Reliability Characterization of Moisture-Induced Degradation of Low-K Dielectric Behavior for Advanced Interconnects
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low-k materials show lower thermal conductivity, higher moisture absorption and permeation and poorer mechanical strength than oxide. Specific steps during low-k IMD processing may also alter reliability through defects induced at active layers level. The aim of this paper is to analyze the way in which moisture stress impacts the insulating properties of the FOX low-k material. FOX is an inorganic material of the hydrogen-silsesquioxane family with k = 2.8 layers about 1 pmr thick are obtained by spin-on coating. Investigated devices are lateral interdigitized comb capacitors (Fig. 1) in a 3-metal level structure (Fig. 2) processed in a conventional way (metal patterning). FOX was used as interline (horizontal) insulator alone or in combination with a 50 nm Si0 2 liner (PECVD/TEOS oxide). Interlevel dielectric is also TEOS-oxide (1 pm), planarized by chemical-mecanical polishing (CMP). Metal thickness t is 0.7 Rm (TiN-Al-TiN), width is 0.4 gm ; line separation in investigated devices is e = 0.3 or 0.4 pm. Metal lines facing length in comb capacitors L is ca. 300 mm. A third split using high-density plasma (HDP) oxide (k = 4) as IMD was processed for comparison. Measurements carried out on a set of capacitors with various metal line separation distances and pitches allowed permittivity derivation, after compensation for fringing capacitance (airinsulated devices were also used for this calibration step). Results are summarized in table I, the permittivity given for FOX+liner being an effective value which corresponds to the composite capacitance of FOX (26 pF, k = 2.8) and oxide liner (74 pF, k = 4.0). 101
Mat. Res. Soc. Symp. Proc. Vol. 565 01999 Materials Research Society
FOX
e 03-0.4 m Linerm
B
PSOO) Fig. 2: SEM image (FIB cross section) of a 3-metal layer stacked comb structure using FOX as IMD, line spacing e = 0.3 im.
Fig. 1I: Schematics of the comb structures investigated in this work: FOX alone (A) and FOX+liner (50 nm plasma SiO2 ). IMD measured capacitance fringing capacitance IMD capacitance permittivity k
HDP Si02 38 pF 13 pF 25 pF 4.0
FOX + liner 32.5 pF 13.3 pF 19.2 pF 3.1 (effect.)
FOX 30.0 pF 13 pF 17 pF 2.8
Table I: Capacitances and permittivities derived after fringing capacitance compensation (comb structures with line spacing e = 0.3 gim, metal thickness 0.7 gtm and facing length L = 300 mm) EXPERIMENTAL All characterizations were performed at wafer-level (8"). The complete set of experiments included thermal shock stressing, moisture stressing and wearout measurement on MOS transistors. No evidence for any critical behavior appeared concerning the first and last subjects. Only moisture stress results are detailed here ; test conditions were 85*C and 85% relative humidity (RH) in all experiments, moisture exposition times ranged from some hours up to 96 hours. Degradation was monitored by measurement (at room temperature) of: (i) 100 kHz capacitance C to evaluate k variations (Fig. 3), (ii) static leakage currents (Fig. 4) and (iii) leakage current transients for stepped voltage (Fig.
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