Cl2 Plasma Research Articles

Optimization of ohmic contact for AlGaNGaN HEMT by introducing patterned etching in ohmic area

In this paper, the ohmic contact of AlGaNGaN HEMT was optimized by introducing patterned etching in ohmic area, and the conventional structure and whole etching structure were investigated for comparison. The contact resistance decreased from 0.46Ωmm for conventional to 0.35Ωmm and 0.18Ωmm respectively for the whole etching and patterned etching structures. The current-voltage characteristics between the ohmic electrodes presented sharper slope, higher saturation current and lower knee voltage on patterned etching structures. After Cl2 plasma etching on ohmic area surface of AlGaN, the surface oxide layers and the pollutants were removed, therefore, the surface roughness of the ohmic metal reduced obviously, and the surface morphology improved. Meanwhile, the side area induced in patterned etching provided more extra contact area, which increased the tunneling current. The different apertures and the duty factor of patterned etching were investigated, and the results indicated that the quantity of side area produced in patterned etching dominated the reduction effect of ohmic contact resistance.

Surface smoothing during plasma etching of Si in Cl2

Effects of initial roughness on the evolution of plasma-induced surface roughness have been investigated during Si etching in inductively coupled Cl2 plasmas, as a function of rf bias power or ion incident energy in the range Ei ≈ 20–500 eV. Experiments showed that smoothing of initially rough surfaces as well as non-roughening of initially planar surfaces can be achieved by plasma etching in the smoothing mode (at high Ei) with some threshold for the initial roughness, above which laterally extended crater-like features were observed to evolve during smoothing. Monte Carlo simulations of the surface feature evolution indicated that the smoothing/non-roughening is attributed primarily to reduced effects of the ion scattering or reflection from microscopically roughened feature surfaces on incidence.

Electrical detection of magnetic domain wall in Fe4N nanostrip by negative anisotropic magnetoresistance effect

The magnetic structure of the domain wall (DW) of a 30-nm-thick Fe4N epitaxial film with a negative spin polarization of the electrical conductivity is observed by magnetic force microscopy and is well explained by micromagnetic simulation. The Fe4N film is grown by molecular beam epitaxy on a SrTiO3(001) substrate and processed into arc-shaped ferromagnetic nanostrips 0.3 μm wide by electron beam lithography and reactive ion etching with Cl2 and BCl3 plasma. Two electrodes mounted approximately 12 μm apart on the nanostrip register an electrical resistance at 8 K. By changing the direction of an external magnetic field (0.2 T), the presence or absence of a DW positioned in the nanostrip between the two electrodes can be controlled. The resistance is increased by approximately 0.5 Ω when the DW is located between the electrodes, which signifies the negative anisotropic magnetoresistance effect of Fe4N. The electrical detection of the resistance change is an important step toward the electrical detection of current-induced DW motion in Fe4N.

Chlorine-based inductively coupled plasma etching of GaAs wafer using tripodal paraffinic triptycene as an etching resist mask

We report the etching properties of tripodal paraffinic triptycene (TripC12) used as a thermal nanoimprint lithography (TNIL) resist mask in Cl2 plasma etching. Using thermally nanoimprinted TripC12 films, we achieved microfabrication of a GaAs substrate by Cl2-based inductively coupled plasma (ICP) etching. Attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy confirmed that the chemical structure of TripC12 remains intact after the ICP etching process using Cl2. We believe that TNIL using TripC12 films is useful for fabricating optical/electrical devices and micro-electro-mechanical systems (MEMSs).

Molecular dynamics simulations of Si etching in Cl- and Br-based plasmas: Cl+ and Br+ ion incidence in the presence of Cl and Br neutrals

Classical molecular dynamics (MD) simulations have been performed for Cl+ and Br+ ions incident on Si(100) surfaces with Cl and Br neutrals, respectively, to gain a better understanding of the ion-enhanced surface reaction kinetics during Si etching in Cl- and Br-based plasmas. The ions were incident normally on surfaces with translational energies in the range Ei = 20–500 eV, and low-energy neutrals of En = 0.01 eV were also incident normally thereon with the neutral-to-ion flux ratio in the range Γn0/Γi0 = 0–100, where an improved Stillinger--Weber potential form was employed for the interatomic potential concerned. The etch yields and thresholds presently simulated were in agreement with the experimental results previously reported for Si etching in Cl2 and Br2 plasmas as well as in Cl+, Cl2+, and Br+ beams, and the product stoichiometry simulated was consistent with that observed during Ar+ beam incidence on Si in Cl2. Moreover, the surface coverage of halogen atoms, halogenated layer thickness, surface stoichiometry, and depth profile of surface products simulated for Γn0/Γi0 = 100 were in excellent agreement with the observations depending on Ei reported for Si etching in Cl2 plasmas. The MD also indicated that the yield, coverage, and surface layer thickness are smaller in Si/Br than in Si/Cl system, while the percentage of higher halogenated species in product and surface stoichiometries is larger in Si/Br. The MD further indicated that in both systems, the translational energy distributions of products and halogen adsorbates desorbed from surfaces are approximated by two Maxwellians of temperature T1 ≈ 2500 K and T2 ≈ 7000–40 000 K. These energy distributions are discussed in terms of the desorption or evaporation from hot spots formed through chemically enhanced physical sputtering and physically enhanced chemical sputtering, which have so far been speculated to both occur in the ion-enhanced surface reaction kinetics of plasma etching.

Key plasma parameters for nanometric precision etching of Si films in chlorine discharges

Ultrathin layered films in new transistors architectures (FinFET and fully depleted SOI) require damage-free plasma etching techniques with unprecedented selectivity between materials. To assist the development of advanced processes, molecular dynamics simulations are performed to quantify modifications (plasma-induced damage, etch rate) of Si films after exposition to various Cl2 plasma conditions, simulated by bombarding the substrate with both ion (Cl+, Cl2+) and neutral (Cl, Cl2) species. All simulations show the formation of a stable SiClx reactive layer and a constant etch yield at steady state. The key plasma parameter to control the etching of ultrathin Si layers is the ion energy (Ei), which lowers significantly both the damaged layer thickness (from 1.8 nm at 100 eV to 0.8 nm at 5 eV when Γ = 100) and the etch yield when it is decreased. The neutral-to-ion flux ratio (Γ) is the second key parameter: its increase reduces the damaged layer thickness (from 1.8 nm for Γ = 100 to 1.1 nm for Γ = 1000 at 100 eV) while the etch rate grows. While maintaining Γ constant, the neutral dissociation rate and the ion composition do not influence significantly the etching process. Quantitatively, simulations suggest that plasmas with low ion energies (<15 eV) and high Γ ratios (>1000) should induce sub-nm thick reactive layers, confirming an interest in low-Te or pulsed plasmas (operating at low duty cycle) to achieve nanometric precision etching.

Nanopore patterning using Al2O3 hard masks on SOI substrates

Aluminum oxide Al2O3, deposited using amorphous atomic layer deposition (ALD), is a very promising material to be utilized as a hard mask for nano-patterning. We used an aluminum oxide hard mask on a silicon-on-insulator (SOI) substrate to implement a sub-100 nm nanopore process. The transfer of nanoscale patterns via dry etching of the Al2O3 thin film was investigated by comparing etch profiles, etch rates, and selectivity of Al2O3 over PMMA resist, using different gas chemistries such as Cl2, Ar, Ar/BCl3 mixtures, and BCl3 plasma. A selectivity of 1:4 was observed using an inductively coupled plasma reactive ion etching (ICP-RIE) tool with BCl3 plasma, and the sub-100 nm nanopore patterns were anisotropically transferred to the alumina layer from a 250 nm PMMA layer. The dense and inert Al2O3 hard mask showed exceptional etch selectivity to Si and SiO2, which allowed the subsequent transfer of the nanopore patterns into the 340 nm-thick Si device layer and made it possible to attempt etching the 1 μm-thick buried oxide (BOX) layer. Using chlorine chemistry, nanopores patterned in the Si device layer showed excellent anisotropy while preserving the original pattern dimensions. The process demonstrated is ideally suited for patterning high aspect ratio nanofluidic structures.

Suppression of plasma-induced damage on GaN etched by a Cl2 plasma at high temperatures

Plasma-induced damage (PID) during plasma-etching processes was suppressed by the application of Cl2 plasma etching at an optimal temperature of 400 °C, based on results of evaluations of photoluminescence (PL), stoichiometric composition, and surface roughness. The effects of ions, photons, and radicals on damage formation were separated from the effects of plasma using the pallet for plasma evaluation (PAPE) method. The PID was induced primarily by energetic ion bombardments at temperatures lower than 400 °C and decreased with increasing temperature. Irradiations by photons and radicals were enhanced to form the PID and to develop surface roughness at temperatures higher than 400 °C. Consequently, Cl2 plasma etching at 400 °C resulted optimally in low damage and a stoichiometric and smooth GaN surface.

Design, fabrication, and optical characteristics of freestanding GaN waveguides on silicon substrate

Freestanding GaN waveguides were fabricated on a silicon substrate by a combination of Cl2 plasma reactive ion etching and XeF2 gas selective etching. The freestanding GaN waveguides ranged from 0.23 to 8 μm in width and were supported in air by bridge structures. The bridge structures were designed via rigorous electromagnetic simulations using the finite-difference time-domain method. The GaN layer was grown epitaxially on a silicon (111) substrate using a buffer layer to compensate for the crystal lattice constant mismatch. Using two types of masks, the GaN layer was etched using a Cl2 inductively coupled plasma. The 625-nm-thick GaN layer was etched by the Cl2 plasma at a substrate temperature of −17 °C to form the GaN waveguide patterns, at the expense of a 92-nm-thick HfO2 mask layer. The etching rate of the GaN layer was 170 nm/min and the etching ratio between the GaN and HfO2 layers was 6.8:1. The silicon substrate was then isotropically etched using XeF2 gas to generate air gaps underneath the GaN waveguides. The transmittance of the fabricated freestanding GaN waveguides was measured using a visible (406 nm) laser and an infrared (1550 nm) laser. The waveguide losses for a 730-nm-wide and 625-nm-thick waveguide were 2.6 dB/mm at 406 nm and 2.2 dB/mm at 1550 nm. These results indicate that the structures are likely to be useful for several visible waveguide devices combined with blue GaN light emitting diodes and for optical telecommunication waveguide devices using the wide transmission window of the GaN crystal.

In-situ etch rate study of HfxLayOz in Cl2/BCl3 plasmas using the quartz crystal microbalance

The etch rate of HfxLayOz films in Cl2/BCl3 plasmas was measured in-situ in an inductively coupled plasma reactor using a quartz crystal microbalance and corroborated by cross-sectional SEM measurements. The etch rate depended on the ion energy as well as the plasma chemistry. In contrast to other Hf-based ternary oxides, the etch rate of HfxLayOz films was higher in Cl2 than in BCl3. In the etching of Hf0.25La0.12O0.63, Hf appeared to be preferentially removed in Cl2 plasmas, per surface compositional analysis by x-ray photoelectron spectroscopy and the detection of HfCl3 generation in mass spectroscopy. These findings were consistent with the higher etch rate of Hf0.25La0.12O0.63 than that of La2O3.

Inductively coupled reactive ion etching studies on sputtered yttria stabilized zirconia thin films in SF6, Cl2, and BCl3 chemistries

Downscaling of yttria stabilized zirconia (YSZ) based electrochemical devices and gate oxide layers requires successful pattern transfer on YSZ thin films. Among a number of techniques available to transfer patterns to a material, reactive ion etching has the capability to offer high resolution, easily controllable, tunable anisotropic/isotropic pattern transfer for batch processing. This work reports inductively coupled reactive ion etching studies on sputtered YSZ thin films in fluorine and chlorine based plasmas and their etch chemistry analyses using x-ray photoelectron spectroscopy. Etching in SF6 plasma gives an etch rate of 7 nm/min chiefly through physical etching process. For same process parameters, in Cl2 and BCl3 plasmas, YSZ etch rate is 17 nm/min and 45 nm/min, respectively. Increased etch rate in BCl3 plasma is attributed to its oxygen scavenging property synergetic with other chemical and physical etch pathways. BCl3 etched YSZ films show residue-free and smooth surface. The surface atomic concentration ratio of Zr/Y in BCl3 etched films is closer to as-annealed YSZ thin films. On the other hand, Cl2 etched films show surface yttrium enrichment. Selectivity ratio of YSZ over silicon (Si), silicon dioxide (SiO2) and silicon nitride (Si3N4) are 1:2.7, 1:1, and 1:0.75, respectively, in BCl3 plasma. YSZ etch rate increases to 53 nm/min when nonoxygen supplying carrier wafer like Si3N4 is used.

Viable chemical approach for patterning nanoscale magnetoresistive random access memory

A reactive ion etching process with alternating Cl2 and H2 exposures has been shown to chemically etch CoFe film that is an integral component in magnetoresistive random access memory (MRAM). Starting with systematic thermodynamic calculations assessing various chemistries and reaction pathways leading to the highest possible vapor pressure of the etch products reactions, the potential chemical combinations were verified by etch rate investigation and surface chemistry analysis in plasma treated CoFe films. An ∼20% enhancement in etch rate was observed with the alternating use of Cl2 and H2 plasmas, in comparison with the use of only Cl2 plasma. This chemical combination was effective in removing metal chloride layers, thus maintaining the desired magnetic properties of the CoFe films. Scanning electron microscopy equipped with energy-dispersive x-ray spectroscopy showed visually and spectroscopically that the metal chloride layers generated by Cl2 plasma were eliminated with H2 plasma to yield a clean etch profile. This work suggests that the selected chemistries can be used to etch magnetic metal alloys with a smooth etch profile and this general strategy can be applied to design chemically based etch processes to enable the fabrication of highly integrated nanoscale MRAM devices.

Evolution of titanium residue on the walls of a plasma-etching reactor and its effect on the polysilicon etching rate

The variation in polysilicon plasma etching rates caused by Ti residue on the reactor walls was investigated. The amount of Ti residue was measured using attenuated total reflection Fourier transform infrared spectroscopy with the HgCdTe (MCT) detector installed on the side of the reactor. As the amount of Ti residue increased, the number of fluorine radicals and the polysilicon etching rate increased. However, a maximum limit in the etching rate was observed. A mechanism of rate variation was proposed, whereby F radical consumption on the quartz reactor wall is suppressed by the Ti residue. The authors also investigated a plasma-cleaning method for the removal of Ti residue without using a BCl3 gas, because the reaction products (e.g., boron oxide) on the reactor walls frequently cause contamination of the product wafers during etching. CH-assisted chlorine cleaning, which is a combination of CHF3 and Cl2 plasma treatment, was found to effectively remove Ti residue from the reactor walls. This result shows that CH radicals play an important role in deoxidizing and/or defluorinating Ti residue on the reactor walls.

Pulsed Cl2/Ar inductively coupled plasma processing: 0D model versus experiments

Comparisons between measurements and spatially-averaged (0D) simulations of low-pressure Ar and Cl2 pulsed-plasmas in an industrial inductively coupled reactor are reported. Our analysis focuses on the impact of the pulsing parameters (frequency f, duty cycle dc) on the chemical reactivity of the plasma and on the ion fluxes to the walls. Charged particle densities and ion fluxes are highly modulated when the plasma is pulsed at 1 kHz < f < 20 kHz. In rare gas Ar plasmas, the ion flux rise time is short (50 μs), therefore the dc has almost no influence on the ion flux value during the pulse. By contrast, in molecular electronegative Cl2 plasmas, both the value and rise/decay time of the ion flux during the on and off-periods depend strongly on the dc. This is because in Cl2 both the plasma chemistry and electronegativity depend on the dc. During the off-period, the electron density drops much faster than the negative ion density, leading to a large increase in plasma electronegativity. A minimum afterglow time (75 µs) is required for an ion-ion plasma to form and for the sheath to collapse, exposing the walls and wafers to a negative ion flux. The positive ion flux is 3 to 10 times smaller in Cl2 than in Ar for the same operating conditions. In contrast with charged species, the radical (Cl) kinetics are slow and thus the radical density is hardly modulated for f > 1 kHz. However, the dc strongly influences the Cl2/Cl density ratio and is an excellent knob for controlling the plasma chemical reactivity: the higher the dc the higher the Cl density. The trends and quantities in the 0D simulation are in close agreement with experiments. This proves the capacity of global models to reproduce the fundamental features of pulsed plasmas in simple chemistries and to assist the development of pulsed processes.

F‐ and Cl‐terminations of (100)‐oriented single crystalline diamond

The surface termination of single crystalline diamond with fluorine and chlorine was investigated. The diamond was exposed to SF6 inductively coupled plasma for fluorination and to Cl2 plasma for chlorination. For the fluorine termination, the plasma treatment caused one monolayer of fluorine (measured with two angle photoelectron detection) and no carbonyl fluorides on the diamond surface. Furthermore no graphitization or roughening of the surface was observed. The plasma treatment with chlorine led to rough surfaces.

Surface properties of indium tin oxide treated by Cl2 inductively coupled plasma

The effects of Cl2 inductively coupled plasma (ICP) treatment on the time dependence of work function (WF) and surface properties of indium tin oxide (ITO) were investigated. Kelvin probe (KP) measurements show that the WF after Cl2 ICP treatment is close to 5.9eV. The WF decrease curve of Cl2 plasma treated ITO is fitted with double exponential functions with an adjusted R-square of 0.99. The mechanism under the decrease process is discussed by X-ray photoelectron spectroscopy (XPS). The ITO WF decrease after Cl2 ICP treatment performs much better than that after O2 ICP treatment and the chlorinated ITO keeps a WF increment of 0.3eV compared with that without plasma treatment after 100 days. Other properties of chlorinated ITO surface such as morphology and transmittance change slightly. The results are significant for the understanding of degradation of Cl2 plasma treated ITO and the fabrication of organic semiconductor devices.

Low inversion equivalent oxide thickness and enhanced mobility in MOSFETs with chlorine plasma interface engineering

High-k gated metal–oxide–semiconductor field-effect-transistors (MOSFETs) with Cl2 and CF4 plasma treatments are studied in this work. A higher-k HfON with more tetragonal phase is formed by the halogen plasma treatment on interfacial layer (IL). A low inversion equivalent oxide thickness in MOSFET is obtained with the Cl2 plasma treated IL. In addition, high mobility and transconductance, and low subthreshold swing are obtained by the Cl2 plasma treatment, which therefore is a promising interface engineering for advanced MOSFETs.

Thermodynamic assessment and experimental verification of reactive ion etching of magnetic metal elements

A thermodynamic analysis of etch chemistries for Co, Fe, and Ni using a combination of hydrogen, oxygen, and halogen gases suggested that a single etchant does not work at 300 K; however, a sequential exposure to multiple etchants results in sufficiently high partial pressure of the reaction products for the process to be considered viable. This sequential dose utilized the two reactions, a surface halogenation followed by the secondary etchant exposure. (MX2 (c) + 3Y →MY(g) + 2XY(g), where M = Co, Fe, Ni; X = F, Cl, Br; Y = O, H) The volatilization reaction induced by sequential plasma exposure changed the equilibrium point, increasing the partial pressure of the etch product. Amongst all combinations, Cl2 or Br2 plasmas followed by H2 plasma were the most effective. From both the gas phase diagnostics and surface composition analysis, H2 plasma alone could not etch metallic Co, Fe, and Ni films but alternating doses of Cl2 and H2 plasmas resulted in more effective removal of chlorinated metals and increased the overall etch rate.

Work function changes of plasma treated indium tin oxide

The work function (WF) changes of indium tin oxide (ITO) treated by O2 or Cl2 plasma were invested. The WF firstly decreases in an exponential way for 2–6h and subsequently in a linear way for days in air after plasma treatment. X-ray photoelectron spectroscopy (XPS) shows the dipole layer formed by O–In and O–O bonds in the ITO surface treated by O2 plasma increases the WF. Exposed in vacuum, the O–O bonds tend to break and the reactive O species reduce in the oxidized ITO surface, accounting for the exponential decrease of WF. The environment preserving the chlorinated or oxidized ITO is important to slow down the decrease of WF. The mechanism for WF changes of plasma treated ITO is significant to investigate the properties of organic optoelectronic devices.

Plasma Treatment of Thin Film Coated with Graphene Flakes for the Reduction of Sheet Resistance

We investigated the effects of plasma treatment on the sheet resistance of thin films spray-coated with graphene flakes on polyethylene terephthalate (PET) substrates. Thin films coated with graphene flakes show high sheet resistance due to defects within graphene edges, domains, and residual oxygen content. Cl2 plasma treatment led to decreased sheet resistance when treatment time was increased, but when thin films were treated for too long the sheet resistance increased again. Optimum treatment time was related to film thickness. The reduction of sheet resistance may be explained by the donation of holes due to forming pi-type covalent bonds of Cl with carbon atoms on graphene surfaces, or by C--Cl bonding at the sites of graphene defects. However, due to radiation damage caused by plasma treatment, sheet resistance increased with increased treatment time. We found that the sheet resistance of PET film coated with graphene flakes could be decreased by 50% under optimum conditions.