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complete daisy chain that can be tested respectively to complete each S-MCM. excess of 50mA at 4.2K. The large number
for continuity and total resistance. Additionally, because of minimum of microbumps per chip, compact bump
These devices are surrogates for a spacing and edge keep-out regions for geometry, high critical current of niobium,
superconducting wafer-scale MCM and dicing and in-line metrology, 2-chip ( and high current-carrying capacity of
the associated superconducting chips to 20mm x 20mm) S-MCMs can have a the microbumps enable this process
be connected. The current microbump significantly higher active circuit area to be suitable for building VLSI flip-
fabrication process uses a single I-line compared to a 16-chip S-MCM for a chip structures and developing complex
liftoff process on the superconducting given lithography process. superconducting computing systems.
MCM. An interconnect layer (1000nm As a case study of larger area flip-
Nb, 20nm Ti, 50nm Pt, 150nm Au, chip interconnections, a 25mm x Stitched S-MCM
20 0 0 n m I n) a n d a n u n d e r b u m p 25mm S-MCM with niobium-indium I n t h i s s e c t ion , we d e t a i l t he
metalization (UBM) layer (1000nm microbumps and a 20mm x 20mm fabrication processes for reticle stitching.
Nb, 20nm Ti, 50nm Pt, 150nm Au) superconducting chip were bonded The previously described single reticle-
are evaporated onto the S-MCM, and together to fabricate a daisy chain based S-MCM fabrication process
superconducting chips, respectively. structure. By alternating daisy chains in was modified to create a larger area
The I-line photo process is important the lay-up prior to bonding, the Nb-In stitched S-MCM. Figure 2 represents
for reducing wafer-scale photoresist microbumps electrically connect the daisy the stitching process where circuit lines
defects, def ining bump diameter, chains. The S-MCMs include a 20mm of individual reticles are stitched at a
and improved alignment to maintain x 20mm flip-chip daisy chain structure stitch boundary. Four I-line photomasks
electrical properties and minimize prepared with approximately 10,000 or (A, B, C, D) were joined together to
bump-bonding issues. 100,000 niobium-indium microbumps. create a stitched field. Photomask A
Figure 1 shows flip-chip S-MCMs To assess the electrical performance of and photomask B were stitched in
bonded with 20mm x 20mm and 5mm the bump bonded full reticle chips, the the X-direction, while photomask A
x 5mm superconducting chips. We have S-MCMs with 20mm x 20mm chips were and photomask C were stitched in the
fabricated 48mm x 48mm S-MCMs with attached to a PCB card and wire bonded Y-direction. Similarly, photomask
niobium-indium (Nb-In) microbumps. to measure I-V characteristics of bump B and photomask D were stitched in
Thermocompression (TC) bonding of interrupted niobium at 4.2K [2]. Niobium- the Y-direction, while Photomask C
Nb-In microbumps provides sufficient indium microbumps not only showed and photomask D were stitched in the
mechanical strength, creates low- very low resistance in the range of 0.05- X-direction. The individual size of
resistance electrical interconnects, and 0.1 milliohm at 4.2K, but also maintained each photomask reticle (A, B, C, D)
produces minimum spacing between the high niobium critical current. We will determine the overall stitched field
chip and S-MCM. Furthermore, large Si- measured 20 mm x 20 mm flip-chip daisy size. For example, a 35mm x 35mm
chip bonding reduces the total number chains ranging in number of interconnect reticle and a 48mm x 48mm reticle will
of bonding cycles and simplifies the segments from a few thousand to tens of produce 70mm x 70mm and 96mm x
assembly process. For example, Figure 1 thousands of microbumps in series. The 96mm stitched S-MCMs respectively,
shows a 2-chip (20mm x 20mm) S-MCM I-V curve of approximately 24,000 Nb- for a four masks per layer process.
and a 16-chip (5mm x 5mm) S-MCM, In microbumps series flip-chip daisy- Figures 2c and d also show an enlarged
which used 2, and 16 TC bonding cycles, chain had a niobium critical current in stitching area between photomask A
and photomask B. Mask A has a circuit
line extending into the chrome area.
The extensions in the chrome area are
defined as the overlap length where the
line will expose twice (i.e., a double
exposure). We optimize the overlap
length as 0.25μm. So, for a stitched line
with a 0.25μm overlap length on each
side, a minimum 0.5μm long line at the
stitch boundary will expose photoresist
twice for stitching. A double-exposed
resist line will distort the linewidth at
the stitch boundary. An optimal 0.25μm
overlap length for each side provided
the least amount of linewidth distortion
at the stitch boundary. Additionally,
corresponding SEM micrographs of
a 0.8μm line going through the stitch
boundary (in the X-direction) shows a
Figure 1: Superconducting multichip modules (S-MCM) with attached superconducting test chips: a) (left) representative example. SEM images
Optical image of a 48mm x 48mm S-MCM bonded with two 20mm x 20mm superconducting test chips; b) (right) (Figure 2e) show an approximately 70-
Optical image of 32mm x 32mm S-MCM bonded with sixteen 5mm x 5mm superconducting test chips. One 110nm linewidth variation within a 1μm
20mm X 20mm superconducting test chip and sixteen 5mm X 5mm chips have the same circuit area.

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