Semitransparent Mo/Si multilayer films with a completely free- standing active area have been developed for use as optical elements in the soft x-ray region. They work not only as a beam splitter, but also as a transmissive polarizer and quarter-wave plate. To achieve a flat, smooth free-standing reflecting surface with a high reflectivity, the following fabrication problems were investigated: stress control of Mo/Si multilayer films, the surface roughness of the initial membrane, and the removal of the initial membrane. A flatness of 1 nm (rms) in the active area was obtained for a fabricated 10-mm-square semitransparent multilayer film. A multilayer film consisting of 50 free-standing pairs of semitransparent Mo/Si were fabricated for transmissive polarizer, and a soft x-ray ellipsometer was developed based on them. The fabricated multilayer polarizer was found to have good polarization performance. Placing two transmissive polarizers in the polarizer/compensator-sample-analyzer configuration enabled full control of the polarization of the probe beam. The modified polarization of light reflected from a sample was analyzed by the rotating-analyzer ellipsometry method. This system was used to measure a multilayer mirror. We verified that the soft X-ray ellipsometer is a very promising tool for the structural evaluation of multilayer films, providing a sensitivity in the sub-angstrom range.

Fabrication of semitransparent multilayer polarizer and its application to soft x-ray ellipsometer

The influence of resist and absorber stress distributions on X-ray mask distortion induced during back-etching preceding subtractive fabrication is analyzed experimentally and simulated. The stress distribution (gradient) in a resist and/or that in an absorber film causes larger pattern displacement rather than the film average stress. Some resists have considerably high stress after coating on a wafer, and this stress also changes during exposure, causing local pattern displacements. However, use of chemically amplified resist systems, in which the reaction after exposure is limited to a small amount acid generation, might solve this problem. Low-stress positive-tone resists should thus be developed.

https://iopscience.iop.org/article/10.1143/JJAP.35.2845/pdf

X-ray mask distortion induced in back-etching preceding subtractive fabrication : Resist and absorber stress effect

We have developed programs to calculate pattern displacement caused by membrane stress, absorber stress distribution and pattern density in X-ray masks. This paper describes the models upon which the calculations were based and shows how the results fit experimental ones. The membrane area shrinkage caused by SiN stress is calculated using a previously reported method. The pattern displacement caused by absorber stress is calculated using the stress gradients estimated for each point from the stress distribution measured in masks used in practice. It is found that the displacement in the membrane with non-patterned absorber is strongly affected by the absorber stress distribution. The influence of pattern density on pattern displacement is calculated using a model in which the absorber thickness is reduced in accordance with the volume etched off during patterning. Comparisons between calculated and experimental results of the pattern displacement in masks with simple patterns reveal that the calculated results agree with the experimental results with an accuracy of about 30 nm.

Simulation of X-ray mask pattern displacement

Stress controllability and stress distribution of Ta4B absorber on polished SiC films have been investigated. Dry etching behaviors of Cr and Si02 films have been characterized as etch-masking and etch-stopping materials. Xe gas sputtering was found to be effective to obtain higher stress controllability and more uniform stress distribution for Ta4B film compared to Ar gas sputtering. Cr film has been found to have high etching selectivity of more than 15 to the Ta4B film during the ECR etching with Cl2 gas, which is proven to be suitable for etch-masking and etch-stopping layers of the Ta4B absorber.

Characteristics of Ta4B/SiC x-ray mask blanks

In order to clarify the influence of multilayer stress distribution on the pattern placement accuracy of an extreme ultraviolet (EUV) mask, the in-plane displacement (IPD) of the mask was calculated using a simulation model based on two-dimensional plane stress theory. In order to deal with multilayer stress distribution, average stress, fluctuation stress and periodic length were defined as parameters. The parameters, which have a significant influence on the mask IPD, are the fluctuation stress and the periodic length. It was found that the average stress of a multilayer film did not affect the mask IPD. An EUV mask with pattern density of 100% makes IPD small by decreasing the fluctuation stress. For EUV masks of 150 mm and 100 mm squares, the periodic lengths that generate the peak IPD in the mask are 200 mm and 150 mm, respectively. In order to improve EUV mask pattern placement accuracy, it is important to achieve the development of uniform multilayer stress distribution.

Estimation of Mask Placement Error Caused by Multilayer Stress Profile

A new system that measures stress in film deposited on Si wafers has been developed to produce highly accurate X-ray masks. The system consists of very rigid air sliders, an electrostatic sensor, and a soft-handling wafer chuck. With the system, wafer warp is precisely measured before and after film deposition, and the stress distribution is calculated from those measurements. Wafer warps can be measured with a repeatability of a few nanometers by this system. The stress distribution of absorber film on 2-mm-thick Si wafers can be determined with an accuracy of ±5 MPa. The stress distribution agrees well with the pattern position shifts in the membrane.

Atsunobu Une

Papers

A Highly Accurate Stress Measurement System for Producing Precise X-Ray Masks