Method of making a thick film low value high frequency inductor

Details Method of making a thick film low value high frequency inductor Method of making a thick film low value high frequency inductor

1999-11-24

2001-03-13

Heinrich, Samuel M. (Department: 1725)

Electric heating

Metal heating

By arc

C219S121850, C029S602100

Reexamination Certificate

active

06201215

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to the design and construction of low value, high frequency inductors. These parts are particularly suited for the communications industry. The current trend is toward continued miniaturization and increased frequency.
BACKGROUND OF THE INVENTION
Typical prior art inductors use one of two manufacturing processes.
1
) Thin Film or
2
) Individually spiraled copper plated ceramic cores.
The Thin Film process requires large capital expenditures. Additionally, the time to build up conductor thicknesses sufficient to meet Q requirements is long, keeping manufacturing costs high.
The second process, spiraled parts, has very high manufacturing costs due mainly to the necessity to handle parts individually. Individual part handling increases manufacturing time and costs dramatically.
Q is a measure of quality in inductors. It is the ratio of inductive reactance, to the sum of all resistive losses. Inductive reactance is desirable. Resistive losses, one of which is skin effect, are undesirable.
Skin effect is the tendency for alternating current to flow near the surface of conductors in lieu of flowing in a manner as to utilize the entire cross sectional area of conductors. This phenomenon causes the resistance of the conductor to increase, thus reducing Q. When the conductor cross section includes sharpened areas at each side, skin effect is even worse. Existing equipment and methods diminish the Q value because the cross sectional cut of conductor layers in fabrication create cross sectional cuts of non-rectangular configuration.
A high frequency chip inductor on ceramic is currently made by screenprinting thickfilm conductor ink forming fine lines on a ceramic substrate. An alternative method is to expose photoimageable conductor ink to UV radiation through a mask, and then develop the pattern via wet chemistry processing. However, these two technologies limit the number of turns in the spiral because they are limited to making line widths no finer than about 4 mils.
It is therefore a principal object of this invention to provide a thick film low value high frequency inductor which is low in resistive losses including skin effect.
A further object of this invention is to provide a thick film low value high frequency inductor, and a method of manufacturing the same which does not have sharpened areas at its side edges whereupon skin effect is reduced.
A still further object of the invention is to use a pulsing laser cutting technique wherein the entire cut is performed simultaneously which shortens the cutting time.
A still further alternative object of-this invention is to provide a method of making an inductor using a photosensitive silver ladened printing ink exposed to ultra violet light in combination with a suitable negative image mask to create the desired image.
A still further object of this invention is to provide a chip inductor comprised of a multi-turn spiral coil on the upper surface of a nonmagnetic dielectric substrate.
A still further object of this invention is a chip inductor comprised of a spiral coil which is formed by ablating a spiral coil pattern into a conductive layer on a substrate by the use of a laser beam.
A still further object of this invention is a chip inductor coil formed by ablating a spiral coil pattern into a conductive layer on a substrate by the use of an excimer laser operating in the ultraviolet region of the electromagnetic spectrum.
A still further object of this invention is to provide for a substrate base layer that has a low dielectric constant of around
4
, which is considerably lower than alumina, which is around
9
.
A still further object of this invention provides for a substrate made of alumina and has a layer of a low dielectric constant dielectric covering the upper surface of said alumina substrate and underlays said conductive layer.
A still further object of this invention provides for an excimer laser that can ablate the conductive layer to provide the shape of a spiral conductive coil without cutting into the low dielectric constant dielectric layer on the upper side of the alumina substrate or into the alumina substrate itself.
A still further object of this invention provides for a second spiral coil to be positioned above the first spiral coil on the substrate with a layer of low K dielectric between the two coils to keep said coils spaced apart but with a via hole positioned to connect one end of each coil to provide for two coils in series.
These and other objectives will be apparent to those skilled in the art.
SUMMARY OF THE INVENTION
A laser pulsing technique of the type disclosed in U.S. Pat. No. 5,091,286 is used to laser cut the coil image in a conductive coil. The low pulsing technique of the '286 patent is incorporated herein by reference. This process yields conductor cross sections that are more rectangular than in conventional thick film screen printing.
This invention involves a new and improved method for manufacturing a high frequency chip inductor on ceramic. This new method allows achieving higher inductance values by making line widths much finer than the 2 mils allowed by the prior art. Finer lines permitting more loops than allowed by the prior art.
This new method consists of using the ultraviolet output of an excimer laser to cut a planar spiral coil pattern in conductor material that is in the form of a flat layer on a substrate. However, rather than making linear cuts with a fine focused, scanned beam (as would be done in a typical industrial metal cutting operation), a beam with a large cross section, approximately 4 cm by 3 cm, is projected through a metal “stencil” mask that contains the coil pattern. This transforms the beam's uniformly intense cross section into a cross section containing the coil pattern. The transformed beam is then optically focused onto a blank metal target into which the coil pattern is burned or permanently transfixed by the process of ablation. The UV excimer laser has a pulsed output so that one or more pulses will be required to produce a clean and clear coil pattern in the conductor material. The number of pulses required will increase with the thickness of the conductor. Suitable conductor targets on ceramic can be made by thickfilm printing, thinfilm deposition, or by bonding metal foil to the ceramic surface.
An alternate form of the invention creates the rectangular cross sectional cuts of conductor layers by a special film screening process which involves the steps of taking a ceramic sheet of high (very hard) alumina which is pre-scribed so that it can be broken into small inductors. An organic layer of silver ladened ink is printed thereon, and then dried. A photo mask that is a negative of the desired image is placed over, and in contact with, the dried photoimagable ink. This configuration is next exposed to UV radiation, polymerizing the ink not masked by the photo mask. Next the photo mask is removed and the “ink” is then chemically developed, the nonpolymerized material is washed away and the desired silver image remains. The device is then fired, the non-developed material is washed away and the silver image remains. Appropriate crossovers between images can thereafter be printed. This process produces a conductor layer with rectangular cross sections and which provides conductors with line, widths and spacings smaller than can be made with conventional thick film screen printing.


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