Approaches to Further Enhance Cut Accuracy
Yield improvement is a critical concern in semiconductor manufacturing and cut accuracy during bevel wet etching has a considerable influence on both yield and the number of retrievable dies. When liquid is dispensed onto the surface of a rotating wafer, a liquid film forms on the hydrophilic surface, and the position of the gas–liquid interface ultimately determines the etch boundary. The more linear this gas-liquid interface is, the more uniform the cut surface obtained. Therefore, stabilizing the gas-liquid interface is indispensable for improving cut accuracy.
High-speed camera observations reveal that when the ratio of wafer peripheral speed to liquid flow velocity exceeds a critical threshold (denoted αc), the gas–liquid interface undergoes non-uniform oscillations (i.e., α > αc).
In general, parameters such as rotational speed and flow rate affect not only the cutting accuracy but also particle generation and etching rate; therefore, it is desirable to improve the peripheral speed to flow velocity ratio without changing these parameters. To raise the local supply velocity while keeping volumetric flow rate, we fabricated a nozzle with a reduced diameter. This modification produced a 33% improvement in cut accuracy.
We considered the mechanism by which improving the peripheral speed-to-flow velocity ratio suppresses interface fluctuations. Liquid dispensed from the nozzle spreads upon impacting the wafer while rotation expels liquid circumferentially. When the ratio of rotational expulsion velocity to supply velocity exceeds a certain value, the liquid film near the impact point thins, and the contact angle can fall below the receding contact angle, causing the interface to move. Interface movement narrows the circumferential width; because supply and expulsion rates are unchanged, volume conservation leads to local thickening of the film. As the film thickens, the contact angle can exceed the advancing contact angle and the interface spreads again. Repeated local cycles of this process are believed to cause the observed interface fluctuations. By reducing nozzle diameter and thereby increasing supply velocity, the contact angle remains above the receding contact angle and the interface is stabilized, explaining the observed improvement in cut accuracy. In conclusion, improving the peripheral speed-to-flow velocity ratio through proper nozzle diameter design effectively suppresses fluctuations of the gas-liquid interface and significantly contributes to the improvement of cut accuracy in the bevel wet etching process.
BIOGRAPHY
Ryotaro Maeda is a process engineer at Tokyo Electron Kyushu Ltd., where he specializes in single-wafer cleaning module targeting the wafer periphery (bevel/edge). In his role, he is dedicated to enhancing process performance through continuous improvements and optimization of the bevel cleaning systems.