Self-assembled Monolayer Passivation Research Articles

Cu-Cu Thermocompression Bonding with a Self-Assembled Monolayer as Oxidation Protection for 3D/2.5D System Integration.

Cu-Cu direct interconnects are highly desirable for the microelectronic industry as they allow for significant reductions in the size and spacing of microcontacts. The main challenge associated with using Cu is its tendency to rapidly oxidize in air. This research paper describes a method of Cu passivation using a self-assembled monolayer (SAM) to protect the surface against oxidation. However, this approach faces two main challenges: the degradation of the SAM at room temperature in the ambient atmosphere and the monolayer desorption technique prior to Cu-Cu bonding. In this paper, the systematic investigation of these challenges and their possible solutions are presented. The methods used in this study include thermocompression (TC) bonding, X-ray photoelectron spectroscopy (XPS), shear strength testing, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDX). The results indicate nearly no Cu oxidation (4 at.%) for samples with SAM passivation in contrast to the bare Cu surface (27 at.%) after the storage at -18 °C in a conventional freezer for three weeks. Significant improvement was observed in the TC bonding with SAM after storage. The mean shear strength of the passivated samples reached 65.5 MPa without storage. The average shear strength values before and after the storage tests were 43% greater for samples with SAM than for the bare Cu surface. In conclusion, this study shows that Cu-Cu bonding technology can be improved by using SAM as an oxidation inhibitor, leading to a higher interconnect quality.

Effect of Head Groups in Self-Assembled Monolayer Passivation on Properties of InSnZnO Thin-Film Transistors

Effect of Head Groups in Self-Assembled Monolayer Passivation on Properties of InSnZnO Thin-Film Transistors

Hexene passivation on a p-type Zn-doped InGaAs surface

Indium gallium arsenide (InGaAs) is a promising candidate for high-performance complementary metal-oxide-semiconductor (CMOS) channel materials. In this study, self-assembled monolayer passivation was performed on the surface of p-type Zn-doped InGaAs to improve the semiconductor/dielectric interfacial electrical properties. In particular, the oxidation behavior and surface state change that occurred with hexene passivation were analyzed. A relatively thin oxide was formed on the hexene-passivated InGaAs surface and compared to the unpassivated surface after exposure to air over time. It was observed that oxidation was effectively suppressed for all the In, Ga, and As elements. It is considered from the relationship between the oxidation time and the oxide thickness change that the initial oxidation surface reaction is hindered by hexene passivation. In addition, the depletion depth was reduced from 28.2 to 24.4 nm and the interface trapped charge density of InGaAs MOS capacitor was decreased from 4.29 × 1013 to 2.24 × 1013 cm−2eV−1 after hexene passivation. The improvement in the interfacial electrical performance of the MOS capacitor may result from the suppressed oxidation caused by the passivation with hexene on the InGaAs surface.

Highly stable lithium anode enabled by self-assembled monolayer of dihexadecanoalkyl phosphate

Li has been considered as the ultimate anode material for high energy density secondary Li batteries. However, its practical application has been limited due to its low Coulombic efficiency (CE) and the formation of lithium dendrites. Recently, we have developed a microspherical Li-carbon nanotube (Li-CNT) composite material passivated with octadecylphosphonic acid (OPA) self-assembled monolayer (SAM) exhibiting suppressed lithium dendrite formation and improved environmental/electrochemical stability. In this work, we demonstrated the significantly enhanced passivation effects of a SAM using dihexadecanoalkyl phosphate (DHP), a molecule that is comprised of double hydrophobic alkyl chains and forms a denser SAM on surfaces with large curvature. As a result, the DHP SAM delivers superior environmental and electrochemical stability to the OPA passivated Li-CNT material. In specific, the DHP passivated Li-CNT composite (DHP-Li-CNT) delivers a high CE of 99.25% under a 33.3% depth of discharge (DOD) at 1 C, when it is paired with a LiFePO4 cathode. The evolution of the SAM during cycling and the effects of DOD and current density on the CE of the DHP-Li-CNT anode have also been investigated. The improved SAM passivation constitutes an important step in achieving the goal of practically applicable Li anodes.

Characterisation of Cu/Cu bonding using self-assembled monolayer

Purpose Cu/Cu diffusion bonding is characterised by high electrical and thermal conductivity, as well as the mechanical strength of the interconnects. But despite a number of advantages, Cu oxidises readily upon exposure to air. To break through the adsorbed oxide-layer high temperature and pressure, long bonding time and inert gas atmosphere are required during the bonding process. This paper aims to present the implementation of an organic self-assembled monolayer (SAM) as a temporary protective coating that inhibits Cu oxidation. Design/methodology/approach Information concerning elemental composition of the Cu surface has been yielded by X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) spectroscopy. Two types of substrates (electroplated and sputtered Cu) are prepared for thermocompression bonding in two different ways. In the first case, Cu is cleaned with dilute sulphuric acid to remove native copper oxide. In the second case, passivation with SAM followed the cleaning step with dilute sulphuric acid. Shear strength, fracture surface, microstructure of the received Cu/Cu interconnects are investigated after the bonding procedure. Findings The XPS method revealed that SAM can retard Cu from oxidation on air for at least 12 h. SAM passivation on the substrates with sputtered Cu appears to have better quality than on the electroplated ones. This derives from the results of the shear strength tests and scanning electron microscopy (SEM) imaging of Cu/Cu interconnects cross sections. SAM passivation improved the bonding quality of the interconnects with sputtered Cu in comparison to the cleaned samples without passivation. Originality/value The Cu/Cu bonding procedure was optimised by a novel preparation method using SAMs which enables storage and bonding of Si-dies with Cu microbumps at air conditions while remaining a good-quality interconnect. The passivation revealed to be advantageous for the smooth surfaces. SEM and shear strength tests showed improved bonding quality for the passivated bottom dies with sputtered Cu in comparison to the samples without SAM.

Reversible Biofunctionalization of Surfaces with a Switchable Mutant of Avidin

Label-free biosensors detect binding of prey molecules (″analytes″) to immobile bait molecules on the sensing surface. Numerous methods are available for immobilization of bait molecules. A convenient option is binding of biotinylated bait molecules to streptavidin-functionalized surfaces, or to biotinylated surfaces via biotin-avidin-biotin bridges. The goal of this study was to find a rapid method for reversible immobilization of biotinylated bait molecules on biotinylated sensor chips. The task was to establish a biotin-avidin-biotin bridge which was easily cleaved when desired, yet perfectly stable under a wide range of measurement conditions. The problem was solved with the avidin mutant M96H which contains extra histidine residues at the subunit-subunit interfaces. This mutant was bound to a mixed self-assembled monolayer (SAM) containing biotin residues on 20% of the oligo(ethylene glycol)-terminated SAM components. Various biotinylated bait molecules were bound on top of the immobilized avidin mutant. The biotin-avidin-biotin bridge was stable at pH ≥3, and it was insensitive to sodium dodecyl sulfate (SDS) at neutral pH. Only the combination of citric acid (2.5%, pH 2) and SDS (0.25%) caused instantaneous cleavage of the biotin-avidin-biotin bridge. As a consequence, the biotinylated bait molecules could be immobilized and removed as often as desired, the only limit being the time span for reproducible chip function when kept in buffer (2-3 weeks at 25 °C). As expected, the high isolectric pH (pI) of the avidin mutant caused nonspecific adsorption of proteins. This problem was solved by acetylation of avidin (to pI < 5), or by optimization of SAM formation and passivation with biotin-BSA and BSA.

Effects of surface treatment on the bonding quality of wafer-level Cu-to-Cu thermo-compression bonding for 3D integration

Various surface treatments are applied for surface oxide removal prior to wafer-level Cu-to-Cu thermo-compression bonding and the bonding quality is systematically analyzed in this work. Three methods are investigated: self-assembled monolayer (SAM) passivation, forming gas annealing and acetic acid wet cleaning. The surface conditions are carefully examined including roughness, contact angle and x-ray photoelectron spectroscopy (XPS) scan. The wafer pairs are bonded at 250 °C under a bonding force of 5500 N for a duration of 1 h in a vacuum environment. The bonding medium consists of a Cu (300 nm) bonding layer and a Ti (50 nm) barrier layer. The bonding quality investigation consists of two parts: hermeticity based on helium leak test and mechanical strength using four-point bending method. Although all samples under test with different surface treatment methods present an excellent hermetic seal and a robust mechanical support, the measurement results show that samples bonded after SAM passivation exhibit the best hermeticity and bonding strength for 3D integration application.

(Invited) Cu Surface Passivation with Self-Assembled Monolayer (SAM) and Its Application for Wafer Bonding at Moderately Low Temperature

In this paper, low temperature (<300 oC) thermo-compression bonding of bump-less Cu-Cu in the context of 3D IC application is discussed. The Cu surface degradation is overcome by the application of a self-assembled monolayer (SAM) of alkane-thiol as a temporary passivation layer. This method is potentially attractive as it is non-UHV and non-corrosive. With SAM passivation, surface oxidation and contamination are controlled and upon SAM desorption, a cleaner Cu surface is available for bonding at low temperature. SAM passivated Cu shows higher degree of interdiffusion and grain growth during bonding. This results in the enhancement of the physical properties of the Cu-Cu bond which is potentially suitable for 3D IC stacking.

Cu–Cu Hermetic Seal Enhancement Using Self-Assembled Monolayer Passivation

Low-temperature Cu–Cu thermocompression bonding enabled by self-assembled monolayer (SAM) passivation for hermetic sealing application is investigated in this work. Cavities are etched to a volume of 1.4 × 10−3 cm3 in accordance with the MIL-STD-883E standard prescribed for microelectronics packaging. The wafer pairs (nonfunctional cavity wafer and cap wafer) are annealed and bonded at 250°C under a bonding force of 5500 N. The encapsulated cavities undergo helium overpressure in a bombing chamber, and the helium leak rate is detected by a mass spectrometer. The measurement results show that the cavities sealed with Cu–Cu bonding after SAM passivation exhibit excellent hermeticity with a leak rate below 10−9 atm cm3/s, which is an improvement of at least 2× compared with the control sample without SAM passivation.

Role of Self-Assembled Monolayer Passivation in Electrical Transport Properties and Flicker Noise of Nanowire Transistors

Semiconductor nanowires have achieved great attention for integration in next-generation electronics. However, for nanowires with diameters comparable to the Debye length, which would generally be required for one-dimensional operation, surface states degrade the device performance and increase the low-frequency noise. In this study, single In(2)O(3) nanowire transistors were fabricated and characterized before and after surface passivation with a self-assembled monolayer of 1-octadecanethiol (ODT). Electrical characterization of the transistors shows that device performance can be enhanced upon ODT passivation, exhibiting steep subthreshold slope (~64 mV/dec), near zero threshold voltage (~0.6 V), high mobility (~624 cm(2)/V·s), and high on-currents (~40 μA). X-ray photoelectron spectroscopy studies of the ODT-passivated nanowires indicate that the molecules are bound to In(2)O(3) nanowires through the thiol linkages. Device simulations using a rectangular geometry to represent the nanowire indicate that the improvement in subthreshold slope and positive shift in threshold voltage can be explained in terms of reduced interface trap density and changes in fixed charge density. Flicker (low-frequency, 1/f) noise measurements show that the noise amplitude is reduced following passivation. The interface trap density before and after ODT passivation is profiled throughout the band gap energy using the subthreshold current-voltage characteristics and is compared to the values extracted from the low-frequency noise measurements. The results indicate that self-assembled monolayer passivation is a promising optimization technology for the realization of low-power, low-noise, and fast-switching applications such as logic, memory, and display circuitry.

Enhancing Cu-Cu Diffusion Bonding at Low Temperature Via Application of Self-assembled Monolayer Passivation

Self-assembled monolayer (SAM) of alkane-thiol is employed to passivate Cu surface in an attempt to preserve its surface cleanliness. Alkane-thiol molecules can self-assemble readily onto the Cu surface forming a densely packed monolayer. SAM formed on Cu surface is found to be able to retard surface oxidation and the degree of protection can be enhanced by alkane-thiol with longer carbon chain length. SAM formed is prone to partial degradation in ambient air and can be desorbed by thermal annealing at moderate temperature (<300°C). SAM passivated Cu wafers are bonded after a pre-bonding anneal step to desorb the SAM. The bonded wafers show enhanced bonding uniformity and efficient Cu cross-diffusion compared to control wafers when shorter chain length of alkane-thiol molecule (n = 6) is used. SIMS analysis shows that there are carbon and oxygen impurities pile-up at the bonding interface when longer chain length alkane-thiol (n = 12) is used, as a result of incomplete desorption prior to bonding. With the selection of the appropriate chain length, clean surface can be conserved after desorption in inert ambient and hence promoting grain growth during Cu bonding. Mechanical shear measurement is performed and enhancement in strength with SAM passivation is confirmed.

Void Density Reduction at the Cu–Cu Bonding Interface by Means of Prebonding Surface Passivation with Self-Assembled Monolayer

A self-assembled monolayer (SAM) of 1-hexanethiol is formed on a copper (Cu) thin layer coated on silicon (Si) wafers with the aim to protect the surface against contamination during storage in room ambient. After 3 days of storage, the SAM is desorbed with an in situ annealing step in inert N 2 ambient to provide a clean Cu surface and a pair of wafers is bonded at 250°C. It is observed with scanning acoustic microscopy that there is a clear reduction in the interfacial void density in the bonded pair compared with the control sample without SAM passivation.

Atomic Force Microscopy Studies of Aluminum Corrosion Inhibition by Adsorbed Self Assembled Monolayers

Abstract Atomic force microscopy (AFM) studies on the microstructure of the phosphonic acid (PA) / Al (with native oxide) self assembled monolayer (SAM) system (a possible corrosion resistant coating interface monolayer) has been evaluated as a function of molecular chain length and processing conditions (i.e. adsorption time) in previous works. It was apparent from the AFM topography and phase images of the monolayer microstructure that the octadecylphosphonic acid (OPA) monolyer was the most conformal and showed little dependence on adsorption time (for times ≤ 24 hours). It was therefore chosen as the most promising monolayer compound of the PA/A1 system with which to study corrosion inhibiting properties. In this present work, AFM is used to study the corrosion inhibiting ability of OPA SAMs on aluminum, and also to investigate the “nucleation” and progression of Al corrosion and oxide formation in water at the nanometer level; with and without SAM passivation.

GaAs Interfaces with Octadecyl Thiol Self-Assembled Monolayer: Structural and Electrical Properties

A passivation layer on chemically etched (100) GaAs surface was synthesized with octadecyl thiol, CH3(CH2)17SH, and its structural and electrical properties were investigated. The layer is a self-assembled monolayer (SAM) in all-trans-planar-zig-zag conformation and is the first conformationally ordered SAM on any semiconductor surface. The barrier height of GaAs Schottky diodes can be modified by the octadecyl thiol SAM passivation without sacrificing good diode characteristics. Octadecyl and other alkane thiol SAM's enable introduction of a nano-scale, controlled structure on a GaAs surface and prove useful in an assessing the GaAs surface electrical properties.