Authors would like to raise a discussion about image intensity for surface exposure, off course, including optical
lithography. As a springboard for the discussion, a novel definition of image "intensity", which expresses local
irradiance associating with optical image, is proposed. An experimental result, which strongly supports the proposed
"intensity", is also obtained.
To describe exposure dose, energy input for unit area with unit of J/m2, is applied as a measure of this amount. A
phrase of "dose-to-clear" is frequently used to show sensitivity of a resist film. In contrast, conventional image intensity
of optical image is defined as a value, which is proportional to volume energy density associating with image. The value
is described with unit of J/m3. In some papers, it is mentioned that number of photochemical reactions in resist film is
proportional to the volume energy density of electromagnetic filed, that is, conventional image intensity. It seems
unclear what physical value is proper measure of surface exposure.
We considered that, in optical lithography, energy flux is proper value to indicate degree of resist exposure from
experience and some former reports. Then, a novel image "intensity", which expresses local irradiance associating with
optical image, is proposed. The proposed image "intensity" is proportional to surface normal component of Poynting
~30nm width isolated line is formed with over 300nm DOF by Single Exposure
process of ArF immersion lithography.
Super-Diffraction-Lithography ("SDL") technique, which utilizes fine dark line image formed between a pair of bright
lines in attenuating non-phase-shifting field and which enables formation of very fine isolated line pattern with single
exposure, is applied with ArF immersion lithography. By simulation study, superior performance of "SDL" is exhibited
for ArF immersion lithography. From view point of mask fabrication, it is shown that requirement for mask technology
is not so severe, such that photo mask for "SDL" in hyper NA ArF immersion era can be fabricated with current mask
technology. By experiments with an optimum quadrupole illumination, ~30 nm width isolated line is successfully
printed by single exposure process with over 300nm DOF by a mature 6% transmission EA-PSM. Moreover, device like
pattern with ~35nm line width is well formed with enough large DOF to industrially fabricate devices.
We believe this technique is one of the promising candidates for advanced logic at 32 nm node and beyond.
Critical processing factors in the lithography process include overlaying the pattern properly to previous layers and
properly exposing the pattern to achieve the desired line width. Proper overlay can only be attained in the lithography
process while the desired line width accuracy is achieved by both lithography and etch processes. Since CD is
substantially influenced by etch processing, therefore, it is possible to say that overlay is one of the most important
processing elements in the lithography process. To achieve the desired overlay accuracy, it is desirable to expose critical
layers with the same exposure tool that exposed the previous or target layer. This need to dedicate a particular exposure
tool, however, complicates the lot dispatching schedule and, even worse, decreases exposure tool utilization. In order to
allow any exposure tool available to print the arriving lot, M&M (Mix and Match) overlay control becomes necessary.
By reducing overlay errors in M&M control, lot dispatching scheduling will become more flexible and exposure tool
utilization will improve.
Since each exposure tool has a unique registration signature, high order errors appear when overlaying multiple layers
exposed with different tools. Even with the same exposure tool, if a different illumination is used, a similar error will be
seen. A correction scheme to make the signature differences has to be implemented, however manually characterizing
each tool's signature per illumination condition is extremely tedious, and is subject human errors. The challenge is to
design a system to perform the corrections automatically.
In the previous paper(1), we have outlined concepts of the system scheme. The system has subsequently been developed
and tested using exposure tools. In this paper test results are shown using automated distortion correction. By analyzing
the results, suggestions for further improvements and further developments are shown.
A novel RET, which enables on-grid sub-50 nm hole pattern formation with ArF immersion lithography, has been
developed. One of the authors has found quasi-iso-focal point image generation at the center of square area of high
transmission embedded attenuating phase shift mask (EA-PSM), where four small openings are laid out at the corners of
the area, utilizing an optimized quadrupole illumination. As an extension of continuous configuration, checker-board
like mask pattern arrangement is created. In the mask, small openings and opaque pads are arranged like as checkerboard,
whose base pitch is around resolution limit of targeted optical system. The mask pattern arrangement is named as
"Checker-Board PSM (CB-PSM)". By eliminating any one opening from "checker-board", very fine point image is
generated at the place. Because four openings around the eliminated one are necessary for the fine imaging
characteristic, minimum distance between the point images is about the double of that for resolution limit. After
simulation study of imaging, experiments are carried out to prove the fine imaging performance utilizing ArF immersion
optics with NA=1.07 and a tri-level resist system. As a result, sub-50nm isolated hole is successfully formed with DOF
larger than 200 nm. Simultaneously, ~ 60 nm semi-dense hole with pitch of 240 nm is printed with over 200 nm DOF.
Moreover, application of conventional mask pattern arrangement, ultimately dense hole of 140nm pitch is well formed.
As a conclusion, we believe that CB-PSM is a promising candidate for hole pattern formation at 32 nm node and beyond.
As Moore's Law drives CD smaller and smaller, overlay budget is shrinking rapidly. Furthermore, the cost of advanced
lithography tools prohibits usage of latest and greatest scanners on non-critical layers, resulting in different layers being
exposed with different tools; a practice commonly known as 'mix and match.' Since each tool has its unique signature,
mix and match becomes the source of high order overlay errors. Scanner alignment performance can be degraded by a
factor of 2 in mix and match, compared to single tool overlay operation. In a production environment where scanners
from different vendors are mixed, errors will be even more significant. Mix and match may also be applied to a single
scanner when multiple illumination modes are used to expose critical levels. This is because different illuminations will
have different impact to scanner aberration fingerprint. The semiconductor technology roadmap has reached a point
where such errors are no longer negligible.
Mix and match overlay errors consist of scanner stage grid component, scanner field distortion component, and process
induced wafer distortion. Scanner components are somewhat systematic, so they can be characterized on non product
wafers using a dedicated reticle. Since these components are known to drift over time it becomes necessary to monitor
them periodically, per scanner, per illumination.
In this paper, we outline a methodology for automating characterization of mix and match errors, and a control system
for real-time correction.
A novel process of OPC-free on-grid fine random hole pattern formation is developed. Any random hole pattern with
~120nm diameter on 240 nm base grid can be printed by KrF exposure. In this technique, double resist patterning
scheme is adopted. Dense hole pattern is delineated with first resist process. Quadrupole illumination is applied with
embedded attenuating phase shift mask (EA-PSM) in imaging on this step. As is well known, fine dense hole pattern
is formed with very large process latitude. After development of the first resist, hardening of the resist film by Ar ion
implantation is carried out so as not to mix with second resist at second coating. This hardening process is very robust
such that rework in second resist process can be performed with stripping the resist by a solvent. Then, second resist
patterning is carried out. In the second exposure, cross-pole illumination is applied with high transmission EA-PSM.
By this imaging, very fine dark spot image is generated. Resultantly, fine random pillar patterns, which plug an
underlying hole, are formed in the second resist film. Because function of the pillar is plugging a hole, no precise CD
control is required. Moreover, pattern connection between adjacent pillars does not cause any problem. Hence, no
OPC is needed in the pillar formation, regardless of printed size variation of the pillars. Undesired holes in the dense
holes are plugged by the pillars. As a result of the double resist patterning, on-grid random hole pattern is successfully
delineated. Due to the robustness of each patterning process, very high process latitude is achieved. Off course, this
technique can be carried out under any wavelength on regard of imaging. In other aspect, this technique utilizes only
positive-tone resist. Hence, this technique can be applied with leading-edge ArF immersion lithography. As a
conclusion, this technique is a promising candidate of hole pattern formation in 32nm era and beyond.
As a promising way to scale down semiconductor devices, 193-nm immersion exposure lithography is being developed
at a rapid pace and is nearing application to mass production. This technology allows the design of projection lens with
higher numerical aperture (NA) by filling the space between the projection lens and the silicon wafer with a liquid
(de-ionized water). However, direct contact between the resist film and water during exposure creates a number of
process risks. There are still many unresolved issues and many problems to be solved concerning defects that arise in
193-nm immersion lithography.
The use of de-ionized water during the exposure process in 193-nm immersion lithography can lead to a variety of
problems. For example, the trapping of microscopic air bubbles can degrade resolution, and residual water droplets left
on the wafer surface after immersion exposure can affect resolution in the regions under those droplets. It has also been
reported that the immersion of resist film in de-ionized water during exposure can cause moisture to penetrate the resist
film and dissolve resist components, and that immersion can affect critical dimensions as well as generate defects.
The use of a top coat is viewed as one possible way to prevent adverse effects from the immersion of resist in water, but
it has been reported that the same problems may occur even with a top coat and that additional problems may be
generated, such as the creation of development residues due to the mixing of top coat and resist. To make 193-nm
immersion lithography technology practical for mass production, it is essential that the above defect problems be solved.
Importance must be attached to understanding the conditions that give rise to residual defects and their transference in
the steps between lithography and the etching/cleaning processes.
In this paper, we use 193-nm immersion lithography equipment to examine the transference (traceability) of defects that
appear in actual device manufacturing. It will be shown that defect transfer to the etching process can be significantly
reduced by the appropriate use of defect-reduction techniques.
A novel mask structure for an alternating aperture phase shift mask (Alt-PSM) to cut mask cost is proposed. By a
mask with structure of an embedded attenuating phase shift mask (Atten-PSM), an Alt-PSM for an isolated line
formation can be well fabricated. Fine image quality is confirmed with optical image calculations. Moreover,
concept of this novel mask is proved by a preliminary experiment. In conclusion, this novel mask can replace
conventional Alt-PSM for logic devices, resulting in considerable cut of mask cost.
Recent integrated circuit (IC) manufacturing processes require smaller critical dimension (CD) in order to facilitate the development of exposure tools with a higher numerical aperture (NA) and shorter wavelength. Consequently, the depth of focus (DOF) has considerably decreased, and the DOF currently required for 45-nm node devices is approximately 150 nm. Hence, the contribution of mask flatness to the total DOF increases. Inoue et al. systematically and precisely investigated the influence of mask flatness by using a free-standing plate and chucked plate interferometer. In this study, we fabricated several back side chrome (BSC) masks for focus monitoring, determined the flatness of these masks by an exposure experiment, and compared the flatness with that directly determined by using a free-standing plate interferometer. Thus, we verified the possibility of predicting the mask flatness component on an image plane by using the mask flatness data obtained using the interferometer.
Top coat process is required for immersion lithography in order to prevent both the chemical contamination of scanner optics with eluted chemicals from resist material and the formation of residual droplet under the immersion exposure with high scanning speed. However, defect density of ArF immersion lithography with alkaline developer soluble type top coat material is much higher than that of ArF dry lithography. Mimic immersion experiments comprised of soaking of exposed conventional dry ArF resist with purified water followed by drying step were performed in order to study the immersion specific defects. It was suggested that the origin of immersion specific defects with alkaline developer soluble type top coat was the remaining water on and in the permeable top coat layer that might interfere the desired deprotection reaction of resist during post exposure bake (PEB). Therefore, application of post exposure rinse process that can eliminate the impact of the residual micro water droplets before PEB is indispensable for defect reduction. Post exposure rinse with optimized purified water dispense sequence was noticed to be valid for defect reduction in mimic immersion lithography, probably in actual immersion lithography.
193 nm lithography is one of the most promising technologies for next-generation lithography and is being actively evaluated for making it practicable <sup>(1,2)</sup>. First, we evaluated an immersion lithography tool (engineering evaluation tool (EET)) <sup>(3)</sup> and a dry lithography tool (S307E) with the same numerical aperture (NA = 0.85), manufactured by Nikon Corporation. As a result, an increase in the depth of focus (DOF) of the EET to 200 nm in comparison with the DOF (110 nm) of the dry exposure tool was confirmed in a 90 nm isolated space pattern. Next, the optical proximity effect (OPE) in this pattern was evaluated. Generally, when an immersion lithography tool is compared with a dry one with the same NA or both the tools, only an increase in the DOF is found. However, we confirmed that the OPE (The OPE of the 90 nm isolated space pattern is defined as the difference in the space width between a dense space and an isolated space.) of the dry exposure tool for the 90 nm isolated space pattern reduced from 33.1 nm to 14.1 nm by immersion lithography. As the effect of the reduction of 19 nm, the OPE reduced to 15.2 nm by the effect of the top coatings (TCs) and to 3.8 nm by the optical characteristics. An impact of about 5 nm on the OPE was confirmed by the process parameters-film thickness and the pre-bake temperature of the TC. In the case that the solvent was replaced with a high boiling point solvent, the impact changed from 5 to 20 nm further, the replacement of the solvent had a considerable impact on the OPE. However, this influence differs considerably according to the kind of resists; further, it was shown that the addition of acid materials and a change in the polymer base resulted in a high impact on the OPE for a certain resist. Thus, it was demonstrated that the selection of TC is very important for the OPE in immersion lithography.
A novel RET, "Super Diffraction Lithography" (SDL), which enable 70 nm any pitch line by single exposure in KrF wavelength, has been studied in order to apply for an actual device pattern formation. In a previous work, the concept of SDL has been described with optical image calculations for 1-dimensional patterns and very superior performance has been proved. In this work, imaging characteristics and printing performance of typical 2-dimension patterns are investigated by optical image calculations and printing experiments to realize an application of SDL technique to fabrication of actual device patterns. As a result, very good performance is achieved for the typical 2-dimentional patterns such as line-end, tee-branch. Moreover, good performance is obtained for general SRAM patterns and standard cell of 65 nm node logic device with a little relaxation of design rule. In conclusion, by the application of SDL, 65 nm SoC patterns with a little relaxed design can be formed by single exposure process in KrF wavelength with a simple Atten-PSM. Then, huge cost reduction can be expected by application of SDL.
A simple and high sensitive focus monitoring has been developed utilizing an aperture in Cr film formed on backside of photomask. A special mask for focus monitoring is developed such that two mark patterns on the front side of the mask are irradiated by different illuminations. The different illuminations for the two marks are generated from usually used illumination with modulation by an aperture on the backside of the mask. In this work, two complementally halves of usually used illumination are effectively generated. Because illumination for each mark pattern on front side of the mask is strongly asymmetric in incident angle such that illumination beam impinged from only one side of the space, imaging of the large size mark pattern is carried out obliquely on the wafer. As a result, image is laterally shifted with focus. The direction of lateral image shift is opposite to that of another mark which is irradiated with illumination beams from opposite side of the space. Thus, the relative displacement between the two mark images may become a measure of focus. Because this focus monitor works under purely geometrical optics, focus monitoring of multiple steppers, which are working under different wavelength, can be performed with the same one photomask. In experiments, the two mark patterns, which are inner and outer box patterns, are printed with overlaying each other by double exposure with stepping of wafer stage. Then, mutual displacement of mark patterns is measured by a commercially available overlay measurement tool whose resolution is a few nm. Very high focus sensitivity (Δx/Δz) of ~0.9 is observed for NA=0.68 optics with strong annular illumination. Because of the high focus sensitivity and high resolution of overlay measurement, focus monitoring with very high resolution of a few nm can be achieved.
For the convenience of practical use of phase shift focus monitor (PSFM), which has been developed by T. Brunner, imaging characteristics of PSFM are investigated under modified illumination by optical image calculations and printing experiments. Although the mechanism of pattern shift with focus offset under modified illumination is different from that for conventional high coherent illumination, sufficient sensitivity for precise focus monitoring is predicted by optical image calculations. Also, it is revealed that reduction of NA, i.e., localizing illumination at the peripheral part of pupil is effective to obtain higher sensitivity. By experiments, predicted characteristics are observed and similar sensitivity to that in conventional high coherent illumination is confirmed both for annular and quadrupole illuminations.
A novel method for monitoring lens aberration in projection optics of a stepper is developed utilizing pinhole aperture formed on backside of photo mask. With the pinhole aperture, illumination beam to a mark pattern on the front side of a photo mask becomes semi-coherent with an incident angle which is determined by lateral distance between the pinhole and the mark. When the mark pattern generates diffraction beams within narrow angle region, imaging is carried out by using localized area of pupil. As a result, Hartmann test structure is effectively realized by this configuration. By elaboration of mask pattern, measurement error is significantly reduced resulting in sufficient accuracy for monitoring lens aberration in current scan stepper. Simulations by optical image calculation reveal that measurement error is less than 10m wave in RMS and 40m wave in maximum local deviation for an aberration which is expressed with first 35 polynomials of Zernike series. In preliminary experiments, measured aberration seems to be reasonable. This method should provide a simple, easy and cost effective tool for monitoring of lens aberration.
Simple focus monitoring method has been successfully developed by application of a special illumination aperture, which generates oblique illumination beam. By this method, very high sensitive focus monitoring has been achieved in a current stepper. In the stop of the illumination aperture, an opening is located at eccentric position near pupil edge. Then, illumination beam obliquely incidents to mark pattern on mask. Because of this configuration of illumination beam, imaging is carried out with oblique beams on wafer. As a result, imaging becomes non-telecentric. That is, image formed by this illumination laterally shifts almost proportional to focal deviation. To measure the lateral pattern shift, box-in-box mark is formed by double exposure. Inner box is formed by the oblique illumination in the first exposure and outer box is formed by conventional low coherent illumination in the second exposure overlaying inner box by stepping of wafer. Then, relative displacement of inner box to outer box is measured by commercially available overlay measurement system. Since sine of landing angle of imaging beams is approximately NA*sigma, which is over approximately 0.50 in a current stepper, the focus sensitivity, which is defined by a ratio of lateral pattern shift per unit defocus, may become approximately 0.50. Because resolution of lateral pattern shift is approximately 2 nm in current overlay measurement, the resolution of focus sensing becomes very high of approximately several nm.