Darren Taylor

Comprehensive defect detection featuring die-to-database reflected light inspection

With ever shrinking k1 lithography, overall reticle quality is paramount to ensure high quality image transfer. State-of-the-art reticle inspection systems play two vital roles in reticle manufacturing: quality assurance and manufacturing process feedback. For quality assurance, the system must be capable of detecting all defects of interest to the end user - defects that repeatedly print on wafer, and also those that may reduce the lithography process window. For process monitoring and improvement, the system must be capable of detecting defects at or near the manufacturing limits of mask manufacturing. In order to meet both needs, an inspection system must detect all defect types including pattern errors and contaminates on all mask surfaces including chrome, quartz, and shifter materials. A new advanced inspection method compares both transmitted and reflected light images to the design database. This comprehensive inspection method detects numerous defects that would be missed in a transmitted or reflected only inspection tool. In this study we have tested a new method for detecting reticle defects. Inspection results will be shown from a programmed defect test vehicle as well as a production reticle.

Implementation of a transparent etch stop layer for an improved alternating PSM

Alternating phase shift masks (alt. PSM) are emerging as an attractive resolution enhancement technique. Although alt. PSM is a technique that clearly improves resolution, there are some inherent disadvantages that are induced by the manufacturing process. Intensity imbalance, phase non-uniformity and quartz defects diminish the performance of an alternating PSM. Many of these disadvantages can be a result of imprecise quartz etching. By implementing a transparent etch stop layer, these deficiencies can be minimized. The etch stop layer ensures that all of the quartz is etched and that over-etching will not induce a phase-shift error. This produces improved phase uniformity and eliminates quartz defects. The etch stop layer also has the ability to improve the image intensity balancing by reducing the intensity through the zero degree region. This paper discusses the advantages and manufacturability of alt. PSM using a transparent etch stop layer.

Dark-field high-transmission chromeless lithography

Dark field (i.e. hole and trench layer) lithographic capability is lagging that of bright field. The most common dark field solution utilizes a biased-up, standard 6% attenuated phase shift mask (PSM) with an under-exposure technique to eliminate side lobes. However, this method produces large optical proximity effects and fails to address the huge mask error enhancement factor (MEEF) associated with dark field layers. It also neglects to provide a dark field lithographic solution beyond the 130nm technology node, which must serve two purposes: 1) to increase resolution without reducing depth of focus, and 2) to reduce the MEEF. Previous studies have shown that by increasing the background transmission in dark field applications, a corresponding decrease in the MEEF was observed. Nevertheless, this technique creates background leakage problems not easily solved without an effective opaqueing scheme. This paper will demonstrate the advantages of high transmission lithography with various approaches. By using chromeless dark field scattering bars around contacts for image contrast and chromeless diffraction gratings in the background, high transmission dark field lithography is made possible. This novel layout strategy combined with a new, very high transmission attenuating layer provides a dark field PSM solution that extends 248nm lithography capabilities beyond what was previously anticipated. It is also more manufacturing-friendly in the mask operation due to the absence of tri-tone array features.

Feasibility study of embedded binary masks

Towards hyper-NA lithography, the mask blank and mask topography have the opportunity to be optimized for imaging performance. At the resolution limit of hyper-NA imaging, depth of focus and MEEF become critical for conventional mask stacks. Although conventional binary masks (BIM) are the simplest and the most cost-effective to manufacture, other mask types can provide better imaging performance. This study explores the feasibility and imaging performance of an embedded binary mask (EBM). The EBM emphasizes the simple binary manufacturing process with the application of an additional transparent layer. Two types of EBMs, topographic and planar, were evaluated. The mask diffraction properties are studied by both measurements using an ellipsometer (Woollam VUV-VASE) and simulations using Solid-E 3.2.0.2 (Sigma-C). In this first phase, the imaging performance is assessed by rigorous simulations for three different illumination conditions (cross-quad, quasar and annular). By comparing metrics such as contrast, NILS, MEEF, and process windows, simulations determined that an optimized topographic EBM has a better overall through-pitch imaging performance than a conventional binary mask. This preliminary investigation suggests that an embedded binary mask may be considered as an RET option for hyper-NA imaging improvement.

Chromeless phase lithography reticle defect inspection challenges and solutions

CPLTM Technology is a promising resolution enhancement technique (RET) to increase the lithography process window at small feature line widths. Successful introduction of a reticle based RET needs to address several reticle manufacturing areas. One key area is reticle inspection. A CPL reticle inspection study has been completed and a best known methodology (BKM) devised. Use of currently available inspection tools and options provides a robust solution for die-to-die inspection. Die-to-database inspection challenges and solutions for optically completed CPL reticles are discussed. Core to the devised BKM is the concept of in-process inspections where the highest sensitivity inspection may not necessarily be performed after the last manufacturing step. The rationale for this BKM is explained in terms of actual manufacturing process flow and most likely defect sources. This rationale also has implications for programmed defect test mask designs in that the choice of defect types need to be linked to a plausible source in the manufacturing process. Often, the choice of a programmed defect type ignores the fact that a naturally occurring defects origin is early in the manufacturing process and would be detected and either repaired or the reticle rejected before subsequent manufacturing steps. Therefore, certain programmed defect types may not be representative of what should be expected on a production mask. Examples such defects are discussed.

Automatic inspection tool sensitivity with characterization of AAPSM defects

As AAPSM becomes more widely utilized, the need for defect inspection sensitivity becomes more critical. In addition, accurate defect characterization must be performed to encompass new effects caused by glass defects. Historically, defect size and position have been the two characteristics that were examined when determining inspection tool sensitivity. Because of the nature of AAPSM defects, phase is a factor that must be taken into account. This experiment utilizes two distinct forms of defect characterization -- SEM sizing, and surface profilometry. Programmed defect test masks were manufactured for phase shifting properties at both 248nm and 193nm exposure wavelengths. The defects were also etched at multiple depths resulting in a variety of phase angle errors. Utilizing the two characterization methods mentioned above, the automatic defect inspection tools sensitivity on multiple programmed defects will be investigated.

Improved phase uniformity control using a new AAPSM etch stop layer technique

One of the major challenges in alternating aperture phase shift mask (AAPSM) production is the variability of the glass etch rate as a function of exposed area (pattern loading) on the mask. The lack of an endpoint system means that the etch is entirely based on time, and the result is increased variability in the mean etch depth as well as decreased yields against ever tightening phase specifications. If a transmissive etch stop layer were placed underneath an appropriate thickness of glass to obtain a 180-degree phase shift, the result is a forced endpoint at exactly 180 degrees every time. Such a film system also leads to many process advantages over conventional AAPSM processes. This paper discusses the film stack deposition and maskmaking at Photronics, Inc. and details the process advantages of using AAPSM blanks with etch stop layers.

Improved method for measuring and assessing reticle pinhole defects for the 100-nm lithography node

With the approach of the 100nm-lithography node, an accurate and reliable method of measuring reticle pinhole defects becomes necessary to assess the capabilities of high-end reticle inspection equipment. The current measurement method of programmed defect pinholes consists of using a SEM. While this method is repeatable, it does not reliably represent the true nature of a pinhole. Earlier studies have suggested that since the SEM images only a top down view of the pinhole, the measurement does not accurately account for edge wall angle and partial filling which both reduce pinhole transmission and subsequent printability. Since wafer lithography and reticle inspection tools use transmitted illumination, pinhole detection performance based on SEM measurements is often erroneous. In this study, a pinhole test reticle was manufactured to further characterize the capabilities of a transmission method to measure pinholes.

Imaging 100-nm contacts with high-transmission attenuated phase-shift masks

This study explores the capability of printing 100 nm contacts through the use of 9% and 15% attenuated phase shift masks and a 0.75 NA 193 nm scanner. The mask designs targeted simultaneous solutions for 100 nm contacts at pitches from 200 nm to 300 nm. The two masks were successfully manufactured from experimental MoSiON embedded-attenuated phase shift mask (EAPSM) blanks. The 100 nm contacts were successfully printed with a depth of focus (DOF) from 0.1-0.7 μm. Overlapping process windows were not achieved but were possible upon adjustment of the mask biases. The observed mask error enhancement factor (MEEF) was approximately 3 for the 220 nm pitch. Side lobe printing was not observed for either mask.

Improved method for measuring and assessing reticle pinhole defects

With the increased resolution of todays lithography processes, reticle pinhole defects are much more printable. Measuring the size of small pinholes using the current SEM method often produces erroneous results when compared to pinhole energy transmission. This is mainly due to the fact that SEMs do not accurately account for edge wall angle and partial filling which can dramatically reduce the pinhole transmission and subsequent printability. Since reticle inspection tools, like wafer steppers and scanners, use transmitted illumination, pinhole detection performance based upon top surface SEM defect sizing is often erroneous for small pinhole diameters. This study first uses simulation to predict printability. Then, a pinhole test reticle is developed with a variety of sub-200nm pinholes. The reticle pinholes are measured with an improved method incorporating transmission and imaged to wafer in order to assess printability.