OPC model error study through mask and SEM measurement error

International audienceMask and metrology errors such as SEM (Scanning Electron Microscopy) measurement errors are currently not accounted for when calibrating OPC models. Nevertheless, they can lead to erroneous model parameters therefore causing inaccuracies in the model prediction if these errors are of the same order of magnitude than targeted modeling accuracy. In this study, we used a dedicated design of hundreds of features exposed through a Focus Exposure Matrix (FEM). We measured the mask bias from target for these structures and investigated its impact on the model accuracy. For the metrology error, we compared the SEM measurements to AFM measurements for as much as 105 features exposed in various process conditions of dose and defocus. These data have then been used in a OPC model calibration procedure. We show that the impact of the metrology error is not negligible and demonstrate the importance of taking into account these errors in order to improve the reliability of the OPC models

Microscale adhesion patterns for the precise localization of amoeba

In order to get a better understanding of amoeba-substrate interactions in the processes of cellular adhesion and directional movement, we engineered glass surfaces with defined local adhesion characteristics at a micrometric scale. Amoeba (Dictyostelium dicoideum) is capable to adhere to various surfaces independently of the presence of extracellular matrix proteins. This paper describes the strategy used to create selective adhesion motifs using an appropriate surface chemistry and shows the first results of locally confined amoeba adhesion. The approach is based on the natural ability of Dictyostelium to adhere to various types of surfaces (hydrophilic and hydrophobic) and on its inability to spread on inert surfaces, such as the block copolymer of polyethylene glycol and polypropylene oxide, named Pluronic. We screened diverse alkylsilanes, such as methoxy, chloro and fluoro silanes for their capacity to anchor Pluronic efficiently on a glass surface. Our results demonstrate that hexylmethyldichlorosilane (HMDCS) was the most appropriate silane for the deposition of Pluronic. A complex dependence between the physicochemistry of the silanes and the polyethylene glycol block copolymer deposition was observed. Using this method, we succeed in scaling down the micro-fabrication of pluronic-based adhesion motifs to the amoebaComment: Microelectronic Engineering (2008) in pres

An inverse ellipsometric problem for thin film characterization: comparison of different optimization methods

International audienceIn this paper, an ill-posed inverse ellipsometric problem for thin film characterization is studied. The aim is to determine the thickness, the refractive index and the coefficient of extinction of homogeneous films deposited on a substrate without assuming any a priori knowledge of the dispersion law. Different methods are implemented for the benchmark. The first method considers the spectroscopic ellipsometer as an addition of single wavelength ellipsometers coupled only via the film thickness. The second is an improvement of the first one and uses Tikhonov regularization in order to smooth out the parameter curve. Cross-validation technique is used to determine the best regularization coefficient. The third method consists in a library searching. The aim is to choose the best combination of parameters inside a pre-computed library. In order to be more accurate, we also used multi-angle and multi-thickness measurements combined with the Tikhonov regularization method. This complementary approach is also part of the benchmark. The same polymer resist material is used as the thin film under test, with two different thicknesses and three angles of measurement. The paper discloses the results obtained with these different methods and provides elements for the choice of the most efficient strategy

Outliers detection by fuzzy classification method for model building

International audienceOptical Proximity Correction (OPC) is used in lithography to increase the achievable resolution and pattern transfer fidelity for IC manufacturing. Nowadays, immersion lithography scanners are reaching the limits of optical resolution leading to more and more constraints on OPC models in terms of simulation reliability. The detection of outliers coming from SEM measurements is key in OPC [1]. Indeed, the model reliability is based in a large part on those measurements accuracy and reliability as they belong to the set of data used to calibrate the model. Many approaches were developed for outlier detection by studying the data and their residual errors, using linear or nonlinear regression and standard deviation as a metric [8]. In this paper, we will present a statistical approach for detection of outlier measurements. This approach consists of scanning Critical Dimension (CD) measurements by process conditions using a statistical method based on fuzzy CMean clustering and the used of a covariant distance for checking aberrant values cluster by cluster. We propose to use the Mahalanobis distance [2] in order to improve the discrimination of the outliers when quantifying the similarity within each cluster of the data set. This fuzzy classification method was applied on the SEM CD data collected for the Active layer of a 65 nm half pitch technology. The measurements were acquired through a process window of 25 (dose, defocus) conditions. We were able to detect automatically 15 potential outliers in a data distribution as large as 1500 different CD measurement. We will discuss about these results as well as the advantages and drawbacks of this technique as automatic outliers detection for large data distribution cleaning

Etude et caractérisation avancées des procédés plasma pour les technologies sub - 0.1 µm

Facing patterns designs below the 0.25µm node, IC manufacturers must replace the historical SiO2 insulator (k~4.4) with new lower dielectric constant materials (“low-K”), such as SiLK™ (purely organic material with k=2.65), to allow lower interconnection delay time. The main issue is that oxygen-based plasma etching of SiLK™ induce a part of isotropic etching component (“bow” or “bowing” effect), proscribed for further integration scheme. To limit this effect, we need to deposit a passivation layer on the features sidewalls during the etching process. We show that this is strongly correlated with both SiLK™ graphitization and presence of low-volatile carbon-based etching by-products in the plasma. Others carbon sources enhance sidewalls passivation. On the origin of bowed profiles, we identify that the charging effects are mainly responsible for ions deflection towards the features sidewalls. Those results can be extended to all low-Ks with low reactive ion etching energy thresholds.Les interconnexions des circuits intégrés sub-0.25µm nécessitent l'intégration d'isolants «low-K» à plus faible permittivité diélectrique que SiO2 (~ 4.4) tel que le SiLK™ (~ 2.65), un matériau organique prometteur. Mais sa gravure plasma conduit à l'obtention de structures en forme de tonneau («bow»), alors que les profils gravés doivent rester anisotropes pour les étapes ultérieures d'intégration. Afin de réduire le bow, cette étude montre que la passivation des flancs des structures gravées est nécessaire, et fortement corrélée à la dégradation («graphitisation») du SiLK et à la présence de résidus carbonés peu volatils dans le plasma. La présence de sources carbonées autres que le SiLK™ permet aussi d'améliorer la passivation. L'étude du phénomène à l'origine du bow montre enfin que les charges électrostatiques jouent un rôle majoritaire dans la déflexion des ions sur les flancs. Ces résultats intéressent également tous les low-Ks à faible seuil de gravure ionique réactive

Etude et caractérisation avancées des procédés plasma pour les technologies sub-0.1 um

L'accroissement des densités d'intégration dans les circuits intégrés nécessite l'utilisation d'isolants (appelés +-low-Ka) à plus faible constante diélectrique (er) que celle de l'oxyde de silicium (er = 4.4). Dans ce cadre, la gravure plasma d'un nouveau polymère hydrocarboné, le SiLK (er = 2.65), a été étudiée : Le problème technologique majeur rencontré avec ces nouveaux matériaux demeure leur très faible seuil de gravure ionique réactive, qui conduit inévitablement à une forte déformation des profils de gravure (+-bowa). Un moyen d'obtenir une gravure anisotrope est de déposer une couche de passivation sur les flancs des motifs gravés au cours du procédé plasma. Dans le cas du SiLK, il a été démontré qu'il existe une très forte corrélation entre : la formation et la présence de résidus de gravure peu volatils dans la phase gazeuse du plasma, l'obtention de profils de gravure plus anisotropes, et la détection d'une couche SiLK graphitée (matériau carboné amorphe pauvre en hydrogène) à la fois sur les flancs, les sommets et au fond des structures gravées. D'autres sources de carbone, comme la présence d'un masque résine ou l'ajout de méthane au mélange gazeux, peuvent être utilisées pour améliorer l'anisotropie de gravure. Ce travail a aussi permis de développer une technique d'analyse topographique chimique avec une orientation du substrat (par rapport aux angles d'irradiation X et de collection des photo-électrons) adaptée à l'analyse de structures totalement isolantes. Par la suite, une recherche de l'origine de la déflexion des ions sur les flancs (responsable de la déformation des profils de gravure au cours du procédé plasma) a été menée parmi les trois phénomènes connus potentiellement responsables du +- bow a : i.e. la réflexion des ions sur la facette du masque, la température transverse des ions ou la présence de charges électrostatiques dans les structures en cours de gravure.ORLEANS-BU Sciences (452342104) / SudocSudocFranceF

Assessment of the critical dimension prediction accuracy for the lumped parameter model for resist lithography

Lithography modelling is widely used to predict the critical dimensions (CD) of patterned features after lithographic processing. A lot of full and simplified resist models are available. Previous works on full resist models have shown that the numerous model parameters are very difficult to set and often have a poor range of validity outside the dataset that have been used to generate them [Proc. SPIE 3678 (1999) 877; Proc. SPIE 4404 (2001) 99]. Simplified resist models are an alternative solution, easier to set, and they often provide a good simulation accuracy [Proc. SPIE 4691 (2002) 1266; Proc. SPIE 5040 (2003) 1536]. Among simplified models, lumped parameter model (LPM) is widely used for CD predictions. In this paper, we study the CD prediction accuracy of the LPM, using a comparison between experimental and simulated data. A systematic method is applied for LPM parameters extraction (contrast γ and effective thickness Deff). This assessment shows that a single parameter set giving reasonable accuracy is not found. Moreover, the critical analysis of the model parameters shows that these LPM parameters have a poor physical meaning. We also point out that there is a fundamental disagreement between the LPM theory and experiments

Comparison of various lithography strategies for the 65- and 45-nm half pitch using simulation

At present, the question of the move from 193 to 157nm lithography is under discussion. There are still several major issues such as the development of 157nm photo-resists and pellicles, as well as calcium-fluoride lens material availability. The extension of the 193nm lithography down to the 65- and 45-nm half pitch technologies is now considered as a serious alternative. This requires several technical challenges with the use of phase shift masks (PSM), optical proximity effects corrections or liquid immersion. Simulation gives information on expected process latitudes and is an important tool to help this technical choice. Previous works [1,2] have shown that the "Diffused Aerial Image Model" (DAIM) [3] is accurate for CD prediction. Reliable process latitudes can be extracted from the simulated focus-exposure matrices (FEM). The model is used for the process latitudes evaluation of the different lithography approaches possibly used to print the 65- and 45-nm half pitches. 193nm illumination in addition to PSM is compared to 157nm lithography associated with conventional or optimized illumination schemes. This work shows that PSM at 193nm gives generally better exposure latitude for all pitches and CD, and confirms that 193nm lithography is a possible alternative to achieve 45nm and 70nm half pitches patterning. The process windows are nevertheless very small, and huge mask error factors (MEEF) are another sign that printing such small features is close to the physical limit (k1 factor close to the quarter)

Validity of the Diffused Aerial Image Model: an Assessment Based on Multiple Test Cases

Lithography modeling is a very attractive way to predict the critical dimensions of patterned features after lithographic processing. In a previous paper[1], we have presented the assessment of three different simplified resist models (aerial image model, aerial image convolved with fixed gaussian noise and aerial image convolved with variable gaussian noise) by using a systematic comparison between experimental and simulated data. It has been shown that the aerial image convolved with fixed gaussian noise, or "diffused aerial image model" (DAIM), exhibits surprisingly good results of CD prediction for lines @ 193nm: using these datasets, the DAIM appears as a fast and accurate model for CD prediction. This approach allows also an easy run, and because it needs only four adjustable parameters, it avoids the difficult task of resist parameters extraction associated to full resist models, with. In this paper, we enlarge the datasets used for the assessment of the DAIM by considering both lines and contact holes of various sizes printed at different wavelengths. The reference wafers have been printed at 248nm, 193nm and 157 nm. The procedure used to extract the model parameters has been improved and now needs less data to provide acceptable values. We will show that the validity of the DAIM extends well outside the results presented in ref. 1. Experimental data printed using various wavelengths, resists and exposure tools can be simulated accurately with CD prediction error ranging within few percents. It is to be noted that the results that will be presented on contact holes data indicate that the model is valid for 2D features. Finally, a comparison with full resist models shows that the accuracy of DAIM is comparable to more sophisticated and heavier models

Assessment of different simplified resist models

Resist modeling is an attractive way to predict the critical dimensions of patterned features after lithographic processing. Unfortunately, previous works have shown that model parameters are very difficult to determine and have often a poor range of validity outside the dataset that have been used to generate them [1, 2]. The goal of this work is to assess different simplified resist models using a systematic method. We have studied the accuracy of aerial image model and aerial image plus gaussian noise convolution model. The approach is based on the comparison between simulated and experimental data for periodic lines of various dimensions at various illumination conditions. We also propose a reliable expression for Bossung curves fitting. Using simple physical considerations, the expression has been made very simple and efficient. After a proper setting of the model parameters to the experimental data, mean CD discrepancies between simulation and experiment are as small as 5% and can be 3% for certain feature types. Moreover, we show that simple gaussian noise convolution models can be predictive with the same accuracy. The method for CD prediction is fully described in this paper. Significant improvements have been made in resists modeling over the last several years, but simplified resist models such as "aerial image + gaussian noise " seems to be an effective tool for CD prediction, which remains the major demand of IC manufacturers