Stop Layer Function
A dielectric layer in a silicon on insulator wafer prevents the advancement of chemical agents during the micromachining process. The buried oxide etch stop provides a chemical barrier that halts the removal of substrate material at a precise depth. Selectivity between the silicon and the silicon dioxide layer ensures that the device layer remains intact while the handle wafer is cleared.
Removal Rate
Chemical selectivity determines how much substrate is removed before the stop layer begins to degrade. Hydrofluoric acid or potassium hydroxide solutions act upon the silicon with high kinetic energy. When the chemistry reaches the buried oxide etch stop, the etch rate drops by several orders of magnitude.
The halt permits the formation of membrane structures with uniform thickness across the entire diameter of the wafer. It allows for batch processing where thousands of chips reach the same depth simultaneously regardless of local concentration gradients in the chemical bath.
Thickness Uniformity
High precision in the final membrane depth depends on the initial growth conditions of the insulator. If the buried oxide etch stop contains pinholes or thickness variations, the chemical agent may penetrate the barrier and damage the active device region. Manufacturers specify the total thickness variation of the oxide to control the mechanical resonance of the resulting sensor.
Thermal oxidation produces a denser barrier than chemical vapour deposition. Measurement of the stop layer thickness occurs through spectroscopic ellipsometry before the bonding of the device layer.
Installation Effect
Mechanical stress often accumulates at the interface between the silicon and the oxide due to differences in lattice constants. While the buried oxide etch stop defines the geometry, it also introduces a residual tension that can warp thin diaphragms. Annealing steps reduce this stress before the final release of the microstructures.
