7 Fundamentals and Failures in Die Preparation for 3D Packaging
169
Mei and Huang have investigated the wrinkling and buckle-delamination of elastic
thin films on elastic substrate, compliant substrate, and viscoelastic substrate [48–
50]. As indicated in Fig. 7.10, the critical compressive stress for wrinkling increases
with stiffness ratio between substrate and film, increases with film thickness, and
decreases with substrate thickness; the wrinkle amplitude increases with film strain
and decreases with substrate modulus; the wrinkle wavelength increases with stiffness ratio. As indicated in Fig. 7.11, the critical compressive stress for buckledelamination increase with stiffness ratio and decreases with initial crack length;
the transition of buckling modes depends on stiffness ratio and initial crack length.
Post TSV device wafer thinning, the stack-up of device wafer, temporary adhesive and carrier wafer can be simplified as elastic film (device wafer) and viscoelastic
substrate (temporary adhesive). During FEOL and BEOL and wafer thinning process,
a residual stress exists on device wafer. Depending on the dielectric deposition condition, metal plating condition, and wafer thinning condition, the residual stress can be
compression or tension. Different metrologies including synchrotron X-ray radiation,
micro-Raman, and bending beam have been employed to check the wafer residual
stress. Based on Figs. 7.10 and 7.11, in order to reduce the risk of wafer buckling
or wrinkling, different approaches can be applied, including (1) reducing compressive stress on device wafer by optimizing Si BEOL design and process and wafer
thinning process, (2) increasing the modulus of temporary adhesive, (3) reducing the
thickness of temporary adhesive, and (4) increasing device wafer thickness.
In addition to wafer level bow, die-level rippling is another effect often exhibit
on bonded device wafers post thinning and high temperature processes, which pose
a major challenge to backside processing steps afterwards. This is because of the
different stack composition of die region versus the street region. The metal and passivation layers end at the kerf boundary, usually with additional crack-stop features
around the die edge. As reported by Tamura et al. (Figure 7.12), up to 13 um rippling
in z was observed on a die size of 15 mm, after Plasma-enhanced chemical vapor
deposition (PECVD) at 250 °C [51]. By increasing the softening temperature of the
bonding adhesive, the magnitude of the rippling is significantly reduced.
7.4 Wafer Debonding and Clean
In the past decade, many different types of debonding systems were explored and
the most studied are thermal slide-off, mechanical release, and laser release.
Thermal slide-off debond requires heating the stacked wafer until the adhesive
softens, and then the two wafers are sheared apart. Inside the debonding chamber,
device wafer and carrier wafer are held by two heated vacuum chucks. After the
device wafer and the carrier wafer are heated up uniformly to debonding temperature,
sliding shear force is applied and device wafer is slid off relative to carrier wafer.
Based on viscosity definition and wafer geometry, Privett et al. investigated the
kinetics of thermal sliding debonding [52]. In the following, Shi further extended
their approach to derive a kinetic model for thermal sliding debonding process and
169
Mei and Huang have investigated the wrinkling and buckle-delamination of elastic
thin films on elastic substrate, compliant substrate, and viscoelastic substrate [48–
50]. As indicated in Fig. 7.10, the critical compressive stress for wrinkling increases
with stiffness ratio between substrate and film, increases with film thickness, and
decreases with substrate thickness; the wrinkle amplitude increases with film strain
and decreases with substrate modulus; the wrinkle wavelength increases with stiffness ratio. As indicated in Fig. 7.11, the critical compressive stress for buckledelamination increase with stiffness ratio and decreases with initial crack length;
the transition of buckling modes depends on stiffness ratio and initial crack length.
Post TSV device wafer thinning, the stack-up of device wafer, temporary adhesive and carrier wafer can be simplified as elastic film (device wafer) and viscoelastic
substrate (temporary adhesive). During FEOL and BEOL and wafer thinning process,
a residual stress exists on device wafer. Depending on the dielectric deposition condition, metal plating condition, and wafer thinning condition, the residual stress can be
compression or tension. Different metrologies including synchrotron X-ray radiation,
micro-Raman, and bending beam have been employed to check the wafer residual
stress. Based on Figs. 7.10 and 7.11, in order to reduce the risk of wafer buckling
or wrinkling, different approaches can be applied, including (1) reducing compressive stress on device wafer by optimizing Si BEOL design and process and wafer
thinning process, (2) increasing the modulus of temporary adhesive, (3) reducing the
thickness of temporary adhesive, and (4) increasing device wafer thickness.
In addition to wafer level bow, die-level rippling is another effect often exhibit
on bonded device wafers post thinning and high temperature processes, which pose
a major challenge to backside processing steps afterwards. This is because of the
different stack composition of die region versus the street region. The metal and passivation layers end at the kerf boundary, usually with additional crack-stop features
around the die edge. As reported by Tamura et al. (Figure 7.12), up to 13 um rippling
in z was observed on a die size of 15 mm, after Plasma-enhanced chemical vapor
deposition (PECVD) at 250 °C [51]. By increasing the softening temperature of the
bonding adhesive, the magnitude of the rippling is significantly reduced.
7.4 Wafer Debonding and Clean
In the past decade, many different types of debonding systems were explored and
the most studied are thermal slide-off, mechanical release, and laser release.
Thermal slide-off debond requires heating the stacked wafer until the adhesive
softens, and then the two wafers are sheared apart. Inside the debonding chamber,
device wafer and carrier wafer are held by two heated vacuum chucks. After the
device wafer and the carrier wafer are heated up uniformly to debonding temperature,
sliding shear force is applied and device wafer is slid off relative to carrier wafer.
Based on viscosity definition and wafer geometry, Privett et al. investigated the
kinetics of thermal sliding debonding [52]. In the following, Shi further extended
their approach to derive a kinetic model for thermal sliding debonding process and