KEYWORDS: Etching, System on a chip, Image processing, Sensors, Lithography, Semiconducting wafers, Distance measurement, Process control, Transistors, Signal to noise ratio
BackgroundThe self-aligned double-patterning (SADP) process is being used extensively to overcome the lithographic resolution limit in the manufacture of integrated circuits. One use case is fin definition in a fin field-effect transistor. Fin cut layers are applied to modify the fins to the requirements of the device designs.AimThe traditional secondary electron (SE) imaging exhibits a disadvantage in the process controlling the fin cut layers, and fin damage defects were observed. This work aims to improve the monitoring and controlling capabilities for the process quality of fin cut layers.ApproachA specially designed fin cut process flow and a backscattered electron (BSE) imaging technique are applied to check the process quality. The patterns formed through the fin cut etch and the fin structures are identified and measured simultaneously in one BSE image.ResultsBy measuring the edge-to-edge distance, pitch walking (PW) of fins, and overlay (OV), the root cause of the fin damage is revealed. The linear fitting model and third-order fitting model are applied to reduce the edge placement error (EPE). The edge distance protecting the “at risk” fin is enlarged from 5.6 to 11.6 nm. The range of the distance is reduced from 11.6 to 8.1 nm, and the improvement in standard deviation is about 33%.ConclusionsThis work shows the capability of the BSE imaging technique in the characterization of fin cut layers and the potential in process window improvement restricted to fin damage defects.
Background: Overlay (OV) is a very important indicator for characterizing the pattern position accuracy between two layers in an integrated circuit in the manufacturing industry. The self-aligned double-patterning (SADP) process, which is limited to the resolution of deep ultraviolet lithography, is being extensively used for the volume manufacturing of integrated circuits. However, compared to the planar process, the SADP process poses new challenges for OV control.Aim: We investigate the bottom grating asymmetry of a microdiffraction-based overlay (μDBO) target in the SADP process and its effect on OV measurement.Approach: The bottom grating of the OV metrology target is analyzed through scanning electron microscopy and transmission electron microscopy. The imbalance of the −1st- and +1st-order diffraction light intensities of the asymmetrical bottom OV target is characterized. Finally, the effects of light polarization and wavelength on the OV accuracy are simulated.Results: Asymmetric behavior of bottom OV grating is identified. OV dependency on the measurement-light polarization is determined. The simulation results indicate that wavelength and polarization affect the OV accuracy simultaneously.Conclusions: We studied the bottom grating asymmetry-induced inaccuracy in DBO measurement and can assist in understanding the mechanism of measurement light-interaction with the OV grating to enhance the DBO accuracy to a new level.
This Conference Presentation, “A holistic study of edge placement error on fin cut layer in self-aligned double patterning process,” was recorded at SPIE Photonics West held in San Francisco, California, United States.
One of the most critical challenges in the lithography process is to effectively control all critical patterns over the full exposure field, across wafer, and from lot to lot consistently. ASML’s advanced dose-control solutions have been widely adopted to control CDU of critical patterns. A new high-order dose-control capability is introduced with extended controllability over a larger number of patterns to mitigate the stochastic effect and optimize dies-in-spec performance. Traditionally, designed marks or patterns are placed in the die for dense metrology sampling required for the advanced high-order dose-control applications. However, this method has a few disadvantages especially for logic foundry use cases. For example, the designed marks are often not identical to random logic critical patterns, thus leading to a situation in which marks are controlled well while device patterns are not. In-die placement of the designed marks normally imposes constrains in device layout, which is not acceptable in some cases such as large-die layouts. A preferred approach would be to measure directly on device-critical and/or weak-point (WP) patterns. But this brings up another challenge in metrology of device WP patterns. With conventional CDSEM the amount of data points is limited by the tool throughput. WP patterns are typically 2D patterns, with normally a high noise contribution from local variations (due to resist stochastics) and metrology. Thus to suppress the local variations, averaging of many local measurements of 2D WP patterns is preferred. This requires a high throughput e-beam metrology tool capable of making massive amount of inline measurements within a given cycle time. To address these challenges, we have developed a method of using yield-limiting device patterns to directly control dose and thus improve CDU. Close to 100 WPs per in-die location have been selected with a dense die coverage to minimize the contribution to global CDU from the local variations and metrology noise. A high-speed e-beam metrology tool is used to measure all the selected WP patterns. A CDU budget breakdown (BB) has been analyzed to identify and quantify CDU contributors, such as reticle fingerprint, OPC error, local CDU, metrology noise, etc. Different in-die WP sampling and dose-control methods are studied in this work to achieve optimal CDU correction while keeping the metrology cycle time under control for HVM implementation.
As technology progress with scaling to meet the market requirements, the patterning characterization of dense features suffers a significant challenge for current optic tools, and measurement accuracy will be an important index and great challenge as well. Patterning can mostly be characterized with index of overlay (OVL) and CDU (critical dimension uniformity) measurement. When you break down the budget of the overlay error, one of the challenges is a gap of measurement results between scribe and device, where provides improper information to be used in overlay correction and causes process anomaly (excursion) detection, resulting in a low yield at the end of the production process. An eBeam tool, using high electron landing energies while utilizing the ElluminatorTM technology[1] for improvement backscattered electrons (BSE) imaging efficiency, can be utilized to directly capture OVL performance of device unit in-die, including local and global level, due to BSE function of eBeam tool[2]. In this paper, we demonstrate overlay measurement of M0 to Poly line in device for advanced logic node (only OVL X measurement), obtaining Overlay gap between in-die and scribe line to capture the actual behavior of device unit in-die. Massive OVL data is measured using eBeam tool with fast speed and high resolution, and local OVL results have been analyzed in detail. We’ve quantified what is the impact of overlay correction by different measurement ways whether it depends on optical tool or eBeam tool and benefits yield improvement.
The requirement of overlay performance, which is determined by alignment process during exposure and overlay measurement process, is getting tighter as technology node shrinks in integrated circuit. Mark design has drawn a lot of attention since appropriately designed marks can guarantee process compatibility and sufficient device performance tracking property. Cut layers are widely used in FinFet to define active area formed by SADP (Self-aligned double patterning) or SAQP (Self-aligned quadruple patterning), of which the mark design is especially challenging for diffusion break layer since it is a cut layer that landing on three dimensional fin structure and will be aligned to.
In this paper, mark design of diffusion break layer is investigated, including alignment marks and overlay marks with various substrates and segmentations. It’s recommended that the whole process from mark definition by lithography to final formation of mark after etch should be well taken into consideration during mark design, along with substrate and segmentation to avoid defect and achieve qualified signal as well.
The reduction of line width and edge roughness (LWR & LER) becomes increasingly challenging with development of integrated circuit manufacturing industry, especially with the application of multi-patterning technology. Recent years, unbiased roughness method was well received and applied in LWR & LER characterization by using power spectral density (PSD) analysis. Measurement noise in scanning electron microscope (SEM) can be identified in the high frequency region of PSD curve. By subtracting electron beam noise effect, the unbiased LWR & LER are gotten. In our research, unbiased LWR & LER under different lithography process conditions, including reflectivity of bottom anti-reflection coating (BARC) materials, photo resists (PR), illuminations, post-apply bake (PAB) and post exposure bake (PEB) temperatures, were investigated by PSD analysis. For some of the above conditions, post-develop and post-etch LWR were also studied.
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