RIE Plasma Research Articles

Hydrophilic Bonding of SiO2/SiO2 and Cu/Cu using Sequential Plasma Activation

To meet the increasing market demand for next-generation electronic devices, 3D system integration is getting greater and greater attention. However, traditional methods such as soldering or bump bonding cannot handle high density interconnection. Therefore, hybrid bonding technology, which simultaneously bonds Cu wiring layer and SiO2 dielectric layer, is indispensable. In the hybrid bonding process, hydrophilic bonding is commonly used to bond SiO2 to SiO2. This technique involves bonding of hydrophilic bonding surfaces, followed by post-bonding annealing to achieve a reliable bonding interface. For Cu bonding, it is necessary to break the oxide layer at the Cu bonding interface and allow Cu atoms to diffuse into each other to achieve electrical connection.However, since post-bonding annealing at high temperatures can damage the device or induce residual stress at the bonding interface, low temperature process below 200°C is preferable. Currently, the most widely adopted process is hydrophilic bonding using N2 plasma, which also achieves hybrid bonding through heating at around 350°C to 400°C.In this study, we propose to apply sequential plasma activation bonding (SPAB), which has been shown to be effective for low-temperature bonding of quartz glass, to the bonding of TEOS SiO2 and Cu. SPAB is a technique that forms a highly reactive oxynitrides on the substrate by continuously irradiating the bonding surface with O2 plasma, N2 plasma, and nitrogen radicals. This then adsorbs a large amount of OH groups by reacting with moisture, leading to a strong bond after bonding and post-bonding annealing at 200°C. Since this technique is effective for quartz, it is expected to be effective for TEOS SiO2 as well, and we also investigate its effect on Cu in this study.For this, we investigated the effect of SPAB for blanket TEOS SiO2 and Cu wafer separately. Cu layers were deposited on 4 inch, 525 µm thick Si wafers, and TEOS SiO2 layers on 6 inch, 625 µm thick Si wafers. The Cu layers were deposited by sputtering with a 1 µm thickness, while the SiO2 layers were deposited using plasma-enhanced TEOS process for a 1 µm thickness. Both types of deposited layers were planarized by CMP.The SPAB process was carried out using RIE plasma and microwave radicals of N2 and O2 gas. The bonding surfaces were activated by O2 plasma, N2 plasma, and N radicals sequentially. After the surface activation, the wafers were exposed to ambient air to adsorb water on the surface, followed by bonding in air and post-bonding annealing at 200°C for 2 hours.For comparison, bonding was also carried out using only N2 plasma and only O2 plasma activation, respectively. Results show that when surface activation was performed using only N2 plasma, the bond strength was 0.75 J/m2 for TEOS SiO2 and 0.33 J/m2 for Cu. In the case of O2 plasma activation, the bond strength was 0.57 J/m2 for TEOS SiO2 and 0.98 J/m2 for Cu.On the other hand, the SPAB technique improves the bond strength to 1.13 J/m2 for TEOS SiO2 and 1.03 J/m2 for Cu. Therefore, it can be concluded that the SPAB technique enhances the bond strengths of both TEOS SiO2 and Cu compared to single gas plasma activation. Since the SPAB improves the bond strength of both SiO2 and Cu in the same process condition with annealing temperature of 200°C, this technique will contribute low temperature hybrid bonding applications.

First-Principles Calculation Guided High-Purity Layer Control of 4 in. MoS2 by Plasma RIE

Two-dimensional molybdenum disulfide (MoS2) is one of the most promising candidates for next-generation semiconductors. Among the advantages offered by MoS2, a tunable bandgap that depends on the thickness is essential for the on-demand manufacturing of nanoelectronics. For this reason, elaborate layer control of MoS2 has been a long-standing research objective. However, prior efforts had several critical issues including surface roughness, poor uniformity/scalability, and impurities. Through this study, we aimed to achieve both ultrahigh precision and purity in large-scale (4 in.) layer control of MoS2 by two consecutive plasma processes: plasma-enhanced chemical vapor deposition (PECVD) and reactive ion etching (RIE). The 4 in. wafer-scale MoS2 was synthesized by PECVD, and the as-grown bulk layers were etched using RIE with a computationally screened gas mixture in the cyclic step. For every RIE cycle, the 4 in. MoS2 wafer was evenly etched to a thickness of 0.3–0.4 nm while there was no damage to the atomic structure and chemical impurities. For the computational screening of candidate gases, first-principles calculations explored the energetics of surface reaction and offered physical insights into the associated electronic interaction. The combination of computational screening and experimentation accelerates optimal process design and provides an in-depth understanding of the plasma–surface interactions.

Towards Long-Term Stable Polyimide-Based Flexible Electrical Insulation for Chronically Implanted Neural Electrodes

For chronic applications of flexible neural implants, e.g., intracortical probes, the flexible substrate material has to encapsulate the electrical conductors with a long-term stability against the saline environment of the neural tissue. The biocompatible polymer polyimide is often used for this purpose. Due to its chemical inertness, the adhesion between two polyimide layers is, however, a challenge, which can lead to delamination and, finally, to short circuits. The state-of-the-art method to improve the adhesion strength is activating the polyimide surface using oxygen reactive ion etching (O2 RIE). However, the influence of the process variations (etching time, bias power) on the long-term stability is still unclear. Therefore, we establish a test method, where the aging of a gold interdigital structure embedded in two polyimide layers and immersed in saline solution is accelerated using an elevated temperature, mechanical stress and an electrical field. A continuous measurement of a leakage current is used to define the failure state. The results show that the variation of the O2 RIE plasma process has a significant effect on the long-term stability of the test samples. Comparing the two different plasma treatments 0.5 min at 25 W and 1 min at 50 W, the long-term stability could be increased from 20.9 ± 19.1 days to 44.9 ± 18.9 days. This corresponds to more than a doubled lifetime. An ideal solution for the delamination problem is still not available; however, the study shows that the fine-tuning of the fabrication processes can improve the long-term stability of chronically implanted neural electrodes.

XPS study of a selective GaN etching process using self-limiting cyclic approach for power devices application

Full recess architecture for GaN based High Electron Mobility Transistor (HEMT) enables high mobility and high density of electrons to be conserved without compromising on voltage threshold. To obtain such architecture, standard RIE plasma etching processes are not suitable due to electrical degradation effects and to the lack of well controlled etch depth. To address these limitations, several Atomic Layer Etching (ALE) processes have been developed in the past. In this work, we study an atomic layer etching (ALE) of GaN based on cyclic steps composed of O2 plasma followed by BCl3 plasma. XPS analysis of the GaN's top surface after each step of the O2-BCl3 process enabled to propose an etching mechanism and to track nitrogen depletion. The cyclic process conserves the surface stoichiometry. Finally, this cyclic process has been validated on patterns, showing a good morphology, a good etch depth control and a slightly lower electrical degradation compared to standard RIE process.

Fabrication of low grass, smooth sidewall InGaAsP by methane–hydrogen inductively coupled plasma RIE through a metal lift-off mask patterned by e-beam lithography

An optimized CH4+H2 inductively coupled-plasma reactive-ion-etching process for fabrication of high quality nanophotonic InGaAsP/InP waveguide device has been developed. Important parameters that influence e-beam lithography and etching have been optimized for a simple metal lift-off masked process. Smooth sidewalls with low grass content has been demonstrated for >3 μm etch depths in InGaAsP/InP using an Al lift-off etch mask.

Exploration of Chemical Bonding Transformation Mapping to Assist Low-k Dielectric Nanostructure Fabrication

In leading edge integrated circuit manufacturing, reduction of RC time delay by incorporation of porous ultra low-k interlayer dielectrics into Cu interconnect nanostructure continues to pose major integration challenges. The main challenge is that porous structure renders interlayer dielectrics mechanically weak, chemically unstable and more susceptible to the RIE plasma etching damages. Besides the challenge of handling weak porous ultra low-k materials, a lack of sensitive metrology to guide systematic development of plasma etching, restoration and cleaning processes is the major stumbling block. We explored Multiple Internal Reflection Infrared Spectroscopy and associated IR techniques as a sensitive (sub-5 nm) characterization tool to investigate chemical bonding modification across fluorocarbon etch residues and low-k dielectric interface after plasma etching, ashing, UV curing and post-etch cleaning. The new insights on chemical bonding transformation mapping can effectively guide the development of clean-friendly plasma etch for creating ultra low-k dielectric nanostructures with minimal dielectric damages.

Exploration of Chemical Bonding Transformation Mapping to Assist Low-k Dielectric Nanostructure Fabrication

In leading edge integrated circuit manufacture, reduction of RC time delay by incorporation of porous ultra-low k (ULK) interlayer dielectrics (ILD) into Cu interconnect nanostructure continues to pose major integration challenges. The main challenge is porosity induced to lower the dielectric constant of the ILD has the undesirable side effect of compromising its mechanical and chemical stability. As a result, these materials are more susceptible to the damages caused by RIE plasma etching processes. Besides the challenge of handling weak porous ULK materials, a lack of sensitive metrology to guide systematic development of plasma etching, restoration and cleaning processes is the major stumbling block. To assist in the overall integration effort, we have been actively exploring Multiple Internal Reflection Infrared Spectroscopy (MIR-IR) and associated IR spectroscopic techniques as a sensitive (sub-5nm) characterization tool to assist the fabrication process of ULK nanostructure with targeted critical dimension. In this paper, we will report new findings obtained on chemical bonding modification across fluorocarbon etch residues and low-k dielectric interface after manufacture relevant processing treatments including plasma etching, ashing, UV curing and post-etch cleaning. MIR-IR metrology showed useful spectroscopic details of chemical bonding transformations taking place during plasma etch and strip processing on both blanket and dielectric trench structures. The observed increase of Si-OH and C=O and the decrease of Si-CH3 IR absorption bands correlated well with increasing plasma induced damage, K increases verified by MIS structure, especially for more aggressive stripping process. With functional group-specific chemical derivatization, chemical bonding structure of 1-2 nm post-etch residue on porous low-k trench structure was established. UV-induced modification of fluorocarbon polymer residue to improve its subsequent wet cleaning removal was studied by FTIR, XPS and SEM. The overall goal is to apply new insights on chemical bonding transformation mapping to achieve cleans-friendly plasma etches on creating ULK dielectric nanostructures with minimal dielectric damages.

Dimensionally controlled complex 3D sub-micron pattern fabrication by single step dual diffuser lithography (DDL)

This article presents a facile single step fabrication method of complex 3D microstructures like microtips, microlenses, microtrapezoids, etc. with controlled dimensions which has been the main focus of researchers over the decades. Each type of above mentioned microstructures individually requires a large number of expensive and complex fabrication processes e.g. plasma RIE, stereo lithography, UV micro stamping and so on. In this study, we have proposed a simple, convenient, cost effective and commercially applicable one step dual diffuser lithography (DDL) method for the fabrication of such complex 3D microstructures by adding a pair of diffusers in conventional photolithography, which diffused the incident beam of ultraviolet (UV) light at wide angles. A conventionally used positive photoresist AZP 4620 was exposed to the diffused light at various exposure energies and the effect of change in exposure energy on the fabricated patterns was studied. Patterns with sub-micron dimensions and microtips with ∼200nm tip size were also fabricated using the proposed DDL process. The morphology of the fabricated patterns was analyzed using the field-emission scanning electron microscope (FE-SEM) images and 3-dimensional (3D) profiler data.

Oxygen Plasma and Humidity Dependent Surface Analysis of Silicon, Silicon Dioxide and Glass for Direct Wafer Bonding

Surface and interface characteristics of substrates are critical for reliable wafer bonding. Understanding the elemental and compositional states of surfaces after various processing conditions is necessary when bonding dissimilar materials. Therefore, we investigated the elemental and compositional states of silicon (Si), silicon dioxide (SiO2) and glass surfaces exposed to oxygen reactive ion etching (O2 RIE) plasma followed by storage in controlled humidity and/or ambient atmospheric conditions to understand the chemical mechanisms in the direct wafer bonding. High-resolution X-ray Photoelectron Spectroscopy (XPS) spectra of O2 RIE treated Si, SiO2 and glass showed the presence of Si(-O)2 resulting in highly reactive surfaces. A considerable shift in the binding energies of Si(-O)2, Si(-O)4 and Si(-OH)x were observed only in Si due to plasma oxidation of the surface. The humidity and ambient storage of plasma activated Si and SiO2 increased Si(-OH)x due to enhanced sorption of hydroxyls. The amounts of Si(-O)2 and Si(-OH)x of Si varied in different humidity storage conditions which are attributed to crystal-orientation dependent surface morphology and oxidation. The O2 RIE plasma induced high surface reactivity and humidity induced Si(-OH)x can play an important role in the hydrophilic wafer bonding with low temperature heating.

Surface and Interface Characterization of Sequentially Plasma Activated Silicon, Silicon dioxide and Germanium Wafers for Low Temperature Bonding Applications

This article reports the sequentially plasma activated bonding (SPAB) of n-Ge with p-Si and SiO2 at low temperature. Surface activation resulted in highest hydrophilicity of Ge compared with Si and SiO2 counterparts. The highest hydrophilicity of Si, Ge and SiO2 induced by O2 RIE plasma was combined with their highest reactivity induced by MW N2 radicals while maintaining smooth surface roughness. Weak bonding strength of Si/Ge and SiO2/Ge in the SPAB at room temperature was improved after heating at 200°C, but they were still lower than that of Si/Si in the SPAB at room temperature, which is due to the unique reactivity of Ge. The deviation of the reverse bias behavior from a typical p-n junction is due to the low doping concentration in Ge. The degradation of current in the sequential heating resulted mainly from the oxidized surfaces of Ge and Si as well as the bonded interface.

Mechanism of porous low- k film damage induced by plasma etching radicals

The etching damage on nano-clustering silica (NCS) film due to etching radicals was investigated using a method of radical treatments in RIE plasma. NCS coated-side of the wafer was turned downward and put at 0.65 mm above the wafer stage to investigate only the influence of radicals. Etching radicals, which comes from CF 4, diffuse into NCS film and reduce Si–CH 3 bonds and Si–CH 3 loss is proportional to the amount of diffused fluorine in NCS film. Several Si–CH 3 bonds are converted to Si–F bonds then. As a result, the low- k performance is degraded and especially the leakage current heavily increases. We proposed a method for estimating the degree of the sidewall damage due to etching radicals using blanket wafers. The degree of sidewall damage is proportional to the value of CR −0.5, where C is the damage diffusion coefficient, which is derived from Si–CH 3 decrement ratio from a radical treatment result and R is the etching rate, which is derived from a RIE treatment result under the same plasma condition. The value of CR −0.5 depends on the etching condition and must be decreased as much as possible in order to reduce the sidewall damage during RIE. For example, lower gas pressure, higher RF power, and higher CF 4/Ar gas flow ratio were desirable for the sidewall damage reduction.

Low Leakage Current Patterned Polymeric Transistors with PAG Assisted Cross-linking PVP as Gate Dielectric and Passivation Layers

AbstractWe have successfully proposed a patterned P3HT thin-film transistor with cross-linked PVP as a passivation material which was cured at low temperature. The active P3HT layer was isolated via photolithographic technique and O2 plasma RIE etching process. In this method, the leakage current could be reduced effectively compared with that of non-patterned device. Although the mobility was degraded 40 %, but the on/off ratio was significantly improved by over three orders and also the subthreshold swing was compatible with the amorphous Si-TFTs (∼1.5 V/decade). Moreover, we also employed this low temperature curing PVP (120 0C) films as the gate dielectrics which exhibited excellent insulating property with high on/off ratio 1.58×104 and good subthreshold swing 1.66 V/decade.

Surface modification of EPDM in r.f. plasma: process optimization and surface characterization

AbstractDue to the excellent thermal and mechanical properties, the EPDM rubber is used as protective film in the aerospace industry. However, its chemical composition and structure do not allow good adhesive properties, making necessary a surface treatment to improve adhesion properties. Most of the conventional chemical methods make use pollutant solvents. Cold plasma is a clean technology that becomes an important alternative for treating these material surfaces. In this work we present a study of the EPDM rubber surface treatment by plasma. The process was conducted in a RIE plasma system excited by r.f. power using N2 and Ar gas mixtures. A significant improvement of the surface properties regarding to the adhesion properties was obtained. An optimization of the plasma parameters, such as gas ratio mixture, rf power, total gas pressure and treatment time is presented. The rubber surface properties were analyzed by goniometry and AFM. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nanolithographic patterning of transparent, conductive single-walled carbon nanotube films by inductively coupled plasma reactive ion etching

The authors report successful patterning of transparent, conductive single-walled carbon nanotube films down to 100nm lateral dimensions by photolithography or e-beam lithography and subsequent O2 plasma etching using an inductively coupled plasma reactive ion etching (ICP-RIE) system. They systematically study the effect of ICP-RIE etch parameters, such as substrate bias power, chamber pressure, and substrate cooling, on the nanotube film etch rate and etch selectivity. They also characterize the effect of the linewidth etched on the nanotube film etch rate for widths ranging from 50μm down to 100nm. Furthermore, by fabricating standard four point probe structures using the patterning capability developed, the authors investigate the effect of different resist processes on the resistivity of patterned single-walled carbon nanotube films and the effect of ICP reactive ion etching on the resistivity of partially etched nanotube films. In addition, they demonstrate that using an ICP-RIE system provides significant advantages, such as faster etch rates and better etch selectivity, over conventional parallel plate RIE plasma systems, making it possible to pattern lateral features as small as 100nm in nanotube films. The simple and efficient “top-down” patterning capability developed in this article could open up many opportunities for integrating single-walled nanotube films into a wide range of electronic and optoelectronic devices.

Transformation of hydrogen silsesquioxane properties with RIE plasma treatment for advanced multiple-gate MOSFETs

Keeping in line with Moore's law requires increasing efforts in the development of alternative electronic devices. Multiple-gate transistors are very promising in order to suppress short-channel effects and to increase the current drive. Nevertheless, the fabrication of such devices represents a strong challenge for silicon process technology. One of the key steps of the process consists in shrouding the silicon fins in an isolating matrix, using a flowable oxide, namely hydrogen silsesquioxane (HSQ). The general objective of this work is to show that the properties of HSQ can be modified by appropriate thermal or plasma treatments to modulate its characteristics in terms of etching selectivity and surface topography. SEM characterization has shown that HSQ exhibits excellent planarization and gap fill capabilities, while AFM analysis, on 100 nm thick HSQ films deposited by spin on, reveals a roughness as low as 3 nm. Various oxygen plasma treatments have been applied to densify the HSQ films. Fourier transform infra-red spectroscopy (FT-IR) has shown very interesting qualitative and quantitative informations. Chemical and physical transformations from a Si–O–Si cage-like structure into an Si–O–Si network one have been observed. It is shown that exposure to oxygen plasma at high power (290 W) for a long time (20 min) or thermal curing at high temperature improves the resistance to wet etching using 1% hydrofluoric acid (HF). This densification technique holds the remarkable property to transform HSQ into an SiO 2-like structure.

Study of preconcentration of non-polar compounds in microchannels with constrictions

The influence of the presence of constrictions in microchannels was investigated in this work, numerically and experimentally, aiming at to obtain preconcentrator microstructures for non-polar compounds that can be present as a contaminant in the air. The flow behavior inside the microstructures was analyzed using the ANSYS FLOTRAN 8.1 and EVOLVER 2.14 simulations packages. The experimental tests for evaluation of preconcentration characteristics were performed using gas mixtures with n-hexane (C6H14) inserted in a microchannel with a length of 30cm, width of 100μm and depth of 30μm. The microchannels were defined using RIE plasma etching and sealed with glass. Preconcentration was effectively observed and it was also demonstrated that the constrictions may favor the occurrence of capillary effects, as compared to adsorption effects, defining the dynamical behavior of the preconcentrator microstructure as a function of geometric and flow conditions. So far, these microstructures can be useful for the preconcentration process at high-contaminated environments, in which the human interference is not safe or small devices with high portability are required.

Self-sustained bridges of a-SiC:H films obtained by PECVD at low temperatures for MEMS applications

In this work PECVD obtained a-SiC:H films are utilized to fabricate self sustained microbridges and microtunnels. The deposition conditions for these films, which present optimized properties for MEMS application, were established elsewhere. The utilized a-SiC:H films are 1 μm thick and are obtained at 320 °C from mixtures of CH 4 and SiH 4, with and without H 2 dilution. The microstructures are fabricated by surface micromachining over silicon substrates utilizing 4 μm thick SiO x N y films as sacrificial layer, also obtained by PECVD. The geometry of the bridges and tunnels is varied from 25 μm up to several hundred microns and was defined by plasma RIE process established after a study also presented here. The final aspect of the microstructures and the evaluation of the fabrication process are investigated by scanning electron microscopy. The results show that bridges fabricated with stoichiometric a-SiC:H grown in `silane starving plasma' condition with H 2 dilution of the gaseous mixture can be very flat, smooth and free of residual stress. Since this material was previously optimized to be a real amorphous counterpart of crystalline SiC, can be doped very efficiently n- and p-type and can be crystallized by thermal annealing, the results presented here are the first step toward the development of MEMS all based on PECVD materials.

Plasma-Induced Fluorine Damage in P-HEMT Caused by C2F6/CHF3 RIE Plasma

Fluorine atoms intruded into the channel layer in P-HEMT and degraded its carrier density and electron mobility during the SiO2 RIE process. Thermal annealing at 300°C for 10 minutes was rather effective for recovering from this plasma damage. Our SIMS investigation revealed that the mechanism of this recovery was the removal of fluorine atoms from the channel layer and their accumulation in the δ-doped Si layer. Ionized impurities in the channel layer were removed, and electron mobility was significantly recovered. However, the carrier density was recovered to a lesser degree than that of the electron mobility because of the neutralization of Si donors in the δ-doped layer caused by the fluorine accumulation.

Surface Evaluation of LiNbO3, LiTaO3 Crystals Etched Using Fluorine System Gas Plasma RIE

Etching characteristic LiNbO3 and LiTaO3 single crystals has been investigated using plasma RIE with mixture gases of CF4/Ar, CF4/H2, CF4/Ar/H2. The etched surface was evaluated by means of atomic force microscopy and X-ray diffraction methods. The in situ surface temperature of the sample during RIE was measuresd. F atoms exist in the contamination layer, such as sediments, on the surface etched using the mixture gases of CF4, Ar and H2 gases. The etch rate was dependent on the orientation of a crystal. The etch rate of LiTaO3 was less than that of LiNbO3.

PMODTM-based Direct Thin Film Imaging, DTFITM, on Flexible Substrates

AbstractPMOD (Photochemical Metal Organic Deposition)-based DTFI (Direct Thin Film Imaging) methodology is a demonstrated means for patterning organic materials on flexible (plastic) substrates. This process is used to pattern 1-micron features at an aspect ratio of 8:1 using contact lithography. Use of oxygen plasma RIE etch to transfer the hard mask pattern to the organic material allows for good sidewall angle control. High etch selectivity between the novolac polymer and PMOD TiOX hard mask (< 800:1) makes the use of very thin hard masks (∼ 200 angstroms) to pattern thick organic films (<10 microns) possible. Selective removal of the PMOD TiOX (TiO2) hard mask makes this process amenable to patterning of functional organic structures fabricated from materials chosen for their desired properties (e.g., glass transition temperature (Tg), etch resisitance, optical properties, mechanical properties, etc.) not their ability to be photopatterned (e.g., photoresist).