ISDRS 2011, December 7- 9, 2011, College Park, MD, USA

ISDRS 2011 – http://www.ece.umd.edu/ISDRS2011

Student Paper

Modeling of a New Liner Stressor comprising Ge2Sb2Te5 (GST): Amorphous-Crystalline Phase Change and Stress induced in FinFET Channel

Ran Cheng, Yinjie Ding, Bin Liu and Yee-Chia Yeo*

Department of Electrical and Computer Engineering, National University of Singapore (NUS), 117576 Singapore. *Phone: +65 6516-2298, Fax: +65 6779-1103, E-mail: yeo@ieee.org

ABSTRACT We report the first simulation study of stress induced in a FinFET channel due to a new stress liner Ge2Sb2Te5 (GST). Volume change in the GST material wrapped around a FinFET was used to induce stress in the channel. We show that a large compressive channel stress can be induced. The channel stress could be further enhanced with device scaling.

INTRODUCTION Silicon nitride (SiN) liner with high intrinsic stress has been used as a stressor to boost transistor performance since the 90 nm technology node. We recently reported a new liner stressor comprising GST that can achieve large drive current enhancement in a FinFET with 4.5 nm gate length (Fig. 1) [1]. When amorphous GST (α-GST) is phase-changed to crystalline GST (c-GST), a volume contraction results. This was exploited for strain engineering. In this paper, we report the first numerical simulation study on the contraction of the GST liner and its impact on channel stress in FinFETs.

MODELING AND SIMULATION PROCESS A numerical modeling tool is employed here for stress simulation. Geometry of the nominal FinFET structure is shown in Fig. 1 (c): gate length LG = 10 nm, gate height HG = 50 nm, fin width WFin = 30 nm, fin height HFin = 40 nm, and GST thickness TGST = 60 nm. Large mesh size (~ 10 nm) was used for Si and SiO2 substrate regions and very small mesh size (~0.4 nm) was used in the FinFET channel region. This gives accurate results for a reasonably short simulation time. The effect of GST contraction during material crystallization is considered by reducing the volume of the GST by an amount which matches experimental data [2].

RESULTS AND DISCUSSION Stress components in x and y directions, i.e. Txx and Tyy, respectively, were obtained. The stress contours of Txx and Tyy in planes A and B are shown in Fig. 2 and 3, respectively. Planes A and B are 2 nm from the fin sidewall and top fin surface. Tyy [in the source (S) to drain (D) direction] is larger than 1.5 GPa (compressive). This indicates that GST could significantly increase drive current in both fin top and sidewall planes. To investigate the impact of geometric parameters on stress in S to D direction, LG, HG, WFin, HFin and TGST are varied individually with the other four parameters maintained at their nominal values. As shown in Fig. 4(a), a reduced LG leads to larger Tyy as less stress from GST would be buffered by the gate. Additionally, as LG decreases, Tyy in Plane B increases more than Tyy in Plane A, indicating Tyy in the sidewall channel is mainly due to the stress transferred from the two sides of the gate. On the other hand, Tyy in Plane A increases more than that in Plane B for a decreasing HG [Fig 4 (b)], showing that Tyy in the fin-top channel region is mainly due to the stress transferred from the gate top. Fig. 5 (a) and (b) show that a lower HFin and a larger WFin could result in a large compressive Tyy. The trends in Fig. 5 (a)-(b) are consistent, showing that a low HFin to WFin ratio could lead to large Tyy. In Fig. 6, Tyy in vertical plane A is less sensitive to the change of TGST than that in horizontal plane B, since GST in z-axis has more mechanical coupling with the channel region in Plane A, as illustrated in Fig. 1(c). Table I summarizes the change of Txx and Tyy as LG, HG, WFin, HFin and TGST are reduced. A higher channel stress could be achieved for GST liner stressed FinFETs with shorter LG, lower HG, and lower HFin to WFin ratio, all of which are favorable for drive current scaling with device miniaturization.

CONCLUSION A novel liner stressor GST integrated on ultra-small scale FinFET is simulated in this work. GST could contribute to more than 1.5 GPa compressive stress in FinFET channel region. This is consistent with experimental observation. In addition, GST also demonstrates capability to increase the channel stress with scaling-down of device geometry, making it a promising candidate for strain engineering in the future technologies. REFERENCES [1] Y. Ding et al., “A new Ge2Sb2Te5 (GST) liner stressor featuring stress enhancement due to amorphous-crystalline

phase change for sub-20 nm p-channel FinFETs,” to be presented at IEDM 2011. [2] T. P. Leervad Pedersen et al., “Mechanical stresses upon crystallization in phase change materials,” Applied Physics

Letters, vol. 79, no. 22, pp. 3597-3599, 2001.

ISDRS 2011, December 7- 9, 2011, College Park, MD, USA

ISDRS 2011 – http://www.ece.umd.edu/ISDRS2011

z

yx

GST

(c)

Plane A

Plane B

(a)

Si

Si�-GST

VolumeContraction

c-GST

(b)

c-GST

50 nmS D

SiO2

SiO2

FIB cut

GST

Fin Sidewall

Gat

e

S

D

S

DG

ateG

ST

Fin Sidewall

Gat

e

S

D

S

D

Gate

y

xz

-2.8

-2.6 -2

.4 -2.6

-2.8

-1.2

-1.4

-1.4

-1.2

(b)

(a)

Fig. 2 Contour lines for (a) Txx and (b) Tyy in Plane B [refer to Fig. 1(c)], showing stress in the sidewall channel region. Large compressive stress is observed in the channel in the x- and y- directions. Up to 1.5 GPa compressive Tyy near the gate region is shown in (b), which could increase hole mobility and drive current on the fin sidewall.

Fig. 1. (a) The volume of Ge2Sb2Te5 (GST) liner stressor is reduced by ~6% upon crystallization, i.e. phase change from amorphous to crystalline state. The volume contraction results in a large compressive strain in the gate and Fin regions. (b) TEM showing c-GST compressively stressed the channel in the S-to-D direction. (Inset: SEM image of our FinFET after a-GST deposition and crystallization) [1]. (c) 3D schematic of the simulated FinFET wrapped around by GST liner stressor. Coordinate axes are also shown. Planes A and B are 2 nm away from Fin sidewall and Fin top, respectively. The directions of stress induced on the Fin and gate are also illustrated.

- 3.0

- 2.5

- 2.0z

yx

S D

(a)

Fin TopGate

GST

Fin

(b)

- 1.0

- 1.5 - 2.0

Gate

GST

Fin Top

FinS D

z

yx

Fig. 3 Contour lines for (a) Txx and (b) Tyy in Plane A [refer to Fig. 1(c)], showing stress in the channel region. Large compressive stress is observed. Up to 2 GPa compressive Tyy near the gate region is observed in (b), which could greatly increase the drive current contirbuted by the top fin surface.

10 15 20 25 30-0.6

-0.8

-1.0

-1.2

-1.4

-1.6

-1.8

20 30 40 50 60-0.6

-0.8

-1.0

-1.2

-1.4

-1.6

-1.8

-2.0(b)

HFin

= 40 nm

Plane A Plane B

Gate Length LG (nm)

Stre

ss T

enso

r T yy

(GPa

)

Plane A Plane B

WFin

= 30 nm

HG = 50 nm

Gate Height HG (nm)

HFin

= 40 nm W

Fin = 30 nm

LG = 10 nm

(a)

Fig. 4. Large compressive Tyy in the channel region is observed in both Planes A and B. (a) Tyy increases as LG decreases. Tyy in Plane B shows more increase than that in plane A, for the same change in LG (b) Tyy increases for lower HG. Tyy in Plane A exhibits more increase than that in plane B, for the same change in HG.

20 30 40 50 60

-0.8

-1.0

-1.2

-1.4

-1.6

-1.8

20 30 40 50 60-0.8

-1.0

-1.2

-1.4

-1.6

-1.8

-2.0

-2.2(b)(a)

Plane A Plane B

Plane A Plane B

Stre

ss T

enso

r T yy

(GPa

)

Fin Height HFin

(nm)

LG = 10 nm

HFin

= 40 nm

HG = 50 nm

WFin

= 30 nm

LG = 10 nm

HG = 50 nm

Fin Width WFin

(nm) Fig. 5. Large compressive Tyy in the channel region is observed in both Planes A and B. (a) Tyy increases as HFin decreases. (b) Tyy increases for structures with larger WFin. A low HFin to WFin ratio is desirable to achieve larger channel stress Tyy.

40 50 60 70 80-1.0

-1.1

-1.2

-1.3

-1.4

-1.5

-1.6

-1.7

-1.8

Stre

ss T

enso

r T yy

(GPa

)

GST Liner Thickness TGST

(nm)

Plane A Plane B

LG = 10 nm H

G = 50 nm

WFin

= 30 nm HFin

= 40 nm

Table I. Summary of stress tensor

change as LG, HG, WFin, HFin and TGST are reduced.

Plane A Plane B

Txx Tyy Txx Tyy

LG

HG

WFin

HFin

TGST

Fig. 6. Tyy from S to D in both Planes A and B, as a function of TGST. Compressive stresses in both planes are not sensitive to the change of liner thickness for TGST larger than 60 nm.