U.S. patent application number 10/932006 was filed with the patent office on 2005-04-21 for wafer cleaning method and equipment.
Invention is credited to Higuchi, Takashi, Matsuo, Hiroyuki, Miyazaki, Kunihiro, Nakajima, Toshiki.
Application Number | 20050081886 10/932006 |
Document ID | / |
Family ID | 34415088 |
Filed Date | 2005-04-21 |
United States Patent
Application |
20050081886 |
Kind Code |
A1 |
Miyazaki, Kunihiro ; et
al. |
April 21, 2005 |
Wafer cleaning method and equipment
Abstract
There is disclosed a wafer cleaning method comprising supplying
a cleaning water to a wafer cleaned with a chemical solution,
measuring the resistivity of a solution including the chemical
solution and cleaning water, and differentiating the measured value
with respect to time, and cleaning the wafer continuously with the
cleaning water until the time differential value of the resistivity
becomes equal to or less than a preset value and is held at that
values for preset time.
Inventors: |
Miyazaki, Kunihiro;
(Yokohama-shi, JP) ; Higuchi, Takashi; (Kyoto-shi,
JP) ; Nakajima, Toshiki; (Suwa-shi, JP) ;
Matsuo, Hiroyuki; (Chino-shi, JP) |
Correspondence
Address: |
Finnegan, Henderson, Farabow,
Garrett & Dunner, L.L.P.
1300 I Street, N.W.
Washington
DC
20005-3315
US
|
Family ID: |
34415088 |
Appl. No.: |
10/932006 |
Filed: |
September 2, 2004 |
Current U.S.
Class: |
134/2 ; 134/113;
134/58R |
Current CPC
Class: |
H01L 21/67057 20130101;
G01N 27/06 20130101 |
Class at
Publication: |
134/002 ;
134/058.00R; 134/113 |
International
Class: |
C23G 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 5, 2003 |
JP |
2003-314513 |
Claims
What is claimed is:
1. A wafer cleaning method comprising: supplying a cleaning water
to a wafer cleaned with a chemical solution; measuring the
resistivity of a solution including the chemical solution and
cleaning water, and differentiating the measured value with respect
to time; and cleaning the wafer continuously with the cleaning
water until the time differential value of the resistivity becomes
equal to or less than a preset value and is held at that values for
preset time.
2. The method according to claim 1, wherein the wafer is cleaned
continuously with the cleaning water until the time differential
value of the resistivity becomes equal to or less than 0.05
M.OMEGA.cm/sec after passing the maximum value, and is held at that
value for equal to or more than 5 seconds.
3. The method according to claim 1, wherein the resistivity of the
solution is subjected to a predetermined smoothing, and the
smoothed value is differentiated with respect to time.
4. The method according to claim 1, wherein the resistivity of the
solution in the cleaning tank which contains the wafer when
cleaning the wafer with the cleaning water is measured as the
resistivity of the solution.
5. A wafer cleaning method comprising: supplying a cleaning water
to a wafer cleaned with a chemical solution; measuring the
conductivity of a solution including the chemical solution and
cleaning water, and differentiating the measured value with respect
to time; and cleaning the wafer continuously with the cleaning
water until the time differential value of the conductivity becomes
equal to or more than a preset value and is held at that values for
preset time.
6. The method according to claim 5, wherein the wafer is cleaned
continuously with the cleaning water until the time differential
value of the conductivity becomes equal to or more than -20
.mu.S/cm.multidot.sec after passing the minimum value, and is held
at that value for equal to or more than 5 seconds.
7. The method according to claim 5, wherein the conductivity of the
solution is subjected to a predetermined smoothing, and the
smoothed value is differentiated with respect to time.
8. The method according to claim 5, wherein the conductivity of the
solution in the cleaning tank which contains the wafer when
cleaning the wafer with the cleaning water is measured as the
conductivity of the solution.
9. A wafer cleaning equipment comprising: a cleaning tank which
contains a wafer cleaned with a chemical solution; a cleaning water
supplying unit which supplies the cleaning tank with a cleaning
water to clean the wafer; an electric characteristic measuring unit
which measure the resistivity of a solution including the cleaning
water and the chemical solution used for cleaning the wafer; an
arithmetic unit which differentiates with respect to time the
resistivity of the solution measured with the electric
characteristic measuring unit; and a control unit which operates
the cleaning water supplying unit and supplies the cleaning water
to the cleaning tank, until the time differential value of the
resistivity calculated by the arithmetic unit becomes equal to or
less than a preset value and is held at that value for preset
time.
10. The equipment according to claim 9, wherein the control unit
operates the cleaning water supplying unit and supplies the
cleaning water to the cleaning tank, until the time differential
value of the resistivity becomes equal to or less than 0.05
M.OMEGA.cm/sec after passing the maximum value, and is held at that
value for equal to or more than 5 seconds.
11. The equipment according to claim 9, wherein the arithmetic unit
smoothes the resistivity of the solution, and differentiates the
smoothed value with respect to time.
12. The equipment according to claim 9, wherein a take-out port to
take out the solution from the cleaning tank is provided at the
middle of the cleaning tank, and the electric characteristic
measuring unit is provided contacting the solution in the cleaning
tank to be taken out through the take-out port.
13. A wafer cleaning equipment comprising: a cleaning tank which
contains a wafer cleaned with a chemical solution; a cleaning water
supplying unit which supplies the cleaning tank with a cleaning
water to clean the wafer; an electric characteristic measuring unit
which measures the conductivity of a solution including the
cleaning water and the chemical solution used for cleaning the
wafer; an arithmetic unit which differentiates with respect to time
the conductivity of the solution measured with the electric
characteristic measuring unit; and a control unit which operates
the cleaning water supplying unit and supplies the cleaning water
to the cleaning tank, until the time differential value of the
conductivity calculated by the arithmetic unit becomes equal to or
more than a preset value and is held at that value for preset
time.
14. The equipment according to claim 13, wherein the control unit
operates the cleaning water supplying unit and supplies the
cleaning water to the cleaning tank, until the time differential
value of the conductivity becomes equal to or more than -20
.mu.S/cm.multidot.sec after passing the minimum value, and is held
at that value for equal to or more than 5 seconds.
15. The equipment according to claim 13, wherein the arithmetic
unit smoothes the conductivity of the solution, and differentiates
the smoothed value with respect to time.
16. The equipment according to claim 13, wherein a take-out port to
take out the solution from the cleaning tank is provided at the
middle of the cleaning tank, and the electric characteristic
measuring unit is provided contacting the solution in the cleaning
tank to be taken out through the take-out port.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from prior Japanese Patent Application No. 2003-314513,
filed Sep. 5, 2003, the entire contents of which are incorporated
herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a wafer cleaning process.
More particularly, the invention relates to a wafer cleaning method
and equipment in a final wafer cleaning process using cleaning
water after cleaning chemical a wafer with a chemical solution.
[0004] 2. Description of the Related Art
[0005] Variety of measures are taken to protect a wafer against
contamination during a semiconductor manufacturing process and
other unexpected contamination for improving the characteristics
and yield of semiconductor elements provided on a wafer. Generally,
a wafer is cleaned with a chemical solution. Common chemical
solutions used for cleaning a wafer include a mixed water solution
of hydrochloric acid and hydrogen peroxide, a mixed water solution
of ammonia and hydrogen peroxide, and a mixed solution of dense
sulfuric acid and hydrogen peroxide. A water solution of
hydrofluoric acid is also commonly used. Recently, a mixed water
solution of hydrofluoric acid and ozone water or a mixed water
solution of hydrofluoric acid and hydrogen peroxide is also
used.
[0006] A wafer cleaning method is roughly divided into the
following two types. One is a method of immersing a plurality of
wafers in a processing tank filled with a chemical solution. This
is so-called a batch cleaning method. The other is a method of
supplying a chemical solution to the surfaces of a plurality of
wafers by rotating one by one. This is so-called a single wafer
cleaning method.
[0007] After the chemical cleaning, eliminate the chemical solution
adhered to a wafer by using ultra-pure water, and dry a wafer.
Then, go to the next semiconductor manufacturing process. If it is
difficult to eliminate the impurities adhered to a wafer with one
kind of chemical solution, use two or more kinds of chemical
solutions and continue cleaning a wafer using each chemical
solution. Insert a rinse step using ultra-pure water into the wafer
cleaning process using chemical solutions. At the end of the
cleaning process, eliminate sufficiently the chemical solution
adhered to a wafer by final rinse with ultra-pure water, and dry a
wafer. This final rise with ultra-pure water aims at eliminating
sufficiently the chemical solution adhered to a wafer.
[0008] However, it is impossible to know directly the end of the
rinsing at which the chemical solution adhered to a wafer is
sufficiently eliminated. In the batch cleaning method, the end of
rinsing (the rinsing time) is generally determined based on the
density of a specified ion included in the chemical solution
existing in the liquid in the processing tank. Concretely, measure
the ion density of a chemical solution by monitoring the
resistivity or the reciprocal number thereof, conductivity of the
solution flowed out from the processing tank during the final rinse
step. When the measured ion density of a chemical solution is equal
to or less than the value indicating that the chemical solution
adhered to a wafer is sufficiently eliminated, the final rinse step
is regarded completed. The value indicating that the chemical
solution adhered to a wafer is sufficiently eliminated is generally
determined by experiments. Though, unlike the wafer cleaning
method, as a method of controlling the resistivity in an equipment
of refining ultra-pure water used for cleaning a wafer, the
technique using the resistivity end point, as well as the method of
deciding the rise time, is disclosed in Jpn. Pat. Appln. KOKAI
Publication No. 9-1138.
BRIEF SUMMARY OF THE INVENTION
[0009] According to an aspect of the invention, there is provided a
wafer cleaning method comprising: supplying a cleaning water to a
wafer cleaned with a chemical solution; measuring the resistivity
of a solution including the chemical solution and cleaning water,
and differentiating the measured value with respect to time; and
cleaning the wafer continuously with the cleaning water until the
time differential value of the resistivity becomes equal to or less
than a preset value and is held at that values for preset time.
[0010] According to another aspect of the invention, there is
provided a wafer cleaning method comprising: supplying a cleaning
water to a wafer cleaned with a chemical solution; measuring the
conductivity of a solution including the chemical solution and
cleaning water, and differentiating the measured value with respect
to time; and cleaning the wafer continuously with the cleaning
water until the time differential value of the conductivity becomes
equal to or more than a preset value and is held at that values for
preset time.
[0011] According to another aspect of the invention, there is
provided a wafer cleaning equipment comprising: a cleaning tank
which contains a wafer cleaned with a chemical solution; a cleaning
water supplying unit which supplies the cleaning tank with a
cleaning water to clean the wafer; an electric characteristic
measuring unit which measure the resistivity of a solution
including the cleaning water and the chemical solution used for
cleaning the wafer; an arithmetic unit which differentiates with
respect to time the resistivity of the solution measured with the
electric characteristic measuring unit; and a control unit which
operates the cleaning water supplying unit and supplies the
cleaning water to the cleaning tank, until the time differential
value of the resistivity calculated by the arithmetic unit becomes
equal to or less than a preset value and is held at that value for
preset time.
[0012] According to still another aspect of the invention, there is
provided a wafer cleaning equipment comprising: a cleaning tank
which contains a wafer cleaned with a chemical solution; a cleaning
water supplying unit which supplies the cleaning tank with a
cleaning water to clean the wafer; an electric characteristic
measuring unit which measures the conductivity of a solution
including the cleaning water and the chemical solution used for
cleaning the wafer; an arithmetic unit which differentiates with
respect to time the conductivity of the solution measured with the
electric characteristic measuring unit; and a control unit which
operates the cleaning water supplying unit and supplies the
cleaning water to the cleaning tank, until the time differential
value of the conductivity calculated by the arithmetic unit becomes
equal to or more than a preset value and is held at that value for
preset time.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0013] FIG. 1 is a flowchart showing a wafer cleaning method
according to a first embodiment;
[0014] FIG. 2 is a simplified block diagram showing a wafer
cleaning equipment according to a first embodiment;
[0015] FIG. 3 is a graph showing the relationship between the time
of cleaning the wafer according to a first embodiment and the time
differential values of the resistivity for each kind of cleaning
chemical solutions and the number of wafers to be cleaned;
[0016] FIG. 4 is a simplified block diagram showing a wafer
cleaning equipment according to a second embodiment;
[0017] FIGS. 5A and 5B are sectional views showing a simplified
wafer cleaning equipment according to a prior art;
[0018] FIG. 6 is a graph showing the relationship between the time
of cleaning a wafer and the resistivity according to a prior art;
and
[0019] FIG. 7 is a graph showing the relationship between the time
of cleaning a wafer and the resistivity according to a prior art
for each kind of cleaning chemical solutions and the number of
wafers to be cleaned.
DETAILED DESCRIPTION OF THE INVENTION
[0020] Hereinafter the present invention will be explained in
detail according to embodiments shown in the accompanying
drawings.
[0021] (1st. Embodiment)
[0022] Before explaining the embodiment, description will be given
on a method of measuring the resistivity of a common solution
according to a prior as an example comparative to the embodiment,
with reference to FIG. 5A-FIG. 7.
[0023] A method of measuring the resistivity of a common
conventional solution uses two types of cleaning equipments 101 and
102 shown in FIG. 5A and FIG. 5B. In the method using the cleaning
equipment 101 shown in FIG. 5A, a resistivity measuring cell (a
resistivity meter) 106, which monitors the resistivity of the
solution 105, is provided in proximity to an upper opening 104a of
a tank 104 containing a wafer 103. The resistivity measuring cell
106 measures the resistivity of the solution 105 overflowed from
the upper opening 104a. In the method using the cleaning equipment
102 shown in FIG. 5B, a port 108 is provided at the middle of the
tank 107 to extract the solution 105 from the tank 107, and the
resistivity measuring cell 106 is provided at the port 108. The
resistivity measuring cell 106 measures the resistivity of the
sampling solution 105 extracted from the tank 107 through the port
108. As the tanks 104 and 107, it is common to use a rinse tank for
rinsing the wafer 103 with adhesion of a chemical solution, or a
processing tank with a mechanism to replace pure water for the
solution supplied to the tank, after the wafer 103 is cleaned with
a chemical solution in the tank.
[0024] FIG. 6 shows an example of the changes with time of the
resistivity of the solution 105 measured by the method shown in
FIG. 5A. Usually, measure the changes with time of the resistivity
at least once and obtain the data as shown in FIG. 6. If the
resistivity rises and becomes stable at a certain value, the
chemical solution in the tank 104 is regarded as almost completely
replaced by pure water. In the example shown in FIG. 6, the final
rising time is set to 10 minutes. In this case, the resistivity of
the solution 105 is substantially stabilized at approximately 16
M.OMEGA.cm about 2 minutes after stop of rising. Namely, the
chemical solution in the tank 104 is regarded as almost completely
replaced by pure water and the chemical solution adhered to the
wafer 103 is regarded sufficiently eliminated. The rinsing time is
usually set with sufficient allowance as described above.
[0025] In recent years, however, semiconductor devices have been
marketed at a low price, and mass production of semiconductor
devices with reduced costs has been demanded. Thus, the rinsing
time has been reduced by decreasing the pure water volume in
cleaning a wafer, or by reducing the time required by cleaning a
wafer. For example, in the above-mentioned cleaning method of
determining the final wafer rinsing time by measuring the
resistivity of solution, the rinsing is finished at the point when
the resistivity reaches a preset value. In FIG. 6, when the
resistivity of the solution 105 rises equal to or more than 16
M.OMEGA.cm, the wafer rinsing is regarded as finished. Therefore,
in this case, the end of rinsing is set at the point when the
resistivity of the solution 105 reaches the point A indicated by
the solid arrow mark in FIG. 6.
[0026] In the wafer cleaning method of finishing the rinsing at the
point when the resistivity of solution reaches a preset value,
there are problems that the rinsing time varies with the kinds and
density of solution or the number of wafers to be processed, and
the resistivity does not reach a preset value. Thus, it has been
practically difficult to use a wafer cleaning equipment which is
incorporated with a system adopting the above cleaning method.
Particularly, in the method (equipment) shown in FIG. 5A, so-called
air involving occurs, and carbonic acid gas, etc. in the air is
easily dissolved in the solution 105 overflowed from the upper
opening 104a of the tank 104. If carbonic acid gas such as carbon
dioxide is dissolved in the solution 105, a noise occurs in the
cleaning system of the cleaning equipment 101, and the resistivity
of the solution 105 is lowered. In addition, in the method
(equipment 101) shown in FIG. 5A, the area of the solution 105
contacting the air varies (the surface fluctuates) and the
dissolved amount of the carbonic acid gas in the solution 105
varies easily, the noise in the cleaning system is easy to
change.
[0027] In the conventional wafer cleaning methods, it is difficult
to measure stably and accurately whether the resistivity
(conductivity) of solution reaches a preset value. Namely, it is
difficult to determine stably and accurately whether a chemical
solution or stains adhered to a wafer is completely eliminated and
a wafer is cleaned to the proper clean state. It is further
difficult to improve the efficiency of cleaning a wafer by
decreasing the volume of pure water used for cleaning a wafer, or
reducing the cleaning time. If semiconductor elements are mounted
on a wafer contaminated by the chemical solution not completely
eliminated, the characteristics and yield of the semiconductor
elements will be lowered. Namely, a semiconductor device using a
contaminated wafer will have low performance, quality, reliability
and yield. Such a semiconductor device will also have low
production efficiency, and increase production costs.
[0028] This embodiment has been made to solve the above problems.
It is an object of the embodiment to provide a wafer cleaning
method and equipment which can clean a wafer to the proper clean
state while increasing the cleaning efficiency regardless of the
number of wafers to be cleaned, and the kinds and density of
chemical solution. It is another object of the embodiment is to
provide a wafer which is completely cleaned to the proper clean
state with no chemical solution remained, and a semiconductor
device which is provided with such a clean wafer and improved in
the performance, quality, reliability and yield. Hereinafter the
first embodiment of the invention will be explained in details with
reference to FIG. 1-FIG. 3.
[0029] FIG. 1 is a flowchart showing a wafer cleaning method
according to this embodiment. FIG. 2 is a simplified block diagram
showing a wafer cleaning (equipment according to this embodiment.
FIG. 3 is a graph showing the relationship between the rinsing
(cleaning) time of the wafer according to this embodiment and the
time differential value of the resistivity for each kind of
cleaning chemical solution and the number of wafers.
[0030] This embodiment defines the end time of the final rinsing
after cleaning a wafer with a chemical solution, to reduce the
volume of cleaning water and net rinsing time (Row Process Time:
RPT) in the wafer cleaning process, and cleans a wafer to the
proper clean state. Concretely, the pure water resistivity
(conductivity) of the solution including the cleaning water is
continuously monitored during the final rinsing in order to define
the end of final wafer rinsing. The obtained resistivity data is
differentiated to obtain the change in the inclination with time.
Then, the end point of rinsing is determined based on the
inclination change with time and the continued final rinsing time.
The method reduces the cleaning water volume and RPT in this way,
and cleans a wafer to the proper clean state. Detailed explanation
will be given below.
[0031] First, explanation will be given on a wafer cleaning
equipment 1 according to this embodiment with reference to FIG. 2.
The cleaning equipment 1 has a cleaning tank 3 which contains one
or more wafers 2 cleaned with a chemical solution. The cleaning
tank 3 may be either a processing tank dedicated to rinsing the
wafer 2 with adhesion of a cleaning chemical solution, or a
processing tank provided with a device to switch the solution
supplied to the wafer 2 from a chemical solution to cleaning water
after the wafer 2 is cleaned with a chemical solution. The bottom
of the cleaning tank 3 is connected to a water supply pipe 4 which
supplies a cleaning water used for rinsing the wafer 2 to the
inside of the cleaning tank 3. At the middle of the water supply
pipe 4, a cleaning water supply valve 5 is provided as a cleaning
water supply device to supply a cleaning water to the inside of the
cleaning tank 3. In this embodiment, ultra-pure water is used as a
cleaning water. Therefore, the cleaning water supply valve can also
be called an ultra-pure water supply valve 5.
[0032] The cleaning tank 3 has an opening 3a at the top. The
chemical solution adhered to the wafer 2 and the solution 6
including the pure water supplied to the inside of the cleaning
tank 3 overflow from the inside to outside of the cleaning tank
through the opening 3a. Provided near the opening 3a of the
cleaning tank 3 is a drain port 7 to drain the solution 6 to the
outside of the cleaning tank 3 after once receiving the solution 6
overflowed from the inside of the cleaning tank 3. An electric
character measuring unit 8 which measures the resistivity or
conductivity of the solution 6 is provided contacting the solution
6 in the drain port 7.
[0033] The resistivity and conductivity are reciprocal to each
other. Therefore, measurement of at least one of the resistivity
and conductivity of the solution 6 corresponds to measurement of
the other. In this embodiment, the resistivity of the solution 6 is
to be measured with the electric characteristic measuring unit 8.
Therefore, in this embodiment, a resistivity meter (resistivity
measuring cell) 8 is used as an electric characteristic measuring
unit. The resistivity measuring cell 8 measures, as the resistivity
of the solution 6, the resistivity of the overflowed water 6a
drained from the inside to outside of the cleaning tank 3 through
the opening 3a at the top of the cleaning tank 3.
[0034] The resistivity of the solution 6 measured with the
resistivity measuring cell 8 is sent to a resistivity measuring
circuit 9 as an electric signal. The resistivity measuring circuit
9 measures the resistivity of the solution 6 that is measured with
the resistivity measuring cell 8, based on the electric signal
output from the resistivity measuring cell 8.
[0035] The resistivity of the solution 6 measured with the
resistivity measuring circuit 9 is sent from the resistivity
measuring circuit 9 to an A/D converter 10 as an electric signal.
In this embodiment, the resistivity measuring circuit 9 is set to
output the measured resistivity of the solution 6 as an analog
signal. An arithmetic control circuit 11 is set to receive a
digital signal. Therefore, in this embodiment, the A/D converter 10
is set to convert an analog signal output from the resistivity
measuring circuit 9 to a digital signal, and send this digital
signal to the arithmetic control circuit 11.
[0036] The resistivity of the solution 6 converted from analog to a
digital signal with the A/D converter 10 is sent to the arithmetic
control unit 11. The arithmetic control unit 11 obtains the
resistivity of the solution 6 measured with the resistivity
measuring circuit 9 at every preset time, holds it for preset time,
differentiates the obtained measured value with respect to time,
and controls open/close of the ultra-pure water supply valve 5. In
this embodiment, the arithmetic control unit 11 consists of an
arithmetic unit (arithmetic section, arithmetic circuit) which
differentiates with respect to time the resistivity of the solution
6 measured with the resistivity measuring cell 8, and a control
unit (control section, control circuit) which is integrated with
the arithmetic unit, and supplies a cleaning water to the cleaning
tank 3 by operating the ultra-pure water supply valve 5 until the
differential value calculated by the arithmetic unit becomes equal
to or less than a preset value and is held at that value for preset
time.
[0037] The cleaning tank 3, water supply pipe 4 and ultra-pure
water supply valve 5 constitute a cleaning system 12 of the
cleaning equipment 1. The resistivity measuring cell 8, resistivity
measuring circuit 9, A/D converter 10 and arithmetic control unit
11 constitute a measuring system 13 of the cleaning equipment
1.
[0038] Next, explanation will be given on a wafer cleaning method
according to this embodiment with reference to FIG. 1. The wafer
cleaning method of this embodiment is concretely a cleaning method
in the final wafer rinsing process, which eliminates stains such as
chemical solution adhered to the wafer 2 cleaned with a chemical
solution, and clean the wafer 2 to the proper clean state. The
wafer cleaning method of this embodiment measures the resistivity
of the chemical solution used for cleaning the wafer 2 and the
solution 6 including the cleaning water used for rinsing the wafer
2 cleaned with the chemical solution, and differentiates the
measured value with respect to time. The wafer 2 is continuously
rinsed until the differentiated value becomes equal to or less than
a preset value and held at that value for preset time. In the wafer
cleaning method of this embodiment, the wafer 2 is rinsed by using
the wafer cleaning equipment 1. Detailed explanation will be given
below.
[0039] First, put one or more wafers 2 in the cleaning tank 3 in
the stat that the wafer is cleaned but the cleaning solution is not
completely eliminated. Next, open the ultra-pure water supply valve
5 by sending a valve control signal to open the ultra-pure water
supply valve 5 from the arithmetic control unit 11 to the
ultra-pure water supply valve 5. The ultra-pure water is supplied
to the inside of the cleaning tank 3, and the wafer 2 is begun to
be cleaned with ultra-pure water (rinsed with ultra-pure water). At
the same time, the resistivity measuring cell 8 starts measuring
the resistivity of the solution 6 (overflowed water 6a) drained
from the cleaning tank 3. The resistivity measuring circuit 9
measures continuously the value (detected value) measured with the
resistivity measuring cell 8. The A/D converter 10 converts
continuously the resistivity value outputted as an analog signal
(analog value) from the resistivity measuring circuit 9, into a
digital signal (digital value). The A/D converter 10 outputs the
digital signal to the arithmetic control unit 11.
[0040] The arithmetic control unit 11 receives the digital signal
output from the A/D converter 10, and performs a predetermined
processing based on the digital signal. The predetermined
processing performed by the arithmetic control unit 11 is indicated
by a dashed line in FIG. 1. Detailed explanation will be given
below.
[0041] First, hold the resistivity value inputted as a digital
signal to the arithmetic control unit 11 at every preset time for
preset time predetermined by the arithmetic control unit 11. Next,
the arithmetic control unit 11 calculates the inclination (change
rate), or the differential value of the resistivity with respect to
the holding time, based on the held number of resistivity values
and the holding time. The differential value can be calculated
after smoothing the resistivity values, if necessary. The
differential value of the resistivity corresponds to the
inclination of the resistivity at a predetermined time. Thus, it is
also permitted to obtain the inclination by smoothing real time the
predetermined number of held resistivity data before holding the
resistivity data held for obtaining the differential value. A
method and degree of smoothing is not specified as long as noises
in the cleaning system 12 and measuring system 13 of the cleaning
equipment 1 are taken into account. A weighted average (a weighted
smoothing), a weighted mean, or Savizky-Golay method is
permitted.
[0042] Next, determine by the arithmetic control unit 11 whether
the differential value obtained by the arithmetic control unit 11
is equal to or less than a preset value and held at that value for
preset time. When the differential value is equal to or less than
the preset value and held at that value for the preset time, the
stains such as a chemical solution adhered to the wafer 2 is
regarded as completely eliminated, and the wafer 2 is regarded as
cleaned to the proper clean state. In this embodiment, the
arithmetic control unit 11 is set to determine whether the
differential value is equal to or less than 0.05 M.OMEGA.cm/sec and
held at that value for equal to or more than 5 seconds after
passing the maximum value. When the differential value is equal to
or less than 0.05 M.OMEGA.cm/sec and held at that value for equal
to or more than 5 seconds after passing the maximum value, the
wafer 2 is regarded as cleaned to the proper clean state, and
rinsing the wafer 2 with ultra-pure water is finished.
[0043] The above differential value measuring condition is set to
an appropriate value according to the cleanness demanded for the
wafer 2. The value of the condition is previously obtained by
experiments. The ideal timing to finish the rinsing with ultra-pure
water is a point when the differential value of the resistivity
reaches 0.00 M.OMEGA.cm/sec, or the inclination of the resistivity
with respect to time becomes zero. However, noises (electric signal
noises) occur in the cleaning system 12 and measuring system 13 of
the cleaning equipment 1, and the differential value of the
resistivity can not practically reach 0.00 M.OMEGA.cm/sec.
According to the experience and experiments done by the inventors,
it is seen that when the differential value of the resistivity is
held equal to or less than 0.05 M.OMEGA.cm/sec for at least 5
seconds after passing the maximum value, the wafer 2 can be cleaned
to the proper clean state regardless of the number of wafers and
the kinds and density of chemical solution used for cleaning.
Therefore, it is set in this embodiment that if the differential
value of the resistivity is held equal to or less than 0.05
M.OMEGA.cm/sec for at least 5 seconds after passing the maximum
value, the rinsing the wafer 2 with ultra-pure water is
finished.
[0044] If the arithmetic control unit 11 determines that the
differential value is not held equal to or less than 0.05
M.OMEGA.cm/sec for equal to or more than 5 seconds after passing
the maximum value, rinsing the wafer 2 with ultra-pure water is
continued and the arithmetic control unit 11 holds the resistivity
data and repeats differentiation of the resistivity based on that
data, until the differential value meets that condition. If the
data is held repeatedly and the data is held for a long time, the
number of held data is increased and the load to the arithmetic
control unit 11 is increased. To avoid this, it is permitted to set
to abandon the data after the preset time passes.
[0045] If the arithmetic control unit 11 determines that the
differential value is held equal to or less than 0.05
M.OMEGA.cm/sec for equal to or more than 5 seconds after passing
the maximum, the arithmetic control unit 11 sends a valve control
signal which closes the ultra-pure water supply valve 5 to the
ultra-pure water supply valve 5, and closes the ultra-pure water
supply valve 5. By this action, supply of ultra-pure water to the
cleaning tank 3 is stopped, and the ultra-pure water rinsing of the
wafer 2 is finished. After the end of ultra-pure water rinsing of
the wafer 2, take out the wafer 2 from the cleaning tank 3, and dry
the wafer. This completes the final wafer rinsing process.
[0046] FIG. 3 is a graph showing that the resistivity data is
obtained at every second and held for a second in the cleaning
method of one example of this embodiment, and the differential
value of the resistivity with respect to the changes with time is
calculated based on the held data. In this example, differentiation
is performed by obtaining the resistivity data at about every
second and holding it for a second, but the data holding time,
differential value calculating interval and differential value
holding time are not limited to about 1 second. They may be the
time sufficiently short against the net time (RPT) required by the
rinsing of the wafer 2 with ultra-pure water.
[0047] HF200/lwf in FIG. 3 indicates the ultra-pure water rinsing
(final rinsing) of one wafer 2 that is cleaned by using the
chemical solution composed of pure water and water solution of 50%
hydrofluoric acid, and diluted to have an about 1:200 volume ratio
of the water solution of 50% hydrofluoric acid to pure water. The
solid line in the graph of FIG. 3 indicates the changes of the time
differential value of the resistivity with respect to the
ultra-pure water rinsing time in HF200/lwf. HF500/lwf indicates the
ultra-pure water rinsing of one wafer 2 that is cleaned by using
the chemical solution composed of pure water and water solution of
50% hydrofluoric acid, and diluted to have an about 1:500 volume
ratio of the water solution of 50% hydrofluoric acid to pure water.
The dashed line in the graph of FIG. 3 indicates the changes in the
time differential value of the resistivity with respect to the
ultra-pure water rinsing time in HF500/lwf. HF200/44wf indicates
the ultra-pure water rinsing of 44 wafers 2 that are cleaned by
using the chemical solution composed of pure water and water
solution of 50% hydrofluoric acid, and diluted to have an about
1:200 volume ratio of the water solution of 50% hydrofluoric acid
to pure water. The chain line in the graph of FIG. 3 indicates the
changes in the time differential value of the resistivity with
respect to the ultra-pure water rinsing time in HF200/44wf.
HF500/44wf indicates the ultra-pure water rinsing of 44 wafers 2
that are cleaned by using the chemical solution composed of pure
water and water solution of 50% hydrofluoric acid, and diluted to
have an about 1:500 volume ratio of the water solution of 50%
hydrofluoric acid to pure water. The chain double-dashed line in
the graph of FIG. 3 indicates the changes in the time differential
value of the resistivity with respect to the ultra-pure water
rinsing time in HF500/44wf.
[0048] As seen from the graph of FIG. 3, the differential value
(inclination) of resistivity generally shows a curve projecting
upward, descending after once rising regardless of the number of
wafers 2 and the kinds and density of the cleaning chemical
solution. Even if the rinsing time is extended under each of the
four conditions, and the differential value 0 of resistivity is not
held various due to noise components. Among the four conditions,
the peak (maximum) position of differential value and sweep time
are largely different. According to the graph of FIG. 3, the
differential value of resistivity can take the same value at
different points except the peak. FIG. 3 indicates that the
resistivity greatly changes until the differential value reaches
its peak. While the resistivity is so changing, the chemical
solution is substituted by ultra-pure water. In view of this, it is
obviously necessary to keep cleaning the wafer 2 until the
differential value reaches the peak in the graph of FIG. 3.
Therefore, when the differential value of resistivity reaches a
preset value after once reaching the peak, the wafer 2 is regarded
as cleaned to the proper clean state.
[0049] The differential value of resistivity at which the wafer 2
is regarded as cleaned to the proper clean state may be set to an
appropriate value according to the cleanness demanded for the wafer
2 as long as it has once reached the peak. As the above
differential value is set smaller, the cleanness of wafer 2 is
increased, but the time required to finish the ultra-pure water
rinsing becomes long. If the rinsing time with ultra-pure water is
long, the row process time (RPT) of rinsing with ultra-pure water
becomes long, decreasing the productivity, and increasing the
production cost with the increased volume of ultra-pure water.
[0050] According to the graph of FIG. 3, it is seen that the part
where the differential value sweeps indicates the state that the
maximum and minimum differential values are repeated due to the
various noise components. If the value that the wafer 2 is regarded
as cleaned to the proper clean state is set small to the state that
the differential value sweeps as shown in FIG. 3, it becomes very
difficult to hold the value for equal to or more than 5 seconds
even if the value is equal to or less than 0.05 M.OMEGA.cm/sec.
Further, it may become impossible to prolong the wafer 2 cleaning
time, and finish rinsing the wafer 2 with ultra-pure water.
Therefore, it is necessary to set the differential value of
resistivity at white the wafer 2 is regarded as cleaned to the
proper clean state to the value that the wafer 2 cleaning time
becomes the shortest within the range satisfying the cleanness
demanded for the wafer 2.
[0051] Because of the above reason, in the embodiment shown in FIG.
3, when the differential value of resistivity is held equal to or
less than 0.05 M.OMEGA.cm/sec for equal to or more than 5 seconds
after passing the maximum value for all the four kinds, the wafer 2
is regarded as cleaned to the proper clean state, and the rinsing
of the wafer 2 is finished. By this method, the final rinsing of
the wafer 2 can be finished in substantially the same state, even
if the resistivity of the solution 6 in the cleaning tank 3 is
different for the processing conditions in cleaning with a chemical
solution. Namely, stains such as chemical solution adhered to the
wafer 2 can be sufficiently eliminated and the wafer 2 can be
cleaned to substantially the same clean state in various
conditions, regardless of the number of wafers 2 to be cleaned, the
kinds and density of cleaning chemical solution, or the resistivity
of the solution 6 in the cleaning tank 3. As shown in FIG. 3, in
this embodiment, the wafer 2 can be cleaned to the proper clean
state and the final rinsing can be finished within about 7 to 8
minutes for all of the four kinds of cleaning solution.
[0052] Next, a brief explanation will be given on an example
comparative to the above embodiment with reference to FIG. 7. FIG.
7 is a graph showing the relationship between the wafer rinsing
time (cleaning time) and resistivity according to a prior art, with
respect to the kinds of cleaning chemical solution and the number
of wafers to be cleaned. Concretely, the graph of FIG. 7 indicates
the resistivity measured by the wafer cleaning method and the
cleaning equipment 101 according to the prior art shown in FIG. 5A,
under the four conditions of HF200/lwf, HF500/lwf, HF200/44wf and
HF500/44wf, as in the embodiment described above. The solid line in
the graph of FIG. 7 indicates the changes in the resistivity with
respect to the ultra-pure water rinsing time in HF200/lwf, or the
resistivity recovery time. The dashed line in the graph of FIG. 7
indicates the changes in the resistivity with respect to the
ultra-pure water rinsing time in HF500/lwf, or the resistivity
recovery time. The chain line in the graph of FIG. 7 indicates the
changes in the resistivity with respect to the ultra-pure water
rinsing time in HF200/44wf, or the resistivity recovery time. The
chain double-dashed line in the graph of FIG. 7 indicates the
changes in the resistivity with respect to the ultra-pure water
rinsing time in HF500/44wf, or the resistivity recovery time.
[0053] According to the prior art, whether a wafer is cleaned to
the proper clean state is determined by whether the resistivity of
solution reaches a preset value. In this comparative example, when
the resistivity of solution reaches 16 M.OMEGA.cm, a wafer is
regarded as cleaned to the proper clean state. Among the four
conditions, in HF200/44wf and HF500/44wf for rinsing 44 wafers, the
ultra-pure water rinsing time is different according to the density
of chemical solution (hydrofluoric acid). The resistivity of the
solution reaches 16 M.OMEGA.cm in both conditions. Therefore, in
HF200/44wf and HF500/44wf, the end time of final wafer rinsing can
be determined (confirmed) also in the above setting. Contrarily, in
HF200/lwf and HF500/lwf for rinsing 1 wafer, the ultra-pure water
rinsing time is different according to the density of chemical
solution, and the resistivity of the solution does not reach 16
M.OMEGA.cm. Therefore, in HF200/lwf and HF500/lwf, the end time of
final wafer rinsing can not be determined (confirmed) in the above
setting.
[0054] The resistivity of the solution at which a wafer is regarded
as cleaned to the proper clean state is set to 13 M.OMEGA., for
example, so as to determine the end time of final wafer rinsing
even in HF200/lwf and HF500/lwf. Then, the final wafer rinsing can
be finished when the resistivity of the solution reaches 13
M.OMEGA.cm I HF200/lwf and HF500/lwf. However, in HF200/44wf and
HF500/44wf, when the resistivity of the solution reaches 13
M.OMEGA.cm, the ion contained in the chemical solution remains in
the solution in the cleaning tank. Namely, in HF200/44wf and
HF500/44wf, if the resistivity of the solution at which a wafer is
regarded as cleaned to the proper clean state is set to 13
M.OMEGA.cm, the final rinsing will be finished before a wafer is
sufficiently rinsed.
[0055] Therefore, in the prior art, the wafer rinsing time is set
long including a sufficient allowance considering variations of the
rinsing time due to the cleaning conditions, so as to clean a wafer
to the sufficiently cleaned state, regardless of the various
conditions such as the number of wafers, the kinds and density of
cleaning chemical solution, and the resistivity of the solution in
the cleaning tank. For example, in the comparative example shown in
FIG. 7, the rinsing time is generally set to about 10 minutes. On
the contrary, in the above-mentioned embodiment, as seen from FIG.
3, the wafer 2 can be cleaned to the proper clan state in 7-8
minutes in all of the four conditions, and the final rinsing can be
finished.
[0056] For example, under the condition of HF500/44wf, the prior
art can reduce the rinsing time by about 200 seconds by applying
this embodiment to the cleaning tank that requires about 600
seconds (10 minutes) for rinsing a wafer. In this case, if the flow
rate per unit time of ultra-pure water supplied to the cleaning
tank is set to about 20L/min, the ultra-pure water can be decreased
by about 67 liters. The rinsing time is about 70 seconds different
between HF200/lwf with the longest rinsing time and HF500/44wf with
the shortest rinsing time in the above example. Namely, according
to this embodiment, the rinsing time can be decreased by about 70
seconds in HF500/44wf compared with HF200/lwf. In this case, if the
flow rate per unit time of ultra-pure water supplied to the
cleaning tank 3 is set to about 20L/min, the ultra-pure water can
be decreased by about 23 liters. On the contrary, in the prior art,
the rinsing time is set to about 600 seconds for both HF200/lef and
HF500/44wf, as described above. Therefore, in the prior art, about
70 seconds of rinsing time and about 23 liters of ultra-pure water
are wasted in HF500/44wf.
[0057] The resistivity recovery time in the final rinsing of the
wafer 2 is easy to be influenced by the number of wafers 2, and the
kinds and density of chemical solution. The resistivity recovery
time is not even. Therefore, in the prior art, the wafer rinsing
time is determined considering the longest rinsing time.
Contrarily, in this embodiment, even if the wafer 2 cleaning
condition is different, the wafer 2 can be cleaned to the same
state while controlling a waste of ultra-pure water, and the wafer
rinsing can be finished. Namely, according to this embodiment, the
wafer 2 can be cleaned to substantially the same proper clean state
regardless of the wafer 2 cleaning conditions. Compared with the
prior art, this embodiment cal also improve the wafer 2 cleaning
efficiency by decreasing the volume of ultra-pure water and
reducing the row process time (RPT) of the wafer 2.
[0058] Further, this embodiment uses the time differential value of
resistivity. This corresponds to using the replacement of chemical
solution by ultra-pure water. Therefore, this is difficult to be
influenced by a final resistivity value attained by the resistivity
of the solution 6 in the cleaning tank 3 when the wafer 2 is
cleaned with ultra-pure water. Namely, this embodiment is little
influenced by the final resistivity difference caused by the
different number of wafers 2 to be cleaned, and the lowered final
resistivity caused by deterioration of the measuring accuracy of a
resistivity meter.
[0059] According to the first embodiment, the rinsing of wafer 2 is
finished at the point when the time differential values of the
resistivity of the chemical solution used for cleaning the wafer 2
and the solution 6 including the cleaning water used for rinsing
the cleaned wafer 2 are equal to or less than preset values, and
held at that values for preset time. This makes it possible to
clean the wafer 2 to the proper clean state while improving the
wafer 2 cleaning efficiency, regardless of the number of wafers 2
to be cleaned and the kinds and density of the chemical solution
used for cleaning.
[0060] The wafer 2 according to this embodiment has been rinsed by
the wafer cleaning method or the wafer cleaning equipment 1
according to this embodiment. Therefore, the wafer 2 of this
embodiment has been cleaned to the proper clean state with stains
of chemical solution removed sufficiently. Further, the wafer 2 of
this embodiment provides high yield (production efficiency), and
reduces the production cost.
[0061] In addition, thought not shown, the semiconductor device
according to this embodiment has the wafer 2 according to this
embodiment. Therefore, the semiconductor device of this embodiment
is improved in the performance, quality, reliability and yield.
Further, the semiconductor device of this embodiment provides high
production efficiency, and reduces the production cost.
[0062] (2nd. Embodiment)
[0063] Now, explanation will be given on a second embodiment of the
present invention with reference to FIG. 4. FIG. 4 is a simplified
block diagram showing a wafer cleaning equipment according to this
embodiment. The same reference numerals are given to the same
components as in the first embodiment, and a detailed explanation
will be omitted.
[0064] Unlike the wafer cleaning equipment according to the first
embodiment, in the wafer cleaning equipment according to this
embodiment, a resistivity meter (resistivity measuring cell) is
provided near the middle part of a cleaning tank. Concrete
explanation will be given below.
[0065] As shown in FIG. 4, in the middle part of a cleaning tank 22
of a wafer cleaning equipment 21 according to this embodiment, a
take-out port (solution extraction port) 23 is provided to take out
the solution 6 from the cleaning tank 22 without exposing to the
air. A resistivity meter (resistivity measuring cell) 8 is provided
contacting the solution 6b taken out from the cleaning tank 3
through the solution extraction port 23. Namely, in this
embodiment, the resistivity measuring cell 8 is set to measure the
resistivity of the solution 6b without contacting the air.
[0066] The wafer cleaning method, wafer, and semiconductor device
according to this embodiment are the same as those of the first
embodiment, and explanation will be omitted.
[0067] The second embodiment can provide the same effects as the
first embodiment. In this embodiment, the resistivity measuring
cell 8 measures the resistivity of the solution 6b without
contacting the air. Therefore, the measured value is difficult to
be influenced by the carbonic acid gas or the like in the air
dissolved in the solution 6 through an upper opening 22a of the
cleaning tank 22 as a result of so-called air involving. Namely,
the measured value of the resistivity in this embodiment is
difficult to be influenced by the noises occurred in a cleaning
system 24 of the cleaning equipment 21 comprising the cleaning tank
22, water supply pipe 4, and ultra-pure water supply valve 5.
Particularly, the measured value is difficult to be influenced by
the changes in the noises in the cleaning system 24 caused by the
changes in the air contacting area of the solution 6 as a result of
the surface fluctuation of the solution 6. Therefore, this
embodiment can measure the resistivity of the solution 6 with a
high accuracy, and clean the wafer 2 to more clean state. Namely,
stains such as chemical solution adhered to the wafer 2 of this
embodiment are sufficiently eliminated, and the wafer 2 is cleaned
to more proper clean state. Further, though not shown, the
semiconductor device of this embodiment is improved in the
performance, quality, reliability and yield.
[0068] The cleaning method and equipment according to the present
invention are not limited to the first and second embodiments. The
invention may be embodied in other specific forms without departing
from its spirit or essential characteristics modifications. The
configurations and processes of the embodiments may be partially
modified, or combined appropriately.
[0069] For example, the A/D converter 10 is provided between the
resistivity measuring circuit 9 and arithmetic control circuit 11
in the first and second embodiments, but the A/D converter 10 is
not always necessary. If the resistivity measuring circuit 9 and
arithmetic control circuit 11 are set to process the same form
analog or digital signal, the A/D converter 10 is unnecessary.
[0070] The arithmetic section (arithmetic circuit) and control
section (control circuit) of the arithmetic control unit 11 are
constructed as one body, but they may not necessarily be one body.
The arithmetic section and control section of the arithmetic
control unit 11 may be configured as separate independent
units.
[0071] The resistivity measuring cell 8 is not necessary provided
near the upper opening 3a of the cleaning tank 3 or at the middle
of the cleaning tank 2. If the solution extracted to the
resistivity cell 8 is not replaced from a chemical solution to pure
water before the atmosphere solution of wafer 2, the resistivity
measuring cell 8 may be provided near the bottom of the cleaning
tanks 3 and 22. In this setting, the measured value of the
resistivity of the solution 6 is more difficult to be influenced by
the noises occurred in the cleaning systems 12 and 24 caused by the
carbonic acid gas dissolved in the solution 6.
[0072] The cleaning tanks 3 and 22 may be either a so-called batch
type capable of cleaning a plurality of wafers 2 at one time, or a
single wafer type for cleaning the wafer 2 one by one.
[0073] As a representative noise in the cleaning systems 12 and 24,
there is carbonic acid gas such as carbon dioxide in the air
dissolved in the solution 6. The carbonic acid gas dissolved in the
solution 6 affects largely the resistivity even if the dissolved
amount is very small. The amount of the carbonic acid gas dissolved
in the solution 6 is changed by the speed of supplying ultra-pure
water to the cleaning tanks 3 and 22, the speed of draining the
solution 6 from the cleaning tanks 3 and 22, or the changes in the
air contact area of the solution 6 by the surface fluctuation of
the solution 6. The change rate of the dissolved amount of carbonic
acid gas is largely influenced by the shapes of the cleaning tanks
3 and 22, the sizes of the upper openings 3a and 22a, or the
installation method and position of the resistivity measuring cell
8. Therefore, the method of smoothing the resistivity values to
eliminate the noises in the cleaning systems 12 and 24 is not
limited to the weighted average (the weighted smoothing), a
weighted mean, or the Savizky-Golay method. Any method suitable for
the noises in the cleaning systems 12 and 24 may be used.
[0074] Actually, the noise components cannot be completely
eliminated only by smoothing the resistivity values. Thus, the
differential value of resistivity that the wafer 2 is regarded as
cleaned to the proper clean state Is not necessarily limited to
0.05 M.OMEGA.m/sec. Any value equal to or less than 0.05
M.OMEGA.cm/sec is usable as a differential value of resistivity
that the wafer 2 is regarded as cleaned to the proper clean
state.
[0075] In the first and second embodiments, the conditions that the
wafer 2 is regarded as cleaned to the proper clean state that the
differential value of resistivity is equal to or less than 0.05
M.OMEGA.cm/sec after passing the maximum value and held at that
value for equal to or more than 5 seconds, but the conditions are
not necessarily limited to them. The conditions that the wafer 2 is
regarded as cleaned to the proper clean state may be determined to
appropriate values according to the number of wafers 2 to be
cleaned, the sizes of the processing tanks 3 and 22, the shapes of
the openings 3a and 22a, or the kinds and density of the chemical
solution used for the cleaning with a chemical solution, and other
various conditions.
[0076] In the first and second embodiment, ultra-pure water as a
cleaning water is supplied to the cleaning tanks 2 and 22 from the
bottom, but the setting is not limited to this. Ultra-pure water
may be supplied from the middle of the cleaning tanks 3 and 22. If
the ultra-pure water is exposed to the air when it is supplied to
the cleaning tanks 3 and 22 through the upper openings 3a and 22a,
for example, air involving occurs and the carbonic acid gas or the
like in the air is dissolved in the ultra-pure water. The carbonic
acid gas or the like dissolved in the ultra-pure water causes a
noise component in the cleaning systems 12 and 24 when measuring
the resistivity and conductivity of the solution 6, and the
measuring accuracy is lowered. Contrarily, if ultra-pure water is
supplied directly to the cleaning tanks 3 and 22 from the bottom or
the middle part without exposing to the air, the possibility of
dissolving carbonic acid gas or the like in the ultra-pure water
will be eliminated to almost zero. Namely, the noise components in
the cleaning systems 12 and 24 can be controlled and the accuracy
of measuring the resistivity and conductivity of the solution 6 can
be improved. Further, the wafer 2 can be cleaned to more clean
state while improving the cleaning efficiency.
[0077] In the first and second embodiments, the cleaning tank 3 is
either a processing tank dedicated to rinsing the wafer 2 with
adhesion of a cleaning chemical solution, or a processing tank
provided with a device to switch the solution supplied to the wafer
2 from a chemical solution to cleaning water after the wafer 2 is
cleaned with a chemical solution. By using the cleaning tanks 3 and
22 as a processing tank dedicated to rinsing, the volume of
chemical solution to be eliminated by a cleaning water can be
decreased. Thus, compared with the case that the cleaning tanks 3
and 22 are used as a processing tank not dedicated to rinsing, the
wafer 2 cleaning efficiency can be improved furthermore.
[0078] In the first and second embodiments, the resistivity of the
solution 6 is measured by using the resistivity measuring cell 8,
but the measurement is not limited to this. It is allowed to
measure the conductivity of the solution 6 instead of the
resistivity. In this case, a conductivity meter may be used instead
of the resistivity measuring cell 8 (a resistivity meter) as an
electric characteristic measuring unit. Cleaning of the wafer 2
with the cleaning water may be continued until reaching the
condition that the time differential value of the conductivity of
the solution 6 becomes larger than a preset value and is held at
that value for preset time.
[0079] Concretely, the cleaning of the wafer 2 with the cleaning
water may be continued until the time differential value of the
conductivity of the solution becomes equal to or more than -20
.mu.S/cm.multidot.sec after passing the minimum value and is held
at that value for equal to or more than 5 seconds.
[0080] Generally, the time differential value of the conductivity
of the solution including the chemical solution used for cleaning a
wafer and the cleaning water used for cleaning a wafer are
substantially zero at the start of measurement, regardless of the
number of wafers to be cleaned and the kinds and density of the
chemical solution used for the cleaning. The time differential
value of the conductivity descends as the measurement time elapses
and reaches the peak at preset time. Thereafter, the time
differential value of the conductivity ascends as the measurement
time elapses and becomes substantially zero. Namely, the value
obtained by differentiating the conductivity with respect to time
traces a curve projecting downward, regardless of the number of
wafers to be cleaned and the kinds and density of the chemical
solution used for the cleaning.
[0081] When using the time differential value of the conductivity
of solution to determine the wafer cleaning time, use the features
of the time differential value of the conductivity. Namely,
continue cleaning a wafer until the time differential value of the
conductivity of cleaning solution becomes larger than a preset
value determined based on the experiment data at which a wafer can
be cleaned to the proper clean state, and is held at that value for
preset time. Thus, the wafer cleaning with a cleaning water can be
finished immediately after a wafer is cleaned to the proper clean
state. As a result, the cleaning water volume used for cleaning a
wafer can be decreased, and a wafer can be cleaned to the proper
clean state while reducing the wafer cleaning time, regardless of
the number of wafers to be cleaned and the kinds and density of the
cleaning solution used for the cleaning.
[0082] Like the time differential value of the resistivity of the
cleaning solution, the time differential value of the conductivity
of the solution is not necessarily limited to -20
.mu.S/cm.multidot.sec. Any values equal to or more than -20
.mu.S/cm.multidot.sec may be used as the differential value of the
conductivity at which the wafer 2 is regarded as cleaned to the
proper clean state. The conditions that the wafer 2 is regarded as
cleaned to the proper clean state are also not necessarily limited
to that the differential value of the conductivity is equal to or
more than -20 .mu.S/cm.multidot.sec after passing the minimum
value, and is held at that value for equal to or more than 5
seconds. The conditions that the wafer 2 is regarded as cleaned to
the proper clean state may be determined to an appropriate value
according to the number of wafers 2 to be cleaned, the size of the
processing tanks 3 and 22, the shapes of the openings 3a and 22a,
the kinds and density of the chemical solution used for the
cleaning, and other various conditions.
[0083] 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.
* * * * *