Patent Application
20060273410
The present invention relates to a Metal Oxide Semiconductor (MOS) device and in particular to a MOS structure having a fully silicided (FUSI) hafnium silicide (HfSi) gate electrode with a work function that is very dose to that of n+ polysilicon.
The demand for greater circuit density and higher performance in Complementary Metal Oxide Semiconductor (CMOS) transistors is driving the critical gate dimension to 65 nm and below. For MOS-based IC manufacturing, a polysilicon gate has been used for many generations in spite of some shortcomings that include the well known poly-depletion effect which was first recognized over 10 years ago. The continued miniaturization of MOS Field Effect Transistor (MOSFET) devices and associated demand for better channel electrostatic control, higher gate capacitance, and higher drive current means that the gate dielectric thickness needs to be scaled down aggressively. For the 65 nm technology node and beyond, the equivalent electrical gate oxide thickness (EOT) must be thinned to below 15 Angstroms. Since poly-depletion has consistently contributed 4 to 10 Angstroms to the total EOT, the poly-depletion effect is no longer acceptable, regardless of advancements made on the high k dielectric layer between the gate and channel. To reduce the high gate resistance and poly-depletion problems, the active dopant density in the polysilicon gate must be increased. However, this practice leads to carrier mobility degradation. Thus, a considerable amount of research and development effort is taking place to identify an alternative gate electrode such as a metal electrode that is much more conductive than conventional heavily doped polysilicon gates and is free of the poly-depletion issue. An important requirement for a gate electrode (gate) is its capability of having a tunable workfunction (Φm) around its mid-gap so that the threshold voltage (Vth) for NMOS and PMOS devices can be obtained symmetrically. One approach for implementing a metal gate in a transistor device is to employ a silicidation process that completely consumes a conventional polysilicon gate electrode which is converted to a metal silicide. A metal gate formed through the process of silicidation and referred to as a Fully Silicided (FUSI) poly gate is a strong contender for gate electrode in advanced technologies because an n-type or p-type dopant that is implanted into polysilicon can modulate the final Φm around the (ΦMid-Gap. In other words, a semiconductor has a certain energy level measured by its Fermi level or EF. An undoped semiconductor has an EF generally at the middle of the bandgap. N-type doping adjusts the EF closer to the conduction band while p-type doping moves the EF nearer the valence band.
A dual metal gate (gate electrode) and a fabrication process to form the same are believed to be imperative for the 50 nm CMOS technology node and beyond [1]. Several candidate metals for a dual metal gate process have recently been proposed [2-8]. Examples of conductive materials being evaluated for metal gate technology are metals such as W and Mo, metal silicides including nickel silicide and cobalt silicide, and metallic nitrides such as TiN and WN. However, most of the thermally stable metals available for CMOS processing have workfunctions near midgap (ΦMid-Gap) or the valence band of silicon. Since the metals with low workfunctions are inherently unstable and reactive, the search for a suitable metal for an n-MOSFET gate electrode with thermal stability compatible with CMOS front end processes is a challenge. Recently, fully silicided (FUSI) metal suicides such as NiSi and CoSi for dual metal gates have drawn considerable attention due to their CMOS compatibility and no-process induced damage to the underlying gate dielectric layer [9-14]. However, the workfunctions for NiSi and CoSi cannot be tuned low enough to satisfy bulk CMOS requirements.
A metal gate is also applicable to metal oxide semiconductor (MOS) capacitors which are often required on integrated circuits to provide certain functions. In one example shown in
Studies have shown that the work function of metal silicide is close to that of the initial metal for silicidation [2, 10] and that the work function of hafnium is as low as about 4.0 eV. Although basic material studies on Hf silicide have been reported [15-18], there has been no successful demonstration in prior art of a Hf silicide gate in an n-MOS device that meets the performance requirements necessary for implementation in the 50 nm technology node.
A means of tuning work function values in n-FET and P-FET devices is described in U.S. Pat. No. 6,373,111. Bilayer stacks of relatively thick Al and thin TiN for n-channel FETs and relatively thick Pd and thin TiN or TaN for p-channel FETs are employed as gate structures. Threshold voltage is modified by changing the thickness of the TiN layer between the Al (or Pd) layer and the gate dielectric layer.
Several references are available that relate to metal gate electrodes and work function engineering and are listed below.
One objective of the present invention is to provide a thermally stable and fully silicided (FUSI) Hf suicide gate electrode with a work function that is very close to that of n+ polysilicon.
A further objective of the present invention is to provide a thermally stable and fully silicided Hf silicide gate electrode in accordance with the first objective that has a negligible change in equivalent oxide thickness (EOT) and flat band voltage even after high temperature annealing as high as 950° C. is performed.
A still further objective of the present invention is to provide a method of forming a MOS device having a thermally stable and fully silicided Hf silicide gate electrode in accordance with the first two objectives.
The first two objectives are achieved in one embodiment with a MOS structure comprised of a semiconductor substrate on which a dielectric layer and overlying gate electrode are formed. The substrate may be a doped or undoped semiconductor material and the dielectric layer may be a thin silicon oxide layer. Optionally, in an SOI embodiment, the substrate may be comprised of an insulating layer with a semiconductor layer formed thereon. There are source and drain regions in the substrate on either side of the gate electrode. A key feature is that the gate electrode is a fully silicided Hf silicide having a thickness of about 600 to 800 Angstroms and a composition ratio wherein the atomic % Hf/atomic % Si is about 0.9. The gate electrode is preferably comprised of an n-type dopant to tune the work function and threshold voltage (Vth) of the MOS structure. The fully silicided Hf silicide gate electrode having an n-type dopant has a work function of about 4.2 eV and is stable to temperatures up to at least 950° C. The Hf silicide gate electrode preferably has a nitride capping layer comprised of TaN that is thermally stable to 950° C. and which suppresses oxidation of the Hf silicide gate and lowers sheet resistance. The MOS structure may be an n-MOSFET or a MOS capacitor. In a MOS capacitor, the source/drain regions are connected to circuit ground and the gate is connected to a voltage bias source. For an n-MOSFET, the gate and the channel below the gate are doped with an n-type dopant and the source/drain regions have a p-type dopant.
The MOS structure of the present invention is fabricated by first providing a semiconductor substrate having the appropriate dopant. A thin dielectric layer is formed on the semiconductor substrate by a conventional method. In the exemplary embodiment wherein an n-MOS device is formed, an in-situ phosphorous doped polysilicon film is deposited on the dielectric layer in a low pressure chemical vapor deposition (LPCVD) furnace. The doped polysilicon film is then annealed in a furnace at 900° C. Native oxides on the surface of the polysilicon layer are removed by a wet etching process using dilute HF or a buffered oxide etchant. Immediately thereafter, a hafnium metal layer about 80 to 200 nm thick is sputter deposited on the polysilicon layer. Next, a tungsten (W) layer about 40 nm thick is deposited on the hafnium layer and prevents Hf from being oxidized during a subsequent silicidation step. The W layer does not react with the Hf layer even at temperatures as high as 750° C. Silicidation is then performed by furnace annealing at 420° C. for approximately 120 minutes or by rapid thermal annealing (RTA) in a temperature range of 600° C. to 750° C. for one minute. After silicidation, the W layer and unreacted hafnium layer are removed by treatment with a SC1 solution followed by treatment with a H2SO4/H2O2 mixture. A permanent capping layer such as a 40 nm thick TaN film is then laid down on the HfSi layer by a conventional physical vapor deposition (PVD) or chemical vapor deposition (CVD) process. Then the substrate is annealed in a RTA chamber at 750° C. to 950° C. for 30 seconds followed by annealing in a forming gas ambient at 420° C. for 30 minutes to complete the formation of a MOS structure having a fully silicided Hf silicided gate.
The present invention is illustrated by way of example and not limitation in the accompanying figures, in which like references indicate similar elements, and in which:
a is a graph that depicts the sheet resistance of a Hf/polysilicon stack of layers after annealing at different temperatures and without stripping unreacted Hf metal.
b is a Rutherford Backscattering (RBS) spectrum of a Hf silicide layer formed by a RTA at 600° C.
a is a plot that shows the variation in EOT and flat band voltage of FUSI HfSi MOS capacitors after annealing at different temperatures.
b is a plot that shows no degradation in gate leakage current characteristics of a FUSI HfSi gate MOS capacitor after high temperature annealing.
a is a high frequency C-V curve of a FUSI HfSi gate n-MOSFET with a 3.2 nm thick gate oxide. Source/drain regions were grounded during measurement.
b is a graph that depicts the Id-Vd characteristics of an n-MOSFET having a FUSI HfSi gate.
The present invention relates to MOS devices having a fully silicided Hf silicide gate electrode with a work function that is essentially the same as that of n+ polysilicon. The MOS device may be a MOSFET, a MOS capacitor, or other devices employing a gate electrode (gate) as appreciated by those skilled in the art. The present invention also encompasses a method of making a thermally stable and fully silicided Hf silicide gate.
In one embodiment, the fully silicided Hf silicide gate is incorporated in an n-MOSFET as depicted in
In the exemplary embodiment, the substrate 11 is comprised of silicon and has an n-well 13 formed therein. There are isolation regions such as shallow trench isolation (STI) regions 12 typically made of silicon oxide or a low k dielectric material formed in the substrate 11 that define the active region 21 of the n-MOSFET 10. STI regions 12 may be coplanar with the substrate 11 as pictured in the drawings, slightly elevated above the substrate, or slightly recessed below the substrate.
A gate stack comprised of a gate dielectric layer 14, a fully silicided Hf silicide gate layer 15 on the gate dielectric layer, and a capping layer 16 on the Hf silicided gate is disposed on the substrate 11 above a channel region 20. The capping layer 16 is preferably comprised of a nitride such as TaN or TiN having a thickness of about 400 to 500 Angstroms which is used to prevent oxidation of the gate layer. On either side of the gate stack are sidewall spacers 17 which may be made of silicon oxide, silicon nitride, or a composite of oxide and nitride. The channel region 20 is defined by source/drain regions 18 and extension regions 19 which are comprised of a p-type dopant. The gate dielectric layer 14 has a thickness of about 5 to 200 Angstroms and preferably 20 to 100 Angstroms (in effective oxide thickness, EOT) and may be comprised of one or more of silicon oxide, silicon oxynitride, or silicon nitride. For instance, an EOT of approximately 15 Angstroms is preferred for the 65 nm technology node. Alternatively, gate dielectric layer 14 may be comprised of one or more high k dielectric materials known to those skilled in the art. A high k dielectric material enables an increase in the physical thickness of the gate dielectric layer 14 to suppress tunneling current and prevents a high gate leakage current while maintaining minimum desired EOT. However, a high k dielectric layer generally has a lower thermal stability than silicon oxide, silicon nitride, or silicon oxynitride, and is employed only when subsequent thermal processes may be restricted to temperatures that do not exceed the thermal stability limit of the high k dielectric layer. In an alternative embodiment, the gate dielectric layer 14 is comprised of a high k dielectric layer formed on an interfacial layer that is SiO2, silicon nitride, or silicon oxynitride.
A key feature of the present invention is the gate layer 15 which is preferably a fully silicided Hf silicide having a composition ratio (atomic % Hf/atomic % Si) of from 0.9 to 1.0. The gate layer 15 is formed as a result of a silicidation process involving an n-doped polysilicon layer and an overlying Hf metal layer as explained in a later section. The gate layer 15 is stable up to temperatures of at least 950° C. and has a thickness of about 400 to 600 Angstroms and a work function of about 4.2 eV that is essentially the same as that of n+ polysilicon (4.17 eV). In one aspect, the fully silicided Hf silicide gate layer 15 is comprised of phosphorous ions with a concentration from about 5×1019 to 5×1020 cm−2. However, other n-type dopants may be used as appreciated by those skilled in the art. Note that the dopant content may be adjusted in the doped polysilicon layer prior to silicidation to tune the work function and threshold voltage (Vth) in the fully silicided Hf silicide gate layer.
In an experiment used to demonstrate the utility of this invention, MOS capacitors having an approximately 30 Angstrom thick silicon oxide dielectric layer were fabricated on p-type (100) silicon substrates (wafers). The silicon oxide dielectric layer was grown by a thermal process. Thereafter, in-situ phosphorous doped (about 5×1019 cm2 concentration) polysilicon films with a thickness from 400 to 2000 Angstroms were deposited on the dielectric layer on a plurality of substrates in a LPCVD furnace. Likewise, in-situ boron doped (5×1015 cm−2 concentration) polysilicon films were deposited on the dielectric layer on a certain number of p-type (100) silicon substrates. Both P- and B-doped polysilicon films were annealed at 900° C. in a furnace with N2 or inert gas ambient. Native oxide was removed from the surface of the polysilicon films using a wet etch process by dipping the polysilicon deposited wafers in a dilute HF solution for about 2 minutes and then rinsing with deionized water for 10 minutes. Finally, the wafers are dried using a spin-rinse dryer or IPA dryer. Immediately thereafter, hafnium metal films with thicknesses ranging from 800 Angstroms to 2000 Angstroms were sputter deposited on the polysilicon films in a sputter deposition chamber of a sputter deposition system at a base pressure of about 1×10−7 torr. Next, a tungsten (W) capping layer about 400 to 500 Angstroms thick and preferably 400 Angstroms thick was deposited on the hafnium layer. The W capping layer may be deposited in the same sputter deposition chamber used for the Hf layer or may be formed in a separate sputter deposition chamber. The W capping layer prevents Hf from being oxidized during a subsequent silicidation process and will be removed together with unreacted Hf after silicidation. A W capping layer can be advantageously used to protect the Hf layer since W does not react with Hf even at 750° C. and is easily removed by treatment with H2O2 or SC1 wet chemical solutions known to those skilled in the art. Silicidation was performed by a RTA at temperatures ranging from 600° C. to 750° C. for 1 minute. Following the silicidation step, W and unreacted Hf films were removed by treatment with a H2O2 solution or with a well known SC1 cleaning solution at room temperature for 5 to 10 minutes followed by treatment with a H2SO4/H2O2 mixture at 130° C. for about 10 minutes.
To evaluate the thermal stability of MOS structures comprised of these experimental Hf suicide films, the samples (substrates) were annealed in a RTA chamber at 750° C. to 950° C. for 30 seconds in a nitrogen ambient. Finally, all the samples were annealed in a forming gas ambient at 420° C. for 30 minutes. For example, a conventional furnace tube may be employed for the forming gas annealing.
Alternatively, when a MOS structure as previously described will be incorporated in a MOS capacitor or MOSFET, a permanent nitride capping layer is laid down on the Hf silicide layer following treatment with the H2SO4/H2O2 mixture. A nitride layer such as TaN or TiN with a thickness of about 400 to 500 Angstroms and preferably 400 Angstroms is formed on the Hf silicide layer by a PVD process. Then the substrate is annealed in a RTA chamber at 750° C. to 950° C. for 30 seconds followed by annealing in a forming gas ambient at 420° C. for 30 minutes.
Returning to the thermal stability study on uncapped Hf silicide films, the amount of silicon consumption by silicidation at different temperatures was determined by etching the Hf silicide films in buffered HF and measuring the thickness of the remaining polysilicon. Results show that the polysilicon thickness consumed by the silicidation reaction with Hf was about 550, 750, and 1150 Angstroms during RTA at 600° C., 650° C., and 750° C. for 1 minute, respectively. The thickness growth of the Hf silicide layer with increasing temperature suggests a decrease in sheet resistance which is consistent with a report by S. P. Murarka in “Silicides for VLSI Application”, Academic Press, New York, p. 16-17, 1983.
The composition of the Hf suicide formed through RTA at 600° C. was analyzed using Rutherford Backscattering Spectrometry (RBS). As shown in
Flat band voltages vs. gate oxide thickness for FUSI Hf silicide gates with various dopants in the initial polysilicon layer are depicted in
As mentioned previously, a permanent nitride capping layer comprised of TaN or the like is preferably formed on the fully silicided Hf silicide after the W and unreacted Hf layers are removed in order to suppress oxidation of the Hf silicide layer and reduce gate resistance. A W capping layer is not used here because unlike TaN, W is oxidized during the subsequent forming gas annealing in a furnace tube. The oxidation of silicide films and grain boundary grooving during subsequent annealing can degrade sheet resistance [20]. It is believed that the use of a nitrided capping layer on a metal silicide can overcome the aforementioned problem.
In our experiments, a TaN capping layer on the Hf silicide layer performed acceptably since there were no signs of sheet resistance degradation even after a high temperature annealing at 950° C. After annealing at 950° C., the sheet resistance of a 400 Angstrom thick TaN capped Hf silicide gate stack was measured to be about 20 ohms/square. Further reduction of sheet resistance can be achieved by increasing the TaN thickness.
Referring to
It should be understood that all of the experiments described herein were performed on unpatterned substrates. Typically, the finished MOS capacitor or MOSFET would be formed by patterning a photoresist layer (not shown) on the TaN capping layer and etching through openings in the photoresist layer to remove exposed portions of the TaN, HfSi, and gate dielectric layers, thereby defining a gate stack having sidewalls. An ion implantation involving p-type dopant may be performed at this point to define lightly doped source/drain extension regions in the substrate. Subsequently, the photoresist layer is stripped to provide a patterned gate stack. Thereafter, spacers that abut the sidewalls of the patterned gate stack are formed by a conventional method. A second ion implantation may then be carried out to form heavily doped source/drain regions in the substrate between the spacers and isolation regions as indicated in
The overall process is highly flexible since the fully silicided gate is compatible with high k gate dielectric layers and can be tuned for (>m and threshold voltage (Vth) control by appropriate polysilicon doping prior to Hf layer deposition and silicidation. The n-HfSi gate electrode described herein has an advantage over other metal gate electrodes because its work function is closer to that of n+ polysilicon than FUSI metal suicides such as NiSi and CoSi.
Referring to
To our knowledge, this is the first successful demonstration of a FUSI n-HfSi gate in an n-MOSFET. The thermal stability and electrical properties of gate electrodes formed according to the present invention will enable advanced MOS devices such as MOS capacitors and MOSFETs to be fabricated that satisfy the requirements of 65 nm and 50 nm technology nodes. Electrical and thermal stability properties of the FUSI n-HfSi gate described herein are also superior to those reported for prior art metal electrodes.
While this invention has been particularly shown and described with reference to, the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of this invention.
