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Chrome migration or aging phenomenon is known for 193nm binary photomasks since a few years. 193nm irradiations and time generate an oxide growth on chrome sidewalls and then cause a non-uniform increase of critical dimensions (CD) [1], [2], [3, [4]. If not prevented or detected early enough, wafer fabs are likely to face process drifts, defectivity issues and even lower yield on wafers in the worst cases. Fortunately, some solutions have been put in place in the industry. A standard cleaning and repel service at the maskshop has been demonstrated as efficient to remove the grown materials and get the mask CD back on target. Some detection methods have been already described in literature, such as wafer CD intrafield monitoring (ACLV) [1], giving reliable results but also consuming additional SEM time with less precision than direct reticle measurement. Another approach is to monitor the CD uniformity directly on the photomask, concurrently with defect inspection for regular requalification to production for wafer fabs [5]. This enables ultimately to trigger the preventive cleanings rather than on predefined thresholds. However, may the 193nm Phase Shift Masks (PSM) be impacted too? In other words, should wafer fabs pay attention to this form of aging? Indeed, some publications [6], [7], [8] report a growth of SiO2, leading to the development of a high duration MoSi (modification of MoSi composition). This study will characterize the aging behaviour on a 193nm PSM contact hole layer, 40nm logic technology node. During this study, the aging phenomenon has been accelerated with the use of a test bench, to reach a CD increase up to 11nm after a cumulated exposure dose of 10kJ/cm2 (equivalent to exposures of >32,000 wafers 300mm). Two dice were compared, one kept as reference without any exposure, whereas the other die was aged on the accelerated test bench. Exhaustive characterization has been performed, with CD measurements on the mask and on wafers, evaluation of lithography process windows for usual patterns and most critical features (Optical Proximity Correction hotspots). It appears that despite a consistent CD increase on the mask, the impact on wafer can be neglected, at least at this amount of exposures. Aerial CD were also analysed through a Zeiss WLCDTM to enable a prediction of wafer impact. An advanced inspection tool (KLA-Tencor X5.2 model) has been challenged as an inline monitoring method to detect the aging degradation on PSM. The Intensity Critical Dimension Uniformity option (iCDUTM) was firstly developed to provide feed-forward CDU maps for scanners intrafield corrections, from arrayed dense structures on memory masks. Due to layout complexity and differing feature types, CDU monitoring on logic masks used to pose unique challenges. CDU monitoring on logic masks is now available, the latest Delta-Die and Delta-Time options gives all the needed information, as shown in this paper. In this study, iCDU has demonstrated its ability to catch a slight degradation of CD uniformity. In the end, this study shows evidences that standard cleanings used in maskshops cannot recover the mask back to its original CD. Finally, Transmission Electron Microscopy (TEM) was used to confirm the chemical nature of the grown material on sidewalls. TEM cuts provide a comparison between a production mask (aging over many years in production) and the test mask (accelerated aging on a test bench).

193nm binary photomasks are still used in the semiconductor industry for the lithography of some critical layers for the nodes 90nm and 65nm, with high volumes and over long periods. However, these 193nm binary photomasks can be impacted by a phenomenon of chrome oxidation leading to critical dimensions uniformity (CDU) degradation with a pronounced radial signature. If not detected early enough, this CDU degradation may cause defectivity issues and lower yield on wafers. Fortunately, a standard cleaning and repellicle service at the mask shop has been demonstrated as efficient to remove the grown materials and get the photomask CD back on target.Some detection methods have been already described in literature, such as wafer CD intrafield monitoring (ACLV), giving reliable results but also consuming additional SEM time with less precision than direct photomask measurement. In this paper, we propose another approach, by monitoring the CDU directly on the photomask, concurrently with defect inspection for regular requalification to production for wafer fabs. For this study, we focused on a Metal layer in a 90nm technology node. Wafers have been exposed with production conditions and then measured by SEM-CD. Afterwards, this photomask has been measured with a SEM-CD in mask shop and also inspected on a KLA-Tencor X5.2 inspection system, with pixels 125 and 90nm, to evaluate the Intensity based Critical Dimension Uniformity (iCDU) option. iCDU was firstly developed to provide feed-forward CDU maps for scanner intrafield corrections, from arrayed dense structures on memory photomasks. Due to layout complexity and differing feature types, CDU monitoring on logic photomasks used to pose unique challenges.The selection of suitable feature types for CDU monitoring on logic photomasks is no longer an issue, since the transmitted intensity map gives all the needed information, as shown in this paper. In this study, the photomask was heavily degraded after more than 18,000 300mm wafers exposed and the cleaning brought it back almost to its original state after manufacture. Wafer CD, photomask CD and iCDU results can be compared, before and after a standard mask shop cleaning. Measurement points have be chosen in logic areas and SRAM areas, so that their respective behaviours can be studied separately. Transmitted maps before and after cleaning were analysed in terms of CD shift and CDU degradation. The delta map shows a nice correlation with photomask CD shift. iCDU demonstrated the capability to detect a reliable CD range degradation of 5nm on photomask by a comparison between a reference inspection and the current inspection. Die to die inspection mode provides also valuable data, highlighting the degraded chrome sidewalls, more in the photomask centre than on the edges. Ultimately, these results would enable to trigger the preventive cleanings rather than on predefined thresholds. The expected gains for wafer fabs are cost savings (adapted cleanings frequency), increased photomask availability for production, longer photomask lifetime, no additional SEM time neither for photomask nor on wafer.

193nm binary photomasks are still used in the semiconductor industry for the lithography of some critical layers for the nodes 90nm and 65nm, with high volumes and over long period. These 193nm binary masks seem to be well-known but recent studies have shown surprising degrading effects, like Electric Field induced chromium Migration (EFM) [1] or chromium migration [2] [3] . Phase shift Masks (PSM) or Opaque MoSi On Glass (OMOG) might not be concerned by these effects [4] [6] under certain conditions. In this paper, we will focus our study on two layers gate and metal lines. We will detail the effects of mask aging, with SEM top view pictures revealing a degraded chromium edge profile and TEM chemical analyses demonstrating the growth of a chromium oxide on the sidewall. SEMCD measurements after volume production indicated a modified CD with respect to initial CD data after manufacture. A regression analysis of these CD measurements shows a radial effect, a die effect and an isolated-dense effect. Mask cleaning effectiveness has also been investigated, with sulphate or ozone cleans, to recover the mask quality in terms of CD. In complement, wafer intrafield CD measurements have been performed on the most sensitive structure to monitor the evolution of the aging effect on mask CD uniformity. Mask CD drift have been correlated with exposure dose drift and isolated-dense bias CD drift on wafers. In the end, we will try to propose a physical explanation of this aging phenomenon and a solution to prevent from it occurring.

In microelectronic industry, mainly from the 65nm node, phase shift photomasks (PSM) are increasingly used for critical layers, optical properties of the shifter (MoSi) giving a better control of critical dimensions (CD) in photoresist. Fabusers and maskshops have committed on specifications that restrict phase and transmission variations within certain limits. The goal of this study is to validate and/or update these previously admitted limits. A specific test reticle has been jointly designed with several structures representative of 65nm and 45nm nodes and then manufactured with a specific process in order to voluntarily degrade the phase and transmission uniformity within the mask. Knowing all CD and their related phase and transmission on mask, CD variations seen on wafers have been directly linked to phase and transmission variations. In parallel, rigorous simulations have been performed using Panoramic software in order to predict effects of phase and transmission variations on wafer. This reticle has been also used for early studies to evaluate the impact of phase and transmission variations on optical proximity correction (OPC) model.

With the optimization of sulfate-free cleaning the issue of haze under pellicle was almost eliminated. In consequence, current reasons for mask repelliclization needs are moving from pattern issues to more gross problems on back glass. Moreover, the longer life of photomasks allows a new problem to appear as growing defects on back glass, commonly ascribed to environmental conditions at user's site. The commonality of these problems is being independent on mask complexity and substrate. In order to avoid the criticalities of pellicle removal and cleaning treatment as well as the cost of necessary inspection after new pellicle application, the best solution is cleaning only the backside of the mask, provided that integrity of pellicle and pattern on front side are preserved In this article we present the results obtained by the use of the Mask cleaner DE050019TM on several cases. The efficiency of the treatment was assessed in terms if removal capability on different kinds of contaminations, either from use or mask aging. Pattern inspections were conducted in order to assess ESD robustness. Ionic residues were checked by IC aimed to compare with standard cleanings. This methodology demonstrated to be capable of maintaining a Particle Removal Efficiency>97% on all kinds of contaminations, without any damage to pellicle or harm to patterns, still maintaining residual ions at the same level as after cleaning by standard tools.

The paper describes a new approach of evaluating isolated opaque defects, as well as CD-like defects on hole layer, using features available on the inspection tool. This eliminates further verifications on specific tools, which would result in their overloading and in time consuming, with a potential negative impact on the delivery time of any product going through such processes. In the first case the method consists of associating the effect of a cluster of assist bars to that of isolated opaque defects, considering their size and position on the layout of the mask. In case of CD-like defects on holes the evaluation is based on a thorough characterization of the performance of the Litho2 detector of the Terascan T576 and its further verification with the AIMS readings.