Method for removal of photoresist residue after dry metal etch

A method is provided for the removal of the surface layer of the residual photoresist mask pattern used for metal subtractive etching which uses the same reactor equipment but employs reactive fluorine-containing gases to form volatile compounds with the surface layer, so that subsequently a conventional oxygen plasma stripping process can be used for complete resist residue removal without requiring excessive temperature exposure of the integrated circuit devices.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the removal of photoresist residues produced in 
the fabrication of semiconductor integrated circuits and more particularly 
to the photoresist residues remaining after the dry subtractive etching of 
metal layer patterns. 
2. Description of the Prior Art 
In the fabrication of semiconductor integrated circuits, metal patterns 
used for interconnection and other purposes are formed by subtractive 
etching through a mask pattern of photoresist. If the etching of the metal 
is done by dry methods involving reactive gases such as halogen compounds, 
the resist mask residue remaining is often difficult to remove. This is 
usually done in a resist stripping chamber, in which an electrical 
plasma-activated oxygen gas environment is employed to react with and 
remove the organic resist material. The resist removal rate is strongly 
temperature-dependent, increasing greatly as temperature increases. 
However, high temperature during striping of resist is undesirable, since 
it promotes surface roughness increase of the remaining metal due to grain 
growth and recrystallization. If the resist residue is not completely 
removed, the manufacturing yield and reliability of integrated circuit 
devices is decreased, and the cost is increased due to the necessity for 
increased visual inspection. Although subtractive etching of metal 
patterns by reactive gases is discussed extensively by authoritative 
sources such as "VLSI Technology" by S. M. Sze, 2nd Ed., McGraw-Hill Book 
Co., Singapore 1988, neither the problem of resist residue removal nor the 
avoidance of excessive temperatures are discussed. There have also been 
disclosed in the art various Reactive Ion Etch (RIE) methods and materials 
for forming patterned aluminum containing conductor layers within 
integrated circuits. For example, Charvat et al. in U.S. Pat, No. 
5,202,291 disclose an RIE method for forming an aluminum containing 
conductor layer through employing a comparatively high carbon 
tetrafluoride (CF4) gas flow within the RIE etchant gas composition. In 
addition, Kanekiyo et al. in U.S. Pat. No. 5,320,707 disclose an RIE 
method for forming an aluminum containing conductor layer with uniform 
sidewall profiles. Finally, Hori et al. in U.S. Pat. No. 5,411,631 
disclose a method for forming a patterned aluminum containing conductor 
layer within an integrated circuit. The method employs both the RIE plasma 
etching characteristics and the sputter etching characteristics of a boron 
trichloride (13C13) containing etchant gas composition. However, none of 
the cited references refer to the difficulty of removing photoresist 
residue after RIE processing or its deleterious effect on the properties 
of the aluminum containing conductor layer in the integrated circuit. 
It has been observed that the most intractable portion of the resist 
residue towards easy removal is the top surface region, which apparently 
interacts with the reactive environment during metal subtractive etching. 
This surface region is thought to be resistant to oxygen during the 
stripping process, forming non-reactive non-volatile substances which 
impede the reaction of the rest of the resist residue and thus interferes 
with stripping. It is further thought that the surface region consists of 
oxidized species derived from the metal during reactive etching which is 
not reactive towards the oxygen plasma until higher temperatures are 
employed. Thus, the present invention is directed towards the goal of 
providing within integrated circuit manufacture a Reactive Ion Etch (RIE) 
method for sequentially forming a patterned aluminum containing conductor 
layer from a blanket aluminum containing conductor layer and subsequently 
stripping a photoresist layer which had defined the said pattern without 
roughening or otherwise damaging the aluminum containing conductor layer 
by such subsequent stripping processes. 
SUMMARY OF THE INVENTION 
It is a principal object of the invention to provide a method for enhancing 
the stripping of photomask residues after subtractive metal etching with 
reactive gases without requiring excessive high temperatures or extensive 
rework cycles. In accordance with the object of the invention, a method is 
provided for the removal of the surface layer of the residual photoresist 
mask pattern used for metal subtractive etching which uses the same 
equipment but employs fluorine-containing reactive gases to form volatile 
compounds with the surface layer, so that subsequently a conventional 
oxygen plasma stripping process can be used for complete resist residue 
removal, without requiring excessive temperature exposure of the 
integrated circuit devices under fabrication.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now more particularly to FIG. 1, there is shown a schematic 
diagram of the photoresist residuel remaining after subtractive etching of 
a metal patern 2 in the fabrication of an integrated circuit device 3. On 
the surface of the remaining photoresist is a layer of material 4 which is 
difficult to remove in the conventional photoresist stripping process, 
which is a reaction to form completely volatile compounds between the 
photoresist and oxygen gas in a plasma-activated reactor chamber. During 
the metal subtractive etching process for which the photoresist pattern 
served as the etching mask, it is thought that certain reaction products 
of the metal etching step and interactions between the reactive plasma and 
the photoresist surface have combined to form thereon a layer of material 
which is not readily converted to volatile substances in the subsequent 
photoresist stripping process in an oxygen plasma. Hence, the removal of 
photoresist residues is not accomplished to the degree of completion 
required in the manufacture of integrated circuit devices, with subsequent 
deleterious impact on device yield and reliability. The removal of 
photoresist residues under these conditions can be expedited by raising 
the power input and temperature of the device, but this causes unwanted 
side effects such as the increased atomic surface migration and 
recrystallization of the metal layers, particularly those of aluminum, 
with the resulting undesirable formation of hillocks or surface 
irregularities on the metal layers. 
The process of this invention employs a pre-stripping step to expose the 
photoresist residue surface to a reactive gas for a short time at low 
radiofrequency (RF) power input to allow the surface material to be 
removed to the extent necessary to permit subsequent photoresist stripping 
in an oxygen plasma. The process found to be particularly useful is shown 
in the process flow given in FIG. 2. The reactive gas is a fluorine 
containing gas such as carbon tetrafluoride (CF4) or sulfur hexafluoride, 
for example. The first step of the process is the metal subtractive 
etching step 20. The second step of the process 22 is carried out in the 
stripping chamber of the reactor used for subtractive metal etching as a 
post-etching step with minimum cost impact on manufacturing. The final 
step of the process 24 is the photoresist stripping step in oxygen gas 
(O2). The details of the postetching step 22 are given in Table I: 
TABLE I 
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Reactive gas 
RF Power input, 
Exposure time, 
Gas Pressure, 
Flow rate, SCCM 
watts seconds milliTorr 
______________________________________ 
20-40 30-500 10-20 30-200 
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The short exposure time in the metal etch chamber results in high 
throughput for this process as well as the maintenance of temperature 
sufficiently low, for example, below 220 degrees C., that no deleterious 
side effects result to the metal and other component portions of the 
integrated circuit device under fabrication. The RF power input is in the 
microwave region, preferably at a frequency of 2.450 gigaHertz (gHz). 
While the invention has been particularly shown and described with 
reference to the preferred embodiment thereof, it will be understood by 
those skilled in the art that various changes in form and details may be 
made without departing from the spirit and scope of the invention.