U.S. patent application number 10/837637 was filed with the patent office on 2004-10-14 for method of machining glass substrate and method of fabricating high-frequency circuit.
This patent application is currently assigned to Masushita Electric Industrial Co., Ltd.. Invention is credited to Hashidate, Yuji, Ogura, Hiroshi, Yajima, Hiroyoshi, Yoshida, Yoshikazu.
Application Number | 20040200067 10/837637 |
Document ID | / |
Family ID | 18677933 |
Filed Date | 2004-10-14 |
United States Patent
Application |
20040200067 |
Kind Code |
A1 |
Ogura, Hiroshi ; et
al. |
October 14, 2004 |
Method of machining glass substrate and method of fabricating
high-frequency circuit
Abstract
A method of machining a glass substrate using a laser, in which
a low-permittivity, low-dielectric-loss glass substrate, capable of
coping with mass production, is made applicable as the substrate of
a high-frequency circuit intended in particular for microwave and
millimeter-wave bands. For that purpose, a glass substrate is
provided in which the amount of air bubbles in glass is arbitrarily
controlled to improve the workability of the substrate. Then, the
glass substrate is machined by being irradiated with a pulsed laser
a plurality of times, thereby improving the machining shape of the
glass substrate. Since glass substrates which are typically
difficult to machine can be easily applied to the fabrication of
high-frequency circuits, it becomes possible to supply
high-performance circuits and apparatuses widely to the public.
Inventors: |
Ogura, Hiroshi; (Tokyo,
JP) ; Hashidate, Yuji; (Tokyo, JP) ; Yajima,
Hiroyoshi; (Sagamihara-shi, JP) ; Yoshida,
Yoshikazu; (Kawagoe-shi, JP) |
Correspondence
Address: |
BROWDY AND NEIMARK, P.L.L.C.
624 NINTH STREET, NW
SUITE 300
WASHINGTON
DC
20001-5303
US
|
Assignee: |
Masushita Electric Industrial Co.,
Ltd.
Osaka
JP
|
Family ID: |
18677933 |
Appl. No.: |
10/837637 |
Filed: |
May 4, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10837637 |
May 4, 2004 |
|
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09878265 |
Jun 12, 2001 |
|
|
|
6772514 |
|
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Current U.S.
Class: |
29/846 |
Current CPC
Class: |
Y10T 29/49156 20150115;
C03C 23/0025 20130101; C03C 17/28 20130101; B23K 26/0624 20151001;
C03C 17/32 20130101; Y10T 29/49155 20150115; Y10T 29/49165
20150115; H01P 11/003 20130101; Y10T 29/49126 20150115; H05K 3/0029
20130101 |
Class at
Publication: |
029/846 |
International
Class: |
H05K 003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 12, 2000 |
JP |
2000-176183 |
Claims
What is claimed is:
1. A glass substrate machining method comprising the steps of: a)
preparing a glass substrate in which an amount of air bubbles in
said glass substrate is controlled to improve the workability of
said glass substrate; b) machining said glass substrate by using a
laser to make a hole or a groove in said glass substrate.
2. A glass substrate machining method comprising the steps of: a)
preparing a glass substrate in which an amount of air bubbles in
said glass substrate is controlled to improve the workability of
said glass substrate; b) forming a thin insulator on a surface of
said glass substrate; c) machining said glass substrate by using a
laser to make a hole or a groove in said glass substrate.
3. The glass substrate machining method according to claim 2,
wherein said thin insulator on said glass surface is glass formed
by coating said surface.
4. The glass substrate machining method according to claim 2,
wherein said thin insulator formed on said glass surface is an
organic insulator film.
5. The glass substrate machining method according to claim 4,
wherein said thin organic insulator on said glass surface is formed
by coating said surface.
6. The glass substrate machining method according to claim 4,
wherein said thin insulator on said glass surface is united with
said glass substrate into a sheet form by lamination therewith.
7. The glass substrate machining method according to claim 1,
wherein a CO.sub.2 laser is used to perform the laser
machining.
8. The glass substrate machining method according to claim 2,
wherein a CO.sub.2 laser is used to perform the laser
machining.
9. A glass substrate machining method comprising the steps of:
machining a glass substrate by using a laser; controlling an amount
of air bubbles in a glass substrate so that said glass substrate,
after said laser machining, has a large surface area on the
machined surface due to bubble traces in glass; and forming a metal
film through simple electroless plating, to improve heat radiation
property of the metal-film-formed portion.
10. The glass substrate machining method according to claim 9,
wherein a CO.sub.2 laser is used to perform the laser
machining.
11. In a method of machining a glass substrate by application of
laser irradiation to said glass substrate from a laser, the
improvement wherein: said glass substrate is a glass substrate
having therein discrete air bubbles; and said laser is a CO.sub.2
laser or an excimer laser.
12. A machined glass product made by the process of claim 11.
13. The method of claim 11, wherein said glass substrate is
provided on a surface thereof with a thin layer selected from the
group consisting of a thin insulator layer and a thin metal
layer.
14. The method of claim 13, wherein said thin layer is an insulator
layer and has a thickness of about 10-70 .mu.m.
15. The method of claim 14, wherein said thin insulator layer is an
organic film formed by lamination or an inorganic substance applied
by spin coating.
16. The method of claim 13, wherein said thin layer is a metal film
having a thickness of approximately 10 .mu.m or less.
17. The method of claim 11, wherein said laser irradiation is
carried out in a plurality of stages.
18. The method of claim 17, wherein a first stage of laser
irradiation is carried out at a first pulse width, and a second
stage of said laser irradiation is carried out with a pulse width
greater than said first pulse width.
19. The method of claim 18, wherein said second stage of laser
irradiation is performed a plurality of times.
20. The method of claim 18, wherein said plural stages of laser
irradiation are carried out with the diameter of the laser beam
remaining substantially unchanged.
21. The method of claim 20, wherein the pulse width of laser
irradiation between said first stage and said second stage is
changed.
22. The method of claim 18, wherein the pulse width of laser
irradiation between said first stage and said second stage is
changed.
23. The method of claim 18, wherein said laser irradiation is
applied to form a through-hole in said substrate, and the focus of
the laser in the depth direction of the through hole is changed
from the first stage to the second stage.
24. The method of claim 11, wherein the bubble diameter of the
discrete air bubbles is greater than or equal to 50 .mu.m.
Description
RELATED APPLICATIONS
[0001] This application is a continuing application of co-pending
application Ser. No. 09/878,265, filed Jun. 12, 2001.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a method and apparatus for
fabricating a high-frequency circuit substrate of which smaller
dielectric losses are desired. In particular, the invention relates
to a method of creating a circuit intended for higher frequency
bands such as microwave bands and millimeter wave bands, and an
apparatus using the same.
[0004] 2. Description of Prior Art
[0005] It is desirable for substrates to be used in microwave,
millimeter-wave, and other high-frequency circuits to be made of
material having low-dielectric-loss properties so as to suppress
dielectric losses in the circuits resulting from the substrates
themselves.
[0006] FIG. 1 is an explanatory diagram showing that the inherent
dielectric loss of a substrate's own affects a signal flowing on a
transmission line. FIG. 1 is a cross-sectional view of the circuit
substrate as cut perpendicular to the signal flowing on the
transmission line. The reference numeral 101 represents the
substrate, 102 the transmission line, 103 a ground electrode, and
104 electric lines of force occurring when an electric signal flows
on the transmission line 102. As shown in FIG. 1, the passage of an
electric signal through the transmission line 102 makes the
electric lines of force 104 run inside the substrate 101. Here, the
electric lines of force 104 undergo the influence of the dielectric
loss (given by the value of dielectric loss tangent) inherent to
the substrate 101. A loss on the transmission line is given by: 1
loss = factor .times. frequency of circuit in question .times. (
permittivity of substrate ) 1 / 2 .times. dielectric
[0007] loss of substrate (dielectric loss tangent).
[0008] The loss caused here converts into thermal energy, causing a
phenomenon of heating the substrate.
[0009] Fabricating a high-frequency circuit involves the phenomenon
described in FIG. 1. Therefore, a substrate having properties of
lower permittivity and lower dielectric loss is selected for use.
Substrates made of typical organic materials, at low frequencies,
show the properties of lower permittivity and lower dielectric
loss. In microwave and millimeter-wave bands of 1 GHz or higher,
however, the substrates significantly deteriorate in permittivity
due to the materials' potential polarization and frequency
response, and thus are not often selected as substrates for use in
high frequencies (approximately 1 GHz or higher). For high
frequencies, it is common to select inorganic materials such as
alumina (permittivity: approximately 9, dielectric loss tangent:
approximately 0.001), zirconia (permittivity: approximately 8,
dielectric loss tangent: approximately 0.001), and aluminum nitride
(permittivity: approximately 8, dielectric loss tangent:
approximately 0.001).
[0010] Glass such as quartz is low in permittivity (permittivity:
approximately 4) and in dielectric loss (dielectric loss tangent:
0.001 or lower) as compared to the inorganic materials. Therefore,
it seems to be a promising material for high-frequency substrates
for microwave and millimeter-wave bands. However, it is difficult
to apply partial machining required of circuit substrates such as
"through hole formation" to glass. Accordingly, it has been seldom
used for high-frequency circuit substrates heretofore.
[0011] When glass is selected as the substrate material, ultrasonic
machining appears to be the effective means for forming through
holes in the glass substrate. The reason why chemical processing
methods such as etching are not used for glass machining is that
glass is a stable material. That is, although glass can be etched
by solutions of hydrofluoric acids, phosphoric acids, alkalis, or
the like, etching rates are extremely low (nearly 1 .mu.m/h or so).
As for sandblasting, sandblasting is capable of in-depth machining
only twice or so the thickness of a mask. For example, in the case
of forming 100-.mu.m-diameter through holes, the holes cannot be
formed beyond 200 .mu.m or so in depth, relative to the mask
pattern having 100-.mu.m openings. Here, no through hole can be
made if the glass substrate is thicker than 200 .mu.m.
[0012] When ultrasonic machining is used to machine a glass
substrate, a 100-.mu.m hole in a 500-.mu.m thick substrate can be
made at a machining rate of 1 sec or faster. Besides, the shape of
the tool (horn) used in the ultrasonic machining can be devised to
make a plurality of holes at a time. Nevertheless, the ultrasonic
machining wears the tool in operation, which requires a replacement
with a new tool after several times of machining to glass
substrates. In addition, there are limitations on the dimensions of
the tools. Therefore, the ultrasonic machining is a method hard to
apply to mass production processes of large area glass
substrates.
[0013] Meanwhile, laser beam machining has been already applied to
mass production processes including the formation of through holes
in alumina substrates and the like intended for high-frequency
circuits, without any limitations on substrate sizes. The laser
beam machining is, thus, suitable for ordinary substrate machining,
whereas its application to glass substrates gives rise to the
following problems. A YAG laser, a typical solid-state laser, has a
laser wavelength (1.06 .mu.m) which is transparent to glass.
Therefore, the YAG laser is hard to apply to glass machining.
Concerning excimer laser machining, the present inventors made
machining experiments on quartz glass of 500 .mu.m in thickness by
using a KrF excimer laser (wavelength: 0.248 .mu.m), and obtained
the following results. That is, through holes of the order of 100
.mu.m in diameter could be made at an energy density of
approximately 25 J/cm.sup.2. Nevertheless, the process conditions
had an extremely narrow range such that any smaller energy
densities preclude the machining while any greater energy densities
create large cracks in glass substrates. This means that the
excimer laser machining is an inappropriate method for machining a
glass substrate, in terms of application to mass production
processes.
[0014] It is conceivable that the use of an F.sub.2 excimer laser
having a wavelength (0.157 .mu.m) shorter than that of the KrF
excimer laser could somewhat relax the narrow range of the process
conditions for glass substrates. F.sub.2 gas is, however, poisonous
to humans and therefore the use of the F.sub.2 excimer laser in
mass production processes is unrealistic.
[0015] Now, turning to the case of machining a glass substrate by
using an ultrashort-pulse laser so-called femtosecond laser which
has a pulse width of no greater than 10.sup.-13 seconds. As
described in e.g. "Interaction Between Light and Glass by
Ultrashort Pulse Laser--Growing Frequency Conversion Crystal in
Glass--" (pp. 67-73), MATERIALS INTEGRATION vol. 13, no. 3 (2000),
the machining to the glass substrate is possible. However, due to
high prices and high running costs of ultrashort-pulse laser
systems, the application to mass production processes is
difficult.
[0016] A machining method using a CO.sub.2 laser that is used for
forming through holes and the like in an alumina substrate can be
adopted to execute the perforation of glass substrates under
process conditions wider than for the excimer lasers. In addition,
since CO.sub.2 laser systems are lower in price and in running
costs than the other systems, it can be said that the CO.sub.2
laser machining is a glass substrate machining method suited to
mass production.
[0017] Nevertheless, machining glass substrates by using the
CO.sub.2 laser produces the problem to be described below.
[0018] FIG. 2 is a diagram showing the problem that arises when a
glass substrate is machined by using a CO.sub.2 laser of variable
pulse width. In FIG. 2, the reference numeral 201 represents the
glass substrate, and 202 a through hole formed by the laser.
Moreover, 203 represents upheaval that occurs upon the formation of
the through hole 202, 204 the hole diameter of the glass substrate
201 on the laser-irradiated side (top hole diameter), and 205 the
diameter on the side opposite to the laser-irradiated side (bottom
hole diameter). As shown in FIG. 2, the through hole 202, being
formed in the glass substrate 201 by using the CO.sub.2 laser of
variable pulse width, has a tapered shape as seen in FIG. 2 with
the upheaval 203 on the top rim. The ratio of the bottom hole
diameter 205 to the top hole diameter 204 and the amount of the
upheaval 203 can be modified by changing the pulse width of the
laser to adjust the pulse energy applied to the glass substrate. It
is impossible, however, to avoid the tapered shape and the upheaval
phenomenon.
SUMMARY OF THE INVENTION
[0019] The present invention has been achieved in view of the
conventional problems described above. It is thus a first object of
the present invention to provide a laser-based glass substrate
machining method capable of coping with mass production
processes.
[0020] A second object of the present invention is to use this
glass substrate machining method to make a low-permittivity,
low-dielectric-loss glass substrate applicable as the substrate of
a high-frequency circuit intended for microwave and millimeter-wave
bands in particular.
[0021] A third object of the present invention is to use the
above-mentioned substrate achieved by the glass substrate machining
method to enhance performances of a radio terminal apparatus and
the like.
[0022] The present invention is to achieve the foregoing objects
through contrivances both to the glass substrate itself and to the
laser beam machining method. Specifically, a glass substrate is
provided in which the amount of air bubbles in glass is
appropriately controlled to improve the workability of the
substrate itself. Then, the glass substrate is irradiated with a
pulsed laser over a plurality of times during machining, thereby
improving the machining shape to the glass substrate.
[0023] To realize the technique as described above, the present
invention features that the amount of air bubbles in the glass
substrate is controlled to improve the laser workability of the
glass substrate itself.
[0024] Moreover, the present invention provides a glass substrate
machining method for machining a glass substrate by using a laser,
comprising the step of controlling the amount of air bubbles in the
glass substrate to improve the workability of the glass substrate,
in which a thin insulator is formed on the glass surface. In this
case, the thin insulator on the glass surface may be glass formed
by coating. Alternatively, the insulator may be an organic
insulator film. When the insulator is an organic insulator film,
the thin organic insulator on the glass surface may be formed by
coating. Furthermore, the thin insulator formed on the glass
surface may be made into a sheet form by using a laminator.
[0025] In addition, the present invention provides a method for
machining a glass substrate by using a laser, comprising the step
of controlling the amount of air bubbles in the glass substrate to
form a vacancy only inside of the glass substrate.
[0026] Moreover, the present invention provides a method of forming
a metal film on a glass substrate, wherein the amount of air
bubbles in a glass substrate is controlled so that the glass
substrate, after laser machining, has a large surface area on the
machined surface due to bubble traces in glass, thereby performing
electroless plating simply.
[0027] The present invention is also characterized in that: the
amount of air bubbles in a glass substrate is controlled so that
the glass substrate, after laser machining, has a large surface
area on the machined surface due to bubble traces in glass; and a
metal film is formed to improve heat radiation property of the
metal-film-formed portion.
[0028] The present invention is also characterized in that a
CO.sub.2 laser is used for laser machining.
[0029] Furthermore, the present invention provides a glass
substrate machining method for using a CO.sub.2 laser of variable
pulse width as machining means, the method comprising: a first step
of executing a single laser irradiation; and a second step of
executing a plurality of laser irradiations.
[0030] The present invention also provides the glass substrate
machining method described above, wherein the pulse width of the
laser in the second step is greater than that in the first
step.
[0031] Furthermore, the present invention provides a method of
fabricating a high-frequency circuit by using the glass substrate
machining method having the characteristics described above.
[0032] Moreover, the present invention provides a radio terminal
apparatus comprising a high-frequency circuit fabricated by using
the glass substrate machining method having the characteristics
described above.
[0033] The present invention also provides a radio base station
apparatus comprising a high-frequency circuit fabricated by using
the glass substrate machining method having the characteristics
described above.
[0034] The present invention also provides a radar apparatus
comprising a high-frequency circuit fabricated by using the glass
substrate machining method having the characteristics described
above.
[0035] As has been described, according to the present invention,
glass substrates which are typically difficult to machine can be
easily applied to the fabrication of high-frequency circuits. This
allows wide, public supply of high-performance circuits and
apparatuses.
[0036] The foregoing objects and advantages of the present
invention will become more apparent from the following description
when read in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1 is a conceptual diagram showing a problem that occurs
when a conventional glass substrate is used;
[0038] FIG. 2 is a conceptual diagram showing a problem that occurs
when laser machining is applied to a conventional glass
substrate;
[0039] FIGS. 3(a) and 3(b) are explanatory diagrams showing a
method of machining a glass substrate according to an embodiment of
the present invention;
[0040] FIGS. 4(a) and 4(b) are explanatory diagrams showing how a
glass substrate is machined with a laser;
[0041] FIG. 5 is an explanatory diagram showing a method of
machining a glass substrate according to an embodiment of the
present invention;
[0042] FIG. 6 is an explanatory diagram showing a method of
machining a glass substrate according to an embodiment of the
present invention;
[0043] FIG. 7 is an explanatory diagram showing a method of
machining a glass substrate according to an embodiment of the
present invention;
[0044] FIG. 8 is an explanatory diagram showing a method of
machining a glass substrate according to an embodiment of the
present invention; and
[0045] FIGS. 9(a) and 9(b) are explanatory diagrams showing a
method of machining a glass substrate according to an embodiment of
the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0046] Hereinafter, an embodiment of the present invention will be
described with reference to the drawings. FIGS. 3(a) and 3(b) are
cross-sectional views of a substrate consisting of glass that bears
air bubbles. FIG. 3(a) shows a state before laser beam machining,
and FIG. 3(b) a state after laser beam machining. In FIGS. 3(a) and
3(b), the reference numeral 1 represents the glass substrate, 2
bubbles in the glass substrate, and 3 a through hole formed by
laser beam machining.
[0047] The method for fabricating the bubble-bearing glass shown in
FIG. 3(a) is publicly known as described in e.g. Japanese Patent
Laid-Open Publication No. Hei 10-29836. Glass substrates with
designated amounts of bubbles are made available by requesting from
substrate suppliers. When a glass substrate bearing such a
controlled amount of bubbles is used as a substrate intended for
high-frequency circuits, it offers even the effect of reducing a
loss during signal transmission since the substrate having a
permittivity yet lower than those of ordinary glass substrates is
in use. When the substrate consisting of bubble-bearing glass as
shown in FIG. 3(a) is perforated by using a laser beam, the ratio
of the hole diameter on the laser-irradiated side (top hole
diameter) to the hole diameter on the side opposite to the
laser-irradiated side (bottom hole diameter) approaches 1 as shown
in FIG. 3(b). The upheaval 203 shown in FIG. 2 is also suppressed
significantly. The reason for this is that while the laser
irradiation thermally machines the glass, minute cracks
successively arise between minute bubbles in the glass along the
direction of the laser irradiation. These cracks propagate
consecutively from the laser-irradiated side to the side opposite
to the laser-irradiated side, thereby causing a machining
phenomenon. As a result, machining with a smaller tendency to taper
is achieved.
[0048] Moreover, the laser pulse energy necessary for forming the
through hole in this glass substrate can be made several tens of
percentages or much lower than that in forming a through hole in
ordinary glass substrates. This can suppress the production of the
upheaval shown in FIG. 2. Furthermore, the bubbles in the glass
function to inhibit the heat inside the glass produced by the laser
irradiation from diffusing in lateral directions of the glass
substrate. This means that the laser-based thermal machining is
performed effectively in the direction of the laser irradiation.
Incidentally, due to the thermal machining by the laser beam, the
laser-machined through hole has a smooth surface with asperities
resulting from bubble traces that have molten away. Concerning the
types of lasers to be used here, a CO.sub.2 laser is more
advantageously used for mass production processes. Nevertheless,
KrF and other excimer lasers can also be used in the perforation to
involve the crack-propagating machining of the bubbles. Here, the
narrow process conditions for ordinary glass substrate machining
are relaxed, and the glass machining itself can be performed
satisfactorily under energy lower than in the ordinary glass
substrate machining.
[0049] Among techniques for further improving the machined shape of
the through hole 3 shown in FIG. 3(b) is a machining method using a
CO.sub.2 laser of variable pulse width, comprising a first step of
performing laser irradiation only once and a second step of
performing laser irradiation over a plurality of times. This method
allows both the suppression of the taper and the suppression of the
upheaval of the machined portion even in ordinary glass machining.
FIGS. 4(a) and 4(b) show how a glass substrate is perforated by
using the CO.sub.2 laser of variable pulse width which can suppress
the taper and the upheaval of the machined portion even in ordinary
glass machining. FIGS. 4(a) and 4(b) both are cross-sectional views
of a glass substrate 201 in which a through hole 202 is formed.
FIG. 4(a) shows a cross section of the glass substrate having
undergone only a single laser irradiation at the first step. FIG.
4(b) shows a cross section of the glass substrate having undergone
a plurality of laser irradiations at the second step. The reason
why such a shape as shown in FIG. 4(b) is obtained through the
execution of a plurality of laser irradiations on the glass
substrate at the second step is that the plurality of laser
irradiations can increase the hole diameter on the side opposite to
the laser-irradiated side (bottom hole diameter). Here, the
diameter of the laser beam is unchanged irrespective of the number
of irradiations; therefore, the hole diameter on the
laser-irradiated side (top hole diameter) remains unchanged.
[0050] The reason why the execution of a plurality of laser
irradiations at the second step reduces the upheaval is that an
annealing effect on the periphery of the through hole evens out the
upheaval. Applications of this machining method include a method of
changing the pulse width of the laser irradiation between the first
step and the second step, or shifting the focus of the laser in the
depth direction of the through hole upon the second laser
irradiation. These methods can be combined to form a more
perpendicular through hole in a glass substrate. In this
connection, the method of making the laser pulse width in the
second step greater than in the first step, if adopted, enhances
the annealing effect of suppressing the upheaval. When these
methods are used to conduct the formation of a through hole in the
substrate consisting of bubble-bearing glass shown in FIG. 3(a), a
through hole of favorable shape can be formed with energy less than
in ordinary glass machining. The machining method described above
can be used to form not only the through holes but also grooves and
vacancies which are difficult to form by ordinary glass
machining.
[0051] FIG. 5 shows an example of the result of groove machining.
In FIG. 5, the reference numeral 4 represents a groove formed in a
section of a substrate 1 that consists of bubble-bearing glass.
This groove can be formed by shifting a laser beam relative to the
bubble-bearing glass substrate. FIG. 6 shows a machining method for
forming a vacancy inside a bubble-bearing glass substrate. In FIG.
6, the reference numeral 1 represents a substrate consisting of
bubble-bearing glass, 5 a vacancy formed inside the glass, 6 a lens
for focusing a laser beam, and 7 the laser beam. The laser beam 7
is condensed by the lens 6 into focus on a portion of 5 in the
bubble-bearing glass substrate 1. In this condition, the laser beam
and the substrate 1 are relatively shifted so that minute cracks
successively occur between minute bubbles inside the glass
substrate 1, along with thermal machining. This makes it possible
to form the vacancy 5 inside the glass substrate 1.
[0052] In this connection, it is desirable to use here a laser
having a higher peak output. Lasers of lower peak outputs, if used,
fail to form vacancies, but the groove shape shown in FIG. 5 would
result.
[0053] The machining methods described in conjunction with FIGS. 3
and 5, when combined with an increase in the diameter of each
bubble mixed in the glass substrate 1, can provide other effects.
Here, in general the bubble diameter is desirably greater than or
equal to 50 .mu.m. FIG. 7 shows in cross section a glass substrate
1 bearing bubbles 2 of somewhat greater diameters, with a through
hole formed therein by a laser. In this glass substrate machining
method, the wall of the through hole 3 undergoing the thermal
effect of the laser becomes an uneven surface, not a completely
smooth one, because the bubbles 2 are of a certain size. The uneven
surface condition of the through hole acts to the advantage of
circuit substrate fabrication. In typical circuit substrate
fabrication, through holes are formed for the purpose of
establishing electric connection between the circuits on the front
and on the back. Metal films are formed over the walls of the
through holes. For example, the metal films for electric connection
are formed by electroless-plating thin metal films, and then
electroplating the metal films up to a certain thickness
(approximately 10 .mu.m or smaller). This electroless plating step
involves roughening, or a step of intentionally coarsening surfaces
to be plated, because sufficient bond strength cannot be obtained
if the surfaces are smooth. The provision of uneven through hole
surfaces as in the present invention eliminates the need for the
roughening step, and thus is advantageous for mass production and
fabrication of circuit substrates.
[0054] FIG. 8 shows an example where a number of grooves are
laser-formed in a glass substrate 1 bearing bubbles 2 of somewhat
greater diameters. This kind of machining can create a surface
having a surface area far greater than in ordinary glass
substrates. Such an uneven surface, if a metal film or other thin
film of high heat conductivity is formed thereon, can make a
surface of high heat radiation. Then, this surface can be applied
to heat-radiation-critical circuit substrates to fabricate
high-performance circuit substrates.
[0055] In fabricating high-frequency circuits on bubble-bearing
glass substrates, and particularly in the cases of adopting larger
bubbles described in conjunction with FIGS. 7 and 8, the surface
roughness of portions for transmission lines to be formed on
sometimes becomes crucial to the circuits which deal with high
frequencies of a microwave band or a millimeter wave band. The
reason is that the transmission lines formed on rough-surfaced
substrates come to have asperities, which can cause losses in
signal transmission and/or increase the actual line lengths because
of increased surface areas. This problem can be avoided by forming
insulator films on the surfaces of the bubble-bearing glass
substrates. FIGS. 9(a) and 9(b) show a method for solving the
above-mentioned problem by forming an insulator on a bubble-bearing
substrate 1.
[0056] FIG. 9(a) is a cross-sectional view of the substrate before
laser beam machining, in which the reference numeral 8 represents
an insulator. Concerning the material of the insulator 8, it is
preferable to select a material having planarization effect to
asperities from among spin-on-glass (SOG) and other insulators
which can be easily formed by coating. Moreover, polyimide,
benzocyclobutene, and other organic insulator films can be formed
with greater thicknesses and improved asperity planarization effect
as compared with SOG and other inorganic substances. The formation
of the polyimide and benzocyclobutene films are typically formed by
spin coating and a following step of hardening, through, but not
limited to, coating-based methods such as the spin coating. The
organic films may be formed in a laminator. FIG. 9(b) shows a state
where a through hole is formed by a laser. There lies an SOG or
other inorganic material film of several micrometers, a polyimide
or benzocyclobutene coating of the order of 10-50 .mu.m, or a
laminator-formed organic film of the order of 20-70 .mu.m. These
substances will not hinder the formation of the through hole 3
since they have far superior laser workability as compared to glass
substrates. Forming transmission lines on the insulator 8 solves
the above-mentioned problem.
[0057] When high-frequency circuits are fabricated by using the
method of machining a glass substrate according to the present
invention, it becomes possible to manufacture circuits of higher
performance and excellent mass productivity. In addition, the
high-frequency circuit according to the present invention is
applicable to radio portable terminals, radio base stations, radar
apparatuses, and the like, and it is possible to produce
high-performance mass-producible apparatuses.
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