Excimer laser projection micromachining of polyimide thin films annealed at different temperatures

Abstract

A KrF excimer laser projection micromachining tool has been designed and implemented aiming to accomplish one-step etching with micron resolution for applications such as chip module packaging and polyimide technology in semiconductor manufacturing. Two polyimide (Probimide 7020) thin films spun on the silicon wafers are annealed at 100 °C and 400 °C, respectively, in order to investigate the effect of the annealing temperature on the laser micromachining process. The micromachined polyimide surface morphology is studied by scanning electron microscopy (SEM), atomic force microscopy (AFM), and a surface profilometer for determination of the etch rate at different laser fluences. Micromachining with micron resolution is achieved by this excimer laser projection tool. Measured on a 25 μm line, the etch rate dependence on laser fluence both below and above the ablation threshold is investigated over the fluence range from 10-1000 mJ/cm2. The polyimide thin film optical properties, i.e., the components of the complex refractive index, n and k at λ = 248 nm are measured by a spectroscopic ellipsometer for both films. The etch rate above the ablation threshold for the film annealed at 100 °C is found 60% higher than that for the film annealed at 400 °C. This behavior originates from the correspondingly higher by 82% absorption coefficient, α in the 100 °C film. The ablation thresholds for both films are found the same at about 100 mJ/cm2. Calculated etch rates agree with experimental values within 15%. Etch rates of the order of nanometer can be achieved near the ablation threshold of 100 mJ/cm2for both films. The linearity between the etched depth and the number of the laser pulses prevails for the micromachining process at and above the threshold.