Integrated circuit semiconductor device having improved isolation region

Abstract: An integrated circuit semiconductor device having first and second circuit elements isolated by an improved isolation region is disclosed. The isolation region includes first and second thick insulating layers at least partly embedded in a semiconductor substrate of one conductivity and an impurity portion of one conductivity having a higher impurity concentration than that of the substrate, provided between the first and second thick insulating layer such that it is separated from both of the first and second circuit elements.

Description: Background of the Invention: The present invention relates to an integrated circuit semiconductor device and method of manufacturing said device having improved isolation region for isolating circuit elements formed in a semiconductor substrate, and more particularly to an improved isolation of insulated-gate type field effect transistors (hereinafter abbreviated as IGFET's) of N-channel type formed in a P-type area of a semiconductor substrate. Such an integrated circuit semiconductor device according to the preamble of claim 1 is known from EP-A-0 090 520. In an integrated circuit semiconductor device, circuit elements are isolated each other by a thick field silicon oxide layer partly embedded in a semiconductor substrate of one conductivity type with a channel stopper region of one conductivity having a higher impurity concentration than that of the substrate being provided under the bottom of the embedded field silicon layer. The isolation region consisting of the field silicon oxide layer and the channel stopper region must realize a high threshold voltage in the field area. For that reason, it is desirable to make the field oxide layer as thick as possible and imparity concentration of the channel stopper region as high as possible. However, the thickness of the field oxide layer is restricted to some extent because too thick field oxide layer sacrifices the required flatness of the surface of the device and produces crystal defects in the active region, that is, circuit element forming region. On the other hand, the impurity concentration of the channel stopper region is also restricted, because an impurity for forming the channel stopper region must be introduced into the substrate prior to thermal oxidation process for forming the thick field oxide layer. The thermal oxidation process is conducted under a high temperature for a long time and therefore, the impurity concentration of the channel stopper region cannot be increased to a level higher than a certain limit. Moreover, even if the impurity concentration of the channel stopper region could be increased, the junction breakdown voltage decreases between the channel stopper region and one region of the circuit element such as source or drain region of IGFET which is in contact with the channel stopper region. Thus, the upper limit of the impurity concentration of the channel stopper region under the partially embedded field oxide layer was 10 18 atoms/cm 3 . Further, when the device of the above-mentioned structure is irradiated with ionizing radiations (γ-rays, α-rays or electron rays), positive holes in the electron-positive hole pairs formed in the silicon oxide layer migrate into the interface between the silicon substrate and the silicon oxide layer, and are trapped near the interface. Therefore, a fixed electric charge of positive polarity accumulates in the silicon oxide layer. In the case of a P-type silicon substrate, the electric field is established in a direction to invert the surface of the silicon substrate and therefore, the leakage current is apt to flow by the irradiation of the ionizing radiations. The leakage current increases with the increase of the irradiation. Further, the ionized positive holes are produced more in a thick silicon oxide layer than in a thin silicon oxide layer and therefore, the leakage current due to the ionizing radiations are increased under the thick silicon oxide layer at the isolation region. Therefore, if the effect by the irradiation of the ionizing radiations is to be considered, the field insulating layer on the P-type substrate area to isolate N-type regions must be thin. Summary of the Invention: An object of the present invention is to provide an integrated circuit semiconductor device having an effective isolation region for isolating circuit elements, which realizes a high threshold voltage in the field area without decreasing the breakdown voltage. Another object of the present invention is to provide an isolation structure which has a low leak current due to the ionizing radiations. These objects of the invention are solved by the features of claims 1 and 3. With the features of the claims it is possible that, when the ionizing radiations are irradiated, the parasitic channeling between the first and second circuit elements can be cut off by the first impurity portion, at the central section of the isolation region. On the other hand, the impurity concentration of the second and third impurity portions favorably ranges from 5 x 10 16 to 1 x 10 18 atoms/cm 3 , and the thickness of the first and second thick insulating layer favorably ranges 0.5 µm to 1.2 µm. These sections of the isolation region are useful for isolating the circuit elements when the ionizing radiations are not irradiated. Further, the thick and at least partly embedded insulating layer is employed to delineate active regions in which the circuit elements are to be formed, and the center section of the isolation region in which the first impurity portion is to be formed. When the circuit elements are N-channel type IGFET's and the N-type source, drain region of 5 x 10 19 atoms/cm 3 is abutted against the thick field insulating layer, the second or third impurity portion of P-type having the impurity concentration of 1 x 10 17 atoms/cm 3 forms a PN junction with the source or drain region. In this case, the breakdown voltage of the PN junction can maintain about 20 V, and any problem does not occur. Whereas, if the first impurity portion of P-type having the impurity concentration of 1 x 10 19 atoms/cm 3 is directly contacted to the source, drain region, the breakdown voltage would become as low as 5 V or less. The area of the substrate may be a P-well. In this case, in other area of the substrate is of N-type conductivity type, and many P-channel type IGFET's are formed therein to constitute CMOS element with the N-channel IGFET in the P-well. In such a device, the first impurity portion of the present invention and P-type source and drain regions of the P-channel type IGFET's may be simultaneously formed by the same process. Breif Description of the Drawings: Fig. 1 is a cross-sectional view of an integrated circuit semiconductor device using a conventional isolation structure for isolating circuit elements; Fig. 2 is a diagram showing the leakage current by a parastic MOS action in a conventional isolation struction that varies depending upon the amount of the ionizing radiations; Fig. 3A is a plan view showing a first embodiment of the present invention, and Fig. 3B is a cross-sectional view taken along line B-B' in Fig. 3A as viewed in the direction of arrows; Figs. 4A to 4C are cross-sectional views showing a manufacturing process in the order of steps according to the first embodiment of the present invention; Description of the Prior Art: Referring to Fig. 1, in a P-type silicon substrate 11 an N + -type source or drain region 15 of a first IGFET and an N + -type source or drain region 25 of a second IGFET are formed. The first and second IGFET's, that is, the N + -type region 15 and the N + -type region 25 is isolated by an isolation region consisting of a thick field silicon oxide layer 13 and a P + -type impurity region 12, that is, a channel stopper region 12. The thick field oxide layer 13 has the thickness of about 1.0 µm and embedded partly from the major surface 18 of the substrate 11 into the substrate, and the channel stopper region 12 is abutted against the bottom 19 of the thick field oxide layer 12. On the thick field oxide layer 13 and on the respective thin gate insulating films 14, 24 an intermediate insulating film 16 of phosphosilicate glass is formed, and a wiring layer 17 is provided on the isolation region. In such a construction, a parastic MOS transistor action is apt to occur. Namely, an inversion layer, in this case N-type layer, is produced under the field layer 13 between the N + -type regions 15, 25 by applying a high voltage of positive polarity to the wiring layer 17, for example, when the thick field oxide layer 13 has the thickness of 1 µm, and the channel stopper region 12 has the impurity concentration of about 10 17 atoms/cm 3 . Further, in this structure, the intermediate insulating film 16 is interposed between the wiring layer 17 and the thick field oxide layer 13. It is estimated that the electric leakage current of a level of smaller than 10 -20 A (ampere) flows between the N + -type regions 15, 25 belonging to the different IGFET's, respectively, when the distance between the N + -type regions 15, 25, that is, the length of the isolation region is 2 µm, and the width of each of the N + -type regions 15, 25 is about 50 µm. With respect to the potentials, a voltage of 5 volts is applied to the wiring layer 17 and the N + -type region 15, and the substrate 11 and the N + -type region 25 are maintained to a ground potential. Over an ordinary range of operation voltage, therefore, the leakage current by the parasitic MOS transistor shown in Fig. 2 is negligibly small. If the device of th above-mentioned structure is irradiated with ionizing radiations (γ-rays, α-rays or electron rays), however, positive holes in the electron-positive hole pairs formed in the oxide layer 13 migrate into the interface between the silicon substrate and the field oxide layer, and are trapped by the positive holes that are distributed in large amounts near the interface. Therefore, a fixed electric charge of positive polarity accumulates in the field oxide layer 13. Accordingly, the leakage current by the parasitic MOS transistor action steadily increases with the increase in the amount of ionizing radiations as shown in Fig. 2. In the case of the parasitic MOS transistor action of the structure described above, for example, the leakage current increases by as much as about 10 10 when the amount of ionizing radiations is 1 x 10 5 rad (Si). The increased leakage current adversely affects not only the characteristics of the active transistors alone but also the operation characteristics of the integrated circuit device. To cope with this problem, it can be attempted to increase the concentration of the impurity region 12 under the field oxide layer 13 so that the surface inversion takes place little. However, the impurity concentration of the impurity region, that is, the channel stopper region is restricted to about 10 18 atoms/cm 3 or less as mentioned above. Such a concentration is not sufficient to prevent the parasitic MOS transistor from leaking the current when it is irradiated with ionizing radiations. Description of the Embodiments of the Present Invention: Referring to Figs. 3A and 3B, the embodiment comprises a P - -type silicon substrate 11 having an impurity concentration of 8 x 10 14 atoms/cm 3 , a first N-channel type IGFET 100 and a second N-channel type IGFET 200. The first and second IGFET's are isolated and separated by an isolation region 300 of the present invention, and an wiring layer 17 of aluminum is provided on the isolation region 300 via an intermediate insulating film 16 of phosphosilicate glass. The intermediate insulating film on the isolation region is not necessary in some case. To avoid complexity, the wiring layer 17 is shown by phantom lines in Fig. 3A. The first IGFET 100 comprises N + -type source and drain regions 15, 15' having the impurity concentration of 5 x 10 19 atoms/cm 3 and a gate electrode 36 of polycrystalline silicon provided on a channel region thereof via a gate insulating film 14. Also, the second IGFET 200 comprises N + -type source and drain regions 25, 25' having the impurity concentration of 5 x 10 19 atoms/cm 3 and a gate electrode 37 of polycrystalline silicon provided on a channel region thereof via a gate insulating film 24. The isolation region 300 isolates one of source and drain regions 15, 15' of the first IGFET 100 from one of source and drain regions 25, 25' of the second IGFET 200. The isolation region 300 comprises a first impurity portion 30 of P ++ -type having the impurity concentration of 1 x 10 19 atoms/cm 3 , a thermally grown thin silicon oxide film 33 provided on the first impurity portion 30, a first thick field silicon oxide layer 34 provided between the first impurity portion 30 and the first IGFET 100 and partly embedded in the major surface 18 of the substrate, and a second thick field silicon oxide layer 35 provided between the first impurity portion 30 and the second IGFET 200 and partly embedded in the major surface 18 of the substrate. As shown in Fig. 3A, the isolation region with the first impurity portion 30 and the first or second insulating layer 34, 35 surrounds the respective IGFET's. The first and second thick field insulating layers 34, 35 having the thickness of 1.0 µm are provided between the surrounding first impurity portion and each of the IGFET's such that they delineate the position of the substrate in which the first impurity portion is to be formed and the positions of the substrate in which the IGFET's are to be formed. Further, in this embodiment, the isolation region 300 comprises second and third impurity portions 31, 32 of P + -type having the impurity concentration of 1 x 10 17 atoms/cm 3 provided under and abutted against the bottoms 19 of the first and second thick insulating layer 34, 35, respectively. According to the isolation structure 300, even when ionizing radiations such as α rays, γ rays or electron rays are irradiated, the inversion layer can be effectively cut off at the center section consisting of the first impurity region 30 and the thin insulating film 33 of the isolation region 300. Further, the first impurity region 30 of high impurity concentration is not contacted to the source, drain regions 15, 25, and therefore, the high breakdown voltage can be expected. In Figs. 3A and 3B, well-known electrodes and lead-out wiring layers for source, drain regions and gate electrodes are omitted for simplicity. Referring to Figs. 4A to 4C, the isolation region 300 of the first embodiment shown in Figs. 3B is formed by the following process steps. First, an antioxidation mask pattern (not shown) of,for example, silicon nitride is selectively formed on the major surface of the P - -type silicon substrate and a P-type impurity such as boron ion is introduced into the substrate through ion-implantation method by using the silicon nitride pattern or a photo resist pattern (not shown) as a mask. Then, by subjecting a heat treatment at a high temperature during a long time, under an oxidizing atmosphere, such as, oxygen or steam, the first and second thick field insulating layers 34, 35 are formed by using the silicon nitride pattern as a mask (see Fig. 4A). The thick field insulating layers 34, 35 are partly embedded from the major surface and thereunder the second and third impurity portions 31, 32 of P + -type are formed, respectively. After removing the silicon nitride pattern, gate insulating films 14, 24 and the thin silicon oxide film 33 are newly formed by a heat treatment. Then, the thermally grown thin film 33 of 80 nm (800 Å) thickness is covered by a first mask member 40 such as of aluminum, as shown in Fig. 4A, and an N-type impurity 41 such as phosphorus or arsenic ion is ion-implanted through the gate insulating films 14, 24 by using the first mask member 40, the field insulating layers 34, 35 and the silicon gate electrodes 36, 37 (Fig. 3A) as a mask to form source, drain regions 15, 15', 25, 25' of the first and second IGFET's. After removing the first mask member, a second mask member 42 of for example, aluminum, as shown in Fig. 4B, is selectively formed so as to expose the thin insulating film 33, and through the thin insulating film 33 a P-type impurity, such as, boron ion 43 is introduced into a substrate between the thick field insulating layers 34, 35 by an ion-implantation method. Then, after removing the second mask member 42, a heat treatment is conducted to activate the implanted boron ion, and the first impurity portion 30 of P ++ -type is formed (Fig. 4C). According to this manufacturing process, the first impurity portion 30 of P ++ -type is not formed prior to effecting the field oxidation for forming the thick insulating layers 34, 35. Therefore, crystal defects do not develop when the field is oxidized, and the impurity concentration of the first impurity portion is not decreased by the thermal diffusion in oxidizing the field at a high temperature for an extended period of time.

Claims: An integrated circuit semiconductor device comprising a P-type semiconductor substrate (11) having an upper surface, a first N-channel type insulated gate field effect transistor (100) having N-type source and drain regions (15,15') formed along said upper surface of said P-type substrate (11), a second N-type insulated gate field effect transistor (200) having N-type source and drain regions (25,25') formed along said upper surface of said P-type substrate, an isolation region (300) for isolating said first and second transistors (100,200) provided between said first and second transistors (100,200), and a wiring layer (17) formed above said isolation region (300), said isolation region comprising a thin silicon oxide layer (33) and a first and second thick silicon oxide layer (34,35) adjacently formed to said thin oxide layer (33) and the N-type source and drain regions of said N-channel type field effect transistors (100,200), a first P-type impurity region (30) having an impurity concentration higher than that of said substrate (11) and being formed in said substrate separately from both of said first and second transistors (100,200), a second P-type impurity region (31) formed under said first thick silicon oxide layer (34) and a third P-type impurity region (32) formed under said second thick silicon oxide layer (35); said thin silicon oxide film being formed on the upper surface of said first P-type impurity region, said first thick silicon oxide layer being formed between one side of said thin silicon oxide film and one of said N-type source and drain regions (15) of said first transistor (100) and partially embedded from said upper surface of said substrate more deeply than said one of said N-type source and drain regions of said first transistor (100), said second thick silicon oxide layer being formed between the other side of said thin silicon oxide film and one of said N-type source and drain regions (25) of said second transistor (200) and partially embedded from said upper surface of said substrate (11) more deeply than said one of said N-type source and drain regions of said second transistor; characterized in that: said first P-type impurity region (30) has an impurity concentration of 5 x 10 18 to 1 x 10 20 atoms/cm 3 , said thin silicon oxide film (33) has a thickness of 10 to 100 nm (100 to 1000 Å), and in that said first and second thick silicon oxide layers (34,35) have a thickness of 0,5 to 1,2 µm, and said second and third P-type impurity regions (31,32) have an impurity concentration of 5 x 10 16 to 1 x 10 18 atoms/cm 3 , and form a PN junction with said one of said N-type source and drain regions (15,25) of said first and second transistors (100,200), respectively. An integrated circuit semiconductor device of claim 1, in which said isolation region (300) surrounds both said first and second transistors (100,200). A method of manufacturing an integrated circuit semiconductor device as claimed in claim 1, said method comprising the following sequence of steps : forming said second (31) and third (32) P-type impurity regions and said first and second thick silicon oxide layers (34,35) by ion-implanting selectively a P-type impurity into said substrate and performing a heat treatment under an oxidizing atmosphere; forming thermally said thin silicon oxide film (33) and gate silicon oxide films (14,24) of said transistors (100,200); forming selectively a first mask member (40) to cover said thin silicon oxide film (33) and to expose said thick silicon oxide films (34,35); ion-implanting an N-type impurity into said substrate through said gate silicon oxide films (14,24) by using said first mask member, said first (34) and second (35) thick silicon oxide layers and the silicon gate electrodes (36,37)as a mask to form said N-type source and drain regions (15,15',25,25') of said first and second transistors; removing said first mask member (40); forming selectively a second mask member (42) to cover said gate silicon oxide films (14,24) and to expose said thin silicon oxide film (33); and ion-implanting a P-type impurity into said substrate (11) through said thin silicon oxide film (33) by using said second mask member (42) and said first and second thick silicon oxide layers (34,35) as a mask to form said first P-type impurity region (30).

Inventors: KOMATSU MICHIO, KOMATSU, MICHIO
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1986
Main classification: H01L21
IPC codes: H01L21, H01L27