Atomic Layer Deposition of SnO₂for Selective Room Temperature Low ppb Level O₃Sensing

Abstract

This work demonstrates ultra-low power ozone sensors for real time, continuous, and portable monitoring. Atomic Layer Deposition (ALD) of SnO 2 enables precise control of ultrathin film thickness on the order of the Debye length to enhance sensitivity at room temperature. Correlation between ozone concentration and the rate of resistance change is used to maintain fast response times and ultraviolet (UV) illumination hastens recovery. ALD SnO 2 ultrathin film sensors realize room temperature operation with highly selective detection of 50 ppb ozone with average power consumption of 150 μW making them well suited for real time, portable environmental monitoring systems. High levels of ozone (O 3 ) have been shown to contribute to respiratory symptoms such as chronic cough, wheeze, and shortness of breath and chest colds with phlegm.1 Individuals suffering with respiratory diseases such as asthma are particularly sensitive to O 3 which can trigger an asthma attack hours after exposure.2 According to the Environmental Protection Agency, O 3 concentrations are higher near urban areas, highways and in general outside on hot sunny days highlighting the need for continuous, portable, real time monitoring of an individual's exposure levels in order to correlate personal health with surrounding environments. Sensors used for these applications must have high sensitivity, appropriate selectivity against other gases, low total power consumption, stability, accuracy, reliability and low cost. Among several sensing materials, SnO 2 has been shown to be sensitive toward gases in a variety of papers however metal oxide sensors typically require high operating temperatures to achieve good sensitivity and fast recovery resulting in mW of power consumption making them unsuitable for portable monitoring.3-6 Several reports exist utilizing metal oxides at room temperature but they typically display response and recovery times greater than 10 minutes which also renders them unsuitable for real time monitoring. [7][8][9] Our work utilizes Atomic Layer Deposition of SnO 2 to tailor film thickness to twice the Debye length which has been shown to provide maximum sensitivity due to electron mobility modulation. [10][11][12] We correlate the derivative of the response to ozone concentration and utilize ultraviolet light to compensate for slow response and recovery times, respectively. These methods enable detection of 10s of ppb of ozone with room temperature operation for average power consumption of just 150 μW. Additionally, our sensors demonstrate selectivity over interfering gases NO 2 and CO at typical atmospheric levels making them good candidates for continuous, portable monitoring. ExperimentalThe fabrication of sensor device started with 500nm thermal oxidation of Si (100) substrate for electrical isolation. After oxidation, the SnO 2 sensing material was deposited in an ALD system (Cambridge Nanotech Savannah 100 model). Tetrakis(dimethylamino)tin precursor and O 3 reactants were used to deposit SnO 2 at 200• C. The films were...

Low-Power Wearable Systems for Continuous Monitoring of Environment and Health for Chronic Respiratory Disease

“…The ALD-fabricated 7 nm ultra-thin film SnO 2 metal-oxide sensors demonstrate room temperature operation for ultra-low power consumption of 150 µW, as previously reported [12]. We have tested responses to three different environmental pollutants: CO (50 ppm), NO 2 (100 ppb), and O 3 (100 ppb) using a NIST-certified ozone generator (T700U, Teledyne, Thousand Oaks, CA) and a custom built testing chamber in order to test the ozone response and false positives from CO and NO 2 .…”

“…By pulsing the UV LED at a 1% duty cycle, the average power consumption of the ozone sensor becomes approximately 150 µW [12]. This is a steep decrease in power consumption when compared to commercial metal-oxide sensors (including the one with our system optimizations) illustrating this sensor has strong potential in wearable systems.…”

“…The SnO 2 sensing layer was deposited on oxidized silicon (100) using an ALD system (Model Savannah S100, Cambridge Nanotech, Waltham, MA) with tetrakis(dimethylamino)tin precursor and O 3 reactants at 200°C The films were annealed at 400°C in N 2 for 30 minutes to achieve rutile crystalline phase. Lastly, interdigitated electrodes were deposited via e-beam evaporation with 10 nm of Ti for adhesion followed by 200-nm Au [12]. …”

Analysis of the Sensing Properties of a Highly Stable and Reproducible Ozone Gas Sensor Based on Amorphous In-Ga-Zn-O Thin Film

“…To clearly observe the R - T curves, the time axis was shifted for the samples with 0.5–4-ppm exposure [4,22,23,24,25,26,27]. …”

Validation of a Terminally Amino Functionalized Tetra‐Alkyl Sn(IV) Precursor in Metal–Organic Chemical Vapor Deposition of SnO2 Thin Films: Study of Film Growth Characteristics, Optical, and Electrical Properties

“…The oxygen deficiency of SnO 2 surfaces facilitates reduction–oxidation (red–ox) reactions at the semiconductor surface/gas phase interface in the course of which adsorbates can reversibly position at eligible surface sites . In the case of often observed adsorbed oxygen, that can be present as , O − , , or O 2 − ions depending on the surface temperature, electrons from the semiconductor surface region are transferred to the incoming oxygen to stabilize the formation and adsorption of the ions. By this electron transfer, the electrical conductivity of the SnO 2 thin‐film surface decreases notably.…”