Atomic Layer Deposition of Nanostructured Tunable Resistance Coatings: Growth, Characterization, and Electrical Properties

We have developed ALD method to synthesize nanocomposite coatings comprised of conducting, metallic nanoparticles embedded in an amorphous dielectric matrix.  These films are nominally composed of M:Al₂O₃ where M= W or Mo, and are prepared using alternating exposures to trimethyl aluminum (TMA) and H₂O for the Al₂O₃ ALD and alternating MF₆/Si₂H₆ exposures for the metal ALD.  By varying the ratio of ALD cycles for the metal and the Al₂O₃ components in the film, we can tune precisely the resistance of these coatings over a very broad range (e.g. 10¹¹-10⁴ Ohm-cm).  These films exhibit Ohmic behavior and resist breakdown even at high electric fields of 10⁷V/m.  Moreover, the self-limiting nature of ALD allows us to grow these films inside of high aspect ratio substrates and on complex, 3D surfaces. 

To investigate the ALD growth mechanism for the nanostructure composite films we employed in-situ quartz crystal microbalance (QCM), quadrupole mass spectrometry, and Fourier transform infrared (FTIR) absorption spectroscopy studies. For M:Al₂O₃ films, QCM showed that the metal ALD inhibits the Al₂O₃ ALD and vice versa.  Despite this inhibition, the relationship between metal content and metal ALD cycle percentage was close to expectations. Depth profiling X-ray photoelectron spectroscopy showed that the M:Al₂O₃ films are uniform in composition and contained Al, O, and metallic Mo or W as expected, but also presence of moderate F and C.  Attempts were also made to investigate the TMA as reducing precursor for MF₆ and resulted in similar kind of resistive coatings but very significant F and C which shows influence on microstructure of the layers. Cross-sectional transmission electron microscopy (XTEM) revealed the film microstructure to be metallic nanoparticles (~1-2 nm) embedded in an amorphous matrix. The transport properties of these M:Al₂O₃ were studied as function of ratio of metal to Al₂O₃ALD cycles.

We have utilized these nanocomposite coatings to functionalize capillary glass array plates to fabricate large-area MCPs suitable for application in large-area photodetectors.  In addition, we have applied these films to serve as charge drain coatings in MEMS devices for a prototype maskless electron beam lithography tool, permitting high resolution electron beam patterns without charging artifacts.  Here we will discuss the ALD growth, characterizations, and applications of ALD tunable resistive coatings.