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Measurement of HFE Reduction Due to Tensile Stresses Within Very Small Bipolar NPN Devices

IP.com Disclosure Number: IPCOM000046832D
Original Publication Date: 1983-Aug-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 2 page(s) / 30K

Publishing Venue

IBM

Related People

Hemmert, RS: AUTHOR [+3]

Abstract

The theoretical value of emitter current gain, HFE, for a bipolar NPN transistor can be reduced by process-induced tensile stress within the emitter region. The effect of the tensile stress can be eliminated by introducing a compressive stress of the same magnitude. On relatively large structures this can be done using a mechanical stylus to introduce the compressive stress. However, on very small and shallow devices it becomes impractical to use this technique. It has been shown by R. S. Hemmert, et al., in J . Appl . Physics 53 6, 4456-4462 (1982), that a compressive stress can result from electromigration-induced pile-up within the emitter contact. Therefore, by passing a current through a transistor the compressive stress monotonically builds-up.

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Measurement of HFE Reduction Due to Tensile Stresses Within Very Small Bipolar NPN Devices

The theoretical value of emitter current gain, HFE, for a bipolar NPN transistor can be reduced by process-induced tensile stress within the emitter region. The effect of the tensile stress can be eliminated by introducing a compressive stress of the same magnitude. On relatively large structures this can be done using a mechanical stylus to introduce the compressive stress. However, on very small and shallow devices it becomes impractical to use this technique. It has been shown by R. S. Hemmert, et al., in J . Appl
. Physics 53 6, 4456-4462 (1982), that a compressive stress can result from electromigration-induced pile-up within the emitter contact. Therefore, by passing a current through a transistor the compressive stress monotonically builds-up. The rate at which the stress builds up is a com plex function of metallurgy lifetime, passivation material, and diffusion profiles. Hence, the current and temperature used to determine the maximum HFE (tensile stress equals compressive stress) must be uniquely determined for each bipolar technology. For the bipolar technology described in the above-cited article, the following procedure was used to determine the maximum HFE: a) Force 0.5 mA emitter current for every um 2 of emitter area, as shown in Fig. 1. b) Maintain 100 C ambient by using an

environmental chamber. c) Periodically remove the transistors from the

environmental...