Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2000-01-13
2001-06-26
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C324S096000, C324S754120
Reexamination Certificate
active
06252222
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to probing of integrated circuit devices with a laser beam.
DESCRIPTION OF RELATED ART
Paniccia et al. U.S. Pat. No. 5,872,360 issued Feb. 16, 1999 and incorporated herein by reference, discloses (see Abstract) a method and apparatus for detecting an electric field in the active regions of an integrated circuit disposed. In one embodiment, a laser beam is provided at a wavelength near the band gap of the integrated circuit semiconductor material such as silicon. The laser beam is focused into a P-N junction such as, for example, the drain region of a MOS transistor. When an external electric field is impressed on the P-N junction such as when, for example, the drain region of the transistor switches, the degree of photo-absorption will be modulated in accordance with the modulation in the electric field due to the phenomena of electro-absorption. Electro-absorption also leads to electro-refraction which leads to a modulation in the reflection coefficient for the laser beam light reflected from the P-N junction/oxide interface.
Wilsher et al. U.S. Pat. No. 5,975,577 issued May 18, 1999 also incorporated herein by reference, discloses dual laser beam probing of integrated circuits. A laser probe beam is used to sample the waveform on an integrated circuit (DUT) during each cycle of an electrical signal test pattern applied to the DUT. For each operating cycle of the test pattern (of the device under test), the probe beam and also a reference laser beam sample the DUT at the same physical location, but at displaced times with respect to each other. Each reference measurement is made at a fixed time relative to the test pattern while the probe measurements are scanned through the test-pattern time portion of interest, in a manner used in equivalent time sampling, to reconstruct the waveform. For each test cycle, the ratio of probe and reference measurements is taken to reduce fluctuations due to noise.
FIG. 6 of Wilsher et al. illustrates a system in which a mode-locked laser source provides the probe pulses. This laser source outputs laser pulses of short time duration with a high frequency laser repetition rate. A reference laser source outputs a laser beam used to form the reference laser pulses. Typically the reference laser source is a continuous wave laser. The laser pulses from the probe laser source and the reference laser source are both optically modulated and guided to a beam combiner by beam deflecting optics. The resulting combined laser pulses are focused through a fiber optic coupler to a laser scanning microscope. Hence, the laser pulses are provided from two separate sources. The resulting combined laser beam is directed onto the DUT, reflected therefrom, and directed onto a photo detector. The probe and reference pulses, which arrive at the photo detector displaced in time, are separately detected and digitized.
Though ratioing reflected probe and reference laser pulses dramatically reduces the sensitivity of the measurement to noise, several factors may limit noise cancellation and prevent the measurement from reaching the shot-noise limit. (Shot noise is the inherent noise in a laser beam.) For example, the modulation of the reflected amplitude of a laser pulse due to electrical activity in the DUT is small compared to the total reflected amplitude. Thus, the modulated signal of interest rides on a large DC offset, which severely limits the effective dynamic range with which the modulated signal is digitized. Also, the noise on the reference and probe laser pulses, which may differ in wavelength, may be imperfectly correlated due to wavelength dependent interactions with the DUT as well as due to the displacement in time between the pulses.
What is needed is an optical probe of integrated circuits less subject to noise.
SUMMARY
The present method and apparatus are directed to, as described above, measuring electrical activity in an integrated circuit. Two laser pulses are derived from the same source, which is a single laser in one embodiment. Alternatively, the two pulses may be derived from an incoherent source. The two pulses sample the electrical activity in the integrated circuit, for example, at two times separated by a time delay &Dgr;t, where &Dgr;t may be zero. The two pulses are then detected separately using suitable identical photo detectors and the resulting two signals are subtracted from each other. The resulting difference cancels out any common mode noise signal, as induced by both mechanical vibration and noise in the amplitude of the beam from the laser source. With suitably accurate photo detectors, the system easily reaches the shot-noise limit set by the number of photons in the laser beam.
Two pulses separated by a nonzero time delay &Dgr;t probe the electrical activity in the DUT at different times. If the two pulses interact with the DUT with similar interaction strengths, the resulting difference signal is proportional to the derivative of the waveform that would have been produced with a single pulse probing approach.
Two pulses coincident in time (&Dgr;t=0) sample the electrical activity in the DUT at the same time. If the pulses interact with the DUT with similar interaction strengths, the resulting difference signal is zero. A nonzero difference signal will result if the pulses interact with the DUT with different interaction strengths. For example, if the two pulses are of orthogonal linear polarizations and the interactions with the DUT are polarization dependent, the resulting difference signal is proportional to the waveform that would have been produced with a single pulse probing approach, but reaches the shot-noise limit. The difference in interaction with the DUT of two pulses of different wavelength may similarly be exploited.
REFERENCES:
patent: 4683420 (1987-07-01), Goutzoulis
patent: 4758092 (1988-07-01), Heinrich et al.
patent: 5847570 (1998-12-01), Takahashi et al.
patent: 5872360 (1999-02-01), Paniccia et al.
patent: 5905577 (1999-05-01), Wilsher et al.
patent: 6114858 (2000-09-01), Kasten
Kasapi Steven A.
Somani Seema
Tsao Chun-Cheng
Klivans Norman R.
Lee John R.
Schlumberger Technologies Inc.
Schmidt Mark E.
Skjerven Morrill & MacPherson LLP
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