Tool Enables Gbit Device Failure Analysis

Alexander E. Braun, Associate Editor -- Semiconductor International, 1/1/1999

In gigabit-level devices, feature sizes go below 0.15 µm. Since some device failures occur in a portion of the gate or interconnecting wire, failure areas are much less than 0.15 µm, under 0.03 µm, or about 100 atoms. Physical and chemical analytical instruments with high resolution for device structure observation and high sensitivity for composition analysis (see Figure) are urgently needed to analyze failures of this scale.

With the reduction of device dimensions, chemical bond analysis of regions in the submicron areas and below has become increasingly important in thin film deposition and etching. Current analysis capability of chemical bonds (spatial resolution) is unsatisfactory for failure analysis. This is due to difficulties in creating a submicron or under X-ray beam using an XPS, which is widely utilized for chemical bond analysis.

Researchers at Hitachi (Tokyo), headed by Dr. Y. Mitsui, have developed a method using a TEM with an electron energy loss spectrometer (TEM-EELS), which produces an electron beam that can be minimized to 0.001 µm f. A chemical bond can be identified by measuring energy loss, since the loss that occurs when an accelerated electron passes through a sample depends on chemical bonds present.

ANALYTICAL INSTRUMENT TRENDS

Current analysis level   Development of large area inspection and in-situ techniques

Fig. 1. Current status and future trends of analytical instruments for failure analysis. (Source: Hitachi)

The need to evaluate electrical characteristics in actual circuits is also increasing because of the difficulty in making test elements with the same electrical characteristics and microstructures in deep submicron devices as in actual circuits. In the work by the Hitachi group, a nanoprober capable of inspecting a transistor I-V and metal resistivity in actual circuits was used.

The group has reported results where these new analytical instruments were used to analyze a failure showing high interconnect resistivity in a sub-0.25 µm DRAM. Using the nanoprober, the failure was pinpointed to one contact between the bit-line and substrate. It had been formed by a CVD-W plug with CVD-TiN and sputtered Ti. Previously, blanket CVD-W plugs with a sputter-deposited Ti contact layer and TiN barrier layer were widely used for contact electrodes. However, deep submicron contacts with high aspect ratios complicated sputter-deposition due to poor step coverage. Therefore, the group investigated use of CVD techniques for Ti and/or TiN, instead of a sputtering technique.

During the analysis, a thin amorphous layer was observed. This layer was identified as mostly SiO₂, using the TEM-EELS. Ti or TiO_x could not be seen due to the low deposition rate of sputtered Ti at the plug's bottom. From TEM observations after every process, it was determined that the amorphous layer was produced in a thermal process following W deposition, despite the fact that no processes with oxygen or water were applied after W deposition.

The SiO₂ film formation was traced to the presence of titanic acid (TiO_xH₂O), which is produced from titanium, water and chlorine at room temperature. Titanic acid has an extreme absorbency for water and dissociates water thermally. It is produced after TiN deposition. Water is drawn from the cleanroom air and/or spin-on-glass (SOG) film, and chlorine is taken from TiCl₄ gas for CVD. The titanic acid absorbs a large amount of water, TiO_x(H₂O)n, where n is a large number. In a thermal process after W deposition, titanic acid dissociates the H₂O. Since W film density is high, and the TiN film is porous, the water moves to the interface between Si and TiN through the TiN film. In this mechanism, resistivity depends on the quantity of chlorine and water present, particularly chlorine. The amount of chlorine was analyzed using a recently developed glow discharge optical emission spectrometer (GDS), which provides high reproducibility and a precise depth resolution in composition analysis.

The results, which indicate that reduction of residual chlorine and/or water is extremely important in the TiN-CVD process, point to additional applications for this method, as dimensions continue to shrink and processes to change.