ABSTRACT
Material /Layer Current limit (A/cm2) Ref.Al 1 × 10 5 [4]Al:Cu 2 × 105Al/Ti layered 2 × 105Au 3 × 105 [5]6 × 105 [4] 18.2.4 Interlevel Dielectric Layer RequirementsSilicon nitride is used extensively in GaAs and other III-V semiconductor fabrication for passivation and as an interlevel insulator. In analog and mixed-signal circuits, MIM capacitors are needed and silicon nitride is a good candidate for these. Silicon nitride deposited by plasma-enhanced chemical vapor deposition (PECVD) has a moderately high dielectric constant (7.0) and has high breakdown strength and moisture resistance. It also has good step coverage and can provide protection against a highly accelerated stress test (HAST)-type environment where moisture could penetrate. It is used as a device passivation layer anyway, so it is a good candidate for MIM layers and for stacked capacitors. These layers are deposited at around 300°C; temperatures over this are not desirable for III-V epilayer structures, contact metallurgies, etc. As an interlayer dielectric, it is not a good candidate because of a high dielectric constant and a lack of planarizing or gap fill properties. Polyimide-type materials have a dielectric constant near 3.0 and have excellent planarizing properties. Gaps and crevices created in the thick metal processes are also easily filled by these. Benzocyclobutane (BCB) and polybenzoxazole (PBO) are other polymers used for this purpose. 18.3 Production Interconnect Processes
18.3.1 Baseline Gold Interconnect ProcessA general-purpose process for mixed-signal circuits may need two or three layers of metals, with thickness of 1 to 2 microns or more.
