Advanced characterization of glass frit bonded micro-chevron-test samples based on scanning acoustic microscopy

Glass frit bonding is a widely used encapsulation technology for micro-electro mechanical systems. In order to guarantee functionality and reliability of a bonding seal, qualified test methods are required for evaluating the quality and strength of the bonding interfaces which are considered key parameters. In the presented work adapting the micro-chevron-test for glass frit bonded samples and arising challenges are discussed. Motivated by the industrial application of glass frit bonding generally used for frame structures an application related guideline for the application of micro-chevron-testing is presented. In addition, high resolution acoustic inspection is used as a key technology for estimating the effective bond strength in combination with further experimental testing and is likewise used for sample pre-selection and defect localization. The presented content provides a sequential overview beginning with sample preparation of glass frit bonded micro chevron samples to mechanical testing and the result analysis as well as a statistical interpretation of a bonded silicon test wafer.     

Research on wafer-lever vacuum package by glass frit at low temoerature

研究了玻璃浆料在低温下真空封装MEMS器件的工艺.封装过程中,采用了丝网印刷技术,丝网的线宽设计为100 μm,印刷后玻璃浆料线宽为160 μm左右,从而能够减小封装器件的尺寸,节省成本;另外,对玻璃浆料键合工艺做了研究,找到了较好的工艺条件,采用该工艺(预烧结温度425℃,键合温度430℃),得到的封装结构具有较高的封接强度(剪切力＞20 kgf)和良好的真空度,测得的漏率为10-9 cm3/s.     

低温玻璃浆料圆片级真空封装的研究

研究了玻璃浆料在低温下真空封装MEMS器件的工艺。封装过程中,采用了丝网印刷技术,丝网的线宽设计为100μm,印刷后玻璃浆料线宽为160μm左右,从而能够减小封装器件的尺寸,节省成本;另外,对玻璃浆料键合工艺做了研究,找到了较好的工艺条件,采用该工艺(预烧结温度425℃,键合温度430℃),得到的封装结构具有较高的封接强度(剪切力>20 kgf)和良好的真空度,测得的漏率为10-9 cm3/s。     

Mechanical properties and fracture analysis of glass substrate for PDPs

Abstract— A thermal shock test was carried out using high‐strain‐point glass substrates for plasma‐display panels. From the fracture analysis, the fracture stress was determined and compared with the initial edge strength. It was observed that the edge strength greatly decreased because of micro cracks that formed on the glass surface during testing. Therefore, it is important to prevent the formation of micro cracks in order to avoid failure as well as to minimize thermal stress in the process. Fatigue is also an important parameter in preventing the failure of the glass substrate when the stress on it lasts for a long period of time.     

BCB contact printing for patterned adhesive full-wafer bonded 0-level packages

Adhesive wafer bonding with a patterned polymer layer is increasingly attracting attention as cheap and simple 0-level packaging technology for microstructures, because the patterned polymer both fulfills the bonding function and determines the volumes between the two wafers housing the devices to be packaged. To be able to pattern a polymer, it has to be cross-linked to a certain degree which makes the material rigid and less adhesive for the bonding afterward. In this paper, a simple method is presented which combines the advantages of a patterned adhesive layer with the advantages of a liquid polymer phase before the bonding. The pattern in the adhesive layer is "inked" with viscous polymer by pressing the substrate toward an auxiliary wafer with a thin liquid polymer layer. Then, the substrate with the inked pattern is finally bonded to the top wafer. Benzocyclobuene (BCB) was used both for the patterned structures and as the "ink". Tensile bond strength tests were carried out on patterned adhesive bonded samples fabricated with and without this contact printing method. The bonding yield is significantly improved with the contact printing method, the fabrication procedure is more robust and the test results show that the bond strength is at least 2 times higher. An investigation of the samples' failure mechanisms revealed that the bond strength even exceeds the adhesion forces of the BCB to the substrate. Furthermore, the BCB contact printing method was successfully applied for 0-level glass-lid packaging done by full-wafer bonding with a patterned adhesive layer. Here, the encapsulating lids are separated after the bonding by dicing the top wafer independently of the bottom wafer.     

Numerical investigation on a laser based localised joining with a glass frit intermediate layer

Localised laser bonding using a glass frit intermediate layer is an ideal technology to hermetically package miniature devices without heating the function components. In this paper, we investigated from a device level packaging perspective, the influence of the laser ring diameter and width, copper boss (heat sink) diameter and misalignment of laser ring in a laser based localised bonding with glass frit intermediate layer. The conclusions are: (1) laser ring diameter and width have slight influence on the bonding process however a system with smaller laser ring diameters achieves better performance compared to that with larger ring diameters. (2) The copper boss (heat sink) diameter has significant influence on the laser power level required to achieve a particular glass frit curing temperature within the glass frit ring. Selection of an appropriate copper boss diameter is determined by the maximum allowable temperature for the temperature-sensitive devices under protection. (3) Misalignment of the laser ring has significant influence on the localised laser bonding and the recommended misalignment deviation for the laser ring should be less than 100 μm.     

Application of micro structured photosensitive glass for the gravure printing process

Photosensitive glasses are well known as materials which are micro structurable with a high aspect ratio of 20:1. Typical applications are micro mechanical, fluidic and optical components due to the very good chemical, thermal and mechanical stability of this material. Currently the aim of the work is the development of micro structured clichés made from photosensitive glass for the gravure printing of electrically functionalised inks on flexible substrates. Glass clichés with a geometrical variation of recessed cells were fabricated and tested regarding the chemical and mechanical stability using process comparable conditions. Printing tests using particle less and particle loaded electrically functionalised inks were carried out to investigate the influence of cell geometry on printed ink layers. It was found that precise dot structures will be printable if cell openings are <80 μm and cell depths are <30 μm. Resulting from this ITO was printed on flexible glass substrates with a thickness of 30 μm for high temperature treatments.     

Bulk metallic glass mold for high volume fabrication of micro optics

Microsystem Technologies - Suitable mold materials are critical for successful high volume replication of micro optical components. As one of the emerging new materials, bulk metallic glass (BMG)...     

Development and reliability analysis of micro gas sensor platform on glass substrate

In this paper, technology for a gas sensor platform with borofloat as the substrate material is presented. Comprehensive characterization of the platform, its comparison with silicon and alumina, fabrication yield improvement and a study of reliability of the micro-heater platform have been carried out. Usually, the chips are suspended in air to reduce power consumption. However, the presented technology is a non-MEMS technique and doesn’t require any complex packaging. Borofloat has much lower thermal conductivity in comparison to silicon and alumina, thereby reducing the thermal losses, making it possible to operate the device with low power consumption. The process adapted for the fabrication of the gas sensor platform has lesser complexities and the process cost is reduced compared to conventional gas sensor fabrication, as it does not require thermal oxidation and bulk micromachining. Different substrates (silicon, alumina and glass) have been simulated using COMSOL to depict the benefit of lower thermal conductivity. Micro-heater has also been fabricted on all the three above mentioned substrates and the power consumption is compared. Various reliability analysis have been carried out on the glass based platform such as maximum temperature test, long term ON test and ON–OFF pulse test.

     

Polysilicon micro beams buckling with temperature-dependent properties

The suspended electrothermal polysilicon micro beams generate displacements and forces by thermal buckling effects. In the previous electro-thermal and thermo-elastic models of suspended polysilicon micro beams, the thermo-mechanical properties of polysilicon have been considered constant over a wide rang of temperature (20–900°C). In reality, the thermo-mechanical properties of polysilicon depend on temperature and change significantly at high temperatures. This paper describes the development and validation of theoretical and Finite element model (FEM) including the temperature dependencies of polysilicon properties such as thermal expansion coefficient and Young’s modulus. In the theoretical models, two parts of elastic deflection model and thermal elastic model of micro beams buckling have been established and simulated. Also, temperature dependent buckling of polysilicon micro beam under high temperature has been modeled by Finite element analysis (FEA). Analytical results and numerical results using FEA are compared with experimental data available in literature. Their reasonable agreement validates analytical model and FEM. This validation indicates the importance of including temperature dependencies of polysilicon thermo-mechanical properties such as Coefficient of thermal expansion (CTE) in the previous models.     

A continuous-membrane micro deformable mirror based on anodic bonding of SOI to glass wafer

As one of the most important components in adaptive optics, the deformable mirror (DM) is required to have a flat surface for better performance. For micromachined DMs, single-crystal-silicon (SCS) membrane is an ideal material for high quality reflective mirror surface owing to its good flatness and small residual stress. In this research, a process was established to realize SCS mirror membrane by DRIE of SOI wafer and anodic bonding of SOI wafer to Pyrex 7740 glass. Using this process, the proof-of-concept for a micromachined DM composed of SCS mirror surface has been successfully demonstrated. The prototype DM shows a stroke of 4.23 μm at 120 V. The P–V and rms of the reflective mirror surface are 492 and 82 nm, respectively. The performance of the prototype DM can somewhat satisfy the need of AO in visible spectrum. Better surface quality is anticipated by employing SOI wafers with strictly controlled residual stress.     

Large area micro hot embossing of Pyrex glass with GC mold machined by dicing

Micro/Nano imprinting or hot embossing is currently a target of interest for industrial production of micro and Nano devices for the low cost aspect. In Fluidic MEMS (Micro Electromechanical Systems) applications, polymer materials have been widely employed for their low cost to fabricate the economical products (Becker and Heim in Sens Acuators A 83:120–135, 2000; Becker and Gaertner in Mol Biotechnol 82:89–99, 2001). However glasses are much more suitable for the higher temperature applications or under the stronger chemical environments. Moreover UV absorption of glass materials is much less than that of polymers, which is the advantage for bio-analysis. In Optical MEMS as well, glasses are good candidate materials for the better optical properties, such as high refractive index, low UV absorption and others. Although wet etching of glasses is widely employed for fabrication of fluidic MEMS devices, the wet etching is not satisfactory for the low machining resolution, the isotropic etched profile and poor roughness of the fabricated structures. Dry etching of glasses is then an alternative for Micro/Nano structuring, but the etching rate is extremely low (order of 0.1 μm/min) and the cost is too high because of the expensive RIE (Reactive Ion Etching) facility. Above mentioned is the reason why we are interested in hot embossing or imprinting of glasses of Micro/Nano scale. In our previous study, Micro/Nano imprinting was developed for Pyrex glasses using GC (Glassy Carbon) mold prepared by FIB machining (Takahashi et al. in Symposium on DTIP 2004 pp 441–446, 2004). The disadvantage of FIB machining is limited area of etching. The typical area of FIB is less than several hundreds micrometer square. This is the reason why we tried the large area of embossing using GC mold fabricated by dicing machine. Micro hot embossed test structures were successfully demonstrated with good fidelity. Fabricated micro structures can be applied for fabrication of microchamber array for PCR (Akagi et al. in Sci Techol Adv Mater 5:343–349, 2004; Nagai et al. in Anal Chem 73:1043–1047, 2001).     

Fabrication,characterization and comparative analysis of mechanical properties of micro features generated by reverse micro EDM

Moderate to high aspect ratio micro features (projected) finds wide application in MEMS components, fuel cells for micro reactors, sensors and actuators, micro optical components etc. Reverse micro electro discharge machining (RMEDM) is one of the extensively used process for fabricating such micro features. The present authors have already developed a technique for generating different shapes and sizes of micro features by suitable modification of tool in RMEDM. Mechanical properties like hardness and elastic modulus of these micro features play a significant role in determining their suitability for various applications. Therefore, the objective of this paper is (a) to generate projected micro features corresponding to tapered blind hole depths of 0.1 mm and 0.3 mm using RMEDM and (b) to determine the effect of tapered blind hole depth on the hardness and elastic modulus of the projected micro features. Hardness and elastic modulus were measured using nano indentation technique. Results indicate that starting from the periphery, both hardness as well as elastic modulus first increase (presence of recast layer) and then decrease (presence of heat affected zone) for projected micro feature corresponding to 0.1 mm hole depth before attaining the properties of the parent material. Due to high amount of debris agglomeration in case of 0.3 mm hole depth, high amount of abnormal discharges occur which do not provide sufficient time for the formation of thermally softened layer as was in the case of micro feature corresponding to 0.1 mm hole depth. Hardness, therefore, is always high starting from recast layer to the HAZ before finally attaining the hardness of the parent material. Debris agglomeration did not have much influence on the elastic modulus and the variation of elastic modulus of the micro feature corresponding to 0.3 mm hole depth remains fairly uniform a compared to the parent material.

     

Organic light‐emitting diodes with enhanced out‐coupling efficiency using high‐refractive‐index glass frit

We have fabricated a novel type of substrate for organic light‐emitting diodes (OLEDs) to improve the light out‐coupling efficiency. It was fabricated by forming an excellent flat layer using a high‐refractive‐index B₂O₃‐SiO₂‐Bi₂O₃ frit glass on the light diffusive glass substrate. By using this substrate, we have sufficiently reduced the total internal reflection of OLEDs, and we successfully obtained more than 1.9 times higher light out‐coupling efficiency without spectral changes and viewing angle dependency. Furthermore, we have also successfully demonstrated 50 × 50 mm large‐area white OLEDs with this novel substrate.     

Study of silicon backside damage in deep reactive ion etching for bonded silicon–glass structures

 For devices of bonded silicon and glass structures fabricated by deep reactive ion etching (DRIE), it is important to avoid damage at the silicon sidewall and backside during through-wafer etching in order to ensure reliability of devices. The silicon damage caused by charge accumulation at the glass surface is inhibited by means of an electrically conducting layer patterned onto the glass and connected with the silicon. In this study, indium tin oxide films were applied in order to identify the positions of silicon damage in the structural layout without destruction of samples. From the results, we report that there exists silicon damage caused by charge accumulation at the silicon islands divided by DRIE and we present important rules for mask layout when utilizing this method. Received: 10 August 2001/Accepted: 24 September 2001 This paper was presented at the Fourth International Workshop on high Aspect Ratio Microstructure Technology HARMST 2001 in June 2001.     

Development of near hermetic silicon/glass cavities for packaging of integrated lithium micro batteries

A technology was developed for fabrication of very thin, chip-sized lithium secondary micro batteries. With help of wafer level processing the batteries can be directly integrated into silicon chips or MEMS devices. The batteries were packaged in 200 μm deep cavities of the silicon wafer and encapsulated with a glass substrate. Battery demonstrators were realized with 7 and 12 mm square and round foot prints. Near hermetic packaging was accomplished with the help of a UV curable epoxy seal that should ensure several years of battery lifetime. Bonding parameters, shear force and the water permeation rate of the adhesive were investigated. A capacity of 3 mAh/cm² and an energy density of 10 mWh/cm² were achieved. The electrical contact between the battery and the contact pads of the housing was investigated in detail. Electrical tests were made with encapsulated micro batteries and compared with macroscopic lithium polymer batteries. A reduction in capacity of approximately 10% was measured after 100 cycles.     

Mechanical testing of micro samples produced by a novel LIGA related process chain

In this paper, a new LIGA related process chain, named MSG process [German acronym for multicomponent injection molding (MS) and electroplating (G)] is presented as a promising production cycle for the replication of micro-testing samples. Structures manufactured by the LIGA process but also e.g. micro-milled structures can be reproduced. The focus of this work was two fold: First, the accuracy of different replicated structures was investigated, and second, the influence of the surface quality on mechanical performance of electrodeposited nanocrystalline materials was tested. Therefore, micro samples were manufactured by the use of different methods resulting in a variation of the measured surface quality. The surface of LIGA and micro-milled samples can be accurately replicated using this process. Furthermore, it can be shown which dimensional variations within the process occur and how this is influenced by the surface of the original structure. Finally, preliminary mechanical tests are presented which show the successful application of the MSG process.     

Applications of laser transmission processes for the joining of plastics, silicon and glass micro parts

Joining processes for microtechnology must fulfil special requirements concerning mechanical and thermal load of the components as well as their alignment accuracy. For these applications laser radiation as an energy source is ideally suited for joining with a minimum of energy load to the parts. In particular, laser transmission joining is used for joining plastics, silicon or glass using the total or partial transmission of the different materials and performing hidden joints inside a component. Since the optical properties of the materials play a decisive role for this joining technology, an optimization of absorbers and material conditions is of crucial importance for high quality joints. Investigations on different absorbent dyes for micro welding of plastics show their applicability, but also the limitations of these materials. For silicon and glass as well as silicon and silicon new fiber lasers with process-adapted wavelengths and strategies have been investigated and qualified as a versatile tool in micro assembly.