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High-precision wafer thinning using ultra-low-TTV

glass carrier and temporary bonding

By Jay Zhang [Corning Incorporated]
W a f e r t h i n n i n g i s in Figure 1. We use stack TTV to Although double-sided polishing

a c r i t i c a l p a r t o f
d ev ic e m a k i ng a nd describe the combined TTV of the carrier techniques have existed for decades,
achieving TTV<1µm on a 300mm
wafer and the adhesive. For emerging
advanced packaging in semiconductor applications that either demand <10µm diameter wafer takes fundamental
manufacturing. Carrier-supported wafer final wafer thickness, or wafer thickness understanding of the entire finishing
thinning becomes attractive when the tolerances <1µm, total thickness variation process a nd opt i m i zat ion of t he
final wafer thickness becomes very thin, of the support structure, namely the stack equipment as well as the consumables.
say <100µm, due to handling difficulties. TTV, must be <1µm, ideally <<1µm. Recent work at Corning has produced
Post-thinning processing (deposition, This requires innovation in both carrier 200mm and 300mm glass wafers with
chemical mechanical polishing [CMP], wafer manufacturing as well as in the TTVs as low as 0.2µm. Figure 2 shows
bonding, etc.) also benefits from having temporary bonding method. TTV performance of a 300mm glass
a rigid and thermomechanically stable wafer. As of May 2022, ultra-low-TTV
support structure. In order to achieve Ultra-low-TTV (ULTTV) glass (ULTTV) wafers are commercially
very low total thickness variation (TTV) carrier wafers available: 200mm or smaller wafers
of the final wafer, both the carrier wafer Glass wafers are typically made promise TTV<0.2µm; 300mm wafers
and the temporary bonding material— using one of two methods. Wafer promise TTV<0.4µm.
typically an adhesive layer—must deliver blanks can either be extracted out of a
sufficiently low TTV. The carrier wafer flat glass sheet, or sliced out of a glass Critical glass wafer attributes for
also must have a suitable coefficient of block using what is called a ganged carrier applications
thermal expansion (CTE). In this paper, wiresaw. While high-precision sheet Wafer shape distortion during bonding
we describe a glass carrier solution and a forming technology, such as our fusion and post-bonding processing can be
temporary bonding method that combine method, can deliver excellent flatness understood from thermomechanical
to enable low TTV wafer thinning. and TTV without additional polishing, modeling. We introduced a simplified
Feasibility demonstration is done using it is not capable of delivering a TTV formula for cases where the carrier wafer
a 150mm diameter single crystal lithium smaller than 1µm with reasonable supports another much thinner wafer to
tantalate wafer, and thinning reaches 5µm yields, let alone a fraction of 1µm. relate magnitude of shape distortion to
with wafer grinding alone. This challenge is tougher the larger the carrier properties [1]:
wafer size. Sliced wafer blanks have
Introduction a rough surface that must go through
Glass wafers are being adopted by lapping and polishing to be usable as a
more and more companies in advanced carrier wafer.
packaging as a carrier. Attractive
attributes of glass include: tailorable
CTE, ability to deliver high-precision
features such as excellent flatness and
very low TTV, stability over a wide
range of processing temperatures,
chemical durability, and compatibility
with mechanical, thermal or light-based
debonding methods. Glass is also highly
manufacturable, delivering cost efficiency.
In a typical use scenario, the glass
carrier wafer is bonded to the wafer to
be thinned using a temporary adhesive
layer that is designed to be releasable
after processing. Both the adhesive layer
and the carrier wafer have thickness
variations that will limit the uniformity
of the thinned wafer. This is illustrated Figure 1: Both carrier TTV and adhesive TTV impact thinned wafer TTV.

6 6 Chip Scale Review March • April • 2024 [ChipScaleReview.com]