Thin film transistors and methods of forming thin film transistors
A method of forming a thin film transistor over a substrate is provided whereby at least one of the source region or the drain region is conductively doped while preventing conductivity doping of the channel region without any masking of the channel region occurring by any separate masking layer. A method includes, a) providing a substrate having a node to which electrical connection is to be made; b) providing a first electrically insulative dielectric layer over the substrate; c) providing an electrically conductive gate layer over the first dielectric layer; d) providing a second electrically insulative dielectric layer over the electrically conductive gate layer; e) 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; f) providing a gate dielectric layer within the contact opening laterally inward of the projecting sidewalls; g) 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 h) 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. Thin film transistor constructions are also disclosed.
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 FIELDThis invention relates specifically to thin film transistor technology.
BACKGROUND OF THE INVENTIONAs 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 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'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 DRAWINGSPreferred embodiments of the invention are described below with reference to the following accompanying drawings.
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.
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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 36 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
Layer 30a can be doped in a single step to form diffusion regions 32a 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 32a 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 32a 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
Yet another alternate preferred embodiment is described with reference to
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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.
Claims
1-35. (canceled)
36. A wafer comprising: a transistor comprising:
- a substrate;
- a gate provided over the substrate and substantially surrounding an opening;
- a first source/drain region;
- a second source/drain region;
- semiconductive material proximate the gate and comprising a channel intermediate the first and second source/drain regions, wherein at least a portion of the channel is formed within the opening; and
- dielectric material formed intermediate the gate and the semiconductive material.
37. The wafer of claim 36, wherein the semiconductive material partially fills the opening within the gate.
38. The wafer of claim 36 further comprising an electrically insulative layer over the substrate and below the gate.
39. The wafer of claim 36, wherein the transistor comprises a thin film transistor.
40. The wafer of claim 36, wherein the dielectric material is formed within the opening, and wherein the dielectric material and the semiconductive material completely fill the opening within the gate.
41. The wafer of claim 36, wherein at least a portion of the first source/drain region is formed above the opening of the gate.
42. The wafer of claim 41, wherein at least a portion of the second source/drain region is formed below the opening of the gate.
43. The wafer of claim 36, wherein the gate laterally surrounds dielectric material.
44. The wafer of claim 36, wherein the dielectric material laterally surrounds the channel formed within the opening.
45. The wafer of claim 36, wherein at least a portion of the first source/drain region is formed within the semiconductive material.
46. The wafer of claim 36, wherein at least a portion of the first source/drain region is formed within the substrate.
47. The wafer of claim 36, wherein the channel is formed elevationally above at least one of the first and second source/drain regions.
48. The wafer of claim 36, wherein the channel comprises an elongated structure extending substantially perpendicularly to an upper surface of the substrate.
49. A wafer comprising: a transistor comprising:
- a substrate;
- a gate provided over the substrate and substantially surrounding an opening, wherein the opening has a lateral cross-sectional dimension which is less than a minimum photolithographic feature size utilized for fabricating the transistor;
- a first source/drain region;
- a second source/drain region; and
- semiconductive material proximate the gate and comprising a channel intermediate the first and second source/drain regions.
50. The wafer of claim 49, wherein the channel is formed within the opening.
51. The wafer of claim 49, wherein at least a portion of the first source/drain region is formed above the opening of the gate.
52. The wafer of claim 49, wherein the gate laterally surrounds the channel.
53. The wafer of claim 49, wherein the transistor comprises a gate dielectric between the gate and the channel, and wherein the gate dielectric laterally surrounds the channel.
54. The wafer of claim 49, wherein at least a portion of the first source/drain region is formed above the opening, and wherein at least a portion of the second source/drain region is formed below the opening.
55. The wafer of claim 49, wherein the transistor comprises a thin film transistor.
56. The wafer of claim 49 further comprising an electrically insulative layer over the substrate and below the gate.
57. A semiconductor device comprising: a transistor comprising:
- a substrate;
- a gate provided over the substrate and substantially surrounding an opening;
- a first source/drain region comprising at least a portion formed partially below the opening;
- a second source/drain region comprising at least a portion formed partially above the opening;
- semiconductive material proximate the gate and comprising a channel intermediate the first and second source/drain regions; and
- a gate dielectric formed intermediate the gate and the semiconductive material.
58. The semiconductor device of claim 57, wherein the channel is formed within the opening.
59. The semiconductor device of claim 57, wherein the transistor comprises a thin film transistor.
60. A semiconductor device comprising: a transistor comprising:
- a substrate;
- a gate provided over the substrate and substantially surrounding an opening, wherein the opening has a lateral cross-sectional dimension which is less than a minimum photolithographic feature size utilized for fabricating the transistor;
- first and second source/drain regions;
- semiconductive material proximate the gate and comprising a channel intermediate the first and second source/drain regions; and
- a gate dielectric formed intermediate the gate and the channel.
61. The semiconductor device of claim 60, wherein the channel is formed within the opening.
62. The semiconductor device of claim 60, wherein the transistor comprises a thin film transistor.
63. A transistor comprising:
- a substrate;
- a gate provided over the substrate and substantially surrounding an opening;
- a first source/drain region comprising at least a portion formed partially below the opening;
- a second source/drain region comprising at least a portion formed partially above the opening;
- semiconductive material proximate the gate and comprising a channel intermediate the first and second source/drain regions; and
- a gate dielectric intermediate the gate and the channel.
64. The transistor of claim 63, wherein the channel is formed in the opening and partially fills the opening.
65. The transistor of claim 63 further comprising a thin film transistor.
66. The transistor of claim 63, wherein the gate dielectric and the semiconductive material completely fill the opening within the gate.
67. The transistor of claim 63, wherein the semiconductive material is provided within the opening and over the gate.
68. The transistor of claim 63, wherein the opening is formed through an entirety of the gate.
69. The transistor of claim 63, wherein at least a portion of the first source/drain region is formed within the semiconductive material.
70. The transistor of claim 63, wherein at least a portion of the first source/drain region is formed within the substrate.
71. A transistor comprising:
- a substrate;
- a gate provided over the substrate and substantially surrounding an opening over the substrate, the opening has a lateral cross-sectional dimension which is less than a minimum photolithographic feature size utilized for fabricating the transistor;
- first and second source/drain regions; and
- semiconductive material intermediate the first and second source/drain regions and comprising a channel.
72. The transistor of claim 71, wherein at least a portion of at least one of the first and second source/drain regions is formed within the opening.
73. The transistor of claim 71, wherein at least a portion of the semiconductive material is formed within the opening.
74. The transistor of claim 71 further comprising insulative material provided intermediate the gate and the semiconductive material.
75. The transistor of claim 71, wherein at least a portion of at least one of the first and second source/drain regions is formed above the opening of the gate.
76. The transistor of claim 75, wherein at least a portion of at least one of the first and second source/drain regions is formed below the opening of the gate.
77. The transistor of claim 71 further comprising a thin film transistor.
Type: Application
Filed: Jan 18, 2005
Publication Date: Jul 21, 2005
Inventor: Monte Manning (Boise, ID)
Application Number: 11/038,601