EUV lithographers have continued to reduce the barriers to high Volume Manufacturing (HVM) introduction. Tool, mask and photoresist manufacturers have made excellent progress on several fronts, including resolution of many EUV source related issues, resists for early imaging characterization, and defect inspection tooling. In this discussion, we will focus on photoresist development. For many years, the team at SUNY Polytechnic Institute (SUNY Poly) has provided results from a neutral photoresist benchmarking study, which has been quite useful in establishing the limits of currently available photoresist systems [1-5]. New photoresist systems are being developed with improving resolution, but they also have lower coated thicknesses. In an effort to continue to point out potential lithographic problem areas, SUNY Poly has been evaluating the ‘etch compatibility’ of the best performing photoresists available in order to determine if the decreasing aspect ratios would prove a detriment to etch performance. In this paper, we will show data from our most recent benchmark study. We will also include smoothing process results, as well as some post-etch results obtained using the NXE:3300B resident on the SUNY Poly campus.
Recently there has been a great deal of effort focused on increasing EUV scanner source power; which is correlated to increased wafer throughput of production systems. Another way of increasing throughput would be to increase the photospeed of the photoresist used. However increasing the photospeed without improving the overall lithographic performance, such as local critical dimension uniformity (L-CDU) and process window, does not deliver the overall improvements required for a high volume manufacturing (HVM). This paper continues a discussion started in prior publications [Ref 3,4,6], which focused on using readily available process tooling (currently in use for 193 nm double patterning applications) and the existing EUV photoresists to increase photospeed (lower dose requirement) for line and space applications. Techniques to improve L-CDU for contact hole applications will also be described.
EUV lithography is needed by the semiconductor industry for both its resolution and for the process simplification it provides compared to multiple patterning. However it needs innovations to make it a success. One area where innovation is needed is resist performance. Resists that are commercially available for EUV use are typically based on conventional chemically amplified resist chemistry. So far, this has not provided the required performance at fast enough photo speed. Many innovative resist systems have been introduced in the last few years that have novel mechanisms and/or incorporate novel chemical elements with high EUV absorbance. These new systems are promising enough for EUV use that work on many of them now needs to shift to characterizing their functional parameters and optimizing their performance. For the future, new systems beyond these will have to focus on reducing the inherent noise in resist imaging. The concept of pixelated resists is introduced and it is suggested pixelated resists are one possible avenue for imaging sub 10nm features with sufficient feature size and profile control.
In this paper, we present the first results of witness sample based outgas resist family test to improve the efficiency of outgas testing using EUV resists that have shown proven imaging performance. The concept of resist family testing is to characterize the boundary conditions of outgassing scale from three major components for each resist family. This achievement can significantly reduce the cost and improve the resist outgas learning cycle. We also report the imaging performance and outgas test results of state of the art resists and discuss the consequence of the resist development with recent change of resist outgassing specifications. Three chemically amplified resists selected from higher outgassing materials are investigated, but no significant improvement in resist performance is observed.
This paper reports on an all-out effort to reduce the intersite gap of the resist outgassing contamination growth in the results obtained under the round-robin scheme. All test sites collaborated to determine the causes of such gaps. First, it was determined that wafer temperature during exposure could impact the amount of contamination growth. We discovered a huge intersite gap of wafer temperatures among the sites by using a wafer-shaped remote thermometer with wireless transmitting capability. Second, whether the contamination-limited regime was attained during testing could have been another primary root cause for such a difference. We found that for one of the model resists whose protecting unit had lower activation energy and molecular weight the contamination-limited regime was insufficient at one test site. Third, the ratio of the exposed area to pumping speed is necessary to equalize contamination growth. We validated the effect of matching the ratio of exposure area to pumping speed on reducing the intersite gap. This study and the protocols put in place should reduce the intersite gap dramatically.
We present an update of the AIS wavefront sensor, a diagnostic sensor set for insertion in the upgraded 0.5 NA SEMATECH Albany and Berkeley METs. AIS works by using offset monopole illumination to probe localized regions of the test optic pupil. Variations in curvature manifest as focus shifts, which are measured using a photodiode- based grating-on- grating contrast monitor, and the wavefront aberrations are reconstructed using a least-squares approach. We present results from an optical prototype of AIS demonstrating an accuracy of better than λ/30 rms for Zernike polynomials Z4 through Z10. We also discuss integration strategies and requirements as well as specifications on system alignment.
In our previous work, various techniques were used to confirm the contamination deposits on the sidewall of extreme ultraviolet (EUV) mask absorbers [1-2]. In order to further understand the effects of contamination topography on mask absorbing features, direct measurements of contaminated features is needed. In this work, we investigated the contamination topography using cross-section transmission electron microscope (TEM) image analysis on four different masks. TEM specimens of contaminated
features from silicon and ruthenium capped EUV masks were prepared using a focused ion beam (FIB). We conducted the contamination experiment with three different exposure sources including EUV, out-of-band, and electron induced processes. Thickness measurements from each contamination experiment were provided. Shadowing effect and geometric analysis on the contamination topography is also discussed.
In support of the Extreme Ultraviolet Lithography (EUVL) roadmap, a SEMATECH/CNSE joint program is underway to produce multiple EUVL (wavelength of 13.5 nm) R&D photolithography tools. The 0.5 NA projection optic magnification (5X), track length and mechanical interfaces match the currently installed 0.3 NA micro-field exposure tools (MET) projection optic [1] [2] [3]. Therefore, significant changes to the current tool platforms and other adjacent modules are not necessary. However, many of the existing systems do need upgrades to achieve the anticipated smaller exposure feature sizes [4]. To date we have made considerable progress in the production of the first of the two-mirror 0.5 NA projection optics for EUVL [5]. With a measured transmitted wave front error of less than 1 nm root mean square (RMS) over its 30 μm × 200 μm image field, lithography modeling shows that a predicted resolution of ≤12 nm and an ultimate resolution of 8 nm (with extreme dipole illumination) will be possible.
This paper will present an update from the 0.5 NA EUVL program. We will detail the more significant activities that
are being undertaken to upgrade the MET and discuss expected performance.
Extreme ultraviolet (EUV) resist outgassing is viewed as one of the factors to be considered in the research and development of EUV resists1-3. Resist outgassing in an EUV exposure tool system can lead to contaminated optics which can cause a decrease in EUV energy reaching the wafer surface, in turn leading to lower throughput. There is a program underway to measure the relative contamination rates from different resists following the ASML approved protocols for witness plate testing4. One of the important steps in this is measuring the residue on the optics after cleaning using X-ray photoelectron spectroscopy (XPS). Anything non-cleanable on the reflective optics could lead to its permanent degradation which is undesirable. Due to the number of resists being developed for EUV, there is a need for rapid testing and optimizing the XPS for throughput, precision and accuracy. In this paper, we discuss the role of XPS in quantification of species that adhere to the witness plate sample, which is a ruthenium-coated silicon wafer, as a result of the resist outgassing upon EUV exposure. XPS is a relatively slow spectroscopic technique when high accuracy in measurements is necessary, as is the requirement for our application. In this study we have attempted to optimize the various XPS parameters such as the beam power, beam spot size as well as the pass energy of the analyzer. We also studied the XPS anode degradation and the impact it has on the measurement accuracy.
We present a new form of optical testing for exposure tools based on measuring localized wavefront curvature. In this method, offset monopole illumination is used to probe localized regions of the test optic pupil. Variations in curvature manifest as focus shifts, which are measured using a photodiode-based grating-on-grating contrast monitor, and the wavefront aberrations are reconstructed using a least-squares approach. This technique is attractive as it is independent of the numerical aperture of the system and does not require a CCD or a separate interferometer branch.
Optics contamination remains one of the challenges in extreme ultraviolet (EUV) lithography. Dependence of
contamination rates on key EUV parameters was investigated. EUV tools have optics at different illumination angles. It
was observed that at shallower angles, the carbon contamination rate and surface roughness was higher on the optics
surface. This is a concern in EUV optics as higher roughness would increase the scattering of the EUV radiation.
Secondary ion time of flight mass spectrometer (TOF-SIMS) data indicated that the carbon contamination film might be
a polymer. Three chemical species were used to investigate the dependence of polymerization and reactivity on the
contamination rate. Acrylic acid was found to have a measurable contamination rate above background compared to
propionic acid and methyl methacrylate. Secondary electron dissociation is one of the mechanisms considered to be a
cause for the growth of the carbon contamination film. Multiple experiments with two substrates having different
secondary electron yields were performed. The substrate with the higher secondary electron yield was found to give a
higher contamination rate.
Optics contamination remains one of the challenges in extreme ultraviolet (EUV) lithography. In addition to the
desired wavelength near 13.5 nm (EUV), plasma sources used in EUV exposure tools emit a wide range of
out-of-band (OOB) wavelengths extending as far as the visible region. We present experimental results of
contamination rates of EUV and OOB light using a Xe plasma source and filters. Employing heated carbon
tape as a source of hydrocarbons, we have measured the wavelength dependence of carbon contamination
on a Ru-capped mirror. These results are compared to contamination rates on TiO2 and ZrO2 capping layers.
The impact of carbon contamination on extreme ultraviolet (EUV) masks is significant due to throughput loss and
potential effects on imaging performance. Current carbon contamination research primarily focuses on the lifetime of the
multilayer surfaces, determined by reflectivity loss and reduced throughput in EUV exposure tools. However,
contamination on patterned EUV masks can cause additional effects on absorbing features and the printed images, as
well as impacting the efficiency of cleaning process. In this work, several different techniques were used to determine
possible contamination topography. Lithographic simulations were also performed and the results compared with the
experimental data.
Carbon contamination of extreme ultraviolet (EUV) masks and its effect on imaging is a significant issue due to lowered
throughput and potential effects on imaging performance. In this work, a series of carbon contamination experiments
were performed on a patterned EUV mask. Contaminated features were then inspected with a reticle scanning electron
microscope (SEM) and printed with the SEMATECH Berkeley Microfield-Exposure tool (MET) [1]. In addition, the
mask was analyzed using the SEMATECH Berkeley Actinic-Inspection tool (AIT) [2] to determine the effect of carbon
contamination on the absorbing features and printing performance.
To understand the contamination topography, simulations were performed based on calculated aerial images and resist
parameters. With the knowledge of the topography, simulations were then used to predict the effect of other thicknesses
of the contamination layer, as well as the imaging performance on printed features.
Typical extreme ultraviolet (EUV) photoresist is known to outgas carbon-containing molecules, which is of particular
concern to the industry as these molecules tend to contaminate optics and diminish reflectivity. This prompted extensive
work to measure these species and the quantities that they outgas in a vacuum environment. Experiments were
performed to test whether the outgassing rate of these carbon-containing molecules is directly proportional to the rate at
which the EUV photons arrive and whether a very high power exposure will cause the same amount of outgassing as a
much lower power exposure with the dose unchanged.
Extreme ultraviolet (EUV) photoresists are known to outgas during exposure to EUV radiation in the vacuum
environment. This is of particular concern since some of the outgassed species may contaminate the nearby EUV optics
and cause a loss of reflectivity and therefore throughput of the EUV exposure tools. Due to this issue, work has been
performed to measure the species and quantities that outgas from EUV resists. Additionally, since the goal of these
measurements is to determine the relative safety of various resists near EUV optics, work has been performed to measure
the deposition rate of the outgassed molecules on Mo/Si-coated witness plate samples. The results for various species
and tests show little measurable effect from resist components on optics contamination with modest EUV exposure
doses.
The Mo/Si multilayer mirrors used for extreme ultraviolet (EUV) lithography can become contaminated during exposure
in the presence of some hydrocarbons [1-3]. Because this leads to a loss in the reflectivity of the optics and throughput
of the exposure tools, it needs to be avoided. Since photoresists are known to outgas during exposure to EUV radiation
in a vacuum environment, the careful choice of materials is important to preserving the EUV optics. Work therefore has
been performed to measure the species and quantities of molecules that outgas from EUV resists when exposed to EUV
radiation [4-7].
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