On Wed, 7 Apr 1999, Shivalik Bakshi wrote:
> Dear MEMS Researchers,
>
> Would you know of a wax material that I can use to protect one side of a wafer
while etching the other side in TMAH (or EDP)?
>
> Thanks,
> Shivalik
Dear Mr.Shivalik,
You could use simple bee wax or use PTFE (teflon). Both
these material easily withstand EDP/TMAH.
A better option is to use 2000 A of Si3N4 and a 4 gram of
pyrazine to your mixture. I have presented below a section of the recipes
compiled by microlab at Univ. of Berkley. You do get a etch rate of Si3N4
of 70 A/ hr. Hence can serve as a mask, and better because deposition,
etc. is done using standard Silicon fabrication techniques.
I have tried many of the recipes mentioned below and they
are quite accurate and with some minor modifications you can have a
excellent backside protection.
**********************Recipe compilation by Univ. of Berkley**********
EDP Etchant for Single Crystal Silicon
EDP etchant can be used on p-type wafers with <100>
orientation, masked with either silicon dioxide or sili-
con nitride. It leaves a cleaner, smoother silicon sur-
face with partial etch than KOH (see below). Heavy boron
doping acts as an etch stop for EDP. Since EDP does not
etch oxide, it is important to remember to dip off any
native oxide from the silicon surfaces to be etched in
HF solution. Etch rates and temperatures are given
below. Complete instructions on the use of EDP are given
in Chapter 1.3 of the lab manual.
* Ethylenediamine NH2O(CH2)2NH2 1 mole = 50.10 g
Pyrocatechol C6H4(OH2) 1 mole = 109.1 g
Water H2O 1 mole = 18.02g
* 1. F & K Etchant - Finne & Klein, Bell Labs @ Murray
Hill. J. Electrochem. Soc., Vol. 114, No. 9, September
1967, pp. 965-970
Ethylenediamine 500 ml 35.1 mole%
Pyrocatechol 88 g 3.7 mole%
Water 234 ml 61.2 mole%
Etch temp: 110C
Etch Rate ratio: <100>:<110>:<111> 50:30:3
Initial Etch Rate: 28 Mm/hr
Oxygen Exposure: up to >50 Mm/hr
Mask Resistance: SiO2 200A/hr.
This is the earliest reported EDP (or EPW) composition.
It is generally used in the temperature range 100-118C.
At lower temperatures it develops insoluble residues.
This composition, as well as other uncatalyzed EDP com-
positions, tends to 'age' rapidly with exposure to oxy-
gen. The etch rate increases with time to 50 Mm/hr and
higher.
The addition of pyrazine increases the <100> etch rate
while making it less sensitive to oxygen exposre. Pyra-
zine has a very small effect on the <111> etch rate so
the <100>/<111> ratio increases with pyrazine content.
The sselectivity to boron content is reported by Reisman
et al. to be similar for the F & K, B, and S etches.
Also, the smoothness of the etching surface is improved
by the asddition of 0-6 g/L pyrazine. 8 g/> has shown
some unevenness, <111> pyramids form, possibly due to
the very high <100>/<111> etching ratio. IBM recommends
4 g of pyrazine to every liter of ethylenediamine for a
smooth surface.
* 2. "B" Etchant E. Bassous, IBM Research Center, Yorktown
Heights, N.Y., U.S. Patent 3,921,916 (1975).
Ethylenediamine 500 ml
Pyrocatechol 80 g
Water 160 ml
Temperature Range: 100-118C
Boiling Point: 118C
<100> Etch Rate (with pyrazine added):
Pyrazine per 500 ml Ethylenediamine
0 g 1.0 g 3.6 g
@ 100C 14 Mm/hr 42 Mm/hr 50 Mm/hr
@ 115C 26 Mm/hr 65 Mm/hr 75 Mm/hr
Mask Resistance:
SiO2 150A/hr
Si3N4 80A/hr
This composition with or without pyrazine provides
residure-free etching above 100C.
* 2. "F" (Fast) Etchant A. Reisman et al., IBM Research
Center, Yorktown Heights, N.Y., J. Electrochemical Soc.,
Vol 126, No. 8, August 1979, pp. 1406-1415.
Ethylenediamine 500 ml
Pyrocatechol 160 g
Water 160 ml
Temperature Range: 100-118C
<100> Etch Rate (with pyrazine added):
Pyrazine per 500 ml Ethylenediamine
0 g 1.0 g 3.0 g
@ 115C 27 Mm/hr 68 Mm/hr 81 Mm/hr
* 3. "S" (Slow) Etchant A. Reisman et al., IBM Research
Center, Yorktown Heights, N.Y., J. Electrochemical Soc., Vol
126, No. 8, August 1979, pp. 1406-1415.
Ethylenediamine 500 ml
Pyrocatechol 80 g
Water 66 ml
Temperature Range: 50-115C
<100> Etch Rate (with pyrazine added):
Pyrazine per 500 ml Ethylenediamine
3.6 g
@ 50C 4.5 Mm/hr
@ 75C 13 Mm/hr
@ 95C 26 Mm/hr
@ 105C 34 Mm hr
@ 115C 45 Mm/hr
* 4. "M" (Medium) Etchant Based on A. Reisman et al., as
above.
This etch is useful for etching below the boiling point in
order to minimize agitation of the wafer. It etches at a
rate between the "F" and "S" etches (hence "M" for medium).
This etch prevents the formation of residues by the "F" etch
by slowing oxidation of the surface through the reduction of
the water content.
Ethylenediamine 500 ml
Pyrocatechol 160 g
Water 125 ml
Temperature Range: 105C
<100> Etch Rate (with pyrazine added):
Pyrazine per 500 ml Ethylenediamine
3.0 g
@ 115C 63 Mm/hr
Other references:
K.E. Petersen, Proc. IEEE, vol. 70, No. 5, May 1982, pp.
420-457.
K.E. Bean, IEEE Trans. ED-25, No. 10, October 1978,
pp.1185-1193.
N.F. Raley etal., J. Electrochemical Soc., vol. 131, No.1,
january 1984, pp.161-171.