Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2001-06-12
2004-08-10
Hail, III, Joseph J. (Department: 3723)
Metal working
Method of mechanical manufacture
Electrical device making
C029S847000, C065S017200, C219S121710
Reexamination Certificate
active
06772514
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
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.
2. Description of Prior Art
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.
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:
loss=factor×frequency of circuit in question×(permittivity of substrate)
1/2
×dielectric loss of substrate (dielectric loss tangent).
The loss caused here converts into thermal energy, causing a phenomenon of heating the substrate.
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).
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.
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 &mgr;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-&mgr;m-diameter through holes, the holes cannot be formed beyond 200 &mgr;m or so in depth, relative to the mask pattern having 100-&mgr;m openings. Here, no through hole can be made if the glass substrate is thicker than 200 &mgr;m.
When ultrasonic machining is used to machine a glass substrate, a 100-&mgr;m hole in a 500-&mgr;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.
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 &mgr;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 &mgr;m in thickness by using a KrF excimer laser (wavelength: 0.248 &mgr;m), and obtained the following results. That is, through holes of the order of 100 &mgr;m in diameter could be made at an energy density of approximately 25 J/cm
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.
It is conceivable that the use of an F
2
excimer laser having a wavelength (0.157 &mgr;m) shorter than that of the KrF excimer laser could somewhat relax the narrow range of the process conditions for glass substrates. F
2
gas is, however, poisonous to humans and therefore the use of the F
2
excimer laser in mass production processes is unrealistic.
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
−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.
A machining method using a CO
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
2
laser systems are lower in price and in running costs than the other systems, it can be said that the CO
2
laser machining is a glass substrate machining method suited to mass production.
Nevertheless, machining glass substrates by using the CO
2
laser produces the problem to be described below.
FIG. 2
is a diagram showing the problem that arises when a glass substrate is machined by using a CO
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 sho
Hashidate Yuji
Ogura Hiroshi
Yajima Hiroyoshi
Yoshida Yoshikazu
Browdy and Neimark , P.L.L.C.
Grant Alvin J
Hail III Joseph J.
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