Sulfur trioxide vapor phase stripping

A method is set forth of treating the surface of an object such as a semiconductor wafer to remove impurities. In particular, the method serves to strip resist in a short period of time, of the order of 30 seconds. The object is positioned with the surface exposed within a treating chamber. Water vapor is contacted with sulfur trioxide vapor adjacent the surface to provide a hot mixture comprising sulfur trioxide, water and sulfuric acid. The hot mixture is impinged onto the surface. The contacting is immediately prior to and/or simultaneous with the impinging. Photoresist is quickly and efficiently removed in accordance with this process. Energy requirements are relatively low since the components are easily vaporized.

BEST MODE FOR CARRYING OUT THE INVENTION 
The present invention is based upon the unexpected observation that if one 
impinges onto the surface of a wafer having a resist material thereon a 
hot mixture comprising sulfur trioxide, water and sulfuric acid, which 
mixture is made by contacting water vapor with sulfur trioxide vapor 
adjacent the surface of the wafer upon which it is being impinged, the 
resist material is stripped completely away, even if the resist material 
is deep UV baked resist. 
Referring to FIG. 1 an embodiment of the method of the present invention is 
illustrated wherein water vapor is flowed under pressure into a tube 10 
and sulfur trioxide vapor is flowed under pressure into a tube 12. The two 
mix together in a chamber 14 defined by a tube 16 from which the combined 
vapors go forward and impinge upon a surface 18 of a semiconductor wafer 
20. If desired an additional material, for example hydrogen peroxide or 
some other oxidizer such as oxygen, can be admixed with the water and 
sulfur trioxide in the chamber 14. The hydrogen peroxide oxidizer or an 
inert carrier gas (e.g., nitrogen) or oxygen can be added via an auxiliary 
tube 22 to the tube 16. An inert carrier gas can also or alternatively be 
introduced along with the water vapor and/or the sulfur trioxide vapor. 
FIG. 2 illustrates a slightly different embodiment of the present invention 
wherein the water, sulfur trioxide, and other components, for example 
hydrogen peroxide, when present, proceed from respective individual tubes 
24,26 and 28, respectively onto the surface 18 of the wafer 20. 
Alternatively, the tubes 24 and 26 may both be used to introduce water 
vapor, e.g., along with an inert gas (or oxygen), and the tube 28 can be 
used to introduce the sulfur trioxide. 
It is preferable if the water-sulfur trioxide mixture, when it impinges 
upon the surface 18 of the wafer 20, is still hot. Thus, it is preferred 
that the water and sulfur trioxide be mixed relatively close to the 
surface 18, or even at the surface 18. Furthermore, it is important that 
the mixture impinge upon the surface 18 since this provides needed 
scrubbing action. 
When water and sulfur trioxide vapor comes together the result is the 
formation of sulfuric acid whereby the mixture of water and sulfur 
trioxide also includes sulfuric acid. Furthermore, the entire reaction is 
quite exothermic whereby any resist on the surface 18 is readily removed. 
Note that it is important that the hot mixture which impinges upon the 
surface 18 of the wafer 20 includes sulfur trioxide and water as well as 
sulfuric acid. The water and sulfur trioxide can continue to react even as 
they impinge upon the surface 18 whereby heat is generated in situ on the 
surface 18, just where such heat is most needed to strip off hard baked 
photoresist. Furthermore, it is believed that the chemical reaction of 
sulfur trioxide is important in stripping off the photoresist as opposed 
to the action of hot sulfuric acid because of the intense process going on 
between SO.sub.3 (v) and H.sub.2 O(v) as sulfuric acid is formed. 
The water vapor which is introduced via the tube 10 is superheated to a 
temperature above 130.degree. C., varying the range from about 100.degree. 
C. to about 200.degree. C., although higher temperatures can be used as 
well. The sulfur trioxide vapor is at a temperature above its boiling 
point of approximately 45.degree. C., and will generally be in the range 
from 45.degree. C. to about 125.degree. C., although higher temperatures 
can also be used. 
While the sulfur trioxide vapor and the water vapor introduced may be at 
relatively low temperatures, for example about 45.degree. C. for the 
sulfur trioxide vapor and just a bit above 100.degree. C. for the water 
vapor, the resulting temperature in the chamber adjacent the surface 18 of 
the wafer 10 is quite high, of the order of 150.degree. C. to 350.degree. 
C., and generally above about 250.degree. C. The temperature elsewhere in 
the chamber 14 can be lower. 
It is important to note that the sulfur trioxide and the water vapor can be 
readily vaporized due to their relatively low boiling points whereas 
vaporization of sulfuric acid, should it be attempted, would require 
heating the compounds to a temperature of above about 330.degree. C., the 
boiling point of sulfuric acid. Thus, the present process is relatively 
low in energy consumption. 
A variety of mass flows can be used depending on the orifice size to 
provide the proper mass velocity impinging on the surface 18 of the wafer 
20. For example, mass flows from 10 or less to hundreds of liters of gas 
per minute can be used. 
It is generally preferable in accordance with the present invention to 
operate at reasonably high temperatures when the sulfur trioxide, water, 
sulfuric acid mixture is impinged upon the surface 18 of the wafer 20. 
Thus, it is preferred that the temperature be above about 150.degree. C., 
and more preferably above about 250.degree. C. Note that the temperature 
referred to is not the overall temperature of the chamber 14 in which the 
stripping operation is being carried out, but rather the temperature of 
the mixture at the surface 18 of the wafer 20. Note also that the 
impinging of the mixture onto the surface 18 of the wafer 20 need not be 
at an extremely high velocity. However, it is preferred that the impinging 
be at sufficient velocity to provide a scrubbing action to dislodge any 
particulate materials as well as to aid in speeding up the removal of the 
resist. 
The entire process is generally carried out in an appropriate chamber and 
the resulting sulfuric acid is recoverable utilizing a tap at the bottom 
of the chamber. 
Hydrogen peroxide or another added cleaning agent, indicated by "X" in 
FIGS. 1 and 2, may also be impinged upon the surface 18 of the wafer 20. 
If hydrogen peroxide is utilized this serves to form H.sub.2 SO.sub.5 by 
the interaction of the sulfuric acid which is formed with the hydrogen 
peroxide. However, it is not yet necessary to include hydrogen peroxide in 
order to obtain complete removal of hard baked resist coatings. 
The present process removes not only photoresist coatings and other organic 
contaminants, but also alkali metals and alkaline earth metals from the 
surface 18 of the wafer 20. 
Following the above set forth process the wafers are generally bulk rinsed 
with high purity and high pressure distilled water vapor by simply 
discontinuing flow through the tube 12, and the tube 22 if that is being 
used, and continuing flow through the tube 10. Thereafter, the wafer can 
be dried, for example with hot ionized nitrogen gas. 
The invention will be better understood by reference to the following 
illustrative example. 
EXAMPLE 
Water and sulfur trioxide vapor, the water vapor at a temperature of about 
105.degree. C. and the sulfur trioxide vapor at a temperature of 
70.degree. C. were flowed through respective tubes 10 and 12 and mixed in 
chamber 14 and then the resulting hot mixture flowed through tube 16 and 
impinged upon the surface 18 of a silicon semiconductor wafer 20. A 
thermometer was in the reaction chamber, which was sealed, near the stream 
exiting the tube 16 but not in that stream. The surface 18 of the wafer 20 
had a hard baked resist layer on it which had been hard baked by heating 
it to 190.degree. C. for 30 minutes. The combined water, sulfur trioxide, 
sulfuric acid stream was impinged on the surface 18 of the wafer 20 for 
approximately 30 seconds, at the end of which time flow was ceased other 
than flow of the water vapor which continued for an additional 300 
seconds. The wafer on removal from the reaction chamber was examined and 
found to be completely free of photoresist. The complete reaction 
conditions were as follows: mass flow ratio of sulfur trioxide to 
water=1.5, mass flow rate of 20 to 40 liters of gas per minute, pressure 
of water vapor=10 psig, pressure of sulfur trioxide vapor=6 to 7 psig. The 
thermometer during the 30 seconds went from an initial temperature of 
60.degree. C. to a final temperature of 185.degree. C. 
Industrial Applicability 
The present invention provides a method for stripping resists and other 
materials from wafer surfaces 18. Reaction is extremely rapid, taking only 
a few seconds as compared to 30 to 60 minutes with state of the art plasma 
and wet stripping processes. The chemicals utilized are relatively easy to 
handle and the waste product is relatively eaily disposed of. 
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.