Radiant energy – With charged particle beam deflection or focussing – Magnetic lens
Reexamination Certificate
2000-06-01
2003-02-18
Lee, John R. (Department: 2881)
Radiant energy
With charged particle beam deflection or focussing
Magnetic lens
C250S505100, C250S398000, C250S310000, C313S361100
Reexamination Certificate
active
06521896
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electron beam systems and more specifically to an electron beam blanker for use in electron beam columns.
DESCRIPTION OF THE PRIOR ART
Beam blanking is a required function for electron beam systems used in lithography, testing, metrology, and inspection. In the field of electron beam lithography, the electron beam is switched on and off by deflecting it against an aperture stop (blanking). The deflection is typically accomplished by an electric field between two parallel plates. The plates form a waveguide structure which conducts electromagnetic blanking signals used to define, in part, the pattern to be exposed by effectively switching the beam on and off.
FIG. 1
shows an example of an electron beam column including such a blanker.
FIG. 1
is taken from Kelly et al., U.S. Pat. No. 4,445,041, incorporated herein by reference, and uses the same reference numbers as does Kelly. Electrons are provided in the column by a cathode
5
which is a field emission electron source. Cathode
5
is supported above an anode
10
with the anode serving to control and effectively collimate the electron beam. As the electrons move down the column from the anode, they encounter a first lens
15
which serves to focus the beam at the center of a beam blanker
25
. Along that path, an alignment deflector/stigmator
20
aligns the beam with the optic axis and stigmates the beam to provide the proper shape before the electrons enter the blanker. Blanker
25
then blanks the beam at the appropriate time to control the exposure when a target
65
at the bottom. A second alignment deflector
30
is also provided to realign the beam after it has passed through blanker
25
.
Following realignment, the beam enters a final lens
35
which focuses it onto target
65
, the object point of the final lens
35
being the beam cross-over at the center of the blanker. Element 40 is located within its final lens
35
, serving as a third alignment deflector and second stigmator. The next element down the column is a dynamic focus coil
45
. This serves as a fine focus for beam
60
as it is being deflected to the appropriate location on the target via high speed deflector
50
and a precision deflector
55
. Element
70
is an electron scintillator which is connected to a light, pipe and multiplier
75
, which are used to accurately monitor the device as it is writing.
FIG. 2
, also from Kelly, shows an enlarged view of the blanker
25
. This blanker has two characteristic U-shaped plates; the rear plate is only partially visible in this view. The deflector plates
260
are spaced apart by 0.10 inch, each having the general shape of a U and being symmetric top to bottom. Electromagnetic energy (the blanking signal) enters this structure through the two leads at point A, traverses a transition region, travels a length of the U and the top half is reflected around the corner, travels back the length of the U on the bottom half, traverses another transition region and exits the leads at point B. The plates have a constant thickness except in the transition regions
270
. Other illustrated dimensions are as indicated in Kelly. There is a knife-edge
280
suspended between the plates by a knife-edge support
290
. This is the beam blanking aperture. The electron beam is seen passing between the plates and through the aperture
280
. Also provided are mounting holes
271
. Angles A
1
and A
2
are designed to optimize performance.
There is a well-known technical problem in conjunction with such blankers. The RF (radio frequency) blanking signal typically propagates near the velocity of light. This velocity is significantly faster than that of the electron beam. It is to be understood that electron beams have no particular characteristic velocity; the velocity of propagation is dependent on the amount of accelerating energy to which they are subject. Typically, even in a very high voltage electron beam column, electron velocity is no more than 0.4 times the velocity of light. In any case, there is typically a mis-match in velocities due to the electron beam propagating much slower than the blanking signal. This leads to non-ideal beam deflections which in turn cause writing anomalies. This problem is recognized in Kelly and compensated for by providing an electrical path length, which is effectively a delay line, incorporated into the blanker. The transit time of the electromagnetic wave of the blanking signal on this path corresponds approximately to the time for an electron in the beam to transverse the width of the top half of the plates. Thus, an electron in the beam entering the blanker will be subject to substantially the same electric field above the beam cross-over as it is below it but delayed in time.
Thus, Kelly uses the delay line approach to match electron beam velocity with the blanking signal propagation velocity. It is to be understood that any mis-match in these velocities results in focusing problems and hence a less precise image being formed on the target.
Other types of electron beam blankers are shown in Gesley et al., U.S. Pat. No. 5,412,281 and Gesley, U.S. Pat. No. 5,276,330 both also incorporated herein by reference and having structures somewhat similar to those of Kelly.
Feuerbaum, U.S. Pat. No. 4,169,229 also incorporated herein by reference, recognizes the same delay issue but solves it differently. Instead of a U-shaped delay line extending along the deflector plate, Feuerbaum uses a meandering conductor as a delay line and also as the deflector plate. This is shown in present
FIG. 3
, identical to
FIG. 2
of Feuerbaum and having the same reference numbers. Here instead of U shaped deflector plates, the plates are planar and rectangular. Only a single plate of the pair is shown in
FIG. 3
; the other plate is not shown for simplicity.
The electron beam
15
is shown as a dotted line transiting the plate. The plate includes the interdigital structure
51
which includes a meander shaped conductor track
52
and a screening or shielding conductor track
53
. Track
53
is grounded. The conductor track
52
has a (schematically indicated) input connection
521
and an output connection
522
likewise grounded via a terminal resistance
523
. A high frequency electromagnetic wave (the blanking signal) enters via the input terminal
521
of the conductor track structure
52
and travels along the meander shape path so as to progress in the direction shown by the arrow
15
, delayed in accordance with the extension of the path. The dimensioning of the meander shaped track is determined as a function of the speed of the electron beam which in turn is determined by its accelerating voltage. Kelly indicates that the interdigital structure
51
may be applied to a plate of dielectric material.
Hence, blanking the beam is an electric deflection of the beam caused by two parallel plates that extend parallel to the beam for a distance L and on either side of the beam. A difficulty arises when the blanking signal applied to the plates changes quickly with respect to the electron beam transit time along the length L. Then the fact that the wave (signal) velocity does not match the electron beam velocity causes non-negligible distortions of the electron beam deflection. Feuerbaum addresses this problem by his meandering delay line. The resulting disadvantage is that the beam is only deflected at discrete points of the line, resulting in not much deflection per volt of signal (the deflection sensitivity is low). A secondary disadvantage is that the structure is complex with many bends in the meandering delay line, which may result in wave energy being partially reflected at the bends and thus distorting the waveform.
The Feuerbaum and Kelly approaches thus have been found by the present inventors to have significant drawbacks. Both have problems with high frequency blanking signals in terms of RF propagation and interference. Also, in both cases, the electron beam is subject to rather uneven influences due to the unusual (non-linear) path taken by the b
DeVore William J.
Penberth Michael John
Applied Materials Inc.
Lee John R.
Sughrue Mion LLP.
Vanore David A.
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