MISFET WITH LIGHTLY DOPED DRAIN AND METHOD OF MANUFACTURING THE SAME

Abstract: The invention provides a semiconductor device and a manufacturing method therefor. The semiconductor device comprises a semiconductor substrate (1) of a first conductivity type, source and drain regions (9, 10) of a second conductivity type provided on the surface of the substrate in a mutually elec­ trically isolated arrangement, and a gate electrode (4) provided on the surface of the substrate on the other side of an insulating film. The substrate surface includes a channel region between the source and drain regions; at least the drain region, of the source and drain regions, consisting of a first impurity diffusion region (5₁, 5₂) provided on the substrate surface in the vicinity of an end portion of the gate electrode (4); a second impurity diffusion region (6₁, 6₂) more highly doped than the first impu­ rity diffusion region and provided in at least a portion of the sur­ face of the first impurity diffusion region (5₁, 5₂) adjacent an end portion of the first impurity diffusion region (5₁, 5₂) and a third impurity diffusion region (8₁, 8₂) more highly doped than the second impurity diffusion region and provided on the sub­ strate surface adjacent to an end portion of the second impurity diffusion region (6₁, 6₂). The first impurity diffusion region (5₁, 5₂) is formed using the gate electrode (4) as a mask and the second and third impurity regions (6₁, 6₂, 8₁, 8₂) are formed using self-alignment techniques.

Description: This invention relates to a MISFET device comprising a drain region of improved structure. Recently, the so-called LDD (light doped drain) structure of the drain region has been developed. This ia a two-layer structure consisting of lightly doped and heavily doped impurity diffusion regions, and was developed as a means of promoting a high degree of circuit integration of MIS type semiconductor devices (e.g. MOS type semiconductor integrated circuits), and of increasing their breakdown voltage. This is achieved by lowering the strength of the high electric field set up in the vicinity of the drain region as a result of miniaturization of such transistors. A MISFET device which corresponds to the pre-characterising portion of claim 1 is disclosed in US-A-4172260. However, while the lightly doped impurity diffusion region suppresses formation of hot carriers by lowering the electric field near the drain junction, the surface of the lightly doped impurity region easily becomes depleted by the electric field set up in the insulating films such as the gate insulating film by the action of the surface state or the trapped charges induced by hot carriers. This in turn results in an increase in the parasitic resistance due to the lightly doped impurity region, with the result that the current drive capacity of transistors of LDD structure undergoes a characteristic deterioration. The drain electric field lowering effect in the LDD structure increases with decreasing concentration of the dopant in the lightly doped impurity diffusion region, but the aforementioned characteristic deterioration also increases with decreasing concentration of the dopant and this produces the problem that the range within which the dopant concentration of the lightly doped impurity region can be selected is restricted by these mutually opposed requirements. An object of this invention is to provide a high-performance MISFET semiconductor device of high reliability with LDD structure, wherein the drain electric field strength lowering effect of the lightly doped impurity region is maintained but lowering of he current drive capacity due to an increase in the parasitic resistance produced by the diffusion regions is prevented. According to one aspect of this invention, there is provided a MISFET semiconductor device comprising: a semiconductor substrate of a first conductivity type; source and drain regions of a second conductivity type, provided in a mutually electrically isolated arrangement on the surface of said substrate; a gate electrode, provided on the surface of said substrate, on the other side of an insulating film, which surface of said substrate includes a channel region between said source and drain regions; at least, said drain region, of said source and drain regions, comprises of: a first impurity diffusion region provided in said substrate surface and positioned in the vicinity of an end portion of said gate electrode; characterised in that said drain region further comprises: a second impurity diffusion region, more highly doped than said first diffusion region; and provided inside of said first impurity region; and a third impurity diffusion region, more highly doped than said second impurity diffusion region, and provided deeper than said first impurity region and adjacent to an end portion of said first and second impurity region. In this manner, the invention makes it possible to obtain high-performance MISFET semiconductor devices of high reliability, wherein the drain electric field lowering effect of the lightly doped impurity diffusion region of the LDD structure is maintained but the lowering of the current drive capacity due to increased parasitic resistance produced by the diffusion region is prevented. Embodiment of this invention will now be described with reference to the accompanying drawings, in which:- Figs. 1 to 4 are cross-sectional views illustrating the process for manufacturing an embodiment of an n-channel MISFET semiconductor device of LDD structure according to this invention; Fig. 5 is a graph showing the doping profile at the boundary surface between the gate oxide film and the diffusion layer; Fig. 6 and 7 are graphs illustrating the effect of this invention. An embodiment of an n-channel MOS transistor having an LDD structure according to this invention will be described below together with its method of manufacture, with reference to Figs. 1 to 4. As shown in Fig.1, a field oxide film 2 of 700 Angstrom thickness is first produced on a p type silicon substrate 1 by the method of local oxidation, followed by thermal oxidation treatment to form a gate oxide film 3 of 250 Angstrom thickness on substrate 1, separated from the field oxide film 2 and forming an island (10Å=1nm). A polycrystalline silicon film of 4000 Angstrom thickness is then deposited over the whole surface, phosphorus diffusion carried out in a POCl₃ atmosphere to P-dope the polycrystalline silicon film, thus lowering its resistance, after which patterning is, performed by the photo lithography technique to form gate electrode 4. Next, as shown in Fig.2, using the field oxide layer 2 and gate electrode 4 as a mask, and applying an accelerating voltage of 40 keV, phosphorus, which has a comparatively large diffusion coefficient, was introduced by ion implantation in a dose of 2 X 10¹³ ions cm⁻², followed by ion implantation of arsenic, which has a comparatively small diffusion coefficient, in a dose of 1 X 10¹⁴ ions cm⁻², by applying an accelerating voltage of 30 keV. This was followed by the activation of the lightly doped n⁻ type diffusion regions (first diffusion regions) 5₁ and 5₂, self-aligned with respect to the gate electrode 4 on the substrate 1, and formation of the n type diffusion regions (second diffusion regions) 6₁ and 6₂ , which are more highly doped than the diffusion regions 5₁ and 5₂. As shown in Fig.3 a SiO2 film of 2000 Angstrom thickness was then deposited over the whole surface and the whole surface was then etched by the reactive ion etching method forming a wall member 7 on the lateral face of the gate electrode 4. The field oxide film 2, the gate electrode 4 and the wall member 7 were then used as mask and arsenic ions were implanted in a dose of 5 X 10¹⁵ ions cm⁻² , using an accelerating voltage of 40 keV. This was followed by activation involving heat treatment for 60 minutes at 900°C, and n + type diffusion regions (third diffusion regions) 8₁ and 8₂, which are more highly doped than the n type diffusion regions 6₁ and 6₂, were then formed in a self-alignment manner with respect to the wall member 7, on the surface of the substrate 1. The different diffusion regions may be activated either individually or all together. In this way, the source region 9 is formed from the n⁻ type diffusion region 5₁, the n type diffusion region 6₁ and the n + type diffusion region 8₁, and the drain region 10 from the n⁻ type diffusion region 5₂, the n type diffusion region 6₂, and the n + type diffusion region 8₂. As shown in Fig.4 a CVD-SiO2 film 11 was then deposited on the whole surface, contact holes 12 were formed in the CVD-SiO2 film 11 and the gate oxide film 3 by photoetching and this was followed by formation of an aluminium film by evaporation, carrying out patterning to form the aluminium distribution wires 13 and 14, connecting the source region 9 and the drain region 10 via the contact holes 12. In this manner an n-channel MOS transistor was produced. Fig. 5 shows a graph of the concentration of impurities at the interface between the gate oxide film and the diffusion layer (ordinate) vs, the position from the gate edge on the semiconductor substrate surface (abscissa). MOS transistors according to the invention thus have an LDD structure as illustrated in Fig. 4, comprising a source region 9 and a drain region 10, consisting of n⁻ type diffusion regions 5₁ and 5₂ provided in the surface of the substrate 1 at positions close to the end portions of the gate electrode 4, n type diffusion regions 6₁, and 6₂, provided on the surface of substrate 1 at positions separated from the end portions of the gate electrode 4, the n type diffusion regions 6₁ and 6₂ having higher concentrations than the n⁻ type diffusion regions 5₁ and 5₂, and n + type diffusion regions 8₁ and 8₂ which have a higher concentration than the n type diffusion regions 6₁ and 6₂. Formation of hot carriers by the n⁻ type diffusion region 5₂ of the drain region 10 can thus be prevented by a lowering of the drain electric field strength. Furthermore, the influence of an electric field set up in the gate oxide film 3 by the action of the surface state or trapped charges induced by hot carriers which would lead to depletion of the n⁻ type diffusion region 5₂ can be prevented by provision of n type diffusion region 6₂ on its surface and this in turn makes it possible to overcome the problem of a lowering of the electric field drive capacity. The conditions applied for the formation of the n⁻, n and n + type impurity diffusion regions (first, second and third diffusion regions) are not limited to the above embodiment, but may be varied freely provided they remain within a range making it possible to attain the objects of the invention. The embodiment discussed above relates to an n-channel MOS transistor, but this invention is equally applicable to p- channel MOS transistors, CMOS transistors or to other MIS type transistors such as MNOS and the like, in which materials other than oxide films are used as gate insulating films. As described in detail above, this invention makes it possible to obtain high-performance MISFET semiconductor devices of high reliability by maintaining the drain electric field lowering effect exerted by the lightly doped diffusion regions of the LDD structure, while preventing the lowering of the current drive capacity produced by the increase in the parasitic resistance due to the diffusion regions. A comparison of the process of the invention with conventional techniques will now be made with the aid of Fig. 6 and Fig. 7. In Fig. 6, the stress time, which is the accumulated time for all the intervals, is plotted along the abscissa and the drain current reduction (percentage change) along the ordinate. The measuring conditions employed to obtain this graph are given below. The stress conditions were that DC current biases were applied as follows: gate voltage VG = 3V, drain voltage VD = 7.5 V and substrate voltage = 0 V. This DC current bias due to this stress condition is applied to the device for a specified timme, for example 10 seconds, 20 seconds and so forth. And for every accumulated stress time, the drain current reduction is measured under the condition of applying gate voltage of 3 V, a drain voltage of 0.1 V, and a substrate voltage of 0 V. The adverse effect represented by the drain current reduction increases in the direction of the ordinate i.e. upwards in the graph. In the Figure, the symbol LDD denotes a device with conventional LDD structure and PLDD a semiconductor device according to this invention. The dose of P ions implanted in the second diffusion layers was fixed as 2 X 10¹³ cm⁻² and the As ion does introduced into the third diffusion layers at 2 X 10¹³, 5 X 10¹³, and 1 X 10¹⁴ ions cm⁻² for the case where source and drain regions 10¹⁴ ions cm⁻² for the case where source and drain regions were formed. Fig. 6 clearly shows that the reduction of the drain current is significantly less than that shown by the conventional device. Fig. 7 shows the results of measuring the reduction in the drain current (percentage change) when varying the drain bias, fixing the stress time at 10⁴ sec. In this graph, the substrate current / channel width (micro ampere / micrometer) under the condition of applying the stress bias is plotted along the abscissa and the drain current reduction along the ordinate. The measurements were carried out using a channel width W = 10 micrometers. In this Figure too, LDD signifies a semiconductor device of conventional structure and PLDD a semiconductor device according to this invention. CONV relates to a semiconductor device containing source and drain but with a conventional n + type diffusion region only. Hereinbelow these device are referred to respectively as LDD, PLDD and CONV. The dose of phosphorus implanted in the second diffusion layers of LDD and PLDD was 2 X 10¹³ ions cm⁻² and the doses of arsenic ions implanted in the third diffusion layers of PLDD were 2 X 10¹³, 2.5 X 10¹³, and 1 X 10¹⁴ ions cm⁻², these being the conditions used to form the source and drain regions. Here, the substrate current is equivalent to the amount of hot carriers formed. From Fig. 11, it is clearly seen that under conditions giving rise to formation of a given amount of hot carriers (same substrate current), the reduction shown by LDD is greater than that given by CONV. The semiconductor device PLDD according to this invention, however, gives a considerably decreased drain current reduction compared with LDD, for a given amount of hot carriers.

Claims: A MISFET device comprising: a semiconductor substrate (1) of a first conductivity type; source and drain regions (9, 10) of a second conductivity type, provided in a mutually electrically isolated arrangement on the surface of said substrate (1); a gate electrode (4), provided on the surface of said substrate (1), on the other side of an insulating film (3), which surface of said substrate includes a channel region between said source and drain regions (9,10); at least said drain region, of said source and drain regions (9,10). comprises: a first impurity diffusion region (5₁ , 5₂) provided in said substrate surface and positioned in the vicinity of an end portion of said gate electrode (4);characterised in that at least said drain region further comprises: a second impurity diffusion region (6₁ , 6₂), more highly doped than said first diffusion region (5₁ , 5₂) and provided on the substrate surface and inside of said first impurity region (5₁ ,5₂); and a third impurity diffusion region (8₁ , 8₂), more highly doped than said second impurity diffusion region (6₁ ,6₂), and provided at a position on said substrate surface and being region (5₁ , 5₂) and adjacent to an end portion of region (5₁, 5₂) and adjacent to an end portion of said first and second impurity region (5₁, 5₂, 6₁, 6₂). A MISFET device according to claim 1, wherein said first and second impurity diffusion regions (5₁, 5₂ ; 6₁, 6₂) are formed in a self-alignment manner with respect to said gate electrode (4) through the use of dissimilar diffusion coefficients. A method of manufacturing a MISFET device comprising the steps of: selectively forming a field insulating film (2) on a semiconductor substrate (1) of a first conductivity type; and forming a gate insulating film (3) on said semiconductor substrate surface, followed by formation of a gate electrode (4); characterised by: forming a first impurity diffusion region from a first impurity (5₁, 5₂) of a second conductivity type on said semiconductor substrate surface, using said gate electrode (4) as a mask: forming a second impurity diffusion region from a second impurity (6₁, 6₂), of the second conductivity type and having a higher concentration than said first impurity diffusion region (5₁, 5₂), in said semiconductor substrate surface, using said gate electrode (4) as a mask, whereby said second impurity has a smaller diffusion coefficient than said first impurity; and forming a first insulating film (7) on the lateral faces of said gate electrode (4), followed by formation of a third impurity diffusion region (8₁, 8₂) of second conductivity type which is deeper than said first impurity diffusion region (5₁, 5₂) and has a higher concentration than said second impurity diffusion region (6₁, 6₂), in said semiconductor substrate surface, using said first insulating film as a mask. A method of manufacturing a MISFET device according to claim 3, wherein said first impurity diffusion region (5₁, 5₂) is formed by implantation of phosphorus ions. A method of manufacturing a MISFET device according to claim 6, wherein said second and third impurity diffusion regions (6₁, 6₂, 8₁, 8₂) are formed by implantation of arsenic ions.

Inventors: TOYOSHIMA, YOSHIAKI C/O PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1985
Main classification: H01L29
IPC codes: H01L21, H01L29