FET WITH NOTCHED GATE AND METHOD OF MANUFACTURING THE SAME

Abstract: An FET has a T-shaped gate. The FET has a halo diffusion self-aligned to the bottom portion of the T and an extension diffusion self aligned to the top portion. The halo is thereby separated from the extension implant, and this provides significant advantages. The top and bottom portions of the T-shaped gate can be formed of layers of two different materials, such as germanium and silicon. The two layers are patterned together. Then exposed edges of the bottom layer are selectively chemically reacted and the reaction products are etched away to provide the notch. In another embodiment, the gate is formed of a single gate conductor. A metal is conformally deposited along sidewalls, recess etched to expose a top portion of the sidewalls, and heated to form silicide along bottom portions. The silicide is etched to provide the notch.

Description: FET WITH NOTCHED GATE AND METHOD OF MANUFACTURING THE SAME Field of the Invention This invention generally relates to integrated circuit chips. More particularly it relates to field effect transistors (FET) . Even more particularly it relates to an improved FET having a T-shaped gate. Background of the Invention The speed of an FET is largely determined by the distance across the gate; transistors with a shorter gate conductor distance have a shorter spacing between source and drain and are generally faster. The industry has moved to photolithography equipment that provides a shorter wavelength of light and a higher numerical aperture lens with each generation of integrated circuits to permit decreasing this dimension of the gate. However, these changes have frequently increased cross chip line width variation. Furthermore, these changes have resulted in higher gate resistance. US patent 5,750,430, to Jeong-Hwan Son describes a gate having curved sidewalls produced by depositing polysilicon for the gate in a window between spacers. The transistor has a larger dimension at the top than at the bottom. It provides a channel length that is shorter than the minimum dimension and reduced overlap capacitance. An FET with a notch at the bottom of the poly gate was described in a paper "lOOnm Gate Length High performance /Low Power CMOS Transistor," by T. Ghani et al, Technical Digest of the 1999 International Electron Devices Meeting, Washington, .DC, 1999, p 415 . The notch offsets the source-drain-extension implant and provides a shorter gate dimension with improved capacitance and also avoids increasing resistance since the gate has a larger overall cross sectional area. The spacer defined gate with the curved sidewalls and the notched gate provide advantage but substantial further device performance improvement is possible. This improvement may be derived by modifying the structures described in the '430 patent and the paper by T. Ghani. The new structures and processes to achieve those new structures are provided by the following invention. Summary of the Invention In one aspect, the present invention is an FET comprising a gate having a top portion having a top dimension and a bottom portion having a bottom dimension. The top dimension is larger than the bottom dimension. The FET further comprises a diffusion defined by the bottom portion. The FET can also include a diffusion defined by the top portion. The diffusion defined by the bottom portion may be a halo implant and the diffusion defined by the top portion may be an extension implant. Another aspect the invention is accomplished by an FET comprising a gate that includes a first conductive material under a second conductive material different from said first conductive material . The first conductive material is notched. Another aspect the invention is a method of fabricating a semiconductor device . The method includes the step of providing a substrate formed of a first material . The substrate has a surface . The next step is forming a gate dielectric on the surface. Then a gate is deposited on the gate dielectric. The gate comprises a first gate layer and a second gate layer, wherein the first gate layer contacts the gate dielectric and the second gate layer is on the first gate layer. The next step is chemically reacting edges of the first gate layer to form a first reaction product. Finally, the method involves selectively removing the first reaction product with respect to remaining portions of the first gate layer and the second gate layer so as to provide a notch in the first gate layer. According to one embodiment an FET has a T-shaped gate. The FET has a halo diffusion self-aligned to the bottom portion of the T and an extension diffusion self aligned to the top portion. The halo is thereby separated from the extension implant, and this provides significant advantages. The top and bottom portions of the T-shaped gate can be formed of layers of two different materials, such as germanium and silicon. The two layers are patterned together. Then exposed edges of the bottom layer are selectively chemically reacted and the reaction products are etched away to provide the notch. In another embodiment, the gate is formed of a single gate conductor. A metal is conformally deposited along sidewalls, recess etched to expose a top portion of the sidewalls, and heated to form suicide along bottom portions. The suicide is etched to provide the notch. Brief Description of the Drawings The foregoing, and other features and advantages of the invention will be apparent from the following detailed description of the invention, as illustrated in the accompanying drawings, in which: FIG. la are doping profiles of extension implants and halos for conventional and notched gate devices showing one side of the device; FIG. lb are net compensated doping profiles of extension implants and halos for conventional and notched gate devices; FIGS. 2a, 2b, 2c are net compensated doping profiles of extension implants and halos for conventional and notched gate devices of different channel lengths showing the full device, including source and drain; FIG. 3a is a cross sectional view of a T-shaped gate and showing that halo and extension implants are defined by different edges of the T; FIGS. 3b, 3c are cross sectional views showing process steps to provide a spacer and the source/drain implants for the a T-shaped gate of FIG. 3a; FIGS. 3d, 3e are cross sectional views showing process steps to provide an air gap behind a spacer for a T-shaped gate of FIG. 3a; FIGS 4-6 show cross sectional views illustrating a process of fabricating the T- shaped gate of FIG. 3a; and FIGS 7-10 show cross sectional views illustrating another process of fabricating a T-shaped gate without a two-layered structure for the gate material. Detailed Description of the Invention Halo, or pocket, implants are well known as a means of moderating short-channel effects in very short MOSFETs. Short channel effects include Vt lowering and subthreshold slope increase as gate length decreases. Halo formation is usually accomplished by implanting a dopant type opposite to the source/drain doping, (e.g. by implanting boron in NFΞTs) . A high energy is used for the halo implant to move it under the gate beyond the extent of the source/drain extension implant, which usually have significantly lower energies and higher doses, as shown in Table 1. As a result the p- dopant of the halo is often placed deeper than the n-dopant of the source/drain diffusion. The n-type source drain diffusion is thus decorated with a p-type halo all around. While the p-type dopant in the channel region is helpful for short channel effect, the halo extending under the source/drain has the unfortunate effect of increasing source/drain junction capacitance. Table 1 Parameters of Standard Halo, Extension, and Source/Drain Implants Implant Species Type dose Dopant energy Purpose ( per cm 2 ) coneentration (kev) ( per cm 3 ) Halo B P lxl0 13 to 8xl0 13 5xlO la to 2x10" 2 to 20 Extension AS n 8xl0 14 to 2xl0 15 5xl0 19 to lxlO 21 0.5 to 5 Source/drain As n 5xl0 15 to lxlO 16 5xl0 19 to lxlO 21 3 to 35 Although a high energy is used to move the halo away from the source/drain and extension, typically the net p-dopant available for the halo is a small fraction of the total implanted p-dopant dose since most of the p implant falls within the more heavily doped source/drain and is compensated. Thus, the standard implant technique allows only limited halo concentrations to be achieved with reasonable control, and this limits the extent to which short channel effects are controlled. Furthermore, control over the halo carrier concentration is difficult because of the small fraction of p-type dopant atoms that are left uncompensated . Therefore small variation in the dose or energy of either the halo or the extension implant can provide large variation in the halo carrier concentration. With the conventional halo, a change in halo carrier concentration of lOx is achieved over a distance of 70 to 100A when the carrier concentration is in the 10el7 to lOe 18 range. This gradual rate of change of carrier concentration is the result of the compensation of the halo implant by the extension and source/drain implants. For improved short channel control a more abrupt change in carrier concentration is needed, such as by providing the same lOx change in carrier concentration over 30 to 40A. In addition, for improved device performance the halo concentration below the source/drain can be reduced so as to reduce source/drain junction capacitance'. The higher halo doping concentration provides a narrower depletion region which provides higher capacitance. α tr cn cn rt u. 91 Ml •d pi H M. H rt rt CQ m Ω >d D. rr Ω H •d & L-I. N Ω tf rr Hi P, μ- O ø" Φ H- Φ pi i d μ- O 9i C M. P- 0 Φ Φ tr ≤ O H o H μ- ø- tf ! M Φ o 9> tr O φ 9i pi P- rt CQ 0 CQ 0 φ o £ ø Dl CQ e Q P o Hl 9 ) 91 - 0 ■ d P 0 H Φ M CQ id M H Φ 91 H- o Ω Φ . Φ tf <! 91 i l μ- CQ M ω rt <! Hi rt 0 Hi " Ω φ < o Ω " < o i Ω_ li Φ S 3 O • ; μ- Ω H 0 rt CQ ø" Ω ι-i μ- 0 i μ- φ rt Φ ø * tr l-i pi Φ CQ <l φ 91 M p ) Ό & H 0- Φ Ω 0 tr 9i 9 ) Φ 0 P- CQ 9i Φ H rr μ- CQ tf 9 ) o μ- ø •d • : ai P- P H 1 μ> O 91 rr 3 μ- 0 CQ 3 H 1 t φ μ- rt ι-3 0 Φ μ- O H 91 rt tr rr K ] P tf I- 1 9 ) P Φ φ P rt Φ o 9 ) PJ Q O ø 1 < O 0 0 H cn 0 ø * 0 tf -s rt CQ pi CQ P Φ O ι-i M IQ to 91 H CQ 0 - o ø φ CQ μ- 9i P - ] & d Ml 91 o 9i φ rt CO rt IQ CQ tr P- 0 0- <1 o o Ω. rt 91 9i μ- Ω ii μ- μ- IQ Ό 0 * o S Ω Φ H 0 rt 3 to Φ φ Ω μ- 0 * μ- ι-i 9 ) Q. 9> o " o CQ H o P. Φ Pi φ p ) Ml rt 9 ) d * C Φ d 9 ) n φ P- tr l-i CQ 9 ) 0 9 ) 1 ø O Φ CQ Ω. 0 tf id 0 φ <! φ μ- H 0 * ■ ; 9 ) CQ M rt Ω H Ω ϋ l-i CQ 0 rt " CQ CQ o CQ μ- 0- μ- μ> <! H μ- rr μ- CQ 0 91 φ « rt Φ P « CQ Ω ø- Φ O 0 Q 9 ) rr o μ- μ- CQ rt Q 91 Φ φ 0- - 3 rr Ml ft X μ- i-f rt P - Φ Φ 0 • 0 φ μ- 9 ) CQ tr O rt rt ø o tf μ- μ> tr ø 0 μ- Φ φ φ i μ- μ- rt Ω- Φ ø * o Φ H- i rr rt . ø 0 Φ rt Φ & o tf rt ø 0 o ~j Φ rt rt ø P. CQ " 0 0 Φ rt 9 ) Φ 91 0 t M 0- H 0 o Φ ≤ ø - i μ- Ω Ω Φ P 0 p- IQ ro Φ " 0 o Q PJ 0 1-3 Ω. 91 £ 0 "d r id p " μ- o tr tr 0 H ø rr • g 9 ) o φ Ω α- rt 0 - P * μ- to μ- CQ CQ μ- Φ 9 ) M tr ø Q ø φ • 0 rr rr o μ- o rt i d 91 rr Ω f- . μ- Mi Ω ø IQ φ Ω ø tr 0 tr -S 91 <1 Q Ω H- Hi O Mi Φ 9 ) 9 ) P 1 Ω & ø" H tr cn Mi ^ Φ " P- C CQ P μ- O tr < • Ω P 4 rT μ> 0 c Φ Ω ø" i-i P *. rt M Φ 0 H Φ Ω o c 91 φ ø μ- 9 ) μ- 9> • s CQ ø 91 9> μ- tf H 9 ) tr P 91 & φ rt ø" 9i CQ rr K rt < id μ- P- id ø Ω Ω, rr 91 μ- rt CQ 0 0 P (D μ- O 9 ) 9 ) rr P rt μ- P * ^ μ" μ- ø- 91 O Φ H Φ rr Ω φ o CQ tf 0 Φ 0 ø μ- CQ q 3 1 P P Φ o M 0 CQ 91 CQ H 0 Φ < ø 0 > ; ø Q μ- Φ ■ d H Ω. π CQ O CQ tr Φ o t) t o- 91 rr o 9i 0 Ω μ- O ø rr Ω- Hi μ> Mi rt l-i ø i μ- 91 0 ø 0" φ CQ cr H 1 o Φ rt n Ω rt 0- O n ≤ n en O 1 H- O rt 91 < tr rt O H P 0 H rt 91 o t Mi P rt P- p" 1 tr rt Φ 0 91 CO rr ≤ l-i " O Φ CQ Hi o CQ μ- O <! tf ≤ ø Ω 0 ) M P 3 t CQ Φ μ- Φ Φ p- D- Φ μ- φ 3 tr Φ rt rr rt < Φ >< rt Φ Φ O 91 rt o Φ Φ D- O- Φ 0 * M Φ v-> 1 o Φ rr Q φ ø Ω H rr rt μ- Ω Ω tf Φ 0 3 CO 0 0 * <! 9i 9 ) 0 < tf CQ rt Ω 91 M O l Φ ø" Ω. o CQ tf μ> 0 9i tr P. rt . 9 ) o 9i Φ iQ < Ω μ- rt C Q tr Φ tf 9 ) rt 0 0 - ϋ 9i Φ φ 0 •d Φ co o " ^ Ω tf CD P rt Hi &. 91 rt o o 0 μ- ø 4 Φ i CQ 0- Φ P rt 0 CQ Ω 1 Ω. rr 0 s; O Φ o S| P- o o rr 0 O o 0" μ- CQ •d O l-S μ- Q P. ø" Φ Ω μ- Ω Ω 4 3 c tr rt rt μ- ^ Φ - H <! Ω 9 ) 0 H »d •d o CQ ø 4 Φ o Hi o CQ tr μ- 91 0 Φ CQ CQ ø" μ- 0 μ- Φ t-i ι-S μ- 13* H- μ Φ o 0 9i O rt I H •d μ- Hi H 1 o CQ rr . 91 Φ 3 Ω n- O 1 3 Φ 3 rt 4 P 0 0 D- " ω Φ 91 0 rf φ H 9 ) ø CQ l-i tf 0 H rt rr P. ■ d •d " Ml Φ 9ι ■ d d Cf 9i rt rt H Ω Mi Φ ι-{ 0 rt rt Ω rt rt Φ 91 91 Hi Φ r 91 l-i -- ^ rt b-> -Q H n_ μ- μ- 91 P to Φ μ- ι-i CQ H tr tr Φ μ- M K rr H Φ CQ Pi M d o Ω- rt 9> 9J 91 CQ o H o 0 M ø 9 ) Ω & μ- -3 o O φ 91 Φ Ω ø H μ- o Ω 9i ø μ- M 3 φ K tf i P rt 0 0 Q 0 C Φ P- ø 1 φ Ω o μ- S. 0 i ø" ø- o o 3 91 •d rr rr ø o X μ- M rt Φ rr 0 μ- . 91 Φ P Ω ø" ø rt Φ 0- 0 φ pi 9 ) • s 0 CQ M Φ CQ <! rr rt μ- m o - P. cn H CQ Φ P ISO 0 " Ω 9i 4 o φ φ H 0 H φ *. rr Φ o $, μ- tf Φ P <! rt Mi t o tr 0 o 0 o ø <l 0 O o H, tf CQ Hi o tf Ω P- Φ S 3 μ- ø" rt M <! P Ω 1 rt ■ g rt - Φ CQ 0 Hi rt Φ μ- rr Φ cn Φ . μ- Mi μ- 0 φ α. φ φ P- Φ p 1 O rt 0 - 91 O 9> Ω -> Φ 0 Φ P 4 0 Ω Ω >d tf " Ω. 3 £ H -> ø * t & CQ μ> M CQ ø μ- X rt Φ Φ H Mi rt Φ Φ ø Φ 0 91 H o Φ Φ CQ Φ rr φ <! o 0 rr tr cn co P Ω rt 0 ■ d Φ Ω μ 1 tf M M rt ø Ω 91 d id CQ X Φ tf Φ rt P Φ o o rr o tr tf CQ l-i ^ P- Φ rr rr φ ø CQ o <! φ o •* μ- 0" rt Ω o Φ Ω CQ φ Ω P P. 91 ø rt tf P Φ o & ø ø * φ o H o Φ rt CQ 91 > ; Φ 0 rt 0 ø Φ 0 Φ 0 Mi ø CQ tf tr <! Φ CQ μ- Q Φ ø ø * rt rt ø" rt H CQ rt g 0 0 μ- H μ> CQ Φ μ- & cn Ω H μ- Ω l-i μ- Hi 9i Hi rt o CQ pi Ω μ- μ 1 μ- 9 ) K tr μ- 0 n fX. rt 0 Λ r rr < P- 0 o 0 id Φ id Φ 0 P 0 Φ P- Ml rt Mi 0" ø t ø" 3 ^ P rt 0 • rt o μ- ø * Ω <! φ CQ P Φ 9 ) -»-. H X P P Φ Φ Φ Φ 9i ø" 0 Φ i — tf ø tr 0 Φ φ rr CQ >r co Ω . 0 r μ- <! tr Ω M rr ** a 9 ) H Φ ■ ; ø μ- ø μ> ø M 9i μ- μ- i-i <l Φ S P Φ D- ct 1 0 9 ) 0 9 ) Ω -S rr o * M i-i ø Φ rr σi P- 91 rt 3 rt id P 91 μ- P μ- CQ 0 CQ μ- μ- 9i o ø P CQ 0 9 ) μ- Hi φ Φ Φ i d μ- X μ- - CQ Φ Pl H in μ- 0- CQ rt rt μ- CQ < 0 tr 0 H- CQ 1 pi rt 0 rr id CQ o 3 rr rr o μ> CQ co tr o P Φ μ- tf rt μ- 0 rt φ o Φ rr 3 ft tf 0 ø μ- μ- μ- id tf Φ ø μ- σ. tr P> <5 tr o o 0 ø μ- CQ • tf CQ o Φ 9i tr μ- rt 9> CQ o ø Φ 0 CQ - Φ n. en μ- Φ φ p ) P rt P μ- 0 μ- ■ d Φ 0 9 ) 0 H ø rr P CQ pi μ- tr o- rt X t* * 3 9i 0 Ω ø ø 1 μ- H H 91 0 μ- α- rr 91 CQ ø K μ- CQ rr μ- H rt -3 rr CQ μ- Φ φ cQ rr 91 ø 0 ø • 3 ø β CQ CQ ] 0 μ- rt Φ o tf ø CQ CQ ø" φ Φ i d 3 M <; <1 tr rt 0 CQ ø- r P" 0" • s 3 ø to iQ ø Mi Φ o rt e Φ rt ro P P Ό P CQ tr rt B * rt Φ rt μ- φ μ- φ Φ 0 φ Φ o Φ μ- ■ fl "• ø * ø 01 Ω o Φ 1 o 9 ) " α Ml 9i μ- tr tQ CQ Φ M o H ii rr 9 ) P. o μ- M. tf H pi Φ μ- φ 9 ) rr μ" Ω α Φ φ o ø φ CQ i d rt 0 P- Φ tf 0 CQ ø r 9i O 9 ) Ω 0 3 0 o μ- a M H r φ Φ 0 μ- Ω. Φ •d3 r Ω_ rt o f . P tf μ" . CQ 4 P rr & φ Q 0 CQ IQ φ o rr l-i M Ω >d 9 ) Q •^ Φ o rr Φ J Φ tr μ- K rt rt 9 ) • a • ? Φ ø μ- tr ro 0" 91 ι-i Φ 0 w μ- a ϋ CQ CQ K. μ- rt H- rt m K rt H ø H rt Φ 0 i Φ Q Φ 0 ø Φ rr ø" 0" d Φ ^ o D. C * Φ < CQ 1 M rr tf P •. -- Ω P 9i Φ >d o Φ φ Φ l-i Φ CQ rt 0 rt 0 M twice, once before and once after spacer formation. It also requires tight control of the width of the spacer to ensure repeatable halo and implant separation. The present invention improves upon this approach. The process of the present invention provides for separating the halo and extension implants, the more abrupt junction, and greater control over the halo carrier concentration, by angle implanting the halo with an inverted T gate while providing the extension implant from a more vertical direction. The present invention thus allows the halo and extension implants to be laterally displaced from each without any masking steps between the implants. Thus, the extension block mask need only be provided once to provide both the halo and the extension. This separation of implants with a single mask is accomplished by first etching T-shaped gate 20 on semiconductor wafer 21. (The T- shaped gate can be formed using the process described herein below.) As shown in FIG. 3a, T-shaped gate 20 has lower part 22 having dimension L x and edge 23 on gate dielectric 24. T-shaped gate 20 also has upper part 26 having dimension L 2 and edge 27 on lower part 22. Notch 28 is thereby defined having height h and lateral extent u where u = (L 2 - L x ) /2. After T-shaped gate 20 has been formed, an extension block mask (not shown) is provided to block PFΞTS while NFET 30 is being formed. P-type halo 32 is then implanted at an angle q given by tan q < h/u where h is the height of the notch and u is the lateral extent of the notch. Thus, halo 32 is defined by lower part 22 of T-shaped gate 20. Typically the angle implant for the halo is provided from all four cardinal directions to provide the implant for devices on the wafer of different orientation. Any particular device receives the implant in active area along two directions, as shown in FIG. 3a. Finally extension diffusion 34 is implanted at an angle normal to semiconductor wafer 21, so extension diffusion 34 is defined by edge 27 of larger upper part 26 of gate 20. Thus, implant edge 36 of halo 32 is displaced from implant edge 38 of extension 34 by a dimension approximately equal to u, which is about equal to the dimension of notch 28, and both implants are accomplished without any masking step between the implant steps. No masking step is required to achieve this separation because the two diffusions 32 and 34 are defined by different edges 23 and 27 of gate 20, and this is accomplished by providing a vertical implant and an angled implant with inverted T- shaped gate 20. In the next step a layer of insulating material 46 is conformally deposited on all surfaces on wafer 21, as shown in FIG. 3b. Insulating material 46 is formed of a material such as silicon dioxide or silicon nitride. Then a directional etch is used to form sidewall spacers 48, as shown in FIG. 3c. Finally, source/drain 50 is implanted defined by spacers 48, as also shown in FIG. 3c. Alternatively, insulating material 46 can be non- conformally deposited so that an air gap 49 is left behind spacer 48 along the notched sidewall of first layer 56, as shown in FIG. 3d. A deposition process such as a plasma- enhanced chemical vapour deposition (PΞCVD) is known to be a more directional deposition process and will thus deposit a thicker film on a horizontal surface compared to a vertical surface. A directional etch is used to form sidewall spacers 48 with air gap 49 as shown in FIG. 3e. The resulting air gap 49 has a dielectric constant of about 1.0 while silicon dioxide sidewall spacers 48 have a dielectric constant of about 3.5. Air gap 49 reduces the effective dielectric constant of sidewall spacers 48 and thus helps to reduce overlap capacitance. While T-shaped gate 20 permits separating halo and extension implants without adding a separate masking step the process described here also provides great advantage in allowing bottom portion 22 to be fabricated with a sub-minimum dimension length, substantially increasing device performance. Furthermore, the method of the present invention allows this shrinking of the gate length without increasing line width tolerance. Furthermore the T-shaped gate provides large area top part 26 that avoids unacceptable increasing of gate resistance when bottom part 22 is shrunk. Thus, performance may be substantially improved without unacceptable negative effects . T-shaped gate 20 is formed by depositing two-layer structure 54 on gate dielectric 24 and on isolation (not shown) on wafer 21, as shown in FIG. 4. Two layer structure 54 includes a first layer, such as germanium layer 56 and a second layer, such as polysilicon layer 58. The two layers are selected from materials for which first layer 56 can be oxidized differently than second layer 58. Next, two layer structure 54 is photolithographically patterned and etched, as shown in FIG. 5, in which etched germanium layer 56 and polysilicon layer 58 have equal lengths and widths. The length or width may be the minimum dimension achievable with the photolithographic system. In the next step, two layer structure 54 is subjected to a chemical reaction step, such as an oxidation at about 500-600 C, as shown in FIG. 6. At this temperature, a germanium oxide 60 grows on exposed edges of germanium layer 56, as shown in FIG. 6. Polysilicon layer 58 will not significantly oxidize at this temperature. Germanium oxide 60 is grown to consume 7-13 nm (lateral extent u) of germanium layer 56. Then germanium oxide 60 is removed with a water rinse, providing notch 28 as shown in FIG. 3a. The oxidation step can be very tightly controlled to provide a very reproducible thickness of germanium that is consumed, and all of the oxide can be removed without further etching either the germanium or the polysilicon. Thus the oxidation and etch process provides a high level of control over the amount of germanium layer 56 that is removed, providing a tight tolerance on length or width of germanium layer 56. Germanium layer 56 thus has a dimension about 14 to about 26nm less than polysilicon layer 58. Germanium layer 56 can now have a dimension that is about 10 to about 50% less than the minimum dimension of the photolithographic system being used while polysilicon layer 58 continues to have the minimum dimension. In another embodiment, first layer 56 can consist of a germanium compound, Ge x Siι_ x , where x is in the range of about 0.5 to about 1.0. Other chemical reaction steps can be provided with good control as well. For example, first layer 56 may be polysilicon and second layer 58 may be a refractory metal. A thin layer of metal, such as platinum, titanium, tantalum, or cobalt, is conformally deposited along sidewalls of two gate layers 56, 58. The substrate is then heated so the thin metal reacts with polysilicon layer 56 to form a metal suicide along sidewall edges of first layer 56. Second layer 58, being a refractory metal, will not react. Then the suicide can be selectively etched without affecting the second layer to form notch 28. Alternatively, a single gate layer of polysilicon 66 can be used for gate 20'. FIG. 7 shows thin metal 68 deposited coating sidewalls 70 of gate polysilicon 66. FIG. 8 shows thin metal 68 is subjected to a directional etch to remove metal along horizontal surfaces and top portion 70a of sidewall 70 of gate polysilicon layer 66. Now substrate 21 is heated so remaining metal 68' reacts with polysilicon 66 to form metal silicide 72 exclusively along lower portion 70b of sidewall 70 that was coated by metal 68', as shown in FIG. 9. This silicide is then removed to form notch 28, as shown in FIG. 10. Metal silicide 72 such as cobalt silicide or titanium silicide can be wet etched in hydrogen peroxide or hot sulfuric/hydrogen peroxide mixtures. T-shaped gate 20 formed by this process has substantial advantages over prior art gates. Since the lower portion of gate 20 determines its effective channel length, this selective oxidation and trimming of germanium layer 56 can provide a higher performance device than a device without trimming with tighter control than previous attempts to form a T-shaped gate. Longer or wider top polysilicon layer 58 can be used to provide higher conductivity for the entire gate conductor. If desired, the gate conductor stack can be composed of a lower layer of germanium and an upper layer of silicon, with a graded layer of SiGe in between the two. This would provide a tapered profile of the final gate conductor structure, rather than a sharply discontinuous profile between the germanium and silicon films. Other conductive materials can be provided for first layer 56 and second layer 58 in which there is selectivity for oxidation and etching of the oxide. For example, a refractory metal, such as tungsten, tantalum, molybdenum, or titanium, or a silicide, such as titanium silicide, cobalt silicide, or platinum silicide can be used for second layer 58. Polysilicon can then be used for first layer 56. In another embodiment first layer 56 can be selectively etched of with respect to second layer 58 to provide T-shaped gate 20, eliminating the oxidation step. However, this is expected to provide somewhat less process control than providing the oxidation and then the etch step. While several embodiments of the invention, together with modifications thereof, have been described in detail herein and illustrated in the accompanying drawings, it will be evident that various further modifications are possible to provide an FET with notched gate without departing from the scope of the invention. Nothing in the above specification is intended to limit the invention more narrowly than the appended claims . The examples given are intended only to be illustrative rather than exclusive.

Claims: CLAIMS 1. An FET comprising: a gate having a top and bottom portion, the top portion having a width that is greater than the width of the bottom portion; and a diffusion self-aligned to the bottom portion. 2. The FET as recited in claim 1, wherein said diffusion comprises a first implant. 3. The FET as recited in claim 2, wherein said first implant comprises a halo implant . 4. The FET as recited in claim 2, wherein said first implant is directed at an angle from the normal to provide said implant self-aligned to said bottom portion. 5. The FET as recited in claim 1, further comprising a second implant defined by said top portion. 6. The FET as recited in claim 5, wherein said second implant comprises an extension implant. 7. The FET as recited in claim 1, further comprising a spacer adjacent said top portion and a third implant defined by said spacer. 8. The FET as recited in claim 7, wherein said third implant comprises a source/drain implant. 9. An FET comprising a gate, said gate comprising first conductive material and a second conductive material different from said first conductive material, said second conductive material on said first conductive material, wherein said second conductive material extends beyond said first conductive material to provide a T-shaped gate. 10. The FET of claim 9, wherein said first conductive material is on a gate dielectric and said gate dielectric is on a substrate. 11. The FET of claim 9 or 10, wherein said first conductive material has a dimension less than a photolithographic minimum dimension. 12. The FET of any of claims 9 to 11, wherein said first conductive material comprises a first semiconductor material. 13. The FET of claim 12, wherein said first semiconductor material comprises germanium. 14. The FET of claim 12, wherein said first semiconductor material comprises a germanium compound Ge x Si ! - x , wherein x is in the range of about 0.5 to about 1.0. 15. The FET of any of claims 9 to 11, wherein said second conductive material comprises polysilicon. 16. The FET of any of claims 9 to 11, wherein said first conductive material comprises polysilicon. 17. The FET of claim 16, wherein said second conductive material comprises a refractory metal . 18. The FET of claim 17, wherein said second conductive material comprises tungsten, tantalum, molybdenum, or titanium. 19. The FET of any of claims 9 to 11, wherein said second conductive material comprises a silicide. 20. The FET of any of claims 9 to 11, further comprising a spacer along sidewalls of said second conductive material. 21. The FET of claim 20, wherein an air gap is left behind said spacer along a notched sidewall of said first conductive material. 22. A method of fabricating a semiconductor device comprising the steps of: providing a substrate formed of a first material, said substrate having a surface; forming a gate dielectric on said surface; forming a gate conductor on said gate dielectric; chemically reacting edges of said gate conductor adjacent said gate dielectric to form a first reaction product; and selectively removing said first reaction product with respect to remaining portions of said gate conductor so as to provide a notch in said gate conductor. 23. The method as recited in claim 22, wherein the step of forming a gate conductor comprises a first gate layer and a second gate layer, wherein said first gate layer contacts said gate dielectric and said second gate layer is on said first gate layer. 24. The method as recited in claim 23, wherein the step of chemically reacting edges of said first gate layer forms said first reaction product. 25. The method as recited in claim 24, wherein the step of selectively removing said first reaction product further includes removing said first reaction product with respect to remaining portions of said first gate layer and said second gate layer so as to provide a notch in said first gate layer. 26. The method as recited in claim 22, further comprising providing a spacer along sidewalls of said second gate layer. 27. The method as recited in claim 26, wherein an air gap is left behind said spacer along sidewalls of said first gate layer. 28. The method as recited in any of claims 23 to 27, wherein said first gate layer comprises germanium. 29. The method as recited in any of claims 23 to 27, wherein said first gate layer comprises a germanium compound Ge x Si ! . x , wherein x is in the range of about 0.5 to about 1.0. 30. The method as recited in any of claims 23 to 27, wherein said second gate layer comprises silicon. 31. The method as recited in claim 22, wherein in said first reaction product comprises germanium oxide, or silicon germanium oxide. 32. The method as recited in claim 22, wherein said first gate layer comprises silicon. 33. The method as recited in claim 32, wherein said second gate layer comprises a refractory metal. 34. The method as recited in claim 33, wherein said second gate layer comprises tungsten, tantalum, or titanium. 35. The method as recited in claim 33, wherein said second gate layer comprises a silicide. 36. The method as recited in claim 22, wherein said first reaction product comprises silicon dioxide. 37. The method as recited in claim 22, wherein said first reaction product comprises a silicide. 38. The method as recited in claim 22, wherein said second gate layer comprises a silicide. 39. The method as recited in claim 38, further comprising the steps of providing metal along sidewalls of said gate conductor; and recess etching said metal. 40. The method as recited in claim 39, wherein the step of chemically reacting further includes the step of : reacting said gate conductor with said metal to form a silicide along edges of said gate conductor adjacent said gate dielectric. 41. The method of claim 40, wherein the step of selectively removing said first reaction product further includes: etching away said silicide. 42. The method of claim 40, wherein said silicide formed along edges of said gate conductor adjacent said gate dielectric is cobalt silicide, or titanium silicide. 43. The method of claim 42, wherein the step of selectively removing said cobalt silicide or said titanium silicide includes using an hydrogen peroxide mixture, or a hot sulphuric/hydrogen peroxide mixture. 44. A method according to claim 22, wherein the step of forming said gate conductor comprises forming a first layer of germanium on said gate dielectric and a second layer of silicon, with a graded layer of SiGe in between the first and second layers . 45. A method according to claim 44, wherein the step of chemically reacting edges of said gate electrode forms said first reaction product with germanium. 46. A method according to claim 45, wherein removing said reaction product produces a narrowing of said first layer and a tapering of said graded layer between said first and second layers . 47. A method according to claim 22, including a first dopant implant step to create a diffusion self-alligned with the portion of the gate conductor on said gate dielectric. 48. A method according to claim 47, wherein said first implant comprises a halo implant. 49. A method according to claim 47 or 48, wherein said first implant is directed at an angle from the normal to said gate dielectric to provide said implant self-aligned to said portion. 50. A method according to any of claims 47 to 49, further comprising carrying out a second implant defined by said upper portion of said gate conductor . 51. A method according to claim 50, wherein said second implant comprises an extension implant. 52. A method according to claim 51, further comprising the step of forming a spacer adjacent said upper portion and the step of carrying out a third implant defined by said spacer. 53. A method according to claim 52 wherein said third implant comprises a source/drain implant .

Inventors: FURUKAWA TOSHIHARU, HAKEY MARK, HOLMES STEVEN, HORAK DAVID, NOWAK EDWARD, FURUKAWA, TOSHIHARU, HAKEY, MARK, HOLMES, STEVEN, HORAK, DAVID, NOWAK, EDWARD
Applicants: IBM, IBM UK, IBM UNITED KINGDOM LIMITED, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 2001
Main classification: H01L21
IPC codes: H01L21, H01L29