1 Confidential Proprietary Application of layers with internal stress for silicon wafer shaping J. Šik 1, R. Lenhard 1, D. Lysáček 1, M. Lorenc 1, V. Maršíková 2, R. Hudec 3,4 1 ON Semiconductor Czech Republic 2 Rigaku Innovative Technologies Europe 3 Astronomical Institute of the Academy of Sciences of the Czech Republic 4 Faculty 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

3 AXRO - 09 Confidential Proprietary WAFER R w D/2 φ For small angle φ: Wafer diameter Warp Radius of curvature What is the relation between R and w? Assuming wafer shape is close to model. Therefore, the Eq. (1) can be rewritten as (1) (2) RADIUS OF CURVATURE and WARP

4 AXRO - 09 Confidential Proprietary RADIUS OF CURVATURE and WARP Wafer diameter Wafer diameter [mm] Warp [um] R = 10 mR = 2 m

5 AXRO - 09 Confidential Proprietary Thermal expansion Intrinsic - growth - misfit - phase transformation Extrinsic - applied stress - plastic deformation (3) ORIGIN of THIN FILM STRESS

6 AXRO - 09 Confidential Proprietary THIN FILM SUBSTRATE Compressive stress in layer Due to mismatch of thermal expansion coefficient between substrate ( ) and film ( ), after temperature ramp down a strain ( ) is built in. DEPOSITION TEMPERATUREROOM TEMPERATURE THERMAL STRAIN and STRESS

7 AXRO - 09 Confidential Proprietary Biaxial stress in thin film on thick substrate is related with strain: (4) Young’s modulus; Silicon (100) – 1.3·10 11 N/m 2 Poisson’s ratio; Silicon (100) – 0.28 Material [1/°C] Silicon2,6·10 -6 Polysilicon2,8·10 -6 Thermal SiO 2 0,35·10 -6 PECVD SiO 2 2,3·10 -6 LPCVD Si 3 N 4 1,6·10 -6 Aluminum25·10 -6 Tungsten4,3·10 -6 THERMAL STRAIN and STRESS

8 AXRO - 09 Confidential Proprietary THIN LAYER w Young’s modulus ; Silicon (100) – 1.3·10 11 N/m 2 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 layer THIN LAYER WAFER (5)(5) INTRINSIC THIN FILM STRESS

9 AXRO - 09 Confidential Proprietary 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/m 2 ] PECVD TEOS1,8·10 8 Thermal SiO 2 3·10 8 PECVD Si 3 N 4 5·10 8 LPCVD Poly Si 2·10 8 *) Compressive stress Layer Stress [N/m 2 ] APCVD SiO 2 2,2·10 8 LPCVD Si 3 N 4 1·10 9 Tensile stress THIN FILM STRESS VALUE

10 AXRO - 09 Confidential Proprietary 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 Compressive stress [MPa]

11 AXRO - 09 Confidential Proprietary 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

12 AXRO - 09 Confidential Proprietary Circular 150 mm wafer, thickness 378  m, warp 181  m was squared to □ 100 mm. Squared wafer keeps axially symmetrical shape. WAFER SHAPE

13 AXRO - 09 Confidential Proprietary Squared wafer has spherical shape. Deviation from ideal sphere is within 1  m. WAFER SHAPE

14 AXRO - 09 Confidential Proprietary 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

15 AXRO - 09 Confidential Proprietary 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.

16 AXRO - 09 Confidential Proprietary 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