In this paper we will present initial results for logic and memory features imaged with the TWINSCAN EXE:5000 at the ASML-imec high NA lab after successful etch pattern transfer. For logic applications random logic metal designs (consisting of tight pitches and aggressive tip-to-tips) and corresponding via structures have been characterized for A14 and A10 nodes. As well, bidirectional designs enabled by high NA will be described. For memory applications, results from BLP/SNLP layer for D1d and D0a nodes will be presented.
As semiconductor features shrink in dimension and pitch, the excessive control of critical-dimension uniformity (CDU) and pattern fidelity is essential for mask manufacturing using electron-beam lithography. Requirements of the electronbeam shot quality affected by shot unsteadiness become more important than before for the advanced mask patterning. Imperfect electron optical system, an inaccurate beam deflector, and imprecise mask stage control are mainly related to the shot unsteadiness including positioning and dose perturbations. This work extensively investigates impacts of variable shaped beam dose and positioning perturbations on local CDU using Monte Carlo simulation for various mask contrast enhancement approaches. In addition, the relationship between the mask lithographic performance and the shot count number correlated with mask writing time is intensively studied.
The inverse polarizing effect of Sub-Wavelength Metallic Gratings (SWMGs) is employed to improve the lithography performance by controlling the polarization. The SWMGs are intentionally created on the top surface of mask. Its polarization selectivity is deliberately designed according to the bottom mask patterns. A series of simulations and optimizations on SWMG structures were done in order to achieve better image quality. We demonstrate that the contrast of aerial image can be improved by designing the inverse polarizer on mask (iPOM) for some specific layout patterns. We also reveal that the double diffraction inevitably occurring in-between the iPOM and layout pattern may damage the image quality in most situations. This leads to narrow usage of iPOM. An alternative to overcome the double diffraction is proposed by optimizing the refractive index and thickness of layout absorber to make the polarization selection feasible without iPOM.
Aerial image measurement system (AIMSTM) has been widely used for wafer level inspection of mask defects. Reported inspection flows include die-to-die (D2D) and die-to-database (D2DB) methods. For patterns that do not repeat in another die, only the D2DB approach is applicable. The D2DB method requires accurate simulation of AIMS measurements for a mask pattern. An optical vectorial model is needed to depict the mask diffraction effect in this simulation. To accurately simulate the imaging results, a rigorous electro-magnetic field (EMF) model is essential to correctly take account of the EMF scattering induced by the mask topography, which is usually called the mask 3D effect.
In this study, the mask 3D model we use is rigorous coupled-wave analysis (RCWA), which calculates the diffraction fields from a single plane wave incidence. A hybrid Hopkins-Abbe method with RCWA is used to calculate the EMF diffraction at a desired accuracy level while keeping the computation time practical. We will compare the speed of the hybrid Hopkins-Abbe method to the rigorous Abbe method.
The matching between simulation and experiment is more challenging for AIMS than CD-SEM because its measurements provide full intensity information. Parameters in the mask 3D model such as film stack thickness or film optical properties, is optimized during the fitting process. We will report the fitting results of AIMS images for twodimensional structures with various pitches. By accurately simulating the AIMS measurements, it provides a necessary tool to perform the mask inspection using the D2DB approach and to accurately predict the mask defects.
KEYWORDS: 3D modeling, Scanning electron microscopy, Diffusion, Data modeling, Lithography, Calibration, Semiconducting wafers, Visual process modeling, 3D image processing, Optical proximity correction
A traditional approach to construct a fast lithographic model is to match wafer top-down SEM images, contours and/or gauge CDs with a TCC model plus some simple resist representation. This modeling method has been proven and is extensively used for OPC modeling. As the technology moves forward, this traditional approach has become insufficient in regard to lithography weak point detection, etching bias prediction, etc. The drawback of this approach is from metrology and simulation. First, top-down SEM is only good for acquiring planar CD information. Some 3D metrology such as cross-section SEM or AFM is necessary to obtain the true resist profile. Second, the TCC modeling approach is only suitable for planar image simulation. In order to model the resist profile, full 3D image simulation is needed. Even though there are many rigorous simulators capable of catching the resist profile very well, none of them is feasible for full-chip application due to the tremendous consumption of computational resource. The authors have proposed a quasi-3D image simulation method in the previous study [1], which is suitable for full-chip simulation with the consideration of sidewall angles, to improve the model accuracy of planar models. In this paper, the quasi-3D image simulation is extended to directly model the resist profile with AFM and/or cross-section SEM data. Resist weak points detected by the model generated with this 3D approach are verified on the wafer.
This paper discusses the CD Bossung tilt phenomena in low-k1 lithography using interference harmonics and rigorous
EM spectrum analysis. Interference harmonics analysis is introduced to explain the interaction of diffraction orders in the
focal region leading to this abnormal CD behavior. This method decomposes the vector image formula into a
superposition of cosine components to describe the interference of diffraction orders. The symmetry properties of
components of an optical projection system were investigated to find out three potential sources for the asymmetric
Bossung behavior, namely mask 3D (M3D) effect, lens aberration, and wafer reflectivity. Under good lens aberration
and substrate reflectivity controls, the M3D effect accounts for most of the CD Bossung tilt. A rigorous EM mask
spectral analysis was performed to reveal the impact of mask topography on the near-field intensity of mask transmission
and the far-field image formation. From the analysis, the asymmetric phase distribution in the mask spectrum is the root
cause for CD Bossung tilt. Using both the interference harmonics and the rigorous EM spectrum analysis, the effect of
various resolution enhancement techniques (RET) to the Bossung tilt is also studied to find the best RET combination for
M3D immunity. In addition, a pupil optimization algorithm based on these two analyses is proposed to generate the
phase compensation map for M3D effect counteraction.
Traditionally, an optical proximity correction model is to evaluate the resist image at a specific depth within the photoresist and then extract the resist contours from the image. Calibration is generally implemented by comparing resist contours with the critical dimensions (CD). The wafer CD is usually collected by a scanning electron microscope (SEM), which evaluates the CD based on some criterion that is a function of gray level, differential signal, threshold or other parameters set by the SEM. However, the criterion does not reveal which depth the CD is obtained at. This depth inconsistency between modeling and SEM makes the model calibration difficult for low k1 images. In this paper, the vertical resist profile is obtained by modifying the model from planar (2D) to quasi-3D approach and comparing the CD from this new model with SEM CD. For this quasi-3D model, the photoresist diffusion along the depth of the resist is considered and the 3D photoresist contours are evaluated. The performance of this new model is studied and is better than the 2D model.
Calibration of mask images on wafer becomes more important as features shrink. Two major types of metrology have
been commonly adopted. One is to measure the mask image with scanning electron microscope (SEM) to obtain the
contours on mask and then simulate the wafer image with optical simulator. The other is to use an optical imaging tool
Aerial Image Measurement System (AIMSTM) to emulate the image on wafer. However, the SEM method is indirect. It
just gathers planar contours on a mask with no consideration of optical characteristics such as 3D topography structures.
Hence, the image on wafer is not predicted precisely. Though the AIMSTM method can be used to directly measure the
intensity at the near field of a mask but the image measured this way is not quite the same as that on the wafer due to
reflections and refractions in the films on wafer.
Here, a new approach is proposed to emulate the image on wafer more precisely. The behavior of plane waves with
different oblique angles is well known inside and between planar film stacks. In an optical microscope imaging system,
plane waves can be extracted from the pupil plane with a coherent point source of illumination. Once plane waves with a
specific coherent illumination are analyzed, the partially coherent component of waves could be reconstructed with a
proper transfer function, which includes lens aberration, polarization, reflection and refraction in films. It is a new
method that we can transfer near light field of a mask into an image on wafer without the disadvantages of indirect SEM
measurement such as neglecting effects of mask topography, reflections and refractions in the wafer film stacks.
Furthermore, with this precise latent image, a separated resist model also becomes more achievable.
Typical OPC models focus on predicting wafer contour or CD; therefore, the modeling approach emphasizes careful
determination of feature and edge locations in the photo-resist (PR) as well as the exposure threshold, so that the 'cut'
model image matches the wafer SEM contours or cut-line CDs most closely. This is an exquisite approach with regard to
the contour-based OPC, for the model is calibrated directly from wafer CDs. However, for other applications such as
hotspot detection or assist feature (AF) printing prediction that might occur at the top or the bottom of the PR, the typical
OPC model approach may not be accurate enough. Usually, these kinds of phenomenon can only be properly described
by rigorous simulation, which is very time-consuming and hence not suitable for OPC.
In this paper, the approach of building the OPC model with multiple image depths will be discussed. This approach
references the images at the bottom and/or the top of the PR. This way, the behavior of the images which are not shown
at the normal image depth can be predicted more accurately without distorting the optical model. This compromised
OPC modeling approach is beneficial for runtime reduction compared to the rigorous simulation, and for better accuracy
compared to conventional model. The applications for AF printing and hotspot predictions using the multiple image
depth approach will be demonstrated.
It is believed that smaller correction segments could achieve better pattern fidelity, however, some unstable OPC results
which are beyond the capability of common OPC correction schemes were found once the segment length is less than a
certain threshold. The dilemma between offering more degree-of-freedom by decreasing the correction segment length at
the cost of longer correction time and the instability induced by the reduced segment length challenges every OPC
engineer.
In this paper, 2 indices are introduced; the segmentation index is proposed to determine a reasonable minimum segment
length while the stability index can be used to examine whether the correction system is a stiff convergence problem. A
compromised correction algorithm is also proposed to consider the OPC accuracy, stability and runtime simultaneously.
The correction results and the runtime are analyzed.
Design rules and the design rule check (DRC) utility are conventional approaches to design for manufacturability
(DFM). The DRC utility is based on unsophisticated rules to check the design layout in a simple environment. As the
design dimension shrinks drastically, the introduction of a more powerful DFM utility with model-based layout
patterning check (LPC) becomes mandatory for designers to filter process weak-points before taping out layouts. In this
paper, a system of integrated hotspot scores consisting of three lithography sensitive indexes is proposed to assist
designers to circumvent risky layout patterns in lithography. With the hotspot fixing guideline and the hotspot severity
classification deduced from the scoring system provided in this paper, designers can deliver much more manufacturable
designs.
As the patterning of IC manufacturing shrinks to the 32-nm node and beyond, high-NA and immersion lithography are
required for pushing resolution to its physical limit. To achieve good OPC performance, various physical effects such as
polarization, mask topography, and mask pellicle have to be considered to improve the model accuracy.
The attenuation and the phase variation of TE and TM wave components induced by the pellicle would impact optical
qualities in terms of resolution, distortion, defocus shift, and high-order aberrations. In this paper, the OPC model
considering pellicle effects is investigated with Jones pupil. The CD variation induced by the pellicle effect can be
predicted accurately. Therefore, the improvement on model accuracy for 32-nm node is demonstrated.
Accurate simulation of today's devices needs to account for real device geometry
complexities after the lithography and etching processes, especially when the channel
length shrinks to 65-nm and below. The device performance is believed to be quite
different from what designers expect in the conventional IC design flow. The
traditional design lacks consideration of the photolithography effects and pattern
geometrical operations from the manufacturing side. In to order obtain more accurate
prediction on circuits, an efficient approach to estimate nonrectangular MOSFET
devices is proposed. In addition, an electrical hotspot criterion is also proposed to
investigate and verify the manufacturability of devices during patterning processes.
This electrical rule criterion will be performed after the regular Design Rule Check
(DRC) or Design for Manufacturing (DFM) rule check. Photolithography and
industrial-strength SPICE model are taken into consideration to further correlate the
process variation. As a result, the correlation between process-windows and driving
current variation of devices will be discussed explicitly in this paper.
Optical proximity correction is the technique of pre-distorting mask layouts so that the printed patterns are as close to the desired shapes as possible. For model-based optical proximity correction, a lithographic model to predict the edge position (contour) of patterns on the wafer after lithographic processing is needed. Generally, segmentation of edges is performed prior to the correction. Pattern edges are dissected into several small segments with corresponding target points. During the correction, the edges are moved back and forth from the initial drawn position, assisted by the lithographic model, to finally settle on the proper positions. When the correction converges, the intensity predicted by the model in every target points hits the model-specific threshold value. Several iterations are required to achieve the convergence and the computation time increases with the increase of the required iterations. An artificial neural network is an information-processing paradigm inspired by biological nervous systems, such as how the brain processes information. It is composed of a large number of highly interconnected processing elements (neurons) working in unison to solve specific problems. A neural network can be a powerful data-modeling tool that is able to capture and represent complex input/output relationships. The network can accurately predict the behavior of a system via the learning procedure. A radial basis function network, a variant of artificial neural network, is an efficient function approximator. In this paper, a radial basis function network was used to build a mapping from the segment characteristics to the edge shift from the drawn position. This network can provide a good initial guess for each segment that OPC has carried out. The good initial guess reduces the required iterations. Consequently, cycle time can be shortened effectively. The optimization of the radial basis function network for this system was practiced by genetic algorithm, which is an artificially intelligent optimization method with a high probability to obtain global optimization. From preliminary results, the required iterations were reduced from 5 to 2 for a simple dumbbell-shape layout.
Optical proximity correction (OPC) is usually used to pre-distort mask layouts to make the printed patterns as close to the desired shapes as possible. For model-based OPC, a lithographic model to predict critical dimensions after lithographic processing is needed. The model is usually obtained via a regression of parameters based on experimental data containing optical proximity effects. When the parameters involve a mix of the continuous (optical and resist models) and the discrete (kernel numbers) sets, the traditional numerical optimization method may have difficulty handling model fitting. In this study, an artificial-intelligent optimization method was used to regress the parameters of the lithographic models for OPC. The implemented phenomenological models were constant-threshold models that combine diffused aerial image models with loading effects. Optical kernels decomposed from Hopkin’s equation were used to calculate aerial images on the wafer. Similarly, the numbers of optical kernels were treated as regression parameters. This way, good regression results were obtained with different sets of optical proximity effect data.
The ripple patterns induced by the lithography process will lead to unpredictable necking or bridging risks on circuit patterns. This phenomenon is particularly severe while using the attenuated-phase-shifting mask combined with the strong off-axis illumination. The CD variation induced by the ripple effect is difficult to be accurately corrected by conventional OPC approaches. In this paper, ripples on patterning for the 65nm node have been studied and their problems solved. One of the dominant root causes of ripples is the optical side-lobes from the surrounding patterns. On the L-shape patterns for example, the ripples that occur on the horizontal lines are induced by the side-lobes of the vertical lines. Based on this study of the ripple effect, the layout types resulting in ripple patterns can be classified and predicted. An advanced OPC approach by the segmentation analysis on polygons as well as the correction algorithm optimization has been developed and applied to solve this ripple problem.
There have been several kinds of resist model proposed for optical proximity correction. The simplest one is the constant threshold resist model. By this method, only area with intensity above a certain threshold value would be developed. Unfortunately, the constant threshold resist model is too simplified to accurately describe the entire resist processes. To solve this problem, variable threshold resist models were proposed thereafter. The printed resist edge is characterized in terms of the aerial image properties, such as intensity, intensity slope and so forth. More parameters and freedoms are required to describe the complicated chemical reactions of the resist during exposure and development processes. However, the computation time for OPC would increase significantly due to the supplementary calculation of the extra aerial image properties. In this paper, the dual model of constant threshold was proposed to enhance the accuracy of constant threshold resist models. Two constant threshold resist models were determined by model fitting process based on different types of pattern structures. During the correction, one-dimensional and two-dimensional edges are identified first and different constant-threshold models were applied for simulation. Good corrections on both of the one-dimensional line/space widths and two-dimensional line-ends could be achieved. The simulation results were also compared with experimental data.
Three important concepts about the mask error enhancement factor (MEEF) are proposed. From the definition of MEEF, it could be derived as a function of the image log slope and the aerial image variation caused by mask critical dimension (CD) errors. Second, a mask error common window indicator (MECWIN) is proposed to evaluate the MEEF and mask CD specification by knowing the wafer CD tolerance. This concept is used to define the mask CD specification without any ambiguity. Finally, we describe the complex 2-D response to the mask-making error around the line end by a mask error enhancement tensor. Both theoretical derivations and experiments to justify the theory are presented.
The control of global critical dimension uniformity (GCDU) across the entire mask becomes an important factor for the high-end masks quality. Three major proceses induce GCDU error before after-developing inspection (ADI) including the E-Beam writing, baking, and developing processes. Due to the charging effect, the fogging effect, the vacuum effect and other not-well-known effects, the E-Beam writing process suffers from some consistent GCDU errors. Specifically, the chemical amplified resist (CAR) induces the GCDU error from improper baking. This phenomenon becomes worse with negative CARs. The developing process is also a source of the GCDU error usually appears radially. This paper reports the results of the study of the impact of the global CD uniformity on mask to wafer images. It also proposes solutions to achieve better masks.
In the IC industry the mask cost and cycle time have increased dramatically since the chip design has become more complex and the required mask specification, tighter. The lithography technology has been driven to 65-nm node and 90-nm product will be manufacturing in 2004, according to ITRS's roadmap. However, the optical exposure tools do not extend to a shorter wavelength as the critical dimension (CD) shrinks. In such sub-wavelength technology generation,
the mask error factor (MEF) is normally higher. Higher MEF means that tighter mask specification is required to sustain the lithography performance. The tighter mask specification will impact both mask processing complexity and cost. The mask is no longer a low-cost process. In addition, the number of wafers printed from each mask set is trending down, resulting in a huge investment to
tape out a new circuit. Higher cost discourages circuit shrinking, thus, prohibits commercialization of new technology nodes.
The yield impact by local non-uniformity of poly-gate CD, edge profile, and transistor performance (between larger pitch area and minimum pitch area) is no longer tolerable in advanced CMOS technology as illustrated in a 2M SRAM vehicle processed by 0.13um flow in this paper. Micro-loading effects shall be minimized for process steps in poly-gate loop (including poly patterning, hard-mask etching, photo-resist (PR) ashing, poly etching, hard-mask removal, wet clean, etc), so that the accumulated local non-uniformity can be minimized. Also additional OPC may be applied locally (on mask) to compensate the remaining local non-uniformity. Significantly higher yield of a vehicle (2M SRAM) is demonstrated by efforts from both minimizing micro-loading effect in process steps as well as applying additional local OPC.
A simple graphic analysis technique named the illumination chart method is introduced to aid the customization of the illumination aperture filter for synergistic combination with a high transmission rim-type attenuated phase-shifting mask (PSM) for deep submicron contact hole printing. This graphic method gives direct visualization of the relationship between the interference condition in the pupil and the incident angle of illumination. The working ranges of oblique illuminations with different numbers of diffraction beams taking part in imaging can be easily clarified by this graphic method, which explains the dependence of depth of focus (DOF) on pattern duty. A customized illumination aperture filter (CIF) is synthesized by collecting the effective source elements for every pattern pitch to remedy the inability of the attenuated PSM for dense patterns. To preserve the merits of off-axis illumination to dense patterns and on-axis illuminations to sparse patterns in a single exposure, the illumination chart suggests a zeroth-order-reduction mask design for dense hole pattern. We applied this integrated resolution enhancement technique to 0.17 μm contact hole printing in 248 nm wavelength, 0.55 numerical aperture optics. The experimental results show our CIF illumination not only balances the DOF enhancement throughout the pattern pitches but also suppresses the best focus shift due to spherical aberration.
Controlling errors of critical dimension (CD) uniformity is crucial to achieving optimal IC performance, high chip yield and long lasting reliability. When the CDs to be resolved are less than the wavelength equipped by a lithographic exposure tool, the chip level CD variations caused by optical proximity effect (OPE) have been found significantly. With the relentlessly reduced CDs in integrated circuits the impact of OPE to chip yield and performance is much more profound and necessitates an inverse correction. In this paper, we report a model-based full-chip OPC on the contact hole layer of 0.13-micrometers logic circuits using 248-nm photo processing and attenuated phase-shifting mask (Att PSM). The final result demonstrates that OPE of random logic contact hole level can be greatly surpassed and controlled even with mask errors and their enhancement factors included of which are typically quite significant with layers of contact holes.
Three important concepts about the mask error enhancement factor (MEEF) are proposed in this paper. From the fundamental assumption, the MEEF is derived to be a function of the image log slope and the aerial image variation caused by mask making error. Secondly, a mask error common window indicator (MECWIN) is proposed to evaluate the MEEF and mask CD specification by knowing the wafer CD tolerance. This concept is used to define the mask CD specification without any ambiguity. Finally, we describe the complex two-dimensional response to the mask making error around the line-end by a mask error enhancement tensor. Both theoretical derivations and experiments to justify the theory are presented in this paper.
High NA illumination system and off-axis illumination (OAI) have been shown as two of the most practical resolution enhancement techniques (RET) available for micro-lithography. However, these two illumination approaches may reduce the DOF of iso-patterns. To overcome this problem, scattering bar (SB) assignment has been wildly used. In this paper, the discussions are focused on SB variables of iso-features. The most important variable of SB usage is where is the suitable assignment position. A simply efficient rule has been found to easily catch the optimal position of SB assignment. For OAI illumination, the optimal SB position is exactly the same with the defocus side-lobe position of iso-line. The effect of the secondary pair of SB is also discussed in this paper, and it is found that if the secondary SB pair was not at the optimal position, the process window would be reduced. Another major topic in this paper is the specification of SB width. Here we design a test pattern to target the specification of SB width. The experimental results might give us a clear specification of SB width.
A completely new concept for designing the illumination aperture filter is suggested. From experimental or simulative methods, we have extracted the performance of every individual beam component on the illumination plane. The optimal apertures are then obtained by superimposing the best components that meet the requirements demanded by the specific photo process. Different kinds of optimal apertures were successfully implanted to deal with different process problems. Therefore, it is called the customized illumination aperture filter (CIF). The zero 1D OPE CIF, as a proof of concept, was designed to eliminate the OPE of low k1 process. Without any OPC, 0.6micrometers DOF of the common ED window was obtained, where k1 equals 0.39 for our NA equals 0.55 stepper to print. 0.18 micrometers line patterns, T push to smaller k1, another CIF was designed to maximize the individual DOF and overcome the reduced power problem accompanied with the typical aggressive OAI. Using this CIF, we achieved 1.1 micrometers common DOF with 7 percent EL for 0.18 micrometers lien patterns. The CIF doubles the power of the Nikon's strong quadrupole, shrine. An ultimate resolution limit of 0.11 micrometers line pattern was reached as well with the CIF. Finally, a contact/via CIF was designed combined with a halt-tone PSM. The CIF gives about 0.8 micrometers common DOF with 7 percent EL for 0.2 micrometers holes and 0.7 micrometers DOF for 0.17 holes using thinner resist. The CIF approach is, therefore, proven to be a cost effective and relatively easy realizable alternative to the alternating PSM for extremely low k1 process applications.
Off-axis illumination (OAI) has been shown as one of the most practical resolution enhancement techniques (RET) available for optical lithography. A customized off-axis illumination aperture filter (CIF) was designed to gain the benefits of OAI and keep the optical proximity effect (OPE) in a manage-able range for sub-0.18micrometers line and space patterns. The performance of the filter comparing with conventional, annular and quadruple illuminations in term of depth of focus, OPE, throughput, dose and power uniformity for both 0.18micrometers and 0.15micrometers NA Nikon KrF excimer laser stepper with a maximum partial coherence factor of 0.8 is presented in the paper. A brief description of the design principle of the filter is also given. A summarized conclusion on the weakness of the filter and possible improvements is also presented in the paper.
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