AVS 46th International Symposium
    Thin Films Division Wednesday Sessions
       Session TF+MM-WeM

Paper TF+MM-WeM8
Investigation and Modeling of Electrical Resistance in Polysilicon Thermal Actuators

Wednesday, October 27, 1999, 10:40 am, Room 615

Session: Thin Films in MEMS and MOEMS
Presenter: J.T. Butler, Air Force Research Laboratory
Authors: J.T. Butler, Air Force Research Laboratory
W.D. Cowan, Air Force Research Laboratory
Correspondent: Click to Email
This paper reports on investigation and modeling of the electrical resistance of micromachined polysilicon thermal actuators. The availability of models compatible with commonly used circuit simulators such as SPICE are extremely useful for design of integrated microsystems which include thermal actuators. The development of a model for thermal actuators necessitated an analysis of the electrical resistance characteristics of the MEMS fabrication process in order to provide an understanding of a key material property. The thermal actuators investigated in this research were fabricated through the DARPA-sponsored Multi-User MEMS Processes (MUMPs). Hence, a TSUPREM model of the MUMPs fabrication process was created to generate polysilicon resistivity parameters which were then fed into the electrothermal SPICE model. Two types of thermally actuated devices were modeled: a lateral thermal actuator and a thermally actuated piston micromirror. The SPICE model exhibits very close agreement with the measured performance of the polysilicon thermal actuators. The MUMPs process used to fabricate the thermal actuators has three structural layers of polysilicon. The resistivity of each of the MUMPs polysilicon layers varies due to differences in fabrication. Moreover, our resistance measurements of test structures and actuators showed that the resistivity of devices formed from the various MUMPs polysilicon layers also varies based on structure linewidth. A TSUPREM fabrication model of the MUMPs process was generated which validated the empirical resistance measurements and the dependence of resistivity on linewidth. The TSUPREM simulation revealed that the diffusion of phosphorus dopant during the anneal cycles in the MUMPs fabrication process were largely responsible for the variations in resistivity due to linewidth. For small ( < 10 (m ) linewidth structures, the presence or absence of lateral diffusion of dopant through the sidewall can significantly alter the electrical resistance. The resistivity dependence on linewidth is significant for our thermal actuators because they are designed with elements having linewidths varying from 2 (m to greater than 20 (m. The electrothermal SPICE model augmented with the TSUPREM resistivity data accurately predicted the I-V performance of both the lateral thermal actuator and the thermal piston micromirror. The use of SPICE allows simulation of both the MEMS device and control electronics in the same analysis package and enables the designer to gain insight into the expected performance of the microsystem prior to fabrication. On-going work includes investigation of adding thermal mechanical modeling to our simulation.