Method of and apparatus for cleaning workpiece

A method of and an apparatus for cleaning a thin disk-shaped workpiece that is required to have a high degree of cleanliness, e.g., a semiconductor wafer, a glass substrate, a liquid crystal display, or the like. The method of cleaning the workpiece includes a plurality of cleaning steps, and comprises the steps of holding a workpiece, and performing a liquid jet cleaning of a surface of the workpiece in a first portion of plural cleaning steps. The method further comprises the step of performing a liquid jet cleaning of the surface of the workpiece in a latter portion of the plural cleaning steps.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of and an apparatus for cleaning 
a workpiece, and more particularly to a method of and an apparatus for 
cleaning a workpiece that is required to have a high degree of 
cleanliness, e.g., a semiconductor wafer, a glass substrate, a liquid 
crystal display, or the like. 
2. Description of the Prior Art 
As semiconductor devices have become more highly integrated in recent 
years, circuit interconnections on semiconductor substrates have become 
finer and the distance between each circuit interconnections have become 
smaller. When semiconductor wafers are processed, small particles such as 
particles of semiconductor material, dust particles, crystalline 
protrusive particles, or the like often tend to be attached to the 
semiconductor wafers being processed. If a particle greater than the 
distance between interconnections exists on a semiconductor substrate, 
then the particle will short-circuit interconnections on the semiconductor 
substrate. Therefore, any undesirable particles on a semiconductor 
substrate have to be sufficiently smaller than the distance between 
interconnections on the semiconductor substrate. Such a problem and a 
requirement hold true for the processing of other workpieces including a 
glass substrate to be used as a mask, a liquid crystal display, and so on. 
To meet the above requirement, there have been practiced various cleaning 
procedures for removing fine particles or submicron particles from 
semiconductor wafers. 
For example, heretofore one practice has been to use a brush of nylon, 
mohair or the like, or a sponge of polyvinyl alcohol (PVA) to scrub a 
surface of a semiconductor wafer. This process is called a scrubbing 
cleaning process. Further, there have been other practices, one of which 
is an ultrasonic cleaning process in which water having ultrasonic 
vibrational energy applied thereto is supplied to a surface of a 
semiconductor wafer, and another of which is a cavitation jet cleaning 
process in which high pressure water containing cavitation (i.e., 
cavitation bubbles) therein is supplied to a surface of a semiconductor 
wafer. Also, a cleaning process which combines two or three of the above 
processes is known in the art. A scrubbing cleaning process using a brush 
of nylon, mohair, or the like is effective to remove particles having 
diameters of 1 .mu.m or larger from semiconductor wafers. However, such a 
scrubbing process fails to produce an appreciable cleaning effect on 
submicron particles smaller than such particle sizes, and adversely 
produces scratches over a surface of the semiconductor wafer. Further, 
when a number of semiconductor wafers are cleaned by the brush, particles 
are attached to the brush which in turn contaminates subsequent 
semiconductor wafers. 
Particles having diameters of submicron size can be removed from 
semiconductor wafers by another scrubbing process sing a sponge of 
polyvinyl alcohol (PVA). The sponge of PVA has a tendency to entrap 
particles therein. This scrubbing process is, however, ineffective where a 
certain amount of particles are entrapped by the sponge. That is, the 
sponge of PVA does not have a long service life without suitable 
regeneration. 
As described above, in case of a scrubbing cleaning process using a brush 
or a sponge, the brush or the sponge is contaminated during the cleaning 
process. Therefore, in the case where a semiconductor wafer to which a 
great number of particles are attached, as in the case of a semiconductor 
wafer which has been polished, is cleaned using the brush or the sponge, a 
brush or the sponge must be cleaned or replaced, resulting in imposing the 
serious burden on the operator. 
Further, in case of cleaning a semiconductor wafer having a circuit pattern 
thereon, as particles are smaller and gaps into which particles are 
entered are smaller, the semiconductor wafer cannot be cleaned only by the 
scrubbing cleaning process. 
On the other hand, an ultrasonic cleaning process and a cavitation jet 
cleaning process do not require cleaning by a brush or a sponge, and can 
remove some particles which cannot be removed by the scrubbing cleaning 
process. 
Further, an ultrasonic cleaning process and a cavitation jet cleaning 
process are effective up to a degree of several ten or several hundred 
particles on a surface of a semiconductor wafer having a diameter of eight 
inches (200 mm). 
However, in case of a semiconductor wafer which has been subjected to CMP 
(chemical mechanical polishing) process, the semiconductor wafer carries 
hundreds of thousands of particles. Thus, the ultrasonic cleaning process 
and the cavitation jet cleaning process are ineffective, because they can 
reduce the number of particles only to a degree of several thousands or 
several tens of thousands of particles. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a method of 
and an apparatus for cleaning a workpiece 10 which has a cleaning member 
having a long service life, and can prolong a maintenance period and 
remove effectively fine particles which cannot be removed only by a 
scrubbing cleaning process. 
According to the present invention, there is provided a method of cleaning 
a thin disk-shaped workpiece including a plurality of cleaning steps, 
comprising the steps of: holding a workpiece; and performing a liquid jet 
cleaning of a surface of the workpiece in the first portion of the plural 
cleaning steps. 
According to the present invention, there is also provided such method of 
cleaning the workpiece, further comprising the step of performing a liquid 
jet cleaning of the surface of the workpiece in a latter portion of the 
plural cleaning steps. 
According to one aspect of the present invention, the liquid jet cleaning 
comprises at least one of a water jet cleaning in which ultrapure water at 
a high pressure is ejected from a nozzle, a cavitation jet cleaning in 
which ultrapure water containing cavitation therein is ejected from a 
nuzzle, and an ultrasonic jet cleaning in which ultrapure water with 
ultrasonic vibrational energy is ejected from a nozzle. 
According to the present invention, there is also provided an apparatus for 
cleaning a thin disk-shaped workpiece by performing a plurality of 
cleaning steps, such apparatus including a chuck for holding and rotating 
a workpiece; and at least one liquid jet nozzle for ejecting a liquid jet 
onto a surface of the workpiece; wherein the liquid jet ejected from the 
liquid jet nozzle performs a liquid jet cleaning of the surface of the 
workpiece in the first portion of plural cleaning steps. 
According to the present invention, liquid jet cleaning in which a liquid 
jet ejected from a nozzle impinges on a surface of the workpiece is 
carried out at a primary stage of the cleaning processes. Even in a 
greatly contaminated semiconductor wafer, having a diameter of eight 
inches (200 mm), which carries on its surface hundreds of thousands of 
particles after a CMP process, since the liquid jet ejected from the 
nozzle is supplied to the semiconductor wafer at a first portion of the 
plural cleaning processes, the semiconductor wafer is not subjected to 
contamination during a cleaning process, unlike a scrubbing cleaning 
process which uses a brush or a sponge of PVA, and the number of particles 
on the semiconductor wafer can be reduced to a desired degree. Further, 
even after a scrubbing cleaning process, since a liquid jet cleaning 
process is carried out, submicron particles can be removed from the 
surface of the semiconductor wafer which has been subjected to at least a 
liquid jet cleaning and has a relatively clean surface. 
The above and other objects, features, and advantages of the present 
invention will become apparent from the following description when taken 
in conjunction with the accompanying drawings which illustrate preferred 
embodiments of the present invention by way of example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A preferred embodiment of the present invention will be described below 
with reference to FIGS. 1 through 6. 
FIG. 1 shows cleaning processes or steps for carrying out a cleaning method 
of the present invention. As shown in FIG. 1, a semiconductor wafer W 
which has been polished is subjected to a plurality of cleaning processes 
(steps) No. 1, No. 2, . . . No. n and cleaned to a high degree of 
cleanliness. 
In a polishing process, a semiconductor wafer W is polished to a flat 
mirror finish while an abrasive liquid containing abrasive grains is 
supplied onto the semiconductor wafer W. Therefore, the semiconductor 
wafer W which has been polished carries on its surface abrasive grains 
contained in the abrasive liquid and ground-off particles of the 
semiconductor wafer, and is contaminated greatly. For example, in the case 
of a semiconductor wafer having a diameter of eight inches (200 mm), it 
carries hundreds of thousands of particles having diameters of 1 .mu.m or 
greater after polishing. 
In the case where a plurality of cleaning processes No. 1, No. 2, . . . No. 
n do not include a scrubbing cleaning process, a plurality of liquid jet 
cleaning processes which use liquid jets such as water jets are provided. 
A great number of particles on the semiconductor wafer is reduced to a 
certain degree by the liquid jet process in the first half of the cleaning 
processes, and the remaining particles are removed by the liquid jet 
process in the latter half of cleaning processes, thus obtaining a high 
degree of cleanliness of the semiconductor wafer. 
To be more specific, for example, at least in the first cleaning process 
No. 1 and the final cleaning process No. n, liquid jet cleaning processes 
in which liquid such as water is ejected from a nozzle and impinges on a 
surface of a semiconductor wafer are carried out. The liquid jet cleaning 
process comprises one of a water jet cleaning process in which ultrapure 
water at a high pressure is ejected from a nozzle, a cavitation jet 
cleaning process in which ultrapure water containing cavitation therein is 
ejected from a nozzle, and an ultrasonic cleaning process in which 
ultrapure water having ultrasonic vibrational energy applied thereto is 
ejected from a nozzle. The above two or three cleaning processes may be 
combined. 
Further, in a plurality of cleaning processes No. 1, No. 2, . . . No. n, in 
the case of carrying out a scrubbing cleaning process, a liquid jet 
cleaning process is provided before the scrubbing cleaning process. By 
such liquid jet cleaning process, a great number of particles on the 
semiconductor wafer is reduced to a certain degree, and thus contamination 
of a brush or a sponge used in the subsequent scrubbing cleaning process 
is reduced. In order to remove particles which have not been removed by 
the scrubbing cleaning process, a liquid jet cleaning process may be 
provided after the scrubbing cleaning process. In this case, although 
liquid jet cleaning processes may be provided before and after the 
scrubbing cleaning process, the latter liquid jet cleaning may use a 
liquid jet which imparts to the semiconductor wafer an impact force 
greater than that of the former liquid jet cleaning because the object of 
the latter liquid jet cleaning process is to remove particles which have 
not been removed by the former liquid jet cleaning process and the 
scrubbing cleaning process. 
FIG. 2 shows a cleaning apparatus for carrying out a cleaning method 
according to the present invention. The cleaning apparatus comprises a 
spinning chuck 10 for holding a semiconductor wafer W and rotating the 
semiconductor wafer W in a horizontal plane at a predetermined speed, a 
liquid jet nozzle 20 for ejecting liquid 21 onto a surface of the 
semiconductor wafer W, and a scrubbing cleaning device 30 for scrubbing 
the surface of the semiconductor wafer W. The spinning chuck 10 has 
clamping members 11 for clamping the outer circumference of the 
semiconductor wafer W, and is rotatable about a shaft 12 in a direction of 
an arrow A. 
The scrubbing cleaning device 30 comprises a shaft 31, an arm 32 supported 
by the shaft 31, and a sponge holder 33 provided at the forward end of the 
arm 32. The shaft 31 is vertically movable as shown by an arrow C, and the 
arm 32 is vertically movable with the shaft 31 and is swingable in a 
horizontal plane by rotation of the shaft 31 as shown by an arrow B. 
A sponge 34 is mounted on the sponge holder 33. A cleaning cup 35 is 
provided adjacent to the spinning chuck 10 and contains pure water 
therein. When the scrubbing cleaning device 30 is not in operation, the 
sponge holder 33 holding the sponge 34 is placed in the cleaning cup 35. A 
nozzle 40 for rinsing the semiconductor wafer is provided adjacent to the 
spinning chuck 10. 
With the above structure, the semiconductor wafer W is clamped by the 
clamping members 11 of the spinning chuck 10 with the surface to be 
cleaned of the semiconductor wafer W facing upwardly. In the first 
cleaning process, while the semiconductor wafer W is rotated in the 
direction of the arrow A by the spinning chuck 10, liquid 21 is ejected 
from the liquid jet nozzle 20 toward the upper surface of the 
semiconductor wafer W, thereby washing away particles such as abrasive 
grains and ground-off particles of the semiconductor wafer. In the second 
cleaning process, the arm 32 is lifted to take the sponge holder 33 
holding the sponge 34 out of the cleaning cup 35 by lifting the shaft 31. 
The arm 32 is turned to move the sponge 34 to a position above the 
semiconductor wafer W. Thereafter, the arm 32 is lowered to press the 
sponge 34 against the upper surface of the semiconductor wafer W. The 
sponge holder 33 starts to rotate in the direction of an arrow D at a 
predetermined speed immediately before the sponge 34 contacts the 
semiconductor wafer W. At this time, the arm 32 is swung about the shaft 
31, the spinning chuck 10 is rotated about the shaft 12, and the sponge 34 
serves to scrub the upper surface of the semiconductor wafer W. Further, 
rinsing liquid such as ultrapure water is supplied from the nozzle 40 onto 
the upper surface of the semiconductor wafer W to wash away abrasive 
grains and ground-off particles of the semiconductor wafer. 
In the second cleaning process of the scrubbing cleaning process, when the 
semiconductor wafer W and the sponge 34 are pressed against each other and 
moved relatively to each other, particles on the semiconductor wafer W are 
scraped off by edges of recesses or micropores of the sponge 34, are 
trapped in the recesses or the micropores, and thus are removed from the 
semiconductor wafer W. In case of scrubbing the polished semiconductor 
wafer W with a high degree of contamination by the sponge 34, since the 
sponge 34 cannot be sufficiently purified in a short time by self-cleaning 
achieved by soaking the sponge 34 in ultrapure water in the cleaning cup 
35, there is a high possibility that the sponge 34 contaminates adversely 
the semiconductor wafer W. 
However, contamination of the sponge 34 can be suppressed to a minimum 
degree by reducing the number of particles in the first cleaning process 
comprising the liquid jet cleaning process and by the self-cleaning which 
is carried out by soaking the sponge 34 in ultrapure water in the cleaning 
cup 35. 
The number of particles on the semiconductor wafer W is reduced to the 
range of tens of thousands to thousands in the first cleaning process, and 
the number of particles having diameters of 1 .mu.m or greater is reduced 
to several tens in the scrubbing cleaning process using the sponge 34. 
In the third cleaning process, the scrubbing cleaning device 30 is stopped, 
the sponge holder 33 holding the sponge 34 is placed in the cleaning cup 
35, and liquid 21 is ejected from the liquid jet nozzle 20 toward the 
upper surface of the semiconductor wafer W, thereby washing away 
particles, including abrasive grains and ground-off particles of the 
semiconductor wafer, which have not been removed by the scrubbing cleaning 
process. 
Recent smaller wiring patterns or interconnections demand a semiconductor 
wafer having a high degree of cleanliness, and hence the number of 
particles having diameters of 0.2 .mu.m or greater is required to be not 
more than ten in a semiconductor wafer having a diameter of eight inches 
(200 mm). However, after the second cleaning process, the semiconductor 
wafer carries on its surface particles, having diameters of 0.2 .mu.m or 
greater, whose number is in the range of several thousands to several 
hundreds. Therefore the cleaning processes up to the second cleaning 
process are not sufficient, and do not satisfy the demand of recent 
smaller wiring patterns or interconnections. Thus, the third cleaning 
process is required to satisfy the above demand. 
After a series of the above cleaning processes, the spinning chuck 12 (see 
FIG. 2) is rotated at a high speed for thereby spin-drying the 
semiconductor wafer W, or N.sub.2 (nitrogen) gas is blown over the 
semiconductor wafer W, and then the dried semiconductor wafer is 
discharged to the outside of the cleaning apparatus. 
Liquid jet processes that may be employed for to the first and third 
cleaning processes are a water jet cleaning process in which ultrapure 
water at a high pressure is ejected from a nozzle, a cavitation jet 
cleaning process in which ultrapure water having cavitation therein is 
ejected from a nozzle, and an ultrasonic cleaning process in which 
ultrapure water having ultrasonic vibrational energy is ejected from a 
nozzle. 
In the embodiment of FIG. 2, the first and third cleaning processes maybe 
carried out by the same liquid jet nozzle 20, however they may be carried 
out by different liquid jet nozzles, for example nozzles 20, 20a, and at 
different locations. In this case, the first and third cleaning processes 
may be carried out by different types of liquid jet processes. Further, 
even in the embodiment of FIG. 2, the state of liquid jets may be changed 
in the first and third cleaning processes by varying the pressure of 
liquid supplied to the nozzle. 
Next, the structures of nozzles which supply various liquid jets will be 
described below. 
FIG. 3 shows a nozzle 25 for ultrasonic cleaning. Nozzle 25 comprises a 
nozzle body 26 and an ultrasonic generator 27 provided in the nozzle body 
26. When the ultrasonic generator 27 is energized and ultrapure water 
having a high pressure is supplied from an inlet 26a into the nozzle body 
26, ultrasonic vibrational energy is imparted to the ultrapure water which 
is in turn ejected from a nozzle port 26b toward the upper surface of the 
semiconductor wafer W. The ultrasonic vibrational energy is imparted to 
the particles on the upper surface of the semiconductor wafer W from the 
ejected ultrapure water. As a result, the particles are vibrated, 
separated from the upper surface of the semiconductor wafer W, and washed 
away by the ultrapure water. 
In the case where the ultrasonic cleaning process is employed in the first 
cleaning process of the embodiment of FIG. 2, although the semiconductor 
wafer has carried hundreds of thousands of particles after polishing, the 
number of particles having diameters of 1 .mu.m or greater is reduced to 
the range of several tens of thousands to several thousands in the 
semiconductor wafer W having a diameter of eight inches (200 mm). The 
frequency of ultrasonic waves used in the ultrasonic cleaning process is 
in the range of 1.0 MHz to 1.5 MHz. 
FIG. 4 shows a nozzle for cavitation jet cleaning. Nozzle 50 comprises a 
low pressure nozzle 51 and a high pressure nozzle 52 whose forward end is 
inserted in the low pressure nozzle 51. The low pressure nozzle 51 has an 
inlet 51a for introducing a low pressure cleaning solution therethrough 
and a nozzle port 51b for ejecting a cleaning solution therefrom. The high 
pressure nozzle 52 has an inlet 52a for introducing a high pressure 
cleaning solution therethrough and a plate 53 provided at an outlet or 
forward end of the nozzle 52. A nozzle port 53a is formed at the central 
part of the plate 53. 
In the nozzle 50 for cavitation jet cleaning thus constructed, a low 
pressure cleaning solution such as ultrapure water and a high pressure 
cleaning solution such as ultrapure water are simultaneously supplied from 
the respective inlets 51a and 52a, and a low speed jet is ejected from the 
nozzle port 51b and a high speed jet is ejected from the nozzle port 53a. 
The high speed jet ejected from the nozzle port 53a passes through the low 
speed jet ejected from the nozzle port 51b, and cavitation (i.e., 
cavitation bubbles) is generated at the boundary of two jets due to the 
speed difference between the high speed jet and the low speed jet. 
By positioning the upper surface of the semiconductor wafer W at the 
location where the cavitation (i.e., cavitation bubbles) is broken, energy 
resulting from such breaking is imparted to the particles which are in 
turn separated from the upper surface of the semiconductor wafer W. For 
example, in FIG. 2, the nozzle 50 for cavitation jet cleaning is 
positioned at the location of the liquid jet nozzle 20, and the position 
of the nozzle 50 is adjusted such that the cavitation is broken at the 
upper surface of the semiconductor wafer W. Such cavitation jet cleaning 
produces an appreciable cleaning effect on submicron particles. In the 
case where this cavitation jet cleaning process is employed for the third 
cleaning process in the embodiment of FIG. 2, the number of particles 
having diameters of 0.2 .mu.m or greater is reduced to several tens or 
less in the semiconductor wafer W having a diameter of eight inches (200 
mm). 
FIG. 5 shows a nozzle 60 for a high pressure jet cleaning. Nozzle 60 
comprises a nozzle body 61 having an inlet 61a and a plate 62 provided at 
the forward or outlet end of the nozzle body. A nozzle port 62a having a 
small inner diameter is formed at the central part of the plate 62. 
In FIG. 2, the nozzle 60 for high pressure jet cleaning is positioned at 
the location of the liquid jet nozzle 20. High pressure cleaning solution 
such as ultrapure water is supplied from the inlet 61a, and the cleaning 
solution is throttled at the nozzle port 62a to become a high speed jet 
having a velocity of several tens of meters per second (m/s). This high 
speed cleaning solution impinges on the upper surface of the semiconductor 
wafer W, particles are separated from the upper surface of the 
semiconductor wafer W by the impact force, and the separated particles are 
washed away with the cleaning solution from the semiconductor wafer W. 
FIG. 6 shows another scrubbing cleaning device comprising a brush cleaning 
device which is used for a cleaning process of the present invention. The 
brush cleaning device comprises a plurality of spindles 71 (six in the 
illustrated embodiment) for supporting the outer circumference of the 
semiconductor wafer W, a cylindrical brush 72 with bristles, a brush 
driving mechanism 73 for moving the brush 72 vertically as shown by an 
arrow G and rotating the brush 72 about its longitudinal axis as shown by 
an arrow F, and a rinsing nozzle 74 for supplying a rinsing solution such 
as ultrapure water onto the upper surface of the semiconductor wafer W. 
In the brush cleaning device thus constructed, rollers 71a provided at the 
respective upper ends of the spindles 71 are pressed against the outer 
circumference of the semiconductor wafer W and rotated, thereby causing 
the semiconductor wafer W to rotate as shown by an arrow E. In this 
embodiment, two of the rollers 71a impart a rotating force to the 
semiconductor wafer W, and the remaining four rollers 71a serve as 
bearings to support the rotating semiconductor wafer W. The brush 72 is 
lowered to contact the upper surface of the semiconductor wafer W, a 
rinsing solution such as ultrapure water is supplied from a rinsing nozzle 
74 toward the upper surface of the semiconductor wafer W, and the 
semiconductor wafer W and the brush 72 are rotated to effect the scrubbing 
cleaning of the semiconductor wafer W. 
Before and after the scrubbing cleaning by the brush cleaning device in 
FIG. 6, one or more of the cleaning processes using the ultrasonic 
cleaning nozzle 25 in FIG. 3, the cavitation jet nozzle 50 in FIG. 4, and 
the high pressure jet nozzle 60 in FIG. 5 is carried out. 
As described above, by providing a liquid jet cleaning process in a first 
portion of plural cleaning processes using the liquid jet nozzle 20 (see 
FIG. 2), the number of particles is reduced to a range of several tens of 
thousands to several thousands by the first liquid jet cleaning process in 
the semiconductor wafer W having a diameter of eight inches (200 mm). 
Thus, the contamination of the sponge or the brush used in the subsequent 
cleaning process is suppressed greatly, and frequency of necessary 
replacement of the sponge or the brush is greatly reduced to the order of 
approximately one-tenth of the conventional method. Further, by providing 
a liquid jet cleaning process in a latter portion of plural cleaning 
processes using the liquid jet nozzle 20, the number of particles having 
diameters of 0.2 .mu.m or greater is reduced to ten or less in the 
semiconductor wafer having a diameter of eight inches (200 mm). 
The present invention has been shown and described as being embodied for 
cleaning a semiconductor wafer. However, the principles of the present 
invention are also applicable to the cleaning of any of other workpieces 
that need to have a high degree of cleanliness, e.g., a semiconductor 
wafer, a glass substrate, a liquid crystal display, or the like. Further, 
the liquid jet nozzle 20 may eject pellets made of ice or dry ice for 
thereby performing a jet cleaning operation in which the pellets impinge 
on the workpiece. 
The present invention offers the following advantages: 
(1) It is possible to remove effectively minute particles or submicron 
particles from a workpiece such as a semiconductor wafer which has been 
contaminated greatly. 
(2) It is possible to suppress contamination of a cleaning member such as a 
sponge or a brush used in an intermediate cleaning process to a minimum 
degree. 
(3) It is possible to increase the service life of a cleaning member such 
as a sponge or a brush used in an intermediate cleaning process. That is, 
the number of times that the cleaning member must be replaced is reduced 
greatly. 
Although certain preferred embodiments of the present invention has been 
shown and described in detail, it should be understood that various 
changes and modifications may be made therein without departing from the 
scope of the appended claims.