Is three is differences between BOE and HF solution (10-48%) for etching native oxide layer?

[Moshe]

Is three is differences between BOE and HF solution (10-48%) for etching native oxide layer?



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[Mantavya Sinha]

Typically, to etch native oxide (approx 10-20 A) on silicon you can use
10% HF (etch rate approx 20-30 nm/min).
BOE has a much higher etch rate (for e.g. approx 80-90 nm/min for 6:1
BOE) and so is used to etch thick thermal oxide layers.
Hope it helps.

Mantavya

-----Original Message-----
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[mailto:mems-talk-***@memsnet.org] On Behalf Of Moshe
Sent: Thursday, November 13, 2008 4:55 AM
To: General MEMS discussion
Subject: [mems-talk] Is three is differences between BOE and HF
solution(10-48%) for etching native oxide layer?

Is three is differences between BOE and HF solution (10-48%) for etching
native oxide layer?
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[James Paul Grant]

Hello Moshe,

Perhaps this is a simplistic view however the major difference between
BOE and 49% HF is their etch rate.

49% HF will etch much faster than 5:1 BOE which in turn will etch much
faster than 10:1 BOE. Might I suggest you have a look at the (in my
view) excellent paper, "Etch Rates for Micromachining Processing -Part
II" by Kirt Williams (Journal od Micromechanical systems Vol. 12, No. 6
December 2003).

The native oxide on silicon is only 20 A thick and 10: 1 BOE will remove
native oxide at around 20 nm/minute so I would use this BOE for around
30 seconds.

Obviously HF etching is isotropic (etches in all directions) so to
reduce the undercut of patterned features one should choose an BOE
concentration and etch time accordingly.

James

Post by Moshe
Is three is differences between BOE and HF solution (10-48%) for etching native oxide layer?

--
Dr. James Paul Grant
Postdoctoral Research Associate
Complex Systems Design Group
76 Oakfield Avenue Room 3
University of Glasgow
Glasgow
Scotland
G12 8LS

Telephone: [phone]

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[汪飞]

Dear James,

"Obviously HF etching is isotropic (etches in all directions) so to reduce
the undercut of patterned features one should choose an BOE concentration
and etch time accordingly."

I think BOE etching is also isotropic. So, why do you think it can help with
better undercut profile?

Post by James Paul Grant
Hello Moshe,
Perhaps this is a simplistic view however the major difference between BOE
and 49% HF is their etch rate.
49% HF will etch much faster than 5:1 BOE which in turn will etch much
faster than 10:1 BOE. Might I suggest you have a look at the (in my view)
excellent paper, "Etch Rates for Micromachining Processing -Part II" by Kirt
Williams (Journal od Micromechanical systems Vol. 12, No. 6 December 2003).
The native oxide on silicon is only 20 A thick and 10: 1 BOE will remove
native oxide at around 20 nm/minute so I would use this BOE for around 30
seconds.
Obviously HF etching is isotropic (etches in all directions) so to reduce
the undercut of patterned features one should choose an BOE concentration
and etch time accordingly.
James

--
Best regards,
Yours sincerely
Fei Wang
______________
Postdoctoral researcher, Dr
MIC - Department of Micro and Nanotechnology
Technical University of Denmark (DTU)
Building 344, 1st floor, Room no. 130
DK-2800, Kgs. Lyngby
Denmark
Tel: [phone]
Fax: [phone]
Email: ***@nanotech.dtu.dk
http://www.nanotech.dtu.dk
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[James Paul Grant]

Of course BOE etchant is also isotropic. Recall it is the HF in BOE that
is responsible for etching the oxide, the ammonium fluoride is just used
to control the pH.

My point was one should use a BOE or HF concentration that is suitable
for the application. In this case the topic creator wanted to wish the
native oxide layer which is only 20 Angstroms thick. If he used 5:1 BOE
etch for one minute then sure he will vertically etch his 20 angstroms
of SiO2 however he will also LATERALLY etch around 20 nm his SiO2.

I hope you now understand my point!

James

Post by 汪飞
Dear James,
"Obviously HF etching is isotropic (etches in all directions) so to reduce
the undercut of patterned features one should choose an BOE concentration
and etch time accordingly."
I think BOE etching is also isotropic. So, why do you think it can help with
better undercut profile?

--
Dr. James Paul Grant
Postdoctoral Research Associate
Complex Systems Design Group
76 Oakfield Avenue Room 3
University of Glasgow
Glasgow
Scotland
G12 8LS

Telephone: [phone]

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[Christopher Tan]

I think James' point was not to imply that BOE is not-isotropic, but
that since BOE etches at a slower rate, it is easier to time and control
the undercut.

More importantly, if one wants to simply strip the oxide layer without
features and without caring about the surface smoothness, then obviously
a higher concentration would work quickly and very well. However, if one
wants to maintain a smooth Si surface without damaging the original
flatness of the wafer (such as for hydrophobic Si fusion bonding), one
should choose a much lower concentration of HF, such as diluted HF (DHF,
~1-6% HF in H2O) or 6:1, 10:1 or 50:1 BOE. High concentrations of HF
will damage the atomic flatness of the wafer.

Christopher Tan

Post by 汪飞
Dear James,
"Obviously HF etching is isotropic (etches in all directions) so to reduce
the undercut of patterned features one should choose an BOE concentration
and etch time accordingly."
I think BOE etching is also isotropic. So, why do you think it can help with
better undercut profile?

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provider of MEMS and Nanotechnology design and fabrication services.
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