Page 56 - Chip Scale Review_January February_2022-digital

Figure 10: Cross-section of the test vehicle assembly
showing the U-groove edge supporting the fiber.
tolerance to relax the alig n ment
accu racy requirement. For these
reasons, the suspended coupler design
is the preferred design over the typical
inverse tapered coupler. The reported
suspended coupler [7] has a 1dB excess
loss of more than 2μm in the cross-
section misalignment, and more than
40μm along the fiber direction. The Figure 11: GSSG TSV model.
performance of the suspended coupler Usually, the f rontside elect r ical to support the solder bump for the
closely matches the requirement of this connection of the interposer is formed assembly of the Si-interposer to the
Si-interposer integration platform. u si n g b a ck- e n d - of-l i n e ( BE OL) substrate or printed circuit board
Both the PIC and the EIC are flip- processing, which supports the bare (PCB). The electrical connection
chip assembled on the Si-interposer. d i e a s s e m b l y between the micro solder bump and
using micro solder the RDL is accomplished by using
bumps. Typically, the TSV. The TSV helps to overcome
up to three metal the bandwidth-limiting inductance of
l a y e r s c a n b e the wire bond and increases the I/O
f o r m e d o n t h e density of the interconnect. For high-
t o p sid e of t he speed electrical connection, a high-
Si-inter poser to resistivity Si wafer is required to
support the front- reduce the electrical loss.
side routing. The For the TSI integration, the length of
minimum metal the TSV has to be much greater than
l i n e w i d t h i s the U-groove depth. The dummy fiber
a r ou n d 0 . 8 μ m , is designed to be supported by the edge
w h i c h i s m o r e of the U-groove, therefore, the depth of
t h an s u f f icie nt the U-groove needs to be deep enough
to support all the so that it can accommodate the epoxy
electrical routing without overflow or pushing up the
between the PIC dummy fiber. Based on the preliminary
a n d t h e EI C . design, the depth of the U-groove
T h e e le c t r i c a l needs to be more than 50μm, but to
c on n e c t i on on prevent the Si-interposer breakage
the backside of during handling, it is recommended to
t h e i n t e r p o s e r increase the height of the Si-interposer
is for med using to about 200μm.
RDL, which has a The EM-simulated response of a
much larger line differential ground-signal-signal-
width resolution. ground (GSSG) TSV with a height of
As most likely the 200μm is shown in Figure 11. The
top side BEOL has typical aspect ratio of the TSV is 1:10,
provided all the so for a 200μm high TSV, the diameter
electrical routing, will be around 20μm. The pitch of
the backside RDL the TSV is set to 100μm, and a high-
i s u s e d m a i n ly resistivity wafer of 700Ω.cm is used.

54 Chip Scale Review January • February • 2022 [ChipScaleReview.com]
54