Electrostatic Discharge (ESD) Breakdown between a Recording Head and a Disk with an Asperity

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Justin Woods
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1 Electrostatic Discharge (ESD) Breakdown between a Recording Head and a Disk with an Asperity Al Wallash and Hong Zhu Hitachi Global Storage Technologies San Jose, CA

2 Outline Background Purpose Experimental Setup Results Breakdown behavior with and without asperity Failure analysis: where does breakdown occur? Conclusions

3 Background Electrical breakdown across spark gap Gap spacing < 5 µm DC pre-breakdown current: ~ na Field emission (or tunneling) Fowler-Nordheim equation Transient discharge current: ~ ma Physical damage melting Lead-shield breakdown V Metal-Insulator-Metal spark gap Conductor Insulator Conductor Factors that affect breakdown voltage Dielectric film properties Thickness and material Electrode Geometry of anode and cathode E-field enhancement at sharp edges Materials: work functions Fowler-Nordheim Equation I = ae 2 e b Φ E 3 / 2

4 Purpose Hard disk drive: Head disk interface = spark gap Thin carbon overcoat (COC) films ~ 3nm head, ~3nm disk Fly height = air gap: V <10nm Head-disk contact possible COC Asperities Load/unload Disk Slider Asperity Study electrical breakdown with head-disk asperity contact Electrical breakdown voltage? Physical damage to slider air bearing surface? Likelihood of ESD damage to read sensor?

5 Experimental Setup Modified Guzik XY Spin stand Float suspension and disk Disk connected to Source/Measure Unit Floating suspension connected to ground through current probe Measurements Voltage on disk, DC current, transient current (2 GHz bandwidth) Acoustic emission (AE) and thermal asperity (TA) to position sensor over asperity GMR resistance (R), track averaged amplitude (TAA), asymmetry Acoustic Emission Sensor Head signals to Preamplifier AE - Transient current probe TEK CT6 Source/Measure Unit Keithley Head Trek P1122 noncontact voltmeter Disk Air Bearing Spindle

6 Experimental Setup Heads and media 40 GMR heads (60 Gb/in 2 ) on a metal AlMag disk; 15k RPM Asperity Indentation: diamond spherical tip with 50 µm radius Pile-up height ~ 30 nm Test Sequence Ramp disk voltage: + or - Dwell time: 1sec Measure R and TAA Breakdown: transient current > 1mA TA signal Disk Voltage (V) Ramped disk voltage 1 second dwell time Time (min) Read back signal Current 4 ma/div Transient current (CT-6) At breakdown

7 Current vs. Voltage DC Current (na) DC Transient Asperity Head 1 Head 2 No asperity Breakdown Transient Current (ma) Increase in DC current, then breakdown Breakdown voltage Without asperity (smooth disk): 3.4V With asperity: 1.6V Disk Voltage (V) 0

8 Breakdown Voltage Frequency Smooth Asperity Average breakdown voltage Asperity Disk Voltage (V) Without asperity With asperity +V V Both + and disk voltage Significant decrease in breakdown voltage with asperity

9 Behavior vs. Voltage: No asperity 53 No asperity (smooth disk) Resistance 1.6 Resistance (Ω) TAA Breakdown at 3.4V 1.5 TAA (mv) Disk Voltage (V) 1.4 TAA increase: electrostatic attraction = spacing decrease No TAA or resistance change after breakdown

10 Behavior vs. Voltage: With asperity 70 Resistance With asperity 1.5 Resistance (Ω) TAA Breakdown at 1.7V Disk Voltage (V) TAA (mv) With asperity positioned over read sensor during breakdown TAA changes Resistance increase Read sensor damaged by breakdown over asperity

11 Slider Damage: No Asperity Sensor area largely unaffected Breakdown to slider body Transient current between slider and disk GMR sensor undamaged Breakdown between disk and conductive TiC grains

current flow to GMR leads Severe magnetic damage similar to ESD damage during handling After breakdown with

12 Slider Damage: With Asperity Breakdown at TiC conductive grains on air bearing surface in center pad Similar to case with no asperity Shields and sensor show damage Breakdown between asperity and shields/reader Direct current flow to GMR leads Severe magnetic damage similar to ESD damage during handling After breakdown with Asperity Mag = 4k X Breakdown between disk and conductive TiC grains After breakdown with Asperity Mag = 25k X

13 Summary and Conclusions Breakdown Voltage (V) Breakdown location Read sensor damage Without asperity With asperity +3.4 / / -2.6 TiC grains on slider No TiC grains and shields/sensor Yes 1. Significant reduction in head-disk breakdown voltage (~50% reduction) 2. Additional damage to shields and sensor area 3. Severe magnetic damage to the read sensor Important to include asperities in head-disk breakdown reliability testing

14 Backup slides

15 Current vs. + and Disk Voltage DC Current (na) Four different heads and disks DC Transient No asperity Asperity Asperity Disk Voltage (V) No asperity Negative voltage, no asperity No measurable pre-breakdown current Transient Current (ma)

16 H45, with asperity, +V on disk, head died H46, with asperity off sensor, +V on disk, no sensor damage

17 Optical Slider Damage: Asperity away from sensor

18 Breakdown damage Step edge

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