Selective etching of phosphosilicate glass

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT In FIG.
1 a silicon wafer 12 has, on a surface 14 thereof, an SiO.
sub.
2 layer 16 having an opening 18 therethrough.
A Si.
sub.
3 N.
sub.
4 layer 20, typically about 60 nm thick, overlies the SiO.
sub.
2 layer 16.
A PSG layer 22, typically about 900 nm thick and deposited by chemical vapor deposition, overlies the Si.
sub.
3 N.
sub.
4 layer 20.
The PSG layer 22 has a surface 24 which is rounded and which tapers to about the same diameter as that of the opening 18 after it is softened.
The Si.
sub.
3 N.
sub.
4 layer 20 undercuts the PSG layer 22 leaving an overhang 26 over the edge of Si.
sub.
3 N.
sub.
4 layer 20.
Typically, the overhang 26 is between about 1 and about 2 times the thickness of the Si.
sub.
3 N.
sub.
4 layer 20.
The overhang 26 is removed by a process which selectively etches the PSG as opposed to the SiO.
sub.
2 and Si.
sub.
3 N.
sub.
4 layers, producing a new surface 28 with no overhang of PSG over the Si.
sub.
3 N.
sub.
4 layer.
The selective etching of the PSG will also remove a small amount of the exposed portion of SiO.
sub.
2 layer 16 in the region of the opening 18, but essentially none of the Si.
sub.
3 N.
sub.
4 or silicon.
The etching process is carried out by immersing the wafer with the layers thereon in an etchant solution held at about 25.
degree.
C.
or above and which contains a carboxylic acid, hydrogen fluoride and water.
Useful members of the group of carboxylic acids include formic acid (HCOOH), acetic acid (CH.
sub.
3 COOH) and propionic acid (C.
sub.
2 H.
sub.
5 COOH).
Typically, hydrogen fluoride is added to the subject etchant solution as an aqueous solution, i.
e.
as hydrofluoric acid.
Commercial hydrofluoric acid, which contains 49% hydrogen fluoride, is conveniently used, but other concentrations are also useful.
The maximum concentration of hydrogen fluoride in water is preferred, however, to minimize the amount of water present in the solution.
Thus, hydrogen fluoride dissolved in the carboxylic acid directly with a small fraction of water would also be useful