Microwave and millimeter wave stripline filter and process for fabricating same

Abstract

Pre-fired ceramic substrates are elected according to the desired electrical performance of the filter. If necessary for enhanced performance, the surfaces of the substrate may be lapped to assure that their top and bottom surfaces are parallel and their surface finish is smooth. The top surface of a lower layer is coated with a conductive film using thick film techniques then patterned to define the filter trace pattern. For precise dimensional control, photolithographic techniques may be used. The bottom and the sides of the lower layer are coated with the same conductive film. A seal glass which has a coefficient of thermal expansion which is matched as closely as possible to that of the ceramic substrate is screen printed onto the top surface of the lower layer. The top of the upper layer is screen printed with the conductive film and the bottom of the upper layer is coated with the seal glass. The upper and lower layers are bonded together by clamping them together and firing the seal glass. The sides of the assembly are then coated with a conductive film to provide groundplane connection.

I claim:  
     
       1. A process for fabricating a stripline filter using pre-fired ceramic technology comprising the steps of: 
       providing a first pre-fired ceramic substrate having a predetermined coefficient of thermal expansion, a first top surface, a first bottom surface and a first plurality of sides;  
       printing a first conductive film onto said first top surface;  
       patterning said first conductive film to define a filter trace pattern;  
       printing a second conductive film onto said first bottom surface;  
       painting a third conductive film onto said first plurality of sides of said first substrate;  
       printing a first seal glass over said first top surface and said filter trace pattern, said first seal glass having a substantially same coefficient of thermal expansion as said predetermined coefficient of thermal expansion;  
       providing a second pre-fired ceramic substrate having said predetermined coefficient of thermal expansion, a second top surface, a second plurality of sides, and a second bottom surface;  
       lapping said second substrate so that said second top surface and said second bottom surface are uniformly spaced across said substrate;  
       printing a fourth conductive film onto said second top surface;  
       printing a second seal glass onto said second bottom surface, said second seal glass having said substantially same coefficient of thermal expansion;  
       forming an assembly by disposing said second bottom surface abutting said first top surface, aligning said first plurality of sides with said second plurality of side  sides;  
       firing said assembly to bond said first substrate to said second substrate by bonding said first seal glass to said second seal glass; and  
       painting a fifth conductive film on said first plurality of sides and said second plurality of sides.  
     
     
       2. A process for fabricating a stripline filter as in  claim 1  further comprising the step of lapping said first substrate so that said first top surface and said first bottom surface are uniformly spaced across said first substrate. 
     
     
       3. A process for fabricating a stripline filter as in  claim 1  further comprising the step of laser machining said first substrate prior to printing said first conductive film. 
     
     
       4. A process for fabricating a stripline filter as in  claim 1  further comprising selecting said first conductive paste, said second conductive paste, said third conductive paste and said fourth conductive paste to be the same type of paste. 
     
     
       5. A process for fabricating a stripline filter as in  claim 1  further comprising the selecting said first seal glass and said second seal glass to be the same type of seal glass. 
     
     
       6. A process for fabricating a stripline filter as in  claim 4  wherein the steps of providing said first substrate and said second substrate including selecting 99.6% alumina. 
     
     
       7. A process for fabricating a stripline filter as in  claim 5  wherein the step of selecting said first seal glass and said second seal glass comprise selecting a seal glass with a coefficient of thermal expansion of 7.2×10 −6 . 
     
     
       8. A process for fabricating a stripline filter as in  claim 1  wherein the step of patterning said first conductive film comprises: 
       spinning a photoresist onto said first conductive film;  
       partially covering said photoresist with a patterned mask;  
       exposing said photoresist to ultraviolet light;  
       developing said photoresist to form a photoresist mask over predetermined portions of said first conductive film;  
       etching portions of said first conductive film which are exposed through said photoresist mask; and  
       removing said photoresist mask.  
     
     
       9. A process for fabricating a stripline filter for use in microwave and millimeter wave applications using a first pre-fired ceramic substrate and a second pre-fired ceramic substrate, each having a first coefficient of thermal expansion, the process comprising the steps of: 
       printingforming a first conductive film onto theon a top surface, a bottom surface, and at least a portion of a plurality of sides of said first substrate;  
       photolithographically  patterning said first conductive film on the top surface of said first substrate to define a filter trace pattern;  
       printing a second conductive film onto the bottom surface of said first substrate;  
       painting a third conductive film onto at least of portion of the sides of said first substrate;  
       printing a first seal glass over the top surface of said first substrate and said filter trace pattern, said first seal glass having a substantially same coefficient of thermal expansion as said first coefficient of thermal expansion;  
       printing a fourthforming a second conductive film onto thea top surface of said second substrate;  
       printing a second seal glass onto the  a bottom surface of said second substrate, said second seal glass having said substantially same coefficient of thermal expansion;  
       forming an assembly by  abutting the bottom surface of said second substrate against the top of said first substrate, aligning the plurality of sides of said first substrate with the  a plurality of sides of said second substrate;  
       firing said assembly to bond said first substrate to said second substrate by  bonding said first seal glass to said second seal glass; and  
       painting a fifthforming a third conductive film on at least a portion of the sides of said assemblyfirst and second substrates.  
     
     
       10. A process for fabricating a stripline filter as in  claim 9  further comprising the step of lapping said first substrate so that its top surface and its bottom surface are uniformly spaced across said first substrate. 
     
     
       11. A process for fabricating a stripline filter as in  claim 9  further comprising the step of lapping said second substrate so that its top surface and its bottom surface are uniformly spaced across said second substrate. 
     
     
       12. A process for fabricating a stripline filter as in  claim 9  further comprising the step of laser machining said first substrate prior to printing said first conductive film to define launch areas. 
     
     
       13. A process for fabricating a stripline filter as in  claim 9  further comprising selecting said first conductive paste, said second conductive paste, and said third conductive paste and said fourth conductive paste  to be the same type of paste. 
     
     
       14. A process for fabricating a stripline filter as in  claim 9  further comprising the selecting said first seal glass and said second seal glass to be the same type of seal glass. 
     
     
       15. A process for fabricating a stripline filter as in  claim 9  wherein said first substrate and said second substrate are 99.6% alumina. 
     
     
       16. A process for fabricating a stripline filter as in  claim 15  wherein the step of selecting said first seal glass and said second seal glass comprise selecting a seal glass with a coefficient of thermal expansion of 7.2×10 −6 . 
     
     
       17. A process for fabricating a stripline filter as in  claim 9  wherein the step of patterning said first conductive film comprises: spinning a photoresist onto said first conductive film; 
       partially covering said photoresist with a patterned mask;  
       exposing said photoresist to ultraviolet light;  
       developing said photoresist to form a photoresist mask over predetermined portions of said first conductive film;  
       etching portions of said first conductive film which are exposed through said photoresist mask; and  
       removing said photoresist mask.  
     
     
       18. A process for fabricating a stripline filter for use in microwave and millimeter wave applications comprising: 
       providing a lower substrate having a first top surface, a first bottom surface and a first plurality of sides, each having a first conductive film formed thereon, having a coefficient of thermal expansion 
       defining filter pattern in the first conductive film on the first top surface; 
       providing an upper substrate having a second top surface with a second conductive film formed thereon, a second bottom surface and a second plurality of sides, having a coefficient of thermal expansion; 
       applying to each of the first top surface and the second bottom surface a material for bonding the upper and lower substrates, the material having a substantially same coefficient of thermal expansion as said coefficient of thermal expansion of the first and second substrates; and 
       forming a third conductive film substantially covering the first and second plurality of sides.  
     
     
       19. A process for fabricating a stripline filter as in  claim 18  wherein the steps of providing the lower substrate and the upper surface include selecting pre-fired ceramics.  
     
     
       20. A process for fabricating a stripline filter as in  claim 18  wherein the step of defining a filter trace pattern comprises photolithographically patterning the first conductive material on the top surface of the first substrate.  
     
     
       21. A process for fabricating a stripline filter as in  claim 18  wherein the step of applying a material includes selecting a material having a second dielectric constant substantially equal to a first dielectric constant of the upper and lower substrates.  
     
     
       22. A process for fabricating a stripline filter as in  claim 18  further comprising lapping the lower substrate for parallelism between the first top surface and the first bottom surface.  
     
     
       23. A process for fabricating a stripline filter as in  claim 18  further comprising lapping the upper substrate for parallelism between the second top surface and the second bottom surface.  
     
     
       24. A stripline filter for use in microwave and millimeter wave applications comprising: 
       a lower substrate having a first top surface, a first bottom surface and a first plurality of sides, each having a first conductive film formed thereon, the lower substrate being fully fired and having a first coefficient of thermal expansion; 
       a filter pattern formed in the first conductive film on the first top surface; 
       a upper substrate having a second top surface, a second bottom surface and a second plurality of sides, the upper substrate being fully fired and having said first coefficient of thermal expansion; 
       a second conductive film formed on said second top surface; 
       a material for bonding the first top surface of the lower substrate to the second bottom surface of the upper substrate, the material having a second coefficient of thermal expansion substantially the same as the first coefficient of thermal expansion;and  
       a third conductive film formed on the first and second pluralities of sides of the upper and lower substrates respectively.  
     
     
       25. A stripline filter as in  claim 24  wherein the lower substrate is lapped for parallelism between the first top surface and the first bottom surface.  
     
     
       26. A stripline filter as in  claim 24  wherein the upper substrate is lapped for parallelism between the second top surface and the second bottom surface.  
     
     
       27. A stripline filter as in  claim 24  wherein the filter pattern has an interdigitated structure.  
     
     
       28. A stripline filter as in  claim 24  wherein the material has a second dielectric constant substantially equal to a first dielectric constant of the upper and lower substrates.