The use of a single figure of merit to judge resist performance
with respect to resolution, linewidth roughness LWR, and sensitivity is
proposed and evaluated. Chemically amplified photoresists used in advanced
lithography nodes need to fulfill stringent requirements for a considerable
number of resist and process characteristics. Along with resolution,
linewidth roughness and resist sensitivity are important examples
where the specifications have become very tight. Previously, it has been
shown that resolution, linewidth roughness, and resist sensitivity are fundamentally
interdependent. Hence, when evaluating or optimizing resist
performance, it is very important to take these three characteristics into
consideration simultaneously. We propose to combine these characteristics
into a single photoresist figure of merit KLUP. This figure of merit,
which is determined from sizing dose, imaging wavelength, resist thickness,
exposure latitude, acid diffusion length, linewidth roughness, and
pitch, allows for a direct comparison of very different resist formulations
independent of the exposure tool used. Thus, KLUP has great potential to
assist in evaluating resist performance for the next lithography nodes, for
both ArF and for EUV wavelengths.
In this paper, the use of a single Figure-of-Merit to judge resist performance with respect to line width roughness, resolution and sizing dose is proposed and evaluated. Chemically amplified photoresists used in advanced lithography nodes need to fulfill stringent requirements for a considerable number of resist and process characteristics. Along with resolution, line width roughness and resist sensitivity are important examples where the specifications have become very tight. Previously, it has been shown that resolution, line width roughness and resist sensitivity are fundamentally interdependent. Hence, when evaluating or optimizing resist performance it is very important to take these three characteristics into consideration simultaneously. We propose to combine these characteristics in a single photoresist Figure of Merit KLUP. This Figure of Merit, which is determined from sizing dose, imaging wavelength, exposure latitude, acid diffusion length, line width roughness and pitch allows for a direct comparison of very different resist formulations independent of the exposure tool used. Thus, KLUP has great potential to assist in evaluating resist performance for the next lithography nodes, for both ArF and for EUV wavelengths.
Since their introduction in the semiconductor industry, chemically amplified resists have proven to offer very valuable benefits to lithography processes, of which improved resist contrast and higher throughput are just two examples. However, the inherent acid diffusion mechanism starts to create some issues. For instance, the reduced chemical contrast due to pronounced acid diffusion during the post-exposure bake will decrease the exposure latitudes and would impact the ultimate resolution. On the other hand, reducing the acid diffusion length will have a negative impact on line edge roughness if one wants to simultaneously keep exposure doses and shot noise effects under control. In this paper, acid diffusion lengths in present-day photoresists at different process conditions are characterized using a lithographic technique. The observed tendencies are correlated with trends in exposure latitude, resolution and the frequency spectrum of line edge roughness. The relationship between acid diffusion length and exposure latitude as well as the relationship between acid diffusion and line edge roughness are addressed in a more fundamental way. The results of this paper highlight the major impact of acid diffusion on important lithographic process characteristics, and the investigated scaling behavior gives guidelines for optimizing exposure latitude and line edge roughness for future technology nodes.
The new high NA (0.3) Micro Exposure Tool at the Advanced Light Source (MET@ALS) at Lawrence Berkeley National Laboratories provides the first opportunity to evaluate the ultimate resolution capabilities of chemically amplified resists using EUV lithography. We characterized the imaging capabilities of a well-known tool-test resist (EUV-2D, XP98248B) and a new high resolution resist (MET-1K, XP3454C). Emphasis was placed on evaluating resists for focus and exposure latitude at 50 nm dense and isolated lines. MET-1K is capable of resolving 30 nm lines and shows modulation in 25 nm dense lines. We describe some early process optimization experiments using MET-1K that show further advances in lithographic capability. Another new series of resists (MET-2A, 2B, 2C, 2D) also show great promise for good resolution, LER and sensitivity.
We previously [Van Steenwinckel and Lammers, Proc. SPIE 5039, 225-239 (2003)] showed that resist effects induced by "overbaking" (enhanced processing) can lead to major improvements in depth of focus and small-line printing capability through improved acid dose contrasts and a balanced optimization of acid diffusion in the presence of quencher. We show how a similar optimization can be attained by changes in resist formulation and how these changes can have a comparable positive impact on ultimate resolution and depth of focus. The formulation changes that are discussed follow the guidelines of the compact resist model that accounts for the overbake performance. The impact of the proposed resist reformulation on line-end shortening, line-edge roughness, and post exposure bake (PEB) sensitivity is discussed as well. The outcome of this work can be used to define which changes to the resist formulation offer the most beneficial improvements in the overall lithography process for printing resist features of 65 nm and smaller using ArF lithography and binary masks.
The shot noise, line edge roughness (LER) and quantum efficiency of EUV interaction with seven resists related to EUV-2D (SP98248B) are studied. These resists were identical to EUV-2D except were prepared with seven levels of added base while keeping all other resist variables constant. These seven resists were patterned with EUV lithography, and LER was measured on 100-200 nm dense lines. Similarly, the resists were also imaged using DUV lithography and LER was determined for 300-500 nm dense lines. LER results for both wavelengths were plotted against Esize. Both curves show very similar LER behavior-the resists requiring low doses have poor LER, whereas the resists requiring high doses have good LER. One possible explanation for the observed LER response is that the added base improves LER by reacting with the photogenerated acid to control the lateral spread of acid, leading to better chemical contrast at the line edge.
An alternative explanation to the observed relationship between LER and Esize is that shot-noise generated LER decreases as the number of photons absorbed at the line edge increases. We present an analytical model for the influence of shot noise based on Poisson statistics that preidicts that the LER is proportional to (Esize)-1/2. Indeed, both sets of data give straight lines when plotted this way (DUV r2 = 0.94; EUV r2 = 0.97). We decided to further evaluate this interpretation by constructing a simulation model for shot noise resulting from exposure and acid diffusion at the mask edge. In order to acquire the data for this model, we used the base titration method developed by Szmanda et al. to determine C-parameters and hence the quantum efficiency for producing photogenerated acid. This information, together with film absorptivity, allows the calculation of number and location of acid molecules generated at the mask edgte by assuming a stochastic distribution of individual photons corresponding to the aerial image function. The edge "roughness" of the acid molecule distribution in the film at the mask edge is then simulated as a function of acid diffusion length and compared to the experimental data. In addition, comparisoins between of the number of acid molecules generated and photons consumed leads to values of quantum efficiencies for these EUV resists.
This paper describes different simplified simulation models which characterize the behavior of the photoresist during lithography processes. The effectiveness of these models is compared with the results of more physics and chemistry containing simulators. The strengths and weaknesses of the simplified models are demonstrated for practical applications. Simplified resist model parameters are calibrated for 193nm chemically amplified resists (CAR). The results are compared with calibration of full simulation models. The validity of the simulation models under different process conditions is investigated.
With the ever-increasing demand for reducing the size of devices on a chip, one frequently meets the lithographic challenge of printing very small lines on a wafer. The rapidly shrinking process latitudes, especially the depth-of-focus (DOF), really become a burden when trying to print sub-80nm resist lines on a wafer with 193nm lithography. In this paper, we report on a method that is capable of enlarging the process windows for printing small lines, while also reducing a line collapse issue, by using an enhanced resist processing procedure. In this procedure, the PEB time duration, in combination with the exposure dose, is used as a tuning process parameter. It is shown that, by using this procedure, a significant increase in DOF is obtained for printing small (down to 40nm) isolated and semi-dense lines, while the pitch is not scaled down. While using a binary mask and annular 193nm illumination with a NA of 0.63 and s of 0.87/0.57, we show that it is possible to print 50nm lines on a 240nm pitch with 0.8 micron DOF, whereas the standard process, using the vendor recommended PEB, can only print 80nm lines with a comparable DOF. The large process enhancement, among which the reduced curvature in the Bossung plots, is explained by the more efficient use of the acid and quencher present in the photoresist, as well as by the peculiarities of acid diffusion near regions where quencher is remaining. The results obtained are explained by a compact resist model in which acid-quencher reaction-diffusion is incorporated.