Removal of MEMS sacrificial layers using supercritical fluid/chemical formulations

Abstract

A method and composition for removing silicon-containing sacrificial layers from Micro Electro Mechanical System (MEMS) substrates having such sacrificial layers is described. The etching compositions include a supercritical fluid, an etchant species, a co-solvent, and optionally a surfactant. Such etching compositions overcome the intrinsic deficiency of SCFs as cleaning reagents, viz., the non-polar character of SCFs and their associated inability to solubilize polar species that must be removed from the semiconductor substrate. The resultant etched MEMS substrates experience lower incidents of stiction relative to MEMS substrates etched using conventional wet etching techniques.

1 . A sacrificial silicon-containing layer etching composition, comprising a supercritical fluid (SCF), at least one co-solvent, at least one etchant species, and optionally at least one surfactant.  
     
     
         2 . The composition of  claim 1 , wherein the SCF is selected from the group consisting of carbon dioxide, oxygen, argon, krypton, xenon, and ammonia.  
     
     
         3 . The composition of  claim 1 , wherein the SCF is carbon dioxide.  
     
     
         4 . The composition of  claim 1 , wherein the co-solvent comprises at least one C 1 -C 6  alcohol.  
     
     
         5 . The composition of  claim 1 , wherein the co-solvent comprises methanol.  
     
     
         6 . The composition of  claim 1 , wherein the co-solvent comprises isopropanol.  
     
     
         7 . The composition of  claim 1 , wherein the sacrificial silicon-containing layer comprises silicon oxide.  
     
     
         8 . The composition of  claim 7 , wherein the etchant species comprises at least one bifluoride compound selected from the group consisting of ammonium bifluoride and tetraalkylammonium bifluoride ((R) 4 NHF 2 ), wherein R is a C 1 -C 4  alkyl group.  
     
     
         9 . The composition of  claim 7 , wherein the etchant species comprises ammonium bifluoride.  
     
     
         10 . The composition of  claim 7 , wherein the surfactant comprises at least one nonionic surfactant.  
     
     
         11 . The composition of  claim 10 , wherein the surfactant is selected from the group consisting of fluoroalkyl surfactants, polyethylene glycols, polypropylene glycols, polyethylene ethers, polypropylene glycol ethers, carboxylic acid salts, dodecylbenzenesulfonic acid, dodecylbenzenesulfonic salts, polyacrylate polymers, dinonylphenyl polyoxyethylene, silicone polymers, modified silicone polymers, acetylenic diols, modified acetylenic diols, alkylammonium salts, modified alkylammonium salts, and combinations comprising at least one of the foregoing.  
     
     
         12 . The composition of  claim 10 , wherein the surfactant comprises a modified acetylenic diol.  
     
     
         13 . The composition of  claim 7 , wherein the etching composition comprises about 75.0 wt % to about 99.5 wt % SCF, about 0.3 wt % to about 22.5 wt % co-solvent, about 0.01 wt % to about 5.0 wt % etchant species, and about 0.01 wt % to about 5.0 wt % surfactant, based on the total weight of the composition.  
     
     
         14 . The composition of  claim 1 , wherein the sacrificial silicon-containing layer consists essentially of silicon.  
     
     
         15 . The composition of  claim 14 , wherein the etchant species is XeF 2 .  
     
     
         16 . The composition of  claim 14 , wherein the etching composition comprises about 75.0 wt % to about 99.5 wt % SCF, about 0.3 wt % to about 22.5 wt % co-solvent, about 0.01 wt % to about 5.0 wt % etchant species, based on the total weight of the composition.  
     
     
         17 . A method of removing silicon-containing substances from a substrate having same thereon, said method comprising contacting the substrate with a SCF-based composition comprising a SCF, at least one co-solvent, at least one etchant species, and optionally at least one surfactant, for sufficient time and under sufficient contacting conditions to remove the silicon-containing substances from the substrate.  
     
     
         18 . The method of  claim 17 , wherein the SCF is selected from the group consisting of carbon dioxide, oxygen, argon, krypton, xenon, and ammonia.  
     
     
         19 . The method of  claim 17 , wherein the SCF is carbon dioxide.  
     
     
         20 . The method of  claim 17 , wherein the contacting conditions comprise pressures in a range of from about 1400 to about 4400 psi.  
     
     
         21 . The method of  claim 17 , wherein said contacting time is in a range of from about 30 seconds to about 30 minutes.  
     
     
         22 . The method of  claim 17 , wherein the co-solvent comprises at least one C 1 -C 6  alcohol.  
     
     
         23 . The method of  claim 17 , wherein the co-solvent comprises methanol.  
     
     
         24 . The method of  claim 17 , wherein the co-solvent comprises isopropanol (IPA).  
     
     
         25 . The method of  claim 17 , wherein the silicon-containing substance comprises a sacrificial silicon oxide layer.  
     
     
         26 . The method of  claim 25 , wherein the etchant species comprises at least one bifluoride compound selected from the group consisting of ammonium bifluoride and tetraalkylammonium, bifluoride ((R) 4 NHF 2 ), wherein R is a C 1 -C 4  alkyl group.  
     
     
         27 . The method of  claim 25 , wherein the etchant species comprises ammonium bifluoride.  
     
     
         28 . The method of  claim 25 , wherein the surfactant comprises at least one nonionic surfactant.  
     
     
         29 . The method of  claim 28 , wherein the surfactant is selected from the group consisting of fluoroalkyl surfactants, polyethylene glycols, polypropylene glycols, polyethylene ethers, polypropylene glycol ethers, carboxylic acid salts, dodecylbenzenesulfonic acid, dodecylbenzenesulfonic salts, polyacrylate polymers, dinonylphenyl polyoxyethylene, silicone polymers, modified silicone polymers, acetylenic diols, modified acetylenic diols, alkylammonium salts, modified alkylammonium salts, and combinations comprising at least one of the foregoing.  
     
     
         30 . The method of  claim 25 , wherein the etching composition comprises about 75.0 wt % to about 99.5 wt % SCF, about 0.3 wt % to about 22.5 wt % co-solvent, about 0.01 wt % to about 5.0 wt % etchant species, and about 0.01 wt % to about 5.0 wt % surfactant, based on the total weight of the composition.  
     
     
         31 . The method of  claim 17 , wherein the silicon-containing substance is selected from the group consisting of silicon, post-ash residue and post-etch residue.  
     
     
         32 . The method of  claim 31 , wherein the etchant species is XeF 2 .  
     
     
         33 . The method of  claim 31 , further comprising dehydrating the substrate prior to contacting the substrate with the SCF-based etching composition.  
     
     
         34 . The method of  claim 31 , wherein the etching composition comprises about 75.0 wt % to about 99.5 wt % SCF, about 0.3 wt % to about 22.5 wt % co-solvent, about 0.01 wt % to about 5.0 wt % etchant species, based on the total weight of the composition.  
     
     
         35 . The method of  claim 17 , wherein the contacting step comprises a etching cycle including (i) dynamic flow contacting of the etching composition with the silicon-containing substance, and (ii) static soaking contacting of the etching composition with the silicon-containing substance.  
     
     
         36 . The method of  claim 35 , wherein said etching cycle comprises alternatingly and repetitively carrying out dynamic flow contacting (i) and static soaking contacting (ii) of the silicon-containing substance.  
     
     
         37 . The method of  claim 17 , further comprising the step of washing the substrate, at a region at which the silicon-containing substance has been removed, with a SCF/methanol/deionized water wash solution in a first washing step, and with a SCF in a second washing step, to remove residual precipitated chemical additives in said first washing step, and to remove residual precipitated chemical additives and/or residual alcohol in said second washing step.  
     
     
         38 . The method of  claim 37 , wherein the SCF is SCCO 2 .