Proc. SPIE. 10149, Advanced Etch Technology for Nanopatterning VI
KEYWORDS: Oxides, Optical lithography, Etching, Image processing, Interfaces, Ions, 3D modeling, Monte Carlo methods, Plasma etching, Chemical elements, Semiconducting wafers, Bromine, Process modeling, Plasma
Increasingly, advanced process nodes such as 7nm (N7) are fundamentally 3D and require stringent control of critical dimensions over high aspect ratio features. Process integration in these nodes requires a deep understanding of complex physical mechanisms to control critical dimensions from lithography through final etch. Polysilicon gate etch processes are critical steps in several device architectures for advanced nodes that rely on self-aligned patterning approaches to gate definition. These processes are required to meet several key metrics: (a) vertical etch profiles over high aspect ratios; (b) clean gate sidewalls free of etch process residue; (c) minimal erosion of liner oxide films protecting key architectural elements such as fins; and (e) residue free corners at gate interfaces with critical device elements. In this study, we explore how hybrid modeling approaches can be used to model a multi-step finFET polysilicon gate etch process. Initial parts of the patterning process through hardmask assembly are modeled using process emulation. Important aspects of gate definition are then modeled using a particle Monte Carlo (PMC) feature scale model that incorporates surface chemical reactions.<sup>1</sup> When necessary, species and energy flux inputs to the PMC model are derived from simulations of the etch chamber. The modeled polysilicon gate etch process consists of several steps including a hard mask breakthrough step (BT), main feature etch steps (ME), and over-etch steps (OE) that control gate profiles at the gate fin interface. An additional constraint on this etch flow is that fin spacer oxides are left intact after final profile tuning steps. A natural optimization required from these processes is to maximize vertical gate profiles while minimizing erosion of fin spacer films.<sup>2</sup>
A theoretical model for describing the propagation and scattering of energetic species in an extreme ultraviolet (EUV) light lithography source is presented. An EUV light emitting XTREME XTS 13-35 Z-pinch plasma source is modeled with a focus on the effect of chamber pressure and buffer gas mass on energetic ion and neutral debris transport. The interactions of the energetic debris species, which is generated by the EUV light emitting plasma, with the buffer gas and chamber walls are considered as scattering events in the model, and the trajectories of the individual atomic species involved are traced using a Monte Carlo algorithm. This study aims to establish the means by which debris is transported to the intermediate focus with the intent to verify the various mitigation techniques currently employed to increase EUV lithography efficiency. The modeling is compared with an experimental investigation.
In extreme ultraviolet (EUV) lithography, plasmas are used to generate EUV light. Unfortunately, these plasmas expel high-energy ions and neutrals which damage the collector optic used to collect and focus the EUV light. One of the main problems facing EUV source manufacturers is the necessity to mitigate this debris. A magnetic mitigation system to deflect ionic debris by use of a strong permanent magnet is proposed and investigated. A detailed computational model of magnetic mitigation is presented, and experimental results from an EUV source confirm both the correctness of the model and the viability of magnetic mitigation as a successful means of deflecting ionic debris.
The presence of Sn on the collector optic of an extreme ultraviolet (EUV) light lithography tool continues to be a
concern for source manufacturers. A mere nanometers deposition results in reduction of EUV light reflectivity to
unacceptable levels. It has been shown previously that hydrogen radical etching of Sn provides a promising technique for
in-situ cleaning of the collector optic. One concern in this technique is the redeposition by radicalized SnH4 breaking
apart after making contact with a surface. To address this concern, large scale etching measurements were made using a
metallic antenna as the substrate. Optimized etch rates approaching 7.5±1 nm/min have been achieved with a flow rate
of 500 sccm at a pressure of 80 mTorr. The effect of variations in the Sn cleaning environment will be investigated with
respect to temperature increases as well as air, oxygen, and methane contamination gasses. Furthermore, the effect of Sn
located away from the cleaning location will also be presented.
Extreme Ultraviolet (EUV) lithography sources expel high-energy ions and neutral particles, which degrade the quality of the collector optic. The mitigation of this debris is one of the main problems facing potential manufacturers of EUV sources. The use of magnetic fields to deflect ionic debris has been proposed and is investigated here. In this paper, we present a detailed computational model of magnetic mitigation, along with experimental results that confirm the correctness of the model. Using a strong permanent magnet, it is experimentally shown that, using high enough fields, magnetic mitigation can be a successful method of deflecting ionic debris from an EUV source. For example, through an orifice centered at 0° from the pinch, we saw a flux of 1.65×10<sup>8</sup> +/- 1.5×10<sup>7</sup> ions/(m<sup>2</sup>*pulse*eV) of 4keV ions without deflection and a negligible flux with deflection.
With the orifice at a 35° angle from the pinch, a negligible 4keV flux was seen without deflection. However, with
magnetic deflection, a 4keV flux of 1.03x10<sup>8</sup> +/- 9.4x10<sup>6</sup> ions/(m<sup>2</sup>*pulse*eV) were seen. The half-angle spread of the orifice was .047° with a tolerance of .008°.
The emission of species that can chemically or physically alter the surface of post intermediate-focus optics will increase the cost of ownership of such an EUV lithography tool past the point of cost effectiveness. To address this
concern, the Center for Plasma-Material Interactions has developed the Sn Intermediate Focus Flux Emission Detector (SNIFFED). The effects of increasing buffer gas, increasing pressure, and chosen buffer gas species will be presented. Furthermore the presence of a secondary plasma, generated by EUV light will be analyzed and exposed as a potential issue in the strive for a contaminant free intermediate focus.
As extreme ultraviolet light lithography matures, critical deficits in the technology are being resolved. Research has largely focused on solving the debris issue caused by using warm (Te ∼ 30 eV) and dense (ne ∼ 1020 cm−3) plasma to create 13.5-nm light. This research has been largely focused on the mitigation of the debris between the plasma and the collector optics. The next step of debris mitigation is investigated, namely the effect of debris mitigation on the transport of undesired contaminants to the intermediate focus (IF). In order to investigate emissions from the IF, the Center for Plasma-Material Interactions at the University of Illinois at Urbana-Champaign has developed the Sn intermediate focus flux emission detector. The effects of a secondary RF-plasma, buffer gas flow rate, chamber pressure, and charged plate deflection are investigated. By increasing the chamber pressure to 10 mTorr, flowing 1000 sccm Ar buffer gas, and utilizing charged particle deflection, it is possible to reduce the measured number of post-IF species by greater than 99%. Furthermore, it is shown that typical debris mitigation techniques lead to the development of a plasma near the IF that can be detrimental to post-IF optics.
One of the major technical hurdles to be overcome before EUV lithography can enter high volume manufacturing is the
amount of defects in EUV mask blanks, many of which occur during the EUV reflector deposition process. The
technology currently used to deposit this reflector is ion beam sputter deposition. Understanding the properties of the
ion beam and the nature of the plasma in the deposition chamber is therefore critical to understanding defect production
mechanisms and subsequently eliminating them.
In this work, we have studied how the source parameters influence ion beam divergence, its footprint on the target, and
the amount of beam that misses the target and hits the shielding. By optimizing the source parameters, we can modulate
certain target- and shield-specific defect types. We have compared our data with models of source performance and
found general agreement, enabling the theory to be fine-tuned based on the results of the measurements. Models are
being developed to better describe actual source performance. We have also investigated the plasma conditions the ion
beam creates in the tool, which is crucial to understanding the transport of defects from their source to the mask. A well
characterized ion beam and plasma will lead to process and tool changes that will ultimately reduce defect levels in EUV
One of the main challenges in extreme ultraviolet lithography (EUVL) is the development of a method for cleaning
collector optics without inhibiting cost-effectiveness. Cost-effectiveness of EUV methods can be increased by in-situ
processes for removing debris placed on the collector optic. This paper focuses on the use of a hydrogen plasma to
remove Sn, a common EUV fuel, from Si surfaces. Sn was deposited on both large and small Si samples via magnetron
sputtering, and optimized hydrogen plasma selectively etched the Sn. Deposition uniformity and thickness are
measured, as are Sn etch rates and cleaning uniformity. Positive results indicate the potential of this method for use in
cleaning EUV mirrors.
The Center for Plasma-Material Interactions has developed a detector capable of diagnosing the energetic ion
and neutral spectrums emanating from extreme ultraviolet light sources. This tool has been used in the past for
high-power output sources, but it is readily evident that actinic inspections tools require the use of debris
mitigation analyzers. Using this tool, manufacturers can optimize the use of debris mitigation techniques, as
well as analyze the effects brightness increases have on tool lifetime.
The development of a successful extreme ultraviolet light source for lithography relies on the ability to
maintain collector optic cleanliness. Cleanliness is required to maintain the reflectivity of the collector optic, thus
maintaining the light power output at the intermediate focus. In this paper, an in-situ method is explored to remove
Sn from a contaminated collector optic. Hydrogen plasma is used to promote Sn etching while maintaining the
integrity of the collector optic's multi-layer structure. The removal rate of Sn is investigated as a function of various
operational parameters including chamber pressure, plasma electron density, as well as plasma electron temperature.
Initial results are presented using an external RF-plasma source. The use of the collector optic as a RF-antenna is
also investigated to optimize the etching rate of the hydrogen plasma. Initial plasma parameter measurements reveal
electron densities on the order of 10<sup>11</sup>-10<sup>12</sup> cm<sup>-3</sup>, with electron temperatures on the order of 1-3 eV. An optimized
etch rate of ~125 nm/min off of Si was observed using 1000 W, 80 mTorr, and a flow rate of 50 sccm of H2. These
initial measurements are used as a basis for optimizing the etching rate off of the collector optic. Such results are
important in allowing the long-term usage of a single collector optic to minimize operating costs involved with
replacing the optic as well as tool downtime.
For extreme ultraviolet light lithography to be a viable process for the future development of computer chips, it is
necessary that clean photons are produced at the intermediate focus (IF). To measure the flux at the IF, the Center
for Plasma-Material Interactiosn (CPMI) at the University of Illinois at Urbana-Champaign has developed a Sn IF
flux emission detector (SNIFFED) apparatus that is capable of measuring charged and neutral particle flux at the IF.
Results will be presented that diagnose debris produced at the IF, as well as methods by which this debris can be
Advanced Materials Research Center, AMRC, International SEMATECH Manufacturing Initiative, and ISMI are
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At CPMI, we built a prototype portable, modified electrostatic spherical sector analyzer (ESA) device incorporating a neutral detector; investigated its capabilities for measuring energetic neutrals; and report results in this paper. This detector at the IF will contain a quartz crystal microbalance (QCM), Si witness plate for ex situ analysis, a set of microchannel plates (MCPs) with corresponding ion-diverting apparatus, Faraday cup as well as triple Langmuir probe. These detectors will be capable of quantifying total particle flux, neutral particle flux, and charged particle flux. To verify the capabilities of the detector, CPMI constructed a mock collector optic, which was placed inside the experimental chamber attached to CPMI's XTS 13-35 EUV source. This mock-up simulates the reflection of debris created by discharge-produced plasma (DPP), although it will not be capable of reflecting the EUV light. Recent results on the neutral, charged particle flux, and the carbon and oxygen contamination on a Si witness plate out of the line of sight of the Z-pinch are reported in this paper.
This paper describes the research done at center for plasma material interactions (CPMI) to address the EUVL (extreme ultra- violet lithography) contamination control to achieve the HVM (high volume manufacturing) requirements in industry. Energetic atom and macro-particle emission are unavoidable when plasmas are used to generate photons in both DPP and LPP based EUV sources. These emitted particles interact first with the collection optics for the EUV radiation. Then some of the low energy sputtered collector material and some of the condensable Sn fuel exit at the intermediate focus (IF). This is undesirable. A critical requirement of stepper manufactures is to have only clean photons at the IF of EUV source-collector module. The very EUV photons that the system is designed to create can have an effect on the projection and illumination optics causing a reduction of mirror reflectivity. Even with advanced mitigation techniques, stepper optics can be damaged due to energetic and thermal neutrals. Particle contamination is problematic at the mask, and resist issues on the wafers themselves have an effect on the masks and optic elements. The efficiency of mitigation schemes is discussed. We present progress on our recent experiments on the measurement of ionic and neutral debris at Intermediate Focus (IF) in the DPP source. We also present progress on cleaning Sn deposition off of a multi-shell collector mock-up using reactive ion etching plasma, particle contamination removal from the mask blanks, and line edge roughness reduction in photoresisit.
Debris generation in EUV sources is a real threat to the lifetime of collector optics. Debris measurements in these
sources are of immense importance to enable source suppliers to estimate collector lifetime. Ion debris measurements
performed so far are not consistent and in part incomplete. To verify lifetime claims from different EUV source
suppliers, SEMATECH, which is leading this investigation, has collaborated with and provided funding to the Center for
Plasma Material Interactions (CPMI) at the University of Illinois to build a fully calibrated and standardized spherical
sector electrostatic energy analyzer (ICE). This device is capable of measuring ion debris flux in absolute units. In
addition to ion flux, the detector is also capable of identifying different ion species present in the plasma, which can be
discriminated based on energy-to-charge ratio. The lifetime of collector optics is calculated using the measured ion flux.
This device was fabricated for SEMATECH with the sole aim of traveling to different EUV source suppliers' sites
around the world and collecting ion debris data. SEMATECH has measured ion debris from different EUV sources
around the world, using a 1 to 14keV ion energy range under different source operational conditions (chamber pressure,
pinch frequency, pinch power, angle). These measurements identify the need for debris mitigation in all the EUV
sources investigated under this project. They also give source suppliers an opportunity to improve and optimize the
performance of their respective sources. The information on absolute ion fluxes is an advantage to source suppliers,
allowing them to design and develop effective debris mitigation schemes, which can again be tested for their
effectiveness using the ion diagnostic tool. As the debris consists of ions and neutrals, the next logical step is to develop
a standardized neutral detector to measure the flux and energy distribution of neutrals present in EUV plasma sources.
Taking into account both ions and neutral fluxes, more definitive conclusions on the performance of a EUV source can
be made and better collector lifetime estimation models can be derived. The Illinois Calibrated ESA (ICE) tool is now
part of the SEMATECH "Flying Circus" equipment set.