Magnetic lens

Abstract

In one embodiment of the present invention, a magnetic lens is provided that can generate a substantially constant amount of average heat power over a pre-selected range of resultant magnetic field strengths. The lens is configured to do this with multiple, asymmetric (different turns) coil sections that can produce a desired range of field strengths, and at the same time, maintain a sufficiently constant temperature signature when the average total power is maintained constant thereby eliminating unreasonable delays in lens operation when the resultant field strength is changed. The asymmetric lens structure allows for the smaller coil to be made with less relative inductance thereby making it more responsive and amendable for an AC drive signal and thus dynamic focusing applications if desired. Thus, a magnetic lens is now provided that can produce a range of magnetic beam-focusing field strengths, implement dynamic focusing, and not impose unreasonable delay for thermal stabilization between changes in magnetic field strength.

Description

TECHNICAL FIELD OF THE INVENTION The present invention generally relates to a magnetic lens and, in particular, relates to a magnetic lens having thermal power dissipation independent of the strength of its magnetic field. BACKGROUND OF THE INVENTION Magnetic lenses are generally known and are often employed to focus a beam of charged particles e.g., electrons. One major use of a magnetic lens is in analytical instruments such as electron microscopes, which utilize focused electron beams to stimulate a reaction from a sample that is being observed or analyzed. In such instruments the magnetic lens is useful as an objective, or final focusing lens. That is, the objective magnetic lens is the last focusing element prior to the beam impinging on the sample. As such, the stability of the beam position as affected by the magnetic lens is quite critical to achieve an accurate characterization of small features on the sample. FIG. 1 is a cross-sectional, schematic drawing of a convention...

AI Technical Summary

Benefits of technology

In one embodiment of the present invention, a magnetic lens is provided that can generate a substantially constant amount of average heat power over a pre-selected range of resultant magnetic field strengths. The lens is configured to do this with multiple, asymmetric (different turns) coil sections that can produce a range of desired field strengths, and at the sane time, maintain a sufficiently constant temperature signature when the average total power is maintained constant thereby eliminating unreasonable delays in lens operation when the resultant field strength is changed over the operating range of field strengths. The asymmetric lens structure allows for the smaller coil to be made with less relative inductance thereby making it more responsive and amenable for AC signal driving and thus dynamic focusing applications if desired. Thus, a magnetic lens is now provided that can produce a range of magnetic beam-focusing field strengths, implement dynamic focusing, and not impose unreasonable delay for thermal stabilization between changes in magnetic field strength.

Problems solved by technology

The heat emitted from the lens coil will raise the temperature in the lens pole assembly, which causes it to expand.

(Likewise, a decrease in field strength causes the temperature to fall and the pole assembly to contract.)

This is problematic because the exact position of the lens pole pieces with respect to each other and the centerline of the electrons define the position where the electrons are focused on the specimen.

This can impose a delay in operating the device after changing the field strength so that the lens coil has sufficient time (e.g., one to seven hours) to thermally stabilize.

Unfortunately, the reaction times of available thermal dissipation mechanisms are typically inadequate to readily keep pace with thermal changes caused by changing the strength of the magnetic field.

While the delay may be decreased, time must still be wasted waiting for the lens to “settle.”

Unfortunately, one drawback with this approach is that the overall response of the magnetic lens is fairly slow.

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