Process for creating a metal etch mask which may be utilized for halogen-plasma excavation of deep trenches

A process for creating a metal etch mask from either cobalt, nickel, palladium, iron or copper which may be utilized for halogen-plasma excavation of deep trenches. The process begins by creating a thin isolation layer of either silicon nitride or silicon dioxide on top of the layer to be trenched. A thin layer of one of the metals selected from the aforementioned list of five is then created on top of the isolation layer. A layer of polysilicon is then blanket deposited on top of the refractory metal layer. Photoresist masking is then performed as though the photoresist were the actual pattern for the trench etch. Exposed portions of the polysilicon layer are then etched away with an anisotropic etch. Following a photoresist strip, the substrate and overlying layers are subjected to an elevated temperature step, which causes the polysilicon to react with the underlying metal layer to form metal silicide. In substrate regions where no polysilicon overlies the metal layer, no silicide is formed. Next, the metal silicide is removed with a wet etch. A metal mask remains that is essentially an exact image of the original photoresist mask. Trenches may be etched to any desired depth with virtually no consumption of the metal mask. Once the trench etch is complete, the metal etch mask may be stripped utilizing a wet etch reagent such as aqua regia.

FIELD OF THE INVENTION 
This invention relates to semiconductor process technology and, more 
specifically, to masking techniques used for plasma or reactive ion 
etches. 
BACKGROUND OF THE INVENTION 
Silicon dioxide has become the preferred hard-mask patterning material for 
plasma etching deep trenches in silicon. In a chemically-halogenated 
plasma (the standard environment for such etches), silicon dioxide has a 
reactivity (also termed "selectivity") approximately 1/40 to 1/10 that of 
silicon. In other words, the silicon dioxide mask is consumed during the 
etch at a rate of 1/40 to 1/10 the rate at which silicon is consumed. For 
a given mask thickness, mask selectivity effectively limits trench depth. 
At the high end of the selectivity range, it is necessary to alter the 
chemistry of the plasma with chemical species that tend to cause 
deposition of unwanted materials on the surfaces of the etch chamber. For 
example, the presence of oxygen radicals and ions, chemical species that 
enhance selectivity of silicon over oxide during a halogenated-plasma 
etch, react with silicon to form silicon dioxide that is deposited as a 
glass layer on etch chamber walls. Deposited materials represent a 
potential source of pollution that must be periodically removed from the 
chamber. In addition, since mask selectivity may also be etch-rate 
dependent, it may be necessary to perform a plasma etch at an 
inconveniently slow rate in order to achieve a desired trench depth with a 
mask of optimal maximum thickness. Furthermore, as silicon dioxide masking 
material is eroded during a plasma etch, it may be redeposited on trench 
sidewalls near the mouth of the trench, thus, further complicating the 
fabrication process. Finally, if MOS gates have been created prior to an 
anisotropic plasma trench etch, removal of a silicon dioxide patterning 
mask with an isotropic oxide etch subsequent to the trench etch may 
compromise the integrity of existing gate oxide. This is especially true 
at gate edges. 
What is needed is a patterning mask material that is essentially impervious 
to halogenated-plasma etches, and that does not require exotic 
modification of a halogen plasma. Ideally, the mask could be removed with 
a chemical wet etch that would not etch existing circuitry components. 
SUMMARY OF THE INVENTION 
This invention provides a process for creating a patterning mask for deep 
trench plasma etches from certain metals which react with silicon at 
elevated temperatures to form a metal silicide, which have a melting point 
appreciably greater than 850.degree. C. (the approximate temperature at 
which polycrystalline silicon is deposited), and which do not react with 
chlorine, fluorine, or bromine to form volatile compounds with boiling 
points below approximately 300.degree. C. (a necessary criterium for a 
material to be immune to erosion during a halogenated-plasma etch). 
Cobalt, nickel, palladium, iron and copper possess these characteristics. 
The masking process begins by creating a thin isolation layer of either 
silicon nitride or silicon dioxide (preferably the latter due to the 
perforation defects common to a deposited nitride layer) on top of the 
layer to be etched (normally a silicon substrate). A thin layer of one of 
the metals selected from the aforementioned list of five is then created 
on top of the isolation layer using one of any number of blanket 
deposition techniques (e.g. sputtering, vapor deposition, etc.). A layer 
of polysilicon is then blanket deposited on top of the refractory metal 
layer. The thickness of the polysilicon layer must be at least sufficient 
to allow complete consumption of subjacent portions of the metal layer 
when silicide-forming temperatures are reached. Photoresist masking is 
then performed as though the photoresist were the actual pattern for the 
trench etch. Exposed portions of the polysilicon layer are then etched 
away, preferably with an anisotropic etch to protect the polysilicon layer 
from undercutting. Following a photoresist strip, the substrate and 
overlying layers are subjected to an elevated temperature step, which 
causes the polysilicon to react with subjacent portions of the underlying 
metal layer to form metal silicide. In substrate regions where no 
polysilicon overlies the metal layer, no silicide is formed. Next, the 
metal silicide is removed with a wet etch. A metal mask remains that is 
essentially an exact image of the original photoresist mask. The metal 
mask may then be utilized as an etch mask in a halogenated plasma etch 
chamber. Trenches may be etched to any desired depth with virtually no 
consumption of the metal mask. Once the trench etch is complete, the metal 
etch mask may be stripped utilizing a wet etch reagent such as aqua regia.

PREFERRED EMBODIMENT OF THE INVENTION 
Referring now to FIG. 1, a silicon dioxide isolation layer 11 has been 
thermally grown on a silicon substrate 12 of an in-process wafer, 
following which a thin metal layer 13 has been deposited on top of 
isolation layer 11. The metallic element for metal layer 13 is selected 
from a group consisting of cobalt, nickel, palladium, iron and copper. A 
polysilicon layer 14 is then deposited on top of metal layer 13, following 
which polysilicon layer 14 is masked with a photoresist mask 15. Trenches 
will ultimately be etched on the substrate 12 that is not subjacent 
segments of photoresist mask 15. 
Referring now to FIG. 2, all portions of polysilicon layer 14 that were not 
protected by photoresist mask 15 have been etched away with an anisotropic 
etch that prevents undercutting of polysilicon beneath photoresist mask 
15. 
Referring now to FIG. 3, the in-process wafer has been subjected to an 
elevated temperature step which has caused the atoms of metal layer 13 to 
react with overlying polysilicon to form metal silicide regions 31. The 
atoms of those portions of metal layer 13 that were not subjacent unetched 
portions of polysilicon layer 14 have not been converted to metal 
silicide. A polysilicon layer remnant 32 remains on top of metal silicide 
region 31. The existence of polysilicon layer remnants 32 is required to 
ensure that all metal subjacent polysilicon is completely consumed during 
the silicidation process. 
Referring now to FIG. 4, polysilicon layer remnants 32 and metal silicide 
regions 31 are removed with a wet etch that leaves unsilicided metal and 
the underlying silicon dioxide isolation layer 11 intact. The unsilicided 
metal now forms a halogen-plasma-resistant etch mask 41 that can be used 
to excavate deep trenches. 
Referring now to FIG. 5, the in-process wafer is subjected to a 
halogenated-plasma etch, which creates trenches 51. Etch mask 41 remains 
largely unaffected by the halogenated-plasma etch. The durability of etch 
mask 41 is no longer a factor in trench-depth limits. 
Referring now to FIG. 6, etch mask 41 has been removed from the substrate 
with a wet etch, using a mix of nitric and hydrochloric acids commonly 
known as aqua regia. Silicon dioxide isolation layer 11 is then removed 
with a wet oxide etch. 
Although only a single embodiment of the process for creating a metal etch 
mask which may be utilized for halogen-plasma excavation of deep trenches 
has been disclosed, it will be apparent to those having ordinary skill in 
the art, that changes may be made thereto without departing from the 
spirit and the scope of the process as claimed.