Introduction to Nanoelectronics and Fabrication

1. Introduction to Nanoelectronics and Fabrication Dr. Sabar D. Hutagalung School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia

2. Nanoscience – working small, thinking big

3. Nano: From the Greek nanos - meaning "dwarf”, this prefix is used in the metric system to mean 10-9 or 1/1,000,000,000.

4. What is Nanotechnology? • Nanotechnology is the creation of functional materials, devices, and systems through control of matter on the nanometer (1 to 100 nm) length scale and the exploitation of novel properties and phenomena developed at that scale. • A scientific and technical revolution has begun that is based upon the ability to systematically organize and manipulate matter on the nanometer length scale.

5. Is this technology new? In one sense there is nothing new… • Whether we knew it or not, every piece of technology has involved the manipulation of atoms at some level. • Many existing technologies depend crucially on processes that take place on the nanometer scale. Ex: Photography & Catalysis Nanotechnology, like any other branch of science, is primarily concerned with understanding how nature works.

6. Why is this length scale so important? There are five reasons: • The wavelike properties of electrons inside matter are influenced by variations on the nanometer scale. By patterning matter on the nanometer length, it is possible to vary fundamental properties of materials (for instance, melting temperature, magnetization, charge capacity) without changing the chemical composition. • The systematic organization of matter on the nanometer length scale is a key feature of biological systems. Nanotechnology promises to allow us to place artificial components and assemblies inside cells, and to make new materials using the self-assembly methods of nature.

7. Why is this length scale so important? • Nanoscale components have very high surface areas, making them ideal for use in composite materials, reacting systems, drug delivery, and energy storage. • The finite size of material entities, as compared to the molecular scale, determine an increase of the relative importance of surface tension and local electromagnetic effects, making nanostructured materials harder and less brittle. • The interaction wavelength scales of various external wave phenomena become comparable to the material entity size, making materials suitable for various opto-electronic applications.

8. How Small We can make the grains? • Because of high surface areas conventional powders methods reach their limits at 10-6 m (1 micron) • Smaller particles can be made but special methods are needed!

9. Working at the nanoscale • Working in the nanoworld was first proposed by Richard Feynman back in 1959. • But it's only true in the last decade. • The world of the ultra small, in practical terms, is a distant place. • We can't see or touch it. • Because, optical microscopes can't provide images of anything smaller than the wavelength of visible light (ie, nothing smaller than 380 nanometres).

10. From “There’s Plenty of Room at the Bottom”, Dec 29, 1959 This image was written using Dip-Pen Nanolithography, and imaged using lateral force microscopy mode of an atomic force microscope.

11. What is Nanoelectronics • Nanoelectronic device? • A very small devices to ovecome limits on scalability • Examples: • Single-Electron Transistors • controlled electron tunneling to amplify current • Resonance Tunneling Device • quantum device use to control current

12. Problem of Making More Powerful Chips • The number of transistors on a chip will approximately double every 18 to 24 months (Moore’s Law). • This law has given chip designers greater incentives to incorporate new features on silicon.

13. Problem of Making More Powerful Chips • Moore's Law works largely through shrinking transistors, the circuits that carry electrical signals. • By shrinking transistors, designers can squeeze more transistors into a chip.

14. Problem of Making More Powerful Chips • However, more transistors means more electricity and heat compressed into a smaller space. • Furthermore, smaller chips increase performance but also create the problem of complexity.

15. Problem of Making More Powerful Chips Band diagram when on A basic MOSFET

16. Problem of Making More Powerful Chips Quantum and coherence effects, high electric fields creating avalanche dielectric breakdowns, heat dissipation problems in closely packed structures as well as the non-uniformity of dopant atoms and the relevance of single atom defects are all roadblocks along the current road of miniaturization.

17. Problem of Making More Powerful Chips Problem 1: • Carrier mobility decreases as channel length decrease and vertical electric fields increase.

18. Eox Problem of Making More Powerful Chips Problem 2: • Tunneling through gate oxide (off state current).

19. Problem of Making More Powerful Chips Problem 3: • Wattage/Area increases as density increases

20. Single-Electron Transistors (SETs) • To solve these problem, the single-electron tunneling transistor - a device that exploits the quantum effect of tunneling to control and measure the movement of single electrons was developed. • Experiments have shown that charge does not flow continuously in these devices but in a quantized way. Fig. A single-electron transistor

21. Single-Electron Transistors (SETs) • SET consists of a gate electrode that electrostaticaly influences electrons traveling between the source and drain electrodes. • The electrons in the SET need to cross two tunnel junctions that form an isolated conducting electrode called the island. Fig. A single-electron transistor

22. Single-Electron Transistors (SETs) • Electrons passing through the island charge and discharge it, and the relative energies of systems containing 0 or 1 extra electrons depends on the gate voltage. • The key point is that charge passes through the island in quantized units. Fig. A single-electron transistor

23. Single-Electron Transistors (SETs) • For an electron to hop onto the island, its energy must equal the Coulomb energy, e2/2C. • When both the gate and bias voltages are zero, electrons do not have enough energy to enter the island and current does not flow. Fig. A single-electron transistor

24. Single-Electron Transistors (SETs) • As the bias voltage between the source and drain is increased, an electron can pass through the island when the energy in the system reaches the Coulomb energy. • This effect is known as the Coulomb blockade, and the critical voltage needed to transfer an electron onto the island, equal to e/2C, is called the Coulomb gap voltage. Fig. A single-electron transistor

25. Here n1 and n2 are the number of electrons passed through the tunnel barriers 1 and 2, respectively, so that n = n1 - n2, while the total island capacitance, C∑, is now a sum of CG, C1, C2, and whatever stray capacitance the island may have. Left: Equivalent circuit of an SET Center: Energy states of an SET. Top Coulomb blockade regime, bottom transfer regime by application of VG=e/2CG Right: I-V characteristic for two different gate voltages. Solid line VG= e/2CG, dashed line VG =0

26. Coulomb Blockade The Coulomb blockade is a single-electron phenomenon, which originates in the discrete nature of electric charge that can be transferred from a conducting island connected to electron reservoirs through thin barriers. The CB allows a precise control of small number of electrons, with important application in switching devices with low power dissipation and a corresponding increased level of circuit integration. Single-electron devices based on the Coulomb blockade.

27. Tunneling & Q Blockade in SET DOT Q transport by single-electron tunneling, but essentially suppressed by Coulomb charging energy: • Ec > kbT (Ec = e2/2CΣ) • Tunneling resistance, Rt > Rk (Junction resistance, Rk = h/e2 = 25.8 K) I-V curve controlled by gate voltage, showing region of QB

28. Silicon SET

29. Silicon nanowire transistors

30. Enhanced Channel Modulation in Dual-Gated Silicon Nanowire Transistors Nano Letters Vol. 5, 2005, 2519-2523

31. Schematic of a NW FET, and (inset) FE-SEM image of a GaN NW FET. • Gate-dependent I–Vsd data recorded on a 17.6 nm diameter GaN NW. The gate voltages for each I–Vsd curve are indicated; • I–Vg data recorded for values of Vsd. (Inset) Conductance, G, vs gate voltage C.N.R. Rao et al. / Progress in Solid State Chemistry 31 (2003) 5–147.

32. ZnO nanorod FETs • Schematic side view and • field-emission scanning electron microscopy (FESEM) image of a ZnO nanorod FET device. ZnO nanorod FETs with backgate geometry were fabricated on SiO2/Si by deposition of Au/Ti metal electrodes for source-drain contacts on nanorod ends. Park et al., APL, 85 (2004) 5052-5054

33. (a) Typical Isd–Vsdcharacteristic curves as a function of Vgfor ZnO nanorod FETs. The linear and symmetric Isd–Vsdcurves were obtained under different Vg, indicating the low resistant ohmic contact formation between ZnO and Ti metal layers. (b) Isd–Vgcurves of ZnO nanorod FETs show that the devices operate in an n-channel depletion mode with gmof ~140 nS for Vsd = 1.0 V. Park et al., APL, 85 (2004) 5052-5054

34. FET fabricated based on In2O3 nanowires: (a) I–V curves recorded on an In2O3 nanowire of 10 nm diameter, (b) I–Vg data of the same device at Vds = 10 mV. Inset shows the SEM image of the nanowire between the source and drain electrodes.

35. Direct Integration of Metal Oxide Nanowire in Vertical Field-Effect Transistor Nano Letters Vol. 4, 2004

36. Carbon Nanotube Transistor

37. Nanoelectronic Fabrication

38. Nanofabrication Top-down Approach Bottom-up Approach

39. Top-down vs Bottom-up • Top-down techniques take a bulk material, machine it, modify it into the desired shape and product • classic example is manufacturing of integrated circuits using a sequence of steps sush as crystal growth, lithography, deposition, etching, CMP, ion implantation… (Microelectronic/Nanoelectronics Fabrication Approach) • Bottom-up techniques build something from basic materials • assembling from the atoms/molecules up • not completely proven in manufacturing yet Examples: • Self-assembly • Sol-gel technology • Deposition (old but is used to obtain nanotubes, nanowires, nanoscale films…) • Manipulators (AFM, STM,….)

40. Top-down • From large items to smaller ones. • The most common method are electron beam lithography(EBL) and scanning probe lithography(SPL). • The approach involves molding or etching materials into smaller components. Making IC? Starting with a thin sheet Si wafer, cleaned, coated, preferentially etched using highly focused optics in as many as 100 separate operations before the final IC is complete.

41. Bottom-up • A  general approach going from small items to bigger ones. • Building larger, more complex objects by integration of smaller building blocks or components. • The sketch shows the essence of bottom-up manufacturing. • Self-assembly from the gaseous phase. • Two principle vapor-phase technologies that are useful and widely practiced: • molecular beam epitaxy (MBE) and vapor-deposition (PVD, CVD).

42. Fabrication of SET SET with a nano particle connected by SWCNs SET with a nano particle

43. Fabrication of SET FIG. (a) Sketch of the SOI nanowire: a metallic top gate is separated from the SiNW by a 55 nm silicon oxide. (b) SEM micrograph of the nanowire with a width below 10 nm and a length of 500 nm. PHYSICAL REVIEW B 68, 075311 (2003)

44. Fabrication of SET

46. Non-Lithographic Positional Control of SiNWs Fig. Patterning of SiNWs. (A) Overview and (B) zoomed in image of patterned lines of vertical SiNWs grown from lines of single nanoparticle catalysts deposited onto a Si substrate. (C) A crosssection SEM image of nanowires that were positionally aligned into lines. The scale bar in images (A), (B), and (C) correspond to 100 µm, 1 µm, and 1µm. Goldberger, Hochbaum, Fan, and Yang, Nano Letters, 6 (2006) 973 - 977

47. Vertically Integrated Nanowire Field Effect Transistors (VINFET) Fig. Si VINFET fabrication. (A) SiNWs are grown vertically from a Si(111) substrate. (B) Thermal oxidation of the Si nanowire is used to form the gate dielectric. (C) The Cr gate material is then sputtered onto the nanowires to achieve a conformal coating. Blue corresponds to the Si substrate and the SiNWs channel, grey is SiO2 dielectric material, and red corresponds to the Cr gate metal. Goldberger, Hochbaum, Fan, and Yang, Nano Letters, 6 (2006) 973 - 977

48. Fig. Si VINFET fabrication. (A) Conformal LPCVD oxide is deposited around the nanowire. (B) The Cr-coated nanowire tips are exposed via chemomechanical polishing and plasma etching of the SiO2 dielectric. (C) The Cr gate material is etched-backed using a Cr photomask etchant. (D) An SEM image taken after the SiO2 deposition. (E) An SEM image showing the exposed Cr-coated tips. (F) an SEM image of the device after the Cr etch back procedure. Goldberger, Hochbaum, Fan, and Yang, Nano Letters, 6 (2006) 973 - 977

49. Fig. Si VINFET fabrication. (A) Another layer of LPCVD SiO2 is deposited onto the nanowire. (B) The nanowire tips are exposed via plasma etching of the SiO2 dielectric. (C) Ni / Pt contacts are sputtered onto the sample to form the drain electrode. Cr gate material is then sputtered onto the nanowires to achieve a conformal coating. Yellow corresponds to the Ni drain material. Goldberger, Hochbaum, Fan, and Yang, Nano Letters, 6 (2006) 973 - 977

50. Threshold Voltage Analysis In the case for our system (p-type nanowires) with a Cr gate electrode, the threshold voltage (Vt); Where VFB is the flatband voltage, NA is the acceptor concentration in Si, C is the oxide capacitance, εs is the oxide permittivity, and φs is the surface potential. Since the onset of accumulation for an metal-oxide-semiconductor system occurs when the surface potential is zero, the threshold voltage is equal to the flatband potential. VFB can be deduced by the following equation; Where ΦM is the gate work function, χ is the electron affinity of Si, and Eg is the band gap of silicon. ΦF is given by the formula; Goldberger, Hochbaum, Fan, and Yang, Nano Letters, 6 (2006) 973 - 977