Ar Beam Research Articles

Combined Surface Activated Bonding Technique for Hydrophilic SiO2-SiO2 and Cu-Cu Bonding

As an evolution of the Cu-Cu bonding and SiO2-SiO2 bonding, Cu/SiO2 hybrid bonding is a promising approach to the emerging three-dimensional (3D) integration of microelectronic/photonic systems, since it obtains both direct metal interconnection and enhanced thermal/mechanical stability with a seamless bonding structure during the single bonding process.1,2 However, because of the different features of Cu-Cu and SiO2-SiO2 bonding, Cu/SiO2 hybrid bonding at low temperatures of no more than 200 °C remains challenging. For instance, the Cu-Cu thermo-compression bonding is typically conducted in vacuum or dry protecting/reducing atmospheres after removal of surface oxides,3–7 while the SiO2-SiO2 bonding needs a humid environment to facilitate termination of Si-OH bonding sites.8–12 It is highly desired to develop a new bonding process that is effective for both Cu-Cu and SiO2-SiO2 bonding in H2O-free ambient, such as vacuum, for improvement of the Cu/SiO2 hybrid bonding. Recently, we proposed a combined surface-activated bonding (SAB) method, which involves a combination of surface bombardment using a Si-containing Ar beam and prebonding attach-detach procedure prior to bonding in vacuum. High SiO2-SiO2 bonding strength of close to the Si bulk fracture strength has been realized at 200 °C. In this paper, we report our recent results of Cu-Cu and SiO2-SiO2 bonding by using the combined SAB method. The mechanism is discussed to understand the present low-temperature bonding technique.

Bonding of SiO2 and SiO2 at Room Temperature Using Si Ultrathin Film

In three-dimensional integration technology, the bonding of metal electrode and insulator hybrid interface is very important technique. The hybrid interface serves as both an electrical connection and mechanical bond. However, the bonding of these hybrid interfaces is a challenging issue. Since the conventional bonding process requires high-temperature annealing, there exist various problems such as thermal damage, low throughput, and low alignment accuracy. As the surface activated bonding (SAB) is a bonding method carried out at room or low temperatures [1], the bonding method is expected to solve these problems. By the SAB method, direct bonding metal materials such as Cu or Al is easy. However, it is very difficult to directly bond insulator materials such as SiO2 or SiN [2], because these surfaces are rendered inactive immediately after being activated. We have reported on the bonding technique at room temperature using only Si ultrathin films for insulator materials, and we have shown that high bonding strength is achieved [3]. The surface of electrode is also covered with Si film by this bonding method. Thus, it is very important to reveal the influence of this Si film thickness on the SiO2/SiO2 bonding. In this report, we have investigated the relationship between the SiO2/SiO2bonding strength and the thickness of Si ultrathin film. Figure 1 shows a schematic illustration of the bonding apparatus (Mitsubishi Heavy Industries, Ltd., MWB-08AX) used in this experiment. Surface activation is carried out by an Ar fast atom beam (FAB). In the normal SAB process, the upper and lower wafers are irradiated with the Ar-FAB at the same time. The bonding procedure which we propose for hybrid bonding is as follows. The one of bonding wafers is held by the electric static chuck (ESC), and Si blanket wafer is placed on the lower side as the sputtering target. First, only the lower wafer is irradiated with FAB1 (first irradiation) after activating electrode surface of the upper wafer. The blanket Si wafer is then exchanged with the other bonding wafer. Only the upper wafer is irradiated with FAB2 (second irradiation) after activating electrode surface of the lower wafer, and the Si film surface on the upper wafer is also etched by FAB2. A Si thin film is then deposited on the lower wafer surface. The upper wafer and the lower wafer are optically aligned and then bonded with high load.Figure 2 shows the relationship between the surface energy and the Si ultrathin film thickness for the bonding of Si blanket wafers (8 in) with a thermal oxide. Applied voltage and current of Ar beam source operated in these experiments were about 1.8 kV and 100 mA, respectively. The background vacuum pressure is about 2×10-6Pa. The surface energy of wafers was about 1 J/m2 for thicker than about 3 nm. A cross-sectional TEM image of SiO2/SiO2bonding is shown in Fig. 3. The bonded wafers were treated with a FAB1 irradiation time of 5 min and a FAB2 irradiation time of 1 min. No micro voids were observed, and Si intermediate layer, of which the thickness is about 3nm, was seen at the bonding interface.In conclusion, we showed that high bonding strength was achieved in SiO2/SiO2 bonding with the Si film thickness of about 3 nm by this bonding method. Reference:[1] H. Takagi, K. Kikuchi, R. Maeda, T. R. Chung, and T. Suga, Appl. Phys. Lett. 68, 2222 (1996).[2] H. Takagi, R. Maeda, T. R. Chung, and T. Suga, Sensors and Actuators, A 70, 164 (1998).[3] J. Utsumi, K. Ide, and Y. Ichiyanagi, Jpn. J. Appl. Phys., 55, 026503 (2016). Figure 1

Combined Surface Activated Bonding Technique for Hydrophilic SiO2-SiO2 and Cu-Cu Bonding

Wafer bonding with high quality (such as high strength, void-less interface, and low-resistance vertical electrical interconnects) obtained at low temperatures is highly desired for 3D integration of microsystems. This paper reports a combined surface activated bonding (SAB) technique for improvement of the bonding quality of SiO2-SiO2 pairs bonded at 200°C. The present bonding technique employs a combination of surface irradiation using a Si-containing Ar beam and prebonding attach-detach procedure prior to bonding in vacuum, followed by postbonding annealing in ambient pressure. The bonding method has also been found effective for SiO2-SiNx and Cu-Cu bonding at 200°C. Results of bonding strength measurements, transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) observations are reported and discussed to understand the present technique. The feasibility of this bonding technique for Cu/SiO2 and Cu/SiO2/SiNx hybrid bonding is also discussed.

Combined surface-activated bonding technique for low-temperature hydrophilic direct wafer bonding

A combined surface-activated bonding technique is studied for surface activation and water management to improve the hydrophilic SiO2–SiO2 bonding quality. Prebonding treatment involving a Si-containing Ar beam bombardment and prebonding attach-detach is employed prior to wafer bonding in vacuum. The results of bonding strength measurement, Monte Carlo simulation, and surface analysis by Fourier transform infrared spectroscopy are reported. A mechanism is proposed to better understand the nature of the hydrophilic bonding at low temperatures of no more than 200 °C. We suggest that the Si-containing Ar beam modifies the SiO2 surfaces by Si enrichment to make them more reactive for OH adsorption, while the prebonding attach and detach facilitate a further increase in the number of OH and the removal of excess trapped H2O prior to bonding, respectively. As a consequence, SiO2–SiO2 bonding strength close to the Si bulk fracture energy can be achieved after low-temperature annealing.

Combined Surface Activated Bonding Technique for Low-Temperature Cu/Dielectric Hybrid Bonding

Cu/dielectric hybrid bonding at low temperatures of no more than 200°C remains challenging because of the different features of Cu-Cu and dielectric-dielectric (such as SiO2-SiO2) bonding. This paper reports a combined surface activated bonding (SAB) technique for low-temperature Cu-Cu, SiO2-SiO2, and SiO2-SiNx bonding. This technique involves a combination of surface irradiation using a Si-containing Ar beam and prebonding attach-detach process prior to bonding in vacuum. Wafer bonding experiments were conducted at either room temperature or 200°C. Results of bonding strength measurements, transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) observations, and X-ray photoelectron spectroscopy (XPS) analysis were reported and discussed to understand the present combined SAB technique.

Etch residue removal of CoFeB using CO/NH3 reactive ion beam for spin transfer torque-magnetic random access memory device

Using a reactive ion beam etching (RIBE) system, the possibility of removing the sidewall residues remaining on etched nanoscale CoFeB features and the W hard mask after using a conventional inductively coupled plasma etching system was investigated. Upon increasing the ion energy of the Ar beam, a similar sputter yield increase was found for both CoFeB side wall residues and the W hard mask. Hence, increasing the ion beam energy to improve etch residue removal efficiency at the same time induces a degradation of the CoFeB profile because of the W hard mask erosion. However, when CO/NH3 was used as the RIBE gas mixture, at ion energy in the range of 90–110 eV, the effective residue removal from CoFeB etched features without etching the W hard mask. When the ion energy of the CO/NH3 RIBE exceeds 140 eV, again similar sputter yields are found for both CoFeB side wall residues and the W hard mask.

Beam imaging in the injection line of the INFN-LNS superconducting cyclotron.

A cheap and efficient diagnostic system for beam monitoring has been recently developed at INFN-LNS in Catania. It consists of a high sensitivity CCD camera detecting the light produced by an ion beam hitting the surface of a scintillating screen and a frame grabber for image acquisition. A scintillating screen, developed at INFN-LNS and consisting of a 2 μm BaF2 layer evaporated on an aluminium plate, has been tested by using (20)Ne and (40)Ar beams in the keV energy range. The CAESAR ECR ion source has been used for investigating the influence of the frequency and magnetic field tuning effects, the impact of the microwave injected power, and of the focusing solenoids along the low energy beam transport on the beam shape and current. These tests will allow to better understand the interplay between the plasma and beam dynamics and, moreover, to improve the transport efficiency along the low energy beam line and the matching with the superconducting cyclotron, particularly relevant in view of the expected upgrade of the machine.

Fabrication and characterization of tunnel barriers in a multi-walled carbon nanotube formed by argon atom beam irradiation

We have evaluated tunnel barriers formed in multi-walled carbon nanotubes (MWNTs) by an Ar atom beam irradiation method and applied the technique to fabricate coupled double quantum dots. The two-terminal resistance of the individual MWNTs was increased owing to local damage caused by the Ar beam irradiation. The temperature dependence of the current through a single barrier suggested two different contributions to its Arrhenius plot, i.e., formed by direct tunneling through the barrier and by thermal activation over the barrier. The height of the formed barriers was estimated. The fabrication technique was used to produce coupled double quantum dots with serially formed triple barriers on a MWNT. The current measured at 1.5 K as a function of two side-gate voltages resulted in a honeycomb-like charge stability diagram, which confirmed the formation of the double dots. The characteristic parameters of the double quantum dots were calculated, and the feasibility of the technique is discussed.

Combined Surface-Activated Bonding Technique for Low-Temperature Cu/SiO2 Hybrid Bonding

Low-temperature wafer bonding is one of key enabling technologies for manufacturing silicon-on-insulator (SOI), micro-electromechanical systems (MEMS), and the emerging three-dimensional (3D) integration. Among many other bonding approaches, low-temperature SiO2-SiO2 bonding and Cu-Cu bonding have been extensively studied because of their high bonding quality and CMOS compatibility. As an evolution of SiO2-SiO2 bonding and Cu-Cu bonding, Cu/SiO2 hybrid bonding provides both ultrafine metal interconnections and enhanced bonding stability during the single bonding process. However, this bonding approach remains difficult because of the challenging simultaneous surface activation of the hybrid surface. For instance, plasma treatment is effective for SiO2 surface activation, but it may induce formation of Cu2O/CuO and increase of surface roughness on Cu surface, which prevents successful hybrid bonding at low temperatures. Therefore, novel bonding techniques with compatibility with hybrid surface are highly desired for low-temperature Cu/SiO2h hybrid bonding. This work develops a combined surface-activated bonding (SAB) technique for low-temperature Cu/SiO2 hybrid bonding at below 200 °C. The technique involves combinations of Ar beam bombardment, Si deposition, and water vapor exposure for prebonding surface activation prior to bonding in vacuum. Homogeneous (i.e., SiO2-SiO2 and Cu-Cu) and heterogeneous (i.e., Cu-SiO2) wafer bonding were carried out with the same bonding technique. Surface properties of the surface-activated wafers have been investigated using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR). Bonding strength has been measured by crack-opening method and tensile test. Bonding interfaces have been inspected by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). Mechanisms of prebonding surface activation and bonding interface evolution during low-temperature postbonding annealing are discussed based on our results.

Type-II Band Profile of GaAs/Si Hetero Junctions by Surface Activated Bonding for Hybrid Tandem Cells

III-V-on-Si tandem cells fabricated in the hybrid, or wafer bonding, approach are promising as high-efficiency (>30%) solar cells. The present authors reported on the characteristics of InGaP-on-Si tandem cells that had been fabricated by surface-activated bonding (SAB) [1]. The key issue in fabricating such tandem cells lies in formation of low-parasitic-resistance tunnelling junctions made of III-V-materials, typically GaAs, and Si. In the present work, we investigate the electrical properties of p-GaAs/n-Si and p-Si/n-GaAs hetero junctions by SAB, which are one of the key factors determining the resistance across the tunnelling junctions.p-GaAs/n-Si and p-Si/n-GaAs junctions were made of (100) p-GaAs (concentration of acceptors (NA) of ~1E19 cm-3), (100) n-Si (concentration of donors (ND) of 4.8E16 cm-3), (100) n-GaAs (ND of 1.1E18 cm-3), and (100) p-Si (NA of 2.4E17 cm-3) substrates by SAB. Ohmic contacts were formed by evaporating AuZn/Ti/Au, Ti/Au, AuGe/Ni/Ti/Au, and Al/Ni/Au films on the respective substrates and annealing (400 deg., 1min. in N2 ambient). Their current-voltage (I-V) and capacitance-voltage (C-V) characteristics were measured at room temperature. The frequency for C-V measurements was 100 kHz.The I-V characteristics of both junctions showed rectifying properties. The onset in the characteristics for forward bias voltages was found to be 0.26 and 0.4 V for the p-GaAs/n-Si and p-Si/n-GaAs junctions, respectively. The flat-band voltage was estimated to be 0.42 and 1.60 V in the p-GaAs/n-Si and p-Si/n-GaAs junctions, respectively, by linearly extrapolating each of 1/C2-V characteristics. The lower flat-band voltage in the p-GaAs/n-Si junction was in accordance with the smaller onset in its I-V characteristics. The extracted flat-band voltage values imply that the conduction band minimum (EC) in GaAs is higher than that in Si by 0.83 and 0.58 eV for the p-GaAs/n-Si and p-Si/n-GaAs junctions, respectively. Such large values of the conduction-band discontinuity (ΔEC) suggest that the band lineup in GaAs/Si junctions reveals type-II features irrespective of their polarity.This result disagrees with the estimation based on a simple model that ΔEC in GaAs/Si hetero junctions is expressed by the difference in the electron affinities of GaAs and Si, both of which are ~4 eV. The disagreement is attributable to the formation of dipole due to the charge imbalance at the hetero interface. Electrical charges in the interface states possibly formed during Ar beam irradiation in the SAB process might explain the difference in ΔECvalues between the two hetero junctions. Practically, the type-II features in the band lineup of GaAs/Si systems, which means that the height of potential barriers that carriers pass through in p-GaAs/n-Si tunnelling junctions is effectively reduced, play an important role in making high-efficiency Si-based tandem cells.This work was supported by "Creative research for clean energy generation using solar energy" project of JST/CREST, Japan.[1] N. Shigekawa, et al. Jpn. J. Appl. Phys. 53, 04ER05 (2014).

Separation efficiency of the MASHA facility for short-lived mercury isotopes

The mass-separator MASHA built to identify Super Heavy Elements by their mass-to-charge ratios is described. The results of the off- and on-line measurements of its separation efficiency are presented. In the former case four calibrated leaks of noble gases were used. In the latter the efficiency was measured via 284 MeV \(^{40}\)Ar beam and with using the hot catcher. The ECR ion source was used in both cases. The \(\alpha \)-radioactive isotopes of mercury produced in the complete fusion reaction \(^{40}\)Ar+\(^{144}\)Sm\(\rightarrow ^{184-xn}\)Hg+xn were detected at the mass-separator focal plane. The half-lives and the separation efficiency for the short-lived mercury isotopes were measured. Potentialities of the MEDIPIX detector system have been demonstrated for future use at the mass-separator MASHA.

Ongoing characterization of the forced electron beam induced arc discharge ion source for the selective production of exotic species facility

An intense research and development activity to finalize the design of the target ion source system for the selective production of exotic species (SPES) facility (operating according to the isotope separation on line technique) is at present ongoing at Legnaro National Laboratories. In particular, the characterization of ion sources in terms of ionization efficiency and transversal emittance is currently in progress, and a preliminary set of data is already available. In this work, the off-line ionization efficiency and emittance measurements for the SPES forced electron beam induced arc discharge ion source in the case of a stable Ar beam are presented in detail.

An innovative experimental setup for the measurement of sputtering yield induced by keV energy ions

An innovative experimental equipment allowing to study the sputtering induced by ion beam irradiation is presented. The sputtered particles are collected on a catcher which is analyzed in situ by Auger electron spectroscopy without breaking the ultra high vacuum (less than 10(-9) mbar), avoiding thus any problem linked to possible contamination. This method allows to measure the angular distribution of sputtering yield. It is now possible to study the sputtering of many elements such as carbon based materials. Preliminary results are presented in the case of highly oriented pyrolytic graphite and tungsten irradiated by an Ar(+) beam at 2.8 keV and 7 keV, respectively.

Room temperature wafer bonding of metal films using flattening by thermal imprint process

This paper reports on the surface flattening of metal films on wafers by the thermal imprint process and its application to wafer direct bonding. The ultraflat surface of a sapphire wafer was pressed onto the surface of a sputter-deposited Au film on a Si wafer for 600s at 200°C with an applied pressure of 5MPa by compressed air. As a result, the Au film surface was successfully flattened by thermal imprint process. The flattened Au films were bonded in a vacuum chamber after surface activation by Ar beam sputtering. In the case of bonding between two Au films on a Si wafer, the surface flattening of the Au films was determined to be effective when the bonding load is small and/or the Au films are relatively thick. In the case of bonding between a Si wafer and a Au film on a Si wafer, defects at the bonding interface were found to be dramatically reduced by the flattening of the Au film surface. Additionally, the Au–Si bonding using the flattened Au film was significantly stronger than that without flattening.

Production of Fe clusters by collisions of metal vapour with supersonic argon beams

The growth of Fe clusters by collisions of Fe atoms with Ar atoms flowing in a supersonic beam was investigated by Fe mass flux measurements and transmission electron (TEM) microscopy. Moderate Ar densities of the order of 1 × 1020 m-3 were sufficient to cause cluster growth which was attributed to the low temperature of the Ar beam. TEM imaging of deposited clusters revealed diameter distributions from 2 to 10 nm depending on the deposition time. Extrapolation to zero deposition time revealed a cluster size of 2.4 nm grown in the gas phase. Growth on the surface was attributed to diffusion of single Fe atoms which are co-deposited with the clusters in the process and which agglomerate when they hit a cluster.

Irradiation effect on PET surface using low energy argon ion beam

A study of various physical properties of a PET film irradiated with an Ar beam is reported. SEM images and UV–VIS and FTIR spectra were obtained for a number of ionic fluxes and three irradiation times. Small changes in the energy gap of degradated samples were found, and the SEM images indicate that the optimum homogeneity and roughness are reached after 30min of irradiation. These results may well be of practical interest. A modified saddle field ion source was used as a preparation tool of the surface of polyethylene terephthalate PET polymer substrate to be ready for coating or thin film deposition. Argon ion beam was used for this purpose, where the scanning electron microscope (SEM) shows that, the best sample is the one which was irradiated to 30min, where this sample is more homogenous and roughness than other irradiated samples. Also the (UV–VIS) spectrum tells us that, there is small change on energy gap and this is meaning that, the change on electric properties is small also. In this case the sample is more homogenous and of higher roughness than other irradiated samples.

Secondary ion mass spectrometry of powdered explosive compounds for forensic evidence analysis

Residual quantities of explosives deposited on, or absorbed in, nearby surfaces can be of forensic value in post-blast analysis. As secondary ion mass spectrometry (SIMS) may be a suitable analytical approach for the screening of such residues, its performance was evaluated. The analyses were carried out in a SIMS instrument fitted with a quadrupole analyzer. The sample was sputtered at a 45º incidence angle with a 100 µm primary Ar(+) beam (3 keV, 500 nA). Surface sample compensation was performed with low-energy electrons (500 eV, 0.75 mA). TNT, RDX, PETN and cloratite were deposited in powdered form on double-sided tape and introduced into the mass spectrometer, without further handling, for SIMS analysis. The analysis conditions including compensation were optimized. A mixture of energetic compounds commonly used for explosive preparation was also analyzed, proving the potential of SIMS in forensic analysis. This study demonstrated the possibility of detecting explosives by SIMS making use of a simple sampling procedure consisting of sticking the sample in powdered form (compatible with the collection performed in forensic post-blast analysis) onto double-sided tape without handling or preparation.

Modeling dynamic cluster SIMS experiments

The underpinnings of two SIMS experimental findings are elucidated using the power of molecular dynamics to provide insight at the molecular level. First, the improvement of depth resolution for C60 bombardment of Irganox delta layers and polymer layers using sample rotation is explained by molecular dynamics simulations of the repetitive bombardment of Ag surfaces with 20 keV Au3, C60, and Ar872 at grazing angles of incidence with both single azimuthal angle and random azimuthal angles. Single azimuthal angle simulations at grazing angles show the formation of trenches and valleys parallel to the cluster beam form and are elongated with increasing incident angle. Cluster bombardment simulations with random azimuthal angles mimic sample rotation and show that under grazing angles of incidence, the surface is smoother because of the random impact angles preventing trench and valley formation. Second, depth profiles using C60 at a 70° angle of incidence have been improved by co‐bombardment with low energy Ar ions emitted at 33° from the C60 beam and at a 45° incident angle from the surface. Molecular dynamics simulations of the bombardment of solid benzene with low energy Ar and 15 keV C60 were used to show the depth of damage created during bombardment. For low kinetic energy (<200 eV), the damage caused by the Ar atom is in the same region as for the C60 cluster. It is believed that for the experimental conditions used during co‐bombardment, the Ar beam is breaking up the ridges created by the grazing C60 beam. Copyright © 2012 John Wiley & Sons, Ltd.

Control of Swelling Height of Si Crystal by Irradiating Ar Beam

It has been found that ion implantation can induce a swelling (step-height) phenomenon on crystal surface. In this paper, we studied about the control of swelling height of Si crystal by irradiating Ar beam under various parameters (fluence, charge and energy). These irradiation parameters were regulated by an irradiation facility that enables to achieve the multiple ionization. For both charges, the swelling height was studied with the various fluencies of two different charges Ar(1+) and Ar(4+). The swelling height increased with increasing the fluence. The swelling height was also studied by changing energy of Ar(4+) beam. The swelling height increased by increasing the energy. The obtained swelling heights are understood base on the contribution of ion-beam induced defect, which is evaluated by SRIM. By comparing with the previous results, it was found that the expansion phenomena also depend on irradiated ion. The swelling structures were found to be stable more than two months. The present results have shown that this method of producing swelling structure indicates the potential application to fabricate 3-D nanostructure.

Molecular Dynamics Simulations Elucidate the Synergy of C60 and Low-Energy Ar Cobombardment for Molecular Depth Profiling

The use of cluster beams in secondary ion mass spectrometry enables molecular depth profiling, a technique that is essential to many fields. The success of the technique often hinges upon the chemical nature of the substrate, the kinetic energy and incident angle of the primary cluster ion beam, and the sample temperature. It has been shown experimentally that the quality of depth profiles can be improved with cobombardment by a C60 cluster beam and a low-energy argon (Ar) beam. We present molecular dynamics simulations to elucidate the mechanistic reasons for the improved molecular depth profiles with an aim of understanding whether this cobombardment approach is generally applicable. We conclude that the low-energy Ar beam breaks up the surface topology created by the C60 beam, increasing the sputtering yield and reducing the buildup of chemical damage. The simulations also suggest that an equivalent result could be achieved without the Ar cobombardment by optimizing the conditions of the C60 beam.