Radiant energy – Means to align or position an object relative to a source or...
Reexamination Certificate
2003-06-25
2004-11-16
Wells, Nikita (Department: 2881)
Radiant energy
Means to align or position an object relative to a source or...
C250S398000, C250S252100
Reexamination Certificate
active
06818906
ABSTRACT:
BACKGROUND OF INVENTION
The field of the invention is that of vacuum technology, and in particular that of electron beam technology.
Electron beam (e-beam) lithography tools are commonly used in semiconductor manufacturing to form sub-micron shapes on a semiconductor wafer. Shapes are formed by directing a beam of electrons from a source at one end of a column onto a photoresistive layer on a substrate at an opposite end of the column. A typical substrate may be 200 mm-300 mm in diameter or larger. These submicron shapes may be formed either by writing the shape directly onto a photoresistive layer on the substrate, wherein the substrate is a semiconductor wafer; or, by writing the shape onto a photoresistive layer on a substrate which is used as a mask, subsequently, to print the shape onto the semiconductor wafer.
Further, there are two broad types of writing modes used in electron beam lithography. The first type is referred to as “blind mode” or a “dead reckoning mode” and is commonly used in mask making. In the blind mode, the substrate is a featureless blank coated with resist and all of the patterns are placed by dead reckoning. The second mode, which may be referred to as the “registered write mode” or a “direct write mode,” is commonly used in direct write applications, i.e. writing directly onto a semiconductor wafer, in what are referred to as device fabrication runs. In the registered write mode case, the patterns must be precisely located relative to previous levels which requires registration targets built into each level and the substrate as well. Regardless of the mode employed, accurately placing or repeating sub-micron shapes at precise locations across a distance of 200-300 mm demands precise beam registration.
However, even if the beam is registered adequately when pattern printing begins, during the course of writing the pattern, the e-beam may exhibit what is referred to as drift, i.e., exhibiting increasing inaccuracy in one direction as time passes. So, in order to maintain adequate precision, pattern writing may be interrupted periodically, depending on the particular tool's inherent e-beam drift, to check tool registration and, whenever registration error exceeds an acceptable tolerance, to adjust the beam.
Normally, the substrate is held on a stage opposite (beneath) the beam source and this registration measurement involves diverting the stage to position a registration target under the beam. Then, the beam is scanned over the registration target, the target's location is measured and the target's measured location is compared against an expected result. Any measured errors are corrected by adjusting the beam or adjusting stage positional controls. Then, the stage is returned to its former position to resume writing the mask pattern. This measurement and reregistration can be time consuming.
Furthermore, for this e-beam registration approach, the registration measurement takes place at a stage location other than where the pattern is actually written. Consequently, even after measuring and correcting errors, moving the stage back into position from the measurement area may actually introduce errors, such as from the stage slipping or from other move related stresses. Also, to assure complete accuracy, the beam should be reregistered, frequently, preferably at each field. However, when throughput is a consideration, as it nearly always is, it is impractical to correct the beam registration prior to printing each field.
U.S. Pat. No. 6,437,347, entitled “Target Locking System for Electron Beam Lithography” to Hartley et al., teaches an e-beam exposure system that may use the invention in its calibration subsystem. This system uses a field locking target that includes alignment marks.
The '347 patent shows an e-beam lithographic system capable of in situ registration. The preferred system is a Variable Axis Immersion Lens (VAIL) e-beam system and is a double hierarchy deflection system. A controllable stage moves a substrate with respect to the beam axis placing the intended substrate writing field within an aperture on a field locking target. The field locking target is located between the optics section and the substrate and the aperture is sized to permit the beam to write the field. The field locking target includes alignment marks around the aperture. A differential interferometric system measures the relative positions of the field locking target and the stage. As the stage is moving into position for writing a field, the beam is swept to hit the alignment marks, checking system alignment. The beam control data (coil currents and electrostatic deflection plate voltages) required to hit the marks are stored, and drift correction values calculated and the field beam control data adjusted accordingly.
FIG. 5
shows a cross-sectional diagram of a typical e-beam lithography system
500
. This system includes an optics section
502
with a registration focus coil
504
a
, an autofocus coil
504
b
, beam deflection coils
506
,
508
, a projection lens axis shifting yoke
510
and beam deflection plates
511
.
An e-beam source
90
emits a beam represented by arrow
512
, which, during writing, travels to a target field on a substrate held on carrier
514
. Autofocus coil
504
b
adjusts beam focus for target height variations resulting from substrate imperfections, thickness variations, etc. In the preferred VAIL lens system, double deflection yokes
506
,
508
magnetically deflect the beam
512
; and axis shifting coil
510
shifts the variable axis of the projection lens to follow the deflected beam
512
. The relatively slow magnetic deflection from coils
506
,
508
determines the subfield location, while within the subfield, the beam
512
is deflected by the high speed electrostatic deflection plates
511
.
A passive field locking target
516
permits the beam
512
to write the pattern in the substrate's target field through an aperture
518
. The preferred aperture is rectangular and is large enough to permit writing an entire field. During normal pattern writing, substrate subfields are placed within the field locking target aperture
518
and electrostatic deflection is used to write spots which form the pattern shapes. During registration, the subfield is defined as being over marks on the field locking target
516
adjacent to the aperture
518
; and, the beam is deflected accordingly, as represented by arrows
512
′. Then, the marks on the field locking target
516
are scanned, in situ, with the electrostatic deflection, to provide near real time positional feedback information.
For tracking and selecting stage location, the e-beam system
500
includes a differential interferometric system
520
. The interferometric system
520
directs a laser, represented by arrows
522
, to laser targets
524
and
524
′ to measure the relative position of the field locking target
516
to the stage mirror assembly
526
. Laser target
524
is mechanically coupled to field locking target
516
and laser target
524
′ is attached to a stage mirror assembly
526
. The carrier
514
is kinematically clamped to the stage mirror assembly
526
at points
528
. The stage mirror assembly
526
, in turn, is flexure mounted to a stage base
530
at points
532
. An x or y drive
534
is attached to an appropriate side of the stage base
530
to drive the stage, typically under computer control, in the x or y direction; and, once in place, to lock the stage in place. A mechanical centering adjustment
536
provides a fine adjustment for the field locking target
516
to precisely place it with respect to the beam.
There is a need for a system in which the position of the electron beam in space is more directly related to the workpiece. In particular, the position of the workpiece being illuminated with a pattern by the beam must be repeatably located in space in order to align the various components of the pattern being written.
SUMMARY OF INVENTION
The present invention addresses the above-described need by providi
Hartley John G.
Kendall Rodney A.
Pinckney David J.
Rieland Richard A.
Anderson Jay H.
Gill Erin-Michael
Wells Nikita
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