Soft Pipes Cause Reliability Problem in Bipolar Integrated Circuits

An investigation of yield-limiting bipolar emitter-collector shorts (commonly called "pipes") uncovered a "soft" pipe reliability problem. For us the major crystal defects causing pipes can be detected by Wright etching test wafers after epitaxial growth. The electrical effects of pipes become apparent during in-process probing and DC testing. Test patterns give a defect density figure which correlates with the corresponding etched wafer defect densities. Wafers which were below an appropriate defect level were shipped. Subsequent AC testing on a product line showed previously undetected, or "soft", pipes caused circuit timing errors and line returns from the customer. Both types of pipes were determined to be the result of secondary implantation of Carbon atoms arising from an ultrathin layer of vacuum pump oil. The Carbon atoms were knocked through the preimplant oxide during an Arsenic buried-layer implant step. Both types of pipes were eliminated by modifying the vacuum system on the implanter.