Flip-chip bonding at STFC-RAL

STFC/RAL are developing a fine pitched Indium flip-chip process. The samples first have an under bump metal (UBM) layer deposited upon them where the bump is to be grown. The UBM used to date is Ti/W, Ni, Au. The Ti/W is an adhesion layer to the Aluminium pad of the device. The nickel is the solderable metal layer and the Au is an oxidation barrier. The indium is deposited in a thermal reactor at RAL. The Indium is heated under vacuum and condenses on the sample that is held at a lower temperature. The samples can then be flip-chipped bonded. The flip-chip process is to work at room temperature or slightly elevated temperatures.

There is some worry that the Indium bumps require more pressure to form a bump than should be the case. One suggestion for this high pressure is that the gold used in the UBM is moving into the Indium and forming an Indium/Gold alloy which requires more force to deform. A SEM/FIB/EDX study of an Indium bump formed on a silicon substrate was performed in Glasgow to try and test the hypothesis that the gold had moved into the Indium. The report is found here:

• Report_on_SEM_Secondary_electron_images_and_EDX_measurements_on_In_bumps_from_RAL.docx: Report on SEM for Indium bump from RAL

First Indium Flip-chip bonded FE-I4

The first indium assembly wafer ID : VMB6NJH die 3

Sensor : CPII Live FE-I4 MPI Guard IBL

Indium_BumpYield_28032014.pptx: Bump Yield Studies March 2013

Indium Flip-Chipped Modules September 2014

Indium bumps require pressure only to make connections so no high temperatures are required to melt alloy, which reduces bowing effects.

Modules under test:

IV Scans

IV Scans on wafer and after processing

Characterisation

USBPix

Bump yield studies performed in August 2014 by Dan Smaranda and Richard Bates

21082014_Indium-DanSmaranda.pdf

RCE System

All modules tuned to and operated at 3000e threshold, 10ToT @ 16ke reference charge.

Tuning

Module 14-07-24_S2:

Tune to 3000e, 10ToT @ 16ke

14-07-24_S2_3000eTuning.pptx

Module 14-07-25_S4:

Tune to 3000e, 10ToT @ 16ke

14-07-25_S4_3000eTuning.pptx

Bump Yield Studies

Bump Yield assessed by performing Threshold Scans with and without sensor bias and performing source scans with Strontium-90 beta source and Americium-241 60keV gamma ray source.

Indium-Characterisation_0914.pptx

Bump yield results

Module	Bond Force	Bump Yield (Sigma plot)	Bump Yield (Source Scans)
14-07-25-S4	5kg	23.37%	22.54%
14-07-24-S2	20kg	82.85%	82.35%
Indium1	?	99.48%	99.38%

Comments