Evaluation of polymer-metal-hybrid bonded wafer-stacks and sensor wafers for ultra-thin hybrid silicon detectors

Abstract

Semiconductor pixel detectors are widely established in High Energy Physics (HEP) and Medical physics for their high spatial resolution and tracking capabilities. Research on both monolithic detectors and hybrid detectors is ongoing. Monolithic detectors, which integrate the sensor and the read-out electronics in the same die, provide the benefit of reduced thickness but the needed intricate imaging process is only offered by a limited number of chip vendors. The hybrid approach instead facilitates the design and fabrication of sensor and read-out chip using different technologies and opens up access to a large market of semiconductor vendors. For the production of silicon pixel detectors, the interconnection between sensor and read-out chip is usually realized on an individual die level. The needed mechanical stability during the handling of the dies limits their possible thinness. The wafer-to-wafer interconnection process being developed in this project uses a polymer underfill layer between the wafers to provide additional mechanical stability. This allows one to thin the wafer stack significantly after interconnection, bringing the total thickness close to that of monolithic detectors. In this paper, we present first results on the bump bonding yield of the process based on daisy-chain wafer measurements. For the first hybrid pixel detectors produced with this technique, a dedicated sensor wafer was designed and fabricated to be bonded to Timepix3 read-out chip wafers. Results of the characterization of the sensor wafer before hybridization are presented. We show that the wafer-to-wafer bonding process is suitable for hybrid semiconductor pixel detectors.

Figures (16)

• Figure 1: Position of the different test structures on a DCW die. In green six single bond structures are shown. In red the corner daisy chains are highlighted. The long daisy chains in the center and top of the die are shown in blue. The probe pads are located at the lower end of the die.
• Figure 2: Photos of the DCW stack. \ref{['fig:DC_Wafer_full']} shows the full 200m m wafer stack with its 50.0 etched windows to access the probe pads with probe needles located on the lower wafer. \ref{['fig:etched_window_to_pads']} shows a close-up of such an etched window in the top wafer of the wafer stack with the square probe pads.
• Figure 3: X-ray image and cross section of DCW stack after wafer-to-wafer bonding. The X-ray image in \ref{['fig:DCW_xray']} shows well aligned solder pillars. \ref{['fig:DCW_crossection']} shows a cross section of a DCW stack with good electrical interconnection (golden) between top and bottom wafer (blue gray). A slight misalignment of approx. 4µ m is visible.
• Figure 4: Wafer maps showing the resistances of the five single bond test structures per die on a common color scale from 0400. The resistance of the die in red in the center of the top left structure \ref{['subfig:SBSTopLeftCommScale']} could not be measured with the chosen resistance range and is considered a broken bond.
• Figure 5: Distribution of single bond resistance values across all five structures. The mean resistance per bond was determined by fitting a Gaussian function to the distribution (dashed line).