Method of cleaning surfaces with water and steam

A method and device for cleaning contaminated surfaces with both water and steam. A water film is applied to the contaminated surface and steam sprayed into the water film while water is continued to be applied to the water film. In this way the steam is directed into the water film and through the water film onto the contaminated surface, whereby contaminates in the contaminated surface are removed.

FIELD OF THE INVENTION 
The present invention relates to a method for super-cleaning surfaces, and 
to a device in accordance with the preamble of for executing this method. 
BACKGROUND OF THE INVENTION 
Conventional cleaning methods are based in immersion of the surface of the 
article or substrate to be cleaned into a wet bath, filled with detergent 
for cleaning, and, for instance reinforced by ultrasound, or by immersion 
of the substrate in vapor atmospheres, or by mechanical treatment of the 
surface, for instance by brushing, spraying of water or detergent at high 
pressure from spray nozzles, or by vibrational excitation vibration 
(megasonic cleaning). 
Particularly in chip manufacture, planarization is a process step that is 
gaining ever greater significance as the miniaturization of structures 
progresses. The structured surfaces of a substrate (for instance of a 
silicon wafer) are ground or lapped flat by a chemical-mechanical 
polishing step (or CMP processing step), using fine grinding agents 
(slurry). These grinding agents have an extremely small particle size, 
with the attendant disadvantage that after the grinding process, 
individual grinding particles still adhere to the machined surface and are 
partly embedded in the surface. After the grinding process, the surface 
must be freed completely of grinding agents and other contaminants so that 
the substrate can be further machined. 
As already noted above, by means of brushes, water and by spraying water at 
high pressures of up to 50 bar, an attempt is made to remove the grinding 
agent residues. The brushes have the substantial disadvantage that they 
must be constantly wetted with water and that they gradually fill up with 
the removed grinding agents. The brushes must therefore be replaced 
regularly, which in turn has the disadvantage that by intervention with 
the cleaning station on replacement of the brushes, the cleaning station 
becomes contaminated. Mechanically treating the surface to be cleaned with 
the brushes also involves the danger that the surface will become damaged 
and in particular scratched. As a rule, the cleaning process with the 
brushes is followed by a further cleaning process in which water is 
sprayed at high pressure. If good results are to be obtained, the surface 
to be cleaned must be exposed to the spray for a certain period of time 
and stopped repeatedly, which makes for a long cleaning time. 
The disadvantages of the conventional method will be listed again below. 
Along with major brush wear and hence high costs for the brushes, frequent 
brush changing involves a necessary intervention into the machine and 
hence a machine shutdown. Because the brushes become filled with 
particles, damage to the surface by scratching must be feared, and the 
fact that the brushes (the brush material is PFA sponge) are kept 
permanently wet means major consumption of deionized water. Moreover, 
designing the cleaning chamber with brush cleaning for cleaning both sides 
of the substrate is complicated and expensive. The contact pressure of the 
sponge brushes cannot be adjusted replicably. In high-pressure cleaning, 
there is the risk of electrostatic charging of the substrate, since 
nonconductive deionized water is used. With megasonic cleaning, the 
surface cannot be cleaned completely, especially after grinding processes. 
From European Patent Disclosure EP0381435A2, a method and a device for 
washing printed circuit boards is known. Steam is sprayed at the boards at 
a temperature and a pressure at which the steam undergoes no change of 
phase. 
Japanese reference JP-A 703 7866 discloses a device with which an article 
is exposed to an atmosphere of steam enriched with NO.sub.2. 
SUMMARY OF THE INVENTION 
It is therefore an object of the invention to provide a method and/or a 
device available, by means of which contaminated surfaces can be cleaned 
more simply, at lower cost and more rapidly, as well as with increased 
protection from damage. 
This object is attained in accordance with the present invention in that in 
the method aspect at least one steam nozzle is brought into the immediate 
vicinity of the surface, and in that the steam is directed in a targeted 
way to the surface to be cleaned via the nozzle, and the surface to be 
cleaned is moistened with water simultaneously with or prior to the 
spraying with steam. 
The device aspect in accordance with the present invention has the 
essential advantage that the surface to be cleaned is not mechanically 
treated, and the adhering particles of the grinding paste which are to be 
removed are on the one hand loosened by means of the heat energy 
introduced via the steam. Because of the different heat absorption of the 
substrate and the particles to be removed, and because of tension caused 
by different coefficients of expansion, the connection between the 
particles and the substrate is weakened, as a result of which the 
particles are loosened from the surface to be cleaned. If wet steam is 
used in place of superheated steam, the particles are additionally 
loosened by the transfer of pulse forces. In the process the kinetic 
energy (ultrasound energy) when the liquid droplets impinge is utilized. A 
further advantage resides in that in the course of introducing the steam 
into a water film present on the surface, the water partially turns into 
steam locally, and the vapor bubbles either collapse again immediately 
afterwards or burst on the surface of the water film. 
The method in accordance with the present invention can be employed not 
only with flat or plane surfaces, but also with deep structures, such as 
occur in the production of micro-mechanical elements, for example. The 
removal of the loosened particles can take place either by means of the 
steam directed on the surface, by the condensate, and/or by additionally 
sprayed-on water. This is of advantage in particular with very thin 
substrates, since only little condensation takes place there. 
In a further development it is provided that the surface can be dried at 
the end, for example by the introduction of foreign gas, such as nitrogen 
or the like, or by employing hot steam. Spots left after drying are thus 
averted. It is considered to be particularly important that the removal of 
the particles takes place by means of the action of pulses (kinetic 
energy), by different heat expansion (thermal energy) and by means of the 
action of pulses (water turning into steam and bursting of the gas 
bubbles). In this connection, blowing of steam into a water film is 
preferred. 
Advantages are seen in the employment of deionized water, to which 
alcohols, detergents, solvents or the like can be added. Foreign gases, 
for example nitrogen or the like, can be added to the steam. One 
particular area of application is considered to be the cleaning which 
follows the CMP process, but others are also in connection with the 
manufacture of LCD products, production of hard disks, and in 
micro-mechanics or other processes requiring super-clean surfaces. 
A transport device is provided in accordance with the present invention, by 
means of which objects, for example substrates whose surface needs to be 
cleaned, are rotated and/or conveyed. In the case of silicon wafers the 
objects are, for example, flat disks, which are transported without 
contact through the cleaning station in a continuous running process. In 
the process both the top and the bottom are cleaned. The two sides can be 
cleaned simultaneously or one after the other. In the process the object 
preferably rotates on a steam and/or water cushion and is maintained 
contactless in this way. The position is fixed by pins or rollers acting 
on the edge of the disk. The substrate is caused to rotate and/or is 
transported through the cleaning station by appropriately disposed nozzles 
on the transport device, which are directed on the underside of the disks. 
The oblique impact angle of the fluids leaving the nozzles has been 
selected such, that disk is moved in any case, possibly even cleaned in 
addition. The impacting fluid is therefore used as a transport medium, 
cleaning medium and removal medium for loosened particles. The runoff of 
the fluids is made easier, for example by transverse grooves or obliquely 
extending grooves in the transport device. 
A drying section, in which the substrate is dried, can follow the section 
of the cleaning station where cleaning takes place. In the process the 
substrate is preferably caused to rotate (1500 rpm), by means of which the 
adhering water film is thrown off, drying can also be aided by hot steam. 
The individual substrates can be inserted into magazines or cassettes at 
the outlet from the cleaning station. 
The advantages of the method in accordance with the present invention and 
the device in accordance with the present invention are essentially the 
following: improved cleaning results at higher eater temperatures; shorter 
process times and attendant higher throughput; contactless cleaning and 
therefore no mechanical stress of the substrate surface; no disposable 
articles such as brushes, etc., and therefore reduced operating costs; no 
interference with the cleaning chamber and therefore no down times and no 
danger of contamination; no cleaning of the process chamber is required, 
since it is automatically also cleaned by the steam; smaller size of the 
installation and lesser need for clean rooms; simpler structure of the 
process chamber, reproducible cleaning results in contrast to the 
customary processes, in which the brushes are used up; easier integration 
in a cluster with other installations; and environmentally friendly 
cleaning, since in contrast to cleaning by the CMP process no solvents are 
involved. 
Further advantages, characteristics and details of the present invention 
will become apparent from the claims and the ensuing description, in which 
especially preferred exemplary embodiments are shown in detail in 
conjunction with the drawing. the description and shown in a drawing may 
each be essential to the invention individually or in arbitrary 
combination.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1, a basic sketch is shown for preparing the steam to be used to 
clean contaminated surfaces. Reference numeral 1 indicates a spray nozzle 
from which steam 16 emerges in the form of superheated steam or wet steam. 
The nozzle 1 communicates via a line 17 with a steam boiler 5. Provided in 
the line 17 in succession are a steam vale 4, a connection 12' for 
introducing foreign gas, for instance for introducing nitrogen (N.sub.2), 
a connection 11' for introducing foreign agents, such as alcohol or the 
like, a superfine filter 3, and a flow reducer 2, for instance in the form 
of a throttle. Located in the steam boiler 5 is deionized water 18, which 
is introduced wither via a direct water inlet 8, for instance in 
non-continuous operation, or via an inflow line 19. For the continuous 
mode of operation, this inflow line 19 communicates with a water 
connection 15 and with a water pressure pump 14 and a water inlet valve 
13. Located in the water bath of the steam boiler 5 is a heater 6, in 
particular an electric heater, by means of which the water bath is heated 
via a heater regulator 7 with a temperature sensor 20. The steam boiler 5 
is also provided with a connection 12 for introducing foreign gas and a 
connection 11 for introducing a foreign agent, as well as a steam pressure 
switch 10 and an overpressure valve 9. In this way, both superheated steam 
and wet steam can be generated, the latter for instance by supplying water 
via the connection 11'. 
In wafer manufacture, the structure surface of a silicon wafer 21 is coated 
with a planarizing layer, which for instance comprises SiO.sub.2. The 
structure surface, which has been produced by exposure to light and 
ensuing etching, is shown in FIG. 2a. FIG. 2b shows the surface coated 
with the planarizing layer 22; it can clearly be seen that the surface of 
this planarizing layer 22 is not plane. Via an ensuing grinding or lapping 
process (CMP process), the planarizing layer 22 is ground down far enough 
that only the indentations 23 in the surface of the silicon wafer 21 are 
filled. In this operation, a very fine grinding agent (slurry) is used, 
but the individual slurry particles still stick to the surface or are 
embedded in indentations of the surface 60 after the grinding process is 
finished. The surface shown at 24 in FIG. 2c must then be decontaminated 
for further processes, or in other words freed of any contamination 
whatever. 
FIG. 3 shows a first exemplary embodiment of the cleaning device according 
to the present invention, which is embodied for instance for holding 
silicon wafers 21, which have a round disklike shape. The silicon wafer 21 
or the substrate 21 is fastened in the region of its edges in at least 
three diabolo-shaped drive rollers 25 which are rotatably supported in the 
direction of the arrow 26. In this way, the substrate 21 can be moved 
without damage to or contact with the surface 24. The two surfaces 24 are 
sprayed with steam 16 via spray nozzles 1 from above and below. The spray 
nozzle 1 can be moved, each via a suitable device, radially in the 
direction of the arrows 27 or in some other way between the center 28 and 
the edge of the surface 21, and they are spaced apart from the surface 24 
by approximately 1 mm. As a result, the entire surface 24 can be treated 
with steam 16. 
The steam 16 emerging from the nozzle 1 strikes the surface 24, so that the 
contaminants adhering to the surface 24 or located in indentations of it 
are loosened by the kinetic energy. If wet steam 16 is sprayed on the 
surface, then the contaminants are loosened by the liquid droplets 
striking them or by the kinetic energy of these droplets. The water vapor 
also condenses on the surface and is partly re-evaporated by the steam 
that follows it, with the vapor bubbles collapsing immediately afterward 
or bursting at the surface of the substrate. These impulsive forces cause 
further loosening of the particles. Moreover, the substrate 21 and the 
individual particles are heated to different extents, causing thermal 
strains that lead to still further loosening. 
The advantage of this method according to the present invention resides in 
the very low consumption of media, and the effect of the pulses, heat and 
impulsive force bring about very effective cleaning. The cleaning force 
effect is also readily controllable by suitable variation of the steam 
pressure, quantity of incident steam, steam temperature, spacing between 
steam nozzle and substrate surface, and thickness of the water film as 
well as by the metering of foreign agent or foreign gas. The cleaning is 
effected by means of deionized water, so that no disposal problems occur 
and the entire method is environmentally friendly. The drying can be done 
by blowing hot steam 16 or by supplying foreign gas. The pressure is 
regulated by the temperature of the water in the temperature range from 
100.degree. C. to approximately 200.degree. C., which is equivalent to a 
pressure range up to about 10 bar. 
FIG. 4 shows another embodiment of the cleaning device according to the 
present invention, in which the substrate 21 is fastened in a rotary mount 
29. The rotary mount 29 can also hold substrates 21 that are not circular 
and engages them in the region of their edge. In this embodiment, the 
upward-pointing surface 24 is cleaned by means of the nozzle 1. 
FIG. 5 shows a first embodiment of a transport device 30 for moving the 
substrate 21 in the direction of the arrow 31. Top this end, the transport 
device 30 has a steam conduit 32, which is provided on its top with many 
steam nozzles 33. These steam nozzles 33 are inclined slightly in the 
direction of the arrow 31, so that steam 16 emerging from the steam 
conduit 32 forms a steam cushion 34 under the substrate 21 and transports 
it in the direction of the arrow 31. In this way, the substrate 21 can be 
moved without being touched in a once-through passage through a cleaning 
station, with the substrate 21 floating on a cushion 34 of steam and 
water. The lateral guidance is effected via boundary strips, as shown in 
FIG. 7. 
The transport device 30, in addition to transport portions shown in FIG. 5 
also has portions 36 in which the substrate 21 is rotated, which is shown 
in FIG. 6. Steam nozzles 33 are also provided in these portions 36, but 
their exit direction is inclined substantially upward and at a tangent 
around the center 28 and optionally extends in inclined fashion outward, 
as shown in FIG. 6, in the direction of the edge of the substrate 21. In 
this way, the substrate is rotated about the center 28 in the direction of 
the arrows 38. The positioning of the substrate 21 is effected via 
boundary pins 39, which are disposed so as to be lowerable in the portion 
36. If the boundary pins 39 have been extended, as shown in FIGS. 6 and 7, 
then the substrate 21 is impeded from being transported further in the 
direction of the arrow 31 and is firmly held in the portion 36. The 
boundary pins 39 may also themselves be rotatably supported. In that case, 
the cleaning of the surface or surfaces 24 of the substrate 21 can take 
place in the portion 36. 
FIG. 8 shows a modified embodiment of a transport portion 40 of a transport 
device 30; the substrate 21 is disposed substantially between two steam 
conduits 32, and both surfaces are acted upon by steam 16 emerging from 
steam nozzles 33. The substrate 21 is thus embedded between two steam 
cushions 34 and is moved in the direction of the arrow 31. 
In FIG. 9, an alternative cleaning portion 41 of the transport device 30 is 
shown, in which the substrate 21 is firmly held via the boundary pins 39 
and rotated by means of the steam nozzles 37, as described for FIGS. 6 and 
7. The steam nozzles 37, as also already shown in the exemplary embodiment 
of FIGS. 6 and 7, may simultaneously act as spray nozzles 1, or in other 
words for cleaning the surfaces 24 of the substrate 21. The detached dirt 
particles and the incident water can be removed, as shown in FIG. 10, via 
conduits 42 that are provided in the lower steam conduit 32. To that end, 
the conduits 32 and/or 42 are inclined. 
FIG. 11 schematically shows one such once-through device, in which the 
substrate 21 is placed on the transport device 30 and transported in the 
direction of the arrow 31. The portion 35 serves to transport the 
substrate 21, and the portion 36 serves to clean it. Reference numeral 43 
schematically indicates a spin dryer, in which the substrate 21 is set 
into rapid rotation (about 1500 rpm) in the direction of the arrow 44. Via 
suitable manipulators 45, the substrate 21 is removed from the spin dryer 
43 and placed for instance in cassettes or magazines (not show) and 
prepared for further transportation in the direction of the arrow 31. 
In FIG. 12, a cleaning method is shown in which the substrate 21 has water 
18 sprayed on it via a water lance 46, thus creating a water film 47 on 
the surface 24 of the substrate 21. Via the water lance 46, the water 18 
is fed onto the surface 24 at the center 28, so that the water 18 runs off 
in the direction of the arrows 48 in the form of a water film 47. At the 
same time, the spray nozzle 1 sweeps over the surface 24 in the direction 
of the arrow 27 and sprays steam 16 directly into the water film 47. Vapor 
bubbles thus form in the water film 47, which either immediately collapse 
again or burst at the surface of the water film 47. Along with the pulsing 
action of the water vapor 16, the particles 49 also undergo the impulsive 
action of these vapor bubbles, and are loosened or detached in this way 
from the surface 24. They are then carried away along with the outflowing 
water film 47. 
In the embodiment of FIG. 13, the substrate 21 is inclined at an angle 
.alpha. and is transported in the direction of the arrow 31 via transport 
rollers 50, only two of which are shown. Once again, water 18 for forming 
a water film 47 is applied to the surface 24 of the substrate 21 via the 
water lance 46. Via the spray nozzle 1, water vapor 16 is blown into this 
water film 47, as in the exemplary embodiment of FIG. 12. In this process, 
the spray nozzle 1 is moved orthogonally to the plane of the drawing, or 
the spray nozzle 1 may be embodied as a wide-slit nozzle, as shown in FIG. 
15. 
In the exemplary embodiment of FIG. 14, the substrate 21 is disposed flat 
and is also transported via rollers 50 in the direction of the arrow 31. 
Once again, via the lance 46, water 18 for forming a water film 47 is 
applied to the surface 24, and steam 16 is blown into the water film 47 
via the nozzle 1. The plan view shown in FIG. 15 shows that both the water 
lance 46 and the nozzle 1 are embodied as wide-slit nozzles. This 
arrangement is excellently suited to the once-through method, and the 
substrate 21 is handled gently. 
In closing it should also be pointed out that this method and a 
corresponding device can be retrofitted easily in existing grinding 
systems and can readily replace existing cleaning device.