Method to control the etching rate of materials
A method for etching materials in which organic solvents are added to the etching mixture and combined in a mixing arrangement. When agitated organic materials mix with the etching agent and interact with the underlying material to form a shield around the etched areas that prevents the additional interaction of water with the newly etched areas and enables the etching of silicon oxides (SiOx) but does not oxidize Si. This method leads to milder reactions with less heat generation and avoids the safety hazards associated with conventional etching methods.
1. A method for etching materials, the method comprising the steps of:
forming a mixture that contains, a silicon containing powder material having a surface that is to be etched and a core portion that is not to be etched and an etchant solution comprising an etchant, water and an organic solvent that can dissolve the etchant but is not miscible with water; the mixture having a ratio of 1 part in weight of Si-containing powder, 5-40 parts in weight of organic solvent, and 5-20 parts in weight of acidic aqueous etchant, and
alternatively stirring the mixture to etch the material whereby the organic solvent with acidic etchant dissolved therein preferentially contacts and etches the surface of material, while the organic solvent preferentially contacts and covers the newly etched area which prevents additional interaction of this newly etched surface with water, and when not stirring the mixture, the etching stops and a phase separation of materials takes place with material covered by organic solvent forming a separate layer.
2. The method of claim 1 wherein the organic solvent comprises an aromatic compound.
3. The method of claim 2 wherein the organic solvent further comprises an aliphatic compound.
4. The method of claim 3 wherein the aromatic compound is selected from the group consisting of such as benzene, toluene, xylene, cumene, and mixtures thereof and the aliphatic is selected from the group consisting of hexane, cyclohexane, pentane, decene, and mixtures thereof.
5. The method of claim 1 wherein the etching agent is selected from the group consisting of hydrofluoric acid (HF), ammonium fluoride (NH 4 F), nitric acid (HNO 3 ) and combinations thereof.
6. The method of claim 1 further comprising the step of recovering the acidic etching agent.
7. The method of claim 1 wherein the stirring takes place for less than 3 hours.
8. The method of claim 1 further comprising the step of pre-treating the Si-powder by heating in an inert atmosphere.
9. The method of claim 1 wherein the Si powder has a granule size of less than 100 μm.
10. A process for making a porous silicon, the process comprising the steps of:
alternatively stirring and not stirring a mixture that contains an organic solvent, an acidic aqueous etchant and a Si-containing powder in a ratio of 1 part Si-powder, 5-20 parts organic solvent, and 5-40 parts acidic aqueous etchant;
whereby etching of the Si-containing powder occurs when the mixture is stirred causing the Si-containing powder and the aqueous acid etchant to be brought into contact while the organic solvent intervenes to form an organic phase that protects newly exposed Si from further reaction with the acidic aqueous etchant thus preventing additional etching and heat generation, when the stirring of the mixture stops the Si containing powder separates into the organic phase away from the acidic aqueous etchant and the etching stops resulting in a powder having a porous silicon structure.
11. The process of claim 10 wherein the resulting porous silicon has a porosity of 10-90%.
12. The method of claim 10 wherein the organic solvent comprises an aromatic compound.
13. The method of claim 11 wherein the organic solvent further comprises an aliphatic compound.
14. The method of claim 12 wherein the aromatic compound is selected from the group consisting of such as benzene, toluene, xylene, cumene, and mixtures thereof and the aliphatic is selected from the group consisting of hexane, cyclohexane, pentane, decene, and mixtures thereof.
15. The method of claim 10 wherein the etching agent is selected from the group consisting of hydrofluoric acid (HF), ammonium fluoride (NH 4 F), Nitric acid (HNO 3 ) and combinations thereof.
16. The method of claim 10 wherein the stirring takes place for less than 3 hours.
17. The method of claim 10 further comprising the step of pre-treating the Si-powder by heating in an inert atmosphere.