Radiation imagery chemistry: process – composition – or product th – Including control feature responsive to a test or measurement
Reexamination Certificate
1999-06-16
2001-10-02
Young, Christopher G. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Including control feature responsive to a test or measurement
C430S296000, C430S942000
Reexamination Certificate
active
06296976
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a method for improving image fidelity on a semiconductor wafer and more particularly, to a method of controlling electron beam intensity of an electron beam projection lithography system in order to provide a uniform feature size throughout a subfield of a resist pattern.
2. Background Description
Semiconductor manufacturers typically use lithographic processes in a highly specialized printing process to put detailed patterns onto silicon wafers. In general, a layer of photosensitive material called “resist” is deposited onto a silicon wafer and an image containing the desired pattern of energy or charged particles is projected onto the silicon wafer. After development, the resist forms a stenciled pattern (e.g., image) across the wafer surface that matches the desired pattern of a circuit.
More specifically, in electron beam lithography, finely focused electron beams are emitted from a cathode surface and through a series of optical lenses and a patterned reticle in order to provide a fine line pattern or image on the resist surface. In common practice, a 4x reticle is electron optically imaged and then demagnified onto the writing surface (e.g., resist). In common practice, the 4x reticle includes a square subfield of 1 mm width which results in a 0.25 mm wide subfield image on the resist. However, 1x, 2x, 3x, etc. reticles may also be used in lithographic systems.
FIG. 1
shows an electron-beam apparatus of common design. Specifically, in the apparatus of
FIG. 1
, electrons are emitted from a hot cathode, for example, approximately 1700° K, in a highly uniform intensity distribution. The electrons are then emitted from a charged particle emitting device
10
, for example, a crystal, forming an electron beam
12
having a uniform intensity distribution. The electron beam
12
passes through a series of optics (e.g., auxiliary lens
14
, condenser
16
and an illuminator doublet
20
) prior to being optically imaged on a reticle
22
and then demagnified onto the target
28
(e.g., resist). Prior to the electron beam being demagnified onto the target
28
, it first passes through a series of optics (e.g., projection doublet
24
and contrast aperture
26
) to ensure proper focus of the electron beam
12
onto the target
28
. The dashed lines
30
of
FIG. 1
represent lateral areas being imaged and the solid vertical line
32
of
FIG. 1
represents an ideal path for the electron beam
10
.
A typical problem presented with the use of electron beam lithography is degradation of image fidelity due to naturally occurring electron optical aberrations occurring within the subfield. Aberrations are typically defined as the variation of pixel size and usually vary symmetrically in the radial direction about the subfield center.
Aberrations usually arise from two sources: (i) geometric field aberrations of the focusing and deflection system, and (ii) space charge interaction within the beam. Geometric field aberrations are independent of the pattern being printed on the wafer surface and are predictable by computation in the limit where the system is well aligned, and a stigmatic image is obtained on the central axis. However, aberrations caused by space charge interaction depend on the pattern being printed on the wafer surface insofar as this determines the local current density within the electron beam path which, in turn, governs the space charge interaction. Aberrations caused by space charge interaction are also predictable by computation. The resultant field aberration arising from both (i) geometric field aberrations and (ii) space charge interaction are measurable for a given set of operating conditions.
Aberrations cause image blurring which effects the intensity distribution or exposure dose (intensity×time) in the image which, in turn, also affects the printed line width on the resist. Blurring is generally manifested as a gray level phenomenon and occurs when the image is not completely developed on the resist.
As the blurring increases, the intensity (or exposure dose) is spread over a wider area and the intensity of the center of the feature decreases. This leads to a narrowing of the printed image because of the dose threshold for full development and the high contrast nature of the preferred resists. If the blurring is sufficiently pronounced, the image may not print at all as the central intensity becomes insufficient to expose the resist.
It is common practice to compensate for image size variations in the reticle by choosing feature sizes in the reticle to compensate for these variations. This necessitates incorporating the image size information at the reticle fabrication step which significantly increases the complexity of the overall process. Furthermore, the reticle can not be changed once it is fabricated, and if the reticle needs to be changed, the entire process must be stopped. This greatly increases manufacturing costs and time.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a method for increasing or decreasing feature size on a resist.
It is a further object of the present invention to provide a method for compensating for naturally occurring aberrations thereby providing increased resist image fidelity.
It is still another object of the present invention to eliminate the deleterious effect of blurring in a resist image.
It is also a further object of the present invention to provide a method for controlling electron beam intensity in order to provide a uniform feature size throughout a subfield of a resist.
The present invention describes a method and apparatus for improving image fidelity on a resist. More specifically, the method of the present invention provides for uniform feature size throughout the subfield of a resist by choosing the incident intensity distribution within the subfield such that the printed size is compensated at each point. This is accomplished by intentionally increasing the incident intensity where the images are small (more pronounced blurring), and intentionally decreasing the incident intensity where the images are large (less pronounced blurring).
In one such embodiment, the energy of the electron beam is selectively increased about the edges of the electron beam causing a non-uniform intensity distribution about the electron beam. This causes the intensity of the electron beam to raise above the threshold intensity level at the edges of the electron beam such that the feature size at the edges of the subfield become equal to the feature size at the center of the subfield. This can be achieved, for example, by maintaining a cathode temperature profile which increases radially by an appropriate amount.
In order to increase or decrease the intensity distribution within an electron beam about the edges or center of the electron beam (e.g., local intensity distribution adjustment), the temperature of the cathode is selectively adjusted about either the edges or center of the cathode, respectively. The temperature of the cathode is selectively adjusted about either the edges or center of the cathode, by bombarding the cathode with bombardment electron beams locally on the edges or center of the backside (non-emitting side) of the cathode, respectively. Due to the local bombardment on the backside of the cathode, the local intensity about the edges or center of the electron beam may thus be adjusted accordingly.
The apparatus of the present invention includes a first cathode emitting an electron beam, having an intensity distribution, on a resist layer. The apparatus further includes an adjusting device for adjusting the intensity distribution of the electron beam such that the intensity distribution of the electron beam is non-uniform in subfields of the resist layer of the electronic device. The adjusting device may include a center and/or annular cathodes which emit bombardment electrons on a backside (non-emitting) side of the first cathode. The emitting of the bombardment electron
Golladay Steven D.
Groves Timothy R.
Pfeiffer Hans C.
McGuireWoods LLP
Nikon Corporation
Young Christopher G.
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