Method and apparatus for aligning electron beams

Claim 2

In a method of precisely positioning a beam of charged particles including continuously moving the beam through a predetermined path and periodically blanking the beam by deflecting the beam to intercept a blanking plate at a first predetermined position offset from a central aperture in the blanking plate, the improvement of precisely aligning the beam of charged particles with respect to said central aperture during those times when the beam is blanked to correct for instabilities due to lateral beam drift, comprising: deflecting the beam toward a second predetermined position on the blanking plate offset from the central aperture; sensing the error in alignment of the beam at the second predetermined offset position; and correcting the alignment of the beam at the second predetermined offset position to cause the beam to be aliGned with respect to the central aperture in the blanking plate when the beam is unblanked.

Claim 3

A method of precisely aligning a beam of charged particles as recited in claim 1 wherein the step of correcting includes: moving the beam in small increments in two orthogonal directions until the error sensed is reduced to a minimum value.

Claim 4

A method of precisely aligning a beam of charged particles as recited in claim 2 wherein the movement of the beam is accomplished by alternate incremental movements in the two orthogonal directions.

Claim 5

A method of precisely aligning a beam of charged particles as recited in claim 2 wherein said incremental movements are smaller than the allowable alignment error.

Claim 6

In an apparatus for controlling the movement of a beam of charged particles including first deflection means for continuously deflecting the beam in a substantially raster fashion and blanking means including a blanking plate having a central aperture aligned along the normal axis of said beam, said blanking means deflecting said beam in a blanking made to intercept said blanking plate at a first predetermined position offset from said central aperture the improvement for precisely aligning said beam of charged particles with respect to said central aperture to correct for instabilities due to lateral beam drift, comprising: second deflecting means for deflecting said beam in said blanking made toward a second predetermined position on said blanking plate offset from said central aperture; means for sensing the error in alignment of said beam at said second predetermined offset position; and servo means responsive to said sensing means and controlling said second deflecting means for correcting the alignment of said beam at said second predetermined offset position with the result that alignment of said beam at said second predetermined offset position causes said beam to be aligned along its normal axis.

Claim 7

In an apparatus for controlling the movement of a beam of charged particles including first deflection means for continuously deflecting the beam in a substantially raster fashion and blanking means including a blanking plate having a central aperture aligned along the normal axis of said beam, said blanking means deflecting said beam in a blanking made to intercept said blanking plate at a first predetermined position offset from said central aperture the improvement for precisely aligning said beam of charged particles with respect to said central aperture to correct for instabilities due to lateral beam drift, comprising: second deflecting means for deflecting said beam in said blanking made toward a second predetermined position on said blanking plate offset from said central aperture; means for sensing the error in alignment of said beam at said second predetermined offset position; and servo means responsive to said sensing means and controlling said second deflecting means for correcting the alignment of said beam at said second predetermined offset position with the result that alignment of said beam at said second predetermined offset position causes said beam to be aligned along its normal axis.

Claim 8

An apparatus for precisely aligning a beam of charged particles as recited in claim 5 wherein said correcting means includes means for moving the beam in small increments in two orthogonal directions until the alignment error sensed by said sensing means is reduced to a minimum value.

Claim 9

An apparatus for precisely aligning a beam of charged particles as recited in claim 6 wherein said moving means is operable to move the beam in alternate incremental movements in the two orthogonal directions.

Claim 10

An apparatus for precisely aligning a beam of charged particles as recited in claim 6 wherein the small increments that the beam is moved by said moving means are smaller than the allowable alignment error.

Claim 11

An apparatus for precisely aligning a beam of charged particles as recited in claim 5 wherein said blanking plate is provided with an auxiliary aperture at said second predetermined offset position and said sensing means comprises: a second plate having a central aperture aligned with the axis of said beam in its normal mode, said second plate being separated from said blanking plate but closely spaced thereto to intercept the beam passing through said auxiliary aperture when the beam is deflected toward said second predetermined offset position; and detecting means connected to said second plate for generating a signal proportional to the beam current intercepted thereby.

Claim 12

An apparatus for precisely aligning a beam of charged particles as recited in claim 9 wherein said correcting means comprises: means for incrementally moving said beam along the line of a first of two orthogonal directions; means responsive to said detecting means for controlling the direction of movement of said beam in said first line; means for incrementally moving said beam along a second line orthogonal to said first line; and means responsive to said detecting means for controlling the direction of movement of said beam along said second line.

Claim 13

An apparatus for precisely aligning a beam of charged particles as recited in claim 10 wherein said means for controlling the direction of movement of said beam in said first line comprises a first bistable device the state of which determines the direction of movement in said first line, and said means for controlling the direction of movement of said beam in said second line comprises a second bistable device the state of which determines the direction of movement in said second line.

Claim 14

An apparatus for precisely aligning a beam of charged particles as recited in claim 11 wherein said sensing means further comprises: storage means for sampling and holding a voltage proportional to the output of said signal generating means, and comparator means connected to both said signal generating means and said storage means and responsive thereto for generating an output which controls the state of both of said first and second bistable devices.

Claim 15

An apparatus for precisely aligning a beam of charged particles as recited in claim 12 wherein said means for controlling includes: first counting means controlled by said first bistable device for counting in one or the other of two directions, second counting means controlled by said second bistable device for counting in one or the other of two directions, first analog to digital converter means connected to said first counting means for generating a first correcting voltage proportional to the counts accumulated by said first counting means, said first correcting voltage being supplied to said correcting means for correcting the alignment in one of said two orthogonal directions, and second analog to digital converter means connected to said second counting means for generating a second correcting voltage proportional to the count accumulated by said second counting means, said second correcting voltage being supplied to said correcting means for correcting the alignment in the other of said two orthogonal directions.

Claim 16

An apparatus for precisely aligning a beam of charged particles as recited in claim 9 wherein said sensing means includes means for generating a signal proportional to the first derivative of the beam current intercepted by said second plate.

Claim 17

An apparatus for precisely aligning a beam of charged particles as recited in claim 14 wherein said correcting means comprises: means for incrementally moving said beam along the line of a first of two orthogonal directions; means responsive to said detecting means for controlling the direction of movement of said beam in said first line; means for incrementally moving said beam along a second line orthogonal to said first line; and means responsive to said deflector means for controlling the direction of movement of said beam along said second line.

Claim 18

An apparatus for precisely aligning a beam of charged particles as recited in claim 15 wherein said means for controlling the direction of movement of said beam in said first line comprises a first phase sensitive detector the output phase of which determines the direction of movement in said first line, and said means for controlling the direction of movement in said second line comprises a second phase sensitive detector the output phase of which determines the direction of movement in said second line.

Claim 19

An apparatus for precisely aligning a beam of charged particles as recited in claim 16 further comprising: first storage means connected to said first phase sensitive detector for storing a voltage indicative of the output thereof and supplying a correcting voltage for aligning said beam in said first direction, and second storage means connected to said second phase sensitive detector for storing a voltage indicative of the output thereof and supplying a correcting voltage for aligning said beam in a second orthogonal direction.