WO2016075242A1 - Method for manufacturing a power semiconductor device - Google Patents

Method for manufacturing a power semiconductor device Download PDF

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Publication number
WO2016075242A1
WO2016075242A1 PCT/EP2015/076447 EP2015076447W WO2016075242A1 WO 2016075242 A1 WO2016075242 A1 WO 2016075242A1 EP 2015076447 W EP2015076447 W EP 2015076447W WO 2016075242 A1 WO2016075242 A1 WO 2016075242A1
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layer
dopant
metal
metal compound
main side
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PCT/EP2015/076447
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French (fr)
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Renato Minamisawa
Vinoth Sundaramoorthy
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Abb Technology Ag
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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 at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer
    • H01L21/0445—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising crystalline silicon carbide
    • H01L21/048—Making electrodes
    • H01L21/0485—Ohmic electrodes
    • 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 at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer
    • H01L21/0445—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising crystalline silicon carbide
    • H01L21/0455—Making n or p doped regions or layers, e.g. using diffusion
    • H01L21/046—Making n or p doped regions or layers, e.g. using diffusion using ion implantation
    • 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 at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer
    • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having at least one potential-jump barrier or surface barrier, e.g. PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic System or AIIIBV compounds with or without impurities, e.g. doping materials
    • H01L21/28—Manufacture of electrodes on semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/268
    • H01L21/283—Deposition of conductive or insulating materials for electrodes conducting electric current
    • H01L21/285—Deposition of conductive or insulating materials for electrodes conducting electric current from a gas or vapour, e.g. condensation
    • H01L21/28506—Deposition of conductive or insulating materials for electrodes conducting electric current from a gas or vapour, e.g. condensation of conductive layers
    • H01L21/28575—Deposition of conductive or insulating materials for electrodes conducting electric current from a gas or vapour, e.g. condensation of conductive layers on semiconductor bodies comprising AIIIBV compounds
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. 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
    • H01L29/45—Ohmic electrodes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. 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
    • H01L29/45—Ohmic electrodes
    • H01L29/452—Ohmic electrodes on AIII-BV compounds
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. 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 System
    • H01L29/1608—Silicon carbide
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices adapted for rectifying, amplifying, oscillating or switching, or capacitors or resistors with at least one potential-jump barrier or surface barrier, e.g. 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
    • H01L29/20—Semiconductor bodies ; Multistep manufacturing processes therefor characterised by the materials of which they are formed including, apart from doping materials or other impurities, only AIIIBV compounds
    • H01L29/2003—Nitride compounds

Definitions

  • the invention relates to the field of power electronics and more particularly to a method for manufacturing a power semiconductor device according to the preamble of claim 1 .
  • a method for manufacturing a power semiconductor device in which a Silicon carbide substrate is provided, on which an epitaxial layer is grown on one main side. On the opposite main side, ions are implanted and a heat treatment is performed at 1300 °C, so that a doping concentration of 10 18 to 10 20 cm 3 is achieved. The implantation depth is about 1 nm. Afterwards, a Nickel layer as a metal layer is deposited having a thickness of at least 30 nm, and a heat treatment is performed at a temperature between 800 and 1000 °C, a temperature high enough to create an ohmic contact at the interface between the metal and the substrate.
  • the employed implantation dopant range R p is much higher than a silicide interface depth X,.
  • SBH Schottky Barrier Height
  • this technique does not work for backgrinded wafers, and thus does not enable thin wafer processing, as it involves annealing at temperatures as high as 800°C.
  • temperatures as high as 800 °C degrade the structures on the front side (e.g. MOS structures, thereby degrading the MOS performance, or a Schottky contact in a SiC Junction Barrier Schottky diode.)
  • US 8,267,507 B1 describes a method for manufacturing a SiC MOSFET.
  • a p well layer and n source regions are formed by implantation and anneal.
  • a p++ dopant is implanted for forming a p well contact region. Both source and well contact region are in amorphous state in order to enhance the following segregation process.
  • a nickel layer is created above the source regions and p++ well contact region and annealed, thereby forming a nickel silicide layer.
  • An n dopant such as phosphorous is implanted above the n source regions and a p dopant such as aluminium is implanted above the p well contact region. These dopant are then segregated by a heating step.
  • Each of the source regions and well contact region are created by two implants followed by a segregation process, resulting in a rather flat profile of these double- implanted layers.
  • the inventive method comprises the following steps: a) providing a wide bandgap wafer made of a wide bandgap material having a base doping concentration;
  • a metal which metal is capable of forming a metal compound layer with a component of the wide band gap material
  • step b) after step b) performing a first heating step, by which the metal layer is transformed into the metal compound layer with the component of the wide bandgap material;
  • the dopant source layer has a dopant range R p from a surface of a semi-fabricated device, which semi-fabricated device at least comprises the wafer and the dopant source layer;
  • step d) after step d) performing a second heating step, by which a segregation is achieved, by which the dopant source layer is transformed into a dopant segregated layer, which is arranged below the metal compound layer, wherein the dopant segregated layer forms a pn junction to a layer below the dopant segregated layer,
  • step e) the metal compound layer forms an ohmic contact up to an interface depth X,, at which depth X, the metal compound layer contacts the dopant segregated layer,
  • the interface depth X is measured form the main sided surface of the semi-fabricated device, which further comprises the dopant segregated layer, wherein the interface depth X, between the metal compound layer and the dopant segregated layer lies deeper in the semi-fabricated device than the dopant range R p of the dopant source layer.
  • dopant segregation Due to the usage of dopant segregation, a highly doped dopant segregated layer and at the same time an ohmic contact can be created, without deteriorating other layers of the semiconductor device. Due to the dopant source layer having such a small dopant range, the dopant is spread during the heating step into both directions, towards the surface and interface, i.e. into the metal layer and the wide bandgap wafer material, so that a metal compound layer is created as an ohmic contact and a dopant segregated layer is created as a highly doped layer.
  • a metal silicide or a metal carbide layer is created as metal compound layer.
  • a metal nitride or a metal gallide layer is created as metal compound layer.
  • An ohmic contact is an electrical junction between two conductors (i.e. for a semiconductor between a metal layer and a semiconductor layer, exemplarily a doped Silicon-carbide layer) that has a linear current-voltage behavior (over all intended operating frequencies).
  • An ohmic contact has low resistance.
  • the temperature applied for the first heating step can be kept low and still a reliable ohmic contact can be achieved (either in the first or in the second heating step c) or e)).
  • a lower temperature can be applied than in prior art methods due to the presence of the highly doped dopant segregated layer.
  • the remaining dopant of the dopant source layer concentrates in a thin dopant segregated layer, because of the small diffusion of dopants in wide bandgap materials. Since the solid solubility in the metal compound material (e.g. metal silicide) is very low and the diffusion of dopants in the wide bandgap material (e.g.
  • SiC SiC
  • the required implantation dose can be significantly reduced compared to the case where Xi>Rp because the dopants are concentrated in the interface. Since atomic vacancies are created at the metal compound layer/wafer interface (e.g.
  • the dopants are incorporated into substitutional sites, resulting in the dipole formation even without thermal activation of dopants, so that the active doping can be improved in the dopant segregated layer compared to prior art methods.
  • Such phenomena is further supported by the decrease of ionization energy at high dopant concentration as stated by charge neutrality equation, which is the case for the dopant segregated layer.
  • Exemplarily pre annealing of dopants for activation purposes prior to silicidation may be performed for further reduction of contact resistance.
  • a high maximum doping concentration of the dopant segregated layer is achievable.
  • the dopant segregated layer is a layer, having a high maximum doping concentration at the surface of the metal compound layer, which goes into a relative minimum and again concentrates to another relative maximum doping concentration deeper in the semi-fabricated device due to the diffusion of the dopant in the direction of the wide bandgap wafer, which another relative maximum doping concentration, however, is lower than the maximum doping concentration at the surface of the wafer on the main side.
  • step d) before step b), i.e. the dopant source layer is created on/in the wide bandgap wafer and a metal layer is created on top of the dopant source layer and afterwards performing the first and second heating steps c) and e) simultaneously, so that the metal compound layer as ohmic contact and the dopant segregated layer are created simultaneously.
  • step d) is performed after step c), i.e. the metal layer is first converted into a metal compound layer in step c) and afterwards a dopant source layer is created on the metal compound layer and transformed into a dopant segregated layer in the second heating step e).
  • the ohmic contact is either created in the first heating step by the formation of the metal compound layer or in the second heating step e) by applying the second temperature and by forming the high doping concentration of the dopant segregated layer.
  • FIG 1 to 4 show manufacturing step for a first inventive method for
  • FIG 5 to 7 show manufacturing step for another inventive method for
  • FIG 8 to 10 show manufacturing step for another inventive method for
  • FIG 1 1 shows the doping concentration of the wafer, the dopant source layer and the dopant segregated layer.
  • An inventive method for manufacturing a power semiconductor device comprises the following manufacturing steps:
  • step b) creating a metal layer 30, 30' on a main side of the wide bandgap wafer by applying a metal, which metal is capable of forming a metal compound layer 3, 3' with a component of the wide band gap material; c) after step b) performing a first heating step, by which the metal layer 30, 30' is transformed into a metal compound layer 3, 3' with the component of the wide bandgap material;
  • step d) after step d) performing a second heating step, by which a segregation is achieved, by which the dopant source layer 20, 20' is transformed into a dopant segregated layer 2, 2', which is arranged below the metal compound layer 3, 3', wherein the dopant segregated layer 2, 2' forms a pn junction to a layer below the dopant segregated layer 2, 2',
  • step e) the metal compound layer 3, 3' forms an ohmic contact up to an interface depth X,, at which the metal compound layer contacts the dopant segregated layer 2, 2', wherein the interface depth X, is measured form the main sided surface of the semi-fabricated device 100, which further comprises the dopant segregated layer 2, 2', wherein the interface depth Xi between the metal compound layer 3, 3' and the dopant segregated layer 2, 2' lies deeper in the semi-fabricated device 100 than the dopant range R p of the dopant source layer.
  • the interface depth X shall be the depth at which the doping concentration of the material of the wide bandgap substrate involved in the metal compound layer formation gets constant from a higher value in the metal compound layer, i.e. the depth at which no material is concentrated any more from the metal compound layer.
  • FIG 1 1 in which a nickel silicide layer has been formed, it is the depth, in which the Si concentration has dropped to a constant value.
  • the metal compound layer is made of the metal(s) of the metal layer and of a component of the wide bandgap material, which exemplarily comprises at least two components.
  • the dopants inside the metal compound layer 3, 3' are not electrically active since the metal on the surface is already very conductive, so that only those particles in the interface contribute to the ohmic contact.
  • the wide bandgap material is silicon carbide and by the segregation a metal silicide or a metal carbide layer is created as the metal compound layer 3, wherein the metal is the material of the metal layer 30.
  • Nickel being the metal
  • a Nickel silicide or a Nickel carbide layer is created as the metal compound layer 3.
  • the wide bandgap material is gallium nitride and by the segregation a metal nitride or metal gallide layer is created as a metal compound layer 3 (e.g. Nickel nitride or Nickel gallide layer).
  • a metal compound layer 3 e.g. Nickel nitride or Nickel gallide layer.
  • the wide bandgap wafer 1 may exemplarily have a thickness of 2 to 10 ⁇ or of 1 to 400 ⁇ .
  • the wide bandgap wafer 1 may have a constant base doping concentration, exemplarily in a range between 1 * 10 14 and 1 * 10 19 cm 3 or between 5 * 10 14 to 1 * 10 18 cm 3 .
  • the semi-fabricated device 100 shall comprise the wafer 1 and all layers and contacts created on or in the wafer, i.e. it is the workpiece at which the further manufacturing steps are performed. I.e. if the dopant source layer 20, 20' is created after the metal layer 30, 30', the semi- fabricated device 1 comprises these layers. If the metal layer is converted into the metal compound layer before the dopant source layer is applied the semi- fabricated device comprises the metal compound layer.
  • the base doping concentration of the wide bandgap wafer 1 may also decrease subsequently from a maximum value of the base doping concentration on a first main side 10 of the wide bandgap wafer to lower base doping concentration on a second main side 15 of the wide bandgap wafer opposite to the first main side 10.
  • the ohmic contact is formed on the first main side, i.e. the side of the higher value of the base doping concentration.
  • the dopant source layer may be created by any method, by which a dopant is applied on or into the wafer, such as implantation, dopant deposition or epitaxial growth.
  • the dopant for the creation of the dopant source layer 20 may be implanted with a dose between 1 * 10 12 and 5 * 10 16 cm 2 .
  • the dopant for the creation of the dopant source layer may be an n-dopant, exemplarily at least one of phosphorous and nitrogen, or a p dopant, exemplarily at least one of boron and aluminium.
  • only one dopant or a combination of two or more dopants may be applied, wherein the two or more dopants are exemplarily all of the same conductivity type.
  • the conductivity type of the dopant is chosen depending on the semiconductor type to be created.
  • the dopant source layer may be of the same conductivity type as the wafer, i.e. n wide bandgap wafer and dopant source layer of n type or both of p type.
  • the dopant source layer may be of different conductivity type than the wafer, one being n and the other of p type.
  • the dopant source layer comprises regions of the first and of the second conductivity type (n and p type) alternating with each other. The ohmic contact may contact the regions of the first and/or second conductivity type.
  • Such regions could be at least one source region of the same conductivity type as the wide bandgap wafer and at least one base region of a different conductivity type and the ohmic contact contacts the at least one source and base region.
  • alternating collector regions of a different conductivity type than the wide bandgap wafer and at least one region of the same conductivity type as the wide bandgap wafer, exemplarily higher doped than the wide bandgap wafer, are created alternatingly on the collector side and the ohmic contact contacts both region types.
  • R p is the dopant range, i.e. the maximum depth of the applied dopant.
  • the dopant range R p is shown in the figures as a dashed line (FIG 2).
  • An exemplary dopant range R p of the dopant source layer 20 may be between 1 to 400 nm. Applying the dopant, e.g. by implantation, may exemplarily be performed at room temperature or at an enhanced temperature, exemplarily at a temperature between 350 to 750 °C.
  • a pre-anneal-step may be performed at a temperature between 500 to 1900 °C in order to activate the dopant.
  • the metal layer 30, 30' is applied on the dopant segregated layer 2, 2' (FIG 3).
  • a first heating step is performed, by which the metal layer 30, 30' is
  • the interface depth X, of the metal compound layer 3, 3' lies deeper in the semi-fabricated device 100 than the location, at which the dopant source layer 20, 20' has had its dopant range R p . That means that the metal compound layer 3, 3' extends deeper into the semi-fabricated device 100 than the dopant range R p would be in this step. Thus, the metal compound layer 3 overlies the region, at which the dopant source layer 20 was or will be created.
  • the dopant segregated layer 2, 2' is shown in the figures as a region having a dotted hachure.
  • the dopant source layer 20, 20' and the metal layer 30, 30' may be created as continuous layers over the whole surface of the wide bandgap wafer on one side, e.g. the first main side 10 or the second main side 15. Thus, also the ohmic contact may be created over the whole surface.
  • either of the dopant source layer 20, 20' or the metal layer 30, 30' may be created only on a partial region on a side of the wide bandgap wafer or heat is only applied in the first heating step on such a partial region, at which an ohmic contact shall be stablished, so that also the ohmic contact is created in such a region, i.e. a laterally limited ohmic contact is created.
  • “Laterally limited” means that an extension of the contact is limited in the plane of the first main side).
  • the ohmic contact may be created only in the central area, leaving the surrounded termination area uncontacted or the ohmic contact may alternate with other contacts (e.g. gate contacts) or with areas, insulated at the surface of the device.
  • the metal layer 30 consists of at least one metal, exemplarily the metal layer consists of or comprises Nickel, Titanium, Tantalum or Tungsten.
  • the metal layer 30 may also consist of or comprise Palladium, Platinum and Aluminium.
  • the metal layer comprises a compound of at least two of the before mentioned metals.
  • the metal layer comprises Nickel and at least one of Palladium, Platinum or Aluminium. These metal may be applied as a compound layer of different metals or as a stack of metal layers, exemplarily with Nickel being the contact layer to the wafer 1 .
  • the first and/or second heating step for the creation of the metal compound layer and/or the segregated layer may be performed at a temperature between 400 to 1200 °C, exemplarily between 600 to 800 °C.
  • the heat is chosen such that an ohmic contact can be created, but other layers which may have been created before are not negatively influenced by such temperature.
  • the thickness of the dopant segregated layer 2 created in the second heating step e) may be between 1 to 100 nm or between 1 and 50 nm or between 1 to 10 nm and the maximum doping concentration between 4 * 10 18 to 1 * 10 20 cm 3 .
  • the dopant segregated layer is a layer, which has a maximum doping
  • the dopant segregated layer has two doping concentration maxima at different depths, wherein one depth is at the interface to the metal compound layer 3.
  • the dopant segregated layer 2 created in the second heating step has a higher maximum doping concentration than the dopant source layer 20.
  • the interface depth X, of the metal compound layer 3 may be between 4 to 500 nm.
  • a metal layer 30, 30' is created (step b; FIG 8) and then a first heating step is performed (step c)).
  • the metal layer 30 converts into a metal compound layer 3 (which may also be called a metal alloy layer) with one material of the wide bandgap wafer 1 , leaving vacancies of this material in the wafer 1 (FIG 9).
  • a metal compound layer 3 which may also be called a metal alloy layer
  • an ohmic contact may be created. It is also possible to perform the first heating step at a lower temperature, at which the metal compound layer 3 is created, but not converted into an ohmic contact yet. In this embodiment, the ohmic contact is created in the second heating step.
  • the presence of the dopant segregated layer improves the formation of the ohmic contact, thus lower temperatures can be applied in this ohmic-contact forming step than in the prior art methods.
  • the first temperature is lower than the second
  • the first temperature may be as low as 400 °C up to 700 °C or up to 600 °C, i.e. below an ohmic contact forming temperature.
  • the second heating step may be performed at higher temperatures, exemplarily up to 1200 °C or up to 800 °C.
  • a dopant source layer 20, 20' is created on the same main side of the wide bandgap wafer, i.e. on the metal compound layer 3, 3', by applying a dopant, wherein the dopant source layer 20, 20' has a dopant range R p (step d); FIG 10).
  • a second heating step is performed, by which a segregation is achieved, which transforms the dopant source layer 20, 20' and the wide bandgap wafer into a dopant segregated layer 2, 2', which layer 2, 2' is arranged below the metal compound layer 3, 3' (resulting in a device as the device shown in FIG 4), because the dopant goes into the vacancies formed in the wide bandgap wafer in the first heating step (step c)).
  • the interface depth X, of the metal compound layer is greater than the dopant range R p of the dopant source layer (the depth X, and range R p compared after having created the metal compound layer 3).
  • the wide bandgap wafer comprises a first main side 10 and a second main side 15 opposite to the first main side 10.
  • the method for the creation of the ohmic contact as disclosed above may be performed on the first and second main side 10, 15, so that a metal compound layer 3, 3' and a dopant segregated layer 2, 2' is created on the first and on the second main side 10, 15.
  • a wide bandgap wafer 1 is provided (FIG 1 )
  • a dopant source layer 20, 20' is created on the first main side 10 and on the second main side 15 (FIG 5).
  • a metal layer 30, 30' is applied on each side 10, 15 (FIG 6).
  • a dopant segregation layer 2, 2' and a metal compound layer 3, 3' are created (FIG 7).
  • first and second main sided ohmic contacts by the process described for the figures 8 to 10, i.e. with applying a metal layer 30, 30' first, then performing a first heating step c), then applying a dopant source layer 20, 20', and then performing the second heating step e) on the first and second main side 10, 15.
  • the dopant segregated layer 2' on the second main side 15 may have a different maximum doping concentration than the dopant segregated layer 2 on the first main side 10 and/or a different conductivity type and/or a different lateral extension.
  • the dopant segregated layer 2' on the second main side 15 has a maximum doping concentration, which is lower than the maximum doping concentration of the dopant segregated layer 2 on the first main side 10, exemplarily lower than at least a factor of 10.
  • the dopant segregated layer 2' on the second main side 15 has a maximum doping concentration between 5 * 10 17 and 1 * 10 20 cm 3 , in particular 5 * 10 17 to 5 * 10 18 cm 3 .
  • the wafer 1 comprises a first main side 10 and a second main side 20 opposite to the first main side and in that the metal compound layer 3 and the dopant segregated layer 2 is formed on the first main side 10 as main side with the process as disclosed above and a Schottky contact is formed on the second main side 15.
  • the Schottky contact contacts the wide bandgap wafer 1 with its unamended doping concentration (or the Schottky contact may contact any other layer formed in or on the wide bandgap wafer 1 ).
  • the Schottky contact may be created before or after the ohmic contact on the first main side 10. If the Schottky contact is created after the ohmic contact, it is ensured that no high temperatures convert the Schottky contact into an ohmic contact.
  • the inventive manufacturing method may be applied to different semiconductor devices, exemplarily to IGBTs (insulated gate bipolar transistor), MOSFETs, thyristors or diodes.
  • the dopant segregated layer 2, 2' forms a pn junction to a layer below the dopant segregated layer 2, 2'.
  • Below shall mean the layer that is arranged in a greater depth from such main side, on which the dopant segregated layer is arranged, and the layer below contacts the dopant segregated layer, thereby forming the pn junction.
  • the dopant segregated layer 2,2 ' may be a collector layer in the case of an inventive IGBT, an anode layer in the case of an inventive thyristor or an anode layer for an inventive diode, which dopant segregated layer (e.g. p doped) forms a pn junction to a below lying buffer or drift layer (n doped in the case of a p doped dopant segregated layer).
  • the dopant segregated layer may also be a source region for an inventive IGBT, an emitter layer for an inventive thyristor or a drain layer for an inventive MOSFET, which dopant segregated layer (e.g. n doped) forms a pn junction to a below lying base layer (p doped in the case of an n doped dopant segregated layer).
  • the wide bandgap wafer and the dopant source layer 20, 20' are of different conductivity types. If ohmic contacts are created on the first and second main side 10, 15, the dopant source layers 20, 20' and thus, the dopant segregated layers 2, 2' may be of the same conductivity type or of different conductivity types (i.e. both n or p doped or one dopant segregated layer 2, 2' being of n type and the other 2', 2 of p type).
  • FIG 1 1 shows the doping concentrations of various layers during or after the manufacturing of the semiconductor device.
  • the wide bandgap wafer is made of Gallium nitride, as a dopant Boron is implanted and Nickel is applied as a metal layer 30.
  • the alternatingly dashed and dotted line shows the boron concentration in the dopant source layer 20.
  • This layer has a small dopant range R p in which Boron is present.
  • the other three layers, shown in FIG 1 1 are layers after finalizing the semiconductor device.
  • the continuous line shows the Boron concentration in the dopant segregated layer.
  • the figure clearly shows the two maximum doping concentrations of this segregated layer with its relative minimum in between.
  • the dashed line shows the Nickel concentration, in which region the ohmic contact is established.
  • the dotted line is the doping concentration of silicon after the segregation. Due to the first heating step (step b)) the Silicon has formed a metal compound layer 3 in form of a Nickel silicide layer, wherein the silicon concentration in the region of the metal compound layer may even be enhanced compared to the silicon concentration in the pure wide bandgap wafer 1 (e.g. for the metal being Nickel and the wide bandgap wafer being made of Silicon carbide, NiSi2 or Ni 2 Si may be formed).

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Abstract

A method for manufacturing a power semiconductor device is provided with the following manufacturing Steps: providing a wide bandgap wafer (1) made of a wide bandgap material and having a base doping concentration; creating a metal layer (30, 30') on a main side of the wide bandgap wafer by applying a metal, which metal is capable of forming a metal Compound layer (3, 3') with a component of the wide band gap material; after step b) performing a first heating step, by which the metal layer (30, 30') is transformed into the metal Compound layer (3, 3'); creating a dopant source layer (20, 20') at the main side of the semi-fabricated device (100) by applying a dopant, wherein the dopant source layer (20, 20') has a dopant range Rp from a surface of a semi-fabricated device (100), which semi-fabricated device (100) comprises the wafer (1), the metal Compound layer (3, 3') and the dopant source layer (20, 20'); after step d) performing a second heating step, by which a segregation is achieved, by which the dopant source layer (20, 20') is transformed into a dopant segregated layer (2, 2'), which is arranged below the metal Compound layer (3, 3'), wherein the dopant segregated layer (2, 2') forms a pn junction to a layer below the dopant segregated layer (2, 2'), wherein in step e) the metal Compound layer (3, 3') forms an ohmic contact up to an interface depth Xi, at which the metal Compound layer contacts the dopant segregated layer (2, 2'), wherein the interface depth Xi, between the metal Compound layer (3, 3') and the dopant segregated layer (2, 2') lies deeper in the semi-fabricated device (100) than the dopant range Rp of the dopant source layer.

Description

Method for Manufacturing a Power
Semiconductor Device
Description
Technical Field
The invention relates to the field of power electronics and more particularly to a method for manufacturing a power semiconductor device according to the preamble of claim 1 .
Background Art
In US 6,909,1 19 B2 a method for manufacturing a power semiconductor device is disclosed, in which a Silicon carbide substrate is provided, on which an epitaxial layer is grown on one main side. On the opposite main side, ions are implanted and a heat treatment is performed at 1300 °C, so that a doping concentration of 1018 to 1020 cm 3 is achieved. The implantation depth is about 1 nm. Afterwards, a Nickel layer as a metal layer is deposited having a thickness of at least 30 nm, and a heat treatment is performed at a temperature between 800 and 1000 °C, a temperature high enough to create an ohmic contact at the interface between the metal and the substrate.
In this prior art technique, the employed implantation dopant range Rp is much higher than a silicide interface depth X,. By shifting the Fermi level of the semiconductor, the band bending is significantly increased and, consequently, the Schottky Barrier Height (SBH) is further reduced such that the Ohmic contact is achieved already by annealing at 800°C for 2 minutes. However, this technique does not work for backgrinded wafers, and thus does not enable thin wafer processing, as it involves annealing at temperatures as high as 800°C. For such backgrinded wafers, temperatures as high as 800 °C, degrade the structures on the front side (e.g. MOS structures, thereby degrading the MOS performance, or a Schottky contact in a SiC Junction Barrier Schottky diode.)
US 8,267,507 B1 describes a method for manufacturing a SiC MOSFET. On an n- drift layer a p well layer and n source regions are formed by implantation and anneal. Between two n source regions, a p++ dopant is implanted for forming a p well contact region. Both source and well contact region are in amorphous state in order to enhance the following segregation process. Then a nickel layer is created above the source regions and p++ well contact region and annealed, thereby forming a nickel silicide layer. An n dopant such as phosphorous is implanted above the n source regions and a p dopant such as aluminium is implanted above the p well contact region. These dopant are then segregated by a heating step. Each of the source regions and well contact region are created by two implants followed by a segregation process, resulting in a rather flat profile of these double- implanted layers.
Disclosure of Invention
It is an object of the invention to provide a method for manufacturing a power semiconductor device having an ohmic contact, which is applicable also for thin wafers, and in which method lower temperatures are applied for the creation of ohmic contacts, so that previously created layers are not deteriorated.
This object is achieved by a method for manufacturing a power semiconductor device according to claim 1 . The inventive method comprises the following steps: a) providing a wide bandgap wafer made of a wide bandgap material having a base doping concentration;
b) creating a metal layer on a main side of the wide bandgap wafer by
applying a metal, which metal is capable of forming a metal compound layer with a component of the wide band gap material;
c) after step b) performing a first heating step, by which the metal layer is transformed into the metal compound layer with the component of the wide bandgap material;
d) creating a dopant source layer at the main side of the semi-fabricated
device by applying a dopant, wherein the dopant source layer has a dopant range Rp from a surface of a semi-fabricated device, which semi-fabricated device at least comprises the wafer and the dopant source layer;
e) after step d) performing a second heating step, by which a segregation is achieved, by which the dopant source layer is transformed into a dopant segregated layer, which is arranged below the metal compound layer, wherein the dopant segregated layer forms a pn junction to a layer below the dopant segregated layer,
wherein in step e) the metal compound layer forms an ohmic contact up to an interface depth X,, at which depth X, the metal compound layer contacts the dopant segregated layer,
wherein the interface depth X, is measured form the main sided surface of the semi-fabricated device, which further comprises the dopant segregated layer, wherein the interface depth X, between the metal compound layer and the dopant segregated layer lies deeper in the semi-fabricated device than the dopant range Rp of the dopant source layer.
Due to the usage of dopant segregation, a highly doped dopant segregated layer and at the same time an ohmic contact can be created, without deteriorating other layers of the semiconductor device. Due to the dopant source layer having such a small dopant range, the dopant is spread during the heating step into both directions, towards the surface and interface, i.e. into the metal layer and the wide bandgap wafer material, so that a metal compound layer is created as an ohmic contact and a dopant segregated layer is created as a highly doped layer.
Exemplarily, for the wide bandgap wafer being silicon carbide, a metal silicide or a metal carbide layer is created as metal compound layer. For the wide bandgap wafer being exemplarily gallium nitride, a metal nitride or a metal gallide layer is created as metal compound layer.
An ohmic contact is an electrical junction between two conductors (i.e. for a semiconductor between a metal layer and a semiconductor layer, exemplarily a doped Silicon-carbide layer) that has a linear current-voltage behavior (over all intended operating frequencies). An ohmic contact has low resistance.
Due to the creation of the metal compound layer, the temperature applied for the first heating step can be kept low and still a reliable ohmic contact can be achieved (either in the first or in the second heating step c) or e)). In the second heating step e) a lower temperature can be applied than in prior art methods due to the presence of the highly doped dopant segregated layer. The remaining dopant of the dopant source layer concentrates in a thin dopant segregated layer, because of the small diffusion of dopants in wide bandgap materials. Since the solid solubility in the metal compound material (e.g. metal silicide) is very low and the diffusion of dopants in the wide bandgap material (e.g. SiC) is negligible, most of the implanted dopant are segregated in the silicide/SiC interface, lowering barrier by increasing net carrier concentration underneath the contact. As a result, the Schottky barrier height can be lowered till the formation of ohmic contacts at very low temperatures. Hence, this method enables to obtain good ohmic contact by silicidation at low temperatures when compared with the prior art methods, wherein higher temperatures have to be applied.
The required implantation dose can be significantly reduced compared to the case where Xi>Rp because the dopants are concentrated in the interface. Since atomic vacancies are created at the metal compound layer/wafer interface (e.g.
silicide/SiC interface) during silicidation, the dopants are incorporated into substitutional sites, resulting in the dipole formation even without thermal activation of dopants, so that the active doping can be improved in the dopant segregated layer compared to prior art methods. Such phenomena is further supported by the decrease of ionization energy at high dopant concentration as stated by charge neutrality equation, which is the case for the dopant segregated layer. Exemplarily pre annealing of dopants for activation purposes prior to silicidation (for Ohmic contact formation) may be performed for further reduction of contact resistance. Thus, a high maximum doping concentration of the dopant segregated layer is achievable.
The dopant segregated layer is a layer, having a high maximum doping concentration at the surface of the metal compound layer, which goes into a relative minimum and again concentrates to another relative maximum doping concentration deeper in the semi-fabricated device due to the diffusion of the dopant in the direction of the wide bandgap wafer, which another relative maximum doping concentration, however, is lower than the maximum doping concentration at the surface of the wafer on the main side.
The method can be performed such that performing step d) before step b), i.e. the dopant source layer is created on/in the wide bandgap wafer and a metal layer is created on top of the dopant source layer and afterwards performing the first and second heating steps c) and e) simultaneously, so that the metal compound layer as ohmic contact and the dopant segregated layer are created simultaneously. Alternatively, step d) is performed after step c), i.e. the metal layer is first converted into a metal compound layer in step c) and afterwards a dopant source layer is created on the metal compound layer and transformed into a dopant segregated layer in the second heating step e). The ohmic contact is either created in the first heating step by the formation of the metal compound layer or in the second heating step e) by applying the second temperature and by forming the high doping concentration of the dopant segregated layer.
Further preferred embodiments of the inventive subject matter are disclosed in the dependent claims. Brief Description of Drawings
The subject matter of the invention will be explained in more detail in the following text with reference to the attached drawings, in which:
FIG 1 to 4 show manufacturing step for a first inventive method for
manufacturing a power semiconductor device;
FIG 5 to 7 show manufacturing step for another inventive method for
manufacturing a power semiconductor device;
FIG 8 to 10 show manufacturing step for another inventive method for
manufacturing a power semiconductor device; and
FIG 1 1 shows the doping concentration of the wafer, the dopant source layer and the dopant segregated layer.
The reference symbols used in the figures and their meaning are summarized in the list of reference symbols. Generally, alike or alike-functioning parts are given the same reference symbols. The described embodiments are meant as examples and shall not confine the invention. Modes for Carrying out the Invention
An inventive method for manufacturing a power semiconductor device is provided, which comprises the following manufacturing steps:
a) providing a wide bandgap wafer 1 made of a wide bandgap material having a base doping concentration;
b) creating a metal layer 30, 30' on a main side of the wide bandgap wafer by applying a metal, which metal is capable of forming a metal compound layer 3, 3' with a component of the wide band gap material; c) after step b) performing a first heating step, by which the metal layer 30, 30' is transformed into a metal compound layer 3, 3' with the component of the wide bandgap material;
d) creating a dopant source layer 20, 20' at the main side of the semi- fabricated device 1 00 by a single application of a dopant, wherein the dopant source layer 20, 20' has a dopant range Rp from a surface of a semi-fabricated device 1 00, which semi-fabricated device 1 00 comprises the wafer 1 , the metal compound layer 3, 3' and the dopant source layer 20, 20';
e) after step d) performing a second heating step, by which a segregation is achieved, by which the dopant source layer 20, 20' is transformed into a dopant segregated layer 2, 2', which is arranged below the metal compound layer 3, 3', wherein the dopant segregated layer 2, 2' forms a pn junction to a layer below the dopant segregated layer 2, 2',
wherein in step e) the metal compound layer 3, 3' forms an ohmic contact up to an interface depth X,, at which the metal compound layer contacts the dopant segregated layer 2, 2', wherein the interface depth X, is measured form the main sided surface of the semi-fabricated device 100, which further comprises the dopant segregated layer 2, 2', wherein the interface depth Xi between the metal compound layer 3, 3' and the dopant segregated layer 2, 2' lies deeper in the semi-fabricated device 100 than the dopant range Rp of the dopant source layer.
The interface depth X, shall be the depth at which the doping concentration of the material of the wide bandgap substrate involved in the metal compound layer formation gets constant from a higher value in the metal compound layer, i.e. the depth at which no material is concentrated any more from the metal compound layer. In FIG 1 1 , in which a nickel silicide layer has been formed, it is the depth, in which the Si concentration has dropped to a constant value.
The metal compound layer is made of the metal(s) of the metal layer and of a component of the wide bandgap material, which exemplarily comprises at least two components. Exemplarily, the dopants inside the metal compound layer 3, 3' are not electrically active since the metal on the surface is already very conductive, so that only those particles in the interface contribute to the ohmic contact. In an exemplary embodiment, the wide bandgap material is silicon carbide and by the segregation a metal silicide or a metal carbide layer is created as the metal compound layer 3, wherein the metal is the material of the metal layer 30.
Exemplarily, for Nickel being the metal, a Nickel silicide or a Nickel carbide layer is created as the metal compound layer 3.
In another embodiment, the wide bandgap material is gallium nitride and by the segregation a metal nitride or metal gallide layer is created as a metal compound layer 3 (e.g. Nickel nitride or Nickel gallide layer).
The wide bandgap wafer 1 may exemplarily have a thickness of 2 to 10 μηι or of 1 to 400 μηι. The wide bandgap wafer 1 may have a constant base doping concentration, exemplarily in a range between 1 * 1014 and 1 * 1019 cm 3 or between 5 * 1014 to 1 * 1018 cm 3. The semi-fabricated device 100 shall comprise the wafer 1 and all layers and contacts created on or in the wafer, i.e. it is the workpiece at which the further manufacturing steps are performed. I.e. if the dopant source layer 20, 20' is created after the metal layer 30, 30', the semi- fabricated device 1 comprises these layers. If the metal layer is converted into the metal compound layer before the dopant source layer is applied the semi- fabricated device comprises the metal compound layer.
The base doping concentration of the wide bandgap wafer 1 may also decrease subsequently from a maximum value of the base doping concentration on a first main side 10 of the wide bandgap wafer to lower base doping concentration on a second main side 15 of the wide bandgap wafer opposite to the first main side 10. Exemplarily, the ohmic contact is formed on the first main side, i.e. the side of the higher value of the base doping concentration.
The dopant source layer may be created by any method, by which a dopant is applied on or into the wafer, such as implantation, dopant deposition or epitaxial growth. Exemplarily, the dopant for the creation of the dopant source layer 20 may be implanted with a dose between 1 * 1012 and 5 * 1016 cm 2. The dopant for the creation of the dopant source layer may be an n-dopant, exemplarily at least one of phosphorous and nitrogen, or a p dopant, exemplarily at least one of boron and aluminium. Thus, only one dopant or a combination of two or more dopants may be applied, wherein the two or more dopants are exemplarily all of the same conductivity type. The conductivity type of the dopant is chosen depending on the semiconductor type to be created. The dopant source layer may be of the same conductivity type as the wafer, i.e. n wide bandgap wafer and dopant source layer of n type or both of p type. Alternatively, the dopant source layer may be of different conductivity type than the wafer, one being n and the other of p type. It is also possible that the dopant source layer comprises regions of the first and of the second conductivity type (n and p type) alternating with each other. The ohmic contact may contact the regions of the first and/or second conductivity type.
Exemplarily for an IGBT with an ohmic contact to be created on the emitter side such regions could be at least one source region of the same conductivity type as the wide bandgap wafer and at least one base region of a different conductivity type and the ohmic contact contacts the at least one source and base region.
Exemplarily for a reverse conducting IGBT with an ohmic contact to be created on the collector side, alternating collector regions of a different conductivity type than the wide bandgap wafer and at least one region of the same conductivity type as the wide bandgap wafer, exemplarily higher doped than the wide bandgap wafer, are created alternatingly on the collector side and the ohmic contact contacts both region types.
Rp is the dopant range, i.e. the maximum depth of the applied dopant. The dopant range Rp is shown in the figures as a dashed line (FIG 2). An exemplary dopant range Rp of the dopant source layer 20 may be between 1 to 400 nm. Applying the dopant, e.g. by implantation, may exemplarily be performed at room temperature or at an enhanced temperature, exemplarily at a temperature between 350 to 750 °C.
In an exemplary embodiment, after creating the dopant source layer 20, 20' and before creating the metal layer 30, 30' a pre-anneal-step may be performed at a temperature between 500 to 1900 °C in order to activate the dopant.
Then the metal layer 30, 30' is applied on the dopant segregated layer 2, 2' (FIG 3). A first heating step is performed, by which the metal layer 30, 30' is
transformed into the metal compound layer 3, 3' shown in the figures as a dotted line. The interface depth X, of the metal compound layer 3, 3' lies deeper in the semi-fabricated device 100 than the location, at which the dopant source layer 20, 20' has had its dopant range Rp. That means that the metal compound layer 3, 3' extends deeper into the semi-fabricated device 100 than the dopant range Rp would be in this step. Thus, the metal compound layer 3 overlies the region, at which the dopant source layer 20 was or will be created. The dopant segregated layer 2, 2' is shown in the figures as a region having a dotted hachure.
The dopant source layer 20, 20' and the metal layer 30, 30' may be created as continuous layers over the whole surface of the wide bandgap wafer on one side, e.g. the first main side 10 or the second main side 15. Thus, also the ohmic contact may be created over the whole surface. Alternatively, either of the dopant source layer 20, 20' or the metal layer 30, 30' may be created only on a partial region on a side of the wide bandgap wafer or heat is only applied in the first heating step on such a partial region, at which an ohmic contact shall be stablished, so that also the ohmic contact is created in such a region, i.e. a laterally limited ohmic contact is created. "Laterally limited" means that an extension of the contact is limited in the plane of the first main side). Exemplarily, the ohmic contact may be created only in the central area, leaving the surrounded termination area uncontacted or the ohmic contact may alternate with other contacts (e.g. gate contacts) or with areas, insulated at the surface of the device.
It is also possible to create a combination of a continuous ohmic contact on one main side and a laterally limited contact on the other main side opposite to the continuous ohmic contacted main side.
The metal layer 30 consists of at least one metal, exemplarily the metal layer consists of or comprises Nickel, Titanium, Tantalum or Tungsten. The metal layer 30 may also consist of or comprise Palladium, Platinum and Aluminium. In another embodiment, the metal layer comprises a compound of at least two of the before mentioned metals. Exemplarily, the metal layer comprises Nickel and at least one of Palladium, Platinum or Aluminium. These metal may be applied as a compound layer of different metals or as a stack of metal layers, exemplarily with Nickel being the contact layer to the wafer 1 .
The first and/or second heating step for the creation of the metal compound layer and/or the segregated layer may be performed at a temperature between 400 to 1200 °C, exemplarily between 600 to 800 °C. The heat is chosen such that an ohmic contact can be created, but other layers which may have been created before are not negatively influenced by such temperature.
The thickness of the dopant segregated layer 2 created in the second heating step e) may be between 1 to 100 nm or between 1 and 50 nm or between 1 to 10 nm and the maximum doping concentration between 4 * 1018 to 1 * 1020 cm 3. The dopant segregated layer is a layer, which has a maximum doping
concentration which decreases from a first doping concentration maximum on or in direction of the surface of the semi-fabricated device to a relative doping concentration minimum in a first depth (depth being measured from the side of the wafer, at which the dopant source layer has been created) (FIG 8). At greater depths than the first depth, the doping concentration further increases to a relative maximum doping concentration in a second depth, beyond which second depth the doping concentration decreases. That means the dopant segregated layer has two doping concentration maxima at different depths, wherein one depth is at the interface to the metal compound layer 3. In an exemplary embodiment, the dopant segregated layer 2 created in the second heating step has a higher maximum doping concentration than the dopant source layer 20.
The interface depth X, of the metal compound layer 3 may be between 4 to 500 nm.
In another inventive method for manufacturing a semiconductor device a metal layer 30, 30' is created (step b; FIG 8) and then a first heating step is performed (step c)). The metal layer 30 converts into a metal compound layer 3 (which may also be called a metal alloy layer) with one material of the wide bandgap wafer 1 , leaving vacancies of this material in the wafer 1 (FIG 9). Depending on the applied temperature in the first heating step, an ohmic contact may be created. It is also possible to perform the first heating step at a lower temperature, at which the metal compound layer 3 is created, but not converted into an ohmic contact yet. In this embodiment, the ohmic contact is created in the second heating step. The presence of the dopant segregated layer (also created in the second heating step e)) improves the formation of the ohmic contact, thus lower temperatures can be applied in this ohmic-contact forming step than in the prior art methods. In another exemplary embodiment, the first temperature is lower than the second
temperature. Exemplarily, the first temperature may be as low as 400 °C up to 700 °C or up to 600 °C, i.e. below an ohmic contact forming temperature. The second heating step may be performed at higher temperatures, exemplarily up to 1200 °C or up to 800 °C.
Afterwards, a dopant source layer 20, 20' is created on the same main side of the wide bandgap wafer, i.e. on the metal compound layer 3, 3', by applying a dopant, wherein the dopant source layer 20, 20' has a dopant range Rp (step d); FIG 10). A second heating step is performed, by which a segregation is achieved, which transforms the dopant source layer 20, 20' and the wide bandgap wafer into a dopant segregated layer 2, 2', which layer 2, 2' is arranged below the metal compound layer 3, 3' (resulting in a device as the device shown in FIG 4), because the dopant goes into the vacancies formed in the wide bandgap wafer in the first heating step (step c)). Again, the interface depth X, of the metal compound layer is greater than the dopant range Rp of the dopant source layer (the depth X, and range Rp compared after having created the metal compound layer 3).
In another exemplary embodiment, the wide bandgap wafer comprises a first main side 10 and a second main side 15 opposite to the first main side 10. The method for the creation of the ohmic contact as disclosed above may be performed on the first and second main side 10, 15, so that a metal compound layer 3, 3' and a dopant segregated layer 2, 2' is created on the first and on the second main side 10, 15. For this case, a wide bandgap wafer 1 is provided (FIG 1 ), then a dopant source layer 20, 20' is created on the first main side 10 and on the second main side 15 (FIG 5). On top of the dopant source layer 20, 20', a metal layer 30, 30' is applied on each side 10, 15 (FIG 6). In the combined first and second heating step c) and e) performed afterwards, a dopant segregation layer 2, 2' and a metal compound layer 3, 3' are created (FIG 7).
Of course, it is also possible to create first and second main sided ohmic contacts by the process described for the figures 8 to 10, i.e. with applying a metal layer 30, 30' first, then performing a first heating step c), then applying a dopant source layer 20, 20', and then performing the second heating step e) on the first and second main side 10, 15.
The dopant segregated layer 2' on the second main side 15 may have a different maximum doping concentration than the dopant segregated layer 2 on the first main side 10 and/or a different conductivity type and/or a different lateral extension. Exemplarily, the dopant segregated layer 2' on the second main side 15 has a maximum doping concentration, which is lower than the maximum doping concentration of the dopant segregated layer 2 on the first main side 10, exemplarily lower than at least a factor of 10. Exemplarily, the dopant segregated layer 2' on the second main side 15 has a maximum doping concentration between 5 * 1017 and 1 *1020 cm 3, in particular 5 * 1017 to 5 * 1018 cm 3. In another exemplary embodiment, the wafer 1 comprises a first main side 10 and a second main side 20 opposite to the first main side and in that the metal compound layer 3 and the dopant segregated layer 2 is formed on the first main side 10 as main side with the process as disclosed above and a Schottky contact is formed on the second main side 15. Exemplarily, the Schottky contact contacts the wide bandgap wafer 1 with its unamended doping concentration (or the Schottky contact may contact any other layer formed in or on the wide bandgap wafer 1 ). The Schottky contact may be created before or after the ohmic contact on the first main side 10. If the Schottky contact is created after the ohmic contact, it is ensured that no high temperatures convert the Schottky contact into an ohmic contact.
The inventive manufacturing method may be applied to different semiconductor devices, exemplarily to IGBTs (insulated gate bipolar transistor), MOSFETs, thyristors or diodes. The dopant segregated layer 2, 2' forms a pn junction to a layer below the dopant segregated layer 2, 2'. Below shall mean the layer that is arranged in a greater depth from such main side, on which the dopant segregated layer is arranged, and the layer below contacts the dopant segregated layer, thereby forming the pn junction. The dopant segregated layer 2,2 ' may be a collector layer in the case of an inventive IGBT, an anode layer in the case of an inventive thyristor or an anode layer for an inventive diode, which dopant segregated layer (e.g. p doped) forms a pn junction to a below lying buffer or drift layer (n doped in the case of a p doped dopant segregated layer). The dopant segregated layer may also be a source region for an inventive IGBT, an emitter layer for an inventive thyristor or a drain layer for an inventive MOSFET, which dopant segregated layer (e.g. n doped) forms a pn junction to a below lying base layer (p doped in the case of an n doped dopant segregated layer).
The wide bandgap wafer and the dopant source layer 20, 20' are of different conductivity types. If ohmic contacts are created on the first and second main side 10, 15, the dopant source layers 20, 20' and thus, the dopant segregated layers 2, 2' may be of the same conductivity type or of different conductivity types (i.e. both n or p doped or one dopant segregated layer 2, 2' being of n type and the other 2', 2 of p type).
FIG 1 1 shows the doping concentrations of various layers during or after the manufacturing of the semiconductor device. In this figure, the wide bandgap wafer is made of Gallium nitride, as a dopant Boron is implanted and Nickel is applied as a metal layer 30. The alternatingly dashed and dotted line shows the boron concentration in the dopant source layer 20. This layer has a small dopant range Rp in which Boron is present. The other three layers, shown in FIG 1 1 are layers after finalizing the semiconductor device. The continuous line shows the Boron concentration in the dopant segregated layer. The figure clearly shows the two maximum doping concentrations of this segregated layer with its relative minimum in between. The dashed line shows the Nickel concentration, in which region the ohmic contact is established. The dotted line is the doping concentration of silicon after the segregation. Due to the first heating step (step b)) the Silicon has formed a metal compound layer 3 in form of a Nickel silicide layer, wherein the silicon concentration in the region of the metal compound layer may even be enhanced compared to the silicon concentration in the pure wide bandgap wafer 1 (e.g. for the metal being Nickel and the wide bandgap wafer being made of Silicon carbide, NiSi2 or Ni2Si may be formed).
Reference List
1 wide bandgap wafer
10 first main side
15 second main side
2, 2' dopant segregated layer
20, 20' dopant source layer
3, 3' metal layer
30, 30' metal compound layer
100 semi-fabricated device
RP dopant range
Xi interface depth of metal compound layer

Claims

C L A I M S
1 . A method for manufacturing a power semiconductor device, characterized in, that
a) providing a wide bandgap wafer (1 ) made of a wide bandgap material having a base doping concentration ;
b) creating a metal layer (30, 30') on a main side of the wide bandgap wafer by applying a metal, which metal is capable of forming a metal compound layer (3, 3') with a component of the wide band gap material;
c) after step b) performing a first heating step, by which the metal layer (30, 30') is transformed into the metal compound layer (3, 3');
d) creating a dopant source layer (20, 20') at the main side of the semi- fabricated device (1 00) by an application of at least onedopant, wherein the dopant source layer (20, 20') has a dopant range Rp from a surface of a semi-fabricated device (100), which semi-fabricated device (100) comprises at least the wafer (1 ), and the dopant source layer (20, 20'); e) after step d) performing a second heating step, by which a segregation is achieved, by which the dopant source layer (20, 20') is transformed into a dopant segregated layer (2, 2'), which is arranged below the metal compound layer (3, 3'), wherein the dopant segregated layer (2, 2') forms a pn junction to a layer below the dopant segregated layer (2, 2'), wherein in step e) the metal compound layer (3, 3') forms an ohmic contact up to an interface depth X,, at which the metal compound layer contacts the dopant segregated layer (2, 2'), wherein the interface depth X, is measured form the main sided surface of the semi-fabricated device (1 00), which further comprises the dopant segregated layer (2, 2'), wherein the interface depth Xi between the metal compound layer (3, 3') and the dopant segregated layer (2, 2') lies deeper in the semi-fabricated device (100) than the dopant range Rp of the dopant source layer.
2. The method according to claim 1 , characterized in, that performing step d) before step b) and afterwards performing step c) and e) simultaneously.
3. The method according to claim 1 , characterized in, that performing step d) after step c).
4. The method according to any of the claims 1 or 3, characterized in, that
performing the first heating step at a lower temperature than the second heating step.
5. The method according to any of the claims 1 to 4, characterized in, that performing the first and second heating step in step c) and e) at a temperature between 400 to 1200 °C or between 600 to 800 °C.
6. The method according to any of the claims 1 to 5, characterized in, that the dopant range Rp of the dopant source layer (20, 20') is between 1 to 400 nm.
7. The method according to any of the claims 1 to 6, characterized in, that
applying the dopant for the creation of the dopant source layer (20, 20') with a dose between 1 * 1012 and 5 * 1016 cm 2.
8. The method according to any of the claims 1 to 7, characterized in, that the dopant segregated layer (2, 2') has a thickness between 1 to 100 nm or 1 to
50 nm or 1 to 10 nm.
9. The method according to any of the claims 1 to 8, characterized in, that the dopant segregated layer (2, 2') has a maximum doping concentration between 4 * 1018 and 1 * 1020 crrr3.
10. The method according to any of the claims 1 to 9, characterized in, that the interface depth X, of the metal compound layer (3, 3') is between 2 to 500 nm.
1 1 . The method according to any of the claims 1 to 10, characterized in, that the metal layer (30, 30') is made of at least one of Nickel, Titanium, Tantalum, Tungsten, Palladium, Platinum and Aluminium.
12. The method according to any of the claims 1 to 10, characterized in, that the metal layer (30, 30') is made of a stack of layers of or a compound comprising at least two of Nickel, Titanium, Tantalum, Tungsten, Palladium, Platinum and Aluminium.
13. The method according to any of the claims 1 to 12, characterized in, that the wide bandgap material is silicon carbide and in step c) a metal silicide or a metal carbide layer is created as the metal compound layer (3, 3'), wherein the metal is the material of the metal layer (30, 30') or in that the wide bandgap material is gallium nitride and in step c) a metal nitride or metal gallide layer is created as a metal compound layer (3, 3').
14. The method according to any of the claims 1 to 13, characterized in, that the wide bandgap wafer comprises a first main side (10) and a second main side (15) opposite to the first main side as main sides and in that performing step a) to e) on the first and on the second main side (10, 15).
15. The method according to any of the claims 1 to 13, characterized in, that the wide bandgap wafer (1 ) comprises a first main side (10) as the main side and a second main side (20) opposite to the first main side and in that forming the metal compound layer (3) and the dopant segregated layer (2) only on the first main side (10) and forming a Schottky contact on the second main side (15).
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