- Nano Express
- Open Access
Performance of Nano-Submicron-Stripe Pd Thin-Film Temperature Sensors
© The Author(s). 2016
Received: 23 March 2016
Accepted: 22 July 2016
Published: 28 July 2016
Dozens of small dual-beam thin-film temperature sensors with a total width down to 430 nm were fabricated and tested. The sensors were all made from 90-nm-thick Pd thin films, where the width of the narrow stripes was 70–100 nm and that of the wide ones was 210–800 nm. Two different calibration methods showed consistent and repeatable sensitivities of 0.7–1.2 μV/K for the sensors, confirming that the sensitivity mainly depended on the width configuration of each sensor. By integrating arrays of such sensors on a practical testing platform using hybrid e-beam lithography and photolithography techniques, we demonstrated that these sensors were capable of detecting a weak surface temperature difference of 0.1–0.2 K at microscale, and they could be scaled up as built-in temperature sensors in many practical devices.
The rapid development in micro/nanoscience and technology and the emerging trend towards Internet of Things (IoT)  have aroused a tremendous demand for a variety of novel sensors with small size, high sensitivity, and accuracy, as well as high stability and reliability. Among them, temperature sensors at micro-/nanoscale are extensively demanded. Temperature measurement techniques are usually classified into two categories depending on their working modes: non-contact techniques and contact techniques. Over the past two decades, a number of non-contact techniques have been reported for temperature sensing at small scales. These techniques employ micro-/nanosized fluorescent indicators for temperature detection, such as quantum dots [2, 3], organic dyes [4, 5], metal-organic frameworks [6, 7], fluorescent proteins [8, 9], nanoparticles , and nanodiamonds [11, 12]. They have the merits of high spatial resolution to nanoscale, quick response time as small as microseconds, and are more suitable for two-dimensional (2D) mapping of local temperatures. However, besides a low temperature resolution, a non-contact technique requires a complicated optical system and exposure of the subject under test to the probing beam. These requirements make them not suitable for applications of most flexible electronics and wearable systems, which are hot issues in fields of fundamental research and industry. Contact sensors are more applicable for wearable electronics and systems, especially built-in sensors made from 2D materials . Among the several currently available built-in thermal sensors, conventional thin film thermocouples have advantages in high spatial resolution around 1 μm , a high temperature resolution better than 10 mK , and fast response time of 1-10 μs [16, 17].
Recently, we developed a novel type of thermal sensor, which had a dual-beam structure and was made from a single layer of metallic thin film [18–20]. By using these sensors with a dual-beam configuration of 30–3 μm, 2D mapping of temperature difference at the 40–100 K level and over an area of 0.7 mm × 0.7 mm was demonstrated . The same working mechanism was found valid in smaller sensors, and the total size of the senor was made down to 900 nm . The submicron sensors have a promising potential for application in novel microelectrical devices  and in microfluidic devices for local chemistry and bio-sensing [22–25]. For example, polymerase chain reaction (PCR) process relies on thermal cycling, i.e., repeated heating and cooling for DNA melting and enzymatic replication, so the thermal detection and control during the process is crucial .
Intracellular temperature sensing has become a hot topic recently. A real-time mapping of the subcellular temperature may greatly help the study of gene expression, enzyme reaction, and other cellular activities [26, 27]. For instance, Okabe et al. found that the temperature of nucleus and centrosome of many cells was higher than that of cytoplasm by 0.7–0.9 K in different cycle stages , indicating that a lot more was unknown in cell biology at the sub-cell level. With more and more doubts rising about the intracellular temperature measured with the non-contact techniques , contact sensors with high temperature sensitivity and high spatial resolution which can verify the intracellular temperature are in great demand. Though solid temperature sensors (working with contact mode) such as thermocouples usually show a high temperature sensitivity, they often have a large size much more than 1 μm and are therefore not considered as the first choice for temperature sensing at the submicron scale. For application at the submicron and nanoscales, solid temperature sensors should be made into smaller size meanwhile kept the high sensitivity. Recently, Wang et al. fabricated a novel tip-shaped thermocouple and successfully detected a weak change in intracellular temperatures of individual cells .
In this work, we report the results on the fabrication and performance of dozens of Pd nano-submicron-stripe thin-film temperature sensors. By using a better fabrication procedure, we have squeezed the total width of the smallest dual-beam sensors down to 430 nm, less than half of the previous record . Calibrated with two different methods, these sensors showed repeated sensitivity, which depended mainly on the width configuration of the two stripes and was consistent with previous results . We also demonstrated that these sensors could be made into practical testing devices as built-in sensors by using a combined fabrication technique, where the submicron sensors made with e-beam lithography could be overlapped on large lead patterns made with photolithography technique. In such a hybrid device, these sensors remained a stable and repeatable sensitivity for sensing a weak surface temperature difference of 0.1–0.2 K over a local area of 10–100-μm scale.
The sensors were made with pure Pd thin films, which were deposited with an electron-beam evaporator (AXXIS, Kurt J. Lesker) on 4-in SiO2/Si wafers. The 2-μm-thick SiO2 served as an insulating layer on the 500-μm-thick Si (100) wafer. For the fabrication of nano-submicron-stripe dual-beam sensors, the device patterns were created with standard electron-beam lithography (EBL) processes on a Raith 150-II system at an operation voltage of 20 kV, an aperture dimension of 20 μm, a step size of 8 nm, and a writing field of 100 μm. EBL resist PMMA solution (Mw = 950 K, A4, Allresist) was spin-coated on the wafer with a programmable spin coater (Solarsemi Easyline 200 TT) at 4000 rpm for 1 min and baked at 170 °C for 3 min. Then, a 90-nm-thick Pd thin film was deposited with e-beam deposition at a speed of 0.2 nm/s. Before that, a 0.5-nm-thick Cr layer was deposited as the adhesive layer. Final device pattern was obtained after standard lift-off process in pure acetone.
To make onsite Cr/Pd control sensors for calibration, a 50-nm-thick Cr layer was deposited with the e-beam evaporator, and the 90-nm Pd layer was patterned and deposited together with the dual-beam Pd sensors. The widths for the Cr and Pd stripes were made the same, and dozens of Cr/Pd control sensors with six different widths were fabricated on each single device. For fabrication of the hybrid testing platform, patterns of the outer leads, onsite heater, and big contact pads (shown in Fig. 4) were all fabricated with standard photolithography processes with AR-P5350 photoresist (Allresist). An oxygen plasma was applied to remove the residual photoresist before deposition of a 90-nm-thick Pd thin film.
The thickness of the Pd thin films was determined with an atomic force microscope (AFM, Veeco D3100). Micrographs of the samples were all taken with a field emission-environment scanning electron microscope (SEM, FEI_XL30 SFEG) operating at an accelerating voltage of 20 kV. Electrical measurements were performed on a standard electrical probe station (EB-6). The direct current was applied with a source meter (Keithley 2400), and the output voltage of each sensor was measured with a nanovoltmeter (Keithley 2182A), both controlled with a homemade computerized data acquisition system using Labview program. The output voltage for each sensor usually stabilized after 10–40 s, depending on its distance to the heating zone, and the average stabilized voltages corresponding to different heating powers were used as the experimental values.
Results and discussion
In this work, dual-beam Pd sensors are made into five different width configurations of W1–W2, where W1 and W2 refer to the widths of the wide and narrow beams, as shown in Fig. 1c. These configurations are 310–100, 310–80, 310–70, 210–80, and 210–70 nm, respectively. Figure 1c–e shows SEM micrographs for the sensing region of individual Pd sensors with the configuration of 210–70, 310–70, and 310–100 nm, respectively. The total width of the 210–70 nm sensor is 430 nm (including the spacing between the two beams).
The onsite Cr/Pd sensor shown in Fig. 1f is fabricated for in situ calibration of the Pd dual-beam sensors. Figure 1g is the magnified image of the marked area with a red frame in Fig. 1f. The shape and material of the onsite heater, as well as the spacing between the heater and the Cr/Pd sensor junction are made exactly the same as those shown in Fig. 1a. In order to obtain reliable calibration results, each beam of the control samples was made into several different widths. The one shown in Fig. 1f, g has a beam width of 500 nm.
Figure 2b shows the measurement results of two Pd sensors with width configurations of 800–80 and 500–80 nm. The data are linearly fitted. The sensitivity (defined as ΔS = ΔV/ΔT) values of the two sensors, ΔS = S1 − S2, where S1 and S2 are the Seebeck coefficients for the wide and narrow beams, have been calculated to be 1.19 and 1.02 μV/K, respectively, in our previous work . Since the materials and film thickness used in this work are the same as our previous work, and Seebeck coefficient is an intrinsic value that is determined by the density of conduction electrons and the scattering rate , we assume here that the two sensors shown in Fig. 2b have the same sensitivities with previous data. With this method, we obtain the corresponding temperature difference (ΔT) between the sensor junction region and the cold ends versus the heating power of the onsite heater. These data are shown with the top x-axis in Fig. 2a.
Comparison between the calculated ΔS values and measured ΔS values of submicron Pd temperature sensors
Width W1–W2 (nm)
Calculated ΔS (μV/K)
Measured ΔS (μV/K)
0.96 ± 0.05
0.84 ± 0.04
0.82 ± 0.04
0.69 ± 0.04
0.50 ± 0.03
1.00 ± 0.05
1.22 ± 0.06
To determine the absolute value of this ΔS Cr-Pd, we have fabricated 4-cm-long 100-μm-wide Cr/Pd thin film thermocouples on 2-in glass wafers and obtained their sensitivity by measuring the output voltage under a large temperature difference between the hot and cold ends of these sensors . The measured data are plotted in Fig. 3b. With a linear fitting of the data, we get the sensitivity ΔS L of these long Cr/Pd thin film sensors to be 9.8 ± 0.5 μV/K. We assume that ΔS Cr-Pd is the same as that for the long Cr/Pd thin film thermocouples, i.e., ΔS Cr-Pd = 9.8 ± 0.5 μV/K. Considering the size effect of Seebeck coefficient observed previously [18, 20], the real value of the onsite small Cr/Pd sensors, ΔS Cr-Pd, could be slightly smaller.
With the sensitivity ΔS of 9.8 ± 0.5 μV/K, the corresponding temperature difference ΔT of each heating power is determined by ΔT = ΔV/ΔS. The results are plotted in Fig. 3c. We also obtain the curve of ΔT versus heating power for the single-layer Pd sensors from Fig. 2b in the same way. Surprisingly, the curve of ΔT versus heating power calibrated with 4-cm-long Cr/Pd thermocouples matches well with the results of the dual-beam Pd sensors of 800–80 and 500–80 nm, as shown in Fig. 3c. This suggests that our dual-beam Pd sensors have the same reliability as the conventional Cr/Pd thin film thermocouples.
The performance of our nano-submicron-stripe dual-beam Pd sensors is further verified on a testing platform. To avoid temperature increase at the cold ends of the sensors while applying heating power at the sensing region, ideally, the cold ends should be far away from the hot zone. Also, on one measurement chip, a sensor array should be fabricated to allow multiple measuring spots. These two requirements lead to a large size of the testing platform. To reduce the cost and fabrication complexity, we have used a hybrid process that combines electron-beam lithography and photolithography techniques for patterning the wafer-size platform with nano-submicron sensors.
Figure 6b plots the raw output voltage data of sensor 1 versus time, for two fixed heating current of 6 and 20 mA. The resistance of the heater is measured to be around 1450 Ω under a small excitation current. The output signals increase first and stabilize after around 20 and 40 s for the current of 6 mA and 20 mA, respectively, where we can see the stabilizing time increase with the current. After stabilization, the voltage has a fluctuation of about ±0.1 μV, corresponding to a temperature fluctuation of ±0.13 K. This is consistent with the physical process of heat transport along the SiO2/Si substrate surface. With further measurement in time, the output does not increase because an equilibrium has been established when the heating power and dissipation are balanced. Therefore, although signal fluctuation is observed in the steady state region, we take the average stabilized output voltage data for each fixed heating powers. In Figure 6c, the experimental data of four sensors, namely sensors 1, 4, 5, and 9 in Fig. 6a, are plotted. With an increasing heating power, sensors 1 and 4 detect the higher temperatures than those by sensors 5 and 9, as sensors 1 and 4 are closer to the heater. Clearly, for sensors 1 and 4, a small ΔT of 0.2–0.4 K is detectable, and the output signal responses linearly to the heating power, showing a good sensing performance. But for sensors 5 and 9, as they are too far away from the heating zone, they show a low signal to noise ratio.
Now we give a brief analysis for the correlation between detected ΔT and the distance to the heater for sensors 4, 5, and 9. The distance from the center of each sensor to the edge of the heater are 22.7, 31.2, and 39.7 μm, respectively. Figure 6d plots the ΔT value versus sensor distance under seven fixed heating powers from 115 to 805 mW. Here we assume the isothermal lines on the surface were the red curves shown in Additional file 1: Figure S1. As the distance increases, Sensors 3, 6 and 9 are expected to show similar results. The colorful diamonds, triangles, and circles represent the experimental data, and the solid curves are fitting results, which are only guidelines to eye for a qualitative analysis. It shows that ΔT values decrease dramatically with the distance. ΔT is almost negligible when the distance is over 40 μm. This result leads to a safe conclusion, that at the cold end region for the sensors where EBL-made cold ends overlap with pre-patterned leads, its 200-μm distance to the heating zone would make ΔT of the cold ends close to 0.
From the experimental data shown in Fig. 2a and Fig. 6c, one can see that our nano-submicron-stripe dual-beam Pd sensors work well in detecting local weak temperature changes on a solid surface. When the sensor-heater distance is as close as 2 μm, as shown in Fig. 2a, a local temperature fluctuation of 0.1 K is detectable. When the sensor heater distance is increased to 20 μm, the detectable ΔT is around 0.2–0.4 K. Therefore, we can expect that when the heating zone is located directly over the sensor, with an insulating space of 20–100 nm, for instance, the present dual-beam Pd sensors may ensure a resolution of 0.1 K with reliable stability.
In this work, we reported the calibration and sensing performance of dozens of dual-beam temperature sensors made from 90-nm-thick Pd thin films. With a clear-cut shape, the total width of the sensors is squeezed to as small as 430 nm. These sub-micron sensors have stable and reproducible sensitivity of 0.7–1.2 μV/K, depending mainly on the width combination of each sansor. We also tested a combined fabrication procedure, where a central array of the Pd sensors was patterned with e-beam lithography and overlapped on patterned long micro-leads made with photolithography technique. The hybrid process remarkably reduced the device cost and showed the scaleup capability of the submicron sensors in a sophisticated device. The testing results showed that over an area of diameter 40–100 μm, the sensors could detect a weak surface temperature difference of 0.1–0.2 K. The merits of simple structure, stable performance, and compatible process to standard semiconductor techniques make the sub-micron sensors applicable in solid electronic devices, microfluidic systems, and even for bio-sensing at the cell level. For instance, it may detect local temperature distribution on an active electronic device with a better temperature resolution than what has been reported , or replace conventional thin-film thermocouples as the built-in sensor arrays in sophisticated lab-on-a-chip systems , and show better performance as they can also be fabricated with their sensing junctions open to the fluid channel .
We thank Haixiao Liu and Weiqiang Sun for valuable discussions. We are grateful to Jia Li and Huarong Xia for technical help in SEM operation. This work was financially supported by NSF of China (Grants 11374016 and 91221202) and partially by MOST of China (Grants 2012CB932702).
XYH carried out most of the sample preparation including device design, e-beam lithography, photolithography and thin film deposition experiments, and conducted electrical measurements of the sub-micron samples. JJX and ZHW performed the large Cr-Pd sample calibration experiments. FY did SEM imaging. SYX conceived the study and coordinated experiments. XYH and SYX did the data analysis and wrote the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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