WO2013019334A1 - Forming sic mosfets with high channel mobility by treating the oxide interface with cesium ions - Google Patents

Forming sic mosfets with high channel mobility by treating the oxide interface with cesium ions Download PDF

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Publication number
WO2013019334A1
WO2013019334A1 PCT/US2012/043699 US2012043699W WO2013019334A1 WO 2013019334 A1 WO2013019334 A1 WO 2013019334A1 US 2012043699 W US2012043699 W US 2012043699W WO 2013019334 A1 WO2013019334 A1 WO 2013019334A1
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Prior art keywords
insulation layer
layer
semiconductor layer
cesium ions
semiconductor
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PCT/US2012/043699
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French (fr)
Inventor
Sarit Dhar
Sei-Hyung Ryu
Anant Agarwal
John Robert Williams
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Cree, Inc.
Auburn University
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Priority to EP12820668.7A priority Critical patent/EP2740148B1/en
Priority to JP2014523931A priority patent/JP6052911B2/en
Publication of WO2013019334A1 publication Critical patent/WO2013019334A1/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
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
    • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
    • H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
    • H01L21/31—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to form insulating layers thereon, e.g. for masking or by using photolithographic techniques; After treatment of these layers; Selection of materials for these layers
    • H01L21/3105—After-treatment
    • H01L21/3115—Doping the insulating layers
    • 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
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
    • H01L21/0445—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising crystalline silicon carbide
    • H01L21/048—Making electrodes
    • H01L21/049—Conductor-insulator-semiconductor electrodes, e.g. MIS contacts
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/40—Electrodes ; Multistep manufacturing processes therefor
    • H01L29/408—Electrodes ; Multistep manufacturing processes therefor with an insulating layer with a particular dielectric or electrostatic property, e.g. with static charges or for controlling trapped charges or moving ions, or with a plate acting on the insulator potential or the insulator charges, e.g. for controlling charges effect or potential distribution in the insulating layer, or with a semi-insulating layer contacting directly the semiconductor surface
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/40—Electrodes ; Multistep manufacturing processes therefor
    • H01L29/43—Electrodes ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/49—Metal-insulator-semiconductor electrodes, e.g. gates of MOSFET
    • H01L29/51—Insulating materials associated therewith
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/66007—Multistep manufacturing processes
    • H01L29/66053—Multistep manufacturing processes of devices having a semiconductor body comprising crystalline silicon carbide
    • H01L29/66068—Multistep manufacturing processes of devices having a semiconductor body comprising crystalline silicon carbide the devices being controllable only by the electric current supplied or the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched, e.g. three-terminal devices
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
    • H01L29/76—Unipolar devices, e.g. field effect transistors
    • H01L29/772—Field effect transistors
    • H01L29/78—Field effect transistors with field effect produced by an insulated gate
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
    • H01L29/68—Types of semiconductor device ; Multistep manufacturing processes therefor controllable by only the electric current supplied, or only the electric potential applied, to an electrode which does not carry the current to be rectified, amplified or switched
    • H01L29/76—Unipolar devices, e.g. field effect transistors
    • H01L29/772—Field effect transistors
    • H01L29/78—Field effect transistors with field effect produced by an insulated gate
    • H01L29/7801—DMOS transistors, i.e. MISFETs with a channel accommodating body or base region adjoining a drain drift region
    • H01L29/7802—Vertical DMOS transistors, i.e. VDMOS transistors
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/06—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions
    • H01L29/08—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by their shape; characterised by the shapes, relative sizes, or dispositions of the semiconductor regions ; characterised by the concentration or distribution of impurities within semiconductor regions with semiconductor regions connected to an electrode carrying current to be rectified, amplified or switched and such electrode being part of a semiconductor device which comprises three or more electrodes
    • H01L29/0843—Source or drain regions of field-effect devices
    • H01L29/0847—Source or drain regions of field-effect devices of field-effect transistors with insulated gate
    • H01L29/0852—Source or drain regions of field-effect devices of field-effect transistors with insulated gate of DMOS transistors
    • H01L29/0873—Drain regions
    • H01L29/0878—Impurity concentration or distribution
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/02—Semiconductor bodies ; Multistep manufacturing processes therefor
    • H01L29/12—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/16—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only elements of Group IV of the Periodic Table
    • H01L29/1608—Silicon carbide

Definitions

  • This invention relates to methods of fabricating power devices and the resulting devices, and more particularly to silicon carbide power devices and methods of fabricating silicon carbide power devices.
  • Power semiconductor devices are widely used to carry large currents and support high voltages. Modern power devices are generally fabricated from monocrystalline silicon semiconductor material.
  • One widely used power device is the power Metal Oxide Semiconductor Field Effect Transistor (MOSFET).
  • MOSFET Metal Oxide Semiconductor Field Effect Transistor
  • a control signal is supplied to a gate electrode that is separated from the semiconductor surface by an intervening insulator, which may be, but is not limited to, silicon dioxide.
  • Power MOSFETs can provide an excellent safe operating area, and can be paralleled in a unit cell structure.
  • MOSFETs may include a lateral structure or a vertical structure.
  • the drain, gate and source terminals are on the same surface of a substrate.
  • the source and drain are on opposite surfaces of the substrate.
  • DMOSFET double diffused MOSFET
  • SiC silicon carbide
  • SiC has a combination of electrical and physical properties that make it attractive for a semiconductor material for high temperature, high voltage, high frequency and high power electronic devices. These properties include a 3.0 eV bandgap, a 4 MV/cm electric field breakdown, a 4.9 W/cm-K thermal conductivity, and a 2.0x10 7 cm/s electron drift velocity.
  • 4H-SiC Power DMOSFETs have the potential to offer significant advantages over conventional high voltage Si power switches. Unfortunately, however, it may be difficult to grow an acceptable gate oxide for these devices. Much effort has been focused on reducing the interface trap density (DIT) at the SiC/SiO 2 interface in order to increase the channel mobility (PCH) of the devices. Nitric Oxide (NO) anneals at 1 75°C have increased the PCH from single digits to -30 cm 2 /Vs. See, e.g., G.Y. Chung, et al., IEEE Electron Dev. Let. 22, 76 (2001). researchers have demonstrated even higher channel mobility ( ⁇ 150 cm 2 /Vs) by oxidizing in an environment containing metallic impurities.
  • Methods of forming a semiconductor structure include providing an insulation layer on a semiconductor layer, and diffusing cesium ions into the insulation layer from a cesium ion source outside the insulation layer.
  • Diffusing cesium ions into the insulation layer may include diffusing cesium ions to an interface between the insulation layer and the semiconductor layer.
  • the methods may further include depositing a cesium ion source on the insulation layer before diffusing cesium ions into the insulation layer.
  • Depositing the cesium ion source may include boiling the insulation layer in a CsCI solution.
  • boiling the insulation layer in a CsCI solution may include boiling the insulation layer in a 0.01 M to 1 M aqueous CsCI solution for 1 to 60 minutes at a temperature of about 90 to 100 °C.
  • the methods may further include nitridating the insulation layer.
  • Nitridating the insulation layer may include forming the insulation layer and/or annealing the insulation layer in an environment containing nitrogen.
  • Diffusing cesium ions into the insulation layer may include annealing the insulation layer at a temperature in excess of about 700 °C.
  • diffusing cesium ions into the insulation layer may include annealing the insulation layer at a temperature between about 700 °C and about 1000 °C.
  • Providing the insulation layer may include thermally oxidizing the semiconductor layer. In other embodiments, providing the insulation layer may include depositing the insulation layer on the semiconductor layer.
  • the semiconductor layer may include silicon carbide and the insulation layer may include silicon dioxide.
  • Methods of forming a field effect transistor device include providing a semiconductor layer, forming spaced apart source and drain regions in the semiconductor layer, the source and drain regions defining a channel region in the semiconductor layer, providing an insulation layer on the semiconductor layer over the channel region, diffusing cesium ions into the insulation layer, forming a gate electrode on the insulation layer, and forming a source contact on the source region.
  • FIG. 1 is a cross sectional illustration of a lateral MOSFET according to some embodiments of the invention.
  • FIG. 2 is a cross sectional illustration of a vertical power MOSFET according to some embodiments of the invention.
  • FIGS. 3A and 3B are band diagrams that schematically illustrate the treatment of an oxide/semiconductor interface according to some embodiments of the invention.
  • FIG. 4 is a graph of a Rutherford backscattering spectrum of a SiC/SiO 2 sample treated in accordance with some embodiments
  • FIG. 5 is a graph of channel mobility versus gate voltage measured at room temperature for lateral MOSFET devices formed in accordance with some embodiments of the invention as well as MOSFET devices formed in accordance with conventional techniques;
  • FIG. 6 is a graph of current-voltage transfer characteristics of a MOSFET device formed in accordance with some embodiments of the invention.
  • FIGS. 7 and 8 are flowcharts illustrating methods according to some embodiments.
  • first, second, third etc. may be used herein to describe various elements, components, regions, materials, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer, material or section from another element, component, region, layer, material or section. Thus, a first element, component, region, layer, material or section discussed below could be termed a second element, component, region, layer, material or section without departing from the teachings of the present invention.
  • Embodiments of the invention are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of the invention.
  • the thickness of layers and regions in the drawings may be exaggerated for clarity. Additionally, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected.
  • embodiments of the invention should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing.
  • an implanted region illustrated as a rectangle will, typically, have rounded or curved features and/or a gradient of implant concentration at its edges rather than a discrete change from implanted to non-implanted region.
  • a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation takes place.
  • the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of the invention.
  • Interface traps can reduce mobility in
  • MOSFETs formed in other types of material systems such as MOSFETs formed in polysilicon, amorphous silicon, gallium nitride, etc.
  • Metal enhanced oxidation such as disclosed in commonly assigned U.S. Patent No. 7,727,904, issued June 1 , 2010 , entitled Methods of Forming SIC Mosfets with High Inversion Layer Mobility, the disclosure of which is incorporated herein by reference, has proven to reduce the density of interface traps, resulting in increased channel mobility.
  • the lattice damage caused by implantation of Cs+ ions may adversely affect the mobility of carriers beneath the oxide, thus potentially rendering such processes unsuitable for MOSFET devices. Implantation of Cs+ ions may also render the oxide unreliable due to implant damage in the oxide.
  • Embodiments of the invention provide MOS devices, including MOSFET devices, having enhanced oxide/semiconductor interfaces.
  • an oxide/semiconductor interface may be enhanced by diffusing Cs+ ions to the interface after formation of the oxide layer.
  • the resulting devices may demonstrate high inversion layer mobility, which may make them suitable for power MOSFET fabrication.
  • the resulting devices may include 4H-SiC power MOSFETs and/or MOSFETs formed using other materials, such as polysilicon, amorphous silicon, gallium nitride, etc.
  • Cs+ ions are relatively immobile, the use of Cs+ ions to treat the oxide/semiconductor interface may offer advantages compared to the incorporation of sodium ions.
  • a gate oxide is treated with an aqueous CsCI solution, after which a high temperature diffusion is performed which results in the diffusion of Cs+ ions form the oxide surface through the oxide and to the gate insulator/semiconductor interface.
  • the positively charged CS ions may offset negatively charged traps at the gate insulator/semiconductor interface, which may result in improved channel mobility.
  • Cs+ ions are relatively immobile in insulators, such as Si0 2 , compared, for example, to sodium ions, the gate voltage of a device treated with Cs+ ions may be more stable at operation temperatures.
  • incorporation of Cs+ ions by diffusion may avoid implant damage that can occur during implantation of Cs+ ions.
  • Embodiments of the invention may increase channel mobility of a 4H-SiC device from about 20 cm 2 /V-s to about 100 cm 2 /V-s.
  • FIG. 1 embodiments of a lateral MOSFET according to some embodiments of the invention are illustrated.
  • an N-channel lateral MOSFET 10 includes a p-type epitaxial layer 14 grown on a substrate 12, which may be an 8° off-axis (0001) conducting 4H p-type SiC crystal. Other polytypes and/or off-axis angles of silicon carbide may also be used for the substrate 12.
  • the substrate 12 may include a material, such as polysilicon, amorphous silicon, and/or gallium nitride.
  • the epitaxial layer 14 may have a thickness of about 5 pm or more and may be formed using, for example, an MOCVD process, and may be doped with p-type impurities such as boron and/or aluminum at a concentration of about 5 x 10 15 - 1 x 10 16 cm "3 .
  • the epitaxial layer 14 may have a thickness less than 5 pm in some cases.
  • the epitaxial layer 14 may have a thickness of about 5 pm and may have a dopant concentration of about 5 x 10 15 cm "3 .
  • the channel region of the epitaxial layer 14 may be doped via ion implantation and may have a dopant concentration of about 1 x 10 6 cm “3 to 1 x 10 19 cm “3 .
  • Nitrogen and/or phosphorus ions may be implanted into the epitaxial layer 14 to form n+ source/drain regions 16, such that the n+ source/drain regions have a dopant concentration of about 1 x 0 19 cm "3 or more. However, if the dopant concentration of the source/drain regions 16 is less than 1 x 0 20 , a thermal anneal may be required to form ohmic contacts thereon.
  • the n+ source/drain regions 16 may be doped with phosphorus at a dopant concentration of about 1 x 10 20 cm "3 .
  • the implants may be activated, for example by a 1650°C Ar anneal in the presence of Si overpressure.
  • a 0.5 ⁇ thick deposited oxide layer may serve as a field oxide (not shown).
  • a gate insulation layer 18 is formed on the epitaxial layer 14 between and extending onto the source/drain regions 16.
  • the thickness of the gate insulation layer 18 may depend on the desired operating parameters of the device. For example, it may be appropriate to select an oxide thickness based on a maximum electric field of 3 MV/cm. In particular, the gate insulation layer 18 may have a thickness of about 300 A.
  • the gate insulation layer may include Si0 2 in some embodiment
  • the gate insulation layer 18 may include a different insulating material, such as silicon nitride.
  • the gate insulation layer 18 may be an Si0 2 layer that is grown, for example, using a multi-stage oxidation process including an oxidation step in dry 0 2 followed by re-oxidation (ReOx) in wet 0 2 as described, for example, in U.S. Patent No. 5,972,801 , the disclosure of which is incorporated herein by reference in its entirety.
  • ReOx re-oxidation
  • the gate insulation layer 8 may be formed, for example, by thermal oxidation of the epitaxial layer 14 for about 3 hours to about 6 hours at a temperature of about 900 °C to about 1300 °C or more.
  • the gate insulation layer 18 may be formed, for example, by thermal oxidation of the epitaxial layer 14 for 3.5 hours at a temperature of 1 75 °C.
  • the gate insulation layer 18 may be grown by a dry-wet oxidation process that includes a growth of bulk oxide in dry 0 2 followed by an anneal of the bulk oxide in wet 0 2 .
  • anneal of oxide in wet 0 2 refers to an anneal of an oxide in an ambient containing both 0 2 and vaporized H 2 0.
  • An additional anneal in an inert atmosphere may be performed between the dry oxide growth and the wet oxide anneal.
  • the dry 0 2 oxide growth may be performed, for example, in a quartz tube at a temperature of up to about 1200 °C in dry 0 2 for a time of at least about 2.5 hours. Dry oxide growth is performed to grow the bulk oxide layer to a desired thickness.
  • the temperature of the dry oxide growth may affect the oxide growth rate. For example, higher process temperatures may produce higher oxide growth rates. The maximum growth temperature may be dependent on the system used.
  • the dry O 2 oxide growth may be performed at a temperature of about 1200 °C in dry 0 2 for about 2.5 hours.
  • the resulting oxide layer may be annealed at a temperature of up to about 1200 °C in an inert atmosphere.
  • the resulting oxide layer may be annealed at a temperature of about 1 75 °C in Ar for about 1 hour.
  • the wet O 2 oxide anneal (ReOx) may be performed at a temperature of about 950 °C or less for a time of at least about 1 hour.
  • the temperature of the wet 0 2 anneal may be limited to discourage further thermal oxide growth at the SiC/Si0 2 interface, which may introduce additional interface states.
  • the wet 0 2 anneal may be performed in wet 0 2 at a temperature of about 950 °C for about 3 hours.
  • the gate insulation layer 8 may be deposited rather than thermally grown.
  • the gate oxide may be deposited using a chemical vapor deposition (CVD) process.
  • the wafer may be boiled in a 0.1 M CsCI solution for 10 minutes at a temperature of about 95 °C.
  • the CsCI solution may range from a 0.01 M CsCI solution to a 1 M CsCI solution.
  • the temperature of the solution may range from room temperature up to 100 °C.
  • the wafer may be boiled in the CsCI solution for a time of 1 minute up to 1 hour.JThis may result in the absorption and/or deposition of CsCI on the surface of the oxide.
  • the concentration of the CsCI solution may be varied to obtain a desired concentration of Cs at the oxide surface.
  • a diffusion (drive-in) anneal may be performed to cause Cs+ ions to diffuse into the gate insulation layer 18.
  • the diffusion anneal may be performed at a temperature of between 700 °C and 1000 °C to control the amount of Cs+ ions that diffuse to the SiC/Si0 2 interface.
  • the diffusion anneal may be performed for a time sufficient to cause a desired
  • concentration of Cs+ ions to diffuse to the SiC/SiO 2 interface may be performed for about 0.5 hours to about 3 hours.
  • the drive-in temperature and time depends on the gate oxide thickness. In general, thicker oxides are treated at higher temperatures and/or for longer times to achieve the same level of Cs incorporation at the
  • the amount of Cs that is diffused to the interface may be selected based on the amount of fixed charge present or expected to be present at the SiC/Si0 2 interface (the interfacial charge).
  • a diffusion anneal may be performed at a
  • the interfacial charge may be reduced, for example, by annealing the gate oxide in NO, NO 2 , N 2 O and/or hydrogen, as described, for example, in commonly assigned U.S. Patent No. 7,067,176, issued June 27, 2006 entitled Method Of Fabricating an Oxide Layer on a Silicon Carbide Layer Utilizing an Anneal in a Hydrogen Environment, U.S. Patent No.
  • a gate contact 20 is formed on the gate insulation layer 18.
  • the gate contact 20 may include, for example, molybdenum, boron-doped polysilicon and/or evaporated aluminum. Boron-doped polysilicon may be used to help adjust the threshold voltage of the device to a desired level. Polysilicon doped with other impurities, including n-type impurities, may also be used as a gate contact 20. In some embodiments, the thermal budget of the process may be a concern. In such cases, the use of evaporated aluminum may help reduce the thermal budget.
  • Nickel source/drain contacts 22, 24 may be formed on the source/drain regions 16. In particular, nickel contacts 22, 24 formed on the n+ source/drain regions 16 may exhibit ohmic behavior without any sintering.
  • FIG. 2 a vertical power MOSFET 30 according to some embodiments of the invention is illustrated.
  • Vertical silicon carbide MOSFETs are generally replicated in a unit cell. For ease of illustration, a single unit cell vertical MOSFET will be described.
  • a MOSFET 30 may include an n+ monocrystalline silicon carbide substrate 32.
  • An n- silicon carbide drift layer 34 is provided on a first face of the substrate 32.
  • the doping and thickness of the drift layer 34 may be determined by taking into consideration the desired blocking voltage of the device. For example, for a high voltage device, the drift layer 34 may have a thickness of about 5 pm to about 100 pm and a doping concentration of about 8 x 10 15 to 1 x 10 16 cm "3 .
  • First and second implanted p-type wells 36 are formed in the n- layer 34.
  • the p-wells 36 may be formed using implanted aluminum, resulting in a dopant concentration of 1 x 10 17 to 1 x 10 19 cm "3 .
  • the p-wells 36 may have a dopant concentration of about 1 x 10 18 cm "3 .
  • Implanted contact regions 38 of p+ silicon carbide may also be provided in the p-wells 36.
  • the implanted contact regions 38 may be formed, for example, by implantation of acceptor ions, such as boron and/or aluminum, to produce regions 38 having a dopant density of about 1 x 10 20 cm "3 .
  • implanted aluminum may be more suitable for the contact regions 38 due to the lower diffusivity of aluminum in SiC.
  • First and second n+ source regions 40 are provided in the p- type wells 36 adjacent the contact regions 38.
  • the implanted source regions 40 may be formed, for example, by implantation of donor ions to produce regions 40 having a dopant density of about 1 x 10 19 cm "3 or more.
  • An n-type silicon carbide JFET region 41 is adjacent to the n+ source regions 40.
  • the JFET region 41 is spaced apart from the source regions 40 by channel regions 43 in the p-type wells 36.
  • the JFET region 41 which extends to the n- layer 34, may have a similar dopant concentration as the drift layer 34. In some embodiments, however, the JFET region 41 may be implanted with n-type impurities to have a higher dopant concentration than the drift layer 34.
  • the JFET region 41 may be implanted with donor ions to have a dopant density of from about 1 x 10 16 to about 1 x 10 17 cm "3 .
  • the actual dopant concentration chosen may depend on the desired operational characteristics of the device.
  • the gate oxide 42 may have a thickness of from about 250 angstroms to about 1000 angstroms. In particular, the gate oxide 42 may have a thickness of about 300 A.
  • the gate oxide 42 may be treated with Cs+ ions as described above.
  • a gate contact 46 is provided on the gate oxide 42 opposite the channel region 43.
  • Source contacts 44 are formed on the n+ source regions 40.
  • the source contacts 44 are also formed on the p+ contact regions 38 to short the n+ source regions to the p-well regions 36.
  • the p-well regions 36 may be relatively highly doped to reduce and/or prevent turning on the parasitic npn transistors formed by source regions 40, well regions 36 and drift layer 34.
  • the p-well regions 36 may have a dopant concentration of about 1 x 10 15 cm "3 to about 1 x 10 18 cm “3 or greater.
  • a drain contact 48 is provided on the face of the substrate 32 opposite the p-type wells 36. The drain contact 48 may be formed, for example, using nickel.
  • FIGS. 3A and 3B are band diagrams that schematically illustrate the treatment of an oxide/semiconductor interface according to some embodiments of the invention.
  • a plurality of fixed negative charges 52 are present at an interface 50 between a silicon carbide layer 14 and an S1O2 layer 18.
  • a plurality of Cs+ ions are present in the oxide layer 18 near the interface 50.
  • the Cs+ ions 54 terminate field lines extending from the fixed negative charges 52, thereby decreasing the impact of the fixed negative charges on mobile charge carriers in an inversion layer formed in the silicon carbide layer 14.
  • FIG. 4 is a graph of a Rutherford backscattering spectrum (RBS) signal 60 of a SiC/Si02 sample treated in accordance with some
  • a simulated RBS signal 61 is also shown.
  • the sample analyzed in the graph of Figure 4 was formed by diffusing Cs+ ions into a silicon dioxide layer of 350 A thickness at a temperature of 800 °C for 0.5 hours.
  • the inset shows the Cs+ peak.
  • the RBS analysis shows that a total amount of about 3.6 x 10 14 cm "2 Cs atoms were incorporated in the oxide. It is believed that some fraction of these Cs atoms were positively charged ions.
  • the lower x-axis in the graph of Figure 4 is the energy channel number (which has no units ) but can be converted to energy (upper x-axis).
  • the energy a peak contains depends on the atomic mass of the element and its depth in the sample. In this case, the O surface peak appears at a lower energy than the Cs peak as oxygen is lighter than cesium.
  • the area under the peak corresponds to the amount of the element in the sample.
  • FIG. 5 is a graph 62 of channel mobility versus gate voltage measured at room temperature for a lateral MOSFET device formed in accordance with some embodiments of the invention as well as a graph 64 of channel mobility versus gate voltage for a MOSFET device formed in accordance with conventional techniques using an NO anneal.
  • the mobility of a device formed in the manner described herein is about twice as much as the conventional device.
  • a device formed according to some embodiments may exhibit a channel mobility in excess of 40 cm 2 /V-s at an applied electric field of 3 MV/cm.
  • FIG. 6 is a graph of current-voltage transfer characteristics of a MOSFET device formed in accordance with some embodiments of the invention. Higher mobility typically results in lower threshold voltages, tending toward normally-off behavior of MOSFET devices, which may be undesirable. However, as shown in Figure 6, a device formed in accordance with some embodiments of the invention may have a positive threshold voltage.
  • an insulation layer is provided on a semiconductor layer.
  • the semiconductor layer may be a silicon carbide layer having a 2H, 4H, 6H, 15R or 3C polytype, an amorphous silicon layer, a polysilicon layer, or a gallium nitride layer.
  • the insulation layer may be an oxide layer that is grown, for example, using a multi-stage oxidation process including an oxidation step in dry O 2 followed by re-oxidation (ReOx) in wet O 2 as described above.
  • the oxide layer may be formed by thermal oxidation of the silicon carbide layer.
  • the oxide layer may be deposited rather than thermally grown.
  • the oxide layer may be deposited using a chemical vapor deposition (CVD) process.
  • a cesium diffusion source may be deposited on the oxide layer (Block 102).
  • the semiconductor layer including the insulation layer may be boiled in a 0.1 M CsCI solution for 10 minutes at a temperature of about 95 °C to deposit CsCI on the oxide layer.
  • cesium ions may be diffused into the insulation layer, and in particular may be diffused to the insulator/semiconductor interface by annealing the semiconductor layer at high temperature (Block 104).
  • the diffusion anneal may be performed at a temperature of between 700 °C and 1000 °C for about 0.5 hours to about 3 hours.
  • a diffusion anneal may be performed at a temperature of 800 °C for 1 hour.
  • an insulation layer is provided on a semiconductor layer.
  • the semiconductor layer may be a silicon carbide layer having a 2H, 4H, 6H, 15R or 3C polytype, an amorphous silicon layer, a polysilicon layer, or a gallium nitride layer.
  • the insulation layer may be an oxide layer that is grown, for example, using a multi-stage oxidation process including an oxidation step in dry 0 2 followed by re-oxidation (ReOx) in wet 0 2 as described above.
  • the insulation layer may be formed by thermal oxidation of the semiconductor layer.
  • the oxide layer may be deposited rather than thermally grown.
  • the oxide layer may be deposited using a chemical vapor deposition (CVD) process.
  • the insulation layer may be nitridated to introduce nitrogen into the oxide layer (Block 202). Nitridation may be performed during or after formation of the insulation layer, and may be performed, for example, by forming the insulation in an environment containing nitrogen and/or annealing the insulation layer in an environment containing nitrogen. For example, the insulation layer may be annealed in an environment containing NO, NO 2 , and/or N 2 0, as described, for example, in above-referenced U.S. Patent Nos. 7,067,176, 6,767,843, and/or 6,6 0,366.
  • a cesium diffusion source may be deposited on the insulation layer (Block 204).
  • the semiconductor layer including the insulation layer may be boiled in a 0.1 M CsCI solution for 10 minutes at a temperature of about 95 °C to deposit CsCI on the oxide layer.
  • cesium ions may be diffused into the insulation layer, and in particular may be diffused to the insulator/semiconductor interface by annealing the semiconductor layer at high temperature (Block 206).
  • the diffusion anneal may be performed at a temperature of between 700 0 C and 1000 °C for about 0.5 hours to about 3 hours.
  • a diffusion anneal may be performed at a temperature of 800 °C for 1 hour.
  • nitridation of the insulation layer may be performed before or after diffusion of cesium ions into the insulation layer.

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Abstract

Methods of forming a semiconductor structure include providing an insulation layer on a semiconductor layer and diffusing cesium ions into the insulation layer from a cesium ion source outside the insulation layer. A MOSFET including an insulation layer treated with cesium ions may exhibit increased inversion layer mobility.

Description

FORMING SIC MOSFETS WITH HIGH CHANNEL MOBILITY BY TREATING THE OXIDE INTERFACE WITH CESIUM IONS
FIELD OF THE INVENTION
[0001] This invention relates to methods of fabricating power devices and the resulting devices, and more particularly to silicon carbide power devices and methods of fabricating silicon carbide power devices.
BACKGROUND
[0002] Power semiconductor devices are widely used to carry large currents and support high voltages. Modern power devices are generally fabricated from monocrystalline silicon semiconductor material. One widely used power device is the power Metal Oxide Semiconductor Field Effect Transistor (MOSFET). In a power MOSFET, a control signal is supplied to a gate electrode that is separated from the semiconductor surface by an intervening insulator, which may be, but is not limited to, silicon dioxide.
Current conduction occurs via transport of majority carriers, without the presence of minority carrier injection that is used in bipolar transistor operation. Power MOSFETs can provide an excellent safe operating area, and can be paralleled in a unit cell structure.
[0003] As is well known to those having skill in the art, power
MOSFETs may include a lateral structure or a vertical structure. In a lateral structure, the drain, gate and source terminals are on the same surface of a substrate. In contrast, in a vertical structure, the source and drain are on opposite surfaces of the substrate.
[0004] One widely used silicon power MOSFET is the double diffused MOSFET (DMOSFET) which is fabricated using a double-diffusion process. In these devices, a p-base region and an n+ source region are diffused through a common opening in a mask. The p-base region is driven in deeper than the n+ source. The difference in the lateral diffusion between the p-base and n+ source regions forms a surface channel region. [0005] Recent development efforts in power devices have also included investigation of the use of silicon carbide (SiC) devices for power devices. Silicon carbide (SiC) has a combination of electrical and physical properties that make it attractive for a semiconductor material for high temperature, high voltage, high frequency and high power electronic devices. These properties include a 3.0 eV bandgap, a 4 MV/cm electric field breakdown, a 4.9 W/cm-K thermal conductivity, and a 2.0x107 cm/s electron drift velocity.
[0006] Consequently, these properties may allow silicon carbide power devices to operate at higher temperatures, higher power levels and/or with lower specific on-resistance than conventional silicon-based power devices. A theoretical analysis of the superiority of silicon carbide devices over silicon devices is found in a publication by Bhatnagar et al. entitled "Comparison of 6H-SiC, 3C--SiC and Si for Power Devices", IEEE Transactions on Electron Devices, Vol. 40, 1993, pp. 645-655. A power MOSFET fabricated in silicon carbide is described in U.S. Pat. No. 5,506,421 to Palmour entitled "Power MOSFET in Silicon Carbide" and assigned to the assignee of the present invention.
[0007] 4H-SiC Power DMOSFETs have the potential to offer significant advantages over conventional high voltage Si power switches. Unfortunately, however, it may be difficult to grow an acceptable gate oxide for these devices. Much effort has been focused on reducing the interface trap density (DIT) at the SiC/SiO2 interface in order to increase the channel mobility (PCH) of the devices. Nitric Oxide (NO) anneals at 1 75°C have increased the PCH from single digits to -30 cm2/Vs. See, e.g., G.Y. Chung, et al., IEEE Electron Dev. Let. 22, 76 (2001). Researchers have demonstrated even higher channel mobility (~150 cm2/Vs) by oxidizing in an environment containing metallic impurities. See, e.g., U.S. Patent No. 6,559,068. However, such a process may result in significant oxide contamination, may provide an uncontrolled oxidation rate (tox > 500 A), and/or may be incompatible with high temperature processing steps such as may be used for ohmic contact anneals. SUMMARY
[0008] Methods of forming a semiconductor structure according to some embodiments include providing an insulation layer on a semiconductor layer, and diffusing cesium ions into the insulation layer from a cesium ion source outside the insulation layer.
[0009] Diffusing cesium ions into the insulation layer may include diffusing cesium ions to an interface between the insulation layer and the semiconductor layer.
[0010] The methods may further include depositing a cesium ion source on the insulation layer before diffusing cesium ions into the insulation layer. Depositing the cesium ion source may include boiling the insulation layer in a CsCI solution. In particular embodiments, boiling the insulation layer in a CsCI solution may include boiling the insulation layer in a 0.01 M to 1 M aqueous CsCI solution for 1 to 60 minutes at a temperature of about 90 to 100 °C.
[0011] The methods may further include nitridating the insulation layer. Nitridating the insulation layer may include forming the insulation layer and/or annealing the insulation layer in an environment containing nitrogen.
[0012] Diffusing cesium ions into the insulation layer may include annealing the insulation layer at a temperature in excess of about 700 °C. In particular, diffusing cesium ions into the insulation layer may include annealing the insulation layer at a temperature between about 700 °C and about 1000 °C.
[0013] Providing the insulation layer may include thermally oxidizing the semiconductor layer. In other embodiments, providing the insulation layer may include depositing the insulation layer on the semiconductor layer.
[0014] The semiconductor layer may include silicon carbide and the insulation layer may include silicon dioxide.
[0015] Methods of forming a field effect transistor device according to some embodiments include providing a semiconductor layer, forming spaced apart source and drain regions in the semiconductor layer, the source and drain regions defining a channel region in the semiconductor layer, providing an insulation layer on the semiconductor layer over the channel region, diffusing cesium ions into the insulation layer, forming a gate electrode on the insulation layer, and forming a source contact on the source region.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate certain embodiment(s) of the invention. In the drawings:
[0017] FIG. 1 is a cross sectional illustration of a lateral MOSFET according to some embodiments of the invention;
[0018] FIG. 2 is a cross sectional illustration of a vertical power MOSFET according to some embodiments of the invention;
[0019] FIGS. 3A and 3B are band diagrams that schematically illustrate the treatment of an oxide/semiconductor interface according to some embodiments of the invention;
[0020] FIG. 4 is a graph of a Rutherford backscattering spectrum of a SiC/SiO2 sample treated in accordance with some embodiments;
[0021] FIG. 5 is a graph of channel mobility versus gate voltage measured at room temperature for lateral MOSFET devices formed in accordance with some embodiments of the invention as well as MOSFET devices formed in accordance with conventional techniques;
[0022] FIG. 6 is a graph of current-voltage transfer characteristics of a MOSFET device formed in accordance with some embodiments of the invention.
[0023] FIGS. 7 and 8 are flowcharts illustrating methods according to some embodiments.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0024] The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. Like numbers refer to like elements throughout.
[0025] As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items. It will be
understood that, although the terms first, second, third etc. may be used herein to describe various elements, components, regions, materials, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer, material or section from another element, component, region, layer, material or section. Thus, a first element, component, region, layer, material or section discussed below could be termed a second element, component, region, layer, material or section without departing from the teachings of the present invention.
[0026] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "includes", "including", "comprises" and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.
[0027] Embodiments of the invention are described herein with reference to cross-section illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of the invention. The thickness of layers and regions in the drawings may be exaggerated for clarity. Additionally, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, embodiments of the invention should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, an implanted region illustrated as a rectangle will, typically, have rounded or curved features and/or a gradient of implant concentration at its edges rather than a discrete change from implanted to non-implanted region. Likewise, a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation takes place. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of the invention.
[0028] It will be understood that when an element such as a layer, region or substrate is referred to as being "on" or extending "onto" another element, it can be directly on or extend directly onto the other element or intervening elements may also be present. In contrast, when an element is referred to as being "directly on" or extending "directly onto" another element, there are no intervening elements present. It will also be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no intervening elements present.
[0029] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly
understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
[0030] As noted above, one factor limiting the development of high voltage SiC MOSFET devices has been the problem of interface traps along the SiC/Si02 interface. The problem of interface traps is not limited to
SiC/Si02 interfaces, however. Interface traps can reduce mobility in
MOSFETs formed in other types of material systems, such as MOSFETs formed in polysilicon, amorphous silicon, gallium nitride, etc.
[0031] Metal enhanced oxidation, such as disclosed in commonly assigned U.S. Patent No. 7,727,904, issued June 1 , 2010 , entitled Methods of Forming SIC Mosfets with High Inversion Layer Mobility, the disclosure of which is incorporated herein by reference, has proven to reduce the density of interface traps, resulting in increased channel mobility.
[0032] More recently, the incorporation of sodium into a SiC/Si02 interface by deliberate contamination of the oxide layer during formation has been investigated and found to reduce interface traps, as discussed in E.O. Sveinbjornsson, et al., Sodium Enhanced Oxidation of Si-face 4H-SiC: a Method to Remove Near Interface Traps, Mater. Sci. Forum, Vol. 556-557, pp. 487-492 (2007). However, sodium is highly mobile in SiO2, which may result in temperature instability in MOSFET devices. In particular, sodium may cause gate instability in devices at operating temperatures of 100 °C to 200 °C.
[0033] The incorporation of Cesium into SiC/SiO2 interfaces by implantation of Cs+ ions into deposited oxides has been investigated and found to decrease the density of interface traps, as described in Y. Wang et al., Optimization of 4H-SiC MOS Properties with Cesium Implantation, Mater. Sci. Forum, Vol. 600-603, p.p. 751-754 (2009). Cesium is less mobile than sodium in SiO2. However, implantation of Cs+ ions may induce implantation damage in the oxide layer, which may be undesirable for a MOSFET gate oxide.lt is noted that Wang is directed to MOS capacitors. The lattice damage caused by implantation of Cs+ ions may adversely affect the mobility of carriers beneath the oxide, thus potentially rendering such processes unsuitable for MOSFET devices. Implantation of Cs+ ions may also render the oxide unreliable due to implant damage in the oxide.
[0034] Embodiments of the invention provide MOS devices, including MOSFET devices, having enhanced oxide/semiconductor interfaces. In particular, an oxide/semiconductor interface may be enhanced by diffusing Cs+ ions to the interface after formation of the oxide layer. The resulting devices may demonstrate high inversion layer mobility, which may make them suitable for power MOSFET fabrication. In particular embodiments, the resulting devices may include 4H-SiC power MOSFETs and/or MOSFETs formed using other materials, such as polysilicon, amorphous silicon, gallium nitride, etc. Furthermore, as Cs+ ions are relatively immobile, the use of Cs+ ions to treat the oxide/semiconductor interface may offer advantages compared to the incorporation of sodium ions.
[0035] In particular, in some embodiments, a gate oxide is treated with an aqueous CsCI solution, after which a high temperature diffusion is performed which results in the diffusion of Cs+ ions form the oxide surface through the oxide and to the gate insulator/semiconductor interface. The positively charged CS ions may offset negatively charged traps at the gate insulator/semiconductor interface, which may result in improved channel mobility. As Cs+ ions are relatively immobile in insulators, such as Si02, compared, for example, to sodium ions, the gate voltage of a device treated with Cs+ ions may be more stable at operation temperatures. Moreover, incorporation of Cs+ ions by diffusion may avoid implant damage that can occur during implantation of Cs+ ions.
[0036] Embodiments of the invention may increase channel mobility of a 4H-SiC device from about 20 cm2/V-s to about 100 cm2/V-s.
[0037] Referring now to FIG. 1 , embodiments of a lateral MOSFET according to some embodiments of the invention are illustrated.
[0038] As illustrated in FIG. 1 , an N-channel lateral MOSFET 10 includes a p-type epitaxial layer 14 grown on a substrate 12, which may be an 8° off-axis (0001) conducting 4H p-type SiC crystal. Other polytypes and/or off-axis angles of silicon carbide may also be used for the substrate 12. In other embodiments, the substrate 12 may include a material, such as polysilicon, amorphous silicon, and/or gallium nitride.
[0039] In some embodiments, the epitaxial layer 14 may have a thickness of about 5 pm or more and may be formed using, for example, an MOCVD process, and may be doped with p-type impurities such as boron and/or aluminum at a concentration of about 5 x 1015 - 1 x 1016 cm"3. The epitaxial layer 14 may have a thickness less than 5 pm in some cases. In particular embodiments, the epitaxial layer 14 may have a thickness of about 5 pm and may have a dopant concentration of about 5 x 1015 cm "3. In some embodiments, the channel region of the epitaxial layer 14 may be doped via ion implantation and may have a dopant concentration of about 1 x 10 6 cm"3 to 1 x 1019 cm"3. [0040] Nitrogen and/or phosphorus ions may be implanted into the epitaxial layer 14 to form n+ source/drain regions 16, such that the n+ source/drain regions have a dopant concentration of about 1 x 019 cm"3 or more. However, if the dopant concentration of the source/drain regions 16 is less than 1 x 020, a thermal anneal may be required to form ohmic contacts thereon. In particular embodiments, the n+ source/drain regions 16 may be doped with phosphorus at a dopant concentration of about 1 x 1020 cm"3. The implants may be activated, for example by a 1650°C Ar anneal in the presence of Si overpressure. A 0.5 μιτι thick deposited oxide layer may serve as a field oxide (not shown).
[0041] A gate insulation layer 18 is formed on the epitaxial layer 14 between and extending onto the source/drain regions 16. The thickness of the gate insulation layer 18 may depend on the desired operating parameters of the device. For example, it may be appropriate to select an oxide thickness based on a maximum electric field of 3 MV/cm. In particular, the gate insulation layer 18 may have a thickness of about 300 A.
[0042] The gate insulation layer may include Si02 in some
embodiments. However, in other embodiments, the gate insulation layer 18 may include a different insulating material, such as silicon nitride.
[0043] The gate insulation layer 18 may be an Si02 layer that is grown, for example, using a multi-stage oxidation process including an oxidation step in dry 02 followed by re-oxidation (ReOx) in wet 02 as described, for example, in U.S. Patent No. 5,972,801 , the disclosure of which is incorporated herein by reference in its entirety.
[0044] The gate insulation layer 8 may be formed, for example, by thermal oxidation of the epitaxial layer 14 for about 3 hours to about 6 hours at a temperature of about 900 °C to about 1300 °C or more. In particular embodiments, the gate insulation layer 18 may be formed, for example, by thermal oxidation of the epitaxial layer 14 for 3.5 hours at a temperature of 1 75 °C.
[0045] In other embodiments, the gate insulation layer 18 may be grown by a dry-wet oxidation process that includes a growth of bulk oxide in dry 02 followed by an anneal of the bulk oxide in wet 02. As used herein, anneal of oxide in wet 02 refers to an anneal of an oxide in an ambient containing both 02 and vaporized H20. An additional anneal in an inert atmosphere may be performed between the dry oxide growth and the wet oxide anneal. The dry 02 oxide growth may be performed, for example, in a quartz tube at a temperature of up to about 1200 °C in dry 02 for a time of at least about 2.5 hours. Dry oxide growth is performed to grow the bulk oxide layer to a desired thickness. The temperature of the dry oxide growth may affect the oxide growth rate. For example, higher process temperatures may produce higher oxide growth rates. The maximum growth temperature may be dependent on the system used.
[0046] In some embodiments, the dry O2 oxide growth may be performed at a temperature of about 1200 °C in dry 02 for about 2.5 hours. The resulting oxide layer may be annealed at a temperature of up to about 1200 °C in an inert atmosphere. In particular, the resulting oxide layer may be annealed at a temperature of about 1 75 °C in Ar for about 1 hour. The wet O2 oxide anneal (ReOx) may be performed at a temperature of about 950 °C or less for a time of at least about 1 hour. The temperature of the wet 02 anneal may be limited to discourage further thermal oxide growth at the SiC/Si02 interface, which may introduce additional interface states. In particular, the wet 02 anneal may be performed in wet 02 at a temperature of about 950 °C for about 3 hours.
[0047] In still further embodiments, the gate insulation layer 8 may be deposited rather than thermally grown. For example, the gate oxide may be deposited using a chemical vapor deposition (CVD) process.
[0048] Following formation of the gate oxide, the wafer may be boiled in a 0.1 M CsCI solution for 10 minutes at a temperature of about 95 °C. The CsCI solution may range from a 0.01 M CsCI solution to a 1 M CsCI solution. The temperature of the solution may range from room temperature up to 100 °C. The wafer may be boiled in the CsCI solution for a time of 1 minute up to 1 hour.JThis may result in the absorption and/or deposition of CsCI on the surface of the oxide. The concentration of the CsCI solution may be varied to obtain a desired concentration of Cs at the oxide surface.
[0049] Following deposition of CsCI on the surface of the gate insulation layer 18, a diffusion (drive-in) anneal may be performed to cause Cs+ ions to diffuse into the gate insulation layer 18. The diffusion anneal may be performed at a temperature of between 700 °C and 1000 °C to control the amount of Cs+ ions that diffuse to the SiC/Si02 interface. The diffusion anneal may be performed for a time sufficient to cause a desired
concentration of Cs+ ions to diffuse to the SiC/SiO2 interface, and in some embodiments may be performed for about 0.5 hours to about 3 hours. The drive-in temperature and time depends on the gate oxide thickness. In general, thicker oxides are treated at higher temperatures and/or for longer times to achieve the same level of Cs incorporation at the
oxide/semiconductor interface. The amount of Cs that is diffused to the interface may be selected based on the amount of fixed charge present or expected to be present at the SiC/Si02 interface (the interfacial charge). In particular embodiments, a diffusion anneal may be performed at a
temperature of 800 °C for 1 hour.
[0050] Before or after treating the gate insulator/semiconductor interface with Cs+ ions, the interfacial charge may be reduced, for example, by annealing the gate oxide in NO, NO2, N2O and/or hydrogen, as described, for example, in commonly assigned U.S. Patent No. 7,067,176, issued June 27, 2006 entitled Method Of Fabricating an Oxide Layer on a Silicon Carbide Layer Utilizing an Anneal in a Hydrogen Environment, U.S. Patent No.
6,767,843; Issued July 27, 2004, entitled Method Of NO Growth Of An Oxide Layer On A Silicon Carbide Layer, and U.S. Patent No. 6,6 0,366, issued August 26, 2003, entitled Method of NO Annealing an Oxide Layer on a Silicon Carbide Layer, the disclosures of which are incorporated herein by reference.
[0051] A gate contact 20 is formed on the gate insulation layer 18. The gate contact 20 may include, for example, molybdenum, boron-doped polysilicon and/or evaporated aluminum. Boron-doped polysilicon may be used to help adjust the threshold voltage of the device to a desired level. Polysilicon doped with other impurities, including n-type impurities, may also be used as a gate contact 20. In some embodiments, the thermal budget of the process may be a concern. In such cases, the use of evaporated aluminum may help reduce the thermal budget. Nickel source/drain contacts 22, 24 may be formed on the source/drain regions 16. In particular, nickel contacts 22, 24 formed on the n+ source/drain regions 16 may exhibit ohmic behavior without any sintering.
[0052] Referring now to FIG. 2, a vertical power MOSFET 30 according to some embodiments of the invention is illustrated. Vertical silicon carbide MOSFETs are generally replicated in a unit cell. For ease of illustration, a single unit cell vertical MOSFET will be described.
[0053] As is seen in FIG. 2, a MOSFET 30 according to embodiments of the present invention may include an n+ monocrystalline silicon carbide substrate 32. An n- silicon carbide drift layer 34 is provided on a first face of the substrate 32. The doping and thickness of the drift layer 34 may be determined by taking into consideration the desired blocking voltage of the device. For example, for a high voltage device, the drift layer 34 may have a thickness of about 5 pm to about 100 pm and a doping concentration of about 8 x 1015 to 1 x 1016 cm"3. First and second implanted p-type wells 36 are formed in the n- layer 34. The p-wells 36 may be formed using implanted aluminum, resulting in a dopant concentration of 1 x 1017 to 1 x 1019 cm"3. In particular embodiments, the p-wells 36 may have a dopant concentration of about 1 x 1018 cm"3. Implanted contact regions 38 of p+ silicon carbide may also be provided in the p-wells 36. The implanted contact regions 38 may be formed, for example, by implantation of acceptor ions, such as boron and/or aluminum, to produce regions 38 having a dopant density of about 1 x 1020 cm"3. In particular, implanted aluminum may be more suitable for the contact regions 38 due to the lower diffusivity of aluminum in SiC.
[0054] First and second n+ source regions 40 are provided in the p- type wells 36 adjacent the contact regions 38. The implanted source regions 40 may be formed, for example, by implantation of donor ions to produce regions 40 having a dopant density of about 1 x 1019 cm"3 or more.
[0055] An n-type silicon carbide JFET region 41 is adjacent to the n+ source regions 40. The JFET region 41 is spaced apart from the source regions 40 by channel regions 43 in the p-type wells 36. The JFET region 41 , which extends to the n- layer 34, may have a similar dopant concentration as the drift layer 34. In some embodiments, however, the JFET region 41 may be implanted with n-type impurities to have a higher dopant concentration than the drift layer 34. For example, the JFET region 41 may be implanted with donor ions to have a dopant density of from about 1 x 1016 to about 1 x 1017 cm"3. The actual dopant concentration chosen may depend on the desired operational characteristics of the device.
[0056] A gate oxide 42 of a suitable dielectric material, such as Si02, extends over the JFET region 41 and the channel regions 43 to the n+ source regions 40. The gate oxide 42 may have a thickness of from about 250 angstroms to about 1000 angstroms. In particular, the gate oxide 42 may have a thickness of about 300 A. The gate oxide 42 may be treated with Cs+ ions as described above.
[0057] A gate contact 46 is provided on the gate oxide 42 opposite the channel region 43. Source contacts 44 are formed on the n+ source regions 40. The source contacts 44 are also formed on the p+ contact regions 38 to short the n+ source regions to the p-well regions 36. The p-well regions 36 may be relatively highly doped to reduce and/or prevent turning on the parasitic npn transistors formed by source regions 40, well regions 36 and drift layer 34. For example, the p-well regions 36 may have a dopant concentration of about 1 x 1015 cm"3 to about 1 x 1018 cm"3 or greater. A drain contact 48 is provided on the face of the substrate 32 opposite the p-type wells 36. The drain contact 48 may be formed, for example, using nickel.
[0058] FIGS. 3A and 3B are band diagrams that schematically illustrate the treatment of an oxide/semiconductor interface according to some embodiments of the invention. Referring to Figure 3A, a plurality of fixed negative charges 52 are present at an interface 50 between a silicon carbide layer 14 and an S1O2 layer 18. Referring to Figure 3B, after treatment of the SiC/Si02 interface 50 with Cs+ ions as described above, a plurality of Cs+ ions are present in the oxide layer 18 near the interface 50. The Cs+ ions 54 terminate field lines extending from the fixed negative charges 52, thereby decreasing the impact of the fixed negative charges on mobile charge carriers in an inversion layer formed in the silicon carbide layer 14.
[0059] FIG. 4 is a graph of a Rutherford backscattering spectrum (RBS) signal 60 of a SiC/Si02 sample treated in accordance with some
embodiments. A simulated RBS signal 61 is also shown. The sample analyzed in the graph of Figure 4 was formed by diffusing Cs+ ions into a silicon dioxide layer of 350 A thickness at a temperature of 800 °C for 0.5 hours. The inset shows the Cs+ peak. The RBS analysis shows that a total amount of about 3.6 x 1014 cm"2 Cs atoms were incorporated in the oxide. It is believed that some fraction of these Cs atoms were positively charged ions. The lower x-axis in the graph of Figure 4 is the energy channel number (which has no units ) but can be converted to energy (upper x-axis). The energy a peak contains depends on the atomic mass of the element and its depth in the sample. In this case, the O surface peak appears at a lower energy than the Cs peak as oxygen is lighter than cesium. The area under the peak corresponds to the amount of the element in the sample.
[0060] FIG. 5 is a graph 62 of channel mobility versus gate voltage measured at room temperature for a lateral MOSFET device formed in accordance with some embodiments of the invention as well as a graph 64 of channel mobility versus gate voltage for a MOSFET device formed in accordance with conventional techniques using an NO anneal. At a typical operating field of 3 MV/cm, the mobility of a device formed in the manner described herein is about twice as much as the conventional device. In particular, a device formed according to some embodiments may exhibit a channel mobility in excess of 40 cm2/V-s at an applied electric field of 3 MV/cm.
[0061] FIG. 6 is a graph of current-voltage transfer characteristics of a MOSFET device formed in accordance with some embodiments of the invention. Higher mobility typically results in lower threshold voltages, tending toward normally-off behavior of MOSFET devices, which may be undesirable. However, as shown in Figure 6, a device formed in accordance with some embodiments of the invention may have a positive threshold voltage.
[0062] Methods according to some embodiments are illustrated in Figure 7. As shown therein, in Block 100, an insulation layer is provided on a semiconductor layer. The semiconductor layer may be a silicon carbide layer having a 2H, 4H, 6H, 15R or 3C polytype, an amorphous silicon layer, a polysilicon layer, or a gallium nitride layer. The insulation layer may be an oxide layer that is grown, for example, using a multi-stage oxidation process including an oxidation step in dry O2 followed by re-oxidation (ReOx) in wet O2 as described above. In some embodiments, the oxide layer may be formed by thermal oxidation of the silicon carbide layer. In still further embodiments, the oxide layer may be deposited rather than thermally grown. For example, the oxide layer may be deposited using a chemical vapor deposition (CVD) process.
[0063] Following formation of the insulation layer, a cesium diffusion source may be deposited on the oxide layer (Block 102). For example, the semiconductor layer including the insulation layer may be boiled in a 0.1 M CsCI solution for 10 minutes at a temperature of about 95 °C to deposit CsCI on the oxide layer.
[0064] Finally, cesium ions may be diffused into the insulation layer, and in particular may be diffused to the insulator/semiconductor interface by annealing the semiconductor layer at high temperature (Block 104). In particular, the diffusion anneal may be performed at a temperature of between 700 °C and 1000 °C for about 0.5 hours to about 3 hours. In particular embodiments, a diffusion anneal may be performed at a temperature of 800 °C for 1 hour.
[006S] Methods according to further embodiments are illustrated in Figure 8. As shown therein, in Block 200, an insulation layer is provided on a semiconductor layer. The semiconductor layer may be a silicon carbide layer having a 2H, 4H, 6H, 15R or 3C polytype, an amorphous silicon layer, a polysilicon layer, or a gallium nitride layer. The insulation layer may be an oxide layer that is grown, for example, using a multi-stage oxidation process including an oxidation step in dry 02 followed by re-oxidation (ReOx) in wet 02 as described above. In some embodiments, the insulation layer may be formed by thermal oxidation of the semiconductor layer. In still further embodiments, the oxide layer may be deposited rather than thermally grown. For example, the oxide layer may be deposited using a chemical vapor deposition (CVD) process.
[0066] The insulation layer may be nitridated to introduce nitrogen into the oxide layer (Block 202). Nitridation may be performed during or after formation of the insulation layer, and may be performed, for example, by forming the insulation in an environment containing nitrogen and/or annealing the insulation layer in an environment containing nitrogen. For example, the insulation layer may be annealed in an environment containing NO, NO2, and/or N20, as described, for example, in above-referenced U.S. Patent Nos. 7,067,176, 6,767,843, and/or 6,6 0,366.
[0067] Following formation of the insulation layer, a cesium diffusion source may be deposited on the insulation layer (Block 204). For example, the semiconductor layer including the insulation layer may be boiled in a 0.1 M CsCI solution for 10 minutes at a temperature of about 95 °C to deposit CsCI on the oxide layer.
[0068] Finally, cesium ions may be diffused into the insulation layer, and in particular may be diffused to the insulator/semiconductor interface by annealing the semiconductor layer at high temperature (Block 206). In particular, the diffusion anneal may be performed at a temperature of between 700 0 C and 1000 °C for about 0.5 hours to about 3 hours. In particular embodiments, a diffusion anneal may be performed at a temperature of 800 °C for 1 hour.
[0069] It will be appreciated that nitridation of the insulation layer may be performed before or after diffusion of cesium ions into the insulation layer.
[0070] In the drawings and specification, there have been disclosed typical embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.

Claims

That which is claimed is:
1. A method of forming a semiconductor structure, comprising: providing an insulation layer on a semiconductor layer; and
diffusing cesium ions into the insulation layer from a cesium ion source outside the insulation layer.
2. The method of Claim 1 , wherein diffusing cesium ions into the oxide layer comprises diffusing cesium ions to an interface between the insulation layer and the semiconductor layer.
3. The method of Claim 1 , further comprising depositing a cesium ion source on the insulation layer before diffusing cesium ions into the insulation layer.
4. The method of Claim 3, wherein depositing the cesium ion source comprises boiling the insulation layer in a CsCI solution.
5. The method of Claim 4, wherein boiling the insulation layer in a CsCI solution comprises boiling the insulation layer in a 0.1 M to 1 M aqueous CsCI solution for 1 minute to 60 minutes at a temperature of about 90 °C to about 100 °C.
6. The method of Claim 1 , further comprising nitridating the insulation layer.
7. The method of Claim 6, wherein nitridating the insulation layer comprises forming the insulation layer and/or annealing the insulation layer in an environment containing nitrogen.
8. The method of Claim 1 , wherein diffusing cesium ions into the insulation layer comprises annealing the insulation layer at a temperature in excess of about 700 °C.
9. The method of Claim 1 , wherein diffusing cesium ions into the insulation layer comprises annealing the insulation layer at a temperature between about 700 °C and about 1000 °C.
10. The method of Claim 1 , wherein providing the insulation layer comprises thermally oxidizing the silicon carbide layer.
11. The method of Claim 1 , wherein providing the insulation layer comprises depositing the insulation layer on the silicon carbide layer.
12. The method of Claim 1 , wherein the semiconductor layer comprises silicon carbide and the insulation layer comprises silicon dioxide.
13. The method of Claim 1 , wherein the semiconductor layer comprises polysilicon, amorphous silicon or gallium nitride.
14. A method of forming a field effect transistor device, comprising: providing a semiconductor layer;
forming spaced apart source and drain regions in the semiconductor layer, the source and drain regions defining a channel region in the semiconductor layer;
providing an insulation layer on the semiconductor layer over the channel region;
diffusing cesium ions into the insulation layer;
forming a gate electrode on the insulation layer; and
forming a source contact on the source region.
15. The method of Claim 14, wherein diffusing cesium ions into the insulation layer comprises diffusing cesium ions to an interface between the insulation layer and the semiconductor layer.
16. The method of Claim 4, further comprising depositing a cesium ion source on the insulation layer before diffusing cesium ions into the insulation layer.
17. The method of Claim 16, wherein depositing the cesium ion source comprises boiling the insulation layer in a CsCI solution.
18. The method of Claim 17, wherein boiling the insulation layer in a CsCI solution comprises boiling the insulation layer in a 0.1 M aqueous CsCI solution for 10 minutes at a temperature of about 95 °C.
19. The method of Claim 14, further comprising nitridating the insulation layer.
20. The method of Claim 19, wherein nitridating the insulation layer comprises forming the insulation layer and/or annealing the insulation layer in an environment containing nitrogen.
21. The method of Claim 14, wherein diffusing cesium ions into the insulation layer comprises annealing the insulation layer at a temperature in excess of about 700 °C.
22. The method of Claim 14, wherein diffusing cesium ions into the insulation layer comprises annealing the insulation layer at a temperature between about 700 °C and about 1000 °C.
23. The method of Claim 14, wherein the semiconductor layer comprises silicon carbide and the insulation layer comprises silicon dioxide.
24. The method of Claim 14, wherein the semiconductor layer comprises polysilicon, amorphous silicon or gallium nitride.
25. A field effect transistor device, comprising:
a semiconductor layer; spaced apart source and drain regions in the semiconductor layer, the source and drain regions defining a channel region in the semiconductor layer;
an insulation layer on the semiconductor layer over the channel region; a gate electrode on the insulation layer; and
a source contact on the source region;
wherein an interface between the semiconductor layer and the insulation layer has a concentration of cesium ions of at least about 3 x 1014 cm"2.
26. The field effect transistor of Claim 25, wherein the channel region has a channel mobility in excess of 40 cm2/V-s at an applied electric field of 3 MV/cm.
27. The field effect transistor of Claim 25, wherein the
semiconductor layer comprises silicon carbide and the insulation layer comprises silicon dioxide.
28. The field effect transistor of Claim 25, wherein the
semiconductor layer comprises polysilicon, amorphous silicon or gallium nitride.
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