US20130260517A1 - Method for fabricating semiconductor device - Google Patents

Method for fabricating semiconductor device Download PDF

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US20130260517A1
US20130260517A1 US13/853,742 US201313853742A US2013260517A1 US 20130260517 A1 US20130260517 A1 US 20130260517A1 US 201313853742 A US201313853742 A US 201313853742A US 2013260517 A1 US2013260517 A1 US 2013260517A1
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film
silicon
layer
larger
nitride
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Tsutomu Komatani
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Sumitomo Electric Device Innovations Inc
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Sumitomo Electric Device Innovations Inc
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Priority to US14/989,462 priority Critical patent/US9396928B2/en
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02107—Forming insulating materials on a substrate
    • H01L21/02109—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates
    • H01L21/02112—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer
    • H01L21/02123—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer the material containing silicon
    • H01L21/0217—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer the material containing silicon the material being a silicon nitride not containing oxygen, e.g. SixNy or SixByNz
    • H01L29/66924—
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02107—Forming insulating materials on a substrate
    • H01L21/02109—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates
    • H01L21/02112—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer
    • H01L21/02123—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer the material containing silicon
    • H01L21/02164—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer the material containing silicon the material being a silicon oxide, e.g. SiO2
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02107—Forming insulating materials on a substrate
    • H01L21/02109—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates
    • H01L21/02112—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer
    • H01L21/02172—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer the material containing at least one metal element, e.g. metal oxides, metal nitrides, metal oxynitrides or metal carbides
    • H01L21/02175—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer the material containing at least one metal element, e.g. metal oxides, metal nitrides, metal oxynitrides or metal carbides characterised by the metal
    • H01L21/02178—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer the material containing at least one metal element, e.g. metal oxides, metal nitrides, metal oxynitrides or metal carbides characterised by the metal the material containing aluminium, e.g. Al2O3
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02107—Forming insulating materials on a substrate
    • H01L21/02225—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer
    • H01L21/0226—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer formation by a deposition process
    • H01L21/02263—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer formation by a deposition process deposition from the gas or vapour phase
    • H01L21/02271—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer formation by a deposition process deposition from the gas or vapour phase deposition by decomposition or reaction of gaseous or vapour phase compounds, i.e. chemical vapour deposition
    • H01L21/0228—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer formation by a deposition process deposition from the gas or vapour phase deposition by decomposition or reaction of gaseous or vapour phase compounds, i.e. chemical vapour deposition deposition by cyclic CVD, e.g. ALD, ALE, pulsed CVD
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
    • H10D30/00—Field-effect transistors [FET]
    • H10D30/01—Manufacture or treatment
    • H10D30/015—Manufacture or treatment of FETs having heterojunction interface channels or heterojunction gate electrodes, e.g. HEMT
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
    • H10D30/00—Field-effect transistors [FET]
    • H10D30/01—Manufacture or treatment
    • H10D30/051—Manufacture or treatment of FETs having PN junction gates
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
    • H10D30/00—Field-effect transistors [FET]
    • H10D30/40—FETs having zero-dimensional [0D], one-dimensional [1D] or two-dimensional [2D] charge carrier gas channels
    • H10D30/47—FETs having zero-dimensional [0D], one-dimensional [1D] or two-dimensional [2D] charge carrier gas channels having 2D charge carrier gas channels, e.g. nanoribbon FETs or high electron mobility transistors [HEMT]
    • H10D30/471—High electron mobility transistors [HEMT] or high hole mobility transistors [HHMT]
    • H10D30/475—High electron mobility transistors [HEMT] or high hole mobility transistors [HHMT] having wider bandgap layer formed on top of lower bandgap active layer, e.g. undoped barrier HEMTs such as i-AlGaN/GaN HEMTs
    • H10D30/4755—High electron mobility transistors [HEMT] or high hole mobility transistors [HHMT] having wider bandgap layer formed on top of lower bandgap active layer, e.g. undoped barrier HEMTs such as i-AlGaN/GaN HEMTs having wide bandgap charge-carrier supplying layers, e.g. modulation doped HEMTs such as n-AlGaAs/GaAs HEMTs
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
    • H10D30/00—Field-effect transistors [FET]
    • H10D30/60—Insulated-gate field-effect transistors [IGFET]
    • H10D30/67—Thin-film transistors [TFT]
    • H10D30/6729—Thin-film transistors [TFT] characterised by the electrodes
    • H10D30/6737—Thin-film transistors [TFT] characterised by the electrodes characterised by the electrode materials
    • H10D30/6738—Schottky barrier electrodes
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
    • H10D30/00—Field-effect transistors [FET]
    • H10D30/60—Insulated-gate field-effect transistors [IGFET]
    • H10D30/67—Thin-film transistors [TFT]
    • H10D30/674—Thin-film transistors [TFT] characterised by the active materials
    • H10D30/675—Group III-V materials, Group II-VI materials, Group IV-VI materials, selenium or tellurium
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
    • H10D62/00—Semiconductor bodies, or regions thereof, of devices having potential barriers
    • H10D62/80—Semiconductor bodies, or regions thereof, of devices having potential barriers characterised by the materials
    • H10D62/85—Semiconductor bodies, or regions thereof, of devices having potential barriers characterised by the materials being Group III-V materials, e.g. GaAs
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
    • H10D64/00—Electrodes of devices having potential barriers
    • H10D64/20—Electrodes characterised by their shapes, relative sizes or dispositions 
    • H10D64/27—Electrodes not carrying the current to be rectified, amplified, oscillated or switched, e.g. gates
    • H10D64/311—Gate electrodes for field-effect devices
    • H10D64/411—Gate electrodes for field-effect devices for FETs
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10D—INORGANIC ELECTRIC SEMICONDUCTOR DEVICES
    • H10D64/00—Electrodes of devices having potential barriers
    • H10D64/60—Electrodes characterised by their materials
    • H10D64/64—Electrodes comprising a Schottky barrier to a semiconductor

Definitions

  • the present invention relates to a method for fabricating a semiconductor device.
  • Nitride semiconductors are used in semiconductor devices such as an FET (Field Effect Transistor).
  • FET Field Effect Transistor
  • an insulating layer that covers the nitride semiconductor layer may be provided.
  • Japanese Patent Application Publication No. 2010-166040 discloses an arrangement in which a protection film made of silicon oxide is provided on a nitride semiconductor layer.
  • the capacitance of the semiconductor device which may include the intrinsic capacitance and the parasitic capacitance, may change due to an oxide layer formed on the surface of the nitride semiconductor layer. Variation in the capacitance may drift the gain. Further, electrons are captured in electron traps in the insulating film, so that the current of the semiconductor device may change. Conventionally, it is difficult to suppress both variation in the capacitance and that in the current.
  • a method for fabricating a semiconductor device including: forming a first film that contact a surface of the nitride semiconductor layer and have a thickness equal to or larger than 1 nm and equal to or smaller than 5 nm, the first film being made of silicon nitride having a composition ratio of silicon to nitrogen larger than 0.75, silicon oxide having a composition ratio of silicon to oxygen larger than 0.5, or aluminum; and forming a source electrode, a gate electrode and a drain electrode on the nitride semiconductor layer.
  • FIG. 1 is a cross-sectional view of a semiconductor device in accordance with a comparative example
  • FIG. 2A is a schematic view that illustrates an example in which an insulating film (silicon nitride) has a large Si composition ratio
  • FIG. 2B is a band diagram of the example
  • FIG. 3 is a schematic diagram of an exemplary structure in which an insulating film has a stoichiometric composition
  • FIG. 4A is a cross-sectional view of a semiconductor device in accordance with a first embodiment
  • FIG. 4B is a diagram that schematically illustrates an enlarged view of a gate electrode and its vicinity;
  • FIGS. 5A through 5C are cross-sectional views of an exemplary method for fabricating a semiconductor device
  • FIGS. 6A through 6C are cross-sectional views of steps of the method that follow the steps of FIGS. 5A through 5C ;
  • FIGS. 7A and 7B illustrate conditions for growing first and second films formed of silicon nitride
  • FIGS. 8A and 8B illustrate conditions for growing first and second films formed of silicon nitride
  • FIGS. 9A and 9B illustrate conditions for growing first and second films formed of silicon nitride
  • FIG. 10A is a schematic diagram of an enlarged view of a gate electrode of a semiconductor device in accordance with a second embodiment, and FIG. 10B illustrates conditions for growing a first film of aluminum;
  • FIG. 11 illustrates conditions for growing first and second films formed of silicon oxide
  • FIG. 12 illustrates conditions for growing first and second films formed of silicon oxide
  • FIG. 13 illustrates conditions for growing first and second films formed of silicon oxide
  • FIG. 14 illustrates conditions for growing first and second films formed of silicon oxide
  • FIG. 15 illustrates conditions for growing first and second films formed of silicon oxide
  • FIG. 16 illustrates conditions for growing first and second films formed of silicon oxide
  • FIG. 17 illustrates conditions for growing a first film of aluminum and a second film of aluminum oxide
  • FIG. 18 illustrates conditions for growing a first film of aluminum and a second film of aluminum nitride.
  • FIG. 1 is a cross-sectional view of a semiconductor device 100 R in accordance with a comparative example.
  • the semiconductor device 100 R includes a substrate 10 , a barrier layer 12 , a channel layer 14 , an electron supply layer 16 , a cap layer 18 , an insulating film 20 , a source electrode 22 , a drain electrode 24 , a gate electrode 26 , and interlayer insulating films 30 and interconnection lines 32 .
  • the barrier layer 12 contacts the upper surface of the substrate 10 , and the channel layer 14 contacts the upper surface of the barrier layer 12 .
  • the electron supply layer 16 contacts the upper layer of the channel layer 14 , and the cap layer 18 contacts the upper surface of the electron supply layer 16 .
  • the insulating film 20 contacts the upper surface of the cap layer 18 .
  • the source electrode 22 , the drain electrode 24 and the gate electrode 26 are formed in openings in the insulating film 20 , and contact the upper surface of the cap layer 18 .
  • the interlayer insulating film 30 contacts the upper layers of the insulating film 20 and the gate electrode 26 .
  • Interconnection lines 32 are formed in openings in the interlayer insulating film 30 , and contact the upper surface of the source electrode 22 or that of the drain electrode 24 .
  • the substrate 10 is formed of, for example, silicon carbide (SiC) or sapphire.
  • the semiconductor layers (the barrier layer 12 , channel layer 14 , electron supply layer 16 and cap layer 18 ) are nitride semiconductor layers.
  • the barrier layer 12 is made of, for example, aluminum nitride (AlN).
  • the channel layer 14 and the cap layer 18 are made of, for example, gallium nitride (GaN).
  • the electron supply layer 16 is made of, for example, aluminum gallium nitride (AlGaN).
  • the insulating film 20 and the interlayer insulating film 30 are made of, for example, silicon nitride (SiN).
  • the source electrode 22 and the drain electrode 24 are made of a metal, each of which may be composed of, for example, a titanium (Ti) layer and an aluminum (Al) layer stacked in this order from the cap layer 18 .
  • the gate electrode 26 is made of a metal and may be composed of a nickel (Ni) layer 26 a and a gold (Au) layer 26 b stacked in this order from the cap layer 18 .
  • FIG. 2A is a schematic view that illustrates an example in which the insulating film has a large Si composition ratio.
  • a portion in the vicinity of the gate electrode 26 is enlarged and hatching is omitted.
  • a denatured layer including gallium oxide (Ga:O in FIG. 2A ) is formed on the upper surface of the cap layer 18 , and dots show electrons.
  • the composition ratio of Si to N (nitrogen) in the insulating film 20 is made larger than a Si/N ratio of 0.75 of silicon nitride (Si 3 N 4 ) having the stoichiometric composition.
  • Silicon in the insulating film 20 has an anti-bonding orbital (the dot on the right side of each Si in FIG. 2A ) in which Si is not bonded to N, and many silicon-hydrogen (Si—H) bonds are formed. As depicted by arrows in FIG.
  • the denatured layer is removed in such a manner that oxygen ions (O ions) in the cap layer 18 are bonded to the anti-bonding orbitals of Si in the insulating film 20 .
  • Si in the insulating films functions as an electron trap, and traps an electron in 2DEG.
  • FIG. 2B is a schematic diagram that illustrates an exemplary band diagram.
  • the horizontal axis denotes the depth of the semiconductor device 100 R, and the vertical axis denotes energy.
  • a reference Ef is the Fermi energy
  • Ec is the energy of the bottom of the conduction band
  • Ev is the energy of the top of the valence band.
  • Solid lines denote energy before the electrons are trapped, and broken lines denote energy after the electrons are trapped.
  • the electrons in 2DEG are captured in the electron traps in the insulating film 20 , and the energy band of the insulating film 20 shifts to the minus side.
  • the energy bands of the channel layer 14 , the electron supply layer 16 and the cap layers 18 shift to the minus side. Since the energy Ec of the conduction band of the channel layer 14 shifts to the minus side, the number of electrons in 2DEG decreases. Thus, the saturation current of the semiconductor device 100 R decreases.
  • FIG. 3 is a schematic diagram of such an exemplary structure.
  • the Si/N ratio is 0.75
  • the Si atoms in the insulating film 20 do not have anti-bonding orbitals.
  • the Si atoms have a difficulty in trapping the electrons in 2DEG. Since the trapping of electrons is limited, the shift of the energy band is limited.
  • Si in the insulating film 20 has a difficulty in bonding with O ions.
  • Ga:O in the cap layer 18 remains.
  • O ions move due to the application of voltages to the source electrode 22 , the drain electrode 24 and the gate electrode 26 or temperature variation.
  • the above movement of O ions varies the capacitance of the semiconductor device 100 R, which may be the gate-source capacitance or the gate-drain capacitance.
  • a variable capacitance Cv is formed between the gate electrode 26 and the channel, layer 14 is generated, and varies.
  • This variation causes a drift in the gain of the semiconductor device 100 R.
  • Such a gain drift may occur even in a case where the composition of the insulating film 20 is not stoichiometric strictly, but is close to the stoichiometric composition.
  • the comparative example has difficulty in suppressing both the capacitance variation and the current variation.
  • a first embodiment has an exemplary structure having a film having a comparatively high composition ratio of Si to N, and a stoichiometric film provided on the large composition film.
  • FIG. 4A is a cross-sectional view of a semiconductor device in accordance with the first embodiment.
  • the semiconductor device 100 is configured to have a first film 34 provided on the cap layer 18 and a second film 36 provided on the first film 34 .
  • the first film 34 contacts the upper surface of the cap layer 18 .
  • the second film 36 contacts the upper surface of the first film 34 .
  • the first film 34 and the second film 36 may be made of SiN.
  • the Si/N ratio of the first film 34 is larger than 0.75.
  • the second film 36 has the stoichiometric composition.
  • the first film 34 has an exemplary thickness that is equal to or larger than 1 nm and is equal to or smaller than 5 nm.
  • the second film 36 has an exemplary thickness that is equal to or larger than 20 nm and is equal to or smaller than 100 nm.
  • FIG. 4B is a diagram that schematically illustrates an enlarged view of the gate electrode 26 and its vicinity. Since the Si/N ratio of the first film 34 is larger than 0.75, Si atoms in the first film 34 have anti-bonding orbitals. The anti-bonding orbitals of Si and the O ions in the cap layer 18 are bonded, so that the denatured layer is removed. Thus, it is possible to suppress variation in the capacitance of the semiconductor device 100 due to the movement of the O ions. Since the second film 36 has the stoichiometric composition, the electron traps are hard to be formed in the second film 36 . Thus, the trapping of the electrons in 2DEG is suppressed and variation in the saturation current is also suppressed. As described above, the use of stacked layers having different Si composition ratios realize the suppression of both the current variation and the capacitance variation.
  • Table 1 shows the standard Gibbs energies of formation of some substances (hereinafter simply referred to as Gibbs energy).
  • the Gibbs energy of SiO 2 is ⁇ 857 kJ/mol, and is lower than the Gibbs energy of Ga 2 O 3 . Since the first film 34 and the cap layer 18 contact each other, the reaction of the Si atoms and the O ions proceeds spontaneously, and the denatured layer is effectively removed. In an exemplary case where the cap layer 18 includes indium (In), the denatured layer includes In 2 O 3 . Since the Gibbs energy of SiO 2 is lower than that of In 2 O 3 , the denatured layer is effectively removed. A case of Al and Al 2 O 3 will be described later.
  • FIGS. 5A through 5C and FIGS. 6A through 6C are cross-sectional views that illustrate an exemplary method for fabricating the semiconductor device 100 .
  • the barrier layer 12 , the channel layer 14 , the electron supply layer 16 and the cap layer 18 are grown by, for example, MOCVD (Metal. Oxide Chemical Vapor Deposition).
  • MOCVD Metal. Oxide Chemical Vapor Deposition
  • the source electrode 22 and the drain electrode 24 are provided on the cap layer 18 by, for example, an evaporation process and a liftoff process.
  • the first film 34 and the second film 36 are provided by, an ALD (Atomic Layer Deposition) method.
  • an opening is formed in the first film 34 and the second film 36 , and the gate electrode 26 is provided in the opening by sputtering, for example.
  • the interlayer insulating film 30 is formed on the gate electrode 26 , the first film 34 and the second film 36 .
  • openings are formed in the interlayer insulating film 30 , and the interconnection lines 32 are provided in the openings by plating. The structure of the semiconductor device 100 illustrated in FIG. 6C is complete.
  • the ALD method forms films as follows.
  • a first source gas is supplied in an ALD apparatus, and a single-atom-thick layer of Si is formed. Then, the first source gas is exhausted from the ALD apparatus.
  • a second source gas is supplied to the ALD apparatus, and the Si layer is converted into nitride. Then, the second source gas is exhausted from the ALD apparatus.
  • a supply time is defined as the time during which the first source gas is supplied.
  • a first exhaust time is defined as the time during which the first source gas is exhausted.
  • the nitridization time is defined as the time during which the second source gas is supplied.
  • a second exhaust time is defined as the time during which the second source gas is exhausted.
  • One cycle is defined as the time from the initiation of supply of the first source gas to the completion of exhaust of the second source gas.
  • FIGS. 7A , 7 B, 8 A, 8 B, 9 A and 9 B are diagrams that show exemplary conditions for growing the first film 34 and the second film 36 made of SiN.
  • the first source gas includes BTBAS (Bis(tertiary-butyl-amino)silane: CH 4 H 4 NH 2 )SiH 2 ), TDMAS (Tris(dimethylamino)silane: SiH(N(CH 2 ) 2 ) 3 ), DMS (dimethyl silane: (CH 3 ) 2 SiH 2 ), BEMAS (Bis(ethyl-methyl-amino)silane: CH 3 (C 2 H 5 )N) 2 SiH 2 ), SiCl 4 and Si 2 Cl 2 , respectively.
  • BTBAS Bis(tertiary-butyl-amino)silane: CH 4 H 4 NH 2 )SiH 2
  • TDMAS Tris(dimethylamino)silane: SiH(
  • the second source gas is common to the conditions shown in FIGS. 7A through 9B , and includes ammonia (NH 3 ) plasma or N 2 plasma.
  • the first film 34 and the second film 36 may be grown by aminosilane (general expression: (R1R2N) n Si 4-n ).
  • the film growing temperature (the temperature in the ALD apparatus) may be equal to or higher than 200° C. and may be equal to or lower than 400° C., for example. Any of the film growing conditions illustrated in FIGS. 7A through 9B may be used or yet another condition may be used.
  • the reason why the first film 34 is formed by the ALD method is now given of the reason why the first film 34 is formed by the ALD method.
  • the first film 34 is too thick, many electron traps are formed since a large number of Si atoms having anti-bonding orbitals exists in the first film 34 .
  • electros in 2DEG may be captured in the electron traps.
  • the removal of the denatured layer has a difficulty because a small number of Si atoms having anti-bonding orbitals exists.
  • the thickness of the first film 34 has a thickness equal to or larger than 1 nm and equal to or smaller than 5 nm, and may be not smaller than 1.5 nm or 2 nm and may be not larger than 4.5 nm or 4 nm.
  • the first film 34 is required to be reliably formed so that the first film 34 is thin and is even or almost even in thickness. This requirement may be preferably achieved by the ALD method.
  • the first film 34 may be formed by plasma CVD, there is a difficulty in reliably forming a film thickness of not larger than 5 nm, which corresponds to a three- or four-atom-thick layer and in removing the denatured layer efficiently, as compared with the ALD method. Another method besides the ALD method may be used as long as the first film 34 is formed so as to be thin and even or almost even in thickness.
  • the composition ratio of Si to N in the first film 34 is larger than 0.75, and may be 0.78, 0.8, 0.85 or 0.9.
  • the second film 36 may be formed to have the substantive stoichiometric composition.
  • the substantive stoichiometric composition includes not only the strict stoichiometric composition but also a composition including an impurity having a difficulty in removal in the fabrication process.
  • the second film 36 may be formed by sputtering.
  • the second film 36 is preferably formed by the ALD method.
  • the second film 36 is preferably equal to or larger than 20 nm and is equal to or smaller than 100 nm in order to make it difficult for the electron traps to be formed and to protect the semiconductor layer from moisture.
  • the thickness of the second film 36 may be not smaller than 25 nm or 30 nm and not larger than 95 nm or 90 nm.
  • the second film 36 has the following effects other than those described above.
  • a parasitic capacitance between the gate electrode 26 and the cap layer 18 is concerned because the first film 34 is thin.
  • the gate electrode 26 is formed in the opening in the first film 34 and the second film 36 on the cap layer 18 in order to increase the distance between the gate electrode 26 and the cap layer 18 and to reduce the parasitic capacitance.
  • it is not essential to form the second film 36 on the first film 34 but the second film 36 may be omitted.
  • a second embodiment has an exemplary structure in which the first film 34 is made of Al.
  • a cross-sectional view of a semiconductor device 200 in accordance with the second embodiment is the same as that of the semiconductor device 100 .
  • FIG. 10A is a schematic diagram illustrating an enlarged view of the gate electrode 26 of the semiconductor device 200 and its vicinity.
  • the Al atoms in the first film 34 are bonded to the O ions in the cap layer 18 .
  • the denatured layer is removed and the trapping of electros is suppressed.
  • the thickness of the first film 34 may be equal to that used in the first embodiment.
  • the semiconductor device 200 may be fabricated as follows. An illustration of the fabrication method of the semiconductor device 200 is omitted, and FIGS. 4A through 5C are referenced.
  • the first film 34 is formed on the cap layer 18 by the ALD method.
  • the second film 36 is formed by the ALD method.
  • the step of forming the first film 34 and the step of forming the second film 36 are successively carried out.
  • the same ALD apparatus may be used to form the first film 34 and the second film 36 .
  • the second film 36 is formed without the first film 34 being exposed to atmosphere. The other steps are the same as those of the first embodiment.
  • FIG. 10B illustrates conditions for growing the first film 34 of Al.
  • the source gas includes TMA (trimethylaluminum), for example.
  • Al is likely to be oxidized.
  • an oxide film is formed on the surface of the first film 34 .
  • the step of forming the first film 34 and that of forming the second film 36 are successively carried out. Since the first film 34 is formed by the ALD apparatus, the first film 34 is thin and even in thickness. Thus, the denatured layer is effectively removed.
  • the Al atoms and O ions are bonded to generate Al 2 O 3 .
  • the Gibbs energy of Al 2 O 3 is ⁇ 791 kJ/mol, and is lower than that of Ga 2 O 3 .
  • the reaction of the Al atoms and the O ions proceeds, and the denatured layer is effectively removed.
  • the Gibbs energy of Al 2 O 3 is lower than that of In 2 O 3
  • the denatured layer is effectively removed even in an exemplary case where the gap layer 18 includes In.
  • the Gibbs energy of Al 2 O 3 is higher than that of SiO 2
  • Al is hard to be bonded to the ions as compared with Si. Therefore, Al is solely arranged to contact the cap layer 18 like the first film 34 , so that the bonding of the Al atoms and the O ions can proceed and the denatured layer can be removed.
  • the second film 36 may be formed of an insulative substance having a stoichiometric composition other than Si 3 N 4 .
  • the second film 36 may include at least one of SiO 2 , Al 2 O 3 and AlN, for example.
  • the first film 34 may be formed of a substance other than SiN and Al, and the second film 36 may be formed of a substance other than Si 3 N 4 .
  • the composition of the second film 36 is a stoichiometric composition (for example, SiO 2 ).
  • the composition ratio of Si to O in the second film 36 is 0.5.
  • the Si/O ratio in the first film 34 is larger than 0.5.
  • the Gibbs energy of SiO 2 is low.
  • FIGS. 11 through 16 illustrate conditions for growing the first film 34 formed by silicon oxide (for example, Si-rich Si 2 O 3 ) and the second film 36 formed by silicon oxide.
  • the first source gases used in the conditions for growing the films in FIGS. 11 through 14 include BTBAS, TDMAS, DMS and BEMAS, respectively, and the second source gases include oxygen (O 2 ), plasma, ozone (O 3 ), water (H 2 O) and a Lewis base.
  • the Lewis base is a complex of the H 2 O coordination of pyridine (C 5 H 5 N).
  • the first source gas includes SiCl 4
  • the second source gas O 2 plasma or O 3 .
  • the first source gas includes Si 2 Cl 6
  • the second source gas includes O 2 plasma or O 3 .
  • it is preferable that the oxidization time is changed in accordance with the type of the second source gas.
  • the composition of the second film 36 has the stoichiometric composition (Al 2 O 3 ).
  • the composition ratio of Al to O in the second film 36 is 0.67.
  • FIG. 17 shows conditions for growing the first film 34 of aluminum and the second film 36 of aluminum oxide.
  • the first source gas includes TMA
  • the second source gas includes O 2 plasma, O 3 , H 2 O or Lewis base.
  • the second film 36 has the stoichiometric composition (AlN).
  • AlN stoichiometric composition
  • FIG. 18 shows conditions for growing the first film 34 of aluminum and the second film 36 of aluminum nitride.
  • the first source gas includes TMA
  • the second source gas includes NH 3 plasma or N 2 plasma.
  • the film growing conditions in FIGS. 11 through 18 include conditions for temperature that range from 200° C. to 400° C., for example.
  • the barrier layer 12 , the channel layer 14 , the electron supply layer 16 and the cap layer 18 may be made of nitride semiconductors other than the above-described semiconductors.
  • the nitride semiconductors are semiconductors including N, and may be indium gallium nitride (InGaN), indium nitride (InN) and aluminum indium gallium nitride (AlInGaN) other than the above-described semiconductors.

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