Note: Descriptions are shown in the official language in which they were submitted.
~5~633
This invention relates to the control of etch
rate of silicon dioxide in boiling phosphoric acid, and
particularly to the substantial reduction in etch rate of
thermally grown silicon dioxide when used as a masking material.
Boiling phosphoric acid is used for etching
silicon nitride (Si3N4). However, when it i5 desired to etch
predetermined patterns in Si3N4, as in the production of semi-
conductor devices, it is necessary to mask the Si3N4 prior to
etching. However, photoresist masking materials are not
resistive ~o etching by boiling phosphoric acid and usually SiO2
is used as a masking material.
The etch rate of SiO2 in boiling phosphoric acid
is still quite high - typically approximately 5A/minute. To
ensure satisfactory etching of the underlying Si3N4 layer, it
is usual to grow a lOOA layer of SiO2, or thicker, but even then
it is possible that the SiO2 layer will be completely removed - ~
at least in some areas, with undesirable etching of the Si3N4. ~`
Thick layers of masking SiO2 are time consuming in production
,
1 and reduce the resolution available.
The present invention provides for the reduction
in etch rate of SiO2 in boiling phosphoric acid by the addition
of a material to the phosphoric acid which will increase the
~ silicate content of the acid above the normal levels of impurity. ~ ;
-~ The invention will be readily understood by -
~ .
,i the following description of certain embodiments, by way of
'A' example, in conjunction with the accompanying drawings in which:-
Figure 1 is a diagrammatic cross-section through
a semiconductor device illustrating the steps in etching Si3N4,
Figure 2 is a curve representative of the
variation in etch rate oF SiO2 with variation in effective
silicate added to the etch solution.
~57633
Considerin~ Fiyure l(a), there is illustrated
a silicon substrate 10, a layer of S;02, 11 on the substrate,
a layer of Si3N4, 12, on the oxide layer 11 and a layer of
thermally grown SiO2, 13, on the Si3N4 layer 12. The oxide
layer 13 is intended to act as a mask for etching the Si3N4
layer 12 and in Figure l(a~ a photoresist mask 14 has been
applied and the SiO2 layer 13 etched through this mask down to
the Si3N4 layer 12, in a conventional manner for example etching
in hydrofluoric acid solution~
The photoresist layer or mask 14 is removed and
the Si3N4 layer 12 etched through the SiO2 masking layer 13 in
boiling phosphoric acid. This is illustrated in ~igure l(b).
In conventional processes the SiO2 masking layer 13 is partly
removed - and may in some cases by completely removed, with loss
of the desired pattern in the Si3N4 layer.
The etch rate of thermally grown SiO2 used for
masking purposes is approximately 5A/minute in boiling phosphoric
acid (180C~. To ensure that mask failure does not occur during
.~ ~
the nitride etching process it is desirable to reduce the SiO2 ~-
etch rate, for example to less than 2A/minute although slightly
higher etch rates can still be useful. The etch rate is decreased,
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in accordance with the present invention1 by increasing the
silicate content of the phosphoric acid. Phosphoric acid is
what is termed a "dirty" acid, and contains several impurities,
including silica. The present invention increases the silicate
content substantially above that which may occur in the commer-
cially available product.
Two ways of increasing the silicate content are
(1~ adding a soluble silicate, for example sodium metasilicate,
to the phosphoric acid, (2) introducing a large surface area of
,
SiO2 into the boiling phosphoric acida as by adding finely divided
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~os7~33
silica powder,silica ~ibre, or silicic acid (a hydrated of silica).
Sodium metasilicate has the advantage of being
soluble in the phosphoric acid but the resulting presence of
sodium in the etchant may be undesirable for some purposes
because of its possible effect - on device stability for example.
The addition of finely divided silica or silicic acid is an
effective and acceptable alternative. The large surface area of
the SiO2 of the added material presented to the boiling acid
results in the required increase in the silicate content of the
phosphoric acid very quickly, for example 1n less than 1/2 hour.
As an example, 2 grams of sodium metasilicate or ;~
~.
-~ 1 gram of finely divided silica or silicic acid per liter of
1 phosphoric acid will decrease the SiO2 etch ra~e from about
O O
SA/minute to about .4A/minute. The etch rate of the silicon
! nitride is not significantly affected by the increased silicate
content of the phosphoric acid.
` Figure 2 is a curve which illustrates the effect
,
` ~ of increasing the silicate content of phosphoric acid etchant ~-
on~ the etch rate of thermally grown SiO2. The particular curve
of Figure 2 was obtained by adding sodium metasilicate to the
p~ho~sphoric acld, the particular example being of approximately
` 1 litre of acid, the etch rate, in boiling phosphoric acid being ``
at about 5A per minute with no additional silicate added by ;
addition of sodium metasilicate down to about 0.35A/0.40A per
minute with 2.0 grams of sodium metasilicate, and with a further `
reduction to about 0.15Aj~minute with 2.S grams of sodium meta~
~ silicate added. Depending upon any particular batch of phosphoric
; acid, the reduction in etch rate, illustrated in Figure 2, can
vary, Figure 2 ;tself being a curve of the mean values for a
3Q number of tests. A particularly useful range would be that
produced by adding between~ about .70 and 2.5 grams of sodium ~`~
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1~5'~633
metasilicate. This corresponds -to from about 0.30 to about 1.25 ;~
gram of silicate added to the acid, giving an etch rate from
O O O
about 3.0A/minute to about 0.15A/minute. A value of around .SA
per minute is a good general rate. As stated, other ways of
adding silicate to the acid can be used. In some cases it is
not readily ascertainable what amount of silicate is actually
added to the acid from the particular additive material used,
~` but if necessary it can be ascertained by chemical testing.
However it is soon ascertained, by actual results9 what desirable
` 10 amounts of additive are needed to give a part;cular etch rate:
that is the amount of actual silicate added.
` Use of the invention enables thinner SiO2 masking
layers to be used without the risk of mask failure, giving
reduced costs and providing improved resolution.
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