IMEC, SEMATECH Show Feasibility of a Sub-2.0 Low-k Dielectric

-- Semiconductor International, 7/1/2000

Researchers from IMEC and SEMATECH reported on the feasibility of an ultralow-k film made by Dow Corning (Midland, Mich.) at the recent IEEE IITC Conference in San Francisco. Changming Jin and Jeff Wetzel of SEMATECH (Austin, Texas) integrated the low-k dielectric, XLK, a hydrogen silsesquioxane (HSQ) film, with copper damascene test structures. Upon parametric test, the devices showed good capacitance and leakage current distributions. SEMATECH evaluated the film's ability to endure etching, CMP, ashing and cleaning processes while avoiding mechanical or chemical damage to the low-k material.

Complementing this work, Ricardo Donaton and his team of researchers at IMEC (Leuven, Belgium) integrated XLK in a 0.2 µm single-damascene copper structure. They determined the refractive index, thermal stability and susceptibility to moisture absorption, among other crucial film properties. With a pore size of 3.5 nm and a tight pore size distribution, XLK appears to offer many of the necessary characteristics needed for device integration.

The XLK product is a relatively simple extension of Dow Corning's HSQ film, tradenamed FOx, a dense inorganic material with a dielectric constant (k) around 2.9. Pores are introduced to the HSQ matrix through a three-step process on the SOD (spin-on dielectric) track system: gellation in the presence of a high boiling point solvent at ambient conditions; removal of the solvent without collapsing the resin structure; and formation of a strong network structure during cure. Gellation occurs when the film is exposed to an ammonia and moisture environment for 30-120 seconds, which causes SiH bonds to react with the H₂O to form SiOH; then the SiOH bonds join together to form a SiOSi gel.

1. Trench with 0.35-0.4 µm lines and spaces etched in 700 nm XLK film with 100 nm oxide cap shows good sidewall etch profiles. (Source: SEMATECH)

SEMATECH determined resulting film characteristics of a 1 µm XLK film: k of 2.19, Young's Modulus of 2.5 GPa, coefficient of thermal expansion of 4 ppm/degC, porosity of 59%, average pore size of 2.4 and pore connectivity of 100%. In single-level copper damascene integration studies, 700 nm of XLK and 100 nm CVD oxide cap were deposited on a wafer with 550 nm thermal SiO₂ and 100 nm nitride. A two-step recipe was used to etch the cap oxide, then the low-k film, resulting in good etch profiles (Fig. 1). Following ashing in an HDP reactor at low power and temperature, a 25 nm TaN barrier, 100 nm Cu seed and 1 µm of electroplated copper were deposited. CMP of the structure showed no delamination, though initial studies with the bulk XLK film did delaminate upon CMP. The fully integrated structure is shown in Figure 2.

2. A completed single-level copper integrated XLK film with an effective k value around 2.2. (Source: SEMATECH)

From this integration work, SEMATECH determined that an O₂-free ashing process is needed to minimize damage to XLK films. Line-to-line leakage current distribution measurements showed leakage decreased from post-CMP to post SiN passivation and after further thermal cycling. Evidently, moisture was outgassing from the XLK film during thermal processing. TEM and EDX analysis showed no copper diffusion into the XLK dielectric. SEMATECH encourages optimizing CMP, etch, ash, clean and other processes to ensure low-k film integrity.

The group from IMEC emphasized the importance of the wet ammonia treatment step as it determines the film's refractive index, modulus and Si-H bonding density. They reported the optimum treatment time depends on many factors, including the ratio of ammonia to moisture in the chamber and chamber conditions. The IMEC researchers used an ACT-8 track system from TEL (Austin, Texas) for processing. The resulting film demonstrated a uniform pore size of 3.5, a fully interconnected network structure with 50% porosity, and direct correlation between % remaining of SiH bonds and increased ammonia treatment time. After the cure, capping layers of silane-based PECVD oxide and SiC (using Dow Corning's trimethylsilane source gas) were deposited on separate wafers, yielding a dielectric constant of the XLK film of 1.85 in both cases. The k value measured from the capacitors with the oxide cap degraded with time, reaching a value of 2.18 after three weeks, though no change in k was observed for the SiC capped film. The researchers did not know why the k value of the low-k film with the oxide cap degraded, and they will be investigating this phenomenon further, according to IMEC's Donaton.

In the IMEC study, the XLK material showed very good resistance to moisture uptake, low film stress (20 MPa) and stability to thermal cycling. After exposing the low-k film to oxygen- and hydrogen-based plasma processes, they measured a decrease in optical and adsorption porosity and film shrinkage of 5-10%. Such shrinkage leads to a decrease in total porosity, but Donaton noted the film maintained its interconnected pore structure. Exposure to a fluorine plasma showed no change in optical porosity, but the adsorption porosity decreased, possibly due to fluorocarbon polymer formation in the C₂F₆ plasma.

The IMEC researchers etched 0.25 µm trenches using an Ar/CF₄/CHF₃/O₂ chemistry to open the SiC hard mask and Ar/CF₄/CHF₃ to etch the XLK, followed by dry strip and electrical evaluation using meander/fork test structures with linewidths down to 0.2 µm. This test confirmed the effectiveness of the etching/stripping processes of the low-k film, good barrier/seed deposition, copper fill and copper CMP of the structure. Interline capacitance at 1 MHz ranged from 0.5 to 1.2 pF for 0.4 to 0.2 µm line spacings.

Together, the SEMATECH and IMEC studies show the feasibility of integrating an ultralow-k film at the first level of copper interconnect. Such studies represent a first step toward moving ultralow-k films, such as XLK, from the research labs to the production line. Ultralow-k films could be required as soon as 2002 for the 0.10 µm generation.•

— Laura Peters

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