UBM, RDL and Bumps

Ni/Cu/Pd, Ni/Cu/Au, Ag:Sn Bumps

Semiconductor manufacturers face many key emerging trends in WLP. These include:

  • Redistribution Layers
  • New UBM Films and Stacks
  • Tighter Pitch Features
  • Smaller Solder Balls
  • Lead-Free Solder Alternatives

These critical trends create associated metrology challenges, and generate the need for:

  • Thickness and composition control
  • High sampling and high throughput
  • Fewer blanket wafers
  • Production wafer sampling

These needs drive the demand for non-destructive thickness measurements on RDL and UBM layers that include a small spot measurement technique with excellent spot placement, with tighter pitch densities (<200μm), and smaller solder balls (<100μm).

Other key considerations are composition measurements on the bulk UBM layer, where Ni(P) composition impacts the quality of the passivation layer, and composition measurements of lead-free solder alternatives:

  • Sn/Ag composition → impacts reliability/prevents “shorts”
  • Sn/Ag has shown to gain Ni or Cu from the UBM during the reflow process

XRF technology from Jordan Valley answers these challenges via energy dispersive (∼150eV) techniques with multi-detector array for fast, high-throughput measurements and 100% detector efficiency for Sn/Ag, plus small spot (poly-capillary) optics capable of measuring down to 50x50μm features. In addition, Jordan Valley XRF technology offers non-destructive real-time measurements that deliver immediate turn around.

Jordan Valley UBM Process Highlights

  • Small bumps using Sn rich solder makes the UBM selection more critical

    • Surface tension is increasingly impacted by wettability

  • Reduced solder ball size increases surface to volume ratio

    • Surface tension is increasingly impacted by wettability

    • Past Cu-Cr-based UBM is not compatible

  • Nickel based UBM materials are desired because of the low reaction rate with Sn

    • Many Ni based UBM stacks are being used

  • Electroless acidic hypophosphite baths are used for nickel deposition

    • If the phosphorus content is high (>9.5 atm%), the Ni(P) will be in the amorphous state

    • Suppresses fast grain-boundary diffusion

XRF data for UBM and RDL stacks.

  • Typical UBM and RDL layers are multi-layer stacks:

    • Barrier (or Under Metal)/Bulk Metal Layer / Passivation Metal Layer

  • Common UBM Film Stack Variations

    • Al/Ni/Au, Al/Ni/Cu

    • Cu/Ni/Au, Cu/Ni/Pd

    • Ti/Ni/Au, Ti/Cu/Ni/Au, Ti-W/Cu/Cu

    • Cr/Cu/Cu, Cr/Cu/Cu/Ni

Why control bump composition?

WLP represents a rapidly growing market segment due to the need for high-density, high-pin-count, smaller size, stacked and high performance devices. This growth gives rise to new metrology challenges with re-distribution and under bump metallization film stacks. In addition, lead-free bumps also require in-line control of material composition. Jordan Valley stands uniquely qualified to address these continually evolving metrology needs with industry-leading small spot, high speed X-ray Fluorescence technology.