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Tri-Layer Lift-off Metallization Process Using Low Temperature Deposited SiNx

Published online by Cambridge University Press:  22 February 2011

J. R. Lothian,
F. Ren,
S. J. Pearton,
U. K. Chakrabarti,
C. R. Abernathy and
A. Katz
J. R. Lothian
Affiliation:
AT&T Bell Laboratories Murray Hiil, NJ 07974
F. Ren
Affiliation:
AT&T Bell Laboratories Murray Hiil, NJ 07974
S. J. Pearton
Affiliation:
AT&T Bell Laboratories Murray Hiil, NJ 07974
U. K. Chakrabarti
Affiliation:
AT&T Bell Laboratories Murray Hiil, NJ 07974
C. R. Abernathy
Affiliation:
AT&T Bell Laboratories Murray Hiil, NJ 07974
A. Katz
Affiliation:
AT&T Bell Laboratories Murray Hiil, NJ 07974
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Abstract

A tri-level resist scheme using low temperature (<50°C) deposited SiNx ratfier than Ge for the transfer layer has been developed. This allows use of an optical stepper for lithographic patterning of the emitter-base junctions in GaAs/AlGaAs heterojunction bipolar transistors (HBTs) where a conventional lift-off process using a single level resist often leads to die presence of shorts between metallizations. The plasma-enhanced chemically vapor deposited (PECVD) SiNx shows a sligtly larger degree of Si-H bonding compared to nitride deposited at higher temperature (275°C), and is under compressive stress (-5 × 1010 dyne · cm−2) which is considerably relieved upor thermal cycling to 500°C (-1.5 × 1010 dyne · cm−2 after cool-down). This final stress is approximately a factor of two higher man conventional PECVD SiNx cycled in the same manner. The adhesion of the low temperature nitride to die underlying polydimediylglutarimide (PMGI) base layer in the tri-level resist is excellent, leading to high yields in the lift-off metallization process. These layers are etched in Electron Cyclotron Resonance (ECR) discharges of SF6 or O2, respectively, using low additional dc bias (≤-100V) on the sample. Subsequent deposition of the HBT base metallization (Ti/Ag/Au) and lift-off of the tri-level resist produces contacts with excellent edge definition and an absence of shorts between metallization.

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References

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