Schottky emission cathode and a method of stabilizing the same

A Schottky emission cathode comprising:

• a filament,
• a needle-shaped piece of single crystal refractory metal having a flat crystal surface at a tip thereof and attached to said filament,
• said needle-shaped piece of single crystal refractory metal being adapted to be heated by passing a current through said filament and to have an electric field applied on said tip so that electrons are extracted from said tip, and
• an adsorbed layer including at least one kind of metal other than said single crystal refractory metal on said flat crystal surface;
• a radius of curvature of a longitudinal cross section of said tip being of a value to produce an energy width among electrons extracted from said tip not exceeding a predetermined value with said electric field being sufficient to prevent said flat crystal surface from collapsing during operation of said cathode, wherein said radius of curvature of said needle-shaped piece of single crystal refractory metal is formed by etching a piece of said single crystal refractory metal into a needle shape in an etching solution and then heating said piece to a temperature higher than 2000 K. in a vacuum to obtain a desired radius of curvature.

A Schottky emission cathode element comprising:

• a filament,
• a needle-shaped piece of single crystal refractory metal having a flat crystal surface at a tip thereof and attached to said filament,
• said needle-shaped piece of single crystal refractory metal being adapted to be heated by passing a current through said filament and to have an electric field applied on said tip so that electrons are extracted from said tip,
• an adsorbed layer including at least one kind of metal other than said single crystal refractory metal on said flat crystal surface,
• leads for supporting said filament and passing said current through said filament, and
• an insulator for embedding and fixing said leads;
• a radius of curvature of a longitudinal cross section of said tip being of a value larger than a radius of curvature at an intersection of a curve of an equilibrium field strength for exerting an electrostatic force balancing with a surface diffusion at said tip vs. a radius of curvature of a longitudinal cross section of said tip and a curve of an electric field strength for extracting electrons of an energy width of a predetermined value among said extracted electrons from said tip vs. a radius of curvature of a longitudinal cross section of said tip, and smaller than 2.5 .mu.m.

A Schottky emission cathode element comprising:

• a filament,
• a needle-shaped piece of single crystal refractory metal having a flat crystal surface at a tip thereof and attached to said filament,
• said needle-shaped piece of single crystal refractory metal being adapted to be heated by passing a current through said filament and to have an electric field applied on said tip so that electrons are extracted from said tip,
• an adsorbed layer including at least one kind of metal other than said single crystal refractory metal on said flat crystal surface,
• leads for supporting said filament and passing said current through said filament, and an insulator for embedding and fixing said leads;
• a radius of curvature of a longitudinal cross section of said tip being of a value to produce an energy width among electrons extracted from said tip not exceeding a predetermined value with said electric field being sufficient to prevent said flat crystal surface from collapsing during operation of said cathode, wherein said radius of curvature of said needle-shaped piece of single crystal refractory metal is formed by etching a piece of said single crystal refractory metal into a needle shape in an etching solution and then heating said piece to a temperature higher than 2000 K. in a vacuum to obtain a desired radius of curvature.

An electron beam apparatus comprising:

• a Schottky emission cathode comprising a filament, a needle-shaped piece of single crystal refractory metal having a flat crystal surface at a tip thereof and attached to said filament, said needle-shaped piece of single crystal refractory metal being adapted to be heated by passing a current through said filament and to have an electric field applied on said tip so that electrons are extracted from said tip, and an adsorbed layer including at least one kind of metal other than said single crystal refractory metal on said flat crystal surface, a radius of curvature of a longitudinal cross section of said tip being of a value larger than a radius of curvature at an intersection of a curve of an equilibrium field strength for exerting an electrostatic force balancing with a surface diffusion at said tip vs. a radius of curvature of a longitudinal cross section of said tip and a curve of an electric field strength for extracting electrons of an energy width of a predetermined value among said extracted electrons from said tip vs. a radius of curvature of a longitudinal cross section of said tip, and smaller than 2.5 .mu.m, a heating power supply for supplying said current through said filament,
• an extraction power supply for supplying said electric field,
• an accelerating power supply for accelerating said extracted electrons,
• a control computer for controlling said extraction power supply for said electric field to maintain a stable electron emission with an energy width not exceeding said predetermined value, and
• an electron lens provided with an aperture plate for focusing a beam of said extracted electrons and illuminating an object with said beam.

An electron beam apparatus for use with a cathode comprising a filament formed of a hairpin-shaped piece of refractory metal, a piece of single crystal refractory metal attached to said filament, and an adsorbed layer including a metal whose work function or electron affinity is lower than that of said single crystal refractory metal and attached to a tip of said piece of single crystal refractory metal comprising:

• a heating power supply for heating said piece of single crystal refractory metal by a current through said filament to a temperature sufficient to maintain said adsorbed layer stably,
• a voltage supply for supplying an electric field to said tip of said piece of single crystal refractory metal to extract electrons therefrom,
• an accelerating power supply for accelerating said electrons extracted from said cathode,
• a lens provided with an aperture plate for focusing said extracted electrons and illuminating an object with said electrons, and
• means for measuring an angular current density distribution of said extracted electrons from said cathode,
• wherein, when two or more local minimums are detected in said angular current density distribution, said electron beam apparatus extracts electrons from said tip of said piece of single crystal refractory metal by heating by passing a current through said filament to a temperature sufficient to maintain said adsorbed layer stably and applying an electric field on said tip, removes said adsorbed layer first, and then applies an electric field appropriate for preventing said tip from blunting due to migration of atoms from said tip until an electron emission current from said cathode saturates.

An electron beam apparatus comprising:

• a Schottky emission cathode comprising a filament, a needle-shaped piece of single crystal refractory metal having a flat crystal surface at a tip thereof and attached to said filament, and an adsorbed layer including at least one kind of metal other than said single crystal refractory metal on said flat crystal surface, a radius of curvature of a longitudinal cross section of said tip being of a value larger than a radius of curvature at an intersection of a curve of an equilibrium field strength for exerting an electrostatic force balancing with a surface diffusion at said tip vs. a radius of curvature of a longitudinal cross section of said tip and a curve of an electric field strength for extracting electrons of an energy width of a predetermined value among said extracted electrons from said tip vs. a radius of curvature of a longitudinal cross section of said tip, and smaller than 2.5 .mu.m, said needle-shaped piece of single crystal refractory metal being adapted to be heated by passing a current through said filament and to have an electric field applied on said tip so that electrons are extracted from said tip,
• a heating power supply for supplying said current through said filament,
• an extraction power supply for supplying an electric field to extract electrons from said Schottky emission cathode,
• an accelerating power supply for accelerating said extracted electrons from said Schottky emission cathode,
• a control computer for controlling said extraction power supply for said electric field to maintain a stable electron emission having an energy width narrower than said predetermined value, and
• an electron lens provided with an aperture plate for focusing said extracted electrons and illuminating an object with said extracted electrons.

A method of forming a cathode facet comprising the steps of:

• providing a cathode comprising a filament formed of a hairpin-shaped piece of refractory metal, a piece of single crystal refractory metal attached to said filament, and an adsorbed layer including a metal whose work function or electron affinity is lower than that of said single crystal refractory metal and attached to a tip of said piece of single crystal refractory metal;
• extracting electrons from said tip of said piece of single crystal refractory metal by heating said tip by passing a current through said filament to a temperature at which said adsorbed layer evaporates and by applying an electric field on said tip;
• returning said tip to a temperature at which said adsorbed layer is maintained stably after an electron emission pattern observable by bombarding an electroluminescent target with said electrons extracted from said tip disappears;
• and
• applying an electric field necessary for preventing said tip from blunting due to migration of atoms from said tip on said tip until said electron emission pattern appears as a nearly uniform circle.

A method of forming a cathode facet comprising the steps of:

• providing a cathode comprising a filament formed of a hairpin-shaped piece of refractory metal, a piece of single crystal refractory metal attached to said filament, and an adsorbed layer including a metal whose work function or electron affinity is lower than that of said piece of single crystal refractory metal and attached to a tip of said piece of single crystal refractory metal;
• continuing to extract electrons from said tip by applying an electric field on said tip and heating said tip by passing a current through said filament to a temperature at which said adsorbed layer evaporates until an emission current decreases to less than 5 .mu.A;
• returning said tip to a temperature at which said adsorbed layer is maintained stably;
• and
• applying an electric field necessary for preventing said tip from blunting due to migration of atoms from said tip on said tip until the electron emission current saturates.

A method of forming a cathode facet comprising the steps of:

• providing a cathode comprising a filament formed of a hairpin-shaped piece of refractory metal, a piece of single crystal refractory metal attached to said filament, and an adsorbed layer including a metal whose work function or electron affinity is lower than that of said single crystal refractory metal and attached to a tip of said piece of single crystal refractory metal;
• extracting electrons from said tip of said piece of single crystal refractory metal by heating said piece of single crystal refractory metal by passing a current through said filament to a temperature at which said adsorbed layer evaporates and applying an electric field on said tip;
• and
• then applying an electric field necessary for preventing said tip from blunting due to migration of atoms from said tip on said tip and controlling the electric field at said tip so that an electron emission current does not exceed a predetermined value at the same time.

A method of forming a cathode facet comprising the steps of:

• providing a cathode comprising a filament formed of a hairpin-shaped piece of tungsten, a piece of single crystal refractory metal of tungsten of crystal orientation <100>
• attached to said filament, and
• an adsorbed layer of zirconium and oxygen attached to a tip of said piece of single crystal refractory metal;
• extracting electrons from said tip by applying an electric field on said tip;
• raising a temperature of said adsorbed layer to more than 1900 K. until an electron emission current reaches an equilibrium state of one of less than 5 .mu.A and an angular intensity of less than 5 .mu.A/sr over a whole useful electron-emissive region of said tip;
• and then
• lowering the temperature of said adsorbed layer to less than 1900 K. next and to apply an electric field of more than 0.15 V/.ANG. on said tip until the electron emission current saturates.

A method of forming a cathode facet comprising the steps of:

• providing a cathode comprising a filament formed of a hairpin-shaped piece of tungsten, a piece of single crystal refractory metal of tungsten of crystal orientation <100>
• attached to said filament, and an adsorbed layer of zirconium and oxygen attached to a tip of said piece of single crystal refractory metal;
• heating said tip of said piece of single crystal refractory metal to more than 1900 K. without applying an electric field on said tip;
• applying and controlling an electric field on said tip so that an emission current does not exceed a predetermined value until the electron emission current reaches a state of one of less than 5 .mu.A and an angular current intensity of less than 5 .mu.A/sr over a whole useful electron-emissive region of said tip;
• and
• then lowering a temperature of said adsorbed layer to less than 1900 K. and controlling the electric field at said tip by applying an electric field of more than 0.15 V/.ANG. at the same time so that the electron emission current does not exceed a predetermined value.

An electron beam apparatus for use with a cathode comprising a filament formed of a hairpin-shaped piece of refractory metal, a piece of single crystal refractory metal attached to said filament, and an adsorbed layer including a metal whose work function or electron affinity is lower than that of said single crystal refractory metal and attached to a tip of said piece of single crystal refractory metal comprising:

• a heating power supply for heating said piece of single crystal refractory metal by a current through said filament to a temperature sufficient to maintain said adsorbed layer stably,
• a voltage supply for supplying an electric field to said tip of said piece of single crystal refractory metal to extract electrons therefrom,
• an accelerating power supply for accelerating said electrons extracted from said cathode,
• a lens provided with an aperture plate for focusing said extracted electrons and illuminating an object with said electrons, and
• means for measuring an angular current density distribution of said extracted electrons from said cathode,
• wherein, when two or more local minimums are detected in said angular current density distribution, said electron beam apparatus continues to extract electrons from said tip by applying an electric field on said tip and heating said tip by passing a current through said filament to a temperature at which said adsorbed layer evaporates until an emission current decreases to less than 5 .mu.A, returns said tip to a temperature at which said adsorbed layer is maintained stably, and applies an electric field necessary for preventing said tip from blunting due to migration of atoms from said tip on said tip until the electron emission current saturates.

An electron beam apparatus for use with a cathode comprising a filament formed of a hairpin-shaped piece of refractory metal, a piece of single crystal refractory metal attached to said filament, and an adsorbed layer including a metal whose work function or electron affinity is lower than that of said single crystal refractory metal and attached to a tip of said piece of single crystal refractory metal comprising:

• a heating power supply for heating said piece of single crystal refractory metal by a current through said filament to a temperature sufficient to maintain said adsorbed layer stably,
• a voltage supply for supplying an electric field to said tip of said piece of single crystal refractory metal to extract electrons therefrom,
• an accelerating power supply for accelerating said electrons extracted from said cathode,
• a lens provided with an aperture plate for focusing said extracted electrons and illuminating an object with said electrons, and
• means for measuring an angular current density distribution of said extracted electrons from said cathode,
• wherein, when two or more local minimums are detected in said angular current density distribution, said electron beam apparatus extracts electrons from said tip of said piece of single crystal refractory metal by heating said piece of single crystal refractory metal by passing a current through said filament to a temperature at which said adsorbed layer evaporates and applying an electric field on said tip, and then applies an electric field necessary for preventing said tip from blunting due to migration of atoms from said tip on said tip and controls the electric field at said tip so that an electron emission current does not exceed a predetermined value at the same time.

An electron beam apparatus for use with a cathode comprising a filament formed of a hairpin-shaped piece of refractory metal, a piece of single crystal refractory metal attached to said filament, and an adsorbed layer including a metal whose work function or electron affinity is lower than that of said single crystal refractory metal and attached to a tip of said piece of single crystal refractory metal comprising:

• a heating power supply for heating said piece of single crystal refractory metal by a current through said filament to a temperature sufficient to maintain said adsorbed layer stably,
• a voltage supply for supplying an electric field to said tip of said piece of single crystal refractory metal to extract electrons therefrom,
• an accelerating power supply for accelerating said electrons extracted from said cathode,
• a lens provided with an aperture plate for focusing said extracted electrons and illuminating on object with said electrons, and
• means for measuring an angular current density distribution of said extracted electrons from said cathode,
• wherein, when two or more local minimums are detected in said angular current density distribution, said electron beam apparatus heats said tip of said piece of single crystal refractory metal to more than 1900 K. without applying an electric field on said tip, applies and controls an electric field on said tip so that an emission current does not exceed a predetermined value until the electron emission current reaches a state of one of less than 5 .mu.A and an angular current intensity of less than 5 .mu.A/sr over a whole useful electron-emissive region of said tip, and then lowers a temperature of said adsorbed layer to less than 1900 K. and controls the electric field at said tip by applying an electric field of more than 0.15 V/.ANG. at the same time so that the electron emission current does not exceed a predetermined value.

A cathode-stabilizing apparatus for use with a cathode comprising a filament formed of a hairpin-shaped piece of refractory metal, a piece of single crystal refractory metal attached to said filament, and an adsorbed layer including a metal whose work function or electron affinity is lower than that of said single crystal refractory metal and attached to a tip of said piece of single crystal refractory metal, said piece of single crystal refractory metal being adapted to be heated by passing a current through said filament and to have an electric field applied on said tip so that electrons are extracted from said tip comprising:

• a heating power supply for supplying said current through said filament,
• an extraction power supply for supplying said electric field,
• an accelerating power source for accelerating said extracted electrons,
• an electron lens provided with an aperture plate for focusing said extracted electrons and illuminating an object with said extracted electrons, and
• a control computer for controlling said three power supplies, wherein, when an electron current adsorbed in said aperture plate decreases by a predetermined value, said control computer controls said heating power supply and said extraction power supply such that said cathode-stabilizing apparatus extracts electrons from said tip of said piece of single crystal refractory metal by heating by passing a current through said filament to a temperature sufficient to maintain said adsorbed layer stably and applying an electric field on said tip, removes said adsorbed layer first, and then applies an electric field appropriate for preventing said tip from blunting due to migration of atoms from said tip until an electron emission current from said cathode saturates.

A cathode-stabilizing apparatus for use with a cathode comprising a filament formed of a hairpin-shaped piece of refractory metal, a piece of single crystal refractory metal attached to said filament, and an adsorbed layer including a metal whose work function or electron affinity is lower than that of said single crystal refractory metal and attached to a tip of said piece of single crystal refractory metal, said piece of single crystal refractory metal being adapted to be heated by passing a current through said filament and to have an electric field applied on said tip so that electrons are extracted from said tip comprising:

• a heating power supply for supplying said current through said filament,
• an extraction power supply for supplying said electric field,
• an accelerating power source for accelerating said extracted electrons,
• an electron lens provided with an aperture plate for focusing said extracted electrons and illuminating an object with said extracted electrons, and
• a control computer for controlling said three power supplies, wherein, when an electron current adsorbed in said aperture plate decreases by a predetermined value, said control computer controls said heating power supply and said extraction power supply such that said cathode-stabilizing apparatus continues to extract electrons from said tip by applying an electric field on said tip and heating said tip by passing a current through said filament to a temperature at which said adsorbed layer evaporates until an emission current decreases to less than 5 .mu.A, returns said tip to a temperature at which said adsorbed layer is maintained stably, and applies an electric field necessary for preventing said tip from blunting due to migration of atoms from said tip on said tip until the electron emission current saturates.

A cathode-stabilizing apparatus for use with a cathode comprising a filament formed of a hairpin-shaped piece of refractory metal, a piece of single crystal refractory metal attached to said filament, and an adsorbed layer including a metal whose work function or electron affinity is lower than that of said single crystal refractory metal and attached to a tip of said piece of single crystal refractory metal, said piece of single crystal refractory metal being adapted to be heated by passing a current through said filament and to have an electric field applied on said tip so that electrons are extracted from said tip comprising:

• a heating power supply for supplying said current through said filament,
• an extraction power supply for supplying said electric field,
• an accelerating power source for accelerating said extracted electrons,
• an electron lens provided with an aperture plate for focusing said extracted electrons and illuminating an object with said extracted electrons, and
• a control computer for controlling said three power supplies, wherein, when an electron current adsorbed in said aperture plate decreases by a predetermined value, said control computer controls said heating power supply and said extraction power supply such that said cathode-stabilizing apparatus extracts electrons from said tip of said piece of single crystal refractory metal by heating said piece of single crystal refractory metal by passing a current through said filament to a temperature at which said adsorbed layer evaporates and applying an electric field on said tip, and then applies an electric field necessary for preventing said tip from blunting due to migration of atoms from said tip on said tip and controls the electric field at said tip so that an electron emission current does not exceed a predetermined value at the same time.

A cathode-stabilizing apparatus for use with a cathode comprising a filament formed of a hairpin-shaped piece of refractory metal, a piece of single crystal refractory metal attached to said filament, and an adsorbed layer including a metal whose work function or electron affinity is lower than that of said single crystal refractory metal and attached to a tip of said piece of single crystal refractory metal, said piece of single crystal refractory metal being adapted to be heated by passing a current through said filament and to have an electric field applied on said tip so that electrons are extracted from said tip comprising:

• a heating power supply for supplying said current through said filament,
• an extraction power supply for supplying said electric field,
• an accelerating power source for accelerating said extracted electrons,
• an electron lens provided with an aperture plate for focusing said extracted electrons and illuminating an object with said extracted electrons, and
• a control computer for controlling said three power supplies, wherein, when an electron current adsorbed in said aperture plate decreases by a predetermined value, said control computer controls said heating power supply and said extraction power supply such that said cathode-stabilizing apparatus heats said tip of said piece of single crystal refractory metal to more than 1900 K. without applying an electric field on said tip, applies and controls an electric field on said tip so that an emission current does not exceed a predetermined value until the electron emission current reaches a state of one of less than 5 .mu.A and an angular current intensity of less than 5 .mu.A/sr over a whole useful electron-emissive region of said tip, and then lowers a temperature of said adsorbed layer to less than 1900 K. and controls the electric field at said tip by applying an electric field of more than 0.15 V/.ANG. at the same time so that the electron emission current does not exceed a predetermined value.