Apparatus for treating wafers utilizing megasonic energy

Apparatus for treating the surfaces of a plurality of wafers, such as semiconductor wafers, which comprises a container of treatment fluid having an open upper side for accepting a cassette containing a plurality of vertically disposed wafers. The cassette is supported in the fluid in said container and a transducer carrier is mounted in the lower portion of the container. The carrier has at least one megasonic transducer arranged to project a beam of ultrasonic energy upwardly over the vertical surfaces of the wafers. The transducer carrier is moved parallel with the wafer surfaces transversely of the container whereby the ultrasonic energy beam contacts and treats the entire surface of all of the wafers therein.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed to the field of surface cleaning and 
etching of wafers such as silicon substrates and, more particularly, to an 
improved apparatus for enhancing the process of treating a plurality of 
semiconductor wafers. 
2. Description Relative to the Prior Art 
The use of ultrasonic energy to enhance the cleaning action of solutions 
used on semiconductor wafers is well established and has been described in 
U.S. Pat. Nos. 3,198,489, 3,240,963, 3,893,869 and 4,401,131. Ultrasonic 
agitation has also been used to enhance the etching action of etching 
solutions on similar semiconductor wafers. Such cleaning and etching 
processes are now commonly used in the production of semiconductor devices 
and have used transducers operating at both intermediate frequencies, 
i.e., 20-50 KHz, and, more recently, frequencies of 0.2 to 5.0 MHz. As 
noted in U.S. Pat. No. 4,602,184, the use of the high frequency, or 
megasonic, agitation of the solutions has resulted in improved cleaning 
and etching, paticularly on wafers with very small, micron-sized 
components or elements thereon. Further, the use of such high frequency 
agitation has resulted in a gentler cleaning action on the wafers than 
that obtained with intermediate frequency agitation. As a result, damage 
to the micro-sized components during the etching and cleaning operations 
during the production of the semiconductor wafers has been significantly 
reduced with the use of high frequency agitation. The productivity of the 
semiconductor production system has thus been improved due to the 
reduction in damage to the elements on the semiconductor wafers. 
Various attempts have been made to provide high frequency agitation to 
tank-type cleaning and etching baths with megasonic-frequency-generating 
transducers mounted to the inner surface of the tank walls. Some of these 
arrangements have employed apparatus to move the wafer-containing 
cassettes through the energy beam created by the transducers to assure 
that all portions of the wafer surfaces have been treated by the energy 
beam to insure that they have been adequately treated. Such arrangements 
have been found to be less than satisfactory due to the fact that the tank 
must be sufficiently large to permit the movement of the cassettes past 
the transducers, requiring larger tanks and more space in the clean rooms 
normally used for such processes, to provide space necessary for such 
cassette movement. Moreover, the larger tank volume requires a 
significantly larger volume of cleaning or etching fluid to fill the tank. 
Further, it has been found that the movement of the cassette containing 
the semiconductor wafers has caused edge damage to the wafers because of 
the motion of the cassettes and the wafers through the cleaning or etching 
bath. Still further, it has been found that due to the high power 
operation of the transducers, they must be replaced regularly. This has 
proven to be a time consuming and costly task with cleaning systems of the 
prior art, often requiring the emptying of the tank of the highly reactive 
fluid therein. 
Thus, a megasonic cleaning and etching device obviating the foregoing 
problems, providing higher wafer yield with less damage to the wafers, 
permitting smaller space requirements and less fluid use, as well as 
easier transducer maintenance, will achieve widespread acceptance in the 
semiconductor manufacturing industry. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention provides apparatus for treating the 
surfaces of a plurality of semiconductor wafers comprising a container 
means arranged to hold a body of a cleaning or etching fluid. The 
container has an open upper side for accepting a cassette containing a 
plurality of vertically disposed semiconductor wafers. Means is provided 
for supporting the cassette with the semiconductor wafers carried thereby 
beneath the surface of the fluid in said container. A transducer carrier 
is mounted in the container. The carrier has at least one megasonic 
transducer arranged to project a beam of ultrasonic energy over the 
vertical surfaces of the semiconductor wafers, and means is provided for 
energizing the megasonic transducer to oscillate at a frequency of between 
about 0.2 and 0.5 MHz. Means is also provided for moving the transducer 
carrier parallel with the semconductor wafer surfaces transversely of the 
container whereby the ultrasonic energy beam contacts and treats the 
entire surface of all of the semiconductor wafers therein. 
Moreover, the present invention provides an apparatus in which the 
transducer may be easily and quickly serviced, without requiring that the 
cleaning or etching fluid be first removed from the container. 
Various means for practicing the invention and other features and 
advantages thereof will be apparent from the following detailed 
description of illustrative preferred embodiments of the invention, 
reference being made to the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1 of the drawing, a preferred embodiment of the novel 
megasonic semiconductor treatment apparatus 10 is illustrated. The 
apparatus comprises a container means or tank 12 arranged to have an open 
upper surface and containing a treatment fluid for treating semiconductor 
wafers. The container means is provided with an inlet 11 and a drain 13 
for adding or removing treatment fluid, as necessary, to achieve a liquid 
level 14. The tank is provided with a flared upper edge forming a ledge 16 
about the upper periphery thereof. The semiconductor wafers 18, shown in 
phantom, are disposed vertically in parallel relationship in a 
wafer-carrying cassette supporting means 20, also shown in phantom. 
(Neither the semiconductor wafers nor the wafer-carrying cassette form a 
part of the present invention, but are shown to illustrate their location 
in the treatment apparatus of the present invention.) A 
cassette-supporting means comprises a plurality of transverse hanger plate 
members 22, arranged to be supported by the ledge 16 at the upper side of 
the container means 12. The hanger plates are spaced apart a distance 
slightly greater than the length of a wafer-carrying cassette and are 
interconnected by longitudinally extending hanger bars 24, only one of 
which is shown. A flange 26 on the outer surface of the cassette 20 is 
arranged to engage and be supported by bars 24, supporting the 
semiconductor wafers 18 below the surface of the treatment fluid in the 
container means. 
A transducer carrier 30 is suspended in the lower portion of the container 
tank 12 and is arranged to be moved beneath the cassettes 20 (two of which 
are illustrated in FIG. 1) and semiconductor wafers 18, as will be 
described hereinafter. The transducer carrier is shown as containing a 
pair of megasonic transducers 32, each disposed beneath a respective 
wafer-carrying cassette 20, substantially as illustrated. A cover member 
34 is disposed over the upper surface of the transducers 32 providing a 
hermetic seal for the components carried by transducer carrier 30. The 
cover 34 is provided with openings 36 over each of the transducers 32 to 
permit the direct radiation of the ultrasonic energy therefrom upwardly 
into the treatment fluid. The transducer carrier 30 extends the full 
length of the container tank 12 but has a width only a fraction of the 
width of the container. 
The transducer carrier 30 is suspended above the bottom of the tank by a 
pair of substantially vertical tubular members 40 and 42 at opposite ends 
thereof. The tubular members are sealed to the carrier and extend upwardly 
to above the top of the tank where they are hung from carriage members 44 
and 46, at either end of the tank. 
A pair of rails 48 and 50 are provided at each end of the tank and are 
arranged parallel with each other and with the bottom of the tank. The 
rails are supported by brackets 54 which extend from the tank to space the 
rails from the end thereof. 
Each carriage member, 44 and 46, comprises an upright member 60 and 61, 
each carrying a set of wheels or rollers, 62 and 63 which are arranged to 
engage and ride on rails 48 and 50, respectively. Each of the upright 
members extends above the upper edge of the tank 12 where it is joined to 
a crossover member 64 and 66. Each of the crossover members extends over 
the top edge of the tank from the upright member 60 and 61, to the tubular 
members 40 and 42 which are joined thereto. The crossover members are 
connected to the upright members with removable fasteners, such as bolts, 
not shown, to permit the disconnection of the crossover members from the 
upright members. 
A drive motor 70 is arranged at one end of the tank 12 and is provided with 
a drive sheave 72 which engages and drives a drive belt 74. The drive belt 
74 extends across the end of the tank 12 to a return sheave 76 and is 
connected to the carriage member 60 by means of a clip 78. 
The megasonic transducers 32 are provided with power via leads, not shown, 
which extend to the transducer carrier through the tubular members 40 and 
42. Since the tubular members 40 and 42 are sealed at their connection 
with the transducer carrier 30, the leads are protected from the 
environment of the bath within tank 12. Suitable instrumentation and 
control leads may also be supplied to the transducer carrier through the 
tubular members. 
In the operation of the present device, the tank is filled with the 
appropriate fluid to a level approximating that indicated at 14, and a 
wafer-carrying cassette 20 is lowered into the bath to be supported by the 
hanger rods 24 which engage the flanges 26 on the outer face of the 
cassette. The semiconductor wafers 18 are thus disposed vertically in the 
bath above the transducer carrier 30. The ultrasonic transducer 32 are 
then actuated along with the drive motor 70 which traverses the transducer 
carrier back and forth beneath the semiconductor wafers in the bath. The 
intense beam of megasonic waves are projected upward from the transducers 
32 and contact the surfaces of the semiconductor wafers 18 as the carrier 
is moved back and forth beneath the wafer-carrying cassette 20. With the 
present invention, the entire surface of the semiconductor wafers can be 
uniformly treated by the megasonic beam generated by transducers 32 
without any intervention of tank walls and without unnecessary movement of 
the wafers in the bath. 
Should it be desirable or necessary to service the transducers, this is 
simply done by removing the cassette supporting means, comprising the 
hanger plates and bars 22 and 24, from the tank 12 by lifting it out. The 
crossover members 64 and 66 are disconnected from the respective upright 
members 60 and 61, and the entire transducer carrier assembly can be 
lifted from the bath. Accordingly, it is not necessary to drain the bath 
or otherwise service the transducers in the bath. 
ALTERNATIVE EMBODIMENTS 
While the preferred embodiment has been described as providing 
substantially uniform treatment to the surfaces of the semiconductors, it 
will be understood that the drive motor 70 may be controlled so as to 
provide non-uniform exposure of the semiconductor wafers to the ultrasonic 
beam. This might be necessary or desirable when it is found that the 
semiconductor wafers contain portions which are more difficult to clean or 
etch than other portions. 
It should also be noted that while only a single drive member 72 is 
illustrated, providing the transducer carrier with a drive force from only 
one end of the tank, it will be apparent that a shaft can be extended from 
the motor 70 to the opposite end of the tank thereby supplying the 
transducer carrier means with driving force uniformly from both ends 
thereof. 
While the foregoing description has been specifically directed to apparatus 
for the treatment of semiconductor wafers, it will be understood that it 
can be also used fro other types of wafer structures or substrates needing 
particle removal, such as photomask plates, memory disks (optical or 
magnetic), solar cells, or optical components. 
It will thus be seen that the present invention provides a semiconductor 
treatment apparatus which reduces the movement of the semiconductor wafers 
and the attendant damage thereto, while still providing uniform exposure 
of the wafers to the action of the ultrasonic energy generated by the 
movement of the megasonic transducers through the bath beneath the wafers. 
Moreover, it will be seen that with the construction of the the present 
invention, service of the transducers is significantly improved with an 
attendant reduction in time and cost. 
The invention has been described in detail with particular reference to a 
presently preferred embodiment, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.