Page 10 - Chip Scale Review_March-April_2024-digital

Figure 4: Schematic of ALoT-based approach to flat panel display (FPD) processing.
Incumbent methods used either mechanical or chemical
thinning with significant environmental and cost challenges.
Figure 4 illustrates the concept. ALoT details are covered
elsewhere [3].
A common feature of the ALoT family is the extremely thin
bond layer, ranging in thickness from sub-nanometer to tens of
nanometers. Such a thin layer would add essentially no TTV to
the stack TTV, which is composed of carrier TTV and bond layer
TTV. Most ALoT recipes use room-temperature pre-bonding
followed by annealing at temperatures <150°C to achieve
sufficient bond strength to support wafer thinning and remain
mechanically debondable using standard debonding equipment.
Low annealing temperature allows materials of different CTEs
to be bondable without delamination or breakage due to stress.
In the demonstration to be described next, ALoT bonding is
done between a piezoelectric single crystal wafer with a CTE of
~14ppm/°C and a glass carrier with a CTE of 3.4ppm/°C.
ALoT bonding requires both the carrier and the wafer to be
thinned to have flat and smooth bonding surfaces. If thinning Figure 5: Thinning of LT using ALoT bonding and ultra-low-TTV glass carrier.
is performed before device making, such as the example After thinning, LT is then transferred to a permanent support substrate, and the
below, wafer surface characteristics can be guaranteed carrier can be reused.
through wafer specifications such as roughness (e.g., Ra) and
local flatness (e.g., LTV). When a device wafer to be thinned
already has surface topography from prior device making
steps, planarization must first be performed to achieve
the required surface flatness and smoothness. The latter
constraints may limit the ALoT application to only certain
device wafer categories.

Wafer thinning demonstration
Single crystal wafers of piezoelectric materials such as lithium
tantalate (LiTaO 3 , LT for short) and lithium niobate (LiNbO 3 ,
LN for short) are used widely in acoustic filter radio-frequency
(RF) applications. Modern wireless systems (4G/5G) use GHz
frequencies that benefit from very thin layers (e.g., <1µm) of
these single crystals [4]. If thinning of LT or LN is supported
with a high TTV substrate, achieving uniform layer thickness
through wafer thinning would be difficult to impossible. Current
practice is to follow mechanical thinning with a point-by-point
technique such as ion beam trimming to fix the nonuniformity
[5]. Ion beam trimming is slow and expensive. One alternative
we set out to demonstrate is to use an ultra-low-TTV glass
carrier with ALoT bonding. We chose LT for its broad adoption
in surface acoustic wave (SAW) devices as well as availability Figure 6: 150mm diameter, ALoT-bonded LT/glass wafer with a small number
up to 150mm diameter size. The concept is shown in Figure 5. of remaining bonding defects. Edge trimming was performed to remove bonding
defectivity near the edge. LT thickness before grinding is 350µm.
8 8 Chip Scale Review March • April • 2024 [ChipScaleReview.com]