Many integrated circuits use thin-film resistors, usually for controlling the current or the voltage of other components of the integrated circuit. Thin-film resistors are typically deposited by evaporation in vacuum conditions or by sputtering and are made of different materials, such as CrSi, TaN, NiCr.
Even though very well controlled processes are used, the initial resistance of these components has a tolerance of 3-15%, according to the desired value of resistance. More accurate values may be obtained by physically removing portions of the resistor in a subsequent trimming operation, carried out via laser during electrical wafer sorting (EWS). For this purpose, laser-beam trimming systems have been developed, which present various advantages, such as speed, accuracy, and cleanliness. These systems may be controlled via computer for modifying and adjusting the electrical parameters of the components during measurement.
Other trimming techniques, such as electrical trimming or reconfiguration of a network of resistors by means of fuses, may be used even after packaging.
In high-precision resistors, another important factor to be taken into consideration is the variation of resistance with temperature. For example, current-reference circuits use load resistors for controlling the current level. As the operating temperature of the circuit changes, also the current changes.
Even though in order to manufacture good resistors high-resistivity materials may be used, they typically have a high temperature coefficient of resistance (TCR), higher than 100 ppm/° C. TCR is defined as the normalized first derivative of the resistance with respect to the temperature and provides an adequate way for measuring the performance of a resistor.
To obtain resistors with an almost zero TCR, various solutions have been proposed, such as resistors connected in series or in parallel, of different materials having different TCRs, and more precisely positive and negative TCRs matched so that the effective temperature coefficient is almost zero (see, for example, U.S. Pat. No. 7,217,981, which is incorporated by reference). These solutions are however rather complex and may require various interconnection levels and the use of costly laser trimming apparatuses.
Italian patent application TO2008A000951, filed on 18 Dec. 2008 (corresponding to EP patent application 09179740.7 and U.S. patent application Ser. No. 12/638,922), all of which are incorporated by reference, describe a resistor of phase-change material formed by two portions with different crystalline phases, so as to have opposite TCRs, and electrically coupled so as to obtain a resistor with an approximately zero global TCR.
In this resistor, the precise value of the resistance is obtained via laser trimming or electrical trimming so as to cause a phase change in one of the two portions. In particular, electrical trimming may be carried out by feeding the resistor with current pulses so as to heat the material and to obtain the phase change envisaged for adjusting the resistance and the temperature coefficient. This technique has proven particularly effective where the resistor has an area of smaller width than the rest of the resistor since, in this case, only in this point there is a current crowding such as to heat the material by the Joule effect up to melting point and to cause the phase and resistance change. Trimming is thus practically confined to this smaller area.
This solution has proven advantageous, but in some cases it may require forcing high currents and/or applying high voltages to the resistor to be trimmed. The application of high electrical quantities, however, may damage the resistor or the device that integrates it. Furthermore, the trimming current depends upon the resistor geometry, which may require remodulating the trimming method (and in particular the current and voltage to be applied to obtain a desired variation of resistance), whenever the resistor geometry is changed.
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