In this paper, we proposed a new design of the test mask to measure the amount of the out-of-band (OOB) light from an extreme-ultraviolet (EUV) light source by detuning the period of the multilayer (ML), rather than changing the material of the absorber, to suppress reflection of EUV light. The new OOB test mask also reflects essentially the same OOB light as that of the production mask at each wavelength in the whole OOB spectral range. With the help of the new OOB test mask, the contributions to the background intensity from in-band flare and OOB light can be correctly separated and an accurate optical-proximity-correction (OPC) model can be established.
Due to the use of reflective optics in extreme-ultraviolet lithography (EUVL), the chief ray angle of incidence at the object (mask) side (CRAO) cannot be zero. If the conventional resolution enhancement technique (RET) of off-axis illumination (OAI) is used, such mask-side non-telecentricity degrades aerial image contrast partly because of asymmetry (w.r.t. the mask) of the two beams in an incident beam pair and partly because of asymmetry (w.r.t. the mask) of the two diffraction orders of either incident beam. The former leads to intensity imbalance of the two incident beams (after leaving the mask) and the latter leads to amplitude and phase imbalance of the two diffraction orders of either incident beam. Solutions proposed previously only alleviate the former and have little help for the latter. In this paper, we introduce n=1 absorber to eliminate the phase imbalance so that the transverse shift between the two aerial images formed by the two incident beams can be minimized and the contrast of the final aerial image (by superposition of the two) can be restored.
In extreme ultraviolet lithography (EUVL), the application of off-axis illumination (OAI) leads to degradation in aerial image contrast, resulting in an unacceptably high mask error enhancement factor as the pattern pitch becomes smaller, even if an attenuated phase-shifting mask (AttPSM) of optimized attenuation is employed. We show that this is an intrinsic problem of OAI and cannot be remedied by adopting a thinner absorber, a smaller chief ray angle of incidence at the object side, or a projection optics box with a higher numerical aperture. Based on simulation results using the best conditions for OAI, we may conclude that single-patterning EUVL will probably end at a technology node with the minimum pitch of 22 nm, unless we can come up with other innovative ways for performing EUVL imaging or designing and formulating resists with blurs less than 5 nm at reasonable exposure dose.
In this paper, the impact of resist on the lithographic process window is investigated. To estimate the resolution
limit of EUVL due to the limitation from resist performance, a simplified resist model, called diffused aerial image
model (DAIM), is employed. In the DAIM, the resist is characterized by the acid diffusion length, or more generally,
resist blur. Lithographic process windows with resists of various blurs are then calculated for different technology nodes.
It is concluded that the resist blur needs to be smaller than 8 nm to achieve a reasonable window for the technology node
with the minimum pitch of 32 nm. The performance of current resists can barely fulfill this requirement. Investigation of
a more refined resist model is also initiated.
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