EXECUTIVE OVERVIEW

Given reduced process windows for cleaning the bottoms of high-aspect-ratio contact holes, particularly in tightly pitched memory structures, traditional batch approaches must be replaced with single-wafer processing to provide greater control. Introducing a single-wafer process also eliminates a signature defect pattern from wet bench processes that consistently created voiding and line damage after metal deposition. A 300mm single-wafer metal contact hole post-etch clean reduces the total number of defects observed and increases die yield.

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Wafer cleaning is becoming an increasingly critical process module in the semiconductor manufacturing flow. As the requirements for integration precision grow, so too do the needs for cleaner and more uniform device structures. The effectiveness of cleans ultimately influences device performance and yield [1-4]. All aspects of cleans processing are being increasingly scrutinized whether in frontend or backend-of-line (BEOL) processing. One area of concern is post-etch and ash residue removal from the sidewalls and bottoms of metal contact holes prior to metallization. An incomplete contact clean is a major concern as it can lead to costly device failures.

In flash memory, metal contacts are the electrodes that connect the bit lines and active areas of the device. These connections are formed by a sequence of dry reactive-ion etching (RIE) through a resist film and the interlayer dielectric (ILD), creating a hole that is subsequently filled with metal. After the ILD etch step, the remaining resist is stripped by plasma ashing. A wet process is then required to remove polymeric residues that are left as artifacts of the dry-strip process.

As semiconductor devices shrink in size and contact holes get narrower and deeper, it is essential yet more difficult to remove the residues, native oxides, and defects from the bottoms of the contact holes as well as from the wafer surface. Otherwise, the gap fill of the upper metal electrode will be unstable, resulting in both performance and functional yield losses.

Flow defects

Wet benches have been the primary tools used for BEOL cleaning steps, including post-etch contact cleans. For device geometries greater than 180nm, conventional wet bench (batch) cleaning has been moderately effective at removing polymer residues and native oxides. However, with the development of smaller device nodes, the inherent cross-contamination associated with processing a batch of wafers immersed in a liquid becomes a greater problem. It is possible to have migration and transfer of defects from various sources and locations to the active area of a wafer. Sometimes this can be from the wafer backside or bevel to the wafer front side, but it can also be from neighboring wafers in the batch. The effects of particle migration during wet bench processing can be seen after different cleaning steps including those for pre-metallization.

Figure 1 shows an overview of the integration process for contact formation and metallization. The metal contact hole is formed by an RIE process through a deposited ILD, which is typically an oxide. After the various clean steps to remove resist and etch polymers, a thin barrier layer of...