Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
1999-04-02
2001-07-10
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S1540PB, C361S762000
Reexamination Certificate
active
06259268
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to integrated circuit structures and, in particular, to voltage stress testable embedded dual capacitor structures.
2. Description of the Related Art
An integrated circuit (IC) includes numerous electronic devices, such as bipolar transistors, metal-oxide-semiconductor (MOS) transistors, diodes, resistors and capacitors. These electronic devices are interconnected by an electrically conducting path, typically a metal line. When a capacitor is incorporated in an IC and is essential to the IC's performance and function, it is often referred to as an “embedded” capacitor. For example, an IC designed to drive a cathode ray tube (CRT), commonly referred to as a “CRT driver IC,” can include such electronic devices as interconnected bipolar transistors, resistors and embedded capacitors. These IC electronic devices are adapted and arranged to provide the electronic signals that control (i.e. “drive”) the CRT's electron gun. The embedded capacitor of such a CRT driver IC provides for positive feedback within the IC and, hence, increases the speed of the CRT driver IC. These embedded capacitors are, therefore, distinct from isolated test capacitors, which are not incorporated in an IC . The isolated test capacitors, consequently, do not affect the IC's performance and function. Such isolated test capacitors are described in U.S. Pat. No. 5,179,433 to Misawa et al., which is hereby incorporated by reference.
After an IC is fabricated, the functionality and reliability of the entire IC and its individual interconnected electronic devices are typically tested under a variety of electrical conditions. For the case of an IC that contains an embedded capacitor, it is frequently desirable to test the reliability of the embedded capacitor by subjecting it to a voltage stress test. In such a voltage stress test, a voltage (commonly referred to as a “applied voltage”) is applied to the embedded capacitor to induce a predetermined electric field therein, while a current flow across the embedded capacitor resulting from that electric field is measured. The ICs exhibiting current flows that are greater than a predetermined current limit are rejected as unreliable, while those exhibiting current flows that are less than the predetermined current limit are considered acceptable for shipment to customers, assuming these ICs pass other appropriate functionality and reliability tests.
An obstacle, however, arises during a voltage stress test, when an embedded capacitor's reliability needs to be assured using an applied voltage at a level that is high enough to damage other electronic devices in the IC. For example, when a CRT driver IC includes a conventional single embedded capacitor that requires a voltage stress test using an applied voltage of 200 volts, but the CRT driver IC also includes interconnected bipolar transistors with breakdown voltages of less than 100 volts. To avoid damage to other electronic devices in the IC (e.g. to avoid breakdown of the interconnected bipolar transistors), conventional single embedded capacitors are regularly subjected to voltage stress tests using an applied voltage that is too low to completely guarantee their reliability.
There is, therefore, still a need in the field for an embedded capacitor structure that can be voltage stress tested using a sufficiently high applied voltage to completely assure its reliability, while not subjecting other electronic devices in the IC to a damaging level of voltage. Also needed is a process for voltage stress testing embedded capacitor structures that does not subject other electronic devices in the IC to a damaging level of voltage.
SUMMARY OF THE INVENTION
A voltage stress testable embedded dual capacitor structure for use in an integrated circuit (IC) in accordance with the present invention includes a semiconductor substrate (e.g., a silicon substrate) with an electrically insulating base layer thereon, as well as a first embedded dual capacitor, a second embedded dual capacitor and a probe pad, all of which are disposed above the electrically insulating base layer. The first embedded dual capacitor and the second embedded dual capacitor are directly electrically connected to each other in series and to electronic devices in the IC. The probe pad is electrically connected directly between the first embedded dual capacitor and the second embedded dual capacitor (e.g. via an electrical connection to an electrically conductive bottom plate of the first embedded dual capacitor; and via an electrical connection to an electrically conductive top plate of the second embedded dual capacitor).
Voltage stress testable embedded dual capacitor structures in accordance with the present invention enable first and second embedded dual capacitors therein to undergo a voltage stress test using an applied voltage high enough to guarantee their reliability, without exposing other electronic devices in the IC to a damaging level of voltage. This is because, by employing two embedded capacitors (i.e. a first embedded dual capacitor and a second embedded dual capacitor) connected in series, rather than employing a conventional single embedded capacitor, the applied voltage required during a voltage stress test for each of the embedded dual capacitors is reduced. If, for example, the first and second embedded dual capacitors have equal capacitance values (e.g. equal capacitor areas, identical dielectric material and equal dielectric material layer thicknesses) and are connected in series, then the applied voltage required during a voltage stress test for each is reduced in half. This reduction in the required applied voltage is due to the inherent splitting of voltages between two capacitors connected in series. In order to assure its reliability, each of two capacitors connected in series is, therefore, adequately voltage stress testable at a lesser applied voltage than a single embedded capacitor. For example, if a conventional single embedded capacitor with a capacitance of 1 picofarad requires an applied voltage of 200 volts during a voltage stress test, then the use of a first embedded dual capacitor and a second embedded dual capacitor that are connected in series (each with a capacitance of 2 picofarads to provide an total capacitance of 1 picofarad) reduces the required applied voltage for each capacitor by one half, to 100 volts.
By employing a probe pad electrically connected directly between the first embedded dual capacitor and the second embedded dual capacitor, a voltage stress test can be independently conducted by placing the required applied voltage on each of the first and second embedded dual capacitors. In the absence of the probe pad or if the probe pad is connected to the first and second embedded dual capacitors at a location other than therebetween, a higher test probe voltage would need to be applied to the probe pad itself, thereby exposing other electronic devices in the IC to that voltage, for a given applied voltage actually placed on each of the first and second embedded dual capacitors. Such an employment of a probe pad at a location between the first embedded dual capacitor and the second embedded dual capacitor, therefore, enables conducting an independent voltage stress test of each of the first and second embedded dual capacitors at a sufficiently high applied voltage that assures their reliability, without subjecting other electronic devices in the IC to a damaging level of voltage. For example, if other electronic devices in the IC cannot withstand voltages above 100 volts, each of two equally sized first and second embedded dual capacitors can be independently voltage stress tested using an applied voltage of 100 volts, thereby subjecting the other electronic devices in the IC to only a voltage of at most 100 volts and, hence, avoiding their breakdown. An independent voltage stress test of each of the embedded dual capacitors, using an applied voltage of 100 volts is equivalent to a voltage stress test o
Crozier James L.
Morrish Andrew J.
Salman Muthanna D.
Girard & Equitz LLP
Metjahic Safet
National Semiconductor Corporation
Nguyen Jimmy
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