We have developed the EBIS (Electron Beam Integrated System), which is a character projection (CP) type low-energy electron-beam direct writing (LEBDW) system. In this system, the proximity effect due to backscattering electrons is very small under the condition that the energy of primary electron is 5 keV. However, there is a serious problem, in that the signal of the mark buried under a thick insulator couldn't be detected. To overcome this problem, we adopted a mark detection method using Voltage Contrast (VC) image with negative charge on the sample surface.
So far, we have detected the signal of alignment mark buried under 600nm-thick (nmt) tri-layer resist using VC image on EBIS. Then we exposed overlay patterns with alignment using the mark detection with VC image. The mark image is very clear with a sufficiently high contrast. The asymmetry originating from VC is mitigated by means of FB scanning. Using this VC mark detection method, EB drawing was performed with alignment with 600nmt tri-layer resist on Si substrate. Moreover, VC mark detection with 600nmt tri-layer resist on the substrates of back-end-of-line (BEOL) of logic device was performed and the mark images with sufficient contrast were obtained. Although the characteristic distortion of VC image exists, mark detection is possible by using X/Y separate scanning, which consists of X-direction scanning to get an X position and Y-direction scanning to get a Y position in non-charged area.