Planar microwave multipole filters using LPE YIG

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Abstract
YIG discs fabricated by LPE on GGG have been utilised in tunable microwave bandpass multipole filters. However, filter realisation in MIC format is complicated by conductor steps in excess of 10 μm. These result from the necessary microwave interconnections of the discrete YIG discs formed by etching the LPE YIG film. This paper describes a planar fabrication procedure which overcomes the conductor step problem without degrading the YIG properties. A photolithographically defined SiO2 mask is used to etch wells into the GGG substrate. LPE YIG is grown over the entire susbtrate to a thickness equal to the depth of the wells. The YIG outside the wells is removed by Syton polishing, leaving a planar surface with discrete YIG islands. YIG discs as thick as 50 μm, and YIG regions as narrow as 10 μm with 5 μm depth have been fabricated. Planar bandstop filters have exhibited linewidths below 1 Oe. Planar bandpass filters have shown external Q’s down to 60 at 2.8 GHz.

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