Tutorial on Chemical Mechanical Polishing (CMP)

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1 Tutorial on Chemical Mechanical Polishing (CMP) Ara Intel Corporation 1999 Arizona Board of Regents for The University of Arizona 1

2 Outline of the Tutorial Section A: Overview Generalized schematics of CMP and Post-CMP Clean Current CMP environment Evolution of CMP The CMP Module The CMP Infrastructure Section B: Polishing equipment trends Section C: Polishing process issues Section D: Consumables (pads & slurries) Quality issues Factors affecting productivity Critical pad and slurry parameters 2

3 Outline of the Tutorial Section E: Industry - University Gaps Section F: Environmental Health and Safety (EHS) considerations Section G: Slurry fluid dynamics Section H: Slurry re-use Section I: Post-CMP cleaning 3

4 Section A: Overview 4

5 Schematic Diagram of Chemical Mechanical Polishing Process Downforce Carrier Pad Conditioner Retaining Ring Slurry Pad Polish Platen 5

6 Schematic Diagram of Post-CMP Scrubbing PVA brush wafer Cleaning Fluid 6

7 CMP Environment CMP has become the widely accepted planarization method of choice for < 0.5 micron technologies The overall CMP market is growing at a rate of ~ 50% per year The current momentum in process integration and scaling far exceeds the fundamental understanding of complex interactions among: Equipment Consumables (i.e. slurry, pad, carrier film) Process parameters IC type and density Processes and consumables are formulated to provide optimum performance for a given equipment and IC product set For a 4 metal layer process with STI, ILD and W CMP steps, approximately 20 polishers are needed ( 60% utilization, 20 wafers per hour, 5000 wafer starts per week factory) 7

8 CMP Environment Protection of intellectual property hinders shared learning among IC, equipment and consumables manufacturers, but also provides a technological advantage: Internally developed equipment, precision parts and sub-systems Morimoto & Patterson, US Patent No. 5,104,828 (1992) Breivogel, Blanchard & Prince, US Patent No. 5,216,843 (1993) Breivogel, Louke, Oliver, Yau & Barns, US Patent No. 5,554,064 (1996) Internal slurry formulations licensed to suppliers for exclusive use Customized pads 3rd party modifications of off-the-shelf consumables and equipment 8

9 Evolution of CMP 9

10 Evolution of CMP 10

11 Total Cost Chemical Expenditure per fully Processed Product Wafer (Disposal and Treatments Costs are Included) a - Negotiate Price b - Insert competition c - Reduce disposal volume d - reclaim and re-use a - Negotiate Price b - Insert competition c - Increase pad life via better QC d - Increase pad life via better chemistry 11

12 The CMP Module Product and Test Wafers Filter Measure & Inspect Water Energy Polish In-Situ Measure Re-work Energy Solid Waste Slurry Clean Pad Liquid Waste Carrier Film Measure & Inspect Product and Test Wafers 12

13 Polishing: Rotary (single or multiple heads and platens) Orbital (single or multiple heads and platens) Linear (multiple heads) Cleaning: Mechanical scrubbing (with & without chemistry or megasonics) Wet cleaning (with and without megasonics) Measurement and inspection: Removal Rate Thickness uniformity (wafer-to-wafer, within-die, die-to-die) Defect density Dishing Erosion Plug recess Planarity Surface Roughness The CMP Infrastructure 13

14 The CMP Infrastructure In-situ Measurement: End-point detection Consumables: Pad (polyurethane, impregnated felt, fixed abrasive) Slurry (silica, alumina or ceria abrasives, organic and inorganic additives) Filter (point-of-use or post-slurry-blending) Conditioning (diamonds) Slurry delivery Water delivery Waste treatment: Off-site disposal Recycling Re-use 14

15 Section B: Polishing Equipment Trends, Morimoto and Cadien, CMP-MIC, Santa Clara, CA (1996) 15

16 Equipment Environment In high-volume manufacturing, the balance between high throughput, size and complexity needs to be maintained 16

17 Equipment Environment Development of automated dry-in-dry-out systems that: Improve throughput Reduce footprint Reduce total cost Reduce ergonomic issues Reduce number of people Polish 1 Polish 2 Polish Clean Robot I/O I/O Clean Ability to polish 300-mm wafers In-situ metrology for device wafers with closed-loop control 17

18 Section C: Polishing Process Issues, Morimoto and Cadien, CMP-MIC, Santa Clara, CA (1996) 18

19 Process Issues Within-Wafer Non-Uniformity (WIWNU): Wafer flatness Carrier film, pad & slurry type (discussed earlier) Carrier design Pad conditioning method Platen & carrier speeds Retaining ring design (i.e. extent of pressure discontinuity between wafer edge and retaining ring) Slurry injection scheme Removal rate: Carrier film, pad & slurry type (discussed earlier) Downforce Platen & carrier speeds Defect density: Pad & slurry type Use of secondary platen Post-CMP cleaning method 19

20 Planarity: Pad type Process Issues Circuit density & structure size Extent of ILD removed Downforce, platen speed & carrier speeds Polish Pre Post Planarization Distance (PD) Step Height Ratio (SHR) = Post Step Height / Pre Step Height The goal is to minimize SHR and maximize PD thereby minimizing Within-Die Non-Uniformity (WIDNU) 20

21 Effect of Structure Size & Density on Post Step Height SHR is greater on metal pads compared to isolated narrow lines Areas with lower circuit density polish faster than areas with dense underlying topography Each circuit design will have a different WIDNU due to variations in size and density of interconnects 21

22 Effect of Downforce on Removal Rate & Planarity Increase in downforce (wafer pressure applied to the polishing pad) results in a linear increase in removal rate (i.e. Preston s Equation) Increase in downforce degrades planarity due to pad deformation and subsequent increase in local pressure at the valley regions (i.e. Hook s Law) 22

23 Effect of Platen Speed on Removal Rate & Planarity Increase in platen speed increases removal rate linearly (i.e. Preston s Equation) Increase in platen speed improves planarity At higher speeds the pad contacts mainly the hill regions since it does not have sufficient time to conform to the valley regions 23

24 Effect of Carrier Speed on Wafer Center & Edge Removal Rates Edge Center Platen speed is maintained at 70 RPM Center-to-edge removal rate difference increases with increasing carrier speed Carrier diameter << platen diameter & at low carrier speeds, the linear velocity vector created by the carrier is much smaller than that created by the platen As carrier speeds approach & exceed platen speed, the linear velocity vector created by the carrier becomes significant 24

25 Effect of Pad Hardness on Post Step Height and Planarization Distance Soft Pad Hard Pad Harder pads deform less under pressure thus leading to: - Lower SHR, higher PD, and improved WIDNU (i.e in mm range) - Poorer WIWNU (i.e. in cm range) Harder pads also result in higher removal rates and higher defect densities 25

26 Effect of Pad Compressibility on Electrical Integrity of ILD Kaufman, Proceedings of Spring MRS, CA (1995) 26

27 Section D: CMP Consumables, Sanaulla, and Moinpour, Semicon West Technical Session on CMP, CA (1998) 27

28 CMP Slurries and Pads Areas of Concern Manufacturability Design Total Cost EHS Legal Supplier Availability Quality & Reliability 28

29 Quality Issues Intel Corporation All Chemicals 29

30 Quality Issues Intel Corporation CMP Slurries 70% Abrasive Issues 20% Foreign Matter 10% Other 30

31 Quality Issues Intel Corporation CMP Pads 40% Texture 30% Foreign Matter 20% Adhesive 10% Other 31

32 Impact of Quality Issues The Quality Indicator (QI) QI = (2) [(a) + (2) (b) + (4) (c) + (8) (d) + (16) (e)] e = No. of factory interrupts (i.e. issues resulting in tool or factory downtime, or product loss) d = No. of near misses (i.e. issues requiring extra Intel resources to keep the factory running) c = No. of repeat SCARs b = No. of SCARs (i.e. issues caused by gross supplier negligence) a = No. of issues (i.e. all issues regardless of impact to Intel) SCAR: Supplier Corrective Action Request Note: The Quality Indicator is measured on a quarterly basis for each supplier 32

33 Supplier Comparison CMP Suppliers vs. Photoresist and Wet Chemical Suppliers (Data Collected Since 1Q96) Challenge 33

34 Factors Influencing Productivity Productivity Process Stability & Manufacturability - RR - WIWNU, WTWNU, WIDNU - Defects - Planarity - Pad life - Pad & slurry quality Equipment - Availability - Reliability - Integrated Run Rate Labor - EHS - Ergonomics - Automation 34

35 Tool Integration and Automation Integrated Run Rate R2 CMP#1 Robot Limited Wafers Robot Robot Cleaner Wafers R1 R1 R4 CMP#2 R3 CMP#1 R2 Cleaner Limited Wafers Robot CMP#2 Robot Cleaner Wafers R1 R3 R1 R5 CMP#3 R4 35

36 Polishing Pad Life Frequency of Changing Pads as a Function of Pad Life Changing pads in high-volume manufacturing poses a serious ergonomic issue: Frequency of change Difficulty of change A compromise must be reached between adhesive strength and its effect on the polishing process: Hardness Compressibility Corrosion resistance Use of chemicals to remove adhesive residues Mechanical pad-pullers are becoming a requirement in factories 36

37 Polishing Pad Life Effect of Pad Life on Tool Availability Availability (%) = Scheduled Downtime - Unscheduled Downtime Scheduled Downtime: Tool PM, facilities PM, monitors, tool qualification and consumables changeout Unscheduled Downtime: Out-of-control conditions, repairs 37

38 5000 WSPW 5 oxide polish steps No. of Polishers vs. Tool Availability Effect of Pad Change Duration (Pad Life & Scheduled and Unscheduled Downtime are Fixed) Pad life of 500 (i.e. number of wafers polished before pad change) Pad change duration: Complexity of process qualification on fresh pad (i.e. pad break-in) Other consumable changes (i.e. wafer carrier & pad conditioner) Ergonomics of pad change (i.e. pad size and adhesive strength) 38

39 No. of Polishers vs. Tool Availability Effect of Un-Scheduled Downtime (Pad Life, Pad Change Duration and Scheduled Downtime are Fixed) 39

40 Oxide Polisher Downtime Pareto Chart C+P C = Consumables P = Process T = Tool C+P+T C+P+T C+P 40

41 Oxide Polisher Downtime Pareto Chart average pad life average POU filter life variability in pad and slurry properties (PSD) average filter life variability in slurry properties (PSD) 41

42 Effect of ph and Abrasive Content on ILD Removal Rate Scherber et al., Proceedings of the Symposium on Planarization Technology: CMP, Semicon West (1994) 42

43 Effect of Trace Metals on ILD Polish Performance - All units in ppm - Slurries F & G are identical except for the metal content - Comparable removal rate and uniformity 43

content - Hydrocarbon contained a polar group - Comparable

44 Effect of Hydrocarbons on ILD Polish Performance - Slurries H & I are identical except for the hydrocarbon content - Hydrocarbon contained a polar group - Comparable removal rate and uniformity - Majority of defects were scratches 44

45 Abrasive Geometry Aggregate Primary Particle 45

46 Effect of Abrasive Geometry on ILD Polish Performance - Fumed silica abrasive - Constant ph and abrasive content - Comparable defect density and planarity 46

47 Effect of Abrasive Geometry on ILD Removal Rate 47

48 Section E: Industry - University Gaps 48

49 Development of Core Competencies (Industry - University Gaps) 49

50 Development of Core Competencies (Industry - University Gaps) 50

51 Section F: EHS Hierarchy and Considerations, Moinpour and Poliak, Proceedings of VMIC, Santa Clara, CA (1998) 51

52 EHS Hierarchy & Issues Replace > Reduce > Re-use > Recycle > Abate Environmental regulations are growing at an amazing rate: Federal and local initiatives & regulations International initiatives Recycling regulations are extremely complex and require detailed understanding and follow-through Many new materials are not designed with EHS in mind. In many cases, suppliers do not even know the potential EHS impact of these materials To find out late in the process that a material has a serious EHS impact can delay technology introduction or increase cost Most chemical suppliers have committed to ownership from cradleto-grave, but follow-through is poor 52

53 Growth of US Environmental Legislation (Cumulative No. of Environmental Laws) Technology & Environment, Washington DC, National Academy Press, p. 101 (1989) CERFA OPA PPA CAAA EPA GCRA GCPA SPA EPCRA WQA HSWA SARA APA NWPA UORA CERCLA SWDAA EAWA NCPA CWA RCRA SWDA HMTA TSCA CZMA SDWA ODA EQIA CAA MVAPCA NPAA AQA NEPA NESA WA IA WL FMLA FWPCA WRA FWCA WA RHAA FIFRA RA PHSA FCA TGA RHA Year 53

54 EHS in CMP (Level - I Considerations) chemical inputs energy outputs ergonomics EHS energy inputs chemical outputs 54

55 EHS in CMP (Level - II Considerations) chemical blending & delivery system publicly owned treatment works post-polish tool slurry type energy outputs film type chemical inputs pad type post-polish consumable IC type energy inputs IC density ergonomics EHS UPW system wafer size chemical outputs process recipe polish tool wafer starts per week fab location in-fab discharge treatment method 55

56 ph abrasive type abrasive size abrasive shape abr. morphology solids content oxidizer type additive type buffer type base type acid type zeta potential ionic strength viscosity color shelf life pot life dispersability EHS in CMP (Level - III Considerations) film type IC type IC density slurry type pad type UPW system chemical blending & delivery system wafer size polish tool post-polish tool post-polish consumable process recipe wafer starts per week fab location in-fab discharge treatment method publicly owned treatment works chemical outputs chemical inputs energy outputs energy inputs ergonomics 56

57 size material stack thickness texture morphology hardness specific gravity compressibility hole pattern groove pattern adhesive strength life shelf life EHS in CMP (Level - III Considerations) film type IC type IC density slurry type pad type UPW system chemical blending & delivery system wafer size polish tool post-polish tool post-polish consumable process recipe wafer starts per week fab location in-fab discharge treatment method publicly owned treatment works chemical outputs chemical inputs energy outputs energy inputs ergonomics 57

58 automation footprint conditioner endpoint detection water inj. scheme slurry inj. scheme effluent segregation POU filtration flow dynamics re-use compatibility carrier design platen design ring design number of platens rotation scheme vent design parts clean req. PPE req. ease of maint. run rate EHS in CMP (Level - III Considerations) film type IC type IC density slurry type pad type UPW system chemical blending & delivery system post-polish tool polish tool wafer size post-polish consumable process recipe wafer starts per week fab location in-fab discharge treatment method publicly owned treatment works chemical chemical outputs inputs energy outputs energy inputs ergonomics 58

59 water flow rate slurry flow rate chemical flow rate dilution flow overlap automation carrier speed platen speed down-force back-pressure number of platens conditioning recipe EHS in CMP (Level - III Considerations) film type IC type IC density slurry type pad type UPW system chemical blending & delivery system wafer size polish tool post-polish consumable post-polish tool process recipe fab location wafer starts per week in-fab discharge treatment method publicly owned treatment works chemical outputs chemical inputs energy outputs energy inputs ergonomics 59

60 Section G: CMP Fluid Dynamics Coppeta, Roger, Racz, Kaufman &, Pad effects on slurry transport beneath a wafer during polishing, CMP-MIC, Santa Clara (1998) 60

61 Goal: Fluid Dynamics Reduce slurry dispense volume Increase slurry utilization efficiency Entrain a uniform layer of new slurry beneath the wafer Prevent polished material from being re-entrained beneath the wafer Key issues which need to be comprehended: Chemical & mechanical factors which influence polishing Slurry film thickness between wafer and the pad Slurry transport mechanism, and factors that influence slurry transport Slurry injection scheme Slurry flow rate Pad type, conditioning and topography Platen and carrier speed 61

62 Dual-Emission Laser-Induced Fluorescence Camera Laser Glass Wafer Polish Platen Pad Slurry with Fluorescence dye Slurry 62

63 63

64 Slurry Transport Wafer Post Interrogation Region Examining: - Mean slurry age - Residence time - Slurry Gradients (flat pads) - Drag on wafer - Fluid thickness measurements Pad 64

65 Slurry Flow Rate Percent New Slurry Flat Pad Grooved Pad Manufacturer: Rodel Slurry Flow Rate: x cc/min Wafer Down Force: 4 psi Platen Speed: 60 rpm X-Y Groove Depth: 20 mils Time (sec) 65

66 Platen Speed Percent New Slurry Flat Pad Grooved Pad Manufacturer: Freudenberg Slurry Flow Rate: 35 cc/min Wafer Down Force: 4 psi Platen Speed: x rpm X-Y Groove Depth: 20 mils Time (sec) 66

67 Static Case Pad deformation: (4 psi, 0 rpm) Image of a single pad Thickness profile as determined by ratiometric technique 67

68 Section H: Slurry Reuse Kodama, A reclaim use of CMP slurry, 29th Symposium on ULSI Ultra Clean Technology, Tokyo, Japan (1996) 68

69 Slurry Re-Use Experimental Setup Secondary Platen Primary Platen Pump & Filter Slurry Capture Tub Spent Slurry Reservoir 69

70 RR & WIWNU vs. Slurry Reclaim fumed 50 / 200 nm colloidal 102 / 212 nm 70

71 Surface Roughness & ph vs. Slurry Reclaim fumed 50 / 200 nm colloidal 102 / 212 nm 71

72 Mean Aggregate Size vs. Slurry Reclaim fumed 50 / 200 nm colloidal 102 / 212 nm 72

73 Section I: Post-CMP Cleaning Moinpour & Burke, Keynote Address, CMP-MIC, Santa Clara (1998) Jankovsky, 3rd CMP Workshop, Lake Placid, NY (1998) Busnaina, 3rd CMP Workshop, Lake Placid, NY (1998) 73

74 Post-CMP Clean Defects & Contamination: Abrasive particle residues (i.e. silica, alumina or ceria) Chemicals on surface (i.e. surfactants, or slurry additives) Alkali metal contaminants (i.e. K or Na) Heavy metals (i.e. Fe) Pad residues Pad conditioner (i.e. diamond) residues Requirements: Quick and repeatable Cause do damage to devices or films (i.e. change roughness or planarity) No residue or redeposition Low cost of ownership (COO) Environment: Mechanical scrubbing (with & without chemistry or megasonics) Wet cleaning (with and without megasonics) 74

75 Step - I Reduce defects during the CMP process: Use slurry additives Step - II Reduce defects further by performing an additional buffing process: Use chemicals on the secondary platen Step - III Reduce defects even further during the post- CMP cleaning process: Post-CMP Clean (Defect Reduction Strategies) Use chemicals in the post- CMP cleaning tool 75

76 Post-CMP Clean (A Sampling of Chemicals or Methods Cited in the Literature) 76

77 Post-CMP Clean (Process Improvement) 0.35 um, 200mm technology Effect of post ILD CMP clean chemistry on end-of-line yield Process 1 and Process 2 are identical polish processes Process 2 uses a different Post-CMP Clean chemistry Improved consumable lifetime No impact on overall run rate 77

78 Particles in liquids: Cleaning Theory Primary cause of adhesion is van der Walls forces (DLVO Theory) Secondary cause of adhesion is Electric Double Layer (EDL) forces (however, they are usually repulsive and can help in particle removal) Particles in solution become charged Stern Layer + Diffuse Layer = EDL Potential at shear plane = Zeta Potential EDL thickness varies as inverse square root of the ionic strength (i.e. 4X increase in ionic strength will reduce EDL thickness by 2X) EDL and the Zeta Potential are a function of ph 78

79 Cleaning Theory ELECTRIC DOUBLE LAYER 79

80 Cleaning Theory DLVO THEORY 80

81 Post-CMP Cleaning 81

82 Brush Cleaning Advantages: Most common cleaning methodology Double-side and edge cleaning capability High energy scrub capability The contact mechanism can help clean wafers with topography Simple integration with dryin-dry-out processing Compatible with wet chemistry Compatible with the recent advances in smartbrushes (zeta-potential engineering) Disadvantages: Contact with wafers may be harmful Brush loading with particle and re-deposition Low throughput High COO (chemicals, DI water, consumables parts) Static build-up which may increase particle adhesion forces Tough for brushes to contact high aspect ratio topography Brush shedding Brush break-in required 82

83 Wet Chemical Cleaning Advantages: More chemically intensive compared to brush cleaning Residues and foreign matter can be readily dissolved and removed from the surface Ability to manipulate zeta potential to remove particles Low COO High throughput Controlled cavitation (formation of gas bubbles by ultrasound) and acoustic streaming (steady flow induced by sound field) can be used to detach and remove particles from the surface Formation of acoustic boundary layer Disadvantages: Particle saturation in the recirculating tank Difficult to integrate with dry-in-dry-out processing Cleaning process must be tailored to each device layer and material Uncontrolled cavitation may cause wafer surface damage 83

CMP Pump Effects on Filter Life

CMP Pump Effects on Filter Life Rakesh K. Singh, Ph.D., P.E. Mykrolis Corporation Mykrolis Corporation, Rakesh K. Singh 1 Acknowledgments Slurry manufacturers for providing CMP slurry and abrasive dispersion