Abstract:
A thin film printed circuit inductive element exhibiting low Q wherein a conductive spiral is deposited on an insulating substrate and resistive links are connected between adjacent turns of the spiral. Inherent resonance is thereby damped out.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to printed circuit inductive elements and more specifically to reducing the inherent resonance of such devices. 
     2. Description of the Prior Art 
     The manufacture of printed circuit inductors is well known. See, e.g., Schweizerhof, U.S. Pat. No. 3,505,569, issued Apr. 7, 1970. The electrical magnitudes of such coils, i.e., inductance and Q, are determined primarily by the length of the winding, the path width, the spacing between paths and the average turn diameter. While the prior art has realized printed circuit coils having higher values of inductance than were initially achievable as well as methods of varying inductance values, an inherent resonance is present in such coils, limiting their usefulness. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to damp out the inherent resonance of a printed circuit spiral which acts as a lumped inductor at microwave frequencies. This and other objects can be achieved by connecting the turns of the spiral with low resistance links to produce a low Q. 
     The foregoing and other objectives, features and advantages of the invention will be more readily understood upon consideration of the following detailed description of certain preferred embodiments of the invention, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B are plan views of a rectangular spiral conductor and a circular spiral conductor, respectively, on an insulating substrate with resistive links connecting adjacent turns of the spiral; 
     FIGS. 1C and 1D are plan views of the rectangular spiral conductor and the circular spiral conductor depicted in 1A and 1B, respectively, also showing shorts across the resistive links. 
     FIGS. 2A to 2E are a series of vertical sectional views of the devices depicted in FIGS. 1A and 1B during their formation. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The word &#34;spiral&#34; as used herein is intended to include a broad class of structures which exhibit a clockwise or counterclockwise outwardly winding path beginning in a substantially centralized location, wherein each winding is successively longer than the previous winding. This definition is intended to include rectangular and circular spirals as well as any other irregular but generally spiraling shape. 
     Two plan views of the invention are shown in FIGS. 1A and 1B. An insulating substrate material 10 forms the base on which a conducting spiral 20 is deposited. The length of the spiral determines the low frequency cutoff of the invention. A number of low resistance links 30 interconnect adjacent turns of the spiral 20. The length of the longest link governs the high frequency resonance of the invention. 
     Referring now to FIGS. 2A to 2E, the structures depicted in FIGS. 1A and 1B are formed by depositing a layer of electrically resistive material 40, such as tantalum nitride, to a thickness of between 400 to 600 Å on top of the insulating substrate 10. The thickness chosen should produce a resistivity of 30 Ψ/□. Next, a layer 42 of titanium (Ti) is sputtered onto the substrate 10 followed by a layer 43 of platinum (Pt). In the preferred embodiment each layer 42 and 43 is approximately 2000 Å thick. (see FIG. 2A). In an alternate embodiment the Ti and Pt may be applied as an alloy in a single layer. 
     To define the pattern of the spiral, a layer 44 of photoresist is deposited over the Pt layer 43 and is exposed for the spiral pattern. The undeveloped, e.g. unexposed photoresist, is removed to reveal a spiral pattern 20 of the Pt layer 43. Gold 46 is electroplated onto the exposed spiral pattern to build up the thickness (see FIG. 2B). In the preferred embodiment the gold layer is approximately 160 minches. The remaining photoresist is then removed. The exposed part of the Pt layer 43 is then removed by a dry etch process. 
     A new layer 48 of photoresist is deposited over the entire surface of the device. This new layer is then exposed through a pattern which allows light to impinge on areas of the photoresist 50 over the locations of the resistors 30 (see FIG. 2C). 
     The undeveloped photoresist 48, i.e. the unexposed photoresist which extends over all of the surface of the device except the area where the resistors 30 are located, is then removed, followed by removal of the Ti layer 42 and the tantalum nitride layer 40 which are not protected by the areas of exposed photoresist 50 or the gold layer 46 (see FIG. 2D). 
     The remaining photoresist 50 is then removed and the Ti layer 42 in the areas not covered by the layer of gold 46 is removed (see FIG. 2E). This leaves the layers 40, 42 and 43 under the gold 46 and the portion of the tantalum nitride layer 40 under the exposed resist 50 intact to define the spiral turns 20 and the inter-turn resistances 30. 
     In a preferred embodiment DC resistance is eliminated by printing shorts 60 across the resistive links 30. (See FIGS. 1C and 1D). The shorts 60 cause an increase in Q at low frequency where self resonance is negligible and cause a decrease in Q at high frequencies by suppressing the resonance. 
     Finally, the device is heat treated at approximately 450° C. to stabilize the resistors 30 to have a resistivity of approximately 60 Ω/□. The resistance of the resistive links 30 between the windings of the spiral 20 is about 2 to 8 Ω. 
     Although the present invention has been shown and described with respect to preferred embodiments, various changes and modifications which are obvious to a person skilled in the art to which the invention pertains are deemed to lie within the spirit and scope of the invention.