Role of carbon dioxide in gas expanded liquids for removal of photoresist and etch residu

Progress in the microelectronics industry is driven by smaller and faster transistors. As feature sizes in integrated circuits become smaller and liquid chemical waste becomes an even greater environmental concern, gas expanded liquids (GXLs) may provide a possible solution to future device fabrication limitations relative to the use of liquids. The properties of GXLs such as surface tension can be tuned by the inclusion of high pressure gases; thereby, the reduced surface tension will allow penetration of cleaning solutions into small features on the nanometer scale. In addition, the inclusion of the gas decreases the amount of liquid necessary for the photoresist and etch residue removal processes. This thesis explores the role of CO2-based GXLs for photoresist and etch residue removal. The gas used for expansion is CO 2 while the liquid used is methanol. The cosolvent serving as the removal agent is tetramethyl ammonium hydroxide (TMAH) which upon reacting with CO 2 becomes predominantly tetramethyl ammonium bicarbonate (TMAB).;First this study determines the feasibility of GXLs for photoresist and etch residue removal by applying GXLs to commercially patterned semiconductor wafers. At 90°C and pressures >1000psi, photoresist and etch residue are removed by a CO2 expanded TMAB solution diluted with methanol. The mechanism of GXL film and residue removal appears to be swelling of the photoresist and subsequent penetration of the photoresist by cosolvent, followed by an etch of the underlying SiO2 by hydroxide ions in equilibrium with the bicarbonate. The etch process releases the photoresist and etch residue from the surface by lift-off.;Besides feasibility of photoresist and etch residue removal by GXLs, this work confirms the compatibility of GXLs with low k materials such as methylsilsesquioxane (MSQ) which is present on state-of-the-art ICs. Low k materials are currently incorporated in industry as an insulation layer between metal layers that form the interconnects between transistors. The dielectric constant of these materials must remain low (∼2.7) to serve as effective insulation between metal layers. Due to the absorption of tetramethyl ammonium, bicarbonate, and hydroxide ions into the MSQ film, the dielectric constant increases. However, a 2200psi CO2/CH3OH is able to solubilize and remove these ions to recover the low k dielectric constant value. These experiments demonstrate that GXLs can be used to remove photoresist and etch residues while remaining compatible with low k materials which makes GXLs a possible choice for these cleaning processes for future IC fabrication.;In certain GXLs, the CO2 gas is replaced with C2H 6 in order to compare the quadrupolar nature of CO2 to that of the nonpolar C2H6. However, C2H6 does not react with TMAH and therefore the concentration of the hydroxide ion removal agent is different as well. Although direct comparisons cannot be made, C2H6 is able to remove the photoresist and etch residue films at a lower temperature of 50°C relative to CO 2 due to the increased hydroxide ion concentration. However, this higher hydroxide ion concentration etches the low k dielectric films.;The expansion gases CO2 and C2H6 are directly compared via their absorption into polyhydroxystyrene (PHOST) and polystyrene (PS), since PS lacks the hydroxyl group of PHOST. A quartz crystal microbalance was used to measure mass uptake of the gases. PHOST absorbed a higher ratio of CO2/C2H6 compared to PS suggesting an interaction of the quadrupole moment of CO 2 with the hydroxyl group in PHOST via a Lewis acid-base interaction.