A well-known method for measuring thermal conductivity is the 3-Omega (3ω) method. A prerequisite for it is the deposition of a metal heater on top of the sample surface. The known design rules for the heater geometry, however, are not yet sufficient. In this work, heaters with different lengths and widths within the known restrictions were investigated. The measurements were carried out on SiO₂ thin films with different film thicknesses as a reference. There was a significant difference between theoretical deposited heater width and real heater width, which could lead to errors of up to 50% for the determined thermal conductivity. Heaters with lengths between 11 and 13 mm and widths of 6.5 μm or more proved to deliver the most trustworthy results. To verify the performance of these newfound heaters, additional investigations on Al₂O₃ thin films were carried out, proving our conclusions to be correct and delivering thermal conductivity values of 0.81 Wm^-1 K^-1 and 0.93 Wm^-1 K^-1 for unannealed and annealed samples, respectively. Furthermore, the effect of annealing on Al₂O₃ was studied, revealing a significant shrinking in film thickness of approximately 11% and an increase in thermal conductivity of 15%. The presented results on well-defined geometries will help to produce optimized heater structures for the 3ω method.