Investigation of NiCr Thin Film Resistors for InP-Based Monolithic Microwave Integrated Circuits (MMICs)

Abstract

We report a systematic study of nickel chrome (NiCr) thin film resistors (TFRs) used as resistors in InP-based monolithic microwave integrated circuits (MMICs). The effects of fabrication process parameters (such as surface treatments or thermal cycles) and substrate materials on the electrical properties of NiCr TFRs are discussed. After completion of MMIC fabrication, the NiCr thin film resistors were found to exhibit lower sheet resistance (R sh ) as compared to the as-deposited films. The origin of this variation has been found to be mainly related to the post-NiCr thermal cycles involved in the IC process technology. In contrast, the R sh values of NiCr TFRs deposited on GaAs or SiN x coated substrates were almost unaffected by thermal annealing. Hence, a thin SiN x passivation layer on the InP substrate has been integrated in the InP-based MMIC process to avoid the NiCr-InP interaction. As a result, NiCr thin film resistors with high sheet resistance uniformity and reproducibility, as well as low temperature coefficient of resistance (TCR) have been achieved. Moreover, the use of a SiN x layer on the InP substrate was also found to improve the inter-device isolation.Although novel material resistors have been developed, nickel chromium (NiCr), and tantalum nitride (TaN) remain the thin film resistors (TFRs) of choice for most applications in the microelectronic industry because of their wide resistivity range, low temperature coefficient of resistance (TCR), good reproducibility and good compatibility with various integrated circuit technologies. In particular, NiCr and related materials have been the subject of numerous investigations, 1-12 covering different aspects of deposition techniques and material properties, as well as potential applications.In monolithic microwave integrated circuit (MMIC) technologies, TFRs are extensively used in feedback circuits, bias circuits, and as terminations. Owing to their properties, they are particularly attractive components for analog and mixed signal circuits, which require precise control of the resistance and thermal stability of the TFR, such as operational amplifiers, and analog to digital converters (ADCs).NiCr resistors are usually deposited as very thin layers (tens of nanometers), directly on the substrate or on dielectric polymer layer, using standard sputtering or e-beam evaporation techniques. As for other thin film resistors, the TCR and resistance of NiCr has, however, been reported to be affected by various process parameters (temperature, composition) and environmental conditions, 2,7,8 which might be at the origin of various performance and reliability problems encountered in integrated circuit.Concerning the properties of NiCr films deposited on III-V substrates, only a few results have, so far, been reported in the literature. Shen et al. 1 compared the characteristics of NiCr, TaN and NiV (nickel vanadium) thin film resistors intended for use in GaAsbased power amplifiers. While TaN and NiV TFRs were directly sputtered on GaAs substrates, NiCr fi...

NiCr thin film resistor integration with InP technology

“…Further, a recently published work [10] reports a 30% decrease in resistance for NiCr resistors on InP after being treated for only 1 min at 300 • C, and a 15% reduction for a treatment at 200 • C for several hours. Reference [10] reports this specific decrease of R sh on InP substrates but not on GaAs, giving a strong hint that specific interface reactions between NiCr and InP are responsible for the decrease in R sh of NiCr. The authors of [10] do not elaborate on the interface reactions themselves.…”

“…Reference [10] reports this specific decrease of R sh on InP substrates but not on GaAs, giving a strong hint that specific interface reactions between NiCr and InP are responsible for the decrease in R sh of NiCr. The authors of [10] do not elaborate on the interface reactions themselves.…”

Fabrication and characterization of thin film Ni–Cr resistors on MMICs

“…Ni-Cr resistors offer specific points of interest for applications with extraordinary demand for high stability as well as low TCR-like chosen circuits for measuring hardware, information handling, media transmission, etc [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. In GaAs monolithic microwave integrated circuits (MMICs), thin film metal resistors are promptly superseding semiconductor resistors where the resistive medium is a mesa etched or an ion implanted GaAs active layer.…”

“…Therefore, there is a limit on the maximum electric field (volts-per-unit-length) allowed in the resistor. This place a limitation on the GaAs resistors when they are used in designing a biasing network, particularly on the drain or collector biasing network where high voltage drop appears [7][8][9][10].…”