Performance of IBM's EL-4 e-beam lithography system

Abstract

IBM's latest electron beam mask maker, EL-41, is online at IBM's Advanced Mask Facility (AMF) in Essex Junction, Vermont. The EL-4 system is a 75KV shaped beam lithography system utilizing a Variable Axis Immersion Lens (VAIL)2 designed to produce lx or NX masks for O.25m lithography ground rules, extendable to O.l3pm. It is currently producing NIST-style X-ray membrane masks with pattern sizes over 30 x 30 mm2. This paper will give a brief description of the EL-4 tool and its operating features, specific measures used to enhance tool stability and accuracy, and measurement data from masks recently produced on the tool.Keywords: e-beam lithography, e-beam systems, electron beam lithography, electron beam systems, microlithography, X-ray masks OVERVIEW OF THE SYSTEMThe EL-4 Lithography system is a shaped beam system with a square spot of 2 x 2 m2. It is capable of being shaped in either axis in increments of 3. 125nm from zero to the full 2pm size. This shaped spot can be electrostatically positioned within a 37.5 x 37.5 .im2 subfield with a resolution of 12.Snm. This subfield is then magnetically positioned within a 2.1 x 2. 1 mm2 field with 8.l3nm resolution. Multiple fields are written sequentially by moving the substrate with a planar stage3 in a step and repeat fashion. The resolution of the stage is limited, in this application, by the laser interferometry system to 4.95nm.A block diagram of the EL-4 lithography system is shown in Figure 1. The column and mechanical subsystems are installed in a class 10 temperature-controlled cleanroom along with the electrostatic deflection drivers and the beam high voltage supply for temperature stability. The rest of the system is in an adjacent control room maintained at an office environment.The electron beam column4 is made up of 8 mechanical sections stacked together. Each section contains various imaging and deflection elements. The electron gun at the top generates a 75KV electron beam from a heated LaB6 cathode assembly. The two sections just below contain intermediate alignment coils and beam shaping and limiting apertures. The next two sections contain the variable-shape beam-forming apertures and electrostatic shaping deflectors. Next is the demagnification section with the beam-limiting aperture and initial spot demagnification lenses. After that is the transfer section that contains the electrostatic subfield position deflectors and dynamic correction coils. Finally, the projection lens assembly contains the magnetic deflection coils and the VAIL to project the composite aperture image onto the target. The spot image is demagnified by a factor of 50 before it reaches the target. Four large diode detectors are mounted at the bottom of the column to collect backscatter electron signals that are used for calibration and registration. The projection lens also contains the autofocus assembly which detects the height of the writing surface with a reflected light beam. Circuits in the Column Control Subsystem automatically adjust the beam focus to compensate for hei...

X-ray imaging: applications to patterning and lithography

“…A feedback system based on a detector placed behind the mask has also been proposed [31]. There are two main strategies for mask writing, based on vector-scan Gaussian [8] or variable shape beams [32]. Typically, Gaussian beam systems have sharper line edge acuity but slower throughput; variable shape beams have larger throughput, but lower edge acuity.…”