Degradation and recovery of SiGe HBTs following radiation and hot ...2011125753

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					                      Degradation and recovery of SiGe HBTs following radiation
                                      and hot-carrier stressing
                                      S.R. Sheng I , S.P. McAlister I , J.P. McCaffrey ' S . Kovacic
                             ' Institute for Microstructural Sciences Institute for National Measurement Standards
                                               National Research Council of Canada, Ottawa, Canada.
                                                     ' SiGe Semiconductor Inc., Ottawa, Canada

                       Device degradation can arise though a variety of stress conditions and mechanisms. Here we
                   focus on SiGe HBTs and the effects of y-radiation and hot-carrier stress on the DC device
                   characteristics, and subsequent annealing 'following the stress. Although the effects of radiation
                   and electrical stressing seem qualitatively the same, the annealing and saturation behavior suggest
                   the mechanisms and spatial distribution ofthe damage are not identical.

                      Self-aligned SiCe HBTs, fabricated in an adapted BiCMOS process, were used in this study.
                  They had two emitters, each of area 0 . 812 pm2. Irradiation was done from either the top or the
                  substrate side at room temperature, using a 1.25 MeV           y-source at a dose rate of -290
                  rad(Si)/min. Hot-carrier electrical stressing was performed at room temperature by applying a
                  constant reverse-bias voltage across the BE junction, with the collector and substrate open. The
                  radiation and hot-carrier induced changes were followed in time during annealing at room and
                  elevated temperatures.
                       In Fig. 1 we show the differences in the forward and reverse Gummel plots of a device
                  irradiated from the top. The base current is increased in the low bias regions, decreasing the.gain,
                  but not the collector current. However, the base currents do not change in the same way. Changes
                  in the forward base current occur across a slightly narrower and lower voltage range than for the
                  reverse case. This reflects the changes in the different junctions in the 2 cases, as shown for the
                  BE and BC diode characteristics in Fig. 2. For the forward bias case the reverse BE current is
                  altered by hot-carrier stress (Fig. 3) but not by the radiation (Fig. 2). This suggests that although
                  the gain in the devices at low bias are reduced in both cases, the origins of the effects are not
                  identical. Further evidence for this is shown in Fig. 4 where the hot-carrier stress continues to
                  cause changes in the base current with prolonged stress, but the radiation stress saturates at a
                  lower level, for the doses we used.
                       When the devices are annealed, by keeping them at room temperature or annealing them at an
                  elevated temperature, the normalized base current evolves with time. Fig. 5 shows that following
                  irradiation only small changes occur for annealing at 160'C. (There may also be some difference
                  between devices radiated from the top compared with those irradiated from the substrate.) For
                  hot-carrier (open-collector) stress at - 2.8 V (Fig. 6) the base current increases with time during
                  the stressing period, but the damage is reduced with annealing. However, after some time, the
                  effects of prolonged thermal stress Stan to appear, in both the stressed and un-stressed devices.
                       All these observations point to different physical damage and origins of the observed changes
                  in the electrical characteristics. Hot-carrier stress normally produces damage at the oxide/silicon
                  interface near the BE junction depletion regions, where the hot-carriers are injected during the
                  electrical stressing. Interface traps are generated at the oxide/silicon interface, which can account
                  for the increases in the base current. However, y radiation produces damage that is more global in
                  its extent and is less likely to be recovered by thermal treatment in the cases we studied.


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                                                                                                 ,        Vw    (V)

Fig. 1 Forward (a), reverse (b) Gummel                          Fig. 2 Dose dependence of the BE (a) and
       plots as a function of radiation dose.                          BC (b) diode characteristics.

                                                                             t        c     .      ,       ,         ,         ,       J
                                                                                 Id       10'      1@     10'       10'       la'      10.
                                                                                                stress charge ( p c )

Fig. 3 Base-emitter diode characteristics                       Fig. 4 Dependence of the normalized base
       for VBE=- j.2 V stress.                                         current on the stress charge.

                                                                          '"Y               '         '   ':    '         '        '

              0'10    2D    40
                                  F w d Gummel

                                 €0"         mu   ua                                               n e (min.)
                     Annealing time (min.)

Fig. 5 Effect of annealing on the base                          Fig. 6 Stress and annealing behavior of the
       current, after radiation stress.                                base current compared with changes
                                                                       induced by thermal stress alone.


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