Ying-Chieh Lee
A Ni-Cr-Mn-Y-Nb resistive thin ï¬lm was prepared in this study using DC and RF magnetron co-sputteringfrom Ni0.45-Cr0.27-Mn0.13-Y0.16 cast alloy and niobium targets. The electrical properties and microstruc-tures of Ni-Cr-Mn-Y ï¬lms with Nb addition under various annealing temperatures were investigated. Thephase evolution, microstructure and composition of Ni-Cr-Mn-Y and Ni-Cr-Mn-Y-Nb ï¬lms were char-acterized using X-ray diffraction (XRD), ï¬eld-emission scanning electron microscopy (FESEM), ï¬eld-emission transmission electron microscopy (HRTEM). All Ni-Cr-Mn-Y-Nb ï¬lms annealed at 300oCexhibited an amorphous structure. The Ni17Y12crystalline phase was observed in Ni-Cr-Mn-Y-Nb ï¬lmswith or without lower Nb content when annealed at 400oC. When the annealing temperature was set to300oC, the Ni-Cr-Mn-Y ï¬lms exhibited a resistivity ~480mU-cm with the temperature coefï¬cient ofresistance (TCR)at þ30 ppm/oC. However, Ni-Cr-Mn-Y ï¬lms with 14 at.% Nb exhibited the smallesttemperature coefï¬cient of resistance (þ5 ppm/oC) with the resistivity ~585mU-cm after annealing at300 C in air.
Introduction
The rapid development and improvement of information andtelecommunication technologies as well as the expansion of digitalindustries are based to a substantial degree on high precision,reliable, integrated, low noise and low power consuming electricalcomponents . The resistor is one of the fundamental componentsused primarily in electronic circuits. The demand for thin ï¬lm re-sistors with low temperature coefï¬cients of resistance (TCR) andhigh precision has dramatically increased in recent years. An important technical parameter of thin ï¬lm resistors is thetemperature coefï¬cient of resistance (TCR). A high TCR will result inthe resistance value drifting, inï¬�?uencing the resistor accuracy asthe temperature changes . The main factors inï¬�?uencing TCRinclude the ï¬lm composition, sputtering process and annealingtemperature. The ï¬lm composition plays a decisive role amongthese three factors. Therefore, employing an appropriate methodfor depositing a suitable ï¬lm composition is essential to obtaininghigh-resistance resistors with a low TCR.Extensive rapid development in high entropy alloy (HEA) ï¬lmwere obtained recent years by Yeh . High entropy alloys aremulticomponent systems composed of elements displaying anearly equiatomic conï¬guration with contents ranging between 5and 35 at.% . It was generally found that high entropy alloys formsimple solid solution structures (rather than manycomplex phases)at elevated temperatures because of their large mixing entropies.However, it is possible to enhance the resistivity of alloy ï¬lms usingthe high entropy alloy method. According to Matthiessen's rule:
ῤtotat= ῤdefects + ῤrimpuritie + ῤrthermal.
2. Experimental procedure
2.1. Ni-Cr-Mn-Y thin ï¬lmNickel (Ni), Chromium (Cr), Manganese (Mn), and Yttrium (Y)powders, as the main raw materials, were chosen to smelt thetarget for high-resistance thin ï¬lm resistors. Alloy ï¬lms weredeposited onto polished alumina (Al2O3) substrates. These aluminasubstrate were scribed for the TCR measurement into 1.6 ×0.8 mmcell sizes using a laser. Glass and Si wafers were used as substratesfor the sheet-resistance measurements and thin ï¬lm thickness,respectively. These substrates were cleaned using a D.I. water-cleaning procedure and dried in nitrogen before loading into thesputtering chamber.Ni-Cr-Mn-Y thin ï¬lms that were 80 nm in thickness weredeposited onto the substrates using a DC magnetron sputteringsystem. A Ni0.45-Cr0.27-Mn0.13-Y0.16 alloy with a diameter 76.2 mmwas used as targets. The DC power was ï¬xed at 50 W. The sput-tering chamber was evacuated to a background pressure of 5 ×10-7torr using a cryo-pump. Sputtering was performed using argon gaswith a purity of 99.999% at ï¬�?ow of 60 sccm using mass ï¬�?ow con-trollers at a working pressure of 3 ×10-3torr for gas introductioninto the chamber.
2.2. Ni-Cr-Mn-Y-Nb thin ï¬lmThe niobium target was made from Nb powders, which ishelpful to improved TCR and stabled ï¬lm structure. Ni-Cr-Mn-Y-Nb thin ï¬lms 80 nm in thickness were deposited onto the substratesusing a DC and RF magnetron co-sputtering system. A Ni0.45-Cr0.27-Mn0.13-Y0.16 alloy and niobium with a diameter 76.2 mm were usedas targets. The Ni-Cr-Mn-Y alloy target was set at the DC position.The niobium target with a diameter 76.2 mm was set at the RFposition. To obtain different niobium contents in the Ni-Cr-Mn-Yï¬lm, the DC power was ï¬xed at 50 W and the RF power waschanged from 20 W to 120 W. The sputtering chamber was evac-uated to a background pressure of 5 ×10-7torr using a cryo-pump.Sputtering argon gas with a purity of 99.999% at ï¬�?ow of 60 sccmwas performed using mass ï¬�?ow controllers with a working pres-sure of 3 *10-3torr.
2.3. Analysis
Thin ï¬lms deposited onto glass plates at room temperature weresubjected to transmission electron microscopy (TEM) and X-raydiffraction (XRD) studies, Thin ï¬lms deposited onto Al2O3sub-strates (size: 400 mm2) were used to measure the electrical prop-erties. The substrate temperature was 25oC. The as-deposited ï¬lmswere annealed at 250-400oC for 2 h, at a heating rate of 5oC/minin air.The sheet resistance Rsof the ï¬lms was measured using thefour-point probe technique. The thickness tof the ï¬lms wasmeasured using FE-SEM (cross-section). The resistivity measuredusing the four-probe method was consistent with the resistivityobtained by the Rsand tsamples. The TCR values of the thin ï¬lmswere measured using thin long strips cleaved from the substrate.Electrical contacts at the two ends of the resistive strips were ob-tained by selectively coating the ends with sputtered silver. The DCresistance of the strips was measured using a digital multimeter(HP 34401A) at different temperatures (25oC and 125oC). The TCRof the thin ï¬lms was measured using the following equation
TCR ¼[(DR/DT) 1/R] *106ppm/K
Conclusion:
This study investigated thin ï¬lms fabricated for the purpose ofpreparing high resistivity with low-TCR thin ï¬lm resistors. Thedependencies of the Ni-Cr-Mn-Y-Nb thin ï¬lm electrical propertieson the annealing temperatures and the ï¬lm compositions were alsoinvestigated. Our conclusions are summarized as follows:A Ni-Cr-Mn-Y resistive thin ï¬lm was prepared using DCmagnetron sputtering from Ni0.45-Cr0.27-Mn0.13-Y0.16cast alloy tar-gets. When the annealing temperature was set to 400oC, Ni-Cr-Mn-Yï¬lms with some amounts of Ni17Y12nanocrystalline phases wereobserved. The Ni-Cr-Mn-Y ï¬lm annealed at 300oC presented anamorphous structure using TEM analysis. The Ni-Cr-Mn-Y ï¬lmsannealed at 300oC exhibited the smallest temperature coefï¬cientof resistance (þ30 ppm/oC) with a resistivity of ~480mU-cm.A Ni-Cr-Mn-Y-Nb resistive thin ï¬lm was prepared using DC andRF magnetron co-sputtering from Ni0.45-Cr0.27-Mn0.13-Y0.16 castalloy and niobium targets. All of the Ni-Cr-Mn-Y-Nb ï¬lms annealedat O400OC exhibited an amorphous structure, except for speci-mens with 0% and 6.3% Nb addition at 400oC. Ni-Cr-Mn-Y ï¬lmswith 6.3 at.% Nb annealed at 400oC presented amount of Ni17Y12crystalline phase. Amorphous structures were obtained for Ni-Cr-Mn-Y ï¬lms with higher Nb content (î�?�?14 at.%), which is attributedto the high entropy alloy effect. The TCR values gradually shiftedfrom positive to negative with increasing in niobium content. Asthe annealing temperature was increased, the TCR shifted fromnegative to close to zero. This shift is attributed to the crystallineand oxidation. The TCR of Ni-Cr-Mn-Y-Nb thin ï¬lms could beadjusted to close to zero using the annealing process and adding Nbcontent. The oxidation layer thickness in the ï¬lms was increasedsigniï¬cantly from 8 nm at 14 at.% Nb to 11 nm at 31.6 at.% Nb. Thisresult indicates that the ï¬lm surface oxidation becomes thickerwith more added Nb. The electrical properties indicated that Ni-Cr-Mn-Y ï¬lms with 14% Nb addition annealed at 300oC exhibited thesmallest temperature coefï¬cient of resistance (þ5 ppm/oC) with aresistivity of ~585mU-cm. For practical purposes, it is importantthat ï¬lms with a small TCR possess high resistivity
This work is partly presented at 19th Nano Congress for Next Generation August 31-01, 2017 Brussels, Belgium