Tin oxide (SnO2) is a critical electronic material which has been widely used in an extensive range of applications such as gas sensor , varistor in electronic devices , electro/photocatalyst [3, 4], transparent conductive oxide films  and optoelectronic devices . The most important application field of SnO2 material is in metal oxide semiconductor (MOS) based gas sensors. The MOS sensors are the most preferred and currently used devices among the other sensing mechanisms such as catalytic, electrochemical, and NDIR (Non-Dispersive Infrared) types to detect the pollutant gases (CO, CO2, NOx, SOx, VOCs, etc.) results in diminishing the air quality.
SnO2 based materials can be deposited into the gas sensing devices in the form of thick or thin films . Thick films are composed of porous SnO2 in a thickness of 1 to 10 microns produced from the slurry form. Thin films, on the other hand, have a thickness of 10 nm to 1 micron. Several methods were used to prepare SnO2 based gas sensors, such as thermal evaporation , dip and spin coating , and hydro/solvothermal method [10, 11]. For industrial aspects, sputtering is a widely accepted route for preparing SnO2 thin films used in gas sensing applications among these methods. Sputtering process can be performed from either metallic or ceramic targets. Ceramic targets are preferred to produce thin films with higher quality and reproductively. They have homogeneous optical and electrical properties along with the material, whereas the metallic one is cheaper and more accessible in the market and has higher thermal conductivity. One of the well-known problems during sputtering is the formation of nodules on the target material [12, 13]. The nodules formed by the arching during the sputtering can result in the uncontrolled highly defective film. The arching is mainly related to the non-homogeneity in the chemical, physical, microstructural, and electrical properties of the sputter target. In addition to that, inhomogeneous grain size, a minimum amount of pores and higher density prevent the formation of nodules. The formation of particles during the sputtering process is one of the significant problems which originate from nodules on the target surface. To overcome the nodule and related causes during the sputtering process, target material should be manufactured with higher density (> 92 % of SnO2 relative density ), higher chemical purity (> 99.95 wt.% of SnO2), homogeneous distribution of cations and oxygen mainly in the surface and higher thermal and mechanical stability to reduce the chance of target cracking. Even ceramics with a density of greater than 98% of its relative density and high electrical conductivity are strongly required for DC magnetron sputtering systems employed in the industry .
It is crucial to produce dense SnO2 sputtering targets with homogenous grain size distribution and high chemical purity which provides low electrical resistivity and high thermal and mechanical stability due to getting relatively higher performance in the sputtering process. However, it is difficult to achieve full densification by pressureless-assisted sintering due to the decomposition of SnO2(s) to SnO(g) above 1200ºC where evaporation-condensation takes place . Therefore, it is concluded with only coarsening of tin oxide ceramics with very low densification (60 % relative density) . Leite et al.  investigated pressureless-assisted sintering kinetics of ultrafine undoped SnO2 powder under atmospheric conditions. The results showed that the decomposition rate of SnO2 increases above 1300ºC with the decreasing particle size of starting SnO2 powder (from approximately 100 to 25 nm).
Possible technique to produce tin oxide-based ceramics with a higher density is pressure-assisting sintering . Spark plasma sintering (SPS, FAST) is one of the most favorable sintering techniques in which dense polycrystalline materials are produced in bulk form through powder metallurgy under applied pressure. In comparison with the conventional methods and other sintering techniques such as hot press (HP) and hot isostatic pressing (HIP), SPS is fundamentally different in terms of working principle, which is based on the generation of high pulsed electric current. Thus, both electrical field and joule heating are provided along with uniaxial pressure. This phenomenon comes with some advantages such as; high-speed diffusion, full densification, elimination of impurities on the surface of particles, reduction of sintering temperature, shorting of sintering time, and improving mechanical and electrical properties [17–19].
A few types of research in the literature focus on the pressure-assisted sintering of undoped SnO2 powders. Yoshinaka et al.  achieved 99.8 % relative density by Hot Isostatic Pressing at 900°C for 2 h under 196 MPa using argon gas following by isostatically cold pressing of synthesized undoped SnO2 powders under 343 MPa. Dense SnO2 ceramic has 103 ohm.cm electrical resistivities and approximately 2.0 µm average grain size, as determined by a linear intercept method. In 2005, Park et al.  focused on the spark plasma sintering of undoped SnO2 powders using commercial undoped SnO2 powder (60 nm crystallite size), and they investigated the variation of the relative density and the weight loss with the sintering temperature. It is reported that weight loss began to occur above 1050ºC. The sintered ceramic by spark plasma sintering at 1050ºC for 5 min under 37.5 MPa exhibited the maximum relative density (around 95 %). That is the only study in the literature based on the spark plasma sintering of undoped SnO2 powders. Unfortunately, the decomposition of SnO2 did not discuss in detail; there was no data regarding the phase evolution and microstructural development. Park et al. only focused on the microstructure analysis of SnO2 ceramic sintered at 1050ºC in terms of the formation of pores and the homogeneity of microstructure. Delorme et al.  stated that undoped SnO2 ceramics were produced by SPS in order to improve transport properties that are sensitive to doping elements of oxides (CoO, MnO2, CuO, ZnO, etc.). In this study, they achieved almost 95 % relative density at 950ºC for 10 min under 100 MPa. Delorme et al. mainly focused on the electrical and thermal conductivity of SnO2 ceramics, which were obtained at different SPS conditions. However, this study does not reflect detailed microstructural development among thermodynamical approaches to understand densification behaviour of SnO2 ceramics under SPS conditions. A correlative assessment of chemical purity, density, homogenously distributed grains, and small grain size of sputtering targets is crucial to both investigate and improve the performance of sputtering targets. To eliminate this shortage in the literature, in the present study, it is aimed to develop a fundamental understanding of defining ideal microstructure and its effect on final target properties based on densification behavior, phase evolution, and microstructural development in order to get higher performance and efficiency during the sputtering process and achieve higher quality of as-produced thin films as well. Therefore, in this paper, the effects of process conditions on densification behavior, phase evolution and microstructural development of SnO2 ceramic sputter targets obtained from undoped submicron commercially available SnO2 powders were systematically investigated as a function of sintering temperature (850-1050°C) and time (1–10 min), while the particle size of starting powder, external pressure, atmospheric conditions, and heating & cooling rates keep constant. Furthermore, the results obtained from SPSed ceramics were compared to conventionally sintered SnO2 ceramics and commercial SnO2 targets.