An Electrical Test Structure for Proximity Effects Measurement and Correction

Abstract

This paper describes the design of a proximity effect test structure and electrical test method for estimating the magnitude of proximity effects in electron-beam lithography. The test structure consists of a van der Pauw cross resistor for measuring sheet resistance, a bridge resistor for measuring electrical linewidth, and a second bridge resistor simulating a close line-space environment for measuring electrical linewidth where proximity exposure effects from nearby patterns may be encountered. In this experiment, test structures were delineated in aluminum on silicon wafers using electron-beam exposure and wet chemical etching. Electrical measurements from these test structures are compared to optical measurements to verify the measurement method. In addition, results from the test structures are used to estimate the parameters for the gaussian model commonly used for proximity correction.Proximity effects in electron-beam (E-beam) lithography are the additional exposure in the resist due to electron scattering in regions not addressed by the electron beam (1-3). This scattering of electrons in the resist and substrate will affect the dimensions of the pattern and additional patterns in close proximity to that being exposed. In both cases, linewidth control for fine-line patterns with high packing density will deteriorate. The magnitude of proximity effects in the exposed resist depends on the beam voltage, beam diameter, resist material, resist thickness, and substrate material. In general, for linewidth and line-to-line spacings of less than 2 t~m, proximity effects can have significant effect on the resultant linewidths (4, 5).Theoretical models and Monte Carlo simulations are the basis for much of the understanding of proximity effects and for predicting the magnitude of these effects on linewidth control (6, 7). However, detailed experimental data for comparison to calculated values are limited by time and by the complexity of obtaining accurate linewidth measurements of resist profiles on wafer.This paper describes an electrical technique for determining the magnitude of proximity exposure on linewidth control. An electrical test structure and a test method were developed for the rapid and precise determination of linewidth of an E-beam-exposed pattern from samples simulating a dense line-space environment. Electrical linewidth measurements were compared to optical linewidth measurements to verify the magnitude of the proximity effects. In addition, a practical application for the measurements from the test structure for proximity correction was investigated. Proximity Effect Test StructureThe proximity effect test structure (PETS) is an extension of the cross-bridge sheet resistor (8). A PETS, shown in Fig. 1, consists of a van der Pauw cross resistor, an upper bridge resistor, and a lower bridge resistor with proximity inducing bars of width T separated from the bridge resistor at a spacing S. The cross-bridge resistor allows electrical measurements of the linewidth of a conducting layer and is sensit...