U.S. patent application number 09/801636 was filed with the patent office on 2002-03-21 for method of and apparatus for drying a wafer using isopropyl alcohol.
Invention is credited to Jung, Jae-Hyung.
Application Number | 20020032973 09/801636 |
Document ID | / |
Family ID | 19681545 |
Filed Date | 2002-03-21 |
United States Patent
Application |
20020032973 |
Kind Code |
A1 |
Jung, Jae-Hyung |
March 21, 2002 |
Method of and apparatus for drying a wafer using isopropyl
alcohol
Abstract
A method of and an apparatus for drying a wafer using the
Marangoni effect quickly forms an isopropyl alcohol layer on a
cleaning liquid in which the wafer is submerged. The isopropyl
alcohol is first heated and then supplied in a fluid state onto the
cleaning liquid. The isopropyl alcohol liquid thus diffuses rapidly
to form the isopropyl alcohol layer. The wafer is thoroughly dried
by removing it from the cleaning liquid through the isopropyl
alcohol while only supplying more of the heated nitrogen gas into
the ambient above the cleaning liquid.
Inventors: |
Jung, Jae-Hyung; (Suwon-si,
KR) |
Correspondence
Address: |
JONES VOLENTINE, L.L.C.
Suite 150
12200 Sunrise Valley Drive
Reston
VA
20191
US
|
Family ID: |
19681545 |
Appl. No.: |
09/801636 |
Filed: |
March 9, 2001 |
Current U.S.
Class: |
34/467 ;
257/E21.228; 34/72 |
Current CPC
Class: |
H01L 21/67028 20130101;
H01L 21/02063 20130101; Y10S 134/902 20130101 |
Class at
Publication: |
34/467 ;
34/72 |
International
Class: |
F26B 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 3, 2000 |
KR |
2000-44984 |
Claims
What is claimed is:
1. A method of drying an object of manufacture after the object has
been washed by being immersed in a cleaning liquid, said method
comprising the steps of: while the object is submerged in the
cleaning liquid, supplying inert gas over the cleaning liquid, and
heating the inert gas, whereby an ambient of the heated inert gas
is produced over the cleaning liquid; while the object remains
submerged in the cleaning liquid, heating a liquid having a smaller
surface tension than that of said cleaning liquid to such a
temperature, and supplying the heated liquid onto the surface of
said cleaning liquid in a fluid state in such an amount that a
stable layer consisting of the heated liquid is formed on the
surface of the cleaning liquid and vapor of the heated liquid mixes
with the inert gas to form an ambient comprising a mixture of the
vapor and the heated inert gas over the cleaning liquid;
subsequently removing the object from said cleaning liquid through
the layer of said heated liquid and into the ambient over said
cleaning liquid; and while the object is exposed to the ambient,
supplying additional heated inert gas into the ambient.
2. A method of drying an object as claimed in claim 1, wherein the
inert gas is nitrogen.
3. A method of drying an object as claimed in claim 1, wherein the
cleaning liquid is de-ionized water, and the heated liquid is
isopropyl alcohol.
4. A method of drying an object as claimed in claim 2, wherein the
step of supplying the heated isopropyl alcohol onto the surface of
the cleaning liquid is carried out until the liquid layer of
isopropyl alcohol has a thickness within a range of about 1 mm to
about 3 mm.
5. A method of drying an object of manufacture after the object has
been washed by being immersed in a cleaning liquid, said method
comprising the steps of: supplying heated nitrogen gas over the
cleaning liquid in which the object is submerged; while the object
remains submerged in the cleaning liquid, supplying a predetermined
quantity of heated liquid isopropyl alcohol onto the cleaning
liquid sufficient to form an isopropyl alcohol layer on the
cleaning liquid and an ambient comprising a mixture of the
isopropyl alcohol and the heated nitrogen gas over said cleaning
liquid; removing the object from the cleaning liquid through the
layer of isopropyl alcohol and into the ambient to thereby dry the
object; and while the object is exposed to the ambient, supplying
additional heated nitrogen gas into the ambient to thereby further
dry the object.
6. A method of drying an object as claimed in claim 5, and further
comprising the step of supplying additional heated nitrogen gas
into the ambient above the cleaning liquid immediately after the
predetermined quantity of the liquid isopropyl alcohol has been
supplied onto the cleaning liquid.
7. A method for drying an object as claimed in claim 6, wherein the
step of supplying additional heated nitrogen gas comprises
supplying heated nitrogen gas into the ambient continuously during
the entire time the object is being removed from the cleaning
liquid.
8. A method of drying an object as claimed in claim 5, and further
comprising the step of draining only some of cleaning liquid from
around the object while keeping the object immersed in the cleaning
liquid, before the heated liquid isopropyl alcohol is supplied onto
the cleaning liquid.
9. A method of drying an object as claimed in claim 5, and further
comprising the step of supplying isopropyl alcohol vapor into the
ambient over the cleaning liquid after the heated liquid isopropyl
alcohol is supplied onto the cleaning liquid, in addition to the
vapor that is derived from the layer of isopropyl alcohol on the
cleaning liquid.
10. A method of drying an object as claimed in claim 9, wherein the
step of supplying the additional isopropyl alcohol vapor comprises
supplying isopropyl alcohol vapor continuously during the entire
time the object is being removed from the cleaning liquid.
11. A method of drying an object as claimed in claim 5, wherein the
step of removing the object from the cleaning liquid comprises
draining the cleaning liquid from around the object until the
object becomes exposed to the ambient.
12. A method of drying an object as claimed in claim 11, wherein
the draining of the cleaning liquid is carried out at such a rate
that the level of the cleaning liquid drops at about 1.5 mm/sec to
about 2.5 mm/sec.
13. A method of drying an object as claimed in claim 5, wherein the
step of removing the object from the cleaning liquid comprises
raising the object up out of the cleaning liquid.
14. A method of drying an object as claimed in claim 5, wherein the
cleaning liquid comprises de-ionized water.
15. A method of drying an object as claimed in claim 5, wherein the
supplying of heated nitrogen gas comprises supplying nitrogen gas
having a temperature within the range of 70.degree. C. to about
90.degree. C. over the cleaning liquid.
16. A method for drying an object as claimed in claim 5, wherein
the liquid isopropyl alcohol is supplied until the layer thereof on
the cleaning liquid has a thickness in the range of about 1 mm to
about 3 mm.
17. An apparatus for cleaning an object, comprising: a cleaning
section including a bath for retaining a cleaning liquid for
cleaning the object; an isopropyl alcohol supply unit including a
source of liquid isopropyl alcohol, a heater operative to heat the
liquid isopropyl alcohol, and a liquid isopropyl alcohol supply
line connected to said source and having at least one outlet at the
top of said bath, wherein the isopropyl alcohol supply unit
supplies heated liquid isopropyl alcohol onto the cleaning liquid
in said bath to form a liquid isopropyl alcohol layer thereon and
an ambient thereover comprising isopropyl alcohol; a separating
unit that removes the object from the cleaning liquid; and a
nitrogen supply unit including a supply of nitrogen gas, a heater
operative to heat the nitrogen gas, and a nitrogen gas supply line
connected to said source of nitrogen and having an outlet located
above the at least one outlet of said isopropyl alcohol supply
unit, wherein the nitrogen supply unit supplies heated nitrogen gas
to a location in the cleaning section above said bath.
18. An apparatus for drying an object as claimed in claim 17,
wherein said bath is an inner bath, and said cleaning section
further includes a source of cleaning liquid, a cleaning liquid
supply line connecting the source of cleaning liquid to the inner
bath, and an outer bath surrounding said inner bath such that any
cleaning liquid over-flowing the inner bath gathers in the outer
bath.
19. An apparatus for drying an object as claimed in claim 17,
wherein said isopropyl alcohol supply unit further comprises a flow
control meter disposed in said isopropyl alcohol supply line for
regulating the rate at which the isopropyl alcohol liquid flows
into the inner bath.
20. An apparatus for drying an object as claimed in claim 17,
wherein said source of isopropyl alcohol is a tank of isopropyl
alcohol, and said isopropyl alcohol supply unit further comprises a
source of pressurized inert gas, and an inert gas supply line
connected to said source of pressurized inert gas and having an
outlet open to the interior of said tank, wherein the inert gas
exerts pressure on the isopropyl alcohol in the tank to force the
isopropyl alcohol into the isopropyl alcohol supply line.
21. An apparatus for drying an object as claimed in claim 17,
wherein said separating unit comprises a cleaning liquid draining
unit for draining said cleaning liquid to separate said object from
said cleaning liquid.
22. An apparatus for drying an object as claimed in claim 21,
wherein said draining unit comprises a drain line connected to said
inner bath, and a pump disposed in said drain line.
23. An apparatus for drying an object as claimed in claim 17,
wherein said separating unit comprises an elevator operatively
associated with said inner bath so as to raise objects up out of
said inner bath.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a method of and an
apparatus for drying a wafer. More particularly, the present
invention relates to a method of and an apparatus for drying a
wafer using isopropyl liquid after the wafer is cleaned following a
semiconductor manufacturing process.
[0003] 2. Description of the Related Art
[0004] Semiconductor devices or semiconductor chips are
manufactured by processing a wafer that is usually formed of
silicon. The wafer is typically subjected to a series of
semiconductor device manufacturing processes such as
photolithography, chemical or physical vapor deposition and plasma
etching.
[0005] After executing these processes, foreign material such as
chemicals or dust remains on the surface of the wafer. In order to
assure the quality of the semiconductor devices, the foreign
material must be removed from the surface of the wafer. The
cleaning process used to remove the foreign material involves both
washing and drying the wafer.
[0006] In particular, the wafer may be washed using de-ionized
water (hereinafter referred to as "DIW"). Because the DIW will
eventually dissolve the silicon, the wafer must be completely dried
after being washed with the DIW or else water spots will be
formed.
[0007] In addition, a method of using isopropyl alcohol to enhance
the drying of a wafer has been developed. For example, Japanese
Patent Laid-Open Publication No. Hei 8-61846 discloses a method of
directly spraying liquid isopropyl alcohol over a wafer, forming a
mixture of the water and the isopropyl alcohol on the surface of
the wafer, and volatilizing the mixture by means of a high
temperature nitrogen gas. In addition, U.S. Pat. No. 6,029,371
issued to Kamikawa et al. discloses a method of drying a wafer by
directly spraying a washed wafer with a drying gas comprising
heated isopropyl alcohol and nitrogen.
[0008] Also, drying apparatuses using isopropyl alcohol are
disclosed in U.S. Pat. No. 5,634,978 issued to Mohindra et al.,
U.S. Pat. No. 5,855,077 issued to Chang-Hyun Nam et al., U.S. Pat.
No. 4,633,893 issued to McConnell et al., and U.S. Pat. No.
4,911,761 also issued to McConnell et al. These drying apparatuses
execute a method in which the isopropyl alcohol is introduced as a
mist over the cleaned wafer to eliminate the water on the
wafer.
[0009] FIG. 1 shows one example of a conventional apparatus 100
that works on the Marangoni effect to dry the wafer using an
isopropyl alcohol mist. The apparatus 100 includes a cleaning
section (or a rinsing section) 110 containing a cleaning liquid (or
rinsing liquid) 113 for cleaning (or rinsing) the wafer. The
cleaning section 110 in turn includes an inner bath 112 having an
upper open end, and an outer bath 116 covered with a lid 117. The
outer bath 116 and lid 117 enclose the inner bath 112. A wafer 101
is immersed in the cleaning liquid 113 of the inner bath 112 for
cleaning. Once the cleaning liquid 113 over-flows the inner bath
112, the over-flown cleaning liquid 113a gathers in the outer bath
116 from where it is drained from the cleaning section 110.
[0010] A cleaning liquid supply tube line 114a is connected to the
inner bath 112 at the bottom thereof. First and second cleaning
liquid drain tube lines 114b and 114c are connected to the inner
bath 112 and the outer bath 116, respectively, at the bottoms
thereof. The cleaning liquid supply tube line 114a supplies the
cleaning liquid 113, such as DIW, to the inner bath 112. The first
cleaning liquid drain tube line 114b drains the cleaning liquid
113a which has over-flown the inner bath 112 into the outer bath
116. The second cleaning liquid drain tube line 114c gradually
drains the cleaning liquid 113 from within the inner bath 112.
[0011] The apparatus 100 for drying the wafer is also equipped with
an isopropyl alcohol mist supply tube line 134 for supplying the
isopropyl alcohol mist from an isopropyl alcohol supply unit (not
shown). The isopropyl alcohol supply unit makes the isopropyl
alcohol bubble by using nitrogen as a carrier gas to form the
isopropyl alcohol mist. Then, the isopropyl alcohol mist and
nitrogen are supplied to the upper portion of the outer bath 116
via the isopropyl alcohol mist supply tube line 134. A diffuser 136
is furnished at the central portion of the lid 117 of the outer
bath 116 for consistently diffusing the isopropyl alcohol mist and
nitrogen throughout the inside of the outer bath 116.
[0012] In addition, a nitrogen gas supply tube line 140 is
connected to the lid 117 for supplying heated nitrogen gas into the
outer bath 116 during the drying process to create a drying
ambient.
[0013] The conventional apparatus 100 for drying a wafer operates
as follows.
[0014] Once a wafer guide 103 loaded with wafers 101 is seated
within the inner bath 112, the cleaning liquid 113, such as the
DIW, is supplied to the inner bath 112 via the cleaning liquid
supply tube line 114a to initiate the cleaning operation. The
cleaning liquid is supplied into the inner bath 112 at such a rate
that it overflows the inner bath 112 during the cleaning operation.
The over-flown cleaning liquid 113a is gathered in the outer bath
116 and drained from the bottom of the outer bath 116 via the first
cleaning liquid drain tube line 114b.
[0015] Once the cleaning operation is complete, the process of
drying the wafer 101 begins. FIGS. 2A, 2B and 2C are schematic
diagrams illustrating the drying process.
[0016] Referring to FIG. 2A, after the cleaning process is
complete, the nitrogen gas and the isopropyl alcohol mist entrained
thereby are introduced via the isopropyl supply tube line 134 and
diffuser 136 into the upper portion of the outer bath 116. Thus,
the ambient in outer bath 116 is converted into a drying ambient.
At this time, approximately 50 cc of the isopropyl alcohol mist is
supplied. Also, at this time, the heated nitrogen gas is supplied
into the outer bath 116 via the nitrogen gas supply tube line 140
connected to the lid 117.
[0017] Referring to FIG. 2B, the cleaning liquid 113 is drained via
the drain tube line 114c while the nitrogen gas is supplied via the
nitrogen gas supply tube line 140. At this time, the cleaning
liquid 113 is drained at a constant rate via the second drain tube
line 114c. The height of the cleaning liquid 113 is decreased at a
rate of about 3 mm/sec. During the draining operation, the water
spots on the wafers 101 are eliminated by means of the Marangoni
effect created by the isopropyl alcohol. Referring to FIG. 2C, once
the cleaning liquid 113 is completely drained to a level below the
wafers 101, the last of the heated nitrogen gas is introduced into
the outer bath 116 via the nitrogen supplying tube line 140,
thereby completing the drying process.
[0018] In addition to this conventional method, Japanese Patent
Laid-Open Publication Nos. Hei 11-87305, Hei10-154689 and
Hei10-22257 disclose methods of drying a wafer in which an
isopropyl alcohol mist is used. In these methods, an isopropyl
alcohol liquid layer is formed by the mist over a cleaning liquid,
and the wafers are raised from the cleaning liquid into the
isopropyl alcohol liquid layer, thereby drying the wafer.
[0019] According to all of these heretofore known methods, the mist
of isopropyl alcohol is supplied to the outer bath using nitrogen
as a carrier gas. Therefore, the amount of isopropyl alcohol
injected into the outer bath is determined by measuring the
reduction in the amount of isopropyl alcohol in the isopropyl
alcohol supply unit. For this reason, the amount of isopropyl
alcohol used for forming the isopropyl alcohol layer cannot be
accurately determined. Furthermore, it is difficult to accurately
control the amount of isopropyl alcohol being supplied.
[0020] Moreover, the isopropyl alcohol sprayed by the diffuser
adheres to the side wall of the outer bath. In this case, the
isopropyl alcohol is likely to fall as drops onto the wafer. These
drops create wafer defects.
[0021] Additionally, a large amount of time is required to form
from the mist a sufficient layer of isopropyl alcohol above the
cleaning liquid within the inner bath for producing the Marangoni
effect. The overall processing time is thus significant. Moreover,
such a mist is likely to be exhausted if the outer bath is not
completely sealed.
[0022] In view of these problems of the conventional art, a method
has been proposed in which the isopropyl alcohol liquid is supplied
directly onto the upper portion of the cleaning liquid. For
example, Japanese Patent Laid-open No. Hei 9-213672 discloses a
method in which an isopropyl alcohol layer is formed over a
cleaning liquid in which a wafer is immersed, and the wafer is
dried while ascending from the cleaning liquid. In this method, the
isopropyl alcohol layer formed on the cleaning liquid has a
thickness of about 5 mm, and the wafer is to raised gradually from
the cleaning liquid.
[0023] However, the above-described method requires a large
quantity of isopropyl alcohol and a relatively long amount of time
for forming the isopropyl alcohol liquid layer. Also, the drying
effect produced by this method is so insufficient that water spots
remain after the wafer is cleaned. The water spots leave too many
particles on the wafer.
SUMMARY OF THE INVENTION
[0024] Therefore, the object of the present invention is to
overcome the above-described drawbacks of the conventional prior
art.
[0025] More specifically, a first object of the present invention
is to provide a highly effective and time-efficient method of
drying a wafer using the Marangoni effect. A second object of the
present invention is to provide an apparatus for drying a wafer
particularly suitable for performing such a method.
[0026] To achieve the first object, the present invention provides
a method of drying an object comprising supplying heated inert gas
over a cleaning liquid in which the object is immersed, heating a
liquid having a smaller surface tension than that of the cleaning
liquid, and then supplying the heated liquid onto the cleaning
liquid in a fluid state. This forms a liquid layer on the cleaning
liquid, and an ambient over the cleaning liquid comprising the
vapor of the heated liquid and the inert gas. Then, the object is
removed from the cleaning liquid while the heated liquid layer is
maintained. The object is dried further by supplying more of the
heated inert gas into the ambient surrounding the object.
[0027] The heated inert gas is preferably nitrogen gas, and the
liquid used to form a layer on the cleaning liquid is preferably
isopropyl alcohol.
[0028] To achieve the second object, the present invention provides
a cleaning apparatus comprising a cleaning section housing the
cleaning liquid for cleaning the object, and an isopropyl alcohol
supplying unit that supplies heated liquid isopropyl alcohol onto
the cleaning liquid to form an isopropyl alcohol liquid layer and
an isopropyl alcohol ambient over the cleaning liquid, and a
nitrogen supply unit that supplies heated nitrogen gas above the
cleaning liquid housed in the cleaning section. A separating unit
can remove the object from the cleaning liquid while the isopropyl
alcohol layer is maintained.
[0029] According to the present invention, to dry the object such
as a wafer, the nitrogen gas is heated and supplied into the
cleaning section at a location above the cleaning liquid in which
the wafer is immersed, and the liquid isopropyl alcohol liquid is
heated and supplied directly onto the cleaning liquid. The
isopropyl alcohol liquid layer diffuses quickly to form a liquid
layer on the cleaning liquid, the liquid isopropyl alcohol liquid
supplied in the fluid state also forms an ambient with the nitrogen
gas that is non-volatile above the cleaning liquid. The wafer is
dried only by removing it from the cleaning liquid into the ambient
while only the heated nitrogen gas continues to be supplied.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] The above and other objects, features and advantages of the
present invention will become more apparent by referring to the
following detailed description of the preferred embodiments thereof
made with reference to the attached drawings, of which:
[0031] FIG. 1 is a schematic diagram of a conventional apparatus
for drying a wafer;
[0032] FIGS. 2A to 2C are schematic diagrams of cleaning sections
of the conventional drying apparatus, illustrating a drying process
executed by the conventional apparatus;
[0033] FIG. 3 is a schematic diagram of a first embodiment of an
apparatus for drying a wafer according to the present
invention;
[0034] FIG. 4 is top view of a cleaning section of the drying
apparatus showing the arrangement of a nitrogen gas supply tube
line and first and second isopropyl alcohol supply tube lines
relative to a bath of the cleaning section;
[0035] FIGS. 5A to 5D are respective side views of the cleaning
section, illustrating a first embodiment of a method of drying a
wafer according to the present invention;
[0036] FIG. 6 is a schematic diagram showing the change in the
level of cleaning liquid and an isopropyl alcohol layer, which
occurs during the draining step shown in FIG. 5B;
[0037] FIG. 7 is a flowchart of the first embodiment of the method
of drying a wafer according to the present invention;
[0038] FIG. 8 is a timing chart of the first embodiment of the
method for drying the wafer according to the present invention;
[0039] FIG. 9 is a graph plotting the change in the number of
particles before and after a wafer is washed and dried in
accordance with the method of the present invention;
[0040] FIG. 10 is a graph plotting the change in the number of
particles before and after a wafer is washed and dried in
accordance with the method of the present invention, using the
average values from FIG. 9, and in accordance with a prior art
process;
[0041] FIG. 11 is a graph plotting the number of particles before
and after a wafer is washed and dried for different amounts of
supplied isopropyl alcohol;
[0042] FIG. 12 is a schematic diagram of a second embodiment of an
apparatus for drying a wafer according to the present invention;
and
[0043] FIG. 13 is a flowchart of the second embodiment of a method
of drying a wafer according to the present invention, which method
can be performed by the apparatus shown in FIG. 12.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0044] The preferred embodiments of the present invention will be
described in detail hereinafter with reference to accompanying
drawings.
[0045] Embodiment 1
[0046] FIG. 3 shows a first embodiment of an apparatus 200 for
drying a wafer according to the present invention.
[0047] The drying apparatus 200 includes a cleaning section 210
containing a cleaning liquid 213 for cleaning wafers 201. DIW may
be used as the cleaning liquid 213. The cleaning section 210, in
turn, comprises an inner bath 212 in which the wafers 201 are
immersed in the cleaning liquid 213 to be cleaned, and an outer
bath 216 enclosing the inner bath 212. The wafers 201 are loaded on
a wafer guide 203 that can be seated in the cleaning bath 210. The
wafer guide 203 comprises two opposing guide plates 203a, three
guide rods 203b (see FIG. 4) fixed between the guide plates 203a,
and a guide arm 203c extending upwardly from one of the guide
plates 203a. The guide arm 203c has a horizontally extending
portion at the top thereof to facilitate the handling of the wafer
guide 203.
[0048] The top of the inner bath 212 is open. The outer bath 216 is
covered with a lid 217. Any of the cleaning liquid that overflows
the inner bath 21 is gathered in the outer bath 216. From there,
the over-flown cleaning liquid 213a is drained from the cleaning
section 210. The lower portion of the outer bath 216 is preferably
funnel-shaped to facilitate the draining of the cleaning liquid
213a.
[0049] Cleaning liquid lines 214 for supplying and draining the
cleaning liquid 213 are connected to the bottom of the inner bath
212. Of these lines 214, a cleaning water supply tube line 214a
constitutes a cleaning water supply unit that supplies the cleaning
liquid to the inner bath 212 from a cleaning water supply source
(not shown).
[0050] A cleaning water draining unit 230 includes an outer bath
cleaning water drain tube line 214b and an inner bath cleaning
water drain tube line 214c. One end of the outer bath cleaning
water drain tube line 214b is connected to the funnel-shaped bottom
of the outer bath 216, and the other end thereof is connected to a
common drain tube line 214d. A first drain valve 214V1 is provided
in-line with the outer bath cleaning water drain tube line 214b.
The inner bath cleaning water drain tube line 214c extending from
the bottom of the inner bath 212 branches into a first inner bath
cleaning water drain tube line 214c1 and a second inner bath
cleaning water drain tube line 214c2. The first inner bath cleaning
water drain tube line 214c1 and the second inner bath cleaning
water drain tube line 214c2 are connected to the common drain tube
line 214d.
[0051] Moreover, a second drain valve 214V2 is provided in-line
with the first inner bath cleaning water drain tube line 214c1, and
a draining pump 214P is provided in-line with the second inner bath
cleaning water draining tube line 214c2.
[0052] The drying apparatus 200 also includes an isopropyl alcohol
supply unit 220 for supplying isopropyl alcohol to the cleaning
section 210. More specifically, the isopropyl alcohol supply unit
220 supplies heated isopropyl alcohol liquid onto the cleaning
liquid 213 to form an isopropyl alcohol liquid layer on the
cleaning liquid 213. Isopropyl alcohol as a liquid has a surface
tension that is smaller than that of the cleaning liquid. The
heated isopropyl alcohol liquid is also partially volatilized to
form an isopropyl alcohol ambient at the upper portion of the outer
bath 216, i.e., over the cleaning liquid 213. In addition, the
isopropyl alcohol supply unit 220 is preferably operated to supply
the cleaning section with isopropyl alcohol vapor while the
cleaning liquid 213 is being drained to thereby maintain the
ambient of isopropyl alcohol in the cleaning section 210.
[0053] The isopropyl alcohol supply unit 220 includes an isopropyl
alcohol tank 224 partially filled with liquid isopropyl alcohol
222. The tank 224 is connected to an isopropyl alcohol supply tube
line 232 by which the tank 224 is continuously supplied with
isopropyl alcohol liquid.
[0054] Also, a pressurizing unit 225 is provided at the upper
portion of the tank 224 for pressurizing the isopropyl alcohol
liquid 222. To this end, the pressurizing unit 225 may exert
hydraulic pressure or pneumatic pressure on the isopropyl alcohol
liquid 222. However, in the preferred embodiment of the present
invention, the pressurizing unit 225 employs nitrogen gas to
pressurize the isopropyl alcohol liquid. In this case, the
pressurizing unit 225 includes a nitrogen gas supply tube line 226
and a nitrogen gas supply source 228. Alternatively, the
pressurizing unit 225 may employ an inert gas such as argon or
helium instead of nitrogen.
[0055] One end of the nitrogen gas supply tube line 226 is
connected with a nitrogen gas supply source 228, and the other end
thereof is disposed over the isopropyl alcohol liquid 222. The
nitrogen gas supplied from the nitrogen gas supply source 228 via
the nitrogen gas supplying tube line 226 exerts pressure on the
surface of the isopropyl alcohol liquid 222. Such pressure causes
the isopropyl alcohol liquid 222 to be forced from the tank
224.
[0056] The isopropyl alcohol liquid 222 is supplied to the upper
portion of inner bath 212 via an isopropyl alcohol supply tube line
234. One end of the isopropyl alcohol supply tube line 234 is
connected to the isopropyl alcohol tank 224. From there, the
isopropyl alcohol supplying tube line 234 branches into a first
isopropyl alcohol supply tube line 234a and a second isopropyl
alcohol supply tube line 234b. The outlets of the first and second
supply tube lines 234a and 234b are disposed over the inner bath
212 of the cleaning section 210. First and second supply tube lines
234a and 234b, as in this embodiment, are preferable in terms of
their ability to supply the isopropyl alcohol efficiently to the
inner bath 212, and to form a layer of isopropyl alcohol liquid
uniformly on the cleaning liquid, etc. However, the isopropyl
alcohol supply tube line 234 may nonetheless extend to the upper
portion of the inner bath 212 without being branched or may branch
into three or more supply tube lines within the purview of the
present invention.
[0057] In addition, FIG. 3 shows the isopropyl alcohol liquid being
supplied into the inner bath from the front and rear directions of
the wafer guide 203. However, this is shown in the drawing for the
sake of simplicity. Actually, the isopropyl alcohol liquid is
preferably introduced into the inner bath 211 from the right and
left sides of the wafer guide 203 as shown in FIG. 4.
[0058] First and second heaters 250a and 250b are disposed around
the outlets of the first and second supply tube lines 234a and
234b, i.e., adjacent locations where the first and second supply
tube lines 234a and 234b enter the outer bath 216. The first and
second heaters 250a and 250b heat the isopropyl alcohol liquid 222
flowing through the first and second supply tube lines 234a and
234b so that heated isopropyl alcohol liquid 222 is supplied to the
inner bath 212.
[0059] The temperature of the first and second heaters 250a and
250b is set to be higher than 50.degree. C. and lower than the
boiling point of isopropyl alcohol (approximately 83.degree. C. at
atmospheric pressure). Preferably, the temperature of the first and
second heaters 250a and 250b is set to be about 60.about.70.degree.
C. Thus, the isopropyl alcohol liquid enters the cleaning section
210 at a temperature of about 40.about.70.degree. C., preferably at
about 50.about.60.degree. C. If the temperature of the first and
second heaters 250a and 250b were lower than 50.degree. C., the
isopropyl alcohol entering the cleaning section 210 would be
unsuitable for forming the isopropyl alcohol ambient. On the other
hand, if the temperature of the first and second heaters 250a and
250b were higher than the boiling point of isopropyl alcohol, the
isopropyl alcohol would enter the cleaning section 210 while
boiling.
[0060] Next, a flow control meter 260 is disposed in-line with the
isopropyl alcohol supply tube line 234 upstream of the first and
second supply tube lines 234a and 234b. The flow control meter 260
allows only a constant amount of isopropyl alcohol liquid 222 to
flow for a prescribed time period through the isopropyl alcohol
supply tube line 234 so that a proper amount of the isopropyl
alcohol liquid 222 is supplied to the inner bath 212.
[0061] For example, in order to satisfactorily dry a silicon wafer
having a diameter of 300 mm, for example, the isopropyl alcohol
liquid layer formed on the cleaning liquid 213 should have a
thickness of approximately 1.about.3 mm, preferably 1.5.about.2.5
mm, for about 4 seconds. If the thickness of the liquid layer of
isopropyl alcohol is less than about 1 mm or larger than 3 mm, too
many particles will remain on the wafer after the drying process.
Therefore, the flow meter 260 regulates the flow of the isopropyl
alcohol to such a rate that the isopropyl alcohol liquid layer
formed on the cleaning liquid 213 acquires a thickness within the
desired range.
[0062] The drying apparatus 200 of the present invention also
includes a nitrogen gas supply unit 240 comprising a nitrogen gas
supply tube line 242 and a nitrogen gas supply source 246. One end
of the nitrogen gas supply tube line 242 is connected to the
nitrogen gas supplying tube line 246, and an outlet thereof is
disposed at the upper portion of the outer bath 216.
[0063] FIG. 4 shows the arrangement of the nitrogen gas supply tube
line 242 and the first and second isopropyl alcohol supply tube
lines 234a and 234b with respect to the cleaning section 210. Note,
though, the outlet of the nitrogen gas supply tube line 242 is
disposed at a level above the outlets of the first and second
isopropyl alcohol supply tube lines 234a and 234b relative to the
outer bath 216. Furthermore, the nitrogen gas supply tube line 242
has a plurality of holes 242a that allow the nitrogen gas to be
supplied uniformly throughout the upper portion of the outer bath
216.
[0064] A heater 244 for heating the nitrogen gas is provided
in-line with the nitrogen gas supply tube line 242 upstream of the
outer bath 216. The temperature of the heater 244 is controlled so
that the nitrogen gas entering the cleaning section 210 produces a
suitable nitrogen gas ambient. To this end, the nitrogen gas should
have a temperature of about 70.about.90.degree. C., and more
preferably, a temperature of about 80.degree. C. To heat the
nitrogen gas to such a temperature, considering the initial
temperature and flux of the nitrogen gas supplied from nitrogen gas
supply source 246, the heater 244 must be regulated to have a
temperature in the range of 100.about.200.degree. C., and
preferably to be about 150.degree. C.
[0065] The method of cleaning the wafer according to the present
invention using the above-described apparatus 200 shown in FIG. 3
will now be described with reference to FIGS. 4, 5A-5D, 6 and
7.
[0066] Referring first to FIGS. 4 and 5A, the wafer guide 203
loaded with the wafers 201 is placed into the inner bath 212 in the
direction of arrows A-A in FIG. 4. Then, the cleaning liquid 213,
such as DIW, is supplied to the inner bath 212 via the cleaning
water supply tube line 214a. During a washing operation, the
cleaning liquid 213 is continuously supplied from the lower portion
of the inner bath 212 so that the cleaning liquid 213 overflows the
inner tub 212. The over-flown cleaning liquid 213a gathers in the
funnel-shaped lower portion of the outer bath 216, and is drained
therefrom via the first cleaning liquid draining tube line 214b and
the common drain tube line 214d. At this time, the first drain
valve 214V1 in the first drain tube line 214b is obviously open. As
shown in FIGS. 7 and 8 (step S10), the wafer is washed using the
DIW for about 300 seconds.
[0067] Once the washing process is completed, the drying process of
the wafer 201 begins. Referring to FIG. 5b, nitrogen gas is
supplied from the nitrogen gas supply source 246 into the outer
bath 216 to create a nitrogen gas ambient within the outer bath
216. Specifically, the nitrogen gas supplied from the nitrogen gas
supply source 246 passes through the nitrogen gas supply tube line
242, whereby it is heated by heater 244 to approximately
70.about.90.degree. C., and preferably to about 80.degree. C. The
heated nitrogen gas enters the upper portion of the outer bath 216
via the holes 242a in the nitrogen gas supply tube line 242 such
that the nitrogen gas ambient is uniform throughout the inside of
outer bath 216. As shown in FIG. 7, the supplied nitrogen gas
maintains a pressure of 2.about.4 kg/cm.sup.2, and preferably 3
kg/cm.sup.2. (step S20) Thereafter, the second drain valve 214V2
disposed in the first inner bath drain tube line 214c1 is opened
for a short time period to drain some of the cleaning liquid 213,
whereby the water level of the cleaning liquid 213 in the inner
bath 212 drops slightly. Alternatively, the drain pump 214P in the
second inner bath drain tube line 214c2 may be operated to drain
the cleaning liquid 213. Then, the second drain valve 214V2 is
closed (or the pump 214P is turned off) before the wafers 201 are
exposed. The difference in water level .DELTA.H of the cleaning
liquid 213 during this process (step S30) is preferably
approximately 5 mm.
[0068] Thereafter, heated isopropyl alcohol liquid 222a is supplied
to the inner bath 212 (step S40). As shown in FIG. 3, nitrogen gas
under pressure is supplied from the nitrogen gas supply source 228
to the isopropyl alcohol tank 224 via the nitrogen gas supply tube
line 226. The nitrogen gas exerts pressure on the surface of the
isopropyl alcohol liquid 222 in the tank 224. Accordingly, the
isopropyl alcohol liquid 222 is forced from the tank and into
isopropyl alcohol supply tube line 234. The isopropyl alcohol
liquid 222 is introduced into the cleaning bath 210 at a constant
rate due to the flow control meter 260. The amount of liquid
isopropyl alcohol 222a allowed to flow into the inner bath 212 is
sufficient to form an isopropyl alcohol liquid layer 215 having a
thickness of 1.about.3 mm, and preferably about 2.1 mm, on the
cleaning liquid 213. Although the total amount of the isopropyl
alcohol 222a admitted into the inner bath 212 depends on the
surface area of the cleaning liquid 213, i.e., the size of the
inner bath 212, the thickness of the isopropyl alcohol liquid layer
215 is independent thereof. In the present invention, only a small
amount of isopropyl alcohol 222a is introduced into the inner bath
212. Accordingly, it takes only a few seconds (four seconds in the
present embodiment) to feed the heated isopropyl alcohol into the
inner bath 212.
[0069] The liquid isopropyl alcohol 222 having passed through the
flow control meter 260 is divided into two parts by the first
supplying tube line 234a and the second supplying tube line 234b.
Then the isopropyl alcohol 222 is heated by the first and second
heaters 250a and 250b just before being introduced into the outer
bath 216.
[0070] The heaters 250a and 250b heat the isopropyl alcohol to a
temperature of 50.degree. C. or so. The heated isopropyl alcohol
liquid 222a spills from the outlets of the first and second
receiving tube lines 234a and 234b onto the upper surface of the
cleaning liquid 213.
[0071] The isopropyl alcohol liquid 222a thus quickly forms an
isopropyl alcohol liquid layer 215 on the upper surface of the
cleaning liquid 213, and is partially evaporated. The isopropyl
alcohol vapor mixes with the nitrogen ambient in the upper portion
of the outer bath 216. Roughly 10 seconds are required for forming
the isopropyl alcohol liquid layer 215 (step S50).
[0072] After the heated liquid isopropyl alcohol 222a is supplied
to the inner bath 212, the heated nitrogen gas is preferably
supplied intermittently to the upper portion of the outer bath 216
via the nitrogen gas supply tube line 242.
[0073] Also, a separate isopropyl alcohol gas supply tube line may
be connected to the upper portion of the outer bath 216. In this
case, isopropyl alcohol vapor is introduced directly into the upper
portion of the outer bath 216 while the nitrogen gas is being
supplied via the nitrogen gas supply tube line 242. Alternatively,
the line may be connected to the nitrogen gas supply tube line 242
so that isopropyl alcohol vapor is mixed with the nitrogen gas so
that the isopropyl alcohol vapor is forced to fill the area between
the outer bath 216 and the inner bath 212.
[0074] The area between the outer bath 216 and the inner bath 212
can alternatively be filled with isopropyl alcohol vapor by
operating the first and second heaters 250a and 250b to heat the
isopropyl alcohol liquid remaining in the first and second
receiving tube lines 234a and 234b. By doing so, the heated liquid
isopropyl alcohol is evaporated to produce isopropyl alcohol vapor
222b. The isopropyl alcohol vapor 222b exits the outlets of the
first and second receiving tube lines 234a and 234b and expands
into the upper portion of outer bath 216.
[0075] Thereafter (refer to FIG. 5C), a process of draining the
cleaning liquid 213 is initiated (step S60). In this process, the
pump 214P is turned on whereupon the cleaning liquid 213 is drawn
through the second inner bath drain tube line 214c2 of the inner
bath drain tube line 214c and into the common drain tube line 214d
at a constant speed. If the cleaning liquid 213 were drained too
fast, the isopropyl alcohol liquid layer 215 would break apart and
the Marangoni effect would be lost. Draining the cleaning liquid
too slowly is disadvantageous in terms of the efficiency of the
cleaning process. Therefore, the pump 214P is operated to drain the
cleaning liquid 213 at such a speed that the water level of the
cleaning liquid 213 in the inner bath 216 drops by about 1.5 to 2.5
mm/sec. During the draining operation, the isopropyl alcohol layer
215 is maintained so that water spots on the wafers 201 are
eliminated by the Marangoni effect.
[0076] FIG. 6 shows how the cleaning liquid 213 and the isopropyl
alcohol layer 215 behave around the wafers 201 during the draining
process illustrated in FIG. 5C. As the cleaning liquid 213 is
gradually drained, portions of the cleaning liquid 213 and the
isopropyl alcohol layer 215 closest to the wafers 201 remain
adhered to the wafers at a level above the other portions of the
cleaning liquid 213 and the isopropyl alcohol layer 21. Because the
isopropyl alcohol has a surface tension that is smaller than that
of the DIW, fluid flows from isopropyl alcohol layer 215 toward the
cleaning liquid 213, thereby preventing the fluid from remaining on
the wafers 201. Consequently, water spots will not form on the
wafers 201 and hence, particles will not be left on the wafers 201
after the cleaning operation. The isopropyl alcohol is preferably
continuously heated while the cleaning liquid 213 is being drained
so that isopropyl alcohol vapor is provided at the upper portion of
the outer bath 216, i.e., over the cleaning liquid 213 and liquid
isopropyl alcohol layer 215. This process stabilizes the isopropyl
alcohol layer 215 on the cleaning liquid 213 as the cleaning liquid
213 is being drained. This, of course, enhances the cleaning
effect.
[0077] Referring now to FIG. 5D, the cleaning liquid 213 is drained
for about 145.about.240 seconds. At some point in time (P1 in FIG.
8) in this process, when the level of the cleaning liquid 213 lies
beneath the wafers 201, the first and second heaters 250a and 250b
are turned off. Thus, the isopropyl alcohol vapor 222b is no longer
supplied into the outer bath 216. Also, the supplying of the heated
nitrogen gas is also stopped. Under this state, the second valve
214V2 disposed in the first inner bath drain tube line 214c1 may be
open. However, the pump 214P in the second inner bath drain tube
line 214c2 remains operating to thereby completely drain the
cleaning liquid 213 from the inner bath 212.
[0078] Finally, the heated nitrogen gas is injected into the outer
bath 216 via the nitrogen gas supply tube line 242 to complete the
drying process. In this way, the lower peripheral portions of the
wafers (that are the last portions to separate from the cleaning
liquid) and any concave portions of the wafers, such as those
defining contact holes, are dried. Once the drying process is
complete, the wafer guide 203 loaded with the wafers 201 is pulled
out of the cleaning bath 210 and is transferred to another
apparatus.
[0079] Embodiment 2
[0080] FIG. 12 shows a second embodiment of a method of and an
apparatus for drying a wafer according to the present
invention.
[0081] This embodiment is similar to that of the first embodiment
described with reference to FIGS. 3, 4 and 5A to 5D, except that in
this embodiment, the wafers are raised to remove them from the
cleaning liquid. Accordingly, parts similar to those of the first
embodiment of the drying apparatus are designated by the same
reference numerals.
[0082] Referring now to FIG. 12, the apparatus 300 for drying a
wafer includes an outer bath draining unit comprising an outer bath
drain tube line 214b connected to the outer bath 216 and a first
drain tube valve 214V1 disposed in the drain tube line 214b. As in
the first embodiment (refer to FIG. 5A), the cleaning liquid 213a
over-flowing the inner bath 212 during the washing process gathers
in the funnel-shaped lower portion of the outer bath 216. The
cleaning liquid 213 is then drained from the outer bath 216 via the
drain tube line 214b.
[0083] The apparatus 300 for drying a wafer also includes an inner
bath draining unit an inner bath drain tube line 214c and a second
drain valve 214V2 disposed in the inner bath drain tube line 214c.
As described with reference to FIG. 5B, after the wafers 203 have
been washed and prior to introducing the heated isopropyl alcohol
222a onto the surface of the cleaning liquid 213, the second drain
valve 214V2 is opened to drain some of the cleaning liquid 213 from
the inner bath. Specifically, the cleaning liquid is drained until
the level thereof in the inner bath 216 drops by approximately 5
mm.
[0084] The drying apparatus 300 further includes a wafer elevating
unit 301 for raising the wafers 201. The wafer elevating unit 301
comprises a support stand 350, a piston support unit 340, a power
transfer mechanism 330, a piston shaft 320, and a coupling unit
310. The support stand 350 is disposed to one side of the lower
portion of the outer bath 216. The piston support unit 340 has
central through-hole and is fixed to the support stand 340. The
piston shaft 320 extends freely through the through-hole of the
piston support unit 340 so as to be capable of reciprocating. The
coupling unit 310 couples the upper end of the piston shaft 320 and
the wafer guide arm 203c. The piston shaft 320 coupled to the wafer
guide arm 203c moves the wafer guide 203 up and down to immerse the
wafers 201 in the cleaning liquid 213 and to raise them out of the
cleaning liquid 213.
[0085] The piston shaft 320 is connected to a power generating unit
332, such as a motor, via the power transfer mechanism 330. The
power transfer mechanism 330 may be a gear box, or a belt and
pulley system configured to transmit the rotary output of the power
transferring unit 332 to the piston shaft 320 as rectilinear
motion. The power transfer mechanism 330 thus can move the piston
shaft 320 vertically in either direction.
[0086] The method of drying the wafers will now be described with
reference to FIG. 13.
[0087] The wafer guide 203 loaded with the wafers 201 is placed
within the inner bath 212. Then, the cleaning liquid 213, such as
DIW, is fed into the inner bath 212 via the cleaning liquid supply
tube line 214a to start the washing process (step S110). The
washing process is identical to that described with respect to step
S10 of FIGS. 7 and 8.
[0088] Once the process of washing the wafers 201 is complete, the
process of drying the wafers 201 begins. First, nitrogen gas is
supplied into the outer bath 216 for producing a drying ambient
(step S120 in FIG. 13). This process is identical to that described
with respect to step S20 of FIGS. 7 and 8.
[0089] Thereafter, the second drain valve 214V2 is opened for a
short period of time to drain the cleaning liquid 213 from the
inner bath 212 and thereby reduce the level of the cleaning liquid
213 in the inner bath 212 slightly (step S130). This process is
identical to that described with respect to step S30 of FIGS. 7 and
8. Next, heated liquid isopropyl alcohol 222a is supplied from the
isopropyl alcohol supplying unit 220 into the inner bath 212 via
the isopropyl supply tube line 234 (step S140). This process is
identical to that described with respect to step S40 of FIGS. 7 and
8.
[0090] The heated isopropyl alcohol liquid 222a quickly forms a
liquid isopropyl alcohol layer 215 on the surface of the cleaning
liquid 213, and at the same time partially evaporates to form an
isopropyl alcohol ambient in the upper portion of outer bath 216
(step S150). This process is identical to that described with
respect to step S50 of FIGS. 7 and 8.
[0091] Thereafter, nitrogen gas is supplied to the upper portion of
the outer bath 216 via the nitrogen gas supply tube line 242. At
the same time, the power generating unit 332 is supplied with
electric power. The output of the power generating unit 332 is
transmitted to the piston shaft 320 as rectilinear motion via the
power transfer mechanism 330, to thereby raise the piston shaft
320. The wafer guide 203 loaded with the wafers 201 is thus raised
until the wafers 201 are removed from the cleaning liquid 213 (step
S160). At this time, the wafer elevating unit is preferably
operated to raise the wafers at a speed of 1.5.about.2.5 mm/sec as
in step S60.
[0092] As denoted by the dashed lines of FIG. 12, the wafers 201
are completely removed from the cleaning liquid 213. Then, the
first and second heaters 250a and 250b are turned off and the
supplying of the isopropyl alcohol vapor 222b ceases. Also, the
supplying of the heated nitrogen ceases. Under this state, the
second drain valve 214V2 is opened to thoroughly drain the cleaning
liquid 213 from the inner bath 212. Thereafter, the heated nitrogen
gas is introduced into the outer bath 216 via the nitrogen gas
supply tube line 242 to complete the drying process (step S170).
After the drying process is completed, the wafer guide 203 loaded
with the wafers 201 is transferred from the outer bath 216 to
another processing apparatus.
[0093] Experiment 1
[0094] Ten lots of the wafers (one lot consists of 24 wafers) were
dried according to the method shown in FIG. 8 using the first
embodiment of the wafer drying apparatus shown in FIG. 3.
[0095] Referring again to FIG. 8, the wafers 201 were washed for
300 seconds using DIW as the cleaning liquid. The heater 244 was
then operated at about 150.degree. C. to heat the nitrogen gas to
about 70.about.90.degree. C. The heated nitrogen gas was supplied
to the upper portion of the outer bath 216 via the nitrogen gas
supply tube line 242 for 60 seconds and at a pressure of 3
kg/cm.sup.2. Thereafter, the second drain valve 214V2 in the first
inner bath drain tube line 214c1 was opened to lower the level of
the cleaning liquid 213 in the inner bath 212 by as much as 5 mm.
Then, the second drain valve 214V2 was closed, and the first and
second heaters 250a and 250b were set at 70.degree. C.
Subsequently, about 30 ml of the heated isopropyl alcohol liquid
222a was supplied via the first and second isopropyl alcohol supply
tube lines 234a and 234b. This process took 4 seconds to complete.
The isopropyl alcohol liquid layer 215 formed on the surface of the
cleaning liquid 213 as a result had a thickness of about 2.1
mm.
[0096] Then, the heated nitrogen gas was supplied into the upper
portion of the outer bath 216 while the first and second heaters
250a and 250b were operated to produce the isopropyl alcohol vapor
222b. After roughly 10 seconds, the cleaning liquid 213 was drained
from the inner bath 212 at a rate of 2 mm/sec using the pump 214P
disposed in the second inner bath drain tube line 214c2. This
draining process was carried out for about 145 seconds or 240
seconds until the level of the cleaning liquid 213 in the inner
bath 212 sank below the wafers 201. At this point in time, the
first and second heaters 250a and 250b were turned off. Also, the
supplying of the heated nitrogen gas was ceased, and the second
drain valve 214V2 disposed in the second inner bath drain tube line
214c2 was opened to thoroughly drain the remaining cleaning liquid
213 and the isopropyl alcohol liquid layer 215 from the inner bath
212. Finally, the heated nitrogen gas was supplied into the outer
bath for 300 seconds to dry the wafers 201 completely. The number
of particles having a diameter of at least 0.12 .mu.m on the wafer
prior to the washing operation was compared to that after the
washing and drying operation. Three wafers were sampled from each
lot of 24 wafers, one each from the front, center and rear of the
lot. Ten lots of the wafers were tested in total.
[0097] FIG. 9 shows the change in the number of particles before
and after the cleaning operation for the sampled wafers of the ten
lots. In FIG. 9, a negative (-) number denotes a decreased number
of particles and a positive (+) number denotes an increased number
of particles. Also, in FIG. 9, the plot using the diamond-shaped
symbol is a result from the wafers sampled at the rear of the wafer
guide, the plot using a square-shaped symbol is a result from the
wafers sampled at the center portion of the wafer guide, and the
plot using triangle-shaped symbol is a result from the wafers
sampled at the front of the wafer guide. The graph plotted using
the character X is the average result from the sampled wafers. The
change in the number of particles before and after the cleaning of
the wafers at the rear of the wafer guide was -4.4 on average, that
at the center of the wafer guide was 2.4 on average, and that at
the rear of the wafer guide was 3.2 on average. The average change
in the number of particles of all of the sampled wafers was
0.4.
[0098] These results show that the drying process according the
present invention hardly leaves any particles that are the result
of water spots. Furthermore, there is little variation in the
change in the number of particles over the different positions that
the wafers occupy in the bath. Accordingly, the drying process
according to the present invention is remarkably uniform and
effective.
Experiments 2, 3 and 4
[0099] In Experiment 2, the isopropyl alcohol liquid was employed
at room temperature instead of being heated. In Experiment 3, the
nitrogen gas was introduced into the outer bath at room temperature
instead of being heated. In Experiment 4, the isopropyl alcohol
liquid was employed at room temperature instead of being heated,
and was introduced into the outer bath at room temperature instead
of being heated. Otherwise, the cleaning and drying operations were
conducted in the same manner as with Experiment 1. And, as with
Experiment 1, the wafers were examined for changes in the number of
particles having a diameter of at least 0.12 .mu.m before and after
the washing and drying operations.
[0100] The results of these first to fourth experiments are
qualified below in Table 1.
1 TABLE 1 Nitrogen Variation of number IPA liquid gas of particles
Evaluation Experiment 1 Heated Heated Varied very little at
Excellent all Experiment 2 Unheated Heated Varied severely Poor
Experiment 3 Heated Unheated Varied slightly Good Experiment 4
Unheated Unheated Varied highly Poor
[0101] As shown in Table 1, when the isopropyl alcohol was heated
to form the isopropyl alcohol liquid layer on the cleaning liquid,
as in Experiments 1 and 3, the drying effect was favorable.
However, when the isopropyl alcohol liquid was injected at room
temperature, as in Experiments 2 and 4, too many particles were
left after the drying operation.
[0102] Moreover, when the nitrogen gas was injected without being
heated, as in Experiment 3, not enough isopropyl alcohol vapor was
produced in the ambient above the cleaning liquid. This resulted in
a much greater variation in the number of particles as compared
with Experiment 1 in which the heated nitrogen gas was used.
[0103] Although the isopropyl alcohol liquid was introduced at room
temperature without being heated in both Experiments 2 and 4,
Experiment 2, in which the heated nitrogen gas was used, showed a
smaller variation in the number of particles than Experiment 4 in
which the unheated nitrogen gas was used. And, although the heated
isopropyl alcohol liquid was used in both Experiments 1 and 3,
Experiment 3, in which the unheated nitrogen gas was used, showed a
greater variation in the number of particles than Experiment 1 in
which the heated nitrogen gas was used. From these observations, it
is clear that the drying effect is improved by using the heated
nitrogen gas. Apparently, the isopropyl alcohol is evaporated
somewhat by the heated nitrogen gas to ensure that the isopropyl
alcohol vapor is present throughout the nitrogen gas ambient formed
over the upper portion of the cleaning liquid.
[0104] Experiment 5
[0105] Experiment 5 was conducted to provide a comparison of the
conventional wafer drying method and that according to the present
invention in terms of the variation in the number of particles on
the wafers.
[0106] More specifically, the conventional cleaning and drying
apparatus shown in FIG. 1 was used to perform a cleaning operation,
an apparatus of the present invention was used to perform the
cleaning operation shown in FIG. 8, and the number of particles
having a diameter of at least 0.16 .mu.m on the wafers was checked
before and after the cleaning operations. FIG. 10 is a graph of the
results. In FIG. 10, the plot using the square-shaped symbol is the
average result of examining lots of the wafers cleaned according to
the present invention, and the plot using the diamond-shaped symbol
is the average result of examining wafers cleaned using the
conventional apparatus of FIG. 1.
[0107] As can be seen from FIG. 10, when the conventional wafer
drying process is executed, the variation in the number of
particles before and after the process was 33 on average. This
means that, on average, the process performed by the conventional
apparatus left 33 particles having a diameter of at least of 0.16
.mu.m on a wafer. On the other hand, the cleaning process performed
according to the present invention left only a negligible number of
particles having a diameter of at least 0.12 .mu.m.
[0108] Experiment 6
[0109] Experiment 6 was conducted to determine how the quantity of
the isopropyl alcohol liquid affects the number of particles left
on the wafers.
[0110] In this experiment, tests were performed in which 20 ml, 30
ml, 50 ml and 70 ml each of the isopropyl alcohol liquid was
supplied to the inner bath. Otherwise, the processes were conducted
according to that shown in FIG. 8. The inner bath was 40 cm long by
35 cm wide. Thus, the isopropyl alcohol liquid layers formed on the
cleaning liquid were approximately 5 mm, 3.6 mm, 2.1 mm and 1.4 mm
thick, respectively. The variation in the number of particles
having a diameter of at least 0.12 .mu.m before and after the
processes was measured. The results of these measurements are shown
in FIG. 11.
[0111] The number of particles increased by 140, 88.9 and 30.2 on
average, when the amounts of isopropyl alcohol liquid supplied were
70 ml, 50 ml and 20 ml, respectively. However, the number of
particles increased by only 0.4, on average, when 30 ml of the
isopropyl alcohol was supplied to the inner bath. This experiment
showed that the amount of isopropyl alcohol liquid supplied needs
to be carefully regulated.
[0112] In fact, these tests revealed that the isopropyl alcohol
liquid layer must have a thickness of about 1 mm to 3 mm if a
satisfactory limit on the particles left by water spots after the
cleaning process is to be met. The preferred thickness of the
isopropyl alcohol liquid layer is 1.5.about.2.5 mm, and more
preferably at least 2 mm and no more than 2.5 mm.
[0113] According to the present invention, in the process of drying
a wafer using the Marangoni effect, heated liquid isopropyl alcohol
is supplied onto the surface of the cleaning liquid. The isopropyl
alcohol diffuses quickly to form an isopropyl liquid layer.
Therefore, as compared with the conventional method in which the
isopropyl alcohol liquid layer is formed by an isopropyl alcohol
mist carried in nitrogen gas, the present invention is more
efficient.
[0114] Furthermore, in the conventional process, it is difficult to
regulate the amount of isopropyl alcohol being supplied because the
isopropyl alcohol is supplied as a mist through the use of the
carrier nitrogen gas. Thus, it is difficult to optimize the process
while it is being carried out. However, in the present invention,
the isopropyl alcohol is supplied in a liquid state. Therefore, the
exactly quantity of isopropyl alcohol required to prevent the
forming of water spots that leave too many particles on the waters
can be supplied.
[0115] Although the present invention has been shown and described
with reference to particular embodiments thereof, various changes
in form and details may be effected therein without departing from
the true spirit and scope of the invention as defined by the
appended claims.
* * * * *