Method for cleaning a substrate

A substrate cleaning method and equipment for removing foreign metters adhered to a substrate, the method comprising the steps of: making hydrophilic a surface of the substrate; causing pure water to diffusively permeate the hydrophilic surface of the substrate, thereby forming a layer of aqueous particles on the substrate surface; removing the foreign matters on the substrate in which the pure water diffusively permeated; and dipping, into hot pure water, the substrate in which the pure water diffusively permeated and then drying the substrate surface by relatively moving the hot water and the substrate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a method and equipment for 
cleaning a substrate and, more particularly, to a substrate cleaning 
method and equipment for removing infinitesimal foreign matters (dusts, 
particles etc.) adhered to a glass substrate such as a photomask and a 
reticle for manufacturing a semiconductor. 
2. Related Background Art 
A line width of a pattern depicted on the surface of a semiconductor wafer 
substrate has become increasingly hyperfine. In the case of obtaining the 
hyperfine pattern by photolithography, if foreign matters such as dusts 
are adhered onto the photomask or the reticle, the foreign matters act as 
light shielding or scattering substances. In general, the circuit pattern 
is formed by opaque material like chromium, or phase shift material on the 
surface of a transparent substrate. 
For this reason, the image of the foreign matters is transferred onto the 
wafer, and it follows that pattern different from the original mask 
pattern is formed on the wafer. 
As a result, there exists a possibility in which an external configuration 
of the mask pattern is deteriorated, and further a defect on the circuit 
is caused. An undesirable influence by the deterioration of the external 
configuration increases when forming a higher integration of the circuit 
or a more hyperfine pattern. For this reason, the deterioration of the 
external configuration due to the infinitesimal foreign matters is not 
ignorable. Hence, an establishment of a method of substantially completely 
eliminating such foreign matters is of a high importance in terms of 
improving quality and yield of the product. 
A conventional method and equipment for cleaning the photomask or the 
reticle are disclosed in U.S. Pat. No. 4,569,695. A brief explanation 
thereof will be given. The mask passes between a pair of rotary brushes 
while the surface and rear face of the mask are wetted with water or an 
electrolyte. Contaminations on the surface and rear face of the mask are 
thereby mechanically eliminated by the rotary brushes. The surface and 
rear face are rinsed with an organic solvent of an alcoholic system and 
thereafter dried in the steam of the alcoholic or freon system. A cleaning 
process is thus completed. This method is remarkably superior as a method 
of removing a foreign matter of 1 .mu.m or larger. 
Further, a method of cleaning the substrate surface after making it 
hydrophilic is known in Japanese Patent Laid-Open Application No. 
1-226156. This cleaning method involves the steps of irradiating the 
substrate surface with ultraviolet light in an oxidative atmosphere 
containing oxygen to make the substrate surface hydrophilic. Thereafter, a 
water-containing layer is formed on the substrate surface. A cleaning 
liquid is brought into close contact with the substrate surface formed 
with the water-containing layer, thus cleaning the substrate surface. 
Thereafter, the cleaned substrate surface is dried. 
As a result of extensive investigation by the present inventors, it has 
been determined that infinitesimal dusts produced from the rotary brushes 
in the first-mentioned technique are re-adhered onto the glass substrate, 
so that foreign matters still remain on the glass substrate after 
cleaning. Hence, a sufficient effect for removing foreign matters as 
infinitesimal as 1 .mu.m or less was not necessarily obtained. It was also 
determined that in the second-mentioned technique, water stains appear 
after drying due to water that cannot be sufficiently removed in the 
drying step. 
SUMMARY OF THE INVENTION 
It is a primary object of the present invention, which has been devised in 
view of the problems described above, to provide a method and equipment 
for cleaning a glass substrate with no water stain and which are capable 
of surely removing foreign matters as infinitesimal as 1 .mu.m or under 
and exhibiting a high-level cleaning ability. 
It is another object of the present invention to enhance a permeability 
into a surface of the substrate so that the surface can be covered with a 
uniform water content (that is, the surface is uniformly moistened.) 
It is still another object of the present invention to perform cleaning 
with no water stain by use of only oxygen of an active group and water, 
without employing harmful organic chemical substances at all. 
According to the present invention, the surface of the glass substrate is 
made sufficiently hydrophilic by irradiating the glass substrate such as a 
photomask or a reticle with ultraviolet light. Then, pure water or hot 
pure water diffusively permeates the surface of the glass substrate. 
Thereafter, cleaning by brush scrubbing or ultrasonic waves is performed. 
Then, after the cleaning has been executed, a pull-up drying process out of 
hot pure water is finally effected. 
The foreign matters can be eliminated from the glass surface on the basis 
of the following principle by the method and equipment for cleaning the 
glass substrate according to the present invention. 
When irradiating the glass surface with the ultraviolet rays, oxygen 
molecules (O.sub.2) existing in close proximity to the glass substrate 
surface are changed into ozone molecules (O.sub.3) by an energy of the 
ultraviolet rays. Further, the ozone molecules are decomposed by the 
energy of the ultraviolet rays or thermal energy, thereby generating an 
oxygen active group (O*) exhibiting high reactivity. This oxygen (O*) 
oxidizes and removes organic substances on the surface of the glass 
substrate. Simultaneously, the oxygen (O*) is coupled with Si--O--H group 
hydrogen atoms (H) on the glass substrate surface, thereby making the 
substrate surface hydrophilic. An interface energy on the hydrophilic 
surface is augmented, and a wetting property (permeability) of the water 
becomes remarkably large. The aqueous molecules (H.sub.2 O) diffusively 
permeate even hyperfine portions on the glass substrate. Cleaning action 
by brush scrubbing or ultrasonic waves with the aid of the water or hot 
water works at the highest efficiency in a state where the glass substrate 
surface is made hydrophilic, and the aqueous molecules diffusively 
permeate. The infinitesimal dusts existing on the glass substrate surface 
are all removed therefrom. Also, the glass surface is covered uniformly 
with the aqueous molecules, which makes it extremely difficult for 
localized residues of the water to be formed in the pull-up step. In 
consequence, A clean, dry surface with no water stain can be obtained.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the present invention will be described with reference to 
the drawings. FIG. 1 is a schematic diagram illustrating an equipment of a 
preferred embodiment of the present invention for cleaning a glass 
substrate. 
The cleaning equipment is, as illustrated in FIG. 1, constructed of: a 
housing unit 50 for housing a plurality of glass substrates 10; a first 
cleaning tank 20 for effecting hydrophilic processing of the glass 
substrate 10; a second cleaning tank 30 for performing pure water (hot 
pure water) cleaning of the glass substrate 10 by brush scrubbing or 
ultrasonic waves; a third cleaning tank (drying unit) 40 for drying the 
glass substrate 10 by dewatering the surface thereof; and a conveying 
system 5 capable of conveying the glass substrates to the respective 
cleaning tanks and vertically moving the glass substrates within the 
respective cleaning tanks. 
Referring now to FIG. 1, the first cleaning tank 20 is disposed in the 
remotest position from the housing unit 50. The second and third cleaning 
tanks are disposed sequentially from the first cleaning tank 20 towards 
the housing unit 50. Note that the first, second and third cleaning tanks 
20, 30, 40 are sequentially disposed along a rectilinear conveying route 
5a for conveying the glass substrates 10 to the respective cleaning tanks. 
The tanks need not necessarily be disposed rectilinearly from the first 
cleaning tank 20 to the housing unit 50, however. For instance, the first 
cleaning tank 20 and the third cleaning tank 40 may be placed adjacently 
to the housing unit 50. 
Turning to FIGS. 2A and 2B, holding portions 12a, 12b of two conveying 
support members (arms) 11a, 11b hold two corners, disposed in the 
direction of a diagonal line, of the glass substrate 10 to be cleaned. The 
glass substrate 10 is thus held obliquely such that all peripheral edges 
of the substrate are inclined with respect to a vertical direction. The 
oblique holding of the glass substrate 10 is intended to make dewatering 
better. The arms 11a, 11b constitute a vertically movable structure. The 
arms bring in and out the glass substrates between the housing unit 50 and 
the respective cleaning tanks 20, 30, 40. Further, the arms move the glass 
substrates 10 between the respective cleaning tanks and the conveying 
system 5. 
The glass substrate 10 is held and conveyed at a right angle to the moving 
direction. When moved in each tank during cleaning, the glass substrate 10 
is conveyed in such way that a pattern surface 10b of the glass substrate 
10 is directed forwards, while a glass surface 10a is directed backwards 
before being processed in each tank. Conversely, after being processed, 
the glass surface 10a of the glass substrate 10 is directed forwards, 
while the pattern surface 10b is directed backwards. This takes into 
consideration both miniaturization of the equipment and an influence of 
adhesion of contaminants if adhered during the conveyance. 
Referring now to FIG. 1, the cleaning tank 20 is constructed of: UV lamps 
21a, 21b for emitting UV rays with which the glass substrate is 
irradiated; an ozone monitor 22 for monitoring the ozone generated when 
the glass substrate 10 is irradiated with the UV rays; UV light shielding 
plates 23a, 23b for preventing direct rays and reflected rays of the UV 
lamp from leaking outside; an ozone exhaust 24; a UV irradiation quantity 
monitor 25; an ozone adsorbent 26; and UV ray reflectors 27a, 27b. The UV 
lamps 21a, 21b are disposed opposite to each other so that glass substrate 
10 is situated therebetween. The UV ray reflectors 27a, 27b are so 
disposed that the UV rays uniformly fall on the glass substrate. A UV ray 
irradiation quantity is measured and controlled by the UV ray quantity 
monitor 25. 
The ozone (O.sub.3) generated is measured and controlled by the ozone 
monitor 22 and exhausted via the ozone exhaust 24 formed under the tank so 
as not to leak outside. Note that the ozone during the exhaust is adsorbed 
and removed by use of the ozone adsorbent 26 provided en route to an 
exhaust system. The ozone is discharged with a safe concentration. A 
wavelength of the UV light incident on the glass substrate 10 is set 
suitably so as to make the glass substrate 10 hydrophilic. In accordance 
with this embodiment, the glass substrate 10 composed of Si--O is 
irradiated with the UV light having a wavelength of 250 nm or shorter. 
This irradiation of the UV rays makes the glass substrate surface 
sufficiently hydrophilic. 
The second cleaning tank 30 consists of: injection type ultrasonic 
oscillators 31a, 31b; pure water shower nozzles 32a, 32b, 33a, 33b; splash 
preventive plates 34a, 34b; rotary brushes 35a, 35b; and a drain port 36. 
Installed at respective equal distances from the top of the cleaning tank 
30 are the pure water shower nozzles 32a, 32b, the injection ultrasonic 
oscillators 31a, 31b; the pure water shower nozzles 33a, 33b and the 
rotary brushes 35a, 35b, whereby the glass substrate 10 can be disposed 
therebetween. These members are located to eliminate foreign matters most 
effectively. 
The splash preventive plates 34a, 34b are interposed between the injection 
ultrasonic oscillators 31a, 31b and the pure water shower nozzles 33a, 
33b, thereby preventing a re-adhesion of the foreign matters to the glass 
substrate 10 during cleaning by the ultrasonic wave and brush scrubbing. 
The drain port and a tank bottom 37 take an oblique structure to maximize a 
recovery efficiency of the waste liquid. Further, the pure water showers 
32a, 32b and the pure water showers 33a, 33b are capable of jetting hot 
pure water as needed. If an electrostatic charge to the glass substrate 10 
is caused, the showers are capable of supplying the water dissolved with 
CO.sub.2 (carbonic acid gas). Note that a cleaning effect is enhanced by 
use of the hot pure water. 
The third cleaning tank 40 comprises a quartz glass tank 41, a heater 42 
for heating the hot pure water, particles (which mainly comprise removed 
foreign matters) measuring device 43, a hot pure water supply port 44 and 
a hot pure water drain port 45. These components are structured such that 
the foreign matters are not re-adhered to the glass substrate 10, and a 
cleaning degree and a temperature of the hot pure water are maintained and 
controlled so as not to produce water stains. More specifically, with 
respect to maintaining the cleaning degree (efficiency), the quartz glass 
tank 41 composed of the quartz glass having a low contamination is 
employed as a tank for drying the glass substrate. An overflow of the hot 
pure water is always effected in a configuration with a small amount of 
residual foreign matters. At the same time, the particle measuring device 
43 measures the number of particles existing in the hot pure water and 
performs the control so that the number of particles does not exceed a 
predetermined number. Speaking of the temperature, the heater 42 for 
heating the hot pure water constantly controls an energy supply quantity 
so as not to cause fluctuations in the temperature of the hot pure water. 
Next, the cleaning operation will be explained. 
[Step 1] 
The glass substrate 10 is conveyed via the conveying system 5 to the first 
cleaning tank 20 remotest from the housing unit 50. The glass substrate 10 
conveyed to the first cleaning tank 20 is shifted to a predetermined 
position in the first cleaning tank 20 by means of the arms 11a, 11b. 
Before insertion of the glass substrate 10, the first cleaning tank 20 
assumes an idling status wherein a lamp intensity drops down. After 
inserting the glass substrate 10, however, the lamp intensity reaches the 
maximum (Full), whereby an intensive irradiation state of the UV rays is 
developed. A UV irradiation light quantity is measured by the UV 
irradiation light quantity monitor 25. The ozone generated is measured by 
the ozone monitor 21. A necessary quantity of UV irradiation is effected. 
The ozone generated at this moment is exhausted with the safety 
concentration from the lower ozone exhaust 24 through the ozone adsorbent 
26. 
[Step 2] 
The glass substrate 10 exhibiting the sufficient hydrophilic property owing 
to the irradiation of the UV rays in [Step 1] is conveyed via the 
conveying system 5 to the second cleaning tank 30. The glass substrate 10 
is moved in the vicinity of the hot water shower nozzles 32a, 32b by the 
arms 11a, 11b. The glass substrate 10 is at first rinsed therein with pure 
water or hot pure water through the hot water shower nozzles 32a, 32b. The 
entire surface of the glass substrate is wetted with the pure water or hot 
pure water uniformly. The glass substrate 10 is sufficiently made 
hydrophilic, and, as stated earlier, the aqueous molecules diffusively 
permeate the glass substrate 10. Thereafter, the arms 11a, 11b shift the 
glass substrate 10 in the vicinity of the rotary brushes 35a, 35b. Then, 
the glass substrate undergoes scrub cleaning by the rotary brushes 35a, 
35b while jetting the pure water or hot pure water from the pure water 
shower nozzles 33a, 33b. The glass substrate 10 is repeatedly 
stroke-operated in the up-and-down directions, thus effecting the scrub 
cleaning over the entire surface of the glass substrate 10. At this time, 
the rotary brushes 35a, 35b rotate in such a direction as to scrub the 
glass substrate 10 from top to bottom. After stopping of the rotary 
brushes, subsequently, the arms 11a, 11b shift the glass substrate 10 in 
the vicinity of the injection ultrasonic oscillators 31a, 31b. Then, the 
entire surface of the glass substrate 10 is, as similar to brush 
scrubbing, subjected to ultrasonic cleaning by use of the injection 
ultrasonic oscillators 31a, 31b. Finally, a finish rinse is effected 
thereon with the pure water or hot pure water through the pure water 
shower nozzles 32a, 32b. 
[Step 3] 
The glass substrate 10 from which the foreign matters are removed is 
conveyed via the conveying system 5 to the third cleaning tank 40. The 
arms 11a, 11b cause the glass substrate 10 to dip into the hot pure water 
within the quartz glass tank 41. Jush when a temperature of the glass 
substrate is equalized to a temperature of the hot pure water, the glass 
substrate is pulled up from the liquid at a speed as low as 2-10 mm/sec. 
As explained earlier, the surface of the glass substrate 10 is covered 
with the aqueous molecules, whereby drying with no water stain is 
attainable. 
[Step 4] 
The glass substrate 10 dried and clean after being pulled up is conveyed 
via the conveying system 5 to the housing unit 50, thus finishing the 
cleaning process. 
In accordance with the embodiment discussed above, the hydrophilic process 
of the glass substrate is performed by the irradiation of the UV rays. It 
was empirically confirmed that ozone (O.sub.3) processing other than the 
irradiation of the UV rays is also effective as a method of making 
hydrophilic the surface of the glass substrate. 
The following is a brief explanation of the ozone processing. In the 
embodiment discussed above, the ozone molecules are generated by the 
irradiation of the UV rays, and oxygen (O*) of an active group is 
produced. In the ozone processing, however, oxygen (O*) of the active 
group is produced by directly supplying the ozone molecules and thermal 
energy without executing the irradiation of the UV rays. 
Although the illustrative embodiments of the present invention have been 
described in detail with reference to the accompanying drawings, it is to 
be understood that the present invention is not limited to those 
embodiments. Various changes or modifications may be effected by one 
skilled in the art without departing from the scope or spirit of the 
invention.