The Influence of Climate Conditions and On-Skin Positioning on InGaZnO Thin-Film Transistor Performance

1 Introduction

The increased interest in flexible and imperceptible electronics has positively impacted on different applications, such as smart textiles Cherenack et al. (2010), epidermal electronics for healthcare Gao et al. (2016), and display technologies Geng et al. (2017). The key element for this rapid evolution is represented by conformable thin-film transistors (TFTs), with stable functionality while a mechanical strain is applied. Among the different TFT technologies, oxide semiconductors, and in particular amorphous Indium-Gallium-Zinc-Oxide (a-IGZO), have proved several advantages to fulfill the requirements for state-of-the-art applications, such as high electrical performance with mobility greater than 10 cm²/Vs, large-area processability, transparency and low deposition temperatures Yabuta et al. (2006), Nomura et al. (2004), Salvatore et al. (2014). The performances of a-IGZO TFTs fabrication on flexible substrate during tensile and compressive strain are extensively reported Park et al. (2009), Munzenrieder et al. (2011), Hasan et al. (2017), Billah et al. (2017), Münzenrieder et al. (2013). In addition to the mechanical stability, the electrical stability at any environmental condition (light exposure, moisture, temperature (T), relative humidity (RH)) is a key aspect for the employment of a-IGZO TFTs on a large scale Costa et al. (2019). Although different studies concerning the effects of the RH on the IGZO TFT operation have proved a reliable variation for the threshold voltage Knobelspies et al. (2018), Lee and Jeong (2018), Kim et al. (2017), no common trend in the variations of the other parameters was observed. The dependence of these variations on several parameters, such as light-excitation Zhou et al. (2014), Dong et al. (2019), Knobelspies et al. (2018), traps density Kim et al. (2011) and defects formation Lee and Jeong (2018), Park et al. (2008), fabrication process Hoshino et al. (2013), Corsino et al. (2020), have been investigated. The results were significantly affected by the active layer thickness, as well as the presence/absence of a passivation layer Park et al. (2008), Chowdhury et al. (2015), Dong et al. (2019), Hoshino et al. (2013) while high T (>150°C) enables the performance recovery Mativenga et al. (2021), Chowdhury et al. (2015), Hasan et al. (2017). On the other hand, the influence of T variation was shown to improve the drain current leading to a negative threshold voltage shift by increasing T Kim et al. (2011), Godo et al. (2010). Despite the impact of T and RH variations on TFTs performance was analysed, all experiments were performed varying one parameter only by keeping the other one constant.

Here, the effects of a simultaneous variation of both temperature and relative humidity were studied. a-IGZO TFTs were first characterized under specific environmental conditions, reproducing the main climate zones of our planet, ranging from Desert (50.0°C, 22%) to Antarctic (−30.0°C, 0%). Additionally, the device response was evaluated by simulating the environmental conditions in proximity of human skin. Here, temperatures associated with three body parts (forehead 35.0°C, arm 32.0°C, and foot 29.0°C) were selected and measurements were performed at three RH levels: normal skin humidity (60%), low humidity (35%), and ultra-high humidity (95%) Cravello and Ferri (2008), Choi and Loftness (2012). The results showed the suitability of IGZO TFTs as active electronics and sensor conditioning platform for both wearable and e-skin applications.

2 TFT Fabrication and Characterization

Figure 1A shows the schematic of the bottom-gate staggered TFT fabricated on a 50-μm thick polyimide foil as substrate. The fabrication process and the experiment are described in the following.

FIGURE 1

FIGURE 1. Bottom-gate staggered IGZO TFTs. (A) Optical picture and schematic. The W/L ratio of the transistor is 560 μm/60 μm. (B) Transfer characteristic and corresponding output (inset) at room conditions: temperature equals to 23.7°C and relative humidity equals to 55%.

2.1 Fabrication

The free-standing polyimide foil was cleaned by sonication in acetone and isopropanol for 5 min, respectively, and cured in a vacuum oven at 200°C overnight. 50-nm thick SiN_x was deposited through plasma-enhanced chemical vapor deposition (PECVD) on both substrate sides as a buffer layer to ensure good mechanical and chemical stability during the fabrication. Then, the process was started with a 35-nm Cr metal gate deposited using electron beam evaporation and patterned through standard photolithography. 25-nm thick Al₂O₃ gate dielectric was deposited using atomic layer deposition (ALD) and 15-nm thick a-IGZO was sputtered at room temperature to form the active layer. Wet etching was performed to structure the semiconductor island and the vias. 10-nm Ti adhesive layer and 60-nm Au were deposited through electron beam evaporation to form the Source/Drain contacts and patterned by lift-off. Finally, an additional 25-nm thick Al₂O₃ was deposited to provide the device passivation. The maximum temperature of the process was 150°C, reached during the ALD. More details on the fabrication process can be found in Munzenrieder et al. (2011). An optical picture of the finalized device is shown in Figure 1A.

2.2 Experiment

The measurements were carried out within an Espec SH-262 climate chamber by setting temperature from −30.0°C to 50.0°C and RH from 0 to 95%. The experiment was conducted in dark to avoid light-induced instability on the IGZO active layer. The flexible TFT was fixed on a rigid glass substrate. The electronics characterization was performed through a Keithley 2614B source meter. The electrical interconnection between the device and the electronics setup was obtained by placing small pieces of Cu tapes at the edge of the polyimide stripe. Here, short Cu wires were glued through a silver conductive paste to connect the TFT pads and the tapes. Similarly, long Cu wires were glued and connected with the source meter. Data were recorded by means of a customized LabVIEW program.

2.3 Performance

The first measurement was performed at room T and RH (23.7°C, 55%) to evaluate the TFT performance. Figure 1B shows the TFT transfer characteristic and output. Note that the gate current (I_g) (not reported here) was in the range of 1 μA and the drain current (I_d) was 0.17 mA for V_GS = −6 V. These non-idealities were caused by the employed measurement setup (long Cu wires and source meter) since a I_g and an I_off in the order of pico-ampere (pA) were obtained by performing the electronics characterization in a probe station using a parameter analyzer (Agilent Technology B1500A).

The extracted parameters were obtained as follows: threshold voltage V_TH = −3.96 V, subthreshold swing SS = 314 mV/dec, on/off current ratio I_on/I_off = 1.39 × 10⁴, linear field-effect mobility μ_lin = 8.10 cm²/Vs. They were evaluated according to standard TFT model equations Sze and Ng (2006). The negative V_TH defined the TFT functionality in depletion mode.

The mechanical stability of the device was analyzed at room conditions Munzenrieder et al. (2011), Münzenrieder et al. (2013). The influence of both tensile and compressive strain on the electronic parameters was evaluated when the IGZO TFTs were bent to a bending radius of 4 mm (corresponding to a tensile strain of 0.63%) Münzenrieder et al. (2013) and 9 mm (corresponding to a compressive strain of 0.27%) Munzenrieder et al. (2011).

3 Results and Discussion

3.1 Climate Zones

The TFT response to weather variations was evaluated by reproducing the most extreme climes on Earth planet, with their typical temperatures and relative humidity conditions as shown in Figure 2A. The transfer characteristics and the corresponding output were grouped for temperature below and above room T, in Figures 2B,C, respectively. For each weather condition, the transfer characteristic was acquired in the linear region (V_DS = 0.1 V) and saturation region (V_DS = 3 V). The gate current was not affected by climatic variations and remained constant in the microampere (μA) range. The electrical performance parameters are shown in Table 1 according to the order of the experiment. First, the TFT performances were evaluated from room T and RH (23.7°C, 55%) to negative temperatures. In particular, different climate conditions were simulated in the following order: Tundra (−2.0°C, 0%), Antarctic (−30.0°C, 0%), Taiga (−13.0°C, 0%), Greenlands (8.0°C, 60%), Rainforest (15.0°C, 80%). This protocol has allowed stable conditions (temperature and humidity) in the climate chamber during the TFT characterization. Within this region, a first positive shift of 0.22 V was observed from the initial condition to −30.0°C, 0% while the other parameters remained stable. Then, T was progressively increased up to 15.0°C as well as the RH from 0% to 80%. Here, an opposite shift of V_TH was observed to a more negative value. This trend of V_TH was in agreement with literature results showing how water vapour absorption induced charges accumulation on the active layer by depleting the channel Kim et al. (2017), Park et al. (2008). Coherently, the gradual reduction of V_TH was observed up to Jungle conditions (26.0°C, 90%). Finally, a major impact of RH was observed when the device was brought to the high temperature zones with a reduction of RH. Specifically, the following geographical locations were mimicked: Savannah (29.0°C, 60%), and Desert (50.0°C, 22%). Despite T increasing up to 50.0°C between these two locations, the overall effect on V_TH was a positive shift from −4.02 V to −3.86 V. An increase of the I_off was denoted at high variations of the RH (ΔRH>30%), in agreement with Park et al. (2008). Accordingly, the I_on/I_off ratio reduced of one order of magnitude down to 6.73 × 10³ and the SS deteriorated. Differently from the other parameters, the linear mobility was not influenced by RH since it increased with T, showing a maximum variation of 10% at the highest T (50.0°C, Desert).

FIGURE 2

FIGURE 2. IGZO TFTs performance at different climate zones. (A) Schematic representation of the simulated geographical locations. The legend reports the associated T and RH conditions. Transfer characteristics and corresponding output (insets) at temperatures (B) lower and (C) greater than room T.

TABLE 1

TABLE 1. TFT electrical performance parameters at different geographical locations.

3.2 On-Skin Applications

The TFT suitability for wearable applications and epidermal electronics was evaluated by testing the device at three RH (low 35%, normal 60%, high 95%) and three temperatures to resemble the different body locations, as highlighted in Figure 3A. In particular, the transfer characteristics and the corresponding output are shown in Figure 3B for the forehead at 35.0°C, in Figure 3C for the arm at 32.0°C, and in Figure 3D for the foot at 29.0°C. The performance parameters are shown in Table 2. The V_TH dependence from both T and RH was coherent with expectations. The smallest value for V_TH (−4.12 V) was measured in the forehead at 35.0°C, 95% by proving the impact of increasing both T and RH on this parameter. On the other hand, the positive shift of the V_TH was observed according to the increase of RH, while keeping T constant Chowdhury et al. (2015). A clear influence on the TFT performance was observed at high humidity. Here, the I_off was dramatically increased, deteriorating the I_on/I_off ratio by two orders of magnitude. Furthermore, the SS increased to 1.5 V/dec at the hottest T and 95% meaning a less efficient TFT switching operation. Regarding the mobility variation, the results showed its dependence on both T and RH. A reduction of the linear mobility was observed when the RH changed from the normal condition (60%) to both high and low levels. This was coherent to previous experiments performed at constant T reporting the electronics characterization of two a-IGZO TFTs stored at ultra-dry condition (5%) and high humidity (95%) Lee and Jeong (2018).

FIGURE 3

FIGURE 3. IGZO TFTs performance for on-skin applications. (A) Simulated body locations. Transfer characteristics and corresponding output (insets) for (B) forehead, (C) arm, and (D) foot at three different RH levels: normal (60%), low (35%), high (95%). W/L equals to 560 μm/60 μm.

TABLE 2

TABLE 2. TFT electrical performance parameters at different body locations.

After all the experiments, the TFT was again characterized at room T and RH (26.0°C, 55%). The results showed a variation of V_TH of 0.2 V (V_TH = −3.76 V), stable I_on/I_off ratio (I_on/I_off = 2.04 × 10⁴) and an increase of the μ_lin less than 5% (μ_lin = 8.37 cm²/Vs) compared to the starting values (V_TH = −3.96 V, I_on/I_off = 1.39 × 10⁴, μ_lin = 8.10 cm²/Vs). The stability of the IGZO TFT to T and RH conditions is linked with the presence of the Al₂O₃ passivation layer, preventing the device degradation due to moisture absorption Corsino et al. (2020), and oxygen vacancies formations Zhou et al. (2014).

4 Conclusion

In this work, the suitability of a-IGZO TFT as active electronics and sensor monitoring device was proved. The electronics parameters coherently changed under extreme T and RH variations while TFT operation was preserved. Evaluating the TFT performance at the corresponding T and RH conditions of several simulated geographical locations as well as human body parts, V_TH has undergone a positive/negative shift according to the reduction/increase of both T and RH. A greater influence on switching operation was also observed at RH > 90%. The sensitivity to epidermal microclimate defined the a-IGZO TFTs as an appropriate candidate for health monitoring sensors. Future works have to consider the combined effect of T and RH with other triggering effects, such as illumination and/or mechanical stress, to support the functionality of the device as a part of a wearable integrated electronic system working at any environmental condition.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

Author Contributions

FC conceived the work and wrote the article. FC, HSO, MCA, and MC carried out the experiments. MCA customized the labVIEW program. SP, NM, and GC supervised the work.

Funding

This work was partially supported by the Free University of Bozen (RTD Call FAST 2020, FERMI), by the Autonomous Province of Bozen-Bolzano/South Tyrol (Provincia Autonoma di Bolzano/Alto Adige–Ripartizione Innovazione, Ricerca, Universitá e Musei) through the International Joint Cooperation between South Tyrol-Switzerland (FLEXIBOTS, grant no.: 2/34), by the Autonomous Province of Bolzano-South Tyrol’s European Regional Development Fund (ERDF) Program (project codes EFRE/FESR 1068-Senslab), and by the Open Access Publishing Fund of the Free University of Bozen-Bolzano.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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