Civil and Mechanical Systems (CMS) NATIONAL SCIENCE FOUNDATION. Lecture 20. Surface Engineering & Materials Design Yip-Wah Chung*, Program Director

Transcription

1 Civil and Mechanical Systems (CMS) NATIONAL SCIENCE FOUNDATION Surface Engineering & Materials Design Yip-Wah Chung*, Program Director (703) Lecture 20 *on leave from MSE, Northwestern University 1

2 Previously Funded Research Projects Related to Data Storage Head-disk interface air bearing design Development of protective overcoats Surface roughness characterization AFM nanotribology studies Boundary lubrication and adhesion Disk architecture Ferroelectrics MEMS/NEMS 2

3 Acknowledgments Funding NSF INSIC People Dong Li - Motorola Mei-Ling Wu (Seagate) Dejun Li- Tianjin Normal Univ Murat Guruz - Hitachi Yanfeng Chen Christina Freyman 3

4 Architecture of a hard disk drive slider lubricant overcoat magnetic media buffer layer Al/Mg or glass substrate 4

5 Typical Hard Disk Drive 5

6 Head-Disk Interface k rpm 6

7 Principle of Magnetic Recording 7

8 Typical Magnetic Recording Slider 8

9 9

10 Areal density versus head-disk disk spacing Areal Density (MBits/in 2 ) 100,000 10,000 1, mid nm, 40 Gb/in 2 ~ nm, 0.1 Gb/in ,000 10,000 Head-Disk Magnetic Spacing (nm) 10

11 Flying Height and Overcoat Thickness vs. Time Flying Height (nm) Overcoat Thickness (nm)

12 Requirements for the overcoat excellent tribological performance ultrasmooth low defect/pinhole density (corrosion) lube compatibility manufacturing compatibility low growth temperature (< 250 o C) Scalable CHx was the material of choice, but slowly ran out of performance in the early 90s. 12

13 Spacing budget for 10 Gb/in 2 head air bearing nm disk pole-tip recession ~ 2-4 nm overcoat 5 nm lube 1 nm overcoat 5 nm 25 nm magnetic spacing 5 nm disk overcoat 13

14 Hypothetical β-c 3 N 4 Scaling studies by Liu and Cohen showed that the bulk modulus B of covalent solids is given by: Nc B = 4 d 3.5 B = bulk modulus (Mbar) i = ionicity d = bond length (A) N C = average coordination number 2.2i 14

15 Bulk modulus vs d 15

16 Bulk Modulus: comparison between theory and experiment Diamond Si β-si 3 N 4 c-bn β-c 3 N 4 Theory Experiment ? 16

17 Standard sputtering setup pulse generator Residual gas analyzer C target DC power supply sputter gas Substrate holder pulse generator DC power supply 17

18 CNx Hardness vs Substrate Bias Hardness (GPa) Pulsed DC Bias (-V) 18

19 CNx Hardness vs Total Pressure Hardness (GPa) Pressure (mtorr) 19

20 30 CNx Hardness vs P(N) 25 Hardness (GPa) N 2 Partial Pressure (mtorr) 20

21 CNx Hardness vs G-band Position 21

22 Nanoindentation on CN X /Si(100) Zero Bias Load (mn) Displacement (nm) 22

23 Nanoindentation on CN X /Si(100) -50V Bias Load (mn) Displacement (nm) 23

24 5 mtorr 0 V bias 250 µn nm µn µn mtorr -200 V bias 250 µn nm µn 50 µn

25 Pin-on-Disk Testing Disk Pin Load 25

26 CNx Coatings (friction vs time) 26

27 Wear performance of CN x 27

28 Amorphous Carbon vs. Carbon Nitride (thickness = 5 nm) 28

29 Spacing budget for 100 Gb/in 2 head air bearing 5 nm disk pole-tip recession ~ 1 nm overcoat 1 nm lube 1 nm overcoat 2 nm 10 nm magnetic spacing 2 nm disk overcoat 29

30 Pulsed DC Magnetron Sputtering Ar + N 2 pulse generator target DC power supply Key Variables pressure/composition pulsed bias (target/substrate) substrate pulse generator DC power supply + 20V -200 V msec 30

31 Advantages of Pulsed DC Power suppressed charging or arcing increased ionization fraction improved ion bombardment of the growing film 31

32 Effects of Pulsed Power From Ashida, Shim and Lieberman, JVST A 14(2) (1996) 32

33 Enhancing Mobility of Surface Species During Growth Ions in the plasma can be extracted to bombard the film surface during deposition low energy ions no effect intermediate energy ions higher mobility of surface species and hence better film quality (denser film/fewer defects) too high energy can damage the surface Strategy 1. low pressure (increase mfp) 2. High ion concentration (decrease dark space width) 33

34 Ion Bombardment of the Substrate Target plasma J Child s Law V M 3 1/ 2 1 d 2 Substrate (-V) potential -V d d = 2 mm mfp = 15 mm mean free path (mfp) >d (dark space width) approx. monoenergetic ions 34

35 MD Simulation 0ps The impact of energetic ions on a surface collapses protruding areas and shrinks trapped void volume. Shown are results of a molecular dynamics simulation of an ion impact on a surface. 0.4 ps 13 ps 35 Figure from Karl-Heinz M 黮 ler, Surf. Sci. Lett. (1987)

36 CNx Surface Roughness vs Substrate Bias 2 r.m.s. roughness (nm) Substrate Bias Voltage (-V) 36

37 CNx Corrosion Data Corrosion Pit Count V s =0 V s =-200V (substrate pulsed 0.2 khz) V s =-200 V (20 khz target, 2 khz substrate) Coating Thickness (nm) 37

38 Desorption Energies Ethers / a-cn x Desorption Energy (E d 0 ) (kj/mole) a-cn x CH 3 CH 2 CH 2 CH 3 δ+ O CF 3 CF 2 CF 2 CF 3 δ+ O 0 (CH 3 CH 2 ) 2 O (CF 3 CF 2 ) 2 O CH 3 OCH 2 OCH 3 CF 3 OCF 2 OCF 3 -CH 2 OCH 2 OCH 2 - -CF 2 OCF 2 OCF 2 - newfigures papers a-cn Ethers & Alcohols fig6 a-cn x Desorption energies of the fluorinated ethers are always lower than those of the hydrocarbon ethers. Desorption energies suggest that the ether linkages in Fomblin interact with the surface through electron donation from the oxygen lone pair. 38

39 Desorption Energies Alcohols / a-cn x 80 Desorption Energy (E d 0 ) (kj/mole) a-cn x - CH 3 CH 2 -O δ- H + CF 3 CH 2 -O δ- H 0 CH 3 CH 2 OH CF 3 CH 2 OH CH 3 CH(OH)CH 3 CH 3 CH(OH)CF 3 CF 3 CH(OH)CF 3 (CH 3 ) 3 COH (CF 3 ) 3 COH newfigures papers a-cn Ethers & Alcohols fig7 a-cn x Desorption energies of the fluorinated alcohols are always higher than those of the hydrocarbon alcohols. Desorption energies suggest that the hydroxyl endgroups of Fomblin Zdol interact with the surface through hydrogen bonding. 39

40 TPD - CF 3 CH 2 OH / 20% a-cn x E des vs. Coverage - CF 3 CH 2 OH / a-cx 60 Desorption Rate (a.u.) Exposure: T p 0.39L 156K 0.33L 155K 0.25L 155K 0.21L 157K 0.15L 167K 0.12L 170K 0.09L 174K 0.06L 180K 0.05L 185K 0.02L 197K 0.01L 206K 0.007L 211K 0.004L 216K 0.003L 224K E des (kj/mole) % a-ch x 10% a-ch x 15% a-ch x 20% a-ch x 10% a-cn x 20% a-cn x CAC 808 CAC 810 CAC 811 Ion beam Temperature (K) Coverage (ML) newfigures papers ach acn Ethers Alcohols fig 3 newfigures papers ach acn Ethers Alcohols fig 1 TPD spectra have been used to determine the desorption energies of fluorinated alcohols on a number of different types of carbon films and over a range of temperatures. 40

41 TPD - (CF 3 CF 2 ) 2 O / 20% a-cn x E des vs. Coverage - (CF 3 CF 2 ) 2 O / a-cx 48 Desorption Rate (a.u.) Exposure: T p 0.66L 114K 0.6L 114K 0.5L 115K 0.4L 116K 0.3L 120K 0.2L 124K 0.1L 131K 0.08L 133K 0.06L 136K 0.04L 140K 0.02L 145K 0.01L 150K 0.004L 160K 0.002L 163K L 178K E des (kj/mole) % a-ch x 10% a-ch x 15% a-ch x 20% a-ch x 10% a-cn x 20% a-cn x CAC 808 CAC 810 CAC 811 Ion Beam Temperature (K) Coverage (ML) newfigures papers ach acn Ethers Alcohols fig 4 newfigures papers ach an Ethers Alcohols fig 5 TPD spectra have been used to determine the desorption energies of fluorinated ethers on a number of different types of carbon films and over a range of temperatures. 41

42 1 Terabit/in 2 : the Next (Final?) Frontier magnetic spacing = 6.5 nm (5.0 nm) head air bearing 3.5 nm* disk overcoat 1.0 nm lube 1.0 nm overcoat 1.0 nm * analogous to flying a B747 at an altitude of 0.14 mm 42

43 1 Terabit/in 2 : Show-Stoppers Stoppers Mechanics and tribology Can we fly at 3.5 nm or less? Tracking (bit size equiv. 25 nm x 25 nm) Continuous overcoat? Media Noise Corrosion Signal processing and error correction. 43

44 Poisson statistics One nm ~ 4 atomic layers If deposition follows Poisson distribution, RMS surface roughness = (average number of layers) = 2 monolayers ~ 0.5 nm Probability of having an open site (pinhole) = e -4 ~ 1.8%! Must deviate from statistics 44

45 Solutions Need energy to remove topographical defects such as pinholes and hillocks Thermal not feasible due to manufacturing limitations Athermal particle bombardment 45

46 Solutions (cont.) Deposition methods that provide particle bombardment: Ion beam deposition cathodic arc decomposition of hydrocarbon gases Sputtering 1 to 5% ions 95 to 99% neutrals 46

47 Oblique angle of incidence Ions Neutrals Oblique angle of incidence of neutrals is more effective in transferring momentum to surface species 47

48 Momentum transfer process Ar + or N + from plasma substrate tilt deposited atoms target N S N reflected or sputtered neutrals 48

49 Particle reflection Argon sputtering of germanium Argon Reflection Probability ev ions 600 ev ions Reflected Particle Energy After calculations by M.A. Ray and J.E. Greene, unpublished ev ions 49

50 Modified sputtering setup Substrate tilted at 45 o, rotatable about the surface normal 50

51 Hardness (GPa) Hardness versus substrate bias Thickness: 110 nm 45 tilt, 20 rpm 200 W Substrate bias (V) 51

52 Roughness at the µm scale versus substrate bias Roughness (nm) Thickness: 50 nm scan size: 10 um 5 um 1 um Substrate bias (V) 45 tilt, 20 rpm, 200 W 52

53 Roughness at the µm scale versus tilt and rotation Roughness (nm) no rotation no rotation 0 0 -rotation rotation Scan size ( µ m) Thickness 50 nm -100 V bias, 200 W 53

54 Roughness at the µm scale versus rotation speed µ m Estimated 50 nm 45 tilt, 200 W -100 V bias Roughness (nm) µ m 10 µ m 5 µ m 1 µ m Substrate rotation speed (rpm) 54

55 Roughness at the µm scale versus power Roughness (nm) W 200 W 300 W Thickness 50 nm 20 rpm, -100V bias Scan size ( µ m) 55

56 Roughness at the µm scale versus P He Roughness (nm) mtorr No He mtorr mtorr 0.06 mtorr Thickness 50 nm 25 rpm, -100V bias, 200 W power Scan size ( µ m) 56

57 Why helium? Dissociation energy of N 2 = 9.8 ev Ionization energy of N 2 = 15.6 ev Ionization energy of N = 14.5 ev Excitation energy of He = 19.6 ev 57

58 Optimum tilt angle Thickness: nm mtorr He 200W, -100V, 25 rpm Roughness (nm) µ m 15 µ m 10 µ m 5 µ m 1 µ m Substrate tilt angle (degree) 58

59 45 tilt with rotation 0 tilt with no rotation 59

60 Ar + or N + from plasma Rotating Magnetron -V s reflected or sputtered neutrals N N N N S N N N 60

61 CNx coating metrology issues Coating thickness Coating density Surface roughness in the mm-cm scale 61

62 Coating thickness Current method: calibrate deposition rate by measuring coating thickness after a known deposition time. Assume deposition rate to be constant. Alternative methods: x-ray photoemission ellipsometry low-angle x-ray reflectivity 62

63 Low-angle x-ray x reflectivity Incident x-rays θ θ Substrate Diffracted x- rays t Thin film Condition for constructive interference: 2 t sin θ = n λ 63

64 Example of x-ray x reflectivity: 50 nm thick CNx Estimated 50 nm Thickness from XRR = 48.5 nm Reflectivity Measured values Calculated values k ( A o -1 ) z,0 = 2π sin θ/λ 64

65 X-ray reflectivity R F ( k z,0 ) = r 2 0, r r 2 1, ,1r1,2 + 2r 0,1 + 2r 0,1 r 1,2 r 1,2 cos(2k cos(2k z,1 z,1 d ) d ) r-- reflection coefficients K-- z component of wave vector d-- film thickness R( k z 2, 0) = R F ( k z,0)exp( 4k z σ σ --surface roughness 2 ) 65

66 Comparison of coating thickness measurement results 12 Measured thickness by XRR (nm) Estimated thickness by growth rate (nm) 66

67 12.0 nm CNx 5 min delay between sputter- cleaning and deposition Calculated thickness = 12.7 nm

68 12.0 nm CNx 1 min delay between sputter- cleaning and deposition Calculated thickness = 12.1 nm

69 Coating density Density is a measure of coating quality Too difficult to measure by weighing Critical angle for total x-ray reflection: below a certain critical angle θ o, x-rays are totally reflected. This angle is directly related to mass density ρ by A 2 ρ = θ A = at. weight o Z Z = at. number 69

70 Coating density example: 10 nm CNx Intensity θ 70

71 CNx surface roughness at the mm-cm scale Size of slider ~ mm Need to measure surface roughness at the millimeter scale X-ray reflectivity measurements give the average surface roughness from the rate of x- ray intensity decay with angle, exp(-4k z,o2 σ 2 ) 71

72 CNx surface roughness at the mm scale: 3 and 10 nm CNx Estimated 10 nm Reflectivity Estimated 3 nm k ( A o -1 ) z,0 72

73 CNx surface roughness at the mm scale: nm CNx Estimated 10 nm X-ray reflectivity 6 nm 3 nm 2 nm 1 nm k z,o (A -1 ) 73

74 Density and surface roughness results Measured roughness by XRR (nm) RBS result (CN 0.26 ) Measured thickness by XRR (nm) Calculated mass density XRR (gm/cc) 74

75 Elastic modulus by laser-induced surface acoustic wave measurements N 2 pulse laser Sample Mirror Piezoelectric transducer Motor drive Surface wave impulse Digitizing oscilloscope 75

76 Elastic modulus obtained from acoustic measurements 76

77 Elastic modulus vs film thickness E(5 nm) = 199 ± 3 GPa E(3 nm) = 185 ± 6GPa E(1 nm) = 32 ± 7GPa 77

78 Oxidation vs. Overcoat thickness Minimum thickness for zero Co oxidation from XPS ~ 1.7 nm Oxidation % CNx CoOx Co Thickness (A) Courtesy: Dr. M. L. Wu, Seagate 78

79 Molecular Dynamics Simulation of Deposition of Carbon Atoms Deposition of a single atom Demonstration of sputtering Deposition of one carbon monolayer at 100 ev Deposition of three carbon layers at 40 ev 79

80 Deposition of a Single Atom 80

81 Deposition of a Single Atom 81

82 Sputtering of Atoms 82

83 Deposition of One Carbon Monolayer at 100 ev 83

84 Deposition of 3 Carbon Layers at 40 ev 84

85 Other possible overcoat candidates Boron carbide hard solid lubricant in normal ambients Doped a-chx hydrophobic to minimize moisture or corrosion problems 85

86 a-chx coatings C. Donnet et al. / Tribology of hydrogenated diamond-like carbon. Tribology Letters 4 (1998)

87 Need to Make Carbon Films Hydrophobic H O H repulsion H O H X X + Carbon film surface 87

88 Sulfur Effect on Hardness Hardness (GPa) Sulfur Content of Film [atomic percent] 88

89 Friction measurements P R Roughness of steel ball ~ 0.5 µ-inch δ ~ 10-20% of CHx thickness Film a 89

90 Doped a-chx a coatings Coefficient of Friction a-chx µ=0.08 RH=25% 9 a/o sulfur µ=0.05 RH=25% 9 a/o sulfur µ=0.008 RH=25% Time a-chx µ=0.003 RH=0% 90

91 Concluding thoughts Are we closer to (a) or (b)? (a) (b) 91

92 Concluding thoughts (cont.) Current disk drive overcoat thickness is ~ 3-4 nm Minimum overcoat thickness = 0 What then? Maximum storage density ~ Tb/in 2 Current storage density ~ 40 Gb/in 2 At 60% compound growth rate, get there in 2017 What then? 92