BY ALAN HUFFMAN AND SIMON KWAN
Scientists at the Fermi National Accelerator Laboratory (Fermilab) are working on a new major high-energy physics project, the BTeV (B physics at the Tevatron) experiment. This experiment is designed to deeply probe several aspects of the Standard Model, the baseline particle physics theory for several decades. At the very least, BTeV will make very precise measurements of many Standard Model parameters. It is hoped that these precise measurements will point the way to a more fundamental theory of particle physics.
At the heart of the BTeV detector apparatus is an array of hundreds of silicon pixel detector modules, geometrically arrayed to track the particles generated by a collision event. Researchers at the U.S. Department of Energy's Fermilab and MCNC Research & Development Institute (MCNC-RDI) are working together to build the detector apparatus using cutting-edge advanced packaging technologies.
MCNC-RDI is performing fine-pitch solder bumping and multichip module assembly. Fermilab is connecting the modules together via high-density interconnections and integrating them into the support structure for the detector. The smallest I/O pitch of the pixilated sensors and readout IC (ROIC) devices is 50 µm, and there are a total of 2,816 interconnects in each ROIC. There are several sizes of sensors, designed to have 4, 5, 6, or 8 ROICs bonded to them.
MCNC-RDI's fine-pitch wafer bumping process uses BCB as a repassivation layer and relies on electroplating to form 25-µm-diameter eutectic Sn/Pb solder bumps on the 200-mm ROIC wafers. The sensor wafers are 250 µm thick, and have structures on both sides. They receive processing similar to the ROICs, but Ni/Au bond pads are deposited for the solder bumps to join to. ROIC wafers are thinned to 200 µm after bumping. Bumps on the ROIC wafers are protected with front-side coatings, applied prior to thinning.
 Figure 1. Examples of single- and multi-chip BteV detector modules. |
Dicing these devices requires a high level of control over the dicing process. The sensor wafer has several different module sizes, resulting in an irregular layout that requires extensive subdicing. The size of the singulated devices from the ROIC wafer must be tightly controlled. The spacing of ROICs when assembled on the sensor module is approximately 100 µm. Two cuts per street are typically needed to remove unneeded silicon from around the active area.
Assembly of the multichip modules (MCM) is achieved using manual flip chip bonders for small numbers of modules, but requires automatic bonders to achieve the throughput needed to build the hundreds of modules needed for the detector. MCNC-RDI uses its plasma-assisted dry soldering (PADS) process to eliminate flux from the assembly and reflow process. PADS alleviates problems with flux residue removal, difficult due to the size of the ROIC (~1 cm2), and the small gap between the sensor and ROIC (< 30µm).
The next step of the assembly is done at Fermilab. Each MCM is mounted on top of a high-density interconnect (HDI) kapton flex circuit. The I/O pads on the ROIC are wirebonded to the corresponding pads on the HDI. A separate wire is glued to the bias pad on the backside of the sensor to allow high voltage (> 100 V) to be applied to the sensor. Preliminary tests have validated the process and close to 100% yield has been obtained.
Testing was done on prototype MCMs to evaluate performance under heating, cooling, vacuum, and radiation environments. Test modules built in an early prototyping phase demonstrated a bump-bonding yield of 99.95%. Recent device modules have bonding yields closer to 100%, which is more than sufficient for the satisfactory performance of the detector. The MCMs have performed well under vacuum and temperature conditions like those in the detector environment and in radiation exposure testing.
A pre-production phase is planned for early 2005, in which 200 modules will be built. The modules produced in this phase will be used for testing and to develop the detector assembly methodologies. Production for the BTeV detector is scheduled to follow in 2006; 2,000 modules will be built during this phase. Commissioning of the experiment is expected to take place in 2008, with data acquisition starting in 2009.
ALAN HUFFMAN, research engineer with the Advanced Electronic Packaging Group, may be contacted at MCNC Research and Development Institute, P.O. Box 13910, 3021 Cornwallis Road, Research Triangle Park, NC 27709; (919) 990-2000; e-mail: [email protected]. SIMON KWAN, senior scientist, may be contacted at Fermi National Accelerator Laboratory, P.O. Box 500, Batavia, IL 60510; (630) 840-2329; e-mail: [email protected].