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) <sup>, , [3, </sup>. 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) <sup></sup>, 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 <sup></sup>. 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<sup> , , </sup> report a growth of SiO<sub>2</sub>, 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/cm<sup>2</sup> (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
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.
The advanced Jeol JBX9000MVII 50kV electron-beam lithography system has been successfully installed at DNP Photomask Europe and timely qualified for the 90nm technology node. The overall performances of this writing tool have thoroughly been assessed on positive and negative tone chemically amplified resists (CARs), fully exploiting the advanced proximity effect correction (PEC) capabilities of the system and carefully optimizing the overall process.
The reported results show the machine capabilities in terms of global and local pattern placement and CD accuracy, CD linearity, pattern fidelity, along with data on some of the most demanding model-based OPC validation patterns. Details on process characterization and tuning effectiveness on resist and chrome are shown, including the PEC approach.
Based on the stringent metrology correlation achieved with DNP Japan manufacturing site, the data show a one-to-one compatibility with the sister tool installed there, even on the most critical OPC structures. Consequently, the complete product interchangeability between the two manufacturing sites has been achieved.