Fabrication and characterization of SOI-MOSFETs with a novel buried body contact

Julie Chien-Hua Chang, Purdue University

Abstract

Silicon-on-Insulator (SOI) MOSFETs with a single crystalline buried body contact has been uniquely fabricated using epitaxial lateral overgrowth (ELO) from selective epitaxial growth (SEG). Unlike other SOI body contacts, this technique incorporates the buried body contact during SOI active area formation using ELO. The resultant cross section is an SOI active area physically connected to a buried body using the same single crystalline silicon. The physical uniqueness of this buried body contact maintains device symmetry and allows topside body contact isolated away from the device source/drain edge. Because the body contact is buried and isolated, additional advantages are reduced contact-area penalty compared to historical SOI body contacts and flexible integration with multilevel interconnects using polysilicon. Electrically, an effective SOI body contact eliminates various floating body effects inherent in partially depleted SOI MOSFETs. In this thesis, electrical characterization of an ELO-SEG buried body contacted SOI MOSFET demonstrates SOI body potential variation by removing the kink effect in its IDSV DS curve with grounded body bias and by shifting its threshold voltage. Here, the ability to change the threshold voltage is particularly important in low power applications where VDD∼1.0V or less because on/off switching of the MOSFET is faster from dynamically lower threshold voltage for larger saturated current in on-state. This fabricated SOI buried body contact further demonstrates body-current collection in IBuried_Contact -VGS curve at large a VDS, due to impact ionization. General partially depleted SOI MOSFET characteristics have measured subthreshold swing of 117mV/dec which is comparable to MEDICI simulated value of 113.8mV/dec. Averaged over 10 devices, the subthreshold slope measured at 137mV/dec, and subthreshold leakage measured at less than 2pA. To further characterize the SOI buried body length, p + n diode (source/drain to body) characteristic is determined to have a diode ideality factor of 1.7, due possibly to additional R-G processes in the buried contact SOI interfaces, and a resistance of 3.3kΩ for thin, narrow, and long single crystalline buried body contact. TSUPREM4 and MEDICI simulations are further discussed in relation to experimental results of the ELO-SEG buried body contact and then followed by conclusion and recommendation.

Degree

Ph.D.

Advisors

Neudeck, Purdue University.

Subject Area

Electrical engineering|Materials science

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