InGaAs Double-Gate Fin-Sidewall MOSFET

Alon Vardi, Xin Zhao and Jesús del AlamoMicrosystems Technology Laboratories, MIT

June 25, 2014

Sponsors: Sematech, Technion-MIT Fellowship, and NSF E3S Center (#0939514)

1

Outline

• Motivation• Process technology• Results and analysis• Conclusions

2

Motivation• III-V MOSFET for sub – 10 nm CMOS• Outstanding planar devices, but need tighter channel

control through 3D designs• Concerns about nano-structure sidewall fabricated by RIE• Curing of plasma etching damage: forming gas annealing,

digital etchSidewall MOS

3

Key technologies

• Digital etch (DE): self-limiting O2 plasma oxidation + H2SO4 oxide removal

40 nm 30 nm

As etched 5 cycles DE

Fins

• BCl3/SiCl4/Ar RIE of InGaAs nanostructures with smooth, vertical sidewall and high aspect ratio (>10).

Zhao, EDL 2014

Lin, EDL 2014

• Shrinks fin width by 2 nm per cycle• Unchanged shape• Reduced roughness

15 nm

240 nm

4

Thick Top Gate OxideG

GS

D

SiO2 Gate dielectric Mo

Gate Patterning

Fin PatterningSI-InP

InAlAs

100 nm thick, n-InGaAsNd=1018 cm-3

Contacts + Pads

InGaAs finFET process flow

Gate stack

5

InGaAs finFETs

• Typical device consists of 100 fins, Lg=5 µm• Process splits:

• Number of digital etch cycles: 1, 2 and 4• HfO2 vs. Al2O3 (same EOT of 4 nm) • w/ and w/o forming gas anneal (FGA): 30 min, 400oC

Wf=

InGaAs

6

Electrical Characteristics

0.0 0.1 0.2 0.3 0.4 0.50

5

10

15

20

Lg=5 µmWf=12 nm

VGS=0 VI D [µ

A/µm

]

VDS [V]

VGS=0.5 V

-1.0 -0.5 0.0 0.5 1.00

5

10

15 ∆Wf=5 nmVDS=50 mV

Wf=37 nmI D [µ

A/µm

]VGS [V]

Wf=12 nm

• Current normalized to active layer thickness (100 nm) and number of fins

• Good electrostatic control over fin sidewalls • Wf ↑ → Vt ↓

Al2O3, 4 cyc DE, FGA

7

Subthreshold characteristics

-1.0 -0.5 0.0 0.5 1.010-6

10-5

10-4

10-3

10-2

10-1

100

101

102

ID

IG

∆Wf=5 nmVDS=50mV

Wf=37 nm

I D,I G

[µA/

µm]

VGS [V]

Wf=12 nm

• Subthreshold characteristics unrelated to IG and governed by Dit on fin sidewalls

Al2O3, 4 cyc DE, FGA

-1.0 -0.5 0.0 0.5 1.00

200

400

600

800

Wf=12 nm

S [m

V/de

c]

VGS [V]

Wf=37 nm

10 15 20 25 30 35 400

200

400

600

800

S min [m

V/de

c]

Wf [nm]8

Smin: Al2O3 vs. HfO2

• Narrow Wf : Smin(HfO2) ≅ Smin(Al2O3)• Wide Wf : Smin(HfO2) > Smin(Al2O3)

10 15 20 25 30 35 400

200400600800

100012001400

Al2O3

HfO2

S min [m

V/de

c]

Wf [nm]

4 cyc DE, FGA

9

Smin: Effect of forming gas annealing

• FGA → S↓ at all Wf on both oxides

10 15 20 25 30 35 400

200

400

600

800

1000

1200

1400

After FGA

Before FGA

S min [m

V/de

c]

Wf [nm]

Al2O3, 4 cyc DE

10 15 20 25 30 35 400

200

400

600

800

1000

1200

1400

After FGAS min [m

V/de

c]

Wf [nm]

Before FGA

HfO2, 4 cyc DE

10

Smin: Effect of digital etching

10 15 20 25 30 35 40 450

200

400

600

800

1000

1200

1400Al2O3, FGA

2 Cycles

4 Cycles

S min [m

V/de

c]

Wf [nm]

0 Cycles

• Narrow Wf : S not affected by digital etch• Wide Wf : #DE cycles ↑ → Smin↓

11

Simulations: Ideal interface

15 20 25 30 35 400

50

100

150

S min [m

V/de

v]

Wf [nm]

• Dit=0 → Smin=60 mV/dec, independent of Wf

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.00

100

200

300

400

Wf=37 nm

S [m

V/de

c]

VG [V]

Wf=17 nm

Vt shift

2D Poisson-Schrödinger (nextnano)

0.02

0.04

0.06

0.08

Wf

n [1018 cm-3]

𝑆𝑆 =𝑑𝑑𝑉𝑉𝑔𝑔

𝑑𝑑𝑙𝑙𝑙𝑙𝑙𝑙10(𝑛𝑛𝑓𝑓)

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

10-6

10-4

10-2

100

102

104

106

108

Wf=37 nm

Wf=17 nm

n f [cm

-1]

VG [V]

60 mV/dec

Onset of sidewall inversion

12

0.0 0.2 0.4 0.6 Ec1E12

1E13

Dit [

cm-2eV

-1]

E-EV [eV]

Simulations: uniform Dit across bandgap

• Dit>0 (uniform) → Smin=60 1 + 𝑞𝑞𝐷𝐷𝑖𝑖𝑖𝑖𝐶𝐶𝑜𝑜𝑜𝑜

, independent of Wf

• Onset of inversion shifts to negative voltage (stretch out)

-1.5 -1.0 -0.5 0.00

100

200

300

400 Wf=37 nm

S [m

V/de

c]

VG [V]

Wf=17 nm

Sidewall Inversion onset

15 20 25 30 35 400

50

100

150

S min [m

V/de

v]

Wf [nm]

60 1 + 𝑞𝑞𝐷𝐷𝑖𝑖𝑖𝑖𝐶𝐶𝑜𝑜𝑜𝑜

1

2

1

2

Dit=0 1

2Dit=3x1012 cm-2eV-1

13

-1.5 -1.0 -0.5 0.00

100

200

300

400

Wf=17nm

Wf=37nm

S [m

V/de

c]

VG [V]15 20 25 30 35 40

6080

100120140160180200

S min [m

V/de

v]

Wf [nm]

Simulations: Dit rising toward VB

0.0 0.2 0.4 0.6 Ec1E12

1E13

Dit [

cm-2eV

-1]

E-EV [eV]

1

2

Dit =0

3

1

2

3

3

12

• Dit rising towards VB: Wf ↑ → Smin ↑• Smin for different Wf probe Dit at different locations in the band gap

14

Simulations: Smin ↔ ϕs

Wf1

qϕt1

Wf2

qϕt2

Wf3

qϕt3

Wf1

Wf2 > Wf1

Wf3 > Wf2

Wf

X X’

• Wf ↑ → |ϕt| ↑ → |Vt| ↑ (negative Vt shift)• Wf ↑ → Smin probe Dit closer to VB 15

Effect of Oxide type on Dit profile

10 15 20 25 30 35 400

200400600800

100012001400

Al2O3

HfO2

S min [m

V/de

c]

Wf [nm]

4 cyc DE, FGA HfO2

Al2O3

ECEV

Dit

Position in bandgap

• Al2O3 yield better Dit than HfO2in lower part of bandgap

16

Effect of FGA on Dit profile

• FGA reduces Dit everywhere in bandgap

FGA

Dit

ECEV Position in bandgap

10 15 20 25 30 35 400

200

400

600

800

1000

1200

1400

After FGAS min [m

V/de

c]

Wf [nm]

Before FGA

HfO2, 4 cyc DE

10 15 20 25 30 35 400

200

400

600

800

1000

1200

1400

After FGA

Before FGA

S min [m

V/de

c]

Wf [nm]

Al2O3, 4 cyc DE

17

10 15 20 25 30 35 40 450

200

400

600

800

1000

1200

1400

2 Cycles

4 Cycles

S min [m

V/de

c]

Wf [nm]

0 Cycles

Effect of DE on Dit profile

Dit

#DE

ECEV Position in bandgap

• DE reduces Dit in lower part of bandgap

18

Towards a more detailed analysis: Mobility extraction

-1.0 -0.5 0.0 0.5 1.00.00.10.20.30.40.50.60.70.8 VDS=0V

Wf=37 nm

Wf=12 nmCg [

µF/c

m2 ]

VGS [V]0.5 1.0

0

200

400

600

800 Wf=27 nm

Wf=12 nm

µ [c

m2 /V

sec]

VGS [V]

• Mobility is 5-7 times smaller than expected• Wf ↓ → µ↓ → sidewall scattering• From ID and µ → extract nf

1MHz

Al2O3, 4 cyc DE, FGA

19

Fitting of Dit profile

-1.0 -0.5 0.0 0.5 1.0101

102

103

104

105

106

107

108

Wf=37 nm

Wf=12 nm

Measurement Simulation: U-shape Dit

n f [cm

-1]

VGS [V]-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

10-6

10-4

10-2

100

102

104

106

108

Ideal Measured

Wf=70 nm

Wf=10 nm

n f [cm

-1]

VGS [V]

60 mv/dec

• U-shape Dit profile provides excellent agreement with measurements

Al2O3, 4 cyc DE, FGA

0.5 Ec 1.01

10

Dit [

1012

cm-2eV

-1]

E-Ev [eV]20

Comparison to planar structures

• Dit distribution on properly prepared dry etched sidewalls approaches that of planar structures

Al2O3/InGaAs

This work

Brammertz, APL 2009

21

Conclusions• Study of Dit on dry etched nano-structure sidewalls• U – shape Dit distribution as in planar MOSFETs• FGA reduces Dit everywhere while DE lowers Dit toward the

valence band• HfO2 and Al2O3 give comparable results near CB, but rise of Dit

toward VB stronger for HfO2

• With all treatments, Dit level approaches level obtained on planar structures

#DE

HfO2

Al2O3

FGA

22