Hello everybody:
  Here are the answer that I received and I submit them to share them
with you. I thank everybody who has helped me.

-----Original Message-----
> Hello everybody:
>   Now I am bothered by the question of etching ZnO on SiO2 film.
> We have tried some solution such as H3PO4 or HF and so on, but we cannot
> get good effect. You are expected to give me some advice. Remember, the
> solution must not attack SiO2 film when we etch ZnO. I will indebted to
> your help. Thank you in advance.
>
> Sincerely,
> Li Gang
> My E-mail: ligang@ime.pku.edu.cn
>            mems@263.net
>
>

check the following references:
1. Chang et al., "Patterning of ZnO Film", IEEE Solid State Sensor and
Actuator, Hilton Head, 1992, p. 41
2. Vellekoop et al., " Compatibility of Zinc Oxide with Silicon IC
Processing", Sensors and Actuators, A21-23, 1990, p. 1027

In the first reference you find the etch solution 5% NH4Cl in water (at
55°C). The etch rate was ~300nm/min and silicon dioxide was not attacked.
The sample was subjected to a short oxygen plasma treatment prior to the
chemical etch to minimize undercutting and to etch more uniform.
If you lower the temperature to RT you can get etch rates of ~ 100 nm/min.
Another "etchant" for ZnO is developer like you use it in lithography. In
our experiments the developer MIF326 (Hoechst) etches ZnO. Maybe this effect
could be explained with the TMAH content in the developer.

I hope these informations could help,

Hartmut Kueppers

___________________________________

Dipl.-Ing. Hartmut Küppers
Institut für Werkstoffe der Elektrotechnik 1
RWTH Aachen
Sommerfeldstr. 24
52074 Aachen

e-mail: kueppers@iwe.rwth-aachen.de
Tel.: 0241-807795
Fax:0241-8888392

___________________________________

Hello,

you could use H3PO4:CH3COOH:DI-H2O with 1:1:160. The mask underetching for
this solution is quite large and can be up to two times the layer thickness
for [002] ZnO. Good luck.