Materials and methods for etching silicides, polycrystalline silicon and polycides

Gas chemistry and a related RIE mode process is described for etching silicides of the refractory metals titanium, tantalum, tungsten and aluminum and for etching composites of these silicides on polycrystalline silicon layers. BCl.sub.3 is added to the HCl/Cl.sub.2 gas chemistry used for the polysilicon etch along with additives selected from fluorinated gases and oxygen to satisfy the multiple requirement of the two-step silicide-polysilicon etch process, including the silicide-to-polysilicon etch ratio requirement.

BACKGROUND OF THE INVENTION 
The present invention relates in general to a process for etching 
conductive layers used in semiconductor integrated circuits (IC). In 
particular, the invention relates to methods for etching metal silicides, 
polycrystalline silicon (polysilicon) and composite silicide-polysilicon 
(polycide) structures and to reactive plasma gas chemistry for use in such 
methods. 
Over the past several years, the silicon integrated circuit technologies 
used in manufacturing conductor-insulator-semiconductor field effect 
transistor (CISFET) devices and bipolar transistor devices have developed 
to the point that they provide very small geometry, highly dense 
integrated circuits. The continued improvement in silicon integrated 
circuit integration has been made possible by advances in the 
manufacturing equipment, as well as in the materials and methods used in 
processing semiconductor wafers and IC chips. At the same time, however, 
the increasingly stringent requirements imposed by the improvements in the 
silicon integrated circuit integration and density have strained much of 
the classic microelectronics processing technology. For example, with the 
trend toward greater device densities and smaller minimum feature sizes 
and smaller separations in integrated circuits, the sheet resistance of 
multi-level interconnects and gate electrodes and other conductors becomes 
a primary factor affecting frequency characteristics and power 
consumption, and in limiting device speed. Thus, to successfully implement 
greater density without adversely affecting such characteristics, it is 
necessary to reduce the sheet resistance of the gate an conductor 
materials. 
Another requirement which must be met to achieve the increasingly small 
minimum feature sizes and minimum separations is that the lithographic 
pattern-transfer process must be very precise. In addition to factors such 
as the lithographic process itself and the wafer topography, satisfaction 
of this requirement necessitates in general the use of an anisotropic 
plasma or dry etching technology that is capable of precisely replicating 
the mask dimensions and size in the etched layer without degradation of 
the mask and loss of line width. 
The two basic types of plasma etching systems--plasma etching itself in 
which the chemical etching component is dominant and reactive ion etching 
in which physical ion bombardment is dominant--are described in commonly 
assigned U.S. Pat. No. 4,376,672, entitled, "Material and Methods for 
Plasma Etching of Oxides and Nitrides of Silicon", filed Oct. 26, 1981 and 
issued Mar. 15, 1983. That description is hereby incorporated by 
reference. Of the different types of plasma etching systems, it is 
believed that reactive ion etching systems are the preferred systems for 
achieving high resolution replication of photoresist patterns, for 
example, in electrically conductive materials. 
FIG. 1 schematically illustrates an etching system 10 that is one presently 
preferred system for reactive ion etching. This system 10 is available 
commercially from Applied Materials, Inc. of Santa Clara, Calif. as the 
8100 Series System. This system 10 utilizes a cylindrical reaction chamber 
11 and a hexagonal cathode 12 which is connected to an RF power supply 13. 
An exhaust port 14 communicates between the interior of the reaction 
chamber and a vacuum pump. The walls of the reaction chamber 11 and the 
base plate 16 form the grounded anode of this system. A supply of reactive 
gas from gas supply 17 is communicated to the interior of the chamber 10 
through an entrance port 18 and by a conduit arrangement 19 to a gas 
distribution ring 20 at the top of the chamber. The reactor 10 is 
asymmetric. That is, the anode-to-cathode ratio is slightly greater than 
two-to-one, resulting in high energy bombardment of the hexagonal cathode 
surface 12 relative to the anode surface 11. Such a design provides lower 
power density and better etch uniformity, decreases contamination of and 
from the chamber walls and promotes a higher anisotropy. Additionally, the 
cathode structure configuration allows all wafers to be vertically 
oriented during the process to minimize wafer exposure to particulates. 
Despite the availability of plasma etching systems such as the AME 8100 
System 10, the microelectronics polycrystalline silicon processing 
technology, like the rest of the classic microelectronics technology, has 
been strained by the increasing levels of silicon integrated circuit 
integration. Polysilicon has been and is widely used in both bipolar and 
CISFET IC technology, for example in conductors, such as gate electrodes; 
in single level and multi-layer interconnects; in resistors; in buried 
contacts; and in the formation of emitter structures such as shallow 
self-aligned emitters and self-aligned emitter-contact structures. 
However, meeting the sheet resistance requirements in very small devices 
and conductors requires very high polysilicon doping levels which are 
obtained at the cost of isotropic etch behavior and precise pattern 
transfer. 
Over the last several years, the microelectronics industry has been 
developing polycide technology as a substitute for polysilicon technology 
in a number of applications, in part because polycides have much lower 
sheet sensitivities than doped polysilicon. Polycide is a layer of metal 
silicide over a layer of polysilicon. Of primary interest here are the 
refractory metal silicides (typically disilicides): titanium silicide, 
TiSi.sub.x ; tantalum silicide, TaSi.sub.x ; molybdenum silicide, 
MoSi.sub.x ; and tungsten silicide, WSi.sub.x. 
Polycide Etch Requirements 
In general, there are certain requirements which must be satisfied when 
etching any material, including conductive layers, during intergrated 
circuit fabrication. The conductive layer should be etched to an 
anisotropic profile (vertical or slopping) with the minimum line width 
loss in the masking layer and etched material. There should be good etch 
selectivity to overlying layers (principally the mask) and to underlying 
layers. Preferably there is a moderate to high etching rate associated 
with the etching step (multiple steps in the case of polycide). The etch 
step(s) should be residue free. In addition, the etch process must provide 
uniform characteristics which are reproducible from run-to-run. Also, 
device damage must be avoided. 
The art has used fluorinated CF.sub.4 /O.sub.2 gas mixtures in a high 
pressure plasma etching mode to provide anisotropic etch profiles of 
titanium silicide. In addition, at lower pressures, typically 3-200 
millitorr, using the reactive ion etching (RIE) mode, tungsten, molybdenum 
and tantalum silicides have been etched anisotropically (as has titanium 
silicide) using fluorinated gas chemistry. However, while fluorinated 
chemistry applied in a low pressure, RIE mode can provide anistropic clean 
etching of silicides, there is a tendency to undercut polysilicon and to 
low selectivity to oxide. 
BCl.sub.3 /Cl.sub.2 gas chemistry has been used to etch tantalum polycide 
anisotropically. In general, chlorinated gas chemistry has a lesser 
tendency to undercut polysilicon and provides higher selectivity to oxide, 
but has a tendency to leave residues. 
In short, it is very difficult to obtain both anisotropy and high 
selectivity to oxide during the silicide etch. At least in part because of 
this difficulty, the art has utilized multi-step processes for polycide 
etching. Using this approach, high selectivity to oxide is a requirement 
during etching of the underlying polysilicon, but not during the silicide 
etch step. In fact, it is desirable that the silicide etch be able to etch 
oxide, to remove any residual native oxide on the silicide surface and at 
the silicide-polysilicon interface. That is, it is actually preferable to 
have a low etch selectivity to oxide, rather than a high selectivity to 
oxide, during the silicide etch step. 
While the use of a multi-step process for etching silicide and polysilicon 
somewhat eases the difficulties associated with single step etching, it 
substitutes a different set of requirements for each step. First, high 
selectivity to the polysilicon is critical during the silicide etch step. 
Also, high selectivity to the underlying layer such as oxide is critical 
only during the polysilicide etch step. In addition, compatibility between 
the silicide and polysilicon etch steps is required in that the silicide 
etch must not undercut the poly or affect the poly etch step performance, 
and the poly etch in turn must no undercut the silicide. 
Topography Etch Requirements 
In addition to the above requirements, which are known in the art, we have 
discovered a less obvious silicide-to-polysilicon etch selectivity 
requirement. This requirement pertains to the step of etching the silicide 
component of polycide structures that are formed on non-planar, stepped 
topography. Stepped topography is associated, for example, with recessed 
or semi-recessed dielectric isolation and with multi-layer metal 
interconnect structures. Specifically, we have determined that the 
filaments or fences frequently formed adjacent the steps in polycide 
etching are eliminated by a sufficiently high silicide:polysilicon etch 
rate ratio, R. We have quantified that etch rate ratio as a function of 
polycide film thickness, topographic step height and angle. 
FIGS. 2-5, 2A-5A and 2B-5B schematically depict the process of sequentially 
etching polycide structures formed over topographical steps of different 
angles, .theta.=30.degree., 60.degree. and 75.degree.. A multiple step 
reactive ion etch process is assumed, involving separate silicide and 
polysilicon etch steps and a silicide etch step which provides a 
silicide:polysilicon etch rate ratio of about 2:1. The structure 21 of 
FIG. 2 is a starting silicide 22-on-polysilicon 23 sandwich formed over a 
topographical step 24 of silicon oxide that forms a relatively shallow 
step angle (.theta.=30.degree.) with the oxide underlayer 25 and substrate 
26. FIGS. 2A and 2B are similar to FIG. 2 except that the topographical 
angles are .theta.=60.degree. and 75.degree., respectively. The step etch 
sequence begins in FIGS. 3, 3A and 3B using reactive ion etching to remove 
the silicide layer 22. 
As shown in FIG. 3, when the polysilicon is just exposed by the silicide 
etch, a thin silicide residue 27 remains on the sidewall or riser of the 
30.degree. step. Then the residue is removed by a subsequent silicide 
overetch, and an RIE mode polysilicon etch is used to remove the 
polysilicon 23. FIG. 4 illustrates when the gate oxide is just exposed by 
the polysilicon etch. At this point, because of the silicide residual 
layer 27 and the step angle, a residual layer of polysilicon 28 remains on 
the sidewall. This residual 28 is removed by a polysilicon overetch of 
about 20 percent, FIG. 5. Because the polysilicon etch exposes the gate 
oxide 25, a high selectivity to oxide is required to prevent 
degradation/destruction of the oxide during the poly overetch. 
Similar etch sequences are employed for the 60.degree. and 75.degree. step 
structures, but result in thicker silicide sidewall residual layers 27A 
and 27B and thicker polycide residual layers 28A and 28B. Assuming a 
sufficiently high selectivity to oxide during the polysilicon etch, these 
residuals can be removed by the polysilicon overetch without destroying 
the gate oxide. 
The result is quite different if, during the silicide etch, the 
silicide-to-polysilicon etch rate ratio is too low to remove the silicide 
residual. That is, if the silicide etching step cannot meet the etch rate 
requirement for a given topography and therefore does not remove the 
silicide residual on the sidewall/riser before etching through the poly 
and exposing the gate oxide, the polysilicon etch step must replace the 
silicide etch step to save the gate oxide. Typically, the polysilicon etch 
step does not effectively etch silicide. The silicide residue along the 
topographic step thus acts as a micromask to form a free-standing 
silicide-polysilicon filament or fence along the step. See filaments 29A 
and 29B shown in phantom in FIGS. 5A and 5B. The filament has a height 
similar to the polysilicon layer thickness and departs from the 
topographic step about the same distance. 
It has been known in the art that a high silicide:polycide etch rate ratio 
is required to provide residue-free removal of silicide over topographical 
steps and to avoid filaments along the steps. However, the prior art has 
not quantified the relationship of the silicide etch selectivity to 
polysilicon and, in fact, is believed not to have recognized the existence 
of a specific relationship between this selectivity and the step geometry 
and thickness. Furthermore, the prior art has not provided an etch 
chemistry capable of eliminating the silicide filaments and satisfying the 
other silicide etch requirements. 
SUMMARY OF THE INVENTION 
In view of the above discussion, it is a primary object of the present 
invention to etch the refractory metal silicide-polysilicon sandwich 
structure while keeping linewidth loss to a minimum and with the required 
selectivity to oxide to maintain gate oxide integrity. 
It is also an object of the present invention to provide an etching gas 
composition and an associated plasma process for etching silicides of 
tungsten, molybdenum, titanium and tantalum with the high selectivity for 
polysilicon which is required to etch topographical structures without 
leaving silicide residue. 
It is a related object of the present invention to provide a gas etching 
composition and an associated plasma etching process for anisotropically 
etching silicides with a high selectivity for overlying photoresist mask 
layers and for underlying polysilicon layers, with a relatively 
low-to-moderate selectivity for oxide. 
It is another related object of the present invention to provide a silicide 
etching gas composition and an associated plasma etch process for 
silicide, of the type described above, in combination with a process for 
etching polysilicon anisotropically, without undercutting of the silicide 
and with a high selectivity to oxide; and in an overall process that 
provides high throughput and utilizes the same base gas chemistry for the 
silicide and polysilicon etch steps. 
In one aspect, the present invention relates in part to the use of 
HCl/Cl.sub.2 gas chemistry as the basic gas chemistry for etching 
refractory metal silicides and polysilicon. The use of HCl/Cl.sub.2 gas 
chemistry in the polysilicon etch step and BCl.sub.3 -containing 
HCl/Cl.sub.2 gas chemistry in the silicide etch step permits an 
essentially continuous silicide and polysilicon etch process in the same 
chamber without breaking vacuum. HCl/Cl.sub.2 gas chemistry provides high 
rate anisotropic etching of the underlying polysilicon with selectivity to 
underlying oxides such as gate oxide layers. BCl.sub.3 -containing 
HCl/Cl.sub.2 gas chemistry selectively doped with relatively small 
volumetric amounts of dopant gas not only etches all four refractory metal 
silicides anisotropically, but also provides the required high etch rate 
ratio of silicide to polysilicon to provide a clean, filament free etch. 
In addition, the BCl.sub.3 /HCl/Cl.sub.2 gas chemistry provides a high 
selectivity to organic photoresist masks and the desired etchability of 
oxide. In more general terms, the process is consistent with requirements 
of anisotropic polycide profiles, minimum linewidth loss, cleanliness, 
uniformity and reproducibility.

DETAILED DESCRIPTION OF THE INVENTION 
1. Determination of R (Silicide:Polysilicon Etch Ratio) 
a. Formulation 
Referring to FIGS. 6 and 7, to quantitatively describe the required etch 
rate ratio, R, of silicide to polysilicon, it is assumed the silicide and 
polysilicon films 22 and 23 (FIG. 2) have conformal coverage; h.sub.1 and 
h.sub.2 are the thickness of the silicide and polysilicon, respectively; 
hs is the topographic height of oxide step 24; and .theta. is the 
topographic angle (O.ltoreq..theta..ltoreq.90.degree.). 
When the polysilicon 23 is just exposed during the silicide etching step, 
the thickest remaining silicide is always located at the foot point C as 
shown in FIGS. 6 and 7. There are two portions on the topographic step: 
one 23C is curved, the other 23L is linear; they are separated at boundary 
point A. 
If the boundary point A is behind the foot point C as shown in FIG. 6, that 
is, when the topographic angle .theta. is low, then the thickest remaining 
silicide will be located in the linear portion and is given by: 
##EQU1## 
The required etch rate ratio of silicide to polysilicon is then equal to 
the remaining silicide to polysilicon thickness ratio: 
##EQU2## 
If the boundary point A exceeds the foot point C, as shown in FIG. 7, that 
is, when the topographic angle .theta. is high, the thickest remaining 
silicide is in the curved portion and the thickness will be: 
##EQU3## 
The required etch rate ratio then will be: 
##EQU4## 
When the topographical step is vertical, i.e. 0 =90.degree.: 
##EQU5## 
b. Discussion of Process Requirements 
Using equation (2) and (4), the required etch rate ratios are plotted as a 
function of .theta. in FIG. 8 and FIG. 9, using h.sub.1 /h.sub.2 (curve 
34, FIG. 8) and h.sub.s /h.sub.1 +h.sub.2 (curve 35, FIG. 9) as discrete 
parameters. 
The required etch rate ratio monatonically increases when topographical 
angle .theta. increases. The steepest rate of increase is around 
45.degree.-60.degree.. For small .theta., the required etch rate ratio is 
independent of topographical step height and is related to the layer 
thickness ratio of silicide to polysilicon. At high .theta., both the step 
height and thickness ratio play roles in the required etch rate ratio. 
Using equation (2) and (4), one can calculate the required etch rate ratio 
for a given device topographical structure. As a typical example, for 
3,000 Angstroms silicide and 2,000 Angstroms doped polysilicon over a 
5,000 Angstroms, .about.90.degree. steep step the required etch rate ratio 
is about 2.3 to etch clean. In general, R of (1-2):1 and, preferably, 
.about.2:1 is required for present topographies. Using BCl.sub.3 
-containing HCl/Cl.sub.2 gas chemistry R values of (1-2):1 are readily 
obtained for tantalum silicide and titanium silicide, but not for 
molybdenum and tungsten silicides. 
2. Gas Chemistry for Required Silicide:Polysilicon R 
a. BCl.sub.3 -Containing Cl.sub.2 Gas Chemistry 
We have found that including BCl.sub.3 in the chlorinated etching gas 
solution promotes cleanliness of the substrate during the etch process and 
maintains the desired profile of the etched material (that is, prevents 
undercutting). Specifically, we have found that this dual purpose is 
satisfied by using BCl.sub.3 and Cl.sub.2 flow rate ratios in which 
BCl.sub.3 :Cl.sub.2 .gtoreq.1:1. 
The BCl.sub.3 does not substantially enhance the etch rate of the 
silicides. This is illustrated by curve 32 of FIG. 10, which depicts the 
molybdenum silicide etch rates which are achieved for Cl.sub.2 /HCl gas 
chemistry by adding increasing amounts of BCl.sub.3 to the Cl.sub.2 and 
HCl flow of 30 sccm and 75 sccm at -15 millitorr pressure and -300 volts 
DC bias. The curve indicates the BCl.sub.3 flow rate has little effect on 
the molybdenum silicide etch rate. If anything, the molybdenum silicide 
etch rate is decreased slightly by increasing the flow of BCl.sub.3. 
Data listed here for silicide and polysilicon polycide films were for films 
which were formed on oxide layers formed on single crystal silicon 
substrates. The polysilicon films were highly doped n-type with phosphorus 
to a concentration which would provide sheet resistivity of about 15-20 
ohms per square for 5000 Angstrom thick films. The silicide and 
polysilicon films were etched by using photoresist masks about one micron 
thick and etching using the above-described Applied Materials, Inc. 8100 
Series reactor 10. The process was also demonstrated on 
silicon-on-sapphire structures. 
In contrast to the effect of BCl.sub.3 alone, the silicide etch rate is 
influenced by the total BCl.sub.3 +Cl.sub.2 flow rate. This is illustrated 
in FIG. 11. Curve 33 depicts the increase in molybdenum silicide etch rate 
which is achieved for BCl.sub.3 /CL.sub.2 /HCl chemistry by adding 
increasing amounts of BCl.sub.3 +Cl.sub.2 to the HCl flow of 75 sccm at 
-25 millitorr pressure and -300 volts DC bias. Using the stated 
conditions, increasing the volume percentage of BCl.sub.3 +Cl.sub.2 from 
20 percent to 50 percent of the total gas flow increased the silicide etch 
rate from about 900 to 1100 Angstroms per minute. 
FIG. 12 illustrates the effect of chlorine on silicide etch rates. In 
addition, the figure illustrates the above-mentioned dichotomy in the etch 
behavior of titanium and tantalum silicides on the one hand, and tungsten 
and molybdenum silicides on the other. That is, this figure depicts the 
increases in the silicide etch rate and in the silicide:polysilicon etch 
rate ratio which are achieved for BCl.sub.3 /Cl.sub.2 gas chemistry by 
adding increasing amounts of chlorine to the BCl.sub.3 flow of 40 sccm. 
Curve 35 shows the variation in polysilicon etch rate as a function of 
chlorine flow rate using 25 millitorr pressure and -350 volts DC bias. 
Curves 36 and 37 illustrate the etch rates of molybdenum silicide and 
tungsten silicide under the same conditions. The molybdenum etch rate 
curve 36 and the tungsten curve 37 nearly parallel the polysilicon curve 
35 and have etch rate ratios, R, relative to the polysilicon of slightly 
greater than 1:1. In contrast, under the same conditions, the tantalum 
curve 38, has R.gtoreq.2:1 for chlorine flows greater than about 20 sccm 
(that is, over the entire range of flow rates of FIG. 12). Similarly, for 
the titanium curve 39, R.gtoreq.2 over the entire range of flow rates of 
FIG. 12 and R.perspectiveto.(3-4) :1 over much of the flow rate range. 
Furthermore, FIGS. 13 and 14 illustrate that these etch rate ratios are 
relatively insensitive to RF power (DC bias voltage) and to chamber 
pressure, at least over a substantial range of conditions. The data of 
FIG. 13 were taken using BCl.sub.3 and Cl.sub.2 flow rates of 40 sccm each 
and a chamber pressure of 25 millitorr. The change in the etch rates of 
the titanium curve 43 and the molybdenum curve 42 as a function of DC bias 
voltage do not differ significantly from the slope of the polysilicon etch 
rate curve 41. Also, the titanium, molybdenum and polysilicon curves 46, 
45 and 44 of FIG. 14, associated with BCl.sub.3 and Cl.sub.2 flow rates of 
40 sccm and -350 volts DC bias, have similar relative slopes. 
Consequently, DC bias and pressure per se cannot be relied upon under 
these conditions to increase the etch rate ratio of molybdenum and, 
presumably, of tungsten silicides. (It should be noted that despite the 
relative insensitivity of the titanium etch rate ratio and the tantalum 
etch rate ratio to power (DC bias voltage) and pressure, those etch rate 
ratios were at an excellent, high level throughout FIGS. 13 and 14.) 
In short, the BCl.sub.3 /Cl.sub.2 chemistry provides excellent 
silicide:polysilicon etch rate ratios for titanium and tantalum silicides 
which are believed sufficient for essentially all present topographical 
geometries. However, the tungsten and molybdenum silicide:polysilicon etch 
rate ratio of about 1:1 is still too low for many topographical IC 
devices. It should also be noted that, even where very high etch rate 
ratios (of about 2:1) are not required because of the particular 
topography (for example, when low .theta. is acceptable), a high etch rate 
ratio of silicide to polysilicon is still preferable to clean up the 
silicide residue with a minimum overetch. 
b. Dopant:Additive Effects 
The molybdenum and tungsten silicides-to-polysilicon etch rate ratios are 
increased by adding small volumetric amounts of oxygen and fluorinated 
dopant/additive gases to the BCl.sub.3 -containing chlorine gas. FIG. 15 
depicts the increases in etch rates and in the relative etch rate ratio of 
tungsten silicide to polysilicon by adding increasing amounts of NF.sub.3 
to BCl.sub.3 /Cl.sub.2 gas chemistry. Curve 47 shows the variation in 
polysilicon etch rate as a function of NF.sub.3 flow rate at 25 millitorr 
chamber pressure, -350 volts DC bias and BCl.sub.3 and Cl.sub.2 flow rates 
of 40 sccm each. Similarly, curve 48 shows the variation in the tungsten 
etch rate as a function of the NF.sub.3 flow rate under the same 
conditions. The silicide:polysilicon etch rate ratio is greater than about 
2:1 for NF.sub.3 flow rates greater than about 20 sccm (greater than about 
20 volume percent of the total BCl.sub.3 +Cl.sub.2 +NF.sub.3 additive gas 
flow rate of 100 sccm). 
Similar results were obtained for other flow rate combinations and other 
fluorinated additives. FIG. 16 depicts the increase in etch rates and etch 
rate ratio of tungsten silicide and polysilicon for BCl.sub.3 /Cl.sub.2 
/HCl gas chemistry by adding increasing amounts of the gas mixture 
CF.sub.4 +8.5% O.sub.2. The etch conditions were 25 millitorr, -350 volts 
DC bias, 45.degree. C. hexode temperature and HCl/BCl.sub.3 /Cl.sub.2 flow 
rates of 75/40/40 sccm. As the (CF.sub.4 +8.5% O.sub.2) flow rate was 
increased, an etch rate ratio of about 2:1 was reached at dopant gas flow 
rates of about 25-30 sccm (about 14-16 percent by volume of the total 
BCl.sub.3 +Cl.sub.2 +HCl + additive gas flow rate of 180-185 sccm). 
Fluorinated additives have also been used to increase the etch rate of 
molybdenum silicide, but oxygen additives have proven more effective than 
fluorinated additives for this purpose. As shown in FIGS. 17-20, the 
oxygen additive effect on molybdenum silicides has proven to be sensitive 
to the hexode temperature, and is sufficient to effectively double the 
molybdenum silicide etch rate without substantially affecting the 
polysilicon etch rate. 
The etching data of FIGS. 17-20 were taken using (2-30) millitorr chamber 
pressure, (-300 to -350) DC bias, and HCl/BCl.sub.3 /Cl.sub.2 flow rates 
of 75/(30-40)/ (25-40) sccm. Referring to FIG. 17, for HCl.sub.3 
/BCl.sub.3 /Cl.sub.2 flow rates of 75/40/40 sccm, 30 millitorr chamber 
pressure, -300 volts DC bias and 70.degree. C. hexode temperature, the 
molybdenum etch data, curve 52, and the polysilicon etch data, curve 51, 
provide an etch rate ratio of about 2:1 at a very low oxygen flow rate 
range of about 4-10 sccm. This is about 2.5-6 volume percent of the total 
gas flow of 159-165 sccm. As mentioned, the FIG. 17 data were taken using 
a hexode temperature of 70.degree. C., which is effectively the 
temperature of the wafers during the etch process. Note, above the oxygen 
flow rate of 10 sccm, the R rapidly decreases to less than 1:1. 
FIG. 18 exhibits a similar, but less additive-sensitive effect, on the 
molybdenum silicide:polysilicon etch rate ratio using a fluorinated-oxygen 
mixture of (CF.sub.4 +8.5% O.sub.2). That is, for HCl.sub.3 /BCl.sub.3 
/Cl.sub.2 flow rates of 75/40/30 sccm, 25 millitorr chamber pressure, -350 
volts DC bias, and 72.degree. C. hexode temperature, the molybdenum 
silicide etch data, curve 54, and the polysilicon etch data, curve 53, 
exhibit an etch rate ratio of about 2:1 for dopant gas flow rates greater 
than about 25 sccm (about 15 volume percent of the total gas flow rate of 
170 sccm). 
FIGS. 19 through 21 indicate that the etch rate ratio-enhancing effect 
shown in FIGS. 17 and 18 is the result of the additive effect of 
temperature as well as the dopant gas. FIG. 19 illustrates that adding 
increasing amounts of (CF.sub.4 +8.5% O.sub.2) may in fact slightly 
decrease the etch rate ratio of molybdenum silicide, curve 59, to 
polysilicon, curve 58. 
The molybdenum and polysilicon etch data shown in curves 62 and 61 of FIG. 
20 illustrate that increasing the hexode temperature over the range 
45.degree. C. to 75.degree. C. in the absence of additive dopant gas 
increases the silicide:polysilicon etch rate ratio. 
Comparing FIG. 21 to FIG. 20 (and to FIG. 17) illustrates the double 
additive effect provided by the presence of a fixed volumetric percentage 
of the dopant gas (CF.sub.4 +8.5% O.sub.2) in the BCl.sub.3 /Cl.sub.2 
etching gas mixture as the hexode temperature is increased. The FIG. 21 
etch data were taken for HCl/BCl.sub.3 /Cl.sub.2 /(CF.sub.4 +8.5% O.sub.2) 
flow rates of 75/40/40/20 sccm, 25 millitorr chamber pressure and -300 
volts DC bias. The etch rate associated with the molybdenum curve 64 
quickly increased relative to the polysilicon etch data associated with 
curve 63 from an etch rate ratio of about 1:1 at 45.degree. C. to about 
2:1 at 80.degree. C. The oxygen component of the additive flow rate 
(8.5%.times.20 sccm=4 sccm) is about 2.3 percent of the total BCl.sub.3 
+Cl.sub.2 +HCl+ additive gas flow of 175 sccm. At a hexode temperature of 
70.degree. C., the etch rate ratio is nearly 2:1, similar to the value 
associated with the FIG. 17 data for 70.degree. C. 
To summarize the effects of temperature and additive gas, small volumetric 
percentage additions of fluorinated gas can double the tungsten 
silicide:polysilicon etch rate ratios of the BCl.sub.3 /Cl.sub.2. 
Fluorinated additives also enhance the etch rate ratio of molybdenum 
silicide to polysilicon, but to a lesser extent. Oxygen additive is more 
effective than fluorinated additives for molybdenum silicide and, like the 
addition of fluorinated gases to the tungsten silicide etching gas, 
effectively doubles the molybdenum silicide etch rate and etch rate ratio 
to polysilicon. While the range of possibilities for varying the hexode 
temperature are limited, the variation of the hexode temperature is 
believed to be effective--certainly is effective over the range of 
variation available in the particular hexode reactor--in increasing the 
etch rate of the molybdenum silicide and the etch rate ratio to 
polysilicon. 
Furthermore, the addition of fluorinated gases and/or oxygen to the 
silicide etching did not degrade the subsequent polysilicon etch process 
provided, that the volume percentage/amount of additives did not exceed 
about 20 volume percent of the total gas flow. When this limit was 
exceeded, polysilicon or silicide undercutting resulted; silicide etch 
selectivity to oxide decreased; the poly etch rate increased, decreasing 
the silicide-to-poly etch rate ratio, R; and residues were left on the 
wafers after etching. 
c. Cleanliness and Profile (Undercutting) 
The desirability of using the flow rate ratio BCl.sub.3 :Cl.sub.2 .gtoreq.1 
for cleanliness and etching profile was discussed previously. 
FIG. 22 depicts the increases in silicide and polysilicon etch rates which 
are achieved for BCl.sub.3 -containing Cl.sub.2 gas chemistry containing 
(CF.sub.4 +8.5% O.sub.2) dopant gas by adding increasing amounts of HCl. 
Curve 65 shows the variation in polysilicon etch rate as a function of the 
HCl flow rate using 25 millitorr chamber pressure, -350 volts DC bias, 
45.degree. C. hexode temperature and BCl.sub.3 /Cl.sub.2 /(CF.sub.4 +8.5% 
O.sub.2) flow rates of 50/40/10. Tungsten curve 66 illustrates that adding 
increasing amounts of HCl under the same conditions did not affect the 
molybdenum silicide etch rate provided by the basic BCl.sub.3 /Cl.sub.2 
gas chemistry and the dopant gas. Also, molybdenum curve 67 shows that 
adding increasing amounts of HCl at 15 millitorr chamber pressure, -300 
volts DC bias, and BCl.sub.3 and Cl.sub.2 flow rates of 40 and 30 sccm, 
respectively, did not substantially affect the silicide etch rate. 
However, inspection of wafers processed under these and similar conditions 
revealed that the HCl was effective in providing clean etching without 
leaving residues on the wafer. In general, the ratio (BCl.sub.3 
+Cl.sub.2): (total gas flow).ltoreq.0.8:l is desirable to provide 
sufficient HCl to maintain clean etching. 
d. Loading Effect 
As shown, for example, in FIGS. 12 and 22, the total BCl.sub.3 and Cl.sub.2 
flow rate (in particular, the Cl.sub.2, flow rate), not the HCl flow rate 
is responsible for the silicide etch rate in the HCl/BCl.sub.3 /Cl.sub.2 
gas chemistry. 
In addition, as shown in FIG. 23, increasing the total flow rate of 
BCl.sub.3 and Cl.sub.2 diminishes the loading effect. FIG. 23 depicts the 
etch rate for tungsten silicide and polysilicon as a function of the 
number of wafers in the etch chamber. Polysilicon etch curve 68 is paired 
with tungsten silicide etch curve 69, whereas polysilicon etch curve 70 is 
paired with tungsten silicide etch curve 71. That is, curves 68 and 70 
show the variation in polysilicon and tungsten silicide etch rate as a 
function of adding wafers to the reactor chamber under the conditions of 
25 millitorr chamber pressure, -350 volts DC bias, and HCl.sub.3 
/BCl.sub.3 /Cl.sub.2 /(CF.sub.4 +8.5% O.sub.2) flow rates of 75/40/40/20 
sccm for a total flow rate of 175 sccm. The tungsten silicide etch rate, 
curve 69, decreased rapidly as wafers were added to the etch chamber. 
Curves 70 and 71 depict the polysilicon and tungsten silicide etch rates 
under the same conditions of pressure, DC bias and total gas flow. 
However, the component gas flow rates for HCl/BCl.sub.3 /Cl.sub.2 
/(CF.sub.4 +8.5% O.sub.2) were 25/60/60/30. As a result, the tungsten 
silicide etch curve 71 was less sensitive to the addition of wafers than 
was tungsten silicide curve 69. This is attributed to the increase of the 
BCl.sub.3 and Cl.sub.2 flow rates from 40 to 60 sccm each. 
e. Selectivity to Photoresist 
Our investigation demonstrated that in addition to enhancing etch 
cleanliness, increasing the hydrogen containing HCl flow rate enhances 
(that is, increases) the overall etch selectivity over photoresist and 
enhances reproducibility. 
In addition, increasing pressure can be effective in increasing the overall 
etch selectivity for photoresist. For example, FIG. 24 illustrates the 
overall selectivity for photoresist during the etching of molybdenum 
silicide. The silicide etch used BCl.sub.3 +CCl.sub.2 F.sub.2 +HCl flow 
rates of 30+5+50 sccm and 1100 watts power. The molybdenum silicide 
etching curve 72 indicates that increasing the pressure over the range 10 
to 70 millitorr increased the silicide etch rate only slightly. However, 
curve 73 indicates that increasing the pressure over the same range 
increased the overall selectivity over photoresist from .ltoreq.1:1 to 
almost 2:1. 
3. Overall Silicide and Polysilicon Etch Process and Summary 
Table 1 summarizes the set of working parameter ranges which have been used 
to etch refractory metal silicide-polysilicon composites using titanium, 
tantalum, tungsten and molybdenum silicides. The basic gas chemistry is 
the HCl/Cl.sub.2 gas chemistry which is used for the polysilicon etch. In 
the polysilicon etch, the HCl serves to improve etching, selectivity for 
resist, whereas the chlorine serves as a reactant. The polysilicon and 
silicide etch steps use similar pressure, DC bias, HCl flow rate and 
Cl.sub.2 flow rate ranges, so that the transition from the silicide to the 
polysilicon etch is primarily a matter of making any needed fine 
adjustments in these parameters and terminating the BCl.sub.3 and the flow 
fluorinated gas and/or the oxygen flow. Thus, the two steps are readily 
implemented in the same reactor, without breaking vacuum, as essentially a 
continuous two-step process. 
The parameters given in Table 1 are for the AME 8100 plasma etching system 
which has a chamber volume of .about.160 liters. However, the present 
invention is readily transferable to other similar plasma etchers capable 
of operating in the reactive ion etching mode by applying the flow rate 
ratios given here. The conversion of flow rate ratios is contemplated in 
the claims defining the invention. Thus, the claims which specify gas flow 
rates in defining the invention are to be interpreted also in accordance 
with the applicable flow rate ratios, and not limited to the specific flow 
rates in sccm. 
TABLE 1 
______________________________________ 
POLYCIDE RIE ETCH 
Process Silicide Polysilicon 
Parameters Etch Etch 
______________________________________ 
Total Gas Flow, sccm 
180 80-140 
HCl Gas Flow, sccm 
40-75 80-100 
Cl.sub.2 Gas Flow, sccm 
60-40 .ltoreq.40 
BCl.sub.3 Gas Flow, sccm 
80-40 -- 
CF.sub.4 Gas Flow, sccm 
.ltoreq.30 
-- 
O.sub.2 Gas Flow, sccm 
.ltoreq.10 
-- 
Pressure, mT 15-45 10-40 
DC bias, -volts 250-400 200-350 
Power, watts 800-1500 300-800 
Cathode Temp., .degree.C. 
45-75 
______________________________________ 
TABLE 2 gives typically presently preferred parameters for etching tungsten 
silicide and molybdenum silicide using CF.sub.4 dopant gas for the 
tungsten silicide and oxygen for the molybdenum silicide. 
TABLE 2 
______________________________________ 
Process Parameters 
Silicide Etch 
WSi.sub.2 
MoSi.sub.2 
______________________________________ 
Total Gas Flow, sccm 
175 165 
HCl Gas Flow, sccm 75 75 
Cl.sub.2 Gas Flow, sccm 
40 40 
BCl.sub.3 Gas Flow, sccm 
40 40 
CF.sub.4 Gas Flow, sccm 
20 
O.sub.2 Gas Flow, sccm 5-10 
Pressure, mT 25 30 
DC bias, -volts 350 300 
Power, watts 1350 1350 
Cathode Temp., .degree.C. 
45 45 
______________________________________ 
To summarize several of the critical parameters and flow rate ratios, 
BCl.sub.3 :Cl.sub.2 .gtoreq.1:1 is used for cleanliness and to prevent 
undercutting. In addition, the ratio of the BCl.sub.3 +Cl.sub.2 flow rate 
to the total flow rate is preferably within the range 0.8-0.4. That is, 
(BCl.sub.3 +Cl.sub.2):(total flow)=0.8-0.4. As mentioned, the limit of 0.8 
is used to ensure sufficient HCl for cleanliness. Below the lower limit of 
about 0.4, insufficient chlorine flow provides an undesirably low etch 
rate ratio of silicide to polysilicon. Preferably, the ratio is toward the 
higher end of the 0.4-0.8 range to decrease the loading effect. Finally, 
but not exhaustively, it is desirable that the additive gas flow rate 
be.ltoreq.20 percent of the total gas flow. That is, (additive 
flow):(total gas flow).ltoreq.0.2. Above this limit undercutting occurs; 
the polysilicon etch rate increases and silicide:polysilicon etch rate 
ratio decreases; and the selectivity to oxide decreases. These flow rates 
and ratios were obtained using a total flow rate of about 180 sccm. The 
above flow rate ratios can be used to determine the constituent flow rates 
for other total flow rates.