Electron optics for multi-beam electron beam lithography tool

Details Electron optics for multi-beam electron beam lithography tool Electron optics for multi-beam electron beam lithography tool

2002-09-12

2003-09-09

Berman, Jack (Department: 2881)

Radiant energy

With charged particle beam deflection or focussing

With target means

C250S3960ML, C250S492200

Reexamination Certificate

active

06617587

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of lithography, and in particular to lenses and other column components, suitable for use in charged particle beam direct-write lithography.
2. Description of the Related Art
At present, there is no practical solution for next generation lithography (NGL) at the ITRS 70 nm resolution node. The leading NGL contenders—Extreme Ultraviolet Lithography, Electron Projection Lithography, X-Ray Lithography and Ion Projection Lithography—all use masks. NGL masks are difficult to fabricate and expensive; and many lithography masks are required for standard IC chips, and in particular for microprocessor chips. The latest Pentium III microprocessor requires approximately 30 masks. These mask costs must be amortized into the cost of fabricating the IC chips. Electron beam direct-write (EBDW) systems offer two particular advantages over other NGL technologies: (1) they are maskless, thus eliminating mask amortization costs and expediting chip development cycles; (2) they have the capabilities of meeting all future ITRS nodes in terms of resolution (out to critical dimensions of 25 nm). The primary disadvantage of the traditional single column and probe-forming or shaped-beam systems is wafer throughput limitation due to space-charge effects. Space-charge effects are electron-electron interactions that occur when there are regions within the column with high electron beam current density. These effects tend to blur the beam and increase the spot size on the wafer. Since most electron optical column designs have a crossover, in which all of the electrons pass through a small area, the current density becomes quite large. In order to achieve writing resolution of less than 50 nm, electron beam currents need to be limited to roughly 1 &mgr;A through a crossover. For a 300 mm diameter wafer and a 10 &mgr;C/cm
2
resist sensitivity, a simple calculation shows that an electron beam current of roughly 80 &mgr;A is required to expose the entire wafer in a time of 90 seconds. Including a writing overhead of 30 seconds, this results in a wafer writing throughput of 30 wafers/hr, which barely meets the chip manufacturers' throughput requirements. A more sensitive resist can be used, but then statistical dose issues become a concern. As can be seen, this amount of electron beam current (80 &mgr;A) is much too high to be used in a single column approach with high resolution. In order to keep the column current to less than 1 &mgr;A/column, a minimum of 80 beams that do not interact with each other are needed. Thus, a multi-beam approach is required.
The straightforward technique to reduce space-charge effects is to spread the current over the wafer by using multiple beams that write simultaneously. Some have proposed multi-beam systems using multiple columns and only a single beam per column, such as Chang, et al. [T. H. P. Chang, D. P. Kern, and L. P. Murray, J. Vac. Sci. Tech. B 10(6), pp. 2743 (1992)] and Groves and Kendall [T. R. Groves and R. A. Kendall, J. Vac. Sci. Technol. B 16(6), 3168, (1998)]. Others, such as Yasuda [H. Yasuda et al., J. Vac. Sci. Technol. B 14(6), 3813 (1996)] and Schneider [J. E. Schneider, P. Sen, D. S. Pickard, G. I. Winograd, M. A. McCord, R. F. W. Pease, W. E. Spicer, A. W. Baum, K. A. Costello, and G. A. Davis., J. Vac. Sci. Technol. B 16(6), 3192 (1998)] propose using multiple beams within a single column. However, these approaches run into another major problem for EBDW systems: data rate. Because the data rate is applied serially to each writing beam, extremely high data rates are required for typical EBDW systems. Assuming that each beam is blanked individually, the data transfer rate of the pattern onto the wafer can be calculated, and from this calculation, we determine an appropriate number of beams required. For a 300 mm wafer with 25 nm×25 nm pixels, there are a total of 1.1×10
14
pixels on the entire wafer. In order to write the wafer in 90 seconds, an overall data rate of 1.26×10
12
pixels/s is required. Blanking rates on the order of 100-300 MHz are presently achievable. Therefore, the minimum number of beams that satisfy the blanking rate requirement is between 4,000 and 12,000. With a practical blanking rate of 250 MHz per beam, roughly 6000 individually controllable beams are required. An approach having 6000 columns per wafer, or 6000 beams per column is not realistic, both in terms of fabrication and electrical interconnects. A multiple column approach, with each column having multiple beams, would solve this problem.
To achieve a compact design with multiple beams per column in a multiple column system is quite challenging. To focus an electron beam to high resolution, without aberrations that cause degradation in the beam shape and size, requires a uniform electrostatic or magnetic field. This level of field uniformity is typically achieved only if the diameter of the electrostatic lens bore is roughly 10 to 100 times larger than the diameter of the electron beam passing through the middle of the lens. Because electron optic imaging systems typically also have a large demagnification factor, this results in a writing field of view on the wafer that is much smaller than the lens diameter. For example, if the writing area required on the wafer is roughly 250 &mgr;m, and the demagnification of the imaging system is roughly 1/50×, then the lens bore diameter must be in the range of 125 mm to 1.25 m in diameter in order to minimize aberrations. Since the wafer diameter itself is only 300 mm, this standard electrostatic lens cannot be used in a multi-column approach. A more compact lens design that can individually focus multiple beams within a single compact column design would overcome this problem.
SUMMARY OF THE INVENTION
This invention includes lenses and other column components, suitable for use in multiple charged particle beam systems, and particularly in multiple column, multiple charged particle beam systems. According to aspects of this invention, an integrated optical element for independent alignment of multiple charged particle beams comprises: a substrate for providing structural support with a multiplicity of apertures and a multiplicity of independently addressable alignment deflectors situated over insulating material of the substrate, such that each of the deflectors is positioned over a corresponding substrate aperture. Further, a multiplicity of object apertures can be situated over the deflectors, such that each object aperture is positioned over and electrically isolated from a corresponding deflector. Furthermore, a multiplicity of independently addressable blankers can be situated over the deflectors, such that each blanker is positioned over and electrically isolated from a corresponding deflector. Further, a multiplicity of spray apertures can be situated between the substrate and the deflectors, such that each spray aperture is positioned below and electrically isolated from a corresponding deflector. The multiplicity of deflectors can be arranged in a regular array, such as a line. In preferred embodiments the integrated optical element comprises spray apertures, deflectors, object apertures and blankers, as described above, with the blankers situated over the object apertures such that each blanker is positioned over and electrically isolated from a corresponding object aperture.
According to further aspects of the invention, an optical column for multiple charged particle probe generation comprises: a charged particle source for generating a multiplicity of charged particle beams; an integrated optical element for independent alignment of each charged particle beam; an accelerating column; a deflector; a blanking aperture; and an immersion lens. Further, the optical column can include a rotator between the integrated optical element and the accelerating column. The charged particle source can be a multiplicity of field emission cathodes; the source can also be an ion

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