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

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/sup (R/) 7020) thin films spun on the silicon wafers are annealed at 100/spl deg/C and 400/spl deg/C, respectively, in order to investigate the effect of the annealing temperature on the laser micromachining process. The micro-machined 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 /spl mu/m 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/cm/sup 2/. The polyimide thin film optical properties, i.e., the components of the complex refractive index, n and k at /spl lambda/=248 nm are measured by a spectroscopic ellipsometer for both films. The etch rate above the ablation threshold for the film annealed at 100/spl deg/C is found 60% higher than that for the film annealed at 400/spl deg/C. This behavior originates from the correspondingly higher by 82% absorption coefficient, /spl alpha/ in the 100/spl deg/C film. The ablation thresholds for both films are found the same at about 100 mJ/cm/sup 2/. 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/cm/sup 2/ for 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.