicated in figure 6 by the reference numeral 13', which completely surrounds the source and drain regions of the N-channel transistor. Due to the presence of the spacers 27, portions 19' of the LDD implanting remain to the sides of the source and drain regions 31, but are still comprised within the wells 13'. The N-channel transistor consequently has P-type wells which surround the source and drain regions and provides protection against punch-through in the N transistor, while the remaining regions 19' allow to increase the breakdown voltage of the N-channel transistors according to the conventional LDD method. The process ends in a conventional manner by forming the contacts and the interconnection lines, with the further final steps for manufacturing integrated circuits in CMOS technology. As can be seen from the previous description, the invention fully achieves the intended aims. In fact, by providing the P-type well surrounding the N⁺ junction of the N-channel transistor, a protection against the punch-through is provided, while the partial compensation of the concentration of the doping agent in the well 14' surrounding the source and drain regions of the P-channel transistors protects this type of transistors against breakdown. It should be noted that these regions are obtained by means of light P implanting on the entire substrate, as shown in figure 3, without using additional masks. In particular, the fact is stressed that in order to provide the protection regions, it is necessary to choose the dose of implanted doping agent P and the subsequent thermal treatment method, in order to provide the well 13' which encloses the entire N⁺ junction of the N-channel transistor, while simultaneously ensuring that the implanted dose is low enough to only partially compensate the doping of the N-well or drain 3 without introducing scarcely doped P regions around the P⁺ junction, thus avoiding the forming of regions with greater resistance. For example, according to a process studied by the applicant, by providing a light P implanting with boron with an implanting dose of approximately 1x10¹² at 100 KeV followed by a thermal process at 900°C for one hour, thus providing LDD regions in the N-channel transistor, an increase of approximately 1.5 V of the breakdown voltage is achieved in the P-channel transistor with a gate length of 1.0 µm, while there is no significant worsening of its resistance to punch-through. In particular it has been noticed that in the indicated conditions, without using additional masks with respect to the conventional CMOS process, it is possible to provide the LDD regions only on the N-channel transistors, while the breakdown voltage of the P-channel transistors can be controlled by compensating the doping agent of the N well 3 without introducing further serial resistances on these transistors, which already have a reduced gain due to the low mobility of the holes. It should be furthermore noted that the N dose of LDD implanted in the P-channel transistors is completely compensated by the high P⁺ dose implanted to form the junctions. Naturally the process may be executed in a specular manner, so as to increase the breakdown voltage on the P-channel transistor by using LDD regions and, in the N-channel transistor, by providing a well with a partial compensation of the doping agent of the region which accommodates said N-channel transistor. In order to implement the above, it is sufficient to initially execute a light N implanting, instead of the P implanting illustrated in figure 3, so as to generate N⁺ doped regions inside the well 3 and regions with a partial compensation of the P doping agent inside the well 2. Then, instead of the N implanting of figure 4, a P implanting is performed to provide the LDD regions for the P-channel transistor. This specular process then provides the implanting of the P⁺ and N⁺ junctions in reverse order with respect to the sequence shown in figure 5, i.e. first the implanting of the N source and drain regions of the N-channel transistor laterally to the gate region of said transistor and then, after providing the spacer structures, the implanting of the P⁺ source and drain regions of the P-channel transistor in the region comprised between the insulations and the spacer structures, in order to keep the LDD regions less doped. With this version of the process an increase of approximately 1 volt in the breakdown voltage of the junction has been noticed in N-channel transistors with a gate length of 0.8 microns, with no significant worsening of its resistance to punch-through, thus obtaining a 10% improvement with respect to the values of 10-12 volts which can be manufactured using the conventional process. In this case the phosphorus dose implanted in the step illustrated in figure 3 is of approximately 1x10¹² cm⁻² at 120 KeV, producing anti-punch-through wells of the halo type on the P⁺ junctions of the P-channel transistor. As can be seen, the invention thus allows a marked improvement of the electric characteristics, without requiring additional masking steps with respect to the conventional process, and therefore with substantially comparable manufacturing costs. Furthermore, the worsening of the electric characteristics due to the presence of resistance in series to the source and drain regions is limited to only one of the two transistors, thus obtaining an improvement with respect to the known LDD process. The invention thus conceived is susceptible to numerous modifications and variations, all of which are within the scope of the inventive concept. In particular the fact is stressed that though the described embodiment is applied to a structure having two drains or wells, a P-type one and an N-type one, both accommodating a respective transistor of a CMOS device, the same process can also be applied to N-well or P-well structures. The described process can furthermore be applied to the production of CMOS devices regardless of the method for defining the active areas and of the method for manufacturing the insulation, regardless of the material used to provide the gate regions (in particular with gates made of polycrystalline silicon, silicide or by superimposing polycrystalline silicon and silicide, etc.), provided that the thickness of the gate allows the self-alignment of the ion implantings which are forming the punch-through protection regions. The process is furthermore independent from the material in which the spacer structures are provided. Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the scope of each element identified by way of example by such reference signs.

Claims: Process for manufacturing a CMOS integrated device with reduced gate lengths, comprising the steps of: defining in a body (1) of semiconductor material having a major surface at least one first region (2) with a first type of conductivity, which has a first level of doping impurities to produce a first MOS transistor with a first type of channel, and a second region (3) with a second type of conductivity opposite to the first one, having a second level of doping impurities to provide a second MOS transistor complementary to the first transistor; defining active areas (6,7) intended to accommodate said MOS transistors in said first region (2) and said second region (3); growing an insulating layer (8) on said large surface; selectively forming gate regions (10,11) of a semiconductor material on said insulating layer (8) at said active regions (6,7); selectively forming source and drain regions (31) with said second type of conductivity for the first transistor in said first region (2) and of source and drain regions (24) with said first type of conductivity for the second transistor in said second (3) region of the body (1) of semiconductor material; defining contacts and interconnection lines; characterized in that after said step of selectively forming gate regions, a first step of introducing doping impurities (12) is performed on the entire larger surface of said body of semiconductor material, said doping impurities (12) being adapted to impart said first type of conductivity to provide first wells (13) with said first type of conductivity around the source and drain regions (31) of said first transistor in said first region (2) and second wells (14) with said second type of conductivity around the source and drain regions (24) of said second transistor in said second region (3), said first wells (13) having a higher level of doping impurities than said first level and said second wells (14) having a lower level of doping impurities than said second level. Process according to claim 1, characterized in that after said first step of introducing doping impurities (12), a second step (18) of introducing further doping impurities is performed on the entire surface of said body (1) of the semiconductor material, said doping impurities being adapted to impart said second type of conductivity, to provided lightly doped regions (19,19') with said second type of conductivity adjacent to said drain and source regions (31) of said first transistor in said first region (2). Process according to any one of the preceding claims, characterized in that said step of selectively forming source and drain regions (24,31) comprises: the selective introduction of doping impurities (23) adapted to impart said first type of conductivity to said second region (3), laterally to the gate region (11) of said second transistor, the provision of spacer structures (27) of insulating material laterally to the gate regions (10) of said first transistor and the selective introduction of doping impurities (30) adapted to impart said second type of conductivity to said first region (2), laterally to said gate region (10) and to said spacer structures (27) of said first transistor. Process according to any one of the preceding claims, characterized in that said first type of conductivity is of the P type, and that said second type of conductivity is of the N type, said first MOS transistor is of the N-channel type with lightly doped regions (19'), and said second MOS transistor is of the P-channel type. Process according to any one of claims 1-3, characterized in that said first type of conductivity is of the N type, and that said second type of conductivity is of the P type, said first MOS transistor is of the P-channel type with lightly doped regions (19'), and said second MOS transistor is of the N-channel type. CMOS device with reduced channel lengths, comprising a body (1) of semiconductor material having a major surface and defining at least one first region (2) with a first type of conductivity which has a first level of doping impurities and accommodates source and drain regions (31) of a first MOS transistor with a first channel type, said source and drain regions having a second type of conductivity opposite to said first type conductivity, and a second region (3) with a second type of conductivity which has a second level of doping impurities and accommodates source and drain regions with said first type of conductivity of a second MOS transistor complementary to the first one, characterized in that it comprises first wells (13) with a first type of conductivity surrounding said source and drain (31) regions of said first transistor in said first region (2) and second wells (14) with said second type of conductivity, surrounding said source and drain regions (24) of said transistor, in said second region (3), said first wells (13) having a higher level of doping than said first level and said second wells (14) having a lower level of doping impurities than said second level.

Inventors: BERGONZONI, CARLO
Applicants: SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1988
Main classification: H01L21
IPC codes: H01L27, H01L21, H01L29