Cleaning processing method of a film forming apparatus

Disclosed herein is a cleaning processing method in which an object to be processed is mounted on a susceptor in a process chamber of a CVD apparatus, a TiCl.sub.4 gas, a H.sub.2 gas, and a Ar gas are introduced, a Ti film is formed on a surface of the object to be processed in a region of a plasma generated, the object to be processed is conveyed out of the process chamber, supply of the H.sub.2 gas and the Ar gas is thereafter stopped without generating a plasma, and the TiCl.sub.4 gas is introduced by means of a carrier gas, to remove unnecessary Ti films sticking to the inside of the film forming apparatus.

BACKGROUND OF THE INVENTION 
The present invention relates to a cleaning processing method of a film 
forming apparatus. 
In general, a semiconductor integrated circuit comprised of a number of 
circuit elements is formed on a substrate such as a silicon substrate or 
the like by repeating film forming processing and pattern etching 
processing. 
In manufacturing steps thereof, metal wires connecting circuit elements 
with each other, contact metals making electric contacts with circuit 
elements, and barrier metals for preventing diffusion of Si from the 
substrate all preferably have a low electric resistance and are also 
preferably made of materials having an excellent corrosion-resistance. 
Ti (titanium), W (tungsten), Mo (molybdenum), and the like are selected as 
those materials. In particular, a Ti film has an excellent characteristic 
such as an electric characteristic, a corrosion-resistance, and the like 
and is therefore used in many cases. 
For example, in case of forming a Ti film by a CVD method using a CVD 
(Chemical Vapor Deposition) apparatus, film formation is carried out by 
plasma processing in an atmosphere in which a TiCl.sub.4 (titanium 
tetrachloride) gas and a H.sub.2 gas are introduced as material gases. 
Not only in film formation of a Ti film but also in film formation on a 
wafer surface by a CVD apparatus, an unnecessary film inevitably sticks to 
the side wall of a process chamber, surfaces of a susceptor (or mount 
table), a shower head which supplies a material gas, and the like. 
If such an unnecessary film is peeled off during film formation on a 
semiconductor wafer, the peeled film becomes particles which stick to the 
semiconductor surface while film formation is being performed, thereby 
causing a factor of a defective element. Therefore, cleaning processing 
for removing a film formed on and sticking to inner structural components 
and the like is sequentially performed on a certain number of wafers, 
e.g., twenty five. 
Generally, in the cleaning processing of an unnecessary Ti film, a 
semiconductor wafer on which a Ti film is formed is conveyed out of a 
process chamber. Thereafter, the temperature of the process chamber is 
decreased to about 200 to 300.degree. C. which is preferred as a cleaning 
temperature, from about 650.degree. C. which is a film forming temperature 
of a Ti film. After the temperature reaches the cleaning temperature, the 
ClF.sub.3 gas, NF.sub.3 gas, and the like are introduced to a processing 
chamber and cleaning processing is carried out. 
In case of using a ClF.sub.3 gas, electric discharging is stopped to 
prevent generation of a plasma (hereinafter called plasmaless cleaning 
processing). In case of using a NF.sub.3 gas, a plasma is generated 
(hereinafter called plasma cleaning processing). 
The temperature in the process chamber is decreased to 200 to 300.degree. 
C. when carrying out cleaning processing as described above, because not 
only unnecessary Ti films but also inner walls of the process chamber and 
the surfaces of the susceptor are removed if the temperature in the 
process chamber is too high. 
After the cleaning processing is thus completed in a predetermined time 
period, the temperature in the process chamber is increased again, for 
example, to 650.degree. C., and formation of a Ti film is continued. 
FIG. 6 shows the temperature in the process chamber in film forming steps 
according to a conventional CVD method. 
In the film forming steps, the period Ti is a film forming processing step 
in which the temperature in the processing chamber is set to 650.degree. 
C. which is the film forming temperature. In this step, for example, film 
formation of Ti films is performed sequentially on twenty five 
semiconductor wafers in an atmosphere in which material gases as described 
above are introduced. 
Upon completion of the film formation, the processing flow goes to a 
cleaning processing step. At first, the temperature is decreased from 
650.degree. C. to 250.degree. C. in a period T2. After the temperature 
reaches the cleaning temperature, a ClF.sub.3 gas, a NF.sub.3 gas, and the 
like are introduced into the process chamber in a period T3 and cleaning 
processing is performed on the Ti films. 
Upon completion of the cleaning processing, the temperature in the process 
chamber is increased to 650.degree. C., and the cleaning processing step 
is completed. 
Further, the processing flow further goes again to a film forming 
processing step and Ti films are formed on a semiconductor wafer, like in 
the period Ti. 
Thus, the film forming processing step (corresponding to the period Ti) and 
the cleaning processing step (corresponding to the periods T2 to T4) are 
repeatedly carried out. 
In a conventional cleaning processing step as described above, the 
temperature of the chamber must be increased and decreased before and 
after the cleaning processing in the period T3, e.g., in the periods T2 
and T4, as shown in FIG. 6. Therefore, film forming processing cannot be 
carried out during the periods T2 and T4, and as a result, a problem 
occurs in that the throughput is decreased. 
Although depending on the number of wafers to be subjected to film forming 
processing, for example, each of the period T2 for decreasing the 
temperature and the period T4 for increasing the temperature requires 
about thirty minutes with respect to the period T3 of the cleaning 
processing of about thirty minutes, so that the time required for the 
cleaning processing step finally occupies ninety minutes, thereby causing 
a significant factor which decreases the throughput. 
In addition, since the increase and decrease of the temperature are 
repeatedly carried between the film forming temperature and the cleaning 
temperature, there is a drawback that metal fatigue is accumulated in 
structural components inside a chamber, such as a mount table or the like, 
and the life-time of the structural components and the like are shortened. 
Further, in plasma cleaning processing using a NF.sub.3 gas, it is 
impossible to sufficiently remove films formed on and sticking to portion 
other than the region where a plasma is formed, e.g., films sticking to 
side surfaces of a shower head. 
BRIEF SUMMARY OF THE INVENTION 
The present invention has an object of providing a cleaning processing 
method for a film forming apparatus, in which an unnecessary portion of a 
Ti film sticking to the inside of the apparatus under a plasmaless 
atmosphere is removed with use of a TiCl.sub.4 gas as a gas for forming a 
Ti film. 
Hence, the present invention provides a cleaning processing method for a 
film forming apparatus, comprising steps of: mounting an object to be 
processed, on a susceptor in a process chamber of the film forming 
apparatus, forming a Ti film on a surface of the object to be processed in 
a plasma region generated in an atmosphere formed by introducing a 
TiCl.sub.4 or other metal halide gas, a H.sub.2 gas, and a Ar gas, and 
conveying the object to be processed, out of the process chamber; and 
thereafter stopping supply of the H.sub.2 gas and the Ar gas while 
continuing supply of the TiCl.sub.4 gas, and performing cleaning to remove 
an unnecessary potion of a Ti film sticking to the film forming apparatus 
under a plasmaless atmosphere. 
Additional objects and advantages of the invention will be set forth in the 
description which follows, and in part will be obvious from the 
description, or may be learned by practice of the invention. The objects 
and advantages of the invention may be realized and obtained by means of 
the instrumentalities and combinations particularly pointed out 
hereinbefore.

DETAILED DESCRIPTION OF THE INVENTION 
In the following, explanation will be specifically made of an embodiment 
according to the present invention. 
FIG. 1 shows an example of a structure of a film forming apparatus (or 
plasma CVD apparatus) and explains a cleaning processing method according 
to the present invention. 
In the present embodiment, explanation will be made of a case of forming a 
Ti (titanium) film as a metal film. 
The CVD apparatus 2 has a process chamber (or reaction chamber) 4 made of, 
for example, stainless steel or the like, and the potential of the process 
chamber 4 is grounded. 
A plurality of exhaust ports 8 are provided at a bottom portion 6 of the 
process chamber 4. The exhaust ports 8 are connected with an exhaust 
system 10 including a vacuum pump and the like. The inside of the process 
chamber is uniformly evacuated or a process gas, a cleaning gas, and the 
like are exhausted uniformly therefrom. 
A support columns 14 made of a conductive material are fixed to the bottom 
surface of the process chamber 4, and a susceptor 16 for mounting, for 
example, a semiconductor wafer W is supported by the support columns 14. 
The susceptor 16 is at the same potential as the process chamber and also 
serves as a lower electrode in the CVD apparatus. The susceptor 16 
consists of a lower base 16A directly supported on the support column 14 
and an upper base 16B joined to the upper surface of the lower base 16A. 
The joining surfaces of the lower base 16A and the upper base 16B are 
joined to each other by welding. 
The shower head 20 is provided so as to have a size enough to cover 
substantially the entire of the upper surface of the susceptor 16, such 
that the head 20 is opposed to the upper surface, forming a processing 
space S (or a plasma region) between the head and the susceptor 16. The 
shower head 20 serves to introduce various gases in form of shower to the 
processing space, and a number of injection holes 28 are formed in the 
injection surface 26 at the lower surface of the shower head 20. 
In addition, a diffusion plate 32 having a number of diffusion holes 30 is 
provided at the inside of the shower head 20 so that a gas can be diffused 
therefrom. 
A gas introduce port 34 for introducing a material gas and the like into 
the head is provided at an upper portion of the shower head 20, and the 
gas introduce port 34 is connected with a material gas supply line 36. 
The gas supply line 36 is branched into a plurality of branch tubes 38 
which are respectively connected to, for example, a TiCl.sub.4 gas source 
40 which stores a TiCl.sub.4 gas, a H.sub.2 gas source 42 which stores a 
H.sub.2 gas, a Ar gas source 44 which stores a Ar gas as a plasma gas, and 
a N.sub.2 gas source 46 which stores an inactive gas, e.g., a N.sub.2 gas 
used as a carrier gas during cleaning. 
The flow rates of the gases are respectively controlled by flow rate 
controllers, e.g., mass-flow controllers 48 provided for the branch tubes. 
In the present embodiment, the gas supply line connected to the shower head 
20 is shared by material gases used for film formation, through branch 
tubes 38. However, the present invention is not limited hitherto, but it 
is possible to adopt a gas supply form called a post mix type, in which 
gas tubes for several parts of gasses such as a carrier gas and a purge 
gas or in which gas tubes for all of the gases may be individually 
connected to the shower head 20 and the parts or all of the gases may be 
mixed in a processing space S. 
In order to form a plasma during formation of a Ti film, the top plate 22 
is connected with a matching circuit 52 and a high frequency power source 
54 for forming a plasma of, for example, 13.56 MHz. 
The side wall of the process chamber 4 is provided with a chamber 
temperature control jacket for selectively flowing a cooled medium upon 
necessity for temperature control of the wall surface. The jacket 55 is 
connected with a temperature controller 56. 
The side wall of the process chamber 4 is also provided with a gate valve 
58 which can be opened and closed when conveying in and out a 
semiconductor wafer W. 
Further, the shower head 20 is provided with a head temperature control 
jacket 60 for selectively flowing a cooled medium upon necessity for 
temperature control of the surface including the injection surface 26. 
Although not shown, it is needless to say that the susceptor 16 may be 
provided with a wafer lifter pin for lifting up and down a semiconductor 
wafer W when conveying in and out the semiconductor wafer W. 
Explanation will now be made of a cleaning processing method according to 
the present invention which is carried out after a film forming processing 
step, using a CVD apparatus constructed as described above. 
The cleaning processing method is carried out in a cleaning processing step 
for removing unnecessary Ti films sticking to the inside of the process 
chamber after a processing object on which a metal film has been formed in 
a film forming processing step is conveyed out. In practice, the film 
forming step and the cleaning processing step are alternately repeated. 
FIG. 2 shows a cleaning processing step according to the present embodiment 
in the film formation using a CVD method, in which the longitudinal axis 
represents the temperature in the process chamber and the lateral axis 
represents the time. 
At first, explanation will be made of a case in which a Ti film is formed 
on the surface of a semiconductor wafer W in the film forming processing 
step of a period T11. 
A semiconductor wafer W is conveyed into the process chamber 4 through an 
opened gate valve 58 from the outside or a load lock chamber not shown. 
The semiconductor wafer W is mounted on a susceptor 16 and the gate valve 
is then closed. A contact hole and the like are formed in the surface of 
the semiconductor wafer W in a previous step, to make contact with 
electrodes of a circuit element on the wafer. 
After the gate valve 58 is closed, evacuation is performed to a 
predetermined vacuum degree. Further, TiCl.sub.4 gas and H.sub.2 gas used 
as material gases and Ar gas for generating a plasma are introduced at 
predetermined flow rates controlled by the mass-flow controller 48, and 
the inside of the process chamber 4 is maintained at a predetermined 
pressure by a vacuum system 10. 
At the same time, a high frequency wave of 13.56 MHz is applied between the 
shower head 20 as an upper electrode and the susceptor 16 as a lower 
electrode, from a high frequency power source 54. 
The Ar gas is thereby changed into plasma. The plasma promotes the 
reduction of TiCl.sub.4 gas with H.sub.2 gas. As a result, a Ti film is 
formed on the surface of the semiconductor wafer W. While the Ti film is 
being formed, the temperature of the semiconductor wafer W is maintained 
at a prescribed value by a heater 18 embedded in the susceptor. 
The side wall of the process chamber 4 and the shower head 20 which tend to 
be heated by a plasma are cooled to predetermined temperatures by 
respectively making cooling media flow through the chamber temperature 
control jacket 56 and the head temperature control jacket 60. 
In this time, the process conditions are arranged, for example, such that 
the wafer temperature (or susceptor temperature) is about 650.degree. C., 
the process pressure is about 1 Torr, and the high frequency power is 
about 700W. 
Thus, upon completion of film forming processing for one semiconductor 
wafer W, the semiconductor wafer W thus processed is conveyed out through 
the gate valve 58 opened, and a next semiconductor wafer W not processed 
is conveyed into the process chamber 4. Film forming processing of a Ti 
film is performed on this wafer in the same manner as described above. 
The film forming processing as described above is sequentially performed on 
a predetermined number of semiconductor wafers W, e.g., twelve to twenty 
five wafers W. In this while, unnecessary Ti films gradually stick to and 
are deposited on the internal structure of the process chamber 4 and the 
inner wall of the chamber. 
Next comes a cleaning processing step according to the present embodiment, 
as indicated by a period T12 in FIG. 2. 
After the last (for example twenty-fifth) semiconductor wafer W on which 
film forming processing is completed is conveyed out of the chamber, the 
gate valve 58 is closed and the inside of the process chamber 4 is 
subjected to evacuation by an exhaust system 10. In this time, the 
temperature in the chamber is maintained at the same temperature as that 
in the film forming processing step. 
Thereafter, with the supply of the Ar gas and H.sub.2 gas stopped, the 
TiCl.sub.4 gas as a material gas and an inactive gas, e.g., an N.sub.2 gas 
as a carrier gas which helps supply of the TiCl.sub.4 gas are introduced 
into the chamber. 
No high frequency wave is supplied from the high frequency power source, 
and cleaning processing is carried out under a plasmaless condition. 
Therefore, a mixture gas of only the TiCl.sub.4 gas and the N.sub.2 gas is 
supplied under a plasamaless condition. 
The cleaning processing is carried out while the temperature of the 
susceptor 16, i.e., the temperature in the process chamber 4 is maintained 
at the same temperature as that when forming a Ti film during the period 
T11, e.g., 650.degree. C. 
Therefore, etching of unnecessary Ti films is performed, i.e., cleaning 
processing is performed without decreasing the temperature in the process 
chamber. 
Process conditions of the cleaning processing are preferably arranged such 
that the pressure is about 0.1 to 10 Torr and the flow ratio of the 
TiCl.sub.4 gas to the N.sub.2 gas is about 5 to 100%, in order to obtain 
an etching rate of unnecessary Ti films. 
The etching processing is performed for a predetermined period T12 and the 
cleaning processing step is thereby completed. Then, introduction of the 
TiCl.sub.4 gas and the N.sub.2 gas is stopped and evacuation is once 
carried out by an exhaust system. 
Further, a next lot of semiconductor wafers W are conveyed into the process 
chamber 4, and the processing flow goes to the film forming processing 
step of forming a Ti film again like in the period T11. 
At the time point when the film forming processing step is thus started 
again, operation for increasing the temperature needs not be carried out 
but the film forming processing step can be immediately started since the 
temperature in the process chamber 4 and the temperature of the susceptor 
16 are maintained at 650.degree. C. in the cleaning processing step, which 
is the same as that used when forming a film. 
In this manner, the film forming step of forming a Ti film and the cleaning 
step thereof are carried out alternately and sequentially, as shown in 
FIG. 2. 
Therefore, as is apparent from the present embodiment shown in FIG. 2 and 
the film forming step including the conventional cleaning processing step 
shown in FIG. 6, it is not necessary to perform operation of increasing or 
decreasing the temperature of the susceptor 16 when the processing flow 
goes to the cleaning processing step from the film forming processing step 
or when the processing flow returns inversely again to the film forming 
processing step from the cleaning processing step, in the present 
embodiment. 
Thus, according to the present embodiment, it is possible to save the time 
required for operation for decreasing or increasing the temperature, which 
is necessary for a conventional method, and the throughput can be improved 
accordingly. 
Next explanation will be specifically made of etching operation for 
unnecessary Ti films with use of a TiCl.sub.4 gas in the cleaning 
processing method according to the present invention. 
FIG. 3 shows data concerning changes of the temperature and the etching 
rate with respect to a Ti film when a Ti film was actually etched. The 
etching conditions were arranged such that the pressure is 0.32 Torr, the 
flow ratio of a TiCl.sub.4 gas to a N.sub.2 gas is 20 cc:650 cc, and the 
TiCl.sub.4 gas has a concentration of about 3%. 
As a result of etching a Ti film with the temperature variously changed 
from 500.degree. C. to 700.degree. C., it has been found that the higher 
the temperature is, the greater the etching rate of the Ti film is. 
When the temperature was 650.degree. C. which is the same as that in the 
step of forming a Ti film exemplified in the present embodiment, an 
etching rate of approximately 60 nm/min was obtained. 
In this cleaning processing method, inner walls of the process chamber 4 
and surfaces of the susceptor 16 are not substantially removed at all but 
maintain original conditions without damages. 
FIG. 4 shows a relationship between the concentration of a TiCl.sub.4 gas 
with respect to a N.sub.2 gas as a carrier gas and the etching rate. 
Etching conditions in this relationship are arranged such that the 
temperature in the process chamber is 650.degree. C. and the pressure is 
0.32 Torr. 
From the relationship, it has been found that the etching rate of a Ti film 
increases in accordance with an increase of the concentration of the 
TiCl.sub.4 gas. 
Generally, in a conventional cleaning processing step using a ClF.sub.3 gas 
and a NF.sub.3 gas, the etching rate is approximately 300 nm/min. Hence, 
it has been found that the concentration of the TiCl.sub.4 gas in the 
present embodiment should be set to about 5% in order to obtain an etching 
rate substantially equal to the etching rate of the conventional cleaning 
processing step. Note that the concentration of a TiCl.sub.4 gas during 
actual cleaning should appropriately be changed in accordance with a 
desired etching rate and the processing pressure is not limited to 0.32 
Torr which is only an example. 
Thus, according to the present invention, a TiCl.sub.4 gas is used as a 
cleaning gas under a plasmaless condition when cleaning Ti films, and 
therefore, it is possible to remove unnecessary Ti films at a high 
temperature equal to the film forming temperature of the Ti films. 
As a result, it is possible to omit operation of decreasing and increasing 
the temperature of the susceptor, which is performed in a conventional 
cleaning processing step, and accordingly, the throughput can be improved. 
Also, the necessity to decrease or increase the temperature in the process 
chamber is lost accordingly. Therefore, the frequency at which a heat 
cycle causes metal fatigue in a structure in a chamber, and accordingly, 
the life-time of the structure in the chamber can be elongated. 
Further, since cleaning processing can be performed without generating a 
plasma, unnecessary Ti films sticking to other portions than the surfaces 
facing a plasma region can be removed. 
In addition, since the gas used for forming films can be used for cleaning 
processing, it is unnecessary to provide an optional equipment such as a 
gas supply system specialized for cleaning, unlike in a conventional 
method. 
Although the present embodiment performs cleaning processing once after 
every sequential film formation of a plurality of semiconductor wafers W, 
e.g., twenty five semiconductor wafers W, cleaning processing can be 
performed even after every film formation of one semiconductor wafer W 
without unacceptably lowering the throughput, as shown in FIG. 5, because 
operation of decreasing and increasing the temperature of the susceptor 
can be omitted. 
In this case, cleaning processing is immediately carried out even after 
only a slight amount of unnecessary Ti film sticks. Therefore, a very 
short time is sufficient for every cleaning processing, and besides, film 
formation can always be carried out under a condition that no unnecessary 
Ti film sticks to the inner walls of the chamber or the like. Accordingly, 
it is possible to prevent generation of particles and to improve the 
yield. 
Although the present embodiment has been explained with reference to a case 
of using a N.sub.2 gas as an inactive gas, the inactive gas is limited 
hitherto but He, Ar, Ne, or the like may be used. 
Also, the above explanation has been made with reference to an example in 
which semiconductor wafers W are used as objects to be processed. Needless 
to say, however, objects to be processed are not limited hitherto but the 
present invention is applicable to glass substrates, LCD substrates, and 
the like. 
Additional advantages and modifications will readily occur to those skilled 
in the art. Therefore, the invention in its broader aspects is not limited 
to the specific details and representative embodiments shown and described 
herein. Accordingly, various modifications may be made without departing 
from the spirit or scope of the general inventive concept as defined by 
the appended claims and their equivalents.