1 Giant Magneto-Resistive Switches & Spin Torque Transfer Switches friendly critic analysis ERD "Beyond CMOS" Technology Maturity Evaluation Workshop San Francisco, California July 12, 2008 Eli Yablonovitch UC Berkeley Electrical Engineering & Computer Sciences Dept.

Si (001) Substrate Ta 5nm Ru 50nm Ta 5nm NiFe 5nm Antiferromagnetic MnIr 8nm CoFe 2nm Ru 0.8nm Ferromagnetic CoFeB 3nm MgO 1.5nm Tunnel Barrier Ferromagnetic CoFeB 3nm Ta 5nm Transpinnor Structure: Ru 15nm 6:1 Resistance Change in Tunnel Magnetoresistive (TMR) stack [1] Current Gate I signal Magnetization Drain Source Insulator Current Gate Drain Source I signal [1] Ikeda et. al., Japanese Journal of Applied Physics, Vol. 44, No 48, pp. L1442-L1445 B Field B Field Device Area 1μm 2 Gate

5μA output5μA input Complementary Transpinnor Logic 500Ω or 2.275kΩ or 500Ω +V +3mV -V -3mV Output Power = 1.6*10 -8 W Total Power = 2.5*10 -8 W Efficiency=65%

80 Efficiency for Complementary Transpinnor Circuit On/Off Ratio Efficiency (%) Best On/Off ratio today, 4.5:1

NAND Gate: input A + output input B Transpinnor Logic Example -

NOR Gate: + input A input B output Transpinnor Logic Example -

Si (001) Substrate Ta 5nm Ru 50nm Ta 5nm NiFe 5nm Antiferromagnetic MnIr 8nm CoFe 2nm Ru 0.8nm Ferromagnetic CoFeB 3nm MgO 1.5nm Tunnel Barrier Ferromagnetic CoFeB 3nm Ta 5nm Transpinnor Structure: Ru 15nm 6:1 Resistance Change in Tunnel Magnetoresistive (TMR) stack [1] Current Gate I signal Magnetization Drain Source Insulator Current Gate Drain Source I signal [1] Ikeda et. al., Japanese Journal of Applied Physics, Vol. 44, No 48, pp. L1442-L1445 B Field B Field Device Area 1μm 2 Gate

I signal Insulator Current Gate B Field B Field What is the minimum current required for switching? 10nm=10 -8 m Ampere's Law: H = J 2 r H = I H needs to be at least H=1 Oersted to switch a GMR device equivalent to B=10 -4 Tesla (private communication from Stuart Parkin of IBM) (This is equivalent to saying the best relative magnetic permeability is =10 4 to generate and effective B=1Tesla) I =2 r H = 2 r (B/ o ) = 2 r /( ) and take r=10nm I = /( ) Amps I = 5 Amps are required for switching! This is really pretty good, but required very optimistic assumptions!

l Repeater l l a RC time = (clock period) /2 aspect ratio of wire Physics of Wires:

= 4kT R f V signal = 0.56 milli-Volts aspect ratio of wire = 4(kT/R) f I signal = 0.25 Amps C = r o a 4800 C 7 femto-Farads

I signal Insulator Current Gate B Field B Field What is the minimum current required for switching? 10nm=10 -8 m I = 5 Amps are required for switching! This is really pretty good, but required very optimistic assumptions! According to the previous slide, operation at 1micro-Amp implies a good noise margin ~ 48kT. Operation at 5 Amps implies 1200kT per bit function, which is at least 100 better than today's technology, and might be worth pursuing, but it still falls 25 short of the practical engineering limit of 48kT.

That was the Giant Magneto-Resistive Effect. What about the Spin Torque effect? Si (001) Substrate Ta 5nm Ru 50nm Ta 5nm NiFe 5nm Antiferromagnetic MnIr 8nm CoFe 2nm Ru 0.8nm Ferromagnetic CoFeB 3nm MgO 1.5nm Tunnel Barrier Ferromagnetic CoFeB 3nm Magnetization Drain Si (001) Substrate Ta 5nm Ru 50nm Ta 5nm NiFe 5nm Antiferromagnetic MnIr 8nm CoFe 2nm Ru 0.8nm Ferromagnetic CoFeB 3nm MgO 1.5nm Tunnel Barrier Ferromagnetic CoFeB 3nm Drain Ta 5nm Ru 15nm Source Magnetization is changed by literally transferring the electrons! Take for a minimum domain size, that 1000 electrons have to be transferred. Current I= 1000e Coul/ seconds I= Amps = 1.6 Amps Slightly better than the GMR case, but not quite to the theoretical goal<1 A. But 1000e- for switching is very optimistic. Further improvements require going slow to keep the current down. Might be interesting at a clock speed <100MHz Magnetization

Summary: 1. Giant Magneto-Resistive Effect Switch: Better than today's technology, but not quite to the level of theoretical goal. 2. Spin-Torque Switch: Slightly better than GMR Switch, and capable of achieving theoretical goal at slow clock speeds, <100MHz.

Backup Slides:

nano-transformer ~1eV A low-voltage technology, or an impedance matching device, needs to be invented/discovered at the Nano-scale: transistor amplifier with steeper sub-threshold slope photo-diode VGVG MEM's switch Cryo-Electronics kT/q~q/C Cu solid electrolyte Electro-Chemical Switch giant magneto-resistance spintronics +

giant magneto-resistance "spintronics" These switches are made of metallic components and are of inherently low impedance +

10μm 1μm1μm 100nm 10nm Moore's Law Critical Dimension Year Technology Gap Gates only Gates including wires Energy per Bit function (kT) The other, for energy per bit function Shoorideh and Yablonovitch, UCLA 2006 Transistor Measurements by Robert Chau, Intel

Recommendations: 1.Milli-Volt powering should be regarded as a Goal for future electronic switching devices. 2.There would be both an immediate power benefit, as well as a benefit at the end of the roadmap. 3.Band edge steepness is poorly known, and should be investigated for a number of semiconductors and semi-metals. 4.The full range of technology options should be included.

Moore You?

! Transistor

Nano-transformers High Impedance Magnetically Loaded Transmission Line

What about very short wires? Johnson Noise: If then the signals could be large enough to be efficiently amplified. If The Coulomb Blockade Capacitance. For wires less than 1 m, a conventional transistor amplifier configuration may be adequate. 10 atto-Farads,

The natural voltage range for wired communication is rather low: The thermally activated device wants at least one electron at ~1Volt. The wire wants 1000 electrons at 1mVolt each. (to fulfill the signal-to-noise requirement >1eV of energy) Voltage Matching Crisis at the nano-scale! If you ignore it the penalty will be (1Volt/1mVolt) 2 = 10 6 The natural voltage range for a thermally activated switch like transistors is >>kT/q, eg. ~ 40kT/q or about ~1Volt

In the future, V dd in digital circuits will drop to 1 milli-Volt, for communication wires.