With constant shrinking of device critical dimensions (CD), the quality of pattern transfer in IC fabrication depends on the etch process and the exposure process fidelities, and the interaction of lithographic and etching processes is no longer negligible. Etch effect correction with accurate models has become an important component in optical proximity correction (OPC) modeling and related applications. It is now commonly accepted that the lithographic and etch effects should be modeled and corrected in a sequential and staged way: a resist (or lithographic) model should be created and used for lithographic effect compensation, and an etch model should be created and used for etch effect compensation. However, there can be various degrees of separation of these two modeling stages. In order to optimally capture the significant variation in the post-development resist patterns and post-etching patterns, it is helpful to integrate these two processes together for the OPC model calibration practice. In this paper, we analyze the integrated simulation approach in OPC modeling where the entire resist model information is made fully accessible in the etch modeling stage to allow the possibility of resist and etch co-optimization, e.g. through adjusting the resist model to optimally fit the etch data. Furthermore, the integrated simulation technique is integrated into a verification flow to simplify the conventional staged flow.
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.
Extreme ultraviolet (EUV) lithography is one of the leading technologies for 16nm and smaller node device patterning.
One patterning issue intrinsic to EUV lithography is the shadowing effect due to oblique illumination at the mask and
mask absorber thickness. This effect can cause CD errors up to a few nanometers, consequently needs to be accounted
for in OPC modeling and compensated accordingly in mask synthesis. Because of the dependence on the reticle field
coordinates, shadowing effect is very different from the traditional optical and resist effects. It poses challenges to
modeling, compensation, and verification that were not encountered in tradition optical lithography mask synthesis.
In this paper, we present a systematic approach for shadowing effect modeling and model-based shadowing
compensation. Edge based shadowing effect calculation with reticle and scan information is presented. Model calibration
and mask synthesis flows are described. Numerical experiments are performed to demonstrate the effectiveness of the
EUV lithography is widely viewed as a main contending technology for 16nm node device patterning.
However, EUV has several complex patterning issues which will need accurate compensation in mask
synthesis development and production steps. The main issues are: high flare levels from optical element
roughness, long range flare scattering distances, large mask topography, non-centered illumination axis
leading to shadowing effects, new resist chemistries to model very accurately, and the need for full reticle
optical proximity correction (OPC). Compensation strategies for these effects must integrate together to
create final user flows which are easy to build and deploy with reasonable time and cost. Therefore,
accuracy, usability, speed and cost are important with methods that have considerably more complexity
than current optical lithography mask synthesis flows.
In this paper we analyze the state of the art in accurate prediction and compensation of several of these
complex EUV patterning issues, and compare that to 16nm node expected production needs. Next we
provide a description of integration issues and solutions which are being implemented for 16nm EUV
process development. This includes descriptions of OPC model calibration with flare, shadowing, and
topography effects. We also propose a realistic (in terms of accuracy and mask area) flare parameter calibration flow to improve short and longer range flare correction accuracy above what can be achieved with only a measured EUV flare PSF.
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.
Now that full-field alpha EUV scanners are available to lithographers at multiple sites around the world,
there is greatly increased demand for full-field EUV circuit and teststructure wafer images. Successful
patterning of these circuit and teststructure wafer images requires mask layout data which accurately
compensates for all expected process transformations occurring in the EUV patterning process. These
process transformations include flare, optical diffraction, resist behavior, mask shadowing, and 3D mask
electromagnetic effects. In this paper, we present a complete fullfield EUV mask data correction flow
which incorporates compensation for patterning transformations due to very long range flare, reflective
multi-layer masks, thick mask absorbers, off-axis EUV scanner illumination, field-dependent shadowing
and orientation dependent shadowing. Optimized algorithms for flare and mask effects now enable both
fast and accurate full-chip process effect compensation. Results are shown for both the 22nm and 16nm
logic device nodes. The results are presented by error component category to highlight the relative
importance of each effect.
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.
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.
In-situ aberration measurement often requires indirect methods that retrieve the pupil phase from the measured images
and presents unique challenges to the engineers involved. Phase wheel monitor allows such in-situ measurement of
aberrations in photolithography systems. The projection lens aberrations may be obtained with high accuracy from
images of phase wheel targets printed in photoresist. As a result, the photolithography tool optics can be characterized
under standard wafer printing conditions. Resulting features are mathematically analyzed to extract information about
the aberrations in optics. We use a detection algorithm and multi-domain modeling to process resist images and
determine the image deviation from the ideal shape, which in turn allow the amount of aberration introduced by the
optical system to be quantified. Experimental results are shown and multiple measurements on the same tool before and
after system corrections are compared.
An automated aberration extraction method is presented which allows extraction of lithographic projection lens' aberration signature having only access to object (mask) and image (wafer) planes. Using phase-wheel targets on a two-level 0/π phase shift mask, images with high sensitivity to aberrations are produced. Zernike aberration coefficients up to 9th order have been extracted by inspection of photoresist images captured via top-down SEM. The automated measurement procedure solves a multi-dimensional optimization problem using numerical methods and demonstrates improved accuracy and minimal cross-correlation. Starting with a detailed procedure analysis, recent experimental results for 193-nm projection optics in commercial full field exposure tools are discussed with an emphasis on the performance of the aberration measurement approach.
Aberration metrology is critical to the manufacture of quality lithography lenses in order to meet strict optical requirements. Additionally, it is becoming increasingly important to be able to measure and monitor lens performance in an IC production environment on a regular basis. The lithographer needs to understand the influence of aberrations on imaging and any changes that may occur in the aberration performance of the lens between assembly and application, and over the course of using an exposure tool. This paper will present a new method for the detection of lens aberrations that may be employed during standard lithography operation. The approach allows for the detection of specific aberration types and trends, as well as levels of aberration, though visual inspection of high resolution images of resist patterns and fitting of the aberrated wavefront. The approach consists of a test target made up of a 180-degree phase pattern array in a “phase wheel” configuration. The circular phase regions in the phase wheel are arranged so that their response to lens aberration is interrelated and the regions respond uniquely to specific aberrations, depending on their location within the target. This test method offers an advantage because of the sensitivity to particular aberration types, the unique response of multiple zones of the test target to aberrations, and the ease with which aberrations can be distinguished. The method of lens aberration detection is based on the identification of the deviations that occur between the images printed with the phase wheel target and images that would be produced in the absence of aberration. This is carried out through the use of lithography simulation, where simulated images can be produced without aberration and with various levels of lens aberration. Comparisons of printed resist images to simulated resist images are made while the values of the coefficients for the primary Zernike aberrations are varied.
Early manufacture and use of 157nm high NA lenses has presented significant challenges including: intrinsic birefringence correction, control of optical surface contamination, and the use of relatively unproven materials, coatings, and metrology. Many of these issues were addressed during the manufacture and use of International SEMATECH’s 0.85NA lens. Most significantly, we were the first to employ 157nm phase measurement interferometry (PMI) and birefringence modeling software for lens optimization. These efforts yielded significant wavefront improvement and produced one of the best wavefront-corrected 157nm lenses to date. After applying the best practices to the manufacture of the lens, we still had to overcome the difficulties of integrating the lens into the tool platform at International SEMATECH instead of at the supplier facility. After lens integration, alignment, and field optimization were complete, conventional lithography and phase ring aberration extraction techniques were used to characterize system performance. These techniques suggested a wavefront error of approximately 0.05 waves RMS--much larger than the 0.03 waves RMS predicted by 157nm PMI. In-situ wavefront correction was planned for in the early stages of this project to mitigate risks introduced by the use of development materials and techniques and field integration of the lens. In this publication, we document the development and use of a phase ring aberration extraction method for characterizing imaging performance and a technique for correcting aberrations with the addition of an optical compensation plate. Imaging results before and after the lens correction are presented and differences between actual and predicted results are discussed.
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.
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.
The onset of lithographic technology involving extreme numerical aperture (NA) values introduces critical technical issues that are now receiving particular attention. Projection lithography with NA values above 0.90 is necessary for future generation devices. The introduction of immersion lithography enables even larger angles, resulting in NA values of 1.2 and above. The imaging effects from oblique angles, electric field polarization, optical interference, optical reflection, and aberration can be significant. This paper addresses polarization considerations at critical locations in the optical path of a projection system, namely in the illuminator, at the mask, and in the photoresist. Several issues are addressed including TE and azimuthal polarized illumination, wire grid polarization effects for real thin film mask materials, and multilayer resist AR coatings for high NA and polarization.
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.
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.
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.
An approach to in-situ wavefront aberration measurement is explored. The test is applicable to sensing aberrations from the image plane of a microlithography projection system or a mask inspection tool. A set of example results is presented which indicate that the method performs well on lenses with a Strehl ration above 0.97. The method uses patterns produced by an open phase figure1 to determine the deviation of the target image from its ideal shape due to aberrations. A numerical solution in the form of Zernike polynomial coefficients is reached by modeling the object interaction with aberrated pupil function using the nonlinear optimization routine over the possible deformations to give an accurate account of the image detail in 2-D. The numerical accuracy for the example below indicated superb performance of the chosen target shapes with only a single illumination setup.
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.
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.
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.
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.
The exposure tool is a critical enabler to continue improving the packing density and transistor speed in the semiconductor industry. In addition to increasing resolution (packing density) a scanner is also expected to provide tight control of the Across Chip Linewidth Variation, ACLV, (transistor speed). An important component of ACLV is lens aberrations. Techniques that measure in-situ the lens aberrations are now available. In a previous paper we reported good agreement between the first 25 Zernike coefficients measured in-situ using one of these techniques ARTEMIS and PMI (Phase Metrology Interferometry) data collected at the lens manufacturer. However questions have arisen as to the practicality of ARTEMIS, especially in view of its heavy reliance on a very large number of SEM images. We have measured the first 25 Zernike coefficients for 13 ASML 500/700 DUV Step & Scan systems in a high-volume wafer fab. In this paper we report on certain enhancements that were made to the best practice of ARTEMIS. We will also present a summary of the measurements taken and our first attempt to cluster the tools according to the aberrations measured.
It is well known that shrinking k1 factors and increasing MEEF are making it more difficult to print contact holes with acceptable latitude and low defectivity. Given the decreasing process latitude this implies, choosing elements of the lithography process independently is becoming less and less of an option. Instead all elements of the lithography process need to be chosen so that a production-worthy process can be rapidly developed. The large number of options available for building a process further complicates the optimization problem. In this study, simulation results are used to explore the tradeoffs between illumination options and reticle substrate choice as applied to contact hole printing. Relative defectivity levels are presented from logic test circuits for selected cases of illumination and reticle type. These selected cases show that what improves defectivity also improves the Normalized Image Log-Slope (NILS). As it has been previously shown that NILS is already an excellent image quality metric NILS improvement will be used as the basis of the work presented in this paper. Extensive simulations will be used to determine the best choice of illumination and mask type to maximize NILS and by implication minimize defect density.
Contact patterning for advanced lithography generations is increasingly being viewed as a major threat to the continuation of Moore's Law. There are no easy patterning strategies which enable dense through isolated contacts of very small size. Lack of isolated contact focus latitude, high dense contact mask error factor and incredibly low defectivity rate requirements are severe issues to overcome. These difficulties mean that new and complex patterning methods for contacts at the 90nm and 65nm device generations are being considered. One possible option for improving the process window of contact patterning is resist reflow. Resist reflow can supplement almost any other optical extension method for contact lithography. Previous results have shown the significant benefits of this method for CD control on semi-dense and isolated contact for the 100nm device generation. This work extends the previous work by investigating very dense pitch through isolated contact patterning at 193nm low K1 lithography regimes. The encouraging overall CD control and process window of reflowed contacts using the ARCH TIS2000 bilayer resist system is analyzed through pitch for different imaging options. An investigation of the capability of resist reflow in combination with optimized reticle and illumination for the 65nm device generation is also presented as are details of defectivity levels for reflowed contacts on 90nm device products.
Due to the rapidly reduced imaging process windows and increasingly stingent device overlay requirements, sub-130 nm lithography processes are more severely impacted than ever by systamic fault. Limits on critical dimensions (CD) and overlay capability further challenge the operational effectiveness of a mix-and-match environment using multiple lithography tools, as such mode additionally consumes the available error budgets. Therefore, a focus on advanced process control (APC) methodologies is key to gaining control in the lithographic modules for critical device levels, which in turn translates to accelerated yield learning, achieving time-to-market lead, and ultimately a higher return on investment. This paper describes the implementation and unique challenges of a closed-loop CD and overlay control solution in high voume manufacturing of leading edge devices. A particular emphasis has been placed on developing a flexible APC application capable of managing a wide range of control aspects such as process and tool drifts, single and multiple lot excursions, referential overlay control, 'special lot' handling, advanced model hierarchy, and automatic model seeding. Specific integration cases, including the multiple-reticle complementary phase shift lithography process, are discussed. A continuous improvement in the overlay and CD Cpk performance as well as the rework rate has been observed through the implementation of this system, and the results are studied.
All optical imaging systems have some amount of stray light, or flare, that detracts from system performance, critical dimension (CD) control, and process latitude. The effects of flare increase when multiple exposure processes, such as complementary phase shift, are used since this doubles the amount of exposure energy going through the optics. Flare was characterized on several modern KrF and ArF exposure tools using a direct method of measurement. Flare is determined by measuring the reduction in the size of a 160 nm line as it is subjected to increasing dose from a second 'flare' exposure. The amount of flare is determined using regression between experimental and modeled data. Lithography modeling was used to quantify the amount of flare responsible for CD variation. This method allows evaluation of CD control degradation on actual features that are sized close to production feature size. The effects of substrate reflectance and mask loading were also studied. The results were compared to a published large pad flare measurement technique in common use.
As optical lithography is pushed to smaller dimensions, methods of resolution enhancement are considered necessary. Illumination modification is getting a good deal of attention, through strong and weak off-axis methods. The shape of an illumination profile does not need to be circular, especially if X/Y feature orientation is considered. This paper describes the improvements in imaging that are possible through use of source shapes that have various degrees of square character. Applications are discussed and interaction with optical proximity correction, aberration, and other imagin factors are addressed.
Lithography at 193nm is the first optical lithography technique that will be introduced for manufacturing of technology levels. where the required dimensions are smaller than the actual wavelength. This paper explores several techniques to extend 193nm to low k1 lithography. Most attention is given to binary mask solution in at 130nm dimensions, where k1 is 0.4. Various strong and Gaussian quadrupole illuminators were designed, manufactured and tested for this application. Strong quadrupoles show that largest DOF improvements. The drawback however, is that these strong quadrupoles are very duty cycle and dimensions specific, resulting in large proximity biases between different duty cycles. Due to their design, Gaussian quadrupoles sample much wider frequency ranges, resulting in less duty cycles specific DOF improvements and less proximity basis. At sub-130nm dimensions, strong phase shift masks provide significant latitude improvements, when compared to binary masks with quadrupole illumination. However, differences in dose to size for different duty cycles were up to 25 percent. For definition of contact holes, linewidth biasing through silylation, a key feature of the CARL bi-layer resist approach, demonstrated significant DOF latitude improvements compared to SLR at 140nm and 160nm contact holes.
As optical lithography below 193 nm is explored, materials issues become more challenging. Thin film coatings that are sufficient for use at wavelengths near or above 200 nm are more likely than not to be problematic at 157 nm, 126 nm, or other potential VUV wavelengths. The situation is a concern for optical coatings, masking films, and for resist/substrate reflectivity control. Potential solutions for several film types are presented, which have been deposited and optically characterized for use as attenuated phase shift masking films, binary masking films, and optical coatings for use at 157 nm.
Off-axis illumination schemes have been developed that can enhance both the resolution and focal depth performance for an optical exposure tool. One approach introduced modifies the illumination profile, filling the condenser lens pupil with weak Gaussian quadrupoles where energy is distributed within and between poles. This method has demonstrated better control of DOF and proximity effect for a variety of feature types. Other possibilities also exist. Presented here are approaches to illumination modification through use of condenser lens masking apertures, fabricated as attenuating fused silica reticles which are inserted at the lens pupil plane. Application of this technique for use in high NA 248 nm and 193 nm exposure tools is shown. For each case, optimization of illumination profiles has been conducted. Optimized source files have been converted to halftone (dithered) masking files for electron beam patterning on fused silica with chromium and anti-reflective (AR) films. Analysis of these modified illumination techniques in terms of resolution, focal depth, throughput, and aberration performance is also presented.
The use of attenuated phase-shift masking materials is being considered as one of the key resolution enhancement techniques for sub-0.18 micrometers lithography. In addition to proper optical performance, films for use at DUV and 193 nm wavelengths require suitable plasma etch characteristics and stability at mask exposure levels of pulse excimer lasers. This may limit practical materials to those which allow suitably volatile etch bi-products and possess stable stoichiometric composition. Several film families have been produced which can deliver a (pi) phase shift and 4 - 15% transmission within a single films thickness including Al/AlN, MoSiO, TaSiO, Zr/ZrN, SixNy, and TaN/Si3N4. Of these materials, TaSiO, SixNy, and TaN/Si3N4, allow for adequate plasma etch performance with selectivity to fused silica and resist. Excimer laser 193 nm exposure at fluences corresponding to mask exposure levels show some degree of optical degradation of materials prone to oxidation.