NANO AND MICROSCALE PARTICLE REMOVAL

Transcription

1 NANO AND MICROSCALE PARTICLE REMOVAL Ahmed A. Busnaina William Lincoln Smith Professor and Director of the oratory Northeastern University, Boston, MA

2 OUTLINE Goals and Objectives Approach Preliminary Results Acoustic Streaming and Boundary Layer Particle Removal Mechanism Double layer Effect of Particle Size and Flow Frequency Key Preliminary Research Results

3 Surface Preparation Technology Requirements -- Long Term ** YEAR TECHNOLOGY NODE 18nm 13nm 1nm 7nm 5nm 35nm FEOL Particle Size(nm FEOL Particles (#/cm BEOL Particle Size(nm BEOL Particles (#/cm Surface Roughness (nm Critical surface metals(* Organics ( *1 13 atoms/cm ** ---- The International Technology Roadmap for Semiconductors, 2

4 Goals and Objectives Develop an effective nanoscale particle removal technique using acoustic streaming. Provide a fundamental understanding of the removal mechanism that will be experimentally verified. Experimentally measure particle removal of particles in the size range of 1-1 nm from semiconductor wafers. Evaluate effect of streaming flow frequency, velocity amplitude and particle size and particle/substrate composition on the removal efficiency experimentally and numerically.

5 Approach Fundamental Approach Experimental and modeling approach to determine, understand and predict: Particle Removal Mechanism Cleaning Efficiency, F{R, V, F ad, etc. Cleaning tank Geometry (single, batch, etc. Optimum cleaning conditions Cleaning technology limits with shrinking particle and defect size

6 Approach Fundamental Approach Key Particle Removal Parameters flow frequency velocity (pressure amplitude Particle size Particle composition Particle shape Particle deformation and contact area Double layer effect on removal Cleaning liquid surface tension Surface and particle surface energy (hydrophilic or hydrophobic

7 MEGASONIC CLEANING MEGASONIC CLEANING sin(, ( sin(, ( F w F w + - = + - = = t kx u t x u t kx p t x p t p c 1 p c 2 p I 2 r = Megasonic Cleaning Mechanism Megasonic sound wave: Megasonic power intensity:

8 ACOUSTIC STREAMING v(cm/s f = 76 khz I =.77W/cm 2 I = 1.55W/cm 2 I = 2.33W/cm 2 I = 3.1W/cm 2 I = 3.88W/cm 2 I = 4.65W/cm 2 I = 5.43W/cm 2 I = 6.2W/cm 2 I = 6.97W/cm 2 I = 7.75W/cm 2 v(cm/s Streaming Velocity vs. Acoustic Power 1 M Hz 85k Hz 76k Hz 36k Hz Distance From Tank Wall (cm Intensity (W/cm 2

9 PARTICLE REMOVAL Nano-scale particles will be a challenge to current cleaning techniques. The most widely used cleaning techniques: non-contact method (megasonic cleaning contact cleaning method (brush scrubbers The two basic elements that need to be understood are : Particle Adhesion Particle Removal

10 Boundary Layer Thickness u u Free stream d(x y d τ τ Velocity boundary layer Acoustic boundary layer thickness: x Velocity boundary layer on a flat plate 1 2ν 2 in water, f=85khz, δ ac =.61µm δ ac = ω f=76khz, δ ac =.65µm f=36khz, δ ac =.94µm The hydrodynamic boundary layer thickness: ν δ H =. 16 Ux 1 7 x in water, u=4m/s, at center of the wafer, δ H =257 µm

11 Velocity Profile in a Boundary Layer y = ~25 micron y = ~1 micron Velocity Profile x = 4 inch, U = 4m/s Velocity Profile x = 4 inch, U = 4m/s 25 Laminar flow Turbulent flow Acoustic Flow ( f = 8kHz 1 9 Laminar flow Turbulent flow Acoustic Flow ( f = 8kHz y (micron 15 1 y (micron Velocity (m/s Velocity (m/s

12 Drag Force Distribution on a Particle F D = C D ρ 2 i u 2 A i f = 8 khz, I = 7.75W/cm 2, U ac = 4.8 m/s Acoutic boundary layer thickness =.63 micron u(y 1 micron u(y (m/s Drag Force (.5 micron particle Drag Force ( 1 micron particle A i.5 micron

13 Effects of Frequency Acoustic Flow Properties I = 7.75 W/cm 2 Boundary layer thickness (micron Acoustic, f=36khz Acoustic, f=76khz Acoustic, f=85khz Boundary layer thickness (micron Streaming Velocity (m/s Drag Force ( 1um particle Drag Force (.1um particle Frequency (k Hz Streaming Velocity (m/s Drag Force (N

14 Ratio of Removal/Adhesion Moment (RM RM: U RM RM = = Removal moment Adhesion resisting moment F d ( R δ F a a + F a When RM >1, most particles are removed. dl 1.399R δ M R F elec. double layer a O M A F Adhesion F drag Rolling removal mechanism

15 Removal Percentage vs. Moment Ratio (Silica Removal Experiment Removal Percentage Moment Ratio Removal Percentage Moment Ratio

16 Effect of Particle Size on Adhesion and Removal Forces Forces vs. Particle Diameter U = 4 m/s U F electrical double layer Drag Force (Acoustic Flow, 8kHz, 7.75W/cm 2, U ac =4m/s, d ac =.63 um Drag Force ( Hydrodynamic Flow, U = 4 m/s, d h = 275 um Double layer force Van der Waals Force, PSL/SiO 2 Van der Waals Force, SiO 2 /SiO 2 F drag Force (N Particle Diameter (micron F Adhesion Drag force, electrical double layer force, and adhesion force all increase with particle size. Acting as removal forces, d>1nm, acoustic flow drag force is dominated; 3nm<d<1nm, drag force and electrical double layer force are on same level; d<3nm, electrical double layer force is dominated;

17 Electrical Double Layer Force Zeta Potential (mv ph Tantalum pentoxide Si3N4 Silica Alumina PVA PSL (99nm Tungsten At the ph of water, silica, PSL, PVA, and W particles are all negatively charged. The high negative zeta potentials are measured at high ph solution for SiO 2, Si 3 N 4, Al 2 O 3, tantalum pentoxide, tungsten, polyvinyl alcohol (PVA, and also for Si and PSL. Using a high ph cleaning solution, electrical double layer force occurs as a strong repulsion between the particle and the substrate.

18 Removal/Adhesion Moment Ratio (RM Removal Moment Ratio (RM vs. Particle Diameter DI water, electrical double layer force is negligible U = 1.8 m/s (Acoustic Flow, 13kHz, 7.75W/cm Removal Moment RM = Adhesion Resisting Moment Removal Moment Ratio (RM vs. Particle Diameter SC-1, electrical double layer force is repulse U = 1.8 m/s (Acoustic Flow, 1.3 MHz, 7.75W/cm Removal Moment RM = Adhesion Resisting Moment RM > 1 Removal > Adhesion Particle will be removed RM > 1 Removal > Adhesion Particle will be removed RM SiO 2 particle on SiO 2, Acoustic ( 1.3 MHz PSL particle on SiO 2, Acoustic ( 1.3 MHz SiO 2 particle on SiO 2, Hydrodynnamic PSL particle on SiO 2, Hydrodynnamic Particle Diameter (micron RM = 1 RM < 1 Removal < Adhesion Particle can not be removed RM SiO 2 particle on SiO 2, Acoustic(1.3MHz+SC1 PSL particle on SiO 2, Acoustic (1.3MHz+SC1 SiO 2 particle on SiO 2, Hydrodynnamic+SC1 PSL particle on SiO 2, Hydrodynnamic+SC Particle Diameter (micron RM = 1 RM < 1 Removal < Adhesion Particle can not be removed Using DI water only, the removal of nano-size particles (1-1 nm can be best accomplished using acoustic streaming at frequencies larger than 1.3 MHz.

19 Effects of Frequency on RM DI water, Electrical double layer force is negligible SC-1, Electrical double layer force is repulsive force Effect of Frequency on RM I = 7.75 W/cm 2 Effect of Frequency on RM I = 7.75 W/cm 2 DI water, Electrical double layerforce isnegligible SC-1, Electrical double layer force is repulse RM no F el (1um SiO 2 /SiO 2 RM no F el (.1um SiO 2 /SiO 2 RM no F el (1um PSL/SiO 2 RM no F el (.1um PSL/SiO 2 RM > 1 Removal > Adhesion Particle will be removed RM (1um SiO 2 /SiO 2 RM (.1um SiO 2 /SiO 2 RM (1um PSL/SiO 2 RM (.1um PSL/SiO 2 RM > 1 Removal > Adhesion Particle will be removed RM 1 RM = 1 RM 1 RM = RM < 1 Removal < Adhesion 1-2 RM < 1 Removal < Adhesion 1-4 Particle can not be removed 1-4 Particle can not be removed Frequency (k Hz Frequency (k Hz The smaller the particles, the higher frequency acoustic flow is needed. Soft particles (PSL are more difficult to remove than hard particle (silica, needing almost an order of magnitude higher frequency.

20 Fast Single Wafer Post-CMP Cleaning Single versus Batch The Removal Efficiency of AL2O3 particles 12 1 Particles >.2 micron Before Dep After Dep After Cleaning 1 min Batch 1 min Batch 2 min Batch 2 min Batch 1 min Single 1 min Single

21 Complete removal of silica particles down to 1nm is achievable by using a single wafer megasonic cleaning with DI water only S in g le M e ga so n ic C lea n in g P rocess, T em p = 3 5 o C R em oval E fficien cy o f S ilica P articles.1 µ m Time (sec P o w e r (W a tts.9 8

22 Megasonic Cleaning of Polished TOX Wafers Using SC Defects # W 54 W min 1 min 5 min 1 min 3 o C 6 o C

23 Key Results from Preliminary Research Megasonics induced acoustic streaming is essential to the removal of submicron and nano-size particles. As the frequency increases, the acoustic boundary layer thickness decreases and streaming velocity increases thereby increasing the removal (drag force. Using DI water, the removal of nano-size particles (1-1 nm can be best accomplished using acoustic streaming at frequencies larger than 1.3 MHz. Utilizing the electrical double layer force as a repulse force, by using basic chemistry, removal of 1nm silica particle can be accomplished using megasonic cleaning above 8 khz. Acting as removal forces, d>1nm, acoustic flow drag force is dominated; 3nm<d<1nm, drag force and electrical double layer force are on same level; d<3nm, electrical double layer force is dominated; Soft particles (such as Polystyrene Latex PSL are more difficult to remove than hard particle (silica, because of adhesion induced deformation, needing almost an order of magnitude higher frequency.