FormFactor, Inc.

Testing for manufacturing defects is considered one of the major challenges for the various types of 3D ICs, including FOWLPs, 2.5D-SICs, and 3D-SICs. Testing itself is a cost-adder, but not testing during the stack assembly flow proves to be even more costly.

FormFactor, Inc. of Livermore, CA provides the industry’s broadest range of wafer test technologies and expertise. FormFactor is the world’s leading supplier of probe cards, and with the recent acquisition of Cascade Microtech, Inc. of Beaverton, OR, they are now also the world’s leading supplier of engineering probe stations. Through both their advanced probe card technologies as well as through their advanced probe stations, FormFactor and Cascade Microtech have greatly contributed to enable testing 3D ICs during crucial moments in the stack assembly flow.

• FormFactor has developed advanced probe cards that were used to test the High Bandwidth Memory (HBM2) DRAMs of SK hynix that were part of AMD’s Radeon R9 Fury GPU product. This is one of the world’s first 3D-SICs produced in high volume, consisting of an AMD processor and four SK hynix HBM2 DRAM stacks on top of a silicon interposer. FormFactor’s MEMS-type probe cards were key in achieving ‘known-good stacks’ (see https://doi.org/10.1109/MDAT.2016.2624282).

• Cascade Microtech’s CM300 probe station was used in conjunction with Cascade Microtech’s Pyramid Probe® technology to probe and test large arrays of fine-pitch micro-bumps. Wafers from research institute imec with micro-bump arrays compliant to the JEDEC standards Wide-I/O (1,200 micro-bumps at 50/40µm pitch) and Wide-I/O2 (1,752 micro-bumps at 40µm pitch) were successfully probed. The contact resistance was within specification, the small probe marks did not have any negative impact on stacking yield, and cost-modeling studies underlined the cost effectiveness of this approach. This work was recently awarded with the National Instruments Engineering Impact Award 2017 in the category ‘Electronics & Semiconductor’ (see http://sine.ni.com/cs/app/doc/p/id/cs-17384#).

• Cascade Microtech’s CM300 probe station has an AlignChip software function that allows to correct small misalignments prior to probing a die. Imec has used this functionality to automatically probe and test die-to-die stacks that were pick-n-placed in a matrix structure on a tape frame or on thermal-release tape (see https://doi.org/10.1109/TEST.2015.7342412).

• Cascade Microtech’s CM300 probe station also has the capability to load large tape frames, for example to test thinned-down wafers with exposed TSVs. Combined with FormFactor’s low-force probe cards, imec has demonstrated that wafers as thin as 50µm can be reliably probed.