From microelectronics down to nanotechnology

sami.franssila@tkk.fi

Contents

• Lithography: scaling x- and y-dimensions• MOS transistor physics • Scaling oxide thickness (z-dimension)• CNT transistors• Conducting wires (metals and CNTs)• Memories • Moore’s law and fabrication economics

Top down nanotechnology

Scaling from micrometer down• Production-proven techniques• Laboratory techniques• Laboratory tricks

Issues to be tackled:• Cost• Area• Speed• Defect density

Writing patterns

By pen• easy to write anything• slow to write long works• easy to change your mind in the middle

By printing press• very fast for large number of copies• expensive to make the first copy• expensive to make changes

Pen vs. printing press

• AFM needle

• electron beam

• optical projection

• nanoimprint

Lithography

Printing press style:• optical: mainstream• EUV/X-ray: small linewidth but otherwise problematic• imprint: partial solution at the moment; only certain

aspects proven

Pen-like:• electron beam: slow writing speed• ion beam: very slow writing speed, ultimate resolution• AFM, dip-pen,...

The goal of lithography is to make lines and spaces small (only this will

increase device packing density)

Contact/proximity lithography

gap

light from light source

quartz mask withchromium pattern

photoresist coveredsilicon wafer

Resolution= line + space

Resolution =3 ( /n)* (gap + ½ thickness ) (Hg-lamp line) 436 nm• gap between mask and resist g 5 µm• d resist thickness d 1

µm• n resist refractive index n 1.6

• 1 µm in production• 100 nm in research

Projection lithography

Sources of radiation(UV 365 nm-436 nm,DUV 193 nm-248 nm,EUV, X-rays, electrons, ions)

Optical system I(lenses, mirrors)

Mask (pattern)

Optical system II(lenses, mirrors)

Numerical aperture NA=sin

Imaging medium (resist)Wafer (with patterns)

Wafer stage (alignment mechanism)

Resolution = k1 /NADepth of focus = k2 /NA2

Resist trimming trick

• Isotropically etch resistthinner & narrower linesline-to-line spacing unchanged

Works best for

narrow initial lines

Used in industry

Phase shift masks (PSM)

binary mask (quartz/chrome) phase shift mask (PSM) shifter

amplitude

intensity

resist exposure threshold

Conditions for phase shifter:

= 2L/

= 2nL/.

L(n-1) = /2

Phase Shift Mask Fabrication

quartz

quartz

quartz

quartz

quartz

quartz

quartz

2-resist way Single resist way

PSM produces /2 lines !

X-ray lithography

• Contact/proximity lithography= 13 nm, resolution very good

• highly penetrating radiation

• not sensitive to particles, but

• need thick metal to block x-rays

• need 1X original because no x-ray mirrors

Optical vs. X-ray masks

-reduction optics -mask is final size

-flat structures -highly 3D structures

40 nm of metal stops UV light; Need >1 µm thick metal to stop X-rays

Electron beam lithography (EBL)

• beam spot size 5 nm easily• beam scattering in resist (in all solids)• 10 nm can be made, but not easily

• use higher energy ( heating, charging)• use thinner resist ( etch resistance down, defects up)

Spot size vs. linewidth

• linewidth typically 3*spot size to ensure reproducibility and reduce roughness

missing pixels

Raster vs. vector scanning

Pixel-by-pixel raster scan; exposure / no exposure decision at each pixel

“Intelligent skipping of empty spaces”

EBL pros and cons

• flexible writing of structures individually

• writing speed very low indeed small areas only

• better resolution lower writing speed

• thinner resist better resolution, worse etch and implant resistance, danger of pin hole defects

Nano imprint (NIL)

• 1X master is pressed against polymer• force is used (pressure, temperature, UV)• release of the master• clearing the bottom residue• feature size limited by master fabrication only

NIL results

NIL problems

Problem 1:• need 1X original pattern (cf. X-ray

lithography)Problem 2:• need 3D original masterProblem 3:• Lifetime of the master ? Does repeated contact

with the polymer damage or contaminate the master ?

Problem 4:• Who is the first one to try something really new

which may not work in production ?

MOS transistor

gate length Lg

source channel drain

field oxide gate polysilicon gate oxide

The goal of silicon processing and thin film technology is to control diffusion depths, film thicknesses and interfaces at atomic precision.

MOS gate oxide seem by TEM

“Metal” gate made of highly doped polycrystalline silicon

Amorphous oxide

Single crystalline silicon substrate

Scaling of gate oxide

• Gate oxide thickness gate length/50

Lg Tox

• 1960’s 30 µm 600 nm

• 1970’s 5 µm 200 nm

• 1980’s 1 µm 20 nm

• 1990’s 0.35 µm 7 nm

• 2000’s 100 nm 2 nm

Oxide thickness limitations

• Leakage current (tunneling)

• Pinhole defects

• Trapped charge

• Interface traps

• Interface structure (dangling bonds)

• Crystallization and grain boundaries (not in SiO2 !)

Leakage current explodes below 2 nm

High-k dielectrics (e.g. HfO2)

Because most high dielectric constant materials (high-k) are oxides, some oxygen is present during deposition, and some SiO2 is formed at the interface. The question is: can you control its formation and thickness with Ångström accuracy ?

EOT: Equivalent Oxide ThicknessEOT = (SiO2/ high) * thigh- + tSiO2

where tSiO2 is the interfacial silicon dioxide thickness, if any.

ZrO2 film of 6 nm physical thickness with 23 has EOT 1 nm

Control of oxide layer

Gate First High-k/Metal Gate Stacks With Zero SiOx Interface Achieving EOT=0.59 nm for 16 nm Application, VLSI Technology Symposium 2009

Half time

CNT transistors

Transistor characteristics

CNT network transistors (TKK)

Random network, many current paths from source to drain.

High performance compared with polymer transistors

CNTN transistors (TKK)

CNT circuitry by IBM (2006)

The five-stage CMOS type nanotube ring oscillator using palladium p-type gates and aluminum n-type gates. The upper right inset shows the nanotube itself with a diameter of ~2 nm.

CNT transistor time scales

• 1998 first CNT transistor (FET)• 2001 logic gate• 2002 Schottky switch• 2002 top gate FET• 2003 ballistic transport demonstrated• 2004 AC characterization• 2006 circuit demo, 72 MHz ring oscillator• 2015? commercial devices (IBM guess)

Metallization

6 levels of metal, cross section IC complexity increase over time

IC metal wire scaling (by n>1)

C’ = (W/n)L/(T/n) = C

R’ =L/(H/n)(W/n) = n2R

RC time delay ’ is then given by

’ = R’C’ = n2RC

LH

T

W

metal

dielectric

While transistor performance improves with downscaling, scaled metal wires are worse !

Electromigration

Hu, C.-K. et al: Electromigration of Al(Cu) two-level structures: effect of Cu kinetics of damage formation,

J.Appl.Phys. 74 (1993), p. 969

Momentum transfer and displacement of lattice atoms by electrons

Depends on bond strength (which can be gauged by melting point)

Aluminum, low melting point, 650oC, low electromigration resistance

Copper 1083oC

Tungsten 3387oCimproved EM resistance

Grain size effects in metals

Mechanical properties scale beneficially with smaller grain size

Thermal properties mostly unchanged

Resistivity increases with decreasing grain size

Erb et al: in The Nano-Micro Interface, Wiley-VCH 2004

Resistivity depends on patterns!

G.B. Alers, J. Sukamto, S. Park, G. Harm and J. Reid, Novellus Systems, San Jose -- Semiconductor International, 5/1/2006

You cannot calculate thickness from resistance

R = ρL/Wt

because thin film resistivity ρ is linewidth and thickness dependent

(use e.g. X-rays to get an independent thickness value)

Grain size affected by:

                      

                                                                    

-underlying film (chemistry and texture)

-deposition process (sputtering vs. plating; & plating A vs. plating B)

-material purity

-thermal treatments

-geometry of structures on wafer

G.B. Alers, J. Sukamto, S. Park, G. Harm and J. Reid, Novellus Systems, San Jose -- Semiconductor International, 5/1/2006

Current density barrier

Electromigration limit of metals

ca. 1 MA/cm2

Vertical CNT connections

Seeded growth in contact holes

CNT’s show ohmic behaviour at current density 4*108 A/cm2

Flash memory: close to limitstunnel oxide 10 nm

interpoly oxide/nitride/oxide 50 nmGate linewidth 100 nm

Limit: thinner tunnel oxide traps charge and does become leaky (10 000 to 100 000 rewrites)

PCM: phase change memory

Chalcogenide materials exhibit

• 100X resistivity difference between crystalline and amorphous states

• factor of 2 difference in reflectivity

Memory programmed and read electrically and/or optically

GST = Ge2Sb2Te5

Chalcogenide PCM

Programming pulse: 100 ns

GST hot spot heated > 620oC

molten GST

rapid cooling amorphous GST

MΩ resistance

Reverse programming: also 100 ns

550oC, crystallization

kΩ resistance

Reliability & problems

Actual GST device

Moore’s law: Intel view• Year Node Lg EOT•• 2005 65 nm 30 nm 0.8 nm• 2007 45 nm 20 nm 0.7 nm• 2009 32 nm 15 nm 0.6 nm• 2011 22 nm 10 nm 0.5 nm• 2013 15 nm not clear how• -carbon nanotubes ?• -III-V on Si ?• 2019 non-electronic devices ?• -spintronics• -optical devices

Linewidth goes down

Chip size vs. wafer size

Chip size determined by:

•how many functions needed

•how small lines used

Wafer size determined by:

•production economics

Chip size goes up !

Lithography cost up exponentially

Cost of top-down nanofabrication

• 28 mask levels with 32 nm minimum lines• cost of finished chips 10$/cm2

• cost per mask level = 30 cents/cm2

• lithography equipment cost 35 M$• need to fabricate > 1 000 000 cm2/year• defects: 1 failed devices in 5 million• cf. DNA self-assembly: ppm error rates

correspond to very slow replication

Continued scaling till 2059

• 2.5 Å minimum linewidth • 0.04 Å gate oxide thickness • 2 mV operating voltage • 64 exabit DRAMs (exa = 1018)

• This is not a scaled MOS transistor but something completely different

• But Moore’s law is general; it is about economics of device manufacturing; not about transistors