Yulia O. Korobko

Effects of soft pellicle frame curvature and mounting process on pellicle-induced distortions in advanced photomasks

Lithography registration errors induced by the attachment of soft pellicles on reticles can significantly affect wafer overlay performance for sub-90 nm lithography chip manufacturing. Intel Corporation, Mitsui Chemicals, and the University of Wisconsin Computational Mechanics Center (UW-CMC) have conducted an extensive experimental study to quantify and minimize the pellicle-induced distortions in order to meet advanced mask manufacturing requirements. A comprehensive design of experiment was elaborated to evaluate the effects of frame curvature, adhesive gasket compliance, and mounting load on pellicle-induced distortions for soft pellicle systems. A frame curvature measurement tool was custom-made at the UW-CMC, employing an MTI Instruments capacitive sensor. A TA Instruments dynamic mechanical analyzer was used to determine the elastic modulus of the adhesive gasket materials. Registration measurements were conducted by Intel on test reticles on a 21 × 21 array of grid points, before and after pellicle attachment, to obtain pellicle-induced distortion results. Results characterize the influence of attachment process, type of adhesive gasket, frame curvature, reticle guiding plate configuration, and attachment load on pellicle-induced distortions.

A SEM-based System for Photomask Placement Metrology

Mask metrology has long been separated into critical dimension (CD) vs. pattern placement (Registration) in terms of both the parametric definitions as well as measurement techniques applied. The combined effect of measured CD and placement errors on mask-to-mask overlay (OL) is hard to model let alone calculate in definitive terms. As device size continues to shrink, novel lithography solutions being considered for 45nm technology node and beyond such as double exposure and patterning techniques are projected to tighten the overlay requirement much faster than originally anticipated. Electron optics is generally the preferred solution for small feature size in-die sampling by virtue of its high image resolution, measurement precision, low cross-field distortion and absence of tool induced shift. In this paper we propose to examine and identify the key elements of a new approach in applying electron optics to a mask metrology system that combines CD and pattern placement. We will then present the results from our experiments with a prototype wide field scanning electron microscope (WFSEM) using reticle with optical proximity correction (OPC) features.

Second level exposure for phase shift mask applications using an SLM-based DUV mask writer

Phase shift mask (PSM) applications are becoming essential for addressing the lithography requirements of the 65 nm technology node and beyond. Many mask writer properties must be under control to expose the second level of advanced PSM: second level alignment system accuracy, resolution, pattern fidelity, critical dimension (CD) uniformity and registration. Optical mask writers have the advantage of process simplicity for this application, as they do not require a discharge layer. This paper discusses how the mask writer properties affect the error budget for printing the second level. A deep ultraviolet (DUV) mask writer with a spatial light modulator (SLM) is used in the experimental part of the paper. Partially coherent imaging optics at the 248 nm wavelength provide improved resolution over previous systems, and pattern fidelity is optimized by a real-time corner enhancement function. Lithographic performance is compared to the requirements for second level exposure of advanced PSM. The results indicate sufficient capability and stability for 2nd level alternating PSM patterning at the 65 nm and 45 nm nodes.

Calibration of the registration metrology systems

Tighter lithography requirements are increasing the challenges for mask registration metrology. Demands on calibration will increase as tool specific calibration need to significantly improve to enable accurate plate quality assessment and adequate matching between multiple writer and metrology system. We present results of calibration study conducted on Leica LMS IPRO 2 and LMS IPRO1. Two different calibration techniques were used to match the tool grid to absolute Cartesian coordinate system. The impact of the two calibration techniques on tool matching is summarized. The results are used to make recommendations on improving calibration methodology.

Resolution limits with 248-nm strong phase-shifting techniques for contact patterning applications

As conventional lithography capability is reaching the limit, resolution enhancement becomes a crucial element to extend the lithography capability. The main purpose of this paper is to examine the capabilities and limitations of various phase shifting techniques. Using contact applications as an example, alternating and phase edge phase shifter were evaluated with single and double exposure techniques. Detailed discussion will include both simulation and experimental results. The PSM performances were predicted with Intel in-house simulator. Analysis was done based on aerial image formation using peak intensity and Modulation Transfer Function to evaluate resolution capabilities. For each type of phase shifter, good resolution enhancement was achieved with optimized pitch range of dense structures. Experimental results of focus-exposure matrices were taken on a DUV stepper with NA of 0.42. Significant resolution enhancement was demonstrated with k1 as low as 0.32 which is far below the limitation a k1 equals 0.5. Small contacts with tight pitch, i.e. 180 nm contact with 380 nm pitch, were demonstrated with reasonable depth of focus by using double exposure technique. By using single exposure alternating phase shifting technique pitches down to 460 nm is steadily resolved. With conventional mask, 350 nm contacts with 700 nm pitch is reported as the minimum printable range for the same stepper.

Technological challenges in implementation of alternating phase-shift mask

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.

Optical DC overlay measurement in the 2nd level process of 65 nm Alternating Phase Shift Mask

Alternating phase shift mask (APSM) techniques help bridge the significant gap between the lithography wavelength and the patterning of minimum features, specifically, the poly line of 35 nm gate length (1x) in Intels 65 nm technology. One of key steps in making APSM mask is to pattern to within the design tolerances the 2nd level resist so that the zero-phase apertures will be protected by the resist and the pi-phase apertures will be wide open for quartz etch. The ability to align the 2nd level to the 1st level binary pattern, i.e. the 2nd level overlay capability is very important, so is the capability of measuring the overlay accurately. Poor overlay could cause so-called the encroachment after quartz etch, producing undesired quartz bumps in the pi-apertures or quartz pits in the zero-apertures. In this paper, a simple, low-cost optical setup for the 2nd level DC (develop check) overlay measurements in the high volume manufacturing (HVM) of APSM masks is presented. By removing systematic errors in overlay associated with TIS and MIS (tool-induced shift and Mask-process induced shift), it is shown that this setup is capable of supporting the measurement of DC overlay with a tolerance as small as +/- 25 nm. The outstanding issues, such as DC overlay error component analysis, DC - FC (final check) overlay correlation and the overlay linearity (periphery vs. indie), are discussed.

Adaptive electrorheological layer for precision control of thin-wall constructions

The data of previous researchers on the structure formation of electrosensitive suspensions under the action of an external electric and on the resulting changes in the mechanical parameters of the medium (viscosity, plasticity, resilience) were used to develop a method of damping in a special cluthes, which were used for fixing thin-wall constraction, that surfaces are to undergo fine turning. As demonstrated, the use of a fluid, the structure of which responds to an electric field, as a cluthing layer improves the technological characteristics of the product and increase its reliability.

Electrorheological vibration system

The present paper is devoted to de3velopment and testing of an active vibration system. The system is intended for providing efficient motion of a piston in a hydraulic channel for creation of shocks and periodic vibrations in a low frequency range by means of the ER-valves based on an electrosensitive working me dium, i.e. electrorheological fluids. The latter manifests the electrorheological (ER) effect, i.e. a reversible change in the rheological characteristics of weak-conducting disperse compositions in the presence of constant and alternating electric fields. As a result of the experimental study of the dependence of viscoelastic properties of the ER-fluid on the magnitude and type of an electric field, the optimum dimensions of the vibrator and the its valves characteristics of the optimal electrical signal are determined. For control of an ER- vibrator having several valves we have designed a special type of a high-voltage two-channel impulse generator. Experiments were conducted at the frequencies ranged from 1- 10 Hz. It has been shown, that a peak force made 70% of the static force exercised by the vibrator rod. A phase shift between the input voltage and the load acceleration was less than 45 degree(s)C which allowed servocontrol and use of the vibrator for attendant operations. It was noted that a response of the vibrator to a stepwise signal has a delay only of several milliseconds.

Thermophysical properties and rheological behavior of electro-rheological fluids at different temperatures

Fluid disperse systems, sensitive to the external electric field-electrorheological fluids, are finding increasing use in various areas of industry and technology. Their physicomechanical, electrophysical characteristics determine the valuable specific properties of the materials with assigned structure, obtainable with everwide use of electric fields, which makes it possible to substantially enhance efficiency and productiveness of technological processes and to improve the control of operational regimes of the equipment which employ fluid disperse media. The present investigations has been undertaken with the aim of studying thermophysical properties and rheophysical behavior of low-concentration ER- fluid (diatomite in transformer oil) at different temperatures. It was shown that the electric field, which changes considerably the structure of electrorheological fluid, influences effective thermal conductivity and diffusivity coefficients. Their increase with electric field intensity and the increase of the effective viscosity with temperature are connected with the increase of the conductive component of the overall heat transfer through the contact spots between the solid particles, and with intensification of electric convection in the spaces between the dispersed particles.