Image analysis – Image transformation or preprocessing – Fourier transform
Reexamination Certificate
1998-07-21
2001-12-04
Boudreau, Leo (Department: 2621)
Image analysis
Image transformation or preprocessing
Fourier transform
C382S278000, C324S617000, C327S003000
Reexamination Certificate
active
06327394
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention disclosed broadly relates to optical tests for measuring switching activity in integrated circuits (“ICs”), and more particularly relates to analyzing data accumulated from such tests.
2. Description of the Related Art
Two disclosures, by the assignee of this application, deal with related technology. The first is U.S. patent application Ser. No. 08/683,837, which was filed on Jul. 18, 1996. That application discloses the use of optical techniques, such as gathering and analyzing photon emissions, for the testing of properly functioning ICs performing normal operations and not merely for testing defective ICs. Such a procedure is referred to as picosecond IC analysis (“PICA”). This first application will be referred to hereinafter as the “Base Application.” The second disclosure is an IBM disclosure number YO9-98-066 and discloses advanced methods of storage and compression of PICA data. This second disclosure shall be referred to hereinafter as the “YO9-98-066 Disclosure.” Both disclosures are included herein by reference, and are more fully explained below.
The Base Application
As part of the design and fabrication of a complex high-speed integrated circuit, it is often essential to observe the logic state switching of the individual devices comprising the circuit. Information on circuit timing is useful to diagnose problems relating to high frequency operation, propagation delays, and critical timing paths. A number of techniques have been developed to make dynamic circuit measurements, such as electron beam testing, electro-optic sampling, photoconductive sampling, and photoemissive sampling. Common to all these techniques is the requirement for providing an external probe (electron beam or laser) to perform the test. This requirement leads to the inability to deternine timing at more than one device at a time, and loads the circuit, etc. For a variety of practical reasons, only electron beam testing has achieved widespread use in commercial chip development. Chief among these reasons is that the other techniques require special structures or materials on the chip which are incompatible with conventional silicon processing. Electron beam testing is limited by the need to access the relevant metal interconnect at the front surface by the electron beam. As logic circuits become more complex, with additional layers of metal interconnects and “flip-chip” bonding, the use of electron beam testing will become problematic.
A summary of the characteristics useful in a diagnostic tool includes: the ability to measure many devices on the chip simultaneously, no special conditions for chip preparation or design, a technique which is non-destructive and non-loading of the device, the ability to measure from the front side or the back side of the wafer, and the ability to measure internal switching speeds exceeding 10 GHz. Such a tool would provide information that would lead to both enhanced device performance and more rapid chip development, prototyping, and debugging. For example, if specific devices or subcircuits of a chip can be identified as limiting the overall speed of the complete circuit, then redesign of, or process modifications for, this portion of the chip can increase the yield of chips that operate at high clock speeds, increasing the performance and the economic value of the chips which are produced.
It has been known for several years that electronic devices, even majority carrier devices such as field effect transistors, including those fabricated from indirect bandgap materials such as silicon, can emit light when in saturation and current is passing through the device. There are a number of inventions, several of which are discussed below, relating to the use of these emissions to probe for failures or long-term degradation in individual devices.
U.S. Pat. No. 4,680,635 addresses the detection of light which is emitted continuously by a defective device on an integrated circuit as a means of failure analysis. This light is emitted as a result of avalanche breakdown, latch-up, current conduction through a damaged dielectric, or electrostatic discharge. Although that patent addresses enabling an image intensifier “for fixed periods of time to provide time resolution” of the images, the purpose of the time resolution is to help identify hot electron-induced long term degradation. In that patent, the term “time varying” refers to the decay or build-up of emissions due to the failure or degradation of a device, and not to the dynamic emissions from a normally-operating circuit which are synchronized with the logic switching of the circuit. The limited scope of that patent can be seen in the specific means chosen to obtain time resolution, that of electronically gating an intensifier. The time resolution obtainable by gating an intensifier is many orders of magnitude too slow for measuring the high speed (>1 GHz) switching of modem operating devices. Such a gating technique also makes such inefficient use of the available photons as to be very difficult to implement.
U.S. Pat. Nos. 4,755,874 and 4,811,090 provide improved means of image processing to aid in detecting the continuous faint emission discussed in U.S. Pat. No. 4,680,635. U.S. Pat. No. 5,006,717 describes a method to estimate the operating lifetime of an integrated circuit by measuring the spectral characteristics and supply voltage dependence of the optical emission associated with hot carriers.
Although each of the above patents considers using the optical emissions from silicon integrated circuits as a diagnostic for circuits, none of them address circuit timing analysis on a circuit with fully functional devices. Instead, those patents disclose the use of continuous or quasi-continuous optical emissions to evaluate circuits which are degrading due to hot carrier effects or which have already failed.
An advance was achieved with the disclosure of the Base Application which disclosed that normally-operating (i.e., filly functional) CMOS devices emit transient pulses of light coincident with logic state switching. Further, these transient pulses of light from normally functioning devices can be used to produce useable information about the timing of such devices.
In the Base Application, optical emission generated by the normal electrical switching of gates in a functioning integrated circuit is used to determine dynamic information about the internal time response of the circuit. By use of a suitable multichannel optical detector which is capable of time resolution of better than 100 psec, temporal information can be obtained from many devices on a chip simultaneously. This temporal information can include, for example, the sequential evolution of the logic state of each device on the circuit. The time resolution is suitable for determining possible timing problems in present and future integrated circuits with switching speeds up to at least 10 GHz.
The optical waveform of the emitted light is used to determine the temporal variation of the electrical voltages in the devices and circuits. Here, the term optical waveform, or time-resolved optical emission data, refers to the time dependence of the optical emission from an individual device which is undergoing periodic variation in its electrical waveform, such as logic state switching. In the common case of MOS circuits, light is emitted mainly when an individual device is in saturation. Light emission from a non-time-varying yet normally operating CMOS logic circuit, which draw very little average current, is essentially undetectable with present detectors. However, individual devices draw significant current and will very briefly be in saturation when the CMOS gate switches logic states.
It was discovered that optical emission from a normally operating CMOS circuit when undergoing switching is detectable by photon counting and other high sensitivity light detectors. Consistent with the above discussion, the emission is found to
Kash Jeffrey Alan
Tsang James Chen Hsiang
August Casey P.
Boudreau Leo
Fleit Kain Gibbons Gutman & Bongini P.L.
Gibbons Jon A.
International Business Machines - Corporation
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