Evaluation of a transmission CD-SEM for EB stencil masks

Abstract

Scanning-electron microscopes designed for critical dimension (CD) measurement (CD-SEMs), which operate in a reflection mode, are commonly used in photomask quality assurance. However, such CD-SEMs are not always suitable for CD measurement of electron beam (EB) stencil masks such as electron-beam projection lithography (EPL) masks and low-energy electron-beam proximity projection lithography (LEEPL) masks. A dual-mode CD-SEM, which can operate in a transmission mode besides in a reflection mode, was recently developed by HOLON. The performance of the tool will be reported. Two EPL test masks with a 2-μm-thick Si scatterer and one LEEPL test mask with about 1-μm-thick absorber were prepared. The EPL masks have 1:1 lines-and-spaces (L&S) patterns and isolated spaces, all varying from 200 to 1000 nm in designed size. The LEEPL mask has 1:1 L&S patterns ranging from 80 to 550 nm. The masks were observed at an acceleration voltage of 5.5 kV with no bias voltage and a current of 10 pA. The dual-mode CD-SEM is found to have the following characters: (1) short-term repeatability and long-term repeatability less than 2 nm in both modes, (2) compatibility with a photomask CD-SEM in the reflection mode, (3) coincidence of the CDs measured in the transmission mode from the front side and back side within 3.0 nm for stencil patterns with a sidewall angle larger than 89.8 deg, and (4) capability of measurement at least down to 80 nm. Therefore we conclude that the dual-mode CD-SEM is applicable for measurement of CDs of EB stencil masks.