Overlay Accuracy Evaluation in Step-and-Repeat X-Ray Lithography

Overlay accuracy in step-and-repeat X-ray lithography is evaluated through MOS device fabrication using an X-ray stepper applied to four lithography levels. Though relatively good overlay accuracies of 0.15 µm (σ) between two levels are obtained, these values are not satisfactory for sub-half-micrometer ULSI fabrication. To improve overlay accuracy, error factors are investigated. It is found that overlay errors are strongly affected by mask pattern placement errors. The standard deviation of mask errors is as high as 0.1 µm. These errors are introduced by e-beam writing errors and pattern displacement caused by stress relief of absorber and membrane layers in the etching process. The influence of these error factors on pattern placement accuracy is discussed.