Plasma-induced Damage Research Articles

Performance evaluation of GaN etching using Cl2-based plasma with bias pulsing

Reducing plasma-induced damage (PID) is one of the most challenging goals for the fabrication of GaN-based MOS-HEMT. In this paper, we propose a performance evaluation of a Cl2-based etching chemistry using bias pulsing mode for GaN applications. The plasma-induced damage using bias pulsing has been compared to conventional reactive ion etching (RIE) and atomic layer etching (ALE) processes using sheet resistance (Rsheet) measurements. This pulsing mode showed low plasma-induced damage, similar to ALE. In addition, it keeps an acceptable GaN etching rate, showing that pulsing mode has potential for industrial applications.

Optical and electrical evaluation methods of plasma-induced damage in InP substrates

Indium phosphide (InP) has been focused on as one of the emerging materials that can be implemented in advanced semiconductor devices. We proposed optical and electrical characterization methods to evaluate plasma-induced physical damage (PPD)—ion bombardment damage—to InP substrates. By introducing a native oxide phase in an interfacial layer, we proposed an optical model of the damaged structure applicable for in-line monitoring by spectroscopic ellipsometry. Gas species dependence was obtained, which suggested that the H2 plasma exposure formed a thicker damaged layer than Ar. Impedance spectroscopy (IS) under various biases (V b) was implemented to reveal the nature of damaged structures. Capacitive and conductive components assigned by the IS were confirmed to depend on incident species from plasma, indicating the difference in the energy profile of created defects. The presented methods are useful to characterize and control PPD in designing future high-performance InP-based devices.

Demystifying metal-assisted chemical etching of GaN and related heterojunctions

GaN and related semiconductors have become an increasingly prominent material for a wide range of active and passive devices from optoelectronics to high frequency and power electronics as well as photocatalysis. Regardless of the application, anisotropic etching is required for micro and nano structuring, currently performed by reactive ion etching (RIE). Alternately, metal-assisted chemical etching (MacEtch) is an open-circuit plasma-free anisotropic etching method that has demonstrated high aspect ratio device structures devoid of plasma-induced damage found in RIE. This paper presents an in-depth study of the ensemble electrochemical mechanisms that govern the photo-enhanced MacEtch process of GaN and related heterojunctions. Through in-depth experimental investigations, modeling and simulations, the effects of local cathode and anode design, energy-band alignments, and solution chemistry on MacEtch are correlated with the underlying electronic mechanisms of carrier generation, annihilation, transport, and extraction, establishing a fundamental framework for parametrized prediction of system behavior. These findings carry profound implications for tailored design of photoelectrochemical processes employed not just for uniformly etching wide/ultrawide bandgap materials but more broadly for semiconductor-based photocatalytic reactions in general. One-pot photo-enhanced MacEtching of AlInGaN multi-heterojunction device structures including superlattices and multi-quantum wells are demonstrated.

Impact of vacuum ultraviolet photons on ultrathin polymethylmethacrylate during plasma etching

State-of-the-art extreme ultraviolet lithography requires the use of ultrathin photoresists (or resists) due to pattern stability concerns and reduced depth of focus of the extreme ultraviolet lithography scanners. Current resists for extreme ultraviolet lithography are less than 50 nm thick. These ultrathin resists further complicate pattern transfer as unintended plasma-induced damage during dry etching is more pronounced. A better understanding of the interaction of plasma species with ultrathin resists is critical for enabling pattern transfer of sub-10 nm features. Here, we study the impact of vacuum ultraviolet photons, argon ions, and argon plasma on a 40 nm thick polymethylmethacrylate film. Using a deuterium lamp, an industrial ion beam etch tool, and an industrial inductively coupled plasma etch tool, we exposed the polymer to photons, ions, and plasma, respectively. The exposed samples were then analyzed for chemical and physical changes using different characterization techniques. It was observed that the vacuum ultraviolet photons interact with the entire bulk of polymer film, while the ions only affect the surface and subsurface region. The photon exposed samples formed smaller polymer fragments at low exposure doses and further started to cross-link at high doses. In contrast, the ion modification leads to carbonization of only the top few nanometers of the polymer film, leaving the bottom bulk intact. The plasma exposed sample showed changes characteristic to both vacuum ultraviolet photons and ions and their synergism. It was stratified with a 1.34 ± 0.03 nm thick ion-caused carbonized layer on top of a 13.25 ± 0.12 nm photon-induced cross-linked layer. By studying the impact of plasma photons on ultrathin polymethylmethacrylate, we were able to establish a baseline for a testing methodology that can be extended to novel ultrathin resist platforms.

Plasma-Assisted N2O Oxidation (PANO) in an Industrial Direct Plasma Reactor for TOPCon Production

Plasma-Enhanced Chemical Vapor Deposition (PECVD) is an attractive tool for TOPCon production, as it enables uniformly in situ doped amorphous silicon (a-Si) and dielectric layer depositions with high throughput. However, a lean process requires in situ interfacial oxide growth in the same tool. In this work, we use Plasma-Assisted N2O Oxidation (PANO) in an industrial kHz direct plasma reactor (centrotherm c.PLASMA) to grow the oxide and deposit in situ phosphorus doped a-Si(n) as well as SiNx on asymmetric lifetime samples. Before optimization, the oxide thickness is non uniform on the wafer, and we show that it correlates with the passivation, the contact resistivity, and the doping profile in n-type TOPCon test structures. The passivation seems to benefit more from moderate in-diffusion in the case with PANO than in the case with thermal oxidation. This is probably due to enhanced field-effect passivation compensating for lower chemical passivation, which likely results from plasma-induced damage. After studying the influence of PANO process parameters on the oxide thickness and uniformity, we optimize them to obtain a non-uniformity as low as ±2% and a recombination current density down to 2.3 fA/cm² on planar wafers.

Polymeric electronic materials for microelectronics manufacturing: A review

This review paper is part of a series of papers dedicated to the centenary celebration of the American Chemical Society Division of Polymeric Materials: Science and Engineering which has been the scientific home of many polymer scientists and engineers in the semiconductor industry. It is a review of parts of the research and development of advanced polymeric electronic materials for the fabrication and manufacturing of modern integrated circuits at the International Business Machines Corporation. This paper covers three types of advanced polymeric electronic materials: (1) photoresist materials used to “print” integrated circuits on silicon wafers, (2) on-chip low dielectric constant (low-κ) materials to insulate electrically conducting copper wires and (3) photo-patternable on-chip low-κ materials which function both as a photoresist and as an on-chip low-κ material. The photoresist materials are thin film imaging material systems of bilayer photoresist materials for 248 nm lithography and trilayer photoresist materials for 193 nm lithography. The low-κ dielectric material part focuses on fundamental understanding of plasma-induced damage to nanoporous organosilicate low-κ dielectric materials and the design of damage-resistant low-κ dielectric materials. Finally, the photo-patternable low-κ dielectric materials part describes the design and the development of dual-functional photo-patternable low-κ dielectric materials for both 193 nm and 248 nm lithography patterning of semiconductor Back-End-Of-the-Line. Several of these materials have been adopted for high-volume manufacturing of semiconductor chips for some of the most popular electronic devices in the world today.

Photo-Enhanced Inverse Metal-Assisted Chemical Etching of α-Ga2O3 grown on Al2O3

Si-based semiconductor devices are not suitable for stable operations in the aerospace environments. Among the ultra-wide bandgap semiconductor materials, gallium oxide (Ga2O3) is attracting attention as the next-generation material for high-power semiconductor devices in extreme condition beyond Si, owing to its large band gap of 4.5-5.3 eV, high breakdown electric fields of 7-10 MV/cm, excellent chemical and thermal stability and radiation hardness. Alpha gallium oxide (α-Ga2O3) has the largest bandgap (4.8-5.3 eV) and the highest breakdown electric fields (~10 MV/cm) among the five Ga2O3 polymorphs, which facilitates the application of α-Ga2O3 as a high power device. However, the research on etching technology for α-Ga2O3 is rare. Etching technology is a crucial step of device fabrication. Chemical etching is free from plasma-induced damage which leads to superior device performance compared with dry etching, and high throughput with low cost is beneficial to industrial implementations. Chemical stability of Ga2O3 hinders the effective reaction with most chemical etchants. However, the photo-enhanced inverse metal assisted chemical (I-MAC) etching, where a noble metal with a high work function is utilized as a catalyst under UV irradiation, has emerged as a candidate for the chemical etching of Ga2O3. Deep-UV irradiation generates electron-hole pairs (EHPs) in the uncovered region of Ga2O3, and the metal electrode withdraws carriers from the photo-generated EHPs. The holes that are accumulated in the uncovered region of Ga2O3 react with gallium ions which produce gallium fluoride (GaF3) in a reaction with Hydrofluoric acid (HF). The etching process continues as the oxidant re-oxidizes the metal. The etch rate can be controlled by various parameter, including concentration and temperature of the etchant. In this work, for the I-MAC etch of α-Ga2O3 on sapphire substrate grown by halide vapor phase epitaxy, Pt was deposited on α-Ga2O3. Etch rate and surface roughness were characterized by atomic force microscopy after each step of the I-MAC etch. The I-MAC etch using HF and potassium persulfate (K2S2O8) solution with 185-nm UV irradiation was performed at constant durations under different etchant temperature conditions. The etch rates obtained at each temperature condition were fitted with the Arrhenius plot to estimate the activation energy of 0.898 eV. The etch depth showed a linear increase with time and the etch rate exhibits a direct dependency on the temperature of the etchant, indicating that the I-MAC etching of α-Ga2O3 is an activation-controlled reaction. As the I-MAC etching proceeded, surface roughness of α-Ga2O3 showed a tendency to increase. The rate at which the surface roughness increased increased with raising the etchant temperature. In the case of Pt there was no significant difference in the surface roughness after etching. After the completion of the I-MAC etch, the remaining Pt can be removed using aqua regia. The I-MAC etch process of α-Ga2O3 can allow us to fabricate α-Ga2O3 without plasma-damage. Figure 1

Mobility Enhancement of BCE-Type Amorphous InGaZnO TFTs Using Triple-Layer Channels

The back-channel-etch-type amorphous InGaZnO (a-IGZO) triple-layer thin-film transistor (TL-TFT) consists of a top barrier and a bottom barrier deposited with oxygen flow (OF) and an a-IGZO main channel deposited without OF in between. The TL-TFT has 1.7 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$</tex-math> </inline-formula> and 1.3 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times$</tex-math> </inline-formula> electron mobilities for the bottom gate (BG) operation as compared to the single-layer and double-layer channel TFTs, respectively. The conduction band difference between the barrier layers and the main channel is high enough to confine the carriers in the main channel. The bottom barrier decreases the Coulomb scattering and the surface roughness scattering to increase the mobility, while the top barrier decreases the plasma-induced damage in the channel and the bottom barrier. However, both the hysteresis and subthreshold swing (S.S.) increase if only the bottom barrier is adopted double-layer TFT (DL-TFT) because the shallow states in the IGZO bottom barrier provide extra tunneling paths for the electrons to be trapped inside the BG oxide. With additional top barrier structure, plasma-induced damage in the main channel and the bottom barrier can be mitigated to reduce the degradation of hysteresis and S.S. Moreover, the TL-TFTs demonstrate better reliability under both positive bias stress and negative bias illumination stress than single-layer TFTs (SL-TFTs) and DL-TFTs.

Defects in Mitochondrial Functions Affect the Survival of Yeast Cells Treated with Non-Thermal Plasma.

Exposure of living cells to non-thermal plasma produced in various electrical discharges affects cell physiology and often results in cell death. Even though plasma-based techniques have started finding practical applications in biotechnology and medicine, the molecular mechanisms of interaction of cells with plasma remain poorly understood. In this study, the involvement of selected cellular components or pathways in plasma-induced cell killing was studied employing yeast deletion mutants. The changes in yeast sensitivity to plasma-activated water were observed in mutants with the defect in mitochondrial functions, including transport across the outer mitochondrial membrane (∆por1), cardiolipin biosynthesis (∆crd1, ∆pgs1), respiration (ρ0) and assumed signaling to the nucleus (∆mdl1, ∆yme1). Together these results indicate that mitochondria play an important role in plasma-activated water cell killing, both as the target of the damage and the participant in the damage signaling, which may lead to the induction of cell protection. On the other hand, our results show that neither mitochondria-ER contact sites, UPR, autophagy, nor proteasome play a major role in the protection of yeast cells from plasma-induced damage.

Sidewall Modification Process for Trench Silicon Power Devices

In this study, trench sidewall modification processes were designed to improve profile uniformity and thereby enhance the electrical performance of silicon power devices in large-scale production. The effects of trench sidewall modification on the morphology, structure and electrical properties were studied. Plasma-induced damage in etching processes was also observed and briefly explained. Straight and smooth sidewall profiles were achieved through adjusting the SF6/CHF3 proportion in a combined etchant gas flow in the main etching procedure. By comparing HRSEM images from different etching protocols, it was evident that an enhanced CHF3 flow formed a proper passivation of the sidewall, eliminating the ion damages that are common in current main etch steps. To address the impurities introduced from the etchant gas and improve the gate oxide uniformity, further steps of depolymerization were applied in a plasma asher chamber, followed by wet clean steps. In the meantime, the plasma-induced charge accumulation effect was reduced by UV curing. Improved trench sidewall profiles and the gate oxide uniformity contributed to a lower leakage current between the gate and source terminals, leading to an overall yield enhancement of device properties in large-scale silicon wafer fabrication.

Novel Mg- and Ga-doped ZnO/Li-Doped Graphene Oxide Transparent Electrode/Electron-Transporting Layer Combinations for High-Performance Thin-Film Solar Cells.

Herein, a novel combination of Mg- and Ga-co-doped ZnO (MGZO)/Li-doped graphene oxide (LGO) transparent electrode (TE)/electron-transporting layer (ETL) has been applied for the first time in Cu2 ZnSn(S,Se)4 (CZTSSe) thin-film solar cells (TFSCs). MGZO has a wide optical spectrum with high transmittance compared to that with conventional Al-doped ZnO (AZO), enabling additional photon harvesting, and has a low electrical resistance that increases electron collection rate. These excellent optoelectronic properties significantly improved the short-circuit current density and fill factor of the TFSCs. Additionally, the solution-processable alternative LGO ETL prevented plasma-induced damage to chemical bath deposited cadmium sulfide (CdS) buffer, thereby enabling the maintenance of high-quality junctions using a thin CdS buffer layer (≈30nm). Interfacial engineering with LGO improved the Voc of the CZTSSe TFSCs from 466 to 502mV. Furthermore, the tunable work function obtained through Li doping generated a more favorable band offset in CdS/LGO/MGZO interfaces, thereby, improving the electron collection. The MGZO/LGO TE/ETL combination achieved a power conversion efficiency of 10.67%, which is considerably higher than that of conventional AZO/intrinsic ZnO (8.33%).

Electron beam-induced etching of SiO2, Si3N4, and poly-Si assisted by CF4/O2 remote plasma

Electron-stimulated etching of surfaces functionalized by remote plasma is a flexible and novel approach for material removal. In comparison with plasma dry etching, which uses the ion-neutral synergistic effect to control material etching, electron beam-induced etching (EBIE) uses an electron-neutral synergistic effect. This approach appears promising for the reduction of plasma-induced damage (PID), including atomic displacement and lateral straggling, along with the potential for greater control and lateral resolution. One challenge for EBIE is the limited selection of chemical precursor molecules that can be used to produce functionalized materials suitable for etching under electron beam irradiation. In this work, we studied a new experimental approach that utilizes a remote plasma source to functionalize substrate surfaces in conjunction with electron beam irradiation by an electron flood gun. Etching rates (ERs) of SiO2, Si3N4, and poly-Si are reported in a broad survey of processing conditions. The parametric dependence of the ER of these Si-based materials on the operating parameters of the flood gun and the remote plasma source is evaluated. We also identified the processing parameters that enable the realization of material selective removal, i.e., the etching selectivity of Si3N4 over SiO2 and poly-Si over SiO2. Additionally, surface characterization of etched materials is used to clarify the effects of the co-introduction of particle fluxes from the remote plasma and flood gun sources on surface chemistry.

Fabrication, and Direct Current and cryogenic analysis of SF6-treated AlGaN/GaN Schottky barrier diodes

Schottky contacts on fluorine implanted AlGaN/GaN heterostructures with the ideality factor close to unity and low on-voltage threshold are presented in this paper. An SF6 plasma anode pretreatment followed by a specific low-temperature annealing is also compared to a nonannealed sample. In addition, physical-model parameters are extracted by means of cryogenic temperature measurements to understand the conduction mechanisms involved in annealed diodes, showing better DC performances than their nonannealed counterparts. Furthermore, annealing induces a decrease of the ideality factor, which sets the field-enhanced thermionic emission as the main conduction mechanism, and reduces the tunneling reverse current leakage. This effect is attributed to the recovery of the plasma-induced damages.

Predicting the effects of plasma-induced damage on p–n junction leakage and its application in the characterization of defect distribution

Understanding the effects of defect creation during plasma exposure is crucial for designing future ultra-low leakage current devices. Created defects play a role as carrier conduction paths, leading to an increase in the p–n junction leakage current (ΔJpn). Herein, we propose a model focusing on the effects of created defects on ΔJpn in combination with technology computer-aided design simulations. Three different defect spatial distributions, ndam(x), (linear, exponential, and Gaussian) were implemented for predicting ΔJpn under various operating conditions. It was confirmed that ΔJpn is strongly dependent on ndam(x), in addition to the total number of defects and generally accepted energy levels. The prediction model was implemented to experimentally assign the profile of defects created by an emerging mechanism—lateral straggling of incident ions—under fluorocarbon-containing plasma exposure, which is commonly employed in electrical contact opening steps. Devices with various lateral p–n junction widths and contact opening areas were exposed to the plasma. The experimental results of ΔJpn revealed that the profile of defects in the lateral direction was exponential. The proposed model prediction scheme is useful for designing plasma processes and circuit layouts to realize future ultra-low leakage current devices.

Revealing low-temperature plasma efficacy through a dose-rate assessment by DNA damage detection combined with machine learning models

Low-temperature plasmas have quickly emerged as alternative and unconventional types of radiation that offer great promise for various clinical modalities. As with other types of radiation, the therapeutic efficacy and safety of low-temperature plasmas are ubiquitous concerns, and assessing their dose rates is crucial in clinical settings. Unfortunately, assessing the dose rates by standard dosimetric techniques has been challenging. To overcome this difficulty, we proposed a dose-rate assessment framework that combined the predictive modeling of plasma-induced damage in DNA by machine learning with existing radiation dose-DNA damage correlations. Our results indicated that low-temperature plasmas have a remarkably high dose rate that can be tuned by various process parameters. This attribute is beneficial for inducing radiobiological effects in a more controllable manner.

Investigation of a facile plasma-driven method for in situ cleaning of metal-based contamination

Self-cleaning of tin contaminants was realized utilizing a self-driven hydrogen plasma. Cleaning rates of 0.7–6 nm min−1 were achieved for removal of discontinuous tin particles at different powers. The analysis of topography and cross-sectional morphology revealed that the removal of tin particles was achieved through top-down cleaning with hydrogen plasma, where the upper part of spherical tin particles was always more intensely cleaned under the synergistic effect of hydrogen atoms and ions due to the vertical incidence of ions to the substrate during the whole cleaning process. Redeposition of tin atoms caused by physical sputtering and its promotion of the chemical cleaning effect was observed for the first time. Reflectance recovery measurements during cleaning and surface analysis of the substrate after cleaning indicated that nondestructive cleaning with a reflectance loss of less than 1% can be achieved at a relatively low power of 120 W. Plasma-induced substrate damage, such as holes and valleys, reduced the reflectance of the substrate when cleaning was performed at a high power greater than 120 W, so this method should only be considered for application under conditions without substrate exposure. This study provides a comprehensive understanding of the removal of discontinuous tin particles using the in situ self-driven plasma cleaning method, and also provides meaningful guidance for the extension of this method in other potential fields of application.

Bactericidal effects of low-temperature atmospheric-pressure air plasma jets with no damage to plant nutrient solutions

The bactericidal effects of air plasma jets produced with a twisted wire-cylindrical electrode configuration were clarified in terms of plasma-induced damage to plant nutrient solutions. The bacterial suspensions were directly irradiated with air plasma jets using a low gas flow rate, which was shown to significantly inactivate the bacteria suspended in the solutions without reducing the nutrient concentrations. However, the plasma irradiation time required for inactivation depended on the type of bacteria; Escherichia coli (E. coli) and Bacillus subtilis (B. subtilis) were inactivated in 20–30 s, while Staphylococcus aureus (S. aureus) required 7 min. The inactivation of E. coli and B. subtilis decreased with increasing air gas flow rate, whereas the inactivation of S. aureus was independent of the rate. The inactivation could be attributed to a greater number of reactive oxygen and nitrogen species (RONS) from the air plasma jet, including O2 molecules in the feeding gas attaching to the bacterial suspension surface, which do not harm the nutrient components. This can be derived from the results; the air-plasma-jet-activated nutrient solutions (RONS introduced in the solutions) and the N2 plasma jets had only a limited inactivation effect on the bacteria suspended in the solutions.

Plasma–induced damage in magnetic tunneling junctions

Plasma–induced damage (PID) in the fabrication of magnetic tunnel junction (MTJ) based magnetic random access memory (MRAM) is studied after the formation of MTJ devices, which excludes process effect directly from the ion beam etching and protection layer deposition. Large degradation in coercivity (Hc) of MTJs is found in comparison with that measured just after the top metal （TM） was formed, which heavily deteriorates MRAM performances. This Hc degradation is found to be mainly introduced in plasma-enhanced chemical vapor deposition (PECVD) for passivation (PA) film after TM formation. The amplitude of the degradation depends on plasma power, but independent of the composition of the PA film. Hc reduction is observed once the plasma is turned on even though no PA deposition is performed. These results suggest that the observed Hc degradation is from PID effect most likely due to the plasma charge damage (PCD) induced in the PECVD process as no direct plasma contact with MTJ devices is involved in the process. It is argued that this PCD effect is due to the charge accumulations at the interface between the free layer and MgO barrier based on electric field–modulated magnetic anisotropy effect. This is further confirmed by an experiment in which Hc degradation is largely cured through partially removing the trapped charges by an annealing process.

The Performance Enhancement of PMOSFETs and Inverter Chains at Low Temperature and Low Voltage by Removing Plasma-Damaged Layers

In this work we report on the improvement in cold temperature characteristics of PMOSFETs and inverter circuits by removing the plasma-damaged layer of the source/drain contacts. We removed the plasma-induced damage on the Si using a simple in situ Si soft treatment technique. We found by transmission electron microscope (TEM) analysis that the damaged amorphous layer reduced from 52 Å to 42 Å and 35 Å with a treatment time of 10 and 20 s, respectively. As a result, the resistances of both the n+ and p+ contacts decreased for all contact sizes and the standard deviations at the cold temperature were suppressed by 45%. At −25 °C, the saturation current of the PMOSFET increased by 3% and the propagation delay time (tPD) decreased by 2%. The tPD increases by 19.3% when the temperature decreases from 85 °C to −25 °C, and the operating voltage decreases from 1.2 V to 0.95 V at the same time. However, this increase can be reduced to 17% by applying the soft treatment for 10 s. This simple and short time process will be considered essential for both mobile applications and automotive applications of dynamic random access memory (DRAM) devices requiring a low-voltage and low-temperature operation.

Plasma-activated GaAs/Si wafer bonding with high mechanical strength and electrical conductivity

Bonded GaAs/Si heterojunctions with high mechanical strength and electrical conductivity are important for low-cost photovoltaics and silicon-based photonics. In this work, we successfully achieved wafer-based (2 in.) GaAs/Si direct bonding structures with high strength by using the plasma-activated bonding (PAB) method. The bonding strength can be close to that of the GaAs/Si samples bonded by the surface-activated bonding method. It is found that the surface roughness and height of GaAs wafers have a greater influence on the bonding strength than Si wafers. More importantly, an ohmic n-GaAs/n-Si interface even with a low doping concentration of wafers was realized. Investigations reveal that the Ar plasma-induced damage in GaAs could be controlled at a significantly low level by using the PAB method. An thin amorphous layer existed between GaAs/Si interface, and no obvious cracks or voids were observed. Our results may provide a promising way to fabricate GaAs/Si heterojunctions for optoelectronic devices.