Active solid-state devices (e.g. – transistors – solid-state diode – Lead frame
Reexamination Certificate
2000-02-29
2001-05-01
Clark, Sheila V. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Lead frame
C257S677000
Reexamination Certificate
active
06225684
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a leadframe for integrated circuits. More particularly, the present invention relates to a resistor element having a low temperature Coefficient of Resistivity (TCR) and configured within a leadframe, such as may be used to facilitate current measuring applications and the like.
BACKGROUND OF THE INVENTION
Although various other types of packaging methodologies, for example, metallic and ceramic packaging, can be used with microelectronic devices, molded plastic encapsulated packaging continues to be greatly used by industry. This packaging is typically low cost, is easy to assemble, and provides adequate protection of the device from water vapor and other contaminants. Typically, the microelectronic device is mounted on a die pad and then electrically connected to a leadframe. The leadframe generally facilitates a number of wiring connections. After encapsulation, the outer leads of the leadframe can be connected, such as by soldering, to a printed circuit board or other external device. Thereafter, the packaged device may be utilized for various applications, such as for voltage or current monitoring and the like, or other conventional integrated circuit applications. Once packaged, however, the microelectronic devices can be difficult to calibrate with traditional techniques.
Many semiconductor devices and integrated circuits are designed to operate over wide temperature ranges. For example, high currents can cause significant changes in temperature for an integrated circuit. However, these temperature ranges can dramatically affect the operation of various elements within these semiconductor devices. In particular, changes in temperature can detrimentally impact resistor elements by changing the resistance thereof. The change in resistance with temperature is generally known as the temperature coefficient of resistance (TCR) and is determined by measuring the incremental change in resistance of a resistive material with temperature change. Typically, the TCR is measured in parts per million variation per degree centigrade, or ppm/° C.
In manufacturing integrated circuits, many attempts have been made reduce the exposure of the circuits to problems caused by higher temperature coefficients and thus increase the precision of the circuits. For example, one solution includes the selection of equal value resistors having matching negative and positive TCR's to facilitate cancellation of the temperature coefficients. However, the selection process is costly to manufacturing and the selected resistors can realize gradual shift that causes the initial matching of the TCR's to become unsuitable. Another approach to reduce the problem caused by temperature coefficients is the use of thin-film resistors that have lower temperature coefficients, such as precision resistors comprising, for example, tantalum, nickel-chromium or Cermet (Cr—SiO). However, a drawback to such prior art resistors has been the associated high costs for the additional processing required during fabrication. Still further, other approaches to the problem caused by temperature coefficients have included the manipulation of the materials by annealing with temperature, or the careful controlling of the deposition of substrates during manufacture of the resistor elements. However, such manipulation and control processes have proven complex, costly and often unpredictable.
To reduce the size and manufacturing expense of packaged integrated circuit devices, resistive elements have been integrated into an integrated circuit package as a part of the leadframe assembly. However, the above problems caused by the TCR in the resistors are still prevalent. Thus, attempts to minimize the effects of TCR have included the providing of temperature compensation capabilities after fabrication of the encapsulated microelectronic device. For example, one technique discloses a method of compensating for measurement fluctuations caused by temperature variations arising the temperature coefficients of resistors. In particular, the above technique discloses the use of additional sensing electronics having post-fabrication temperature compensation capabilities within a current sensing application. Generally, the sensing electronics include a sensing resistor comprising various metals, such as copper, which has a temperature coefficient of approximately 3800 ppm/° C., Alloy 42, which has a temperature coefficient of approximately 2500 ppm/° C., or nickel alloys, which have a temperature coefficient of approximately 3300 ppm/° C. Accordingly, the temperature compensation circuit is designed to measure the temperature variations caused by the temperature coefficient of the sensing resistor and then essentially cancel out the variations with an offset temperature compensation reference voltage. While such techniques may provide adequate results, the additional complexity and associated manufacturing costs can prove undesirable in many applications.
Accordingly, a long-felt need exists for a leadframe having a low temperature coefficient resistor element, for example under 500 ppm/° C., which does not require complex schemes for reducing the temperature coefficient, such as the temperature coefficient cancellation or other resistor manipulation processes described above. Moreover, a long-felt need exists for a current monitoring circuit having a self-contained current sensor within a leadframe and coupled to a microelectronic device in which the current monitoring circuit does not require post-fabrication temperature compensation or other complex techniques.
SUMMARY OF THE INVENTION
A leadframe in accordance with the present invention addresses many of the shortcomings of the prior art. In accordance with one aspect of the present invention, the leadframe is configured to provide a resistive element incorporated within the leadframe. The resistive element suitably comprises a material that can provide a temperature coefficient of under 500 ppm/° C., preferably approximating 100 ppm/° C. or less, while also being readily configurable for integrated circuits. Exemplary embodiments of the resistive material may include Constantan, Manganin, or Evanohm.
The leadframe and integrated resistive element may be utilized in a variety of integrated circuit applications. An exemplary embodiment includes the incorporation of the leadframe and integrated resistive element with a microelectronic device in a current monitoring circuit. Accordingly, variations in temperature will not dramatically affect the accuracy of the leadframe current sensing resistor during operation. Additionally, after encapsulation of the leadframe and integrated resistive element and any such integrated circuit device, the gain of the encapsulated circuit may be suitably calibrated or adjusted by various calibration techniques.
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Hobson Larry D.
Stitt, II R. Mark
Clark Sheila V.
Snell & Wilmer L.L.P.
Texas Instruments Tucson Corporation
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