Abstract:
A thin film structure is provided with the capability of low-power laser tuning and trimming. Trim windows in a plated ground plane are incorporated to allow automated tuning procedures using relatively low-power lasers. The trim windows are conductive, but the heavier layer of plated copper is omitted from those areas, leaving only the titanium/evaporated copper/ gold layers. The method can also provide for areas of bare substrate if desired.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the field of metallized substrates for communications applications and, more particularly, to a means of tuning thin film structures such as stripline filters, using automated low-power laser trimming. 
     Metallization on non-conductive substrates such as ceramics is well known in the art for such circuit components as stripline filters. Such filters typically include a configuration or pattern of conductive elements on one surface of the ceramic plate, these elements interacting with a relatively large area of ground plane on the opposing substrate surface. For electrical reasons, the ground plane is required to be a very low conductivity layer, typically including a heavy (one mil) layer of plated copper. Filters or other such components may require removal of a portion of the conductive ground plane for tuning or other adjustment. Such adjustments have been made by abrasion; i.e., mechanically removing small portions of the ground plane in the appropriate areas, using a diamond grinding wheel or sand trimmer. It will be obvious that such a method does not lend itself to automated trimming procedures, where the circuitry is monitored as it is tuned and the monitoring equipment controls an extremely precise laser beam. Thick film resistors on substrates can be trimmed precisely by low-power (1.8 kw input power) laser beams, but that power is not sufficient to trim a heavily plated ground plane. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a means of trimming a ground plane or the like with a low-power laser beam. 
     It is a particular object to provide such a means utilizing standard process steps. 
     It is another object to provide additionally for removal of all conductive layers on other portions of the substrate. 
     These objects, and others which will become apparent, are provided in a structure and accompanying method in accordance with the present invention. On a non-conductive wafer, one or more conductive circuit components are formed on a first surface. A conductive layer or ground plane is formed on a substantial portion of the second surface of the wafer opposite at least one element of a circuit component which requires a fine tuning or trimming adjustment. The ground plane will consist of a thin conductive layer, such as copper evaporated over titanium, a heavy conductive layer such as plated copper, and a thin protective layer of a noble metal such as gold. The heavy copper layer will have an aperture opposite at least a portion of the component to be adjusted. Therefore, tuning or trimming can be accomplished by removing discrete areas of the thinner layers within the aperture in the heavy layer, using a relatively low-power laser beam, such as those used to trim thick film resistors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a bottom view of a stripline filter substrate. 
     FIG. 2 is a cut-away, perspective view of a portion of the substrate of FIG. 1, along the line 2--2. 
     FIG. 3 shows the successive steps in one preferred embodiment of the invention. 
     FIG. 4 shows the successive steps in another embodiment, which provides for areas of bare substrate. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For purposes of clarity, the existing thin film process will be briefly described. On a non-conductive substrate, a very thin layer of titanium is deposited first to provide adhesion for the subsequent layers of metal. Next, a thin layer of copper is evaporated over the titanium. A photo-resist is applied and exposed to ultraviolet light through a mask, the unexposed areas resulting eventually in conductive areas. After the resist is developed, the unhardened portions are stripped off and a thick layer of copper is plated on the areas of bare copper. A thin layer of gold is plated on over all, then the hardened resist is stripped from the substrate. Etching subsequently removes all thin layers down to the bare substrate in the desired areas. 
     As may be seen in the drawing, the invention is used for the fine tuning (trimming) of a stripline filter on a substrate 10 (see FIGS. 2-4) such as a ceramic wafer. In this example, the visible surface of the substrate is covered by a ground plane 12 having three apertures or &#34;trim windows&#34; 14. The other surface of the substrate 10 has on it (as indicated by dashed lines in FIG. 1) elements 16A, 16B and 16C of the filter circuit. 
     Stripline filters are well known in the art, and have previously been constructed using a layer of heavy copper throughout the ground plane. Tuning or other fine adjustment has been accomplished by use of an abrasion technique using a diamond grinding wheel. No further description of the circuit aspects of the filter will be given here, since the present invention is only concerned with the capability of fine tuning by the removal of discrete areas of the ground plane with a low-power laser beam. 
     In the cut-away, perspective view of FIG. 2, the filter of FIG. 1 is shown cut along the line 2--2 with the edges of circuit elements 16A, B and C visible on the lower surface (normally considered the top side of the filter). The ground plane 12 and windows 14 are on the upper side of the ceramic wafer 10 in this view and it is apparent that the copper layer within the window 14 is thinner than it is in the adjacent areas. It is to be noted that no attempt has been made to scale any of the drawing figures; instead, typical dimensions are given as appropriate in the text. 
     FIG. 3 shows a series of progressive steps in producing the ground plane only of one embodiment of the invention. FIG. 3A shows the bare substrate 10 and FIG. 3B shows a very thin (500 A) added layer 18 of deposited titanium. In FIG. 3C, a thin (10,000 A) layer 20 of copper has been evaporated over the titanium layer 18. In FIG. 3D, a film 22 of a photo-resist (such as that sold commercially as Riston™ 218R) has been added, and a mask 24 is shown in position to expose the area 14&#39; of the photo-resist film 22 for providing one window 14 (FIGS. 1 and 2). After the photo-resist has been exposed to ultraviolet light through the mask, and developed, the unhardened areas are removed as shown in FIG. 3E. A heavy (0.7 to 1.2 mils) layer 25 of copper is plated on all areas except area 14, as shown in FIG. 3F. The hardened resist 22 is then stripped off, as shown in FIG. 3G. As shown in FIG. 3H, a thin (5000 A) layer 26 of gold is plated over all exposed copper, both thick and thin layers. In the tuning operation, a low-powered laser beam can be manually or automatically controlled to remove as much of the thin layers of gold and copper in any of the windows 14 as is required; e.g., for tuning a stripline filter. 
     In FIG. 4 is seen the series of progressive steps in an alternate embodiment which can provide both the trim windows 14 as shown in FIG. 3 and areas of bare substrate which may be desired for other purposes than tuning. 
     FIGS. 4A-4D are the same as FIGS. 3A-3D except that the mask 24-1 has an area 14&#39; for providing the window or aperture 14 and another area 28&#39; for providing an area 28 (see FIG. 4M) of bare substrate. After the photo-resist 22 has been exposed to ultraviolet light through the mask 24 and developed, the unhardened resist is removed as in FIG. 4E, leaving areas 22-1 and 22-2. The heavy copper layer 25 is then plated on (FIG. 4F) followed by the thin layer 26 of gold (FIG. 4G). The hardened areas of photo-resist 22 are then stripped off as shown in FIG. 4H, and another layer 30 of the photo-resist is applied over all (FIG. 4I). All areas except area 14&#39; of the photo-resist are exposed through a second mask 24-2, then the photo-resist is developed. After the unhardened resist is removed (FIG. 4J); i.e., the portion in the area 14&#39;, that area is plated with gold (layer 26) as shown in FIG. 4K. When the hardened resist is removed (FIG. 4L), all exposed surfaces have been plated with gold except for the area 28&#39;. An etching process (FIG. 4M) will remove the thin copper and titanium layer to provide bare substrate in the area 28&#39;. 
     Thus there has been shown and described a means of providing a tunable circuit element on a substrate using only the standard process steps, and of tuning the said element with a low-power laser beam. It is intended to cover all modifications and variations which fall within the spirit and scope of the appended claims.