Reliability of Au bump––Cu direct interconnections fabricated by means of surface activated bonding method

As a room temperature bonding method, surface activated bonding (SAB) method has been introduced to be one of the most appropriate interconnection methods for the next generation of electronic packaging. Thus it is important to study the reliability of SAB interconnection in long term life test.In this paper, interconnections of Au bump and Cu film bonded by SAB method were performed in high temperature thermal aging test. Degradation of properties such as electrical resistance, shear strength of bump and interface microstructure during aging process were studied to investigate the failure mechanism of the interconnection. Intermetallic compound Cu₃Au was found formed at the interface during thermal aging, and it causes evolvement of the properties and failure mode of the interconnection changing in shear test. Results reveal that SAB is suitable for the interconnection between Au bump and Cu film and it is reliable in thermal reliability test.     

Investigation of fluorine containing plasma activation for room-temperature bonding of Si-based materials

Wafer direct bonding of Si based materials, such as silicon, silicon oxide and silicon nitride, is a generic technique enabling realization of innovative structures in semiconductor industry. In this paper, a fluorine containing plasma activated bonding method is developed to achieve sufficient bonding at room temperature in air ambient with no heating process. The whole process is facile and cost effective without requiring high-vacuum system. It does work well for bonding of Si-based materials except for Si₃N₄/Si₃N₄ bonded pairs. The bonding strengths of specimens prepared by fluorine containing oxygen plasma are significantly improved at room temperature compared with those by oxygen plasma. The improved bonding strength is possibly attributed to the formation of fluorinated oxide layers on wafer surfaces after the plasma treatment.     

The influence of surface profiles on leakage in room temperature seal-bonding

We have developed a characterization method based on measured surface profiles for room temperature seal-bonding using surface activated bonding (SAB) technique and investigated the quantitative influence of surface profiles on leakage. We used the model for the bonding between the perfect flat surface which is not deformed and the metal surface with sub-nanometer rms surface roughness based on AFM surface profiles. The leak rates of the model were calculated by the Monte–Carlo method. Specific critical contact ratio P_sc gives the necessary contact ratio to disappear leak paths at a bonding interface. The P_sc was also calculated by the simulation with a basic plastic deformation law. Surface profiles of as-sputtered Au and chemical mechanical polished Cu surfaces were used as the metal surfaces. Investigating the relationship between the contact ratio and the leak rate, it was found that leak rates decrease drastically at a contact ratio close to the P_sc and leak rates decrease as the surface roughness decreases. Because of the drastic decrease of the leak rate, the vacuum sealing using SAB technique needs the contact ratio which is higher than P_sc. We also found that in the case of the as-sputtered Au deposited by the same conditions (only the deposition time change), P_sc has the tendency to decrease as the surface roughness decreases: if the rms roughness of the as-sputtered Au surface decreased from 1.5 to 0.5, the P_sc decreased from 0.6 to 0.52.     

Mechanochemical Polishing of Silicon Carbide Single Crystal with Chromium(III) Oxide Abrasive

In order to clarify the mechanism of mechanochemical polishing of SiC with Cr₂O₃ abrasive, 6H-wurtzite single-crystal specimens were dry-polished. A significant anisotropic polishing rate difference was found between Si(0001) and C(000 1 ) surfaces. The C(000 1 ) surface was removed 10 times as fast. Polished surfaces were observed from cross-sectional and plan-view directions by high-voltage TEM. There was no trace of mechanical effects such as residual strain or scratches. The polishing debris was analyzed by X-ray diffraction, high-resolution TEM, and analytical TEM. No crystalline phases were identified from X-ray diffraction patterns except for Cr₂O₃, while it was found from TEM observation that a large amount of an amorphous phase consisting of Si, C, and O was contained in the debris. These results indicated that the surface of SiC was removed mechanochemically by the aid of a catalytic oxidation effect of Cr₂O₃.     

Low-temperature bonding of surface-activated polyimide to Cu Foil in Pt-catalyzed formic acid atmosphere

Journal of Materials Science: Materials in Electronics - In this study, a two-step process involving oxygen plasma surface activation and thermos-compression in Pt-catalyzed formic acid gas was...     

Investigation of anti-stiction coating for ohmic contact MEMS switches with thiophenol and 2-naphthalenethiol self-assembled monolayer

Anti-stiction coating with a conductive self-assembled monolayer (SAM) formed by π-conjugated bonds was investigated for ohmic contact microelectromechanical system (MEMS) switches with low-load contacts. SAMs of thiophenol (C₆H₅SH, TP) or 2-naphthalenethiol (C₁₀H₇SH, 2NT) were coated on Au samples with different surface roughness to investigate the effects of the surface asperities on the adhesion force and contact resistance. The adhesion force was measured using a silicon tipless cantilever in the relative humidity range of 10-85% and the contact resistance was measured in the contact force range of 0-70 μN using a conductive tipless cantilever coated with Au for the SAM coated samples and compared with those for a Au sample surface. The adhesion force measurements indicate that the TP and 2NT coatings can prevent a liquid meniscus from forming on device surfaces due to their hydrophobic character caused by the protruding aromatic group. In addition, it was confirmed that these coatings could reduce van der Waals forces more than Au coating. Contact resistance measurements revealed that an electric current begins to flow with smaller contact force for TP and 2NT coated samples than for Au coated samples. The measured contact resistances of the SAM and Au coated samples were comparable. Based on these results, SAMs of TP and 2NT have excellent potential as anti-stiction coating for MEMS switch contacts.     

表面活化连接/SAB微电子系统超高密度互连技术

介绍一种微电子系统用新型超高密度、高可靠互连方法及表面活化连接(SAB)技术。同种或者不同材料之间,经过洁净和表面活化处理之后,在一定条件下,适当的压力下可以获得非常可靠的连接,同传统的引线键合、焊料球倒装焊、导电胶连接等相比,SAB具有明显的优势,特别适合于开发高集成度、微小型的电子装置。并介绍相关的SAB室温连结装置以及低温SAB倒装焊机,SAB技术用于开发新型微电子系统的一些具体例子。     

Pt催化甲酸气氛下的碳化硅低温键合

为实现碳化硅（SiC）在300 ℃以下的低温键合,研究了以Cu和Au作为键合中间层,利用Pt催化甲酸气氛预处理金属膜表面并为键合提供还原性保护气氛的低温键合方法。首先,研究了带有Cu金属层的SiC晶片在氮气、甲酸气氛以及Pt催化甲酸气氛下的键合过程,通过对比键合强度来验证Pt催化甲酸气氛下完成带有Cu层的SiC晶片键合的可能性。其次,采用扫描声学显微镜（SAM）和扫描电子显微镜（SEM）对键合后样品进行表征,并通过改变键合温度及键合加载的压力,研究关键工艺参数对于键合强度的影响。最后,对比了Cu,Au两种中间层金属在不同气氛下的SiC-SiC键合效果。研究结果表明:相比不含甲酸的N₂气氛以及未经过Pt催化的甲酸气氛,经过Pt催化的甲酸气氛能够更好地还原Cu表面的氧化物,实现带有Cu金属层的SiC-SiC低温键合;SEM、SAM测试表明键合界面无明显空洞,键合效果良好;与使用Au层的SiC样品相比,在低温下基于Cu中间层的SiC样品键合强度显著高于Au中间层的样品,显示了本方法对还原表面主要为氧化物的金属层键合的优越性。本方法利用Pt催化甲酸气氛预处理Cu表面并为键合提供还原性保护气氛,在200 ℃时获得了12.5 MPa的剪切强度,实现了高强度的SiC-SiC低温键合。     

Vapor-Assisted Surface Activation Method for Homo- and Heterogeneous Bonding of Cu, SiO2, and Polyimide at 150°C and Atmospheric Pressure

Homo- and heterogeneous bonding of Cu, SiO₂, and polyimide, by using a single vapor-assisted surface activation method at 150°C and atmospheric pressure, is highly feasible and will be of practical use in three-dimensional heterointegration of thin, flat interconnection layers. Since it is necessary to achieve good bondability to diverse materials in a single process in order to maintain bumpless structures, we have to create a compatible bridging layer. Bridging layers, based on Cu hydroxide hydrate and silanol and hydroxyl groups formed from SiO₂ and polyimide, respectively, were prepared by introducing water onto the activated surfaces at atmospheric pressure. The growth rate of the bridging layers was tunable via absolute humidity, and exposure of 8 g/m³ was used. Heating at 150°C, after exposure to humidity, caused tight adhesion between the mating surfaces for all combinations of starting materials with voidless amorphous interfacial (bridging) layers. Because of the well-controlled layer thickness, low electrical resistivity of ～4 × 10^?8 Ω m was obtained at the Cu–Cu interface.