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Electron inelastic scattering in thin films is briefly discussed. It is found that in thin films of interest the plasmon generation by an energetic electron is the dominant process and the electron energy loss spectra (EELS) can be described by an analytical function of the membrane thickness and the material characteristics. A straightforward procedure using the analytical approximation of EELS and the results obtained from it for the SCALPEL mask-membrane in terms of chromatic aberration evaluation and control are presented.

The global space charge (SC) effect in SCALPEL electron beam lithography system is investigated. First order properties of the SC lensing action (defocus and magnification change) in SCALPEL type projection systems are analyzed using a simple analytical technique. Aberrations induced by the lenses and SC in the projection optics are evaluated numerically using a Monte Carlo code developed to calculate the combined effect of Coulomb interactions and lens aberrations in the charge particle projection systems. We found that the defocus and the magnification change induced by SC are functions of two parameters, the beam perveance and the SCALPEL aperture size, that are critical for the system performance. The strong correlation identified between the best focus plane location and the aberrations induced by SC indicates that the SC lensing action can be effectively compensated by simply adjusting either the wafer plane position or excitations of projection lenses.

A method for optimizing the electron optics of the SCALPEL exposure tool is described. The method uses the SCALPEL mark detection method with a grating mark as an aerial image monitor. The root-mean-square deviation of the recorded backscattered electron signal from an ideal triangle waveform was used as a measure of the image fidelity, scale and orientation. The resolution of the technique is limited only by signal-to-noise and the fidelity of the marks. Experiments were performed using 2 @mm period grating marks that were fabricated in a SiO"2/WSi"2 structure using SCALPEL lithography and plasma processing. The projector lenses and magnification/rotation coils were optimized. For these experiments the measured resolutions for determining focus (@df), magnification (@dM), and rotation (@d@q) of a 250 @mm X 250 @mm field were @df ~ +/-1 @mm, @dM ~ +/-15 ppm and @d@q ~ +/- 0.1 mrad. A straightforward path to improving these results is described.

SCALPEL alignment marks have been fabricated in a SiO 2 /WSi 2 structure using SCALPEL lithography and plasma processing. The positions of the marks were detected through e-beam resist in the SCALPEL proof of lithography (SPOL) tool by scanning the image of the corresponding mask mark over the wafer mark and detecting the backscattered electron signal. Single scans of line space patterns yielded mark positions that were repeatable within 30 nm 3σ with a dose of 0.4 μC/cm 2 and signal-to-noise of 16 dB. An analysis shows that the measured repeatability is consistent with a random noise limited response. The mark detection repeatability limit, that can be attributed to SPOL machine factors, was measured to be 20 nm 3σ. By using a digitally sequenced mark pattern, the capture range of the mark detection was increased to 13 μm while maintaining 36 nm 3σ precision. The SPOL machine mark detection results are very promising considering that they were measured under electron optical conditions that were not optimized.

Parametric image-forming models were used to demonstrate the imaging of potential mask defects over ranges of descriptive parameters, either isolated or coincident with a critical object. Beyond basic defect printability effects, CD errors due to defects near CD objects cause serious constraint on allowable pattern defects or contaminants, and define post-repair requirements. The models can provide scaling laws based on defect type, size, contrast, and proximity to a CD object, as well as rules describing tolerable defects using a threshold value for the local CD anomaly they cause.

Electrostatic charge generation and accumulation in a:SiN"x thin films irradiated by energetic (100 keV) electrons is investigated and kinetic equations describing the dynamic process are formulated. It is found that the incident electrons, inelastically scattered in the membrane, primarily generate plasmons. The plasmon decay creates electron-hole pairs and secondary electrons (SE). The escape of the SEs from the target leads to a positive electrostatic charge accumulation in the membrane. It is shown that SCALPEL^(R) mask-membrane charging is defined by the balance of the SE escape, hole trapping and transport processes.

Coulomb interactions in high-throughput charged particle lithography systems lead to an uncorrectable image blur, and can also results in image placement errors. Throughput is ultimately limited by the increasing loss of process latitude with increasing beam current, or by the loss of critical dimension or overlay control due to placement errors. We previously developed an analytical model of the stochastic space-charge induced beam blur for our SCALPEL (SCattering with Angular Limitation Projection Electron Lithography) system, and used it to optimize the design of our experimental tool; principally by constraining the column length and optimizing the numerical aperture to achieve the best balance between electron-optical aberrations and the stochastic blur. We have attempted to validate this model with experimental measurements. We have also begun to quantify pattern distortions due to the global space-charge effects. Preliminary measurements show dose latitudes of ∼ 15%, consistent with a total blur of ∼ 150 nm, and a space-charge component no larger than 30 nm. No evidence for current dependent intrafield distortions has been observed.