Method for precisely controlled masked anodization

Abstract

The present invention is related to a method for masked anodization of an anodizable layer on a substrate, for example an aluminum layer present on a sacrificial layer, wherein the sacrificial layer needs to be removed from a cavity comprising a Micro or Nano Electromechanical System (MEMS or NEMS). Anodization of an Al layer leads to the formation of elongate pores, through which the sacrificial layer can be removed. According to the method of the invention, the anodization of the Al layer is done with the help of a first mask which defines the area to be anodized, and a second mask which defines a second area to be anodized, said second area surrounding the first area. Anodization of the areas defined by the first and second mask leads to the formation of an anodized structure in the form of a closed ring around the first area, which forms a barrier against unwanted lateral anodization in the first area.

1 . A method for masked anodization, comprising:
 Providing a substrate;   providing an anodizable layer on the substrate;   providing, on the anodizable layer, at least one first mask defining one or more first structures to be anodized and an additional second mask defining a second structure to be anodized, the second structure surrounding the one or more first structures;   anodizing the anodizable layer in regions defined by the first and second mask in order to create the one or more first anodized structures and the second anodized structure; and   removing the at least one first mask and the additional second mask.   
     
     
         2 . The method according to  claim 1 , wherein anodizing the anodizable layer is performed by inserting the substrate in an electrolyte and applying a voltage or electrical current to the substrate. 
     
     
         3 . The method according to  claim 1 , wherein anodizing the anodizable layer is performed using a first anodizing step to form a first anodized layer, followed by a second anodizing step after etching away the first anodized layer. 
     
     
         4 . The method according to  claim 1 , wherein anodizing is performed by inserting the substrate in an electrolyte comprising at least one chemical selected from the group consisting of sulfuric acid, phosphoric acid, oxalic acid, hydrofluoric acid, ethanol, and isopropyl alcohol. 
     
     
         5 . The method according to  claim 1 , wherein the anodizable layer is an Al layer and wherein anodizing is performed by inserting the substrate in a sulfuric acid-based electrolyte at a temperature of from 20° C. to 40° C. and applying a voltage of about 20V. 
     
     
         6 . The method according to  claim 1 , wherein anodizing produces elongated pores in the anodized structures, the pores extending from a front surface to a back surface of the anodizable layer. 
     
     
         7 . The method according to  claim 6 , wherein the anodizable layer is a metal layer which forms a metal oxide layer upon anodization, wherein anodizing is conducted in an anodization bath comprising an etchant configured for etching the metal oxide layer, and wherein anodizing is conducted for a first time period during which the voltage is applied, the voltage being removed at an end of the first time period, and for a second time period following the first time period, during which the substrate is left in the anodization bath, whereby a barrier layer at the bottom of the elongated pores. 
     
     
         8 . The method according to  claim 7 , wherein the metal is aluminum and the etchant is phosphoric acid. 
     
     
         9 . The method according to  claim 1 , wherein providing a substrate comprises depositing a sacrificial layer onto a base substrate, wherein the anodizable layer is provided on the sacrificial layer, wherein anodizing produces elongated pores in the one or more first structures, the pores extending from a front surface to a back surface of the anodizable layer, wherein at least a portion of the sacrificial layer is removed through the pores so as to form a cavity, and wherein a sealing layer is applied onto the anodized structure to seal off the cavity. 
     
     
         10 . The method according to  claim 9 , wherein the sacrificial layer comprises a material selected from the group consisting of polycrystalline SiGe, an oxide-based film, a nitride-based film, a polymer, single crystal Si, and polycrystalline Si. 
     
     
         11 . The method according to  claim 9 , wherein the substrate is a NEMS/MEMS device encapsulated in the cavity. 
     
     
         12 . The method according to  claim 11 , wherein removing at least some of the sacrificial material is performed using a selective wet or dry etching that does not damage the encapsulated NEMS/MEMS device. 
     
     
         13 . The method according to  claim 9 , wherein the sacrificial layer is silicon oxide, and wherein removal of the portion of the sacrificial layer is performed in a reduced-pressure chamber using a vapor-phase hydrofluoric acid mixed with at least one other component selected from the group consisting of nitrogen, ethanol, and water vapor. 
     
     
         14 . The method according to  claim 9 , wherein the sacrificial layer is a polymer-based material and wherein removal of the portion of the sacrificial layer is performed by dry plasma etching in a low-pressure chamber in the presence of oxygen ions. 
     
     
         15 . The method according to  claim 9 , wherein the sealing layer is selected from the group consisting of a conductive layer, an Al layer, a Cu layer, a Ni layer, a polycrystalline Si layer, a polycrystalline SiGe layer, a dielectric material, an oxide-based silicon compound, a nitride-based silicon compound, a polymer, combinations thereof, and stacks thereof, wherein the sealing layer is deposited is deposited in a low-pressure chamber using an evaporation technique, chemical vapor deposition, or physical vapor deposition. 
     
     
         16 . The method according to  claim 1 , wherein the anodizable layer is a metal layer or a semiconducting layer. 
     
     
         17 . The method according to  claim 16 , wherein the anodizable layer is selected from the group consisting of Al, Ta, Ti, Cu, Ni, polycrystalline Si, and polycrystalline SiGe. 
     
     
         18 . The method according to  claim 1 , wherein at least one of the first mask and the second mask are formed by photolithographic patterning of a photosensitive layer.