Abstract:
A thin film transistor includes a thin film transistor layer having a source region, a channel region and a drain region. In one implementation, a gate of the transistor is disposed laterally proximate the thin film channel region and comprises an annulus which laterally encircles the laterally proximate thin film channel region. In another implementation, a channel region of a thin film transistor extends elevationally away from a substrate. Source and drain regions are operatively associated with the channel region and are elevationally spaced therealong and apart from one another. A gate is disposed over the substrate and laterally proximate the channel region.

Description:
RELATED PATENT DATA 
     This patent resulted from a continuation application of U.S. patent application Ser. No. 08/679,955, filed Jul. 15, 1996, entitled “Thin Film Transistors and Methods of Forming Thin Film Transistors”, naming Monte Manning as inventor, and which is now U.S. Pat. No. 5,804,855 the disclosure of which is incorporated by reference. That patent resulted from a divisional application of U.S. patent application Ser. No. 08/506,084, filed Jul. 24, 1995, by Monte Manning, titled “Thin Film Transistors And Methods of Forming Thin Film Transistors”, which is now U.S. Pat. No. 5,700,727. 
    
    
     PATENT RIGHTS STATEMENT 
     This invention was made with Government support under Contract No. MDA972-92-C-0054 awarded by Advanced Research Projects Agency (ARPA). The Government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     This invention relates specifically to thin film transistor technology. 
     BACKGROUND OF THE INVENTION 
     As circuit density continues to increase, there is a corresponding drive to produce smaller and smaller field effect transistors. Field effect transistors have typically been formed by providing active areas is within a bulk substrate material or within a complementary conductivity type well formed within a bulk substrate. One additional technique finding greater application in achieving reduced transistor size is to form field effect transistors with thin films, which is commonly referred to as “thin film transistor” (TFT) technology. These transistors are formed using thin layers which constitute all or a part of the resultant source and drain regions, as opposed to providing both regions within a bulk semiconductor substrate. 
     Specifically, typical prior art TFT&#39;s are formed from a thin film of semiconductive material (typically polysilicon). A central channel region of the thin film is masked by a separate layer, while opposing adjacent source/drain regions are doped with an appropriate p or n type conductivity enhancing impurity. A gate insulator and gate are provided either above or below the thin film channel region, thus providing a field effect transistor having active and channel regions formed within a thin film as opposed to a bulk substrate. 
     It would be desirable to improve upon methods of forming thin film transistors and in improving thin film transistor constructions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention are described below with reference to the following accompanying drawings. 
     FIG. 1 is a diagrammatic sectional view of a semiconductor wafer fragment at one processing step in accordance with the invention. 
     FIG. 2 is a view of the FIG. 1 wafer fragment at a processing step subsequent to that shown by FIG.  1 . 
     FIG. 3 is a view of the FIG. 1 wafer fragment at a processing step subsequent to that shown by FIG.  2 . 
     FIG. 4 is one example of a possible top view of FIG.  3 . 
     FIG. 5 is a view of the FIG. 1 wafer fragment at a processing step subsequent to that shown by FIG.  3 . 
     FIG. 6 is a diagrammatic sectional view of an alternate embodiment wafer fragment in accordance with the invention. 
     FIG. 7 is a diagrammatic sectional view of yet another alternate embodiment wafer fragment at one processing step in accordance with the invention. 
     FIG. 8 is a view of the FIG. 7 wafer fragment at a processing step subsequent to that shown by FIG.  7 . 
     FIG. 9 is a view of the FIG. 7 wafer fragment at a processing step subsequent to that shown by FIG.  8 . 
     FIG. 10 is a view of the FIG. 7 wafer fragment at a processing step subsequent to that shown by FIG.  9 . 
     FIG. 11 is a view of the FIG. 7 wafer fragment at a processing step subsequent to that shown by FIG.  10 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This disclosure of the invention is submitted in furtherance of the constitutional purposes of the U.S. Patent Laws “to promote the progress of science and useful arts” (Article 1, Section 8). 
     In accordance with one aspect of the invention, a method of forming a thin film transistor over a substrate comprises the following steps: 
     providing a layer of semiconductive material from which a channel region and at least one of a source region or a drain region of a thin film transistor are to be formed; and 
     conductively doping the at least one of the source region or the drain region of the semiconductive material layer while preventing conductivity doping of the channel region of the semiconductive material layer, such doping being conducted without any masking of the channel region by any separate masking layer. 
     In accordance with another aspect of the invention, a method of forming a thin film transistor comprises the following steps: 
     providing a substrate having a node to which electrical connection is to be made; 
     providing a first electrically insulative dielectric layer over the substrate; 
     providing an electrically conductive gate layer over the first dielectric layer; 
     providing a second electrically insulative dielectric layer over the electrically conductive gate layer; 
     providing a contact opening through the second dielectric layer, the electrically conductive gate layer and the first dielectric layer; the contact opening defining projecting sidewalls; 
     providing a gate dielectric layer within the contact opening laterally inward of the contact opening sidewalls; 
     providing a layer of semiconductive material over the second dielectric layer and within the contact opening against the gate dielectric layer and in electrical communication with the node; the semiconductive material within the contact opening defining an elongated and outwardly extending channel region the electrical conductance of which can be modulated by means of the adjacent electrically conductive gate and gate dielectric layers; and 
     conductively doping the semiconductive material layer lying outwardly of the contact opening to form one of a source region or a drain region of a thin film transistor. 
     In accordance with still another aspect of the invention, a thin film transistor comprises: 
     a thin film transistor layer having a source region, a channel region and a drain region; the thin film channel region comprising an annulus; and 
     a gate in proximity to the thin film channel annulus, the gate comprising an annulus which surrounds the thin film channel annulus. 
     These and other aspects of the invention will be more readily appreciated from the following description with proceeds with reference to the accompanying drawings. 
     Referring to FIG. 1, a semiconductor wafer fragment in process is indicated generally with reference numeral  10 . Such comprises a bulk substrate  12  of lightly doped p or n type monocrystalline silicon, having a diffusion region  13  provided therein. A first electrically insulative dielectric layer  14  (typical SiO 2 ) is provided over bulk substrate  12 . An example and preferred thickness range for layer  12  is from 50 Angstroms to 2000 Angstroms, with 100 Angstroms being more preferred. An electrically conductive layer  16  is provided over first dielectric layer  14 . Layer  16  will ultimately comprise the conductive gate of the thin film transistor, and preferably comprises a heavily doped (greater than 1×10 20  ion/cm 3 ) layer of polysilicon. An example and preferred thickness range is from 3000 Angstroms to 10,000 Angstroms, with 8000 Angstroms being more preferred. A second electrically insulative dielectric layer  18  is provided over electrically conductive gate layer  16 . Such can be considered as a base layer over which a thin film transistor layer will be provided. An example and preferred material is SiO 2  deposited to a thickness range of from 300 Angstroms to 3000 Angstroms, with 1000 Angstroms being more preferred. 
     Referring to FIG. 2, a contact opening  20  is provided through second dielectric layer  18 , electrically conductive gate layer  16 , and first dielectric layer  14  to outwardly expose diffusion region  13 . Alternately, diffusion region  13  could be provided after forming contact opening  20 . One method of doing this is by using ion implantation through contact opening  20 , thereby making diffusion regions  13  self-aligned to contact opening  20 . Contact opening  20  defines projecting sidewalls  22  which in the preferred embodiment are provided to be substantially perpendicular relative to the expanse of bulk substrate  12 . A dielectric layer  24 , which will serve as gate dielectric layer, is deposited over second dielectric layer  18  and within contact opening  20  to a thickness which less than completely fills contact opening  20 . An example diameter for contact opening  20  is 3500 Angstroms, with an example layer  24  being SiO 2  deposited to a thickness of 200 Angstroms in such instance. 
     Referring to FIG. 3, gate dielectric layer  24  is anisotropically etched to define a resultant gate dielectric layer  26  within contact opening  20  laterally inward of sidewalls  22 . When anisotropically etching gate dielectric layer  24 , some of second dielectric layer  18  is removed during a desired overetch. If layer  18  is 1000 Angstroms thick and layer  24  is 200 Angstroms thick, a preferred over-etch would be 200 Angstroms, reducing  18  to 800 Angstroms. In the illustrated and preferred embodiment, such gate dielectric layer takes on the shape or appearance of conventional insulative sidewall spacers, and in the depicted embodiment is in the form or shape of a longitudinally elongated annulus. Thus, electrically conductive gate layer  16  also is comprised of an annulus which surrounds contact opening  20 . 
     FIG. 4 illustrates one example of a possible patterned top construction of FIG.  3 . Such illustrates gate dielectric annulus  26  encircling within contact opening  20 . Electrically conductive gate layer  16  has been patterned to comprise a ring portion and an extension  27 . Regardless, the bulk mass of layer  16  constitutes an annulus which encircles contact opening  20 . The above described process provides but one example of a manner in which a gate dielectric layer is provided within contact opening  20 . 
     Referring to FIG. 5, a layer  30  of semiconductive material is provided over second dielectric layer  18  and within contact opening  20  against gate dielectric layer  26 , and in electrical communication with diffusion region  13 . In this particular described embodiment, layer  30  is provided to completely fill the remaining open portion of contact opening  20 . Semiconductive material layer  30  constitutes a layer from which a channel region and at least one of a source region or a drain region of a thin film transistor are to be formed. The semiconductive material of layer  30  within contact opening  20  defines an elongated and outwardly extending channel region  31  the electrical conductance of which can be modulated by means of the laterally adjacent electrically conductive gate and gate dielectric layers  18  and  26 , respectively. 
     Field effect transistor channel regions typically utilize some minimum conductivity doping, less than the doping concentrations of the source and drain, to provide desired conductance when modulated by the gate. Such can be provided in this example by in situ doping of layer  30  during its deposition. Alternately, an ion implant can be conducted with subsequent processing providing desired diffusion of the dopants. 
     The semiconductive material layer  30  is then conductively doped such that its portion lying outwardly of contact opening  20  forms one of a source or a drain region  32  of a thin film transistor. The doping results in an interface  34  being created relative to the outermost portions of layer  30  and that portion within channel region  31 , such that portion  32  constitutes a highly doped electrically conductive region, while channel region  31  constitutes a semiconductive layer capable of being rendered conductive by applying suitable voltage to gate layer  16 . Note that advantageously in accordance with the preferred process, conductive doping of layer  30  is conducted using its thickness to effectively prevent conductivity doping of channel region  31 , with such doping being conducted without other masking of the channel region by any separate masking layer. The effective thickness and doping conditions for the outer portion of layer  30  effectively can be utilized to prevent undesired conductivity enhancing doping of channel region  31 . 
     In the above described embodiment; one of doped regions  32  of layer  30  or diffusion region  13  of bulk substrate  12  constitutes a source region of a thin film transistor, while the other of such constitutes a drain region. Region  31  constitutes a channel region, with gate layer  16  comprising an annulus which encircles thin film channel region  31 . Both of channel region  31  and diffusion region  32  are elongated, with diffusion region  32  being oriented substantially perpendicular relative to channel region  31  and also substantially parallel with bulk substrate  14 . Elongated channel region  31  and gate dielectric annulus  26  are perpendicularly oriented relative to bulk substrate  14 . 
     If region  13  constitutes the drain region, then the thickness of oxide layer  14  defines the gate-drain offset dimension of the thin film transistor. As well known to those of skill in the art, a drain offset is a region used in thin film transistors to reduce off current caused by thermionic field emission in the channel region near the drain. If region  32  is the drain, then the thickness of layer  18  defines the offset dimension. The thickness of gate polysilicon layer  16  defines the channel length of the thin film transistor. 
     An alternate embodiment is shown and described with reference to FIG.  6 . Like numerals from the first described embodiment are utilized where appropriate, with differences being indicated by the suffix “a” or with different numerals. In the depicted embodiment of wafer fragment  10   a,  semiconductive material  30   a  is provided to only partially fill the remaining portion of contact opening  20 . Such forms an annulus  33  within contact opening  20 , with such annulus being utilized to comprise the channel region of the resultant thin film transistor. 
     Layer  30   a  can be doped in a single step to form diffusion regions  32   a  and  35 , one of which constitutes a drain region and the other of which constitutes a source region of the resultant thin film transistor. Accordingly, channel annulus  33  is elongated and oriented substantially perpendicularly relative to bulk substrate  12  and diffusion regions  32   a  and  35 . In this described embodiment, gate layer  16  comprises an annulus which surrounds thin film channel annulus  33 . Again, the elongated and substantially vertical nature or orientation of channel region  33  prevents conductivity doping from occurring therein when regions  32   a  and  35  are doped by a highly directional perpendicular ion implantation doping. In this embodiment, diffusion region  13  constitutes a node to which electrical connection of a thin film transistor is to be made, while in the first embodiment example region  13  comprised an inherent part of the thin film transistor. Diffusion region  13  might alternately be provided by out-diffusion of dopant material from region  35  from subsequent heating steps. 
     Desired minimum doping for the channel region of FIG. 6 can be provided by in situ doping or by ion implanting, such as angled implanting. 
     Yet another alternate preferred embodiment is described with reference to FIGS.  7 - 11 . Like numerals from the first described embodiment have been utilized where appropriate, with differences being indicated with the suffix “b” or with different numerals. Referring first to FIG. 7, second electrically insulative dielectric layer  18  is provided with an initial contact opening  50  therethrough to electrically conductive gate layer  16 . A preliminary electrically insulative layer  52  is provided over second dielectric layer  18  and to within initial contact opening  50 , with such layer less than completely filling contact opening  50 . 
     Referring to FIG. 8, preliminary electrically insulative layer  52  is anisotropically etched to define an insulative annulus spacer  54  within initial contact opening  50 . Such facilitates or enables producing a contact opening inwardly of the spacers which is less than the minimum photolithographic feature size which can be useable to produce the smallest possible initial contact opening  50 . For example, where a minimum available photolithographic width for contact opening  50  were 0.32 micron, the resultant width of the opening after spacer etch can be reduced to 0.1 micron. As examples, if layer  18  is 1500 Angstroms thick and opening  50  is 3200 Angstroms in diameter, layer  52  is preferably provided to a thickness of from 500 Angstroms to 1200 Angstroms, with 1000 Angstroms being most preferred. An anisotropic etch of a 1000 Angstrom thick layer  52  will preferably be conducted as an over-etch of 300 Angstroms, leaving layer  18  1200 Angstroms thick. 
     Referring to FIG. 9, a secondary contact  56  is etched through electrically conductive gate layer  16  and first dielectric layer  14 . During such etching, insulative annulus spacer  54  and second dielectric layer  18  are used as an etching mask. Diffusion regions  13   b  is provided as shown. 
     Referring to FIG. 10, a secondary electrically insulative layer  58  is provided over second dielectric layer  18  and insulative annulus spacer  54  to within secondary contact opening  56 , with such layer being provided to less than completely fill secondary contact opening  56 . 
     Referring to FIG. 11, secondary electrically insulative layer  58  has been anisotropically etched to define a gate dielectric layer annulus  26   b  within secondary contact opening  56 . A subsequent semiconductive layer  30   b  is provided and doped as shown to provide diffusion region  32   b,  and to provide channel region  31   b.  An example thickness for layer  58  is 200 Angstroms. Anisotropic etching of such a layer preferably includes a 200 Angstrom over-etch, resulting in a final preferred thickness of layer  18  of 1000 Angstroms. 
     The above described embodiments utilizing an annulus gate essentially enables provision of a channel region which is gated about all sides, thus enabling provision of smaller field effect transistors. Such results in a reduced consumption of substrate area, with such example thin film transistors enabling the required area to be that of the contact and the associated anisotropic spacer-like constructions. Conventional horizontal thin film transistors require additional area for the channel, source and drain regions. Such also provides for improved thin film transistor characteristics, due to gating of the channel region on all sides which provides greater controllable on/off currents. 
     The above described method and embodiment further reduce overall mask count in semiconductor processing. Since in the preferred embodiment the channel region is substantially vertical, masks are not required to protect the desired channel from the thin film transistor source and drain implants. Depending on implementation, the channel region may even be completely sealed from the surface providing even greater protection, thus eliminating at least two masks. 
     In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.