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Background: In extreme ultraviolet lithography, the printable feature density is limited by stochastic defectivity, which can be reduced by increasing the optical contrast. The photomask induces pole-specific aerial image offsets. Consequently, illumination settings with multiple poles lead to contrast loss and focus offsets between different features.

Aim: We aim to mitigate the contrast loss and best focus offsets between different features.

Approach: Illumination was decomposed into monopoles. Each monopole was exposed separately using a fraction of the total dose. Each exposure was shifted by its pole-specific image offset to mitigate 3D mask effects.

Results: Single monopoles mitigate contrast loss and best focus shifts, but in defocus, they suffer from aerial image shifts and distortions. Multiple aligned monopole exposures conserve these advantages but mitigate the problems in defocus. Because each monopole is exposed with only a fraction of the dose, the throughput penalty is limited to the scanner overhead.

Conclusions: A multiple monopole exposure scheme can increase contrast, align the best foci, and mitigate single monopole exposure constraints. Additionally, it offers an improved pattern placement control through dose control knobs.

Abbe’s resolution formulas for a grating imaged under on-axis, off-axis, and incoherent illuminations are obtained with the help of the line-spread function. A rule-of-thumb relationship applicable to the extreme ultraviolet mask for patterning a grating near the resolution limit of the imaging optics is given.

The progressive degradation of transmission reticles used in semiconductor production occurs via several mechanisms, the most prevalent being haze formation in 193 nm lithography. A less frequently observed yet more significant problem involves the migration of the chrome from the features in chrome-on-glass (COG) reticles onto the clear areas. All these critical dimension degradation mechanisms can result in yield loss but only the effect of haze can be corrected by cleaning the reticle. Chrome migration is caused by exposure to 193 nm UV and electric field. To differentiate between the two causes, different acronyms are used: here, PIM for photon-induced migration, and EFM for electric field-induced migration. The characteristics of both mechanisms are described and compared. A common explanation is proposed for PIM and EFM type 1, whereas EFM type 2 involves a physical process that is not present in PIM. These types of damage have only been observed in COG reticles to date, but the physical processes causing them are common to all materials. It is, therefore, concluded that ensuring the prevention of these progressive forms of reticle damage in all types of transmission reticle requires both the elimination of humidity and the exclusion of electric field from the reticle’s environment.

We present experimental evidence for an empirical correlator (or model) for stochastic local CD uniformity (LCDU) of contact holes printed in resist with 0.33-NA EUV lithography. The key component of this correlator is a term that has the “traditional” inverse proportionality to the image log-slope (ILS). We also show that, when the contact CD is close to (one of) the stochastic pattern-failure cliffs (i.e., missing contacts or merging contacts), additional terms need to be added. These terms represent the steep rise in LCDU that is typically observed in the vicinity of these cliffs. We also demonstrate an approach for generating the image-intensity function from which ILS is calculated to obtain the best LCDU correlation result (a procedure which we call “image calibration”). Several experimental LCDU data sets that support the validity of the proposed correlator are presented.

Background: Overlay (OV) is a very important indicator for characterizing the pattern position accuracy between two layers in an integrated circuit in the manufacturing industry. The self-aligned double-patterning (SADP) process, which is limited to the resolution of deep ultraviolet lithography, is being extensively used for the volume manufacturing of integrated circuits. However, compared to the planar process, the SADP process poses new challenges for OV control.

Aim: We investigate the bottom grating asymmetry of a microdiffraction-based overlay (μDBO) target in the SADP process and its effect on OV measurement.

Approach: The bottom grating of the OV metrology target is analyzed through scanning electron microscopy and transmission electron microscopy. The imbalance of the −1st- and +1st-order diffraction light intensities of the asymmetrical bottom OV target is characterized. Finally, the effects of light polarization and wavelength on the OV accuracy are simulated.

Results: Asymmetric behavior of bottom OV grating is identified. OV dependency on the measurement-light polarization is determined. The simulation results indicate that wavelength and polarization affect the OV accuracy simultaneously.

Conclusions: We studied the bottom grating asymmetry-induced inaccuracy in DBO measurement and can assist in understanding the mechanism of measurement light-interaction with the OV grating to enhance the DBO accuracy to a new level.

Background: With the adoption of extreme ultraviolet (EUV) lithography in the semiconductor manufacturing, actinic EUV mask metrology has become a crucial technology to ensure the required defect sensitivity and throughput for high-volume manufacturing. Reflection-mode EUV scanning microscope (RESCAN) is a lensless actinic microscope dedicated to EUV mask metrology based on coherent diffraction imaging (CDI) as an alternative approach for EUV mask metrology and inspection.

Aim and Approach: In CDI, the complex-amplitude image of the sample is obtained through its measured diffraction. Though this approach can overcome the disadvantages and limitations of conventional imaging systems, the quality of the recorded diffraction data is crucial for the reliable reconstruction of a high-resolution image. Ultimately, the signal-to-noise ratio of the recorded diffraction data depends on several parameters, such as the sample’s reflectance, the quantum efficiency of the detector, its full well capacity, and the intensity of the illumination.

Result: We investigate the optimal photon flux for RESCAN and provide a systematic study on the relation between the image resolution and the illumination intensity for CDI-based imaging at EUV wavelength.

Conclusions: The insights provided will be helpful for the optimization of CDI for EUV imaging, in particular for increasing the throughput of EUV mask inspection with low power sources.

Pattern collapse and photoresist scumming are major limiting factors to achieve a failure-free process window in extreme ultraviolet lithography. Previous works on this topic have empirically proven the importance of matching photoresist and underlayer surface energy, and the role played by developer liquid in wet development. In this work, we extend those concepts and formulate a figure of merit for the free energy at the exposed and unexposed photoresist-underlayer-developer interfaces. This figure of merit provides a tool to optimize the underlayer surface energy components that best match a given photoresist and developer process. The model is tested against experimental patterning of a chemically amplified resist at pitch 32 nm and pitch 80-nm line spaces, successfully predicting the likelihood of pattern collapse and photoresist scumming. Moreover, we write a quantitative expression for the peeling force acting on photoresist lines owing to unbalanced capillary forces and the threshold energy at which film delamination onsets. It is shown that adhesion and scumming are two manifestations of the same phenomenon at these interfaces.

Background: An overlay target design is essential before performing overlay metrology and has a great impact on overlay performance. In addition, accurate overlay metrology is required to achieve high product yields in semiconductor manufacturing. When asymmetry of the overlay target occurs, it is necessary to increase the area of the pattern existing in the target and stabilize the optical aberration for stable measurement. For this reason, we propose an image-based overlay target design that can secure a larger area compared with the existing overlay target and stabilize the optical aberration by overlapping and exposing the upper and lower patterns of the overlay target.

Aim: The effective area of the existing mark design is improved by proposing an overlay target and a measurement method used in image-based overlay measurement.

Approach: The proposed overlay target design is configured by arranging different marks orthogonally to the upper and lower layers, and different two-dimensional information can be inserted in the same area. When necessary, single-dimensional information desired by the user can be obtained through a technique called projection, and it has the advantage of using a larger effective area than the existing overlay target.

Results: Overlay can be measured using projected signal components, and the possibility and performance of the proposed target can be confirmed through simulation and test wafer. In addition, it was possible to bring about a 15% improvement in total measurement uncertainty performance compared with the existing advanced imaging metrology (AIM) target by taking a wider effective area.

Conclusions: An overlay target design is proposed to solve the technical problems of the existing overlay target design. The proposed overlay target has a method that is capable of minimizing the size of the overlay target while maximally maintaining the signal density, a way of reducing the influence of aberration, and various other advantages.

In addition, the proposed method provides a design rule that breaks the “lower pattern and upper pattern must not overlap” framework.

The erratum corrects an error in the parameter units in the original article.

The erratum corrects an error in parameter unit in the original article.