Application of layers with internal stress for silicon wafer shaping

1 Confidential Proprietary

Application of layers with internal stress for silicon wafer shaping

J. Šik1, R. Lenhard1, D. Lysáček1, M. Lorenc1, V. Maršíková2, R. Hudec3,4

1ON Semiconductor Czech Republic

2Rigaku Innovative Technologies Europe

3Astronomical Institute of the Academy of Sciences of the Czech Republic

4Faculty of Electrical Engineering, Czech Technical University in Prague

2 • AXRO - 09 Confidential Proprietary

OUTLINE

• Theory

–Radius of curvature and warp

–Thin film stress

• Experiment

–LPCVD Poly-Si Films

–Squared wafer shape

• Multilayer stack design proposal

• Summary & Acknowledgements


WAFER

R

w

D/2

φ

For small angle φ:

R

Wafer diameter

Warp

Radius of curvature

w

What is the relation between R and w? Assuming wafer shape is close to model.

R

D

RRw

2

)cos(

D

21)cos(

2

Therefore, the Eq. (1) can be rewritten as

(1)

R

Dw

8

2

(2)

RADIUS OF CURVATURE and WARP


RADIUS OF CURVATURE and WARP

1,0

10,0

100,0

1000,0

10 100 1000

Warp [um]

Rad

ius

of

Cu

rvat

ure

[m

]

100mm

150mm

200mm

Wafer diameter

Wafer diameter [mm]

Warp [um]

R = 10 m R = 2 m

100 120 630

150 280 1400

200 500 2500


• Thermal expansion

• Intrinsic

- growth

- misfit

- phase transformation

• Extrinsic

- applied stress

- plastic deformation

extthtot int (3)

ORIGIN of THIN FILM STRESS


THIN FILM

SUBSTRATE

Compressive stress in layer

sf

Due to mismatch of thermal expansion coefficient between substrate ( ) and film ( ), after temperature ramp down a strain ( ) is built in.

s fth

DEPOSITION TEMPERATURE ROOM TEMPERATUREdepT roomT

0th ))(( roomdepsfth TT

THERMAL STRAIN and STRESS


Biaxial stress in thin film on thick substrate is related with strain:

thth

E

1

(4)

Young’s modulus; Silicon (100) – 1.3·1011 N/m2

Poisson’s ratio; Silicon (100) – 0.28E

Material[1/°C]

Silicon 2,6·10-6

Polysilicon 2,8·10-6

Thermal SiO2 0,35·10-6

PECVD SiO2 2,3·10-6

LPCVD Si3N4 1,6·10-6

Aluminum 25·10-6

Tungsten 4,3·10-6

THERMAL STRAIN and STRESS


THIN LAYER

w

Young’s modulus ; Silicon (100) – 1.3·1011 N/m2

Poisson’s ratio; Silicon (100) – 0.28

Wafer thickness

Radius of curvature after film depo

Radius of curvature before film depo

WAFER

COMPRESSIVE STRESS in layer

R

Thin film with residual stress on the top of silicon wafer deform wafer according stress value and stress type [S.Timoshenko, J. Opt. Soc. Am., 11, 233 (1925) ] (compressive or tensile)

Therefore the warp is proportional to the residual stress and film thickness and inversely proportional to the wafer thickness squared.

TENSILE STRESS in layerTHIN LAYER

WAFER

f

st

(5)

E

0

2 11

)1(6 RRt

tE

f

sf

R

0R

INTRINSIC THIN FILM STRESS


Example of residual stress in different depo and thermal growth layers are in tables. Values are just indicative as the intrinsic stress may vary with the process conditions.

Layer Stress[N/m2]

PECVD TEOS 1,8·108

Thermal SiO2 3·108

PECVD Si3N4 5·108

LPCVD Poly Si 2·108 *)

Compressive stress

LayerStress

[N/m2]

APCVD SiO2 2,2·108

LPCVD Si3N4 1·109

Tensile stress

THIN FILM STRESS VALUE


Heat treatment of poly-Si films can cause the atoms to move to low-energy positions. Poly-Si thickness (THX) is proportional to the depo time, which can impact the stress in poly-Si films.

LPCVD Poly-Si FILMS

Co

mp

ress

ive

stre

ss [

MP

a]


BACK SIDE LAYER

After depo of poly-Si (THX 1.5 m) and for wafer thickness 507 m the warp 110 m (R = 25.6 m) was achieved.

Wafer deformation map Warp profile perpendicular to the facet


Circular 150 mm wafer, thickness 378 m, warp 181 m was squared to □ 100 mm. Squared wafer keeps axially symmetrical shape.

WAFER SHAPE

-60 -40 -20 0 20 40 600

20

40

60

80

100

120

140

160

1

2

3

4


-60 -40 -20 0 20 40 600

20

40

60

80

100

120

140

160

180 measured data spherical R=11.7m

D

evi

atio

n (m

)

Position (mm)

Squared wafer has spherical shape. Deviation from ideal sphere is within 1 m.

WAFER SHAPE

-60 -40 -20 0 20 40 60

-1

0

1 deviation from sphere

Dev

iatio

n (

m)

Position (mm)


MULTILAYER STACK DESIGN

• To get low R we need to combine layers with high tensile stress on the front side and

compressive stress on the back side.

• All process steps have to keep high surface quality of the polished front side.

Layer with tensile stress

WAFER THX ?

Layer with compressive stress

R < 10m


LAYER STACK AND WAFER THICKNESS

• For designed stack we can calculate the wafer thickness to achieve expected radius of curvature.

• As we can see in chart the wafer thickness 195 m would be needed for R ~ 2 m.

• That thin wafer is sensitive for handling and also it is affected by gravity sag.

0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

100 150 200 250 300 350 400

Wafer THX [um]

R [

m]


• Impact of thin film stress on wafer shaping has been reviewed.

• Layers with internal stress uniformly shape silicon wafer w/o deterioration of high quality of the polished front side (surface RMS ~ 0.1 nm ).

• Stress in thin film is supposed to be constant regarding to the film thickness, which is valid for most of dielectric thin films used in microelectronics, except of poly silicon.

• Stress in poly silicon layer is reduced with film thickness due to atoms migration into low energy position.

• The circular wafer keeps the original axially symmetrical spherical shape after squaring. The solid area can be build from squared segments.

• Multilayer stack has been designed to decrease the radius of wafer curvature to R ~ 2 m.

• For other than spherical shape photolithography has to be used. Suitable technology is available in semiconductor industry.

• Research was partially supported by Projects MŠMT KONTAKT ME09028 & MŠMT ME0918.

SUMMARY & ACKNOWLEDGEMENTS

Application of layers with internal stress for silicon wafer shaping

Documents

Transcript of Application of layers with internal stress for silicon wafer shaping