UBM

•  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.

RDL & 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.