Etching of deep trenches in silicon explained

Dutch researcher Michiel Blauw has described the physical limitations of the plasma-etching of deep, narrow microstructures in silicon. His results have led to such an improvement in the etching process that trenches with a depth more than 30 times their width can now be made. This is important for the production of sensitive sensors.

Blauw investigated fluorine-based plasma etching processes. A plasma with a high ion-density ’burns’ a small hole in silicon. Many applications require narrow, deep holes. Blauw studied how the plasma reacts with the silicon and how the sidewalls must be treated so as to make the trench as deep and as straight as possible.

The researcher came up with two ways to improve the profile of the trenches in the so-called Bosch process. During this process, a polymer layer ensures that the sidewalls are not etched by the plasma. However, the thin polymer layer is also deposited onto the bottom of the trench and this hinders the etching of deep, narrow trenches.

Firstly the researcher added a third plasma pulse to the Bosch-process after the etching and passivation pulses. This efficiently removed the polymer layer from the bottom of the trench. A patent has been granted for this method. Secondly he optimised the passivation pulse used to treat the sidewalls so that no polymer deposition occurred on the bottom of the trenches.
This made a maximum depth-width ratio of more than 30 possible.

In principle, the etching of silicon occurs at the same speed in all directions. To obtain the deep, narrow trenches necessary for accurate sensors, the sidewalls must be made insensitive to the plasma. This is termed passivating. After a variety of experiments in which he added oxygen to the plasma or deposited a polymer layer, Blauw found an effective passivating technique. A plasma with a high ion-density removes the passivating layer from the surface. This results in deep, narrow trenches because the ions are accelerated perpendicular to the substrate. He also found that the etch rate as a function of the depth-width ratio can be controlled by tuning the ion-density.

Plasma-etching provides considerable advantages for the manufacture of inertial sensors such as accelerometers and gyroscopes. This is because the manufacturing processes for the sensor and the electronics for signal processing are compatible, allowing both parts to be integrated onto a single chip. Furthermore, increasing the depth-width ratio of the etched microstructures considerably improves the integration density and accuracy of these devices.

The research was funded by the Technology Foundation STW.