Wafer-level manufacturing technology of glass microlenses

In high-tech products, there is an increasing demand to integrate glass lenses into complex micro systems. Especially in the lighting industry LEDs and laser diodes used for automotive applications require encapsulated micro lenses. To enable low-cost production, manufacturing of micro lenses on wafer level base using a replication technology is a key technology. This requires accurate forming of thousands of lenses with a diameter of 1–2 mm on a 200 mm wafer compliant with mass production. The article will discuss the technical aspects of a lens manufacturing replication process and the challenges, which need to be solved: choice of an appropriate master for replication, thermally robust interlayer coating, choice of replica glass, bonding and separation procedure. A promising approach for the master substrate material is based on a lens structured high-quality glass wafer with high melting point covered by a coating layer of amorphous silicon or germanium. This layer serves as an interlayer for the glass bonding process. Low pressure chemical vapor deposition and plasma enhanced chemical vapor deposition processes allow a deposition of layer coatings with different hydrogen and doping content influencing their chemical and physical behavior. A time reduced molding process using a float glass enables the formation of high quality lenses while preserving the recyclability of the mother substrate. The challenge is the separation of the replica from the master mold. An overview of chemical methods based on optimized etching of coating layer through small channels will be given and the impact of glass etching on surface roughness is discussed.     

紫外/臭氧法PDMS表面亲水改性机理研究

紫外/臭氧改性法是一种操作简单、成本低廉的PDMS表面亲水改性方法。采用该方法对PDMS表面进行亲水改性,利用接触角测量仪对改性效果进行评价,并与PDMS无臭氧紫外法进行了比较。测试表明PDMS表面经紫外/臭氧法处理12小时后,表面接触角达到60°左右,在空气中放置两周后仍保持较好的亲水性。其改性机理可以通过多种表征手段进行分析。红外光谱测试可以看出,PDMS在经过紫外/臭氧改性后,其表面官能团变化明显,随改性时间延长,疏水基团—CH₃逐渐减少,亲水基团Si—OH和—OH逐步增加,二氧化硅典型红外光谱峰也同时出现。通过扫描电镜和能谱测试可以看出,PDMS表面经过改性产生了二氧化硅为主的硅的氧化物。综合上述结果,紫外/臭氧处理法能够使PDMS表面亲水基团增多,同时生成类玻璃态SiO_x薄层,既改善了PDMS表面的亲水性,又阻止了PDMS表面疏水性的完全恢复,亲水性可以长时间保持。     

High-density-plasma (HDP)-CVD oxide to thermal oxide wafer bonding for strained silicon layer transfer applications

Direct wafer bonding between high-density-plasma chemical vapour deposited (HDP-CVD) oxide and thermal oxide (TO) has been investigated. HDP-CVD oxides, about 230 nm in thickness, were deposited on Si(0 0 1) control wafers and the wafers of interest that contain a thin strained silicon (sSi) layer on a so-called virtual substrate that is composed of relaxed SiGe (∼4 μm thick) on Si(0 0 1) wafers. The surfaces of the as-deposited HDP-CVD oxides on the Si control wafers were smooth with a root-mean-square (RMS) roughness of <1 nm, which is sufficiently smooth for direct wafer bonding. The surfaces of the sSi/SiGe/Si(0 0 1) substrates show an RMS roughness of >2 nm. After HDP-CVD oxide deposition on the sSi/SiGe/Si substrates, the RMS roughness of the oxide surfaces was also found to be the same, i.e., >2 nm. To use these wafers for direct bonding the RMS roughness had to be reduced below 1 nm, which was carried out using a chemo-mechanical polishing (CMP) step. After bonding the HDP-CVD oxides to thermally oxidized handle wafers, the bonded interfaces were mostly bubble- and void-free for the silicon control and the sSi/SiGe/Si(0 0 1) wafers. The bonded wafer pairs were then annealed at higher temperatures up to 800 °C and the bonded interfaces were still found to be almost bubble- and void-free. Thus, HDP-CVD oxide is quite suitable for direct wafer bonding and layer transfer of ultrathin sSi layers on oxidized Si wafers for the fabrication of novel sSOI substrates.     

Fabrication of patterned gold electrodes with spin-coated-and-fired Au(1 1 1) film by the soft lithography

We prepared high quality Au(1 1 1) film on Si wafer through the spin coating and thermal decomposition of a gold ink, spin-coated-and-fired (SCAF) Au film. The X-ray measurements, XRD and pole-figure analysis, showed that the SCAF Au film has a (1 1 1) out-of-plane orientation with a random in-plane orientation. In order to confirm the chemical activity of the SCAF Au film, we demonstrate the formation of patterned structures with the film by using soft lithography technique. The chemical activities of this physically stable SCAF Au film to the alkanethiols were at least equivalent those of physically deposited the Au films. The possibility of the mass production of micro patterned structure with the SCAF Au film was also demonstrated over the wide region on Si wafer by the microcontact lithography. These suggest that the Au film will help the easy fabrication of various nanosized devices on Si wafer and other substrates.     

溶胶-凝胶光学薄膜的激光损伤研究

 采用溶胶-凝胶工艺制备了SiO₂与ZrO₂单层介质膜，用输出波长1.06μm，脉宽15ns的电光调Q激光系统产生的强激光进行辐照实验。观察了光学薄膜经强激光辐照后的损伤情况，讨论了溶胶 凝胶光学薄膜在强激光照射下的损伤机理，提出了溶剂替换、紫外光处理、添加有机粘接剂等提高溶胶 凝胶光学薄膜激光损伤阈值的方法。     

热透镜技术中反射和透射两种探测方式的比较分析

热透镜技术按照测量方式的不同可以分为反射式和透射式两种。这两种方式的灵敏度强烈地依赖于样品的热物理性质,对于不同的样品,选择合适的测量构型尤为重要。采用高斯激光束作为加热光源,分别从理论和实验上比较分析了模式不匹配的反射式和透射式两种热透镜探测构型在测量以BK7玻璃和石英为基底的高反射光学薄膜弱吸收中的应用,结果表明,对以BK7玻璃为基底的样品,采用反射式探测构型具有较高的灵敏度,而对以石英为基底的样品而言,采用透射式探测构型则具有更高的灵敏度,为实际光学薄膜吸收损耗测试灵敏度的提高提供了理论和实验依据。     

Selective gold nano-patterning on flexible polymer substrate via concurrent nanoimprinting and nanotransfer printing

We report a simple technique of selective gold nano-patterning on non-planar polycarbonate substrate by combining nanoimprinting and gold nanotransfer printing techniques in a single concurrent nano-patterning process: thermal nanoimprinting directly patterns a MPTMS (3-mercaptopropyl trimethoxysilane)-coated polycarbonate sheet to form the non-planar nanostructures using a gold-film deposited mold. Simultaneously, the gold-film from the mold is selectively transfer-printed either onto the protrusion or the base of the imprinted non-planar structures. This high nano-patterning selectivity is achieved due to a combined effect of a thiol-terminated MPTMS-gold surface chemistry, more importantly, aided by a surface area dependent differential work of adhesion. We show the high delineation of the patterned gold on the non-planar polycarbonate substrate of various geometries such as pillars, dimples, and gratings, down to nano-scale resolution (130 nm) as well as over micro-scale resolution (10 μm), thus demonstrating that this can be a generic metal patterning technique. Using this technique, we fabricate a metal nanowire grid polarizer to demonstrate a potential device application. The main advantage of this technique lies in its inherent self-aligned process that simplifies selective Au nano-patterning on non-planar polymer surfaces, which is otherwise difficult to be obtained using conventional patterning techniques.     

Single Crystal Si Layers on Glass Fabricated by Hydrophilic Fusion Bonding and Smart-Cut Technology

A single crystal Si thin film on a glass substrate has been obtained successfully by hydrophilic fusion bonding and the smart-cut technology. Tensile strength testing shows that the bonded interface has strong adhesion and the bonding strength is about 8.7MPa. Crystallinity and microstructure of the samples have been characterized by transmission electron microscopy (TEM). Electrical properties have also been investigated by Hall measurements and four-point probe. The mobility of the transferred Si layer on glass is about 122cm^2//V.s. The results show that the single-crystal silicon layer transferred onto glass by direct bonding keeps good quality for the applications of integrated circuits, transducers, and fiat panel display.     

Micropatterning and transferring of polymeric semiconductor thin films by hot lift-off and polymer bonding lithography in fabrication of organic field effect transistors (OFETs) on flexible substrate

Micropatterning and transferring of polymeric semiconductor thin films by hot lift-off and polymer bonding lithography in fabrication of OFETs with polymeric dielectric on the flexible substrate was proposed. The desired polymeric semiconductor patterns were fabricated on the flat polydimethylsiloxane (PDMS) surface with a selective lift-off method we proposed previously. The isolated and well defined polymeric semiconductor patterns left on the flat PDMS surface can be further transferred to the gate polymeric dielectric surface by polymer bonding lithography due to the low interfacial energy of PDMS. The transistor fabricated with this ‘dry’ process has a higher field-effect mobility compared with that using spin coated semiconductor layer.     

Si layer transfer to InP substrate using low-temperature wafer bonding

Using a low-temperature wafer bonding process, InP substrates are bonded to silicon-on-insulator (SOI) substrates at 220 °C. A combination of oxygen plasma and chemical treatment results in a direct contact bonding at room temperature. After the bonding process at 220 °C for 45 min, removal of the Si handle substrate by sacrificial etching of the buried oxide layer in SOI, results in a thin membrane of Si robustly bonded to InP. The thin Si membrane bonded to InP shows uniformly bonded interface under high-resolution electron microscopy. Micro-Raman analysis has also been carried out to study the bonded interface. I-V characteristics of the bonded structures suggest that such bonding and layer transfer processes are suitable for device integration.     

Mechanics of plasma exposed spin-on-glass (SOG) and polydimethyl siloxane (PDMS) surfaces and their impact on bond strength

Silicone polymer (PDMS), widely used for micro-fluidic and biosensor applications, possesses an extremely dynamic surface after it is subjected to an oxygen plasma treatment process. The surface becomes extremely hydrophilic immediately after oxygen plasma exposure by developing silanol bond (SiOH), which promotes its adhesion to some other surfaces like, silicon, silicon dioxide, glass, etc. Such a surface, if left in ambient dry air, shows a gradual recovery of hydrophobicity. We have found an identical behavior to occur to surfaces coated with a thin continuous film of SOG (methyl silsesquioxane). The chemistry induced by oxygen plasma treatment of a spin-on-glass (SOG) coated surface provides a much higher density of surface silanol groups in comparison to precleaned glass, silicon or silicon dioxide substrates thus providing a higher bond strength with polydimethyl siloxane (PDMS). The bonding protocol developed by using the spin coated and cured SOG intermediate layer provides an universal regime of multi level wafer bonding of PDMS to a variety of substrates. The paper describes a contact angle based estimation of bond strength for SOG and PDMS surfaces exposed to various combinations of plasma parameters. We have found that the highest bond strength condition is achieved if the contact angle on the SOG surface is less than 10°.     

Fabrication and characterization of a Cu seed layer on a 60-nm trench-patterned SiO2 substrate by a self-assembled-monolayer (SAM) process

Continuous electroless deposition of a 10-nm thick layer of Cu was successfully performed on a SiO₂/Si substrate coated with a 3-nm Au catalytic layer. The Au catalytic layer was formed by a self-assembled monolayer (SAM) process terminated with NH₂ headgroups, upon which negatively charged Au particles were deposited via electrostatic interaction with the positively charged NH₂-SAM. The Au and NH₂-SAM layers were analyzed by X-ray photoelectron spectroscopy (XPS) and contact angle analysis. Atomic force microscopy, field emission scanning electron microscopy, and XPS revealed that the Cu layer formed by this electroless processes had good step-coverage, small grain size, and excellent adhesion to the substrate. The proposed process is a very promising method for fabrication of a conductive Cu seed layer in a 60-nm trench-pattern.     

Optimization and improvement of TIPS–pentacene transistors (OTFT) with UV–ozone and chemical treatments using an all-step solution process

We fabricated an organic thin-film transistor (OTFT) using an all-step solution process. The printed layers, in which the electrode (silver), dielectric layer (BaTiO₃–PMMA), source–drain layer, and semiconductor 6,13-Bis(triisopropylsilylethynyl)pentacene(TIPS–pentacene), were optimized using roll-to-roll, an inkjet printer, and drop-casting. After coating the source–drain layer, we applied ultraviolet (UV)–ozone and self-assembled monolayer (SAM) treatments to the composite layer. The OTFTs treated with the UV–ozone and SAM treatments were found to exhibit excellent performance and good properties in comparison to silicon-based OTFTs.     

Heterogeneous integration of GaSb layer on (100) Si substrate by ion-slicing technique

Integration of the high-quality GaSb layer on an Si substrate is significant to improve the GaSb application in optoelectronic integration. In this work, a suitable ion implantation fluence of 5×10¹⁶-cm^-2 H ions for GaSb layer transfer is confirmed. Combining the strain change and the defect evolution, the blistering and exfoliation processes of GaSb during annealing is revealed in detail. With the direct wafer bonding, the GaSb layer is successfully transferred onto a (100) Si substrate covered by 500-nm thickness thermal oxide SiO₂ layer. After being annealed at 200 ℃, the GaSb layer shows high crystalline quality with only 77 arcsec for the full width at half maximum (FWHM) of the x-ray rocking curve (XRC).     

Effects of different binders on microstructure and phase composition of hydroxyapatite Nd-YAG laser clad coatings

The laser clad coating technique can help to produce metallurgical bonding with high bonding strength between the coating layer and the substrate, which has been gradually applied for hydroxyapatite (HA) coating on metallic substrates. In this study, HA powder is mixed with two different binders, namely water glass (WG) and polyvinyl alcohol (PVA), respectively, and is then clad on Ti-6Al-4V substrates using an Nd:YAG laser system under various processing conditions. The microstructure, chemical composition and hardness of the coating layer and transition layer of the various samples are then systematically explored. The experimental results show that the coating layers of the various samples all contain both cellular dendrites and rod-like piled structures, while the transition layers contain only cellular dendrites. For all samples, the coating layer consists mostly of CaTiO₃, Ca₂P₂O₇, CaO and HA phases, whereas the transition layer contains primarily CaTiO₃, Ca₂P₂O₇, Ti₃P, Ti and HA phases. In addition, the transition layer of the WG samples also contains SiO₂ and Si₂Ti phases. In all of the specimens, the transition layer has a higher average hardness than the substrate or coating layer. Moreover, the transition layer in the WG sample is harder than that in the PVA sample.     

Transfer bonding technology for batch fabrication of SMA microactuators

Currently, the broad market introduction of shape memory alloy (SMA) microactuators and sensors is hampered by technological barriers, since batch fabrication methods common to electronics industry are not available. The present study intends to overcome these barriers by introducing a wafer scale transfer process that allows the selective transfer of heat-treated and micromachined shape memory alloy (SMA) film or foil microactuators to randomly selected receiving sites on a target substrate. The technology relies on a temporary adhesive bonding layer between SMA film/foil and an auxiliary substrate, which can be removed by laser ablation. The transfer technology was tested for microactuators of a cold-rolled NiTi foil of 20 μm thickness, which were heat-treated in free-standing condition, then micromachined on an auxiliary substrate of glass, and finally selectively transferred to different target substrates of a polymer. For demonstration, the new technology was used for batch-fabrication of SMA-actuated polymer microvalves.     

Synthesis of free-standing Ga_2O_3 films for flexible devices by water etching of Sr_3Al_2O_6 sacrificial layers

Flexible electronic devices have attracted much attention due to their practical and commercial value. Integration of thin films with soft substrate is an effective way to fabricate flexible electronic devices. Ga_2O_3 thin films deposited directly on soft substrates would be amorphous mostly. However, the thickness of the thin film obtained by mechanical exfoliation method is difficult to control and the edge of the film is fragile and easy to be damaged. In this work, we fabricated free-standing Ga_2O_3 thin films using the water-soluble perovskite Sr_3Al_2O_6 as a sacrificial buffer layer. The obtained Ga_2O_3 thin films were polycrystalline. The thickness and dimension of the films were controllable. A flexible Ga_2O_3solar-blind UV photodetector was fabricated by transferring the free-standing Ga_2O_3 film on a flexible polyethylene terephthalate substrate. The results displayed that the photoelectric performances of the flexible Ga_2O_3 photodetector were not sensitive to bending of the device. The free-standing Ga_2O_3 thin films synthesized through the method described here can be transferred to any substrates or integrated with other thin films to fabricate electronic devices.     

A sandwiched flexible polymer mold for control of particle-induced defects in nanoimprint lithography

Particle related defects are one of the key concerns for nanoimprint lithography, since the particle can amplify the defect to become much larger than the particle itself. We developed a flexible tri-layer mold for control of particle-induced defects. The mold was composed of a PDMS cushion layer sandwiched between a rigid imprint pattern layer and a plastic polyethylene terephthalate (PET) backplane. The PET foil was used as the backplane of the mold to protect the sticky PDMS surface. The PDMS as a cushion layer could locally deform to conform the shape of substrate due to its high elasticity. The multifunctional epoxysiloxane was used for the formation of an imprint layer because of its insensitivity toward oxygen during curing, high transparency, excellent mechanical strength and high resistance to oxygen plasma after cross-linking. Nanostructures with different geometries and sizes were faithfully duplicated by this mold through a UV-curing imprint process. The particle-induced defectivity was dramatically improved by the deformation of the PDMS cushion layer with a slight external pressure. 500 nm pitch grating structures were successfully imprinted on a microposts array surface, both the top and the intervening bottom portions between the microposts.     

Effect of substrate curvature on thickness distribution of polydimethylsiloxane thin film in spin coating process

The polymer spin coating is critical in flexible electronic manufaction and micro-electro-mechanical system(MEMS)devices due to its simple operation, and uniformly coated layers. Some researchers focus on the effects of spin coating parameters such as wafer rotating speed, the viscosity of the coating liquid and solvent evaporation on final film thickness.In this work, the influence of substrate curvature on film thickness distribution is considered. A new parameter which represents the edge bead effect ratio(re) is proposed to investigate the influence factor of edge bead effect. Several operation parameters including the curvature of the substrate and the wafer-spin speed are taken into account to study the effects on the film thickness uniformity and edge-bead ratio. The morphologies and film thickness values of the spin-coated PDMS films under various substrate curvatures and coating speeds are measured with laser confocal microscopy. According to the results, both the convex and concave substrate will help to reduce the edge-bead effect significantly and thin film with better surface morphology can be obtained at high spin speed. Additionally, the relationship between the edge-bead ratio and the thin film thickness is like parabolic curve instead of linear dependence. This work may contribute to the mass production of flexible electronic devices.     

Molecular dynamics study of interfacial bonding strength of self-assembled monolayer-coated Au-epoxy and Au-Au systems

Interfacial adhesion between metals and organic polymers plays a crucial role in the mechanical properties and reliability performance of multiplayer thin film structures. To improve their interfacial bonding strength and so the reliability, the self-assembled monolayer (SAM) method is considered as an effective means. The present study is devoted to studying the effects of SAM coating on the interfacial bonding strength of the Au-epoxy and the Au-Au bonding structures through molecular dynamics (MD) simulation. Three different types of functionalized alkanethiol SAMs (SH(CH₂)_nX, X = CH₃, OH, NH₂) chemisorbed onto two different Au crystal planes, i.e., (1 0 0) and (1 1 1), are considered. The study starts from the characterization of the interfacial bonding strength of both the SAM-coated Au-epoxy and Au-Au systems, followed by the investigation of the dependence of the interfacial bonding strength on the chain lengths and tail groups of the n-alkanethiolates. A comparative study of the effects of the crystal orientation of Au substrate on the bonding strength is reported, and the elastic moduli of these SAMs through uniaxial tensile simulation are also examined. The calculated results are compared with the published experimental data, and also with each other to identify the optimal SAM candidate.Results show that the interfacial bonding strength of the SAM-coated Au-epoxy and Au-Au systems exhibits a strong dependency on the crystal orientation of Au substrate and also on the chain length of the monolayer where it tends to increase with an increasing SAM chain length. In specific, the interfacial bonding strength of the SH(CH₂)_nCH₃ SAM-coated Au-Au joint would reach a maximal value at the chain length n = 8 while that of the SAM/epoxy interface in the SH(CH₂)_nCH₃ SAM-coated Au-epoxy system attains a minimal value at n = 4 and becomes the maximum at n = 10, regardless of the crystal orientation of the Au substrates. Besides, the Au substrate with (1 1 1) crystal orientation would outperform the Au(1 0 0) substrate in the SAM/epoxy interfacial bonding strength of the SAM-coated Au-epoxy system while there is a totally opposite result for that of the SAM-coated Au-Au joint.