| A novel isolation technology for RF applications based on unoxidized porous Si (PS) is demonstrated. This study examines all the important issues pertinent to incorporating PS with Si very-large-scale integration (VLSI) technology, where PS is used as a semi-insulating material. Specifically, the etching profile generated from selective incorporation of PS regions into Si substrate by anodization and the issues on RF isolation performance of PS as a function of porosity (from coplanar waveguide (CPW) line measurements), PS width (from crosstalk suppression) and PS thickness (from on-chip inductors) are discussed in detail.; Optimum mask material for selective formation of PS in heavily doped Si substrates is LPCVD Si3N4. Secondary undercutting of PS, as looks like bird's beak, in low doped Si is primarily due to formation of hole accumulation layer underneath Si3N4 film. From simulation, such secondary undercutting can be suppressed by n+ ion implantation underneath Si3N4 film or reverse bias application on top of Si3N4 film.; CPW line measurements show that the relative dielectric constant of PS films decreases from 9 to 3 with increasing porosity from 24% to 78%. PS is a very low loss material with loss tangent <0.001 at 20 GHz when its porosity is above 51%. RF crosstalk through a Si substrate can be reduced to that through air by inserting a PS trench between noise generating circuit and noise sensing circuit. On-chip spiral inductors fabricated on top of PS regions of through-the-wafer thickness have Qmax of about 29 at 7 GHz and resonant frequency higher than 20 GHz. With the additional advantage of planar topography and mechanical integrity, we show that unoxidized PS is an outstanding material for RF isolation in Si VLSI. |
