A new dry de-veil process has been developed for via etch de-veil. The via is stripped of photoresist and etch residues are treated using a fluorinated, oxygen plasma. Residues are then removed using only a room temperature DI water rinse. This process has been ramped into production and has resulted in equivalent or better device yields and performance. With this process, the use of costly solvents or acids is eliminated, thus providing a substantial cost and environmental burden savings. Based on a modeled 10,000 wafer starts/week factory, this process can save over $5M per year in solvent chemical costs.

Introduction

In the manufacturing of IC devices, multi-layer interconnects are formed through repeated depositions of dielectric layers on metal stacks. These metal stacks are often formed by subsequent depositions of metal films such as TiN/AlCu/TiN/Ti. Metal lines are connected through holes, or “vias,” which have been etched through the dielectric silicon oxide to the underlying metal stack. The via layer is patterned by photolithography, etched using reactive ions, and finally stripped of both photoresist and etch residues. The cleaned via is then filled with tungsten or other conductive metal. It is critical to device performance and reliability that the vias are residue free prior to fill. Figure 1 shows a schematic of the via process integration as well as the generation of via veil.

The removal of post via etch photoresist and etch residues can be quite challenging. The via etch process is designed to use polymer-forming gases to create a vertical etch profile and to selectively etch oxide while using the TiN barrier film as an etch stop layer. These chemistries are commonly a combination of CF₄/CHF₃/Ar.¹ Post-etch residues, often called via veils, form along the via sidewalls as a byproduct of the etch chemistry. These veils contain entrapped species of the materials etched, generally siliconoxycarbon compounds plus metal from the etch stop layer. The via veil may also contain fluorine. The photoresist is commonly removed by reaction of molecular oxygen at elevated wafer temperatures (>200°C). This “ashing” process serves to volatize the carbon chains of the photoresist, while oxidizing the exposed metal.^2,3 The combination of polymer-forming via etch chemistries and high temperature ash processing can create via veils which can be removed only in aggressive, wet chemical solvents.

Hydroxylamine-based chemical strippers have been found to be effective at removing via veils.⁴ But the use of these solvents has been found to be problematic. These chemicals are most effective at elevated temperature, generally >70°C. Water concentration must also be maintained to guarantee veil removal. At elevated temperature and under equipment exhaust, however, water is evaporated from the chemical mixture. The bath must therefore be periodically refreshed to maintain both temperature and water concentration. Spray tools are often used to dispense solvents due to throughput and floor space considerations.