Jan P. Heumann

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.

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.

Alternating phase-shift mask inspection using multiple simultaneous illumunation techniques

This paper discusses the challenges to alternating phase shift mask defect inspection and new approaches for phase defect detection using multiple illumination methods in conjunction with defect detection algorithm modifications. Die-to-die inspection algorithms were developed for the KLA-Tencor 365UV-HR (APS algorithm) and TeraStar SLF27 (TeraPhase algorithm) inspection systems based upon the use of simultaneous transmitted and reflected light signals. The development of an AltPSM programmed test vehicle is described and defect sensitivity characterization results from programmed phase defect reticles are presented. A comparison of the two approaches used for the different inspection systems is discussed. A comparison of TeraPhase to transmitted light only results from a programmed phase defect test mask shows improved phase defect detection results.

AltPSM Inspection Capability and Printability of Quartz Defects

Alternating phase shift masks (altPSMs) are a promising resolution enhancement technique to realize smaller design rules at the same lithography wavelength. Quartz defect inspection of altPSMs is challenging, as the optical contrast for defects within the quartz substrate is very small. AltPSM inspection capability was studied with different types of programmed test masks. The programmed quartz defects were characterized with a scanning electron microscope, an atomic force microscope and an aerial imaging microscope system. Finally a defect printability study was done. With the programmed test masks the performance of two altPSM inspection techniques was evaluated. Quartz defect detection was studied with respect to different pattern types and sizes. Quartz defect sensitivity was measured with respect to defect size as well as defect printability. It was found that quartz defect sensitivity with respect to defect size is constant for different pattern types, but decreases for decreasing line widths on 1:1 pitch line and space patterns. Whereas defect detection for the altPSM algorithms studied is governed by the defect’s extension perpendicular to the pattern line, defect printability is determined by the defect’s lateral area.

Defect printability and inspectability of halftone masks for the 90nm and 70nm node

This paper presents first results of a defect printability study for the 70nm and 90nm technology. Two 6% halftone test masks with dense line/space (l/s) and contact hole (CH) structures, containing programmed defects were exposed at different production illumination conditions. The resultant data was compared with respect to the mask defect sizes, the Aerial Image Measurement System (AIMS) values and the mask defect inspection sensitivity. As expected over-and under-sized features exhibity the highest printability and AIMS value intensity deviation. No difference was found in the lithographic behavior of dark and clear extension. Additionally (to the determination of the print critical AIMS values) the programmed defect masks were used for the evaluation of a KLA 52x inspection system. The performances of two detection pixels named P125 and P90 in combination with two inspection modes named die-to-die transmission (d2dT) and die-to-die reflective (d2dR) were investigated on 90nm and 70nm dense l/s and contact hole areas with respect to the print results. Over and under-sized small dense structures as well as dark and clear defects centered in a clear or dark structure are challenging for the new inspection tool. For dense contact hole arrays d2dR shows a better performance than d2dT.

Defect inspection of quartz-PSMs: taking a leap forward

Defect inspection of Quartz-PSMs is challenging, as the optical contrast for defects within the quartz substrate is small. The performance of three phase contrast algorithms is studied with a variety of defect test masks. For alternating phase shift masks key parameters such as optimum focus offset, defect sensitivity for different feature sizes, as well as defect sensitivity with respect to defect printability criteria are studied. In the studied range for two of the algorithms the defect sensitivity is independent of the feature size, whereas the third algorithm exhibited a decrease in sensitivity with decreasing feature sizes. In focus runs performed on large feature sizes a single optimum focus offset is observed, whereas for small feature sizes a two-path inspection using a positive and negative focus offset is found necessary. With respect to defect printability all critical 180° defects were found. For the newest of the three algorithms good inspectability of chrome-less PSMs is achieved.

Defect printability study with programmed defects on halftone reticles

Small structure sizes in the order of half the exposure wavelengths on wafers are nowadays accomplished with optical enhancement methods. Instead of COG the semi-transparent halfton reticles are used to reach a sufficient process window for the production of smaller memory products at low k1. In the semitransparent halftone material (MoSi) the intensity of the incident light is reduced to 6% and the phase is shifted by half of the wavelength (180 degree(s)). In this study halftone PSM for 248nm and 193nm wavelength with programmed defects of different sizes in lines/spaces (l/s) and brick stone structures were examined. With inspection, repair and print tests valid criteria for critical defect sizes were found. The defects were all analyzed with a Zeiss Aerial Image Measurement System (AIMS) and characterized with a mask SEM. Several defects were repaired using a FIB. Finally, this halftone PSM was printed and the defects were analyzed by a wafer SEM. The sizes of the programmed defects were distributed from printing to not printing. Critical defect sizes were clearly defined and the sensitivity of inspection tools for photomasks (KLA and Orbot Aris-i) could be checked.

Inspectability and printability of lines and spaces halftone masks for the advanced DRAM node

With decreasing pattern sizes the absolute size of acceptable pattern deviations decreases. For mask-makers a new technology requires a review, which mask design variations print on the wafer under production illumination conditions and whether these variations can be found reliably (100%) with the current inspection tools. As defect dispositioning is performed with an AIMS-tool, the critical AIMS values, above which a defect prints lithographically significant on the wafer, needs to be determined. In this paper we present a detailed sensitivity analysis for programmed defects on 2 different KLA 5xx tools employing the pixel P90 at various sensitivity settings in die-to-die transmitted mode. Comparing the inspection results with the wafer prints of the mask under disar illumination it could be shown that all critical design variations are reliably detected using a state-of-the-art tool setup. Furthermore, AIMS measurements on defects with increasing defect area of various defect categories were taken under the same illumination conditions as for the wafer prints. The measurements were evaluated in terms of AIMS intensity variation (AIV). It could be shown that the AIMS results exhibit a linear behavior if plotted against the square-root area (SRA) of the defects on the mask as obtained from mask SEM images. A consistent lower AIV value was derived for all defect categories.