Self-assembled monolayers (SAMs) of alkanethiols and dialkanethiols on gold are key elements for building many systems and devices with applications in the wide field of nanotechnology. Despite the progress made in the knowledge of these fascinating two-dimensional molecular systems, there are still several “hot topics” that deserve special attention in order to understand and to control their physical and chemistry properties at the molecular level. This critical review focuses on some of these topics, including the nature of the molecule–gold interface, whose chemistry and structure remain elusive, the self-assembly process on planar and irregular surfaces, and on nanometre-sized objects, and the chemical reactivity and thermal stability of these systems in ambient and aqueous solutions, an issue which seriously limits their technological applications (375 references).

Self-assembled monolayers of thiols and dithiols on gold: new challenges for a well-known system

Qualitative and quantitative studies of the oxidation of polycrystalline copper (Cu) thin films upon exposure to ambient air conditions for long periods (on the order of several months) are reported in this work. Thin films of Cu, prepared by thermal evaporation, were analyzed by means of X-ray photoelectron spectroscopy (XPS) to gain an understanding on the growth mechanism of the surface oxide layer. Analysis of high-resolution Cu LMM, Cu2p3/2, and O1s spectra was used to follow the time dependence of individual oxide overlayer thicknesses as well as the overall oxide composite thickness. Transmission electron microscopy (TEM) and spectroscopic ellipsometry (SE) were used to confirm the results obtained from XPS measurements. Three main stages of copper oxide growth were observed:  (a) the formation of a Cu2O layer, most likely due to Cu metal ionic transport toward the oxide−oxygen interface, (b) the formation of a Cu(OH)2 metastable overlayer, due to the interactions of Cu ions with hydroxyl groups pr...

Oxidation of Polycrystalline Copper Thin Films at Ambient Conditions

Using standard single damascene type techniques on bulk-Si, combined on one hand with extreme wafer thinning and on the other with Cu-Cu thermo-compression bonding technology, the paper demonstrate yielding 10k through-wafer 3D-via chains with a via pitch of 10μm for a via diameter of 5μm. The bonded contacts exhibit shear strengths exceeding 40MPa. Measurements indicate there is no significant contact resistance at the Cu-Cu bonded interface: within measurement accuracy, the 4-point via chain resistance is consistent with bulk Cu resistivity

3D integration by Cu-Cu thermo-compression bonding of extremely thinned bulk-Si die containing 10 μm pitch through-Si vias

A review article on fundamental aspects of thiolate self-assembled monolayers (SAMs) on the (111) and (100) surfaces of the Cu and Ni groups is presented. In particular this work is focused on two important points that remain poorly understood in most of these metals: the chemistry of the S-metal interface, which strongly depends on the nature of the metallic surface, and the role of the interaction forces that not only guide the self-assembly process but also influence the surface structure of SAMs. In addition to recent experimental and theoretical data on these issues we present new density functional calculations including van der Waals forces for an important number of known thiolate surface structures as a function of the hydrocarbon chain length.

Self-assembled monolayers of thiolates on metals: A review article on sulfur-metal chemistry and surface structures

Self-assembled monolayer (SAM) of 1-hexanethiol is applied on copper (Cu) surface to retard surface oxidation during exposure in the ambient. This SAM layer can be desorbed effectively with an annealing step in inert N2 ambient to provide a clean Cu surface. Using this passivation method with SAM, wafers covered with thin Cu layer are passivated, stored, desorbed, and bonded at 250 °C. The bonded Cu layer presents clear evidence of substantial interdiffusion and grain growth despite prolonged exposure in the ambient. This method of passivation is proven to be effective and can be further optimized to enable high quality Cu–Cu direct bonding at low temperature for application in three-dimensional integration.

Cu-Cu diffusion bonding enhancement at low temperature by surface passivation using self-assembled monolayer of alkane-thiol

Corrosion inhibition by self-assembled monolayers for enhanced wire bonding on Cu surfaces

Formation and stability of thiol-derived self-assembled monolayers (sAMs) on Cu surfaces have been investigated using a multitechnique characterization approach, including contact angle measurements, X-ray photoelectron spectroscopy, ellipsometry, and electrochemistry. High-quality SAMs were formed on Cu surfaces under various conditions of substrate preparation, thiol molecular structure, adsorption from the gas and liquid phases, concentration, solvent effects, and self-assembly time. Stability studies exploring SAM resistance to aging, processing chemicals, temperature, and electrochemically induced oxidation-reduction reactions demonstrate promising corrosion inhibition properties. The feasibility of exploiting SAMs in microelectronics applications was demonstrated by the enhancement of Cu wire bonding onto thiol-passivated Cu bond pads.

https://iopscience.iop.org/article/10.1149/1.1635387/pdf

Corrosion Inhibition by Thiol-Derived SAMs for Enhanced Wire Bonding on Cu Surfaces

Direct gold and copper wires bonding on copper

The treatment of oxidized Cu surfaces using an alkanethiol as a reducing agent has been investigated. Exposure to a dilute solution of 1-decanethiol resulted in the complete removal and/or conversion of CuO and subsequent formation of a passivating thiolate film, a so-called self-assembled monolayer (SAM), on the underlying Cu/Cu2O surface as evidenced by x-ray photoelectron spectroscopy (XPS) analysis. Morphological changes, monitored by scanning electron microscopy (SEM) and atomic force microscopy (AFM), revealed transformation of the rough, porous CuO layer into a comparatively smooth Cu/Cu2O surface. Experiments performed on integrated circuit back-end-of-line (BEOL) die structures, comprising Cu/SiO2 bond pads used as substrates for Cu wire bonding, demonstrate the potential application of a thiol-based in-situ cleaning-passivation procedure in microelectronics.

In-situ cleaning and passivation of oxidized Cu surfaces by alkanethiols and its application to wire bonding

The development of a new copper wire bonding process for fine pitch applications is currently in progress. The bond pad cleanliness and oxidation levels are important for the ball bond (first bond). By applying an organic coating layer it is possible to protect the copper bond pad from oxidation. Copper wire in an unopened sealed spool already has a thin layer of copper oxide on the surface and does not readily weld (stitch bond) to silver spot plated lead frames and BGA substrates with Au metallization. The intent of this study is to establish the effects of the process parameters for a wire bond process with copper wire with an initial bond pad of 70 /spl mu/m. The copper ball bond on copper bond pad and copper wire stitch bond on silver spot plated lead frame and BGA substrate as well the materials were studied.

Hong Meng Ho

Papers

Corrosion inhibition by self-assembled monolayers for enhanced wire bonding on Cu surfaces

Corrosion Inhibition by Thiol-Derived SAMs for Enhanced Wire Bonding on Cu Surfaces

Direct gold and copper wires bonding on copper

In-situ cleaning and passivation of oxidized Cu surfaces by alkanethiols and its application to wire bonding

Fine pitch copper wire bonding on copper bond pad process optimization