Etching molydbenum with ferric sulfate and ferric ammonium sulfate

A molybdenum etching process which reduces hazardous treatment waste is disclosed. Etchants which can be used are ferric sulfate and ferric ammonium sulfate. Waste products resulting from this etch are Fe(OH).sub.3 and CaSO.sub.4.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to methods for etching of 
molybdenum for fabricating a metal mask to be used in electronic packaging 
and semiconductor fabrication processes and, more particularly, to a 
method of etching molybdenum with an etchant which contains no cyanide and 
which does not emit noxious fumes when heated. 
2. Description of the Prior Art 
Current practice in molybdenum etching is to use ferricyanide. See, for 
example, A. F. Bogenschfitz, W. Braun, and J. L. Jostan, 
Metalloberflaiche, 29(9), 451-455 (1975), M. Beyer, A. F. Bogenschiitz, 
and J. L. Jostan, Metalloberflache, 29(10), 506-511 (1975), and J. D. 
David and M. T. Kurdziel, Metal Finishing, 86(5) 47-49 (1988). The 
ferricyanide mask etching operations for molybdenum is performed at 
pH.apprxeq.13: 
EQU 6Fe(CN).sub.6.sup.3- +Mo+8OH.sup.- .fwdarw.MoO.sub.4.sup.2- +4H.sub.2 
O+6Fe(CN).sub.6.sup.4- 
Ferrocyanide is then recycled with ozone: 
EQU 2Fe(CN).sub.6.sup.4- +O.sub.3 +H.sub.2 O.fwdarw.2Fe(CN).sub.6.sup.3- 
+2OH.sup.- +O.sub.2 
Little etch waste results, but considerable quantities of rinse waste 
result, however, which are difficult to deal with because of the cyanide 
content of the iron complexes. 
In addition, the etchant pH must be maintained around 13 to avoid 
polymerizing molybdate to molybdenum oxide and polymolybdate gels. This 
high pH, however, often results in resist delamination and resist 
stripping from the molybdenum. This limits the choice of photoresists to 
those that are not resistant to bases. 
An etchant containing ferricyanide has been devised at neutral pH 
(.about.4) for molybdenum and tungsten etching is described in U.S. Pat. 
No. 4,995,942, but the cyanide bearing waste is still a disposal problem. 
Ferricyanide produces chemical wastes which are difficult and expensive to 
dispose because it contains cyano complexes. 
Ferric nitrate is an alternative etchant which has been documented as an 
effective molybdenum etch. See D. M. Allen, 
The Principles and Practice of Photochemical Machining and Photoetching, 
81, Adam Hilger, Boston (1986). Ferric nitrate etches anisotropically with 
respect to grain boundaries of the crystal structure of the molybdenum 
material. The molybdenum that meets the thickness uniformity and flatness 
criteria necessary for metal evaporation and screening masks has a high 
degree of orientation in its grain structure. This results in diamond 
shaped holes, as shown in FIG. 1, when the molybdenum is etched with 
ferric nitrate, regardless of the shape of the artwork. This is 
unacceptable for molybdenum masks. In addition, during the etch process 
ferric nitrate produces nitric acid, which has a boiling point of 
83.degree. C., making it very volatile in the processing environment. 
The need thus exists for a molybdenum etchant, which contains no cyanide, 
even complexed cyanide, which yields good etch rates, etch profiles, and 
etch quality. The etchant and its waste must be easily treatable by 
conventional neutralization methods to yield nonhazardous effluent and 
sludge cheaply. The etchant also must not attack conventional polyphenolic 
photoresists. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a molybdenum 
etchant which reduces intensive hazardous treatment waste yet is capable 
of satisfying mask fabrication requirements. 
It is another object of this invention to provide an etchant for molybdenum 
which is less volatile, provides a safer environment for operators, and is 
noncorrosive to stainless steel. 
It is a further object of this invention to provide flexibility in 
selecting photoresists which can be subjected to etching operations on 
molybdenum. 
It is yet another object of this invention to provide isotropic etching of 
molybdenum. 
It is still another object of this invention to provide faster etch rates 
on molybdenum than can be accomplished using ferricyanide. 
According to the invention, ferric sulfate Fe.sub.2 
(SO.sub.4).sub.3.nH.sub.2 O and ferric ammonium sulfate NH.sub.4 
Fe(SO.sub.4).sub.2 are used as etchants for molybdenum. Ferric sulfate and 
ferric ammonium sulfate produce wastes which can be readily treated using 
simple and inexpensive neutralization methods. Ferrous ions can be 
oxidized to ferric ions and then precipitated out as ferric hydroxide by 
raising the pH. Sulfate ion can be precipitated as calcium sulfate with 
lime. Both precipitates are neither toxic nor corrosive. During the 
etching process, ferric nitrate produces nitric acid while ferric sulfate 
produces sulfuric acid. Nitric acid with a boiling point of 83.degree. C. 
is much more volatile than sulfuric acid, which has a boiling point of 
330.degree. C. Consequently, ferric sulfate provides a much safer 
environment for the operators. Ferric sulfate does not corrode 316 
stainless steel, which is commonly used in manufacturing tools. Ferric 
sulfate and ferric ammonium sulfate give isotropic etching, producing hole 
shapes dependent on the shape of the artwork. Ferric sulfate also uses 
acid resistant photoresists, thereby providing flexibility for selecting 
different photoresists. 
The substitution of both ferric sulfate and ferric ammonium sulfate for 
ferricyanide yields comparable or superior etch rates, and no resist 
attack. Wastes can be easily treated using simple neutralization methods. 
The etchant is recyclable by reoxidyzing with oxygen or ozone. Round 
shaped holes in molybdenum can be obtained regardless of the crystal 
structure/grain boundaries of the metal.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
Ferric sulfate, Fe.sub.2 (SO.sub.4).sub.3.nH.sub.2 O, and ferric ammonium 
sulfate, NH.sub.4 Fe(SO.sub.4).sub.2, have been found to efficiently etch 
molybdenum metal. Ferric sulfate, in concentration ranging from 20 to 40 
weight percent, and pH in the range +0.50 to -0.50, will etch 1-3 
mils/hour of molybdenum at temperatures up to 60.degree. C. Higher 
temperatures are feasible, but not necessary. The etch rate can be speeded 
up by doing the process in a spray etches from two sides. Masks patterned 
with resist have exhibited etch rates in 30% Fe.sub.2 (SO.sub.4).sub.3 at 
50.degree. C. comparable to ferrous cyanide. 
It is necessary to maintain the low pH of the etchant for two reasons. 
First, the ferric ions must be prevented from gelling due to the formation 
of hydroxo- and oxo- bridged species: 
EQU Fe(H.sub.2 O).sub.6.sup.3+ .revreaction.Fe(OH)(H.sub.2 O).sub.5.sup.2+ 
+H.sup.+ 
EQU 2[Fe(OH)(H.sub.2 O).sub.5 ].sup.2+ .revreaction.[H.sub.2 O).sub.5 
FeOFe(H.sub.2 O).sub.5 ].sup.++++ +H.sub.2 O 
See F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry, 5th Ed., 
p. 717, Wiley-Interscience (1988). 
The other reason is that during the spraying, the etchant picks up oxygen 
from the air and recycles the Fe.sup.2+ product of etching back to 
Fe.sup.3+. In this process acid is consumed: 
EQU 2Fe.sup.2 +1/2O.sub.2 +2H.sup.+ .fwdarw.2Fe.sup.3+ +H.sub.2 O 
The pH is thus maintained throughout by metering in 50-100% H.sub.2 
SO.sub.4. 
The etching reaction consumes six moles of ferric ion per mole of 
molybdenum etched as is seen from the following equation: 
EQU 6Fe.sup.3+ +Mo+6MoO.sub.4.sup.2- .fwdarw.Mo.sub.7 O.sub.24.sup.6- 
+6Fe.sup.2+ 
Although MoO.sub.4.sup.2- does not exist at pH&lt;11, the formation of 
polymolybdates, such as Mo.sub.7 O.sub.24.sup.6- and other higher soluble 
polyions, is certain because no insoluble molybdenum bearing precipitates 
form during the etching, and the polymolybdates are the only soluble 
Mo(VI) species at these pH values. 
Ferric sulfate waste is easily treated. Merely raising the pH to greater 
than 2 precipitates iron as hydrogel oxide: 
EQU Fe.sup.3+ +3OH.sup.- .fwdarw.Fe(OH).sub.3 .dwnarw. 
and sulfate is precipitated by a lime bed: 
EQU Ca.sup.2+ SO.sub.4.sup.2- .fwdarw.CaSO.sub.4 .dwnarw. 
No poisonous or corrosive residues can thus pass an ordinary waste 
treatment facility. This offers economically superior handling ease over 
any cyano complex based etchant. 
The choice of ferric sulfate based etchants is not obvious. Not only is 
etch rate important, but also the quality of the etch profiles, 
particularly of hole walls through a mask. The holes should be uniform in 
shape. In contrast to the diamond-shaped holes shown in FIG. 1 obtained 
using ferric nitrate etch, ferric sulfate etch, yields round holes, as is 
shown in FIG. 2. Thus, ferric salts will etch molybdenum in different 
ways, depending on the identity of the counterion and the crystal 
structure/grain boundaries of the metal. Round holes, however, are 
required for the molybdenum masks. 
Another advantage of ferric sulfate is that unlike ferric nitrate and 
ferric chloride, this etchant can be heated to 9.degree. C. without any 
outgassing. Ferric nitrate emits acrid fumes of nitric acid even at 
50.degree. C., and ferric chloride outgasses HCl above 60.degree. C. This 
feature facilitates the constructions of ventilation for etching stations 
which can use ferric sulfate. 
The low pH of ferric sulfate does not cause any observed resist degradation 
or delamination in the conditions evaluated. Thus, by changing over the 
etchant from ferricyanide to ferric sulfate the resist delamination 
problems in molybdenum mask etching are solved. 
As mentioned before, the etch is recycled with adventitious atmospheric 
oxygen. Ozone can also be used if the need arises to recycle etchant more 
quickly due to heavier throughput. 
The oxidation/reduction potential (ORP) is thus controlled with ozone or 
oxygen throughput to maintain this at a constant level, thus ensuring 
constant [Fe.sup.3+ ]/[Fe.sup.2+ ] ratio, and thus uniform etching. pH is 
monitored and leveled simultaneously, with H.sub.2 SO.sub.4 additions. The 
etchant thus does not become depleted and is immortal. 
EXAMPLES 
EXAMPLE 1 
A ferric sulfate solution was made by dissolving 1,486 g Fe.sub.2 
(SO.sub.4).sub.3.nH.sub.2 O in 1,620 ml H.sub.2 O. The solution was 
.apprxeq.36 weight percent Fe.sub.2 (SO.sub.4).sub.3, corrected for the 
waters of hydration. A molybdenum foil weighing 18.8 g with a surface area 
of 233 cm.sup.2 was immersed in a beaker of the solution while it was 
maintained between 63.degree. C. and 69.degree. C. After 45 minutes, the 
foil weighed 16.6 grams. Thus, an etch rate of 3.4 mils/hour, or 1.7 
mils/hour/side was achieved. 
EXAMPLE 2 
A 5.1 mil thick molybdenum foil with photoresist covering all but a 
repeating pattern of approximately 2 mil square openings was suspended in 
the ferric sulfate solution. The solution temperature was held between 
57.degree. and 63.degree. C. After two hours in the solution the 
molybdenum foil was etched through with two repeating patterns of round 
holes. There was no evidence of preferential etching with respect to 
crystal or grain orientation, as evidenced by the uniform roundness of the 
holes. 
EXAMPLE 3 
A 5.1 mil thick molybdenum foil with the same pattern as in Example 2 was 
placed in a rotary spray etcher. A ferric sulfate solution, of .apprxeq.25 
weight percent maintained at 50.degree. C., was sprayed from the rotating 
manifold onto the stationary foil. After 45 minutes, a pattern had been 
etched through the molybdenum foil corresponding to the photoresist 
pattern. Thus, as expected, spray etching accelerated the etch rate about 
three times compared to beaker etch (at a higher temperature or 
57.degree.-63.degree. C.). 
EXAMPLE 4 
A 2.1 mil thick molybdenum foil was placed in the spray etcher with the 25 
weight percent Fe.sub.2 (SO.sub.4).sub.3 solution. The foil was patterned 
with 2.4 mil diameter vias and 1.6 mil lines. After 15 minutes of spraying 
at 50.degree. C., the pattern defined by the photoresist was etched 
through the foil. This demonstrated the etch feasibility of a foil 
.apprxeq.40% as thick as one in Example 3. 
In alternative embodiments, molybdenum articles including molybdenum masks, 
molybdenum thin films, molybdenum coatings on substrates, and the like may 
be substituted for the molybdenum foil of the examples. In additional, 
alternative embodiments, resistive coatings which are resistant to the 
etching solution of the invention, including refractory or unreactive 
metal coatings, polymer coatings, glass coatings, silicon oxide coatings, 
silicon nitride coatings, and the like, may be substituted for the 
photoresist of the examples. 
While the invention has been described in terms of a preferred embodiment 
and illustrated by several specific examples, those skilled in the art 
will recognize that the invention can be practiced with modification 
within the spirit and scope of the appended claims.