MIS device with auxiliary gate

Abstract: In a MOSFET of an LDD structure, a side wall (51) is made conductive and connected to a gate (3) through a resis­ tance (13) thereby to cause hot carriers taken into the side wall (51) to be discharged to a gate (3) through the resistance (13), whereby a channel resistance is prevented from being increased by an effect of carriers accumulated in the side wall (51). As a result, a reliable MOSFET with a short channel can be provided which is not degraded even if it is used for a long time.

Description: Background of the Invention The present invention relates to a semiconductor device of a MIS or MOS structure such as a MOSFET or the like formed on a semiconductor substrate, and more specifically, to this kind of a highly reliable semiconductor device. Recently, the gate length of a semiconductor device is made very fine with the progress of a fine processing technology for the semiconductor device such as a MOSFET or the like, which permits the degree of integration and the performance of an integrated circuit composed of the MOSFET to be greatly improved. However, as the gate is made fine, various problems arise. These problems include, for example, reduced reliability such as a degradation and the like caused by a hot carrier effect, a reduction in a breakdown voltage between a source and a drain, a short channel effect and the like. They are very disadvantageous in a MOSFET having a gate length of 2 µm or less. The problem of the degradation due to the hot carrier effect arises especially in an n channel MOSFET, and then it will be described below with reference to the n channel MOSFET as an example. There is, for example, a MOSFET of an LDD (Lightly Doped Drain) structure as a conventional technology which is known best as a means to solve the problem of the degradation of the MOSFET caused by this hot carrier effect, the technology being proposed at the National Convention of the Institute of Electronics and Communication Engineers of Japan, 1978 (paper No. 270 of National Convention Report, April 1978). Patent Abstracts of Japan vol. 11, us. 296, (2843) [E-568] & JP-A-62 156 783 discloses an LDD MOSFET device having a gate electrode and sidewall spacers of doped polycrystalline silicon, the spacers being insulated and separated from the substrate and the gate by an insulating film and connected to the drain region and the source region, respectively. Fig. 1 shows a diagram of a structure of a MOSFET of the conventional technology, wherein reference numeral 1 designates a p type substrate, reference numeral 2 designates a gate oxide film, reference numeral 3 designates a gate, reference numeral 4 designates an n type low concentration layer, reference numeral 5 designates a side wall, reference numeral 6 designates a source or drain diffusion layer, reference numeral 7 designates a passivation film, and reference numeral 8 designates source and drain electrodes. The MOSFET of the LDD structure (referred to as an LDDMOSFET, hereinafter) is characterized in that an offset region of an n type low concentration layer 4 is provided between the source diffusion layer 6 or the drain diffusion layer 6 and a channel region formed on the lower layer of the gate 3. The LDDMOSFET is manufactured by forming an n type high concentration layer in such a manner that ions are implanted to form the n type low concentration layer 4, using as a mask the gate 3 provided on the gate oxide film 2 extending on the p type substrate 1 and then ions are implanted for the source and drain diffusion layers 6 after the side wall 5 has been formed by a silicon oxide film. With the LDDMOSFET, since the introduction of the n type low concentration layer 4 enables an electric field effect in the channel region in the vicinity of the drain to be eased and a width of a depletion layer extending in the direction of the channel region from the drain to be reduced, the hot carrier effect is prevented, a breakdown voltage between the source and the drain is increased, and the short channel effect is prevented. More specifically, the LDDMOSFET has a less, degradation caused by the effect of hot carriers and can establish higher reliability (prevention of malfunction) in a MOSFET with a shorter gate length. The LDDMOSFET according to the conventional technology, however, has a problem that when it is provided with a shorter gate length, it is subject to a serious degradation caused by hot carriers and cannot maintain reliability. Some models are proposed with respect to a mechanism of the degradation caused by the hot carriers and a degradation mechanism characteristic to the LDDMOSFET will be described with reference to drawings. Fig. 2A and Fig. 2B are diagrams explanatory of a degradation mechanism of the LDDMOSFET, wherein reference numerals 9, 10, 12 designate electrons, respectively, and reference numeral 11 designates a hole. Like reference numerals used in Fig. 1 describe similar or corresponding parts. A strong electric field exists at the drain end in the MOSFET, and as shown in Fig. 2A, an electron-hole pair composed of the electron 10 and the hole 11 is created by the electron 9 accelerated there and the electron passes over a potential barrier of the gate oxide film 2, and as shown in Fig. 2B, it advances into the side wall 5 to be caught. The side wall 5 becomes negatively charged by the caught electron 12 and electrons in the n type low concentration layer 4 are reduced in the vicinity of the gate oxide film 2, i.e., in the vicinity of the substrate surface, and thus a resistance of the portion is increased. Then, as described above, the LDDMOSFET according to the conventional technology has the problem that it has a limit in establishing reliability for hot carriers when it is provided with a shorter gate length, and when the gate has a certain length (0.5 µm) or a length less than it, the LDDMOSFET cannot establish the reliability. As shown in Japanese Patent Publication No. 62-156873, there is a method to connect a spacer (side wall) insulated and separated from a gate electrode to source and drain regions to solve the problem of the hot carriers, but with the method a capacity between a gate and a source or between the gate and a drain is increased and a circuit speed is reduced. Further, as shown in Japanese Patent Publication 62-122273, there is a method to integrally construct a gate electrode and a conductive side wall structurally or electrically, but this method increases a circuit delay. Summary of the Invention An object of the present invention is to provide a MOSFET semiconductor device having a larger hot carrier withstanding amount while maintaining a high speed as compared with an LDDMOSFET of the conventional technology. The present invention achieves the above object by the characteristics of claim 1. Brief Description of the Drawings The above and other objects, features and advantages of the present invention will be described hereinafter with respect to the accompanying claims and drawings, in which like reference numerals are used to describe similar parts throughout the several views of the invention, and wherein: Fig. 1 is a cross-sectional view of a structure of a MOSFET according to a conventional technology; Fig. 2A and Fig. 2B are cross-sectional views of a device explanatory of a degradation mechanism of an LDDMOSFET. Fig. 3 is a cross-sectional view of a structure of a MOSFET of an embodiment of the present invention; and Fig. 4 is a schematic view explanatory of operation of the present invention. Detailed Description of the Preferred Embodiment The present invention will be now described in detail with respect to drawings. According to the present invention a side wall of a gate electrode of a MOSFET is made conductive and is connected to the gate electrode through a resistance member so that electrons taken into the side wall can be discharged through the resistance member and the gate electrode. When a resistance of the resistance member is suitably set to a resistance of 1 x 10³ ohm to 1 x 10¹² ohm in the case, almost no effect is given by a parasitic capacitance existing between the side wall and an offset region (low concentration region) forming a portion of a drain diffusion layer. More specifically, when the resistance of the resistance member is selected to a value larger than a certain degree and a time constant by the resistance and the above parasitic capacitance is set longer than a cycle at which the MOSFET element actually operates, a delay of a circuit operation caused by the parasitic capacitance can be ignored. On the other hand, electrons as hot carriers are injected only when a potential of the gate is at a high level, and when the period is long, the electrons taken into the side wall are increased. However, since the side wall is made conductive and connected to the gate through the resistance member, the electrons taken in are gradually absorbed by the gate and not accumulated in the side wall, and thus a degradation such as a gm drop of a channel conductance of an LDDMOSFET or the like is not caused. Fig. 3 is a diagram of a structure of a MOSFET as an embodiment of a semiconductor device according to the present invention and Fig. 4 is a diagram explanatory of a mechanism capable of reducing a degradation by the present invention, wherein reference numeral 13 designates a resistance member, reference numeral 51 designates a side wall and other reference numerals are similar to those of the conventional technology shown in Fig. 1. Like the conventional LDDMOSFET, the MOSFET according to the present invention comprises a gate 3, source and drain diffusion layers 6 (2 x 10²⁰ cm⁻³), an offset region of an n type low concentration layer 4 (1 x 10¹⁸ cm⁻³) and the side wall 51, as shown in Fig. 3. The MOSFET is different from the LDDMOSFET of the conventional technology in that the side wall 51 has conductivity (the same degree with the gate electrode) and that the resistance member is formed between the side wall 51 and the gate 3. The resistance of the resistance member is set to a degree of 1 x 10³ ohm - 1 x 10¹² ohm and a material thereof is a silicon oxide film, a silicon nitride or a non-doped polysilicon having a thickness of several to several tens of angstroms. A polysilicon doped with n type impurities or the like can be used as a material for the side wall 51. The side wall may have a p type conductivity. For example, the structure of the MOSFET of this embodiment can be produced as follows. First, ions are implanted (dosing amount of phosphorus; 1 x 10¹³ cm⁻² at 50 Kev) using as a mask the gate 3 provided on a gate oxide film 2 extending on a p type substrate 1 to form an n type low concentration layer 4 (concentration: 1 x 10¹⁸ cm⁻³). The gate 3 is formed of a so-called polyside composed of a polysilicon or a polysilicon with a siliside layer added thereto. Next, the surface of the gate 3 is oxidized and provided with a silicon oxide film of several to several tens of angstrom to form the resistance member 13. Alternatively, the surface of the gate 3 may be provided with a silicon oxide film or a silicon nitride film by a chemical vapor phase reaction. Then, after a polysilicon doped with intrinsic or n type impurities has been deposited by a chemical vapor phase reaction, an anisotropic etching such as an RIE etching or the like is effected to form the side wall 51 on a side wall of the gate 3. Next, an ion implantation of n type impurities (dosing amount; 5 x 10¹⁵ cm⁻³) is effected by a self-aligning system using the gate 3 and side wall 51 as masks to form source and diffusion layers. At the time, an ion implantation of n type impurities (concentration; 2 x 10²⁰ cm⁻³) is also effected to the side wall 51 so that the side wall 51 is provided with a low resistance (the same degree as the gate electrode). The MOSFET of this embodiment is produced by the above processes. As described above, since the MOSFET according to the present invention makes the side wall 51 conductive and connects it to the gate 3 through the resistance member 13, electrons taken into the side wall 51 are externally discharged through the resistance member 13 and the gate 3. The reason why the resistance member 13 has a resistance of 1 x 10³ ohm - 1 x 10¹² ohm is to eliminate the effect of the parasitic capacitance (drop of a circuit speed) existing between the side wall 51 and the n type low concentration layer 4 on the operation an element so as not to cause the side wall 51 to operate as a gate. More specifically, as shown in Fig. 4, assuming that the resistance of the resistance member 13 is R₁ and the parasitic capacitance is C₁, the gate is 3 is connected to the n type low concentration layer 4 through a series circuit of the resistance R₁ and the parasitic capacitance C₁. When the resistance of the resistance member 13 is selected and a time constant of the resistance R₁ and the parasitic capacitance C₁ is made longer than a cycle at which the MOSFET actually operates (a figure or two figures larger than the cycle), a delay of the circuit operation can be ignored. In addition, since the electrons taken into the side wall 51 are not accumulated in the side wall 51 and discharged through the resistance R₁, a degradation such as a drop of gm is not caused. Although the present invention is described above with respect to the example of the n channel MOSFET, the present invention can be applied to a p channel MOSFET by changing the conductivity of the impurities. Since the above side wall 51 has a conductivity, it is thought to be a gate electrode from a view point that it is provided through a passivation film, and then it may be assumed that this embodiment of the present invention has a structure comprising first, second, and third gate electrodes (the first and third gate electrodes are the side wall 51), and the first and third electrodes are connected to the second gate electrode through the resistance member, respectively. As described above, since the present invention has such a simple structure that the resistance member 13 is interposed between the conductive member 51 (also referred to as a conductive side wall) formed on the low concentration (1 x 10¹⁸ cm⁻³) impurity region, it can be easily achieved. In addition, a MOSFET with a short channel of 0.5 µm or less can be provided without lowering a power voltage (e.g., to lower it from 5V to 3V) or degrading a circuit performance. According to the present invention, as described below, the side wall in an LDDMOSFET is provided with a conductivity and the side wall is connected to the gate through a resistance member to discharge hot carriers taken into the side wall, and thus an increase in a resistance a low concentration impurity layer interposed between a drain diffusion layer and a channel region can be restricted. As a result, a reliable MOSFET with a short channel can be provided which is not degraded even if it is used for a long time.

Claims: A semiconductor device having: two impurity regions (6) of a first conductivity type formed in a semiconductor region (1) of a second conductivity type opposite to said first conductivity type, a gate electrode (3) formed on a gate insulator film (2) over at least a region between said two impurity regions (6) on a main surface of said semiconductor region (1), wherein each of said two impurity regions (6) has a predetermined low impurity concentration region (4) in the vicinity of said gate electrode (3), and a conductive member (51) is provided on said gate insulator film (2) over said predetermined low impurity concentration region (4), characterized in that said conductive member (51) is connected to said gate electrode (3) through a resistance (R1, 13) having a predetermined resistance value of 1 x 10³ ohm to 1 x 10¹² ohm. A semiconductor device according to claim 1, wherein said conductive member (51) and said gate electrode (3) are composed of the same material. A semiconductor device according to claim 1, wherein said conductive member is composed of a polycrystal semiconductor. A semiconductor device according to any one of claims 1, 2 and 3, wherein said resistance (R1, 13) is composed of a semiconductor oxide.

Inventors: MINAMI MASATAKA, NAGANO TAKAHIRO, WAKUI YOUKOU, MINAMI, MASATAKA, NAGANO, TAKAHIRO, WAKUI, YOUKOU
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1989
Main classification: H01L29
IPC codes: H01L21, H01L29