As semiconductor feature sizes and pitches shrink to ever-decreasing dimensions, Line Edge Roughness (LER) becomes
and increasing important problem. The LER is transferred from the photoresist to the substrate through the subsequent
processing steps, causing variations in, eg, gate length. This leads to mismatch in device performance and leakage.
Thus, an efficient and cost effective way to reduce the LER in the semiconductor photoresist is needed in order to keep
the imperfections from affecting processing steps further down the line. At the CPMI a new technique to reduce LER
from patterened photoresist has been developed in conjunction with INTEL. Results obtained using our technique
showed significant LER reduction from 6.9±0.47 nm to 3.9±0.61 nm for 45 nm lines and spaces. Recent results on 40
nm lines and spaces showed significant LER reduction from 5.9±0.50 nm to 4.1±0.63nm. LER reduction results on 40
nm lines and spaces reveal the fact that our technique is superior to other available techniques such as etching, vapor
smoothing, hardbake, ozonation and rinse.
For the continued advancement of lithography, specifically extreme ultraviolet lithography (EUVL), particle contamination on the photomask and the subsequent removal of these particles is of critical importance. Particle contamination on the photomasks can result in defects printed on devices and their subsequent failure and/or process throughput reduction. A new idea for the removal of these particles is to utilize the energy in metastable species in a plasma. In a laboratory or processing plasma where ionization fraction is relatively low, there exists metastable species with long lifetimes that have significant energy, in some cases on the order of ~20 eV. Through a combined process of ion bombardment as well as the energy transferred from the neutralization of the metastable species, particles on a surface can be reduced to volatile compounds which can be pumped off of the surface thus reducing the particle contamination on the surface. Preliminary results for the removal of polystyrene latex (PSL) nano particles on low resistivity silicon wafers have shown approximately 20 nm/min removal rates. The removal rate obtained through the use of the PACMAN technique is much faster compared to just metastable cleaning alone. The current results of the removal of particles via the PACMAN technique will be presented as well as a damage assessment if any caused by this process.
Despite a higher conversion efficiency of Sn for extreme ultra violet (EUV) light generation at 13.5 nm, Sn contamination on collector optics in EUV source systems must be overcome before adopting Sn as EUV fuel. Considerable portion of debris from Sn source can be suppressed by various debris mitigation techniques. However, debris mitigation technique alone will not be sufficient for high volume manufacturing (HVM) scale light production. Sn contamination affects not only the light output but also cost of ownership because of costly and time-consuming cleaning or replacing. In order to solve this contamination issue, Center for Plasma Material Interactions (CPMI) at University of Illinois at Urbana-Champaign(UIUC) had been working on cleaning Sn from EUV collector mirror surface using inductively coupled plasma-reactive ion etching (ICP-RIE) method. Previously, our group showed the fast cleaning rate of >100±10 nm/min and the dependence of cleaning on plasma-source location. Atomic force microscopy (AFM) surface roughness scan after cleaning showed almost 95% recovery in root-mean-square roughness compared to before-cleaning. Sn debris contamination can also be cleaned by halogen gas at high pressure (several hundreds mTorr). However, cleaning rate is much slower so that longer cleaning time is needed and other components in the system can be harmed by high pressure of corrosive gas. In this study, a remote plasma cleaning method is newly investigated. We designed and fabricated a remote plasma cleaning system which operates with 13.56MHz RF. A residual gas analyzer is used to quantify the chlorine radicals generated in a remote plasma system. A comparative study on the chlorine radicals generated in ICP and remote plasma is carried out. The initial result with gas temperature control shows that more chlorine radicals generate by remote plasma than ICP. It is also reported that high power can produce more chlorine radicals as expected.
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.
For extreme ultraviolet lithography (EUVL) to become a high volume manufacturing technology for integrated
circuit manufacturing, the cleanliness of the system, especially the photomask, is of high importance. For EUV
photomasks, which cannot be protected from contamination by the use of a pellicle, an effective and quick
cleaning technology needs to be ready in order to maintain wafer throughput. There are challenges to extend
current wet cleaning technologies to meet the future needs for damage-free and high efficiency mask cleaning.
Accordingly, a unique process for cleaning particulates from surfaces, specifically photomasks as well as wafers,
has been evaluated at the University of Illinois Urbana-Champaign. The removal technique utilizes a high density
plasma source as well as pulsed substrate biases to provide for removal. Helium is used as the primary gas in the
plasma, which under ionization, provides for a large density of helium metastable atoms present in the plasma.
These metastable helium atoms have on the order of 20 eV of energy which can transfer to particles on the
substrate to be cleaned. When the substrate is under a small flux of ion bombardment, these bonds then remain
broken and it is theorized that this allows the particles to be volatilized for their subsequent removal. 100 %
particle removal efficiency has been obtained for 30 nm, 80 nm, and 200 nm polystyrene latex particles. In
addition, removal rate has been correlated with helium metastable population density determined by optical
As semiconductor feature sizes continue to decrease, the phenomena of line edge roughness (LER) becomes more
disruptive in chip manufacturing. While many efforts are underway to decrease LER from the photoresist, postdevelop
smoothing techniques may be required to continue shrinking chip features economically. This paper
reports on one such method employing the use of an ion beam at grazing incidence along the features. This
method smooths relatively long spatial length LER, a potential advantage over other smoothing techniques that
focus on just small-scale LER. LER reduction numbers using Ne and Ar beams are reported at both short
and long spatial wavelength. Variables include beam energy, length of time and angular dependence. LER
measurements are taken using Hitachi image analysis software on top-down analytical SEM measurements. Line
profile data are taken from cross sectional SEM photographs. Tests have achieved a reduction in LER from
9.8±0.67 nm to 5.5±0.86 nm for 45 nm 1:1 lines using an Ar beam at 500 eV for 6 s at an 85o angle of incidence.
A reduction from 10.1±1.07 nm to 6±1.02 nm was shown using an Ar beam at 1000 eV for 4 s at a 60o angle of
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.
Particle contamination on surfaces used in extreme ultraviolet (EUV) mask blank deposition, mask fabrication,
and patterned mask handling must be avoided since the contamination can create significant distortions and
loss of reflectivity. Particles on the order of 10nm are problematic during MLM mirror fabrication, since the
introduced defects disrupt the local Bragg planes. The most serious problem is the accumulation of particles
on surfaces of patterned blanks during EUV light exposure, since > 25nm particles will be printed without an
out-of-focus pellicle. Particle contaminants are also a problem with direct imprint processes since defects are
printed every time. Plasma Assisted Cleaning by Electrostatics (PACE) works by utilizing a helicon plasma as
well as a pulsed DC substrate bias to charge particle and repel them electrostatically from the surface. Removal
of this nature is a dry cleaning method and removes contamination perpendicular from the surface instead of
rolling or sweeping the particles off the surface, a benefit when cleaning patterned surfaces where contamination
can be rolled or trapped between features. Also, an entire mask can be cleaned at once since the plasma can cover
the entire surface, thus there is no need to focus in on an area to clean. Sophisticated particle contamination
detection system utilizing high power laser called DEFCON is developed to analyze the particle removal after
PACE cleaning process. PACE has shown greater than 90 % particle removal efficiencies for 30 to 220 nm PSL
particles on ruthenium capped quartz. Removal results for silicon surfaces and quartz surfaces show similar
removal efficiencies. Results of cleaning 80 nm PSL spheres from silicon substrates will be shown.
Extreme ultraviolet lithography (EUVL) is a potential candidate for the next generation lithography techniques, which
will use Xe or Sn as a main fuel to produce EUV light. However, the industry has favored to use Sn as main fuel in
EUVL systems because of its high conversion efficiency over Xe. Sn has an advantage of producing more light, but on
the other hand its condensable nature is a real threat to the reflective mirrors which are used to collect the EUV light at
intermediate focus. Center for Plasma Material Interactions (CPMI) at the University of Illinois has studied plasma
etching as a potential method of Sn removal from collector optics. RF-driven chlorine plasma is used to etch Sn from
mirror samples. Previously we reported high selectivity of Sn over several EUV compatible mirror materials. The
increased confidence in this technology had led us to perform cleaning experiments on real Sn contaminated samples
exposed in an EUV source and the results obtained have been very encouraging. Small mock up shells (same as in the
grazing incidence collector optics system) were constructed at CPMI and chlorine etching was performed at different
samples placed at different locations on multi-shell collector mock up in ICP-RIE chamber. Post cleaning material
characterization results of samples shows that chlorine can potentially clean Sn off of collector optics (Ru was used in
this study as a mirror sample). Realizing this as a viable cleaning solution, we have stepped further and performed a full
size cleaning test in the Xtreme's XTS 13-35 EUV source. Large mock up with appropriate dimension was placed in the
EUV source chamber and the cleaning system was installed to etch Sn away from Ru surface. This study compares the
cleaning results in a real system scale with the previous simulated system. The comparison shows how to improve the Sn
cleaning system in the EUV source chamber. Results are encouraging and may enable source suppliers to integrate this
technology in their respective sources. Cleaning rate was measured as >100nm/min using ion sputtered Sn samples.