Martin Verbeek

Alternating phase-shifting masks: phase determination and impact of quartz defects--theoretical and experimental results

In optical lithography balancing the aerial image of an alternating phase shifting mask (alt. PSM) is a major challenge. For the exposure wavelengths (currently 248nm and 193nm) an optimum etching method is necessary to overcome imbalance effects. Defects play an important role in the imbalances of the aerial image. In this contribution defects will be discussed by using the methodology of global phase imbalance control also for local imbalances which are a result of quartz defects. The effective phase error can be determined with an AIMS-system by measuring the CD width between the images of deep- and shallow trenches at different focus settings. The AIMS results are analyzed in comparison to the simulated and lithographic print results of the alternating structures. For the analysis of local aerial image imbalances it is necessary to investigate the capability of detecting these phase defects with state of the art inspection systems. Alternating PSMs containing programmed defects were inspected with different algorithms to investigate the capture rate of special phase defects in dependence on the defect size. Besides inspection also repair of phase defects is an important task. In this contribution we show the effect of repair on the optical behavior of phase defects. Due to the limited accuracy of the repair tools the repaired area still shows a certain local phase error. This error can be caused either by residual quartz material or a substrate damage. The influence of such repair induced phase errors on the aerial image were investigated.

Fourier analysis of AIMS images for mask characterization

Mask characterization and qualification are becoming more and more difficult for high-end and alternating phase shifting masks. One choice is to use indirect methods like as SEM/AFM measurements to characterize mask performance. The indirect measurement has the disadvantage that the measured CD is not the true merit function. It is therefore tempting to use optical methods to characterize masks. The Zeiss AIMS (Aerial Image Measurement System) microscope is particular appealing for this task since it simulates the lithographic imaging optics. The key problem is the reliability and repeatability of the resulting AIMS measurements. The quality of the measurement depends strongly on the tool characteristic such as illumination stability and operator skill, e.g. for focus adjustment. In this paper we discuss the application of image processing and Fourier analysis techniques to AIMS images of periodic structures. By computing the Fourier series coefficients one obtains a very compact but complete description of the AIMS image over defocus. This computation and interpretation of the series coefficients allows to compensate many error influences such as mask rotation, tool magnification and focus adjustment. The algorithm is demonstrated on COG and alt. phase shifting mask measurements for a wavelength of 193nm and compared to the results obtained by simulation.

Use of nanomachining as a technique to reduce scrap of high-end photomasks

Nanomachining is a new technique to the semiconductor industry. This technique combines the positional control of an AFM and RAVE LLCs proprietary nanomachining head to perform material removal at nanometer levels. This is the first commercial application of this technology. Benefits of this technology as applied to photomask repair are discussed. Specifically, we address the use of this technique for removal of defects in quartz, chrome, MoSi, and various exotic materials. Emphasis is given to materials that provide significant challenges for the current industry standard repair techniques. Specifically, quartz bumps on alternating phase shift masks, trimming of carbon patches, and repairs within tight lines and spaces. Several advantages of the technique are illustrated, including the abilities to machine in tight geometries, approximate line edge slope for both wet and dry etched material, image with resolution exceeding todays needs, extend to future mask sizes and technologies NGL, develop custom repair processes and tips quickly, image and repair quartz bumps of nearly any size and shape, minimize surface damage, and achieve superior edge control and transmission. Statistical analysis is conducted on performance of the technique with respect to edge placement, transmission, and surface damage performance to specifications.

Qualification of alternating PSM: defect inspection analysis in comparison to wafer printing results

With alternating phase shift masks (altPSM) an enhancement technique is available to realize smaller design rules. Meanwhile the basic production process for this mask type is well known and established for 193nm technology development. The qualification of the masks is now in the focus of development work. Sensitive defect inspection is essential for the qualification of altPSMs. In addition accurate phase and transmission balancing measurement technique has to be applied. In this paper we are presenting a detailed defect printability study for sub-100nm feature size technology at 193nm wavelength. Programmed quartz defects with different shapes and sizes were designed. They were implemented in a lines/spaces altPSM design. The processed quartz defects were characterized with a scanning electron microscope and an arial imaging microscope system. The printing behavior of the defects was analyzed after wafer exposures. In addition the required sensitivity for the altPSM inspection was evaluated. Finally the inspection sensitivity was characterized and optimized with programmed and production like defects.

Defect printability and repair of alternating phase-shift masks

This paper will start with an overview of the different defect types which can occur on alternating phase shifting masks. A test mask with programmed defects of these different types was fabricated. The defect printability was investigated using an AIMS system. These results were correlated to first printing results in the wafer-fab. The results give an overview of the requirements for an inspection and repair system for alternating phase shifting masks. In order to get a better understanding of this printability behavior first simulations of defects using a 3D mask simulation tool were carried out and compared to the measurements. Several examples of quartz-repairs with different qualities are presented together with the influence on the aerial image.

Influence of 157-nm specific cleaning procedures on the quality of FIB repair depositions on reticles

An UV lamp cleaning system (172 nm) and a 157 nm laser have been used to irradiate Ga-based focused ion beam (FIB) repair depositions on 193 nm and 157 nm reticles. The thickness change of the depositions due to the irradiation has been measured using an atomic force microscope. For the 193 nm reticle additional transmission measurements were realized. These depositions are found to be highly resistant to UV lamp cleaning treatments of up to 18h (2mW/cm2, 1% O2). Extended tests were also done with 157 nm irradiation (9kJ/cm2, 1 - 10ppm O2). At the end of these tests a film loss of the depositions of 0.4nm /kJ/cm2 and a transmission change of 10% could be determined.

Investigation of fast and accurate reticle defect assessment methods using STARlight for chrome-on-glass reticle defects

Fast and accurate reticle defect assessment becomes increasingly important because wafer critical dimensions continue to shrink and mask inspection equipment has moved into the UV range thereby increasing the number of detected reticle defects. Defect size is not sufficient in determining if a defect prints or does not print and the threshold size for printing defects can vary broadly between 0.35 (lambda) /NA. At the low k1 factors required to print current technology feature sizes, correlation between reticle and wafer CDs ceases to be linear. The impact of reticle defects on CDs therefore, is more critical than for previous technologies and defect size, shape, and proximity to other features must be taken into consideration. Presented in this paper is an evaluation of different methods to determine the accuracy of imaging prediction for reticle defects, decreasing the time to results in a prediction environment by accelerating the decision process. These methods include printability based on aerial image and the in-line STARlight Contamination Printability Index.