CMP Developers Take Aim at STI Applications

Staff -- Semiconductor International, 2/1/1998

Shallow-trench isolation (STI) has several advantages compared to traditional local oxidation of silicon (LOCOs). Not only does it offer improved electrical isolation, but it enables greater packing density, smaller channel-width encroachment, superior latch-up immunity and better planarity.

  Fabricating STI structures typically involves the following steps: silicon trench etch, nitride deposition and etch, CVD dielectric trench fill, planarization etch back and CMP dielectric planarization. CMP, without an assist from RIE etch back processing, has been unable to meet the planarization needs for STI (see SI , July 1997, p. 184). This adds complication to the STI isolation processing by introducing an additional photolithography step to form a resist block and an additional RIE etch.

  In efforts to eliminate these two steps associated with etch back, CMP researchers have been working to develop new slurries and new CMP processes that improve the removal rate selectivity of oxide to nitride, while dealing with the strong dependence of the planarization results on the pattern density of the active area.

  At the upcoming CMP for ULSI Multilevel Interconnection Conference (CMPMIC 8), to be held Feb. 19-20 in Santa Clara, Calif., several solutions to these challenges will be presented.

  Rodel, Cybeq and UMC, for example, are scheduled to report results from studies using a newly formulated slurry designed specifically for STI applications (Rodel - XSHD3562). The new slurry, used with a concentric grooved polishing pad and a multihead CMP tool, is said to provide high oxide removal rates and very low nitride removal rates, allowing the nitride film to be "truly a polish stop layer."

  Rodel (Dover, Del.) also teamed up with Lam Research Corp. (Fremont, Calif.) to investigate various combinations of rotary and linear polishing technologies, as well as conventional and high-selectivity polishing slurries. They found that the best results were obtained with a linear planarization technology and a highly selective slurry. Less than 150 Å dishing of oxide in the filed region with complete removal of oxide from isolated active regions 10-4000 m m in size, with a total nitride thickness variation of <1000 Å, was achieved.

  In other work, researchers from IMEC (Leuven, Belgium) have developed a novel approach that introduces an additional nitride stopping layer. In this dual nitride concept, the trenches are first filled with oxide so that the surface of the oxide remains lower than the nitride on the active areas. Subsequently, a thin nitride (<80 nm) is deposited, and the wafers are patterned to remove the second nitride layer on the active areas together with a small border around the active areas. Using a high-selectivity slurry, it is now possible to remove all the oxide on top of the dense active areas without any risk of overpolishing the small isolated active areas. This is because the remaining part of the second nitride layer acts as a very efficient polishing stop. A larger overpolish can be tolerated in this approach to make sure that all the oxide on the large active areas is removed.

Traditionally, CMP is performed by applying the wafer against a porous pad soaked with an abrasive slurry. The main drawbacks of this process are the rounding and dishing, which can occur on very large patterns, leading to low planarization efficiency.

In a paper to be presented later this month at the CMPMIC '98 Feb. 19-20, researchers from 3M, LETI and SGS-Thomson will describe experimental results based on the new process. (Table 1).

Experiments were carried out on a PRESI Mecapol 550 polisher using a two-step polishing process. The first step (polishing) was performed on a 3M slurry-free pad using either KOH or water as liquid. The second step (buffing) was conducted on a Rodel politex pad. Oxide removal rate and planarization efficiency were evaluated for each liquid. Planarization was determined by using CMP-specific patterns. The initial step height was 7000 Å, and polishing was performed by removing 7500 Å on the top area. The degree of planarization, residual step, rounding and dishing were measured on different critical patterns.

The main advantage of the new technique, according to researchers Fayolle, Lugand, Wiemar and Bruxvoort, is its planarization efficiency on larger patterns. While both CMP techniques (conventional and slurry-free) gave similar results on 100 µm wide patterns, the slurry-free technique was superior on larger patterns (e.g., 1-3 mm wide). As a result of the improved efficiency, the polishing time was half that of the conventional CMP process.