Abstract:
A method for making a MOSFET device (20) in a semiconductor body (10) includes the step of forming source and drain contact electrodes (12.1, 12.2) prior to growth of the gate oxide (10.3) and after formation of a high conductivity surface region (10.5). The exposed mutually opposing sidewall edges of each of the contact electrodes (12.1, 12.2) are coated with a sidewall silicon dioxide layer (15.1, 15.2), and the then exposed surface of the semiconductor body (10) between these sidewalls is etched to depth beneath the high conductivity surface region (10.5) in order to separate it into the source and drain regions (10.1, 10.2). 
     Formation of the high conductivity region may be omitted by using Schottky barrier or impurity doped material for the contact electrodes (12.1, 12.2).

Description:
Field of the Invention 
     This invention relates to the field of semiconductor device fabrication, and more particularly to methods for fabricating metal oxide semiconductor field effect transistor (MOSFET) devices. 
     BACKGROUND OF THE INVENTION 
     It is known in the art of semiconductor transistor devices that short channels are desirable in MOSFET devices, in order to achieve high frequency operation of the order of GHz. By &#34;short channels&#34; is meant those with source-to-drain separations of less than about 2 microns. 
     In the fabrication of short channel MOSFET devices with polycrystalline silicon gate electrodes, it is difficult to control the length of the polycrystalline silicon (&#34;polysilicon&#34;) layer defining the self-aligned gate and hence the source-to-drain separation and the channel length: This length is thus subject to fluctuations from device to device in the usual mass fabrication techniques in which many such devices are simultaneously fabricated at a major surface of a single crystal semiconductive silicon body. Specifically, the actual channel length is so small in many of the devices whose channels are intended to be a micron in length that undesirable &#34;punch-through&#34; or &#34;reach-through&#34; of the depletion region of the drain to the source occurs during operation; thereby, device performance is severely degraded. Moreover, the electrical resistance between the source terminal and the semiconductor region controlled by the gate, as well as the electrical resistance between the drain and the load, tends to be excessive, especially in those devices having shallow junctions which are desirable to minimize &#34;punch-through&#34; or &#34;reach-through&#34; effects. 
     In a paper by S. Nishimatsu et al., entitled &#34;Grooved Gate MOSFET&#34;, published in Japanese Journal of Applied Physics, Vol. 16 (1977), Supplement 16-1, pp. 179-183, at page 181, a method for making a MOSFET device characterized by a grooved gate is disclosed, using a first polysilicon layer for the source and drain contact electrodes and a second polysilicon layer for the gate electrode. These electrodes are mutually insulated from each other by a silicon dioxide layer which is grown on the first polysilicon layer prior to deposition of the second polysilicon layer. Such a device, however, requires the use of a maskless photolithography operation which becomes impractical at channel lengths less than about 2 microns. As a consequence of this limitation, the distance from source-to-drain cannot be made as small as desired, such as 0.5 micron to within ±5%, and the breakdown voltage tends to be unduly low. Moreover, the use of polysilicon as source and drain electrode material results in undesirably high source and drain access resistances. 
     It would, therefore, be desirable to have a method for making short-channel devices that alleviates these problems of the prior art. 
     SUMMARY OF THE INVENTION 
     This invention is for a method of fabricating a semiconductor device (20) having a pair of spaced-apart source and drain contact electrodes (12.1, 12.2) contacting spaced-apart surfaces of source and drain regions (10.1, 10.2), respectively, at a top surface of a semiconductor body (10) and having a gate oxide layer (10.3) grown on a first portion of the top surface characterized by the step of depositing, prior to growth of the gate oxide layer (10.3), a sidewall insulating layer (15.1, 15.2) on an exposed edge of each of the contact electrodes (12.1, 12.2) located on mutually opposing sidewalls of these contact electrodes followed by the step of thermally growing the gate oxide layer (10.3) on the exposed first portion of the top surface of the body extending between the mutually opposed extremities of the sidewall insulating layers (15.1, 15.2). The contact electrodes (12.1, 12.2) are essentially metal, metal-silicide, or silicon rich metal-silicide. Moreover, another electrode layer (16), typically polysilicon, can be deposited on both the gate oxide layer (10.3) and the sidewall insulating layers (15.1, 15.2), in order to provide a gate electrode (16.3) after suitable patterning of this electrode layer (16). Further, the sidewall insulating layers (15.1, 15.2) advantageously extend to other sidewalls (22.1, 22.2) of the source and drain contact electrodes (12.1, 12.2) removed from the mutually opposing sidewalls, in order to insulate the gate electrode (16.3) from the source and drain contact electrodes (12.1, 12.2) extending to these other sidewalls (22.1, 22.2). 
     In one specific embodiment, a top surface region (10.5) of body (10) is doped with impurities suitable for forming source and drain regions, each of the contact electrodes (12.1, 12.2) is formed together with its corresponding sidewall insulating layer (15.1, 15.2) and an insulating layer (13, 14) covering its top surface, and semiconductor material at the first portion of the top surface of the body (10) is removed to form a recess of depth sufficient to separate the top surface region (10.5) into the source and drain regions (10.1, 10.2). In another specific embodiment, source and/or drain regions are formed, typically during growth of the gate oxide (10.3), by diffusion of impurities from the corresponding source and/or drain contact electrode(s) (12.1, 12.2). In yet another specific embodiment, doping of both source and drain regions is omitted, the source and drain contact electrodes (12.1, 12.2) forming Schottky barrier contacts to the body (10). 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     This invention, together with its features, advantages, and objects, may be better understood from the following detailed description when read in conjunction with the drawings in which: 
     FIGS. 1 through 5 depict in cross section a sequence of successive stages in the fabrication of a MOSFET device in accordance with a specific embodiment of the invention; and 
     FIG. 6 shows a top view of the device shown in FIG. 5, further illustrating a typical metallization cross-under for interconnection. 
    
    
     Only for the sake of clarity, none of the drawings is to any scale. Although the detailed description is largely in terms of fabricating but a single MOSFET device, it should be understood that many such similar devices, together with their interconnections, can be simultaneously fabricated in a single crystal semiconductor body. 
     DETAILED DESCRIPTION 
     As indicated in FIG. 1, the starting material is a P-type single crystal semiconductive silicon body 10, typically of surface orientation &lt;100&gt; with an excess boron acceptor impurity concentration of the order of 10 16  per cm 3 . An N+ doped surface region 10.5 is formed, typically by arsenic ion implantation, over a whole major surface of this body 10; and substantially planar field oxide portions 11 are then formed at selected locations of the surface, for example, by using conventional methods or the technique described in a pending application Ser. No. 128,841, filed on Mar. 10, 1980 by D. Kahng et al. (35-8). Thereby, the N+ doped surface region 10.5 remains, extending between neighboring field oxide portions 11. Alternatively, a conventional non-planar field oxide can first be thermally grown in accordance with a desired pattern and an N+ implant of arsenic could then be performed after the field oxide has been patterned. Ordinarily, a channel stop diffusion of acceptor impurities is performed, in order to provide a P+ channel stop region (not shown) underlying the field oxide 11. 
     Next, a refractory metal or a refractory metal-like layer 12, is deposited over the entire top surface of the body 10. For example, a layer of the metal-like titanium (or tantalum) silicide, typically about 2000 Angstroms in thickness, is deposited by co-sputtering of titanium (or tantalum) and silicon onto the silicon body 10. Alternatively, a silicon rich metal-silicide can also be used for the layer 12, such as silicon rich titanium silicide containing preferably about 25 percent extra silicon or silicon rich tantalum silicide containing preferably about 5 percent to 10 percent extra silicon, either of them formed by co-sputtering of silicon and the corresponding metal. Then, a relatively thick (typically about 0.5 to 1 micron) first intermediate insulating layer 13, typically of silicon dioxide, is deposited, typically by a chemical vapor deposition step at a low pressure of about 1 torr. Next, a relatively thin second intermediate insulating layer 14, typically of silicon nitride about 500 to 2000 Angstroms thick, is deposited over the intermediate oxide film 13, to serve as an end-point or etching-stopper during a subsequent (FIG. 3) anisotropic oxide etching step. The structure just after the formation of this nitride layer 14 is illustrated in FIG. 1. 
     Then the silicon nitride layer 14 and the silicon dioxide layer 13 are successively patterned and etched anisotropically, typically using a dry neutral or ion beam or plasma etching technique, to expose the metal or metal-like layer 12. The thus exposed layer 12 is then etched, typically by anodization or other dry techniques suitable for the particular metal or metal-like material of the layer 12, to form spaced-apart electrode contact layers 12.1 and 12.2 each contacting the N+ region 10.5. The semiconductor material at the surface of the body 10 at the central portion of the gap between these electrode contacts 12.1 and 12.2 will ultimately form the active channel region of the completed MOSFET device. 
     Then, as indicated in FIG. 2, another oxide layer 15 is deposited over the top surface of the whole structure, in a manner similar to the previously deposited intermediate oxide layer 13. The oxide layer 15 typically is about 2000 Angstroms thick, but can be varied to control the width of the channel over which an electrode layer 16 is to be deposited. This oxide layer 15 is then subjected to anisotropic etching, typically dry etching with CFH 3  and ammonia until the end-point silicon nitride layer 14 is reached; the top surface of the silicon nitride 14 and the top surface of the N+ region 10.5 thus serve as etching stoppers. Thereby (FIG. 3) only sidewall oxide layers 15.1 and 15.2 of the material of the originally deposited oxide layer 15 remain in the structure; that is, this oxide remains only on the vertical sidewall edges of the nitride layer 14, of the intermediate oxide layer 13, and of the contact electrodes 12.1 and 12.2. These sidewall oxide layers 15.1 and 15.2 serve to passivate the edges of the contact electrodes 12.1 and 12.2; and, in addition, the sidewall oxide layers 15.1 and 15.2 serve both to adjust the electrical channel length of the MOSFET device 20 (FIG. 5) being fabricated and to insulate the metallization to be formed, as discussed more fully below. 
     Next, the silicon material at the portion of the top surface of the N+ region 10.5 thus exposed by the anisotropic etching is itself etched by a plasma or other etching process for a time sufficient to form a recess in the silicon deep enough to penetrate beneath the N+ surface region 10.5; thereby this N+ region is separated into a pair of spaced-apart N+ source and drain regions 10.1 and 10.2. The structure is then thermally oxidized to grow a gate oxide layer 10.3, either by a dry or wet oxidation, including also appropriate cleaning steps, as necessary, as known in the art. 
     If desired for such purposes as adjusting the threshold voltage of the transistor being fabricated, an impurity ion implantation may be performed, either just before or after growth of this gate oxide layer 10.3, in order to dope the channel portion with impurities in a suitable concentration profile (not shown) for the completed device 20 (FIG. 5). 
     Then, a relatively thin electrode layer 16, typically polysilicon in the range of about 500 to 300 Angstroms thick, is deposited and impurity doped, as by diffusion therein of phosphorus, suitably for making the thus doped polysilicon useful as a gate electrode. The electrode layer 16 should be characterized by good step coverage over the rather steep exposed surface of the sidewall oxide layers 15.1 and 15.2. For depositing the polysilicon layer 16, a chemical vapor deposition step can be used, for example, a deposition using a system of 30 percent to 100 percent silane in a nitrogen carrier, preferably at a low pressure, typically of about 0.25 to 0.5 torr. Thus, the electrode layer 16 supplies a good foundation for electrical interconnections. 
     Apertures are then etched through the electrode layer 16, the silicon nitride layer 14, and the intermediate oxide layer 13, to expose the contact electrode layers 12.1 and 12.2, for electrical access thereto. Finally (FIG. 5) for the purpose of interconnect metallization, a metal layer is deposited, illustratively an aluminum layer by evaporation, over the top surface of the structure; and this metal layer is patterned, as by conventional masking and etching, to form the desired gate electrode interconnect layer 17.3 and the source and drain electrode interconnect layers 17.1 and 17.2. The electrode layer 16 is also similarly patterned into a gate electrode segment 16.3 and source and drain electrode segments 16.1 and 16.2. Thereby, the electrode combinations 16.1-17.1, 16.2-17.2, and 16.3-17.3 form double-layer interconnect metallizations for the source, drain, and gate, respectively, of the MOSFET device 20. 
     The gate electrode interconnect layer 17.3 need not cover the polysilicon gate electrode segment 16.3 in regions overlying the sidewall oxide layers 15.1, 15.2, or the gate oxide 10.3; the purpose of the gate electrode interconnect layer 17.3 is to lower the resistance of interconnection with the gate electrode segment 16.3. 
     It should be noted that the length of the source-to-drain channel in the device 20 is defined along a U-shaped line, and thus this channel length is determined by the depth of the recess in the semiconductor body 10 as well as by the thickness of the sidewall oxide 15.1 and 15.2. The resulting channel length can thus be made to be about a micron or less. 
     In an illustrative example, the doping level resulting in the source and drain regions 10.1 and 10.2 is of the order of 10 18  per cm 3  to a depth of about 0.25 microns. The recess in the N+ region 10.5 separating the source from drain has a depth of about 0.40 microns, that is, about 0.15 microns deeper than the source and drain regions themselves; thus the depth of the recess is about 1.5 times or more the depth of the N+ region 10.5. 
     It should be noted that the impurity doping to form the N+ region 10.5 may be omitted; and the contact electrodes 12.1 and 12.2 are then made of materials which form a Schottky barrier with the silicon body 10, or the electrode layer 12 may be initially itself doped with impurities which later (particularly during thermal growth of the gate oxide 10.3) diffuse out into the silicon body to form the source and drain regions 10.1 and 10.2. In such cases, it may not be necessary to form such a deep recess, if any at all, in the silicon body. 
     FIG. 6 shows the top view of device 20, together with an illustrative metallization cross-under 40 with respect to its gate electrode 17.3 as an example. Other elements in FIG. 6 not previously described, and which are fabricated simultaneously with those of FIGS. 1-5, are labeled with reference numerals which are equal to those of FIGS. 1-5 plus 20. A similar type of cross-under can obviously be made with respect to other metallization lines than the gate electrode. It should be noted that the gate electrode 16.3 is also insulated from the source and drain contact electrodes 12.1 and 12.2 in such other regions as sidewall edge surfaces 22.1 and 22.2 of the contact electrodes where this gate electrode 16.3 is formed and that these contact electrodes 12.1 and 12.2 overlie field oxide portions 11 removed from the source and drain regions 10.1 and 10.2. 
     Although the invention has been described in terms of specific embodiments, various modifications can be made without departing from the scope of the invention. For example, instead of depositing the silicon dioxide layer 15 by chemical vapor deposition, such other techniques as plasma deposition also be used. Moreover, P and N-type semiconductive material may be interchanged, in order to form P-channel MOSFET devices rather than N-channel as described above. Also, instead of a metal-like silicide, the contact electrodes 12.1 and 12.2 can be made of a metal, such as tungsten, and can be initially deposited in accordance with the desired spaced-apart configuration as by a photodeposition process, as known in the art, thereby avoiding the need for masking and etching of the electrode layer 12. Moreover, the second intermediate layer 14 can be a material chemically different from silicon nitride, such as a metal, polysilicon, or an organic polymer, providing such material is resistant to the etch employed to pattern the silicon dioxide layer 15. Finally, the silicon nitride layer 14 can be eliminated, provided that the oxide layer 13 is made sufficiently thick for providing a suitable margin in stopping the etch of the silicon dioxide layer 15.