Yen-Cheng Lu

Printability of buried mask defects in extreme UV lithography

A programmed-defect mask consisting of both bump- and pit-type defects on the LTEM mask substrate has been successfully fabricated. It is seen that pit-type defects are less printable because they are more smoothed out by the employed MLM deposition process. Specifically, all bump-type defects print even at the smallest height split of 1.7 nm whereas pit-type defects print only at the largest depth split of 5.7 nm. At this depth, the largest nonprintable 1D and 2D defect widths are about 23 nm and 64 nm, respectively.

An accurate method to determine the amount of out-of-band light in an EUV scanner

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.

On the extensibility of extreme UV lithography

In this paper, definition of line/space patterns at 44-, 32-, and 22-nm pitches using extreme-ultraviolet lithography (EUVL) is investigated by aerial image simulation. The results indicate that extending EUVL to the 22-nm pitch requires reducing the mask shadowing effect, which implies reducing the mask absorber thickness as well as maintaining the 6-degree angle of incidence on the mask, if the reduction ratio of the imaging system is to be kept at 4. Reduction of the mask absorber thickness can be realized by implementing attenuated phase-shifting masks. Otherwise, all critical patterns must be laid out in single orientation.

Mitigation of image contrast loss due to mask-side non-telecentricity in an EUV scanner

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.

Limitation of OAI + AttPSM in EUVL

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.

The role resist plays in EUVL extensibility

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.