Reflectometry, a simple whole-field curvature measurement system using a novel computer aided phase shift reflection
grating method has been improved to certain extend. The similar system was developed from our earlier works on
Computer Aided Moiré Methods and Novel Techniques in Reflection Moiré, Experimental Mechanics (1994) in which
novel structured light approach was shown for surface slope and curvature measurement. This method uses similar
technology but coupled with a novel phase shift system to accurately measure surface profile, slope and curvature.
In our previous paper, "Stress Measurement of thin wafer using Reflection Grating Method", the surface curvature and
residual stresses were evaluated using the versatility of the proposed system.. The curvature of wafers due to the deposition
of backside metallization was evaluated and compared with a commercially stress measurement system from KLA-Tencor.
In this paper, some aspects of the work are extended. Our proposed system is calibrated using a reference flat mirror and
spherical mirror certified by Zygo Corporation. The mirrors together with the camera calibration toolbox allow the system
to acquire measurement accuracy that is demanded by semiconductor industry. Finally, the results obtained from
Reflectometry are compared and contrast with results from KLA Tencor System.
In modern field of microelectronics and MEMS, wafer bonding has emerged as an important processing step in wide
range of manufacturing applications. During the manufacturing process, even in the modern clean room, small defects
result from trapped particles and gas bubbles exist at bonded interface. Defects and trapped particles may exist on the
top and bottom of the wafers, or at the interface of bonded wafer pair. These inclusions will generate high stress around
debond region at the wafers bonded interface. In this paper, inspection at the bonded interface will be the interest of
investigation.
Since silicon wafer is opaque to visible light, defect detection at the bonded interface of silicon wafer is not possible.
Due to the fact that silicon wafer is transparent to wavelength greater than 1150nm, an Near Infrared Polariscope which
has showed some promises on residual stress measurement on silicon devices has been adapted and developed. This
method is based on the well known photoelastic principles, where the stress variations are measured based on the
changes of light propagation velocity in birefringence material. The results are compared and contrast with
conventional Infrared Transmission Imaging tool (IRT) which is widely used to inspect the bonded silicon wafer.
In this research, the trapped particles that are not visible via conventional infrared transmission method are identified via
the generated residual stress pattern. The magnitude of the residual stress fields associated with each defect is examined
qualitatively and quantitatively. The stress field generated at the wafers bonded interface will looks like a 'butterfly'
pattern. Wafer pairs Pyrex-Si and Si-Si bonded interface will be examined.
Flatness/Curvature measurement is critical in many Si-wafer based technologies ranging from micro-electronics to
MEMS and to the current PV industry. As the thickness of the wafer becomes smaller there is an increased tendency for
it to warp and this is not conducive to both patterning as well as dicing. Monitoring of curvature/flatness is thus
necessary to ensure reliability of device and its uses.
A simple whole-field curvature measurement system using a novel computer aided phase shift reflection grating method
has been developed and this project aims to take it to the next step for residual stress measurement. The system was
developed from our earlier works on Computer Aided Moiré Methods and Novel Techniques in Reflection Moiré,
Experimental Mechanics (1994) in which novel structured light approach was shown for surface slope and curvature
measurement. This method uses similar technology but coupled with a novel phase shift system to accurately measure
slope and curvature.
In this research, the system is calibrated with reference to stress measurement equipment from KLA-Tencor. Some initial
results based on a joint project with Infineon Technologies are re-examined. The stress distribution of the wafers are
derived with the aid of Stoney's equation. Finally, the results from our proposed system are compared and contrasted
with data obtained from KLA-Tencor equipment.
Presently, most defective problems in silicon based devices can be traced ultimately to stresses developed during various
fabrications process stages. If the process induced stresses exceed the critical stress magnitude, dislocations occur to
relax the stain. Subsequently, other defects such as cracks, chips and void related to stresses may surface. Hence an
understanding of the development, effects and possible avoidance of residual stress is vital and even more important for
the semi-conductor industries as it goes into the nanometer technology. Whether process-induced, residual stress is a
significant factor in high wafer breakage rates during microelectronic fabrication. Identifying the cause and initiation
point of brittle fracture in processed wafers is an important first step towards the goal of making this and other newer
technologies based on silicon commercially competitive.
Infrared Phase Shift Gray Field which has showed some promises on residual stress measurement on silicon devices
ranging from microelectronics to photovoltaic industry will be employed and developed. This method is based on the
well known photoelastic principles, where the stress variations are measured based on the changes of light propagation
velocity in birefringence material. In this research, the residual stress patterns that are not visible via conventional
infrared transmission method will be used to locate defects. The magnitude of the residual stress fields associated with
each defect is examined qualitatively and quantitatively. Firstly, the inspection of defects on wafer surface in early stage
of wafer fabrication processes will be carried out. Next, an investigation of trapped particles and defects in bonded wafer
with using the associated residual stress field will be also be covered.
Flatness/Curvature measurement is critical in many Si-wafer based technologies ranging from micro-electronics to
MEMS and to the current PV industry. As the thickness of the wafer becomes smaller there is an increased tendency for
it to warp and this is not conducive to both patterning as well as dicing. Monitoring of curvature/flatness is thus
necessary to ensure reliability of device and its uses. However, due to the prevalence of surface flatness measurement
systems that flooded the market, the cycle time for curvature measurement system has become one of the critical factors
for the user to consider.
A simple and rapid whole-field curvature measurement system using a novel a computer aided phase shift reflection
grating method has been developed and discussed in the previous publications. Laterals gratings in horizontal has
vertical directions are needed in order to realize the curvature information on the wafer in both directions. In this paper,
with same system setup, circular grating is being projected on to the specimen to measure the curvature distribution of
the wafer. With the aid of coordinate-transform method and the digital phase-shifting technique, the digital images of
reflected gratings are processed automatically and analyzed in the polar coordinate system. Unlike vertical or horizontal
line gratings, the utilization of the circular gratings in radial shearing method provides curvature information in all
directions, not only in one. Further, only four phase shifted images are captured and the measurement cycle time is thus
reduced by half.
In our previous paper, "Warpage of thin wafers using computer aided reflection moire method," the surface curvature
and residual stresses were evaluated using the versatility of computer aided reflection grating method to manipulate and
generate gratings in two orthogonal directions. A very good agreement between the theory and experimental results was
established. The bending stresses of wafers due to the deposition of backside metallization were evaluated without the
aid of reference grating.
In this paper, some aspects of the work is extended. An optical flat with flatness λ/10 is used as a reference plate to
extract the residual stress of the wafers with different backside metallization. By utilizing the phase information from the
moiré pattern between deformed grating (wafer) and undeformed grating (optical flat), the surface deformation of the
wafer and residual stresses are investigated quantitatively and numerically.
This technique, with satisfactory sensitivity and accuracy, can be used to characterize the residual stress of wafer due to
warpage that may lead to the crack issues in semiconductor manufacturing industry.
To cope with advances in the electronic packaging industry, thinner wafers are being widely employed to produce
thinner packages. However, this has lead to an increase in random cracks during the wafer singulation process, thus
reducing the yield of the overall production.
Large stresses are induced particularly during backside metal deposition. The wafers bend due to these stresses. This
residual stress due to warpage lead to cracks which will severely re-orient the residual stress distributions, thus,
weakening the mechanical and electrical properties of the singulated die.
In this study, Computer aided reflection moiré technique is adapted to further investigates the warpage induced on
wafers with different backside metallization (bare silicon, AuY, AuX). The backside metal on the wafer is then etched
to remove the residual stress. Residual stress due to the effect of warpage caused by different backside metallization has
been experimentally investigated and compared. Applicability of this technique to correlate with the random crack in
the die is further validated.
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