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With the continuous reduction of layout dimension, critical dimension control becomes more and more tricky. Being one contribution of critical dimension variation, etch bias correction needs to get more accurate. The correction by rule-based optical proximity correction, which only takes into account the distance to the facing outer and inner edges, is reaching its limit at 65nm node. Indeed, its found to be not accurate enough in some particular configurations. Thus, applying etch correction by model-based optical proximity correction looks like a good solution. The etch bias to correct for can be approximated by a function of the local density, obtained by convoluting the design with Kernels. However, it is found to be still not accurate enough in some configurations if the Kernels used for etch modeling are symmetrical, as for optical part. In this study, several etch models are tested to correct etch bias variations of poly layer patterning for 65nm node.

International audience; Assist features (AF) are an essential component of reticle enhancement techniques. Their use is indispensable in sub-100 nm technologies to ensure a maximum process window (PW) across chip, especially for critical levels. Indeed, AF, which can be binary, attenuated or phase-shifted, help in providing a larger PW to the features they assist when they are used in conjunction with off-axis illumination. The depth of focus (DOF) of an isolated structure is improved by the presence of AF by providing to the optical system a diffraction pattern close to the diffraction pattern of a dense structure. The resulting DOF and exposure latitude (EL) are dependent on the relative position of the AF from the main feature. Moreover, the PW varies while inserting one, two or more AF. The relative position of each influences the results. In this paper, a method will be detailed to optimise the placement of the AF. For the purpose of this study, the diffraction pattern induced by the insertion of one or several AF is analysed in frequency space. This analysis details the evolution of the intensity of even and odd orders during the insertion of AF. The calculation of the optimum placement is detailed, and the DOF resulting from the insertion of one or more AF is also presented.

In advanced technology nodes, due to accuracy and computing time constraint, OPC has shifted from discrete simulation to pixel based simulation. The simulation is grid based and then interpolation occurs between grid points. Even if the sampling is done below Nyquist rate, interpolation can cause some variations for same polygon placed at different location in the layout. Any variation is rounded during OPC treatment, because of discrete numbers used in OPC output file. The end result is inconsistency in post-OPC layout, where the same input polygon will give different outputs, depending on its position and orientation relative to the grid. This can have a major impact in CD control, in structures like SRAM for example, where mismatching between gates can cause major issue. There are some workarounds to minimize this effect, but most of them are post-treatment fix. In this paper, we will try to identify and solve the root cause of the problem. We will study the relationship between the pixel size and the consistency of post OPC results. The pixel size is often set based on optical parameters, but it might be possible to optimize it around this value to avoid inconsistency. One can say that the optimization will highly depend on design and not be possible for a real layout. As the range of pitch used in a design tends to decrease, thanks to fix pitch layouts, we may optimize pixel size for a full layout.

In the continuous battle to improve critical dimension (CD) uniformity, especially for 45-nanometer (nm) logic advanced products, one important recent advance is the ability to accurately predict the mask CD uniformity contribution to the overall global wafer CD error budget. In most wafer process simulation models, mask error contribution is embedded in the optical and/or resist models. We have separated the mask effects, however, by creating a short-range mask process model (MPM) for each unique mask process and a long-range CD uniformity mask bias map (MBM) for each individual mask. By establishing a mask bias map, we are able to incorporate the mask CD uniformity signature into our modelling simulations and measure the effects on global wafer CD uniformity and hotspots. We also have examined several ways of proving the efficiency of this approach, including the analysis of OPC hot spot signatures with and without the mask bias map (see Figure 1) and by comparing the precision of the model contour prediction to wafer SEM images. In this paper we will show the different steps of mask bias map generation and use for advanced 45nm logic node layers, along with the current results of this new dynamic application to improve hot spot verification through Brion Technologies' model-based mask verification loop.

At 45 and 32 nm nodes, one of the most critical layers is the Contact one. Due to the use of hyper NA imaging, the depth of focus starts to be very limited. Moreover the OPC is rapidly limited because of the increase of the pattern density. The limited surface in the dark field region of a Contact layer mask enforces the edges movement to stop very quickly. The use of SRAF (Sub Resolution Assist Feature) has been widely use for DOF enhancement of line and space layers since many technology node. Recently, SRAF generated using inverse lithography have shown interesting DOF improvement1. However, the advantage of the ideal mask generated by inverse lithography is lost when switching to a manufacturable mask with Manhattan structures. For SRAF placed in rule based as well as Manhattan SRAF generated after inverse lithography, it is important to know what their behavior is, in term of size and placement. In this article we propose to study the placement of scatter-trenches assist features for the contact layer. For this we have performed process window simulation with different SRAF sizes and distance to the main OPC. These results permit us to establish the trends for size and placement of the SRAF. Moreover we have also take a look of the advantages of using 8 surrounding SRAF (4 in vertical - horizontal and 4 at 45°) versus 4 surrounding SRAF. Based on these studies we have seen that there is no real gain of increasing the complexity by adding additional SRAF.

The perpetual shrinking in critical dimensions in semiconductor devices is driving the need for increased resolution in optical lithography. Increasing NA to gain resolution also increases Optical Proximity Correction (OPC) model complexity. Some optical effects which have been completely neglected in OPC modeling become important. Over the past few years, off-axis illumination has been widely used to improve the imaging process. OPC models which utilize such illumination still use the thin film mask approximation (Kirchhoff approach), during optical model generation, which utilizes a normal incidence. However, simulating a three dimensional mask near-field using an off-axis illumination requires OPC models to introduce oblique incidence. In addition, the use of higher NA systems introduces high obliquity field components that can no longer be assimilated as normal incident waves. The introduction of oblique incidence requires other effects, such as corner rounding of mask features, to be considered, that are seldom taken into account in OPC modeling. In this paper, the effects of oblique incidence and corner rounding of mask features on resist contours of 2D structures (i.e. line-ends and corners) are studied. Rigorous electromagnetic simulations are performed to investigate the scattering properties of various lithographic 32nm node mask structures. Simulations are conducted using a three dimensional phase shift mask topology and an off-axis illumination at high NA. Aerial images are calculated and compared with those obtained from a classical normal incidence illumination. The benefits of using an oblique incidence to improve hot-spot prediction will be discussed.

Patterning isolated trenches for bright field layers such as the active layer has always been difficult for lithographers. This patterning is even more challenging for advanced technologies such as the 45nm node where most of the process optimization is done for minimum pitch dense lines. Similar to the use of scattering-bars to assist isolated lines structures, we can use inverse Sub Resolution Assist Features (SRAF) to assist the patterning of isolated trenches structures. Full characterization studies on the C45 Active layer demonstrate the benefits and potential issues of this technique: - Screen Inverse SRAF parameters (size, distance to main feature) utilizing optical simulation - Verify simulation predictions and ensure sufficient improvement in Depth of Focus and Exposure latitude with silicon process window analysis - Define Inverse SRAF OPC generation script parameters and validate, with accurate on silicon, measurement characterization of specific test patterns - Maskshop manufacturability through CD measurements and inspection capability Finally, initial silicon results from a 45nm mask are given with suggestions for additional optimization of inverse SRAF for trenches.

Resolution Enhancement Techniques (RET) are inherently design dependent technologies. To be successful the RET strategy needs to be adapted to the type of circuit desired. For SOC (system on chip), the three main patterning constraints come from: -Static RAM with very aggressive design rules specially at active, poly and contact -transistor variability control at the chip level -random layouts The development of regular layouts, within the framework of DFM, enables the use of more aggressive RET, pushing the required k1 factor further than allowed with existing RET techniques and the current wavelength and NA limitations. Besides that, it is shown that the primary appeal of regular design usage comes from the significant decrease in transistor variability. In 45nm technology a more than 80% variability reduction for the width and the length of the transistor at best conditions, and more than 50% variability reduction though the process window has been demonstrated. In addition, line-end control in the SRAM bitcell becomes a key challenge for the 32nm node. Taking all these constraints into account, we present the existing best patterning strategy for active and poly level of 32nm : -dipole with polarization and regular layout for active level -dipole with polarization, regular layout and double patterning to cut the line-end for poly level. These choices have been made based on the printing performances of a 0.17mm2 SRAM bitcell and a 32nm flip-flop with NA 1.2 immersion scanner.

Several qualification stages are required for new maskshop tools, first step is done by the maskshop internally. Taking a new writer for example, the maskshop will review the basic factory and site acceptance tests, including CD uniformity, CD linearity, local CD errors and registration errors. The second step is to have dedicated OPC (Optical Proximity Correction) structures from the wafer fab. These dedicated OPC structures will be measured by the maskshop to get a reticle CD metrology trend line. With this trend line, we can: - ensure the stability at reticle level of the maskshop processes - put in place a matching procedure to guarantee the same OPC signature at reticle level in case of any internal maskshop process change or new maskshop evaluation. Changes that require qualification could be process changes for capacity reasons, like introducing a new writer or a new manufacturing line, or for capability reasons, like a new process (new developer tool for example) introduction. Most advanced levels will have dedicated OPC structures. Also dedicated maskshop processes will be monitored with these specific OPC structures. In this paper, we will follow in detail the different reticle CD measurements of dedicated OPC structures for the three advanced logic levels of the 65nm node: poly level, contact level and metal level. The related maskshop's processes are - for poly: eaPSM 193nm with a nega CAR (Chemically Amplified Resist) process for Clear Field L/S (Lines & Space) reticles - for contact: eaPSM 193nm with a posi CAR process for Dark Field Holes reticles - for metal1: eaPSM 193nm with a posi CAR process for Dark Field L/S reticles. For all these structures, CD linearity, CD through pitch, length effects, and pattern density effects will be monitored. To average the metrology errors, the structures are placed twice on the reticle. The first part of this paper will describe the different OPC structures. These OPC structures are close to the DRM (Design Rule Manual) of the dedicated levels to be monitored. The second part of the paper will describe the matching procedure to ensure the same OPC signature at reticle level. We will give an example of an internal maskshop matching exercise, which could be needed when we switched from an already qualified 50 KeV tool to a new 50 KeV tool. The second example is the same matching exercise of our 65nm OPC structures, but with two different maskshops. The last part of the paper will show first results on dedicated OPC structures for the 45nm node.

The semiconductor industry will soon be putting >=1.07NA 193nm immersion lithography systems into production for the 45nm device node and in about three years will be putting >=1.30NA systems into production for the 32nm device node. For these very high NA systems, the maximum angle of light incident on a 4X reticle will reach ~16 degrees and ~20 degrees for the 45nm and 32nm nodes respectively. These angles can no longer be accurately approximated by an assumption of normal incidence. The optical diffraction and thin film effects of high incident angles on the wafer and on the photomask have been studied by many different authors. Extensive previous work has also investigated the impact of high angles upon hard (e.g., F-doped silica) thick (>700μm) pellicles for 157nm lithography, e.g.,. However, the interaction of these high incident angles with traditional thin (< 1μm) organic pellicles has not been widely discussed in the literature. In this paper we analyze the impact of traditional thin organic pellicles in the imaging plane for hyper-NA immersion lithography at the 45nm and 32nm nodes. The use of existing pellicles with hyper-NA imaging is shown to have a definite negative impact upon lithographic CD control and optical proximity correction (OPC) model accuracy. This is due to the traditional method of setting organic pellicle thickness to optimize normally incident light transmission intensity. Due to thin film interference effects with hyper-NA angles, this traditional pellicle optimization method will induce a loss of high spatial frequency (i.e., high transmitted angle) intensity which is similar in negative impact to a strong lens apodization effect. Therefore, using simulation we investigate different pellicle manufacturing options (e.g., multi-layer pellicle films) and OPC modeling options to reduce the high spatial frequency loss and its impact.

The quality of model-based OPC correction depends strongly on how the model is calibrated in order to generate a resist image as close to the desired shapes as possible. As the k1 process factor decreases and design complexity increases, the correction accuracy and the model stability become more important. It is also assumed that the stability of one model can be tested when its response to a small variation in one or several parameters is small. In order to quantify this, the small-variation method has been tested on a variable threshold based model initially optimized for the 65nm node using measurements done with a test pattern mask. This method consists of introducing small variations to one input model parameter and analyzing the induced effects on the simulated edge placement error (EPE). In this paper, we study the impact of small changes in the optical and resist parameters (focus settings, inner and outer partial coherent factors, NA, resist thickness) on the model stability. And then, we quantify the sensitivity of the model towards each parameter shift. We also study the effects of modeling parameters (kernel count, model fitness, optical diameter) on the resulting simulated EPE. This kind of study allows us to detect coverage or process window problems. The process and modeling parameters have been modified one by one. The ranges of variations correspond to those observed during a typical experiment. Then the difference in simulated EPE between the reference model and the modified one has been calculated. Simulations show that the loss in model accuracy is essentially caused by changes in focus, outer sigma and NA and lower values of optical diameter and kernel count. Model results agree well with a production layout.