Layout engineering is the link in the design flow with the highest degree of freedom for manufacturability optimization.
Generating correct lithography-ready mask data as part of layout design would significantly shorten product’s time to
market. Until recently, however, a layout designer has no way of evaluating the manufacturability beyond design rules.
In this paper, we propose to integrate a comprehensive manufacturability preview capability into the layout design
environment. Based on the feedback from process simulation, a correct by construction approach for generating
manufacturing friendly layout is proposed. It consists of selective mask optimization and process weak spot detection.
We demonstrate the advantage of correct process construction using MOSIS standard cell library and discuss its future
Variations in manufacturing process introduce uncertainties optical proximity correction. Discrepancies may arise between model extraction and the actual manufacturing conditions. An optimally constructed mask should minimize the sensitivity of line width variation in lithography and prevent pattern failure such a line pinch-off. In this paper, the effect of defocus on OPC mask and wafer patterning is investigated using a physical pattern transfer simulator, LithoScope. We evaluate the impact of defocus on a set of special test patterns and on a real circuit layout. We propose to control defocus effect by a combination of proper design centering and physical model-based data verification.
Variations in manufacturing process introduce uncertainties in model based optical proximity correction. Discrepancies may arise between the model description and the actual manufacturing condition. Optimal mask correction should minimize the sensitivity of line width variation as the lithography process variables change within the accepted range. In this paper, the effect of defocus on OPC mask and wafer patterning is investigated using a physical pattern transfer simulator, LithoScope. We evaluate the impact of defocus on a set of test patterns and on real circuit layout. We propose to control defocus effect by design centering and physical model-based verification.
The complexity in sub-130 nm mask layout often obscures its correctness and true lithography performance. A cost effective solution to ensure high mask performance in lithography is to apply simulation based mask layout verification. Because mask layout verification serves as a gateway to the expensive manufacturing process, the moel used for verification must have superior accuracy across the process window than models used upstream. In this paper, we demonstrate, for the first time, a software system for mask layout verification and optical proximity correction that employs a full resist development model. The new system, LithoScope, predicts wafer pattern by solving optical and resist processing equations on a scale that is until recently considered unpractical. Leveraging the predictive capability of the physical model, LithoScope can perform mask layout verification and optical proximity correction under a wide range of processing conditions and for any reticle enhancement technology without the need for multiple model development. We discuss hotspot detection, line width variation statistics, and chip level process window prediction using a practical cell layout. We show that LithoScope model can accurately describe the resist-intensive poly gate layer patterning by iso-focal optimization. This system can be used to pre-screen and fix mask data problems before manufacturing to reduce the overall cost of the mask and the product.
In this paper, we apply image-processing techniques to the task of metrology on complex shapes. We describe techniques for noise reduction, edge detection, and feature contour extraction on scanning electron microscope images. We present a novel capability for calculating the area of an arbitrary shape. We demonstrate imaged based metrology capabilities using examples encountered in photomask processing.
Simulation based mask layout verification and optimization is a cost effective way to ensure high mask performance in wafer lithography. Because mask layout verification serves as a gateway to the expensive manufacturing process, the model used for verification must have superior accuracy than models used upstream. In this paper, we demonstrate, for the first time, a software system for mask layout verification and optical proximity correction that employs a physical resist development model. The new system, LithoScope, predicts wafer patterning by solving optical and resist processing equations on a scale that is until recently considered unpractical. Leveraging the predictive capability of the physical model, LithoScope can perform mask layout verification and optical proximity correction under a wide range of processing conditions and for any reticle enhancement technology without the need for multiple model development. We show the ability for physical resist model to change iso-focal bias by optimizing resist parameters, which is critical for matching the experimental process window. We present line width variation statistics and chip level process window predictions using a practical cell layout. We show that LithoScope model can accurately describe the resist-intensive poly gate layer patterning. This system can be used to pre-screen mask data problems before manufacturing to reduce the overall cost of the mask and the product.
Simulated wafer images for Attenuated Phase Shift Mask (ATTPSM) features are performed by the Virtual Stepper System. The ATTPSM test reticles were prepared with programmed defects (hard defects and phase defects) on line/space patterns, contact hole patterns, and rectangle patterns for 150-nm design rules. Each defect area was inspected using KLA-Tencor's UV-HR365 and SLF27 inspection systems. Virtual Stepper simulations are compared with Aerial Image Measurement System (AIMSTM) simulation at best focus and at multiple defocus levels. In addition, simulation accuracy from different inspection images is compared.
In this paper, we demonstrate new simulation capabilities for defect dispositioning of alternating aperture phase shift masks (AAPSM). A defect mask for use in a 248 nm exposure tool was fabricated with programmed phase defects. Inspection images of the defects were taken on Lasertec's MD3000 and KLA-Tencor's SLF27 inspection systems. The simulation tool takes defect images as input and simulates photolithography performance via aerial image modeling. We present preliminary modeling results that show good agreement between simulated CDs and the CDs from Aerial Image Measurement System (AIMSTM) measurements. This work shows the potential for extending Virtual Stepper?System to AAPSMs on a variety of inspection platforms.
In this paper, the simulation of wafer images for Alternating Aperture Phase Shift Masks is addressed by comparing wafer printing image with simulation. This is the first accuracy study for Virtual Stepper's newly developed AAPSM simulation module. The test reticle used includes 70 nm gate structures with three types of programmed phase defects: edge, corner, and center defects on rectangular shifter patterns. Wafer exposures are performed using 193 nm imaging technology and inspection images generated on a KLA-Tencor's SLF27 system. These images are used by the Virtual Stepper System to provide simulated wafer images using the specified stepper parameters. The results are compared to the simulation results from the Aerial Image Measurement System (AIMSTM) and SEM images of resist patterns.
In this paper the simulation of wafer images for Attenuated Phase Shift Masks (ATTPSM) and repaired binary masks are performed by Virtual Stepper System in a real production environment. In addition, the Automatic Defect Severity Scoring module in Virtual Stepper is also used to calculate the defect severity score for each defect. ADSS provides an overall score that quantifies the impact of a given defect on the surrounding features. For the binary masks, the quality of reported defects is studied. For the ATTPSM three types of programmed defects on both line/space and contact hole patterns are assessed. Wafer exposures are performed using 248 nm imaging technology and inspection images generated on a KLA-Tencor's SLF27 system. These images are used by the Virtual Stepper System to simulate wafer images under the specific stepper parameters. The result are compared to SEM images of resist patterns and Aerial Image Measurement System simulated results.
Alternating Phase Shift Mask (APSM) reticles is critical to achieve sub 0.1 um poly gate lithography. Intrinsic APSM image inbalance can be resolved with various methods such as isotropic etch and aperture sizing, where positional line-shift can be reduced to within 5nm of final CD target. Defect reduction of APSM fabrication is addressed with multiple-option strategy to achieve high manufacturing yield. After Develop Inspection (ADI) capability was demonstrated with partial and complete missing 180 deg apertures, detected at post-develop with correlation to Qz defect after dry etch. Feasibility of APSM inspection and repair was demonstrated with existing toolsets and critical gap versus APSM defect specification remained to be bridged.
In this paper, we present a comparative simulation study of mask defect impact in a high MEEF process and its detectability under a mask inspection too. A simulation mode for a mask inspection system is constructed and validated by comparing the simulated signal with data collected from an inspection tool. With this calibrated mode, defect images and scan signals from programmed defect pattens are studied. The corresponding wafer CD variations caused by programmed defects are simulated using a photolithography simulator. We find that for a mask defect of a given size, its impact on wafer varies greatly from location to location, depending on the MEEF of the host patterns surrounding the defect. In comparison, the signal from a high-resolution inspection tool varies linearly with defect size and is nearly independent of the host patterns. Once the MEEF starts its sharp increase, the sensitivity of the inspection tool is required to increase at the same rate. An inspection tool operating at its resolution limit generally could not follow the sharp increase in MEEF once the wafer process starts to degrade. It is therefore important to control the MEEF in the original pattern design to ensure that residual defect does not cause circuit malfunction. Extra margins may have to be introduced in the design rule to account for the impact of residual defects.
In this paper, we present a method for linking a finite element Maxwell's equation solver with a scalar lithography simulator, iPHOTO-II. The combined simulator takes the mask topography and the stepper parameters as input and simulates the resist profile on the wafer plane. The accuracy of the simulator is demonstrated by comparing simulation results with experimental data over a wide range of focus, exposure and mask dimensions. The simulator is used to predict the performance of a phase edge phase shift mask. It is revealed that the true position of the line center in a phase edge PSM is shifted slightly from the location given by geometric projection. Biasing rules for compensating for this location shift are presented.
In this paper, the effect of laser ablation induced carbon residue and quartz damage near the mask repair region in a sub-half-micron DUV wafer printing process is discussed. In the study, we found that the laser ablation induced carbon residue and quartz damage during a clean-up process of a clear intrusion mask defect repair could cause both phase and transmission errors near the repaired region. As a result, the printing characteristics of the resist in the repaired region are different than that of the defect-free region, especially at defocus conditions. At zero defocus, the resist critical dimension (CD) difference between the repaired and defect-free regions is mainly determined by the repair edge error and the amount of transmission loss which is due to the quartz damage and carbon residue in the clear mask region. At positive defocus, the repaired region tends to print narrower than that of defect-free region and vice versa for the negative defocus conditions. This phenomenon is the result of quartz damage induced phase error in the clear mask area near the repair. This quartz damage induced effect is more pronounced at 0.25 micrometer regime than that of 0.4 micrometer regime. In the study, we also compared wafer level results of laser repaired features to that of focused ion beam repaired features to identify the carbon residue and quartz damage induced effects in the laser repair. Our simulations also predicted the above observed experimental results.
Partial coherent imaging in a high NA stepper is treated with the source integration method. Image formation in 3D is accomplished by the propagation and interference of plane waves. This approach allows the extensive use of FFT and leads to efficient computation of the latent image. In order to further reduce the computation time, we propose a sufficient condition for the grid density in an image plane based on the sampling theorem. Finally, we present a semi-analytical method for the modeling of post exposure bake process in 3D. With these enhancements in the algorithm, a typical 3D latent image problem can be solved in a few second on a workstation.
KEYWORDS: Deep ultraviolet, Data modeling, Signal processing, Semiconducting wafers, Process modeling, Photoresist processing, Picture Archiving and Communication System, Lithography, In situ metrology, Systems modeling
A study of the dissolution behavior of acid-hardened resists (AHR) was undertaken for spray and spray/puddle development processes. The Site Services DSM-100 end-point detection system is used to measure both spray and puddle dissolution data for a commercially available deep-ultraviolet AHR resist, Shipley SNR-248. The DSM allows in situ measurement of dissolution rate on the wafer chuck and hence allows parameter extraction for modeling spray and puddle processes. The dissolution data for spray and puddle processes was collected across a range of exposure dose and postexposure bake temperature. The development recipe was varied to decouple the contribution of the spray and puddle modes to the overall dissolution characteristics. The mechanisms involved in spray versus puddle dissolution and the impact of spray versus puddle dissolution on process performance metrics has been investigated. We used the effective-dose-modeling approach and the measurement capability of the DSM-100 and developed a lumped parameter model for acid-hardened resists that incorporates the effects of exposure, postexposure bake temperature and time, and development condition. The PARMEX photoresist-modeling program is used to determine parameters for the spray and for the puddle process. The lumped parameter AHR model developed showed good agreement with experimental data.
In this paper, detailed simulation and some experimental studies on stepper lens aberration effect in the case of oblique illumination source are presented. The results are compared to that of conventional illumination source. Due to the unique feature of oblique illumination source imaging, i.e., imaging by using only zero and first diffraction order light, both stepper resolution limit and depth of focus (DOF) are extended. As a result, the effect of lens aberration in resist printing are also different from that of conventional illumination source. Unlike the conventional illumination source, the net effect of stepper lens aberration in resist printing depends not only on both the amount and type of the lens aberration, but also on the mask feature pattern. In the case of lens distortion, unlike the other types of lens aberration, the oblique illumination source does not show any improvement as compared to that of conventional illumination source. It does not show pattern dependent distortion either. In the experiment, an effect of a stepper lens aberration in resist printing for both conventional illumination and quadrapole illumination sources (mostly astigmatism) were measured. The results were in agreement with our simulation results.
Using etched quartz as a phase shifter for i-line phase shift masks requires an etched depth of 385 nm referenced from the quartz surface. Recent work shows a direct relationship of focus offset as a function of the phase angle as it deviates from 180 degree(s). Knowing the etched depth and phase in transmission becomes critical to the production and verification of these masks. A method of interferometrically evaluating a phase shift mask is proposed. The method calculates the phase shift from the surface profile of the etched shifter assuming that a good optical surface is maintained on the unetched regions. This surface measurement method possesses high spatial resolution at the expense of only knowing the amount of phase shift from the profile of the etched quartz shifter. Correlations between this method and mechanical stylus measurements establish the validity and advantages of this measurement technique.
The Rayleigh criteria for minimum resolution and acceptable depth of focus (DOF) in the case of finite excimer laser bandwidth with a chromatic lens design were re-evaluated both by experiment and by simulation. As a result of chromatic lens design combined with a narrow spectral bandwidth (BW) excimer laser, both resolution and DOF are not only determined by source wavelength and the numerical aperture (NA) of the lens, as predicted by Rayleigh criteria, but also by the laser spectral BW. To fully understand the role of laser spectral BW in chromatic projection printing, the resolution as a function of lens NA, laser spectral BW, and defocus were studied through simulation. The relations between resolution and its corresponding DOF for different laser spectral BW and NA were also obtained. The results were compared to that obtained by Rayleigh criteria. Unlike the single wavelength case, resolution at larger NA is basically limited by the spectral BW of the laser rather than NA of the lens system. The DOF is limited by both laser spectral BW and lens NA. The optimum NA for a given BW for different defocus cases were predicted. The effect of chromatic focus spread due to finite laser spectral BW was compared to the case of single wavelength with small amount defocus. Intel’s rigorous bulk image model has been used for process window simulations1. In the experiment, the effect of finite laser spectral BW to the pattern resolution and DOF were studied for the case of 0.42 NA chromatic lens with finite laser spectral full width at half maximum (FWHM) of 2 pm, 3 pm, 4 pm and 5 pm, respectively. The experimental results were compared to the simulation results.
Proc. SPIE. 1674, Optical/Laser Microlithography V
KEYWORDS: Lithography, Optical lithography, Data modeling, Deep ultraviolet, Signal processing, Semiconducting wafers, Statistical modeling, Systems modeling, Process modeling, Picture Archiving and Communication System
An investigative study of the dissolution behavior of acid hardened resists (AHR) was undertaken for spray and spray-puddle development processes. A unique tool, the Site Services DSM-100 End-point detection system, is used to measure both spray and puddle dissolution data for a commercially available deep ultra-violet AHR resist, Shipley SNR-248. The DSM allows in- situ measurement of dissolution rate on the wafer chuck and hence allows parameter extraction for modeling spray and puddle processes. The dissolution data for spray and puddle processes was collected across a range of exposure dose and PEB temperature. The development recipe was varied to decouple the contribution of the spray and puddle modes to the overall dissolution characteristics. The mechanisms involved in spray versus puddle dissolution and their impact on process performance metrics have been investigated. The PARMEX photoresist modeling program is used to determine parameters for the spray and for the puddle process. A lumped parameter AHR model developed at Intel was used in iPHOTO for simulation studies.
The utility of exposure margin, defined as the ratio between the 1:1 mask reproduction exposure energy and the open frame threshold exposure energy (E0), as an indicator of process latitude, is probed using extensive computer simulations and some experimental photolithography. The correlation is shown to be excellent for latitudes which depend primarily on critical dimension such as mask linearity and exposure latitude: a high exposure margin implies a high process latitude. Qualitative physical arguments are offered to explain this. For sidewall angle constrained latitudes such as defocus, the correlation is also good if comparing photoresists with similar optical absorption characteristics. These results are potentially significant because exposure margin is easily measured and therefore provides an efficient means for process optimization (at zero mask bias). As part of this work a simulation procedure which reduces the focus: exposure: mask dimension latitude to a single process latitude volume was developed and is described.
This paper describes a three-dimensional computer modeling technique for alignment system simulation, and some example calculations. The technique has been developed to address issues of alignment and overlay accuracy for future generation VLSI technology. The analytical basis is a general finite element electromagnetic wave propagation code, EMFlex, that rigorously simulates light scattering from the 3-D alignment mark. Using the Nikon Laser Step Alignment (LSA) system as a model instrument, the overlay error and signal shape are simulated. Examples of an idealized asymmetric metal mark are studied. Preliminary results suggest that the rigorous simulations are substantially different from the one-dimensional Fresnel approximations that have been used previously.