Radiant energy – With charged particle beam deflection or focussing – With target means
Reexamination Certificate
2000-10-20
2003-03-04
Berman, Jack (Department: 2881)
Radiant energy
With charged particle beam deflection or focussing
With target means
C250S492220, C250S492230, C430S296000
Reexamination Certificate
active
06528799
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an illumination system for an electron beam lithographic apparatus and an electron beam lithographic apparatus having such an illumination system, and more particularly to such apparatuses used for the manufacture of micro-devices, including semiconductor integrated circuits.
2. Discussion of Related Art
Electron beam exposure apparatuses have been used for lithography in semiconductor processing for more than two decades. The first e-beam exposure devices were based on the flying spot concept of a highly focused beam, raster scanned over the object plane. In this approach, the electron beam is modulated as it scans so that the beam itself generates the lithographic pattern. These devices have been widely used for high precision tasks, such as lithographic mask making, but the raster scan mode is too slow to achieve the high throughput required in semiconductor wafer processing. The electron source in this type of device is similar to that used in electron microscopes, i.e., a high brightness source focused to a small spot beam.
More recently, a new electron beam exposure apparatus has been developed based on the SCALPEL (Scattering with Angular Limitation Projection Electron-beam Lithography) technique. In this approach, a wide-area electron beam is projected through a lithographic mask onto the object plane. Since relatively large areas of a semiconductor wafer can be exposed at a time, throughput is increased. The high resolution of this device makes it attractive for ultra-fine line lithography, i.e., sub-micron.
The requirements for the electron beam source in SCALPEL exposure devices differ significantly from those of a conventional focused beam exposure device, or a conventional transmission electron microscope (TEM) or scanning electron microscope (SEM). While high resolution imaging is still a primary goal, this must be achieved at relatively high (10-100 &mgr;A) gun currents in order to realize improved wafer throughput. Relatively low axial brightness is required, e.g., 10
2
to 10
4
Acm
−2
sr
−1
, as compared with a value of 10
6
to 10
9
Acm
−2
sr
−1
for a typical focused beam source. However, the beam flux over the larger area must be highly uniform to obtain the required lithographic dose latitude and CD control.
The development of SCALPEL apparatuses has been directed to the development of an electron source that provides uniform electron flux over a relatively large area, that has relatively low brightness, and high emittance. (Emittance is defined as D×&agr; and is typically expressed in units of micron×milliradian, where D is beam diameter, and &agr; is divergence angle.) Conventional, state-of-the-art electron beam sources generate beams having an emittance in the 0.1-400 micron×milliradian range, while SCALPEL-like devices require emittance in the 1000 to 5000 micron×milliradian range. Conventional SCALPEL illumination system designs have been either Gaussian gun-based or grid-controlled gun-based. A common drawback of both types is that beam emittance depends on actual Wehnelt bias, which couples beam current control with inevitable emittance changes. From a system viewpoint, independent control of the beam current and beam emittance is beneficial.
Furthermore, particle projection system throughput is dependent on the resolution due to the Coulomb interactions between the charged particles. Coulomb interactions induce beam blur that increases with increasing beam current, thus limiting the system performance for a given resolution.
One can distinguish three separate effects based on Coulomb interactions. The first is a global, or average effect, also called the global space charge (SC) effect. The global SC effect induces only image defocus and magnification change if the current density distribution is uniform in every cross-section of the beam along the projection column. In this case SC acts like an ideal negative lens and its effect can be corrected simply by changing the lens focal length or the image plane position relative to the Gaussian plane. The second effect is also a global SC effect, but one that creates image aberrations if the beam current density profile is not uniform, in addition to the above-noted defocus and magnification.
The third effect is a stochastic space charge effect resulting from the random nature of the mutual Coulomb interaction force between particles in a flux consisting of discrete charged particles. Stochastic interactions generate beam blur either directly by inducing trajectory deflections of a particle moving along the column, or indirectly through lens chromatic aberrations by generating particle energy spread, that is, the so-called Boersch effect.
Unlike the deterministic global space charge effects, neither trajectory displacement nor the Boersch effect can be corrected, because they are stochastic. They can only be controlled by careful design.
There is currently a need for devices and techniques to control the global space charge induced aberrations which are dominant in high throughput projection systems (the second above-noted effect). Global SC induced aberrations can, in principle, be corrected or avoided if the system design provides a laminar flux of the charged particles with a uniform current density profile in any cross-section of the beam along the column. In beams with low and medium currents used in lithography and metrology, laminar flux (i.e., particle trajectories that do not cross each other) cannot be realized. This has the implication that a uniform illumination intensity or current density achieved, for instance, in several critical planes such as the mask and wafer planes, will not necessarily stay such in any cross-section of the column. Accordingly, in the areas of the column where the current density becomes nonuniform, the space charge effect will induce beam blur.
The global space charge effects are, in general, well characterized by the beam perveance, P=I/V
{fraction (3/2)}
, a function of the beam current, I, and the beam voltage, V. While the space charge induced defocus, which is completely correctable, might also be proportional to the column length, L, the blur induced by the space charge lensing action, is defined by both the degree of the beam current nonuniformity and the effective length of only the portion of the column, L
eff
, where the current density is not uniform.
In particle projection systems, Koehler illumination is utilized to achieve a uniformly illuminated sub-field in the mask plane. Koehler illumination optics project a bundle of rays coming out from any single point of the cathode or the virtual source into the entire sub-field in the mask plane. In projection devices with fixed size and shape of the sub-field, the sub-field is formed using one or two perpendicular rectangular shaping apertures installed in the planes optically conjugate with the mask or wafer plane in variable shape systems.
In current electron projection systems, the electron gun has a relatively large size uniform-emission, low-brightness cathode, and appropriately designed electrodes, and provides spatially uniform intensity of the energetic (typically 100 keV) electrons leaving the source uniformly in a given angular range (about NA/4 after the shaping aperture to match with the limiting aperture angular size, where NA is the numerical aperture). Note that in probe forming systems, sources with a high-brightness LaB
6
cathode and electrodes that form a narrow crossover of about tens of micrometers with essentially nonuniform Gaussian current density are used. The smaller the NA, and the lower the degree of the beam current density nonuniformity, the smaller the global SC induced blur, as opposed to the stochastic blur increasing with decreasing NA. This is why in the early designs of particle projection systems with small NA, and, therefore, low throughput, the major concerns were the stochastic blurring and the lens aberrations. However, the current
Katsap Victor
Liddle James A.
Mkrtchyan Masis
Stanton Stuart T.
Berman Jack
Lucent Technologies - Inc.
Pillsbury & Winthrop LLP
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