Nanolink-based thermal devices: Integration of ALD TiN thin films

Alfons W. Groenland
Abstract & Cover

In this work, a new fabrication process is investigated for ultralow power, microelectronic hotplates. These hotplates are based on a small surface (0.0012 -0.12 mm2 ) that is heated by a heater to temperatures in the range of 300-400 °C. These hotplates can be used for instance as gas or flow sensors. Applied as gas sensor, an (exothermic) combustion reaction will take place at the (catalytic) surface in the presence of a flammable gas during which heat is generated. This reaction heat is detected by a change in the (temperature dependent) heater resistance. This means that in a hotplate the heater simultaneously acts as a heat source and thermal sensor. The research, discussed in this work, is part of the ‘Hot Silicon’ project in which ultralow power (10-6 W) hotplates are studied; the power consumption is a factor 1000 less than state-of-the-art ‘low power’ heaters. The ultralow power enables integration of hotplates as for instance gas sensors in portable (battery powered) systems for industrial or domestic applications, for instance for the detection of hazardous (harmful and/or flammable) gases. A combination of various sensors (in a ‘sensor array’ for instance) can be used for the detection of multiple gases. Furthermore, low power gas sensors are generally safer. In this work, the hotplate is based on a small high ohmic conducting cylinder (the ‘link’). The link is embedded in an insulating (glass) layer and positioned between two crossing electrodes. Fabrication of the link is based on etching a hole in the glass layer and coating it with an ultrathin (7-15 nm) titanium nitride (TiN). Due to the excellent step coverage of the ALD process, a perfectly hollow and conducting cylinder is created. In the first part of this work, the material properties of ALD TiN thin films are studied. In the second part, the realized hotplates are discussed, as well as the integration of ALD TiN in the fabrication process. In chapter 2, the resistivity (ρ) and the temperature dependence of the resistance (i.e. the temperature coefficient of resistance, TCR) of ultrathin ALD TiN films are measured using special test structures. The values of ρ and TiN are important parameters for the sensor design, as they can be used for an accurate temperature measurement of the hot surface. A relation is established between the TiN layer thickness and the resistivity and between the TCR and the resistivity: thin TiN layers have a higher ρ and TiN with a high resistivity have a lower TCR. Furthermore, it is shown that the TCR, measured between 25 and 175 °C, remains constant up to 600 °C. In chapter 3, the oxidation behaviour of thin TiN layers is investigated. During the fabrication process and operation hotplate, the ALD TiN layer should not oxidize. Despite the fact that TiN is considered as a very oxidation-proof material, little oxidation is necessary to modify the properties of a 7-15 nm thin film significantly in dry (O2) and wet (H2O) atmosphere at temperatures between 300 and 500 °C and during exposure to oxygen containing plasmas. The kinetics of the oxidation process have been studied. The composition of the ALD TiN film and the generated TiO2 is comparable to sputtered stoichiometric TiN layers. Using a suitable protection layer, the oxidation of ultrathin TiN layers can be prevented effectively. In chapter 4, the design, the novel fabrication process and the electrical characterization of link-based microelectronic hotplates are shown. Hotplates were fabricated with different link sizes: microlinks (∅ 2-6 μm) and nanolinks (∅ 100 nm) and for some hotplates, the silicon underneath the device is removed. By doing so, a suspended membrane is released for thermal insulation. Microlink-based hotplates have a low ohmic link. They reach a temperature of 250 °C with a power dissipation of 2.7 mW and cannot be heated without a suspended membrane. Nanolink-based hotplates can reach a temperature of 280 °C with a power consumption of 5.5 μW. Without a suspended membrane, only factor 2 more power is required for the same temperature. This makes a nanolink-based device without a suspended membrane and interesting candidate for a mechanically robust hotplate. In chapter 5, two techniques are investigated for measuring the temperature of a device using an alternative method than the temperature dependent resistance of the heater. The employed infrared (IR) thermometry method can be applied successfully to microlink-based hotplates. The employed polymer melting method can be applied successfully to measuring large (> 100×100 μm2 ) areas. However, it turned out that both methods, as employed in this work, are not sensitive enough to detect the small amount of heat that is generated by the small (< 1×1 μm2 ) surface of the nanolink based hotplates. Finally, in chapter 6, the unexpectedly high leakage current through the glass around the nanolink is discussed (together with a low breakdown field of the glass) that is observed for some hotplates. The high leakage current is related to a fundamental property of the ALD TiN process, required for manufacturing of the link. The excellent step coverage has the disadvantage that the sensitivity for process related errors in the device increases dramatically. Structural defects in the insulating glass layer are filled in with conducting TiN, leading to a low ohmic conducting path parallel to the link. Experiments using sputtered TiN layers with a worse step coverage show that these structural defects remain unnoticed otherwise. These experiments show the importance of the process integration for the introduction of new process steps and/or materials.

Source of Information
Fred Roozeboom
University of Twente
(Enschede, Netherlands)
External Link
Read Thesis
linkedin invite