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TiW Etch
Titanium (Ti), titanium-tungsten (TiW), titanium tungsten nitride (TiW (N)) and other UBM metals possess dual properties that can double as the adhesion layer and the barrier layer. Due to their capabilities for direct contact with the final metall layer and passivation layer, the etching solutions should provide fast etching rate as part of the requirement; meanwhile, they should be designed to avoid damaging the metal layer, the passivation layer, and bump material.
Many leading bump fabricators have chosen GPTC's 300 mm Auto Wet Bench systems for TiW and Ti etching in copper pillar bumping (Figure 1). Etching solutions composed primarily of hydrogen peroxide (H2O2) are the popular choice for etching TiW and TiW (N) in UBM layers. Etch rate must be monitored and controlled closely during TiW etching, because the etch rate is heavily affected by process temperature, etching chemical lifetime and metal trace concentration. An in-depth understanding of etching reaction mechanism is necessary for designing the machines. Time needed for etching TiW, the depth of the undercut and the shape of the bump reflow have to be taken into consideration for modifying the compositions of the etching solution and etching temperature to meet manufacturing requirements.
TiW etching endpoint control is absolutely crucial: when TiW etching is completed, the wafers must be rinsed with DI water ASAP to avoid over-etching and undercut. Hydrogen peroxide and the stabilizer would lose their etching efficiency over time during TiW etching; this is why the compositions of the etching solution must be rigorously monitored to ensure process yield.
Many leading bump fabricators have chosen GPTC's 300 mm Auto Wet Bench systems for TiW and Ti etching in copper pillar bumping (Figure 1). Etching solutions composed primarily of hydrogen peroxide (H2O2) are the popular choice for etching TiW and TiW (N) in UBM layers. Etch rate must be monitored and controlled closely during TiW etching, because the etch rate is heavily affected by process temperature, etching chemical lifetime and metal trace concentration. An in-depth understanding of etching reaction mechanism is necessary for designing the machines. Time needed for etching TiW, the depth of the undercut and the shape of the bump reflow have to be taken into consideration for modifying the compositions of the etching solution and etching temperature to meet manufacturing requirements.
TiW etching endpoint control is absolutely crucial: when TiW etching is completed, the wafers must be rinsed with DI water ASAP to avoid over-etching and undercut. Hydrogen peroxide and the stabilizer would lose their etching efficiency over time during TiW etching; this is why the compositions of the etching solution must be rigorously monitored to ensure process yield.
| Item | Factors about TiW etch uniformity and undercut |
|---|---|
| 1 | Etch rate control |
| 2 | Process temperature control |
| 3 | Etchant compositions and lifetime |
| 4 | Trace metal concentration |
| 5 | Etching endpoint control |
Figure 1. TiW etching for UBM in copper pillar bumping (Source: GPTC)
Detta and fellow developers used hydrogen peroxide-based etchants when they refined TiW etching process. They removed TiW in the etching process at 50℃in the presence of PbSn, CrCu, Cu, and Al. The etchant contains the following：
(1) hydrogen peroxide as etchant.
(2) potassium sulfate as the passivating agent, to protect PbSn.
(3) Potassium - EDTA serves as a stabilizer for hydrogen peroxide. It also acts as a buffering agent and complex compound for the etching process.
(2) potassium sulfate as the passivating agent, to protect PbSn.
(3) Potassium - EDTA serves as a stabilizer for hydrogen peroxide. It also acts as a buffering agent and complex compound for the etching process.
The TiW (N) etching solution is comprised of hydrogen peroxide and ammonium hydroxide. At present, the optimum etching solution formula for TiW and (TiW (N)) documented in research is as follows: 30% of H₂O₂ at 6% in volume, and 30% of NH4OH at 0.75% in volume, with operating temperature set at 37 ℃.