Precision resistor

Sunday, 21 January 2007

A resistor is a common circuit element that provides a specified electrical resistance under specified conditions. Electrical resistance, in turn, is defined as the ratio of the potential difference between the ends of a conductor and the current flowing through the conductor. Electronic devices are made up of a variety of components including, for example, transistors, resistors, capacitors and inductors. The characteristics of these components may depend on the material used to make them, the physical dimensions of the components and the operating conditions. Advances in integrated circuit technology have produced a trend toward the use of low voltage differential signaling and high-speed terminated buses. This development has placed greater importance on the implementation of precise electronic components and circuit designs capable of driving signals with an output impedance that matches the characteristic impedance of the terminated buses. The ability to integrate high accuracy passive components such as resistors and capacitors with active devices has become increasingly important. In semiconductor industry, resistors are increasingly integrated in integrated circuits as passive elements. Integrated circuits that require passive resistors often have tight tolerances on the resistance value of these resistors. These resistors are required to be very precise, as small as possible and they are required not to heat up themselves and the surrounding traces and systems beyond a specified range of temperature. Under this circumstance, high precision chip resistors have been demanded as alternatives to conventionally used trimming volumes. In particular, the demand for chip resistors having a low resistance and a small temperature coefficient of resistance used in a power supply circuit has been increased. Precision resistors are critical components in applications such as analog and mixed signal integrated circuits. Reducing the variation of the resistance values of precision resistors over the operational temperature range is critical to maintaining the stability of an analog or mixed signal circuit. In RF circuit applications, precision resistors are needed for I/O circuitry implementing both radio frequency CMOS an RF SiGe technology. High tolerance resistors are important for accurate prediction of models and statistical control. In RF devices and circuits, high tolerance resistors are needed that have good linearity; a low temperature coefficient of resistance (TCR) which is the normalized first derivative of resistance and temperature, and provides an adequate means to measure the performance of a resistor; a high quality factor (Q); and are suitable for high current applications.

The resistance of a resistor may depend on the size of the resistor, the composition of materials used to make the resistor and the operating temperature. The resistance, R, of a resistor is proportional to the length, L, of the resistor and the reciprocal cross sectional area, 1/A, of the resistor; the L and A are measured in the direction of current flow. The basic equation for resistance of a resistor is thus: R=L/A. In order to meet these requirements, conventional resistors used in integrated circuits have large-area dimensions or take up a large chip area, respectively, so that the heat can be readily dissipated downwards to the substrate usually made of silicon. Due to the increasing miniaturization in the field of integrated circuits these space requirements must be reduced. As CMOS technology continues to allow gate sizes to shrink, the potential for an increase in circuit density and therefore the total number of semiconductor devices allowed per semiconductor circuit chip has increased dramatically. In devices and circuits where high component precision is usually not required, such as digital circuits, resistor area size is more easily managed. However, in many analog devices where resistor precision, large resistance values and even resistance matching are required, the resistor area required may be the controlling factor in determining device density. For example, in integrated circuit products that oscillators are used a high precision current source is used to charge the capacitor to establish a precision frequency. The precision current source is typically controlled with a precision resistor. Because of the precision required, the high precision resistor is typically provided as an external resistor to an integrated circuit that includes the current source. It would be desirable to obtain a completely integrated current source to obtain a precision oscillator output. In high-performance, high-speed integrated circuit devices the signal paths between integrated circuit chips are effectively transmission lines. The impedances of the input and output circuits at the chip are different from the impedances of the signal paths, which causes reflections and degrade the signal. On-chip resistors to match the resistance on the chip with that of the signal path are the most effective way of reducing these signal-degrading reflections. Accordingly, precision resistors are required to allow adequate resistance matching even for large resistance values. They must have an accurately adjustable and readily reproducible resistance value. The TCR (temperature coefficient of resistance) of such a precision resistor must also be accurately adjustable and readily reproducible.

As previously mentioned, analog circuits require a resistor containing a precise resistance value. Moreover, it is highly desirable that resistors can be manufactured to have predetermined or tailor-made resistance values for differing applications. Thus, precision resistors that have a predetermined resistance and that are made of materials having low temperature and voltage coefficients are desired to accommodate the precision needed in manufacturing analog circuits. Resistors commonly used in complementary metal-oxide-semiconductor (CMOS) integrated circuits are constructed using polysilicon above the semiconductor surface or implanted layers below the surface of the silicon. Polycrystalline silicon resistors have been used in the electronic circuit industry for many years. Polysilicon resistors used in modem CMOS circuits are normally constructed as a pattern of squares by depositing a uniform polysilicon layer on the field oxide of the integrated circuit. Polysilicon resistors are simple and inexpensive to fabricate, but their resistance values vary with applied voltage and temperature, they exhibit high parasitic capacitance, and often have poor matching characteristics. Polysilicon resistors generally have high sheet resistance tolerances ranging from 15 to 20%. This means the sheet resistance changes by +/-15 to 20%. Diffused resistors have a dopant diffused therein to give it the desired resistance. Such resistors are doped to achieve the desired resistance value, which is controlled by the length and width of the resistor used, depth of diffusion and the resistivity of the dopant used. The use of diffused resistors is popular because of their compatibility with the remainder of the semiconductor manufacturing process. They can be formed at the same time as the other circuit elements, and hence, do not add to the fabrication cost. They are desirable because they may be tailored to the space available and the resistivity needed. However, while a diffused resistor's resistance can be tailored for a specific application, its resistance may change depending on the temperature or operating voltage, which can lead to diminished transmission or reception quality. The resistance value of diffused resistors can change as the result of subsequent processing temperatures, operating temperatures or applied operating voltages. In addition, diffused resistors may also suffer from parasitic junction capacitance associated with the resistor and the underlying region, and exhibit high temperature and voltage coefficients.

To overcome the drawbacks associated with above mentioned resistors, thin-film deposited resistors, such as thin metal film transistors, have been used. Thin film resistors are very attractive components for high precision analog and mixed signal applications. Thin-film resistors are generally considered to be more precise than resistors made by diffusion or by deposited polysilicon. This is due to the superior temperature coefficient of resistivity, and voltage coefficient of thin-film resistors, when compared to diffused resistors and polysilicon resistors. In addition to a low thermal coefficient of resistance and low voltage coefficient of resistance, thin film resistors provide good resistor matching and good stability under thermal stress. Thin film resistors are used in integrated circuits to implement the desired functionality of the circuit, including biasing of active devices, serving as voltage dividers, assisting in impedance matching, etc. They are typically formed by deposition of a resistive material on a dielectric layer, and subsequently patterned to a desired size and shape. Numerous resistive materials, including lightly-to-heavily doped polysilicon, silicon chrome (SiCr), nichrome (NiCr), tantalum, and cermet (Cr-SiO), have been used to form thin-film resistors. These resistor materials are generally evaporated, sputtered, or CVD deposited onto a substrate wafer at a metal interconnect level and subsequently patterned and etched. Precise resistance control of the thin film resistor is essential for high precision analog circuits such as analog-to-digital converters and digital-to-analog converters. The performance of thin-film resistors is defined by a number of parameters which include the resistor value, the resistor tolerance, and the temperature coefficient of resistance (TCR). In order to achieve a higher level of precision than that achievable by fabrication processes, it is known to use a laser to trim a thin-film resistor fabricated on a silicon substrate. The laser alters the shape of a resistor and thereby brings its resistance to a desired value. Alternatively, the resistor may be severed altogether, if used as part of a resistor trimming network. For ease of laser trimming, the thickness of the thin film resistor is made as thin as possible consistent with the requirement of the sheet resistance need for the particular circuit design.

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