U.S. patent application number 11/954282 was filed with the patent office on 2008-05-08 for resistive heater including wire resistor.
This patent application is currently assigned to NEC CORPORATION. Invention is credited to Morio TAKAHASHI.
Application Number | 20080107370 11/954282 |
Document ID | / |
Family ID | 33487302 |
Filed Date | 2008-05-08 |
United States Patent
Application |
20080107370 |
Kind Code |
A1 |
TAKAHASHI; Morio |
May 8, 2008 |
RESISTIVE HEATER INCLUDING WIRE RESISTOR
Abstract
A resistor heater includes an anode (10) arranged along one side
and a cathode (20) arranged along the other side of a
thin-line-shaped resistor (30). The anode (10) is connected to the
resistor (30) at connections points (P2, P3) by a plurality of
branches (13, 14) arranged at a certain interval along the resistor
(30). The cathode (20) is connected to the resistor (30) at
connection points (P1, P4) by branches (23, 24) arranged at a
certain interval along the resistor (30). The connection points
(P1, P4) are located at positions shifted from one another along
the resistor (30). A portion (31) of the resistor (30) located
between the connections points (P1, P2) and a portion (32) of the
resistor (30) located between the connection points (P3, P4)
function as effective regions of the resistor (30).
Inventors: |
TAKAHASHI; Morio; (Tokyo,
JP) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W.
SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
NEC CORPORATION
Tokyo
JP
|
Family ID: |
33487302 |
Appl. No.: |
11/954282 |
Filed: |
December 12, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10547479 |
Aug 30, 2005 |
7335862 |
|
|
PCT/JP2004/007738 |
May 28, 2004 |
|
|
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11954282 |
Dec 12, 2007 |
|
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Current U.S.
Class: |
385/1 |
Current CPC
Class: |
G02F 2203/50 20130101;
G02F 1/0147 20130101; H05B 3/24 20130101; G02F 2201/12 20130101;
H05B 2203/013 20130101; H05B 2203/017 20130101 |
Class at
Publication: |
385/001 |
International
Class: |
G02F 1/035 20060101
G02F001/035 |
Foreign Application Data
Date |
Code |
Application Number |
May 30, 2003 |
JP |
2003-153903 |
Claims
1. A thermooptic phase shifter comprising: an optical waveguide;
and a resistive heater comprising: a wire resistor; a first
electrode, placed on a side of the resistor, extending along the
resistor; and a second electrode, placed on the side opposite to
the first electrode, extending along the resistor, wherein the
first electrode is connected to a plurality of first nodes placed
on the resistor with branches spaced along the resistor, the second
electrode is connected to a plurality of second nodes placed on the
resistor with branches spaced along the resistor, the second nodes
are spaced from the first nodes in the longitudinal direction of
the resistor, and the resistor has effective regions each
sandwiched between one of the first nodes and one of the second
nodes that is adjacent to the first connection, for heating the
optical waveguide, wherein the resistor included in the resistive
heater extends along the optical waveguide.
2. A thermooptic phase shifter according to claim 1, wherein said
resistive heater further comprising the first, wherein the first
and second nodes are alternately arranged in the longitudinal
direction of the resistor.
Description
TECHNICAL FIELD
[0001] The present invention relates to resistive heaters for
electrically generating Joule heat and particularly relates to a
resistive heater including a wire resistor. The apparent (or
superficial) electrical resistance of the resistive heater can be
arbitrarily adjusted without changing the shape of the wire
resistor.
BACKGROUND ART
[0002] "Resistive heaters" for generating Joule heat by applying
currents to thin-film resistors are widely used for various
applications. Examples of such resistive heaters include
micro-sized resistive heaters placed on circuit substrates or
semiconductors such as silicon. A large number of attempts have
been made to solve problems due to the size of the micro-sized
resistive heaters. See, for example, Japanese Unexamined Patent
Application Publication Nos. 58-134764, 3-164270, and 61-219666 and
Japanese Patent No. 2811209. Techniques relating to these resistive
heaters are usually used to heat specific micro-regions (several
micrometers square) or relatively large-area regions which are
several millimeters to several centimeters square such that
semiconductor devices mounted on such regions are heated.
[0003] In a case where a square region or a rectangular region
which has a small aspect ratio and which is therefore close to a
square shape is heated, the shape of a resistive heater placed in
the region is not particularly limited. Therefore, a desired object
can be readily achieved by allowing the resistive heater to have
such a shape that the temperature distribution in the region can be
desirably adjusted. For the electrical resistance of resistive
heaters, since a large number of holes can be bored in a
sheet-shaped resistive heater, the electrical resistance of the
heater can be readily adjusted by varying the size and/or number of
the holes as disclosed in Japanese Unexamined Patent Application
Publication No. 58-134764.
[0004] For the resistive heater for heating the square or
rectangular region, the temperature distribution obtained by the
resistive heater and the electrical resistance of the resistive
heater can be adjusted by varying the shape of the resistive
heater.
[0005] The electrical resistance of the resistive heater is a key
factor to determine the necessary performance, for example, the
maximum voltage, of an external circuit for driving the resistive
heater. If the resistive heater has a large electrical resistance,
an extremely high voltage must be applied to the driving circuit.
In consideration of the voltage (about 5 to 12 V) of a power
supply, connected to a control circuit (usually including
semiconductor devices), for controlling the temperature, there is a
problem in that these circuits cannot be connected to a common
power supply. Thus, it is necessary to adjust the electrical
resistance of the resistive heater.
[0006] On the other hand, an optical component, for example, "a
thermooptic phase shifter", used for optical communication includes
a resistive heater (see, for example, Japanese Unexamined Patent
Application Publication No. 6-34926). This resistive heater
includes a resistor having a width of several micrometers to
several tens micrometers and a length of about 2 to 5 mm. The
length of the resistor is extremely greater than the width thereof.
Therefore, this resistive heater is different from that resistive
heater in that the resistor has a narrow line shape (a narrow
stripe shape). The thermooptic phase shifter includes an optical
waveguide section having a width of about 5 .mu.m and a length of
about 2 to 5 mm. In order to selectively heat the optical waveguide
section having such a shape using this resistive heater, the
resistor must also have a narrow line shape.
[0007] Since the resistor has a width of several micrometers, it is
difficult to arbitrarily adjust the electrical resistance of the
resistor by varying the shape thereof in the same manner as that
described above. This is because a micromachining technique is
necessary to shape the resistor.
[0008] The resistor is allowed to have a thickness of up to several
hundreds nanometers because of the reason due to a semiconductor
process used to form the thermooptic phase shifter. That is, the
thickness of the resistor is limited. The number of materials for
forming the optical waveguide section is not very large because
such materials must have good machinability, high stability, and
high adhesion to a glass material for forming the optical waveguide
section.
[0009] As described above, in the resistive heater included in the
optical component, there is a limitation that the resistor must
have a narrow line shape; hence, it is very difficult to prepare a
heating element (in particular, a heating element with low
electrical resistance) with desired electrical resistance
properties by improving the shape of the resistor. Furthermore, it
is not easy to adjust the thickness of the resistive heater or
change a material for forming the resistive heater as required
because of process and material constraints.
[0010] There are known techniques relating to the present invention
as described below.
[0011] Japanese Unexamined Patent Application Publication No.
2001-301219 discloses a thermal print head including a wire
resistor. The thermal print head, as specified in claim 1 of this
patent document, includes "a linear resistor, a power supply line,
a grounding line, and an integrated circuit device, wherein the
integrated circuit device includes a plurality of transistors each
including respective first electrodes connected to the power supply
and respective second electrodes connected to the grounding line
and also includes a plurality of pads for connecting the second
electrodes to the grounding line and the resistor generates heat
when a current is applied to the resistor by bring the transistors
into conduction". According to such a configuration, the following
advantages can be achieved: "the second electrodes can be connected
to the pads with short wires, the wires therefore have low
resistance, a difference in wiring resistance between the
transistors is small, electricity consumption is low, the life of a
battery included in the thermal print head is long if the thermal
print head is of a portable type, the thermal print head can be
driven with a low-voltage battery because a voltage drop due to the
wiring resistance is small, the quality of an image formed by the
thermal print head is high because a difference in wiring
resistance between the transistors is small and because a
difference in temperature between portions of the resistor is
small".
[0012] In the thermal print head disclosed in Japanese Unexamined
Patent Application Publication No. 2001-301219, the resistor and
the power supply line are connected to each other with a plurality
of spaced wires and the first electrodes of the transistors are
connected to the resistor with wires. The first and second
electrodes of the transistors correspond to the drains and sources
of MOS transistors, respectively. If one of the transistors in the
integrated circuit device is turned on, a current flows from the
power supply line to the grounding line through the resistor and
the transistor. Since the current flows in two wires for connecting
the power supply line to the resistor and flows in a portion of the
resistor that is sandwiched between the two wires, the resistor
portion can be selectively heated.
[0013] Japanese Unexamined Patent Application Publication No.
2002-008901 discloses a thin-film resistor, a hybrid IC, and a
microwave monolithic integrated circuit (MMIC). In the thin-film
resistor, "a first electrode and second electrode connected to
thin-film resistor portions have narrow, irregular sections
extending in the direction that the first and second electrodes
face to each other; sides of the irregular sections of the first
and second electrodes are arranged at predetermined intervals; and
the thin-film resistor portions are arranged between the sides
facing to each other". That is, in the thin-film resistor, an end
section of the first electrode is shaped so as to have an
interdigital shape so that the irregular electrode sections are
formed, an end section of the second electrode is shaped so as to
have an interdigital shape so that the irregular electrode sections
are formed, and the electrode sections are engaged with each other
in such a manner that the interdigital electrode sections of the
second electrode are placed in spaces between the interdigital
electrode sections of the first electrode. The thin-film resistor
portions are separately placed in spaces between the interdigital
electrode sections engaged with each other.
[0014] According to such a configuration, the following advantages
can be achieved: "the thin-film resistor can be shaped so as to
have a size close to the width of wires and a region for forming
the thin-film resistor can therefore be formed so as to have a
desired characteristic impedance".
[0015] If the operational stability and reliability of resistive
heaters are regarded as most important, tantalum nitride (TaN) is
usually used to prepare resistors. Thin-film heaters, made of TaN,
for semiconductor circuits have a large electrical resistance
because the resistivity of TaN is usually high, 200 to 300
.mu..OMEGA.cm, under conditions for stably forming layers. If, for
example, a TaN layer is processed into fine wires having a
thickness of 200 nm a width of 10 .mu.m, and a length of 2 mm, the
wires have an electrical resistance of 2 to 3 k.OMEGA.. In order to
allow a wire resistor, made of TaN, having such an electrical
resistance to generate 300 mW of heat, the voltage necessary to
energize the wire resistor is very high, 17 to 30 V.
[0016] An attempt to prepare a small-sized, precisely controllable
heating element including a TaN wire resistor causes a problem,
i.e., an increase in the size of a driving power supply. Hence, the
attempt is impossible. This can be applied to titanium nitride
(TiN), as well as TaN, having a relatively large resistivity.
DISCLOSURE OF INVENTION
[0017] It is an object of the present invention to provide a
resistive heater including a wire resistor and a thermooptic phase
shifter including such a resistive heater. If the wire resistor is
made of a material, such as tantalum nitride or titanium nitride,
having a relatively large resistivity, the apparent electrical
resistance of the resistive heater (the superficial electrical
resistance of the resistive heater) is less than the electrical
resistance estimated from the material.
[0018] It is another object of the present invention to provide a
resistive heater of which the apparent electrical resistance can be
arbitrarily adjusted and which includes a wire resistor and a
thermooptic phase shifter including such a resistive heater.
[0019] It is another object of the present invention to provide a
resistive heater, including a wire resistor, for producing heat of
which the amount can be controlled with a simple electronic circuit
and a thermooptic phase shifter including such a resistive
heater.
[0020] In the thermal print head disclosed in Japanese Unexamined
Patent Application Publication No. 2001-301219, a current flows
through the resistor portion sandwiched between the two wires for
connecting the power supply line to the resistor having a narrow
line shape, whereby the resistor portion is selectively allowed to
generate heat. However, such a technique is not useful in achieving
the following object of the present invention: "if the narrow
resistor is made of a material, such as tantalum nitride or
titanium nitride, having a relatively large resistivity, the
apparent electrical resistance of the resistive heater is less than
the electrical resistance estimated from the material". The thermal
print head is quite different from a resistive heater according to
the present invention.
[0021] The thin-film resistor disclosed in Japanese Unexamined
Patent Application Publication No. 2002-008901 does not have a wire
shape and is therefore different from "a resistive heater including
a wire resistor" as specified herein. An object (purpose) thereof
is as follows: "when the first and second electrodes have a size
significantly greater than the width of lines, the first and second
electrodes have a characteristic impedance (for example, 50.OMEGA.)
unsuitable for transmission lines; hence, desired operations cannot
be performed due to miss-matching". An effect thereof is as
follows: "the resistive heater can be formed so as to have a size
close to the line width and a region for forming the thin-film
resistor can therefore be formed so as to have a desired
characteristic impedance". As is clear from the object and effect,
the thin-film resistor is quite different from a resistive heater
of the present invention.
[0022] Further other objects of the present invention will become
apparent from descriptions below and the accompanying drawings
although the objects are not described above.
[0023] (1) A resistive heater of the present invention
includes:
[0024] a wire resistor;
[0025] a first electrode, placed on a side of the resistor,
extending along the resistor; and
[0026] a second electrode, placed on the side opposite to the first
electrode, extending along the resistor,
[0027] wherein the first electrode is connected to a plurality of
first nodes placed on the resistor with branches spaced along the
resistor,
[0028] the second electrode is connected to a plurality of second
nodes placed on the resistor with branches spaced along the
resistor,
[0029] the second nodes are spaced from the first nodes in the
longitudinal direction of the resistor, and the resistor has
effective regions each sandwiched between one of the first nodes
and one of the second nodes that is adjacent to the first
connection.
[0030] (2) In the resistive heater of the present invention, the
first electrode is placed on a side of the resistor and extends
along the resistor and the second electrode is placed on the side
opposite to the first electrode and extends along the resistor. The
first electrode is connected to the first nodes placed on the
resistor with the branches spaced along the resistor and the second
electrode is connected to the second nodes placed on the resistor
with the branches spaced along the resistor. The second nodes are
spaced from the first nodes in the longitudinal direction of the
resistor and the resistor has effective regions each sandwiched
between one of the first nodes and one of the second nodes that is
adjacent to the first connection.
[0031] (3) In a preferable example of the resistive heater of the
present invention, the first and second nodes are alternately
arranged in the longitudinal direction of the resistor.
[0032] (4) A thermooptic phase shifter of the present invention
includes:
[0033] an optical waveguide; and
[0034] the resistive heater, according to Items (1) to (3)
described above, for heating the optical waveguide,
[0035] wherein the resistor included in the resistive heater
extends along the optical waveguide.
[0036] (5) The thermooptic phase shifter of the present invention
includes the resistive heater, according to Items (1) to (3)
described above, for heating the optical waveguide and the resistor
of the resistive heater extends along the optical waveguide.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1A is a plan view showing a resistive heater according
to a first example of the present invention.
[0038] FIG. 1B is a diagram showing an equivalent circuit of the
resistive heater shown in FIG. 1A.
[0039] FIG. 2A is a plan view showing a resistive heater according
to a second example of the present invention.
[0040] FIG. 2B is a diagram showing an equivalent circuit of the
resistive heater shown in FIG. 2A.
[0041] FIG. 3A is a plan view showing a resistive heater according
to a third example of the present invention.
[0042] FIG. 3B is a diagram showing an equivalent circuit of the
resistive heater shown in FIG. 3A.
[0043] FIG. 4A is a plan view showing a resistive heater according
to a fourth example of the present invention.
[0044] FIG. 4B is a diagram showing an equivalent circuit of the
resistive heater shown in FIG. 4A.
[0045] FIGS. 5A to 5E are sectional views showing principal parts
of the resistive heater of the fourth example in the order of
manufacturing steps.
[0046] FIG. 6 is a plan view showing a configuration of a
thermooptic phase shifter according to a fifth example of the
present invention.
[0047] FIG. 7A is a plan view showing a known resistive heater.
[0048] FIG. 7B is a diagram showing an equivalent circuit of the
known resistive heater shown in FIG. 7A.
BEST MODE FOR CARRYING OUT THE INVENTION
[0049] A resistive heater according to the present invention will
now be described.
[0050] According to the present invention, the wire resistor is
divided into a plurality of effective regions and non-effective
regions other than the effective regions with the first and second
nodes. If the resistor is made of a material, such as tantalum
nitride or titanium nitride, having a relatively large resistivity,
the apparent electrical resistance (the superficial electrical
resistance of the resistive heater) is less than the electrical
resistance of the material because the effective regions are
connected to the first and second electrodes in parallel.
Therefore, the amount of heat generated by the resistive heater can
be controlled with a simple electronic circuit.
[0051] The number, position, and/or length of the effective regions
can be varied by changing the number and/or position of the
branches of the first or second electrode. Therefore, the apparent
electrical resistance of the resistive heater (the superficial
electrical resistance of the resistive heater) can be readily
adjusted to any value.
[0052] In a preferable example of the resistive heater according to
the present invention, the first electrode and the second
electrode, i.e., a positive electrode and a negative electrode, are
alternately arranged in the longitudinal direction of the resistor;
hence, heat is generated from substantially the whole of the
resistor. Therefore, there is an advantage in that the temperature
of the resistive heater is uniform in the longitudinal direction
thereof. Furthermore, since the temperature of the resistive heater
is uniform in the longitudinal direction, the resistor can be
prevented from being deteriorated. Therefore, there is an advantage
in that the resistive heater has high long-term reliability.
[0053] In another preferable example of the resistive heater
according to the present invention, the first and second nodes are
arranged such that the effective regions extending in the
longitudinal direction of the resistor have the same length. In
this example, the effective regions generate the same amount of
heat (calorific power); hence, the temperature of the resistive
heater is uniform in the longitudinal direction. Therefore, there
is an advantage in that a stress applied to the resistor can be
greatly reduced. Furthermore, the effective regions sandwiched
between the first and second electrodes (for example, a positive
electrode and a negative electrode) have the same electrical
resistance; hence, there is an advantage in that the resistive
heater can be readily designed, controlled, and operated.
[0054] In another preferable example of the resistive heater
according to the present invention, any two of the first and second
nodes are each placed at one of both ends of the resistor. In this
example, since the whole of the resistor can be effectively used,
the apparent electrical resistance R' of the resistive heater is
determined depending only on the number n of the effective regions
of the resistive heater. This means that the apparent electrical
resistance R' can be designed using only the number n of the
effective regions; hence, there is an advantage in that the
resistive heater can be readily designed.
[0055] In another preferable example of the resistive heater
according to the present invention, the resistor is made of a
material principally containing tantalum nitride or titanium
nitride. In this example, the material for forming the resistor has
high reliability and the tantalum nitride and titanium nitride have
relatively high resistivity; hence, there is an advantage in that
advantages of the present invention can be maximized.
[0056] In another preferable example of the resistive heater
according to the present invention, the first and second electrodes
are made of a material containing at least two selected from the
group consisting of gold, platinum, chromium, titanium, copper,
aluminum, titanium nitride, and tantalum nitride. In this example,
the first and second electrodes have an electrical resistance
sufficiently less than that of the resistor; hence, there is an
advantage in that the resistor can efficiently generate heat.
[0057] Thus, the same advantage as that of the resistor of the
present invention can be achieved. Furthermore, there is an
advantage in that it is preferable to efficiently use heat
generated by the resistive heater of the present invention.
[0058] Examples of the resistor according to the present invention
will now be described in detail with reference to the accompanying
drawings.
FIRST EXAMPLE
[0059] FIG. 1A is a plan view showing a configuration of a
resistive heater according to a first example of the present
invention and FIG. 1B is a diagram showing an equivalent circuit of
the resistive heater.
[0060] With reference to FIG. 1A, the resistive heater of the first
example is placed on an insulating substrate (not shown) and
includes a wire resistor 30 having a predetermined length; a
positive electrode 10, placed on a side (the upper side in FIG. 1A)
of the resistor 30, extending along the resistor 30; and a negative
electrode 20, placed on the side (the lower side in FIG. 1A)
opposite to the positive electrode 10, extending along the resistor
30.
[0061] The resistor 30 extends straight on the substrate and has a
uniform width of, for example, 10 .mu.m. The resistor 30 has a
length of, for example, 2 mm and a thickness of, for example, 200
nm. The resistor 30 is made of TaN or TiN.
[0062] The positive electrode 10 extends along the resistor 30 and
they are spaced from each other. The negative electrode 20 extends
along the resistor 30 and they are spaced from each other. The
positive electrode 10 and the negative electrode 20 are parallel to
the resistor 30. The positive electrode 10 and the negative
electrode 20 each include a conductive body having a resistivity
sufficiently less than that of the resistor 30. The conductive body
has a triple layer structure consisting of an aluminum (Al) layer,
a titanium (Ti) layer, and a gold (Au) layer.
[0063] The positive electrode 10 includes a connection 11, an
extension 12, and two branches 13 and 14. The connection 11 is
connected to an external circuit, that is, the connection 11 is
used as a bonding pad. The extension 12 has a stripe shape and
extends from the connection 11 in parallel to the resistor 30. The
branches 13 and 14 are arranged between the extension 12 and the
resistor 30. The branches 13 and 14 have a stripe shape, are
narrower than the extension 12, meet the resistor 30 and the
extension 12 at right angles, and are spaced along the resistor 30.
The branch 13 is connected to a node P2 placed on the resistor 30.
The branch 14 is connected to a node P3 placed on the resistor 30.
The positive electrode 10 has an electrical resistance sufficiently
less than that of the resistor 30.
[0064] The negative electrode 20 as well as the positive electrode
10 includes a connection 21, an extension 22, and two branches 23
and 24. The connection 21 is connected to an external circuit, that
is, the connection 21 is used as a bonding pad. The extension 22
has a stripe shape and extends from the connection 21 in parallel
to the resistor 30. The branches 23 and 24 are arranged between the
extension 22 and the resistor 30. The branches 23 and 24 have a
stripe shape, are narrower than the extension 22, meet the resistor
30 and the extension 22 at right angles, and are spaced along the
resistor 30. The branch 23 is connected to a node P1 placed on the
resistor 30. The branch 24 is connected to a node P4 placed on the
resistor 30. The negative electrode 20 also has an electrical
resistance sufficiently less than that of the resistor 30.
[0065] The node P2 connected to the branch 13 of the positive
electrode 10 is spaced from the node P3 connected to the branch 14
of the positive electrode 10 in the longitudinal direction of the
resistor 30. The node P1 connected to the branch 23 of the negative
electrode 20 is spaced from the node P4 connected to the branch 24
of the negative electrode 20 in the longitudinal direction of the
resistor 30. The node P1 connected to the negative electrode 20 is
spaced from the nodes P2 and P3 placed on the positive electrode 10
in the longitudinal direction of the resistor 30. The node P4
placed on the negative electrode 20 is spaced from the nodes P2 and
P3 placed on the positive electrode 10 in the longitudinal
direction of the resistor 30. That is, the nodes P1 to P4 are
located at different positions.
[0066] In the resistive heater according to the first example of
the present invention, since the nodes P1 to P4 are arranged as
described above, the resistor 30 has an effective region 31
sandwiched between the node P2 placed on the positive electrode 10
and the node P1 placed on the negative electrode 20 and an
effective region 32 sandwiched between the node P3 placed on the
positive electrode 10 and the node P4 placed on the negative
electrode 20 (see FIGS. 1A and 1B). Regions other than the
effective regions 31 and 32 do not function as "resistive regions"
and are therefore referred to as non-effective regions.
[0067] If a predetermined voltage is applied to the positive
electrode 10 from a power supply and the negative electrode 20 is
grounded, currents flow from the positive electrode 10 to the
negative electrode 20. In this situation, a current flows from the
branch 13 of the positive electrode 10 to the branch 23 of the
negative electrode 20 through the effective region 31 of the
resistor 30 and a current flows from the branch 14 of the positive
electrode 10 to the branch 24 of the negative electrode 20 through
the effective region 32 of the resistor 30. No current flows
between the branches 13 and 14 of the positive electrode 10. This
is because the branches 13 and 14 have the same potential. As a
matter of course, no currents flow in both end regions of the
resistor 30, that is, one end region located on the left side of
the branch 23 of the negative electrode 20 and the other end region
located on the right side of the branch 24 of the negative
electrode 20.
[0068] If the electrical resistances of the positive and negative
electrodes 10 and 20 are negligible, an equivalent circuit of the
resistor 30 is as shown in FIG. 1B, wherein R1 represents the
electrical resistance of the effective region 31 and R2 represents
the electrical resistance of the effective region 32. The apparent
electrical resistance of the resistive heater (the superficial
electrical resistance of the resistive heater) according to the
first example of the present invention is equal to the electrical
resistance of a circuit including two resistors, connected to each
other in parallel, having an electrical resistance equal to R1 or
R2. Therefore, the apparent electrical resistance of the resistive
heater is greatly less than the electrical resistance estimated
from the resistivity of the resistor 30.
[0069] As described above, although the resistor 30 is made of a
material, such as TaN or TiN, having a relatively large
resistivity, the resistive heater (see FIGS. 1A and 1B) according
to the first example of the present invention has an apparent
electrical resistance less than that estimated from the material.
Therefore, the amount of heat generated by the resistive heater can
be controlled with a simple electronic circuit.
[0070] The number, position, and/or length of the effective regions
of the resistor 30 can be varied by changing the number and/or
position of the branches of the positive or negative electrode 10
or 20. Therefore, the apparent electrical resistance of the
resistive heater can be adjusted to any value.
[0071] Thus, a control circuit, a driving circuit, and other
circuits can be connected to a common power supply; hence, a
small-sized, user-friendly heating element can be achieved.
[0072] FIG. 7A is a plan view showing a configuration of a known
resistive heater, which is a comparative example, and FIG. 7B is a
diagram showing an equivalent circuit of the known resistive
heater. The known resistive heater shown in FIG. 7A includes a wire
resistor 130, a positive electrode 110 connected to one end of the
resistor 130, a negative electrode 120 connected to the other end
thereof. The equivalent circuit thereof is as shown in FIG. 7B and
the apparent electrical resistance of this resistive heater is
equal to the electrical resistance R of the resistor 130.
Therefore, once the resistor 130 is formed, the apparent electrical
resistance of this resistive heater cannot be adjusted.
[0073] The configuration of the resistive heater according to the
first example of the present invention can be generally described
as below.
[0074] The following equation (1) holds:
(1/R')=(1/R).times.{(1/m.sub.1)+(1/m.sub.2)+ . . . +(1/m.sub.n)}
(1) wherein R' represents the apparent electrical resistance of the
resistive heater observed from an external driving circuit,
m.sub.1, m.sub.2, . . . , and m.sub.n represent the percentages of
effective regions in the resistive heater and are less than 1, and
n represents the number of the effective regions and is not equal
to 0.
[0075] Since m.sub.1 to m.sub.n are less than 1, R' is less than R.
Accordingly, the resistive heater has an electrical resistance less
than that of the known resistive heater shown in FIGS. 7A and
7B.
SECOND EXAMPLE
[0076] FIG. 2A is a plan view showing a configuration of a
resistive heater according to a second example of the present
invention and FIG. 2B is a diagram showing an equivalent circuit of
the resistive heater.
[0077] With reference to FIG. 2A, the resistive heater of the
second example is placed on an insulating substrate (not shown) and
includes a wire resistor 30A having a predetermined length; a
positive electrode 10A, placed on a side (the upper side in FIG.
2A) of the resistor 30A, extending along the resistor 30A; and a
negative electrode 20A, placed on the side (the lower side in FIG.
2A) opposite to the positive electrode 10A, extending along the
resistor 30A.
[0078] Other components of the resistor 30A are the same as those
of the resistor 30 of the first example and the description of the
resistor 30A is therefore omitted.
[0079] The positive electrode 10A extends along the resistor 30A
and they are spaced from each other. The negative electrode 20A
extends along the resistor 30A and they are spaced from each other.
The positive electrode 10A and the negative electrode 20A are
parallel to the resistor 30A. The positive electrode 10 and the
negative electrode 20 each include a conductive body having a
resistivity sufficiently less than that of the resistor 30A.
[0080] The positive electrode 10A includes a connection 11A, an
extension 12A, and three branches 13A, 14A, and 15A. The connection
11A is connected to an external circuit. The extension 12A has a
stripe shape and extends from the connection 11A in parallel to the
resistor 30A. The branches 13A, 14A, and 15A are arranged between
the extension 12A and the resistor 30A. The branches 13A, 14A, and
15A have a stripe shape, are narrower than the extension 12A, meet
the resistor 30A and the extension 12A at right angles, and are
spaced along the resistor 30.
[0081] The branch 13A is connected to a node P11 placed on the
resistor 30A. The branch 14A is connected to a node P13 placed on
the resistor 30A. The branch 15A is connected to a node P15 placed
on the resistor 30A. The positive electrode 10A has an electrical
resistance sufficiently less than that of the resistor 30A.
[0082] The negative electrode 20A as well as the positive electrode
10A includes a connection 21A, an extension 22A, and three branches
23A, 24A, and 25A. The connection 21A is connected to an external
circuit. The extension 22A has a stripe shape and extends from the
connection 21A in parallel to the resistor 30A. The branches 23A,
24A, and 25A are arranged between the extension 22A and the
resistor 30A. The branches 23A, 24A, and 25A have a stripe shape,
are narrower than the extension 22A, meet the resistor 30A and the
extension 22A at right angles, and are spaced along the resistor
30A.
[0083] The branch 23A is connected to a node P12 placed on the
resistor 30A. The branch 24A is connected to a node P14 placed on
the resistor 30A. The branch 25A is connected to a node P16 placed
on the resistor 30A. The negative electrode 20A also has an
electrical resistance sufficiently less than that of the resistor
30A.
[0084] The nodes P11, P13, and P15 connected to the branches 13A,
14A, and 15A, respectively, on the positive electrode 10A are
spaced from each other in the longitudinal direction of the
resistor 30A. The nodes P12, P14, and P16 connected to the branches
23A, 24A, and 25A, respectively, on the positive electrode 10A are
spaced from each other in the longitudinal direction of the
resistor 30A. The node P12 on the negative electrode 20A is spaced
from the nodes P11, P13, and P15 on the positive electrode 10A in
the longitudinal direction of the resistor 30A. The node P14 on the
negative electrode 20A is spaced from the nodes P11, P13, and P15
on the positive electrode 10A in the longitudinal direction of the
resistor 30A. The node P16 on the negative electrode 20A is spaced
from the nodes P11, P13, and P15 on the positive electrode 10A in
the longitudinal direction of the resistor 30A. That is, the nodes
P11 to P16 are located at different positions.
[0085] In the resistive heater according to the second example of
the present invention, since the nodes P11 to P16 are arranged as
described above, the resistor 30A has effective regions 31A, 32A,
33A, 34A, and 35A (see FIGS. 2A and 2B). Regions other than the
five effective regions 31A, 32A, 33A, 34A, and 35A do not function
as "resistive regions" and are therefore referred to as
non-effective regions. The region 31A is a portion of the resistor
30A that is sandwiched between the node P11 on the positive
electrode 10A and the node P12 on the negative electrode 20A. The
region 32A is a portion of the resistor 30A that is sandwiched
between the node P13 on the positive electrode 10A and the node P12
on the negative electrode 20A. The region 33A is a portion of the
resistor 30A that is sandwiched between the node P13 on the
positive electrode 10A and the node P 14 on the negative electrode
20A. The region 34A is a portion of the resistor 30A that is
sandwiched between the node P15 on the positive electrode 10A and
the node P 14 on the negative electrode 20A. The region 35A is a
portion of the resistor 30A that is sandwiched between the node P15
on the positive electrode 10A and the node P 16 on the negative
electrode 20A.
[0086] If a predetermined voltage is applied to the positive
electrode 10A from a power supply and the negative electrode 20A is
grounded, currents flow from the positive electrode 10A to the
negative electrode 20A. In this situation, a current flows from the
branch 13A of the positive electrode 10A to the branch 23A of the
negative electrode 20A through the effective region 31A of the
resistor 30A, a current flows from the branch 14A of the positive
electrode 10A to the branch 23A of the negative electrode 20A
through the effective region 32A of the resistor 30A, a current
flows from the branch 14A of the positive electrode 10A to the
branch 24A of the negative electrode 20A through the effective
region 33A of the resistor 30A, a current flows from the branch 15A
of the positive electrode 10A to the branch 24A of the negative
electrode 20A through the effective region 34A of the resistor 30A,
and a current flows from the branch 15A of the positive electrode
10A to the branch 25A of the negative electrode 20A through the
effective region 35A of the resistor 30A. No currents flow in a
portion located outside the branch 13A (the node P 11) of the
positive electrode 10A and a portion located outside the branch 25A
(the node P 16) of the negative electrode 20A.
[0087] If the electrical resistances of the positive and negative
electrodes 10A and 20A are negligible, an equivalent circuit of the
resistor 30A is as shown in FIG. 2B, wherein R1, R2, R3, R4, and R5
represent the electrical resistance of the effective regions 31A,
32A, 33A, 34A, and 35A, respectively. The apparent electrical
resistance R' of the resistive heater according to the second
example of the present invention is equal to the electrical
resistance of a circuit including five resistors, connected to each
other in parallel, having an electrical resistance equal to R1, R2,
R3, R4, or R5. Therefore, the apparent electrical resistance R' of
the resistive heater is greatly less than the electrical resistance
estimated from the resistivity of the resistor 30A.
[0088] The resistive heater of the second example has the same
advantages as those of the resistive heater of the first example.
Furthermore, the apparent electrical resistance R' of the resistive
heater of the second example is less than that of the resistive
heater of the first example.
[0089] In the resistive heater of the second example, since the
nodes P11, P13, and P15 of the positive electrode 10A and the nodes
P12, P14, and P16 of the negative electrode 20A are alternately
arranged in the longitudinal direction of the resistor 30A, each
region of the resistor 30A that is sandwiched between the nodes
adjacent to each other effectively functions. Therefore, the
resistor 30A has no non-effective regions except for both end
regions thereof. This means that available regions of the resistor
30A can be fully used. Therefore, almost all regions of the
resistor 30A are allowed to generate heat; hence, the temperature
thereof is uniform. Furthermore, since the resistive heater of the
second example generates heat from the effective regions having a
larger area as compared to the resistive heater of the first
example, a load applied to the resistor 30A is distributed; hence,
there is an advantage in that this resistive heater can be
prevented from being deteriorated.
THIRD EXAMPLE
[0090] FIG. 3A is a plan view showing a configuration of a
resistive heater according to a third example of the present
invention and FIG. 3B is a diagram showing an equivalent circuit of
the resistive heater.
[0091] With reference to FIG. 3A, the resistive heater of the third
example as well as the resistive heater of the first example is
placed on an insulating substrate (not shown) and includes a wire
resistor 30B having a predetermined length; a positive electrode
10B, placed on a side (the upper side in FIG. 3A) of the resistor
30B, extending along the resistor 30B; and a negative electrode
20B, placed on the side (the lower side in FIG. 3A) opposite to the
positive electrode 10B, extending along the resistor 30B.
[0092] Other components of the resistor 30B are the same as those
of the resistor 30 of the first example and the description of the
resistor 30B is therefore omitted.
[0093] The positive electrode 10B has the same configuration as
that of the positive electrode 10 of the first example except that
the positive electrode 10B includes three branches 13B, 14B, and
15B. Reference numeral 11B represents a connection and reference
numeral 12B represents an extension.
[0094] The branch 13B of the positive electrode 10B is connected to
a node P21 placed on the resistor 30B. The branch 14B is connected
to a node P23 placed on the resistor 30B. The branch 15B is
connected to a node P24 placed on the resistor 30B.
[0095] The negative electrode 20B has the same configuration as
that of the negative electrode 20 of the first example. Reference
numeral 21B represents a connection, reference numeral 22B
represents an extension, and reference numerals 23B and 24B
represent branches.
[0096] The branch 23B of the negative electrode 20B is connected to
a node P22 placed on the resistor 30B. The branch 24B is connected
to a node P25 placed on the resistor 30B.
[0097] In this example, an effective region 31B is present between
the nodes P21 and P22 of the resistor 30B, an effective region 32B
is present between the nodes P22 and P23, and an effective region
33B is present between the nodes P24 and P25. The nodes P21 to P25
are arranged such that the three effective regions 31B, 32B, and
33B have the same length. Therefore, the effective regions 31B,
32B, and 33B have the same electrical resistance.
[0098] If the electrical resistances of the positive and negative
electrodes 10B and 20B are negligible, an equivalent circuit of the
resistor 30B is as shown in FIG. 3B, wherein R1, R2, and R3
(R1=R2=R3) represent the electrical resistance of the effective
regions 31A, 32A, and 33A, respectively. The apparent electrical
resistance R' of the resistive heater according to the third
example of the present invention is equal to the electrical
resistance of a circuit including three resistors, connected to
each other in parallel, having an electrical resistance equal to
R1, R2, or R3. Therefore, the apparent electrical resistance R' of
the resistive heater is greatly less than the electrical resistance
estimated from the resistivity of the resistor 30B.
[0099] As described above, the resistive heater of the third
example has the same advantage as that of the resistive heater of
the first example.
[0100] The following equation (2) holds: 1/R'=(1/R).times.(n/m) (2)
wherein R' represents the apparent electrical resistance of the
resistive heater observed from an external driving circuit, n
represents the number of the effective regions of the resistive
heater and is not equal to 0, and m represents the percentage of
the effective regions in the resistive heater and is less than
1.
[0101] Therefore, there is an advantage in that the apparent
electrical resistance R' of the resistive heater can be readily
determined using the number n of the effective regions of the
resistive heater and the percentage m of the effective regions in
the resistive heater.
FOURTH EXAMPLE
[0102] FIG. 4A is a plan view showing a configuration of a
resistive heater according to a fourth example of the present
invention and FIG. 4B is a diagram showing an equivalent circuit of
the resistive heater.
[0103] With reference to FIG. 4A, the resistive heater of the
fourth example, as well as that of the first example, is placed on
an insulating substrate (not shown) and includes a wire resistor
30C having a predetermined length; a positive electrode 10C, placed
on a side (the upper side in FIG. 4A) of the resistor 30C,
extending along the resistor 30C; and a negative electrode 20C,
placed on the side (the lower side in FIG. 4A) opposite to the
positive electrode 10C, extending along the resistor 30C.
[0104] The resistor 30C has no region extending out of a node P31
nor P36. In other words, the resistor 30C has substantially the
same configuration as that of the resistor 30A, shown in FIGS. 2A
and 2B, according to the second example except that the nodes P31
and P36 are each placed at one of both ends of the resistor 30C;
hence, the description of the resistor 30C is omitted.
[0105] The positive electrode 10C has the same configuration as
that of the positive electrode 10A of the second example except
that the positions of three branches 13C, 14C, and 15C included in
the positive electrode 10C are different from those of the branches
of the positive electrode 10A of the second example. Reference
numeral 11C represents a connection and reference numeral 12C
represents an extension.
[0106] The branch 13C of the positive electrode 10C is connected to
the node P31 of the resistor 30C. The branch 14C is connected to a
node P33 placed on the resistor 30C. The branch 15C is connected to
a node P35 placed on the resistor 30C.
[0107] The negative electrode 20C has the same configuration as
that of the negative electrode 20A of the second example except
that the positions of three branches 23C, 24C, and 25C included in
the negative electrode 20C are different from those of the branches
of the negative electrode 20A of the second example. Reference
numeral 21C represents a connection and reference numeral 22C
represents an extension.
[0108] The branch 23C of the negative electrode 20C is connected to
a node P32 placed on the resistor 30C. The branch 24C is connected
to a node P34 placed on the resistor 30C. The branch 25C is
connected to the node P36 of the resistor 30C.
[0109] In this example, an effective region 31C is present between
the nodes P31 and P32 of the resistor 30C, an effective region 32C
is present between the nodes P32 and P33, an effective region 33C
is present between the nodes P33 and P34, an effective region 34C
is present between the nodes P34 and P35, and an effective region
35C is present between the nodes P35 and P36. The nodes P31 to P36
are arranged such that the five effective regions 31C, 32C, 33C,
34C, and 35C have the same length. Therefore, the effective regions
31C, 32C, 33C, 34C, and 35C have the same electrical
resistance.
[0110] If the electrical resistances of the positive and negative
electrodes 10C and 20C are negligible, an equivalent circuit of the
resistor 30C is as shown in FIG. 4B, wherein R1, R2, R3, R4, and R5
(R1=R2=R3=R4=R5) represent the electrical resistance of the
effective regions 31A, 32A, 33A, 34A, and 35A, respectively. The
apparent electrical resistance R' of the resistive heater according
to the fourth example of the present invention is equal to the
electrical resistance of a circuit including five resistors,
connected to each other in parallel, having an electrical
resistance equal to R1, R2, R3, R4, or R5. Therefore, the apparent
electrical resistance R' of the resistive heater is greatly less
than the electrical resistance estimated from the resistivity of
the resistor 30C.
[0111] As described above, the resistive heater of the fourth
example has the same advantage as that of the resistive heater of
the first example.
[0112] Above equation (2) holds for the apparent electrical
resistance R' of the resistive heater observed from an external
driving circuit, the number n (n.noteq.1) of the effective regions
of the resistive heater, and the percentage m (m<1) of each
effective region in the wire resistor. In the resistive heater of
the fourth example, since the branch 13C of the positive electrode
10C and the branch 25C of the negative electrode 20C are each
connected to one of both ends of the resistor 30C, the whole of the
resistor 30C can be effectively used. Therefore, the following
equation holds: m.times.n=1 (3)
[0113] The following equation (4) can be obtained by substituting
equation (3) into equation (2): R'=R/(n.sup.2) (4)
[0114] That is, the apparent electrical resistance R' of the
resistive heater can be determined using only the number n of the
effective regions of the resistive heater. This means that the
apparent electrical resistance R' can be designed based only on the
number n of the effective regions; hence, there is an advantage in
that the resistive heater of the fourth example can be readily
designed in addition to the advantages described in the third
example.
[0115] A method for manufacturing the resistive heater (see FIGS.
4A and 4B) according to the fourth example of the present invention
will now be described.
[0116] FIGS. 5A to 5E are sectional views showing principal parts
of the resistive heater in the order of manufacturing steps. The
resistor 30C is made of a material, such as TiN, having a
resistivity of 200 .mu..OMEGA.cm. TiN as well as TaN is very
chemically stable as disclosed in Japanese Unexamined Patent
Application Publication Nos. 2000-294738 and 6-34925; hence, TiN is
useful in achieving high long-term reliability.
[0117] First of all, as shown in FIG. 5A, the following layers are
formed on an insulating substrate 50 in this order by a sputtering
process: a TiN layer 51 having a thickness of, for example, 200 nm;
an aluminum (Al) layer 52 having a thickness of, for example, 200
nm; a titanium (Ti) layer 53 having a thickness of, for example,
100 nm; and a gold (Au) layer 54 having a thickness of, for
example, 500 nm. Examples of the insulating substrate 50 include a
glass substrate, ceramic substrate, and silicon substrate having a
silica layer thereon. The following process can be used instead of
the sputtering process: a reactive sputtering process, an electron
beam vapor deposition process, a resistance-heating vapor
deposition process, or another process. The resistor 30C is
prepared by processing the TiN layer 51 and may be made of TaN or
another material. The aluminum layer 52, the titanium layer 53, and
the gold layer 54 form a conductive layer having a triple layer
structure. The conductive layer is patterned into the positive
electrode 10C and the negative electrode 20C. The conductive layer
may have a triple layer structure consisting of a copper (Cu)
layer, a chromium (Cr) layer, and a platinum (Pt) layer without
including the aluminum layer 52 and the titanium layer 53 and
another type of conductive layer may be used.
[0118] As shown in FIG. 5B, a photoresist layer 60 is formed on the
Au layer 54 and then patterned by a photolithographic process. The
resulting photoresist layer 60 is used as a mask when the Al layer
52, the Ti layer 53, and the Au layer 54 are etched into the
positive electrode 10C and the negative electrode 20C.
[0119] The Al layer 52, the Ti layer 53, and the Au layer 54 are
etched using the photoresist layer 60 as a mask, whereby the
positive electrode 10C and negative electrode 20C each including
portions of these three layers are formed as shown in FIG. 5C. In
this etching step, a wet etching process or a dry etching process
such as a milling process or a reactive ion etching process may be
used.
[0120] The photoresist layer 60 is removed and the surfaces of the
positive electrode 10C, the negative electrode 20C, and the TiN
layer 51 are then cleaned. As shown in FIG. 5D, another photoresist
layer 61 is formed on the TiN layer 51 and then patterned by a
photolithographic process. This photoresist layer 61 is used as a
mask when the TiN layer 51 is etched so as to have a wire shape
identical to the shape of the resistor 30C.
[0121] The TiN layer 51 is etched using the photoresist layer 61 as
a mask, whereby the resistor 30C made of TiN layer 51 as shown in
FIG. 5E. In this etching step, a wet etching process or a dry
etching process such as a milling process or a reactive ion etching
process may be used. According to this procedure, the resistive
heater, shown in FIGS. 4A and 4B, according to the fourth example
is completed.
[0122] The resistor 30C made of TiN layer 51 has a width of 10
.mu.m, a length of 2 mm, and a thickness of 200 nm. If the known
resistive heater shown in FIGS. 7A and 7B includes the resistor
30C, this known resistive heater has an electrical resistance of 2
k.OMEGA. because the resistivity of TiN is 200 .mu..OMEGA.cm. In
order to allow this known resistive heater to generate 300 mW of
heat, a power supply with a voltage of 17 V or more must be used.
However, the voltage of a power supply for electronic circuits is
about 3 to 12 V; hence, it is useless to connect this known
resistive heater and an electronic circuit to a common power
supply. Therefore, a power supply devoted to this known resistive
heater must be designed.
[0123] On the other hand, the resistive heater, shown in FIGS. 4A
and 4B, according to the fourth example includes the resistor 30C
made of TiN layer 51 and the number of the effective regions of the
resistive heater is equal to five, that is, n=5. Therefore, the
apparent electrical resistance R' can be determined using equation
(4) as follows: R'=2 k.OMEGA./(5.sup.2)=80.OMEGA. In order to allow
the resistive heater to generate 300 mW of heat, the voltage of a
necessary power supply is 4.9 V. Therefore, it is useful to
commonly connect the resistive heater and an electronic circuit to
this power supply.
[0124] If the number of effective regions of a resistive heater is
equal to eight, that is, n=8, the apparent electrical resistance R'
thereof can be determined as follows: R'=2
k.OMEGA./(8.sup.2)=31.25.OMEGA. In order to allow this resistive
heater to generate 300 mW of heat, the voltage of a necessary power
supply is about 3.1 V. Therefore, it is more useful to commonly
connect this resistive heater and the electronic circuit to this
power supply.
[0125] A material for forming the positive electrode 10C and the
negative electrode 20C is will now be described. Such a material
preferably contains at least two selected from the group consisting
of gold, platinum, chromium, titanium, copper, aluminum, titanium
nitride, and tantalum nitride. Other conductive elements or
compounds other than these elements and compounds may be used.
[0126] For the resistive heater of the present invention, the
resistivity of the positive, electrode 10C and that of the negative
electrode 20C are critical. Suppose that an imaginary resistive
heater has the same configuration as that of the known resistive
heater shown in FIGS. 7A and 7B and includes a positive electrode
110, a negative electrode 120, and a resistor 130 and a material
for forming this positive electrode 110 and this negative electrode
120 has a resistivity equal to that of a material for forming this
resistor 130. In general, it is not rare that a positive electrode
and a negative electrode have a length greater than or equal to
that of this resistor 130. If this positive electrode 110 and this
negative electrode 120 have a length equal to that of this resistor
130, this positive electrode 110 and this negative electrode 120
consume half of the electricity input to the imaginary resistive
heater. That is, in order to allow this resistor 130 to generate
300 mW of heat, 600 mW of electricity must be input to the
imaginary resistive heater.
[0127] Furthermore, suppose that another imaginary resistive heater
has the same configuration as that of the resistive heater, shown
in FIGS. 4A and 4B, according to the fourth example and includes a
positive electrode 10C, a negative electrode 20C, and a resistor
30C. Since these positive and negative electrodes 10C and 20C have
a length greater than that of those positive and negative
electrodes included in the known resistive heater shown in FIGS. 7A
and 7B, this resistor 30C has an apparent electrical resistance R'
less than that of that resistor included in the known resistive
heater; however, the sum of the electrical resistances of these
positive and negative electrodes 10C and 20C is greater than the
sum of the electrical resistances of those positive and negative
electrodes of the known resistive heater. Therefore, there is a
problem in that the amount of heat generated from this resistor 30C
is greater than the sum of the amount of heat generated from this
positive electrode 10C and that from this negative electrode 20C.
Hence, these positive and negative electrodes 10C and 20C must have
an electrical resistance sufficiently less than that of this
resistor 30C.
[0128] In the positive and negative electrodes 10C and 20C,
described in the fourth example, having the triple layer structure
consisting of the Al layer 52, the Ti layer 53, and the Au layer
54, each section between the connection 11C (bonding pad) of the
positive electrode 10C and the node P31, P33, or P35 thereof has an
electrical resistance of about 1 to 3.OMEGA. and each section
between the connection 21C (bonding pad) of the negative electrode
20C and the node P32, P34, or P36 thereof has an electrical
resistance of about 1 to 3.OMEGA.. Since the number n of the
effective regions of the resistor 30C is five (see FIGS. 4A and
4B), the positive and negative electrodes 10C and 20C are allowed
to have an electrical resistance less than 4% of that of the
resistor 30C. If the number n of the effective regions of the
resistor 30C is eight, the positive and negative electrodes 10C and
20C are allowed to have an electrical resistance less than 10% of
that of the resistor 30C.
FIFTH EXAMPLE
[0129] FIG. 6 is a plan view showing a configuration of a
thermooptic phase shifter according to a fifth example of the
present invention. The thermooptic phase shifter includes a
resistive heater, which has substantially the same configuration as
that of the resistive heater of the fourth example.
[0130] The thermooptic phase shifter further includes an insulating
substrate (not shown) and a straight optical waveguide extending
along the insulating substrate. The optical waveguide has a core
70, which is simply shown in FIG. 6. The core 70 is surrounded by a
clad layer, which is not shown.
[0131] The resistive heater included in the thermooptic phase
shifter includes a wire resistor 30D having a predetermined length;
a positive electrode 10D, placed on a side (the upper side in FIG.
6) of the resistor 30D, extending along the resistor 30D; and a
negative electrode 20D, placed on the side (the lower side in FIG.
6) opposite to the positive electrode 10D, extending along the
resistor 30D.
[0132] The resistor 30D has the same configuration as that of the
resistor 30C, shown in FIGS. 4A and 4B, according to the fourth
example. The resistor 30D is placed above the clad layer
surrounding the optical waveguide core 70 and extends in parallel
to the optical waveguide core 70.
[0133] The positive electrode 10D includes a connection 11D, an
L-shaped extension 12D, and three straight branches 13D, 14D, and
15D. The branch 13D is connected to a node P41 placed at one end of
the resistor 30D. The branch 14D is connected to a node P43 placed
on the resistor 30D. The branch 15D is connected to a node P45
placed on the resistor 30D.
[0134] The negative electrode 20D includes a connection 21D, a
straight extension 22D, and three straight branches 23D, 24D, and
25D. The branch 23D is connected to a node P42 placed on the
resistor 30D. The branch 24D is connected to a node P44 placed on
the resistor 30D. The branch 25D is connected to a node P46 placed
at the other end of the resistor 30D.
[0135] An effective region 31D is present between the nodes P1 and
P42 of the resistor 30C, an effective region 32D is present between
the nodes P42 and P43, an effective region 33D is present between
the nodes P43 and P44, an effective region 34D is present between
the nodes P44 and P45, and an effective region 35D is present
between the nodes P45 and P46. The nodes P41 to P46 are arranged
such that the five effective regions 31D, 32D, 33D, 34D, and 35D
have the same length. Therefore, the effective regions 31D, 32D,
33D, 34D, and 35D have the same electrical resistance.
[0136] If the electrical resistances of the positive and negative
electrodes 10C and 20C are negligible, an equivalent circuit of the
resistor 30C is as shown in FIG. 4B, wherein R1, R2, R3, R4, and R5
(R1=R2=R3=R4=R5) represent the electrical resistance of the
effective regions 31A, 32A, 33A, 34A, and 35A, respectively. The
apparent electrical resistance R' of the resistive heater according
to the fifth example of the present invention is equal to the
electrical resistance of a circuit including five resistors,
connected to each other in parallel, having an electrical
resistance equal to R1, R2, R3, R4, or R5.
[0137] The thermooptic phase shifter according to the fifth example
of the present invention can vary the phase of light propagated
through the optical waveguide in such a manner that the resistor
30D is allowed to generate heat by applying a current to the
resistive heater and the refractive index of the optical waveguide
core 70 is varied by heating the optical waveguide core 70 using
the heat.
[0138] In order to minimize the amount of electricity consumed by
the resistor 30D, the optical waveguide core 70 must be efficiently
heated. Since the heat generated from the resistor 30D is
transmitted to the optical waveguide core 70 through the clad layer
made of glass, the distance between the optical waveguide core 70
and the resistor 30D for generating heat is preferably small as
long as optical properties of the core 70 are not deteriorated. In
this example, the resistor 30D is placed close to the optical
waveguide core 70 and extends in parallel to the optical waveguide
core 70; hence, the distance therebetween is minimum and the heat
generated from the resistor 30D can therefore be efficiently
transmitted to the core 70. Furthermore, the temperature of a
section, extending in the direction that light travels, for heating
the core 70 is uniform; hence, optical properties of the core 70
can be prevented from being deteriorated due to thermal stress.
MODIFICATION
[0139] The first to fifth examples described above are intended to
illustrate the present invention. Therefore, the present invention
is not limited these examples and various modifications may be made
within the scope of the present invention. For example, the number,
position, and shape of connections, extensions, and branches of
positive and negative electrodes may be arbitrarily varied as
required.
[0140] As described above in detail, according to the present
invention, although a resistor is made of a material, such as
tantalum nitride or titanium nitride, having a relatively large
resistivity, the apparent electrical resistance (the superficial
electrical resistance of the resistive heater) is less than the
electrical resistance estimated from the material. Therefore, the
amount of heat generated from the resistive heater can be
controlled with a simple electronic circuit. The apparent
electrical resistance of the resistive heater can be adjusted to
any value. Accordingly, the present invention is exceedingly useful
in manufacturing a wire-shaped resistive heater controllable with a
simple electronic circuit.
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