We have developed a very simple source optimization (SO) method for L/S and C/H critical layers patterning of
advanced NAND FLASH. Starting from the strong off-axis illumination shape which is optimized for the finest
structure of the mask pattern, a systematic procedure is performed to extract the optimum parameters of additional assist
sources to balance the imaging performance (DOF, contrast and optical proximity effect, etc.) of dense/sparse/rough
patterns. Performance equations (linear optimization) with performance map (sensitivity) are utilized to search the best
combination of intensity for each assist source. For C/H pattern, the optimization procedure is modified to solve the
non-linearity and non-continuity problems on the relationship between assist source intensity and each imaging
performance. Finally, optimized source shapes have been successfully demonstrated and verified on 40 nm node NAND
FLASH L/S and C/H critical patterns despite the simplicity of the optimization method, without utilizing SO dedicated
We have developed the new technology to measure focus variations in a field or over the wafer quickly for exposure tool
management. With the new technology, 2-dimensional image(s) of the whole wafer are captured with diffraction optics,
and by analyzing the image signal(s), we are able to get a focus map in an exposure field or over the entire wafer.
Diffraction-focus curve is used instead of a CD-focus curve to get the focus value from the image signal(s). The
measurements on the production patterns with the production illumination conditions are available. We can measure the
field inclination and curvature from the focus map. The performance of the new method was confirmed with a test
pattern and production patterns.
Double patterning (DP) has now become a fixture on the development roadmaps of many device manufacturers for half pitches of 32 nm and beyond. Depending on the device feature, different types of DP and double exposure (DE) are being considered. This paper focuses on the requirements of the most complex forms of DP, pitch-splitting (where line density is doubled through two exposures) and spacer processes (where a deposition process is used to achieve the final pattern). Budgets for critical dimension uniformity and overlay are presented along with tool and process requirements to achieve these budgets. Experimental results showing 45-nm lines and spaces using dry ArF lithography with a k1 factor of 0.20 are presented to highlight some of the challenges. Finally, alternatives to DP are presented.
Double patterning (DP) has now become a fixture on the development roadmaps of many device manufacturers for half
pitches of 32 nm and beyond. Depending on the device feature, different types of DP and double exposure (DE) are
being considered. This paper focuses on the requirements of the most complex forms of DP, pitch splitting, where line
density is doubled through two exposures, and sidewall processes, where a deposition process is used to achieve the final
pattern. Budgets for CD uniformity and overlay are presented along with tool and process requirements to achieve these
budgets. Experimental results showing 45 nm lines and spaces using dry ArF lithography with a k1 factor of 0.20 are
presented to highlight some of the challenges. Finally, alternatives to double patterning are presented.
The OPE signature of a lithographic stepper or scanner has become a very important characteristic of the tool, as it determines the OPC correction to be applied to reticles exposed on that tool. The signature depends on a variety of detailed information about the scanner lens and illuminator, which in turn depend on the characteristics of the illumination light from the laser. Specifically, changes in the laser bandwidth should impact OPE as the lens exhibits some chromatic aberration. Tool-to-tool differences and time fluctuation of the laser bandwidth could cause variations in OPE tool matching and stability. To assess this, a detailed study of laser bandwidth effects on OPE was performed. A sensitive spectrometer was connected to a litho laser, allowing careful measurements of both the FWHM and <i>E</i><sub>95</sub> parameters of the laser spectral profile. Lithographic modeling using the chromatic response of the lens was run in order to predict effects. Exposures of CD through pitch were made to test the modeling. Finally, the bandwidth data was correlated with litho sensitivity to create a "bandwidth effect", put in context with the other common scanner parameters affecting OPE.
Nikon has developed an illuminator with special options for RET (Resolution Enhancement Technique). For one of the solutions of RET, Nikon has pursued the development of a <i>loss-less</i> polarized illumination system. When the polarization direction is the same as the direction of the printed pattern, this technique improves image contrast and extends the process margin. We have simulated the impact of the RET with polarized illumination, in the case of dipole illumination and phase-shift masks, and we have estimated the dominant parameters for high performance polarized illumination.
In addition, we have constructed a polarized-light illuminator and installed it in an ArF full-field scanner. We have measured and optimized the degree and distribution of polarization at the wafer plane with a special tool, and we have investigated image performance with polarized dipole illumination. Results show that the new polarized-light illuminator has extended the process margin, especially with respect to dose latitude. The results of the image simulations and experiments will be reported.
Immersion lithography is becoming a realistic method of high resolution pattern generation for semiconductor manufacturing. Nikon has a roadmap of full-field immersion exposure tools starting with an Engineering Evaluation Tool (EET, NA=0.85), succeeded with production models of S609B (NA=1.07) and S6xx (NA=1.30). EET was constructed in 2004, and is being used for evaluation of immersion technology and process development. With EET, focus, stepping, overlay and across-wafer CD uniformity data are demonstrated to be better or equivalent to dry tools, while the depth of focus (DOF) is significantly improved as expected. A remarkable point is the defectivity result with EET. We have detected no bubbles and a negligible level of “immersion specific” defects even with hydrophobic top coat. A production model S609B will have the NA=1.07 optics, which will be the highest NA of “all refractive optics”, and will be shipped at 2005/4Q. S6xx, with planned shipment timing is 2006/2H, will have NA=1.30 catadioptric optics, whose NA will be the highest NA of “water-immersion”. Both S609B and S6xx will be equipped with loss-less polarized illuminators, which will enable 50nm L/S with S609B and 42nm L/S with S6xx. Resist and top coat are studied from the viewpoints of chemical contamination and scanning properties. Tentative specifications are proposed for leaching of PAG and amines against chemical contamination. As for scanning properties, static contact angle was found to be not a good parameter; instead, sliding angle is proposed.
The X Architecture is a novel on-chip interconnect architecture based on the pervasive use of diagonal wiring. This diagonal wiring reduces total chip wire length by an average 20% and via count by an average of 30%, resulting in simultaneous improvements in chip speed, power, a cost. Thirty percent or greater reduction in via counts is a compelling feature for IC design - but can chips with massive amounts of diagonal wiring be manufactured without some other penalty? This paper presents the result of a project, collaborated by Cadence Design Systems, Numerical Technologies, DuPont Photomasks, and Nikon, aimed at optimizing each step of the lithography supply chain for the Architecture from masks to wafers at 130 nm.
We introduce a projection lens adjustment procedure that is customer application oriented. This technique is based on the simulated imaging performance using Zernike sensitivity, the measurement results of wavefront aberration and wavefront change by lens element position change. This system finds the optimum combination of lens position where the amount of specific imaging performance error is in tolerance. In this paper, the idea of optimization and some optimization results are shown.
Photolithography simulation has become a common methodology used in engineering tasks such as critical level patterning analysis and process design, patterning tool qualification to meet the process control requirements, selection of the patterning tools capable of delivering requisite patterning performance, and projection lens tuning for optimum patterning performance. Such diverse use of simulation is motivated by the need to quantify the patterning tradeoffs, when the performance margins collapse around the fundamental process constraints. These complex analysis and design tasks relay on various photolithography simulators available as commercial or proprietary software. The diversity of the available simulators poses two issues: what is the role of numerical methodologies in modifying the simulation analysis otherwise limited by the image formation fundamentals, and to what extend the results obtained with different simulators are similar to each other. In this paper, we present the results of the comparison involving three simulators, two of them commercial. The comparison involved image formation simulations of the current generation of the critical IC designs. The comparison was a basis of a judgment on the portability of simulation analyses obtained by various photolithography simulation tools.
The wavefront aberration that is generally represented with Zernike polynomials is an approximate representation. The reasons are that, in actual optics, the wavefront has such frequency component that cannot be represented with Zernike polynomials. Moreover, it can never be scalar or monochromatic. Instead, it must be vector and polychromatic. Higher frequency component beyond Zernike representation could cause a local flare that will be observed in the surrounding area of nominally bright patterns. Vector aspect of light leads to imaging degradation combined with birefringence of the material. Even with a narrowed spectral bandwidth of excimer lasers, chromatic aberration could be a factor that impacts imaging performance. Lateral, rather than axial, chromatic aberration can be critical because it is influential to CD uniformity across the field. This paper describes the factors that deteriorate imaging performance based on Nikon’s optics, and finally concludes hat our optics is well balanced among these factors.
Wavefront aberrations of the projection lens are measured in many situations. The results are expressed by coefficients of Zernike polynomials that can be used as a basis for critical dimensions (CD) performance evaluation. Here we investigated several methods to obtain the CD distribution from the Zernike coefficients. We present three unique methods, called “Transformation of CD-Focus", "Conversion of Defocus into Aberrations" and “Response Surface of Aerial Image". These methods calculate CD distribution faster than direct simulations. Five tests with different cases were conducted to compare the three methods. Their accuracies are reported.
Recent introduction of phase measurement interferometer (PMI), to measure wavefront aberrations brought about rapid reduction of residual aberrations in stepper and scanner projection lenses. Zernike sensitivity method (ZSM) is useful to capture and to understand the lens aberrations impact on the imaging performances, and to guide the improvements in patterning performance of the projection tools. In this paper we present ZSM for CD-Focus curve capable of precisely predicting CD at any focus position. We found that cross-term interactions of several Zernike combinations impact ZSM for CD-Focus. We present an example of V-H difference for which the cross-term interactions dominate. Aerial image simulation results presented here are illustrated by corresponding to experimental results.
The impact of lens aberration on linewidth control and pattern shift is investigated with aerial image simulation using Zernike sensitivity method through focus to secure sufficient depth-of-focus (DOF). We found 0 and even theta component has large amount of impact on linewidth control in defocused condition due to best focus shift. This phenomenon makes degradation of DOF performance. For actual phase shift mask (PSM) application, DOF performance is very important, so as to reduce the focus shift by lens aberration, and the lens is controlled with pre-correction of best focus deviation in the field. This method is effective to obtain large DOF with PSM.
A novel technique to characterize variations of the spatial (partial) coherence (sigma) across the image field in modern steppers and scanners has been developed and experimentally tested. It is based on the high sensitivity of the length L of macroscopically large diamond-shaped marks printed in photoresist to (sigma) variation. Variations in the (sigma) value across the image field lead to variations in the length of marks printed at different image field locations. The mark lengths are measured rapidly with high accuracy by a built-in optical system and then converted into (sigma) values using the calibration dependence L((sigma) ) measured in the same exposure tool. Simulation and experimental studies show that the level of projection lens aberrations in modern Nikon tools have practically no effect on (sigma) measurements obtained with this technique. Our results demonstrate that in the conventional illumination scheme, (sigma) distribution can be measured with an accuracy of 2.5%. The main advantage of the presented method is that (sigma) variation over the image field is characterized by the exposure tool itself, avoiding expensive and time-consuming SEM measurements. Moreover, since the measurement procedure is based on the wedge-shaped marks and laser scanning system currently used in Nikon tools for automated focus detection, implementation of the technique does not require any hardware or software modification.
Two types of new optical system for 150 nm lithography are studied. One is the system with KrF source and high numerical aperture (NA), the other is the system with ArF source. By aerial image simulation, the adequate NA of each projection lens is searched, and the value was 0.68 for KrF source and 0.60 for ArF source. Then the projection lens is fabricated and evaluated. The results are almost same as those of simulation.
An innovative method of determining best focus with an optical exposure tool has been developed. The method uses wedge shaped marks in photoresist that can be measured automatically. The results show that best focus can be measured with a repeatability of 20 nm. The automatic focus measurement system can be used to characterize lens astigmatism, field curvature and tilt. Data shows good correlation with conventional methods using SEM linewidth measurement.
Two types of innovative imaging systems are studied. One is the system with non-incoherent effective source and the other is the system with non-coherent pupil function. When the optics are configured in such a way that a certain pattern would be illuminated by a group of waves diffracted by the pattern itself or an exactly the same pattern, the effective source can be made no longer incoherent. Then the coherence of the effective source can be a new parameter to improve image quality. The simulation shows that the contrast of L/S pattern imaging under the off-axis illumination, for instance, can be boosted up to 99.5% with this system, while to only 90.6% with conventional optics. A new pupil filter is proposed; the area of which is divided into some annular zones to be made mutually incoherent. Then the phase difference becomes less crucial and large DOF can be achieved. The simulation shows that this filter is effective for the exposure of contact hole patterns.
We have established a new photolithographic technique called SHRINC ( Super High Resolution by I I tumi-Nation Control ) which is based on an innovative illumination system. SHRINC improves the resolution and depth-of-focus ( DOF ) by optimum arrangement of the illumination system in respect of the angle of the Ist-order of diffraction generated by the reticle pitch.
The capabilities of SHRINC have been studied by computer simulation. Results from phase shift, annular illumination, and conventional illumination are compared with those of SHRINC. The results show that using SHRINC with 0.35? m line and space patterns, the DOF, defined as the distance over which the aerial image contrast exceeds 60%, is 2. 5x larger than that obtained with conventional illumination, and almost the same as that with phase shift techniques.
In our experiments we have obtained a critical resolution of 0. 275 ? m and more than 2.8 ? m DOF with 0.35? m L/S patterns, using an i-line stepper and SHRINC illumination.
Moreover SHRINC is effective not only for simple line and space patterns, but also for complicated patterns with 0.30 or 0.35?m design rules, such as memory cell patterns or peripheral circuit patterns in the DRAM.
From these results we conclude that i-line steppers with SHRINC will make possible mass production of 64M-DRAMs with single layer resist.