Tantalum film for x-ray lithography mask deposited by electron cyclotron resonance plasma source coupled with divided microwaves

Abstract

An advanced electron cyclotron resonance (ECR) plasma source coupled with divided microwaves has been applied to Ta film formation. The feasibility of the Ta film as an x-ray mask absorber is investigated. The stress in an x-ray absorber material must be low, uniform, and stable. According to the results of an investigation on the stress-depth distribution in Ta film, the stress can be described as the sum of interfacial stress (that retained near the film–substrate interface) and bulk stress (that retained in the film bulk). It is found that these individual stresses can be independently controlled. The interfacial stress depends on the substrate temperature, working-gas pressure, and deposition rate. The bulk stress becomes compressive by adding Ar to the sputtering gas (Xe). A low stress film of 23 MPa (400 nm thick film) with no stress-depth distribution is obtained using optimized deposition parameters of 3.3×10−2 Pa of Xe, 1.75 sccm of Ar, and 280 °C substrate temperature. The ECR Ta film showed excellent stability against stress with a change in stress of less than 5 MPa after the x-ray mask fabrication process and synchrotron radiation irradiation. A highly accurate x-ray mask is fabricated by a wafer process using an ECR-deposited Ta film. The maximum image placement (IP) error of the x-ray mask is less than 60 nm, which includes the IP error induced by membrane shrinkage during back etching.