U.S. patent application number 10/420042 was filed with the patent office on 2004-01-08 for substrate cleaning apparatus and substrate cleaning method.
This patent application is currently assigned to Dainippon Screen Mfg. Co., Ltd.. Invention is credited to Hirae, Sadao.
Application Number | 20040003829 10/420042 |
Document ID | / |
Family ID | 29720272 |
Filed Date | 2004-01-08 |
United States Patent
Application |
20040003829 |
Kind Code |
A1 |
Hirae, Sadao |
January 8, 2004 |
Substrate cleaning apparatus and substrate cleaning method
Abstract
A plurality of substrate holding pins 13 extend upright from a
holding stage 12, and a substrate W is mechanically held in a
circumferential direction with a back surface Sb of the substrate W
faced up. Between the substrate W and the holding stage 12, a
drip-proof plate 14 whose shape is approximately the same as that
of the substrate W is disposed with a distance from the substrate W
in such a manner that the drip-proof plate 14 covers a front
surface (pattern-bearing surface) Sf of the substrate W from below.
The drip-proof plate 14 disposed so as to cover the front surface
Sf of the substrate W blocks the mist from splashing upon the front
surface Sf of the substrate.
Inventors: |
Hirae, Sadao; (Kyoto,
JP) |
Correspondence
Address: |
OSTROLENK FABER GERB & SOFFEN
1180 AVENUE OF THE AMERICAS
NEW YORK
NY
100368403
|
Assignee: |
Dainippon Screen Mfg. Co.,
Ltd.
|
Family ID: |
29720272 |
Appl. No.: |
10/420042 |
Filed: |
April 18, 2003 |
Current U.S.
Class: |
134/1.3 ;
134/153; 134/157; 134/184; 134/33; 134/6 |
Current CPC
Class: |
H01L 21/67051 20130101;
H01L 21/67046 20130101 |
Class at
Publication: |
134/1.3 ; 134/6;
134/33; 134/153; 134/157; 134/184 |
International
Class: |
C25F 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 3, 2002 |
JP |
2002-194418 |
Claims
What is claimed is:
1. A substrate cleaning apparatus for cleaning one major surface of
a substrate, comprising: substrate holding means which holds a
substrate with one major surface of said substrate faced up;
cleaning liquid supplying means which includes a cleaning nozzle
and which supplies a cleaning liquid at a discharge outlet of said
cleaning nozzle onto one major surface of said substrate which is
held by said substrate holding means; ultrasonic wave applying
means which is disposed on one major surface side of said substrate
and applies ultrasonic vibrations upon said cleaning liquid; and
drip-proof means which is disposed with a distance from said
substrate so as to cover other major surface of said substrate from
below.
2. The substrate cleaning apparatus of claim 1, further comprising
a brush which is disposed adjacent to said discharge outlet of said
cleaning nozzle and which is capable of rubbing one major surface
of said substrate.
3. The substrate cleaning apparatus of claim 2, further comprising
moving means which moves said cleaning nozzle relative to one major
surface of said substrate.
4. The substrate cleaning apparatus of claim 1, further comprising
rotating means which rotates said substrate and said drip-proof
means as one integrated unit.
5. The substrate cleaning apparatus of claim 1, further comprising
gas flow generating means which supplies gas into a region-in-space
which is located between said other major surface of said substrate
and said drip-proof means, to thereby generate a gas flow which
travels outward from said region-in-space.
6. The substrate cleaning apparatus of claim 5, wherein said gas
flow generating means supplies gas into said region-in-space from
an approximately center portion of said drip-proof means.
7. The substrate cleaning apparatus of claim 1, wherein said other
major surface of said substrate is a front surface of said
substrate which bears a predetermined pattern and one major surface
of said substrate is a back surface of said substrate which is to
be cleaned.
8. The substrate cleaning apparatus of claim 7, wherein said
ultrasonic wave applying means comprises: a transducer which is
disposed in the vicinity of one major surface of said substrate;
and a high-frequency oscillator which applies an alternating
current signal whose frequency is lower than 1.2 MHz upon said
transducer.
9. The substrate cleaning apparatus of claim 8, wherein said
high-frequency oscillator applies an alternating current signal
whose frequency is 400 KHz or lower upon said transducer.
10. A substrate cleaning method of cleaning a back surface of a
substrate whose front surface bears a pattern, comprising the steps
of: disposing a substrate in a condition that said back surface of
said substrate is faced up and that a gas layer is adjacent to a
front surface of said substrate; and supplying a cleaning liquid
upon said back surface of said substrate while ultrasonic
vibrations are applied upon said cleaning liquid, to thereby clean
said back surface of said substrate with an ultrasonic wave.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a substrate cleaning
apparatus for and a substrate cleaning method of cleaning various
types of substrates (hereinafter referred to simply as
"substrate(s)"), such as a semiconductor wafer, a glass substrate
for photomask, a glass substrate for liquid crystal display, a
glass substrate for plasma display and an optical disk
substrate.
[0003] 2. Description of the Related Art
[0004] At steps for manufacturing semiconductors, manufacturing
liquid crystal display apparatuses, etc., a step of cleaning a
substrate is indispensable in order to form a pattern through a
photolithographic process. Known as an apparatus for cleaning
substrates is the substrate cleaning apparatus which is described
in Japanese Patent Application Laid-Open Gazette No. H7-86218, for
instance. This substrate cleaning apparatus positions a substrate
with one major surface of the substrate faced up. The apparatus
rubs one major surface of the substrate with a brush while
supplying a cleaning liquid applied with ultrasonic vibrations upon
one major surface of the substrate, to thereby clean the substrate.
Even fine particles, which can not be removed with a brush alone,
are removed using a cleaning liquid to which ultrasonic vibrations
are applied.
[0005] To be more specific, an ultrasonic transducer is disposed to
a cleaning nozzle which is for supplying a cleaning liquid upon one
major surface of a substrate. The ultrasonic transducer is
electrically connected to a high-frequency oscillator. A high radio
frequency whose frequency is 1.5.+-.0.3 MHz is applied to the
ultrasonic transducer, whereby ultrasonic vibrations are applied to
the cleaning liquid. And the vibrated cleaning liquid is injected
from the cleaning nozzle toward the substrate.
[0006] By the way, there is a problem that during ultrasonic
cleaning of one major surface of a substrate as described above, a
cleaning liquid splashes as a mist and adheres to the other major
surface of the substrate.
[0007] When a back surface of a substrate is to be cleaned using a
substrate cleaning apparatus which has such a structure above, it
is necessary to consider the following problem. That is, a surface
bearing a circuit pattern and the like of a substrate among the
both major surfaces of the substrate is mainly a surface to be
cleaned according to a conventional technique. More and more people
have started to recognize that one of causes of a deterioration in
yield is particles adhering to a back surface of a substrate during
steps for manufacturing electronic components of semiconductors,
liquid crystal display apparatuses and like. Therefore, a technique
for efficiently removing the particles adhering to the back surface
of the substrate is desired. An approach noting this may be to use
the substrate cleaning apparatus described above as it is also for
the purpose of cleaning back surfaces.
[0008] However, a conventional apparatus is set up with a purpose
of cleaning and accordingly removing particles which adhere to a
substrate front surface having a pattern. Hence, the conventional
apparatus can not clean and accordingly remove particles which
strongly adhere to a back surface of the substrate. Ultrasonic
cleaning permits to enhance the cleaning capability by means of
cavitations and improve the cleaning efficiency as the frequency of
the ultrasonic cleaning becomes lower. On the contrary, in a
conventional apparatus which mainly aims at cleaning a front
surface of a substrate, it is necessary to set the frequency of a
ultrasonic wave high just to such an extent that the pattern will
not be destroyed (For instance, the frequency is set to 1.2 MHz or
higher in the conventional apparatus.). Hence, the conventional
apparatus suppresses the cleaning capability of ultrasonic cleaning
relatively low such that such a pattern destruction will not occur.
This results that particles, which adhere to a back surface of a
substrate, fail to be efficiently cleaned, and therefore, it is not
possible to obtain a sufficient cleaning efficiency.
[0009] The frequency of an ultrasonic wave may be set low when only
a back surface of a substrate is to be cleaned. However, cleaning
in such a condition will lead to a problem that a mist adheres to a
front surface of the substrate (pattern-bearing surface) during
cleaning of the back surface of the substrate as described above.
Meanwhile, if a water film is formed on the front surface of the
substrate in an effort to solve this problem, there may arise
another problem. It is a problem that the ultrasonic wave applied
upon the back surface of the substrate is transmitted by the front
surface of the substrate (pattern-bearing surface) and destroys the
pattern.
SUMMARY OF THE INVENTION
[0010] A principal object of the present invention is to provide a
substrate cleaning apparatus which prevents a mist of a cleaning
liquid from adhering to other major surface of a substrate while
one major surface of the substrate is cleaned through ultrasonic
cleaning, to thereby clean the substrate with a ultrasonic wave at
an excellent cleaning efficiency.
[0011] Another object of the present invention is to provide a
substrate cleaning apparatus and a substrate cleaning method with
which it is possible to clean a back surface of a substrate whose
front surface bears a pattern at an excellent cleaning
efficiency.
[0012] To achieve the objects described above, the present
invention is directed to a substrate cleaning apparatus for
cleaning one major surface of a substrate, comprising: substrate
holding means which holds a substrate with one major surface of the
substrate faced up; cleaning liquid supplying means which supplies
a cleaning liquid at a discharge outlet of a cleaning nozzle onto
one major surface of the substrate which is held by the substrate
holding means; ultrasonic wave applying means which is disposed on
one major surface side of the substrate and applies ultrasonic
vibrations upon the cleaning liquid; and drip-proof means which is
disposed with a distance from the substrate so as to cover other
major surface of the substrate from below.
[0013] With such a structure according to the present invention,
the cleaning liquid to which ultrasonic vibrations are applied is
supplied upon one major surface of the substrate, and ultrasonic
cleaning is performed. A mist of the cleaning liquid is created
during the cleaning and partially splashes toward the other major
surface of the substrate. However, the drip-proof means, disposed
to cover the other major surface of the substrate, blocks the mist
from splashing upon the other major surface of the substrate and
sneaking of the mist.
[0014] The drip-proof means is disposed with a distance from the
substrate so as to cover the other major surface of the substrate
from below. On the account, it is possible to perform ultrasonic
cleaning of one major surface of the substrate with a
region-in-space (gas layer) created between the other major surface
of the substrate and the drip-proof means. As the region-in-space
is created adjacent to the other major surface of the substrate, it
is possible to dramatically reduce an ultrasonic wave which passes
through the other major surface. And therefore, it is not virtually
necessary to consider an influence of an ultrasonic wave upon the
other major surface and it is possible to increase the freedom with
respect to settings regarding the ultrasonic wave. Hence, setting
of the frequency of ultrasonic vibrations applied to the cleaning
liquid makes it possible to enhance the cleaning capability and
improve the cleaning efficiency.
[0015] The substrate cleaning method according to the present
invention is a method of cleaning a back surface of a substrate
whose front surface bears a pattern. In order to achieve another
object described above, the method comprises the steps of:
disposing a substrate in a condition that the back surface of the
substrate is faced up and that a gas layer is adjacent to a front
surface of the substrate; and supplying a cleaning liquid upon the
back surface of the substrate while ultrasonic vibrations are
applied upon the cleaning liquid, to thereby clean the back surface
of the substrate with an ultrasonic wave.
[0016] Such a structure according to the present invention requires
to supply a cleaning liquid upon a back surface of a substrate
while applying ultrasonic vibrations upon the cleaning liquid.
Since there is a gas layer adjacent to a front surface of the
substrate, an ultrasonic wave transmitted by the surface of the
substrate is dramatically reduced. This permits to apply ultrasonic
vibrations which are suitable to cleaning of the back surface of
the substrate upon the cleaning liquid without considering an
adverse affect of an ultrasonic wave over the surface of the
substrate (pattern-bearing surface) almost at all, and hence, to
easily enhance the cleaning efficiency.
[0017] The above and further objects and novel features of the
invention will more fully appear from the following detailed
description when the same is read in connection with the
accompanying drawing. It is to be expressly understood, however,
that the drawing is for purpose of illustration only and is not
intended as a definition of the limits of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a drawing which shows the preferred embodiment of
the substrate cleaning apparatus according to the present
invention;
[0019] FIG. 2 is a partially enlarged cross sectional view of the
substrate cleaning apparatus of FIG. 1; and
[0020] FIGS. 3A through 3C are drawings which show the testing
method of verifying a relationship between the frequency of the
alternating current signal and the sound pressure (ultrasonic
cleaning capability) and the result of the experiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] FIG. 1 is a drawing which shows a preferred embodiment of a
substrate cleaning apparatus according to the present invention,
while FIG. 2 is a partially enlarged cross sectional view of the
substrate cleaning apparatus of FIG. 1. This substrate cleaning
apparatus is for cleaning and drying a back surface of a substrate
W such as a semiconductor wafer. The apparatus comprises: a
substrate holding/rotating mechanism 1 which is for rotating the
substrate W while holding the substrate W; and a brush cleaning
mechanism 2 which slides against the back surface of the substrate
W and cleans the back surface of the substrate W. In this preferred
embodiment, the "back surface of the substrate" refers to a surface
which is on the opposite side to a front surface of a substrate on
which a pattern such as an electronic circuit is formed.
[0022] The substrate holding/rotating mechanism 1 comprises: a
rotation shaft 11 which is driven into rotations by a drive
mechanism 10 such as a motor; and a holding stage 12 which is
shaped like a disk and whose center is fixed at a top edge of the
rotation shaft 11. In this preferred embodiment, the diameter of
the holding stage 12 is set to be slightly larger than the diameter
of the substrate W. There are a plurality of, four in this
preferred embodiment, substrate holding pins 13, disposed to a top
surface circumference of the holding stage 12. The pins 13
mechanically hold the substrate W at a circumferential edge
portion. This makes it possible to mechanically hold the substrate
W in a circumferential direction with the back surface Sb of the
substrate W faced up. The plurality of substrate holding pins 13
extending upright from the top surface of the holding stage 12 thus
function the "substrate holding means" of the present invention, in
this preferred embodiment.
[0023] Further, between the holding stage 12 and the substrate W
which is held by the plurality of substrate holding pins 13, a
drip-proof plate 14 is disposed with a distance from the substrate
W. A shape of the drip-proof plate 14 is approximately the same as
that of the substrate W. The drip-proof plate 14 covers the front
surface (pattern-bearing surface) Sf of the substrate W from below.
Hence, as shown in FIG. 2, a region-in-space R is created in a gap
space SP between the substrate W and the drip-proof plate 14. The
drip-proof plate 14 may be formed by a metal plate, a ceramic
plate, a resin plate or the like for instance. An appropriate
selection may be made in accordance with the type of a cleaning
liquid, etc. The drip-proof plate 14 is received by the plurality
of substrate holding pins 13 at the circumferential edge portion of
the drip-proof plate 14 and accordingly fixed to the holding stage
12 in this preferred embodiment. A method of fixing the drip-proof
plate 14 is not limited to this fixing method but may be any
desired method.
[0024] The rotation shaft 11 is formed by a hollow shaft, and a
pipe 15 is disposed in the hollow portion of the rotation shaft. A
top edge of the pipe 15 penetrates the holding stage 12 and is
attached to the drip-proof plate 14 through an approximately center
portion of the drip-proof plate 14. Further, a bottom edge of the
pipe 15 is linked to a nitrogen gas supplier 30. The nitrogen gas
supplier 30 may be a utility of a factory where the substrate
cleaning apparatus is installed, for instance. Hence, as denoted at
the broken-line arrow in FIG. 2, nitrogen gas is supplied from the
nitrogen gas supplier 30 through the pipe 15 to the approximately
center portion of the region-in-space R, thereby developing a gas
flow which moves outward from the region-in-space R. The pipe 15
thus forms the "gas flow generating means" of the present
invention, in this preferred embodiment.
[0025] In the substrate holding/rotating mechanism 1 having such a
structure described above, as the drive mechanism 10 operates,
rotations of the rotation shaft 11 rotate the substrate W together
with the holding stage 12 and the drip-proof plate 14 as one
integrated unit. The drive mechanism 10 thus functions as the
"rotating means" of the present invention. In addition, a cup 3 is
disposed outside the substrate holding/rotating mechanism 1 in such
a manner that the cup 3 covers outer circumferential edges of the
holding stage 12, the drip-proof plate 14 and the substrate W from
the side toward above. This permits to collect the cleaning liquid
which splashes from the back surface Sb of the substrate W as
described later.
[0026] A structure of the brush cleaning mechanism 2 will now be
described. The brush cleaning mechanism 2 comprises an arm 21. A
rear end portion of the arm 21 is a base portion and axially
supported for free revolutions at a top edge of a column 22 which
is disposed outside the cup 3 in such a manner that the column can
freely move up and down. As a revolution driver 20 disposed to a
top edge portion of the column 22 operates, the front edge of the
arm 21 reciprocally whirls in the horizontal direction from the
center of the substrate W and a rim, about a link with the column
22. The revolution driver 20 thus functions as the "moving means"
of the present invention, and can move a cleaning nozzle which is
attached to the front edge of the arm 21 along the substrate W as
described below. The column 22 is capable of moving in the vertical
direction as described above, and as the columns 22 moves up and
down, the arm 21 and the cleaning nozzle can move in the vertical
direction as one integrated unit closer to and away from the
substrate W.
[0027] A cleaning unit 23, which corresponds to the "cleaning
liquid supplying means" of the present invention, is disposed to
the front edge of the arm 21. In the cleaning unit 23, as shown in
FIG. 2, a cleaning nozzle 25 is attached to a lower portion of a
front edge of a cleaning body 24. And it is therefore possible to
gush out a cleaning liquid 26 such as pure water supplied from a
cleaning liquid supply source not shown toward the back surface Sb
of the substrate W. Further, a brush 28 is attached to the bottom
surface of the cleaning nozzle 25 in such a manner that the brush
28 surrounds a discharge outlet 27 of the cleaning nozzle 25. As
the brush 28 slides against the back surface Sb of the substrate W
which rotates as described above, the back surface Sb is cleaned.
Used as the brush 28 in this preferred embodiment is one implanted
with a large number of fibers of a nylon resin or the like.
[0028] As shown in FIG. 2, disposed inside the cleaning nozzle 25
is a channel 29 for guiding the cleaning liquid 26 from the
cleaning liquid supply source to the discharge outlet 27. An
ultrasonic transducer 41 is disposed inside the cleaning nozzle 25
in such a manner that the ultrasonic transducer 41 faces the
channel 29. The ultrasonic transducer 41 is electrically connected
with a high-frequency oscillator 42. Receiving an alternating
current signal outputted from the high-frequency oscillator 42, the
ultrasonic transducer 41 vibrates and applies ultrasonic vibrations
upon the cleaning liquid 26 which flows through the channel 29
inside the cleaning nozzle 25. As ultrasonic vibrations are applied
upon the cleaning liquid 26 in this manner, cavitations are created
in a near-surface-area 43 close to the back surface Sb of the
substrate W. And an impulse wave develops when the cavitations are
crushed. Particles adhering to an area of the back surface facing
the near-surface-area 43 are stripped off by the impulse wave thus
develops from the substrate W and dispersed within the cleaning
liquid. The ultrasonic transducer 41 and the high-frequency
oscillator 42 thus function as the "ultrasonic wave applying means"
of the present invention which ensures an ultrasonic cleaning
effect.
[0029] Operations of the substrate cleaning apparatus having such a
structure described above will now be described. Although not shown
in the drawings, around the substrate cleaning apparatus, there is
a cassette in which a number of the substrates W are stacked one
atop the other. A substrate transporting robot unloads one
substrate at a time and transports the same to the substrate
cleaning apparatus. In the substrate cleaning apparatus, the
substrate W transported by the substrate transporting robot is
positioned by the substrate holding pins 13 with the back surface
Sb faced up and mechanically held so as to prevent idling of the
substrate. As the substrate transporting robot retracts away from
the substrate cleaning apparatus, a control unit not shown controls
the respective parts of the apparatus, thereby executing cleaning
and drying which will be described in the following.
[0030] First, the arm 21 reciprocally whirls in the horizontal
direction to a center position of the substrate W, and then moves
down to such a position at which a bottom edge of the brush 28
abuts on the back surface Sb of the substrate W. The cleaning
liquid is injected at the cleaning nozzle 25 toward the back
surface Sb of the substrate W, concurrently with which the
high-frequency oscillator 42 applies an alternating current signal
upon the ultrasonic transducer 41 and ultrasonic vibrations are
accordingly applied upon the cleaning liquid. Cleaning of the back
surface Sb of the substrate is initiated in this manner. In this
preferred embodiment, the frequency of the alternating current
signal mentioned above is set to 400 KHz based on an experiment
which will be described later in relation to an example.
[0031] The cleaning liquid 26 thus injected at the cleaning nozzle
25 flows upon the substrate W through the discharge outlet 27.
Owing to the function of cavitations which are created in the
near-surface-area 43, particles adhering to the back surface area
receiving the cleaning liquid are stripped off from the back
surface Sb of the substrate and dispersed within the cleaning
liquid. In this preferred embodiment in particular, since the
alternating current signal whose frequency is lower than that used
in a conventional apparatus is applied upon the ultrasonic
transducer 41 to create cavitations, a stronger cleaning effect is
obtained. In addition, the rotation shaft 11 rotates to thereby
rotate the substrate W while moving the arm 21 in the horizontal
direction in this condition. Hence, the cleaning nozzle 25 moves
along the back surface Sb of the substrate and the back surface Sb
of the substrate as a whole is accordingly subjected to ultrasonic
cleaning.
[0032] Further, not only such ultrasonic cleaning but sliding of
the brush 28 against the back surface Sb of the substrate as well
realize removal of the particles by means of the brush 28 at the
same time, which in turn makes it possible to effectively clean and
accordingly remove the particles which adhere to the back surface
Sb of the substrate. Still further, since brush-based cleaning and
ultrasonic cleaning are simultaneously performed by the same arm
21, a cleaning time is shortened as compared to where the
ultrasonic cleaning is executed after the brush-based cleaning.
Moreover, even when particles adhere to the brush 28 during
brush-based cleaning, application of ultrasonic vibrations upon the
brush 28 removes the particles from the brush 28 and keeps the
brush 28 clean.
[0033] After the cleaning finishes, the arm 21 moves slightly up to
such a position at which the bottom edge of the brush 28 stays
clear of the back surface Sb of the substrate W and the brush-based
cleaning is stopped, which is followed by the ultrasonic cleaning
alone and rinsing of the back surface Sb of the substrate. As a
predetermined period of time elapses, the substrate W is rotated at
a high speed with the ultrasonic cleaning, too, stopped and the
substrate W is accordingly dried.
[0034] After the series of cleaning and drying described above, the
substrate transporting robot unloads the substrate W from the
substrate cleaning apparatus and houses the substrate W in a
different cassette. As all substrates W are thus transferred from
the supplying cassette to the receiving cassette, the series of
processing completes.
[0035] As described above, in this preferred embodiment, the
cleaning liquid develops a mist while the cleaning liquid applied
with ultrasonic vibrations is supplied upon the back surface Sb of
the substrate W and ultrasonic cleaning is executed and the mist
partially splashes toward the front surface Sf of the substrate W.
However, the drip-proof plate 14 blocks the mist from splashing
upon the front surface Sf of the substrate W and effectively
prevents sneaking of the mist. Hence, it is possible to clean the
substrate in an excellent manner.
[0036] In addition, since nitrogen gas is supplied between the
substrate W and the drip-proof plate 14, i.e., in the
region-in-space R and develops a gas flow which flies outward from
the region-in-space R. This results that the gas flow serves as a
resistance against splashing of the mist toward the front surface
Sf of the substrate and more effectively prevents sneaking of the
mist the front surface Sf of the substrate. While this embodiment
uses nitrogen gas, it is needless to mention that air, other inert
gas or the like may be used instead of nitrogen gas. Further,
supplying of gas to the region-in-space R to thereby create a gas
flow is not an essential and indispensable element of the present
invention but is optional. That is, in the event that the
drip-proof plate 14 can sufficiently prevent adhesion of the mist
to the front surface Sf of the substrate, the gas supply may be
omitted.
[0037] It is possible to prevent the mist from adhering to the back
surface Sb of the substrate W in this preferred embodiment. Hence,
this is particularly effective to substrate processing for which it
is desirable that moisture does not adhere to the front surface Sf
of the substrate W, e.g., processing for forming a water repellent
film or a film which reduces a dielectric constant on the front
surface Sf of the substrate for example. Further, adhesion of the
mist to the front surface Sf of the substrate is prevented and
ultrasonic vibrations are applied upon the cleaning liquid in a
condition that the front surface Sf of the substrate is always
adjacent to the region-in-space R of the gas layer. On this
account, there is less danger that an ultrasonic wave applied upon
the back surface Sb of the substrate W will be transmitted by the
front surface (pattern-bearing surface) Sf of the substrate and
destroy the pattern. In fact, in this preferred embodiment, the
frequency of the alternating current signal is set to 400 KHz which
is lower than a set value (1.5 MHz.+-.0.3) which is used in a
conventional apparatus, to thereby enhance the cleaning effect.
Still further, with the frequency of the alternating current signal
set low, it is possible to stably ensure a high ultrasonic cleaning
effect as described in relation to the following example.
[0038] The present invention is not limited to the preferred
embodiment described above but may be modified to the extent not
deviating from the intention of the invention. For instance, while
the foregoing has described that gas such as nitrogen gas is
supplied to the region-in-space R from the approximately center
portion of the drip-proof plate 14 to thereby create a gas flow
which flies outward from the region-in-space R, the position for
introduction of the gas is not limited to this but may be any
desired position at which it is possible to introduce gas to the
region-in-space R.
[0039] In the preferred embodiment described above, the rotation
shaft 11 rotates to thereby rotate the substrate W while moving the
arm 21 in the horizontal direction and the cleaning nozzle 25
accordingly moves along the entire back surface Sb of the
substrate. An alternative structure may be used that the cleaning
nozzle 25 alone scans the entire back surface of the substrate. The
alternative structure may be also used that the ultrasonic
transducer 41 whose size is the same or larger than the radius of
the substrate is fixed in the horizontal direction and the
substrate W is rotated relative to the ultrasonic transducer
41.
[0040] Further, although the preferred embodiment described above
is directed to a substrate cleaning apparatus which cleans the back
surface Sb of the substrate, applications of the present invention
are not limited to this. The present invention is applicable also
to a substrate cleaning apparatus which cleans the front surface Sf
of the substrate. In short, the substrate may be held with the
front surface Sf of the substrate faced up as "one major surface"
of the present invention and the drip-proof plate may be disposed
with a distance from the substrate W in such a manner that the
drip-proof plate covers the back surface of the substrate which
serves as the other major surface of the substrate from below. In
this case as well, it is possible to block the mist which develops
during cleaning of the front surface of the substrate from
partially splashing toward the back surface of the substrate and
effectively prevent sneaking of the mist.
[0041] An example of the present invention will now be described.
The present invention however is not restricted by the following
example but may be implemented after modified appropriately to the
extent conforming to the intention of the invention. Any such
modifications are within the technical scope of the present
invention.
[0042] The cleaning capability by means of cavitations improves and
the cleaning efficiency becomes better as the frequency of the
alternating current signal decreases as described earlier. Further,
it was found through various experiments that when a gap G between
the ultrasonic transducer 41 and the substrate W changes, the
cleaning capability also changes. To be more specific, as shown in
FIG. 3A, while changing a space, namely, the gap G between the
ultrasonic transducer 41 and one major surface S1 of the substrate
W, an alternating current signal whose frequency was 1 MHz was
applied from the high-frequency oscillator 42. And with a
piezoelectric element 5 bonded to the other major surface S2 of the
substrate W, a voltage value corresponding to a sound pressure upon
the other major surface S2 of the substrate W was measured. FIG. 3B
is a graph which shows one example of the measurement result.
[0043] As FIG. 3B clearly shows, a variation in gap G changes the
sound pressure. A sound pressure level serves as an index value for
the cleaning capability. For the purpose of attaining a high
cleaning capability, it is desirable that the gap is set such that
the sound pressure level will be ensured. The gap G is therefore
preferably set to the gaps G1 and G2 which correspond to sound
pressure peaks. While a plurality of sound pressure peaks generally
appear, which peak the gap should be set may be determined in
accordance with the thickness of the brush and the frequency.
[0044] It is necessary to control vertical movements of the column
22 in the substrate cleaning apparatus described above in order to
set the gap G to the predetermined gap G1. On the contrary, there
is a limitation upon the positioning accuracy for this purpose. It
is therefore desired that the sound pressure level is ensured even
despite a variation in gap G to a certain extent. In addition,
there is a similar demand to that described above, as the optimal
gap G1 varies to a certain extent for each apparatus. In other
words, it is advantageous to execute substrate cleaning under the
condition that a selective peak sound pressure width, e.g., a half
width value H is wide. Noting this, for the purpose of verifying a
relationship between the frequency of the alternating current
signal and the selective peak sound pressure width H, the selective
peak sound pressure width H was measured at each frequency while
changing the frequency of the alternating current signal among
three types of frequency which were 400 KHz, 1 MHz, 1.5 MHz and 3
MHz. FIG. 3C is a graph which shows one example of the measurement
result.
[0045] As FIG. 3C clearly shows, the selective peak sound pressure
width H increases as the frequency of the alternating current
signal becomes lower. The curve indicates the change in selective
peak sound pressure width H corresponding to the frequency. There
is a an inflection point of the curve in the vicinity of 1.2 MHz,
which shows that a relatively wide selective peak sound pressure
width is obtained as the alternating current signal is set to a
lower frequency that this. Hence, when the frequency of the
alternating current signal is set to lower than 1.2 MHz, it is
possible not only to obtain a higher cleaning capability than that
realized by a conventional apparatus but also to widen the
selective peak sound pressure width, enhance the tolerance for the
gap accuracy and perform stable cleaning. For stabilization of
cleaning and the cleaning capability, the frequency of the
alternating current signal is preferably set further lower, and
more preferably to 400 KHz or lower.
[0046] Although the invention has been described with reference to
specific embodiments, this description is not meant to be construed
in a limiting sense. Various modifications of the disclosed
embodiment, as well as other embodiments of the present invention,
will become apparent to persons skilled in the art upon reference
to the description of the invention. It is therefore contemplated
that the appended claims will cover any such modifications or
embodiments as fall within the true scope of the invention.
* * * * *