Questions about the Si wet etching process in SOI wafer

[Donkoun Lee]

Hi all:

Recently, I used the 25% TMAH in order to wet-etch the Si layer in SOI wafer.
Before etching a SOI wafer, when I used a Si wafer for the reference, the Si wafer was well etched.
However, when I used the SOI wafer, the Si layer (device layer) in SOI wafer was not etched at all:

The condition I used is like below:
Temperature: 45 oC
Concentration of TMAH: 25%
The Si thickness in SOI wafer: 30 ~ 50 nm

I also used the higher temperature, like ~ 70 oC. But, I could not etch the Si layer in SOI wafer.
Si layer is P-type. However, the doping concentration is around 10^15/cm2. 
Furthermore, I double checked out the orientation of the Si layer and the orientation was (001)

Could you give me a tip for solving this problem?

Thank you.
Kind Regards,
Donkoun Lee