High Stress Silicon Nitride Films for Strained Silicon Technology

Transcription

1 High Stress Silicon Nitride Films for Strained Silicon Technology Bhadri Varadarajan*, Jim Sims PECVD Business Unit Novellus Systems, Inc.

2 Effect of Strain on Carrier Mobility! Strain lifts degeneracy, reducing the inter-valley and inter-band scattering. Effect of tensile strain on Si conduction band Effect of compressive strain on Si valence band! Strain also distorts the electron/hole - lattice interaction in a way that reduces the electron/hole s effective mass.! Tensile strain increases electron mobility. Both compressive and tensile strain increase hole mobility compared to unstrained Si, but tensile strain does it to a lesser extent. P. 2

3 Methods For Strain Generation! Strain Options: Embedded SiGe Contact Etch Stop Layer Shallow Trench Isolation Stress Memorization! CESL involves depositing a layer of high stress nitride (either tensile or compressive depending on whether it is a NMOS or PMOS device) on top of the silicide. High Stress Nitride Film Spacer NiSi P. 3

4 High Stress Tensile SiN (HTN) for NMOS devices Key requirements - High stress and low thermal budget! Increasing tensile channel strain results in increased drive current gains. Channel strain depends on film stress and thickness. Uni-axial strain Intel, IEDM 2004 Intel, IEDM 2003 HTN liner deposited over a NMOS device Intel IEDM paper, 2003! NiSi thermal budget requirement is! 400 C. NiSi undergoes a highresistivity phase transition at elevated thermal budgets.. M.C..Poon, et al, Microelectronics Reliability, 1998 P. 4

5 Silicon Nitride - Structure and Hydrogen Incorporation! SiN has a rigid structure. N joins three tetrahedrons, forming a nitrogen triangle (N(-Si") 3 group). Undistorted Si- N 4 tetrahedron N-Si 3 Trigonal planar arrangement! Very difficult for valence of each Si and N to be filled due to this rigid structure. These vacant sites are readily occupied by hydrogen atoms.! The Si-NH-Si group is isoelectronic to Si-O-Si. Addition of the NH group adds flexibility to the nitride network, lowering the stress. P. 5

6 Tensile Stress Generation During Film Growth! Three step reaction sequence results in creation of tensile stress: Surface reaction of gas phase disilane and aminosilane radicals. Subsequent reduction of H via H 2 and NH 3 elimination / condensation reactions. Stretched Si-N bond formation.! Rigidity of trigonal planar N-centered network elements forming low-energy bonds with tetrahedral Si-centered elements causes stress. P. 6

7 As-Deposited Film - PECVD vs LPCVD! For a high as-deposited stress, need to maximize hydrogen removal during deposition.! In PECVD SiN films, lower substrate temperatures (! 400 C) result in: Lower surface mobility of the deposited species. Fewer condensation reactions on the substrate. Increased H incorporation, fewer stretched Si-N bond formation and a porous micro-structure - LOWER STRESS.! In LPCVD films, higher substrate temperatures ( C) and lower deposition rates allow increased condensation reactions during film growth. Results in lower H and higher stress films. Films with stress ~ 1 GPa 400 C P. 7

8 Alternate Methods to Generate High Tensile Stress Films! NiSi thermal budget constraints prevent use of higher substrate temperatures.! Alternate sources of energy required to hydrogen from the film in order to generate high tensile stress.! Novellus has developed two techniques to achieve this: Multi-layer deposition technique - Plasma as the energy source. Ultra-Violet Assisted Thermal Processing (UVTP ) - UV photons as the energy source. Hydrogen Concentration Film Shrinkage Stress (GPa) Standard 27% 0% ~ 1 Plasma Treated 20% NA 1.2 UV Cured 13% 12% 1.6 P. 8

9 Multi-Layer Deposition Technique (MLD)! Methodology: Very thin layers of film deposited at a time, followed by a plasma posttreatment. Above process is repeated to get required film thickness.! Plasma provides energy to break Si-H and N-H bonds in the film, resulting in hydrogen removal. Plasma affect sonly the first few layers of the film; multi-layered approach provides accessibility to entire film.! Layer thickness, post-treatment time and film stoichiometry mainly determine the film stress. Highest stress was on a near-stoichiometric film (SiN = 0.77). P. 9

10 MLD - Effect of post-treatment time and film thickness Effect of post-treatment time on hydrogen reduction and stress increase Stress (GPa) 1.10 Effect of layer thickness on film stress - (post-treatment time = 20s in all cases) A Layer Thickness (A) P. 10! Amount of hydrogen removal correlates with stress increase Films with stress ~ GPa 400 C with MLD technique! Optimal layer thickness of ~ 30 Å provides highest stress. For thicker layers, inner portion of film remains untouched since plasma affects only the first few surface layers. Stress relaxation occurs on thinner layers with long post-treatment time.

11 Ultra-Violet Absorption of SiN Films! Absorption edge shifts to higher energies with increasing N/Si ratio. Band gap increases with increasing N/Si ratio.! The absorption coefficient, photon energy and band gap are related through the Tuac formula: 1.20 UV UV Absorption Vis Spectra of Spectrum High Stress Tensile of SiN SiN Films Absorption Absorbance (Arb. Units) BKM films Si Rich SiN Film Ref Silica As-Dep BKM 15min 480% UV 15min 100% UV High Si BKM Silicon Reference (nm) Wavelength (nm) P. 11

12 Response of SiN Film to UV Curing! UV photon energy results in significant bond breaking.! Hydrogen atoms from neighboring broken bonds combine to form molecular H 2 that diffuses out. The dangling bonds cross-link to form extended Si-N bonds.! Assuming 2 nd order kinetics for hydrogen removal, R = k [x-h][x-h] we get varying rate constants (rate decreases with time). Hydrogen reduction correlates with stress increase. P. 12

13 Bond Evolution With UV Cure Effect of UV Cure time on SiH and N-H Concentration and SiH Peak Wave number SiH and NH concentr ation (bonds /cm3) SiH Concentration SiH Peak Wave number NH Concentration Cure time (minutes) H 2 SiN 2 SiH peak Wave number (cm -1 ) HSiN 2 Si SiH bonding environment changes from predominantly HSiN 2 Si in the as-deposited film to H 2 SiN 2 after UVTP. P. 13! Significant reduction in both SiH and NH concentrations.! H atom recapture occurs as a competing process to H 2 formation.! The following reaction is thermodynamically preferred : Si Si + N H # Si N + Si H Si-N and Si-H bonds are energetically favored over Si-Si and N-H bonds.! H transfer from N-H site to Si-H site occurs as long as Si-Si bonds are available in the film.

14 Correlation of Stress and Temperature for UVTP Stress (GPa) Effect of Cure Temperature on Stress Cure Time (Minutes) 480 ºC 420 ºC 380 ºC Same as deposited film for all cure conditions! UV cure stress can be modulated based on thermal budget. P. 14

15 Effect of SiN Film Composition on Post-Cured Stress Si-H Rich Film Stress = 0.8 GPa Balanced N-H/SiH Film Stress=1.6 GPa Absorbance (Arb. Units) Wavenumber (cm-1) N-H Rich Film Stress = 1.2 GPa N-H Wavenumber (cm-1) 1300! One reactant is prematurely exhausted in unbalanced films. Bond energy requirements favor hydrogen recapture in such films Si-H Maximum Stress Requires Optimal Film Composition Si-N P. 15

16 Comparison of UVTP to Thermal Only Process High efficiency hydrogen elimination with UVTP % Hydrogen from FTIR Evolution of Hydrogen Content With Time For Thermal Only Anneals and UVTP Treatment 400 ºC Thermal Only 450 ºC Thermal Only 500 ºC Thermal Only ºC Thermal Only 400 ºC UVTP Anneal Time (Minutes)! Significant bond breaking due to UV photons enables considerable hydrogen removal even at 400 C. P. 16

17 Comparison of UVTP to Thermal Only Process Film Shrinkage Evolution Densification with very little hydrogen removal Evolution of Thickness With Time For Thermal Only Anneals and UVTP Treatment 400 ºC UVTP Film Thickness Shrinkage (%) ºC Thermal Only 500 ºC Thermal Only 450 ºC Thermal Only 400 ºC Thermal Only Anneal Time (Minutes)! UV Cure results in a much higher film shrinkage than a thermal only process. P. 17

18 Comparison of UVTP to Thermal Only Process Stress Evolution Evolution of Film Stress With Time For Thermal Only Anneals and UVTP Treatment 550 ºC Thermal Only 500 ºC Thermal Only Tensile Stress (GPa) ºC UVTP 450 ºC Thermal Only ºC Thermal Only Anneal Time (Minutes)! Stress relaxation mechanism occurs with UVTP for extended cure times. P. 18

19 Correlation Between Hydrogen Removal and Stress Increase Stress Increase (GPa) 1.4 Stress Change as a function of hydrogen evolution High Temp > 600 C 1.2 < 600 C UVTP data Deviation from linearity % Hydrogen removed! Non-linear regime characteristic of stress relaxation. Behavior observed on UVTP and also high temperature anneals.! Stress reaches a maximum even though hydrogen is being liberated. P. 19