Electron-assisted photoresist etching in an inductively coupled oxygen plasma via low-energy electron beam

An electron-assisted photoresist etching (as known as ashing) was proceeded in an inductively coupled plasma (ICP) with grid system. The low-energy electron beam (< 40 eV) was generated in oxygen plasma by using the grid, and beam energy was controlled by varying the DC grid voltage. As DC voltage increase from 0 V to 40 V, PR etch rate decreases and then increases. This etch rate variation is related with the thickness of the sheath on the grid. When the grid is open due to thin sheath, source plasma can diffuse freely to the electron beam extraction region, so that plasma potential become high (≈25 V). Therefore, the main mechanism of PR etching in this case is ion-assisted etching due to high ion bombardment energy. However, since the effect of ion decreases because sheath becomes thicker as increasing DC voltage, PR etch rate also decreases and have a minimum value. When the grid is fully closed, electron beam starts to be generated and the PR etch rate increases due to increment of electron beam energy by increasing DC voltage. This relation between the sheath and the PR etch rate was discussed in detailed by measuring electron energy distribution function (EEDF). The electron-assisted PR etching was clarified by measuring the ion bombardment energy and the optical emission spectroscopy. Since the relative O radical density is nearly constant and the ion bombardment energy decreases as increasing DC voltage, the effect of O radical and ion can be neglected. Further, PR etching was proceeded as decreasing DC voltage from 40 V to 0 V to prevent thermal effect on the PR etch rate. Therefore, the increment of PR etch rate is regarded as the result of electron-assisted PR etching.