# Patent

## Title
Field effect transistor with narrow bandgap source and drain regions and method of fabrication

## Abstract

A transistor having a narrow bandgap semiconductor source/drain region is described. The transistor includes a gate electrode formed on a gate dielectric layer formed on a silicon layer. A pair of source/drain regions are formed on opposite sides of the gate electrode wherein said pair of source/drain regions comprise a narrow bandgap semiconductor film formed in the silicon layer on opposite sides of the gate electrode.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of field effect transistors and more particularly to a field effect transistor having a pair of source/drain regions formed from a narrow bandgap semiconductor film and its method of fabrication.
2. Discussion of Related Art
Integrated circuits, such as microprocessors, digital signal processors, and memory devices are made up of literally millions of transistors coupled together into functional circuits. An example of a conventional metal oxide semiconductor field effect transistor (MOSFET) 100 is illustrated in FIG. 1. Transistor 100 includes a gate electrode 102 formed on a gate dielectric layer 104 which in turn is formed on a monocrystalline silicon substrate. A pair of sidewall spacers 108 are then formed along laterally opposite sidewalls of the gate electrode 102. A pair of source/drain regions 110 are then formed along opposite sides of the gate electrode 102 as shown in FIG. 1. The source and drain regions comprise heavily doped portions of the silicon substrate 106. Typically, a silicide layer 112, such as titanium silicide or nickel silicide, is used to couple contacts 120 formed in a interlayer dielectric 140 to the source and drain regions 110. Silicide regions 112 are generally formed by alloying a metal, such as titanium, nickel or cobalt with the silicon substrate 106 to form the metal silicide. Additionally, contacts 120 are generally formed from a relatively high resistance film such as tungsten which can be conformally deposited so that it fills contact opening formed in the into dielectric layer 140.
The dimension of transistor 100 are continually being scaled down in order to increase packing density and thereby increase the computational power of the fabricated integrated circuits. Unfortunately, as transistor 100 is continually scaled down, the external resistance of the device (Rext) is increased degrading device performance, such as its drive current. Presently, the problem of increased Rext is solved by high active doping of the source and drain region and fully siliciding the source and drain regions. High active doping of the source and drain regions can decrease the electron mobility in the source and drain regions. Fully siliciding the source and drain regions results in a schkotty barrier transistors resulting in ambipolar conduction. Additionally, formation of silicide films by alloying a metal and the semiconductor substrate together can increase the thermal budget of the device which can decrease device performance.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an illustration of a cross-sectional view of a standard MOS nonplanar field effect transistor.
FIGS. 2A and 2B are illustrations of cross-sectional views of nonplanar field effect transistors having a pair of source/drain regions formed from a narrow bandgap semiconductor in accordance with embodiments of the present invention.
FIGS. 3A-3F illustrate a method of fabricating a field effect transistor having a pair of source/drain regions comprising a narrow bandgap semiconductor in accordance with embodiments of the present invention.
FIG. 4 is a plot showing the electron mobility verses sheet carrier concentration for indium antimonide (InSb).
FIG. 5 is an illustration of a nonplanar transistor having a pair of source/drains formed from a narrow bandgap semiconductor.
FIG. 6 illustrates a system in accordance with one embodiment.



DETAILED DESCRIPTION OF THE PRESENT INVENTION
The present invention is a field effect transistor having a pair of source/drain regions formed from a narrow bandgap semiconductor film. In the following description numerous specific details are set forth in order to provide a thorough understanding of the present invention. In other instances, well known semiconductor processing techniques and equipment have not been forth in particular detail in order to not unnecessarily obscure the present invention.
Embodiments of the present invention include a field effect transistor having a pair of source/drain regions formed from a narrow energy bandgap (e.g., less than 0.75 eV) semiconductor film. In an embodiment of the present invention, the silicon substrate adjacent to the gate electrode is etched out and refilled with a selective epitaxial regrowth of a narrow bandgap high mobility compound semiconductor material. By forming the source and drain regions from a narrow bandgap high conductivity semiconductor, such as indium antimonide (InSb), indium arsenide (InAs), indium gallium arsenide (In1-xGaxAs(x>50%)) and indium phosphide (InP), a significant reduction in the parasitic series resistance in extremely scaled sub-50 nanometer MOSFETs can be obtained. Additionally, in embodiments of the present invention, non-alloyed ohmic contacts are used to make contact to the narrow bandgap semiconductor film which results in a very low thermal budget in the front end process thereby improving device performance. Additionally, in embodiments of the present invention, the transistor is formed in a thin silicon layer of silicon-on-insulator (SOI) substrate in order to decrease or remove the possibility of increased junction leakage current. Source and drain regions formed from a narrow bandgap semiconductor material can be used to decrease the parasitic resistance in both planar and nonplanar devices.
FIG. 2A is an example of a metal oxide semiconductor field effect transistor (MOSFET) 200 in accordance with an embodiment of the present invention. Transistor 200 has a gate electrode 202 formed on a gate dielectric layer 204 formed on a silicon layer 206. A pair of sidewall spacers 208 are formed along laterally opposite sidewalls of gate electrode 202 as shown in FIG. 2A. Transistor 200 has a pair of source/drain regions 210 formed on opposite sides of gate electrode 202. A channel region 206 is located in the silicon layer between the source and drain regions 210.
In an embodiment of the present invention, the source and drain regions 210, or a portion of the source and drain regions 210, are formed from a low or narrow bandgap energy semiconductor film 212. In an embodiment of the present invention the narrow bandgap semiconductor is an epitaxial or single crystalline film. In an embodiment of the present invention, the source and drain regions 210 are formed from a narrow bandgap semiconductor 212 having an energy bandgap of less than 0.75 eV and in embodiments less than 0.36 eV. In an embodiment of the present invention, the narrow bandgap semiconductor film 212 is a compound semiconductor film having a bandgap between 0.75 eV and 0.18 eV. In an embodiment of the present invention, the source and drain regions 210 include a epitaxial narrow bandgap, high mobility compound semiconductor material 212, such as but not limited to indium antimonide (InSb), indium arsenide (InAs), indium gallium arsenide (In1-xGaxAs(x>50%)) and indium phosphide (InP). In an embodiment of the present invention, the source and drain region 210 are formed from a compound semiconductor 212 having bulk mobility between 10,000-80,000 μ (cm2V−1s−1). An advantage of using a narrow bandgap semiconductor in the source and drain regions, is that they have low sheet resistivities due to their inherently high mobilities and superior electron transport properties as compared to silicon. FIG. 4 is a plot which illustrates the electron mobility in InSb vs. the carrier concentration. The higher mobility results in a low sheet resistance (Rsd). Another advantage of using a narrow bandgap semiconductor 212 with very high mobility is that it provides a lower band offset, Øb, and a lower effective electron mass (m*) which results in a lowering of the contact resistivity, ρc, which results in a lower contact resistance, Rc, compared to conventional n+implanted silicon source and drain regions with nickel silicide contacts.
In an embodiment of the present invention, transistor 200 has raised source and drain regions. Raised source and drain regions are formed by having the narrow bandgap semiconductor film 212 extend above the top surface 201 of the silicon film upon which the gate dielectric layer 204 is formed. In an embodiment of the present invention, the narrow bandgap semiconductor film 212 of the source and drain regions 210 extends at least 200 Å above the silicon surface 201 and generally between 200-300 Å above the surface 201. The dielectric sidewall spacers 208 isolate the raised portion of the source and drain regions 210 from the gate electrode 202. As such, the spacers 208 are formed to a width sufficient enough to isolate the narrow bandgap semiconductor films from the gate electrode 202 as well as wide enough to reduce the parasitic Miller capacitance which can develop between the raised source and drain regions and the gate electrode 202. Additionally, spacers 208 should not be formed too wide so that current traversal path is increased and packing density decreased. In an embodiment of the present invention, the sidewall spacers are formed from an insulating film, such as silicon nitride or silicon oxide, having a width of between 100-500 Å.
In an embodiment of the present invention, as illustrated in FIG. 2A, the narrow bandgap semiconductor 212 laterally extends completely beneath spacer 208 and slightly extends beneath or undercuts the gate dielectric/gate electrode 204/202. In an embodiment of the present invention, the narrow bandgap semiconductor 212 extends beneath the outside edges of the gate electrode 210 by approximately 50-70 Å.
Additionally, when forming an n type field effect transistor (FET) where the majority carriers are electrons, the narrow bandgap semiconductor is doped to an n type conductivity and when forming a p type field effect transistor where the majority carriers are holes, the narrow bandgap semiconductor 212 is doped to a p type conductivity. A narrow bandgap semiconductor 212, such as indium antimonide (InSb), indium arsenide (InAs), indium gallium arsenide (In1-xGaxAs(x>50%)) and indium phosphide (InP), can be doped to an n type conductivity with, for example, tellurium (Te), silicon (Si) and sulfur (S) and doped to a p type conductivity with, for example, carbon (C), cadmium (Cd), zinc (Zn) and chromium (Cr).
In an embodiment of the present invention, the source and drain regions can include a pair of source/drain extensions or tip regions 214 as illustrated in FIG. 2B. Source/drain extensions are a portion of the source/drain regions 210 and are formed by doping the silicon layer and extend beneath the gate electrode and spacers. The extensions 214 are doped to a p type conductivity for a p type device and to an n type conductivity for a n type device. If the source/drain extensions 214 are provided, then the narrow bandgap semiconductor portion 212 of the source and drain regions 210, need not extend beneath the outside edges of the gate electrode because the laterally tip regions 214 extend the source and drain regions 210 beneath the spacers 208 and gate electrode 202 as shown in FIG. 2B. In an embodiment of the present invention, the narrow bandgap semiconductor 212 extends only slightly beneath the sidewall spacers 208 where it contacts the source/drain extensions 214. In yet another embodiment of the present invention, the narrow bandgap semiconductor 212 contacts the source/drain extensions 214 in alignment with the outside edges of the sidewall spacers 208.
In an embodiment of the present invention, transistor 200 is formed in a silicon layer which is part of a monocrystalline silicon substrate. When transistor 200 is formed in a monocrystalline silicon substrate, the channel region 206 is formed in a doped region of the monocrystalline silicon substrate and the narrow bandgap semiconductor film 210 is formed in recesses formed in the monocrystalline silicon substrate. When a transistor is formed in a monocrystalline silicon substrate, the transistor is sometimes referred to as “a bulk” transistor. Unfortunately, forming the source and drain regions 210 from a low bandgap semiconductor, a bulk device can cause an increase the junction leakage current. Accordingly, in an embodiment of the present invention, in order to reduce the junction leakage current, transistor 200 is formed in a thin epitaxial silicon film 220 of a silicon-on-insulator (SOI) substrate 224. A silicon-on-insulator (SOI) substrate 224 includes a thin silicon film 220 formed on a buried oxide layer 222 which in turn is formed on a monocrystalline silicon substrate 226. In an embodiment of the present invention, the narrow bandgap semiconductor portions 212 of the source and drain regions 210 are formed in recesses formed in the epitaxial silicon film 220 of the SOI substrate 226. In an embodiment of the present invention, a small portion 230 of the epitaxial silicon film 220 remains between the buried oxide layer and the bottom of the narrow bandgap semiconductor 212 so that an epitaxial narrow bandgap semiconductor film can be selectively grown on the epitaxial silicon film 220. In an embodiment of the present invention, the portion 230 of the epitaxial silicon film remaining between the buried oxide layer and the narrow bandgap semiconductor film 212 is between 10-100 Å thick.
Additionally, in an embodiment of the present invention, an interlayer dielectric 240, such as but not limited to a silicon dioxide film or a low k dielectric, such as a fluorine doped silicon dioxide (SiOF), a carbon doped oxide (CDO) or a zeolite dielectric, is formed over and around transistor 200 as shown in FIGS. 2A and 2B. The interlayer dielectric layer 240 isolates transistor 200 from the upper levels of metallization 260 used to interconnect the various transistor 200 formed on substrate 224 into function circuits, such as microprocessors, digital signal processors and memory devices. Metal contacts 250 are formed through the interlayer dielectric and directly contact the narrow bandgap semiconductor material 212 of the source and drain regions 210 to provide electrical connection between the first level of metallization and the source and drain regions 210 as illustrated in FIG. 2A. In an embodiment of the present invention, the metal contacts 250 form a low resistance non-alloyed ohmic contact with a narrow bandgap semiconductor 210. In an embodiment of the present invention, metal contacts 250 comprise a lower titanium adhesion layer 252 and a bulk gold (Au) layer 254. In an embodiment of the present invention, there is no silicide or alloyed films formed between the narrow bandgap semiconductor film 210 and the contact 250. The use of a non-alloyed metal contact 250 results in a very low thermal budget in the front end of the process flow and its associated benefits.
It is to be appreciated that the present invention is not limited to planar devices and can be used in nonplanar devices, such as tri-gate transistors and dual gate transistors. FIG. 5 illustrates a tri-gate transistor 500 having source and drain regions comprising a narrow bandgap semiconductor film 212. In a nonplanar device, instead of forming the gate dielectric layer 204 on a single surface of a silicon film, the silicon film is first patterned into a silicon body having a top surface opposite a bottom surface formed on buried oxide and a pair of laterally sidewalls as shown in FIG. 5. The gate dielectric layer 204 is then formed on the top surface and sidewalls of the silicon body as shown in FIG. 5. A gate electrode is then formed on the gate dielectric layer on the top surface of a silicon body and directly adjacent to the gate dielectric layer 204 on the sidewalls of the semiconductor body so that the gate electrode 202 surrounds the channel region of the silicon body on three sides. In a FINFET or dual gate device, the gate electrode 202 can be isolated from the silicon body by a thick dielectric layer (not shown) so that the gate only controls two sides of the body. The portion of the silicon body on opposite sides of the gate electrode used to form the source and drain regions can then be partially etched away so that the narrow bandgap semiconductor film 212 can be regrown to form the source and drain regions 210.
FIGS. 3A-3F illustrate a method of forming a field effect transistor having source and drain regions formed from a narrow bandgap semiconductor film in accordance with embodiments of the present invention. The fabrication of a transistor in accordance with the present invention begins with a substrate. In an embodiment of the present invention, the substrate is a monocrystalline silicon substrate, for example, when forming a bulk semiconductor device. In an embodiment of the present invention, the monocrystalline silicon substrate may include an epitaxial silicon film formed on the monocrystalline substrate as is well known in the art. In yet another embodiment of the present invention, the substrate is a silicon-on-insulator (SOI), such as substrate 300 as illustrated in FIG. 3A. In an embodiment of the present invention, the silicon-on-insulator substrate 300 includes an epitaxial silicon film 302 formed on a buried oxide layer 304 which in turn is formed on a monocrystalline silicon substrate 306. In an embodiment of the present invention, the buried oxide layer has a thickness between 200-2000 Å. Additionally, in an embodiment of the present invention, the epitaxial silicon film or layer 302 has a thickness of between 10-400 Å.
Isolation regions, not shown, are typically formed in the silicon film 302 or substrate adjacent to the device fabrication area to isolate the fabricated transistor from adjacent transistors. The epitaxial silicon film 302 is then doped to a conductivity type and concentration desired for the channel region of the device. In an embodiment of the present invention, when fabricating a p type transistor, the silicon film can be doped to an n type conductivity and when fabricating an n type device the silicon film 302 can be doped to a p type conductivity. Typically, the semiconductor film 302 will be doped to an n type or p type conductivity with a concentration level between 1×1016-1×1019 atoms/cm3. In an embodiment of the present invention, the silicon film 302 is left undoped or intrinsic in order to fabricate a transistor with an undoped or intrinsic channel region.
Next, as shown in FIG. 3B, a gate dielectric layer 308 is formed on the epitaxial silicon film 302 and a gate electrode 310 formed on the gate dielectric layer 308. The gate dielectric layer can be a deposited or grown dielectric. In an embodiment of the present invention, the gate dielectric layer is a silicon dioxide or a silicon oxynitride dielectric film grown with a wet/dry oxidation process. In an embodiment of the present invention, the silicon oxide film is grown to a thickness between 5-15 Å. In an embodiment of the present invention, the gate dielectric layer is a deposited dielectric, such as but not limited to a high dielectric constant film (high k), such as a metal oxide dielectric, such as tantalum pentaoxide, titanium oxide, hafnium oxide, zirconium oxide, aluminum oxide, and various silicates or other high k dielectric, such lead zirconium titanate (PZT) and barium strontium titanate (BST). A high dielectric constant film can be formed by any well known technique, such as but not limited to chemical vapor deposition (CVD) and atomic layer deposition (ALD).
The gate electrode 310 can be formed by any well known technique, such as by blanket depositing a gate electrode material over substrate 300 and then utilizing well known photolithography and etching techniques to pattern the gate electrode material into a gate electrode 310. In an embodiment of the present invention, the gate electrode 310 has a pair of laterally opposite sidewalls separated by a distance which defines the gate length (Lg) of the transistor. In an embodiment of the present invention, gate electrode 310 has a gate length (Lg) of less than 50 nanometers. The gate electrode 310 may be formed from any suitable gate electrode material. In an embodiment of the present invention, the gate electrode material may comprise a polycrystalline silicon. In an embodiment of the present invention, the electrode material comprises a polycrystalline silicon germanium alloy. In yet another embodiment of the present invention, the gate electrode material may comprise a metal film, such as tungsten, tantalum and their nitrides. In an embodiment of the present invention, the gate electrode has a thickness or height between 200-3000 Å.
It is to be appreciated that when forming a nonplanar transistor, such as a tri-gate or dual gate transistor, epitaxial silicon film 302 would first be patterned into a silicon body having a top surface opposite the buried oxide layer 304 and a pair of laterally opposite sidewalls as is well known in the art. When forming a tri-gate device, the gate dielectric layer would be formed on the top surface and sidewalls of the fabricated semiconductor body and then the gate electrode formed on and adjacent to the gate dielectric layers so that the gate electrode surrounds the semiconductor body on three sides as is well known in the art. When forming a dual gate or FINFET device, a thick insulating layer may be left on the top surface of a semiconductor body to isolate the gate electrode from the top surface of a semiconductor body so that the gate electrode controls only the two sidewalls of the body as is well known in the art.
Next, after the formation of gate dielectric layer 308 and gate electrode 310, source/drain extensions 309 may be formed in semiconductor film 302 on opposite sides of gate electrode 310 as shown in FIG. 3B, if desired. Source/drain extensions 309 can be formed by, for example, ion implanting dopants into the silicon layer 302. The gate electrode 310 can act as a mask to prevent the doping of a channel region 320 during the ion implantation process resulting in the formation of source/drain extensions 309 which are formed in alignment with the outside edges of gate electrode 310. A subsequent anneal can be used to cause the dopants to slightly diffusion beneath the outside edges of the gate electrode 310 as shown in FIG. 3B. Then gate electrode 310 is a polysilicon gate electrode the source/drain extension implant process can be used to dope the polysilicon film to reduce its resistance. The source/drain extensions 309 can be used, for example, when the narrow bandgap semiconductor film for the source and drain regions is not extended beneath the gate electrode. When fabricating a transistor where the narrow bandgap semiconductor laterally extends beneath the gate electrode 310, the source/drain extension formation process may not be necessary. Omitting the source/drain extension process can help reduce the thermal budget of the front-end process and thereby improve device performance.
Next, as shown in FIG. 3C, a pair of thin sidewall spacers 312 are formed adjacent to the laterally opposite sidewalls of gate electrode 310. Sidewall spacers 312 can be formed by blanket depositing a conformal insulating film, such as silicon nitride, silicon oxynitride, or silicon oxide or a combination thereof over substrate 300 and gate electrode 310 and then anisotropically etching back the dielectric film to form spacers 312. The thickness of the deposited dielectric film determines the thickness of the spacers 312. In an embodiment of the present invention, the spacers 312 are formed to a thickness between 100-500 Å. In an embodiment of the present invention, the spacers 312 are formed from a silicon nitride film formed by a hot wall low pressure chemical vapor deposition process (LPCVD).
Next, a pair of recesses 316 are formed in silicon film 302 on opposite sides of gate electrode 308 as shown in FIG. 3D. In an embodiment of the present invention, recesses 316 extend beneath the outside edge of gate electrode 310 and gate dielectric layer 308 as shown in FIG. 3D. In an embodiment of the present invention, recesses 316 extend between approximately 50-70 Å beneath the outside edges of the gate electrode 310. In an embodiment of the present invention, the recesses are formed to a depth of approximately 5-350 Å beneath the surface 301 of silicon film 302 upon which the gate dielectric layer 308 is formed. In an embodiment of the present invention, a portion of the silicon film 302 remains in recess 316 above buried oxide layer 304 so that narrow bandgap semiconductor film can be subsequently selectively epitaxially grown thereon. Any well known and suitable etching technique can be utilized to form recesses 316. In an embodiment of the present invention, a selective etch process is used to form recesses 316. In a selective etch process, an etchant is used which only etches silicon films, such as epitaxial silicon layer 302 and not insulating films such as spacers 310, gate dielectric 308 and isolations regions. (If gate electrode 310 is formed from polycrystalline silicon, an insulating hard mask can be used to form gate electrode 310 then left on during the etching of recesses 316 to protect the silicon electrode from being etched). In an embodiment of the present invention, an isotropic etching processes, such as a wet etch, is used to form recesses 316 so that the recess 316 can laterally undercut spacers 312 and gate electrode 310. In an embodiment of the present invention, recesses 316 are formed with a wet etch comprising an ammonium hydroxide or asymmetric or symmetric tetra (alkyl) ammonium hydroxides (alkyl=methyl, ethyl, propyl, isopropyl, butyl, tert-butyl; symmetric meaning all 4 alkyl groups are the same).
Next, as shown in FIG. 3E, recesses 316 are filled with a narrow bandgap semiconductor film, such as but not limited to indium antimonide (InSb), indium arsenide (InAs), indium gallium arsenide (In1-xGaxAs(x>50%)) and indium phosphide (InP). In an embodiment of the present invention, the narrow bandgap semiconductor 318 is deposited to a thickness to fill recess 318 and in another embodiment of the present invention, is deposited to a thickness sufficient to extend above the top surface 301 of silicon film 302 in order to form raised or elevated source and drain regions. In an embodiment of the present invention, the narrow bandgap semiconductor film extends at least 200 Å above top surface 301 of silicon film 302 and in an embodiment of the present invention, extends between 200-500 Å.
Any well known and suitable technique can be used to deposit narrow bandgap semiconductor film 318. In an embodiment of the present invention, the narrow bandgap semiconductor film 318 is grown with an epitaxial process, such as but not limited to, molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD), and low pressure chemical vapor deposition (LPCVD).
In embodiment of the present invention, the narrow bandgap semiconductor film 318 is insitu doped (i.e., doped while it is deposited) to an n type or p type conductivity. Semiconductor film 318 can be doped to an n type conductivity utilizing dopant atoms, such as silicon (Si) or tellurium (Te) and can be doped to a p type conductivity atoms utilizing, for example, carbon (C), cadmium (Cd), zinc (Zn) and chromium (Cr). In an embodiment of the present invention, the narrow bandgap semiconductor film has a melting temperature between 550-500° C. so that it can be deposited at a relatively low temperature, such as less than 500° C., and still enable dopant atoms to substitute into the narrow bandgap semiconductor lattice without requiring an activation anneal. Eliminating an activation anneal reduces the thermal budget of the process used to fabricate the transistor.
In an alternative embodiment of the present invention, the narrow bandgap semiconductor film 318 can be deposited undoped and then subsequently doped by, for example, ion implantation or solid source diffusion.
In an embodiment of the present invention, the narrow bandgap semiconductor film 318 is selectively grown into recesses 316. In a selective deposition process, the narrow bandgap semiconductor forms only on silicon locations, such as epitaxial silicon film in recesses 316 and not on insulating or dielectric layers, such as sidewall spacers 312 and isolation regions. If gate electrode 310 is formed from a polycrystalline silicon or silicon germanium alloy, then an insulating hard mask can be used during the patterning of the gate electrode and left on during the growth of the narrow bandgap semiconductor 318 in order to prevent the formation of a narrow bandgap semiconductor film on the gate electrode. The epitaxial silicon film 302 in the lower parts of recesses 316 provide a single crystalline lattice from which an epitaxial narrow bandgap semiconductor film can be grown.
In an embodiment of the present invention, an InSb epitaxial silicon film 318 is grown in recess 316 utilizing a MOCVD process utilizing an equimolar gaseous mixture of volatized trimethylindium and stibane (SbH3) with about 5% each by mass in a carrier gas such as argon. In an embodiment of the present invention, an indium antimonide (InSb) epitaxial film is grown utilizing molecular beam epitaxial (MBE) utilizing a solid source.
In embodiment of the present invention, non-alloyed ohmic contacts 340 are used to electrically couple the source and drain region to the upper levels of metallization 350. Accordingly, first an interlayer dielectric (ILD) 330, such as but not limited to silicon dioxide (SiO2), fluorine doped silicon dioxide (SiOF) and carbon doped oxide (CDO), is blanket deposited over substrate 300 including gate electrode 310, spacers 312 and narrow bandgap semiconductor 318. The interlayer dielectric 330 is formed to a thickness sufficient to isolate the gate electrode 310 and narrow bandgap semiconductor 318 from the subsequently formed upper level of metallization 350. In an embodiment of the present invention, the interlayer dielectric is formed to a thickness between 500 Å-2000 Å. It is to be appreciated that an interlayer dielectric need not necessarily be a single layer dielectric and can be a composite dielectric layer formed from multiple insulating layers. After depositing the interlayer dielectric, the interlayer dielectric may be planarized to provide a planar top surface.
Next, contact openings can then be formed through interlayer dielectric 330 utilizing well known photolithography and etching techniques at locations where contacts 340 are desired to contact the narrow bandgap semiconductor 318. Conductive contacts 340 are then formed into the openings and make direct contact with the narrow bandgap semiconductor film 318. In an embodiment of the present invention, contacts 340 are formed by blanket depositing a conductive contact film by, for example, chemical vapor deposition (CVD) or atomic layer deposition (ALD) into the openings and onto the top surface of ILD 330. The conductive film is then removed from the top surface of the ILD by, for example, plasma etch back or chemical mechanical planarization, to form contacts 340. In an embodiment of the present invention, the contacts are formed from a high conductivity metal or metals having a low work function, such as a work function less than 4 eV. In an embodiment of the present invention, the contacts 340 are bi-layer contacts including a thin adhesion layer 342 and a bulk metal 344 having a high conductivity such as illustrated in FIG. 3F. In an embodiment of the present invention, contacts 340 include a thin titanium (Ti) adhesion layer of less than 150 Å and a gold (Au) bulk material 344. In an embodiment of the present invention, the contacts 340 do not include any alloyed metals or suicides (i.e., metal silicon alloy, such as nickel silicide and titanium silicide). In this way, the thermal budget can remain low and device performance reliability improved. Next, upper levels of metallization 350 can then be formed in contact with contacts 340 to electrically couple the fabricated transistor with other transistors into functional circuits.
This completes the fabrication of a field effect transistor having source and drain regions formed from a narrow bandgap semiconductor.
FIG. 6 illustrates a system 600 in accordance with one embodiment. As illustrated, for the embodiment, system 600 includes computing device 602 for processing data. Computing device 602 may include a motherboard 604. Motherboard 604 may include in particular a processor 606, and a networking interface 608 coupled to a bus 610. More specifically, processor 606 may comprise the device 200 or 500 that has the earlier described narrow bandgap source and drain regions.
Depending on the applications, system 600 may include other components, including but are not limited to volatile and non-volatile memory, a graphics processor, a digital signal processor, a crypto processor, a chipset, mass storage (such as hard disk, compact disk (CD), digital versatile disk (DVD) and so forth), and so forth. One or more of these components may also include the earlier described narrow bandgap source and drain regions.
In various embodiments, system 600 may be a personal digital assistant (PDA), a mobile phone, a tablet computing device, a laptop computing device, a desktop computing device, a set-top box, an entertainment control unit, a digital camera, a digital video recorder, a CD player, a DVD player, or other digital device of the like.

## Claims

1. A transistor comprising:
a gate electrode formed on a gate dielectric layer formed on a silicon layer;
a pair of sidewall spacers formed along opposite sides of said gate electrode; and
a pair of source/drain regions formed on opposite sides of and extending beneath said gate electrode, said pair of source/drain regions comprising a narrow bandgap semiconductor film, wherein said narrow bandgap semiconductor film has a bulk mobility between 10,000 to 80,000 cm2V−1s−1 and wherein said narrow bandgap semiconductor film is doped to an n type or p type conductivity;
wherein said pair of source/drain regions comprising said narrow bandgap semiconductor film is formed adjacent to said sidewall spacers and above a top surface of said gate dielectric layer.

2. The transistor of claim 1 wherein said narrow bandgap semiconductor film has energy bandgap of less than 0.75 eV.

3. The transistor of claim 2 wherein said narrow bandgap semiconductor film is selected from the group consisting of indium antimonide (InSb), indium arsenide (InAs), and indium gallium arsenide (In1−xGaxAs(x>50%)).

4. The transistor of claim 1 wherein said narrow bandgap semiconductor film has a melting point of less than 550° C.

5. The transistor of claim 1 wherein said pair of source/drain regions comprise source/drain extensions formed in said silicon layer and beneath said gate electrode.

6. The transistor of claim 1 wherein said pair of source/drain regions have bottom surface in contact with said silicon layer.

7. The transistor of claim 1 wherein said narrow bandgap semiconductor film is doped to an n type conductivity.

8. The transistor claim 1 wherein said narrow bandgap semiconductor film is doped with a dopant selected from the group consisting of Te or Si or S.

9. The semiconductor device of claim 1 further comprising a metal contact in direct contact with said narrow bandgap semiconductor film wherein said metal contact comprises a Ti/Au bi-layer contact.

10. The transistor of claim 1 wherein said silicon layer is a silicon body having a top surface opposite a bottom surface formed on an insulating substrate, and wherein said gate dielectric is formed on said top surface and sidewalls of said silicon body and said gate electrode is formed on said gate dielectric on the top surface of said silicon body and adjacent to said gate dielectric layer on said sidewalls of said silicon body.

11. A transistor comprising:
a gate dielectric layer formed on a first surface of a silicon layer;
a gate electrode having a pair of laterally opposite sidewalls formed on said gate dielectric layer;
a pair of sidewall spacers formed adjacent to said pair of laterally opposite sidewalls of said gate electrode;
a pair of source/drain regions formed from an epitaxial narrow bandgap semiconductor film having a bandgap of less than 0.75 eV and a electron mobility greater than 10,000 cm2V−1s−1 wherein said narrow bandgap semiconductor film of said pair of source/drain regions extends beneath said gate dielectric layer and has a top surface extending above a top surface of said gate dielectric layer; and
a pair of non-alloyed contacts in direct contact with said narrow bandgap semiconductor.

12. The transistor of claim 11 wherein said narrow bandgap semiconductor is selected from the group consisting of indium antimonide (InSb), indium arsenide (InAs), and indium gallium arsenide (In1−xGaxAs(x>50%)).

13. The transistor of claim 11 wherein said pair of non-alloyed contacts comprise a bi-layer titanium/gold contact.

14. The transistor of claim 11 wherein said narrow bandgap semiconductor is doped to an n type conductivity with a dopant selected from the group consisting of Te or Si.

15. The transistor of claim 11 wherein said narrow bandgap semiconductor is doped to a p type conductivity and wherein said dopant is carbon.

16. The transistor of claim 11 wherein said silicon layer is a portion of a monocrystalline silicon substrate.

17. The transistor of claim 11 wherein said silicon layer is an epitaxial silicon film formed on a buried oxide layer formed on a silicon substrate.

18. The transistor of claim 11 wherein said silicon layer is a silicon body having a top surface opposite a bottom surface formed on an insulating substrate, and wherein said gate dielectric layer is formed on said top surface and sidewalls of said silicon body and said gate electrode is formed on said gate dielectric layer on the top surface of said silicon body and adjacent to said gate dielectric layer on said sidewalls of said silicon body

19. A transistor comprising:
a gate electrode formed on a gate dielectric layer formed on a silicon layer; and
a pair of source/drain regions formed on opposite sides of and extending directly beneath said gate electrode , said pair of source/drain regions comprising a narrow bandgap semiconductor film, wherein said narrow bandgap semiconductor film has a bulk mobility between 10,000 to 80,000 cm2V−1s−1 and is doped to a p type conductivity or a n type conductivity.

20. The transistor of claim 19, wherein said pair of source/drain regions comprising said narrow bandgap semiconductor film is formed adjacent to said sidewalls spacers and above a top surface of said silicon layer.

21. The transistor of claim 19 wherein said narrow bandgap semiconductor film has a melting point of less than 550° C.

22. The transistor of claim 19 further comprising a pair of non-alloyed contacts in direct contact with said narrow bandgap semiconductor film.