Kaustuve Bhattacharyya

Process window impact of progressive mask defects, its inspection and disposition techniques (go / no-go criteria) via a lithographic detector

Progressive mask defect problem is an industry wide mask reliability issue. During the start of this problem when the defects on masks are just forming and are still non-critical, it is possible to continue to run such a problem mask in production with relatively low risk of yield impact. But when the defects approach more critical state, a decision needs to be made whether to pull the mask out of production to send for clean (repair). As this problem increases on the high-end masks running DUV lithography where masks are expensive, it is in the interest of the fab to sustain these problem masks in production as long as possible and take these out of production only when absolutely necessary; i.e., when the defects have reached such a critical condition on these masks that it will impact the process window. During the course of this technical work, investigation has been done towards understanding the impact of such small progressive defects on process window. It was seen that a small growing defect may not print at the best focus exposure condition, but it can still influence the process window and can shrink it significantly. With the help of a high-resolution direct reticle inspection, early detection of these defects is possible, but fabs are still searching for a way to disposition (make a go / no-go decision) on these defective masks. But it is not an easy task as the impact of these defects will depend on not only their size, but also on their transmission and MEEF. A lithographic detector has been evaluated to see if this can predict the criticality of such progressive mask defects.

Investigation of reticle defect formation at DUV lithography

Defect formation on advanced photomasks used for DUV lithography has introduced new challenges at low k1 processes industry wide. Especially at 193-nm scanner exposure, the mask pattern surface, pellicle film and the enclosed space between the pellicle and pattern surface can create a highly reactive environment. This environment can become susceptible to defect growth during repetitive exposure of a mask on DUV lithography systems due to the flow of high energy through the mask. Due to increased number of fields on the wafer, a reticle used at a 300-mm wafer fab receives roughly double the number of exposures without any cool down period, as compared to the reticles in a 200-mm wafer fab. Therefore, 193-nm lithography processes at a 300-mm wafer fab put lithographers and defect engineers into an area of untested mask behavior. During the scope of this investigation, an attenuated phase shift mask (attPSM) was periodically exposed on a 193-nm scanner and the relationship between the number of exposures (i.e., energy passed through the mask during exposures) versus defect growth was developed. Finally, chemical analysis of these defects was performed in order to understand the mechanism of this “growth”.

Formation and detection of subpellicle defects by exposure to DUV system illumination

As DUV lithography becomes more ubiquitous in the manufacture of semiconductors, the importance of detecting mask anomalies that can be attributed to the exposure of mask materials to 248 nm exposure becomes necessary. The requirement to find and eliminate the sources of these types of defects becomes even more important with low k1 lithography. The authors wish to report a new class of defects that can significantly impact mask performance and semiconductor chip manufacturing yields. This paper will discuss the techniques and defect detection systems used to identify the presence of these sub-pellicle (or pellicle-related) defects. Additionally, the mechanism of defect formation and micro-analytical results identifying both the composition and possible sources of the defects will be presented.

Reticle surface contaminants and their relationship to sub-pellicle defect formation

DUV lithography induced sub-pellicle particle formation continues to be a significant problem in semiconductor fabs. We have previously reported on the identification of various defects detected on reticles after extended use. This paper provides a comprehensive evaluation of various molecular contaminants found on the backside surface of a reticle used in high-volume production. Previously all or most of the photo-induced contaminants were detected under the pellicle. This particular contamination is a white “haze” detected by pre-exposure inspection using KLA-Tencor TeraStar STARlight with Un-patterned Reticle Surface Analysis, (URSA). Chemical analysis was done using Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) and Raman spectroscopy.

A New Generation of Progressive Mask Defects on the Pattern Side of Advanced Photomasks

The progressive mask defect problem is an industry-wide mask reliability issue. Even if masks are determined to be clean upon arrival from the mask supplier, some of these masks can show catastrophic defect growth over the course of production usage in the fab. The categories of defects that cause reticle-quality degradation over time are defined as progressive defects, commonly known as crystal growth, haze, fungus or precipitate. This progressive defect problem has been around for more than a decade and was observed at almost every lithographic wavelength. This problem is especially severe at 193nm lithography. Triggering the increased severity are shorter wavelength lithography - where the photons are highly energized - and the concurrent transition to 300 mm wafers, which require photomasks to endure more prolonged exposure as compared to 200 mm wafers. Both embedded phase shift masks (EPSMs) and chrome-on-glass masks are affected by progressive defects. These defects are generally found on the patterned surface underneath the pellicle (on clear, half-tone or chrome patterns), as well as on the backside surface of the masks. Past cases have indicated that this problem mainly starts on the scribes and borders, with emerging semi-transmissive contamination of ~100nm. These defects then propagate into the die area while growing in both size and opaqueness. Compositional analysis has shown that the majority of these defects are ammonium sulfate. However, since significant effort focused on the elimination of ammonium sulfate a new trend has emerged. Current studies show severe defect growth consists of organic contaminants (ammonium oxalate, cyanuric acid etc.) on half-tone edges and on chromium edges. The sources for progressive defect mechanisms are under investigation, though several candidates have been considered: maskmaking materials and process residues (mainly ammonium or sulfate ions), the fab environment, or the stepper environment. Controlling or balancing these sources may help to reduce the frequency at which these defects occur, but thus far has been unable to eliminate the problem. With each successive device shrink, the resultant changes in lithographic wavelength and processing within the mask fabrication facility and IC fab disrupt the fine balance among the above suspected defect sources, resulting in the return of catastrophic progressive defect growth. Due to this uncertainty, strict mask quality monitoring in the fab is essential. The ideal reticle quality control goal in a fab should be to detect any nascent progressive defects before they become yield limiting. Hence, the masks should be monitored on an established frequency that allows problem masks to be removed from production and sent for rework prior to impacting device performance and fab yield.

An investigation of a new generation of progressive mask defects on the pattern side of advanced photomasks

DUV lithography has introduced a progressive mask defect growth problem widely known as crystal growth or haze. Even when incoming mask quality is high, there is no guarantee that the mask will remain clean during its production usage in the wafer fab. These progressive defects must be caught early during production in the fabs. In the absence of a solution for the defect’s root cause, the ideal reticle quality control goal should be to detect and monitor any nascent progressive defects before they become yield limiting. Most of the work published so far has been focused on crystals on clear area (on the pattern surface) and on the backglass of the mask. But there is a new generation of growing defects: crystals that grow on the half tone (MoSi) film or on the chrome film, on the pattern side of the mask. It is believed that the formation mechanisms and rates are different for these new types of crystals. This work becomes more important with the impact of such defects’ instability on masks in volume production. The purpose of this investigation is to improve manufacturability of PSM’s through haze contamination reduction and to understand the impact and dependency of this contamination on die yield, on reticle lifetime, and on usage patterns.

High-resolution mask inspection in advanced fab

High resolution mask inspection in advanced wafer fabs is a necessity. Initial and progressive mask defect problem still remains an industry wide mask reliability issue. Defect incidences and its criticality vary significantly among the type of masks, technology node and layer, fab environment and mask usage. A usage and layer based qualification strategy for masks in production need to be adopted in wafer fabs. With the help of a high-resolution direct reticle inspection, early detection of critical and also non-critical defects at high capture rates is possible. A high-resolution inspection that is capable of providing necessary sensitivity to critical emerging defects (near edge) is very important in advanced nodes. At the same time, a way to disposition (make a go / no-go decision) on these defective masks is also very important. As the impact of these defects will depend on not only their size, but also on their transmission and MEEF, various defect types and characteristics have to be considered. In this technical report the adoption of such a high-resolution mask inspection system in wafer fab production is presented and discussed. Data on this work will include inspection results from advanced masks, layer and product based inspection pixel assignment, defect disposition and overall wafer fab strategies in day-to-day production towards mask inspection.

Evaluation of mask quality control methods addressing progressive haze issues

A traditional method of mask quality control in a fab has been wafer image qualification i.e., wafer inspection on printed monitor wafer or wafer inspection on production wafer. But recently many fabs that are using DUV lithography for low k1 process are experiencing the progressive defect growth challenge (such as crystal growth, haze, fungus, precipitate etc.) on their photomasks. The quality of some reticles will worsen over time due to this progressive defect problem on the mask. Hence it is important to detect such problems before they start impacting the device yield. An evaluation was constructed in a Japanese advanced logic fab to compare the performance between traditional image qualification methods and direct reticle inspection using TeraStar STARlight. The goal was to determine if the TeraStar STARlight inspection could provide the sensitivity required to give early warning of progressive defects before image qualification can detect these defects. Evaluation results show that TeraStar STARlight is the most effective method in a fab to provide early warning to a progressive defect growth on reticle that is likely to print later during mask life.

Printability impact of progressive defects: ammonium sulfate emulation study

In the relentless pursuit of device miniaturization and sustainable yield performance, resolution enhancement techniques (RET) such as optical proximity correction (OPC) and sub-resolution assist feature (SRAF) are identified as enabling technologies that fuel the industry. The introduction of advanced reticles, however, considerably augments the mask error enhancement factor (MEEF) where the growth of progressive defects or haze is accelerated by repeated laser exposure, and continues to be a source of reticle degradation threatening device yield. Previous investigations have identified ammonium sulfate, cyanuric acid and ammonium oxalate as the primary and most concerning species found in both mask shop and wafer fabs. In this work, magnesium sulfate is used to emulate crystal growth due to its identical optical properties to ammonium sulfate. A technique has been developed to deposit magnesium sulfate of varying concentrations onto chemically cleaned reticle surfaces. These defects are then inspected with a high resolution reticle inspection system enabled with MEEF detector Litho3. Upon inspection, defects are classified and analyzed with respect to their location relative to device geometry, optical transmission loss as well as the residing surface. Ammonium oxalate crystals are also deposited separately onto reticle surface to comprehend the impact of crystal type and population on defect printability. Compositional analysis are carried out using Raman spectroscopy and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) to correlate the amount of magnesium sulfate and ammonium oxalate crystals with transmission loss. Such emulation study of various crystal formulation mimics progressing stages of crystallization and allows a mechanistic understanding of crystal congregation, transmission loss and defect printability.

A Cost Model Comparing the Economics of Reticle Requalification Methods in Advanced Wafer fabs

Progressive mask defect (such as crystal growth, haze etc.) continues to threaten the industry (especially at 193 nm lithography) [1]. This drives the need for wafer fab mask inspection [2] which can be achieved via two methods. The first method is indirect, commonly known as image qualification[4], where a mask is being exposed followed by the inspection of the printed wafer to detect if there is any repeater on the wafer or not. The other method of mask inspection is direct mask inspection (such as STARlighttrade). Understanding the economics involved in wafer fab mask inspection is important as litho- cluster cycle time will drive the economics for fabs even harder at nodes 65 nm and below and any methods or techniques that can reduce this litho- cluster cycle time need to be looked at seriously. It was one of the goals for this technical report to evaluate the impact (if any) of these wafer fab mask inspection methods on litho-cluster cycle time.