Electron beam apparatus and method for detecting secondary electrons

Claim 1

Electron beam apparatus including an electron gun for producing an axially directed, focused electron bean; a magnetic electron objective lens for focusing the electron bean to a spot on a body when located on a support beyond the objective lens; electrostatic means located beyond the magnetic objective lens and close to the support for accelerating secondary electrons emitted by the body and directing them back into and through the magnetic field within the magnetic electron objective lens, a filter electrode placed so as to be approached by secondary electrons after passage through the objective lens and means for biasing the filter electrode to such a potential as to allow to pass only those secondary electrons having energies greater than a predetermined value, electron collecting means for receiving the secondary electrons passing through the filter electrode, the objective lens producing an axial magnetic field which has a region of maximum field strength close to the end of the lens nearer the support, with a high gradient rising from the end of the lens nearer the support to the region of maximum field strength, and an adiabatically falling, lower gradient between the region of maximum field strength and the end of the lens further from the support, the electrostatic accelerating means arranged to accelerate the secondary electrons to a high speed at which they pass through the high gradient part of the objective lens field to the region of its maximum field strength, and means provided at the region of maximum magnetic field strength of the objective lens to decelerate the electrons to approach the filter electrode at speeds of the same order as those at which they were emitted from the body.

Claim 12

A method of detecting secondary electrons emitted from a body by directing a primary electron beam in an axial produced by an objective lens direction through an axial magnetic field to impinge on the body at a location outside the magnetic field comprising the steps of accelerating the secondary electrons in an axial direction away from the body by means of a substantially planar electrostatic field, maintaining the electrons at high speed as they enter the axial magnetic field, passing through a magnetic field region of high gradient to a region of maximum field strength, retarding the secondary electrons while they are in the region of maximum field strength to a lower speed of the same order as that at which they were emitted from the body, passing the secondary electrons at a speed close to the lower speed through a region of the magnetic field in which it weakens adiabatically at a lower gradient, applying the secondary electrons at a speed substantially equal to that at which they were emitted from the body to a biased planar filter electrode placed transverse to the axis, beyond the objective lens detecting secondary electrons passing the filter electrode, and controlling the bias on the filter electrode in response to the detected secondary electrons so as to limit the number of detected secondary electrons such that the bias on the filter electrode substantially represents the energies of the secondary electrons emitted from the body.

Claim 13

A method of detecting secondary electrons emitted from a body by directing a primary electron beam in an axial produced by an objective lens direction through an axial magnetic field to impinge on the body at a location outside the magnetic field comprising the steps of accelerating the secondary electrons in an axial direction away from the body by means of a substantially planar electrostatic field, maintaining the electrons at high speed as they enter the axial magnetic field, passing through a magnetic field region of high gradient to a region of maximum field strength, retarding the secondary electrons while they are in the region of maximum field strength to a lower speed of the same order as that at which they were emitted from the body, passing the secondary electrons at a speed close to the lower speed through a region of the magnetic field in which it weakens adiabatically at a lower gradient, applying the secondary electrons at a speed substantially equal to that at which they were emitted from the body to a biased planar filter electrode placed transverse to the axis, beyond the objective lens detecting secondary electrons passing the filter electrode, and controlling the bias on the filter electrode in response to the detected secondary electrons so as to limit the number of detected secondary electrons such that the bias on the filter electrode substantially represents the energies of the secondary electrons emitted from the body.

Claim 14

A method of detecting secondary electrons emitted from a body by directing a primary electron bean in an approximately axial direction through an axial magnetic field to impinge on the body at a location outside the magnetic field; in a direction extending away from the body, the magnetic field increasing over a short distance with a high gradient to a maximum value and then falling adiabatically with a low gradient over a longer distance; electrostatically accelerating secondary electrons emitted from the body to a high speed to pass initially into the high gradient portion of the magnetic field, then decelerating the secondary electrons at the region of maximum magnetic field strength to speeds of the same order as those at which they were emitted from the body, to pass through the low gradient, falling magnetic field causing the secondary electron paths assume substantially axial directions, and then passing those of the axially directed secondary electrons having selected energies through an energy selection filter for collection.

Claim 15

A method of detecting secondary electrons emitted from a body by directing a primary electron bean in an approximately axial direction through an axial magnetic field to impinge on the body at a location outside the magnetic field; in a direction extending away from the body, the magnetic field increasing over a short distance with a high gradient to a maximum value and then falling adiabatically with a low gradient over a longer distance; electrostatically accelerating secondary electrons emitted from the body to a high speed to pass initially into the high gradient portion of the magnetic field, then decelerating the secondary electrons at the region of maximum magnetic field strength to speeds of the same order as those at which they were emitted from the body, to pass through the low gradient, falling magnetic field causing the secondary electron paths assume substantially axial directions, and then passing those of the axially directed secondary electrons having selected energies through an energy selection filter for collection.