Experiment and analysis of the effect of BCB sealing ring flatness on BCB cap transfer packaging

Microsystem Technologies - This paper presents the effect of BCB sealing ring flatness on BCB bonding for wafer-scale BCB cap transfer packaging. BCB sealing ring has shown partial bonding or full...     

Laser Bonding of Glass to Silicon Using Polymer for Microsystems Packaging

Laser joining is a promising technique for wafer-level bonding. It avoids subjecting the complete microelectromechanical system (MEMS) package to a high temperature and/or the high electric field associated with conventional wafer-level bonding processes, using the laser to provide only localized heating. We demonstrate that a benzocyclobutene (BCB) polymer, used as an intermediate bonding layer in the packaging of MEMS devices, can be satisfactorily cured by using laser heating with a substantial reduction of curing time compared with an oven-based process. A glass-on-silicon (Si) cavity bonded with a BCB ring can be produced in a few seconds at a typical laser intensity of 1 W/mm² resulting in a local temperature of ~300degC. Hermeticity and bond strength tests show that such cavities have similar or better performance than cavities sealed by commercial substrate bonders. The influence of exposure time, laser power, and applied pressure on the degree of cure, bond strength, and hermeticity is investigated. The concept of using a large area uniform laser beam together with a simple mirror mask is tested, demonstrating that such a mask is capable of protecting the center of the cavity from the laser beam; however, to prevent lateral heating via conduction through the Si, a high-conductivity heat sink is required to be in good thermal contact with the rear of the Si.     

Wafer-level BCB bonding using a thermal press for microfluidics

Benzocyclobutene (BCB) is a thermosetting polymer that can form microfluidics and bond top and bottom layers of the microfluidics at the same time, and yields high repeatability and high bonding strength. This paper reports using photosensitive BCB to fabricate microfluidics and to bond with a thermal press for 4 in. wafers. By optimizing the parameters for pattern development and using a three-stage temperature and pressure increment BCB bonding, we realize the whole wafer glass–Si or glass–glass bonding in thermal press without any crack. The wafer-level bonding shows a bonding percentage above 70%, a tensile stress above 4.94 MPa, and a bonding repeatability over 95%. Furthermore, the bonding is compatible with thick electrode integration, that microfluidics with 380 nm thick electrodes underneath can be well-bonded. Our bonding method much reduces the cost compared with bonding BCB in a wafer bonding machine. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

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.     

MEMS光敏BCB硅—硅键合工艺研究

光敏BCB作为粘结介质进行键合工艺实验研究。实验中选用XUS35078负性光敏BCB,提出了优化的光刻工艺参数,得到了所需要的BCB图形层,然后将两硅片在特定的温度与压力条件下完成了BCB键合。测试表明:该光敏BCB具有较小的流动性和较低的塌陷率。键合后的BCB胶厚约为11.6μm,剪切强度为18MPa,He细检漏率小于5.0×10-8atm·cm3/s。此键合工艺可应用于制作需要低温工艺且不能承受高电压的MEMS器件。     

A study on effect of wafer bow in wafer-level BCB cap transfer packaging

This paper investigates the effects of wafer bow of Si carrier wafer to achieve high-yield BCB cap transfer in wafer-scale packaging. BCB caps are built-up on Si carrier wafer and then they are bonded and transferred to a target wafer. The height of BCB cap is 25 μm and the thickness of Si carrier wafer is 380 μm. Through several experiments, it is found that BCB cap transfer rate is mainly dependent on wafer bow of Si carrier wafer rather than that of the target wafer due to relatively large thickness of BCB caps. Therefore, Si carrier wafer bow with the BCB layers is investigated as a function of temperature. It is to figure out the effect of the wafer bow at certain temperature and applying pressure on BCB cap transfer rate. Through the study, it is found that zero wafer bow is very important for the cap transfer. Hence, aluminum metal layer is introduced to compensate the existing wafer bow of the Si carrier wafer.     

Bonded thick film SOI with pre-etched cavities

We have studied direct bonding and thinning of pre-etched silicon wafers. Silicon-on-insulator (SOI) substrates with pre-etched cavities provide freedom to MEMS design and enable manufacturing of advanced sensor structures (sensor structures that would be difficult or impossible with conventional substrates). Cavities with different shapes and sizes were etched on to the handle wafers. The etched handle wafers were bonded to unpatterned cap wafers in air or in vacuum. The bonding quality was evaluated with scanning acoustic microscopy and with HF-etching test. After bonding, the cap wafers were thinned down with grinding and polishing. The thickness variation of silicon diaphragm over the cavities was evaluated with cross-sectional SEM. The deflection of the Si film was measured with surface profilometry. To decrease the deflection and the thickness variation of the film, different support structures were placed inside the cavities.The bonding experiments carried out with patterned wafers showed that vacuum bonding results in slightly higher bonding energy than bonding in air. With large cavity fraction (80% of total wafer area), the air bonded samples had large void on the bonded interface. With smaller cavity fractions or with vacuum bonded samples, no such voids were found. Thinning studies showed that the thickness variation of the silicon diaphragm increases with increasing cavity dimensions and with decreasing SOI layer thickness. Thickness variation can be reduced with support structures under the Si membrane.     

High performance suspended spiral inductor and band-pass filter by wafer level packaging technology

Suspended inductors and 2.45 GHz BPF with patterned ground shields on the lossy silicon substrate by using Cu/BCB based wafer level packaging and bulk Si etching technologies were fabricated. Thick BCB interlayer is used as the supporting dielectric and the backside cavity is easily removed by using a two-step back-etching process. The fabricated 2.7 nH inductor has a maximum Q factor 49 at 8.2 GHz and high Q factors more than 22 in the broadband frequency range from 1 to 10 GHz. And the realized 2.45 GHz BPF has the insertion loss of 3.0 dB and the return loss of more than 14 dB at the pass band. It is also featured by more than 48 and 25 dB attenuation at 0.9 and 1.8 GHz respectively, with the second harmonic rejection being 33 dB.

     

基于正交试验的光纤传感器金属化连接工艺优化

为了克服光纤传感器有机胶封装带来的可靠性差、应变传递效率低的问题,采用粒子扩散系统对光纤传感器进行金属化连接以实现光纤传感器的无胶封装;为提高金属粘接层与基体的结合强度,设计了以工作距离、驱动电压、进给速度、粒子场气压为试验素的4水平正交试验方案,并用划痕法对金属粘接层的结合强度结果进行评估。通过统计分析,获得了影响金属粘接层与基体结合强度的主要因素和次要因素,优化了光纤传感器金属化连接工艺。     

Adhesive bonding with SU-8 in a vacuum for capacitive pressure sensors

This paper describes a method for fabricating capacitive pressure sensors through the use of adhesive bonding with SU-8 in a vacuum. The influence of different parameters on the bonding of structured wafers was investigated. It was found that pre-bake time, pumping time, and the thickness of the crosslink layer are the most important factors for successful bonding. Bonding quality was evaluated by inspection through the transparent glass of the sensor and through the use of an SEM photograph, with 90% of the area successfully bonded and an ultimate yield of 70% of the sensors. The measured bonding strength was 17.15 MPa and 19.6 MPa for wafers bonded in 80 °C and 100 °C, respectively. The pressure–capacitance characteristic test results show that this bonding process is a viable micro electro mechanical systems (MEMS) fabrication technology for cavity sealing in a vacuum.     

Bonding of planar poly (methyl methacrylate) (PMMA) nanofluidic channels using thermal assisted ultrasonic bonding method

Because of extremely small dimensions of nanochannels and low rigidity of polymer, most bonding techniques which are suitable for sealing polymer microscale channels (width and depth in tens to hundreds microns) are not competent for bonding of polymer nanochannels. In this study, a new thermal assisted ultrasonic bonding method for sealing poly (methyl methacrylate) (PMMA) nanochannels was presented. Substrates were preheated to 30–40°C lower than glass transition temperature (T _g ) of the material by hot plate. Then low amplitude ultrasonic vibration was employed to generate facial heat at the interface of the substrates. Influences of preheating temperature on bonding strength and dimension loss were studied. The nanochannels were successfully bonded with depth loss less than 5.3% (10.6 nm) and bonding strength of 0.21 J/cm² at the preheating temperature of 70°C. Thermal assisted ultrasonic bonding is proven to be competent for bonding of polymer nanochannels with high bonding strength, low dimension loss and short bonding time.     

Low temperature transient liquid phase bonding of Au/Sn and Cu/Sn electroplated material systems for MEMS wafer-level packaging

This paper presents the development of a low temperature transient liquid phase bonding process for 8″ wafer-level packaging of micro-electro-mechanical systems. Cu/Sn and Au/Sn material systems have been investigated under varying bonding temperatures from 240 to 280 °C and different dwell times from 8 to 30 min. The used bond frame had a width of 80 μm and lateral dimensions of 1.5 mm × 1.55 mm. The sealing frame of the cap wafer consisted of Au and Cu, respectively, and Sn. The MEMS wafer only holds the parent metal of Au or Cu. High quality bonds were confirmed by shear tests, cleavage analysis, polished cross-section analysis using optical and electron microscope, energy dispersive X-ray spectroscopy and pressure cocker test. The samples showed high shear strength (>80 MPa), nearly perfect bond regions and no main failure mode in the cleavage analyses. Non-corroded Cu test structures confirmed the hermeticity.     

Recessed ground microstrip line at 60 GHz and its application of band‐pass filter

The effect of finite‐size recessed ground on characteristic features of a microstrip transmission line is investigated and verified experimentally on alumina substrate of height 0.127 mm with ε_r = 9.8 at 60 GHz. A measured characteristic impedance of 238 Ω, effective dielectric constant of 3.09, and attenuation constant of 3.4 Np/m is achieved by using a recessed ground of dimensions (width × depth) 4.5 mm × 0.95 mm, below a 50‐Ω (on conventional ground plane) microstrip line. The effect of recessed ground on lumped equivalent circuit elements of microstrip line discontinuities including series‐gap, open‐end, and step discontinuities is also studied. To show the usefulness of recessed ground microstrip line, a prototype of fifth‐order Chebyshev‐type recessed ground end‐coupled band‐pass filter is designed and fabricated at 60 GHz. The filter exhibits measured insertion loss lower than 2.2 dB and return loss better than 13 dB over 3‐dB passband of 6% centered at 60 GHz. The measured results show good consistency with simulated results and confirm the usefulness of recessed ground plane microstrip line.     

MEMS packaging process by film transfer using an anti-adhesive layer

A low cost and low temperature thin film packaging process based on the transfer of an electroplated Nickel 3D cap is proposed. This process is based on adhesion control of a thick molded cap Ni film on the carrier wafer by using a plasma deposited fluorocarbon film, on mechanical debonding and on adhesive bonding of the microcaps on the host wafer with BCB sealing rings. Mechanical characterizations show that the transferred microcaps have a high stiffness, a low stress and a high adhesion. Because this process is simple and only involves a low temperature (250°C) heating of the host wafer, it is highly versatile and suitable for the encapsulation of micro and nano devices, circuits and systems elaborated on a large range of substrate materials.     

Thin-Film Encapsulated RF MEMS Switches

A wafer-level thin-film encapsulation process has been demonstrated to package radio-frequency (RF) microelectromechanical systems (MEMS) switches in this paper. Individual shunt capacitive switches were packaged in a ~1nL inorganic enclosure with process temperatures not exceeding 300 degC. A shell covering the switch consisted of 10 nm of sputtered alumina and 1.67 mum of sputtered silicon nitride dielectric film. The switch and dielectric shell were simultaneously wet-released through access channels in the shell. Following release, access channels were sealed with 10 nm of sputtered alumina and 2-4 mum of either plasma-enhanced chemical vapor deposited silicon dioxide or silicon nitride. Electromagnetic simulation and RF test results before and after sealing show minimal RF degradation of switch performance. Before sealing, the insertion loss and isolation at 10 GHz averaged 0.12 and 10.7 dB, respectively. After sealing, the same devices had an average insertion loss and isolation of 0.12 and 10.1 dB, respectively. Complete characterization of the package atmosphere was not completed due to challenges in assessing nanoliter-scale volumes     

Non-destructive in situ test for anodic bonding

New test structures have been designed, fabricated and tested to monitor the quality of the anodic bonding between silicon and glass. The main advantage of the described test is that it is not destructive and allows the bond quality to be monitored in processed wafers. This test is very easy to implement in a chip or in a wafer because of its simplicity. Test structures consist of a matrix of circular and rectangular cavities defined by reactive ion etching (RIE) on the silicon wafer, with different sizes and depths. The bonding process and quality can be monitorized by the measurement of the size of the smallest bonded cavity and the distance between the bonded area and the cavity border. These structures give information about the level of electrostatic pressure that has been applied to pull together into intimate contact the surfaces of the two wafers. The higher the electrostatic pressure, the better the bond. We have applied these test structures to study the influence of the voltage and the temperature on the anodic bonding process. Results are in good agreement with finite-element method (FEM) simulations.     

A simple thermo-compression bonding setup for wire bonding interconnection in pressure sensor silicon chip packaging

In the process of piezo-resistive pressure sensor packaging, a simple thermo-compression bonding setup has been fabricated to achieve the wire bonding interconnection of a silicon chip with printed circuit board. An annealed gold wire is joined onto a pad surface with a needle-like chisel under a force of 0.5?C1.5?N/point. The temperature of the substrate was maintained in the range of 150?C200°C and the temperature of the chisel was fixed at around 150°C during wire bonding operation. The tensile strength of the wire bonding was measured with a bonding tester by the destructive-pulling experiment and was found to be at the average of 132?mN/mm². The microstructure of the bonding point was examined by scanning electron microscopy. The interface of the thermo-compression boning was shown to possess an acceptable level of reliability for a micro-electromechanical system (MEMS)-based device. The results showed that this setup can be easily operated for fabrication and is suitable for fabricating not only low-cost pressure sensors, but also other MEMS devices.     

Water glass bonding

The author developed a low-temperature (80°C), low-external loads (electric field, magnetic field, load, etc.) bonding technology using water glass which is used in making molds (silica sand). The adhesive area ratio attained by this bonding technology was more than 95% and the bonding strength was about 290 kgf/cm². As this water glass bonding technology is applicable at comparatively low-temperatures, the residual stress of the bond is so small as to be able to bond eight 4-inch wafers together in the 3rd dimensional direction. In addition, as the thickness of a bonding layer is as small as several nanometers, the precision bonding of a tolerance of ±3 μm was also possible through alignment, and we could successfully fixed a cap with having a bonding seal of 0.32 mm wide in the vacuum. The leak rate was less than the detection limit of the He leak detector (1×10⁻¹⁰ Pa m³/s), showing excellent air-tightness. Using this bonding technology, the author made a self-package type IR microsensor on an experimental basis, and carried out an accelerated environmental test. As a result, its MTTF (mean time to failure) was estimated to be 6 years.     

Microrivets for MEMS packaging: concept, fabrication, and strengthtesting

Microriveting is introduced as a novel and alternative joining technique to package MEMS devices. In contrast to the existing methods, mostly surface bonding, the reported technique joins two wafer pieces together by riveting, a mechanical joining means. Advantages include wafer joining at room temperature and low voltage, and relaxed requirements for surface preparation. The microrivets, which hold a cap-base wafer pair together, are formed by filling rivet holes through electroplating. The cap wafer has a recess to house the MEMS devices and also has through-holes to serve as rivet molds. The seed layer on the base wafer becomes the base of the rivet. The process requires only simple mechanical clamping of the wafer pair during riveting, compared with the more involved procedures needed for wafer bonding. Directionality of electroplating in an electric field is what makes this process simple and robust. Strength testing is carried out to evaluate the joining with microrivets. Different modes of rivet failure under different loading conditions are identified and investigated. Effective strength between 7 and 11 MPa was measured under normal loading with nickel microrivets. Joining strengths comparable to conventional wafer bonding processes, ease of fabrication with repeatability, and compatibility with batch fabrication show that microriveting is a feasible technique to join wafers for MEMS packaging, especially when hermetic sealing is not essential     

Solder bonding with a buffer layer for MOEMS packaging using induction heating

In this paper, the selective induction heating technology is applied to glass–glass and glass–silicon solder bonding for MOEMS (optical MEMS) packaging. The Ni bumping with a buffer layer is successful to release the thermal stress for avoiding delamination. The Au wetting layer must be thick enough to prevent from being solved entirely into Sn, and it will improve bonding strength. The bonding specimens are soaked into 25°C water and placed into 85°C/85% RH oven, respectively. No moisture penetrates into the cavity after 1 day in both test conditions. In the test condition of 125°C leakage-test liquid (Galden HS260), no bubble is observed. The lowest bonding strength is 3 MPa.