Frontiers of Engineering Reports on Leading-Edge Engineering from the 2003 NAE Symposium on Frontiers of Engineering (2004) / Chapter Skim
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Fundamental Limits of Nanotechnology: How Far Down is the Bottom?
Fundamental Limits of Nanotechnology: How Far Down is the Bottom?
Pages 25-74
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From page 27... ...
transistor density, integration, switching speed and energy, and cost per electronic function have driven the $160-billion semiconductor industry, one of the most dynamic industries in the world. These faster and cheaper technologies have led to fundamental changes in the economies of the United States and other countries around the world.
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From page 28... ...
To enable low-power chips, MOSFETs must be excellent switches and have very small suicide suicide n+ source Gate gate oxide 1 Cop - type \ channel 14 G silicide n+ drain \ FIGURE 1 Basic structure of a traditional planar nMOSFET.
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From page 29... ...
Figure 2 shows the intrinsic switching time of nMOSFETs plotted against their physical gate lengths LG. The switching time is essentially the time it takes a first transistor carrying a current ID to charge the input gate capacitance CG of an identical transistor to the supply voltage VDD.
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From page 30... ...
It is likely that today's research devices with intrinsic switching times under 500 Is will be produced in volume before the end of the decade. Figure 3 shows the switching energy CGVDD2, a simple metric that leaves out second-order effects.
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From page 31... ...
have been demonstrated with gate lengths as small as 15 nm (Hokazono et al., 2002; Yu et al.,2001~. Although these devices have record-breaking intrinsic switching times and switching energies, they must still undergo significant optimization over the next five to seven years to have a shot at meeting the ITRS performance targets.
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From page 32... ...
Another significant challenge to continued scaling is the rapid increase of the quantum mechanical tunneling current that passes through the gate oxide as it is progressively thinned. This gate current is rapidly approaching the size of the subthreshold leakage.
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From page 33... ...
The frontiers of research on Si technology feature a wide spectrum of work aimed at enhancing CMOS performance, solving (or at least postponing) some of the major issues, such as gate leakage, and providing options in case the traditional scaling path falters.
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From page 34... ...
(Inumiya et al., 2003; Rotondaro et al., 2002~. Because the main objective of scaling the gate oxide is to increase the capacitance density of the gate insulator, high-k dielectrics are attractive because the same capacitance density can be achieved with a physically thicker (and if the new material's barrier height is sufficient, a lower gate-leakage)
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From page 35... ...
enhanced electrostatic scalability because two gates are in close proximity to the current-carrying channel; (2) reduced-surface electric fields that can significantly improve the carrier mobilities and the CGVDD/ID performance; and (3)
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From page 36... ...
The gate of the device wraps around both sides and the top of the fin to provide excellent control of the channel potential and resistance to short-channel effects. Within the past year, aggressively-scaled FinFETs with 10 nm gate lengths and intrinsic switching times (CGVDD/ID)
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From page 37... ...
International Electron Devices Meeting Technical Digest 1032-1034. Hobbs, C., L
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From page 38... ...
International Electron Devices Meeting Technical Digest 411-414.
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From page 39... ...
double-gate MOSFET with a 25nm thick Si channel. International Electron Devices Meeting Technical Digest 427430.
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From page 40... ...
2001. 13 am gag length plow COOS star ~~on~ Beckon Devices ~eeUng Technics D1gest 937-939.
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From page 41... ...
In those cases, molecular-electronics components may have the advantages of less manufacturing complexity, lower power consumption, and easier scaling. The field of molecular electronics is evolving rapidly, and even though there are no commercial applications as yet, the science coming out of this research is spectacular.
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From page 42... ...
For molecular electronics, it is vastly improved energy efficiency per bit operation, as well as continued device scaling to true molecular dimensions. For true neural networks, it is greatly increased connectivity and, therefore, a greatly increased rate of information flow through a circuit.
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From page 43... ...
In the crossed-wire circuit shown in Figure 1 (called a crossbar circuit) , the molecular component is typically sandwiched between the intersection of two crossing wires.
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From page 44... ...
Typical data from one of our molecular switches is shown in Figure 4 (we have incorporated additional data from various control molecules in this figure)
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From page 45... ...
Perhaps the most fundamental challenge facing scientists constructing solid-state molecular-electronic devices and circuits is developing an intuition for guiding the design of the molecular components. Charge transport through molecules has been known and studied for a long time, but it has traditionally been a solution-phase science (Joachim et al., 2000; Kwok and Ellenbogen, 2002; Mujica and Ratner, 2002; Nitzan, 2001; Ratner, 2002~.
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From page 46... ...
A third electrode (called the gate) provides an electric field for tuning the molecularelectronic energy levels into and out of resonance with the Fermi energies of the source and drain electrodes.
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From page 47... ...
All measurements are carried out at 2 Kelvin. application that sets this field apart from traditional electronics, is the potential construction of an electrical interface to a single biological cell (Cud et al., 2001~.
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From page 48... ...
2002. Introducing molecular electronics.
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From page 49... ...
Because the head applies localized magnetic fields to write data, an actuator and armature swivels the head to access different tracks of data recorded at specific radii on the disk surface. Localization of both the write fields and read head sensitivity is achieved by positioning both the write head and read head sensor in very close proximity of the disk surface.
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From page 50... ...
One of the astounding dimensions associated with mechanical disk drives is the extremely thin air bearing that separates the media from the head. The reason for such small head/media spacing is quickly apparent when you consider the frequency components for magnetic fields in free space generated by an arbitrary magnetization distribution in a plane at z = 0.
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From page 51... ...
This perturbation energy, acting on the ferromagnetic order parameter, imbues the ferromagnet with its hysteretic properties (Stoner and Wohlfarth, 1948~. To understand the role of anisotropy in the magnetic recording process, we need only consider the lowest order term in the anisotropy expansion of Eq.2, also referred to as uniaxial anisotropy energy density, Ku.
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From page 52... ...
The fluctuations in the uniform state manifest themselves as random variations in the magnetization orientation ~ relative to the anisotropy axis. If, however, the thermal fluctuations cause the magnetic orientation to move to a position where the total magnetic energy is maximized, we assume that the magnetic state will change its ground state configuration, so that the probability distribution for fluctuations will now be referenced to a new ground state oriented 180 degrees relative to the original magnetization direction with energy density Us; the system "hops" over the energy barrier imposed by the rotational anisotropy of the magnetic energy.
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From page 53... ...
The Onsager regression hypothesis states that the correlation time of thermal fluctuations for a system in thermal equilibrium is identical to the time scale at which the same system relaxes back into equilibrium after perturbation by an external stimulus (Chandler, 1987~. For example, suppose that a sudden magnetic field pulse is applied to a magnetic system and that the ground state orientation is rotated by an angle All.
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From page 54... ...
For most magnetic materials, damping values are experimentally measured to lie in the range 0.1 > or > 0.01, and the magnetic moment for data storage media is typically on the order of poMS ~ 0~5 T Thus, damping times for media are expected to range over 0.2 < ~0 < 2 ns.
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From page 55... ...
To understand why this is the case, we must consider the various noise sources in the magnetic recording process. There are three principle contributions to the noise in disk drives: (1)
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From page 57... ...
57 FIGURE 3 Probability for reversal of a lithographically patterned magnetic element, as a function of pulse duration. Different curves are for varying pulse amplitudes.
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From page 58... ...
(The write head is essentially a ferrous-core electromagnet that uses the soft magnetic properties of high-moment alloys to convert weak currents of a few hundred mA into large magnetic fields.) At this point, we are now limited by the excess spin that can exist in a ferromagnet.
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From page 59... ...
In this case, however, the thermal fluctuations are not detected as irreversible jumps in the ground state configuration of the sensor, but simply as voltage fluctuations in the sensor output. The read head in a modern disk drive uses the giant magnetoresistance (GMR)
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From page 60... ...
60 FRONTIERS OF ENGINEERING different magnetic eigenmodes (also referred to as magnons) is given by a Planck distribution.
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From page 61... ...
(19) The noise power spectrum is proportional to | %(co)
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From page 62... ...
k = Unlock is the effective gyromagnetic frequency of the anisotropy. Notice that the noise power is proportional to the ratio of the thermal energy to
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From page 63... ...
, but to date none has demonstrated the predictive power necessary to qualify as a true physical theory for damping. This remains a serious deficiency in the theory of ferromagnetic materials that deserves more attention in the physics community.
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From page 64... ...
Current recording technology relies upon intrinsic relaxation processes induced by large applied magnetic fields. Borrowing terminology from nuclear magnetic resonance, large
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From page 65... ...
Physical Review Letters 83(23)
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From page 67... ...
becomes impossible to deny. Once we consider systems small enough that we can distinguish individual molecules or atoms, thermal fluctuations become important.
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From page 68... ...
Here the words average and variance refer to many repetitions of the pulling experiment; because of thermal noise, the exact amount of work performed in stretching the RNA molecule differs from one repetition to the next, as illustrated in Figure 1. By dissipated work, we mean the amount by which the total work performed (during a single realization)
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From page 69... ...
WdisS—W—Wrev (2) (As discussed below, this is equivalent to W- /\F, where AF is the free energy difference between the initial and final states of the system.)
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From page 70... ...
What is the relationship between this distribution and the free-energy difference /\F = FB - FA between the equilibrium states corresponding to the initial and final volumes? Based on our knowledge of macroscopic thermodynamics (Eq.3 and 4)
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From page 71... ...
(The derivation of Eq.7 from Eq.6 is left as an exercise for the reader.) If perchance we change the volume of the container relatively slowly so that the gas remains close to thermal equilibrium at all times, then p(W)
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From page 72... ...
, originally derived in the context of systems in nonequilibrium steady states and recently confirmed by the Australian bead-dragging experiment mentioned earlier (Wang et al., 2002~. All of these predictions have potentially important design implications for nanomachines, for which thermal fluctuations are bound to play an important role.
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From page 73... ...
1999. Entropy production fluctuation theorem and the nonequilibrium work relation for free energy differences.
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