Method for patterning aluminum metallizations

A process for the fine replication of aluminum-based metallizations on semiconductor devices. A layer of material, such as silicon dioxide or oxynitride, that is resistant to the chlorine-based etchants that readily attack aluminum and aluminum alloys is deposited upon an aluminum-based metallization layer. A relatively thin layer of photoresist is deposited thereover and developed. Etchant gases attack the dielectric layer, creating a patterned hard mask for subsequent etching of the underlying metal.

BACKGROUND 
1. Field of the Invention 
The present invention relates to apparatus and methods for use in 
patterning the metallization of a semiconductor device. More particularly, 
this invention pertains to the utilization of a "hard" mask for patterning 
of metallizations that include aluminum alloys. 
2. Description of the Prior Art 
Semiconductor devices generally require a network of metallizations at 
their upper surfaces that serve as gates and interconnecting conductors 
for device control, activation and output. For example, in a so-called 
charge coupled device (CCD) conductive metallizations provide means for 
"reading out" charge accumulated in localized regions of the device as a 
consequence and measure of incident radiation. The arrangement or 
"patterning" of metallizations must be extremely accurate. This is 
especially true in the case of VLSI (very large scale integration) devices 
that may possess densities in excess of 10.sup.5 transistors per chip. 
In addition to accuracy of location, the optimum operation of such devices 
requires metallizations of consistent line widths and the absence of 
"undercutting". The former is exceedingly difficult to attain in the 
fabrication of VLSI devices that commonly require gate widths of less than 
two (2) microns. An extremely small deviation from nominal width may 
result in significant and undesired localized effects. The latter 
characteristic can introduce deleterious stray capacitances that produce 
device and system failures. 
The "standard" metal for use in present day devices is aluminum. This metal 
and its selected alloys possess the lowest resistivity of all materials 
that have been found to be practical. That is, the selected aluminum-based 
alloys have proven to be amenable to commercial etching and like 
processes. 
In the past, patterning has generally been accomplished by etching 
processes in which a layer of metallization is deposited upon a 
semiconductor substrate. A layer of photoresist is then deposited atop the 
metallization and photolithographically developed and removed by means of 
an acetate bath or the like. Thereafter a (liquid or gaseous) etchant is 
introduced that attacks the exposed portions of metallization, leaving the 
desired pattern atop the substrate. 
A relatively long time is required to etch the metallization, since a 
relatively thick layer of photoresist must be employed to protect the 
pattern throughout etching since present-day etchants are highly corrosive 
and will tend to dissolve the photoresist that remains to protect the 
underlying metallization causing corrosion of the underlying 
metallization. For example, a 1.5 micron (15,000 Angstrom) thick layer of 
photoresist is normally required to protect a 7,000 Angstroms thick layer 
of metallization during etching. The photoresist traps highly corrosive 
etchant (solution or gas) ions. Therefore, while a thick layer of 
photoresist protects the metallization, the very thickness of the layer 
materially increases the risk of corrosion after the pattern has been 
formed. Further, the thicker the layer of photoresist, the greater the 
difficulty of maintaining a very narrow physical aperture for etching a 
correspondingly-narrow line of metallization. 
SUMMARY OF THE INVENTION 
The present invention addresses the preceding shortcomings of the prior art 
by providing a novel process of forming a preselected pattern of 
aluminum-based metallizations on a semiconductor device of the type that 
includes a substrate of semiconductor material. This process includes the 
deposition of a layer of aluminum-based metallization on comprising alloy 
of copper and aluminum on the substrate. A layer of Tiw is deposited at 
the top of the metallization. Thereafter, a layer of material that is 
resistant to chlorine-based etchants is deposited. A layer of photoresist 
is then deposited, developed and regions thereof removed in accordance 
with a predetermined pattern to thereby expose predetermined regions of 
the layer of resistant material. A fluorine-based etchant is then applied 
to the exposed regions of the layer of resistant material to create a hard 
mask having a predetermined pattern of apertures for exposing 
predetermined underlying regions of the Tiw The fluorine-based etchant is 
continued to be applied to etch the exposed regions of the TiW layer. 
Thereafter, a chlorine-based etchant is applied until the exposed regions 
of the metallization layer are removed to form a preselected pattern of 
metallizations on the substrate. 
The foregoing and additional features of this invention will become further 
apparent from the detailed description that follows. This written 
description is accompanied by a set of drawing figures. Numerals of the 
drawing figures, corresponding to those of the written description, point 
to the various features of the invention, like numerals referring to like 
features throughout.

DETAILED DESCRIPTION 
Turning now to the drawings, FIG. 1 is an elevation view in cross-section 
of a semiconductor wafer 10 with an overlying layer of metallization 14 
that is to be patterned in accordance with the process of the invention. 
The layer 14, when appropriately patterned, provides a network of 
conductors for communicating with various input, output and control 
devices (e.g. gates) to thereby effect and measure the operation of the 
resultant semiconductor device. Various diffusions (not shown) within the 
substrate 10 will, to a large extent, dictate the proper geometry of the 
network of conductors. 
The wafer 10 preferably comprises a substrate 11 of single crystalline 
silicon and an overlying oxide layer 12. The oxide layer 12, which 
provides passivation, may be grown atop the silicon 11 in a high 
temperature epitaxial growth process or may be deposited by low pressure 
chemical vapor deposition (LPCVD) or like conventional process. 
The metallization 14 preferably comprises three distinct sub-layers, each 
of which is suitably sputter deposited. A lower barrier layer 16 is 
interposed for preventing interdiffusion of the aluminum atoms within the 
main conducting middle layer 18 (discussed below) and the silicon atoms of 
the substrate 10. An appropriate layer 16 can be formed of titungsten (an 
alloy of titanium and tungsten also known as TiW) composition. TiW 
generally comprises about 12.5 per cent titanium and 87.5 per cent 
tungsten and is suitably deposited to a depth of about 1500 Angstroms. 
The middle layer 18 may be an aluminum-copper (about 4 per cent copper) 
alloy that serves as the main conductor of the metallization pattern. An 
appropriate thickness of this portion of the metallization 14 to 
facilitate conduction is approximately 5000 Angstroms. 
A top layer 20 comprises a TiW film of about 500 Angstroms in thickness. 
Such a top layer acts to suppress the formation of undesired "hillocks" 
during patterning. Additionally, the top layer 20 reduces the reflectivity 
of the underlying aluminum alloy and thereby facilitates patterning. 
FIG. 2 is an elevation view in cross-section of the workpiece after 
subsequent formation steps during which (1) a layer 22 of material that 
will act as a hard mask and (2) an overlying layer 24 of photoresist have 
been formed. The hard mask dielectric layer 22 is preferably of either 
silicon dioxide or oxynitride composition and has a thickness of about 
3500 Angstroms. The material of the hard mask is chosen for compatibility 
with the main portion of the underlying layer of metallization 14. That 
is, the composition of the layer 22 is chosen for resistance to the 
preferred etchant of the aluminum-based layer 14. Generally, a 
chlorine-based gaseous etchant such as chlorine or boron trichloride 
serves to etch an aluminum based alloy both accurately and relatively 
rapidly and each of the above-named dielectric materials is quite 
resistant thereto. The photoresist 24 may be spun deposited to a depth of 
only 5000 Angstroms, significantly thinner than photoresist layers 
employed for similar purposes in the prior art. 
As mentioned earlier, the top 20 of the metallization 14 preferably 
includes a non-aluminum based alloy such as TiW which may not be readily 
etched by a chlorine-based etchant. This particular material is more 
readily etched with a fluorine-based etchant such as carbon tetrafluoride 
or the compound that is more commonly known by the trademark "FREON". 
Unlike chlorine-based etchants, the identified materials of the hard 
masking layer 22 are also quite subject to (relatively slow) etching by 
such fluorine-based compounds. As will be described below, this factor, 
which may arise from a desire to enhance the resultant conductors (i.e. by 
minimizing reflectivity, removing hillocks, etc.) is accounted for in the 
processing of the pattern of metallizations. 
FIG. 3 is a cross-sectional elevation view of the structure after 
photolithographic development and removal of predetermined regions of the 
layer of photoresist 24 in an acetone bath or like process. Unlike the 
prior art, the layer of photoresist 24 is patterned only as a precursor to 
etching the hard mask layer 22. As will be seen, the formation of a hard 
mask of appropriate composition enables one to reduce the thickness of the 
photoresist layer 24 to a significant extent, the attendant advantages 
thereof being apparent from the discussion of the prior art above. 
FIG. 4 is a cross-sectional view that discloses the effect of the 
introduction of a controlled atmosphere including an appropriate etchant. 
As can be seen, the patterned photoresist 24 defines an aperture of 
pre-selected width d. After the introduction of the etchant gas, the 
exposed region of the hard mask layer 22 underlying the aperture in the 
photoresist 24 is attacked. As mentioned above, this layer is attacked by 
a fluorine-based etchant, as is the TiW film at the upper surface of the 
metallization 14, leaving a depression of approximately 500 Angstroms or 
less in the top of the metallization. As can be seen, the depth of the 
photoresist layer 24 is also thinned by continuing exposure to the etchant 
gases. 
The depth of the upper TiW treatment layer 20 is only a small fraction of 
that of the middle layer 18 that provides the main conductor of the 
metallization 14. After this layer has been partially etched by the 
flourine-based etchant as shown in FIG. 4, a chlorine-based etchant is 
used for the purpose of continuing the etching of the metallization 14 
through the aluminum-based main conductor layer. The chlorine-based etch 
is continued through the aluminum-copper middle layer 18 eventually 
producing the structure of FIG. 5. As mentioned earlier, the composition 
of the hard mask layer 22 is chosen for resistance to the chlorine 
etchants that readily attack aluminum-based alloys. 
As is shown in FIGS. 4 and 5, the layer of photoresist 24 is continuously 
attacked by (both) etchant gases throughout the process. Gradually the 
layer 24 is reduced to a minimal depth at the time that the removal of the 
predetermined region of the middle layer 18 of the metallization followed 
by removal of the TiW barrier layer 16 by means of a further 
fluorine-based etching step has been completed as shown in FIG. 5. The 
apparatus as shown in this figure may then be removed from the etchant 
atmosphere, rinsed to remove the remaining photoresist and baked until 
dry. The etched hard mask layer 22 remains for passivation of the 
underlying pattern of metallization as shown in FIG. 6. 
The process of the invention relies in large measure upon the relative 
resistance of the dielectric hard mask material to commonly employed 
aluminum etchants. The oxide materials discussed above are, in fact, 
subject to etching by chlorine-based etchants. Hence the term "etch 
resistant" is qualified. This refers to the fact that a significantly 
greater amount of time is required to etch the preferred materials by 
means of such a chlorine-based etchant than is required to etch a layer of 
conventional photoresist to the same extent or depth. The disparity is 
roughly on the order of four times. That is, it requires a chlorine-based 
etchant appropriate for etching an aluminum-based metallization 
approximately four times the length of exposure time to etch a layer of 
silicon dioxide or oxynitride to the same depth as is required to etch a 
conventional photoresist by the same amount. This quality permits the use 
of thinner masking layers, a smaller volume of etchant-absorbing 
photoresist and the other advantages that follow from the use of a thinner 
layer of photoresist. 
As can be seen, the present invention provides an improved method for 
forming a preselected pattern of aluminum-based metallizations on a 
semiconductor device of the type that includes a substrate of 
semiconductor material. By utilizing the teachings of the invention one 
may reliably pattern the metallizations required for operation of a large 
variety of semiconductor devices, including those falling into the VLSI 
range, and avoid the degree of imperfection and risk of corrosion 
associated with prior art methods. While this invention has been described 
with reference to its presently preferred embodiment it is not limited 
thereto. Rather its scope is limited only insofar as defined by the 
following set of claims and includes all equivalents thereof.