KEYWORDS: Sensors, Signal detection, Photomasks, Algorithm development, 3D metrology, Polonium, Metrology, Scanning electron microscopy, Electron beams, Detection and tracking algorithms
A new metrology method for CD-SEM has been developed to measure the side wall angle of a pattern on photomask. The
height and edge width of pattern can be measured by the analysis of the signal intensity profile of each channel from multiple
detectors in CD-SEM.
The edge width is measured by the peak width of the signal intensity profile. But it is not possible to measure the accurate
edge width of the pattern, if the edge width is smaller than the primary electron beam diameter. Using four detectors, the
edge width can be measured by the peak width which appears on the subtracting signal profile of two detectors in opposition
to each other. Therefore, the side wall angle can be calculated if the pattern height is known.
The shadow of the side wall appears in the signal profile from the detector of the opposite side of the side wall.
Furthermore, we found that there was the proportional relation between pattern height and the shadow length of the signal on
one side.
This paper describes a method of measuring the side wall width of a pattern and experimental results of the side wall angle
measurements.
Influence of the prominent charging effect on the precision of measuring EUV mask features using CD-SEM was studied.
The dimensions of EUV mask features continuously measured by CD-SEM gradually varied because of the charging.
The charging effect on the measured CD variation mainly consists of three factors: 1) shift of the incident points of
primary electrons deflected by the surface charge, 2) distortions of the profiles of secondary electron signal intensity
caused by the deflection of the secondary electrons, 3) deviation of the maximum slope points of the secondary electron
signal intensity due to the variation of the image contrast. For those three factors described above, how the material
constant affect the CD variation measured by CD-SEM is discussed.
KEYWORDS: Sensors, Atomic force microscopy, Signal detection, Photomasks, Etching, 3D metrology, Tantalum, Critical dimension metrology, Scanning electron microscopy, Extreme ultraviolet
The Multiple Detector CD-SEM acquires the secondary electron from pattern surface at each detector. The 3D shape
and height of mask patterns are generated by adding or subtracting signal profile of each detector. In signal profile of the
differential image formed in difference between left and right detector signal, including concavo-convex information of
mask patterns. Therefore, the 3D shape of mask patterns can be obtained by integrating differential signal profile. This
time, we found that proportional relation between pattern height and shadow length on one side of pattern edge. In this
paper, we will report experimental results of pattern height measurement. The accuracy of measurement and side wall
angle dependency are studied. The proposal method is applied to OMOG masks.
During the development of optical lithography extensions for 32nm, both binary and attenuated phase shift Reticle
Enhancement Technologies (RETs) were evaluated. The mask blank has a very strong influence on the minimum feature
size and critical dimension (CD) performance that can be achieved on the finished reticle and can have a significant
impact on the ultimate wafer lithographic performance. Development of a suitable high resolution binary mask making
process was particularly challenging. Standard chrome on glass (COG) binary blanks with 70 nm thick chrome films
were unable to support the required minimum feature size, linearity, and through pitch requirements. Two alternative
mask blank configurations were evaluated for use in building high resolution binary masks: a binary (BIN) mask blank
based on the standard attenuated PSM blank and an Opaque MoSi on Glass (OMOG) mask blank consisting of a newly-
developed opaque MoSi [1]. Data comparing the total process bias, minimum feature size, CD uniformity, linearity,
through pitch, etch loading effects, flatness, film stress, cleaning durability and radiation durability performance of the
different binary and attenuated PSM mask blanks are reported. The results show that the new OMOG binary blank offers
significant mask performance benefits relative to the other binary and attenuated PSM mask blanks. The new OMOG
blank was the opaque mask blank candidate most capable of meeting 32nm binary mask fabrication requirements..
As optical lithography is extended for use in manufacturing 45 nm devices, it becomes increasingly important to
maximize the lithography process window and enable the largest depth of focus possible at the wafer stepper.
Consequently it is very important that the reticles used in the wafer stepper be as flat as possible. The ITRS roadmap
requirement for mask flatness for 45 nm node is 250 nm. To achieve this very tight reticle flatness requirement, the stress
of each film present on the mask substrate must be minimized. Another key reticle specification influenced by film stress
on the mask blank is image placement. In this paper, we will describe the development and detailed characterization of a
new low stress Molybdenum Silicide (MoSi) film for use in manufacturing 45 nm node critical level attenuated phase
shift masks to be used in 193 nm immersion lithography. Data assessing and comparing the cleaning durability, mask
flatness, image placement, Critical Dimension (CD) performance, dry etch properties, phase performance, and defect
performance of the new low stress MoSi film versus the previous industry standard A61A higher stress MoSi attenuator
film will be described. The results of our studies indicate that the new low stress MoSi film is suitable for 45 nm mask
manufacturing and can be introduced with minimal changes to the mask manufacturing process.
The application of aggressive optical proximity correction (OPC) has permitted the extension of advanced lithographic
technologies. OPC is also the source of challenges for the mask-maker. Sub-resolution features, small shapes between
features and highly-fragmented edges in the design data are difficult to reproduce on masks and even more difficult to
inspect. Since the inspection step examines every image on the mask, it is required to guarantee the total plate quality.
The patterns themselves must be differentiated from defects, and the ability to recognize small deviations must be
maintained. In other words, high inspectability at high defect sensitivities must be achieved simultaneously. This must be
done without restricting necessary OPC designs features. Historically, transmitted light has been deployed for mask
pattern inspection. Recently, the inspection challenge has been both enhanced and complicated by the introduction of
reflected light pattern inspection. Reflected light reverses the image contrast of features, creating a new set of design
limits. This paper introduces these new reflected inspection limits. Multiple platform capabilities will be incorporated
into the study of reflected and transmitted inspection capability. The benefits and challenges of integrating a combination
of transmitted and reflected light pattern inspection into manufacturing will be explored. Aerial Image Measurement
System (AIMS) analysis will be used to help understand how to leverage the enhanced inspection capability while
avoiding unnecessary restrictions on OPC.
Previous work has shown that photomask blank flatness as well as photomask patterning and pelliclization all play an
important role in finished photomask flatness. Other work has shown that pellicle mounting techniques and pellicle
adhesives play a role as well. In this work, a comparison of the impact of various pellicle types, frame flatness, frame
shape and pellicle mounting tools on final photomask flatness will be shown. Pellicles with various adhesives, frame
shapes and flatness were mounted on blanks and completed photomasks using several mounting tools and the pellicle
induced flatness change was measured. These data will be discussed with the objective of demonstrating the effects of
pellicle type and mounting tool on photomask flatness.
Three types of high transmission attenuated phase shift masks were evaluated. The attenuating materials were obtained from commercial and non-commercial sources. Various key performance metrics were investigated. Blanket film transmission and reflection was measured at various wavelengths. Laser durability and cleaning durability were
measured. Standard dry etch processes were used for each film and the profile and surface properties were compared. Final mask transmission and phase were also measured. The summarized results show clear benefits of using some high transmission materials relative to others.
The phase shift mask (PSM) is one of the most effective approaches to improve ArF lithography performance. Recently, the quartz dry etching technology plays an important role to fabricate the PSM, such as space bias type Alternating (Alt.) PSM and chrome-less phase lithography (CPL) mask. The quartz etching profiles seems to be affected the lithography performance. In this paper, preliminary, we evaluate the nominal influences of quartz profile by rigorous electromagnetic field simulation. Then influence of the quartz profile is investigated by measuring the real masks. In this experiment, we intentionally fabricate Alt. PSM and CPL masks with the tapered side-wall and deeper micro-trench. Lithography performances of the real masks are measured by the aerial image measurement system (AIMS fab193). We compare the result of AIMS with simulation. We investigate the AIMS measurement well corresponds to the simulation. Side-wall angle and corner rounding strongly affect the lithography performance. However, micro-trench doesn’t affect a lot.
Proximity effect and foggy effect correction is performed to obtain an ideal CD distribution of resist patterns within a mask plate. However, gobal loading effect in dry etching causes an additional CD distribution of Cr patterns. In order to satisfy the CD distribution specification in 65nm node, CD distribution in global loading effect should be improved to be 2nm or less. To accomplish the goal, a correction system of dry etching loading effect has been developed. The correction is performed by sizing patterns in each writing field (1mm x 1mm). The sizing amount, minimum step of 1nm, is calculated according to the parameters, which are defined by measuring the test patterns. The loading effect is evaluated by measuring the CD difference of 1 micron lines and spaces in 80mm x 40mm clear area and that in completely dark area, which is an extremely severe case. The writer is JEOL/JBX-3030, and the dry etcher is Unaxia/VLR700GIII in the experiment. By applying this correction, CD uniformity caused by the global loading effect can be reduced to 2nm or less.
The slow progress of the 157nm-F2 laser exposure tool development results in broad adaptation of high numerical aperture (NA>0.8) 193nm-ArF lithography for the 65nm-node production solution. This decision, however, forces lithographers to increase dependency on very aggressive RET technologies. This in turn demands mask making capabilities the industry has never faced before such as 100nm (@4X on mask scale) size Sub Resolution Assist Features (SRAF). This report covers our early work on our mask making capability development for the 65nm-node process technology development cycle for production in 2005. Our report includes the 65nm node mask technology capability development status for mask CD and registration dimensions control, current inspection capability/issues and development efforts for critical layer masks with aggressive RET (especially of EAPSM with SRAF).
The KrF12% tri-tone PSM (phase shift mask) was designed with the programmed defects on the chrome (Cr) and phase shift (PS) layers. From the lithography simulation, the PS defects, generated on the PS layer, were estimated to fluctuate the CD of the contact holes on the wafer more than the defects in the same size on the conventional EAPSM (half-tone PSM). The printability of the PS defects and Cr defects on the contact holes were investigated by the print-test on the wafer. The Cr residues on the PS layer slightly changed the CD of the contact holes on the wafer. The PS defects showed the great influence to the CD variation of the contact holes. The light calibration of the defect inspection was optimized to detect the PS and Cr defects. For the detection of the PS defects in the die-to-die inspection mode, the UV inspection system SLFX7 showed the high performance with the PS/SiO2 calibration, in which the boundary of the PS layer and SiO2 substrate was used as the light calibration point. The SLFX7 also available to detect the Cr defects in the die-to-die mode. For the die-to-database mode to detect the Cr defect, the KLA351, the visible light inspection system, was available by the Cr/PS calibration. The sensitivity of the SLFX7 and KLA351 was adequate for the Cr defects, however, that was not enough to the specification of the PS defects estimated from the print-test. The sensitivity of the SLFX7 showed a slight difference between the tri-tone and binary layout in the specific defect types.
The new repair of the clear defects on the half-tone PSM (EAPSM) was proposed. The Ga (gallium) ions were implanted by the FIB on the area adjacent to the carbon films formed on the clear defects. The Ga ion implanted area on the SiO2 substrate showed the semi-transparency at the KrF and ArF wavelengths. The lithography simulations of the layouts designed for the defect-repaired area endorsed the concept of the new repair. The Ga ion implantation was optimized to the new repair by using the AIMS and AFM measurements for the transmittance and etched depth of the SiO2 surface. The authors applied this method to the clear defects programmed on the KrF and ArF EAPSMs. The AIMS analysis showed that the deviation of the CD (critical dimension) of the defect-repaired regions on the wafer was within +/-5% to the non-defective reference at every defocused point. The new repair moderated the specification of the edge placement accuracy of the FIB processing compared to the conventional carbon film deposition.
The critical dimensions (CD) change by the process delay is the most critical issue to apply the chemically amplified resists (CAR) for photomask fabrication. In the photomask fabrication processes, the resist should have both post coating delay (PCD) and post exposure delay (PED) stability, while keeping higher sensitivity. To achieve this requirement, overcoat process has been examined for the purpose of CD stabilization in CAR process for photomask manufacture. The material, which consists of hydrophobic polymer and PAG, was used for the overcoat in this study. Consequently, it has become clear that pattern formations have been possible without unnecessary thickness loss. Moreover, it has been proved that the overcoat shows the effect of controlling CD change and improvement of CD uniformity. From these results, it is thought that the overcoat process is promising for the size stabilization in photomask manufacture for devices less than 90 nm.
The critical dimensions (CD) change by the process delay is the most critical issue to use the chemically amplified resists (CAR) for photomask fabrication. In the photo-mask fabrication processes, the resist should have both post coating delay (PCD) and post exposure delay (PED) stability, while keeping higher sensitivity. To achieve this requirement, overcoat process has been examined for the purpose of CD stabilization in CAR process for photomask manufacture. The material, which consists of hydrophobic polymer and photo acid generator (PAG), was used for the overcoat in this study. It has been proved that the overcoat shows the effect of controlling CD change, and applying the overcoat does not generate a fatal number of defects and pinholes. From these results, it is thought that the overcoat process is promising for the size stabilization in photomask manufacture for 100 nm devices.
We are focusing on a high-performance cleaning process with minimum use of chemicals. For the substitution of chemicals, the refined cleaning tools and process have been developed, which use the high-concentration ozonic together with hydrogen water. To optimize a cleaning process, we have evaluated the removal and decomposition efficiency of organic compounds on the mask surface, the optical degradation of Cr and Suicide materials and so on. In conclusion, the substitution of sulfuric acid, ammonia and other chemicals is available for practical cleaning process by combining their functional cleaning steps. Especially in the ArF generation, this cleaning technique was found to be promising for the reduction of optical-damage and chemical residues for mask patterns and as well as high-efficiency particle removal.
In this paper, we focused on the refined cleaning process with minium use of chemicals. We developed a cleaning tools and process using high-concentration ozonic water generated by the high-efficiency ozonizing apparatus (OW00345, Mitsubishi Electric Corp. Industrial Systems), as chemicals substitution. To optimize a cleaning process, we have evaluated the removal and decomposition efficiency of organic compounds on the mask surface, the optical degradation of Cr and Siliside materials and so on.
Currently various techniques, such as OPC, Beam proximity correction, are under development aiming at volume production of 180 nm logic devices. 180 nm lithography requires to handle critical dimensions below the light wavelength of stepper, HT- PSM is considered to be a potential solution for securing a certain process margin, and it is the case not only for 'hole' patterns but for 'line' patterns. On 180 nm device, since the CD on reticle is sub-micron, uniformity control across iso- dense area and CD linearity become very difficult compared with simple cell-repeating patterns like memory devices. Here, under the assumption that we apply MoSiON HT-PSM to 'line' pattern of 180 nm device, we will report various evaluation results which are mainly related to mask making process. The conclusion is that HT-PSM has advantages over binary mask when it is applied for 'line' patterns, and we could fulfill those reticle requirements by optimizing conditions of materials, dry-etcher, writing tools and beam/resist combination.
The halftone phase shift mask (PSM) gives a significant improvement on DOF for hole pattern printing on wafer. However, we need to be careful when we use it on other patterns such as line and space or patterns surrounded by bright field, because sub-peak effect could cause deterioration of those patterns when they are printed on wafer. Toppan presents a new structure in which Cr film is partially placed on the half tone surface. With this structure, so called Tri-tone PSM, we can block harmful light transmission for non-hole patterns, while keeping the full characteristics of PSM effect on targeted patterns. This structure could also prevent the line shortening problem. Using this structure we can get good printability on smaller geometry without using optical proximity correction (OPC) patterns (serif, etc.). Of course this method can offer a perfect shielding of the opaque ring which prevent the wafer damage caused by light leakage. By using this Tri-tone structure, the application of half tone PSM, which is so far limited to hole patterns, would be extended to line patterns.
Attenuate phase shift masks have been developed for practical use because there are less limitations on the design, and mask defect repairs are easier. However, phase shift mask manufacturing technology improvement is needed not only for control of phase shift or transmission, but also higher accuracy and more precise patterns such as KrF application or for 4X reticle applications. This report shows the manufacturing method of the MoSiON attenuate phase shift mask with Cr border and its quality encourages its evaluation as a promising technique for finer patterns.
We have been developing the technology for phase shift masks since 1991 in order to
supply the reticles for 64 MDRAM generation and after. We are still developing both the
alternative and embedded types fori-line.
The embedded type is suitable for ASIC and contact hole. Now, we are reaching an
adequate supply of embedded type PSMs for practical use.
Currently the embedded types is relatively popular because the burden on designing is
small, the process is comparatively short, and defect repair is easier.
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