A low cost alternative lithographic technology is desired to cope with the challenges in decreasing feature size of semiconductor devices. Nano-imprint lithography (NIL) is one of the viable candidates.  NIL has been a promising solution to overcome the cost issue associated with expensive process and tool of multi patterning and EUVL. The challenges of NIL implementation for mass-production are overlay, defects, throughput, template life, and template patterning. The overlay and defects must satisfy the requirements of the products applied. The throughput needs to provide adequate cost of ownership (CoO). Since NIL is a contact process, its template damage by the particles on a wafer is inescapable and a longer template life is required for mass production.- In our previous study, we have reported that the hp2xnm NIL process performance is getting closer to the requirement for the high volume manufacturing. We focused on the process overlay accuracy and demonstrated dramatic reduction of process overlay error by using CVA(controlled viscosity alignment) and HODC(high order distortion control) function of FPA-1200 NZ2C.  Currently, we have further developed a nanoimprint lithography (NIL) technology including NIL system, template, and resist process for half pitch 14 nm direct pattering. The hp14 nm template was fabricated by a self-aligned double patterning (SADP) on a template. Using this template, we fabricated hp 14 nm dense Si lines with a depth of 50 nm on a 300 mm wafer. In this paper, we report on the latest lithography performance of NIL including hp14nm pattering with single mask exposure.
A low cost alternative lithographic technology is desired to cope with the challenges in decreasing feature size of semiconductor devices. Nano-imprint lithography (NIL) is one of the viable candidates. NIL has been a promising solution to overcome the cost issue associated with expensive process and tool of multi patterning and EUVL. NIL is a simple technology and is capable of forming critical patterns easily. On the other hand, the critical issues of NIL are defectivity, overlay, and throughput. In order to introduce NIL into the High Volume Manufacturing (HVM), it is necessary to overcome these three challenges simultaneously.- In our previous study, we have reported improvement in NIL overlay, defectivity and throughput by the optimization of resist process on a pilot line tool, FPA-1200 NZ2C. In this study, we report recent evaluation of the NIL performance to judge its applicability in semiconductor device HVM. We have described that the NIL is getting closer to the target of HVM for 2x nm half pitch.Defectivity level below 1pcs/cm2 has been achieved for the 2x nm half pitch L/S. The overlay accuracy of the test device is being improved down to 6nm or lower by introducing high order distortion correction.
Nanoimprint lithography (NIL) is a promising technique for fine-patterning with a lower cost than other lithography techniques such as EUV or immersion with multi-patterning. NIL has the potential of "single" patterning for both line patterns and hole patterns with a half-pitch of less than 20nm. NIL tools for semiconductor manufacturing employ die-by-die alignment system with moiré fringe detection which gives alignment measurement accuracy of below 1nm.
In this paper we describe the evaluation results of NIL the overlay performance using an up-to-date NIL tool for 300mm wafer. We show the progress of both "NIL-to-NIL" and "NIL-to-optical tool" distortion matching techniques. From these analyses based on actual NIL overlay data, we discuss the possibility of NIL overlay evolution to realize an on-product overlay accuracy to 3nm and beyond.
A low cost alternative lithographic technology is desired to meet the decreasing feature size of semiconductor devices. Nano-imprint lithography (NIL) is one of the candidates for alternative lithographic technologies. NIL has such advantages as good resolution, critical dimension (CD) uniformity and low line edge roughness (LER). On the other hand, the critical issues of NIL are defectivity, overlay, and throughput. In order to introduce NIL into the HVM, it is necessary to overcome these three challenges simultaneously.- In our previous study, we have reported a dramatic improvement in NIL process defectivity on a pilot line tool, FPA-1100 NZ2. We have described that the NIL process for 2x nm half pitch is getting closer to the target of HVM. In this study, we report the recent evaluation of the NIL process performance to judge the applicability of NIL to memory device fabrications. In detail, the CD uniformity and LER are found to be less than 2nm. The overlay accuracy of the test device is less than 7nm. A defectivity level of below 1pcs./cm2 has been achieved at a throughput of 15 wafers per hour.
CD-SEM is now attracting attention as a tool that can accurately measure positional error of device patterns. However, the measurement accuracy can get worse due to pattern asymmetry as in the case of image based overlay (IBO) and diffraction based overlay (DBO). For IBO and DBO, a way of correcting the inaccuracy arising from measurement patterns was suggested. For CD-SEM, although a way of correcting CD bias was proposed, it has not been argued how to correct the inaccuracy arising from pattern asymmetry using CD-SEM. In this study we will propose how to quantify and correct the measurement inaccuracy affected by pattern asymmetry.
Thermal aberration becomes a serious problem in the production of semiconductors for which low-k1 immersion lithography with a strong off-axis illumination, such as dipole setting, is used. The illumination setting localizes energy of the light in the projection lens, bringing about localized temperature rise. The temperature change varies lens refractive index and thus generates aberrations. The phenomenon is called thermal aberration. For realizing manufacturability of fine patterns with high productivity, thermal aberration control is important. Since heating areas in the projection lens are determined by source shape and distribution of diffracted light by a mask, the diffracted pupilgram convolving illumination source shape with diffraction distribution can be calculated using mask layout data for the thermal aberration prediction. Thermal aberration is calculated as a function of accumulated irradiation power. We have evaluated the thermal aberration computational prediction and control technology “Thermal Aberration Optimizer” (ThAO) on a Nikon immersion system. The thermal aberration prediction consists of two steps. The first step is prediction of the diffraction map on the projection pupil. The second step is computing thermal aberration from the diffraction map using a lens thermal model and an aberration correction function. We performed a verification test for ThAO using a mask of 1x-nm memory and strong off-axis illumination. We clarified the current performance of thermal aberration prediction, and also confirmed that the impacts of thermal aberration of NSR-S621D on CD and overlay for our 1x-nm memory pattern are very small. Accurate thermal aberration prediction with ThAO will enable thermal aberration risk-free lithography for semiconductor chip production.
As feature size shrinkage in semiconductor device progress, process fluctuation, especially focus strongly affects device performance. Because focus control is an ongoing challenge in optical lithography, various studies have sought for improving focus monitoring and control. Focus errors are due to wafers, exposure tools, reticles, QCs, and so on. Few studies are performed to minimize the measurement errors of auto focus (AF) sensors of exposure tool, especially when processed wafers are exposed. With current focus measurement techniques, the phase shift grating (PSG) focus monitor 1) has been already proposed and its basic principle is that the intensity of the diffraction light of the mask pattern is made asymmetric by arranging a π/2 phase shift area on a reticle. The resist pattern exposed at the defocus position is shifted on the wafer and shifted pattern can be easily measured using an overlay inspection tool. However, it is difficult to measure shifted pattern for the pattern on the processed wafer because of interruptions caused by other patterns in the underlayer. In this paper, we therefore propose "SEM-PSG" technique, where the shift of the PSG resist mark is measured by employing critical dimension-scanning electron microscope (CD-SEM) to measure the focus error on the processed wafer. First, we evaluate the accuracy of SEM-PSG technique. Second, by applying the SEM-PSG technique and feeding the results back to the exposure, we evaluate the focus accuracy on processed wafers. By applying SEM-PSG feedback, the focus accuracy on the processed wafer was improved from 40 to 29 nm in 3σ.
Liquid immersion lithography tools with NA=1.3 are being applied for hp4x node device and beyond. As the typical
DOF has been reduced to 100 nm or less for this hyper-NA exposure, precise consideration of the error factors that lead
to DOF reduction is required. The BFD (Best Focus Difference between pattern kinds) induced by resist stack is one of
the error factors yet to be solved. In this work, the BFD induced by resist stack was studied through simulation and
experiment by decomposing it into the following three components: One induced by the "refraction" effect, one induced
by the "reflection" effect and one induced by the "process" effect. Each BFD was evaluated by simulation to make a
BFD budget. Experimental verification was also performed, which confirmed BFD of 25 nm and it was found that over
30 % of the observed BFD was induced owing to resist stack. Based on this result, we discuss a method for controlling
the BFD by utilizing projection lens aberration and review the design flow of resist stacks with antireflection coatings.
Flow of fixing of hot spot induced by optical variation among exposure tools is discussed for quick ramp-up of high volume products. To achieve robust pattern formation for optical variation, following hot spot detection and fixing approaches are introduced: i) at the design stage, hot spot detection within the optical variation space and hot spot fixing by layout modification or OPC optimization, ii) in order to efficiently detect hot spots within the optical variation space, lithography simulation by combinations of optical parameters determined by the design of experiment (DoE), iii) at the manufacturing stage, hot spot fixing by adjustment of optical parameters using the multi-variable optimization to match OPE between the primary and secondary exposure tool.
The purpose of this work was to identify the specific effects of mask topography by analyzing in the Fourier domain.
Our focus patterns extend from a simple contact hole (CH) with a fixed pitch and bias to ones that have a variety of
different pitches and hole sizes. We also attempt to predict phases and amplitudes of diffraction on the pupil plane
without a rigorous mask topography approximated model. Intensities of CH patterns are simulated using three mask
models. We had determined that there are serious differences among the three mask models concerning the contrast of
the intensity and the qualitative interpretation of the trend of contrast varies according to pitch and hole sizes.
The mask topography effects can be classified into waveguide and shadowing effects simply by using the diffraction
decomposition diagram. We clarify how much and when the mask topography influences imaging under hyper-NA
lithography by the diagram. From 1D near-field phase distribution, it is clarified that phase distribution has also been
decided by the MoSi width between holes so that waveguide effects are not only from hole but also from MoSi area.
It has been determined that the influence of the real 3D structures of the mask under the hyper-NA condition cannot
be disregarded. However, use of the rigorous EMF calculation costs much more and requires more time than using a
non-EMF calculation. We have also clarified the mechanism of 3D mask effects based on the amplitude and the phase
of the diffraction light in the Fourier-domain diagram and examined whether the 3D mask effects can be predicted by
thin mask approximation (TMA) and found that once we have values of amplitude and phase of the 0th and the 1st
diffraction in TMA, it will be possible to predict the values of the other pitch and the other hole size.
Robust optical proximity correction (OPC) and design for manufacturability (DFM) methodology for optical
variation among exposure tools is proposed. It is demonstrated that application of the methodology improves standard
deviation of CD difference for target CD by 33% compared with the case of using the conventional methodology. Under
the low-k1 lithography condition, hot spots induced by optical variation among exposure tools delay ramp-up of
production of high-volume products. To realize robust pattern formation for all exposure tools, the following new
methodologies are introduced : i) OPC modeling methodology using actual optics of primary tool, ii) OPC processing
methodology using averaged or designed optics, iii) at the design stage, hot spot detection within the optical variation
space centered on average or designed optics and hot spot fixing by layout modification or OPC optimization, iv) at the
manufacturing stage, hot spot detection using actual optics and hot spot fixing by optical adjustment of troubled tool.
In the case of hyper-NA (NA>1) imaging with the lens magnification keeping 1/4, the angle
of light incidence on pellicle becomes bigger. For example, it is up to 19 degrees for NA=1.3 lens. It
is already known that the effect of multiple reflections of the light inside the pellicle film becomes
obvious, in that the effect contains transmission variation across the light incidence angle on the
pellicle. For normal pellicle, transmission of oblique incidence light is lower than the normal
incidence light and the difference is about 10% as intensity changes. And pellicle thickness error
affects the transmission characteristics. Thus, pellicle thickness error causes change of iso-dense
bias (or optical proximity effect; OPE) and dense line CD variation.
Specs for CD uniformity in below half pitch (hp) 45nm imaging become tighter, and
therefore, pellicle should not be a new root cause of CD error. The solutions for the issue are (1)
tighter specs for pellicle thickness or (2) selection of optimal pellicle thickness. The latter is more
effective for suppressing CD variation across the exposure field than the former.
In our paper, we describe the pellicle effect for through-pitch imaging including below hp45
nm dense L/S using hyper-NA lens. We discuss pellicle thickness optimization for better CD
uniformity and the results of simulation for some pellicle conditions.
A hyper-NA lithography tool is used in production of the latest devices. In the next generation immersion lithography,
issues that had so for neglected had to be considered because NA of illumination optics is larger than conventional tools.
Here, items were listed up for accurate prediction of imaging by optical simulation. These were transmittance of
illumination rays to the mask, mask induced effects such as polarization and aberration, and pellicle induced effect. These
were depending on incident angle. Therefore consideration of angle dependency of these effects was necessary for more
accurate imaging simulation. We presented the requirements for simulation to facilitate discussion of the imaging
performance of below 40 nm hp pattern node immersion lithography.
The purpose of this work was to find the specific effects of hole patterns in 32nm node logic by analyzing in the Fourier domain and to clarify the mechanism of mask topography effects. Our focus patterns extend from the lines and spaces (LS) to the contact hole (CH). We also attempt to perform factor analyses of mask topography effects.
Intensities of LS and CH patterns are simulated using three mask models. For each of the three models, the method of approximating the mask topography effect is different. As a result, a serious difference among the three mask models has been found with respect to the intensity profile for 32nm node and beyond, though the mask sizes for all models are the same.
As the accuracy of mask model improved, it was found that the image contrast tends to decrease on LS patterns while increasing on CH patterns. The qualitative interpretation of the trend of contrast variations can be described by analyzing in the Fourier domain.
The mask topography effects can be separated into waveguide and shadowing effects using scatter graphs.
It is concluded from the result that one of the major differences between LS and CH is attributable to phase differences between 0th order and 1st order diffractions, because the size of effects for CH have been larger than that for LS.
In the exposure using ArF immersion exposure tool, under the conditions in which the mask
pattern pitch is smaller than several times the exposure wavelength, diffraction light distribution
cannot be predicted correctly by the Kirchhoff approximation mask model, and therefore, rigorous
electromagnetic field analysis, or 3D mask model, is required. In particular, in the dense line and
space (L/S) formation using oblique illumination and an attenuated phase shifting mask (att-PSM),
the intensity of 0th and 1st diffraction lights changes as pitch shrinks.
In dense L/S formation, it is necessary to reduce a mask error enhancement factor (MEF) and
to obtain sufficient exposure latitude. We consider the following three contrast control "knobs" (CCKs):
(1) Mask bias, (2) Transmittance of attenuating mask material (absorber), (3) Thickness of absorber.
We also considered the effect of illumination angle of incidence on 3D mask.
We performed a simple optimization for exposure latitude of dense L/S pattern, reflecting
consideration of the mask 3D model for half pitch (hp) 45nm L&S imaging using att-PSM and oblique
illumination. The important image characteristics are normalized image log slope (NILS) and dose-
MEF for obtaining sufficient exposure latitude.
We carried out an experiment of attenuated PSM exposure using hyper-NA exposure tools and
compared the results with the 3D mask simulation. The degree of agreement between the experiment
and the 3D mask simulation, and the practical effectiveness of the CCKs are discussed in this paper.
In the exposure using ArF immersion exposure tool, under the conditions in which the mask pattern pitch is smaller than a several times of the exposure wavelength, diffraction light distribution cannot be predicted correctly by the Kirchhoff approximation mask model, and therefore, rigorous Electromagnetic Field (EMF) analysis is required. In particular, in the dense L&S formation using oblique illumination and an attenuated phase shift mask (att-PSM), the intensity of 0th and 1st diffraction lights changes as pitch shrinks.
In high density L&S formation, it is necessary to reduce a mask error enhancement factor (MEF) and to obtain sufficient exposure latitude. We consider the following two contrast control knobs: (1) optimizing the transmittance of attenuated mask material, (2) optimizing mask bias. The important image characteristics are normalized image log slope (NILS) and dose-MEF. Dose-MEF means a dose to size change per mask critical dimension (CD) change.
We performed a simple optimization for exposure-defocus window of dense L&S pattern reflecting consideration of the mask EMF model for half pitch 45nm L&S imaging using att-PSM and oblique illumination. We explain the characteristics of the contrast control knobs and their effectiveness. An optimized combination of contrast control knobs depends on the capability of mask CD process as a smallest limit of mask CD and mask CD uniformity.
In recent low-k1 lithography, the size of a mask pattern is becoming close to wavelength of the light source. The light intensity through the mask pattern is depending on polarization. TM polarization light is higher transmission than TE polarization light for a MoSi mask. This effect influences not only the zeroth-order light but the first-order light. On the other hand, TE polarization imaging makes higher contrast than TM polarization in two beam interference. Effects of
polarization to resolution are not simple. Since an attenuated phase shift mask is used in order to obtain high contrast, it is necessary to take into consideration the influence of that. It is also taken into consideration that illumination light is not perpendicular incidence but oblique incidence for an ArF hyper-NA tool. We will perform a rigorous simulation in consideration of the above conditions. Hereby influence of the to the utmost resolution will be clarified by the rigorous simulation.
Mid-range flare (MRF) of an ArF exposure tool induces resist critical dimension (CD) variation depending on local mask (or resist) pattern density. CD correction techniques such as mask CD modification are needed to compensate MRF-induced CD variation and obtain sufficient resist CD uniformity. For this purpose, MRF should be accurately characterized and distinguished from other factors of similar influence caused by photoresist, mask, and so on. We have investigated a method of measuring magnitude and affected range of MRF in an exposure tool easily. In this method, double exposure of L/S with large area and a small square "window" in a widely shielded area is executed on a positive photoresist layer. After bake and development, resist CDs of the L/S are measured from near the window to far from the window using scanning electron microscope. An overcoat layer is used to exclude the influence of acid evaporation from photoresist and re-sticking. L/S mask shows small CD variation because local pattern density is uniform, and so influence of mask error on MRF measurement is small. Influence of MRF is shown in the graph of distance from window edge versus CD. From the distance-ΔCD curve, point spread function (PSF) which represents the characteristics of MRF can be obtained. Comparison of experimentally obtained MRF-induced CD variation, which appears in the periphery of a memory cell area, and calculated CD variation using the PSF obtained in the exposure tool showed good agreement.
We propose a methodology that separates critical dimension (CD) variation depending on local pattern density into error sources; CD error on exposure mask, mid-range flare of exposure tool, and acid evaporation during post exposure baking (PEB). This methodology consists of two particular processes. One is over-coating process onto resist film before exposure, and the other is double exposure process using test reticles. The test reticles have line-and-space (L/S) array region and peripheral region that contains opaque area to avoid overlapping with L/S array region in double exposure process. Over-coating process allows separation of acid evaporation from other error sources. On the other hand, double exposure process enables elimination of CD error on exposure mask depending on local pattern density since double exposure process can make various pattern densities around fixed L/S mask pattern. In fact, the experimental CD results could provide the good agreement with estimated CD results using mid-range flare model. It has become clear that the influence of local pattern density increases with design rule shrinkage. Furthermore, the methodology revealed that the influence of each error source is greatly dependent on local pattern density. Consequently, the methodology is effective to separate CD variation on local pattern density into error sources.
In 45nm-node CMOS, the k1 value is around 0.35. In the low-k1 lithography, the robust design for lens aberration and process fluctuation such as mask CD error is required for manufacturing. The technologies of robust design for 45nm-node CMOS are proposed. The alternating phase shift mask has been applied to obtain high accurate CD controllability for gate level. Since the sensitivity to lens aberration is high, design rule is restricted. Immersion lithography with hyper NA over 1.0 is necessary for contact hole level to get large DOF margin. Since the mask enhanced error factor is large, high accurate CD uniformity on mask is necessary. Using hyper NA immersion tool, high density SRAM whose area is 0.25um2 can be clearly resolved.
The double line and space (L&S) formation method with L&S masks and dipole illumination was found to have high capability to fabricate ~0.3 k1 contact hole (C/H) pattern. The procedure was as follows. The first L&S pattern was formed and was hardened to avoid the dissolution and mixing during the second resist coating. The second L&S pattern perpendicular to the first one was formed on the first resist pattern. The common space area of the two patterns became 1:1 C/H pattern. Simulation results showed that the double L&S formation method has much wider lithography latitude than other methods, such as single exposure of a C/H mask with quadrupole illumination, single exposure of a vortex mask with conventional illumination, and double exposure of L&S masks with dipole illumination to a single-layer resist. A 75 nm (0.30 k1) 1:1 C/H pattern was fabricated. An 80 nm (0.32 k1) 1:1 C/H pattern had 280 and 600 nm depth of focus in each resist layer at 8% exposure latitude. Moreover, a new method, in which a C/H mask replaces the L&S masks, is proposed to achieve cost reduction and the same high performance as the L&S masks.
An Illumination intensity distribution of an exposure tool varies CD in simulation. In order to obtain reliable resist parameters, we studied the influence of the illumination intensity distribution in tuning the resist parameters and the accuracy of the simulation using the tuned resist parameters under different illumination conditions from in tuning. We tuned resist parameters with two models of illumination intensity to experimental FEM data. One model, "Nominal", was assumed to be uniform intensity and a nominal shape of an exposure tool. Another model, "Measured", was measured illumination intensity distribution with grating-pinhole mask.
Under the same illumination condition to tuning, RMS of CD difference between experiment and simulation using "Measured" in tuning and simulation was 0.7nm smaller than that using "Nominal". But under the different illumination condition from tuning, RMS using "Measured" was 1.4 - 1.6nm smaller in total of 1D-pattern than that using "Nominal". In the specific pattern RMS using "Measured" was rather smaller than RMS using "Nominal". These results indicate that, in order to gain accurate simulation result, the accurate illumination intensity distributions is need in tuning and simulation.
If using "Nominal" in tuning and in simulation, CD difference between experiment and simulation will enlarge in fine patterns.
The double line and space (L&S) formation method with L&S masks and dipole illumination was found to have high capability to fabricate about 0.3-k1 contact hole (C/H) pattern. The procedure was as follows. The first L&S pattern was formed and was hardened to avoid the dissolution and mixing during the second resist coating. The second L&S pattern perpendicular to the first one was formed on the first resist pattern. The common space area of the two patterns became 1:1 C/H pattern. Simulation results showed that the double L&S formation method has much wider lithography latitude than other methods, such as single exposure of a C/H mask with quadrupole illumination, single exposure of a vortex mask with conventional illumination, and double exposure of L&S masks with dipole illumination to a single layer resist. 75-nm (0.30-k1) 1:1 C/H pattern was fabricated. 80-nm (0.32-k1) 1:1 C/H pattern had 280 nm and 600 nm depth of focus (DOF) in each resist layer. Moreover, a new method, in which a C/H mask replaces the L&S masks, is proposed to achieve cost reduction and the same high performance as the L&S masks.
Transmittance of projection lenses used in DUV exposure tools changes depending on the exposure light path. We studied the impact of this phenomenon (Across Pupil Transmittance Variation, APTV) on fine device pattern imaging. Zernike polynomials, which are commonly used for analysis of wavefront aberration, were applied for inspecting the influence of APTV. We investigated influences of each Zernike component on fine pattern imaging, and clarified that a significant component of APTV can be extracted by Zernike polynomial series expansion. Even components of APTV cause CD variation depending on the pattern pitch, that is, change of optical proximity effect (OPE), while odd components of APTV have little influence. In this paper, we present a method of decomposing APTV in Zernike coefficients together with the results of Zernike sensitivity analysis of the influence on OPE change. APTV was found to differ from tool to tool, and varies across the exposure field. Impact of tool dependence, across-field variation, and vertical-horizontal deviation of APTV are discussed.
A new method of measuring across pupil transmittance variation (APTV) is proposed. APTV measurement can be performed using a grating-pinhole (GP) mask, which comprises grating stricture in a small circular area, small-sigma illumination, and common wafer process. A measurement mark consists of twelve kinds of GPs with the same duty ratio, different pitches, and different orientations. First-order beams generated at GPs with the same intensity travel through the lens and reach the wafer, in which the relative intensities of first-orders are measured and compared. Transmittance distribution of 24 points across the pupil plane associated with approximately one image point is measured. In addition, exposure field dependency of APTV is measured by arranging some identical sets of GPs across the exposure field. We have successfully measured APTV for an argon fluoride excimer laser (ArF) exposure tool as a demonstration of this method. The measurement error of relative intensity due to mask fabrication error has been estimated to be less than 0.4%. The influences of APTV on lithography have been investigated. Simulation results indicated that unbalanced APTV causes asymmetric modification of a symmetric pattern or periodic pattern edge.
A methodology for measuring the effective illumination source shift in exposure tools has been established. A grating-pinhole mask is placed upside-down on mask stage, and exposed. This mask consists of square pinholes with 80 micrometers square and 2D square lattices in these pinholes. The pitch of the grating pattern is suitably designed so that the 1st-order diffraction beams can illuminate the edge of the pupil of the projection optics. Both the shape of illumination source and the silhouette of the pupil of the projection optics are projected on the wafer located by normal photoresist. A conventional optical microscope is available for easily observing the photoresist patterns. The grating-pinhole consisting of attenuated phase-shifting structure has found to be also effective to measure both effective coherence factor and intensity non-uniformity of effective illumination source.