7.5: Massive parallel Electron Beam lithography based on a planar type Si nanowire array ballistic electron source with large surface

Abstract

The parallel Electron Beam (EB) lithography based on a Surface Electron Emission Lithography system (SEL) for high resolution and high throughput is demonstrated in this study. The exposure results indicated that the submicron mask patterns on a planar type silicon nanowire array ballistic electron emitter (PBE) were well reproduced in large area. The capability of ballistic electron emission from the PBE allows us to exposure fine pattern in large region The parallel EB lithography employs a prototype EB stepper with the PBE1 which holds a patterned mask as shown in Fig.1(a) and (b). The PBE has a metal / Si nanowire array / semiconductor structure. The nanowire layer was composed of massive amounts of silicon nanowires. The nanowire is the one dimensional interconnected Si quantum dots. When a bias voltage is applied to the Si nanowire layer, the electrons are accelerated via cascade tunneling between Si quantum dots, and emitted to the vacuum space. The electrons are emitted with uniform intensity in the surface of PBE. The emitted electrons also show small energy distribution because the scattering probability of the electron-phonon interaction was reduced in the Si nanowire. When the dot size in the nanowire becomes 3 nm, calculated minibandgaps in the nanowire exceed 0.06 eV corresponding to the maximum phonon energy. Since the energy of phonon which is the vibration of Si atomic lattice is limited up to 0.06 eV, the phonon scattering is reduced in the nanowire2. In the massive parallel EB lithography, small emission angle dispersion and small energy dispersion provides higher spacial resolution as attractive features for the parallel lithography, because the characteristics of ballistic electron emission of the PBE provide small chromatic aberration in the 1:1 parallel EB lithography. The PBE projects the pattern on the target wafer in the electron optics of parallel electric and magnetic fields. If all emitted electrons have same initial velocity, they are focused at the same distance from the surface electrode of PBE. The pattern of the mask on the PBE is reproduced on the target wafer at the distance of the n (n=1, 2, …) cycles of the spiral trajectory of the electron. Practical resolution is limited by the chromatic aberration. The experiments of 1:1 parallel EB lithography were performed on the prototype EB stepper. The vacuum level in the chamber was about 10−4 Pa. The system was composed of the PBE as a surface electron source, a target wafer parallel to the PBE, and vertical electromagnetic fields to the surface of the wafer as shown in Fig. 1(a) and (b). The target wafer coated with resist film (ZEP520: t50 nm) was located on the stage. An acceleration voltage of 5kV was applied between the PBE as the electron emitter and the target wafer. The strength of the electric field E was approximately 5 kV / 20 mm. In order to focus the electron image projected on the target wafer two magnet poles provided a magnetic field B of 0.3 T. The direction of the electromagnetic fields between the emitter and the target wafer was controlled to obtain the parallelism of E//B. The distance between the emitter and the target wafer was adjusted so that the focus was produced at any distance of n cycles of the trajectory of the emitted electron. When a bias voltage was applied to the surface electrode with respect to the substrate as a rear electrode, the PBE projected a patterned electron image on the target. The exposure time was 10 sec so that the electron dose was 30 μC/cm2. Figure 2(a) shows the whole view of the exposed patterns. The sub-micron patterns are uniformly reproduced all over the exposure area of 2.8 mm square. The magnified image of the exposed pattern indicated in Figure 2(b) consists of the repetition of lines of 0.5 μm, 1 μm, 2 μm and 4 μm in width at interval of 20 μm in 2.8 mm square. It is confirmed that the patterns of the mask is reproduced. This homogeneity of 1 shot exposure in the large area results from the uniformity in the surface electron emission current from PBE. We performed the parallel lithography by prototype EB stepper with the patterned PBE. The exposure results indicated that the submicron mask patterns on the PBE were well reproduced in large area. The capability of ballistic electron emission from the PBE allows us to exposure fine pattern in large region without complicated electron optics. We think that the high homogeneity of the exposure will be available to produce next generation MEMS of fine patterns with lower cost than that of optical stepper. Considering the equation of the resolution in 1:1 electron projection lithography, the resolving power of the prototype EB stepper will be improved to be sub-100 nm by increasing accelerating voltage.