Mutumi Tuda

Numerical study of the etch anisotropy in low-pressure, high-density plasma etching

Evolution of etched profiles has been numerically studied during low-pressure, high-density (LPHD) plasma etching of Si in Cl2. The surface etch rates were calculated using a reaction model of synergism between incoming ions and neutral reactants, including the spread of ion angular distributions due to their thermal motions and the transport of neutrals arising from the reemission on surfaces in a microstructure. Etched profiles were then simulated using a so-called two-dimensional string algorithm to examine the effects of ion temperature kTi and energy (or sheath voltage) eVs on the etch anisotropy for different neutral-to-ion flux ratios Γn/Γi toward the substrate. Numerical results indicated that in typical Cl2 LPHD plasma etching environments, where the neutral-to-ion flux ratio is Γn/Γi∼1 and the ratio of sheath voltage to ion temperature is eVs/kTi∼100, the chlorinated surface coverage is microscopically nonuniform in etched features: The coverage is very low at the bottom (α∼0.1), whereas the sid...

Effects of etch products and surface oxidation on profile evolution during electron cyclotron resonance plasma etching of poly‐Si

Evolution of etched profiles has been numerically investigated during electron cyclotron resonance (ECR) Cl2/O2 plasma etching of polycrystalline Si. The calculations included the processes of chemically enhanced ion etching and passivation layer formation, which are important in such low‐pressure, high‐density plasma etching environments. The sidewall passivation was modeled by taking account of the redeposition of etch products on sidewalls of etched features. In addition, the effect of surface oxidation was also included in the model. Etched profiles were then simulated to examine effects of the neutral‐to‐ion flux ratio onto the substrate, sticking coefficient of etch products, and additional incident fluxes of etch products and oxygen atoms. Numerical results indicated that in typical ECR plasma etching environments, where the gas pressure is P0≲1 mTorr and the ion current density is Ji∼10 mA/cm2 onto the substrate, the chlorinated surface coverage α is microscopically nonuniform on sidewalls and bot...

Profile evolution during polysilicon gate etching with low-pressure high-density Cl2/HBr/O2 plasma chemistries

Profile evolution during polysilicon gate etching has been investigated with low-pressure high-density Cl2/HBr/O2 plasma chemistries. Etching was performed in electron cyclotron resonance Cl2/HBr/O2 plasmas as a function of HBr percentage in a Cl2/HBr mixture, using oxide-masked poly-Si gate structures. The linewidth was nominally 0.18 μm, and the spacing between the two neighboring poly-Si lines was varied in the range ∼0.2–10 μm. In addition, the macroscopic open space of the oxide-masked sample was also varied over a wide range from ≈28% to ≈76%. As the HBr percentage in Cl2/HBr is increased from 0 to 100%, the linewidth shift ΔL of poly-Si relative to the mask width (or the degree of sidewall tapering of poly-Si lines) first decreased linearly, passed through a minimum, and then increased considerably at above ∼80%. In Cl2/O2 plasmas without HBr addition, ΔL was almost independent of the microscopic and macroscopic poly-Si open spaces although its value was relatively large; on the contrary, in HBr/O2...

Simulation of Ion Trajectories near Submicron-Patterned Surface Including Effects of Local Charging and Ion Drift Velocity toward Wafer

Ion trajectories near a submicron-patterned surface were investigated using numerical simulations including the effects of local charging on the patterned surface and ion drift velocity toward the wafer. The simulation results were also discussed relative to the etched profile characteristics in electron cyclotron resonance (ECR) plasmas with a divergent magnetic field. Since the pattern size was much smaller than the Debye length, charge neutrality was not satisfied on the submicron-patterned surface. The simulated ion trajectories were largely deflected at the inside of the outermost lines of line-and-space patterns. Moreover, the ion trajectory deflection was reduced with increasing ion drift velocity. These simulation results showed a similar tendency as the etching characteristics.

Chemical Kinetics of Chlorine in Electron Cyclotron Resonance Plasma Etching of Si.

Electron cyclotron resonance (ECR) plasma etching of Si in Cl2 has been studied from the viewpoint of plasma chemistry. Experiments were performed over a wide pressure range (0.2?10 mTorr), using a divergent magnetic-field ECR plasma reactor supplied with 2.45-GHz microwaves; a floating electrode or substrate holder was located ?30 cm downstream (B?150 G) of the 875-G ECR resonance region, and samples of polycrystalline Si were etched with no additional wafer biasing. Several diagnostics were employed to characterize the plasma around the wafer position, including two-photon laser-induced fluorescence (LIF) for detection of Cl atoms and Fourier transform infrared (FTIR) absorption spectroscopy for etch products or SiClx (x=1?4) molecules. The Si etch rate behavior obtained is interpreted in terms of a modified adsorption-reaction-ion-stimulated desorption process model based on these diagnostics.

Measurements of the Cl atom concentration in radio‐frequency and microwave plasmas by two‐photon laser‐induced fluorescence: Relation to the etching of Si

Atomic chlorine concentrations in Cl2 plasmas have been measured by two‐photon laser‐induced fluorescence (LIF). Experiments were performed over a wide pressure range in rf, rf magnetron, and microwave electron cyclotron resonance (ECR) discharges. Absolute calibration was achieved by generating Cl atoms through UV photolysis of CCl4 with the same laser radiation. The LIF measurements showed that in the respective discharges, the Cl concentrations decreased and the corresponding Cl fractions increased with decreasing pressure: the measured fractions ranged from 0.3% to 0.7% in rf discharges, from 0.8% to 8% in magnetron, and from 1% to 2% in ECR. These results, together with electrical measurements of ion and electron energies and densities, demonstrated that the ratio of neutral Cl flux to ion flux toward the substrate decreased from 400:1 to 1:1, almost linearly with decreasing pressure in the three discharges. These results of plasma diagnostics are compared with the etching characteristics of heavily ...

Mechanisms for Microscopic Nonuniformity in Low-Pressure, High-Density Plasma Etching of Poly-Si in Cl2 and Cl2/O2 Mixtures

Etch anisotropy and microscopic uniformity have been investigated in low-pressure, high-density plasma etching environments. Polycrystalline Si films masked with a photoresist pattern of lines and spaces were etched in electron cyclotron resonance (ECR) Cl2 and Cl2/O2 plasmas with additional rf biasing. Experiments were performed by varying the gas pressure, substrate temperature, and percentage of O2 added. Moreover, the profile evolution during etching was simulated taking into account the transport of neutral and ionic species in microstructures and the following surface reactions: adsorption of neutral reactants, ion-stimulated desorption of reaction products, surface oxidation, and redeposition of etch products. The etched profiles obtained in ECR Cl2 and Cl2/O2 plasmas were compared with simulated results, and interpreted in terms of the deposition of etch products and the surface oxidation that occur competitively during etching. Etch products desorbed from the surface in microstructures have a sticking coefficient S p\lesssim0.1, while etch products arriving from the plasma have a much larger coefficient S p\gtrsim0.5. It was also shown that the competitive surface oxidation by incoming oxygen atoms plays a more important role in achieving aspect-ratio-independent etching than the deposition of etch products: the increased deposition of etch products resulted in enhanced sidewall tapering with little change in vertical etch rate; the surface oxidation resulted in inverse reactive-ion-etching lag without significant change in sidewall tapering.

Effects of O2 addition on BCl3/Cl2 plasma chemistry for Al etching

Effects of low level (0.5–20%) O2 addition on BCl3/Cl2 plasma chemistry have been investigated using several diagnostic tools: optical emission spectroscopy, microwave interferometry, and mass spectrometry. Experiments were performed using a magnetically enhanced planar 13.56-MHz rf plasma reactor, where aluminum etching was also performed using samples masked with a photoresist pattern of lines and spaces. As O2 was added into a BCl3/Cl2 plasma, the Al etch rate first increased and then dropped above ≈3% O2 added; a transition from reactive-ion-etching (RIE) lag to inverse RIE lag occurred at an O2 percentage of ~8%. Optical and mass spectrometric measurements indicated that the Cl concentration increases as O2 is added into a BCl3/Cl2 plasma, and that above a critical O2 percentage (~6% O2) Bx Oy species are formed in the plasma through a reaction between boron chlorides and oxygen and then deposit onto the wafer surface during etching. The Al etching characteristics obtained in BCl3/Cl2/O2 plasmas are interpreted in terms of competitive effects of increased concentrations of Cl and Bx Oy .

Profile Evolution during Cold Plasma Beam Etching of Silicon

Processing characteristics of cold, directional plasma beams have been studied by numerical simulation of the profile evolution of trenches etched in silicon with chlorine, where the cold plasma beams were characterized by highly directional fluxes of neutrals as well as ions with their low temperatures or random thermal energies. The model included ion and neutral transport in microstructures and ion-assisted surface chemistry for the chlorine-silicon system. The numerical results demonstrate that the cold plasma beams achieve high etch anisotropy and microscopic uniformity: a tapered profile, inversely tapered profile, and reactive-ion-etching (RIE) lag, which often occur in usual plasma etching environments, are significantly suppressed owing to preferential incidence of neutral reactants as well as ions onto the bottom of microstructural features. In addition, the increased directionality of neutral fluxes not only offsets the RIE lag, but also further causes a weak inverse RIE lag without relying on surface inhibitors. Experimental techniques are also discussed for generation of such cold, directional beams of reactive plasmas.

New-type microwave plasma source excited by azimuthally symmetric surface waves with magnetic multicusp fields

A new-type microwave plasma source has been developed for materials processing. The plasma reactor employed a launcher of azimuthally symmetric surface waves at a frequency of 2.45 GHz and also magnetic multicusp fields around the reactor chamber walls. This configuration yielded high-density (Ne≳1011 cm−3) plasmas sustained by surface waves even at low gas pressures below 10 mTorr, following easy plasma ignition by electron cyclotron resonance (ECR) discharges. Electrical and optical diagnostics were made to obtain the plasma properties in Ar. It was shown that a transition occurs from ECR exited to surface-wave excited plasmas under conditions where the plasma electron density exceeds a critical value of Ne∼1×1011 cm−3.