SEMICONDUCTOR DEVICE WITH REDUCED CAPACITIVE LOAD AND MANUFACTURING PROCESS THEREOF

Abstract: There is provided a semiconductor device which comprises device regions (X) and isolation regions (Y) to isolate the device regions (X) from each other on a semiconductor substrate (1), wherein field insulators (7) are formed in the isolation regions (Y) and conductive layers (10) for wiring are formed above the field insulators (7). An additional impurity buried layer (11) having an opposite conductivity to the semiconductor substrate is formed under the field insulators (7). Therefore a first capacitance element is composed of the conductive layer (10), the impurity buried layer (11), and the field insulator (7) therebetween. A second capacitance element is composed of the impurity buried layer (11), the semiconductor substrate (1), and a PN junction layer therebetween. Thus the first capacitance element and the second capacitance element are connected in series. This series connection reduces a wiring capacitance of the device and improves the operation speed thereof.

Description: BACKGROUND OF THE INVENTION This invention relates to a semiconductor device and, more particularly, to a semiconductor device in which a reduction of a parasitic capacitance between a substrate and a conductive layer utilized for wiring is needed in order to improve the operation speed. Fig. 1 illustrates a structure of a general MOS (Metal Oxide Semiconductor) transistor device according to the prior art. Fig. 1(a) is a sectional view of an N channel transistor device and Fig. 1(b) is a sectional view of a CMOS (Complimentary Metal Oxide Semiconductor) transistor device. In both the devices, device regions X's and isolation regions Y's are defined on a P-type substrate 1. In the device region X of the device shown in Fig. 1(a), an N channel MOS transistor is constructed which comprises N⁺-buried layer 2 ( a layer implanted with an N⁺-type impurity), a gate oxide 3, and a gate electrode 4. On the other hand, in the device region X of the device shown in Fig. 1(b), a P channel MOS transistor is constructed in an N-well 5 as well as an N channel MOS transistor. The P channel MOS transistor comprises a P⁺-buried layer 6, a gate oxide 3, and a gate electrode 4. In order to isolate these device regions X' s, field oxides 7's are constructed in the isolation regions Y's. The transistors and the field oxides 7's are coated with insulative protection layers 8's. After forming contact windows, interconnection layers 9's are formed through protection layers 8's. Then conductive layers 10's are formed on the protection layers 8's in the isolation regions Y's so as to wire the transistors. In the operation mode of the above described MOS transistors, it is considered that there are three different types of capacitance element which behaves as a load in a circuit, that is, (i) a junction capacitance between the substrate 1 and the N⁺-buried layer 2 which corresponds to a source or a drain; (ii) a capacitance between the gate 4 and the substrate 1; and (iii) a capacitance between the substrate 1 and the conductive layer 10. When the size of a transistor becomes smaller due to an integration of an LSI, the above mentioned capacitances (i) and (ii) are reduced. However, the capacitance (iii) tends to increase because the integration of the LSI lengthens the conductive layer and increases the thickness of the field oxides. Since the capacitance behaves as a load in a circuit, an operation speed of the circuit becomes low with increasing capacitance. Therefore the increase in the capacitance described above causes a serious problem in the LSI. SUMMARY OF THE INVENTION An object of the present invention is to provide a semiconductor device which can operate at a high speed by reducing a circuit load caused by a wiring capacitance. The foregoing object is accomplished by providing a semiconductor device in which device regions and isolation regions adapted to isolate the device regions from each other are defined on a surface of a semiconductor substrate, and which comprises field insulators formed in the isolation regions and conductive layers for wiring formed above the field oxides, characterized by comprising an impurity buried layer having an opposite conductivity to the semiconductor substrate under the field insulators, so as to form a first capacitance element composed of the conductive layer, the impurity buried layer, and the field insulator therebetween, and a second capacitance element composed of the impurity buried layer, the semiconductor substrate, and a PN junction layer therebetween, the first capacitance element and the second capacitance element being connected in series so that a circuit load based on the wiring capacitance can be reduced and the circuit can operate with a high speed. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Fig. 1(a) is a cross sectional view of an N channel MOS transistor device according to the prior art; Fig. 1(b) is a cross sectional view of a CMOS transistor device according to the prior art; Fig. 2 is a cross sectional view of an embodiment of CMOS transistor device according to this invention; and Figs. 3(a) ∼ (c) are cross sectional views showing a manufacturing process of a CMOS transistor device according to this invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Figs. 2 and 3, a preferred embodiment of this invention will be described in detail. Fig. 2 shows a structure of a CMOS transistor according to the invention as an embodiment, where the same elements as shown in Fig. 1 are indicated by the same reference numerals. An element added to the device shown in Fig. 1 is an N-well 11 which is an impurity buried layer having an opposite conductivity to the substrate 1 and is formed under the field oxide 7. In a CMOS device, another N-­ well 5 is formed in the device region X. The N-well 11 in the isolation region Y need not have the same impurity concentration, depth and width as those of the N-well 5. It is preferable to isolate the N-well 11 from the N-well 5, N⁺-buried layer 2 or other layers in order not to fix the potential of the N-well 11. In the above described structure, two capacitance elements are connected in series in the isolation region Y. The first capacitance element is composed of the conductive layer 10, the N-well 11, and the field oxide 7 therebetween, which corresponds to an intrinsic capacitance element caused by a wiring in the device according to the prior art. The second capacitance element is composed of the N-well 11, the substrate 1, and the PN junction layer therebetween, which is connected in series to the first capacitance element. Since the second capacitance element is connected in series to the first capacitance element, a total circuit load caused by a wiring capacitance is reduced than that of the prior device. For example, when the capacitance value of the second capacitance element is equal to that of the first capacitance element, the capacitance value of the total wiring capacitance becomes half that of the prior device. Fig. 3 shows a manufacturing process of the CMOS transistor device described in Fig. 2. First of all, as is shown in Fig. 3(a), a photoresist layer 12 is coated on a semiconductor substrate 1 and windows 13a and 13b are opened by a photo etching process in the regions where wells are to be formed later. In other words, the window 13a is located in a region corresponding to an N-­ well 11 and the window 13b is located in a region corresponding to an N-well 5. Then an impurity having an opposite conductivity to the substrate 1, for example phosphor, is implanted into the substrate 1 using the photoresist layer 12 as a mask. Next, the impurity is diffused by thermal treatment so as to form an N-well 11 and an N-well 5 with a predetermined concentration of an impurity and a predetermined depth as is shown in Fig. 3(b). After removing the photoresist layer 12, oxidation protecting layers 14, such as silicon nitride layers, are formed in only the device region X's on the substrate 1 by a photo etching process. By selective oxidation of the exposed surface of the substrate, which belongs to the isolation region Y, a field oxide 7 is formed as is shown in Fig. 3(c). Then the succeeding processes to construct a MOS transistor in the device region X are taken place in the same manner as in the prior method. Since the N-well 11 and the N-well 5 can be simultaneously formed in the same process, the producing method according to this invention can be carried out with the same steps as the prior method. It is not required to form the N-well 11 under every field oxide 7. It is enough to simply form the N-well 11 in some broad isolation regions in which a conductive layer 10 utilized to form later a wiring. Especially, when the N-well 11 is formed in the specific region called "a critical path", in which a special high speed operation is required for a device, a remarkable effect can be expected. The effect of the invention will be numerically considered as follows. In the prior device shown in Fig. 1(b), defining W L as the width of the wiring conductive layer 10, ε ox as the dielectric constant of the field oxide 7 and T f as the sum of the thickness of the field oxide 7 and the thickness of the protection layer 8, then a wiring capacitance C L per unit length is approximately represented by eq. (1). On the other hand, in the device shown in Fig. 2, that is, in the embodiment of this invention, defining T D as the thickness of the depletion layer caused by the PN junction between the N-well 11 and the substrate 1, W w as the width of the N-well 11 and ε s as the dielectric constant of the semiconductor, then a capacitance C D of the depletion layer per unit length is represented by eq. (2). Therefore the total wiring capacitance C T per unit length in the whole device shown in Fig. 2 is represented by eq. (3). Actual values of wiring capacitances C L and C T are calculated as follows. For example, supposing that the impurity concentration in the substrate 1 is equal to 10¹⁵cm⁻³ and that in the N-well 11 is equal to 10¹⁶∼¹⁷cm⁻³ then the thickness of the depletion layer T D becomes 1.2 µm. Thus substituting the value 1.2 µm into the variable W L , using values of 0.8 µm as T f and 2 µm as W w , which are typical values in a CMOS device, and substituting the actual values of dielectric constants in silicon oxide and silicon into ε ox and ε s , respectively, the following results are obtained through the calculation using eqs. (1) ∼ (3). C L ≈ 60 fF/mm, and C T ≈ 40 fF/mm. These results clearly show that the wiring capacitance in the device according to the invention shown in Fig. 2 becomes lower than that in the prior device shown in Fig. 1 by more than 30%. As described above, according to this invention, since an additional capacitance element is connected in series to the intrinsic wiring capacitance element in a semiconductor device, the total wiring capacitance can be reduced and the operation speed can be improved.

Claims: 1. Semiconductor device in which device regions (X) and isolation regions (Y) adapted to isolate said device regions (X) from each other are defined on a surface of a semiconductor substrate (1), said semiconductor device comprising field insulators (7) formed in said isolation regions (Y) and conductive layers (10) for wiring formed above said field insulators (7), characterized by comprising an impurity buried layer (11) having an opposite conductivity to said semiconductor substrate (1) under said field insulator, so as to form a first capacitance element composed of said conductive layer (10), said impurity buried layer (11), and said field insulator (7) therebetween, and a second capacitance element composed of said impurity buried layer (11), said semiconductor substrate (1), and a PN junction layer therebetween, said first capacitance element and said second capacitance element being connected in series. 2. Semiconductor device according to claim 1, wherein the semiconductor device comprises another impurity buried layer (5) having an opposite conductivity to the semiconductor substrate (1) in the device region (X), and said impurity buried layer (5) in the device region (X) is isolated from the impurity buried layer (11) in the isolation region (Y). 3. Semiconductor device according to claim 1, wherein the semiconductor substrate (1) is a P-type silicon substrate and the impurity buried layer (5, 11) is an N-­ type well layer. 4. Semiconductor device according to claim 3, wherein a CMOS transistor is formed in the device region (X). 5. Manufacturing process of a semiconductor device comprising device regions (X) and isolation regions (Y) to isolate said device regions (X) from each other on a semiconductor substrate (1), characterized by comprising the steps of forming a first impurity buried layer (5) in said device region (X) of said semiconductor substrate (1) and a second impurity buried layer (11) in said isolation region (Y) of said semiconductor substrate (1) so that said first and second impurity buried layers are isolated from each other and have an opposite conductivity to said semiconductor substrate (1), forming a field insulator (7) on said second impurity buried layer (11) in said isolation region (Y), forming individual integrated layers to constitute the device in said device region (X), and forming a conductive layer (10) for wiring said individual integrated layers on said field insulator (7). 6. Manufacturing process according to claim 5, wherein the semiconductor substrate (1) is a P-type silicon substrate and the impurity buried layer (5, 11) is an N-­ type well layer. 7. Manufacturing process according to claim 5, wherein the individual integrated layers constitute a CMOS transistor.

Inventors: NOGUCHI, TATSUO C/O TOSHIBA HIYOSHI RYO
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1986
Main classification: H01L29
IPC codes: H01L21, H01L23, H01L27, H01L29