As the design rule of the semiconductor shrinks, the CD MTT (Critical Dimension Mean-to-Target) specification for photomask becomes tighter. So, more precise control of CD MTT is required. We have investigated the CD MTT control and applied it to the attenuated PSM (Phase Shift Mask) successfully for several years. We can control the CD MTT of MoSi pattern by measuring Cr/MoSi pattern to estimate MoSi pattern CD and additional etch to shrink MoSi pattern as reported in previous study. At first, the MoSi pattern CD can be estimated with the Cr/MoSi pattern CD because the CD gap between MoSi pattern and Cr/MoSi pattern is relatively constant. Additional MoSi etch is performed to shrink the MoSi pattern CD after then. The CD gap alwasys exists and the variation of the CD gap is enough small to be not considered in conventional photomask production until now. However, the variation of the CD gap is not ignorable in case of sub-20 nm tech.
In this study, we investigated new method to measure MoSi pattern CD before Cr strip process to eliminate the CD gap between MoSi pattern and Cr/MoSi pattern. To eliminate the CD gap, we attempt three solutions – 1) Optimize etch process to perform perfect Cr/MoSi pattern profile without the CD gap, 2) Improve CD measurement accuracy by developing new SEM measuring mechanism, 3) Develop of new process to modify Cr/MoSi pattern profile to be measured without the CD gap. It was found that the CD gap can be eliminated and MoSi pattern CD can be measured perfectly. Finally, MoSi pattern CD control was improved because of CD gap elimination.
EUV (Extreme Ultraviolet) Lithography has been delayed caused by several technical problems such as
EUV mask, source power and etc. So ArF immersion lithography has been continued with adopting new
technology. Especially, the wafer lithography tends to increase rapidly NTD(Negative Tone Develop) process
for overcoming high resolution such as small hole type patterns. For wafer NTD process, the pattern shape in
mask has changed from hole pattern to dot pattern. Also the local CD uniformity of aerial image is getting
more important. In this paper, we studied local CD uniformity with analyzing aerial images of high
transmittance HT-PSM (attenuated phase-shift mask) and conventional 6% HT-PSM from AIMS (Aerial Image
Measurement System) tool. Additionally, several cell sizes were analyzed to find an optimum target cell size
which has good wafer performance and AIMS aerial image. And we analyzed NILS(Normalized Image Log
Slope) factor which represent wafer photolithographic performance. Furthermore, we analyzed not only AIMS
NILS simulation, but also wafer lithographic performance.
Influence of phase defect on printed images of mask pattern was experimentally investigated by printing contact hole (CH) pattern of hp 32 nm on wafer. And the experimental results were compared with the simulation results. A test mask prepared for this experiment contained programmed phase defects of 92 nm ~ 34.8 nm in width and of around 0.68 nm ~ 1.65 nm in depth. The defects were arrayed in a way such that the pitch of the array would differ from the pitch of the absorber contact hole pitch. Therefore, the phase defects were placed at different positions relative to those of the CH patterns. Mask patterns were printed on wafer using an exposure tool NXE3100 with a numerical aperture (NA) of 0.25 and a reduction of 4X. To evaluate the printed patterns affected by the phase defects, circular illumination was employed. The incident angle of mask illumination chief
ray was 6 degrees. The printed CH patterns were measured by SEM. An influence of resolution limit of the resist pattern did seem to appear in this experiment, to be a quantitative difference between the simulation and experimental results, the relative location dependence was quite noticeable and the effect of a phase defect was mitigated by covering the defect with an absorber pattern.
Advanced photomasks exploit complex patterns that show little resemblance to the target printed wafer
pattern. The main mask pattern is modified by various OPC and SRAF features while further complexity is
introduced as source-mask-optimization (SMO) technologies experience early adoption at leading
manufacturers. The small size and irregularity of these features challenge the mask inspection process as well
as the mask manufacturing process.
The two major concerns for mask inspection and qualification efficacy of advanced masks are defect
detection and photomask inspectability. Enhanced defect detection is critical for the overall mask
manufacturing process qualification which entails characterization of the systematic deviations of the pattern.
High resolution optical conditions are the optimal solution for manufacturing process qualification as well as
a source of additional information for the mask qualification. Mask inspection using high resolution
conditions operates on an optical image that differs from the aerial image. The high resolution image closely
represents the mask plane pattern. Aerial imaging mode inspection conditions, where the optics of the
inspection tool emulates the lithography manufacturing conditions in a scanner, are the most compatible
imaging solution for photomask pattern development and hence mask inspectability. This is an optimal
environment for performing mask printability characterization and qualification.
In this paper we will compare the roles of aerial imaging and high resolution mask inspection in the mask
As design rule of memory device is shrinking, the various errors obtained by process, such as line edge roughness, local CD variation and electron beam shot placement error, are significant to CD measurement results on mask and wafer. Reliable CD measurement is needed to represent real feature size of mask and wafer results in high accurate CD target and uniformity by various CD correction techniques before mask fabrication and after. Recently light transmittance control technique on mask has been introduced, which reduce the field CD variation of wafer . To correct the wafer field CD uniformity by selective control of the light transmittance of mask, good correlation of mask CD and wafer field CD is important . AIMS (aerial image measurement and simulation) or light intensity uniformity of inspection tools or other light intensity measurement tools are generally used to measure mask CD uniformity on mask. In this study, mask CD uniformity measured by CD-SEM was used to compensate the field CD variation on wafer, by enhancing the correlation between wafer field CD uniformity and mask using spatial filtering of SEM image and area CD measurement concept. Expected residual error of wafer field CD error using correction of mask CD uniformity were compared to wafer CD variation by selective light correction using wafer CD uniformity map.
In order to realize the effect of pattern-specific off-axis illumination under the conventional circular illumination, the
illumination method using a mask grating formed on the top side of a photo mask was evaluated and improved. Contrary
to an off-axis illumination, it could provide the locally different off-axis illumination depending on the pattern shape
defined on the bottom side of a mask. The structure of the mask grating was determined from the feature characteristics
of the mask pattern and its performance was evaluated with the simulated Bossung curves.
A new inspection system with DUV laser beam and high NA optic for EUV mask has been developed to inspect defects
on EUV blank mask and defects by process and handling. The development of new reflective image and optics has
increased inspection speed on EUV mask before absorber etch and after absorber etch. Defect classification and
operation has increased the productivity of inspection and particle control on EUV mask process. With this new
inspection system, defects on blank mask, after resist develop and after etch processed mask were classified and
evaluated to install EUV mask process. And defect sensitivities according to various pattern size and process steps were
evaluated with required defect size of simulated printing effect on wafer. Designed defect pattern of 46nm node were
prepared. Blank masks from Hoya were used. Patterns were exposed using 50KeV electron beam writer. After resist
develop, patterns with program defect were inspected. After absorber etching, defects were inspected and evaluated.
According to sub film, inspection condition was optimized. Using simulation tool, defects printability were simulated
and compared with sensitivity of this inspection tool. Our results demonstrate that this inspection tool is very effective
to detect and identify defects and their sources on EUV mask process. In this paper, mask inspection performance of
high NA, DUV optic with short working distance was evaluated and described on programmed EUV mask.
Sensitivity of newly developed photo mask inspection tool with reflective optic was evaluated for 45nm DRAM device.
To get the required defect sensitivity of mask, printability of mask defect on wafer were simulated using in house
simulation tool. Simulation results were compared with inspection results. Characteristic and sensitivity comparison
between conventional transmissive and reflective optic tools were evaluated for several types of mask layer of 45nm and
55nm DRAM according to pixel size of detector of inspection tools. This reflective optic with short working distance
was equivalent in sensitivity to transmissive optic tool. Mask for 45nm DRAM can be qualified by current status of the
art inspection tools.
As mask feature size is shrinking, required accuracy and repeatability of mask CD measurement is more severe. CD-SEM which is usually used to measure below 0.5um pattern shows the degradation of repeatability by the sparkle noise. To reduce this, larger ROI (range of interest) is recommended on line and space patterns. But this wide ROI is difficult to use on Hole or isolated patterns. In this paper, anisotropic diffusion filtering method will be introduced to replace the ROI, and evaluated on various patterns such as holes and isolated patterns. It can also reduce the effects of defocus of CD-SEM and enhance the repeatability of CD-SEM. And multi-point CD measurement technique is described to reduce the local CD errors on CD uniformity of mask which is usual on one dimensional CD measurement conventionally. Using these methods, local CD uniformity and global CD uniformity of masks which is the key performance of mask quality can be measured more exactly compared to old CD measurement method. And we can give correct information of mask to reduce global CD uniformity by process tuning such as FEC (Fogging Effect Correction) or development process.
As feature size is shrinking and MEEF (Mask error enhancement factor) is increasing, CD measurement accuracy is more important, and CD SEM is widely used to replace optic tools because of their resolution. But CD-SEM is not representing the effect of Cr profile or transmittance of light which is transferred to wafer. Recently, new OCD (optic CD) tool which use scatterometry (Spectroscopic Ellipsometry) *1) is introduced to compensate the demerit of SEM of low through-put and reflected surface information of mask. This scatterometry tool can be used only on periodic pattern like DRAM. And this tool must be calibrated on each pattern type and shape. This calibration is the barrier to use this scatterometry method to mask process where all masks are processed one time.
In this work, new optical CD measurement method which use conventional optic microscope of transmitted and reflected light with high resolution lens of DUV on periodic patterns is introduced. To enhance the accuracy of measurement, interpolating method and FFT (Fast Fourier Transform) are used. CD measurement results of linearity by optic CD, SE and CD-SEM were compared on several patterns. And CD variations on full field of image were evaluated on L/S patterns and active layer of DRAM.