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.     

Enhancement of bonding strength of packaging based on BCB bonding for RF devices

This paper presents a Si cap zero-level packaging technique based on a double-layer BCB sealing ring. The BCB ring is defined before the housing cavity etching to achieve high BCB bonding strength. It is found that the non-uniformity of the BCB ring defined on a Si cap with housing cavity prevents the package having high bonding strength. Three different packages have been prepared for shear test; a Si cap without cavity, a recessed Si cap with a conventional BCB ring and a recessed Si cap with pre-defined BCB ring. The three samples for each type of package are measured. The average measured bonding strengths of the test samples are 71, 16 and 42?MPa, respectively, and hence the proposed BCB sealing ring process provides 60?% of bonding strength of Si cap package without cavity for Si cap package with cavity. In addition, the insertion loss change of the packaged CPW is less than 0.1?dB up to 67?GHz while the return loss better than 15?dB at the measured frequency range.     

Wafer-level bonding and direct electrical interconnection of stacked 3D MEMS by a hybrid low temperature process

The presented fabrication technology enables the direct integration of electrical interconnects during low temperature wafer bonding of stacked 3D MEMS and wafer-level packaging. The low temperature fabrication process is based on hydrophilic direct bonding of plasma activated Si/SiO₂ surfaces and the simultaneous interconnection of two metallization layers by eutectic bonding of ultra-thin AuSn connects. This hybrid wafer-level bonding and interconnection technology allows for the integration of metal interconnects and multiple materials in stacked MEMS devices. The process flow is successfully validated by fabricating test structures made out of a two wafer stack and featuring multiple ohmic electrical interconnects.     

Localised laser joining of glass to silicon with BCB intermediate layer

The use of a laser to provide localised heating is an ideal solution to the problem of packaging micro-electro-mechanical-systems (MEMS) whilst maintaining a low device temperature to avoid changes in temperature-sensitive materials in the device. In this paper we present localised laser bonding of glass to silicon (normally used as the MEMS substrate) by using a fibre-delivered high power laser diode array to cure an intermediate layer of the thermosetting polymer Benzocyclobutene (BCB). In our experiments, we use two techniques to realise localised heating: one is to use an axicon together with a conventional positive lens to generate a ring focus; the other is to use a scanning focused laser beam. In both cases localised cooling is required to confine the elevated temperatures to the bonding area. Finite Element (FE) simulation indicates that both techniques should keep the temperature in the centre of the package to approximately that of the ambient environment (300 K) during the process. However, experiments show that the temperature in the centre of the package rises to a value of around 500 K, likely due to poor contact between silicon and cooling sink. Experimentally, we confirmed that either technique can be used to obtain excellent bonding of glass to silicon with leak rate at a level of 10⁻¹⁰ mbar l s⁻¹, whilst keeping the centre of device at a lower temperature.     

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。     

Sealing of adhesive bonded devices on wafer level

In this paper, we present a low temperature wafer-level encapsulation technique to hermetically seal adhesive bonded microsystem structures by cladding the adhesive with an additional diffusion barrier. Two wafers containing cavities for MEMS devices were bonded together using benzocyclobutene (BCB). The devices were sealed by a combined dicing and self-aligning etching technique and by finally coating the structures with evaporated gold or PECVD silicon nitride. The sealing layer was inspected visually by SEM and helium leak tests were carried out. Devices sealed with silicon nitride and with known damage of the sealing layer showed a helium leak rate of about 7–14 times higher than the background level. Devices of the same size without damage in the sealing layer had a leak rate of only 1.5 times higher than the background level. Experiments with evaporated gold as cladding layer revealed leaking cracks in the film even up to a gold thickness of 5 μm. The sealing technique with silicon nitride shows a significant improvement of the hermeticity properties of adhesive bonded cavities, making this bonding technique suitable for applications with certain demands on gas-tightness.     

Characterization of wafer-level thermocompression bonds

Thermocompression bonding joins substrates via a bonding layer. In this paper, silicon substrates were bonded using gold thin films. Experimental data on the effects of bonding pressure (30 to 120 MPa), temperature (260 and 300/spl deg/C), and time (2 to 90 min) on the bond toughness, measured using the four-point bend technique, are presented. In general, higher temperature and pressure lead to higher toughness bonds. Considerable variation in toughness was observed across specimens. Possible causes of the nonuniform bond quality were explored using finite element analysis. Simulation results showed that the mask layout contributed to the pressure nonuniformity applied across the wafer. Finally, some process guidelines for successful wafer-level bonding using gold thin films are presented.     

CMOS: compatible wafer bonding for MEMS and wafer-level 3D integration

Wafer bonding became during past decade an important technology for MEMS manufacturing and wafer-level 3D integration applications. The increased complexity of the MEMS devices brings new challenges to the processing techniques. In MEMS manufacturing wafer bonding can be used for integration of the electronic components (e.g. CMOS circuitries) with the mechanical (e.g. resonators) or optical components (e.g. waveguides, mirrors) in a single, wafer-level process step. However, wafer bonding with CMOS wafers brings additional challenges due to very strict requirements in terms of process temperature and contamination. These challenges were identified and wafer bonding process solutions will be presented illustrated with examples.     

Pyrex 7740玻璃深刻蚀掩模研究

玻璃湿法深刻蚀掩模常采用低压化学气相沉积(LPCVD)多晶硅、Cr/Au金属层+光刻胶等,但往往会在玻璃中引入应力,影响后期应用(如阳极键合),而且Cr/Au金属层价格昂贵。为避免以上缺点,引入了SX AR—PC 5000/40保护胶+WBR2075干膜作为玻璃的刻蚀掩模,在HF︰NH4F,HF︰HCl,HF︰HCl︰NH4F刻蚀溶液中进行了大量实验。实验结果表明:SX AR—PC 5000/40抗腐蚀能力强,且成功实现了对Pyrex 7740玻璃131μm的深刻蚀。整个工艺过程与IC工艺兼容,可以进行圆片级批量加工。实验结果对圆片级封装和其他MEMS器件的制作有一定参考作用。     

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.     

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.     

Characterization of localized laser assisted eutectic bonds

An innovative bonding process for silicon and single crystal quartz has been developed and investigated using various material science characterization methods, such as TOF-SIMS, SEM, EDX and XRD. The bonding process combines the principles of laser transmission welding, eutectic bonding and bonding by localized heating. A focused laser beam (low power, max. 0.83 W) is transmitted through a quartz medium to intermediate layers of chromium, gold and tin at the silicon–quartz interface to provide localized heating and bonding. This bonding process is particularly suitable for bonding wafers containing temperature sensitive devices as it confines the temperature increase to a small area. Bond strength of over 15 MPa is comparable to most localized bonding schemes. This process provides a simple yet robust bonding solution with rapid processing time, selectivity of bonded area and corrosion resistant joints.     

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.     

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.     

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.

     

Wafer-Scale Manufacturing of Bulk Shape-Memory-Alloy Microactuators Based on Adhesive Bonding of Titanium–Nickel Sheets to Structured Silicon Wafers

This paper presents a concept for the wafer-scale manufacturing of microactuators based on the adhesive bonding of bulk shape-memory-alloy (SMA) sheets to silicon microstructures. Wafer-scale integration of a cold-state deformation mechanism is provided by the deposition of stressed films onto the SMA sheet. A concept for heating of the SMA by Joule heating through a resistive heater layer is presented. Critical fabrication issues were investigated, including the cold-state deformation, the bonding scheme and related stresses, and the titanium–nickel (TiNi) sheet patterning. Novel methods for the transfer stamping of adhesive and for the handling of the thin TiNi sheets were developed, based on the use of standard dicing blue tape. First demonstrator TiNi cantilevers, wafer-level adhesively bonded on a microstructured silicon substrate, were successfully fabricated and evaluated. Intrinsically stressed silicon dioxide and silicon nitride were deposited using plasma-enhanced chemical vapor deposition to deform the cantilevers in the cold state. Tip deflections for 2.5-mm-long cantilevers in cold/hot state of 250/70 and 125/28 ${rm mu}hbox{m}$ were obtained using silicon dioxide and silicon nitride, respectively. The bond strength proved to be stronger than the force created by the 2.5-mm-long TiNi cantilever and showed no degradation after more than 700 temperature cycles. The shape-memory behavior of the TiNi is maintained during the integration process.$hfill$[2009-0085]      

Wafer direct bonding with ambient pressure plasma activation

Ambient pressure plasma processes were applied for surface activation of semiconductor (Si, Ge and GaAs) and other wafers (glass) before direct wafer bonding for MEMS and engineered substrates. Surface properties of activated wafers were analysed. Caused by activation high bond energies were obtained for homogeneous (e.g. Si/Si) as well as for heterogeneous material combinations (for instance Si/Ge) after a subsequent low temperature annealing process at 200°C. The resulting bond energies are analogous or higher as obtained for low-pressure plasma activation processes. The advantages of the ambient pressure plasma processes are described; a technical solution is discussed demonstrating the low risk for contamination and radiation damage.     

Room-Temperature Sealing of Microcavities by Cold Metal Welding

In this paper, we present a wafer-to-wafer attachment and sealing method for wafer-level manufacturing of microcavities using a room-temperature bonding process. The proposed attachment and sealing method is based on plastic deformation and cold welding of overlapping metal rings to create metal-to-metal bonding and sealing. We present the results from experiments using various bonding process parameters and metal sealing ring designs including their impact on the resulting bond quality. The sealing properties against liquids and vapor of different sealing ring structures have been evaluated for glass wafers that are bonded to silicon wafers. In addition, wafer-level vacuum sealing of microcavities was demonstrated when bonding a silicon wafer to another silicon wafer with the proposed room-temperature sealing and bonding technique.$hfill$ [2008-0053]      

Fabrication of curved sub-micron Si structures by a combination of anisotropic etching using surfactant-added TMAH solution and FIB direct-drawn mask

Silicon anisotropic wet etching is applied for fabricating round-shaped micro-structures in a size range of sub-microns. In this work, we demonstrate that arbitrary 2-D mask patterns having curved profile can be successfully transferred to deep-etched cavity profiles on a Si {100} wafer. The sub-micron mask is directly drawn on the Si wafer by irradiating focused ion beam to the wafer surface. Anisotropy in etch rate of Si using tetra-methyl ammonium hydroxide solution was modified and controlled by adding a surfactant Triton X-100 to the solution. Etched profile was conformal to etch mask patterns having smooth curvatures.     

Fabrication of a Fabry–PÉrot Cavity in a Microfluidic Channel Using Thermocompressive Gold Bonding of Glass Substrates

This paper presents a simple, low-cost, and reliable process for the fabrication of a microfluidic Fabry–PÉrot cavity in a Pyrex glass substrate. The microfluidic channels were etched in HF solution on a glass substrate using a Cr/Au/photoresist etching mask resulting in a channel bottom roughness of 1.309 nm. An effective thermocompressive gold–gold bonding technique was used to bond the photolithographically etched glass substrates inside a 350$^circ C $oven in a$ 10^ - 3~ torr $vacuum. Pressure was applied to the glass pieces by using two aluminum blocks with intermediate copper sheets. This method takes advantage of using Cr/Au layers both as a wet etching mask and as intermediate bonding layers, requiring only one lithography step for the entire process. The fabrication method is also compatible with the incorporation of dielectric mirror coatings in the channels to form a high-finesse Fabry–PÉrot cavity. A parallelism of 0.095 degrees was measured, and a finesse as high as 30 was obtained using an LED. The microfluidic cavity developed here can be used in electrophoresis and intracavity spectroscopy experiments.hfillhbox[1375]