With the continuously increasing level of integration for microelectronics and microelectromechanical systems (MEMS) devices, such as gyroscopes, accelerometers and bolometers, metal wafer bonding becomes progressively more importance. In the present work common metal wafer bonding techniques were categorized, described and compared. While devices produced with metal thermo-compression wafer bonding ensure high bonding quality and a high degree of reliability, the required bonding temperatures are very often close to the maximum complementary metal oxide semiconductor (CMOS) compatible process temperature (400-450°C). Based on a thermodynamic model of increasing the Gibbs free energy prior wafer bonding, in-situ ComBond(R) surface activation was applied to enable low-temperature Au-Au, Al-Al and Cu-Cu wafer bonding. Different aspects, such as bonding quality, dicing yield, bond strength, grain growth and elemental analysis across the initial bonding interface, were investigated. Based on these parameters successful wafer bonding was demonstrated at room temperature for Au-Au and Cu-Cu, and at 100°C for Al-Al wafer bonding.
Various MEMS devices are incorporated into consumer electronic devices. A particular category of MEMS require vacuum packaging by wafer bonding with the need to encapsulate vacuum levels of 10-2 mbar or higher with long time stability. The vacuum requirement is limiting the choice of the wafer bonding process and raises significant challenges to the existing investigation methods (metrology) used for results qualification. From the broad range of wafer bonding processes only few are compatible with vacuum applications: fusion bonding, anodic bonding, glass frit bonding and metal-based bonding. The outgassing from the enclosed surfaces after bonding will affect the vacuum level in the cavity: in some cases, a getter material is used inside the device cavity to compensate for this outgassing. Additionally the selected bonding process must be compatible with the devices on the wafers being bonded. This work reviews the principles of vacuum encapsulation using wafer bonding. Examples showing the suitability of each process for specific applications types will be presented. A significant challenge in vacuum MEMS fabrication is the lack of analytical methods needed for process characterization or reliability testing. A short overview of the most used methods and their limitations will be presented. Specific needs to be addressed will be introduced with examples.
The impact of process parameters on final bonding layer quality was investigated for Transient Liquid Phase (TLP)
wafer-level bonding based on the Cu-Sn system. Subjects of this investigation were bonding temperature profile,
bonding time and contact pressure as well as the choice of metal deposition method and the ratio of deposited metal layer
thicknesses. Typical failure modes in Inter-Metallic Compound (IMC) growth for the mentioned process and design
parameters were identified and subjected to qualitative and quantitative analysis. The possibilities to avoid
abovementioned failures are indicated based on experimental results.
A pre-bond cleaning process was developed utilizing a unique, radially uniform, large area proximity type Megasonic
transducer. In prior work this new cleaning method was investigated for PRE (particle removal efficiency) as well as
particle neutrality. These tests yielded higher values than those achieved with the processes of record. Subsequently, this
process was integrated into an industrial volume low temperature fusion bonding process and enabled higher bonding
In the above process flow the process fluid was dispensed to fill the gap between the Megasonic transducer surface and
the substrate using an atmospheric free flow stream applied to the substrate. Current work describes development, testing
and operational verification of a process fluid management device used in conjunction with the wide area proximity
Megasonic transducer. The goals of this development were reduction of process fluid amount required, increase the
operating substrate rotation speed, and provide better control of process fluid parameters. The design criteria and process
flow as well as test results demonstrating the benefits of the new system are presented.
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.
Adhesive wafer bonding is a technique that uses an intermediate layer for bonding (typically a polymer). The main
advantages of using this approach are: low temperature processing (maximum temperatures below 400°C), surface
planarization and tolerance to particles (the intermediate layer can incorporate particles with the diameter in the layer
thickness range). Evaporated glass, polymers, spin-on glasses, resists and polyimides are some of the materials suitable
for use as intermediate layers for bonding. The main properties of the dielectric materials required for a large field of
versatile applications/designs can be summarized as: isotropic dielectric constants, good thermal stability, low CTE and
Young's modulus, and a good adhesion to different substrates.
This paper reports on wafer-to-wafer adhesive bonding using SINR polymer materials. Substrate coating process as well
as wafer bonding process parameters optimization was studied. Wafer bonds exceeding the yield strength of the SINR
polymer were accomplished on 150 mm Si wafers. Features of as low as 15 μm were successfully resolved and bonded.
A unique megasonic-enhanced development process of the patterned film using low cost solvent was established and
proven to exceed standard development method performance. Statistical analysis methods were used to show
repeatability and reliability of coating processes.
We summarize the results of a European Project entitled WAPITI (Waferbonding and Active Passive Integration Technology and Implementation) dealing with the fabrication and investigation of active/passive vertically coupled ring resonators, wafer bonded on GaAs, and based on full wafer technology. The concept allows for the integration of an active ring laser vertically coupled to a transparent bus waveguide. All necessary layers are grown in a single epitaxial run so that the critical coupling gap can be precisely controlled with the high degree of accuracy of epitaxial growth. One key challenge of the project was to establish a reliable wafer bonding technique using BCB as an intermediate layer. In intensive tests we investigated and quantified the effect of unavoidable shrinkage of the BCB on the overall device performance. Results on cw-operation, low threshold currents of about 8 mA, high side-mode suppression ratios in the range of 40 dB and large signal modulation bandwidths of up to 5 GHz for a radius of 40 μm shows the viability of the integration process.
Manufacturing and integration of MEMS devices by wafer bonding often lead to problems generated by thermal
properties of materials. These include alignment shifts, substrate warping and thin film stress. By limiting the thermal
processing temperatures, thermal expansion differences between materials can be minimized in order to achieve stress-free,
aligned substrates without warpage.
Achieving wafer level bonding at low temperature employs a little magic and requires new technology development. The
cornerstone of low temperature bonding is plasma activation. The plasma is chosen to compliment existing interface
conditions and can result in conductive or insulating interfaces. A wide range of materials including semiconductors,
glasses, quartz and even plastics respond favorably to plasma activated bonding. The annealing temperatures required to
create permanent bonds are typically ranging from room temperature to 400°C for process times ranging from 15-30
minutes and up to 2-3 hours. This new technique enables integration of various materials combinations coming from
separate production lines.
Wafer bonding became in the last decade a very powerful technology for MEMS/MOEMS manufacturing. Being able to offer a solution to overcome some problems of the standard processes used for materials integration (e.g. epitaxy, thin films deposition), wafer bonding is nowadays considered an important item in the MEMS engineer toolbox. Different principles governing the wafer bonding processes will be reviewed in this paper. Various types of applications will be presented as examples.
Wafer bonding became a key technology in various MEMS devices manufacturing. In this respect, wafer bonding is a
very important technology as far as it enables not only 3D structure building but also wafer level packaging.
Plasma activated wafer bonding is a surface activation method in which by applying a plasma treatment to the wafers
prior to bringing them in contact for bonding, the surface chemistry can be tailored in order to obtain maximum bond
strength for low temperature thermal annealing. A major advantage of this process is that it makes possible some
bonding applications which are not possible using standard bonding processes due to different materials characteristics
(e.g. high thermal mismatch of the two bonding partners, low Tg for polymer bonds, etc.)
Plasma activated bonding was successfully applied for different types of materials: silicon, compound semiconductors,
oxides and polymers (e.g. PMMA). The present paper presents experimental results demonstrating the benefits of this
new technology and shows examples on how plasma activated wafer bonding can be an alternative to standard wafer
Low temperature wafer bonding is a powerful technique for MEMS/MOEMS devices fabrication and packaging. Among the low temperature processes adhesive bonding focuses a high technological interest. Adhesive wafer bonding is a bonding approach using an intermediate layer for bonding (e.g. glass, polymers, resists, polyimides). The main advantages of this method are: surface planarization, encapsulation of structures on the wafer surface, particle compensation and decrease of annealing temperature after bonding. This paper presents results on adhesive bonding using spin-on glass and Benzocyclobutene (BCB) from Dow Chemicals. The advantages of using adhesive bonding for MEMS applications will be illustrated be presenting a technology of fabricating GaAs-on-Si substrates (up to 150 mm diameter) and results on BCB bonding of Si wafers (200 mm diameter).
Further applications of MEMS require the combination of different materials in order to combine different functions in one and the same system. Besides conventional techniques (layer deposition methods) semiconductor wafer direct bonding is expected to be an effective method to produce heterogeneous materials. Different examples for optical and high-temperature applications are presented (Si-based heterostructures, Si/GaAs heterostructures).
The ferroelectric/semiconductor heterostructures were fabricated by sol-gel deposition of lead titanate (PT) thin films on a single-crystalline p-type Si wafers. The PT films were crystallized by a conventional thermal annealing for 30 min at temperatures ranging from 575 degree(s)C to 675 degree(s)C. Current-voltage and capacitance-voltage characteristics show a hysteresis which can be due to the spontaneous polarization of PbTiO3. The current-voltage characteristics exhibit a diode behavior while the capacitance-voltage exhibits a large memory window, up to 3.5 V, for the films annealed at 600 - 650 degree(s)C. At the illumination with modulated light, a.c. photovoltage was detected on a broad range of wavelengths (0.35 divided by 4 micrometers ) for all samples. The spectral distribution of the photoelectric signal in the UV-Vis- IR domain shows two local maxima. A model is proposed to explain the observed experimental results.