Apparatus and method for spreading resist on a wafer and detecting bubbles in the resist

A station for coating a semiconductor wafer with a photoresist is provided with a detector for bubbles that may occur in the resist that is supplied to the wafer. The resist is carried to the coating apparatus by a plastic tube. A commercially available capacitance detector is positioned to detect the dielectric constant of the combination of the tube, the resist, and any bubbles in the resist. The dielectric constant of the bubbles is lower than the dielectric constant of the resist, but the difference is not sufficient for detecting the bubbles with the capacitance detector alone. A metal backing plate is located on the side of the tube opposite the detector and enhances the operation of the detector sufficiently to detect bubbles of various sizes.

FIELD OF THE INVENTION 
This invention relates to apparatus for the manufacture of semiconductor 
devices, and more specifically to apparatus for detecting bubbles in a 
photoresist that is supplied to a wafer coating station. 
INTRODUCTION 
The apparatus and related process for coating a semiconductor wafer are 
well known, but it will be helpful to review the features and terminology 
that particularly apply to this invention. The resist station has a chuck 
for holding the wafer and a nozzle for applying a measured amount of 
resist to the exposed surface of the wafer. While the resist is being 
applied, the chuck is spun and the resist is spread over the wafer surface 
by centrifugal force. 
The resist is held in any suitable container and it is carried to the 
resist spreader nozzle by any suitable means, commonly by a plastic tube. 
From a more general standpoint, the tube is a dielectric. 
THE BUBBLE PROBLEM 
Air bubbles can appear in the resist, and these bubbles can damage the 
resist coating. The common practice is to stop the process when the 
effects of bubbles are noticed, to clear the system of bubbles, and to 
restart or continue the process. 
Therefore it would be advantageous to detect the bubbles as close to the 
bubble source as possible. 
THE PRIOR ART 
U.S. Pat. No. 4,899,686 discusses the bubble problem. 
SUMMARY OF THE INVENTION 
The dielectric constants of the resist and the tube are sufficiently 
different from the dielectric constant of air bubbles that it would seem 
to be possible to detect the bubbles with a commercially available 
capacitance sensor. However, these detectors have been found to be unable 
to detect bubbles sufficiently well for use in a semiconductor 
manufacturing line. 
According to this invention, a conductive backing plate is located close to 
the supply tube on the opposite side of the tube from the capacitive 
sensor. The plate enhances the operation of the capacitive tester, 
probably by its effect on the electrostatic field of the capacitive 
sensor, as will be discussed later. 
Other objects and features of the invention will be apparent from the 
description of the preferred embodiment of this bubble detector.

THE PREFERRED EMBODIMENT 
The Conventional Resist Supply System 
FIG. 1 shows a conventional wafer coating station with a wafer 5, apparatus 
6 for holding and spinning the wafer, a nozzle 7 for applying a 
photoresist to the wafer surface, a container 8 for the resist, a resist 
pump 9 and a plastic tube 16 that carries the resist to the nozzle. 
The tube 16 is conventionally made of PTFE ("TEFLON"). The tube commonly 
has an outside diameter of about 1/4, and this tubing size is preferred in 
the method and apparatus of this invention. 
The Bubble Problem 
A single bubble might be 5-6 mm in diameter and will almost fill the tube. 
Alternatively, the resist may have a mass of small bubbles that almost 
fill the tube or only a single small bubble. Thus the bubbles present a 
difficult problem for a capacitance sensor, and a capacitance sensor alone 
has been unable to detect bubbles satisfactorily 
The conventional capacitive sensor 
Capacitance sensors are commercially available and many of these sensors 
will operate satisfactorily in the detector of this invention. These 
bubble detectors operate by producing an electrostatic field in the region 
that is to be tested. The results of the test can be interpreted for 
example as the dielectric constant of the material in which the field is 
established. 
The operation of the apparatus of this invention can be understood by 
considering a simplified description of the conventional capacitance 
sensor. The sensor has an electrode located in its tip. The electrode is 
connected to a circuit that supplies an oscillatory voltage and senses the 
oscillatory current in the electrode circuit. 
The effect of the dielectric constant 
The bubble detector responds to the difference in dielectric constant 
between the bubbles and the resist. The resist bubbles are air, which has 
a dielectric constant of 1, which is low in relation to the other 
materials that are relevant to this explanation. The dielectric constant 
of the PTFE tube is about 2, and the dielectric constant of the resist is 
about 3.5 to 5. 
To simplify the explanation, the tube and the resist (without any bubbles) 
can be thought of as a series of dielectrics with these values. Similarly, 
the simple case in which a bubble fills the tube can be thought of as the 
series of dielectrics that have dielectric constants of about 2 for the 
tube and 1 for the bubble. 
In a hypothetical situation that does not correspond to the wafer coating 
situation, the commercial capacitance sensor would produce one signal for 
the tube and resist without the bubbles and it would produce a 
distinguishable signal for the tube and the bubble. 
The backing plate 
The combination of the backing plate and the commercial capacitance sensor 
does produce this distinguishable signal for bubbles in the delivery line 
16. 
FIG. 1 shows a section of tubing 14, the capacitance sensor 12, and a 
housing 13 that supports the sensor and the tube section 14 for the 
detector to sense the dielectric constant of the tube and its contents. A 
backing plate 15 is mounted in the housing on the side of the tube 
opposite the detector and close to the tube section 14. 
FIG. 2 shows the preferred housing structure. The housing 13 preferably is 
made with rectangular sides. A section 14 of tubing in the housing is 
preferably connected to the rest of the delivery line 16 by removable 
fittings so the sensor 12, its housing 13, and the associated section 14 
of line 16 can be removed from the coating station. The capacitance sensor 
12 is held in place by suitable means such as the surrounding wall of the 
corresponding side of the housing. 
The backing plate is a metal plate that is mounted on the back wall of the 
housing to be close to the tube. The backing plate has a suitable 
thickness to enhance the operation of the capacitance sensor sufficiently 
to detect bubbles in the resist. The tube is sufficiently thick to provide 
the conductance to serve as an electrode in the detector system, but a 
size that is suitable for construction purposes will ordinarily be 
sufficient for electrical purposes. As FIG. 1 shows, the metal backing 
plate is flat and is located parallel to the tubing where bubbles are to 
be detected. From an electrical standpoint, the backing plate is thick 
enough to form a ground for the capacitance detector. Some bubbles that 
cannot be distinguished from resist without the backing plate can be 
detected with the detector using the backing plate. 
Other Embodiments 
The detector with the backing plate should operate satisfactorily with 
tubing of a different size, for example with 1/2 tubing and a larger 
capacitive sensor. Those skilled in the art will recognize other 
modifications of the preferred embodiment within the spirit of the 
invention and the intended scope of the claims.