Dear All,
Could you please any one tell me why specially any chemical etches the Silicon?
Example TMAH,KOH these chemicals etches the silicon why not other chemical etches the Silicon?
Thank You
Manoj
KOH is non toxic.
I have a some little experience with Silicon bulk micromachine, and can share the following information.
Typically the variation of the etch rate with KOH concentration shows minimal dependence at lower temperatures (say around 40 to 55 deg C) in comparison to higher temperatures ( 60 to 70 deg C).with the higher one (70 °C). Temperature is seen to have a profound influence on the etch rate, rather than the concentration of KOH.
The etch rate increases with increasing reaction temperature, for all examined range of KOH concentrations. At 60 deg C to 70 deg C, a maximum etching rate is obtained for 30wt % KOH + 10% of IPA.
Considering the Si etching reaction.: Si + 2OH- + 2H2O → Si (OH4) - + H2(g) (1)
When KOH reacts with Si, soluble Si (OH4) – is formed along with H2 (Hydrogen bubbles.
At a low KOH concentration, OH ions are in low concentration, whereas the numbers of H2O molecules are low at high KOH concentration. Conseuqnetly, the reaction can vary at high and low KOH concentrations. Furthermore the reaction rates, or concentrations can be affected by adding different IPA percentage. Above all the etching temperature has its own added influence in speeding up the reaction, and has a has much more significant effect on etch rate than changing KOH concentration,
K. Sreenivas
It is well known that KOH as anisotropic silicon etchants used to produce three dimensional mechanical and electronic microstructures and it produce a perfect crystal plane at a lower temperature when compared by another chemical bath like NaOH:H2O or acid etchant like (HF:HNO3:CH3COOH). So the KOH is not the unique silicon etchant. Depending on the desired silicon surface structure which we like to obtain and the final application.
· Isotropic Etching - etches at equal rate in all directions
· Anisotropic Etching - etches faster vertically than horizontally
Also there are wet cleaning processes to remove organic inorganic &metallic contaminants from the silicon surface: NH4OH-H2O2-H2O, HCl-H2O2-H2O, H2SO4:H2O2.
The reason:
The most of these chemicals react with silicon to form SiO2 layer and etch it simultaneously. The final results from point of view of morphology depend on several parameters:
· Chemical composition: KOH+IPA (Iso-Propanol- Alcohol), TMAH, HF:HNO3:CH3COOH, …etc.
· Temperature
· Etch time
· Crystallographic orientation of the silicon surface (111) , 100, …etc.
Other chemicals etch the silicon like CrO3 and K2Cr2O7 mixed with HF and H2O for defects delineation in silicon.
In conclusion; these chemicals are mixed with others at optimized conditions (Parts, temperature and time) to etch silicon. The same chemicals can be used to etch oxides like SiO2, Si3N4 or metals, but with a specified mixtures and conditions.
I also have one question how you define the 30 wt% or 3 wt% KOH. Is there any formula to calculate it ?
I hope your etching process went alright.
I have a KOH etching related question, too. I am etching Si through Si3N4 mask from the back side of the wafer to suspend Si3N4 membranes on the front side. After I have finished etching, I see some particles/contaminants on the back of the membranes. They do not go with water and I am advised to not use HF to clean it.
How did you clean up your substrate after the KOH etching?
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