Atomic Layer Deposition of SnO2 for Selective Room Temperature Low Ppb Level O3 Sensing

Ozone sensing is particularly interesting as exposure has been shown to be very harmful to human health. Ozone exposure has been associated with an increased risk of developing respiratory disease, inflammation of airways and increased response to allergens. Individuals suffering from respiratory diseases such as asthma are particularly sensitive to ozone and both short and long term exposures can induce undesirable effects on the human condition [1-4] and ozone concentrations have increased strongly in highly populated areas over the past decade [2,5]. Key challenges in metal oxide sensors are high power consumption associated with high operating temperatures and lack of selectivity as the sensors tend to respond broadly to many different gases. In this study we demonstrate low power and selective ozone sensors using SnO₂ metal oxide thin film gas sensors produced by Atomic Layer Deposition (ALD). ALD enables precise film thickness control and conformal coating tailoring of film thickness to the Debye length, enhancing depletion region effects and increasing surface to volume ratios for enhanced sensitivity at lower temperatures.

SnO₂ films were deposited via ALD using tetrakis dimethylamino tin precursor and ozone reactant on SiO₂/Si substrates. The films were annealed in atmosphere or inert ambient at 400C for 30 minutes and were then patterned for lift off using standard photolithography. Ten nm of titanium were used for an adhesion layer followed by 150 nm of Au.

The SnO₂ deposition rate was characterized with variable angle spectroscopic ellipsometry and determined to be 1.1 A/cycle. X-ray photoelectron spectroscopy and X-ray diffraction were used to evaluate the film properties.  Ozone testing was done using custom built testing chamber and a Teledyne T700U gas calibration system equipped with a UV ozone generator and photometer feedback for precision concentrations. Resistance measurements were made during gas exposure using a Keithley 4200 semiconductor parameter analyzer.

We have characterized our ALD SnO₂ process and film properties.  XRD confirmed rutile phase of the film after 400C anneal for 30 minutes as shown in Figure 1. Ozone exposure resulted in increased resistance of the SnO₂ films and response was demonstrated for various concentrations at room temperature resulting in ozone detection at several microwatts of power. Response to concentrations from 50-150 ppb can be seen in figure 2, and to 400 ppb in figures 3 and 4. It is found that the recovery of the SnO₂ sensors is related to the film thickness. Films of 12 nm of SnO₂ demonstrated extremely slow recovery while films of 6 nm recovered more quickly even at room temperature as shown in Figures 3 and 4. The recovery process can be expedited by the addition of thermal energy to aid in ozone desorption but low power applications at room temperature can take advantage of both of these modes of operation. Thicker sensors with slow recovery may be useful in measuring dose of ozone over time for asthmatics and other health conscious users while the thinner films may be better suited toward instantaneous concentration quantification.

Both sensors offer faster response and recovery with increasing temperature enabling trading off of power needs with sensor response but room temperature operation offers yet another advantage. Without any added thermal energy, our films respond to tens of ppb of ozone while not responding to hundreds of ppb of NO₂ or ppm of CO, typical concentration found in the environment. Response to each of these gases can be seen in Figures 5 and 6. Sensor reset can be accomplished with periodic thermal cycles to maintain low power levels and selectivity advantages of room temperature operation.

In summary, we have demonstrated room temperature SnO₂ gas sensor deposited by ALD method. It was observed that the ALD SnO₂ gas sensor enables sensitivity to ozone in the 50-400ppb range with selectivity toward NO₂ and CO, common interfering gases in the environment. It was also shown that our SnO2 sensor operates at room temperature and hence the total power consumption reduces to mW range that is feasible for wearable sensor system.