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Description  |
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BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to a positive photoresist developer system and
improved method for developing positive photoresist layers, and more
particularly, to an improved two-step developer system used in the
processing of alkali soluble resin--diazo ketone photoresists to increase
the contrast of the developed photoresists. The process of the invention
involves the use of a two bath system wherein the substrate coated with
positive photoresist is exposed, then immersed in a "predip" bath, rinsed,
and then immersed in the second of the two bath system developer baths.
Positive photoresists typically consist of a novolac resin and a
diazonaphthaquinone sulfonic acid ester dissolved in an organic solvent.
The resist is usually applied by a spin casting technique to silicon
wafers which may have a thin coating of silicon dioxide, aluminum, silicon
nitride, glass or other material typically used in the fabrication of
integrated circuits.
The coated wafers are exposed to light through a mask which provides a
pattern for building the various circuit components on the wafer. The
pattern is developed by dipping, spraying or placing a puddle of developer
solution on the wafer. During spray or puddle development the wafer may be
stationary or spun, but excess developer is usually spun off by
accelerating the wafer to about 5000 rpm.
The base used in preparing the developer formulation may be selected from
the general class of water soluble bases known for use for this purpose
and include, for example, metal hydroxides, e.g. sodium hydroxide,
potassium hydroxide, sodium silicate, potassium carbonate, and the like.
It is highly desirable that during development there be as little attack as
possible on the unexposed resist under conditions which lead to complete
removal of exposed resist layer down to the substrate. The lower the rate
of dissolution of the unexposed resist relative to the exposed resist, the
higher the contrast. High contrast gives a pattern with well defined
vertical walls resulting in an accurate reproduction of the mask
dimensions in the photoresist coating.
The present invention has application in the "lift-off" operation which is
a simplified process during semiconductor manufacture for putting aluminum
metal interconnects into place. The undercut resist profile comprises a
shadow mask which permits aluminum vapor to be deposited on the substrate
in a pattern determined by the developed openings in the resist film. The
aluminum deposited on the resist is removed when the resist is dissolved
away using a suitable solvent. The remaining aluminum is left firmly
bonded to the substrate in a predetermined pattern of interconnects.
The "lift-off" process is described in U.S. Pat. No. 4,212,935. Control of
the wall profile, according to the teaching of that patent, is achieved by
predipping the resist coated wafer, after exposure but prior to
development, into an organic solvent such as chlorobenzene. The use of
toxic, combustible organic solvents is undesirable because of health,
safety, and environmental concerns. The present invention involves the use
of an aqueous solution containing a small amount of fluorochemical or
carboxylated surfactant. Such aqueous solutions are preferred, from a
health and safety viewpoint to organic solvents.
An unexpected feature of the double dip process using aqueous metal
developers is that it permits control of the wall profile from vertical to
undercut depending on the exposure; this ability to produce a profile
which is broader at the top and narrower at the bottom is particularly
important for use in the "lift-off" process.
In co-pending U.S. patent application,, Ser. No. 505,571 filed on June 17,
1983, now abandoned and entitled "High Contrast Photoresist Developer," it
is disclosed that the incorporation of a fluorochemical surfactant in an
aqueous alkali metal development bath provides high contrast. In that
system, which involves a single dip development with alkali metal
developers, it has been observed that as the additional exposed wafers are
developed, the contrast and sensitivity of the resist tend to deteriorate
with prolonged use of the bath. It is thus apparent that a need exists for
a safe and efficient system wherein the contrast and sensitivity of the
photoresist remains essentially unchanged over the life of the bath.
SUMMARY OF THE INVENTION
In accordance with the invention, a positive photoresist metal ion aqueous
base developer is provided that gives high contrast to the photoresist.
The gamma obtained is greater than 5 and usually in excess of 7. The high
contrast provides good linewidth control and process latitude in
photoresist imaging.
In essence the invention resides in a novel process using metal ion
developer compositions in a two bath system which provides high contrast
images and long developer bath life that does not decrease significantly
over the life of the development bath. The elements of this invention
comprise a two bath system, i.e. (i) a predip solution containing aqueous
alkali metal base and a fluorochemical or carboxylated surfactant adjusted
to a concentration that does not necessarily give development, and (ii) a
developer solution containing aqueous alkali metal hydroxide and,
optionally, a fluorochemical or carboxylated surfactant adjusted to a
concentration that provides a substantially complete development.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The photoresists employed with the developer of the present invention are
those sensitizer-resin compositions in which the exposed portions of the
composition become more soluble upon exposure.
Suitable sensitizers used in positive photoresists of this kind are diazo
ketones having diazo and keto groups at adjacent positions on the
molecule, such as the quinone-diazide sulfonic acid derivatives which are
described in U.S. Pat. Nos. 2,958,599; 3,046,110; 3,046,114; 3,046,116;
3,046,118; 3,046,119; 3,046,121; 3,046,122; 3,046,123; 3,106,465;
3,148,983; 3,635,709; 3,711,285; 4,174,222 which are hereby incorporated
by reference. Examples of typical photosensitive compounds used in
positive photoresists are shown in Table I.
TABLE I
__________________________________________________________________________
##STR1##
##STR2##
##STR3##
##STR4##
##STR5##
##STR6##
##STR7##
##STR8##
##STR9##
##STR10##
##STR11##
##STR12##
##STR13##
##STR14##
##STR15##
##STR16##
##STR17##
##STR18##
__________________________________________________________________________
The photoactive compound acts to decrease the solubility of the resin. Upon
exposure to light, the photoactive diazonapthoquinone undergoes a chemical
reaction to form a carboxylic acid which increases the rate of
solubilization of the photoresist in the exposed areas.
##STR19##
In general, photosensitizers which contain more than one
diazonaphthoquinone group are preferred because they appear to provide
higher contrast photoresists. Suitable alkali soluble resins may be
employed in the positive photoresists. Those contemplated by this
invention are the prepolymerized phenolic-aldehyde resins, e.g., phenol
formaldehyde, which are known as novolac resins and are available
commercially. Resins of this kind are disclosed, for example, in U.S. Pat.
Nos. 3,201,239; 3,868,254; 4,123,219 and 4,173,470, the disclosures of
which are incorporated herein by reference. These phenolic-aldehyde
resinous compounds must be soluble in organic solvents and aqueous
alkaline solutions.
A number of phenolic compounds and aldehyde or aldehyde producing compounds
will yield novolac resins through well-known syntheses. Phenolic compounds
that may be used include, but are not limited to, phenol, xylenol, cresol,
resorcinol, naphthol, hydroquinone, alkyl phenols and halogenated phenols.
Illustrative of the aldehydes and aldehyde producing compounds that may be
used, but not limited to, are formaldehyde, actaldehyde, paraformaldehyde,
formaline, acrolein, crotonaldehyde and furfural.
The predip bath applied in accordance with the invention comprises an
aqueous basic solution containing as an essential constituent a
fluorochemical or carboxylated surfactant.
Illustrative of the various bases that can be used in adjusting the
alkalinity of the basic solution include, for example, potassium
hydroxide, potassium carbonate, sodium hydroxide, sodium silicate, lithium
hydroxide, tetramethyl ammonium hydroxide,
(2-hydroxyethyl)-trimethyl-ammonium hydroxide, tetra-(2-hydroxide, hydroxy
ethyl)-ammonium hydroxide, tetraethylammonium hydroxide or ammonium
hydroxide and the like. The use of potassium carbonate and potassium
hydroxide in particular afford practical advantages.
The fluorocarbon surfactants that can be used in the predip bath are those
characterized by the formula R.sub.f -Y-(CH.sub.2 CH.sub.2 O).sub.m R,
wherein Y is a radical selected from the group --CH.sub.2 CH.sub.2 O--,
--SO.sub.2 NR', SO.sub.3, SO.sub.2 N(R')CH.sub.2 CO.sub.2, CO.sub.2 and
--CO--NR' wherein R.sub.f is either a straight or branched chain of the
formula C.sub.p F.sub.2p+1 where p is an integer from 3 to 17; and wherein
R is hydrogen or an acyl or alkyl radical of 1 to 30 carbon atoms and m is
an integer of 2 to 26 and preferably where m is an integer of 5-26, and R'
is hydrogen or an alkyl radical of 1-6 carbon atoms. Illustrative of
fluorocarbon surfactants of this kind are those included in Table II as
well as, more mixtures thereof.
TABLE II
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Example Fluorosurfactant
______________________________________
a CF.sub.3 (CF.sub.2).sub.6CH.sub.2 CH.sub.2 O(CH.sub.2 CH.sub.2
O).sub.4 H
##STR20##
c CF.sub.3 (CF.sub.2).sub.3SO.sub.2NH(CH.sub.2 CH.sub.2 O).sub.14CH
.sub.3
d CF.sub.3 (CF.sub.2).sub.8SO.sub.2O(CH.sub.2 CH.sub.2 O).sub.8CH.s
ub.3
e CF.sub.3 (CF.sub.2).sub.5COO(CH.sub.2 CH.sub.2 O).sub.9C.sub.4
H.sub.9
f
##STR21##
g
##STR22##
______________________________________
The carboxylated surfactants which may be employed in the invention
generally conform to the formula:
R--O--(C.sub.n H.sub.2n O).sub.m R.sub.1 COOX
wherein R is a fatty group of 6 to 18 carbon atoms, R.sub.1 is a C.sub.1 to
C.sub.3 alkyl substituent, n has a value of 2 to 4, m has a value of 1 to
100 and X is a cationic substituent select from the group consisting of
H.sup.+, Na.sup.+, K.sup.+, Li.sup.+, NH.sub.4.sup.+, diethanolamine,
triethanolamine Al.sup.++, Cu.sup.++, Ca.sup.++, Mg.sup.++ and Sr.sup.++.
Typical carboxylated surfactants are identified in Table III in which the
values for R and N for the Above formula are set out.
TABLE III
______________________________________
R N
______________________________________
1 C13 7
2 C12-C15 5
3 C12 12
4 C13 11
5 C13 18
6 C16-C18 4
7 C16-C18 8
8 C16-C18 24
9 C18 12
10 C16 12
11 i-C18 5
12 i-C18 10
13 C6 1
14 C2 4
15 C4 4
______________________________________
The surfactants used in the development bath may be selected from the same
group. The concentrations of surfactant in the developer in the range from
about 0.0001% to 1.0% of the developer may be used with advantage. The
more effective level of the fluorocarbon or carboxylated surfactant ranges
from 0.005 to 0.5%. The preferred range is 0.001 to 0.1%. The
concentration of alkali metal hydroxide must be varied accordingly to
maintain the sensitivity level. The more surfactant, the more concentrated
the developer needs to be.
The bases useful in the developer both are the metal hydroxide type.
Illustrative of these bases are sodium hydroxide, potassium hydroxide,
sodium silicate, and lithium hydroxide. The developer bath operates at a
pH of at least 9 and generally at a pH above about 10.5. Preferably a pH
above 12 is employed.
The concentration of surfactant used in the predip bath may range from
0.0001 to 1.0% of the predip bath. In general, the concentration of
surfactant is greater in the predip bath than in the developer bath. The
more effective level is between 0.001 to 1%. The disadvantage of higher
levels of surfactant in the predip bath is primarily loss of excess
surfactant.
The concentration of base of the kind referred to above in preparing the
aqueous predip bath solution containing the fluorochemical or carboxylated
surfactant should be sufficiently low that there is no significant
dissolution of the photoresist film resulting in measurable film loss.
Since photoresists vary in the ease with which they dissolve in aqueous
alkali solutions, it will be necessary to adjust the base concentration to
the solubility characteristics of the resist. The concentration of base in
the predip solution may range from about 0.1% to 100% of the concentration
of base in the developer. The preferred range is between 10% and 25% of
the base strength in the developer.
The following examples are illustrative of the invention. The enumeration
of details in the examples should not be interpreted as limitations except
as may be expressed in the appended claims.
TYPICAL APPLICATION OF THE PHOTORESIST TO THE SUBSTRATE
A photoresist coating was prepared by spin coating a novolac resin and the
photosensitizer of the kind identified by formula 15 of Table I. The
substrate used was a silicon wafer that had been subjected to a
200.degree. C. dehydration bake for at least sixteen (16) hours; and then
treated with a 50% hexamethyldisilazane solution in xylene for twenty
seconds immediately prior to coating. The wafers were spun so as to
provide a 1 micrometer (.mu.m) thick film. The coated wafers were baked at
100.degree. C. for 30 minutes in a forced air convection oven. After
baking, the wafers were exposed to ultraviolet light through an Opto-Line
step tablet resolution mask with windows of various optical density so as
to provide various exposure levels on the same wafer. The incident
exposure was such that a range of exposure from no exposure to exposure
sufficient for the resist to develop to the substrate was used.
EXAMPLE I
METAL ION DEVELOPER AND METAL ION PREDEVELOPMENT SOLUTION
The photoresist is coated, prebaked, and exposed in the conventional manner
such as that described in the above spin coating procedure. The wafers are
then immersed in a predip (predevelopment) solution of 0.135N KOH with
0.026% Zonyl FSN surfactant for 30 seconds, rinsed with water for 1
minute, then developed with a solution of 0.271N KOH, 0.0125% Zonyl FSN,
and 0.0045% of novolac resin for 1 minute.
Immersion of the wafers in the 0.135N KOH with 0.026% Zonyl FSN surfactant
predip solution alone does not develop the resist. Rather, the predip step
coats or modifies the surface allowing the developer to remove the exposed
photoresist and maintain a constant sensitivity and high contrast
throughout the life of the developer. The sensitivity throughout the bath
life was 16-18 mJ/cm.sup.2. The gamma or contrast was 8-7.
EXAMPLE II (Comparative )
The procedure of Example I was repeated except that the predevelopment
solution is not used. The gamma or contrast is found to change from 8 to 3
during the bath life of the developer.
SEM photographs of the image show a profile that can be used in a metal
lift-off process when the metal ion pretreatment solution is used with the
metal ion developer. The profile is similar to that obtained using
chlorobenzene to pretreat the photoresist.
EXAMPLE III
The photoresist is coated, prebaked, and exposed in the conventional manner
such as that desribed in the typical application above. The wafers were
then immersed in a predip solution of 0.036N potassium hydroxide with 83
ppm of alkyl polyoxyethylene carboxylate surfactant (Formula 1, Table III)
for 30 seconds, rinsed with water for 30 seconds, then developed in 0.2N
potassium hydroxide for 75 seconds.
The sensitivity was 18 mj/cm.sup.2 and the contrast was 5.0 throughout the
bath life of the developer.
EXAMPLE IV
The procedure of Example III was repeated except that surfactant (Formula
2, Table III) is used in the predip solution.
The sensitivity was 25 mj/cm.sup.2 and the contrast was 7 throughout the
life of the developer.
In addition to the alkyl polyoxyethylene carboxylate employed in Examples
III and IV, various other carboxylated surfactants, the chemical formulas
of which are represented in Table III, may be used, e.g. lauryl
dodeca(ethyleneoxy) 2-acetic acid, 2[hexylpoly(ethylene)]acetic acid and
the like.
While the invention has been described with reference to positive
photoresists sensitive to ultraviolet light (290-500 nm), the novel
developer is applicable also to positive electron beam, X-ray, ion beam,
deep ultraviolet (220-290 nm) light and other radiation sensitive resists.
The invention has been particularly described with reference to preferred
embodiments thereof; it will be understood by those skilled in the art,
however, that changes in form and details may be made therein without
departing from the spirit and scope of the invention.
* * * * *
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