# Patent

## Title
Method of forming a field effect transistor

## Abstract

A method of forming a field effect transistor includes forming a channel region within bulk semiconductive material of a semiconductor substrate. Source/drain regions are formed on opposing sides of the channel region. An insulative dielectric region is formed within the bulk semiconductive material proximately beneath at least one of the source/drain regions. A method of forming a field effect transistor includes providing a semiconductor-on-insulator substrate, said substrate comprising a layer of semiconductive material formed over a layer of insulative material. All of a portion of the semiconductive material layer and all of the insulative material layer directly beneath the portion are removed thereby creating a void in the semiconductive material layer and the insulative material layer. Semiconductive channel material is formed within the void. Opposing source/drain regions are provided laterally proximate the channel material. A gate is formed over the channel material. Integrated circuitry includes a bulk semiconductor substrate. A field effect transistor thereon includes a gate, a channel region in the bulk semiconductor substrate, and source/drain regions within the substrate on opposing sides of the channel region. A field isolation region is formed in the bulk semiconductor substrate and laterally adjoins with one of the source/drain regions. The field isolation region includes a portion which extends beneath at least some of the one source/drain region. Other aspects are contemplated.

## Background
RELATED PATENT DATA
This patent resulted from a continuation application of U.S. patent application Ser. No. 10/236,282, filed Sep. 5, 2002, entitled “Integrated Circuitry”, naming Todd R. Abbott, Zhongze Wang, Jigish D. Trivedi and Chih-Chen Cho as inventors, the disclosure of which is incorporated by reference; which patent resulted from a divisional application of U.S. patent application Ser. No. 09/713,844, filed Nov. 15, 2000, entitled “Method of Forming a Field Effect Transistor”, naming Todd R. Abbott, Zhongze Wang, Jigish D. Trivedi and Chih-Chen Cho as inventors, now U.S. Pat. No. 6,599,789 B1, the disclosure of which is incorporated by reference.


TECHNICAL FIELD
This invention relates to methods of forming field effect transistors, to methods of forming integrated circuitry, and to integrated circuitry.
BACKGROUND OF THE INVENTION
Semiconductor processors continue to strive to reduce the size of individual electronic components, thereby enabling smaller and denser integrated circuitry. One typical circuitry device is a field effect transistor. Such typically includes opposing semiconductive source/drain regions of one conductivity type having a semiconductive channel region of opposite conductivity type therebetween. A gate construction is received over the channel region. Current can be caused to flow between the source/drain regions through the channel region by applying a suitable voltage to the gate.
The channel region is in some cases composed of background doped bulk semiconductive substrate or well material, which is also received immediately beneath the opposite type doped source/drain regions. This results in a parasitic capacitance developing between the bulk substrate/well and the source/drain regions. This can adversely affect speed and device operation, and becomes an increasingly adverse factor as device dimensions continue to decrease.
The invention was principally motivated in overcoming problems associated with the above-identified parasitic capacitance in bulk field effect transistor devices. However, the invention is in no way so limited, nor limited to solving or reducing this or any other problem whether identified/identifiable herein or elsewhere, with the invention only being limited by the accompanying claims as literally worded and as appropriately interpreted in accordance with the doctrine of equivalents.
SUMMARY
This invention includes methods of forming field effect transistors, methods of forming integrated circuitry, and integrated circuitry. In but one implementation, a method of forming a field effect transistor includes forming a channel region within bulk semiconductive material of a semiconductor substrate. Source/drain regions are formed on opposing sides of the channel region. An insulative dielectric region is formed within the bulk semiconductive material proximately beneath at least one of the source/drain regions.
In one implementation, a method of forming a field effect transistor includes providing a semiconductor-on-insulator substrate, said substrate comprising a layer of semiconductive material formed over a layer of insulative material. All of a portion of the semiconductive material layer and all of the insulative material layer directly beneath the portion are removed thereby creating a void in the semiconductive material layer and the insulative material layer. Semiconductive channel material is formed within the void. Opposing source/drain regions are provided laterally proximate the channel material. A gate is formed over the channel material.
In one implementation, integrated circuitry includes a bulk semiconductor substrate. A field effect transistor thereon includes a gate, a channel region in the bulk semiconductor substrate, and source/drain regions within the substrate on opposing sides of the channel region. A field isolation region is formed in the bulk semiconductor substrate and laterally adjoins with one of the source/drain regions. The field isolation region includes a portion which extends beneath at least some of the one source/drain region.
In one implementation, integrated circuitry includes a substrate having a field effect transistor formed thereon. The transistor includes a gate, a channel region, and source/drain regions on opposing sides of the channel region. First and second dielectric insulative material masses are received beneath and contact the source/drain regions. The dielectric insulative material masses do not extend to beneath the channel region.



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 an aspect of the invention.
FIG. 2 is a view of the FIG. 1 wafer at a processing step subsequent to that shown by FIG. 1.
FIG. 3 is a view of the FIG. 1 wafer at a processing step subsequent to that shown by FIG. 2.
FIG. 4 is a view of the FIG. 1 wafer at a processing step subsequent to that shown by FIG. 3.
FIG. 5 is a view of the FIG. 1 wafer at a processing step subsequent to that shown by FIG. 4.
FIG. 6 is a view of the FIG. 1 wafer at a processing step subsequent to that shown by FIG. 5.
FIG. 7 is a view of the FIG. 1 wafer at a processing step subsequent to that shown by FIG. 6.
FIG. 8 is a view of the FIG. 1 wafer at a processing step subsequent to that shown by FIG. 7.
FIG. 9 is a view of the FIG. 1 wafer at a processing step subsequent to that shown by FIG. 8.
FIG. 10 is a diagrammatic top view of the FIG. 9 wafer.
FIG. 11 is a diagrammatic sectional view of an alternate embodiment semiconductor wafer fragment in accordance with an aspect of the invention.
FIG. 12 is a diagrammatic sectional view of still another alternate embodiment semiconductor wafer fragment at a processing step in accordance with an aspect of the invention.
FIG. 13 is a view of the FIG. 12 wafer at a processing step subsequent to that shown by FIG. 12.
FIG. 14 is a view of the FIG. 12 wafer at a processing step subsequent to that shown by FIG. 13.
FIG. 15 is a view of the FIG. 12 wafer at a processing step subsequent to that shown by FIG. 14.



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).
A method of forming integrated circuitry, including a field effect transistor, is initially described in but only some aspects of the invention in connection with FIGS. 1–10. Referring initially to FIG. 1, a semiconductor substrate is indicated generally with reference 10. In the context of this document, the term “semiconductor substrate” or “semiconductive substrate” is defined to mean any construction comprising semiconductive material, including, but not limited to, bulk semiconductive materials such as a semiconductive wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term “substrate” refers to any supporting structure, including, but not limited to, the semiconductive substrates described above. Also in the context of this document unless otherwise indicated, the term “layer” includes the singular and the plural.
Substrate 10 comprises a bulk semiconductor substrate 12. An example preferred material is monocrystalline, such as monocrystalline silicon lightly doped with p-type material. In the context of this document, the term “bulk” also includes doped well regions within such substrates. Bulk substrate 12 comprises a channel region 14 which is shown as being masked by a pad oxide layer 13 and a patterned block of masking material 16. For an exemplary 0.15 micron transistor gate width, an exemplary thickness for layer 13 is 100 Angstroms. An exemplary preferred material for mask 16 is silicon nitride deposited to an exemplary thickness of 900 Angstroms. An example width is 0.25 micron. In the illustrated and preferred embodiment, masking material 16 extends laterally beyond the lateral confines of channel region 14. Such provides but one example of forming a channel region within bulk semiconductive material of a semiconductor substrate, and of masking the same.
Referring to FIG. 2, at least one trench is formed into the bulk semiconductor substrate on at least one side of the channel region received within the bulk semiconductor substrate. Preferably and as shown, two trenches 18, 19 are formed into bulk semiconductor substrate 12 on opposing sides of masked channel region 14. Such preferably occurs by any existing or yet-to-be developed substantially anisotropic etching technique. An exemplary preferred depth for the trench etching is 2500 Angstroms.
Referring to FIG. 3, an insulative dielectric material 20 is deposited over masking material 16 and to within and overfilling trenches 18 and 19. Exemplary and preferred processing includes sidewall oxidation either before or after deposition of layer 20. An example and preferred material for layer 20 is high-density plasma deposited oxide. The insulative dielectric material is preferably initially deposited to overfill the trenches and then subsequently planarized at least to masking material 16 to provide the preferred illustrated FIG. 3 construction. Example planarizing techniques-include chemical-mechanical polishing and etch back.
Referring to FIG. 4, portions of insulative dielectric material 20 are removed from within trenches 18 and 19 effective to form at least one, and preferably two as shown, source/drain voids 22 and 24 on the respective sides of channel region 14. Such removal as shown is also preferably effective to expose bulk semiconductive material 12. An example preferred depth of voids 22 and 24 within insulative dielectric material 20 is 1700 Angstroms. The preferred removal technique is a timed anisotropic etch, and with a photolithographic patterned mask being received over the non-etched portions of layer 20. In the preferred embodiment, such effectively defines the outlines of the source/drains of the transistor(s) being formed. Preferably and as shown, such removing forms an outer surface of insulative dielectric material 20 to be planar at the base of such voids 22 and 24.
Referring to FIG. 5, source/drain semiconductive material 26 is formed within source/drain voids 22 and 24. Material 26 in but one embodiment comprises monocrystalline material (by way of example only via epitaxial silicon growth), and in but another embodiment comprises polycrystalline material, and in but another embodiment a mixture of monocrystalline and polycrystalline. An exemplary preferred material is polycrystalline silicon, preferably in situ conductively doped with an n-type conductivity enhancing impurity during a chemical vapor deposition. Accordingly, in the preferred embodiment, source/drain material 26 covers and physically contacts the previously-exposed bulk semiconductor substrate material 12.
Referring to FIG. 6, deposited material 26 is planarized at least to masking material 16. Example and preferred techniques include chemical-mechanical polishing and etch back.
Referring to FIG. 7, channel region 14 is unmasked preferably by etching away all of the masking material 16 and all of pad oxide layer 13. Further preferably as shown, some and only some of semiconductive material 26 is etched from the substrate. Such might occur in a single or more etching step(s) depending on the chemistry utilized and the desires of the processor, as readily determinable by the artisan. By way of example only, an example etch chemistry which will etch polysilicon and silicon nitride in a substantially nonselective manner includes plasma CF4, CH2F2 and He. In the subject example, the preferred amount of semiconductive material left is 900 Angstroms thick. Such provides but one example of forming source/drain regions 30 and 32 on opposing sides of channel region 14. The upper surface of channel region 14 in FIG. 7 is preferably approximately 200 Angstroms beneath the upper surfaces of regions 30 and 32, which are also preferably substantially planar.
Referring to FIG. 8, a gate 34 is formed over channel region 14. Preferably as shown, a gate dielectric layer 36, for example silicon dioxide, is first formed over channel region 14. A gate stack is then formed thereover, preferably comprising a conductively doped polysilicon layer 38 and a conductive silicide layer 40 (for example WSiX) and a nitride capping layer 42. Thereafter, at least one pocket implanting is conducted to provide at least one pocket implant region intermediate source/drain semiconductive material 26 and channel region 14. In the illustrated and preferred example, exemplary pocket implants include source/drain extension (SDE) implant regions 44 having a thickness of 500 Angstroms, and halo implant regions 46 provided therebeneath having an approximate thickness of 500 Angstroms and to extend below source/drain regions 30, 32. Insulative spacers are subsequently added as shown. Rapid thermal processing is preferably conducted at some point, as is conventional.
Referring to FIGS. 9 and 10, subsequent exemplary processing is illustrated. Depicted is the provision and planarizing of an insulative dielectric layer 48, for example borophosphosilicate glass (BPSG). Contact openings have been formed therethrough and plugged with conductive material to form source/drain contacts 50.
The above-described embodiment provides but one example of so providing an insulative dielectric region within bulk semiconductive material 12 proximately beneath at least one of the source/drain regions. Preferably and as shown, such insulative dielectric region is formed beneath both source/drain regions and physically contacts the subject source/drain regions. Further in the described and preferred embodiment, forming of the insulative dielectric region beneath the source/drain regions occurs prior to forming the source/drain regions, and includes at least some depositing of an insulative dielectric layer. Such preferred processing also depicts the formation of gate 34 after forming the source/drain semiconductive material.
Further, the illustrated construction provides but one example of novel integrated circuitry independent of the method of fabrication. Such comprises a bulk semiconductor substrate including a field effect transistor comprising a gate, a channel region in the bulk semiconductor substrate, and source/drain regions within the substrate on opposing sides of the channel region. At least one field isolation region is formed in the bulk semiconductor substrate and laterally adjoins with one of the source/drain regions. The field isolation region includes some portion 54 which extends beneath at least some of the source/drain region (FIG. 9). In the illustrated and preferred embodiment, the field isolation region portions 54 contact the source/drain regions therebeneath. Further preferably, field isolation region portion 54 extends beneath at least a majority of the one source/drain region and even more preferably extends beneath at least 90% of the source/drain regions. The illustrated example shows greater than 95% coverage by portions 54 beneath the source/drain regions. Further preferably and as shown, each field isolation region portion 54 extends beneath less than all of the source/drain region. Further, at least one pocket implant region is received intermediate the source/drain region and the channel region.
FIG. 11 illustrates but one exemplary alternate embodiment which includes forming the insulative dielectric region after forming the source/drain regions. Like numerals from the first-described embodiment are utilized where appropriate, with differences being indicated by the suffix “a” or with different numerals. Substrate 10a comprises source/drain regions 30a and 32a formed within a bulk monocrystalline silicon substrate 12a. An implant masking construction 58 is formed over gate 34. Substrate 10a is then subjected to a suitable ion implantation whereby material is ion implanted into bulk semiconductive substrate material 12a which is either insulative dielectric material or a material which reacts with the bulk semiconductor material to form an insulative dielectric material. FIG. 11 depicts regions 59 formed thereby. Such processing might occur either before or after forming source/drain regions 30a and 32a. An example implant would be of oxygen atoms, for example at a dose of 4×1017 atoms or ions per cubic centimeter at a suitable energy to achieve desired depth, and preferably followed by an anneal.
Yet but one additional alternate embodiment of forming integrated circuitry, including the forming of a field effect transistor, is described with reference to FIGS. 12–15. FIG. 12 illustrates a semiconductor-on-insulator substrate 60. Such comprises, in the preferred example, a bulk monocrystalline silicon substrate wafer 61 having a layer 62 of insulative material formed thereover. An example material is silicon dioxide. A layer 64 of semiconductive material is formed over layer 62. An example preferred material for layer 64 is silicon, preferably elemental silicon, such as monocrystalline or polycrystalline silicon.
Referring to FIG. 13, all of a portion of semiconductive material layer 64 and all of insulative material layer 62 immediately therebeneath are removed, thereby creating a void 65 in semiconductive material layer 64. Such removing preferably occurs by photolithographic patterning of a masking layer and subsequent conventional or yet-to-be developed etching thereof. Such removing preferably exposes bulk monocrystalline silicon 61 of substrate 60 as shown.
Referring to FIG. 14, semiconductive channel material 66 is formed within void 65. The example and preferred technique, where substrate material 61 comprises monocrystalline silicon, is conventional or yet-to-be-developed epitaxial silicon growth within void 65 from bulk monocrystalline silicon 61. Deposited polysilicon is but one alternate example.
Referring to FIG. 15, a gate dielectric layer 68 and a gate construction 70 are provided over channel material 66. Opposing source/drain regions 72 and 74 are provided laterally proximate channel material 66. As shown, such preferably constitute a portion of semiconductive material layer 64. Such might be formed by ion implantation or other doping with or without masking. Further, such doping might occur prior to forming gate 70, prior to forming semiconductive channel material 66 or prior to forming void 65. One example preferred process would be to dope semiconductor material layer 64 to a desired source/drain concentration prior to forming void 65, whereby the removing to form such void removes semiconductive material previously subjected to such doping.
Such provides but one exemplary alternate method embodiment, and as well depicts integrated circuitry construction in accordance with aspects of the invention independent of the method of fabrication. Such integrated circuitry comprises a substrate having a field effect transistor formed thereon. FIG. 15 illustrates dielectric insulative material masses 75 and 76 received beneath and contacting source/drain regions 72 and 74, with such dielectric insulative material masses not extending to beneath channel region 66. FIG. 9 also illustrates but one additional exemplary embodiment comprising first and second dielectric insulative material masses which are received beneath and contact source/drain regions, with such masses not extending to beneath the channel region.
By way of example only, one or more of the above embodiments may achieve one or more benefits. However, no one or combination of these benefits constitutes a requirement or subject matter of the accompanying claims. A first exemplary benefit includes reduction or essential elimination of junction capacitance beneath the source/drain regions, particularly in bulk semiconductor processing. Junction leakage is also reduced or effectively eliminated, preferably. Further with respect to bulk processing, junction capacitance can be significantly reduced compared to semiconductor-on-insulator processing. Further, the above-described processing can result in a reduction of short channel effects compared to other bulk semiconductor field effect transistor processing.
Another hopefully achieved advantage is improvement in active area isolation. Such can effectively occur by an essential lateral extension of the isolation region into what previously was a total bulk active area beneath the source/drain regions in bulk wafer processing.
Further, the above processing and structure can provide for reduction or elimination of floating body effects, which still can occur in fully depleted semiconductor-on-insulator structures. Further, the above processing can be used to fully integrate with borderless/low leakage contacts where the risk of over etch into underlying substrate can be effectively eliminated by the provision of the dielectric region immediately and contacting the source/drain junctions.
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. A method of forming a field effect transistor comprising:
forming a channel region within bulk semiconductive material of a semiconductor substrate;
forming source/drain regions within the bulk semiconductive material on opposing sides of the channel region; and
after forming the source/drain regions within the bulk semiconductive material, providing an insulative dielectric region within the bulk semiconductive material proximately beneath at least one of the source/drain regions, said providing comprising ion implanting a material through material of the previously formed source/drain regions.

2. The method of claim 1 comprising providing an insulative dielectric region within the bulk semiconductive material proximately beneath both source/drain regions.

3. The method of claim 1 comprising forming the at least one source/drain region and providing the insulative dielectric region to physically contact one another.

4. The method of claim 1 wherein the bulk semiconductive material comprises monocrystalline silicon.

5. The method of claim 1 comprising forming both the source/drain regions and the channel regions to comprise crystalline elemental silicon.

6. The method of claim 1 comprising forming a gate construction over the channel region prior to the ion implanting.

7. The method of claim 1 comprising forming a gate construction over the channel region prior to the ion implanting, and forming an implant mask over the gate construction prior to the ion implanting.

8. The method of claim 1 comprising forming a gate over the channel region prior to the ion implanting, the gate construction comprising insulative sidewall spacers, and forming an implant mask over the gate construction including the insulative sidewall spacers prior to the ion implanting.

9. The method of claim 1 comprising forming a gate over the channel region prior to the ion implanting, the gate construction comprising an insulative cap, and forming an implant mask over the gate construction including the insulative cap prior to the ion implanting.

10. The method of claim 1 comprising forming a gate over the channel region prior to the ion implanting, the gate construction comprising an insulative cap and insulative sidewall spacers, and forming an implant mask over the gate construction including the insulative cap and the insulative sidewall spacers prior to the ion implanting.