Room temperature Cu–Cu direct bonding using surface activated bonding method

In situ study of thin film growth by internal stress measurement under ultrahigh vacuum conditions: Silver and copper under the influence of oxygen

The aim of this work is to further the understanding of the important parameters in the formation process of 2D nanostructures and therewith pioneer for novel applications. Such 2D nanostructures can be composed of specially designed organic molecules, which are adsorbed on various surfaces. In order to study true 2D structures, monolayers were deposited. Their properties have been investigated by scanning tunneling microscopy (STM) under ultra-high vacuum (UHV) conditions as well as under ambient conditions. The latter is a highly dynamic environment, where several parameters come into play. Complementary surface analysis techniques such as low-energy electron diffraction (LEED), X-Ray photo-emission spectroscopy (XPS), and Raman spectroscopy were used when necessary to characterize these novel molecular networks. In order to conduct this type of experiments, high technical requirements have to be fulfilled, in particular for UHV experiments. Thus, the focus is on a drift-stable STM, which lays the foundation for high resolution STM topographs. Under ambient conditions, the liquid-solid STM can be easily upgraded by an injection add-on due to the highly flexible design. This special extension allows for adding extra solvent without impairing the high resolution of the STM data. Besides the device, also the quality of the tip is of pivotal importance. In order to meet the high requirements for STM tips, an in vacuo ion-sputtering and electron-beam annealing device was realized for the post-preparation of scanning probes within one device. This two-step cleaning process consists of an ion-sputtering step and subsequent thermal annealing of the probe. One study using this STM setup concerned the incorporation dynamics of coronene (COR) guest molecules into pre-existent pores of a rigid 2D supramolecular host networks of trimesic acid (TMA) as well as the larger analogous benzenetribenzoic acid (BTB) at the liquid-solid interface. By means of the injection add-on the additional solution containing the guest molecules was applied to the surface. At the same time the incorporation process was monitored by the STM. The incorporation dynamics into geometrically perfectly matched pores of trimesic acid as well as into the substantially larger pores of benzentribenzoic acid exhibit a clearly different behavior. For the BTB network instantaneous incorporation within the temporal resolution of the experiment was observed; for the TMA network, however, intermediate adsorption states of COR could be visualized before the final adsorption state was reached. A further issue addressed in this work is the generation of metal-organic frameworks (MOFs) under ultra-high vacuum conditions. A suitable building block therefore is an aromatic trithiol, i.e. 1,3,5-tris(4-mercaptophenyl)benzene (TMB). To understand the specific role of the substrate, the surface-mediated reaction has been studied on Cu(111) as well as on Ag(111). Room temperature deposition on both substrates results in densely packed trigonal structures. Yet, heating the Cu(111) with the TMB molecules to moderate temperature (150 °C) yields two different porous metal coordinated networks, depending on the initial surface coverage. For Ag(111) the first structural change occurs after annealing the sample at 300 °C. Here, several disordered structures with partially covalent disulfur bridges were identified. Proceeding further in the scope of increasing interaction strength between the building blocks, covalent organic frameworks (COFs) were studied under ultra-high vacuum conditions as well as under ambient conditions. For this purpose, a promising strategy is covalent coupling through radical addition reactions of appropriate monomers, i.e. halogenated aromatic molecules such as 1,3,5-tris(4-bromophenyl)benzene (TBPB) and 1,3,5-tris(4- iodophenyl)benzene (TIPB). Besides the correct choice of a catalytic surface, the activation energy for the scission of the carbon-halogen bonds is an essential parameter. In the case of ultra-high vacuum experiments, the influence of substrate temperature, material, and crystallographic orientation on the coupling reaction was studied. For reactive Cu(111) and Ag(110) surfaces room temperature deposition of TBPB already leads to a homolysis of the C-Br bond and subsequent formation of proto-polymers. Applying additional heat facilitates the transformation of proto-polymers into 2D covalent networks. In contrast, for Ag(111) just a variety of self-assembled and rather poorly ordered structures composed of intact molecules has emerged. The deposition onto substrates held at 80 K has never resulted in proto-polymers. For ambient conditions, the polymerization reaction of 1,3,5-tri(4-iodophenyl)benzene (TIPB) on Au(111) was studied by STM after drop-casting the monomer onto the substrate held either at room temperature or at 100 °C. For room temperature deposition only poorly ordered non-covalent arrangements were observed. In accordance with the established UHV protocol for halogenated coupling reaction, a covalent aryl-aryl coupling was accomplished for high temperature deposition. Interestingly, these covalent aggregates were not directly adsorbed on the Au(111) surface, but attached on top of a chemisorbed monolayer comprised of iodine and partially dehalogenated TIPB molecules. For a detailed analysis of the processes, the temperature dependent dehalogenation reaction was monitored by X-ray photoelectron spectroscopy under ultra-high vacuum conditions.

Reflection–absorption infrared spectroscopy, Auger electron spectroscopy (AES), temperature programmed desorption, and reactive scattering were used to investigate the adsorption and desorption behavior of PEt3 on copper and chlorinated copper surfaces under ultrahigh vacuum conditions. No reaction was observed between PEt3 and clean Cu(100) or between PEt3 and a c(2×2)–Cl overlayer. At temperatures above 320 K, PEt3 reacted rapidly with a heavily chlorinated copper surface (10 000 L exposure of Cl2 at 300 K). Subsequent examination of the substrate by AES showed removal of chlorine and presumably copper. The remaining chlorine corresponded roughly to 0.5 monolayer coverage. This is consistent with reactive scattering, infrared and AES experiments carried out on thin chlorinated layers (8 L Cl2 exposure), where essentially no reaction of PEt3 with the surface was observed. The reaction between PEt3 and a heavily chlorinated Cu(100) surface yields both ClCu(PEt3)2 and Cl2PEt3. These data are consistent with the etching of copper under ultrahigh vacuum conditions at low temperatures by reaction with Cl2 and PEt3 to form CuCl(PEt3)2.

Metallization plays a key role in the production process of integrated devices. Recently, copper (Cu) has been proposed as an alternative material to aluminum to address the need for metallic thin films with low resistivity and high electromigration resistance. 1 As a consequence, great attention has been paid to electrochemical processes, which enable the deposition of metallic copper at low temperatures with low costs. Copper electroless deposition has been shown to selectively plate silicon (Si) substrates and structures with high aspect ratios and large structural heights. 2,3 However, this method requires a pretreatment to activate the surface. Selectivity is achieved only to the extent that the activating treatment can be made selectively. In addition, many studies have also been carried out on the galvanic deposition of copper from fluoride containing solutions.4-11 In contrast to electroless deposition, galvanic displacement deposition requires no prior activation of the surface and is truly selective to silicon surfaces. Thus, it provides an attractive deposition method for copper interconnects or seed layers for subsequent metallization.4 Galvanic displacement is also a promising avenue for the integration of metals in micromechanical devices, due to its conformal nature and high substrate selectivity. Despite these attractive features, a crucial issue in the aforementioned processes is the lack of adhesion of copper to the Si substrate, which may severely constrain their application. 12 In particular, copper films deposited by galvanic displacement fail the qualitative Scotch tape test. In this paper, we report a process for the galvanic deposition of copper onto silicon from fluoride-containing solutions. Thin copper films with reflective and smooth surfaces and excellent adhesion to silicon are obtained. Results on plating of microelectromechanical systems (MEMS) after release are also presented. Polycrystalline silicon and single crystalline Si(100) and Si(111), p- or n-type, and analytical grade chemicals were used. Samples were ultrasonicated in acetone and, after drying with nitrogen flux, etched in concentrated hydrofluoric acid for 10 min. The hydrogenterminated surfaces thus obtained were rinsed, dried, and immersed in the plating solution. The following additives were used to prepare the aqueous solution: ammonium fluoride (NH 4 F 40%) 50 vol %, copper sulfate (CuSO 4 ·5H 2 O) 0.01 M, ascorbic acid (C 6 H 8 O 6 ) 0.01 M, sodium potassium tartrate (KNaC 4 H 4 O 6 ·4H 2 O) 0.005 M, and methanol 30 vol % (percentages are referred to the final solution volume).13 The pH of the solution was 7.5. Room temperature (25°C) and gentle agitation of samples were used. Samples were finally rinsed in deionized water and dried with nitrogen flux. The adhesion of the copper film to the substrate is strongly related to the presence of ascorbic acid in solution. The absence of the acid results in the formation of a copper film which fails the standard scotch tape test. Because we have observed a similar effect when ascorbic acid was substituted with fumaric acid, and both acids are

Surface processes in GaAs digital etching using alternating incidence of Cl radicals and a low energy Ar ion beam are discussed. For Cl feed time greater than 0.3 s, etching of GaAs does not proceed for short Ar ion irradiation times. This etching delay time is about 3 and 7 s for Cl feed time of 0.5 and 0.7 s, respectively. After etching starts, the etch rate increases and finally saturates during Ar ion irradiation. Since the same saturation value was obtained for Cl feed times between 0.5 and 0.7 s, digital etching involves a self-limiting mechanism. Rate equation analysis can explain these unique characteristics of digital etching on the assumption that physisorbed Cl layers prevent the sample surface from etching during short Ar ion irradiation times.

The vapour deposition of antimony on silver {1 1 1} under ultra-high vacuum conditions has been studied by means of reflection high-energy electron diffraction (RHEED) and Auger electron spectroscopy (AES). At room temperature, no evidence for sub monolayer structures was obtained and a deposit of an apparent thickness equivalent to six monolayers was necessary before any diffraction features typical of the overlayer could be detected. With increasing film thickness, the formation of islands with random rotational orientation was detected with Sb 〈100〉 normal to the substrate surface. Annealing of the deposit at 573 K led to the formation of the intermetallic compound Ag3Sb in epitaxial relation to the substrate. Lattice parameters for pseudo-cubic antimony (a=0.613 nm) and hexagonal Ag3Sb (a=0.295 nm andc=0.482 nm) were obtained.

Thin copper films were deposited in an ultrahigh vacuum on carbon and silicon monoxide substrates at room temperature. Their morphological changes after annealing and oxidation were investigated by electron microscopy and electron diffraction. The changes during annealing of very thin deposits (<50 Å mean thickness) are strongly influenced by the substrate. The copper on carbon substrates grows into large crystallites during heat treatment at 500°C whereas on silicon monoxide substrates very little recrystallization occurs. During the annealing of thicker deposits of copper, self-diffusion of the metal can take place and the change in structure of the heat-treated film is independent of the substrate. The structures of the cuprous oxide particles formed during exposure to different pressures of oxygen at 150° to 200°C indicate that different transport mechanisms are involved in the oxidation process at very low oxygen pressures from those at pressures above 10−2 Torr.

Alkanethiol self-assembled monolayer on copper polycrystalline thin films: Influence on resistivity

Fluorine ions were implanted at an energy of 40 keV at room temperature into Cu-implanted (160 keV, 6×1016 ions/cm2) SiO2 glass substrates in which implanted coppers occur in the form of nanosized metallic colloid particles with spherical shape. After the F implantation, the peak position of a band due to plasma oscillation shifted from 2.21 eV (562 nm) to 2.10 eV (591 nm) and the color changed from bulk copperlike to deep violet, visually. The bimodal distribution of depth Cu concentrations became distinct after the implantation. Cross-sectional transmission electron microscopy observation revealed that the distribution of copper colloid particles remains bimodal in shape and the particles coalesce with each other to form larger colloid particles with nonspherical shape. Fourier transform IR attenuated total reflection spectra showed that implanted F ions are incorporated in the form of O3Si-F units breaking continuity of the Si-O-Si network, suggesting that reduction of local viscosity in collision cascade regions during the implantation plays an important role in the observed coalescence.

Crystalline quality of low-temperature-grown InxGa1−xAs coherently grown on InP(0 0 1) substrate

Optical, electrical, and microstructural properties of thermally evaporated Cr thin films assisted by the low-energy Ar ion beam were investigated. The result shows that the optical and electrical properties are close to those of the corresponding bulk Cr: both refractive index and extinction coefficient increase, reflectance increases, and electrical resistivity decreases. The tensile stress decreases while the grain size changes slightly. From this experimental study, it is found that the low-energy Ar ion beam bombardment on growing Cr films modifies the microstructure of Cr films to improve the optical and electrical properties.

Interface adhesion enhancement by condensed aromatic ring expansion of naphthalene imide derivatives for microvia metallization by copper electroplating

This paper presents a room temperature bonding technique of glass substrates with a transparent bonding interface. As a room temperature bonding method, surface activated bonding (SAB) is applied for the bonding of glass using Si intermediate layer. However, the bonding interface is colored by the deposited Si layers, which leads to lower applicability to optical devices. In this study, a new concept of SAB is developed using aluminum oxide intermediate layer. Glass substrates are successfully bonded at room temperature via the aluminum oxide intermediate layers activated by Ar ion beam irradiation, showing a bond strength of 1.32 J m−2. It is also shown that the optical characteristics at the interface such as transparency and reflectance are not degraded, while the bonding interface of the SAB using Si intermediate layer deteriorates. The XPS and STEM‐EDX analyses indicate that the bonding is achieved through the amorphous and O‐deficient aluminum oxide interface. This technique will provide a transparent interface for glass bonding by a room temperature process, which is highly desired in optical device fabrication.

The value of the room-temperature copper-gold interdiffusion coefficient derived by extrapolating from high-temperature measurements is an underestimate by several orders of magnitude. Plating copper with a gold film has several disadvantages. Two specimens were analyzed by using Auger electron spectroscopy. Once the full thickness of the gold film is penetrated, copper accumulates on the surface, and a layer of high concentration of copper exists immediately below the gold-air interface. Electrical resistivity of an alloy is much higher than the resistivity of either component. This high-resistivity layer may be localized within the skin depth of propagating electromagnetic waves; in cases where copper has reached the surface, it is permanently within the skin depth at any frequency. A nickel diffusion barrier, commonly applied between copper and gold, is unsuitable in many microwave and millimeter-wave applications because of ferromagnetism of nickel at room temperature. The compound that forms on the surface of untreated copper at room temperature in a reasonably clean atmosphere is cuprous oxide. Its properties make it a better alternative to gold in microwave and millimeter-wave engineering. >

Cellular structure formed by ion-implantation-induced point defect