Method of preventing photoresist residue on metal lines

A method of preventing photoresist residue on metal lines is disclosed herein. A strip recipe with a preheat step has been developed for use with the Applied Materials Mxp Centura. The preheat step is performed before the strip step. The preheat step can rapidly shorten the temperature balance time between the wafer and the strip chamber and make the photoresist flow to increase photoresist surface area. Therefore, the strip photoresist rate will be improved by higher wafer temperature in the first few strip cycles.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates to semiconductor fabrication, and more 
particularly, to a method of preventing photoresist residue from forming 
on metal lines. 
BACKGROUND OF THE INVENTION 
In semiconductor integrated circuit (IC) fabrication, metal lines are 
deposited to interconnect IC components and to connect IC components to 
pads. The metal lines are formed by physical deposition (such as by 
sputtering) of a layer of metal (such as aluminum). Photoresist is applied 
to the metal layer to define a pattern for forming lines that interconnect 
the desired components of the IC. Referring to FIG. 1, metal lines 10 are 
etched according to the pattern defined by the photoresist 14. One common 
apparatus used for such etching is the Applied Materials MxP Centura. 
The Mxp Centura has four chambers: two chambers are etch chambers and two 
chambers are strip chambers which are called ASP chambers. A typical metal 
etching process is performed in the etch chamber at about 80 degrees 
centigrade. During a metal line etching process, the metal lines are 
etched using a chloride plasma, which will result in chloride formed on 
the surface of the metal lines. Furthermore, polymers 12 are also usually 
formed on the sidewalls of metal lines 10. 
Referring to FIG. 3, after the metal lines 10 are etched in the etch 
chamber at etch step 30, the wafer is transferred from the etch chamber to 
the strip chamber (ASP chamber). The photoresist is then stripped in the 
strip chamber at a strip step 31, usually with a dry etching process at 
about 250 degrees centigrade. Afterward, the wafer is taken out of the 
strip chamber to cool the wafer. To remove the photoresist residue and 
polymer, two polymer strip steps (PRS) 32, 34 and one "Mattson" step 33 
between the two polymer strip steps are then performed. 
Referring to FIG. 4, a detailed flow diagram of strip step 31 of FIG. 3 is 
shown. A stabilization step 40 is performed, and gases, such as N.sub.2, 
H.sub.2 O and O.sub.2, are inserted into the strip chamber to stabilize 
the response condition. In the strip chamber, at a temperature of about 
250 degrees centigrade, an etching chloride process 41 and a stripping 
photoresist process 42 (together referred to as an strip cycle 43) are 
repeatedly performed several times to remove the chloride and the 
photoresist. Then, a pump is applied to take the response gases out of the 
strip chamber in a pumping step 44. 
A typical removing chloride recipe is described as follows: 500 sccm 
H.sub.2 O/1400 watts/2 Torrs/10 seconds (a gas flow of H.sub.2 O at about 
500 sccm; an energy of etching at about 1400 watts; a gas pressure of the 
chamber at about 2 Torrs; an removing time of about 10 seconds). A typical 
strip photoresist recipe is described as follows: 300 sccm H.sub.2 O/3500 
sccm O.sub.2 /200 sccm N.sub.2 /1400 watts/2 Torrs/20 seconds (a gas flow 
of H.sub.2 O at about 300 sccm; a gas flow of O.sub.2 at about 3500 sccm; 
a gas flow of N.sub.2 at about 200 sccm; an energy of etching at about 
1400 watts; a gas pressure of chamber at about 2 Torrs; an removing time 
of about 20 seconds). When the wafer is transferred form the etch chamber 
(80 degrees centigrade) to the strip chamber (250 degrees centigrade), the 
temperature difference between the wafer and the strip chamber is large. 
Indeed, there is a large temperature differential of about 170 degrees 
centigrade. Therefore, the time for the temperature to balance between the 
wafer and strip chamber is relatively long. Further, the strip cycle may 
be repeated a number of times, and may be repeated up to seven times. 
Even with this cleaning technique, photoresist residue is sometimes 
observed on the metal lines. With the more densely packed metal layout for 
sub-micron devices, the impact of photoresist residue will be more 
noticeable. The photoresist residue 16 on densely packed metal lines 10 
may remain on the metal lines 10, regardless of the number of times the 
stripping step is performed, as shown in FIG. 2. Further, the photoresist 
residue is difficult to detect during after-etch inspection (AEI). Also, 
in processing after the strip cycles, removal of the polymer 12 from the 
metal lines 10, as shown in FIG. 2, typically includes two stripping 
polymer steps (PRS) 32, 34. Additionally, it is necessary for a stripping 
photoresist residue step (Mattson) 33 to be added between the two PRS 
steps to remove photoresist residue remaining on the metal lines. Thus, 
there is an unmet need in the art for a method for preventing photoresist 
residue from forming on metal lines. 
SUMMARY OF THE INVENTION 
A method of preventing photoresist residue on metal lines is disclosed. The 
method comprises the steps of: providing a metal layer over a wafer; 
defining a pattern on the metal layer using a photoresist layer; etching 
the metal layer to form metal lines in an etch chamber using the 
photoresist layer as an etching mask; performing a preheat step in a strip 
chamber, the preheat step shortening the temperature balance time between 
the wafer and the strip chamber; and performing a strip step in the strip 
chamber to strip the photoresist layer.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Photoresist residue on metal lines is found occasionally by quality control 
(QC) inspection of ICs. The photoresist residue typically formed on dense 
metal lines, and it is difficult to strip even with multiple strip cycles. 
Further, it is often hard to find in after etch inspection (AEI). 
A method of preventing photoresist residue on metal lines is provided. A 
strip recipe with a preheat step has been developed for use with the 
Applied Materials MxP Centura to prevent the formation of photoresist 
residue. The method provides a preheat step after the etching metal step 
and before the strip step. The primary difference between the present 
invention and the prior art is the addition of a preheat step 50 before 
the strip recipe, as shown in FIG. 5. 
When a wafer is transferred from the etch chamber to the strip chamber, the 
preheat step 50 is performed first at 240 degrees centigrade. The recipe 
of the preheat step 50 is described as follows: 500 sccm H.sub.2 O/500 
sccm N.sub.2 /8 Torrs/10 seconds (a gas flow of H.sub.2 O at about 500 
sccm; a gas flow of N.sub.2 at about 500 sccm; a gas pressure of chamber 
at about 8 Torrs; a time of preheating of about 10 seconds). Afterward, in 
a stabilizing step 51, gases, such as N.sub.2, H.sub.2 O and O.sub.2, are 
inserted into the strip chamber to stabilize the response condition. An 
removing chloride step and a stripping photoresist step are then 
performed. 
The recipe of the removing chloride step 52 is described as follows: 500 
sccm H.sub.2 O/1400 watts/2 Torrs/10 seconds (a gas flow of H.sub.2 O at 
about 500 sccm; an energy of etching at about 1400 watts; a gas pressure 
of the chamber at about 2 Torrs; an removing time of about 10 seconds). 
The recipe of said stripping photoresist step 53 is described as follows: 
300 sccm H.sub.2 O/3500 sccm O.sub.2 /200 sccm N.sub.2 /1400 watts/2 
Torrs/20 seconds (a gas flow of H.sub.2 O at about 300 sccm; a gas flow of 
O.sub.2 at about 3500 sccm; a gas flow of N.sub.2 at about 200 sccm; an 
energy of etching of about 1400 watts; a gas pressure of the chamber at 
about 2 Torrs; an removing time of about 20 seconds). The removing 
chloride step 52 and the stripping photoresist step 53 are collectively an 
strip cycle 54 that is repeatedly performed several times to remove the 
chloride and photoresist. Then, a pump is applied to remove the response 
gases out of the strip chamber in a pumping step 55. After the process of 
FIG. 5, the result is as shown in FIG. 6 where the metal lines 60 have no 
photoresist residue. 
In the prior art, the temperature of the ASP chamber is at 250 degrees 
centigrade higher than the photoresist melting point. When a wafer is 
transferred from the etch chamber at 80 degrees centigrade to the strip 
chamber, the temperature difference between the wafer and the strip 
chamber is large. Therefore, the temperature balance time is very long, 
which in turn will increase the number of strip cycles needed to remove 
the photoresist. In the present invention, adding a preheat step can 
shorten the temperature balance time between the wafer and the strip 
chamber and make the photoresist to flow which increases the photoresist 
surface area. 
Furthermore, the stripping rate of the photoresist is increased with the 
increased temperature. Therefore, the ability of the photoresist stripping 
step is increased, and the strip rate will be improved by the higher wafer 
temperature in the first few strip cycles. Thus, the number of strip 
cycles will be reduced. However, it is important that the preheat time 
should be controlled carefully to avoid photoresist hardening. The preheat 
step is typically suitably performed between about 5 and 15 seconds, and 
preferably about 10 seconds. The lower end of the time range of about 5 
seconds allows throughput to be increased over the 10 seconds preheating 
embodiment. The upper end of the time range prevents the photoresist from 
excessive hardening. 
Additionally, with the temperature increased, polymers will harden. In the 
prior art strip recipe, the temperature of the strip chamber is at 250 
degrees centigrade. However, with the strip recipe of the present 
invention adding the preheat step, the temperature of the strip chamber is 
at 240 degrees centigrade. The temperature of the new strip recipe is 
lower than the prior art strip recipe. Therefore, the preheating step 
reduce the risk of polymer hardening and polymers and polymers are easily 
removed after the strip cycles are completed. Referring to FIG. 3, in the 
conventional method, after the etching metal step 30 and the stripping 
photoresist step 31 are performed, an etching polymer step (PRS) 32 with 
typically a wet etching process is performed. Afterwards, photoresist 
residue is stripped via a Mattson step 33 with typically a wet etching 
process. Another PRS step 34 is performed. In the conventional method, the 
strip recipe is at a higher temperature of about 250 degrees centigrade. 
This will cause the polymers to be harder. Therefore, it is necessary for 
two PRS steps 32, 34 to remove the polymers, and requires a Mattson step 
33 between the two PRS steps to remove the photoresist residue. 
Referring to FIG. 7, according to the present invention, after the etching 
metal step 70, the preheat step 71 is performed. Afterwards, the strip 
step 72 is performed. Because of the present invention, photoresist 
residue is nearly nonexistent on the metal lines. Therefore, the polymers 
can be removed merely by two stripping polymer steps (PRS) 73, 74, and it 
is not necessary to add the extra Mattson strip step to remove photoresist 
residue. 
Additionally, in the new strip recipe of the present invention, the 
temperature of the strip chamber is 240 degrees centigrade, lower than the 
prior art strip recipe temperature of 250 degree centigrade. While the 
temperature of the strip chamber is suitably reduced in the new strip 
recipe, polymers will be more easily removed than that in the old strip 
recipe. Thus, it may be that only one PRS step 83 is required to remove 
the polymers after the etching metal step 80 and the strip step 82, as 
shown in FIG. 8. 
Accordingly, the present invention teaches that a preheat step is added 
after the etching metal step and before the strip step. The preheat step 
is performed at about 240 degrees centigrade in the strip chamber. The 
preheat step serves to rapidly balance the temperature between the wafer 
and ASP chamber. Therefore, the ability of the photoresist stripping step 
can be increased in the first few strip cycles. The preheat step allows 
the strip time to be shortened and throughput to be increased. 
Additionally, the strip recipe with the preheat step can effectively 
simplify the process of removing polymers and photoresist residue after 
the strip cycles are completed. The preheat step enables substantially all 
photoresist residue to be removed in about five strip cycles. In the 
present invention, the process of removing the polymers and photoresist 
residue can be performed only by two PRS steps. Further, the preheat step 
oftentimes permits the polymers to be preferably removed merely by one PRS 
step. The specific recipe is just an example in the embodiment of the 
present invention. The main point is the concept of preheating. Therefore, 
a suitable temperature of preheating and a suitable preheating time can 
effectively remove photoresist residue in the strip step. Also, preheating 
can simplify the process of removing the polymers after the strip cycles 
are completed. 
Although specific embodiments including the preferred embodiment have been 
illustrated and described, it will be obvious to those skilled in the art 
that various modifications may be made without departing from the spirit 
and scope of the present invention, which is intended to be limited solely 
by the appended claims.