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Single-wafer Approach For 300mm

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With the increasing penetration of flip-chip technology into the 300 mm wafer-level packaging (WLP) market, requirements for process performance of solder bump technology have become more stringent. One of the biggest challenges for 300 mm WLP processing is the control of undercut and uniformity during etching of under-bump metal (UBM). Lucy Chen, David Chen, Jackie Chen of SEZ Taiwan present the results of UBM etching for Ti/NiV/Cu, TiW/Cu, Ti/Cu based bumps using a single-wafer spin processor that can provide a solution for stringent undercut control.

Wafer-level packaging is becoming more critical as the semiconductor industry becomes increasingly driven by economic and technology challenges. Undercut uniformity is a critical variable that cannot be successfully solved using a batch approach. The within-wafer and wafer-to-wafer reproducibility is not controlled in batch systems as the wafers are immersed in a large tank of chemicals allowing little control over how the chemicals interact with the wafers in the tank. This can lead to a large variation in underbump metal (UBM) etch. SEZ has successfully demonstrated UBM etch processing for stacks that include:

  • Cu deposited on titanium (Ti) or Ti-tungsten
    (TiW) stacks (with eutectic solder bumps)
  • Cu deposited on nickel-vanadium on titanium
    (NiV/Ti) stacks (with high lead solder bump)

Demonstration
Tests were performed using an SEZ SP304 single wafer spin processor configured for 300mm wafers. This system uses a Bernoulli chuck to hold the wafer. The etchant is supplied from an external chemical tank through a point-of-use heat exchanger and delivered to the wafer surface via a radial arm media dispenser. Figure 1 shows a representation of the process chamber configuration.

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The Bernoulli chuck uses a nitrogen cushion to allow wafers to be processed on one side without contacting the other (non-processed) side. The chuck design permits a small amount of chemistry to dispense around the wafer bevel edge to clean a defined area on the non-processed side of the wafer. The chemistry can be run in a re-circulation or single pass modes.

Initially, etch rate and etch uniformity of 1000Å metal film stacks were determined using different chemical mixtures. The residual metal was inspected with an optical Nikon Olympus microscope and the undercut profile (under-bump metal layer) was measured using a LEO 1530 scanning electron microscope. Energy dispersive spectroscopy (EDX) was also used to confirm residual metal under the bump.

Etch uniformity is dependent on the process parameters and etch agent used. The initial etch rate and etch uniformity evaluations were performed on wafers of various metal layers. (Figure 2)

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Successful under bump metal layer etching must be achieved without solder bump damage. This requires high etch selectivity between different layers. The exact nature of the chemical used in these experiments is confidential. However, it can be said that two chemicals must be used. SEZ spin processors can implement up to three chemical blends. One chemistry was used for etching the copper (Cu) and a second was used for the Ti, TiW or NiV/Ti layers. The processing is sequential and includes an intermediate deionised (DI) water rinse in order to avoid cross contamination of media.

It has also been shown to enhance the process performance. Figures 3 through 6 show the results of the implementation of the novel UBM process with excellent undercut control (less than 3µm) and without residual metal. Figure 3 shows an optical image of the wafer surface pre and post spin etch process. It appears visually that the Cu metal has been removed from the wafer surface without noticeable damage to the TiW film.

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Similar results are observed in Figure 4 for a Ti/NiV/Cu stack. These images also highlight the within-wafer consistency and uniformity of the process. Respectively Figures 5 and 6 show SEM analysis results of the UBM structures for both Ti/NiV/Cu and TiW. The UBM etch profiles for both structures are minimal, with values below 2µm and 3µm, respectively. Representative EDX spectroscopy data are also shown for the TiW/Cu. The area examined appears to be metal-free to within the limits of detection.

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Conclusion
Under bump metal layer wafers containing TiW/Cu, Ti/Cu or Ti/NiV/Cu can be successively wet etched under controlled chemical and process conditions afforded by an SEZ single wafer spin processor. The process allows for minimal undercut of less than 3µm and enhanced process controls relative to batch spray and bench systems while delivering comparable throughput. Optical and spectroscopic techniques confirm the removal of metallic residues while maintaining excellent undercut and within-wafer and wafer-to-wafer control.

ACKNOWLEDGMENTS
Leo Archer, Ph.D. of SEZ AG, Villach, Austria and Shea Stickler of SEZ America, Phoenix, AZ, USA.



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