Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2000-02-25
2002-06-11
Pyo, Kevin (Department: 2877)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S239000, C324S754120
Reexamination Certificate
active
06403946
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electro-optic sampling probe (or prober) in which an electric field generated by a target signal to be measured is applied to an electro-optic crystal, and an optical pulse signal generated based on a timing signal is incident onto the electro-optic crystal, and the waveform of the target signal is observed according to the polarization state of the incident optical pulse signal. In particular, the present invention relates to a technique for improving the S/N ratio of the probe.
This application is based on Patent Application No. Hei 11-80543 filed in Japan, the contents of which are incorporated herein by reference.
2. Description of the Related Art
In a conventional technique, the waveform of a target signal (to be measured) can be observed by applying an electric field generated by the target signal to an electro-optic crystal; inputting a laser beam to the electro-optic crystal, and observing the waveform of the target signal according to the polarization state of the laser beam. If a pulsed laser beam is used for sampling the target signal, the measurement can be performed with a very high temporal resolution. The electro-optic sampling probe (abbreviated to “E
0
S probe”, hereinbelow) employs an electro-optic probe having the above function.
In comparison with the conventional probes employing known electric probes, the above EOS probe has the following characteristics and thus has received widespread notice:
(1) A ground line is unnecessary for measuring the signal; thus, the measurement can be easily performed.
(2) The tip of the electro-optic probe is insulated from the circuit; thus, a high input impedance can be obtained and (the state of) the point to be measured is not significantly disturbed.
(3) An optical pulse signal is used; thus, wide-band measurement of a GHz order can be performed.
(4) An electro-optic crystal can be made to contact a wafer of an IC (or the like), and the laser beam can be made to converge on the wiring printed on the IC wafer, thereby enabling the measurement of thin wiring which a metallic pin cannot physically contact.
In the following explanations, the units for the optical wavelength are “nm”.
The structure of a conventional EOS probe will be explained with reference to FIG.
11
. In
FIG. 11
, reference numeral
1
indicates an IC wafer connected to an external device via a power supply line and a signal line. Reference numeral
2
indicates an electro-optic element using an electro-optic crystal. Reference numeral
3
indicates an objective used for converging the beam incident on the electro-optic element
2
. Reference numeral
4
indicates the (main) body of the probe, comprising a dichroic mirror
4
a
, a half mirror
4
b
, and a reflecting mirror
4
c
. Reference numeral
6
indicates an EOS optical system module (called “EOS optical system”, hereinbelow) comprising a photodiode, a polarized beam splitter, a wave plate, and so on.
Reference numeral
7
indicates an optical fiber, to one end of which fiber collimator
7
a
is connected. Reference numeral
8
indicates a laser light source for supplying a laser beam to the EOS optical system. The wavelength of the outgoing laser beam having a maximum light intensity is 1550 nm. Reference numeral
9
indicates a halogen lamp for irradiating IC wafer
1
to be measured. The halogen lamp
9
may be replaced with a xenon or tungsten lamp, or the like.
Reference numeral
10
indicates an infrared camera (abbreviated to “IR camera”, hereinbelow) for confirming the positioning for converging a beam onto the target wiring provided on the IC wafer
1
. The image obtained by the camera is displayed on monitor
10
a
. The IR camera
10
has a light receiving sensitivity within a wavelength range from 500 to 1800 nm. Reference numeral
11
indicates a suction stage for fixing the IC wafer
1
, which can be finely moved in the x, y, and z directions (perpendicular to each other).
FIG. 12
is a diagram showing the general structure of the EOS optical system
6
. The basic structural elements of the EOS optical system
6
are a polarized beam splitter, a wave plate, and a photodiode. However, the structure as shown in
FIG. 12
can reduce the noise and improve the sensitivity, thus is preferable in practical use.
As shown in
FIG. 12
, in the EOS optical system
6
, optical path
13
is provided inside main frame
12
, and half-wave plates
14
and
15
, a quarter-wave plate
16
, polarized beam splitters
17
and
18
, and a Faraday element
19
are arranged along the optical path
13
.
In addition, photodiodes
22
and
23
are provided in a manner such that they respectively face polarized beam splitters
17
and
18
, as shown in FIG.
12
. These photodiodes
22
and
23
are attached to the main frame
12
.
Below, the optical path of the laser beam emitted from the laser light source
8
will be explained with reference to FIG.
11
. In
FIG. 11
, the laser optical path inside the probe body
4
is indicated by reference numerals A, B, and C.
The laser beam emitted from the laser light source
8
is transmitted through optical fiber
7
, and is collimated by fiber collimator
7
a
. This collimated beam then passes through optical path
13
in the EOS optical system
6
(see FIG.
12
), and is input into the probe body
4
(refer to optical path A in FIG.
11
). This input beam is deflected by 90 degrees by reflecting mirror
4
c
(refer to optical path B in FIG.
11
). The reflecting mirror
4
c
used here is a total reflection mirror manufactured by depositing aluminum (or the like) on a surface of a glass material.
The laser beam deflected by 90 degrees by reflecting mirror
4
c
is further deflected by 90 degrees by dichroic mirror
4
a
(refer to optical path C in FIG.
1
), and then converged by objective
3
onto a face of the electro-optic element
2
disposed on the wiring on the IC wafer
1
, the face facing the IC wafer
1
.
FIG. 13
shows an optical characteristic of dichroic mirror
4
a
. In this figure, the x-axis indicates wavelength, and the y-axis indicates transmittance. As shown in
FIG. 13
, the dichroic mirror
4
a
has the characteristic of transmitting 5% (and reflecting 95%) of a beam having a wavelength of 1550 nm. Therefore, 95% of the beam emitted from the laser source is reflected and deflected by 90 degrees.
A dielectric mirror
2
a
(functioning as a reflecting film) is deposited on the face (which faces the IC wafer
1
) of the electro-optic element
2
. The laser beam reflected by this face is again collimated by objective
3
and returns to the EOS optical system
6
through the optical paths C, B, and A (in this order). Some portions of the reflected beam are then isolated by isolator
20
, and they are incident on photodiodes
22
and
23
and converted into electrical signals.
Below, the operation of measuring a target signal (to be measured) using the EOS probe having the above structure will be explained.
When a voltage is applied to the target wiring on the IC wafer
1
, the corresponding electric field is applied to the electro-optic element
2
, and the refractive index thereof is then changed due to the Pockels effect. As explained above, the laser beam emitted from the laser light source
8
is incident on the electro-optic element
2
, and is reflected by dielectric mirror
2
a
and returned through the same optical path. According to the above effect, the polarization state of the beam output from the electro-optic element
2
is changed. This laser beam having a changed polarization state is again incident on the EOS optical system
6
via optical paths C, B, and A.
In the EOS optical system
6
, the change of the polarization state of this incident laser beam is converted into a change of light intensity, which is detected by photodiodes
22
and
23
so as to convert them into electric signals. These electric signals are processed by a signal processing section (not shown), thereby measuring the electric signal applied to the target wiring on the IC waf
Akikuni Fumio
Nagatsuma Tadao
Ohta Katsushi
Shinagawa Mitsuru
Yamada Junzo
Ando Electric Co. Ltd.
Darby & Darby
Pyo Kevin
LandOfFree
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