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Description  |
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TECHNICAL FIELD
The invention is concerned with a method of treating a surface of an object
to remove impurities. The method is particularly useful for treatment of
the surfaces of semiconductor wafers to remove resist and Group I and
Group II metals.
BACKGROUND ART
In the processing of wafers used in the manufacture of integrated circuits
and the like, it is necessary in various processing steps to accomplish
etching of certain areas of the wafer, to remove photoresist coating which
may be applied to the wafer, and to otherwise clean the wafer. Prior art
resist stripper processes are generally relatively slow, taking 30 to 60
minutes to remove the resist.
An apparatus which is directed toward these processing steps is disclosed
in U.S. Pat. No. 3,990,462, issued to Elftmann, et al. As disclosed
therein, wafers are disposed in cassettes which are in turn mounted on a
rotary turntable. A central post defines a plurality of pairs of ports,
each port being positioned to direct fluid therefrom onto an adjacent
wafer as that wafer passes the port due to the rotary motion of the
turntable. As disclosed in that patent, one port of each pair carries acid
or water, while the other port carried gas, such as gaseous nitrogen, or
air. Each pair of ports is positioned so that, with continuous fluid flow
from that pair, an atomized fog-spray is provided to the wafer.
It will be understood that processing of such wafers must be done in an
extremely even manner so that all parts of the wafer are processed
properly and uniformly. This has been a problem in the type of apparatus
disclosed in the above-cited patent. Furthermore, the processing of a
wafer in such a system is relatively slow.
In U.S. Pat. No. 3,970,249 to Singer, secondary streams are alternatively
applied to a stream of atomized particles by applying those particles to a
substrate. The system thereof, however, is not applicable to processing
semiconductor wafers, in particular the removal of material from a
semiconductor wafer through etching, stripping or cleaning.
It is also known to utilize a semiconductor wafer processing apparatus
which comprises a body defining first and second adjacent ports through
which fluid may flow from the body, the ports being positioned so that
fluid flow from the first port influences fluid flow from the second port,
means for varying fluid flow from the first port, and means for supporting
the wafer such that at least a portion of the fluid flow moves generally
across a major surface of a so-supported wafer.
Of more general interest in this area are U.S. Pat. Nos. 3,769,992, to
Wallestad and 3,727,620 to Orr.
Another process for cleaning semiconductor wafers is to contact them with a
liquid solvent of the desired composition. The composition may be stirred
or otherwise caused to flow across or against the surfaces of the wafers
to provide somewhat of a washing action.
In the past, resist materials have been removed by utilizing one or more of
the following: halogenated hydrocarbons, such as methylene chloride,
sulfides such as dimethylsulfide, amines and their derivatives such as
dimethylformamide, N-methyl-2-pyrrolidinone, glycol ethers such as
ethylene glycol monomethyl ether, ethynol and the acetates thereof,
ketones such as methyl ethyl ketone and acetone and materials such as
isopropyl alcohol, sulfuric acid-hydrogen peroxide mixtures, ammonium
persulfate and mixtures of caustic and phenol derivatives as well as by
various other materials. Cleaning of the wafer utilizing such liquid
solvents takes a good deal of time, often from 30 minutes to an hour
residence time in a bath of the liquid.
Other methods of stripping resist include O.sub.2 plasmas, U.V./ozone, plus
combinations, e.g., two operative processes. First a plasma step and then
a liquid acid dip is the most common practice and the only way to do a
complete resist stripping job.
There are several drawbacks with the use of the aforementioned resist
material removing compositions. One of the major disadvantages with the
aforementioned methods of stripping resist is that they do not remove deep
UV baked, implant-hardened or plasma-hardened photoresist, or they leave
behind traces of resist film and non-dissolved yield limiting contaminants
originating from the liquid stripping compositions. Other disadvantages
are undesirable flammability, volatility, odor or toxicity, drainage of
large quantities of possibly dangerous chemicals in city sewer systems and
attack of underlying metal films. Additionally, such strippers are not
effective against resist materials that are subject to a severe post
baking operation thereby limiting their usefulness. A disadvantage in
instances where plasmas or U.V./ozone is used is that particulates are
formed which must be liquid acid stripped. The plasma systems of choice
are very complex (involving complicated vacuum systems and automation) and
are also quite expensive.
The present invention is directed to solving one or more of the problems as
set forth above.
DISCLOSURE OF INVENTION
In accordance with the present invention, a method is set forth of treating
a surface of an object to remove impurities. The treating method comprises
positioning the object, with the surface exposed, within a treating
chamber; contacting ammonia vapor with water vapor adjacent the surface to
form a hot mixture of water, ammonia and ammonium hydroxide; and impinging
the hot mixture on the surface, the contacting being immediately prior to
and/or simultaneous with the impinging.
When one operates in accordance with the present invention, organic
materials, including resists (photo-, electron-beam, etc.), and including
hard UV baked and other particularly removal resistant resists, are
extremely efficiently and quickly (generally in less than about 5 minutes,
e.g., in about 30 seconds) stripped from wafers. And, the necessary
components, ammonia and water, can be readily vaporized and floated to
contact with one another adjacent the surface of the wafer whereby
operation is both reasonably easy and inexpensive. Furthermore, the final
waste product, ammonium hydroxide, can be readily disposed of, or consumed
in other processes.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood by reference to the figures of the
drawings wherein like numbers denote like parts throughout and wherein:
FIG. 1 illustrates, schematically, an embodiment in accordance with the
present invention; and
FIG. 2 illustrates, schematically, an alternate embodiment in accordance
with the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 illustrates an embodiment of the present invention wherein water via
tube 10 is admixed with ammonia which enters via a tube 12 into a mixing
chamber 14 and the resulting mixture proceeds via a tube 16 and therefrom
is impelled, as illustrated by the arrows, onto a surface 18 of a
semiconductor wafer 20, for example a silicon wafer. Hydrogen peroxide or
an oxidizer such as oxygen may be introduced via a tube 22 to the mixing
chamber 14 and, in accordance with the preferred embodiment of the present
invention, is so introduced.
FIG. 2 illustrates another embodiment of the present invention wherein the
water, ammonia and hydrogen peroxide vapors are introduced respectively by
individual tubes 24,16 and 28 and wherein the mixing occurs at the surface
18 of the wafer 20.
In operation it may be desirable to first impinge the ammonia alone onto
the surface 18 of the wafer 20. This can provide an initial solvating
action for resist materials and complexing of the Group I and Group II
species which may be present. Generally the temperature of the
water-ammonia mixture during impingement of the mixture upon the surface
18 of the wafer 20 will be from about 65.degree. C. to about 185.degree.
C. Preferably the impinging is carried out at a temperature of at least
about 100.degree. C., said temperature being the temperature at the
surface 18 of the wafer 20 rather than the general temperature within the
chamber in which the stripping occurs.
It is very desirable in the present invention to include oxygen or hydrogen
peroxide, preferably the latter, along with the ammonia and water. This
greatly increases the cleansing ability of the mixture and also
significantly increases the temperature of the mixture due to the
exothermicity of mixing of hydrogen peroxide and ammonia.
A significant scrubbing action is provided by the direct impingment of the
hot mixture on the surface 18 of the wafer 20. This serves to sweep away
any particulate impurities as well as significantly improving the
dissolution and reaction time with the resist.
The spent hydrogen peroxide, water and ammonia (generally in the form of
ammonium hydroxide by then) falls to the bottom of the chamber in which
the stripping is being carried out and can be readily disposed of.
Also in accordance with an embodiment of the present invention the hydrogen
peroxide is used along with the ammonia in the absence of water. Again,
very efficient stripping of photoresist results. The temperature of the
resulting mixture is generally in the range from about 65.degree. C. to
about 185.degree. C. although other temperatures can be used as well.
Generally it is preferred that the temperature be above about 100.degree.
C.
It should be noted that ammonia is a gas having a boiling point below
0.degree. C., and that water and hydrogen peroxide are easily vaporized
having boiling points of 100.degree. C. and about 152.degree. C.,
respectively.
The preferred temperatures for the ammonia, the water and hydrogen peroxide
gases are as follows: H.sub.2 O.sub.2 --(100.degree.-110.degree.) C.,
H.sub.2 O--(130.degree.-165.degree.) C. and NH.sub.3 about 100.degree. C.
The impinging is generally carried out at a pressure which falls in a range
from about ambient to about 15 PSIG, although the pressure is not critical.
The complete stripping of a hard baked resist layer from a surface 18 of a
wafer 20 generally takes from about 30 seconds to about 300 seconds. In
any event, it is usually completed in less than 5 minutes. Of course,
longer times can be utilized but such is unnecessary.
It is further possible to utilize a mixture of hydrogen peroxide and water
and to admix that mixture with ammonia. Again the reaction is exothermic
and cleaning of the surface 18 of the wafer 20 is efficiently carried out.
Generally high pressure distilled water may be utilized, for example,
introduced via the tube 10, to rinse off any residual cleaning chemicals
from the surface 18 of the wafer 20.
The invention will be better understood by reference to the following
example.
EXAMPLE
Referring to FIG. 1, hydrogen peroxide, water, and ammonia, the hydrogen
peroxide at a source temperature of 110.degree. C., the water source at a
temperature of 130.degree. C. and the ammonia at a temperature of about
100.degree. C., are flowed through tubes 10 and 12 and mixed in chamber
14, and then the resulting hot mixture flows through tube 16 and impinges
upon the surface 18 of a silicon wafer 20. A thermometer is in the
reaction chamber, which is seated, near the stream exiting the tube but
not in that stream. The surface 18 of the wafer 20 has a hard-baked resist
layer on it which has been hard-baked by heating it at 190.degree. C. for
30 minutes. The combined hydrogen peroxide, water, ammonia stream is
impinged on the surface 18 of the wafer 20 for approximately 180 seconds.
At the end of this time, flow is ceased other than flow of the water
vapor, which continues for an additional 300 seconds. The wafer is then
dried with ionized dry nitrogen for an additional 300 seconds. The wafer
on removal from the reaction chamber is completely free of photoresist.
The complete reaction conditions are as follows: mass flow ratio of
hydrogen peroxide, water and ammonia vapors=1:5:5, pressure of water vapor
30 psig, and pressure of ammonia 15 psig. Hydrogen peroxide is bubbled in
to the reaction chamber through a quartz bubbler capable of delivering the
proper mass flow at a bubbler pressure of 10 psig. The thermometer during
the 300 seconds shows a reading in the range of 65.degree. to 185.degree.
C.
INDUSTRIAL APPLICABILITY
The present invention provides very high speed cleaning of photoresist and
Group I and Group II metals from the surfaces 18 of semiconductor wafers
20.
While the invention has been described in connection with specific
embodiments thereof, it will be understood that it is capable of further
modification, and this application is intended to cover any variations,
uses, or adaptations of the invention following, in general, the
principles of the invention and including such departures from the present
disclosure as come within known or customary practice in the art to which
the invention pertains and as may be applied to the essential features
hereinbefore set forth, and as fall within the scope of the invention and
the limits of the appended claims.
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