Method of dry etching A1Cu using SiN hard mask

A new method of etching AlCu or AlSiCu lines is described. Semiconductor device structures are provided in and on a semiconductor substrate. The semiconductor device structures are covered with an insulating layer. A layer of AlCu or AlSiCu is deposited overlying insulating layer. A silicon nitride or titanium nitride/silicon dioxide layer is deposited overlying the metal layer wherein a hard mask is formed. The hard mask is covered with a layer of photoresist which is exposed to actinic light wherein the hard mask prevents reflection of the actinic light from its surface. The photoresist layer is developed and patterned to form the desired photoresist mask. The hard mask is etched away where it is not covered by the photoresist mask leaving a patterned hard mask. The AlCu or AlSiCu layer and the barrier layer not covered by the patterned hard mask are etched away to form metal lines having an outwardly tapered profile.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The invention relates to a method of photolithographic etching of AlCu 
lines, and more particularly, to a method of photolithographic etching of 
AlCu lines having excellent critical dimension control and resulting in a 
tapered profile of the AlCu lines in the manufacture of integrated 
circuits. 
(2) Description of the Prior Art 
Due to the limitation of poor etch rate selectivity to photoresist, a thick 
photoresist mask is often needed during AlCu etching. As a result, the 
depth of focus in photolithography processes becomes even more 
challenging. Conventionally, a layer of titanium nitride is deposited over 
the AlCu layer as a barrier and anti-reflective coating layer (BARC). 
Conventional etchant gases are BCl.sub.3 and Cl.sub.2. These gases have 
been found to be insufficient in producing polymer on the sidewalls of the 
AlCu lines which would prevent undercutting during ethching. 
U.S. Pat. No. 4,915,779 assigned to Motorola uses a protective oxide layer 
over AlCu and etches both the oxide and the AlCu layers under vacuum. U.S. 
Pat. No. 5,449,639 to Wei et al teaches the use of a disposable titanium 
nitride anti-reflective coating in etching an underlying metal layer. U.S. 
Pat. No. 4,412,885 to Wang et al shows a method of etching an Al layer 
using BCl.sub.3 and Cl.sub.2 with a dopant gas containing oxygen and a 
fluorocarbon. U.S. Pat. No. 4,511,429 to Mizutani et al teaches dry 
etching of Al using a hydrogen-containing gas. U.S. Pat. No. 4,919,748 to 
Bredbenner et al shows a method for tapered etching of metal layers using 
chlorine and trifluoromethane gases. 
SUMMARY OF THE INVENTION 
A principal object of the present invention is to provide an effective and 
very manufacturable method of etching AlCu or AlSiCu lines. 
Another object of the present invention is to provide a method of etching 
AlCu or AlSiCu lines wherein photolithographic depth of focus is not a 
problem. 
A further object of the present invention is to provide a method of etching 
AlCu or AlSiCu lines wherein there is no undercutting of the AlCu or 
AlSiCu. 
Yet another object of the present invention is to provide a method of 
etching AlCu or AlSiCu lines wherein a tapered AlCu or AlSiCu line profile 
is achieved. 
In accordance with the objects of this invention a new method of etching 
AlCu or AlSiCu lines is achieved. Semiconductor device structures are 
provided in and on a semiconductor substrate. The semiconductor device 
structures are covered with an insulating layer. A layer of AlCu or AlSiCu 
is deposited overlying insulating layer. A silicon nitride or titanium 
nitride/silicon dioxide layer is deposited overlying the metal layer 
wherein a hard mask is formed. The hard mask is covered with a layer of 
photoresist which is exposed to actinic light wherein the hard mask 
prevents reflection of the actinic light from its surface. The photoresist 
layer is developed and patterned to form the desired photoresist mask. The 
hard mask is etched away where it is not covered by the photoresist mask 
leaving a patterned hard mask. The AlCu or AlSiCu layer and the barrier 
layer not covered by the patterned hard mask are etched away to form metal 
lines having an outwardly tapered profile.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now more particularly to FIG. 1, there is illustrated a portion 
of a partially completed integrated circuit. Semiconductor substrate 10 is 
preferably composed of monocrystalline silicon. Field oxide regions 12 
have been formed as is conventional in the art in the semiconductor 
substrate 10. Semiconductor device structures, including gate electrode 14 
and source and drain regions 16, are fabricated in and on the 
semiconductor substrate. A thick insulating layer of silicon dioxide or 
borophosphosilicate glass (BPSG), or the like, 18 is blanket deposited 
over the semiconductor device structures. Next, a barrier layer 20 is 
deposited over the insulating layer. This may be titanium and titanium 
nitride with a combined thickness of between about 500 and 1500 Angstroms. 
Referring now to FIG. 2, the metal layer 22 is deposited over the barrier 
layer 20. The metal layer comprises AlCu or AlSiCu and is deposited by 
sputtering to a thickness of between about 4000 and 8000 Angstroms. 
Next, the hard mask o f the present invention is formed. Referring to FIG. 
3, a layer of silicon nitride or a multilayer of titanium nitride and 
silicon dioxide 24 is deposited over the metal layer 22 to a thickness of 
between about 2000 to 4000 Angstroms to form a hard mask. The hard mask is 
opaque to the actinic light used in photolithography so that light will 
not be reflected from it. The hard mask also acts as an anti-reflective 
coating. 
A layer of photoresist is coated over the hard mask 24 and is exposed and 
developed to form the photoresist mask 26. In the process of the present 
invention, the photoresist layer has a thickness of between about 7000 and 
14,000 Angstroms. Conventionally, the photoresist mask must be as thick as 
between about 12,000 and 18,000 Angstroms. The thinner photoresist layer 
of the present invention does not cause depth of focus problems during 
photolithography to form the photoresist mask. Using DUV photolithography 
instead of conventional i-line photolithography allows the use of a 
thinner photoresist layer. 
The silicon nitride layer is etched away where it is not covered by the 
photoresist mask 26, as illustrated in FIG. 4. The silicon nitride layer 
is etched using a high silicon nitride etch rate recipe, such as SF.sub.6 
/HBr/He or CF.sub.4 /Ar chemistry. These etch recipes include high 
fluorine atom plasma which maintains a near vertical silicon nitride 
profile and excellent critical dimension control. For example, using 
CF.sub.4 /Ar chemistry, the gases are flowed at the rate of about 50 sccm 
for CF.sub.4 and 500 sccm for Ar. A radio frequency of about 1300 watts 
and pressure of about 150 mTorr are maintained. The high Ar flow allows a 
low concentration of fluorine radicals to prevent sidewall undercutting of 
the silicon nitride. The high radio frequency power also helps the 
directional etching. 
At this point, the photoresist mask 26 may be removed as is conventional in 
the art, such as by oxygen ashing. Alternatively, the photoresist mask may 
be left on until after the metal lines are etched. If the silicon nitride 
loss during metal etching can be kept within the minimum remaining 
thickness (typically about 500 Angstroms), the photoresist may be removed 
before metal etching. If the silicon nitride loss would be too high, the 
photoresist should be kept on as part of the etching mask. 
Now, the metal layer and the underlying barrier layer 20 are to be etched 
away where they are not covered by the hard mask, leaving metal lines 22 
as shown in FIGS. 5A and 5B. An outwardly tapered profile of the metal 
lines is preferred with good sidewall passivation. A polymer 28 is formed 
on the sidewalls of the metal lines during etching to provide the sidewall 
passivation. This polymer prevents chlorine radicals from attacking the 
metal lines (so-called corrosion) during and after etching. 
The tapered profile of the metal lines is achieved by increasing the 
operating pressure during etching and introducing a passivation gas, such 
as CH.sub.4 or CHF.sub.3. The etching recipe includes a total processing 
gas flow of 300 to 700 sccm, where the processing gases include Argon 15 
to 45%, CH.sub.4 or CHF.sub.3 1 to 10%, BCl.sub.3 20%, and Cl.sub.2 50%, 
at a pressure of 1.5 to 8 Pascals, which is about 11 to 62 mTorr. 
FIGS. 5A and 5B illustrate the resulting metal lines 22 having the desired 
outwardly tapered profile. FIG. 5A illustrates the alternative in which 
the photoresist mask remains rn the hard mask during etching. FIG. 5B 
illustrates the alternative in which the photoresist mask has been removed 
before metal etching. 
Now, the photoresist mask 26 is removed if it has not been removed already. 
The polymer is removed such as by oxygen ashing and wet chemical 
stripping. Referring to FIG. 6, an insulating layer 30 of silicon oxide or 
silicon nitride, or the like, is deposited over the metal lines 22. 
Because of the tapered profile of the metal lines, formation of voids in 
the gaps between the metal lines is avoided. 
The process of the invention uses a silicon nitride or titanium 
nitride/silicon dioxide hard mask. The thinner photoresist layer allowed 
by the invention prevents depth of focus problems in forging the hard 
mask. The etching recipes used to form the hard mask have been developed 
and used experimentally to provide a near vertical profile of the hard 
mask with excellent critical dimension control. The etching recipe of the 
present invention used for etching the metal lines provides an outwardly 
tapered profile of the metal lines with no undercutting. 
While the invention has been particularly shown and described with 
reference to the preferred embodiments 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.