Effects of Substrates on the Performance of Pt Thin-Film Resistance Temperature Detectors

Abstract

Pt thin-film resistance temperature detectors (RTDs) have been fabricated by magnetron sputtering on various substrates, including silica, polyimide (PI) and LaAlO₃ (LAO) (100) single crystal. The influences of different substrates on the performance of Pt thin-film RTDs have been studied. It is revealed that the substrates exhibit a significant dependence on the temperature coefficient of resistance (TCR). Silica, PI and LAO substrates yield TCRs of 3.2 × 10⁻³, 2.7 × 10⁻³ and 3.4 × 10⁻³ /K, respectively. The Pt thin-film RTDs on LAO substrate exhibit a significantly larger TCR, compared to most of the other reported values. These devices also demonstrate a fast response time of 680 μs, which is shorter than that of many other reported RTDs. Furthermore, Pt thin-film RTDs on PI substrates could serve as flexible detectors, maintaining a consistent linear relationship between resistance and temperature even when bent.

1. Introduction

The measurement of temperature is a key aspect in many fields, including the electronics industry, environmental monitoring and medical treatment. Platinum (Pt) resistance temperature detectors (RTDs) have been utilized in various regions to measure the temperature, due to their capability to withstand oxidation, the large temperature coefficient of resistance (TCR) and their wide measurement range [1,2,3,4,5]. Traditional wire-wound types of Pt RTDs (such as birdcage or helical RTDs) composed of Pt wires cannot perform well in the in situ temperature measurement of operational components (like rotating blades) due to their sensitivity on vibration [4]. The high cost of Pt wires also prohibits the wide application of such wire-wound RTDs. In this case, Pt thin-film RTDs are fully of interest, as they may possess a fast response time due to their small volume [6,7]. Furthermore, thin-film sensors with quite a small size can be easily integrated with microchips [8]. In addition, Pt thin-film RTDs can be directly fabricated on special test parts without significant influences on the environments being tested, such as with rotating engine blades [2,9,10].

TCR and response time have been considered as two of the key parameters of Pt thin-film RTDs. These two parameters strongly correlate with each other. The large thickness and size of Pt may be beneficial for achieving a large TCR, which may suppress the response time due to the rise of thermal capacity. On the contrary, the small thickness and size of Pt thin film could reduce the thermal capacity, yielding a fast response time, while the small grain size of Pt thin film with its small thickness may result in the small TCR. However, many of the reported TCR values of Pt thin-film RTDs with a thickness of one hundred nanometers remain less than 3.2 × 10⁻³ /K [11,12,13], which is notably smaller than 3.9 × 10⁻³ /K at 20 °C for bulk Pt [14]. The detailed mechanisms for the small TCR of Pt thin-film RTDs remain unclear. Various sources have been proposed to be related with such a small TCR, such as the microstructure, chemical composition, grain size, the interdiffusion of the adhesion layer, and the oxidation of the adhesion layer [15,16,17,18,19,20]. In addition, the response time of the Pt thin-film RTDs has rarely been investigated. In practical applications, the chemical inertness of Pt makes it challenging for its thin film to adhere to the surface of common substrate materials, such as silicon dioxide and glass [21,22]. Consequently, some metals are often interposed between the Pt thin film and the substrate to act as adhesion layers. Given its exceptional chemical and physical properties, Ti stands as a notable selection for the adhesion layer [23].

In this work, the effects of substrates on the performances of Pt thin-film RTDs have been studied. Silica, PI and LAO substrates have been used to manufacture the Pt thin-film RTDs by magnetron sputtering. It is demonstrated that a LAO single-crystal substrate gives the maximum TCR compared to the other two substrates. Additionally, the fast response time has been identified in such Pt thin-film RTDs grown on LAO single-crystal substrates.

2. Experimental Details

Figure 1 shows the schematic device structure of Pt thin-film RTDs. In order to study the effects of various substrates on the performances of Pt thin-film RTDs, a series of Pt thin films were grown on 5° vicinal cut LaAlO₃ (LAO) (100) single-crystal, silica and PI substrates by RF magnetron sputtering (MIS-560B, 13.56 MHz) in Ar (with a gas purity of 99.999%) atmosphere without intentional heating on the substrate. Ti layers with a thickness of about 5, 13 and 26 nm were first deposited on the substrates to enhance the adhesion. Following this, Pt layers with a thickness of approximately 106 nm were deposited on the top of Ti layers as the sensing units. The thickness of Ti and Pt layers was measured by a surface profilometer (Ambios Technology Company, Santa Cruz, CA, USA). Metal masks were employed and fixed on the surface of substrates. The background vacuum of the sputtering chamber was around 2 × 10⁻⁴ Pa, while the total sputtering pressure was fixed at 0.4 Pa. RF powers of 70 and 45 W were applied in Ti (3 inches, a purity of 99.99%) and Pt targets (3 inches, a purity of 99.99%), respectively. The morphology and roughness of Pt thin-film RTDs have been checked by atomic force microscopy (AFM, Shimadzu SPM-9700, Kyoto, Japan) and scanning electron microscopy (SEM, FEI Nova Nano SEM 450 FE-SEM, Hillsboro, OR, USA). The phase structures of thin films have been checked by X-ray diffraction (XRD, Rigaku D/MAX 2500V/PC X, Tokyo, Japan). Rapid thermal annealing was performed in air with a heating up rate of 90 °C/s and cooling down rate of 15 °C/min for such Pt thin films by a furnace (RTP-500, Beijing, China). Pt thin films on both silica and LAO substrates were annealed in air at 900 °C for 3 min, while Pt thin film on PI substrate is annealed in air at 300 °C for 10 min. After the rapid thermal annealing treatment, Pt wires (a diameter of 100 μm) were bonded to the Pt thin-film RTD electrodes by high-temperature silver pastes. The resistance–temperature (R-T) curves of Pt thin-film RTDs are determined by a multimeter (Keithley DMM6500 6½, Solon, OH, USA) via a four-wire method in a cooling system from 80 to 300 K (Lakeshore 331, Columbus, OH, USA).

3. Results and Discussion

The effects of the Ti layer’s thickness (5, 13 and 26 nm) on the roughness of Pt layers have been studied. Figure 2 shows the AFM images of Pt/Ti bilayer thin films on silica substrates after air annealing with a Ti thickness of 5 nm in Figure 2a, 13 nm in Figure 2b and 26 nm in Figure 2c. The root mean square (RSM) roughness values calculated for the surfaces are 28.392 nm, 17.728 nm and 36.368 nm, respectively, for the Ti thickness of 5 nm, 13 nm and 26 nm. This means a Ti thickness of 13 nm gives the relatively low roughness for a Pt layer. Thus, a Ti thickness of 13 nm is employed in this work.

Figure 3 shows the R-T curves (80–300 K) of Pt thin-film RTDs grown on silica, PI and LAO substrates, respectively. In Figure 3, HR and CR represent the resistance recorded in the heating up and cooling down processes, respectively. As shown in Figure 3, the R-T curves identified in HR and CR processes overlap well, which indicates a good stability for these Pt thin-film RTDs. Furthermore, the average TCR of these RTDs is calculated by the following formula [15]:

$$TCR=\alpha ={\displaystyle \frac{R_T-R_0}{R_0\left(T_1-T_2\right)}}$$

(1)

where R₀ is the resistance value measured at 80 K and R_T is the resistance measured at 300 K; T₂ and T₁ correspond to a temperature of 300 and 80 K, respectively. The TCR of Pt thin-film RTDs on silica, PI and LAO substrates is 3.2 × 10⁻³, 2.7 × 10⁻³ and 3.4 × 10⁻³/K, respectively. Such a TCR of 3.4 × 10⁻³ /K for Pt thin-film RTDs on LAO substrates is much larger than in most of other reported ones grown on various substrates (see

Table 1. The TCR of Pt thin-film RTDs obtained in this work is compared with other reported ones of Pt thin-film RTDs on various substrates.

Substrates of RTDs	TCR(10−3/K)	Reference
Alumina	2.400	[12]
Si/SiO2/Si4N3/ SiO2	2.810	[20]
Si and Al2O3 ceramics	<3.200	[24]
Flexible hastelloy tapes	2.790	[2]
Glass (b270)	2.800	[25]
Silicon	2.885	[26]
Glass	2.090	[27]
Polyimide (PI)	2.700	This work
Silica	3.200	This work
LaAlO3 single crystal	3.400	This work

).

In order to understand the mechanisms behind the different TCRs of these Pt thin-film RTDs, the resistance at 80 K, the slope of $\mathsf{\Delta }R/\mathsf{\Delta }T$ and their TCR have been listed in

Table 2. Pt thin-film RTDs on silica, PI and LAO substrates.

Substrates of Pt Thin Film RTD	Silica	PI	LAO
Resistance at 80 K (Ω)	83	143	33
Slope $\mathsf{\Delta }R/\mathsf{\Delta }T$ (Ω/K)	0.99	0.98	0.45
TCR (10−3/K)	3.2	2.7	3.4

. It is observed that LAO substrates yield the smallest resistance of 33 Ω, while the silica and PI substrates give a resistance of 83 and 143 Ω, respectively. The large resistance of Pt thin film on PI substrates may come from its low annealing temperature of 300 °C, which may produce the relatively poorer crystallization and smaller grain size. This has been evidenced by the XRD analyses. The XRD patterns of Pt thin-film RTDs on silica, LAO and PI are shown in Figure 4, which shows the relatively large full width at half maximum (FWHM) of the Pt (111) diffraction peak for Pt thin-film RTDs on PI. The much smaller resistance of Pt thin-film RTDs on LAO than that on silica may be due to the better crystallization with a smaller FWHM of the diffraction peak (see Figure 4). However, the slope of ∆R/∆T of Pt thin film on LAO substrates is much smaller than the ones for silica and PI substrates. Therefore, the largest TCRs of Pt thin film on LAO substrates mainly come from its small resistance.

Since LAO substrates yield the larger TCR, the response time of Pt thin-film RTDs on LAO substrates has been evaluated. A pulsed infrared laser with a wavelength of 1080 nm, a pulsed duration of 1 ms and a power of 15 W was radiated on the surface of Pt RTDs. Figure 5 shows the resistance vs. time curve, for when the Pt thin-film RTDs were exposed to the laser irradiation. The corresponding temperature as a function of time is also obtained by considering the relationship between resistance and temperature, which is linearly fitted from Figure 3. As shown in Figure 5, a response time of 680 μs (from 10 to 90% of peak value) has been determined. The response time is much smaller than many other reported response times of various temperature detectors, including RTD, thermocouple (TC) and fiber optic temperature sensors (FOTS) (see

Table 3. The response times of Pt thin-film RTDs on LAO in this work are compared with those from other reported temperature detectors.

Materials	Categories	Response Time (ms)	References
Ti/Pt/Ti thin film	RTD	80	[6]
Ni/Cr thin film	RTD	7500	[28]
Graphene	TC	240	[29]
Indium oxide and indium tin oxide	TC	4–5	[33]
NiCr/NiSi thin film	TC	0.8	[30]
Polymer fluorescent fiber	FOTS	240,000	[31]
MWCNT-Ag-PVDF	RTD	11,000	[32]
Pt/Ti thin film on LAO	RTD	0.68	This work

) [6,28,29,30,31,32,33].

Although the Pt thin-film RTDs on PI substrates exhibit a relatively small TCR, the flexibility of PI substrates may offer the opportunity to manufacture the flexible Pt thin-film RTDs. Thus, the electrical properties of Pt thin-film RTDs on PI substrates in bending states have been studied. Figure 6 shows the R-T curves of Pt thin-film RTDs (from 300 to 500 K) on PI substrates with 150° compressive bending. It is observed that the resistance still shows a linear relationship with the temperature, even in the 150° compressive bending states. However, the slope of ∆R/∆T in the bending states drops down. As seen in Figure 6, even after being in bending states for 60 min, the performances of Pt thin-film RTDs on PI substrates can recover well. This means Pt thin-film RTDs on PI may possess great potential applications for flexible temperature detectors.

4. Conclusions

Pt thin-film RTDs composed of Ti/Pt bilayers have been fabricated by magnetron sputtering on silica, PI and LAO substrates. The performances of these RTDs have studied. It is demonstrated that the TCR of Pt thin-film RTDs exhibits notable dependence on the substrates. Silica, PI and LAO substrates yield a TCR of 3.2 × 10⁻³/K, 2.7 × 10⁻³/K and 3.4 × 10⁻³ /K, respectively. Such a large TCR of 3.4 × 10⁻³ /K for Pt thin-film RTDs on LAO single-crystal substrate exceeds most of the other reported values, which may mainly come from its small resistance. Further, these RTDs on LAO show a fast response time of 680 μs, which has been identified by a pulse laser. Such a response time is smaller than most of the other reported ones. Additionally, the flexible performances of Pt thin-film RTDs on PI substrates have been checked. The Pt thin-film RTDs on PI substrate in the bending states also possess a good linear relationship between resistance and temperature.