Electro-optic sampling prober

Details Electro-optic sampling prober Electro-optic sampling prober

1999-11-30

2002-05-14

Sherry, Michael J. (Department: 2829)

Electricity: measuring and testing

Fault detecting in electric circuits and of electric components

Of individual circuit component or element

C324S1540PB

Reexamination Certificate

active

06388454

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electro-optic sampling probers that are used to measure waveforms of measured signals by using electro-optical crystals.
This application is based on Patent Application No. Hei 10-340823 filed in Japan, the content of which is incorporated herein by reference.
2. Description of the Related Art
The electro-optic sampling probers utilize electro-optic probes, which operate as follows:
Electric fields caused by measured signals are applied to electro-optical crystals, on which laser beams being produced based on timing signals are incident. Waveforms of the measured signals are measured for observation in response to polarization states of the laser beams, which are changed in response to the electric fields. Herein, by making the laser beams in a pulse-like form for sampling of the measured signals, it is possible to measure the waveforms with a very high resolution with respect to time.
As compared with the conventional probers using electric probes, the electro-optic sampling probers (abbreviated by “EOS probers”) draw considerable attention of engineers and scientists because of some advantages as follows:
(1) At measurement of signals, the EOS probers do not require ground lines. So, it is possible to perform measurement with ease.
(2) Tip ends of the EOS probers are insulated from the circuitry, so it is possible to realize high input impedance. Therefore, the EOS probers do not substantially disturb states of measuring points.
(3) Because the EOS probers utilize optical pulses for measurement, it is possible to perform measurement of a broad band, a frequency range of which is in Giga-Hertz (GHz) order.
(4) The EOS probers are designed such that electro-optical crystals are brought in contact with wafers of integrated circuits having wires, on which laser beams are converged. So, the EOS probers are capable of performing measurement with respect to fine wires, which cannot be brought in “physical” contact with metal pins.
For convenience sake, the following description uses a specific unit of nano-meter (nm) for dimension of the wavelength of light.
Now, an example of a construction of an EOS prober will be described with reference to FIG.
6
. In
FIG. 6
, an IC wafer
1
is connected to the external (i.e., device or system, not shown) by way of power lines and signal lines. A electro-optical element
2
is configured using an electro-optical crystal. An objective lens
3
converges beams to be incident on the electro-optical element
2
. A prober unit
4
is equipped with a dichroic mirror
4
a
, a half mirror
4
b
and a reflecting mirror
4
c
. An EOS optical module (or EOS optical system) is constructed using photodiodes, polarization beam splitters and wavelength plates, all of which are not shown.
An optical fiber
7
is equipped with a fiber collimator
7
a
at a terminal end thereof. A light source (i.e., laser)
8
supplies the EOS optical system
6
with laser beams. Herein, outgoing beams of the laser
8
have a wavelength of 1550 nano-meter (nm) at maximum intensity. A halogen lamp
9
illuminates the IC wafer
1
being subjected to measurement. Incidentally, the EOS prober of
FIG. 6
does not necessarily use the halogen lamp
9
for illumination of the IC wafer
1
. That is, it is possible to use other lamps such as the xenon lamp and tungsten lamp.
An infrared camera (or IR camera)
10
makes confirmation with respect to the positioning for convergence of beams on the IC wafer
1
. Images created by the IR camera
10
are displayed on a screen of a monitor
10
a
. The IR camera
10
has light-receiving sensitivity in a range of wavelengths between 500 nm and 1800 nm. An absorption stage
11
absorbs the IC wafer
1
to be fixed in position. The absorption stage
11
is capable of making small movements in x-axis, y-axis and z-axis directions, which are perpendicular to each other.
Next, optical paths of laser beams radiated from the laser
8
will be described with reference to FIG.
6
. In
FIG. 6
, optical paths of laser beams in the prober unit
4
are designated by reference symbols A, B and C respectively.
First, laser beams output from the laser
8
pass through the optical fiber
7
, in which they are converted to parallel beams by the fiber collimator
7
a
. The parallel beams pass through the EOS optical system
6
and are then introduced into the prober unit
4
as its incoming beams. In the prober unit
4
, the incoming beams propagate along the optical path A. The incoming beams are reflected by the reflecting mirror
4
c
, by which they are changed in propagation direction by an angle of 90 degrees. So, reflected beams propagate along the optical path B. Herein, the reflecting mirror
4
c
is a surface mirror of a full reflection type, which is made by depositing aluminum material on a glass surface.
The reflected beams, corresponding to the laser beams reflected by the reflecting mirror
4
c
, are further reflected by the dichroic mirror
4
a
, wherein they are changed in propagation degree by an angle of 90 degrees. So, further reflected beams being reflected by the dichroic mirror
4
a
propagate along the optical path C. The objective lens
3
converges such further reflected beams onto an opposite surface of the electro-optical element
2
that is placed being opposite to face with a surface of the IC wafer
1
, wherein the electro-optical element
2
is arranged on wiring (or wires) of the IC wafer
1
.
FIG. 7
shows an example of an optical characteristic in transmission of the dichroic mirror
4
a
with respect to wavelength, wherein a horizontal axis represents “wavelength” while a vertical axis represents “transmission factor”. According to the optical characteristic shown in
FIG. 7
, the dichroic mirror
4
a
allows only 5% of light to transmit therethrough while reflecting 95% of light with respect to wavelength of 1550 nm. For this reason, 95% of the laser beams radiated from the laser
8
are reflected by the dichroic mirror
4
a
, wherein they are changed in optical path by an angle of 90 degrees.
A dielectric mirror is deposited on the opposite surface of the electro-optical element
2
, which faces with the IC wafer
1
. Laser beams being reflected by the dielectric mirror
4
a
are converted to parallel beams by the objective lens
3
. Then, the parallel beams propagate back along the optical paths C, B, A in turn and are returned to the EOS optical system
6
. Those beams are incident on photodiodes (not shown) within the EOS optical system
6
.
Next, a description will be given with respect to operations in positioning of the IC wafer
1
in connection with the halogen lamp
9
and the IR camera
10
. Specifically, the following description is given with respect to optical paths for light radiated from the halogen lamp
9
and positioning operations of the IC wafer
1
. In
FIG. 6
, the optical paths along which the light of the halogen lamp
9
propagate are designated by reference symbols D, E and C respectively.
The light radiated from the halogen lamp
9
propagate along the optical path D and is incident on the half mirror
4
b
. The half mirror
4
b
reflects the light by an angle of 90 degrees, so that reflected light propagate along the optical path E. The reflected light propagate straight through the dichroic mirror
4
a
along the optical path C so as to illuminate the IC wafer
1
. Herein, the half mirror
4
b
is designed such that reflected light and transmitted light coincide with each other in intensity.
FIG. 8
shows an optical characteristic in radiation of the halogen lamp
9
with respect to wavelength, wherein a horizontal axis represents “wavelength” while a vertical axis represents “light intensity”.
FIG. 8
shows that the halogen lamp
9
radiates light in a range of wavelengths between 400 nm and 1650 nm.
The IR camera
10
picks up an infrared image with respect to a part of the IC wafer
1
within a visual field of the objective lens
3
being subjected to illumination by the halogen lamp
9
. Such an infrared image

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