A Host of Challenges for Copper CMP Cleaning

SEMATECH tests a promising new chemistry for copper cleaning.

Laura Peters, Senior Editor -- Semiconductor International, 3/1/1998

Together with SEMATECH (Austin, Texas), OnTrak Systems Inc., a subsidiary of Lam Research (San Jose, Calif.), is evaluating copper (Cu) cleaning capabilities using OnTrak's Synergy system. Dr. John de Larios, vice president of cleaning technology for OnTrak, summarized the most significant issues associated with copper CMP and post-CMP cleaning: 'During CMP, copper pitting and scratching can occur, and corrosion must be prevented.' He continued, 'In the area of cleaning, the alumina (Al₂O₃) slurry is difficult to remove, trace metals removal is important and a passivation layer may be required to suppress corrosion.' Dr. de Larios recently presented his findings at a conference at SEMICON Korea.

Perhaps the greatest challenge for the developers of general CMP and post-CMP cleaning processes for copper is the variety of materials and deposition methods used to form the metal barrier (likely TaN, T, TiN or WN_x), copper seed layer (PVD, CVD), copper fill (likely electroplating) and the interlayer dielectrics (TEOS-based SiO₂ or low-k dielectrics). 'Currently, we are developing general process solutions,' de Larios said, but he admitted that application-based CMP and cleaning processes may be required in the future. For example, some customers may choose a two-step polish using the alumina slurry for the first step to remove the metal layers, followed by a silica slurry second step to clean up the oxide and reduce surface defects. Other customers will opt for a higher throughput process, using only the alumina slurry.

1. The complexity of post-CMP cleaning increases with the use of new materials.

'Fortunately, CMP engineers are already familiar with the alumina slurry as it is used in tungsten CMP,' de Larios said. OnTrak's copper cleaning development project has been under way for approximately eight months.

This post-CMP copper cleaning project, which will collaborate with SEMATECH's widely publicized copper interconnect project, is focused on a proprietary copper cleaning process developed by OnTrak. The Synergy double-sided scrubber (DSS) will be utilized for process development, with possible modifications to the current tool set to allow for process integration and materials compatibility between the slurry, acid or base, and the proprietary cleaning solutions. The copper CMP cleaning process is designed to remove slurry particles and reduce the level of surface trace metals. He added that as users begin to use low-k dielectric films, a TEOS-based capping layer may be used, as CMP of SiO₂ is a more mature and understood process.

CMP cleaning chemistries are application-specific. Oxide CMP cleaning has evolved from using deionized water only to using NH₄OH for longer brush life and HF for reduction of trace metals (Fig. 1). The post-CMP cleaning of shallow-trench isolation layers uses NH₄OH, polysilicon uses a standard clean 1 solution and often HCl. However, despite its advantages in cleaning alumina slurries, the high pH of NH₄OH is relatively incompatible with copper, as even dilute solutions (0.1-0.05%) will etch the copper surface. Dr. de Larios showed that surface roughness when using a 0.1% NH₄OH scrub to a 0.5% NH₄OH scrub more than doubles from 4.4 nm to 11 nm (5 x 5 mm scan size). The same polished copper surface has a 0.9 nm roughness when cleaned with the proprietary copper cleaning scrub. In addition, this process removes trace metals, as shown in Table 1. In this experiment, 20 Å of TEOS-based oxide was removed during the copper scrub cleaning process, and trace metals remaining in the dielectric were measured by TXRF. As shown, less than 1x10¹⁰ copper atoms/cm² remained on the wafer frontside following the proprietary clean.