Dimensions on mask continue to shrink to keep up with the ITRS roadmap. This has implications on the material of
choice for the blanks. For example, the new binary OMOG stack (Opaque MOSi on Glass) was successfully introduced
to meet the mask specifications at the 32nm technology node. Obviously 193-nm optical lithography will be further used
in production at even higher NA and lower k1 emphasizing, for example, the impact on wafer of any electromagnetic
field migration effects. Indeed, long term radiation damage inducing CD growth and consequently, device yield loss, has
already been reported [1, 2]. This mechanism, known as Electric Field induced Migration of chrome (EMF) often
shortens the mask's lifetime.
Here, a study was conducted to investigate the impact of intensive ArF scanner exposure both on final wafer and mask
performances. The Si printed wafers measured with top-down CD-SEM were characterized with respect to CD
uniformity, linearity, Sub Resolution Assist Feature (SRAF) printability through process window, MEEF, DOF, and
OPC accuracy. The data was also correlated to advanced mask inspection results (e.g. AIMSTM) taken at the same
location. More precisely, this work follows a preliminary study [1] which pointed out that OMOG is less sensitive to
radiation than standard COG (Chrome On Glass). And, in this paper, we report on results obtained at higher energy to
determine the ultimate lifetime of OMOG masks.
As mask dimensions continue to shrink to meet the ITRS roadmap and with the extension of 193 nm immersion
lithography, the masks are affected by electromagnetic field at high NA. Absorber degradation is regularly reported
under long term 193 nm exposures in the subwavelength diffraction regime. The damage mechanism known as Electric
Field induced Migration of chrome (EFM) partly contributes to the lifetime reduction of advanced masks. The EFM
results in a progressive alteration of the Critical Dimension (CD), CD uniformity (CDU) degradation and assist features.
This study evaluates the impact and the rate of absorber degradation due to an intensive ArF irradiation on assist features
and its influence on the through pitch process window for sub-45 nm technology nodes. Lithographic performance is
characterized after cumulative reticle aging stages. The aging test exposures are carried out directly on 193 nm scanner
to duplicate the production environment. The analysis of printed wafers is correlated to advanced mask inspection
(AIMSTM). This paper reports results on irradiation damage sensitivity on two types of reticles: conventional 6%
attenuated PSM and new binary material OMOG (Opaque MoSi On Glass) reticle. Test patterns have been generated
with and without a set of Optical Proximity Corrections (OPC) model calibration structures based on 45nm down to
28nm half-pitch design. The combination of metrology measurements used in this work between printed wafers and
reticles enables to define accurately the impact of mask damage caused by EFM effects on various test patterns and CD
evolution and highlight some trends about advances masks aging phenomenon.
The mask error budget continues to shrink with shrinking DRAM half pitch and MPU gate size year by year. The ITRS
roadmap calls for mask CDU to be cut in half by 2014[1]. Both mask maker and mask user must take advantage of
various mask properties, OPC strategies and resolution enhancement techniques to drive improvements. Mask material
selection impacts both lithographic performance and mask manufacturability. In turn mask material properties and
manufacturing techniques impact our ability to meet the technology roadmap. Studies have shown the advantages of
polarized light[2,3] as well as the impact of various mask materials on high NA lithography[4]. In this paper we select the
recently introduced binary mask material made from a MoSi absorber called Opaque MoSi On Glass (OMOG) for
comparison with the conventional 6% att. PSM and 20% att. MoSi PSM. Through simulation and wafer prints, we
optimized mask feature from viewpoint of MEEF and maximum exposure latitude (EL). The MoSi att. PSMs suffer from
higher MEEF, which is attributed to the negative effect of TE polarization for mask duty cycle of 50% for 50 nm half
pitch and below. Therefore a lower mask duty cycle is required for att. PSM to bring the MEEF performance back to
acceptable levels. Experimental results confirm simulation results. As a result of the lower mask duty cycle, the att. MoSi
PSMs exhibit poor Sub Resolution Assist Feature (SRAF) printability. On the contrary, the MoSi binary mask delivers
both acceptable MEEF and acceptable SRAF printing performance. Moreover, we found that the mask structure impact
of OMOG to wafer CD is smallest among three masks. OMOG gives the best combination of lithographic performance
and delivery compared to the MoSi att. PSMs.
Full-field printing on the ASML Alpha Demo Tool, followed by wafer inspection on a KLA-T 2800, is used to qualify
typical defectivity levels of EUV reticles. Mask defects are found as repeaters among multiple dies on wafer. The
uniform pattern consists of dense lines and spaces. In a first reticle with 40nm linewidth, high levels of natural defects
have been found of which a relatively large share was considered as multilayer (ML) type defects, because they printed
as rings. Simulation of ML defects could explain this printing behavior as a function of height, size and slope. The main
parameter determining the printing behavior of a ML defect is its height. A local distortion of the upper part of the ML,
as thin as ~2nm can already print. On-reticle analysis of the ring defects by SEM showed that the defects are present on
the absorber, which already explains the printing result. Yet, still several other defects were found to print on the wafer,
whereas they were not visible on the reticle and considered local distortions of the ML. Printing results with a second
version of the mask that additionally includes programmed multilayer defects with 3nm height confirmed the
pronounced printing impact of ML defects as they were simulated. Encouragingly low numbers of natural defects have
been found on a third reticle. With this reticle also a first correlation has become possible between the defect maps
obtained from wafer inspection, (direct) mask inspection, and blank inspection. This is a viable method to highlight
potential gaps between the capability of these tools and printability of defects.
The development of semiconductor process for 32nm node is in progress. Immersion lithography has been introduced as
an extension of 193nm lithograpy. In addition, DPL (Double patterning lithography) is becoming a strong candidate of
next generation lithography. The extension of optical lithography increases more mask complexity and tighter
specification of photomasks.
CD performance is the most important issue in the advanced photomask technology. However, it is expected that
conventional mask cannot satisfy the required mask specifications for 32nm node and beyond. Most of CD errors are
contributed to the dry etching process. Mask CD variation is greatly influenced by the loading effect from dry etching of
the absorber.
As the required accuracy of the mask arises, Cr absorber thickness has been gradually thinner. CD linearity with the
thinner Cr absorber thickness has better performance. However, it is difficult to apply thinner Cr absorber thickness
simply under the condition of OD > 3, which is needed for wafer printing. So, we adopted MoSi absorber instead of
conventional Cr absorber, because MoSi absorber has less micro and global loading effect than that of Cr absorber. By
using MoSi absorber, we can reduce Cr thickness as a hardmask. The thinner Cr hardmask allows for reduce resist
thickness and become same condition for conventional EB resist lithography.
The lithography performances were confirmed by the simulation and wafer printing. The new MoSi absorber mask
behaves similar to the conventional Cr absorber mask.
The adoption of super thin Cr as a hardmask made it possible to reduce resist thickness. By the application of the thin
resist and the latest tools, we'll improve the mask performance to meet the 32 nm generation specification.
With the use of 193nm lithography, haze growth has increased and become a critical issue for photomask suppliers and wafer fabs. Currently, the industry uses various test methods to measure known contributions to crystal growth, such as ion chromatography of cleaning residues and environmental monitoring in steppers. The understanding of the conditions that create haze is limited to end user photomask lifetime experience, which is gathered under varying environmental conditions. A better method to understand the formation of haze is to create a controlled environment and vary experimental conditions. Once experimental factors are understood, product reliability can be verified through end-user feedback. A custom excimer laser test system capable of 193nm and 248nm wavelengths was built to accelerate haze growth and to better understand haze formation. A photomask is enclosed in a test chamber where the environmental atmosphere and exposure conditions are controlled and monitored throughout testing. The system is used to test various elements important to mask fabrication and use, including materials, mask fabrication processes, and environmental operating conditions. This paper details the investigation of haze performance with commercially available pellicles using controlled environmental conditions and varying exposure parameters, such as pulse rate, energy density, and exposure dose. Using this methodology, the conditions that create haze growth were identified.
In the recent past Deep Ultra Violet (DUV) Laser generated photomasks have gained widespread acceptance for critical and semi-critical applications in semi-conductor lithography. The advent of stable, highly capable, single-layer Chemically Amplified Resist (CAR) processes has made fabrication of this type of mask very robust in today's mask manufacturing environment. This platform affords mask makers benefits of the highly parallel architecture available in today's DUV Laser pattern generators - providing excellent cost and cycle time advantages when compared with alternative leading-edge processes using 50 KeV VSB e-beam systems. To date literature on this topic has focused mostly on characterization and optimization of DUV mask making processes. Meanwhile treatment of the resultant aerial image for critical litho applications has been largely ignored. In this paper details of the aerial image produced using DUV Laser generated photomasks will be detailed. Both 248nm and 193nm source printing with multiple types of illumination will be discussed. Details of a print test comparison performed on photomasks from two popular mask lithography platforms in use today; DUV, and 50 KeV VSB, will be documented. Finally, the most recent process improvements achieved in DUV Laser mask fabrication will be detailed. Special attention will be given to the impact of these enhancements on image quality.
Reticle defects are one leading source of yield loss. This paper is a continuation of work begun to track defect sources to the process steps that generate them. Prior work was done on a common electron beam resist. This study will examine a DUV optical chemically amplified resist.
The semiconductor industry continues to aggressively shrink linewidths and manufacture more closely spaced patterns to improve power and speed performances, as well as increase die per wafer counts. Current development is near 100 nm and high volume manufacturing is commonly near 150 nm. These smaller linewidths and more dense patterns are hampered with new populations of defects that were previously unimportant second or third order effects. As a result, new defects of interest must specifically be investigated, detected, and prevented on the reticles that hold master images of what is being printed. This is often difficult because reticle manufacturing, reticle defect detection, and reticle end-usage (manufacturer) typically spans three corporations. If defects occur on wafers that were not detected with current reticle inspection capabilities, there must be correlation from the printed wafer back to the reticle to assist the reticle supplier in locating the defect and working towards eliminating the problem. In some cases, photo defects may not impact device functionality. As a result closed-loop analyses and actions, which determine whether an event actually causes yield loss, must be developed into the business practices of multiple companies. This paper reports on an inter-company process (ICP) involving Cypress Semiconductor Corporation, Dupont Photomask Incorporated, and KLA-Tencor. The ICP evaluates reticle defects on wafers, their impact on yield, and the transfer of the defect information back to the reticle vendor in an effort to improve overall reticle quality for a 150 nm and beyond semiconductor manufacturing fab.
Repair and printability of 193nm alternating aperture phase shift masks have been studied in detail in an effort to understand the overall production capability of these masks for wafer production at the 100nm node and below.
As reticle enhancement techniques (OPC, PSM) become more commonly used in multi-masking levels for 130nm node and below, the need for a better and more precise reticle specification will be even greater. OPC and sub-nominal assist feature like scattering bar represent a challenge for mask metrology tools. Consequentiy, defect inspection for advanced reticles has become a gating step for both Mask shops and Wafer Fabs alike. In this paper, a binary defect test reticle with and without OPC/SB features, manufactured by Dupont Photomask’s Reticle Technology Center, was used for the study. The reticle was made using the ALTA 3500 Laser Writer Tool and dry chrome etch process. Reticle inspection results from KLA363UV for 0. 13 um design rule patterns in both Clear and Dark polarities, with bump, divot and pinhole programmed defects between 0.1 and 0.6 um sizes (at 4x) were shown. These results were then compared to the wafer printability using ArF 193nm lithography. The impact of each defect type on 0.13 um Isolated, Semi-Dense and Dense lines was evaluated for 10% CD tolerance to provide an assessment on the KLA363UV inspection tool capability. A set of edge defects (bump and divot) and point defect (hole between line ends) from 0.1 to 0.6 um on the reticle (4x) were repaired using the Seiko SIR-3xxx tool at DPI-RTC. The printability of repairs on wafer at 193nm exposure wavelength was evaluated. CD of repaired features was compared to that of nominal feature for both reticle and resist wafer to evaluate the repair effectiveness. Finally, a defect specification for 0.13 um design rule binary reticle using ArF lithography is discussed.
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