Roie Volkovich

Spectral tunability for accuracy, robustness, and resilience

In overlay (OVL) metrology the quality of measurements and the resulting reported values depend heavily on the measurement setup used. For example, in scatterometry OVL (SCOL) metrology a specific target may be measured with multiple illumination setups, including several apodization options, two possible laser polarizations, and multiple possible laser wavelengths. Not all possible setups are suitable for the metrology method as different setups can yield significantly different performance in terms of the accuracy and robustness of the reported OVL values. Finding an optimal measurement setup requires great flexibility in measurement, to allow for high-resolution landscape mapping (mapping the dependence of OVL, other metrics, and details of pupil images on measurement setup). This can then be followed by a method for analyzing the landscape and selecting an accurate and robust measurement setup. The selection of an optimal measurement setup is complicated by the sensitivity of metrology to variations in the fabrication process (process variations) such as variations in layer thickness or in the properties of target symmetry. The metrology landscape changes with process variations and maintaining optimal performance might require continuous adjustments of the measurement setup. Here we present a method for the selection and adjustment of an optimal measurement setup. First, the landscape is measured and analyzed to calculate theory-based accurate OVL values as well as quality metrics which depend on details of the pupil image. These OVL values and metrics are then used as an internal ruler (“self-reference”), effectively eliminating the need for an external reference such as CD-SEM. Finally, an optimal measurement setup is selected by choosing a setup which yields the same OVL values as the self-reference and is also robust to small changes in the landscape. We present measurements which show how a SCOL landscape changes within wafer, wafer to wafer, and lot to lot with intentionally designed process variations between. In this case the process variations cause large shifts in the SCOL landscape and it is not possible to find a common optimal measurement setup for all wafers. To deal with such process variations we adjust the measurement setup as needed. Initially an optimal setup is chosen based on the first wafer. For subsequent wafers the process stability is continuously monitored. Once large process variations are detected the landscape information is used for selecting a new measurement setup, thereby maintaining optimal accuracy and robustness. Methods described in this work are enabled by the ATL (Accurate Tunable Laser) scatterometry-based overlay metrology system.

Overlay Accuracy Investigation for advanced memory device

Overlay in lithography becomes much more challenging due to the shrink of device node and multi-patterning approach. Consequently, the specification of overlay becomes tighter, and more complicated overlay control methods like high order or field-by-field control become mandatory. In addition, the tight overlay specification starts to raise another fundamental question: accuracy. Overlay inaccuracy is dominated by two main components: one is measurement quality and the other is representing device overlay. The latter is because overlay is being measured on overlay targets, not on the real device structures. We investigated the following for accurate overlay measurement: optimal target design by simulation; optimal recipe selection using the index of measurement quality; and, the correlation with device pattern’s overlay. Simulation was done for an advanced memory stack for optimal overlay target design which provides robustness for the process variation and sufficient signal for the stack. Robustness factor and sufficient signal factor sometimes contradicting each other, therefore there is trade-off between these two factors. Simulation helped to find the design to meet the requirement of both factors. The investigation involves also recipe optimization which decides the measurement conditions like wavelength. KLA-Tencor also introduced a new index which help to find an accurate measurement condition. In this investigation, we used CD-SEM to measure the overlay of device pattern after etch or decap process to check the correlation between the overlay of overlay mark and the overlay of device pattern.

Overlay measurement accuracy enhancement by design and algorithm

Advanced design nodes require more complex lithography techniques, such as double patterning, as well as advanced materials like hard masks. This poses new challenge for overlay metrology and process control. In this publication several step are taken to face these challenges. Accurate overlay metrology solutions are demonstrated for advanced memory devices.

Lithography focus/exposure control and corrections to improve CDU

As leading edge lithography moves to advanced nodes which requires better critical dimension (CD) control ability within wafer. Current methods generally make exposure corrections by field via factory automation or by sub-recipe to improve CD uniformity. KLA-Tencor has developed a method to provide CD uniformity (CDU) control using a generated Focus/Exposure (F/E) model from a representative process. Exposure corrections by each field can be applied back to the scanner so as to improve CD uniformity through the factory automation. CDU improvement can be observed either at after lithography or after etch metrology steps. In addition to corrections, the graphic K-T Analyzer interface also facilitates the focus/exposure monitoring at the extreme wafer edge. This paper will explain the KT CDFE method and the application in production environment. Run to run focus/exposure monitoring will be carried out both on monitoring and production wafers to control the wafer process and/or scanner fleet. CDU improvement opportunities will be considered as well.

In-line focus monitoring and fast determination of best focus using scatterometry

Persistently shrinking design rules and increasing process complexity require tight control and monitoring of the exposure tool parameters [1, 2]. While control of exposure dose by means of resist single metric measurements is common and widely adopted. Focus assessment and monitoring are usually more difficult to achieve. A diffused method to determine process specific dose and focus conditions is based on plotting Bossung curves from single CD-SEM measurements and choosing the best focus setting to obtain the desired target CD with the widest useful window. With this approach there is no opportunity to build a data flow architecture that can enable continuous focus monitoring on nominal production wafers [3-5]. KLA-Tencor has developed a method to enable in-line monitoring of scanner focus on production wafers by measuring resist profile shapes on grating targets using scatterometry, and analyzing the information using AcuShapeTM and K-T AnalyzerTM software. This methodology is based on a fast and robust determination of best scanner focus by analyzing focus-exposure matrices (FEMs). This paper will demonstrate the KT CDFE and FEM Analysis methods and their application in production environment.

Photoresist qualification using scatterometry CD

As the semiconductor industry advances to smaller design rules, Photoresist performance is critical for the tight lithography process. Critical Dimension (CD), Side Wall Angle (SWA) and Photoresist height, which are critical for the final semiconductor patterning, depend on the Photoresist chemistry. Each Photoresist batch has to be qualified to verify that it can achieve the required quality specifications. Photoresist qualification is done by exposing Photoresist and monitoring outcome after developing. In this work, Archer 300LCM scatterometry-based Optical CD (OCD) was evaluated using Dow 193 Immersion Top Coat Free Photoresist and Anti Reflection Layers (ARL). As part of the sensitivity analysis, changes in Photoresist thickness, ARL thickness and Photoresist formulation were evaluated. Results were compared to CD-SEM measurements. The CD sensitivity was evaluated on two grating dense line and space features with nominal Middle CD (MCD) values of 37nm and 75nm. Sensitivity of the OCD for Photoresist parameters was demonstrated.