The extension of optical lithography to 7 nm node and beyond relies heavily on multiple litho-etch patterning technologies. The etch processes in multiple patterning often require progressively large bias differences between litho and etch as the target features become smaller. Moreover, since this litho-etch bias has strong pattern dependency, it must be taken into consideration during the Optical Proximity Correction (OPC) processes. Traditionally, two approaches are used to compensate etch biases: rule-based retargeting and model-based retargeting. The rule-based approach has a turn-around-time advantage but now has challenges meeting the increasingly tighter critical dimension (CD) requirements using a reasonable etch-bias table, especially for complex 2D patterns. Alternatively, model-based retargeting can meet these CD requirements by capturing the etch process physics with high accuracy, including the etch bias variability that arises from both patterning proximity effects and etch chamber non-uniformity. In the past, empirical terms have been used to approximate the etch bias due to pattern proximity effects but sometimes empirical models are known to have compromised model accuracy so a physical based approach is desired. This paper’s work will address the etch bias variability due to patterning proximity effects by using a physical approach based simplified chemical kinetics. It starts from a well calibrated After-Development-Inspection (ADI) model and the subsequent etch model is based on the ADI model contour. By assuming that plasma chemical species in the trenches are maintained in an equilibrium state, the plasma species act on the edges to induce etch bias. Methods are developed to evaluate plasma collision probability on trench edges for random layouts. Furthermore, the impact of resist materials on etch bias are treated with Arrhenius equation or as a second order reaction. Equations governing plasma collision probabilities on trench edges as a function of time are derived. An etch bias model can be calibrated based on those equations. Experimental results have shown that this physical approach to model etch bias is a promising direction to applications for full-chip etch proximity corrections.
Strong resist shrinkage effects have been widely observed in resist profiles after negative tone development (NTD) and therefore must be taken into account in computational lithography applications. However, existing lithography simulation tools, especially those designed for full-chip applications, lack resist shrinkage modeling capabilities because they are not needed until only recently when NTD processes begin to replace the conventional positive tone development (PTD) processes where resist shrinkage effects are negligible. In this work we describe the development of a physical resist shrinkage (PRS) model for full-chip lithography simulations and present its accuracy evaluation against experimental data.
3D Resist profile aware OPC has becoming increasingly important to address hot spots generated at etch processes
due to the mass occurrence of non-ideal resist profile in 28nm technology node and beyond. It is therefore critical to
build compact models capable of 3D simulation for OPC applications. A straightforward and simple approach is to
build individual 2D models at different image depths either based on actual wafer measurement data or virtual
simulation data from rigorous lithography simulators. Individual models at interested heights can be used by
downstream OPC/LRC tools to account for 3D resist profile effects. However, the relevant image depths need be
predetermined due to the discontinuous nature of the methodology itself. Furthermore, the physical commonality
among the individual 2D models may deviate from each other as well during the separate calibration processes. To
overcome the drawbacks, efforts are made in this paper to compute the whole bulk image using Hopkins equation in
one shot. The bulk image is then used to build 3D resist models. This approach also opens the feasibility of
including resist interface effects (for example, top or bottom out-diffusion), which are important to resist profile
formation, into a compact 3D resist model. The interface effects calculations are merged into the bulk image
Hopkins equation. Simulation experiments are conducted to demonstrate that resist profile heavily rely on interface
conditions. Our experimental results show that those interface effects can be accurately simulated with reference to
rigorous simulation results. In modeling reality, such a 3D resist model can be calibrated with data from discrete
image planes but can be used at arbitrary interpolated planes. One obvious advantage of this 3D resist model
approach is that the 3D model is more physically represented by a common set of resist parameters (in contrast to
the individual model approach) for 3D resist profile simulation. A full model calibration test is conducted on a
virtual lithography process. It is demonstrated that 3D resist profile of the process can be precisely captured by this
method. It is shown that the resist model can be carried to a different lithography process with same resist setup but
a different illumination source without model any accuracy degradation. In an additional test, the model is used to
demonstrate the capability of resist 3D profile correction by ILT.
Mask topography (Mask3D) effect is one of the most influential factors in sub-28 nm technology node. To build a successful Mask3D compact model, the runtime efficiency, accuracy and the flexibility to handle various geometry patterns are the three most important criterion to fulfill. In the meanwhile, Mask3D modeling must be able to handle the off-axis illumination (OAI) condition accurately. In this paper, we propose our full chip Mask3D modeling method which is an extension to the edge-based Mask3D model. In our modeling flow, we first review the edge-based Mask3D model and then analyze the impact from the off-axis source. We propose a parameter-based extension to characterize the off-axis impact efficiently. We further introduce two methods to calibrate the OAI-aware parameters by using rigorous or wafer data as the reference. Our experimental results show the great calibration accuracy throughout the defocus range with OAI sources, and validate the accuracy of our two parameter calibration approach.
As the technology node keeps shrinking down to sub-28 nm, mask topography (Mask3D) effect is one of the most influential factors to draw intensive research lately. To build a successful Mask3D compact model, the runtime efficiency, accuracy and the flexibility to handle various geometry patterns are the three most important criterion to fulfill. Different approaches have been tried to resolve the difficulties in the full-chip modeling, but so far none of the existing Mask3D modeling methods have succeeded in meeting all the three criterion at the same time. It is often seen that an existing Mask3D model to succeed in one or two criteria, but fails in the rest. In this paper, we propose our innovative full chip Mask3D modeling method to successfully handle the above criterion at the same time. To our best of knowledge, it is the first ever Mask3D modeling in literature that is be able to achieve this goal. In our modeling flow, we first analyze the Mask3D effect by using rigorous simulation as the reference and generate edge-based kernels to mimic the Mask3D effect near the feature boundaries. The flexibility of handling the kernel helps us enable the support for all-angle patterns and be extendable for edge coupling effect and off-axis illumination. Our experimental results show that with only less than 30% runtime overhead compared to the conventional Mask2D model, we are able to achieve less than 0.8 nm CD RMS on the flexible feature patterns. An ILT-based OPC and simulation result is provided to validate the capability of all-angle support of our proposed model.
While critical lithographic feature size diminishes, resist profile can vary significantly as image varies. As a consequence, the final etch results are becoming more dependent on 3D resist profile rather than only a simple 2D resist image as an etch mask. Therefore, it has become necessary to build resist profile information into OPC models, which traditionally only contain 2D information in the x-y plane. At the same time, rigorous lithographic simulators are capable of modeling 3D resist profiles on a small chip area. In this work, one approach is investigated to account for 3D resist profile characteristics in full-chip OPC models with the assistance of rigorous simulation. With measurement data collected from experimental wafers, a rigorous resist model is first calibrated and verified. Then individual compact models are built to match the rigorous resist model profile at specified resist heights. The calibrated compact model for bottom resist line width corresponds to a conventional OPC model while resist profile is described by multiple models specified for certain resist heights, with each model being in the form of conventional compact models. In practice, the bottom model along with one or two models at critical heights are usually sufficient to detect sites where etch results become sensitive to resist profile. It has been found that the rigorous resist profile model can be well matched by the suggested compact models. For a quick application demonstration, hot spots of the etch results in the test case have been shown to be successfully captured by the calibrated compact models.
Traditional rule-based and model-based OPC methods only simulate in a very local area (generally less than 1um) to identify and correct for systematic optical or process problems. Despite this limitation, however, these methods have been very successful for many technology generations and have been a major reason for the industry being able to tremendously push down lithographic K1. This is also enabled by overall good across-exposure field lithographic process control which has been able to minimize longer range effects across the field. Now, however, the situation has now become more complex. The lithographic single exposure resolution limit with 1.35NA tools remains about 80nm pitch but the final wafer dimensions and final wafer pitches required in advanced technologies continue to scale down. This is putting severe strain on lithographic process and OPC CD control. Therefore, formerly less important 2nd order effects are now starting to have significant CD control impact if not corrected for. In this paper, we provide examples and discussion of how optical and chemical flare related effects are becoming more problematic, especially at the boundaries of large, dense memory arrays. We then introduce a practical correction method for these systematic effects which reuses some of the recent long range effect correcting OPC techniques developed for EUV pattern correction (such as EUV flare). We next provide analysis of the benefits of these OPC methods for chemical flare issues in 193nm lithography very low K1 lithography. Finally, we summarize our work and briefly mention possible future extensions.
As modern photolithography feature sizes reduce, the use of sub-resolution assist features (SRAFs) to improve the
manufacturing process window has become more prevalent. Beyond the assist features placement based on rules, a
model based assist feature (MBAF) flow is needed to optimize the shape and the size of SRAFs, so that the process
margin of the main features (MFs) is maximized. In the MBAF flow, a vital component is to build an accurate model
that specifically checks the printability of SRAFs, which are supposed to leave no trace on wafer. Compared to the
traditional optical proximity correction (OPC) model, the SRAF printability check model faces extra challenges, for
example, the small size of SRAFs makes their direct transfer to the mask pattern more difficult, the SRAFs are usually
not measurable on wafer and the worst-case SRAFs printability is typically at off-nominal conditions. In this paper, we
propose an innovative binary modeling method for SRAF printability check model, which does not require the
measurement of SRAFs' size on wafer and yet provides accurate prediction of SRAFs printing on wafer. In this
modeling method, the binary determination of whether an SRAF prints/does not print (i.e., clean) on wafer was acquired
by inspecting the SEMs taken from real wafer measurements. Then the local extrema of the signal intensity around the
SRAFs was simulated and used to classify print/clean groups of SRAFs, and a special cost function was designed to
separate the print SRAFs and clean SRAFs as much as possible during model calibration.
This paper presents a novel mask corner rounding (MCR) modeling approach based on Synopsys' Integrated Mask and
Optics (IMO) modeling framework. The point spread functions of single, double, and elliptical Gaussians are applied to
the IMO mask kernels to simulate MCR effects. The simulation results on two dimensional patterns indicate that the
aerial image intensity variation is proportional to the MCR induced effective area variations for single type corners. The
relationship may be reversed when multiple types of corners exist, where the corners close to the maximum intensity
region have a greater influence than others. The CD variations due to MCR can be estimated by the effective area
variation ratio and the image slope around the threshold. The good fitting results on line-end patterns indicate that the
ΔCD is the quadratic function of the Gaussian standard deviations. OPC modeling on 28nm-node contacts shows that
MCR has significant impact on model fitting results and process window controls. By considering the real mask
geometry effects and allowing in-line calibration of model parameters, the IMO simulation framework significantly
improves the OPC model accuracy, and maintains the calibration speed at a good level.
As semiconductor manufacturing moves to 32nm and 22nm technology nodes with 193nm water immersion
lithography, the demand for more accurate OPC modeling is unprecedented to accommodate the diminishing
process margin. Among all the challenges, modeling the process of Chemically Amplified Resist (CAR) is a
difficult and critical one to overcome. The difficulty lies in the fact that it is an extremely complex physical and
chemical process. Although there are well-studied CAR process models, those are usually developed for TCAD
rigorous lithography simulators, making them unsuitable for OPC simulation tasks in view of their full-chip
capability at an acceptable turn-around time. In our recent endeavors, a simplified reaction-diffusion model capable
of full-chip simulation was investigated for simulating the Post-Exposure-Bake (PEB) step in a CAR process. This
model uses aerial image intensity and background base concentration as inputs along with a small number of
parameters to account for the diffusion and quenching of acid and base in the resist film. It is appropriate for OPC
models with regards to speed, accuracy and experimental tuning. Based on wafer measurement data, the parameters
can be regressed to optimize model prediction accuracy. This method has been tested to model numerous CAR
processes with wafer measurement data sets. Model residual of 1nm RMS and superior resist edge contour
predictions have been observed. Analysis has shown that the so-obtained resist models are separable from the effects
of optical system, i.e., the calibrated resist model with one illumination condition can be carried to a process with
different illumination conditions. It is shown that the simplified CAR system has great potential of being applicable
to full-chip OPC simulation.
The mechanism of chemically amplified resist plays a critical role in the modeling of the latent image. To achieve a
practical model which can fit into the time frame of OPC, some simplifications and assumptions have to be made. We
introduced regression kernels that take into account best exposure focus difference between isotropic pitch, dense, and
line end features for the evaluation of image intensity. It compares the image intensity (signal) over small changes
above and/or below the regressed "nominal" image position, which in principle corresponds to evaluating the intensity
signal at various depths of a fixed resist profile thus can also be regressed for optimization during model development.
Our calibration has shown that the model brought a great improvement in prediction for difficult structures such as dense
features at or near the optical resolution limit and 2-dimensional features, which are the limiter of the overall model
fitting accuracy for 45nm node and below. By replacing other existing techniques, total number of output kernels used
for OPC operation is actually reduced with improvement of model accuracy. This model is proven to be a very effective
yet accurate addition to the current OPC technology.
A precise lithographic model has always been a critical component for the technique of Optical Proximity Correction
(OPC) since it was introduced a decade ago . As semiconductor manufacturing moves to 32nm and 22nm technology
nodes with 193nm wafer immersion lithography, the demand for more accurate models is unprecedented to predict
complex imaging phenomena at high numerical aperture (NA) with aggressive illumination conditions necessary for
these nodes. An OPC model may comprise all the physical processing components from mask e-beam writing steps to
final CDSEM measurement of the feature dimensions. In order to provide a precise model, it is desired that every
component involved in the processing physics be accurately modeled using minimum metrology data. In the past years,
much attention has been paid to studying mask 3-D effects, mask writing limitations, laser spectrum profile, lens pupil
polarization/apodization, source shape characterization, stage vibration, and so on. However, relatively fewer studies
have been devoted to modeling of the development process of resist film though it is an essential processing step that
cannot be neglected. Instead, threshold models are commonly used to approximate resist development behavior. While
resist models capable of simulating development path are widely used in many commercial lithography simulators, the
lack of this component in current OPC modeling lies in the fact that direct adoption of those development models into
OPC modeling compromises its capability of full chip simulation. In this work, we have successfully incorporated a
photoresist development model into production OPC modeling software without sacrificing its full chip capability. The
resist film development behavior is simulated in the model to incorporate observed complex resist phenomena such as
surface inhibition, developer mass transport, HMDS poisoning, development contrast, etc. The necessary parameters are
calibrated using metrology data in the same way that current model calibration is done. The method is validated with a
rigorous lithography process simulation tool which is based on physical models to simulate and predict effects during the
resist PEB and development process. Furthermore, an experimental lithographic process was modeled using this new
methodology, showing significant improvement in modeling accuracy in compassion to a traditional model. Layout
correction test has shown that the new model form is equivalent to traditional model forms in terms of correction
convergence and speed.
As the semiconductor industry moves to the 45nm node and beyond, the tolerable
lithography process window significantly shrinks due to the combined use of high NA
and low k1 factor. This is exacerbated by the fact that the usable depth of focus at 45nm
node for critical layer is 200nm or less. Traditional Optical Proximity Correction (OPC)
only computes the optimal pattern layout to optimize its lithography patterning at
nominal process condition (nominal defocus and nominal exposure dose) according to an
OPC model calibrated at this nominal condition, and this may put the post-OPC layout at
non-negligible patterning failure risk due to the inevitable process variation (mainly
defocus and dose variations). With a little sacrifice at the nominal condition, process
variation aware OPC can greatly enhance the robustness of post-OPC layout patterning in
the presence of defocus and dose variation. There is also an increasing demand for
through process window lithography verification for post-OPC circuit layout. The corner
stone for successful process variation aware OPC and lithography verification is an
accurately calibrated continuous process window model which is a continuous function
of defocus and dose. This calibrated model needs to be able to interpolate and extrapolate
in the usable process window.
Based on Synopsys' OPC modeling software package-ProGen and ProGenPlus, we
developed an automated process window (PW) modeling module, which can build
process variation aware process window OPC model with continuously adjustable
process parameters: defocus and dose. The calibration of this continuous PW model was
performed in a single calibration process using silicon measurement at nominal condition
and off-focus-off-dose conditions. Through the example of several process window
models for layers at 45nm technology nodes, we demonstrated that this novel continuous
PW modeling approach can achieve very good performance both at nominal condition
and at interpolated or extrapolated off-focus-off-dose conditions.
Production optical proximity correction (OPC) tools employ compact optical models in order to accurately
predict complicated optical lithography systems with good theoretical accuracy. Theoretical accuracy is
not the same as usable prediction accuracy in a real lithographic imaging system. Real lithographic
systems have deviations from ideal behavior in the process, illumination, projection and mechanical
systems as well as in metrology. The deviations from the ideal are small but non-negligible. For this study
we use realistic process variations and scanner values to perform a detailed study of useful OPC model
accuracy vs. the variation from ideal behavior and vs. theoretical OPC accuracy. The study is performed
for different 32nm lithographic processes. The results clearly show that incorporating realistic process,
metrology and imaging tool signatures is significantly more important to predictive accuracy than small
improvements in theoretical accuracy.
As alternative approaches (i.e. EUV) to pattern smaller device geometries are being explored but none has showed
maturity for large volume production, 193nm ArF lithography continues to be the workhorse for semiconductor
manufacturing industry. The extension of 193nm lithography heavily depends on the application of Resolution
Enhancement Technologies, driving k1 factor closer to the theoretical limit of 0.25. One effective way to drive k1
lower is the combination of dipole illumination with embedded phase-shifting mask (EPSM). However, one of the
disadvantages of dipole illumination is the presence of forbidden pitches due to that the illumination conditions are
only optimized for the critical pitches. One obvious solution to address this issue is the double exposure strategy.
With the critical pitches are patterned using dipole illumination, the looser pitches are addressed by a less aggressive
illumination condition. One concern of this double exposure strategy is that the geometries from the first exposure
and the geometries from the second exposure need be seamlessly stitched together for certain device designs. This
paper discusses the OPC optimization for image stitching. Three stitch OPC schemes are studied to stitch two resist
space features together. The results show that the reticle registration tolerance for image stitching depends on how
the stitch OPC is done. The registration error tolerance is maximal when the OPC is performed on the low resolution
Immersion interferometric lithography has been applied successfully to semiconductor device applications, but its potential is not limited to this application only. This paper explores this imaging technology for the production of three-dimensional nano-structures using a 193 nm excimer laser and immersion Talbot interferometric lithographic tool. The fabrication of 3-D photonic crystals for the UV spectrum is still considered to be a challenge. A systematic analysis of immersion lithography for 3-D photonic crystal fabrication will be provided in this paper. Significant progress has been made on optical immersion lithography since it was first proposed. Two-beam immersion interferometric lithography can provide sub-30nm resolution. By changing the exposure parameters, such as the numerical aperture of the exposure system, the polarization states and wavelength of the illumination source, 30 nm polymeric nanospheres with different crystal structures can be fabricated.
New applications of evanescent imaging for microlithography are introduced. The use of evanescent wave lithography (EWL) has been employed for 26nm resolution at 1.85NA using a 193nm ArF excimer laser wavelength to record images in a photoresist with a refractive index of 1.71. Additionally, a photomask enhancement effect is described using evanescent wave assist features (EWAF) to take advantage of the coupling of the evanescent energy bound at the substrate-absorber surface, enhancing the transmission of a mask opening through coupled interference.
In a photolithographic system, the mask patterns are imaged through a set of lenses on a resist-coated wafer. The image of mask patterns physically can be viewed as the interference of the plane waves of the diffraction spectrum captured by the lens set incident on the wafer plane at a spectrum of angles. Two-beam interference fringe is the simplest format of the image. Consequently, two-beam interferometric lithography is often employed for photolithographic researches. For two-beam interferometric lithography, beam pointing instability of the illumination source can induce fringe displacement, which results in a loss of fringe contrast if it happens during the exposure. Since some extent of beam pointing instability is not avoidable, it is necessary to investigate its effects on the contrast of the interference fringe. In this paper, the effects of beam pointing instability associated with a two-beam interferometric lithography setup are analyzed. Using geometrical ray tracing technique and basic interference theory, the relationship between the beam tilt angle and interference fringe displacement is established. For a beam pointing instability with random distribution, the resulted fringe contrast is directly proportional to the Fourier transform of the pointing distribution evaluated at 1/(2π). The effect of a pointing instability with normal distribution on interference contrast is numerically investigated.
Degradation in image contrast becomes a concern at higher numerical apertures (NAs) due to mask-induced polarization effects. We study how different photomask materials (binary and attenuated phase shift), feature sizes and shapes, pitch values, duty ratios (line to space), and wavelengths effect the polarization of transmitted radiation. Rigorous coupled-wave analysis (RCWA) is used to simulate the polarization of radiation by the photomask. The results show that higher NA leads to greater polarization effects in all cases. Off-axis illumination increases polarization in one of the first orders, decreasing it in the other. Nonvertical sidewall angles and rounded corners can also impact polarization, but the wavelength of incident radiation has no effect on polarization effects at the same NA values. In general, materials with higher refractive indices and lower extinction coefficients tend to pass more of the TM polarization state, whereas materials with lower refractive indices and a relatively wider range of extinction coefficients pass more TE polarized radiation. These properties can provide new design considerations for the development of next-generation masking materials.
The objective of this paper is to study the polarization induced by mask structures. Rigorous coupled-wave analysis (RCWA) was used to study the interaction of electromagnetic waves with mask features. RCWA allows the dependence of polarization effects of various wavelengths of radiation on grating pitch, profile, material, and thickness to be studied. The results show that for the five different mask materials examined, the material properties, mask pitch, and illumination all have a large influence on how the photomask polarizes radiation.
As an emerging technique, immersion lithography offers the capability of reducing critical dimensions by increasing numerical aperture (NA) due to the higher refractive indices of immersion liquids than that of air. Among the candidates for immersion liquids, water appears to be an excellent choice due to its high transparency at a wavelength of 193 nm, as well as its immediate availability and low processing cost. However, in the process of forming a water fluid layer between the resist and lens surfaces, air bubbles are often created due to the high surface tension of water. The presence of air bubbles in the immersion layer will degrade the image quality because of the inhomogeneity induced light scattering in the optical path. Therefore, it is essential to understand the air bubble induced light scattering effect on image quality. Analysis by geometrical optics indicates that the total reflection of light causes the enhancement of scattering in the region where the scattering angle is less than the critical scattering angle, which is 92 degrees at 193 nm. Based on Mie theory, numerical evaluation of scattering due to air bubbles, polystyrene spheres and PMMA spheres was conducted for TE, TM or unpolarized incident light. Comparison of the scattering patterns shows that the polystyrene spheres and air bubbles resemble each other with respect to scattering properties. Hence polystyrene spheres are used to mimic air bubbles in studies of lithographic imaging of “bubbles” in immersion water. In direct interference lithography, it is found that polystyrene spheres (2 μm in diameter) 0.3 mm away from the resist surface would not image, while for interferometric lithography at 0.5NA, this distance is estimated to be 1.3 mm. Surprisingly, polystyrene spheres in diameter of 0.5 μm (which is 5 times larger than the interferometric line-width) will not image. It is proposed that “bubbles” are repelled from contact with the resist film by surface tension. The scatter of exposure light can be characterized as “flare”. This work shows that microbubbles are not a technical barrier to immersion lithography.
A Talbot interference immersion lithography system that uses a compact prism is presented. The use of a compact prism allows the formation of a fluid layer between the optics and the image plane, enhancing the resolution. The reduced dimensions of the system alleviate coherence requirements placed on the source, allowing the use of a compact ArF excimer laser. Photoresist patterns with a half pitch of 45 nm were formed at an effective NA of 1.05. In addition, a variable NA immersion interference system was used to achieve an effective NA of 1.25. The smallest half-pitch of the photoresist pattern produced with this system was 38 nm.
As immersion nanolithography gains acceptance for next generation device applications, experimental data becomes increasingly important. The behavior of resist materials, fluids, coatings, sources, and optical components in the presence of a water immersion media presents conditions unique compared to convention “dry” lithography. Several groups have initiated fundamental studies into the imaging, fluids, contamination, and integration issues involved with water immersion lithography at 193nm. This paper will present the status and results of the next stage of the development efforts carried out at RIT. The status of two systems are presented; a small field projection microstepper utilizing a 1.05 catadioptric immersion objective lens and a 0.50 to 1.26NA interferometric immersion exposure system based on a compact Talbot prism lens design. Results of the fundamental resolution limits of resist materials and of imaging optics are presented. Additionally, an exploration into the benefits of increasing the refractive index of water is addressed through the use of sulfate and phosphate additives. The potential of KrF, 248nm immersion lithography is also presented with experimental resist imaging results.
The aerial image attained from an optical projection photolithography system is ultimately limited by the frequency information present in the pupil plane of the objective lens. Careful examination of the frequency distribution will allow the operation of such a system to be synthesized experimentally through the use of interferometric lithography. Synthesis is accomplished through single beam attenuation in a two-beam interference system, which is equivalent to adjusting the relative intensities of the primary diffraction orders in a projection system. Typical lithography conditions, such as defocus and partial coherence, can be synthesized efficiently using this technique. The metric of contrast has been utilized to assess the level of correlation between defocus in a projection system and interferometric synthesis. Simulations have shown that interferometric lithography can approximate the performance of a variety of projection system configurations with a significantly high degree of accuracy.
Recent advances in immersion lithography have created the need for a small field microstepper to carry out the early learning necessary for next generation device application. Combined with fluid immersion, multiple-beam lithography can provide an opportunity to explore lithographic imaging at oblique propagation angles and extreme NA imaging. Using the phase preserving properties of Smith Talbot interferometry, the Amphibian XIS immersion lithography microstepper has been created for research and development applications directed toward sub-90nm patterning. The system has been designed for use at ArF and KrF excimer laser wavelengths, based on a fused silica or sapphire prism lens with numerical aperture values up to 1.60. Combined with a chromeless phase grating mask, two and four beam imaging is made possible for feature resolution to 35nm. The approach is combined with X-Y staging to provide immersion imaging on a microstepper platform for substrates ranging up to 300mm. The Amphibian system consists of single or dual wavelength sources (193nm and 248nm), a 2mm exposure field size, stage accuracy better than 1 um, polarization control over a full range from linear polarization to unpolarized illumination, full control of exposure dose and demodulation (to synthesize defocus), and the ability to image both line patterns as well as contact features. A fluid control system allows use of water or alternative fluids, with the ability to change fluids rapidly between wafers. The Amphibian system is fully enclosed in a HEPA and amine controlled environment for use in fab or research environments.
The physical limitations of lithographic imaging are ultimately imposed by the refractive indices of the materials involved. At oblique collection angles, the numerical aperture of an optical system is determined by nsin(θ) , where n is the lowest material refractive index (in the absence of any refractive power through curvature). For 193nm water immersion lithography, the fluid is the limiting material, with a refractive index of near 1.44, followed by the lens material (if planar) with a refractive index near 1.56, and the photoresist, with a refractive index near 1.75. A critical goal for immersion imaging improvement is to first increase the refractive indices of the weakest link, namely the fluid or the lens material. This paper will present an approach to immersion lithography that will allow for the exploration into the extreme limits of immersion lithography by eliminating the fluid altogether. By using a solid immersion lithography (SIL) approach, we have developed a method to contact the last element of an imaging system directly to the photoresist. Furthermore, by fabricating this last element as an aluminum oxide (sapphire) prism, we can increase its refractive index to a value near 1.92. The photoresist becomes the material with the lowest refractive index and imaging becomes possible down to 28nm for a resist index of 1.75 (and 25nm for a photoresist with a refractive index of 1.93). Imaging is based on two-beam Talbot interference of a phase grating mask, illuminated with highly polarized 193nm ArF radiation. Additionally, a roadmap is presented to show the possible extension of 193nm lithography to the year 2020.
It is important to understand how a photomask will polarize incident radiation. This paper presents data collected on binary mask and various attenuated phase shifting mask materials, feature sizes, duty ratios, and illumination schemes via rigorous coupled wave analysis, extinction spectroscopy, and 193nm lithographic evaluation. Additionally, the result of polarization effects due to the photomask on imaging has been studied. It was found that in the majority of the cases, higher NA led to greater polarization effects. All mask materials predominantly pass the TM polarization state for the 0 order, whereas different materials and duty ratios affect the polarization of the first diffracted orders differently. The polarization effects contributed by mask materials being considered for use in high NA imaging systems need to be examined. The degree of polarization as a function of n and k is presented, providing an introduction to the desirable properties of future mask materials. Materials with higher refractive indices and lower extinction coefficients tend to pass more of the TM polarization state, which is undesirable. Materials with lower indices and relatively wide range of extinction coefficients pass more TE polarized radiation. The duty ratio, critical dimension, mask material, material thickness, and illumination scheme all influence mask induced polarization effects.
Interference imaging systems are being used more extensively for R&D applications where NA manipulation, polarization control, relative beam attenuation, and other parameters are explored and projection imaging approaches may not exist. To facilitate interferometric lithography research, we have developed a compact simulation tool, ILSim, for studying multi-beam interferometric imaging, including fluid immersion lithography. The simulator is based on full-vector interference theory, which allows for application at extremely high NA values, such as those projected for use with immersion lithography. In this paper, ILSim is demonstrated for use with two-beam and four-beam interferometric immersion lithography. The simulation tool was written with Matlab, where the thin film assembly (ambient, top coat, resist layer, BARC layers, and substrate) and illumination conditions (wavelength, polarization state, interference angle, demodulation, NA) can be defined. The light intensity distributions within the resist film for 1 exposure or 2-pass exposure are displayed in the graph window. It also can optimize BARC layer thickness and top coat thickness.
Interference lithography has been widely utilized as a tool for the evaluation of photoresist materials, as well as emerging resolution enhancement techniques such as immersion lithography. The interferometric approach is both simple and inexpensive to implement, however it is limited in its ability to examine the impact of defocus due to the inherently large DOF (Depth-of-Focus) in two-beam interference. Alternatively, the demodulation of the aerial image that occurs as a result of defocus in a projection system may be synthesized using a two pass exposure with the interferometric method. The simulated aerial image modulation for defocused projection systems has been used to calculate the single beam exposure required to reproduce the same level of modulation in an interferometric system through the use of a “Modulation Transfer Curve”. The two methods have been theoretically correlated, by way of modulation for projection illumination configurations, including quadrupole and annular. An interferometric exposure system was used to experimentally synthesize defocus for modulations of 0.3, 0.5, 0.7 and 1.0. Feature sizes of 90nm were evaluated across dose and synthetic focus.
Photoresist modulation curves are introduced as a quantitative way to characterize the photoresist process performance when used as a detector in a microlithographic system. The new method allows predicting exposure latitude of the photoresist process across a wide range of resolutions and modulation levels of the aerial image. The data collection process is demonstrated using an immersion interference system, capable of variable resolution and full control over the modulation of the delivered aerial image.
Immersion lithography has become attractive since it can reduce critical dimensions by increasing numerical aperture (NA) beyond unity. Among all the candidates for immersion fluids, those with higher refractive indices are desired. However, for many of the fluids, the strong absorption at 193nm becomes a serious problem. Therefore, it is essential to find a fluid that is transparent enough (with absorbance less than 0.5mm-1) and has high refractive index (above water, 1.44) at 193nm. Characterization of various fluid candidates has been performed and the absorbance of these fluids has been measured. To measure the absolute refractive index, a prism deviation angle method was developed. This method offers the possibility of measuring fluid refractive indices accurately. This paper also presents the obtained refractive indices of these fluids. Several candidates have been identified for 193nm application with refractive indices near 1.55, which is about 0.1 higher than that of water at this wavelength. Cauchy parameters of these fluids were generated and approaches were investigated to tailor the fluid absorption edges to be close to 193nm. The effects of these fluids on photoresist performance were also examined with 193nm immersion lithography exposure at various NA's. 1.5 NA was obtained to image 32nm lines with phosphoric acid as the immersion medium. These fluids are potential candidates for immersion lithography technology.
Historically, the application of immersion optics to microlitho-graphy has not been seriously pursued because of the alternative technologies available. As the challenges of shorter wavelength become increasingly difficult, immersion imaging becomes more feasible. We present results from research into 193-nm excimer laser immersion lithography at extreme propagation angles. This is being carried out in a fluid that is most compatible in a manufacturable process, namely water. By designing a system around the optical properties of water, we are able to image with wavelengths down to 193 nm. Measured absorption is below 0.50 cm−1 at 185 nm and below 0.05 cm−1 at 193 nm. Furthermore, through the development of oblique angle imaging, numerical apertures approaching 1.0 in air and 1.44 in water are feasible. The refractive index of water at 193 nm allows for exploration of the following: k1 values near 0.25 leading to half-pitch resolution approaching 35 nm at a 193-nm wavelength; polarization effects at oblique angles (extreme NA); immersion and photoresist interactions with polarization; immersion fluid composition, temperature, flow, and micro-bubble influence on optical properties (index, absorption, aberration, birefringence); mechanical requirements for imaging, scanning, and wafer transport in a water media; and synthesizing conventional projection imaging via interferometric imaging.
It is possible to extend optical lithography by using immersion imaging methods. Historically, the application of immersion optics to microlithography has not been seriously pursued because of the alternative solutions available. As the challenges of shorter wavelength become increasingly difficult, immersion imaging becomes more feasible. We present results from research into 193nm excimer laser immersion lithography at extreme propagation angles (such as those produces with strong OAI and PSM). This is being carried out in a fluid that is most compatible in a manufacturable process, namely water. By designing a system around the optical properties of water, we are able to image with wavelengths down to 193nm. Measured absorption is below 0.50 cm-1 at 185nm and below 0.05 cm-1 at 193nm. Furthermore, through the development of oblique angle imaging, numerical apertures approaching 1.0 in air and 1.44 in water are feasible. The refractive index of water at 193nm (1.44) allows for exploration of the following: 1. k1 values approaching 0.17 and optical lithography approaching 35nm. 2. Polarization effects at oblique angles (extreme NA). 3. Immersion and photoresist interactions with polarization. 4. Immersion fluid composition, temperature, flow, and micro-bubble influence on optical properties (index, absorption, aberration, birefringence). 5. Mechanical requirements for imaging, scanning, and wafer transport in a water media. 6. Synthesizing conventional projection imaging via interferometric imaging.