Abstract:
A vertical-conduction and planar-structure MOS device having a double thickness gate oxide includes a semiconductor substrate including spaced apart active areas in the semiconductor substrate and defining a JFET area therebetween. The JFET area also forms a channel between the spaced apart active areas. A gate oxide is on the semiconductor substrate and includes a first portion having a first thickness on the active areas and at a periphery of the JFET area, and a second portion having a second thickness on a central area of the JFET area. The second thickness is greater than the first thickness. The JFET area also includes an enrichment region under the second portion of the gate oxide.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates in general to vertical-conduction MOS power devices and to a method for forming the same on a commercial scale and with a high integration density. More specifically, the present invention relates to a vertical-conduction and a planar-structure MOS device with a double thickness gate oxide.  
         [0002]     The present invention also relates to a process for forming vertical-conduction MOS power devices starting from corresponding planar MOS structures. Even more specifically, the present invention relates to a method for reducing the capacitance of MOS devices of the type concerned, and operating simultaneously and in a self-aligned way on corresponding planar MOS structures.  
       BACKGROUND OF THE INVENTION  
       [0003]     In a VDMOS device ( FIG. 1 ) the resistive component due to the JFET area that is created between body wells limits an increase in the packing density, and thus an improvement in the static and dynamic performances of the device. The JFET resistance depends not only on the epitaxial layer resistivity, but also on the distance between the two adjacent body wells. The capacitances associated with the gate oxide, and thus the charge storage capacity of the gate region depend, in an opposite sense, on this distance.  
         [0004]     Improving the output resistance by increasing the distance between the body wells means penalizing the dynamic features of the device. This problem occurred up to now mainly in high voltage MOS devices (˜500V) because of the high epitaxial layer resistivity (˜20Ω/cm), while in low voltage devices (30-60V) the problem is considered almost negligible because of the low epitaxial layer resistivity (&lt;1Ω/cm).  
         [0005]     As far as PMOS devices are concerned, particularly low voltage PMOS devices, higher and higher integration densities (i.e., scaling down) and the formation of submicron channel lengths, although obtained with other known technologies, led to a considerable reduction of the output resistance components. This is due to the channel and to a greater focus on the component due to the JFET and to the need to reduce capacitances.  
         [0006]     By way of example of this trend, the following TABLE 1 shows a comparison between the relative weight of the output resistance components of a 30-volt P-channel device.  
                           TABLE 1                                   Output resistance components for a 30-   Relative           volt P-channel PMOS device   weight                           Die-package connection   25%           Substrate contribution   12%           Epi layer contribution   12%           Contribution of the JFET between body   33%           wells           Component due to the channel   18%           Ron   100%                       
 
         [0007]     To preserve the dynamic features of a PMOS device, and improve in parallel the output resistance thereof, methods based on the reduction of the distance between body wells have been implemented. These prior art methods essentially provide either the surface resistivity modification ( FIG. 2 ) or the following technology change: a) surface enrichment between body wells to locally reduce resistivity (U.S. Pat. Nos. 4,376,286 and 4,974,059); b) use of a low-resistivity surface epitaxial layer; and c) use of trench technology.  
         [0008]     The methods a) and b) applied to low voltage PMOS devices involve known technical drawbacks linked to the influence of the enriched layer on the channel. In fact, in a low voltage PMOS structure the drain is characterized by an epitaxial layer whose concentration ranges between 1 and 5*10 16  at/cm 3 , and gate oxides, because of the low threshold voltage (driving with Vg&lt;4.5V) and the low capacitances required by the applications, have a thickness not lower than 200 Å and peak concentrations in the channel do not exceed 10 17  at /cm 3 .  
         [0009]     This means that the highest concentration in the channel is higher by almost an order of magnitude than the epitaxial layer concentration ( FIG. 3 ). Consequently, as soon as attempts are made to enrich uniformly the whole surface, channel features (e.g., threshold voltage and channel length) are significantly changed which jeopardizes function of the PMOS device. This phenomenon is known as premature punch through.  
         [0010]     With the third mentioned method, point (c), the JFET component can be eliminated. However, the capacitance problem is not solved since in most cases the area whereon the gate oxide rests is increased, as shown by the comparison between the two gate structures represented in  FIG. 4 . In the case of a planar structure VDMOS the gate area is proportional to the distance L (1.5-4 μm), while in the case of a trench technology device it is proportional to the sum of the double depth and of the trench width (2h+1˜2.5-4 μm).  
       SUMMARY OF THE INVENTION  
       [0011]     An object of the present invention is to provide a method for forming vertical-conduction MOS power devices having improved static and dynamic performances with respect to prior art devices, as well as being capable of reducing the capacitances of these devices for the same static performances.  
         [0012]     This and other objects, advantages and features in accordance with the present invention are provided by using a double thickness gate oxide comprising a thin layer in the channel region and a thicker layer on inactive areas. This provides an enrichment of the JFET region to reduce the distance between the body regions and to increase the integration area, which reduces the extension of the gate oxide surface.  
         [0013]     More particularly, the method for forming vertical MOS devices of the type concerned comprises the steps of forming on a planar MOS structure a double thickness for the gate oxide comprising a thin layer in the channel region of the structure and a thicker layer in the inactive areas thereof. The method may further comprise and enriching, simultaneously and in a self-aligned way, the JFET area to reduce the distance between two adjacent body wells. This reduces the gate oxide surface and allows for an increased packing density. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The features and advantages of the method according to the present invention will be apparent from the following description of an embodiment thereof given by way of non-limiting examples with reference to the attached drawings.  
         [0015]      FIG. 1  is a vertical-section and enlarged schematic view of a prior art PMOS electronic device with the current flow direction during operation being highlighted;  
         [0016]      FIG. 2  is a vertical-section and enlarged schematic view of a prior art surface-enriched PMOS electronic device;  
         [0017]      FIG. 3  is a diagram showing concentration vs. channel width between the PMOS devices of  FIGS. 1 and 2 , and in particular, showing the surface enrichment influence on the concentration profile along the channel;  
         [0018]      FIG. 4  is a schematic diagram comparing the planar structure with the trench structure in an intermediate step, both formed according to the prior art, for illustrating the typical magnitudes on which the surface covered by the gate oxide depends;  
         [0019]      FIG. 5  is a vertical-section and enlarged schematic view of a vertical MOS electronic device underlining the specific features introduced by the present invention, e.g., oxide thickening and a corresponding enrichment of the epitaxial layer between body wells;  
         [0020]      FIGS. 6   a ,  6   b  and  6   c  are vertical-section and enlarged schematic views of the PMOS device according to a first method of the invention illustrating different steps of the manufacturing method;  
         [0021]      FIGS. 7   a ,  7   b  and  7   c  are vertical-section and enlarged schematic views of the PMOS device according to a second method of the invention illustrating different steps of the manufacturing method; and  
         [0022]      FIG. 8  is a diagram illustrating the vertical section of a transistor at an intermediate step according to the invention obtained by simulations. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     With reference to the figures, and particularly to the examples of  FIG. 5 , a MOS transistor electronic device, particularly of the PMOS type and according to the method of the present invention, is schematically indicated. The process steps and the structures described below do not form a complete process flow for manufacturing integrated circuits. In fact, the present invention can be implemented together with the integrated circuit manufacturing techniques presently used in this field, but only those commonly used process steps necessary to understand the invention are described.  
         [0024]     The figures representing cross sections of portions of an integrated circuit during the manufacturing are not drawn to scale. Instead, they are drawn to show the important features of the invention.  
         [0025]     The transistor device  1  is obtained from a semiconductor substrate  2  covered by an oxide protective layer. According to a preferred embodiment, described with reference to  FIGS. 6   a  to  6   c , the method of the present invention is characterized in that it comprises the following steps.  
         [0026]     The active areas  4  of the transistor  1  and a channel region between them are defined, in a conventional way, by opening the oxide protective layer on the substrate  2 . A pad oxide  5  is grown on the active areas  4  and on the channel at a thickness of about 100-500 A. A gate region according to the method of the invention will be formed on the channel. A nitride layer  6  is deposited on the oxide layer  5 . The thickness of this nitride layer  6  can reach 300-900 A. A photomasking step is now provided to define the inactive areas  11  which are to have a thicker oxide layer. This step is implemented by a photoresist layer  7 . An etching step is performed to remove the nitride layer  6  on the channel to expose the oxide layer  5 , as shown in  FIG. 6   a.    
         [0027]     A further implant step allows an enrichment of a central area of the channel  9  underlying the area  11  to be formed with P or As ions for an N-channel transistor, and with B or Al ions for a P-channel transistor. The implant energy can be regulated between 60-500 KeV while ion doses can range from 1E12 and 1E13 ions/cm 2 . The resist layer  7  is then removed. The following step provides the growth of a first gate oxide layer  8  of variable thickness between 800 and 3000 A. This step can provide a possible dopant “drive in”, as shown in  FIG. 6   b . A wet etching of the nitride layer  6  is performed with H 3 PO 4  acid, or some other convenient method is performed.  
         [0028]     A selective wet etching with HF acid allows the oxide layer  5  on the active areas  4  to also be removed. The etching time is such to remove the whole pad oxide layer  5 , but without removing the first gate oxide layer  8 . The greater the difference between the two oxide thicknesses results in the margins of the etching times being the widest.  
         [0029]     A sacrificial oxide layer of about 100-600 A is then formed. This sacrificial oxide layer is immediately wet-etched with an HF acid to expose the gate region of the transistor  1  on the channel but at the periphery of the area  11 . In this case too, the etching time is such to remove the whole sacrificial oxide but without removing the first gate oxide layer  8 . The greater the difference between the two oxide thicknesses results in the margins of the etching times being the widest.  
         [0030]     The method continues with the growth of a second gate oxide layer having a variable thickness ranging from 100 to 1000 A. A conductive layer  12 , for example, polycristalline silicon (i.e., poly) is deposited on the gate region, which is doped to change the conductivity.  
         [0031]     The obtained MOS transistor structure can be completed with further process steps continuing according to the prior art and leading to define traditional body and source wells housing the transistor active areas  4 , as shown in  FIG. 6   c . These body wells can advantageously be formed in accordance with the present invention with a reduced mutual distance.  
         [0032]     In accordance with a second preferred embodiment, described with reference to  FIGS. 7   a  to  7   c , the method of the present invention is characterized in that it comprises the following operative steps.  
         [0033]     The active areas  4  of the transistor  1  and a channel region between them are defined in a conventional way by opening the oxide protective layer on the substrate  2 . The oxide layer  5  is grown on the active areas  4  and on the channel for the thickness required for the device to correctly operate (100-1500 A).  
         [0034]     A polysilicon layer  13  having a thickness lower or equal to half the thickness of a portion of the thicker oxide layer  5   a  is deposited. A nitride layer  6  of 300-900 A is deposited on the polysilicon layer  13 . The inactive areas  11 , which have a thicker oxide layer, are defined using photomasking. The nitride layer  6  is etched to remove the same on the channel, as shown in  FIG. 7   a , to expose the polysilicon layer  13 .  
         [0035]     Further implanting is performed to form the enrichment region  9  in the channel central area underlying the area  11  using the photoresist as an implant window. The implant can be performed with P or As ions for an N-channel transistor, and with B or Al ions for a P-channel transistor. The implant energy can be regulated between 60-500 KeV while ion doses can range from 1E12 and 1E13 ions/cm 2 .  
         [0036]     The photoresist is removed and the polysilicon not covered by the nitride layer  6  is completely oxidized, as shown in  FIG. 7   b . The nitride layer  6  is then etched. A conductive layer  12  is deposited to form the gate electrode. For example, polycristalline silicon, i.e., poly, which is doped to change conductivity is deposited.  
         [0037]     The obtained MOS transistor structure can be completed with further process steps continuing according to the prior art, while defining traditional body and source wells housing the transistor active areas  41  as shown in  FIG. 7   c . These body wells can advantageously be formed in accordance with the present invention with a reduced mutual distance. It is also possible to advantageously use a metal layer (for example, Ti, W or Co) as a conductive layer  12  to form the gate electrode.  
         [0038]     The previous detailed descriptions show how the transistor obtained with the method according to the invention is a planar MOS device with a double thickness gate oxide, i.e., with a thinner oxide layer in the channel area and a thicker oxide layer in the central channel inactive area. In particular, the final MOS device comprises a gate oxide layer having thin oxide side portions  5   a  and a thick oxide central portion  5   b . This thick oxide central portion  5   b  is in correspondence with the channel central inactive area.  
         [0039]     According to the alternative embodiment of the method according to the invention, the final MOS device comprises a polysilicon layer  13  on the side portions  5   a  of the gate oxide layer. The enrichment region  9  in the JFET area underlying and self-aligned with the thicker oxide central portion  5   a  allows the distance between two adjacent body wells, and thus the gate oxide surface to be reduced. This allows an increased integration or packing density of the transistors to be obtained.  
         [0040]      FIG. 8  shows the vertical section of a transistor formed according to the invention. This section has been obtained by process simulations for illustrating the gate oxide thickening and the enrichment layer position.  FIG. 8  also shows the concentration profile along the central line between two body wells.