Bond characterization
Encyclopedia
The wafer bond characterization is based on different methods and tests. Considered a high importance of the wafer are the successful bonded wafers without flaws. Those flaws can be caused by void formation in the interface due to unevenness or impurities. The bond connection is characterized for wafer bond development or quality assessment of fabricated wafers and sensors.
The bond strength can be evaluated using double cantilever beam or chevron respectively micro-chevron tests. Other pull tests as well as burst, shear or bend tests enable the determination of the bond strength. The packaging hermeticity is characterized using membrane, He-leak, resonator/pressure tests.
Three additional possibilities to evaluate the bond connection are optical, electron and acoustic measurements. At first, optical measurement techniques are optical microscopy, IR transmission microscopy and visual inspection. Secondly, the electron measurement is commonly applied using electron microscopy, e.g. scanning electron microscopy (SEM), high voltage transmittance electron microscopy (HVTEM) and high resolution scanning electron microscopy (HRSEM). And finally, typical acoustic measurement approaches are scanning acoustic microscope (SAM), scanning laser acoustic microscope (SLAM) and C-mode scanning acoustic microscope (C-SAM).
The specimen preparation is sophisticated and the mechanical, electronic properties are important for the bonding technology characterization and comparison.
Infrared transmitted light is based on the fact that silicon is translucent at wavelength ≥ 1.2 µm. The equipment consists of a infrared lamp as light source and a infrared video system (compare to figure "Schematic infrared transmission microscopy setup").
The IR imaging system enables the analysis of the bond wave and additionally micro mechanical structures as well as deformities in the silicon. This procedure allows also to analyze multiple layer bonds. The image contrast depends on the distance between the wafers. Usually if using monochromatic IR the center of the wafers is display brighter based on the vicinity. Particles in the bond interface generate highly visible spots with differing contrast because of the interference fringes. Unbonded areas can be shown if the void opening (height) is ≥ 1 nm.
is a non-destructive hermeticity characterization method. The radiation absorption enables the analysis with a specific wavelength for gases.
This method works with an ultrasonic transducer scanning the wafer bond. The reflected sound signal is used for the image creation. The lateral resolutions depends on the ultrasonic frequency, the acoustic beam diameter and the signal-to-noise ratio (contrast).
Unbonded areas, i.e. impurities or voids, do not reflect the ultrasonic beam like bonded areas, therefore a quality assessment of the bond is possible.
equals the distance between the blade tip and the crack tip and is determined using IR transmitted light. The IR light is able to illuminate the crack, when using materials transparent to IR or visible light. Is the fracture surface toughness very high, it is very difficult to insert the blade and the wafers are endangered to break at the slide in of the blade.
The DCB test characterizes the time dependent strength by mechanical fracture evaluation and is therefore well suited for lifetime predictions. A disadvantage of this method is, that between the entering of the blade and the time to take the IR image, the results can be influenced. In addition, the measurement inaccuracy increases with a high surface fracture toughness resulting in a smaller crack length or broken wafers at the blade insertion as well as the influence of the fourth power of the measured crack length. The measured crack length determines surface energy
in relation to a rectangular, beam-shaped specimen.
Thereby is
the Young's modulus, 
the wafer thickness, 
the blade thickness and 
the measured crack length.
In literature different DCB models are mentioned, i.e. measurement approaches by Maszara, Gillis and Gilman, Srawley and Gross, Kanninen or Williams. The most commonly used approaches are by Maszara or Gillis and Gilman.
The compliance is determined out of the crack length
, the width 
and the beam thickness 
. 
defines the Young's modulus. The surface fracture energy 
is:
with
as load-point displacement.
The first term
describes the strain energy in the cantilever due to bending. The second term is the contribution from elastic deformations in the un-cleaved specimen part and the third term considers the shear deformation. Therefore 
and 
are dependent on the conditions of the fixed end of the cantilever. The shear coefficient 
is dependent on the cross-section geometry of the beam.
of brittle construction materials. The fracture toughness is a basic material parameter for analyzing the bond strength.
The chevron test uses a special notch geometry for the specimen that is loaded with an increasing tensile force. The chevron notch geometry is commonly in shape of a triangle with different bond patterns. At a specific tensile load the crack starts at the chevron tip and grows with continuous applied load until a critical length is reached. The crack growth becomes unstable and accelerates resulting in a fracture of the specimen. The critical length depends only on the specimen geometry and the loading condition. The fracture toughness commonly
is determined by measuring the recorded fracture load of the test. This improves the test quality and accuracy and decreases measurement scatter.
Two approaches, based on energy release rate
or stress intensity factor 
, can be used for explaining the chevron test method. The fracture occurs when 
or 
reach a critical value, describing the fracture toughness 
or 
.
The advantage using chevron notch specimen is due to the formation of a specified crack of well-defined length. The disadvantage of the approach is that the gluing required for loading is time consuming and may induce data scatter due to misalignment.
and the critical fracture toughness 
. It is commonly used to characterize the wafer bond strength as well as the reliability. The reliability characterization is determined based on the fracture mechanical evaluation of critical failure. The evaluation is determined by analyzing the fracture toughness as well as the resistance against crack propagation.
The fracture toughness allows comparison of the strength properties independent on the particular specimen geometry. In addition, bond strength of the bonded interface can be determined. The chevron specimen is designed out of bonded stripes in shape of a triangle. The space of the tip of the chevron structure triangle is used as lever arm for the applied force. This reduces the force required to initiate the crack. The dimensions of the micro chevron structures are in the range of several millimeters and usually an angle of 70 ° chevron notch. This chevron pattern is fabricated using wet or reactive ion etching.
The MC test is applied with special specimen stamp glued onto the non-bonded edge of the processed structures. The specimen is loaded in a tensile tester and the load is applied perpendicular to the bonded area. When the load equals the maximum bearable conditions, a crack is initiated at the tip of the chevron notch.´
By increasing the mechanical stress by means of a higher loading, two opposing effects can be observed. First, the resistance against the crack expansion increases based on the increasing bonding of the triangular shaped first half of the chevron pattern. Second, the lever arm is getting longer with increased crack length
. From the critical crack length 
an instable crack expansion and the destruction of the specimen is initiated. The critical crack length 
corresponds to the maximum force 
in a force-length-diagram and a minimum of the geometric function 
.
The fracture toughness
can be calculated with maximum force, width 
and thickness 
:
The maximum force
is determined during the test and the minimal stress intensity coefficient 
is determined by FE Simulation. In addition, the energy release rate 
can be determined with 
as modulus of elasticity and 
as Poisson's ratio in the following way.´
The advantage of this test is the high accuracy compared to other tensile or bend tests. It is an effective, reliable and precise approach for the development of wafer bonds as well as for the quality control of the micro mechanical device production.
A test device as shown in the image is commonly used to determine the shear strength. Two skids are pressed together. The top tool provides the applied force. The bottom tool holds the specimen and is mounted on a movable skid to ensure that only shear forces effect the specimen.
For the white light interferometer the position of zero order interference fringe and the spacing of the interference fringes needs to be independent of wavelength.
White light interferometry is utilized to detect deformations of the wafer. Low coherence light from a white light source passes through the top wafer to the sensor. The white light is generated by a halogen lamp and modulated. The spectrum of the reflected light of the sensor cavity is detected by a spectrometer. The captured spectrum is used to obtain the cavity length of the sensor. The cavity length corresponds to the applied pressure and is determined by the spectrum of the reflection of the light of the sensor. This pressure value is subsequently displayed on a screen. The cavity length
is determined using
with
as refractive index of the sensor cavity material, 
and 
as adjacent peaks in the reflection spectrum.
The advantage of using white light interferometry as characterization method is the influence reduction of the bending loss.
Overview
Wafer bonds are commonly characterized by three important encapsulation parameters: bond strength, hermeticity of encapsulation and bonding induced stress.The bond strength can be evaluated using double cantilever beam or chevron respectively micro-chevron tests. Other pull tests as well as burst, shear or bend tests enable the determination of the bond strength. The packaging hermeticity is characterized using membrane, He-leak, resonator/pressure tests.
Three additional possibilities to evaluate the bond connection are optical, electron and acoustic measurements. At first, optical measurement techniques are optical microscopy, IR transmission microscopy and visual inspection. Secondly, the electron measurement is commonly applied using electron microscopy, e.g. scanning electron microscopy (SEM), high voltage transmittance electron microscopy (HVTEM) and high resolution scanning electron microscopy (HRSEM). And finally, typical acoustic measurement approaches are scanning acoustic microscope (SAM), scanning laser acoustic microscope (SLAM) and C-mode scanning acoustic microscope (C-SAM).
The specimen preparation is sophisticated and the mechanical, electronic properties are important for the bonding technology characterization and comparison.
Infrared (IR) transmission microscopy
Infrared (IR) void imaging is possible if the analyzed materials are IR transparent, i.e. silicon. This method gives a rapid qualitative examination and is very suitable due to its sensitivity to the surface and to the buried interface. It obtains information on chemical nature of surface and interface.Infrared transmitted light is based on the fact that silicon is translucent at wavelength ≥ 1.2 µm. The equipment consists of a infrared lamp as light source and a infrared video system (compare to figure "Schematic infrared transmission microscopy setup").
The IR imaging system enables the analysis of the bond wave and additionally micro mechanical structures as well as deformities in the silicon. This procedure allows also to analyze multiple layer bonds. The image contrast depends on the distance between the wafers. Usually if using monochromatic IR the center of the wafers is display brighter based on the vicinity. Particles in the bond interface generate highly visible spots with differing contrast because of the interference fringes. Unbonded areas can be shown if the void opening (height) is ≥ 1 nm.
Fourier transform infrared (FT-IR) spectroscopy
The Fourier transform infrared (FT-IR) spectroscopyFourier transform infrared spectroscopy
Fourier transform infrared spectroscopy is a technique which is used to obtain an infrared spectrum of absorption, emission, photoconductivity or Raman scattering of a solid, liquid or gas. An FTIR spectrometer simultaneously collects spectral data in a wide spectral range...
is a non-destructive hermeticity characterization method. The radiation absorption enables the analysis with a specific wavelength for gases.
Ultrasonic microscopy
Ultrasonic microscopy uses high frequency sound waves to image bonded interfaces. The DI water is used as acoustic interconnect medium between the acoustic transducer and the wafer.This method works with an ultrasonic transducer scanning the wafer bond. The reflected sound signal is used for the image creation. The lateral resolutions depends on the ultrasonic frequency, the acoustic beam diameter and the signal-to-noise ratio (contrast).
Unbonded areas, i.e. impurities or voids, do not reflect the ultrasonic beam like bonded areas, therefore a quality assessment of the bond is possible.
Double cantilever beam (DCB) test
Double cantilever beam test, also referred to as crack opening or razor blade method, is a method to define the strength of the bond. This is achieved by determine the energy of the bonded surfaces. A blade of a specific thickness is inserted between the bonded wafer pair. This leads to a split-up of the bond connection. The crack length equals the distance between the blade tip and the crack tip and is determined using IR transmitted light. The IR light is able to illuminate the crack, when using materials transparent to IR or visible light. Is the fracture surface toughness very high, it is very difficult to insert the blade and the wafers are endangered to break at the slide in of the blade.The DCB test characterizes the time dependent strength by mechanical fracture evaluation and is therefore well suited for lifetime predictions. A disadvantage of this method is, that between the entering of the blade and the time to take the IR image, the results can be influenced. In addition, the measurement inaccuracy increases with a high surface fracture toughness resulting in a smaller crack length or broken wafers at the blade insertion as well as the influence of the fourth power of the measured crack length. The measured crack length determines surface energy
in relation to a rectangular, beam-shaped specimen.Thereby is
the Young's modulus, the wafer thickness, the blade thickness and the measured crack length.In literature different DCB models are mentioned, i.e. measurement approaches by Maszara, Gillis and Gilman, Srawley and Gross, Kanninen or Williams. The most commonly used approaches are by Maszara or Gillis and Gilman.
Maszara model
The Maszara model neglects shear stress as well as stress in the un-cleaved part for the obtained crack lengths. The compliance of a symmetric DCB specimen is described as follows:The compliance is determined out of the crack length
, the width and the beam thickness . defines the Young's modulus. The surface fracture energy is:with
as load-point displacement.Gillis and Gilman model
The Gillis and Gilman approach considers bend and shear forces in the beam. The compliance equation is:The first term
describes the strain energy in the cantilever due to bending. The second term is the contribution from elastic deformations in the un-cleaved specimen part and the third term considers the shear deformation. Therefore and are dependent on the conditions of the fixed end of the cantilever. The shear coefficient is dependent on the cross-section geometry of the beam.Chevron test
The chevron test is used to determine the fracture toughness of brittle construction materials. The fracture toughness is a basic material parameter for analyzing the bond strength.The chevron test uses a special notch geometry for the specimen that is loaded with an increasing tensile force. The chevron notch geometry is commonly in shape of a triangle with different bond patterns. At a specific tensile load the crack starts at the chevron tip and grows with continuous applied load until a critical length is reached. The crack growth becomes unstable and accelerates resulting in a fracture of the specimen. The critical length depends only on the specimen geometry and the loading condition. The fracture toughness commonly
is determined by measuring the recorded fracture load of the test. This improves the test quality and accuracy and decreases measurement scatter.Two approaches, based on energy release rate
or stress intensity factor , can be used for explaining the chevron test method. The fracture occurs when or reach a critical value, describing the fracture toughness or .The advantage using chevron notch specimen is due to the formation of a specified crack of well-defined length. The disadvantage of the approach is that the gluing required for loading is time consuming and may induce data scatter due to misalignment.
Micro chevron (MC) test
The micro chevron (MC) test is a modification of the chevron test using a specimen of defined and reproducible size and shape. The test allows the determination of the critical energy release rate and the critical fracture toughness . It is commonly used to characterize the wafer bond strength as well as the reliability. The reliability characterization is determined based on the fracture mechanical evaluation of critical failure. The evaluation is determined by analyzing the fracture toughness as well as the resistance against crack propagation.The fracture toughness allows comparison of the strength properties independent on the particular specimen geometry. In addition, bond strength of the bonded interface can be determined. The chevron specimen is designed out of bonded stripes in shape of a triangle. The space of the tip of the chevron structure triangle is used as lever arm for the applied force. This reduces the force required to initiate the crack. The dimensions of the micro chevron structures are in the range of several millimeters and usually an angle of 70 ° chevron notch. This chevron pattern is fabricated using wet or reactive ion etching.
The MC test is applied with special specimen stamp glued onto the non-bonded edge of the processed structures. The specimen is loaded in a tensile tester and the load is applied perpendicular to the bonded area. When the load equals the maximum bearable conditions, a crack is initiated at the tip of the chevron notch.´
By increasing the mechanical stress by means of a higher loading, two opposing effects can be observed. First, the resistance against the crack expansion increases based on the increasing bonding of the triangular shaped first half of the chevron pattern. Second, the lever arm is getting longer with increased crack length
. From the critical crack length an instable crack expansion and the destruction of the specimen is initiated. The critical crack length corresponds to the maximum force in a force-length-diagram and a minimum of the geometric function .The fracture toughness
can be calculated with maximum force, width and thickness :The maximum force
is determined during the test and the minimal stress intensity coefficient is determined by FE Simulation. In addition, the energy release rate can be determined with as modulus of elasticity and as Poisson's ratio in the following way.´The advantage of this test is the high accuracy compared to other tensile or bend tests. It is an effective, reliable and precise approach for the development of wafer bonds as well as for the quality control of the micro mechanical device production.
Shear test
Shear testing is a method to determine the average strength and local stress the bonding layer can withstand. It is used to determine the integrity of materials and procedures and evaluate the overall performance of the bonding frame and to compare various bonding technologies with each other. It is based on measuring the applied force, the failure type due to the applied force and the visual appearance of the residual medium used. The contact tool is required to apply uniform force over the whole contact area. The top tool applies a force on the test medium that is sufficient to shear the specimen. The applied force used equals the maximum shear strength the connection can withstand.A test device as shown in the image is commonly used to determine the shear strength. Two skids are pressed together. The top tool provides the applied force. The bottom tool holds the specimen and is mounted on a movable skid to ensure that only shear forces effect the specimen.
White Light Interferometers
White light interferometry is commonly used for detecting deformations of the wafer surface based on optical measurements. Low-coherence light from a white light source passes through the optical top wafer, e.g. glass wafer, to the bond interface. Usually there are three different white light interferometers:- diffraction grating interferometers
- vertical scanning or coherence probe interferometers
- white light scatter plate interferometers
For the white light interferometer the position of zero order interference fringe and the spacing of the interference fringes needs to be independent of wavelength.
White light interferometry is utilized to detect deformations of the wafer. Low coherence light from a white light source passes through the top wafer to the sensor. The white light is generated by a halogen lamp and modulated. The spectrum of the reflected light of the sensor cavity is detected by a spectrometer. The captured spectrum is used to obtain the cavity length of the sensor. The cavity length corresponds to the applied pressure and is determined by the spectrum of the reflection of the light of the sensor. This pressure value is subsequently displayed on a screen. The cavity length
is determined usingwith
as refractive index of the sensor cavity material, and as adjacent peaks in the reflection spectrum.The advantage of using white light interferometry as characterization method is the influence reduction of the bending loss.
See also
- Wafer bondingWafer bondingWafer bonding is a packaging technology on wafer-level for the fabrication of microelectromechanical systems , nanoelectromechanical systems , microelectronics and optoelectronics, ensuring a mechanically stable and hermetically sealed encapsulation...
- Direct bondingDirect bondingDirect bonding describes a wafer bonding process without any additional intermediate layers. The bonding process is based on chemical bonds between two surfaces of any material possible meeting numerous requirements....
- Plasma activated bondingPlasma activated bondingPlasma activated bonding is a derivative, directed to lower processing temperatures for direct bonding with hydrophilic surfaces. The main requirements for lowering temperatures of direct bonding are the use of materials melting at low temperatures and with different coefficients of thermal...
- Anodic bondingAnodic bondingAnodic bonding is a wafer bonding procedure without any intermediate layer. This bonding technique, also known as field assisted bonding or electrostatic sealing, is mostly used for connecting silicon/glass and metal/glass through electric fields...
- Eutectic bondingEutectic bondingEutectic bonding, also referred to as eutectic soldering, describes a wafer bonding technique with an intermediate metal layer. Those eutectic metals are alloys that transform directly from solid to liquid state at a specific composition and temperature without passing a two phase equilibrium, i.e...
- Glass frit bondingGlass frit bondingGlass frit bonding, also referred to as glass soldering or seal glass bonding, describes a wafer bonding technique with an intermediate glass layer. It is a widely used encapsulation technology for surface micro-machined structures, i.e. accelerometers or gyroscopes. This technique utilizes low...
- Adhesive bondingAdhesive bondingAdhesive bonding describes a wafer bonding technique with applying an intermediate layer to connect substrates of different materials. These produced connections can be soluble or insoluble. The commercially available adhesive can be organic or inorganic and is deposited on one or both substrate...
- Thermocompression bondingThermocompression bondingThermocompression bonding describes a wafer bonding technique and is also referred to as diffusion bonding, pressure joining, thermocompression welding or solid-state welding. Two metals, e.g. gold -gold , are brought into atomic contact applying force and heat simultaneously. The diffusion...
- Reactive bondingReactive bondingReactive bonding describes a wafer bonding procedure using highly reactive nanoscale multilayer systems as an intermediate layer between the bonding substrates. The multilayer system consists of two alternating different thin metallic films. The self-propagating exothermic reaction within the...