Paper PS2+MS-MoM3
High Frequency Capacitively Coupled Plasma for Low Ion Energy Dual Damascene Etching

Monday, October 15, 2007, 8:40 am, Room 607

Capacitively coupled discharges are widely used for semiconductor processing, especially in the area of dielectric etching. With a wide range of film materials and complex stacks including multiple mask layers, advanced dielectric etch processes require tight control of plasma parameters, such as ion flux, radical composition, and ion energy distribution (IED). This paper presents IED measurements and patterned-wafer etch results as a function of RF bias excitation frequency applied to the wafer electrode. The aim is to produce the optimal IED for etching of soft materials, such as low-k dielectrics commonly used in Dual Damascene interconnect schemes. One key challenge arises in the trench etch step, which requires a vertical etch profile in the low-k film. But the process must avoid corner faceting of any exposed via holes or of the hard mask layer which defines the initial trench pattern. Faceting would cause an increase in via or trench critical dimension, respectively. The competing requirements of vertical profile and minimal faceting define a fairly narrow range of acceptable ion energies for the process. If power is held constant, higher RF driving frequencies typically produce plasmas with higher densities and lower potentials, and enable operation at lower pressure. Both the mean ion energy and width of the IED reaching the wafer tend to decrease as the wafer bias frequency increases. For frequencies too low, the width of the IED is too large and faceting is induced by the high energy ions. For frequencies too high, the mean IED is too low to etch the low-k film with a vertical profile and acceptable rate. Results show that the necessary IED for these applications can be obtained by applying 60MHz to the wafer electrode.