Selective etching of refractory metal nitrides

A method of selectively etching a layer of a refractory metal nitride, with application to formation of TiN local interconnects for VLSI integrated circuits, and particularly a method of selectively etching TiN relative to a refractory metal silicide. The method comprises the step of heating surfaces of the substrate to a selected etch temperature between 50.degree. C. and 200.degree. C. in a non-reactive gas and then exposing the heated substrate to reactive halogen species of a plasma having ion energies substantially less than 100 eV, and preferably below 30 eV. The etch selectivity is controlled by selecting a relatively low ion energy to reduce ion bombardment and heating effects during etching, and independently controlling the etch temperature in the heating step. The reactive species of the plasma are preferably generated by electron cyclotron resonance (ECR) excitation of a halocarbon containing gas, and heating comprises ion bombardment with a non-reactive gas. For etching TiN, preferred halocarbon containing gases are CF4 and C2F6, and mixtures thereof, whereby optimum etch selectivity is obtained with reactive species having ion energies less than 30 eV, and at an etch temperature in the range 100.degree. to 120.degree. C.

BACKGROUND OF THE INVENTION 
In fabrication of VLSI integrated circuits, the use of local interconnects 
provides for direct local connection between, for example, the source and 
drain junctions and/or gate electrodes of transistors, without additional 
contacts and metal layers, and allows for more compact layouts and circuit 
design. With the widespread use of self-aligned silicidation for source 
and drain contact metallization, a conductive film of a refractory metal 
nitride, for example titanium nitride (TIN), is a preferred material for 
local interconnects. Whereas silicides allow diffusion of boron and 
phosphorus, resulting in interdiffusion and counterdoping problems, a thin 
conductive film of TiN, or other refractory metal nitride such as tungsten 
nitride, is also an effective diffusion barrier. 
For example, a known process for forming local interconnects with TiN may 
include the following process steps: 
a. deposition overall of a titanium layer; 
b. self aligned silicidation of exposed source, drain, and gate regions in 
a nitrogen atmosphere to form titanium silicide with overlying layer of 
titanium nitride (TIN); 
c. removal of the TiN layer formed during silicidation; 
d. deposition of a layer of conductive TiN; 
e. coating and patterning of photoresist to mask selected areas of TiN for 
interconnect; and 
f. etching of the exposed titanium nitride layer to leave TiN interconnects 
in selected areas. 
A conductive titanium nitride film for interconnect may comprise 
stoichiometric TiN, but the composition may also contain oxygen and/or 
other elements. Thus, in this application, titanium nitride is denoted by 
the formula TiN to reflect a composition of approximately 50% titanium 
with nitrogen, and is not intended to be limited only to stoichiometric 
TiN. An etch process for defining the local interconnects must have 
selectivity to interconnect material, e.g. TiN, relative to underlying 
silicide to prevent degradation of the latter during etching. Commonly 
used silicides include TiSi.sub.2, CoSi.sub.2, and PtSi.sub.2. Etch 
selectivity is also required relative to other substrate materials 
including oxides, photoresist encountered in semiconductor processing. 
It is well known that a simple halogen containing discharge, and in 
particular a fluorine discharge, is effective for reactive ion etching of 
refractory metal nitrides, particularly TiN. However, a halogen discharge 
does not provide etch selectivity relative to TiSi.sub.2 and other 
refractory metal silicides, such as WSi.sub.2 and MoSi.sub.2 or 
Various processes are known which have sought to provide improved etch 
selectivity for titanium nitride relative to silicides and other 
integrated circuit substrate materials using plasma or reactive ion 
etching with fluorine containing gases. 
For example, a known method of selectively etching TiN relative to titanium 
silicide is disclosed in U.S. Pat. No. 4,675,073 to Douglas, which 
describes a dry etch process using a conventional plasma etching system 
providing a glow discharge plasma containing reactive fluorine species, 
provided by a feed gas comprising CF.sub.4 in helium. The process takes 
advantage of the passivation effect of adsorption of CF.sub.2 .cndot. 
radicals, formed by dissociation of CF.sub.4, which are preferentially 
adsorbed on the silicide so as to hinder etching of the silicide by 
fluorine radicals. However, it was found that selectivity between TiN and 
TiSi.sub.2 is low, and both oxide and photoresist were rapidly etched. It 
was also found that the high energy ions generated in a conventional 
plasma etch system, with ion energies typically .about.100-250 eV, may 
cause surface damage to exposed silicide and other substrate materials 
during overetching near the end point. Thus, to avoid substrate damage and 
improve control of the endpoint, a dry etch step using a fluorine plasma 
is typically used to etch about 90% of the total thickness of TiN, and the 
remaining thickness of TiN is removed to the endpoint, with a subsequent 
wet etch. The latter comprises, for example an aqueous solution of dilute 
H2O.sub.2 and NH.sub.4 OH. Nevertheless, this wet etch solution also 
attacks photoresist, necessitating stripping of photoresist prior to the 
wet etch. Consequently, the wet etch step is not selective and after 
removing the photoresist the overall thickness of the exposed TiN, i.e. 
that for forming the interconnect, is also reduced during the wet etch. 
Furthermore, while NH.sub.4 OH/H.sub.2 O.sub.2 is suitable for stripping 
or cleaning TiN, the etching rate is not sufficiently controllable for 
repeatably removing a small thickness of TiN. 
Other fluorine containing feed gases, for example, CHF3, C2F6 and SF6, have 
been used for non-selective dry etching of titanium nitride, as described 
in U.S. Pat. No. 4,877,482 to Knapp et al. However, the '073 patent to 
Douglas teaches that a gas which is a copious fluorine source, e.g. 
SF.sub.6, is unsuitable for selective etching, because the etch 
selectivity of TiN relative to TiSi2 is reduced in the presence of excess 
fluorine. Consequently, Douglas found that it was advantageous (see 
Douglas '073 patent discussed above) for improved selectivity to use a low 
flow rate of a non-copious fluorine source gas, i.e. CF.sub.4 in helium 
with a reducing electrode to scavenge fluorine and maintain fluorine 
deficiency in the plasma. 
In two other recent U.S. Pat. Nos. 4,863,559 and 4,793,896 to Douglas, a 
method is described which provides improved etch selectivity using a 
plasma photo-generated from a gas mixture of CCl.sub.4 and helium, in 
which excess chlorine is reduced by use of a consumable power electrode or 
by introducing a chlorine scavenger gas such as chloroform into the 
reactor. 
In U.S. Pat. No. 4,878,994 to Jucha, there is described another chlorine 
based process for selective etching of Ti containing film, such as TiN, 
with respect to silicide which utilizes a plasma generated from a helium 
and CCl.sub.4, in a plasma etching system which provides for a two stage 
plasma generation process to improve control of the etch anisotropy, 
selectivity and etch rate, compared to the method of the Douglas '073 
patent, so that a subsequent wet etch step is unnecessary. 
However, in use of a chlorine bearing gas for etching TiN, the reaction 
product is titanium chloride, which is less volatile than the 
corresponding fluoride. Thus a fluorine plasma is preferred to reduce 
residues and increase etch rate. There may also be problems associated 
with residual interfacial deposits left on surfaces parallel to the ion 
flux and caused by chlorine plasma reactions with SiO.sub.2. Further, 
plasma etching with CCl.sub.4 causes increased polymerization of resist 
material which necessitates a special resist strip. Use of a graphite 
electrode to scavenge chlorine may result in carbon contamination and 
further polymerization. Thus, it is preferred for microelectronics 
applications to avoid an etchant gas containing reactive chlorine species, 
which may result in contamination problems, as well as for reasons of 
health and environmental concerns associated with chlorinated gases. 
Thus although the use of refractory metal nitrides as interconnect 
materials provides a significant advantage in simplifying circuit layout 
in VLSI integrated circuits, it is believed that the lack of selectivity 
of known etch processes may be a significant factor limiting more 
widespread use of refractory metal nitride local interconnects. 
SUMMARY OF THE INVENTION 
The present invention seeks to provide a method of selectively etching 
refractory metal nitrides, which avoids or reduces the above mentioned 
problems. 
According to one aspect of the present invention, there is provided a 
method of selectively etching a layer of refractory metal nitride relative 
to an underlying layer of refractory metal silicide comprising part of a 
substrate of an integrated circuit, the method comprising: 
heating surfaces of the substrate in a non-reactive gas to a selected etch 
temperature between 50.degree. C. and 200.degree. C. in an etching 
chamber; 
generating a plasma from a halocarbon feed gas, the plasma comprising 
chemically reactive halogen species having ion energies in a predetermined 
range substantially below 100 eV; 
exposing the heated surface of the substrate to said chemically reactive 
halogen species of the plasma at said selected etch temperature, thereby 
selectively removing the layer of refractory metal nitride from the 
underlying refractory metal silicide and the underlying substrate, 
the etch selectivity for the refractory metal nitride relative to the 
refractory metal silicide and substrate being controlled by selecting a 
relatively low ion energy whereby ion bombardment and heating effects 
during etching are reduced, and independently controlling the etch 
temperature by said heating step. 
Thus, an etch process is provided with improved selectivity by restricting 
the ion energy of the incident plasma to substantially less than 100 eV, 
so that ion bombardment and heating effects of the plasma are reduced, 
while etch temperature is controlled independently in a heating step. Ion 
energies of the reactive species are beneficially selected in the range 
below 50 eV and preferably below 30 eV. A satisfactory etch rate is 
maintained by heating the substrate to a selected etch temperature above 
50.degree. C., in an independent heating step before exposure to the 
reactive halogen species of the plasma. 
The substrate is heated conveniently in the etching chamber by ion 
bombardment of exposed surfaces with a non reactive gas, e.g. nitrogen, 
prior to the etching steps, and then the etch process is carried out while 
the substrate surface is still at an elevated temperature. Alternatively, 
other conventional heating means such as a heated chuck may be used to 
provide an selected substrate temperature during etching. However, the 
former method has the advantage of heating the surfaces where etching 
takes place while maintaining a low bulk temperature of the substrate. 
Thus, improved etch selectivity is provided by preheating the substrate and 
selecting the energy of the ions of the etchant plasma incident on the 
substrate so that the ion energy is sufficient preferentially attack and 
etch the nitride at a significantly higher rate than the silicide. 
Advantageously, the process of generating the plasma is carried out in an 
ECR excited plasma etching system in which the ion energy of the reactive 
species of the plasma incident on the substrate can be controlled 
independently of the ion flux, for example by applying an RF bias to the 
substrate. The RF bias may be reduced, or turned off during the etching, 
i.e. the ECR etcher is operated in a plasma stream mode, so that the ion 
energy of reactive species incident on the substrate during etching is 
minimized. However, during the heating step, by increasing the RF bias, 
ion bombardment with relatively high energy unreactive gas ions is 
feasible, to provide for rapid heating. 
A plasma comprising reactive species with selected low ion energy range may 
also be generated in other known systems, for example magnetron reactive 
ion etch system. Alternatively the plasma may be generated in a barrel 
type etcher, e.g. of the type used as a photoresist stripper which is 
provided with means for heating the substrate, i.e. a heated wafer support 
chuck. 
According to another aspect of the present invention there is provided a 
method of forming a refractory metal nitride interconnect structure for an 
integrated circuit, the method comprising: 
providing a substrate of an integrated circuit comprising a layer of a 
refractory silicide and an overlying layer of refractory metal nitride; 
selectively masking the refractory metal nitride layer with a masking 
layer; 
exposing the substrate in an etching chamber; 
preheating surfaces of the substrate to a selected etch temperature between 
50.degree. C. and 20.degree. C. in an unreactive gas; 
generating a plasma from a halocarbon feed gas, the plasma comprising 
reactive halogen species having ion energies in a predetermined range 
substantially less than 100 eV; 
and exposing the heated substrate to said reactive halogen species of the 
plasma at the selected etch temperature, thereby selectively removing 
unmasked areas of the layer of refractory metal nitride from the 
underlying layer of refractory metal silicide and underlying substrate. 
the etch selectivity of the reactive species of the plasma for the 
refractory metal nitride relative to the refractory metal substrate, the 
underlying substrate and the masking layer being controlled by selecting a 
relatively low ion energy whereby ion bombardment and heating effects 
during etching are reduced and independently controlling the selected etch 
temperature in said preheating step. 
A method of etching a refractory metal nitride relative to a refractory 
metal silicide which also provides improved etch selectivity relative to 
other materials used integrated circuits fabrication, is beneficial for 
fabrication of conductive refractory metal nitride interconnect structures 
for integrated circuits. 
According to another aspect of the present invention there is provided a 
method of etching titanium nitride relative to an underlying layer of a 
refractory metal silicide comprising part of a substrate of an integrated 
circuit, comprising: 
exposing the substrate in an etching chamber and heating surfaces of the 
substrate to a selected etch temperature between 50.degree. C. and 
200.degree. C.; 
generating a plasma from a halocarbon gas selected from the group 
consisting CF.sub.4, C2F.sub.6, and mixtures thereof, the plasma 
comprising reactive fluorine species having ion energies in a 
predetermined range substantially below 100 eV, and exposing the heated 
substrate at the selected etch temperature to said reactive fluorine 
species, thereby selectively etching the layer of titanium nitride 
relative to the refractory metal silicide, the etch selectivity increasing 
at lower ion energies whereby an optimum etch selectivity is controlled by 
selecting a lower range of ion energy whereby heating effects and ion 
bombardment effects during etching are reduced, and independently 
controlling the etch temperature in said heating step. 
Where the refractory metal nitride is titanium nitride, a preferred etchant 
plasma comprises a chemically reactive fluorine species produced from a 
feed gas containing CF.sub.4, C2F.sub.6 or mixtures thereof. These 
fluorocarbon gases may be obtained with high purity for microelectronics 
applications. 
Using an etch gas comprising CF4, and with suitable control of the ion 
energy and temperature, selective etch rates of titanium nitride relative 
to titanium silicide of 6:1 and for titanium nitride relative to cobalt 
silicide of 15:1 were obtained. Good etch selectivity for TiN with respect 
to photoresist (.about.7:1) and oxide of (.about.4:1) respectively was 
also achieved. Consequently the method is particularly applicable for 
selective etching in microelectronics applications. 
Thus the present invention provides a method of selectively etching 
refractory metal nitrides, a method of selectively etching TiN, and a 
method of formation of a refractory metal nitride interconnect structure, 
which overcome or reduce the above mentioned problems.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A cross sectional view of a part of an integrated circuit structure 10 at 
successive process steps of a method of forming TiN local interconnects 
for CMOS device structures, including process steps for selective etching 
of TiN according to a first embodiment of the present invention, is shown 
in FIG. 1a) to f). The partially fabricated integrated circuit structure 
10 forms part of a substrate comprising a semiconductor silicon wafer 12 
on which are defined parts of a conventional CMOS device, i.e. a 
transistor, which include doped silicon source and drain regions 12, a 
thin gate oxide 14, and a polysilicon gate electrode 16 having a 
dielectric sidewall spacer 18. 
After formation of device structures on the substrate wafer, self-aligned 
silicidation of selected silicon source and drain regions 12 and 
polysilicon gate regions 16 is performed by a known method, by deposition 
of a film of titanium 20 and heating of the titanium film in a nitrogen 
atmosphere, which results in formation of titanium silicide 22 contact 
areas where the titanium film overlies exposed silicon of the source, 
drain and gate regions, and some nitridation of the titanium film occurs 
to form a film of titanium nitride TiN 24 overall. (FIG. 1a and 1b) This 
latter TiN film 24 is removed by wet etching in a mixture of NH.sub.4 OH 
and H.sub.2 O.sub.2 (FIG. 1c). A thin layer 26 of a controlled thickness 
(.about.800 .ANG.) of conductive TiN is then deposited on the wafer by a 
suitable method, i.e. by reactive sputtering (FIG. 1e). The conductive TiN 
layer 26 is selectively masked by coating and patterning of photoresist 28 
(FIG. 1e). Exposed parts of the TiN layer 26 are then selectively removed 
by etching in an ECR excited plasma etching system by exposure of the 
substrate wafer to a plasma containing a reactive fluorine species, as 
will be described in detail below, to leave desired regions of conductive 
TiN 26 forming local interconnects between the source and drain regions 
and the gate electrode (FIG. 1f). 
A suitable known apparatus for carrying out the method according to a first 
embodiment of the invention comprises an ECR excited plasma etching system 
of known prior art structure (not shown), having a first chamber for 
generation of a plasma by ECR excitation of a suitable feed gas. 
Chemically reactive species of the plasma including radicals and ions are 
then directed into an adjoining etching chamber. The etching chamber 
provides a substrate holder which includes means for applying an RF bias 
to the substrate for controlling the ion energy of reactive species 
directed from the plasma excitation chamber and incident on a sample held 
on the substrate holder in the adjoining etching chamber. Typically, the 
system may be evacuated to a base pressure of .about.10- 3-10- 4 Torr. 
Feed gas inlets into the plasma excitation chamber provides for one or 
more gases to be introduced at a controlled flow rate. While the flux of 
ions is dependent on the power supplied to the ECR plasma excitation 
chamber, which generates a high flux of highly electronically excited 
radicals and ions, the ion energy of the ions incident on the substrate 
may be independently controlled by adjusting the RF bias on the substrate. 
The ECR etcher may operated in a plasma stream mode, i.e. in the absence 
of RF bias on the substrate, to minimize the energy of ions incident on 
the substrate, or with an RF bias applied to the substrate, to accelerate 
ions towards the substrate and thereby increase the ion energy of reactive 
species of the plasma incident on the sample substrate. 
Selective etching of the TiN layer was carried out as will now be 
described. After formation of the conductive TiN layer, the resist 
patterned wafer was placed in the ECR etching chamber, which was then 
evacuated. The sample was heated to a predetermined temperature between 
.about.50.degree. C. and .about.150.degree. C. by exposure to ion 
bombardment with a non-reactive gas, e.g. nitrogen. The nitrogen flow 
rate, and power levels were selected to obtain a controlled rate of 
heating. For example, ion bombardment using a nitrogen flow rate of 15 
sccm N2, ECR excitation with microwave power of 600 W, magnet current of 
16 Amp, and RF bias of 100 volts on the substrate holder heated the 
surfaces of the wafer to 120.degree. C. in 3 minutes. The rate of 
temperature rise was controlled by altering the RF bias on the substrate 
during nitrogen ion bombardment. Immediately after heating the wafer, the 
nitrogen flow and the RF bias were turned off, the feed gas was changed to 
a desired fluorocarbon containing gas, that is, pure CF.sub.4, and a 
plasma was generated which contained reactive fluorine species for etching 
TiN. The heated substrate was exposed to the reactive fluorine species of 
the plasma while the wafer remained at the selected etch temperature, to 
selectively remove TiN from the underlying silicide and other substrate 
materials. Etching was carried out over a range of parameters including 
gas flow rates, microwave power and magnet current, RF bias, and substrate 
temperature. Advantageously the etch selectivity may be controlled by 
appropriate selection of the ion energy and the etch temperature, so that 
etching of the TiN layer may proceed to the endpoint, to expose the 
underlying silicide layer. 
Ion flux depends on ECR microwave excitation and magnet current which 
produces ions and radicals with a high electron temperature, but a low ion 
energy, 10-20 eV. The RF field on the substrate holder was used to control 
the ion energy independently of ion flux. Etch rates for TiN were measured 
in the absence of an applied RF bias, when the energy of ions incident on 
the substrate was 10-20 eV, and over a range of ion energies up to 
.about.100 eV, comparable with ion energies of a conventional plasma etch 
system, by increasing the applied RF bias on the substrate. 
While a range of flow rates of pure CF.sub.4 and CF.sub.4 in nitrogen were 
be used, it was found that at low gas flow rates up to .about.20 sccm, the 
supply of CF4 was apparently rate limiting, and an inert diluent gas, such 
as nitrogen, was not required. For example, a flow rate of 20 sccm pure 
CF.sub.4, microwave excitation power of 250 W, and magnet current was of 
16 A found to provide a high TiN etch rate of .about.150 nm/min when the 
RF bias on the substrate was turned off during etching. 
The etch rate of TiN had a strong dependent on the RF power. The etch rate 
of TiN increased with increasing RF power on the substrate up to 100 W. 
However, as illustrated by the results shown in FIG. 2, the etch rate of 
titanium silicide increased more rapidly with increasing RF bias (FIG. 2). 
Thus, increasing the ion energy was found to increase the etch rate of 
silicide relative to titanium nitride, and thus the etch selectivity to 
TiN is reduced. (FIG. 2). 
In an ECR plasma, the ion energy is related to the applied RF power. Ion 
energy increases as the RF power is increases. The ion energy is 10-20 eV 
when the RF bias is off. At RF power close to 100 watts, the ion energy is 
similar to that of a typical capacitatively coupled plasma systems, (&gt;100 
eV) The etch selectivity for TiN relative to TiSi.sub.2 at an RF power or 
100 W is similar (2:1) to that obtained in a conventional capacitatively 
coupled plasma system with ion energies &gt;100 eV. In order to increase the 
selectivity, the ion energy is lowered, e.g. by using an ECR excited 
plasma with RF bias at the lower range shown figure two, or with no RF 
bias. 
The etch rate was 150 nm/min for TiN with an etch rate ratio of 5:1 for 
TiN:TiSi.sub.2 at ion energies of 10-20 eV, compared with .about.2:1 at 
ion energies .about.100 eV. The nitride is thus preferentially attacked 
and etched relative to the silicide at lower ion energies with increasing 
selectivity at lower ion energies. Thus improved selectivity is obtained 
at lower ion energies e.g. below 50 eV compared with etching at 100 eV or 
more, and even better selectivity at ion energies less than 30 eV when the 
RF bias is off. Thus, to increase the selectivity to increase the 
selectivity, the ion energy is lowered by operating an ECR etcher with low 
or no RF bias and pre-heating the substrate above 50.degree. C. 
The requirement for heating of the substrate is in contrast to a 
conventional plasma etch or reactive ion etch in which ion energies may be 
.about.100 eV and where, typically, the substrate must be cooled on a 
chilled chuck to prevent overheating of the sample and loss of etch 
selectivity. On the other hand, it was found that etch selectivity in the 
present method was further enhanced by heating the substrate to 
.about.100.degree. C. to .about.120.degree. C. Increasing the temperature 
from 50.degree. C. to 120.degree. C. provided an increase in selectivity 
by a factor of about 2. Temperatures above 120.degree. C. were not found 
to be advantageous in the present examples. It is believed that several 
temperature dependent factors may contribute to the latter effect: 
increased desorption of reactants from the surface competing with the TiN 
etch reactions; increased polymerization of TiN; and, lack of reactant 
supply, i.e. supply of fluorine radicals may be rate limiting on the 
etching reactions. Heating of the substrate by molecular nitrogen ion 
bombardment provided effective surface heating of the sample wafer while 
maintaining a low bulk wafer temperature and without causing significant 
sputtering damage to the surface. 
Thus, reducing the ion energy by an order of magnitude compared with a 
conventional reactive ion etch, (i.e. from .about.100 eV to .about.10 eV) 
and increasing the etch temperature in an independent step produced a five 
fold improvement in etch selectivity for nitride over silicide. Superior 
selectivity of the etch process for TiN relative to TiSi.sub.2 was 
obtained at low RF bias resulting in ion energies of less than 30 eV, or 
with the RF bias was turned off, ion energies 10-20 eV. Improved 
selectivity to other materials including oxide and photoresist was also 
obtained at lower RF bias or in the absence of RF bias. 
Conveniently, the end point for TiN etching was detected by a change in 
optical emission from the plasma on completion of etching of exposed TiN. 
The high etch rate of TiN, 150 nm/min, resulted in a significant change in 
optical emission from the plasma when etching of TiN was completed, which 
facilitated end point detection. 
In use of a ECR plasma excitation system, it is known the high efficiency 
of plasma generation results in efficient dissociation of CF.sub.4 into 
F.cndot. and CF.sub.x.sup..cndot. (x=1,2,3) radicals. ECR plasma 
excitation of CF.sub.4 was found to produce a plasma containing high 
concentrations of F.cndot. radicals for etching TiN, with a significant 
proportion of CF.sub.2.sup..cndot. and CF.sub.4.sup..cndot. as well as 
CF.sub.3.sup..cndot. radicals which are adsorbed on the silicide and 
function as passivant. 
The plasma composition is completely different to a CF.sub.4 plasma used 
for conventional reactive ion etching, and in contrast to degradation of 
selectivity by use of a plasma from a `copious` fluorine or chlorine 
source, significantly higher selectivity between etch rates of TiN and 
TiSi.sub.2 were found with plasma having a very high concentration of F 
from highly dissociated CF.sub.4 when the ion energy was maintained below 
30 eV. It is believed that this result is explained by a correspondingly 
high concentration of CF.sub.x radicals, which are produced by efficient 
dissociation of CF.sub.4 by ECR excitation, so that the [F.sup..cndot. 
]/[CF.sub.x.sup..cndot. ] ratio remains low, combined with the high 
adsorption coefficient of CF.sub.x.sup..cndot. radicals on silicide to 
provide an effective passivation layer. Thus the silicide is masked from 
the etching effect of reactive F.sup..cndot. radicals. Furthermore, when 
the energy of the ions incident on the substrate is less than 30 eV, the 
ion energy is not high enough to significantly displace the passivating 
film of CF.sub. x radicals adsorbed on the silicide film. In contrast, in 
a conventional reactive ion etching plasma the degree of dissociation of 
the feed gas is lower, and hence the concentration of the CF.sub.x 
radicals is limited so that the passivation effect to the silicide film is 
relatively poor. Furthermore, the passivating layer of CFx is continually 
removed by bombardment with high energy ions from the plasma. 
Moreover, since the concentration of reactive gas in the above described 
embodiments was not rate limiting, an inert carrier gas is not required, 
and further reduce ion bombardment effects during etching. 
The method according to the first embodiment using a fluorine plasma 
generated from CF.sub.4 may be used for selective etching of refractory 
metal nitrides including TiN, TiN containing oxygen or other elements, and 
tungsten nitride when the underlying silicide comprises a refractory metal 
silicide such as TiSi.sub.2, CoSi.sub.2, PtSi.sub.2 or other silicides 
having volatile fluorides. An etch rate selectivity of 6:1 for TiN 
relative to an underlying film of TiSi.sub.2 and 15:1 for TiN relative to 
an underlying film of CoSi.sub.2 or PtSi.sub.2 was obtained. The etch 
selectivity between TiN and photoresist was .gtoreq.7:1 and between TiN 
and SiO.sub.2 was .gtoreq.4:1. 
Thus, improved selectivity of a fluorine plasma for selective etching of 
TiN relative to photoresist and oxide, as well as TiSi.sub.2, is 
advantageous in microelectronics applications, such as patterning of TiN 
local interconnect on underlying films of self aligned silicide contact 
regions for VLSI integrated circuits. 
In a method according to a second embodiment of the present invention, the 
method is similar to that of the first embodiment, except that the plasma 
containing a reactive fluorine species is generated from a mixture of 
CF.sub.4 and C.sub.2 F.sub.6 and resulted in improved selectivity of the 
etch for TiN relative to TiSi.sub.2. 
In modifications of the embodiments of the invention, refractory metal 
nitrides are selectively etched relative to refractory metal silicides by 
exposing the substrate to reactive halogen species of low ion energy from 
plasmas generated from pure or mixed fluorocarbon gases including 
CF.sub.4, CHF.sub.3 and C2F.sub.6, or chlorocarbon gases, such as 
CCl.sub.4. However, since the fluoride of titanium is more volatile than 
the chloride, reactant gases containing fluorine are preferred for etching 
TiN. Fluorine species are also preferred to provide better selectivity 
with respect to photoresist and oxide. Chlorine containing etch gases may 
be avoided to reduce residue formation and contamination. For example, 
CF.sub.4 is preferred over CCl4 for microelectronics applications as it is 
readily obtainable in higher purity form, and is less toxic. Furthermore, 
use of CCl4 has been discontinued in this laboratory in view of concern 
over environmental problems and health hazards associated with some 
chlorinated halocarbons. 
In heating of the substrate wafer prior to etching by ion bombardment with 
an unreactive gas, nitrogen was preferred to minimize or reduce sputter 
damage to the substrate. The energy of ions used for heating was 
controlled by RF bias on the substrate holder to provide a suitable rate 
of heating to the desired substrate temperature for etching. 
In an alternative embodiment, a conventional heating means comprising an 
electrically heated chuck in thermal contact with the substrate is used 
for heating the substrate. 
Although an ECR excited plasma etch system is advantageous in providing 
independent control of ion energy and ion flux, a method according to the 
invention may be carried out in other types of etching apparatus capable 
of generating low energy reactive ions in a selected range of energies, 
that is in an inductively coupled (helicon or transversely coupled) plasma 
with low or no RF bias applied. 
Thus in a method of selectively etching titanium nitride local interconnect 
according to a third embodiment of the present invention, the substrate 
placed in a magnetron etching system, and pre-heated the substrate to 
120.degree. C. by means of a heated chuck. Then a reactive fluorine plasma 
was generated from a halocarbon gas consisting of CF.sub.4 which was 
introduced into the chamber, at a flow rate about 50 sccm, a pressure of 
40 mTorr, 80 Gauss magnetic field, and 100 Watts power. The resulting etch 
rate was about 1000 .ANG. per minute. The ion energy of the reactive 
fluorine species in this system was less than 50 eV. 
In a method of etching titanium nitride according to a fourth embodiment of 
the present invention, instead of using a magnetically confined plasma 
chamber, the sample is etched using a down stream plasma etcher, e.g. a 
photoresist stripper, with or without application of RF bias to the 
substrate. The substrate is pre-heated to about 100.degree. C. to 
120.degree. C. as in the other embodiments, and the ion energy is 
advantageously restricted to a range less than 50 eV, and preferably to 
less than 30 eV by applying low RF or zero RF bias to the substrate. The 
etch may be carried out in a downstream plasma etch mode in a barrel type 
etch apparatus, which is equipped with independent heater control on 
heated substrate holders, to preheat the substrate to about 100.degree. C. 
before exposure to a reactive plasma species generated from CF.sub.4, 
C.sub.2 F.sub.6 or mixtures thereof. 
Thus there is provided an improved etch process for selective etching of 
refractory metal silicides comprising controlling etch selectivity by 
selecting the ion energy of the reactive halogen species in the plasma in 
a predetermined low energy range, and independently controlling the etch 
temperature in a separate heating step. 
Consequently, a method of etching refractory metal nitrides with improved 
selectivity relative to refractory metal silicides and other integrated 
substrate materials. The feasibility of using refractory metal nitrides, 
and in particular TiN for local interconnect is much improved. Restriction 
of the reactive ion energy to lower ion energies than found in 
conventional RIE and plasma etch systems improves selectivity and reduces 
plasma damage, and ion bombardment effects, for improved reliability of 
the conductive interconnect material and underlying materials.