Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2000-01-14
2002-02-12
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S254000, C324S754120, C324S096000
Reexamination Certificate
active
06347005
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electro-optic (hereinafter, abbreviated as EO) sampling probe, which is used for observing an waveform of a test signal based on a change of the polarization state of a light pulse caused when the light pulse generated based on the timing signal is admitted into an electro-optic crystal which is coupled with an electric field generated by the test signal, and particularly relates to an electro-optic sampling probe provided with an improved probe optical system.
2. Background Art
First, the principle of the electro-optic (hereinafter, abbreviated as EO) sampling measurement will be described with reference to
FIGS. 4 and 5
.
In
FIG. 4
, the reference numeral
10
denotes a test object such as a semiconductor IC wafer etc., which is connected to an electric signal probe
33
, and to which electric signals from a signal generator
34
are supplied. The test object
10
is operated based on the electric signals.
The reference numeral
1
denotes the electro-optic crystal (hereinafter, called the EO crystal). The numeral
5
denotes an object glass for-condensing light incident on the EO crystal
1
. The numeral
6
denotes an observing optical system comprising a dichroic mirror
29
, a half mirror
23
, and an observing light source
26
. The numeral
22
denotes an EO sampling optical system comprising a photo-diode (not shown) and a light isolator (not shown), and a fiber collimator
30
is disposed at one end of the EO sampling optical system
22
. The numeral
21
denotes a pulse laser light source for supplying light to the fiber collimator
30
.
The reference numeral
27
denotes an infrared camera for localization of the laser light in order to condense the light onto a wiring pattern on the test object
10
, and the alignment is confirmed by use of an image monitor
28
. The numeral
35
is an XY stage for aligning the test object. The numeral
24
denotes a differential amplifier for differentially amplifying an output signal obtained when the change of the polarization state of the laser light is converted into an electric signal by means of the EO sampling optical system
22
. The numeral
25
denotes a waveform indicator for indicating the electric signal.
Next, referring to
FIG. 4
, the path of the laser light emitted from the pulse laser source
21
is described.
First, the laser light admitted into the EO sampling optical system
22
through an optical fiber from the pulse laser source
21
is collimated by the fiber collimator
30
and admitted into the observing optical system
6
after rectilinearly propagating in the EO sampling optical system
22
. The laser light further rectilinearly propagates in the observing optical system
6
and is then condensed on the test object
10
through the EO crystal
1
by the objective lens
5
.
The wavelength used in this sampling optical measurement is 1550 nm. The dichroic mirror
29
is characterized by the transmittance and reflectance for the light of the wavelength of 1550 nm being 95% and 5%, respectively, and the transmittance and reflectance for a light having an wavelength of 1300 nm is 20% and 80%, respectively. Accordingly, 95% of the laser light emitted by the pulse laser source
21
is transmitted and irradiated on the test object
10
.
The light beam reflected by the test object is collimated into a parallel beam again by the objective lens
5
, and returns to the EO sampling optical system
22
through the same light path as that for entering the photodiode (not shown) in the EO sampling optical system.
An explanation is described hereinafter of the path of light emitted by the observing light source
26
in the case of executing alignment of the test object by use of the observing light source
26
and the infrared camera
27
. The lamp used for the observing light source
26
is a halogen lamp which emits light having wavelengths ranging from 400 to 1650 nm.
The light beam, emitted from the observing light source
26
, passes the half mirror
23
, advances rectilinearly, and irradiates the test object after being turned at a right angle by the dichroic mirror
29
. The half mirror
23
used in this optical system is a mirror, in which both transmittance and reflectance are equal to 50%.
The infrared camera
27
takes a photograph of a part of the test object, irradiated by the observation light source
26
in the view field of the objective lens
5
, and the obtained infrared image is displayed on the monitor
28
.
An operator moves the XY stage
35
slowly while watching the image displayed on the monitor
28
, and adjusts the position of the wiring to be tested on the test object so as to enter into the view field.
Furthermore, the laser beam, entering into the EO sampling optical system from the pulse laser source
21
through the optical fiber, is reflected at the side of the EO crystal facing to the wiring. The reflected light, then reflectively bent at a right angle by the dichroic mirror
29
, and again bent at an right angle by the half mirror
23
, is verified by the operator from the images of the infrared camera
27
, and the operator moves the XY stage such that the laser beam is condensed on a point on the wiring surface to be tested. At this time, the laser beam can be verified by the infrared camera
27
because the dichroic mirror
29
has a reflectance of 5% for light having the wavelength of the laser light.
Next, a test operation for measuring the test signal by use of an apparatus of the EO sampling measurement is described.
When a voltage is applied to the wiring on a test object, the phenomenon occurs that, due to the Pockels effect, the index of the double refraction of the EO crystal changes depending on the electric field applied to the EO crystal.
Thereby, when the laser light is entered into the EO crystal and propagates through the EO crystal, the polarization state of the laser light changes. The laser beam, after being subjected to the change of its polarization state, is reflected at the EO crystal surface facing to the wiring, and again impinges on the EO sampling optical system
22
.
The light beam impinging on the EO sampling optical system
22
is isolated by a light isolator (not shown) in the EO sampling optical system
22
. The isolated light is received by a diode (not shown), which converts it into an electric signal. The electric signal is amplified by the differential amplifier
24
and is then displayed on the waveform indicator
25
, so that the measurement of the electric signal applied to the wiring on the test object
10
can be executed.
The principle of the EO sampling is shown in FIG.
5
. When a signal is applied on the EO crystal, the change of the electric field in the crystal by the applied signal causes a change of the index of the double diffraction of the EO crystal due to the Pockels effect. When a laser beam passes through the thus changed crystal, the polarization state of the laser light changes according to the change of the double refractive index, so that it becomes possible to measure the change of the electric field, that is, the change of the signal, by measuring the change of the polarization state of the laser light.
In order to execute the above described measurement, it is necessary to place the EO crystal on the test object. Conventionally, an operator has placed the EO crystal
1
on the test objects
10
, such as a semiconductor wafer or the like, using tweezers. However, the problem arises that the placement must be executed very carefully so as not to damage the test object, so that the conventional measurement has been difficult and can be successfully executed only by experienced operators.
There is a conventional method reported in a paper entitled “An on-wafer EO probing system for internal diagnosis of the ultra high speed IC” (C-309, Proceeding of Electronic Information Communication Society, 1992) The constitution of the probing system reported in the above paper is shown in
FIGS. 6 and 7
.
FIG. 6
is a perspective view of the whole
Nagatsuma Tadao
Shinagawa Mitsuru
Toriyama Noriyuki
Yamada Junzo
Ando Electric Co. Ltd.
Darby & Darby
Epps Georgia
Thompson Tim
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