by Christophe Lécuyer
Hoerni developed a new manufacturing process which relied heavily on the masking and passivating properties of sillicon oxide. Under the mesa process the oxide layer was deposited on after the making of the base in order to mask the emitter diffusion and was later removed. Hoerni grew an oxide layer on top of the wafer at the very beginning of the process and used it to mask both the base and the emitter. More importantly, Hoerni, in a very innovative move, left the oxide layer on top of the wafer after transistor processing. This went against all accepted knowledge in the silicon community: It is widely believed at the time among practitioners of the silicon art that the oxide layer used to mask dopants was dirty and, as a result, had to be etched away. Instead, Hoerni, a self professed “contrarian,” left the oxide layer on top of the wafer. He then made the startling discovery that far from contaminating the wafer, it passivated the crystal’s surface and protected the transistor junctions from outside contaminants. Exploiting these basic ideas, Hoerni built his process around seven basic steps. Starting with an N-type wafer (a), Hoerni carefully oxidized it by exposing it to an oxygen carrier gas in a high temperature furnace. Using the photolithographic techniques developed for Fairchild’s first mesa transistor, he then selectively etched the oxide (b). In the next step (c), Hoerni exposed the wafer to a boron vapor, another standard process at Fairchild, to create the transistor’s base. As the boron atoms diffused both vertically and horizontally, the junction between the collector and the base moved laterally and was protected from outside contaminants by the oxide layer. Hoerni later reoxidized the exposed area. Applying another masking and etching process (d), he opened a window into the regrown oxide of the base area. He then diffused phosphorus to form the emitter (e), creating a base-emitter junction. This junction, like the base to collector junction, was protected from outside contamination by the silicon oxide. After diffusing the emitter, Hoerni again selectively etched the oxide layer to provide for alloy contacts (f). Finally (g), he deposited contacts on the etched areas and alloyed them in.
Source: Jean Hoerni, “Planar Silicon Transistors and Diodes,” Paper presented at the 1960 Electron Devices Meeting, Washington, D.C. – October 1960, Bruce Deal Papers, 88-033, Stanford Archives and Special Collections.