Fabrication And Tests Of Ohmic Contacts For MEMS Accelerometers

The electrical signal extraction of micro accelerometers, micro gyroscopes, CMOS, transistors and other semiconductor devices is one of the important issues affecting their performances. There are many choices to achieve the transition of the metal-semiconductor contacts from Schottky rectifying characteristics to linear ohmic dependence, according to needs and functions of respective applications. For the two types of MEMS accelerometers under on-going investigation in our group, we have to realize good ohmic contacts between metal electrodes and silicon substrates in order to shield them by grounding the silicon substrates.We investigate the formation mechanisms, fabrication processes and measurement methods developed for ohmic contacts in the last decades of years. It is chosen to make Au/Sb ohmic contacts to the n-type silicon substrate by low temperature alloying, according to the needs of both MEMS accelerometers above-mentioned. This ohmic contact formation process is easy and rapid to implement, and is compatible with other steps in the process flow. The rectangle transmission line model and circular transmission line model are employed to measure the specific contact resistivity. The AuSb/NiCr/Au metal stack is deposited and patterned on n-Si followed by an anneal process above AuSi eutectic temperature (363?) to realize the ohmic contact. This thesis systematically studies the parameters which affect the fabrication of ohmic contacts, including anneal temperatures, the anneal times, film thicknesses, coating materials, heating rates, cooling rates and so on. Based on these results, we have obtained deep insights into the formation mechanism of the alloying process.It is found that a silicon substrate resistivity of 0.02 Q-cm should be utilized for a electrode size of 0.5mmx5mm for the sake of improving the contact resistivity measurement accuracy. The role is verified and analyzed which Sb atoms play in the ohmic contact formation. It is concluded that the intermediate layer of NiCr in the stack can block the mutual diffusion and promote the adhesion between layers. The thickness of AuSb barely affects the resultant resisvity, but it is strongly dependent on the anneal the resistivity improves from 10-1?·cm2 to 10-3?·cm2 in the order of magnitude. In addition, the alloying process of AuSb and Si can be sufficiently completed within a few minutes. Prolongation of the anneal time shows no significant influences on the results. The heating rates and cooling rates have invisible impacts when ranging from 10?/min to 40?/min in our experiments. The temperature experiments demonstrate that the process window of this ohmic contact scheme is wide to work with. The resistivity remains the same after thermal cycling or long-term storage, which illustrates the stability of this ohmic contact. To conclude, ohmic contacts with a specific contact resistivity of (0.0025 ± 0.0005) ?·cm2 can be routinely achieved for the (100) n-type Si substrate with a sheet resistivity of 0.014 ?· cm by annealing the AuSb/NiCr/Au metal stack at 425?.