Diamond wire sawing Influence of sawing parameters on wafer surfaces and decreasing wire consumption

Transcription

1 Diamond wire sawing Influence of sawing parameters on wafer surfaces and decreasing wire consumption Paul Brooker Florida Solar Energy Center University of Central Florida A Research Institute of the University of Central Florida

2 Overview of Diamond Wire Slicing Wire speed (m/s) Time (sec) Table speed (mm/min) Table speed Wire speed

3 Wafering Issues Wire doubling adjacent wires overlap Loss of wafers, increased wire wear Uniform web Overlapping wires

4 Wafering Issues Skew non-uniform bow in web Potential loss of wafers Nearly % of wafers are lost Doubled wires Skew

5 Wafering Issues Wire breakage Significant loss of wafers and time Wire break near end of cut Wire break mid-cut Partial recovery

6 Overview of Diamond Wire Slicing Key parameters Wire tension: avoid doubling without breaking Table speed: related to cutting efficiency Fresh wire feed: balance consumption and wear Parameter requirements change during cut Entrance region: establish well-defined channels Bulk region: minimize cutting time Exit region: minimize skew

7 Wafer Surface Analysis Pilgrim waves present on DW-sawn wafers Increase processing requirements Can decrease wafer performance Analyzed wafers from three cuts Height (µm) Distance (mm) - Wire Speed (m/s) Table Position (mm)

8 Ingot Alignment and Regions of Exit SIDE VIEW Interest Direction of table movement Fresh Wire TOP VIEW Web Bulk Ingot Ingot Entry Fresh Wire Region Web Table Speed (mm/min) Forward/Backward Length (m) Wire feed rate (m wire /mm table ) Entrance. /. Bulk. /. Increasing wafer # Exit. /. o Used Wire

9 Entrance Bulk Exit Wire # Height (µm) Height (µm) Height (µm) Average peak height Distance (mm) Distance (mm) Distance (mm) Average peak frequency Wire # Height (µm) Height (µm) Height (µm) Distance (mm) Distance (mm) Distance (mm) Wire # Height (µm) Distance (mm) Height (µm) Distance (mm) Distance (mm) Height (µm)

10 Wafer surface vs. wafer number Average Peak Height (µm) Wire # Entrance Bulk Exit Severe doubling and wire break Increased wire wear Average Peak Height (µm) Wire # Entrance Bulk Exit Wire break and repair Exit may have less wear Wafer Number Wafer Number Average Peak Height (µm) Clean run Wire # Wafer Number Entrance Bulk Exit Lower peak height should reduce wafer processing time

11 Origin of Pilgrim Wave Amplitude Lateral motion of wire results in pilgrim waves Extent of motion appears to depend on wire wear Higher wafer number = greater wire wear, lower amplitude Diamond particle size/shape influences? Average Peak Height (µm) Wire # Entrance Bulk Exit Wafer Number

12 Wire # Wire # Wire # Entrance Bulk Exit % lower peak height

13 Origin of Pilgrim Wave Frequency K. Sunder, R. Buchwald and O. Anspach. Origin of the Periodic Structures on Silicon Wafers Sawn with Diamond Wires. EU PVSEC

14 Wire Wear Modeling Developed model for wire wear during slicing Linearize entire length of wire Ingot Work roller Web Work roller Tension roller Wire spool Top View Side View. On the feed spool. Between the feed spool and work roller. On the work roller, but not under silicon. On the work roller and under silicon. Between the work roller and take-up spool. On the take-up spool

15 Wire Wear Modeling Developed model for wire wear during slicing Linearize entire length of wire Consider wire wear along entire length of wire that is in active use Block Block Block Block N. On the feed spool. Between the feed spool and work roller. On the work roller, but not under silicon. On the work roller and under silicon. Between the work roller and take-up spool. On the take-up spool

16 Wire Wear Modeling B m during forward stroke A B ~m in web A % B.% Wire wear (%) A Position on wire (m)

17 Wire Wear vs. Wafer Number Wire wear changes as wafer number increases Wafers near fresh wire feed experience wire with lower wear Wafer Wafer Wafer # Wire wear (%) Wafer # Wire length (m)

18 Wire Wear vs. Wafer Number Wire wear changes as wafer number increases Wafers near fresh wire feed experience wire with lower wear Wafer Thickness (µm) Wire wear (%) Wafer Wafer Wafer # DW DW DW Increased wafer thickness due to thinner wire Wafer # Wire length (m) Wafer #

19 Wire Wear vs. Wafer Number Wire wear changes as wafer number increases Wafers near fresh wire feed experience wire with lower wear Wafer Wafer Wire wear (%) Wafer # Skew is a result of lower cutting efficiency at higher wafer numbers Increasing wafer # Wafer # Wire length (m)

20 Evolution of Wire Wear Wire wear increases with each cycle Initial wire wear = % Distribution of wear on active wire increases Cyc Cyc Cyc Cyc Cyc Cyc Cyc Cyc Wire wear (%) Position on wire (m)

21 Final Wafer Wire Wear Distribution Different regions of the wafer will experience different levels of wire wear Cyc Cyc Cyc Exit Bulk Entrance

22 Wire Wear Modeling Minimizing wire consumption requires maximizing wire wear Proper wire management is required to ensure full utilization of the wire Less than / of wire experiences max wear % complete % complete % complete % complete % complete Wire wear (%) Position on wire (m)

23 DW Wear Summary Sawing parameters must be optimized to reduce doubling, skew and wire breaks Pilgrim waves are related to sawing parameters and wire wear Understanding pilgrim wave evolution requires greater understanding of wire wear evolution

24 Near-Term Challenges (- Years) Challenge : Minimize pilgrim waves in DW sawn wafers Challenge : Decrease diamond wire consumption through improved wire monitoring Challenge : Decrease diamond wire thickness without compromising wafer quality

25 Long-Term Challenges (+ Years) Challenge : Reduce doubling in thin-wafer (<µm) DW sawn processes Challenge : Improve thin wafer handling Challenge : Develop kerf recycling technology

26 Acknowledgements Crystalline Silicon USPVMC Feedstock and Wafering working group Slicing Tech

27 Back-up Slides

28 Work Roller Trace Measured work roller grooves before and after slicing Red = before A Feed side Web Ingot Take-up side Tension roller Wire spool B Feed side work rollers A B Work roller Work roller C D Top View Side View C D Take-up side work rollers