In situ measurement of mechanical properties of polyimide filmsusing micromachined resonant string structures

Two in situ measurement schemes, using micromachined resonant string structures, for the measurement of the polyimide residual stress and polyimide/metal adhesion durability have been developed. The residual stress of polyimide films, DuPont PI-2555 and PI-2611, have been measured using a bulk micromachined string structure. According to the Rayleigh's method, the resonant frequency of a polyimide string can be related to the film stress. By measuring the resonant frequency of these polyimide strings, the residual stresses have been calculated. The measurement results of various strings have been compared with conventional measurement results, which shows that they are in good agreement. Also, a noble scheme to quantize the adhesion durability between a polyimide film and a metal film has been developed. This scheme is based on a polyimide/metal bimorph string structures, fabricated using a surface micromachining technique, vibrating with an alternating potential. The change of resonance profile of this string structure can be related to the degradation of adhesion strength at the polyimide/metal interface. Various polyimide/gold string structures have been fabricated using a surface micromachining with Cu sacrificial layers, and the resonant qualities have been monitored. Notable changes of resonant Q-factor and resonant frequency, due to the degradation of adhesion between the metal and polyimide, have been observed after 10 ⁸ cycles (string vibration) for the polyimide/gold bimorph strings. The changes of resonant Q-factor and resonant frequency over a time period (vibration cycles) have been monitored     

Fabrication and adhesion strength of Cu/Ni-Cr/polyimide films for flexible printed circuits

The adhesion strength of a Cu/Ni-Cr/polyimide flexible copper clad laminate (FCCL), was evaluated according to the thickness of the Ni-Cr (Ni:Cr = 95:5 ratio) seed layer using the 90° peel test. The changes in the morphology, chemical bonding and adhesion properties were characterized by SEM, AFM and XPS. The peel strength of the FCCL increased with increasing thickness of the Ni-Cr seed layer, due to the increase in the ion bombardment caused by the higher power used in the Ni-Cr sputtering process. This increase in the FCCL peel strength was attributed to the lower proportion of C-N bonds and higher proportion of C-O bonds in the polyimide surface. The adhesion strength between the metal and polyimide was mostly attributed to the chemical interaction between the metal layer and the functional groups of the polyimide.     

Copper/Polyimide Heterojunctions: Controlling Interfacial Structures Through an Additive‐Based,All‐Wet Chemical Process Using Ion‐Doped Precursors

Heterojunctions comprising copper thin films and polyimide underlayers are exploited as an important system for generating flexible microelectronic circuit elements. A fully additive‐based chemical method that allows metallization of polyimide films with copper by the in situ reduction of copper ions doped in surface‐modified polyimide precursors is reported. It is shown that dimethylamine borane is a good reducing agent for copper ions initially complexed with carboxylate anions in the hydrolyzed polyimide layers. This reduction allows diffusion of copper ions towards the film surface to form copper thin films, and simultaneously controls the fabrication of interfacial microstructures between the copper and underlying polyimide. The formation of copper thin films and composite layers is elucidated by glow‐discharge optical emission spectrometry depth profiling, scanning electron microscopy, and cross‐sectional transmission electron microscopy studies, and it is shown that the final microstructure at the copper/polyimide interface is dependent upon experimental variables: a larger amount of copper ions incorporated into the modified layers and a higher reduction rate result in the formation of a granular layer containing smaller copper nanoparticles near the film surface. The granular layers thus formed are found to play a critical role in achieving strong adhesion between metal thin films and the substrate, owing to the increased contact area and hence the increased work of adhesion between them. These results have important implications for realizing a novel adhesion scheme between deposited metals and underlying dielectrics based on nanoscale interlocking through metal nanoparticles.     

Three-dimensional interconnect with excellent moisture resistance for low-cost MMICs

The moisture resistance of three-dimensional (3-D) interconnects using organic insulator films and Au metals has been investigated to ascertain the feasibility of housing monolithic-microwave integrated circuits with these interconnects in inexpensive nonhermetic packages. By comparing polyimide and benzocyclobutene (BCB) for organic insulator films, it was found that although polyimide has higher moisture absorption than BCB, it has a greater moisture resistance. This suggests that moisture absorption is not the dominant factor in moisture resistance and that BCB has higher water permeability than polyimide. As an adhesion layer between Au metal and insulator film, W and WN have better moisture resistance than WSi and WSiN; adhesion layer compositions containing Si oxidize easily. Further, the metal patterning method has an effect on moisture resistance in terms of leakage current. Reactive ion etching (RIE) with SF/sub 6/ gas is necessary in order to completely remove the metal atom residue left after ion milling with Ar gas. An interconnect using polyimide insulator film, W or WN adhesion metal, and metal patterned by ion milling and RIE, did not fail in terms of contact resistance and leakage current under stress of 85/spl deg/C and 85% relative humidity with a bias for 1000 h.     

A novel technology to form air gap for ULSI application

In this letter, we develop a novel process to fill the interline space with air (dielectric constant=1). A silicon wafer, whose face is downward, patterned with metal lines, is placed on the top of another silicon wafer coated with dry polyimide. Hydrogen silsesquioxanes (HSQ), FOx-16, is diluted and trickled through the slit between the metal lines and the polyimide. Then the HSQ forms an ultrathin liquid layer on the dry polyimide and contacts with the top of the metal lines. After the liquid HSQ becomes dry, the air gap is formed. The dry polyimide has good adhesion to silicon substrate but not to the dry FOx-16, so we can separate the polyimide from the dry FOx-16 and get the air gap. The liquid property of HSQ and its high selective adsorption between the metal lines and the polyimide are utilized to form the air gap     

Adhesion of aluminum films on polyimide by the electromagnetic tensile test

The electromagnetic tensile test has been adapted to the measurement of the adhesion of thin aluminum alloy films on polyimide. The adhesion per unit area was found to be weakly dependent on linewidth for lines as small as 5 μm. Processing conditions affected the adhesion with plasma ash and annealing steps improving the adhesion. Al 4% Cu was somewhat more adherent than Al-0.5% Cu.     

Interfacial Adhesion Characteristics Between Electroless-Plated Ni and Polyimide Films Modified by Alkali Surface Pretreatment

The effects of KOH + ethylenediamine (EDA) and subsequent KMnO₄ + KOH treatments on the interfacial adhesion characteristics of an electroless-plated Ni/polyimide system were investigated for flexible printed circuit board applications. The interfacial adhesion energy, evaluated from 180° peel tests during the steady-state peeling process, showed a twofold decrease after subsequent KMnO₄ + KOH treatment of the pyromellitic dianhydianiline- oxydianiline polyimide surface. Atomic force microscopy and x-ray photoemission spectroscopy results clearly reveal that this decrease can be attributed to both mechanical interlocking and chemical bonding effects.     

Adhesion improvement of silicon/underfill/polyimide interfaces by UV/ozone treatment and sol–gel derived hybrid layers

Adhesion and mechanical reliability improvement is an important issue for flexible electronics due to weak bonds between silicon/underfill/polyimide interfaces. These interfaces are bonded with weak hydrogen and ester bonds which are vulnerable to humidity. Therefore, in this study, adhesion and reliability of silicon/underfill/polyimide interfaces are enhanced by using UV/Ozone treatment and sol–gel derived hybrid layers. In order to examine the effectiveness of those surface treatment methods, double cantilever beam (DCB) test and subcritical crack growth test were applied to accurately measure the adhesion energy and subcritical crack growth rate. The results showed that the adhesion and reliability against humidity were enhanced by more than 300% and 1000% when both surface treatment methods were applied. Also, the adhesive failure path was altered to mixed mode failure of both cohesive and adhesive failure paths.     

Optimal design toward enhancement of thermomechanical reliability of polyimide layers in a flip-chip-on-lead-frame dual flat no-leads package with copper pillar bumps

Thermomechanical reliability of polyimide layers in a flip-chip-on-lead-frame dual flat no-leads package subjected to thermal cycling test condition was studied by the finite element method and the Taguchi method. Different control factors were considered for optimal design toward enhancement of the thermomechanical reliability of polyimide layers, including diameter of the Cu pillar bumps, polyimide opening, and size of the Al pad. Conforming to design rules, the largest Al pad diameter, the smallest size of bump diameter, and the largest polyimide opening were found to be beneficial to enhance the thermomechanical reliability of polyimide layers. And the optimal design was experimentally verified.     

埋容电路用高介电常数热塑性聚酰亚胺的性能研究

通过填料预分散法和原位聚合法合成了一种BaTiO3填充的热塑性聚酰亚胺树脂。考察不同填充量的填料对热塑性聚酰亚胺的力学性能,热性能和电性能的影响。结果发现:随着BaTiO3添加量的增大,TPI的介电常数和介电损耗都增大,力学性能急剧下降,尤其是柔韧性急剧降低;玻璃化转变温度不变,热分解温度相应增加,热膨胀系数减小,吸潮率降低,TPI与铜箔之间的粘接力减少。     

Metal Conductive Surface Patterning on Photoactive Polyimide

Current fabrication methods for metal interconnects and contacts are generally based on conventional photoresist fabrication procedures that require expensive equipment and multiple material/time‐consuming steps. In this work, a photopatternable polyimide is synthesized via the copolymerization of a functional diamine monomer with a 1,4‐dihydropyridine side‐chain which can decompose under UV irradiation into a pyridine group—a promising ligand for palladium ions. After the absorption of palladium ions, the electroless copper plating is carried out to form metal patterns of copper. Copper patterns with smooth boundaries are confirmed by scanning electron microscope and atomic force microscope. Robust interfacial bonding between the copper and the polyimide film is evidenced by Scotch tape adhesion tests. The photopatternable polyimide has the advantages of low Pd consumption, easy operation without expansive equipment. The linear thermal expansion coefficient of the photopatternable polyimide remains close to the one of copper wire, demonstrating the adaptability of the photopatternable polyimide for integrated circuit application. This work presents the approach of (i) the synthesis of a novel photopatternable polyimide and (ii) its application for making flexible conductive metal structures and patterned metal interconnects, which can be expected to have tremendous potential in the field of flexible electronics.     

Characterization of interfacial adhesion damage induced byaccelerated life testing

The purpose of this effort was to characterize statistically and experimentally the interfacial adhesion damage of generic dual in-line packages (DIP's) exposed to accelerated stress conditions. DIP's, consisting of three parallel gold conductors in a meander pattern on a 24 pin Al₂O₃ ceramic substrate spin-coated with Pyralin PI-2555 polyimide with and without an adhesion promoter (APS), were chosen as the test vehicles. Leakage current measurements and optical microscopy were employed to monitor the degradation process at the interface between the polyimide and the substrate. In order to characterize the damage adequately, the entire surface of each DIP was analyzed. A two-parameter Frechet cumulative distribution function (cdf) was found to be excellent for characterizing the statistical scatter in the geometrical features, such as, the cross-sectional area of the damage located over the interface. It was observed that the adhesion damage tends to be larger and clustered for more severe testing conditions. It was demonstrated that APS statistically improves long-term reliability. Furthermore, interfacial adhesion damage was not initiated solely by contamination at the interface     

Polyimide-planarized vertical-cavity surface-emitting lasers with 17.0-GHz bandwidth

High-speed oxide-confined polyimide-planarized 850-nm vertical-cavity surface-emitting lasers exhibit -3-dB modulation bandwidths up to 17.0 GHz. The devices are fabricated using a reproducible, simple process incorporating polyimide with good adhesion that does not require implantation or semiinsulating substrates to achieve low capacitance.     

Degradation of moulding compounds during highly accelerated stress tests – A simple approach to study adhesion by performing button shear tests

High temperature storage can degrade moulding compounds for chip encapsulation to such an extent that the adhesion to surfaces like copper (lead frames) or polyimide (chip coating) decreases drastically causing delamination. Also during normal operation of electronic components heat is generated locally (bond wire or chip surface) degrading the moulding compound and reducing the adhesion which in extreme cases can destroy the metallisation or the bond wires.     

Processing thick multilevel polyimide films for 3-D stacked memory

This paper discusses thick polyimide film processing for a three-dimensional (3-D) semiconductor chip-stacking application. The formation of a complex, multilevel via structure is demonstrated. The issues that arise in forming these vias relate to apply, develop, profile modification, and integration. Apply issues include “outgassing” defects, edge-bead effects, as well as the planarity and leveling of both resist and polyimide over deep-via structures. Develop issues pertain to the implementation of a thick resist process that increases the structural integrity of the resist and controls its breakage, and to a vacuum bake before applying resist, which reduces solvent absorption into the resist. Profile modification issues include rounding via edges while minimizing bulk polyimide loss and maintaining image-size control. Developer attack of the metal pads during wet processing is discussed and a solution is proposed. Finally, additional process-integration issues relating to polyimide-to-metal adhesion and composite stress levels of the multilayer thick films are presented     

聚酰亚胺衬底CIGS薄膜附着性的改善

为改善聚酰亚胺(PI)衬底Cu(In,Ga)Se2(CIGS)薄膜的附着性,提出在NaF沉积前预先在Mo层上蒸发沉积100nm厚的In-Ga-Se(IGS)薄膜的新掺Na工艺。结果表明:这种IGS-NaF-CIGS式新工艺可显著改善CIGS薄膜的附着,而且CIGS薄膜材料和器件特性没有显著退化;新工艺促进了NaInSe2的生成,减少了In-Se二元相的残余,但也造成薄膜电阻率的升高和电池填充因子的下降,进而导致制备的PI衬底CIGS电池的转换效率由9.8%降至9.0%。综合考虑附着性的改善和器件效率的轻微下降,新工艺利大于弊,有很好的应用前景。     

Feature size limit of liftoff metallization technology

The feature size limits for liftoff metallization technology are evaluated both experimentally and by computer simulation following Blech's model. The mechanism producing a smoothly sloped metallization pattern profile was also clarified. The simulation reveals that the average slope angle of the metallization pattern sidewalls is determined by the reverse mask topology, metallization thickness, and the maximum incident angle of the evaporation system used for the metallization deposition. Simulation results showed good coincidence with experimental results. It is shown that the average slope angle can be controlled between 20° and 70° with polyimide liftoff technology. Feature size limit, i.e., the minimum pitch of metallization patterns, is determined by the reverse mask topology and the maximum incident angle as well as by mechanical and chemical properties of the polyimide layer, but is independent of metallization thickness. In a sample application of the technology in the fabrication of interconnections on rugged LSI surfaces, the minimum pitch of the polyimide liftoff metallization patterns was estimated to be 2.6 µm.     

Feature Size Limit of Liftoff Metallization Technology

The feature size limits for liftoff metallization technology are evaluated both experimentally and by computer simulation following Blech's model. The mechanism producing a smoothly sloped metallization pattern profile was also clarified. The simulation reveals that the average slope angle of the metallization pattern sidewalls is determined by the reverse mask topology, metallization thickness, and the maximum incident angle of the evaporation system used for the metallization deposition. Simulation results showed good coincidence with experimental results. It is shown that the average slope angle can be controlled between 20/spl deg/and 70/spl deg/ with polyimide liftoff technology. Feature size limit, i.e., the minimum pitch of metallization patterns, is determined by the reverse mask topology and the maximum incident angle as well as by mechanical and chemical properties of the polyimide layer, but is independent of metallization thickness. In a sample application of the technology in the fabrication of interconnections on rugged LSI surfaces, the minimum pitch of the polyimide liftoff metallization patterns was estirriated to be 2.6 /spl mu/m.     

Electrical properties of ZnO Nano-particles embedded in polyimide

The ZnO nano-particles were made in the polyimide dielectric matrix by using the chemical reaction between the zinc metal film and polyamic acid. The concentration of the ZnO particle is about 1.5×10¹² cm⁻², with average size below 10 nm, and its shapes are almost spherical. Then, the polyimide layer is a stable dielectric material with a dielectric constant of 2.9. To investigate the electrical properties of ZnO particles in the polyimide insulator film, we fabricated a metal-insulator-semiconductor (MIS) structure and measured capacitance-voltage (C-V) with temperature modulation. At room temperature, C-V hysteresis with a voltage gap of 2.8 V appeared in the MIS structure using SiO₂/Si substrate. As the measuring temperature decreased, the C-V curves were shifted slightly to the accumulation region with gate bias. It was considered that the electrical charging may occur dominantly in nanoparticles, having only a few defects at the interface of the polyimide/SiO₂ and the polyimide/ZnO.     

Direct metallization of gold patterns on polyimide substrate by microcontact printing and selective surface modification

Patterned gold microstructures were fabricated on a polymer substrate by a novel method involving selective electroless plating and microcontact printing. The micro-sized gold patterns were made by the site-selective chemical modification of polyimide substrate films using aqueous potassium hydroxide solution and microcontact printing with a pitch size in the range of 20-200 μm. The base-treated area of the polyimide film became hydrophilic in the regions where the ion-exchange reactions took place for the subsequent metallization. The hydrophilic patterns were sensitized by placing the film in a solution of PdCl₂ and, subsequently, the activated substrate was immersed in an electroless plating solution of Ni and Au to provide well-developed gold patterns on polyimide substrate films.