Method for thermally oxidizing polycide substrates in a dry oxygen environment and semiconductor circuit structures produced thereby

A method of thermally oxidizing polycide substrates in a dry oxygen environment as well as a MOSFET structure provided by the method are disclosed. The method includes heating a plurality of polycide substrates to temperatures greater than about 800 degrees Centigrade in a dry oxygen environment, and introducing into the environment an amount of a halogenated alkane gas sufficient to induce oxidation.

FIELD OF THE INVENTION 
The present invention relates generally to the field of integrated circuit 
fabrication, and more particularly concerns a method for thermally 
oxidizing polycide substrates in a dry oxygen environment. 
BACKGROUND OF THE INVENTION 
The design and manufacture of semiconductor integrated circuits is well 
known in the art. With the many recent advances in integrated circuit 
technology, device dimensions are continuously decreasing while the 
packing density and complexity of these devices are correspondingly 
increasing. Coincident with these advances are also increasingly stringent 
requirements for electrical interconnection materials. Briefly, these 
requirements are low resistivity, the ability to withstand the chemicals 
and high temperatures used in fabrication processes, and the capability of 
being patterned into fine lines. 
In a typical MOSFET structure, for example, an epitaxially grown 
single-crystal silicon layer provides a base or substrate, while 
polycrystalline silicon ("polysilicon") is the standard material for both 
gate and interconnect structures. The polysilicon is insulated from 
electrically conductive overlayers and the single crystal silicon 
substrate by layers of silicon dioxide. Although polysilicon provides the 
requisite stability to processing chemicals and high temperatures, a major 
limitation now restricting its utility as an interconnection material in 
high performance devices is its limited conductivity. Even heavily doped 
polysilicon has a conductivity of only about 300 micro-ohm.multidot.cm., 
thus imposing a serious limitation on circuit performance. One proposed 
solution has been to replace polysilicon with pure metals such as 
aluminum, tungsten or titanium, which have a conductivity far higher than 
that of polysilicon. However, these materials are also limited in that 
they may react with the silicon substrate at the high temperatures used in 
integrated circuit fabrication and may additionally be unable to withstand 
the chemical reagents used in processing. 
An alternative solution has been the incorporation of refractory metal 
silicides into integrated circuit fabrication technology. Silicides offer 
several advantages over single-layer doped polysilicon. In contrast to 
doped polysilicon, which at a typical thickness of 4500 Angstroms has a 
sheet resistivity of 15 ohms per sheet or more, silicides provide on the 
order of 2 ohms per sheet or less. Tantalum and tungsten silicides each 
have sheet resistivities of about 2 ohms per sheet, molybdenum silicide 
about 1.5 to about 2.0 ohms per sheet, and titanium silicide about 0.5 
ohms per sheet. Silicides are also compatible with MOSFET and other 
integrated circuit fabrication processes as they can generally withstand 
high temperatures and caustic processing chemicals. Finally, providing 
there is sufficient silicon underlying the silicide layer, a 
self-passivating silicon dioxide layer can be thermally grown over the 
silicide without any degradation of chemical or electrical properties of 
the silicide film. 
Metallic silicides provide the desired resistivity and the chemical and 
thermal stability necessary for use as interconnects, and they function 
well as FET gates. A layered structure having a polysilicon layer 
sufficiently thick to serve as a transistor gate, underlying the silicide, 
is often used. These "polycide" structures have resistivities on the order 
of 4 ohms per sheet or less where the combined thickness of both layers is 
about 4500 Angstroms. The use of such polycide structures is fairly recent 
but is known in the art. U.S. Pat. No. 4,180,596 to Crowder et al., for 
example, discloses a method of providing a silicide layer on a polysilicon 
substrate by means of sputtering and a subsequent annealing process. U.S. 
Pat. No. 4,468,308 to Scovell et al. shows a method of providing a 
silicide layer on a semiconductor substrate using a vapor deposition 
technique. Other semiconductor circuit structures having a silicide layer 
include those disclosed in the following: U.S. Pat. No. 4,276,557 to 
Levinstein et al., which shows a tantalum or titanium silicide layer 
sandwiched between a layer of doped polysilicon and a vapor-deposited 
layer of silicon dioxide; U.S. Pat. Nos. 4,332,839 to Levinstein et al. 
and 4,337,476 to Fraser et al., which show a silicide layer interposed 
between a layer of polysilicon and a thermally grown layer of silicon 
dioxide; and U.S. Pat. No. 4,450,620 to Fuls et al., which shows an MOS 
integrated circuit device having both silicide and polysilicon layers. 
After deposition of a silicide layer on polysilicon to form a polycide 
structure, an insulating silicon dioxide layer is normally provided 
thereon, by oxidation of the silicide or low pressure chemical vapor 
deposition (LPCVD) or both. A problem encountered in the thermal oxidation 
of metallic silicides and noted in the prior art is the poor oxidation of 
silicides in dry oxygen (see, e.g., U.S. Pat. No. 4,332,839 to Levinstein 
et al., and Murarka, et al., "Refractory Silicides of Titanium and 
Tantalum for Low-Resistivity Gates and Interconnects," IEEE Transactions 
on Electronic Devices, ED-27(8), pp. 1409-1417 (1980)). This problem has 
been particularly noted with respect to tantalum disilicide, which is 
almost completely resistant to a dry oxygen environment; no silicon 
dioxide can be grown even at temperatures of up to 1000 degrees 
Centigrade. The problem has also been noted with regard to tungsten 
disilicide; although evaporated tungsten disilicide may be oxidized in 
either a pure O.sub.2 or an H.sub.2 O environment, tungsten disilicide 
deposited by sputtering behaves as tantalum disilicide in an O.sub.2 -only 
ambient and will not oxidize. 
While several references have suggested the alternative of steam oxidation 
(notably Murarka et al., and Crowder, et al., 1 .mu.M MOSFET VLSI 
Technology: Part VII--Metal Silicide Interconnection Technology--A Future 
Perspective," IEEE Journal of Solid-State Circuits, SC-14 (2), pp. 291-293 
(1979)), the prior art does not suggest a feasible method of dry 
oxidation. In an integrated circuit manufacturing process, steam oxidation 
is often undesirable. Oxide growth rate in a steam oxidation process can 
be uncontrollably high, and growth rates may be very different on 
single-crystal silicon, on polysilicon and on polycide. Oxidation in a dry 
environment is not only more uniform, but is slower and therefore easier 
to control. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a method 
of thermally oxidizing polycide substrates in controlled and uniform 
manner. 
It is another object of the invention to provide a method of thermally 
oxidizing polycide substrates in a dry oxygen environment. 
It is still another object of the invention to provide a method of 
thermally growing a silicon dioxide layer on a polycide substrate in a dry 
oxygen environment, using a relatively small amount of a halogenated 
alkane gas such as trichloroethane and temperatures greater than about 800 
degrees Centigrade. 
It is a further object of the invention to provide a semiconductor 
integrated circuit structure including a silicide layer and a layer of 
silicon dioxide thermally grown in a dry oxygen ambient thereon. 
Additional objects, advantages and novel features of the invention will be 
set forth in part in the description which follows, and in part will 
become apparent to those skilled in the art on examination of the 
following. 
To achieve the foregoing and other objects, and in accordance with the 
purpose of the present invention, the method includes heating to a 
temperature greater than about 800 degrees Centigrade a polycide substrate 
in a dry oxygen environment, and introducing into the environment an 
amount of a halogenated alkane gas sufficient to induce oxidation. 
In a further aspect of the present invention, the halogenated alkane gas is 
a chlorinated alkane, and a particularly preferable compound is 
1,1,1-trichloroethane. The polycide substrate is emplaced in a 
substantially enclosed chamber including a gas inlet means and a gas 
outlet means, whereby the chamber may be filled with gas and maintained at 
atmospheric pressure. The chamber is filled with dry oxygen, heated to a 
temperature preferably between about 900 and 950 degrees Centigrade, and 
an amount of 1,1,1,-trichloroethane between about 1% and 10% by volume of 
the total volume of the gas flowing through the chamber is introduced. The 
substrate is heated for a length of time sufficient to provide a desired 
thickness of silicon dioxide thereon.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIGS. 1 and 2, a known technique for forming polycide 
conductors is described. The example given is the formation of a 
metal-oxide-silicon (MOS) field effect transistor (FET). A silicon 
semiconductor substrate 11 initially has thick field oxide (FOX) thermally 
grown in all areas other than where active devices are to be formed. This 
field oxide is usually grown after those same areas are doped to isolate 
the active devices from one another. The ionization may be done by 
implanting boron ions in such regions. The top surface of the wafer 11 is 
initially flat but growth of the field oxide layers 13 consumes a layer of 
silicon at the top surface, thus forming the depressed regions shown in 
FIG. 1. 
The next step in standard integrated circuit formation techniques is to 
grow a thin layer 15 of very high quality silicon dioxide over the wafer 
surface. This oxide layer will serve as the gate oxide of the active 
devices. 
Next, a layer 17 of polysilicon is formed by standard techniques over the 
entire surface of the wafer. Next, a layer 19 of a refractory metal 
silicide is formed in a known manner, preferably by sputtering. The 
silicide layer 19 may be any one of a number of specific compounds 
satisfying the formula MSi.sub.x, where x ranges from about 2.2 to about 
2.6, and M is a metal selected from the group consisting of tantalum, 
tungsten, titanium, molybdenum and mixtures thereof. 
After the silicide layer 19 is deposited, that layer and the polysilicon 17 
are removed from all areas of the wafer where conductors are not to be 
permanently formed. The layers 17 and 19 are removed in unwanted areas by 
the use of standard photoresist masking and etching techniques. As shown 
in FIG. 2, this leaves a polycide gate structure formed of the layer 17' 
of polysilicon and layer 19' of selected metal silicide. The total 
thickness is preferably approximately 4500 Angstroms with the polysilicon 
layer 17' being approximately 2000 Angstroms of that total thickness. This 
provides enough polysilicon for the necessary gate function and enough 
silicide for the necessary low resistivity conductivity for connecting 
that gate with other areas of the integrated circuit chip being formed. 
Source and drain regions 21 and 23 (FIG. 2) are usually formed at this 
point by ion implantation techniques. In the example being described, an 
NMOS process, N+ regions are formed as shown. 
After the intermediate structure of FIG. 2 is formed, it is desired to 
cover the entire wafer with an insulating layer, usually silicon dioxide. 
This is to provide protection to the device as formed in a silicon wafer 
substrate 11, and also to allow conductors to be formed over this 
insulating layer without substantially interfering electrically with the 
devices formed in the substrate below. Accordingly, as shown in FIG. 3, 
layers 25 and 27 of silicon dioxide are formed. The layer 25 is thin, 
usually around 1000 angstroms, and is of very high quality. The thicker 
layer 27 is most conveniently formed by standard chemical vapor deposition 
(CVD) techniques. But since the quality of the CVD-deposited dioxide is 
not good enough, the initial layer 25 is formed. 
It is the formation of the layer 25 that is the subject of the improved 
technique of the present invention. For most of the wafer surface, the 
layer 25 is easily formed on top of previously formed gate oxide layer 15 
but the oxide layer 25 also needs to be grown over the metal silicide 
layer 19' that is part of the gate electrode. The layer 25 is formed in a 
dry oxygen environment with an appropriate amount of halogenated 
hydrocarbon gas, preferably 1,1,1-trichloroethane, sufficient to induce 
growth of oxide over the silicide layer 19'. The amount of trichloroethane 
introduced preferably constitutes about one percent (1%) to ten percent 
(10%) of the total gas flowing through the chamber, and more preferably 
constitutes about six percent (6%) to seven percent (7%) of the total gas 
flow. Oxidation temperatures should be at least 800 degrees Centigrade, 
and preferably between about 900 and 950 degrees Centigrade. 
Referring to FIG. 4, a well-known high temperature furnace is shown in 
schematic form. A quartz tube 31 forms a reaction chamber 33 that is 
closed at one end by a removable cover 35 through which a plurality of 
wafers 37 may be introduced and removed. An appropriate heating element 39 
is provided on the outside of the quartz tube 31, usually an ohmic heating 
source that induces heating in the wafers 37 when they are emplaced within 
the tube. An elongated holder 41 adapted to be inserted into tube 31 
contains wafers 37. 
Gases are introduced into the chamber 33 through inlet port 49 and are 
removed by means of either outlet port 67 or, if holder 41 has been 
emplaced in tube 31, through vent 69. A quantity of dry nitrogen is held 
by container 43 and is connected through flow meter 45 to inlet port 49. A 
quantity of dry oxygen is similarly held by a container 51 that is 
connected through flow meter 53 to port 49. A second quantity of dry 
nitrogen is held by container 47, connected through flow meter 55 and 
directed into a quantity of liquid 57 enclosed in container 59. A 
temperature-controlled housing 63 encases container 59, which additionally 
contains a quantity of halogenated hydrocarbon gas 61. Gas 61, carried by 
dry nitrogen from source container 47, is directed through check valve 65 
into inlet port 49. Gas flow is preferably continuous at about 1 to about 
5 liters per minute. 
The following examples illustrate certain embodiments of the present 
invention, and are not intended to limit the scope of the invention as 
defined in the appended claims. 
EXAMPLE 1 
A dry oxidation process was carried out at 950 degrees Centigrade using 
silicon-rich tungsten disilicide (WSi.sub.x where x was approximately 2.3) 
polycide substrates. The oxidation step was effected by use of dry oxygen 
and 1,1,1-trichloroethane gas. 
The reaction chamber was first purged with nitrogen gas for approximately 
ten minutes. The end cap was removed, and the polycide substrates were 
then gradually pushed into the chamber over a period of about five 
minutes. The substrates were annealed at 950 degrees Centigrade for about 
ten minutes, ensuring bonding of the tungsten to silicon and equilibrium 
of temperature. Flow of nitrogen gas through the chamber was held constant 
at a rate of approximately 2.0 liters per minute for each of these 
processes. 
Oxidation of the substrates was then carried out in a mixture of dry oxygen 
and 1,1,1-trichloroethane, for variable lengths of time. Flow rate of 
oxygen was held constant at about 1.5 liters per minute, while the 
concurrent flow of 1,1,1-trichloroethane was maintained at a rate of about 
0.1 liters per minute. 
The oxidized substrates were annealed in nitrogen for about fifteen minutes 
at a temperature of about 950 degrees Centigrade. The substrates were then 
gradually removed over a period of about five minutes, and the chamber was 
purged with nitrogen gas for about ten minutes. Flow rate of nitrogen 
throughout these latter three processes was about 2.0 liters per minute. 
The thicknesses of silicon dioxide layers grown over the substrates were 
measured in several areas over the surfaces of the substrates and 
averaged. As expected, the average thicknesses of the SiO.sub.2 layers 
obtained varied in approximately a linear fashion with oxidation time. 
Results are shown in Table 1, below. 
EXAMPLE 2 
For purposes of comparison with the dry oxidation method of the invention, 
a steam oxidation process was carried out in a similar manner, using 
silicon-rich tungsten disilicide substrates identical to those used in 
Example 1. 
Over a period of about fifteen minutes, the reaction chamber was first 
purged with nitrogen gas, and polycide substrates were gradually pushed 
into the chamber. The substrates were then annealed at 900 degrees 
Centigrade for about ten minutes. As in Example 1, flow of nitrogen gas 
through the chamber was held constant at a rate of approximately 2.0 
liters per minute throughout each of these processes. 
Oxidation of the substrates was then carried out in steam for variable 
lengths of time. Gas flow rate during this step was maintained at about 
1.5 liters per minute. 
The oxidized substrates were annealed in nitrogen for about ten minutes at 
a temperature of about 900 degrees Centigrade and then gradually removed 
over a period of about ten minutes. The chamber was then purged with dry 
nitrogen for about five minutes. Flow rate of nitrogen throughout these 
last three processes was held constant at about 2.0 liters per minute. 
Average silicon dioxide thicknesses were measured as in Example 1. Results 
are shown in Table 1, below. As may be seen from the table, the inventive 
method using an O.sub.2 trichloroethane mix provided a far more gradual 
oxidation. 
TABLE 1 
______________________________________ 
Dry Oxidation* Steam Oxidation 
Time, Average SiO.sub.2 
Time, Average SiO.sub.2 
min. Thickness, angstroms 
min. Thickness, angstroms 
______________________________________ 
80 877 .+-. 14 4 634 .+-. 47 
90 900 .+-. 20 8 779 .+-. 56 
160 1105 .+-. 25 16 1095 .+-. 66 
______________________________________ 
*O.sub.2 /1,1,1trichloroethane mix. 
EXAMPLE 3 
Two tungsten disilicide polycide substrates were oxidized by the dry 
oxidation method set forth above in Example 1. Oxidation time was 
approximately 160 minutes. After oxidation and removal of the substrates, 
measurements of indices of refraction were made at various points on the 
substrates as a check of SiO.sub.2 layer uniformity. 
Results are set forth in Table 2. 
EXAMPLE 4 
For purposes of comparison, indices of refraction were likewise measured on 
wafers prepared using steam oxidation. Two tungsten disilicide polycide 
substrates were thus oxidized by the steam oxidation method set forth in 
Example 2. Oxidation time was approximately 90 minutes, and indices of 
refraction were measured at various points on the substrates as in Example 
3. Results are listed in Table 2. 
A comparison of the results yielded by steam oxidation and the dry 
oxidation process of the invention shows that the inventive method 
provides a purer and more uniform SiO.sub.2 layer, as indicated by the 
higher indices of refraction obtained. 
TABLE 2 
______________________________________ 
Dry Oxidation* Steam Oxidation 
Wafer # 
Index of Refraction 
Wafer # Index of Refraction 
______________________________________ 
1 1.45 1 1.37 
1.45 1.33 
1.45 1.37 
2 1.48 2 1.37 
1.45 1.33 
1.43 1.37 
3 1.37 
1.33 
1.37 
4 1.37 
1.33 
1.37 
______________________________________ 
*O.sub.2 /1,1,1trichloroethane mix.