Optimization of picosecond laser processing for microscopy sample preparation prior to Ion milling polishing

Microchips are more and more complex and designed in thick 3 dimensional packages. In order to access and characterize by electron microscopy (SEM and TEM) any detected defect responsible for malfunction of the device, a large quantity of matter needs to be removed without damaging the surrounded area. The available techniques, such as plasma Focused Ion Beam (FIB), allow achieving high quality surfaces but are limited by their low matter removal rate (∼10
⁴
μm
³
/s). In order to accelerate the process, other techniques such as laser micromachining of the sample prior to FIB polishing are envisioned [1, 2]. In this work, picosecond laser micromachining has been investigated at 3 wavelengths (343, 515 and 1030 nm) in silicon and tested in integrated circuits. In order to minimize the FIB polishing time, the laser induced damaged zone should be the least extended and the micromachined sidewalls should be as vertical and smooth as possible. It is shown that by using sufficiently high fluences and number of pulses, almost vertical and smooth sidewalls can be obtained (see Fig. la) [3]. Moreover, according to the TEM images, sidewalls are only covered by a few hundred nm thick debris layer with limited heat affected zones. These results, coupled with the high matter removal rate (∼10
⁶
μm
³
/s) demonstrate that picosecond laser machining fulfills the requirements for sample preparation. The processing method was successfully tested on microchips as shown by SEM imaging which reveals clean exposed interfaces of underlayers (see Fig. 1b). In addition, using a simple model allowing a better understanding of laser absorption in silicon for picosecond pulses, the conditions (wavelength and fluence) to achieve optimal matter removal rate and ablation efficiency were identified and validated by experimental results.