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Analysis of Mechanical Stress Effects in Short Channel MOSFETs

Analysis of Mechanical Stress Effects in Short Channel MOSFETs C. Gallon 1 , G. Reimbold 1 , G. Ghibaudo 2 , R.A. Bianchi 3 and R. Gwoziecki 1,3 . 1 CEA-Leti, 38054 Grenoble Cedex 9, France . Tel. : +33 (0)4 38 78 49 93. E-mail: cgallon@cea.fr

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Analysis of Mechanical Stress Effects in Short Channel MOSFETs

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  1. Analysis of Mechanical Stress Effects in Short Channel MOSFETs C. Gallon1, G. Reimbold1, G. Ghibaudo2, R.A. Bianchi3 and R. Gwoziecki1,3. 1CEA-Leti, 38054 Grenoble Cedex 9, France. Tel. : +33 (0)4 38 78 49 93. E-mail: cgallon@cea.fr 2IMEP, BP257, 38016 Grenoble Cedex 9, France. 3STMicroelectronics, Central R&D, 38921 Crolles, France. ULIS 2003 - Udine (Italy) - C. Gallon

  2. OUTLINE • Introduction • Experimental Method • Four point bending technique • Tested devices • Experimental Results: • Stress Influence on Long Channel Devices • Stress Influence on Short Channel Devices • Conclusion and Perspectives ULIS 2003 - Udine (Italy) - C. Gallon

  3. INTRODUCTION (1) • Context: • Generation of mechanical stress at various process steps. • These effects are more important in scaled CMOS devices • A key role of mechanical stress in MOSFETs devices: • Performance improvements (SiGe, SiGe:C… ) • or Performance reductions (STI,…) • Needs: • Improve extraction methodologies versus mechanical stress. • Better understanding and evaluation of stress effects on MOSFETs devices. ULIS 2003 - Udine (Italy) - C. Gallon

  4. INTRODUCTION (2) • Purpose of this work: • Analysis of external mechanical stress effects on MOSFETs from advanced 0.13µm CMOS technology: • Relative variations of mobility with external stress, • Extraction of Piezoresistive Response (PR), • Simple approach proposed to extract PR on short devices accounting for Rsd influence. • Objectives: Try to provide data for device simulators and a betteranalysis of stress effects. ULIS 2003 - Udine (Italy) - C. Gallon

  5. F F a 2a a EXPERIMENTAL METHOD: 4-POINT BENDING • Principle of a four-point bending technique: • Interest: Application of an uniform uniaxial stress between the two central fulcrums. ULIS 2003 - Udine (Italy) - C. Gallon

  6. F F a 2a a ydis EXPERIMENTAL METHOD: 4-POINT BENDING • Evaluation of stress : • Estimated error: - Uncertainty on L, a , y measurements and mainly on E value. - Global accuracy: 7% ULIS 2003 - Udine (Italy) - C. Gallon

  7. compressive L 2a a a tensile L Micrometer screw a a 2a EXPERIMENTAL METHOD: 4-POINT BENDING ULIS 2003 - Udine (Italy) - C. Gallon

  8. EXPERIMENTAL METHOD: 4-POINT BENDING • Specific characteristics: • Rectangular strip are cut from a saw technique. • With an appropriate preparation of the strips, mechanical stress: • longitudinal direction (// to current flow) • or transversal direction ( to current flow). • Most strips fail for 150-200MPa • Keep the applied mechanical stress below 100MPa. ULIS 2003 - Udine (Italy) - C. Gallon

  9. Gate Gate Source Drain Source Drain Buried oxide Substrate Substrate Bulk Technology SOI technology DEVICES TESTED ON 4-POINT BENDING • Bulk and SOI similar technologies - nMOS and pMOS fabricated on (100) substrates - Tox=2nm; W=10µm;Long (L=10µm) or short (L=0.13µm) channel length; - Important point on our short devices: a long distance between STI and gate  limit parasitic internal stress. - Mechanical stress ranging from 0 to 100MPa was applied. ULIS 2003 - Udine (Italy) - C. Gallon

  10. 5 5 Id@100MPa Id@100MPa 4 4 S) S) 3 3 5 5 - - Id@0MPa Id@0MPa (10 (10 2 2 Gm Gm 1 1 0 0 - - 0,5 0,5 0 0 0,5 0,5 1 1 1,5 1,5 Vg Vg (V) (V) EXPERIMENTAL RESULTS: Long Channel Devices • Effects of mechanical stress on transfer characteristics - Linear region characteristics of nMOS BULK: - Use of standard expressionsto extract VTand µ. - Note the invariance of VTand mobility variations. ULIS 2003 - Udine (Italy) - C. Gallon

  11. 8 8 8 8 8 8 8 8 nMOS pMOS 7 7 7 7 6 6 6 6 SOI SOI L=10µm W=10µm L=10µm W=10µm 4 4 4 4 6 6 6 6 BULK 2 2 2 2 5 5 5 5 BULK µ/µ (%) Longitudinal µ/µ (%) 0 0 0 0 Transversal 4 4 4 4 -2 SOI 3 3 3 3 -4 BULK 2 2 2 2 BULK -6 Longitudinal 1 1 1 1 -8 SOI Transversal 0 0 0 0 0 0 0 0 25 25 25 25 50 50 50 50 75 75 75 75 100 100 100 100 0 0 0 0 25 25 25 25 50 50 50 50 75 75 75 75 100 100 100 100 Tensile Stress (MPa) Tensile Stress (MPa) EXPERIMENTAL RESULTS: Long Channel Devices • Normalized mobility variations versus applied stress: - Excellent linear dependence for both n and p MOS devices. ULIS 2003 - Udine (Italy) - C. Gallon

  12. EXPERIMENTAL RESULTS: Long Channel Devices • Mobility variations and piezoresistance response: L longitudinal coefficient T transverse coefficient Majors coefficients of cubic structure for silicon ULIS 2003 - Udine (Italy) - C. Gallon

  13. Bradley&al. IEEE2001 0.3µm techno. Ref. Si EXPERIMENTAL RESULTS: Long Channel Devices • Piezoresistance coefficients (.10-12Pa-1): Bulk&SOI results 0.13µm techno. ULIS 2003 - Udine (Italy) - C. Gallon

  14. 8 nMOS Longitudinal Stress 6 L=10µm 4 µ/µ (%) D’ D’ 2 R R L=0.13µm D D 0 G G 0 25 50 75 100 S S R R Tensile Stress (MPa) S’ S’ EXPERIMENTAL RESULTS: Long&Short Channel Devices • Comparison between Long and Short devices: Bradley&al.: “Reduction is only due to the influence of Rsd". (IEEE 2001) ULIS 2003 - Udine (Italy) - C. Gallon

  15. EXPERIMENTAL RESULTS: Short Channel Devices • Bradley approach: • Problems of this approach: - Extraction of Ron is Vg dependent - Choice in Vg extraction results in significant variation on Ron • Significant uncertainty on piezoresistive coefficients. • A novel approach is proposed. ULIS 2003 - Udine (Italy) - C. Gallon

  16. EXPERIMENTAL RESULTS: Short Channel Devices • New approach proposed: 1. Correction from Rsd influence on Id0: • Calculation of equivalent Vg shift to get the same Id with and without stress: • Vg is related to Vt and mobility change by: [Roux-dit-Buisson, IEEProceedings-G, 1993] ULIS 2003 - Udine (Italy) - C. Gallon

  17. 2. Normalizedmobility change versus applied uniaxial stress. 8 8 No Rsd Correction Rsd correction 6 6 4 4 Transversal 2 2 µ/µ (%) 5 0 0 100MPa - 2 Longitudinal 4 - 4 75MPa - 6 Vg=Id/Gm (10-2V) 3 - 8 50MPa 2 0 0 25 25 50 50 75 75 100 100 25MPa Tensile Stress (MPa) 1 0 3. Piezoresistive coefficients extraction. 0 0 0,1 0,1 0,2 0,2 0,3 0,3 0,4 0,4 0,5 0,5 0,6 0,6 Id/Gm (V) EXPERIMENTAL RESULTS: Short Channel Devices 1. Experimental variations after various stress levels for a 0.13µm pMOS/SOI. Note excellent linearity. ULIS 2003 - Udine (Italy) - C. Gallon

  18. EXPERIMENTAL RESULTS: Short Channel Devices • Example of calculations including Rsd corrections: • Agreement between  coefficients for both short and long L. • Local or 2D stress do not affect significantly short devices, however a slight longitudinal effect may exist. ULIS 2003 - Udine (Italy) - C. Gallon

  19. CONCLUSIONS & PERSPECTIVES • Study of mechanical stress effects on long and short channels. • Proposition of a simple approach to determine directly Vt and µ • Vt is independent of stress, • Mobility variations dominate the piezoresistive response, • Bulk & SOI: similar piezoresistive response both n and p MOS, slightly higher for SOI. • After Rsd corrections, comparable results on short and long devices:  2D or local effects are small for a 0.13µm technology. • A first step to provide piezoresistive data for device simulators and a better analysis of mechanical stress effects. ULIS 2003 - Udine (Italy) - C. Gallon

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