H. Shin

Control of ion energy distributions using a pulsed plasma with synchronous bias on a boundary electrode

Ion energy distributions (IEDs) on a grounded substrate in a Faraday-shielded argon inductively coupled plasma were measured with a retarding field energy analyzer. A Langmuir probe was also used to measure space- and time-resolved plasma parameters. IEDs and plasma parameters were studied with continuous or pulsed positive dc bias voltage on a ‘boundary electrode’ in contact with the plasma. For continuous wave plasmas without applied bias, the IED exhibited a single broad peak at the plasma potential. Applying a continuous positive dc bias on the boundary electrode shifted the peak of the IED to higher energy. Application of a synchronous dc bias on the boundary electrode during the afterglow of a pulsed plasma resulted in a double-peaked IED. The mean energies of the two peaks, as well as the peak separation, were controlled by adjusting the applied dc bias and the discharge pressure. The full width at half maximum (FWHM) of the peak corresponding to the synchronous dc bias diminished with decreasing electron temperature. The FWHM was controlled by varying the time window in the afterglow during which dc bias was applied. (Some figures in this article are in colour only in the electronic version)

Surprising importance of photo-assisted etching of silicon in chlorine-containing plasmas

The authors report a new, important phenomenon: photo-assisted etching of p-type Si in chlorine-containing plasmas. This mechanism was discovered in mostly Ar plasmas with a few percent added Cl2, but was found to be even more important in pure Cl2 plasmas. Nearly monoenergetic ion energy distributions (IEDs) were obtained by applying a synchronous dc bias on a “boundary electrode” during the afterglow of a pulsed, inductively coupled, Faraday-shielded plasma. Such precisely controlled IEDs allowed the study of silicon etching as a function of ion energy, at near-threshold energies. Etching rates increased with the square root of the ion energy above the observed threshold of 16 eV, in agreement with published data. Surprisingly, a substantial etching rate was observed, independent of ion energy, when the ion energy was below the ion-assisted etching threshold. Experiments ruled out chemical etching by Cl atoms, etching assisted by Ar metastables, and etching mediated by holes and/or low energy electrons ...

Ion energy distributions in inductively coupled plasmas having a biased boundary electrode

In many plasma materials processing applications requiring energetic ion bombardment such as plasma etching, control of the time-averaged ion energy distributions (IEDs) to surfaces is becoming increasingly important to discriminate between surface processes having different threshold energies. Inductively coupled plasmas (ICPs) are attractive in this regard since the plasma potential is low and so the energy of ion fluxes can be more finely tuned with externally applied biases. In these situations, pulsed plasmas provide another level of control as the IEDs from different times during the pulse power period can be combined to create the desired time-averaged IED. A recent development in controlling of IEDs in ICPs is the use of a boundary electrode (BE) in which a continuous or pulsed dc bias is applied to shift the plasma potential and modify the IEDs to surfaces without significant changes in the bulk plasma properties. Combinations of pulsing the ICP power and the BE bias provide additional flexibility to craft IEDs. In this paper we discuss results from a computational investigation of IEDs to a grounded substrate in low-pressure (a few to 50 mTorr) ICPs sustained in argon. Results are compared with experimental measurements of plasma properties and IEDs. We demonstrate the ability to customize IEDs consisting of three energy peaks corresponding to the plasma potential during the plasma active glow, plasma afterglow and the plasma potential with the applied boundary voltage.

Ion energy distributions, electron temperatures, and electron densities in Ar, Kr, and Xe pulsed discharges

Ion energy distributions (IEDs) were measured near the edge of Faraday-shielded, inductively coupled pulsed plasmas in Ar, Kr, or Xe gas, while applying a synchronous dc bias on a boundary electrode, late in the afterglow. The magnitudes of the full width at half maximum of the IEDs were Xe > Kr > Ar, following the order of the corresponding electron temperatures in the afterglow, Te(Xe) > Te(Kr) > Te(Ar). The measured decays of Te with time in the afterglow were in excellent agreement with predictions from a global model. Measured time-resolved electron and positive ion densities near the plasma edge did not decay appreciably, even in the 80 μs long afterglow. This was attributed to transport of ions and electrons from the higher density central region of the plasma to the edge region, balancing the loss of plasma due to diffusion. This provides a convenient means of maintaining a relatively constant plasma density in the afterglow during processing using pulsed plasmas.

In situ plasma diagnostics study of a commercial high-power hollow cathode magnetron deposition tool

Using a newly designed and built plasma diagnostic system, the plasma parameters were investigated on a commercial 200mm high-power hollow cathode magnetron (HCM) physical vapor deposition tool using Ta target under argon plasma. A three dimensional (3D) scanning radio frequency (rf)-compensated Langmuir probe was constructed to measure the spatial distribution of the electron temperature (Te) and electron density (ne) in the substrate region of the HCM tool at various input powers (2–15kW) and pressures (10–70mTorr). The Te was in the range of 1–3eV, scaling with decreasing power and decreasing pressure. Meanwhile, ne was in the range of 4×1010–1×1012cm−3 scaling with increasing power and decreasing pressure. As metal deposits on the probe during the probe measurements, a self-cleaning plasma cup was designed and installed in the chamber to clean the tungsten probe tip. However, its effectiveness in recovering the measured plasma parameters was hindered by the metal layer deposited on the insulating probe tube which was accounted for the variation in the plasma measurements. Using a quartz crystal microbalance combined with electrostatic filters, the ionization fraction of the metal flux was measured at various input power of 2–16kW and pressure of 5–40mTorr. The metal ionization fraction reduced significantly with the increasing input power and decreasing gas pressure which were attributed to the corresponding variation in the ionization cross section and the residence time of the sputtered atoms in the plasma, respectively. Both the metal neutral and ion flux increased at higher power and lower pressure. The 3D measurements further showed that the ionization fraction decreased when moving up from the substrate to the cathode.Using a newly designed and built plasma diagnostic system, the plasma parameters were investigated on a commercial 200mm high-power hollow cathode magnetron (HCM) physical vapor deposition tool using Ta target under argon plasma. A three dimensional (3D) scanning radio frequency (rf)-compensated Langmuir probe was constructed to measure the spatial distribution of the electron temperature (Te) and electron density (ne) in the substrate region of the HCM tool at various input powers (2–15kW) and pressures (10–70mTorr). The Te was in the range of 1–3eV, scaling with decreasing power and decreasing pressure. Meanwhile, ne was in the range of 4×1010–1×1012cm−3 scaling with increasing power and decreasing pressure. As metal deposits on the probe during the probe measurements, a self-cleaning plasma cup was designed and installed in the chamber to clean the tungsten probe tip. However, its effectiveness in recovering the measured plasma parameters was hindered by the metal layer deposited on the insulating prob...

Selective etching of TiN over TaN and vice versa in chlorine-containing plasmas

Selectivity of etching between physical vapor-deposited TiN and TaN was studied in chlorine-containing plasmas, under isotropic etching conditions. Etching rates for blanket films were measured in-situ using optical emission of the N2 (C3Πu →B3Πg) bandhead at 337 nm to determine the etching time, and transmission electron microscopy to determine the starting film thickness. The etching selectivity in Cl2/He or HCl/He plasmas was poor (<2:1). There was a window of very high selectivity of etching TiN over TaN by adding small amounts (<1%) of O2 in the Cl2/He plasma. Reverse selectivity (10:1 of TaN etching over TiN) was observed when adding small amounts of O2 to the HCl/He plasma. Results are explained on the basis of the volatility of plausible reaction products.

Tin removal from extreme ultraviolet collector optics by inductively coupled plasma reactive ion etching

Tin (Sn) has the advantage of delivering higher conversion efficiency compared to other fuel materials (e.g., Xe or Li) in an extreme ultraviolet (EUV) source, a necessary component for the leading next generation lithography. However, the use of a condensable fuel in a lithography system leads to some additional challenges for maintaining a satisfactory lifetime of the collector optics. A critical issue leading to decreased mirror lifetime is the buildup of debris on the surface of the primary mirror that comes from the use of Sn in either gas discharge produced plasma (GDPP) or laser produced plasma (LPP). This leads to a decreased reflectivity from the added material thickness and increased surface roughness that contributes to scattering. Inductively coupled plasma reactive ion etching with halide ions is one potential solution to this problem. This article presents results for etch rate and selectivity of Sn over SiO2 and Ru. The Sn etch rate in a chlorine plasma is found to be much higher (of the or...

Plasma cleaning of nanoparticles from EUV mask materials by electrostatics

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.

Remote plasma cleaning of Sn from an EUV collector mirror

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.

Contamination removal from collector optics and masks: an essential step for next-generation lithography

Tin is the preferred fuel in EUV sources due to its higher conversion efficiency (3%) compared to Xe (1%.) However, there are several critical challenges to overcome before Sn can be used. Sn is a condensable fuel, which deposits on nearby surfaces. The light is collected in this technology using reflective collector mirrors, which are placed near to the plasma pinch area. Collection efficiency of these mirrors and their ability to direct EUV light to the intermediate focus depends heavily on its reflectivity, which in turn depends on the surface morphology and composition. Tin contamination reduces the reflectivity of the mirror surfaces. High energy tin ions or neutrals, contaminate the surface, makes it rougher and also erode it away. Due to these effects mirrors would need to be changed frequently, which increases the cost of ownership. The Center for Plasma Material Interactions at the UIUC is expanding efforts to develop cleaning methods for Sn off of EUV compatible surfaces. Reactive ion etching methods are developed as an effective tool for this process. An in-house RIE chamber is used to investigate Sn etching by Ar/Cl2 plasma. Gas flow rates, chuck bias, sample temperatures and the chamber geometries are being analyzed to optimize the etching. Results are very promising and encouraging towards an extended collector life time. Etch rates are measured for Sn and its selectivity is studied over SiO2 and Ru, which shows that the method adopted at UIUC for Sn etching is a potential solution to this problem. Additional experiments for cleaning Sn off a mock collector mirror geometry, shows the potential to integrate this method in real technology.