Method of in-situ cleaning of a chuck within a plasma chamber

A method for in-situ cleaning of a chuck that bears a semiconductor wafer in a semiconductor manufacturing machine maintains a processing chamber in a sealed condition with the chuck inside the chamber. A wafer bearing surface of the chuck is exposed upon determining that the chuck requires a cleaning. A cleaning gas is then injected into the chamber and RF power is applied to the chamber to create a plasma that cleans the wafer bearing surface. Since the processing chamber is maintained in a sealed condition during the in-situ cleaning of the chuck, the time required to clean the chuck and prepare the chamber for continued production runs is greatly reduced.

FIELD OF THE INVENTION 
The present invention relates to the field of chemical vapor deposition 
systems, and more particularly, to methods of cleaning deposition left by 
the process gas on a wafer bearing surface of a chuck. 
BACKGROUND OF THE INVENTION 
Chemical vapor deposition (CVD) systems normally employ a chamber in which 
gaseous chemicals react. From these reactions, a substance is deposited on 
a surface of a workpiece e.g., a semiconductor wafer or dielectric plate 
or metal sheet to form dielectric, conductor, and semiconductor film 
layers that constitute an integrated circuit, for example. (Throughout, 
most of the remainder of this document, the workpiece is assumed to be a 
semiconductor wafer but it is to be understood that the invention is 
applicable to other types of workpieces.) In a chemical vapor deposition 
system, a process gas is injected into the plasma chamber in which a 
plasma is formed. Due to the ion bombardment within the plasma of the 
process gas (SiH.sub.4 (silane), SiF.sub.4 for example), silicon is 
deposited on a wafer which has been previously placed on a chuck in the 
chamber. 
The chuck uses electrostatic force, for example, to hold the wafer securely 
in place during its processing. Since the workpiece covers the chuck 
during the processing of the semiconductor wafer, deposition of 
materials., e.g. silicon, onto the wafer bearing surface of the chuck is 
substantially prevented. However, in practice, a wafer may occasionally 
slide on the chuck, in which case a portion of the wafer bearing surface 
is exposed during the deposition process, causing deposition of material 
on the chuck wafer bearing surface. Alternatively, over time, deposition 
material may seep in under a wafer and leave a deposit on the chuck wafer 
bearing surface. 
The presence of deposition material on the wafer bearing surface creates 
problems in the wafers that are manufactured in production runs. For 
example, deposition on the wafer may not have uniform thickness throughout 
the wafer as a result of hot spots created by the deposited material on 
the chuck wafer bearing surface. 
Since creating deposition on wafers in a production run normally must have 
uniform thickness within precise tolerances, deposition on a chuck wafer 
bearing surface is a problem that must be corrected before additional 
wafers are processed. In the prior art, the process of removing the 
deposition from the chuck involved opening the sealed process chamber, 
removing the chuck, and cleaning the chuck with some material, such as 
hydrofluoric acid. This is a very expensive process since it shuts down 
the semiconductor manufacturing machine for a large number of hours. Time 
is required both to clean the chuck, as well as to re-prepare the chamber 
for continued processing. Since the chamber is a high vacuum chamber, it 
may take six or more hours to return the chamber to its operating 
condition. Also, if a chamber is opened for cleaning, a conditioning run 
of approximately 75 wafers must be executed before the processing can 
again be performed on production wafers. 
It is known to remove deposition material (such as SiO.sub.2 or 
flourine-doped SiO.sub.2) in a sealed chamber by injecting a cleaning gas 
(such as NF.sub.3) and then applying RF power to the chamber. The walls 
are cleaned of oxide deposition. This chamber cleaning is normally 
performed periodically during a production run, such as after every five 
wafers have been processed. In this known process, however, the chuck is 
kept covered since prolonged NF.sub.3 exposure may damage the chuck and 
degrade the clamping force of the chuck. Another reason for covering the 
chuck in the prior art method is that temperature probes used in the chuck 
to measure the temperature of the wafer or for calibrating have been black 
body temperature probes that are damaged by exposure to NF.sub.3 and 
plasma. As the chuck is covered during the cleaning of the process 
chamber, the only method of cleaning the deposition material from the 
chuck according to the prior art has been to open the chamber and remove 
the chuck. As stated earlier, this is a very expensive and time-consuming 
process. 
SUMMARY OF THE INVENTION 
There is a need for a method of cleaning the deposition material of a chuck 
in a semiconductor wafer manufacturing machine without opening the 
processing chamber of the machine. 
This and other means are met by the present invention which provides a 
method of in-situ cleaning of a semiconductor wafer chuck. In this method, 
the processing chamber is maintained in a sealed condition with the chuck 
inside the chamber. The wafer bearing surface of the chuck is exposed and 
a cleaning gas is injected into the chamber. RF power is applied to the 
chamber to create a plasma that cleans the wafer bearing surface. The RF 
power energizes the plasma and creates reactions on the wafer bearing 
surface to dissociate the deposition material on the chuck. 
In certain embodiments of the present invention, the cleaning gas is 
NF.sub.3. While an excessive amount of exposure of NF.sub.3 may damage a 
chuck, a limited amount of exposure does not degrade the operation of the 
chuck to a great extent. Furthermore, the use of temperature probes in the 
chuck that are not sensitive and damaged by either plasma or NF.sub.3 
allow the chuck to be exposed and cleaned in-situ. 
The in-situ cleaning of the chuck has a number of advantages over the prior 
art methods of cleaning the chuck. For example, the mean time to clean the 
chuck and recondition the chamber is reduced drastically, from about 10-12 
hours to approximately one hour, including the conditioning procedure. 
Also, the chuck is cleaned completely and uniformly with the method of the 
present invention. Since no operator contact is necessary, the in-situ 
NF.sub.3 cleaning produces a consistent chuck surface. As there is no need 
to open the process chamber to the atmosphere, the reactor condition as a 
whole is not disrupted. The uniformity of the thickness and other film 
properties is improved by the in-situ cleaning of the chuck. Furthermore, 
the temperature probes in the chuck do not need to be re-calibrated after 
the cleaning, since their position is not moved. A "first wafer effect" 
that occurs on the first wafer processed after each in-situ clean is also 
reduced. 
The foregoing and other features, aspects and advantages of the present 
invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
A schematic depiction of a cross-section of an electron cyclotron resonance 
(ECR) chemical vapor deposition (CVD) system which performs the method 
according to the present invention is provided in FIG. 1. The system 
includes an electron cyclotron resonance plasma chamber 30 having a 2.45 
GHz microwave power supply 32. The microwaves pass through a microwave 
window 34 into a plasma chamber 30. A quartz liner 36 lines the interior 
of the plasma chamber 30. 
Plasmas are generated by the ionization of gas molecules. This is 
accomplished by an energetic electron striking a neutral molecule. 
Electrons also cause dissociation and other excitations. The electrons are 
excited by electric fields such as RF and microwaves. As such, these are 
the conventional methods of generating processing plasmas. 
In contrast to conventional plasmas that typically operate at pressures 
greater than 70 millitorr, high density plasmas generally operate at 
pressures in the range of 0.5 to 10 millitorr. The ion to neutral ratio 
can be as high as one in a hundred, as compared to less than one in a 
million in low density plasmas. Ion densities can be more than 
1.times.10.sup.12 per cubic centimeter. Such plasmas require sophisticated 
plasma generation techniques such as electron cyclotron resonance. 
In an electron cyclotron resonance plasma chamber, electron angular 
frequency due to the magnetic field matches a microwave frequency, so that 
electron cyclotron resonance occurs. In this state, electrons gain energy 
from the microwave source and accelerate in a circular motion. The 
cross-section for ionization is therefor effectively increased, allowing 
for the creation of high density plasma at low pressure. 
The magnetic field is also used to extract the ions out of the plasma 
source. The ions follow the lines of induction toward the wafer. The 
plasma tends to be cone-shaped due to the divergent magnetic field. This 
divergent magnetic field creates a force that pulls the electrons out of 
the magnetic field. The resulting potential extracts ions to form a varied 
directional plasma stream. 
The divergent magnetic field is created by a primary coil 38 that supplies 
the 875 Gauss needed for the electron cyclotron resonance condition. This 
primary coil 38 also provides the divergent magnetic field for ion 
extraction. 
Auxiliary magnetic coils 40 behind a wafer holder assembly 42 shape the 
plasma into the desired shape. The wafer holder assembly 42 holds the 
wafer, not illustrated in FIG. 1. The wafer holder 42 includes a 13.56 MHz 
RF power supply (up to 3000 W). The RF power accelerates the ions from the 
plasma to cause sputtering on the wafer. It also provides, along with the 
microwaves, the electric fields that excite the electrons to generate the 
processing plasma. 
An electrostatic chuck 46 is provided to hold the wafers within a reactor 
chamber 48 (also referred to as a "process chamber"). The use of an 
electrostatic chuck 46 obviates the need for mechanical clamping of the 
wafer. Wafer cooling is provided by helium, for example, to the underside 
or backside of the wafer through a helium supply line 50. Closed-loop 
control of the helium pressure regulates the wafer temperature during 
deposition. An in-situ wafer temperature monitoring is provided through a 
first temperature probe 52 which sends its sensor signals to a controller 
(not depicted). A 3200 l/sec turbo molecular pump which can achieve a base 
pressure less than 1.times.10.sup.-6 Torr is used to control the pressure 
within the plasma chamber 30 and the reactor chamber 48. 
A conventional wafer transport mechanism, not depicted, is transports 
wafers into and out of the reactor chamber 48. 
Oxygen and argon gas flows into the plasma chamber 30 through an injection 
port 60 from a gas supply source. Plasma is generated in the plasma 
chamber by applying the microwave energy and RF energy by the microwave 
generator 32 and the RF generator 44 to the gas flowing into chamber 30. 
Once a wafer has been transported into the reactor chamber 48 by the wafer 
transport system and placed onto the electrostatic chuck 46, and a plasma 
has been generated within the plasma chamber 30, the deposition gas 
(SiH.sub.4 or SiF.sub.4, for example) is introduced into the plasma 
chamber 30 through one or more gas injection ports 62 that are separate 
from the gas injection port 60 through which the gas to form the plasma is 
provided. 
The deposition process results in the surfaces of the plasma chamber 30 and 
the reactor chamber 40 becoming coated with a residue (SiO.sub.2 or 
flourine-doped SiO.sub.2). This residue is cleaned periodically from the 
surfaces of the chamber so that each wafer encounters the same 
environment, thereby making the process repeatable. Accordingly, the 
cleaning gas (NF.sub.3, in the exemplary embodiment of the present 
invention) is introduced into the plasma chamber and RF power is applied 
from the RF generator 44 to the horn 61. This may be done after each 
wafer, or is typically done after the processing of every five wafers. 
During the cleaning of the chamber 48 in a production run, a cover, i.e., 
dummy, wafer is transferred to the reactor chamber 48 and placed over the 
electrostatic chuck 46. The cover wafer is a standard silicon wafer coated 
with aluminum. The cover wafer protects the chuck water bearing surface 
from the plasma cleaning and conditioning step. It is desirable to cover 
the electrostatic chuck water bearing surface during routine cleaning of 
the process chamber since repeated exposure of the chuck wafer bearing 
surface to NF.sub.3 damages the chuck wafer bearing surface and degrades 
the chuck clamping force. 
In the processing of many wafers, however, occasionally a wafer slides from 
its position on the water bearing surface of electrostatic chuck 46 so 
that deposition occurs on the wafer bearing surface of the electrostatic 
chuck 46. Also, over time, deposition can seep in under the wafer and 
deposit on the water bearing surface of the electrostatic chuck 46 to the 
point where the electrostatic chuck 46 does not function properly and 
needs cleaning. 
The determination of when a chuck 46 needs cleaning is made by visual 
inspection of the chuck through a port in the chamber, or by inspection of 
wafers that have been manufactured. Wafers are thin in certain sections; 
there is a hot area on the water bearing surface of chuck 46, indicating 
that the chuck 46 contains deposition material in that area and needs 
cleaning. In the prior art, once it had been determined that a chuck 
needed to be cleaned, the process chamber of the semiconductor 
manufacturing machine was opened and the chuck was removed. The chuck was 
cleaned with hydrofluoric acid or some other fluid to remove the 
deposition material. Since the chamber is operated in a high vacuum, 
opening therefor is highly detrimental. Once the chamber is opened, at 
least six hours must elapse to restore the process chamber back to its 
former condition after the chuck has been cleaned and replaced into the 
chamber. Also, after the vacuum has been restored 75 to 100 wafers must be 
processed before production wafers can be manufactured through the 
processing chamber. 
The present invention overcomes the problems involved in the opening of a 
process chamber by cleaning the electrostatic chuck 46 in-situ. A 
three-dimensional cross-section of an exemplary embodiment of an 
electrostatic chuck 46 which can be used in the semiconductor 
manufacturing machine is depicted in FIG. 2. RF injection is provided 
during normal deposition to facilitate sputtering while depositing, as is 
known in the art. Two temperature probes 52 and 64 for monitoring the 
temperature of processed wafers are located on the wafer processing 
surface of chuck 46 are also provided. The temperature probe 52 is a 
phosphorous dot probe that is an optical fiber having a face abutting the 
bottom face of a wafer that is placed on the electrostatic chuck 46. A 
light pulse injected into the fiber is incident on the back face of the 
wafer on the water processing surface electrostatic of the chuck 46, the 
light pulse is reflected back into the fiber with a certain delay rate 
that is based on the water temperature. Since the fiber does not have a 
phosphor dot as provided in the black body temperature probes used in the 
prior art, the temperature probe 52 is not corrupted by occasional 
exposure to NF.sub.3 and plasma. The temperature probe 52 acts as a wafer 
temperature probe and is used in the closed-loop control of the pressure 
of helium that is supplied through chuck 46 to the backside of the wafer 
to regulate the wafer temperature during deposition. The second 
temperature probe is an infrared probe 64 that is provided for calibration 
purposes. It is also not sensitive to the NF.sub.3 or plasma. 
Various grooves, not depicted, in the wafer bearing surface 66 of the 
electrostatic chuck 46 distribute the helium on the wafer bearing surface 
66. Channels within the body of electrostatic chuck 46 distribute cooling 
water in the electrostatic chuck 46 to maintaining an even temperature 
throughout the electrostatic chuck 46. 
Because the temperature probe 52 and the infrared probe 64 are not 
particularly adversely affected by infrequent exposure to NF.sub.3 gas and 
plasma the electrostatic chuck 46 and its wafer bearing surface 66 can be 
exposed without ill effects to NF.sub.3 gas (the cleaning gas) and a 
generated plasma in order to clean the wafer bearing surface 66. This is 
done without opening the reactor chamber 48 ("process chamber") so that 
down time caused by the requirement to clean the chuck 46 is reduced from 
10-12 hours to approximately one hour. 
An exemplary embodiment of a method of cleaning an electrostatic chuck 
according to the present invention is depicted in the flow chart of FIG. 
3. Throughout this process, the processing chamber 48 is maintained in a 
sealed condition with the electrostatic chuck 46 inside the chamber 48. In 
step 70, an operator determines whether the wafer bearing surface 66 of 
the chuck 46 requires cleaning. This is done by a direct visual inspection 
of the chuck 46, or by inference through an inspection of the wafers that 
have been manufactured. If the chuck requires cleaning, the wafer bearing 
surface 66 of the chuck 46 is then exposed in step 72. A cleaning gas, 
such as NF.sub.3 is then injected into the chamber 48 in step 74. RF power 
is applied in step 76 to the chamber whereby the cleaning gas is ionized 
to create a plasma that cleans the wafer bearing surface 66. In this case, 
the RF power is applied my source 32 to horn feed 61 (FIGS. 1 and 3) while 
all the other surfaces except for those of the electrostatic chuck 46 are 
grounded (step 76). The RF power energizes cleaning gas to form the plasma 
and creates reactions on the wafer bearing surface 66 of the electrostatic 
chuck 46. The application of a turbo pump removes from the chamber 48 the 
deposition material that has been dissociated from the chuck 46. 
The application of the RF power and exposure of the wafer bearing surface 
66 of the electrostatic chuck 46 to the plasma is typically for 
approximately 100 to 300 seconds. Determination that the wafer bearing 
surface has been adequately cleaned is preferably made by a visual 
inspection of that surface while the process chamber 46 is still sealed. 
Following the cleaning of the wafer bearing surface 66, a pump/purge cycle 
is performed to remove any particles present in the chamber (step 77). 
Then, in step 78 a small number of dummy wafers (e.g., 5-10 wafers) are 
processed in the reactor. This step is performed since the chucking force 
exerted by the electrostatic chuck 46 is increased due to the in-situ 
cleaning and therefor the cooling provided to the wafers is also 
increased. As a result, the dummy wafers do not reach the regular 
operating temperature, but through the course of 5-10 dummy wafers, the 
chuck 46 returns to its normal condition and the reactor is ready to 
continue or initiate its production runs (step 80). 
The in-situ cleaning of the chuck dramatically degreases the mean time to 
clean the chuck 46 and recondition the chamber 48. It is also used to 
condition new chucks by removing residual films from the manufacturing 
process. This process, however, is not limited to electron cyclotron 
resonance chemical vapor deposition machines, but is also applicable to 
other semiconductor manufacturing machines. For example, the process can 
be applied to etching machines, or any other manufacturing machines in 
which a plasma is generated and a workpiece chuck is used that may become 
contaminated. An example of such a machine is an etching machine. 
Although the present invention has been described and illustrated in 
detail, it is to be clearly understood that the same is by way of 
illustration and example only and is not to be taken by way of limitation, 
the spirit and scope of the present invention being limited only by the 
terms of the appended claims.