CMOS DEVICE

Abstract: In a semiconductor device comprising: a semiconductor body having a first conductivity type; a first impurity region being formed in the semiconductor body and having said first conductivity type, the first impurity region having higher impurity concentration than the semiconductor body; a first transistor formed in the first impurity region; a second impurity region being formed in the semiconductor body and having a second conductivity type opposite to the first conductivity type; a second transistor formed in the second impurity region; according to the present invention, a third transistor is formed in the semiconductor body having lower impurity than the first impurity region.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a semiconductor device, more particularly to a complementary metal oxide semiconductor (CMOS) integrated circuit (IC) having a p-type island and an n-type island. Description of the Prior Art In addition to its inherent advantage of low power consumption operation, a CMOS IC offers the advantage of high speed operation, it is therefore drawing attention as a semiconductor device suited for large-scale integration and for reducing element sizes. However, the CMOS IC has a problem with its operation characteristics, i.e., -withstand voltage. In an n-channel MOS field-effect transistor (FET) according to a prior art, which, together with a p-channel MOS FET constitutes a CMOS IC, the operation voltage is limited by the following five factors: 1. The punch-through effect between the source and the drain by the electric field between the source and the drain. 2. The breakdown oft he gate insulation film by the electric field between & e gate and the drain. 3. The junction#bre'akdown of the drain in the vicinity of gate by the electric fieldbetween the gate and the drain. 4. The junction breakdown in the diffusion layer by the electric field between the source and drain diffusion layers and the substrate. 5. The threshold voltage in the field region. Countermeasures for the above-mentioned five factors in the prior art will be explained with reference to Fig. 1, which is a schematic cross-sectional view of an n-channel MOS field-effect transistor (FET) according to a prior art. Two n-channel MOS FET's, indicated by A and B, are formed on a p-type silicon substrate 1. In the silicon substrate 1, p-type channel cut diffusion layers 3, n type diffusion layers 4a and 4b, n type diffusion layers 5, and p type diffusion layers 6 are formed. Reference numeral 2 is a silicon dioxide film, on which polycrystalline silicon gates 7 are formed. Now, referring to the MOS FET indicated by A in Fig. 1, to prevent the punch-through effect caused between the source and drain region, n type diffusion layers 4a, which act as source and drain layers are formed remote from the gate. Further, the n -type high resistance diffusion layer 5 is formed between the gate region 7 and the n type diffusion layer 4a. To improve the withstand voltage against breakdowns at the gate insulation film and junction breakdowns of the drain, the depletion region of the n#-type diffusion layer 5 is expanded so as to decrease the electric field relative to the gate electrode. To improve the withstand voltage against junction breakdowns of the diffusion layer, the silicon substrate 1 is used as is without forming a channel cut diffusion region. To improve the threshold voltage of the field region, the p+ diffusion layer 6 i###r#med#with a high impurity concentration. I With a MOS FET which does\#ot#r#quire such a high withstand voltage, the¸c#anne# cut diiffusion region may be connected to the@n+-type diffusion layers 4b, as indicated by B, to enhance the degree of integration. The above-mentioned'rpethods of increasing the withstand voltage, however,#cannot be easily applied to MOS FET's formed on conventional CMOS IC's since the limit of the operation voltage is low, i.e., the limit of the operation voltage mentioned in 4 above is low, e.g., 20 odd volts with an n-channel MOS FET formed in a p-well. CMOS IC's, furthermore, are subject to the following two adiditional factors. 6. The punch-through effect between the source and drain and the silicon substrate by the electric field between the source and drain and the silicon substrate of the MOS FET in the well region. 7. Latching up. The limit on the operation voltage of 4 varies depending upon the impurity concentration in the vicinity of source and drain diffusion layers, i.e., the withstand voltage increases with a decrease in the impurity concentration in the vicinity of the source and drain diffusion layers. With regard to 6 and 7, however, the withstand voltage generally decreases with a decrease of the impurity concentration. To increase the integration density, it is often required to form transistors in the regions of high impurity concentrations. SUMMARY OF THE INVENTION It is an object of the present invention to provide a semiconductor device in which a MOS FET is formed with CMOS transistors, the MOS FET having a withstand voltage high enough in regard to junction breakdown between the source and the drain. According to the present invention, there is provided a semiconductor device comprising: a semiconductor body having a first conductivity type; a first impurity region being formed in said semiconductor body and having said first conductivity type, said first impurity region having higher impurity concentration than said semiconductor body; a first transistor formed in said first impurity region; a second impurity region being formed in said semiconductor body and having a second conductivity type opposite to said first conductivity type; a second transistor formed in said second impurity region; and a third transistor formed in said semiconductor body having lower impurity than said first impurity region. It is preferable that the first and third transistor are p channel MOS transistor and the second transistor is n channel MOS transistor. BRIEF DESCRIPTION OF THE DRAWINGS Further advantages and details of the present invention will now be explained with reference to the accompanying drawings; in which: Fig. 1 was the schematic cross-sectional view of an n-channel MOS FET according to a prior art; Figs. 2A to 2D are sectional views of major portions of a CMOS IC in manufacturing steps according to an embodiment of the present invention; Figs. 3A to 3D are sectional views of major portions of a CMOS IC in manufacturing steps according to another embodiment of the present invention; and Fig. 4 is a sectional view of major portions of a CMOS IC according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Figures 2A to 2D illustrate steps of manufacturing a MOS FET in a region of low impurity concentration on the surface of a silicon substrate. In Figs. 2A to 2D and the subsequent drawings, the same portions are denoted by the same reference numerals. Referring to Fig. 2A, a silicon dioxide (SiO2) film 22 is first formed on the surface of an n-type silicon substrate 21 by the thermal oxidation method. Then, a resist film 23 for patterning marks 24 of a mask for ion injection is formed. The positioning marks 24, which have a criss-cross shape viewed from above, are next formed by etching. Referring to Fig. 2B, the resist film 23 is removed and the silicon dioxide film 22 is grown again by the thermal oxidation method. Another resist film 25 is formed and an n-type well region is patterned. Phosphorus ions (P+) are injected by the ion implantation method under an acceleration energy of 20 to 200 KeV and a dosage of 2 x 1012 to 2 x 1014cm-2. Referring to Fig. 2C, a resist film 26 is formed and patterning is effected to form a p-type well region. Boron ions (B+) are then implanted by the ion implantation method. The boron ions are implanted under an acceler 12 ation energy of 20 to 200 KeV and a dosage of 2 x 10 to 2x10 cm Referring to Fig. 2D, impurity diffusion layers 27 and 28 are next formed by heat treatment of about 12000C. Then, as in the case of producing conventional CMOS IC's, channel-cutting diffusion layers 43, 46 and 46A are formed by the ion implantation method, as shown in Fig. 4, an oxide film 22A is formed on the field regions, and a gate oxide film 22B is formed after an oxide film 22 has been removed. Polycrystalline silicon gates 41, an oxide film 22C, phosphosilicate glass (PSG) film 47, and aluminum electrodes 42 are formed. Finally, the device is covered with a PSG film 48 to complete the CMOS IC. Referring to Fig. 3A, a silicon dioxide film 22 is formed by the thermal oxidation on the surface of an n-type silicon substrate 21. A nitride film 29 is formed to a thickness of about 1,000 nm. Then, as in Fig. 3B, the nitride film 29 is patterned to form active regions, a pattern for forming an n-type well region is formed by a resist film 34, and phosphorus ions are implanted by the ion implantation method under an acceleration energy of 100 to 300 KeV and a dosage of 2 x 1012 to 2 x 1014cm 2. Similarly, as shown in Fig. 3C, a pattern 35 for forming a p-type well region is formed. Boron ions are implanted by the ion implantation method under an acceleration energy of 50 to 200 KeV and a dosage of 2 x 1012 to 2 x l014cm#2. Then, as shown in Fig. 3D, impurity diffusion layers 27 and 28 are formed by heat treatment. Therefore, the MOS FET is formed in the same manner as the case of producing conventional CMOS IC's, thereby completing the CMOS IC illustrated in Fig. 4. Referring to Fig. 4, which is a sectional view of major portions of a CMOS IC produced according to the above-mentioned embodiments of the present invention, a central MOS FET among three MOS FET's is formed on a portion of the silicon substrate 21 where no well region is formed and where the impurity concentration is low. Further, the central MOS FET has no channel-cut diffusion layer 43 or 46, unlike the two other MOS FET's, and, hence, exhibits increased withstand voltage. Compared with the two other transistors, furthermore, the central transistor can have an increased gate width, although the drawing illustrates the same width. The gate width can in practice be increased from the conventional width of 2 to 3 vm (microns) to as great as about 6 pm. Accordingly, the central transistor exhibits improved characteristics against the punch-through effect. Furthermore, the central MOS FET of Fig. 4 may be used as a high withstand voltage MOS FET indicated by A in Fig. 1.

Claims: CLAIMS 1. A semiconductor device comprising: a semiconductor body having a first conductivity type; a first impurity region being formed in said semiconductor body and having said first conductivity type, said first impurity region having higher impurity concentration than said semiconductor body; a first transistor formed in said first impurity region; a second impurity region being formed in said semiconductor body and having a second conductivity type opposite to said first conductivity type; and a second transistor formed in said second impurity region characterized in that a third transistor is formed in said semiconductor body having lower impurity than said first impurity region. 2. A semiconductor device according to claim 1 characterized in that said first and third transistor are p channel MOS transistor and said second transistor is n channel MOS transistor.

Inventors: INABA, TOURU
Applicants: FUJITSU LIMITED
Application year: 1983
Main classification: H01L27
IPC codes: H01L21, H01L27, H01L29