Application of advanced 100-kV EB writer EB-X3 for 100-nm node x-ray mask fabrication

Most important issues in a precise X-ray mask fabrication are the mask materials and EB writing to achieve good accuracy in critical dimension (CD) control and image placement (IP). However, 1-to-1 X-ray mask is required severe accuracy in comparison with photo reticles. The following discussions focus on how to realize the precise IP accuracy. We installed and evaluated 100-kV electron-beam (EB) mask writer (EB-X3), and developed the writing process on a thin membrane. Key factors in accurate EB mask writing include not only EB positioning accuracy but also mask distortions caused by mask holding and the temperature change of the mask and a mask holder. This paper presents mask distortion characteristics due to the holding, temperature change, and then, good results of mask accuracy of 4-Gb dynamic random access memory (DRAM) test patterns (gate and contact layers) and 90 nm SRAM test patterns. In addition, we employed the advanced PAT method with 4-multi-pass writing that adequately compensates the process-induced mask distortion and the beam drift. These improvements resulted in IP accuracy of better than 10 nm (3(sigma) ), 100 nm CD uniformity within 8 nm (mean shift +/- 3(sigma) ) and the overlay accuracy within 10 nm for 4-Gb gate and contact layers with a 24 mm x 24 mm area on the X-ray membrane mask. These results demonstrate that we can actually fabricate precise X-ray membrane masks that meet our final target of IP accuracy corresponding to the 100 nm technology node.