Madhura Nataraju

Analysis of Coulomb and Johnsen-Rahbek electrostatic chuck performance for extreme ultraviolet lithography

The successful implementation of extreme ultraviolet lithography (EUVL) requires the use of an electrostatic chuck to both support and flatten the mask during scanning exposure. The EUVL Mask and Chucking Standards, SEMI P37 and P40, specify the nonflatness of the mask frontside and backside, as well as the chucking surface, to be on the order of 50 nm peak-to-valley. Thus, characterizing and predicting the capability of the electrostatic chuck to reduce mask nonflatness to meet this specification are critical issues. Details describing the performance of the Coulomb electrostatic chuck have been presented in earlier publications. In this paper, the governing equation identifying the force-gap relationship for a Johnsen-Rahbek (J-R) chuck is described and compared to the Coulomb response. Using finite element techniques, numerical models of Coulomb and J-R electrostatic chucks have been constructed and evaluated for their clamping performance. The models include the effects of reticle and chuck nonflatnes...

Electrostatic chucking for extreme ultraviolet lithography: Simulations and experiments

The purpose of this research is to assess the effectiveness of electrostatic chucks in reducing low-spatial frequency mask (or reticle) flatness variations and to validate finite element (FE) models of the chuck-mask interaction. The flatness of a sample extreme ultraviolet lithography reticle and an electrostatic pin chuck were measured using a Zygo interferometer. The measured flatness data were entered into the FE models, and electrostatic chucking was simulated by applying an area-weighted average pressure on the reticle. The shape of the mask when clamped by the electrostatic chuck was then predicted using the FE model. To validate these predictions, experiments were conducted in which the previously measured reticle was electrostatically clamped using the pin chuck. These experiments were conducted in a vacuum chamber to minimize the effects of humidity. Interferometric plots of the chucked reticle surface were obtained and compared with the FE predictions. It was found that the measured and predict...

EUV mask and chuck analysis: simulation and experimentation

Extreme ultraviolet (EUV) masks and mask chucks require extreme flatness in order to meet the performance and timing specified by the International Technology Roadmap for Semiconductors (ITRS). The EUVL Mask and Chucking Standards, SEMI P37 and SEMI P40, specify the nonflatness of the mask frontside and backside, as well as the chucking surface, to be no more than 50 nm peak-to-valley (p-v). Understanding and characterizing the clamping ability of the electrostatic chuck and its effect on the mask flatness is a critical issue. In the present study, chucking experiments were performed using an electrostatic pin chuck and finite element (FE) models were developed to simulate the chucking. The frontside and backside surface flatness of several EUV substrates were measured using a Zygo large-area interferometer. Flatness data for the electrostatic chuck was also obtained and this data along with the substrate flatness data was used as the input for the FE modeling. Data from one substrate was selected for modeling and testing and is included in this paper. Electrostatic chucking experiments were conducted in a clean-room facility to minimize contamination due to particles. The substrate was chucked using an electrostatic pin chuck and the measured flatness was compared to the predictions obtained from the FE simulation.

Electrostatic chucking and EUVL mask flatness analysis

Successful implementation of Extreme Ultraviolet Lithography (EUVL) depends on advancements in many areas, including the quality of the mask and chuck system to control image placement (IP) errors. One source of IP error is the height variations of the patterned mask surface (i.e., its nonflatness). The SEMI EUVL mask and chucking standards (SEMI P37 and SEMI P40) describe stringent requirements for the nonflatness of the mask frontside and backside, and the chucking surfaces. Understanding and characterizing the clamping ability of the electrostatic chuck and the effect on the mask flatness is therefore critical in order to meet these requirements. Legendre polynomials have been identified as an effective and efficient means of representing EUVL mask surface shapes. Finite element (FE) models have been developed to utilize the Legendre coefficients (obtained from measured mask and chuck data) as input data to define the surfaces of the mask and the chuck. The FE models are then used to determine the clamping response of the mask and the resulting flatness of the pattern surface. The sum of the mask thickness nonuniformity and the chuck surface shape has a dominant effect on the flatness of the patterned surface after chucking. The focus of the present research is a comprehensive analysis of the flatness and interaction between the nonflat chuck and the mask. Experiments will be conducted using several sample masks chucked by a slab type electrostatic chuck. Results from the study will support and facilitate the timely development of EUVL mask/chuck systems which meet required specifications.

Compensating for image placement errors induced during the fabrication and chucking of EUVL masks

With the stringent requirements on image placement (IP) errors in the sub-65-nm regime, all sources of mask distortion during fabrication and usage must be minimized or corrected. For extreme ultraviolet lithography, the nonflatness of the mask is critical as well, due to the nontelecentric illumination during exposure. This paper outlines a procedure to predict the IP errors induced on the mask during the fabrication processing, e-beam tool chucking, and exposure tool chucking. Finite element (FE) models are used to simulate the out-of-plane and in-plane distortions at each load step. The FE results are compiled to produce a set of Correction Tables that can be implemented during e-beam writing to compensate for these distortions and significantly increase IP accuracy. A previous version of this paper appeared in the Proceedings of the European Mask and Lithography Conference (EMLC), SPIE, 6533, 653314 (2007). The paper has been updated, retitled, and published here as a result of winning the Best Paper Award at the EMLC.

Electrostatic chucking of EUVL reticles

Characterizing the effect of electrostatic chucking on the flatness of Extreme Ultraviolet Lithography (EUVL) reticles is necessary for the implementation of EUVL for the sub-32 nm node. In this research, finite element (FE) models have been developed to predict the flatness of reticles when clamped by a bipolar Coulombic pin chuck. Nonflatness measurements of the reticle and chuck surfaces were used to create the model geometry. Chucking was then simulated by applying forces consistent with the pin chuck under consideration. The effect of the nonuniformity of electrostatic forces due to the presence of gaps between the chuck and reticle backside surfaces was also included. The model predictions of the final pattern surface shape of the chucked reticle have been verified with chucking experiments and the results have established the validity of the models. Parametric studies with varying reticle shape, chuck shape, chuck geometry, and chucking pressure performed using FE modeling techniques are extremely useful in the development of SEMI standards for EUVL.

Predicting and correcting for image placement errors during the fabrication of EUVL masks

With the stringent requirements on image placement (IP) errors in the sub-65 nm regime, all sources of mask distortion during fabrication and usage must be minimized or corrected. For extreme ultraviolet lithography, the nonflatness of the mask is critical as well, due to the nontelecentric illumination during exposure. This paper outlines a procedure to predict the IP errors induced on the mask during the fabrication processing, e-beam tool chucking, and exposure tool chucking. Finite element (FE) models are used to simulate the out-of-plane and in-plane distortions at each loading step. The FE results are compiled to produce a set of Correction Tables that can be implemented during e-beam writing to compensate for these distortions and significantly increase IP accuracy.

Measuring and characterizing the nonflatness of EUVL reticles and electrostatic chucks

According to the International Technology Roadmap for Semiconductors, meeting the strict requirements on image placement errors in the sub-45-nm regime may be one of the most difficult challenges for the industry. For Extreme Ultraviolet Lithography (EUVL), the nonflatness of both the mask and chuck is critical as well, due to the nontelecentric illumination during exposure. To address this issue, SEMI Standards P37 and P40 have established the specifications on flatness for the EUVL mask substrate and electrostatic chuck. This study investigates the procedures for implementing the Standards when measuring and characterizing the shapes of these surfaces. Finite element simulations are used to demonstrate the difficulties in supporting the mask substrate, while ensuring that the measured flatness is accurate. Additional modeling is performed to illustrate the most appropriate methods of characterizing the nonflatness of the electrostatic chuck. The results presented will aid in identifying modifications and clarifications that are needed in the Standards to facilitate the timely development of EUV lithography.

Modeling LEEPL mask fabrication processes

The challenges in fabricating next-generation lithography (NGL) masks are distinct from those encountered in optical technology. The masks for electron proximity lithography, as well as those for ion and electron projection, use freestanding membranes incorporating layers that are different from the traditional chrome-on-glass photomask blanks. As a promising NGL technology, low-energy electron-beam proximity-projection lithography (LEEPL) will be subject to strict error budgets, requiring high pattern placement accuracy. Meeting these stringent conditions will necessitate an optimization of the design parameters involved in the mask fabrication process. Consequently, comprehensive simulations can be used to characterize the sources of the mechanical distortions induced in LEEPL masks during fabrication, pattern transfer, and mounting. For this purpose, finite element (FE) structural models have been developed to identify the response of the LEEPL mask during fabrication and chucking. Membrane prestress, which is used as input in the FE models, was measured on a 200-mm test mask and found to low in magnitude with excellent cross-mask uniformity. The numerical models were also validated both analytically and experimentally considering intrinsic and extrinsic loading of the mask. Finally, simulations were performed to predict the response of the LEEPL mask during electrostatic chucking. FE results indicate that the mask structure is sufficiently stiff to remain relatively flat under gravitational loadings. The results illustrate that mechanical modeling and simulation can facilitate the timely and cost-effective implementation of the LEEPL technology.

Experimental verification of finite element model prediction of EUVL mask flatness during electrostatic chucking

Stringent flatness requirements have been imposed for the front and back surfaces of extreme ultraviolet lithography masks to ensure successful pattern transfer within the image placement error budget. During exposure, an electrostatic chuck will be used to support and flatten the mask. It is therefore critical that the electrostatic chucking process and its effect on mask flatness be well-understood. The current research is focused on the characterization of various aspects of electrostatic chucking through advanced finite element (FE) models and experiments. FE models that use flatness measurements of the mask and the chuck to predict the final flatness of the pattern surface have been developed. Pressure was applied between the reticle and chuck to simulate electrostatic clamping. The modeling results are compared to experimental data obtained using a bipolar Coulombic pin chuck. Electrostatic chucking experiments were performed in a cleanroom, within a vacuum chamber mounted on a vibration isolation cradle, to minimize the effects of particles, humidity, and static charges. During these experiments, the chuck was supported on a 3-point mount; the reticle was placed on the chuck with the backside in contact with the chucking surface and the voltage was applied. A Zygo interferometer was used to measure the flatness of the reticle before and after chucking. The FE models and experiments provide insight into the electrostatic chucking process which will expedite the design of electrostatic chucks and the development of the SEMI standards.