Structural and electrical properties of CuAlMo thin films prepared by magnetron sputtering
Introduction
Thin film resistors (TFR) are used extensively in electronic circuits due to their high accuracy and excellent long term stability [1], [2], [3]. One of the most widely used TFR materials is nickel–chromium (NiCr) which has a temperature coefficient of resistance (TCR) of <± 50 ppm/°C (ppm/°C) and can cover a wide sheet resistance range of 10 to 500 Ω/□ [4]. However in recent years there has been an increase in demand for lower value TFR in the range 0.1 to 10 Ω, especially in portable devices for the purpose of saving battery power [5]. This requirement is difficult to meet with NiCr due to the associated increase in film thickness, which is costly to deposit and can also cause problems at the subsequent laser trimming stage, where it becomes difficult to ablate.
In a bid to overcome this problem a TFR material of copper–aluminium–molybdenum (CuAlMo) has been developed which possess a lower resistivity of ~ 80 μΩ cm yet retains electrical performance in line with the established NiCr film [6], thus allowing lower resistance films to be deposited in shorter timescales with reduced material usage.
Initial investigations have shown that electrically stable films of CuAlMo possessing sheet resistances of < 1 Ω/□ can be sputtered at a pressure of 0.13 Pa using a cathode power of 1000 W [7]. Following heat treatment at 450 °C in an air atmosphere for 5 h the negative as-grown TCR property of the films was shifted to near zero. The aim of this work is to further investigate the effect of sputtering and annealing process parameters on the structural and electrical properties of the CuAlMo films.
There have been numerous studies undertaken recently in this area [8], [9], [10], [11], [12]. A reduction in sheet resistance and increase in TCR with sputtering time can be largely attributed to increasing film thickness.
In addition to film thickness, the deposition rate is also known to influence the grain size and the number of residual gas molecules or impurities captured within the growing film. Increasing the deposition rate generally leads to the formation of fine grained structures with a low concentration of impurities [13], provided the substrate is cooled. However for un-cooled substrates, increasing the sputtering rate through the energy of the bombarding Ar ions can in turn lead to an increase in the energy of the ejected atoms from the sputtering target. On reaching the substrate these atoms have a higher surface mobility, thus raising the temperature of the substrate and resulting in a larger grained film structure [14]. Furthermore it has also been shown that exceeding a critical deposition rate can lead to the burial of these mobile surface atoms, thereby impeding grain growth [15]. This situation can also result in an increase in the density of vacancies captured in the film, since they have less time to escape.
In addition to deposition rate, sputtering pressure and substrate temperature are also known to play significant roles in determining the structure of the growing film [16]. The combined influences of substrate temperature and sputtering pressure have been considered in a now classical article by Thornton [17].
In this model an increase in sputtering gas pressure allows more Ar atoms to be adsorbed at the substrate surface. This limits the mobility and hence the surface diffusion of arriving adatoms resulting in a structure with porous grain boundaries. Conversely, an increase in substrate temperature enhances surface mobility and conventional bulk diffusion, producing a film structure consisting of columnar grains with fully dense boundaries.
The current study will focus specifically on the combined effect of sputtering pressure and annealing time and temperature on the electrical performance of these CuAlMo films. In particular the influence of these process parameters on the important film properties of sheet resistance, TCR and long term stability is investigated.
Section snippets
Experimental details
The films were sputtered from a CuAlMo 69/24/7 wt.% target using a Circuit Processing Apparatus 900 (CPA) load locked deposition plant which had been modified to give fully automatic control. All films were deposited at the previously optimised cathode power and sputtering time of 1000 W and 24 min, respectively [7]. The sputtering pressure was varied between 0.13 and 0.80 Pa for each experimental run through control of the Ar gas flow rate.
Prior to each experimental run the plant was conditioned
Results and discussion
The electrical properties of the as-grown CuAlMo films with variation in sputtering pressure are shown in Fig. 2. All results are based on the measurement of 20 film samples. The as-grown thickness of the films was measured to be in the range 900 to 1100 nm.
In the higher pressure range of 0.4 to 0.80 Pa an increase in sputtering pressure causes an increase in sheet resistance (Rs) coupled with a decrease in TCR, whilst both of these properties are constant in the lower pressure of 0.13 to 0.4 Pa.
Conclusions
This work has shown that the electrical properties of the CuAlMo thin films are not only a function of their chemical composition but also strongly depend on the sputtering and heat treatment process settings. To obtain films of low sheet resistance with near zero TCR and good long term stability, a low sputtering pressure in the range 0.13 to 0.40 Pa is required.
The development of the CuAlMo film structure during sputtering follows the classical Thornton's model [17]. For films sputtered at
Acknowledgements
This work was funded and supported by TT Electronics, Welwyn Components Ltd and an Industrial Fellowship awarded by The Royal Commission for the Exhibition of 1851.
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Low resistivity and near-zero temperature drift ZrB<inf>2</inf>-Ag composite films prepared by DC magnetron co-sputtering
2022, Materials LettersCitation Excerpt :Thin-film resistors (TFR), as basic electronic components used in microelectronic devices, are facing an increasing demand such as with lower resistivity to save battery power as used in portable devices, near-zero temperature coefficient of resistivity (TCR) for high accuracy and high reliability [1–3]. Cu-Ni or Ni-Cr metallic based thin films with a resistivity of ∼102 μΩ·cm are usually used as TFR materials for low ohmic applications, but most of the films had to endure post-deposition annealing to approach the state of zero TCR, in the process there exists a risk of degrading other devices or components [4–5]. Our latest research found that ZrB2 film exhibits a negative TCR character and relatively lower resistivity [6].
Electrical resistivity of CuAlMo thin films grown at room temperature by dc magnetron sputtering
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