Fabrication and characterization of room temperature operating single electron transistors using focused ion beam technologies

The single electron transistor (SET) is a Coulomb blockade device, whose operation is based on the controlled manipulation of individual electrons. Single electron transistors show immense potential to be used in future ultra lowpower devices, high density memory and also in high precision electrometry. Most SET devices operate at cryogenic temperatures, because the charging energy is much smaller than the thermal oscillations. The room temperature operation of these devices is possible with sub- 10nm nano-islands due to the inverse dependance of charging energy on the radius of the conducting nano-island. The fabrication of sub-10nm features with existing lithographic techniques is a technological challenge. Here we present the results for the first room temperature operating SET device fabricated using Focused Ion Beam deposition technology. The SET device, incorporates an array of tungsten nano-islands with an average diameter of 8nm. The SET devices shows clear Coulomb blockade for different gate voltages at room temperature. The charging energy of the device was calculated to be 160.0 meV; the capacitance per junction was found to be 0.94 atto F; and the tunnel resistance per junction was calculated to be 1.26 G Ω. The tunnel resistance is five orders of magnitude larger than the quantum of resistance (26 k Ω) and allows for the localization of electrons on the tungsten nano-island. The lower capacitance of the device combined with the high tunnel resistance, allows for the Coulomb blockade effects observed at room temperature. Different device configurations, minimizing the total capacitance of the device have been explored. The effect of the geometry of the nano electrodes on the device characteristics has been presented. Simulated device characteristics, based on the soliton model have been discussed. The first application of SET device as a gas sensor has been demonstrated.

Integration of room temperature single electron transistor with CMOS subsystem

The single electron transistor (SET) is a charge-based device that may complement the dominant metal-oxide-semiconductor field effect transistor (MOSFET) technology. As the cost of scaling MOSFET to smaller dimensions are rising and the the basic functionality of MOSFET is encountering numerous challenges at dimensions smaller than 10nm, the SET has shown the potential to become the next generation device which operates based on the tunneling of electrons. Since the electron transfer mechanism of a SET device is based on the non-dissipative electron tunneling effect, the power consumption of a SET device is extremely low, estimated to be on the order of 10^-18J. The objectives of this research are to demonstrate technologies that would enable the mass produce of SET devices that are operational at room temperature and to integrate these devices on top of an active complementary-MOSFET (CMOS) substrate. To achieve these goals, two fabrication techniques are considered in this work. The Focus Ion Beam (FIB) technique is used to fabricate the islands and the tunnel junctions of the SET device. A Ultra-Violet (UV) light based Nano-Imprint Lithography (NIL) call Step-and-Flash- Imprint Lithography (SFIL) is used to fabricate the interconnections of the SET devices. Combining these two techniques, a full array of SET devices are fabricated on a planar substrate. Test and characterization of the SET devices has shown consistent Coulomb blockade effect, an important single electron characteristic. To realize a room temperature operational SET device that function as a logic device to work along CMOS, it is important to know the device behavior at different temperatures. Based on the theory developed for a single island SET device, a thermal analysis is carried out on the multi-island SET device and the observation of changes in Coulomb blockade effect is presented. The results show that the multi-island SET device operation highly depends on temperature. The important parameters that determine the SET operation is the effective capacitance Ceff and tunneling resistance Rt . These two parameters lead to the tunneling rate of an electron in the SET device, Γ. To obtain an accurate model for SET operation, the effects of the deviation in dimensions, the trap states in the insulation, and the background charge effect have to be taken into consideration. The theoretical and experimental evidence for these non-ideal effects are presented in this work.

Development of room temperature operating single electron transistor using FIB etching and deposition technology

The single-electron transistor (SET) is one of the best candidates for future nano electronic circuits because of its ultralow power consumption, small size and unique functionality. SET devices operate on the principle of Coulomb blockade, which is more prominent at dimensions of a few nano meters. Typically, the SET device consists of two capacitively coupled ultra-small tunnel junctions with a nano island between them. In order to observe the Coulomb blockade effects in a SET device the charging energy of the device has to be greater that the thermal energy. This condition limits the operation of most of the existing SET devices to cryogenic temperatures. Room temperature operation of SET devices requires sub-10nm nano-islands due to the inverse dependence of charging energy on the radius of the conducting nano-island. Fabrication of sub-10nm structures using lithography processes is still a technological challenge. In the present investigation, Focused Ion Beam based etch and deposition technology is used to fabricate single electron transistors devices operating at room temperature. The SET device incorporates an array of tungsten nano-islands with an average diameter of 8nm. The fabricated devices are characterized at room temperature and clear Coulomb blockade and Coulomb oscillations are observed. An improvement in the resolution limitation of the FIB etching process is demonstrated by optimizing the thickness of the active layer. SET devices with structural and topological variation are developed to explore their impact on the behavior of the device. The threshold voltage of the device was minimized to ~500mV by minimizing the source-drain gap of the device to 17nm. Vertical source and drain terminals are fabricated to realize single-dot based SET device. A unique process flow is developed to fabricate Si dot based SET devices for better gate controllability in the device characteristic. The device vi parameters of the fabricated devices are extracted by using a conductance model. Finally, characteristic of these devices are validated with the simulated data from theoretical modeling.