Insitu detection of tube sagging in semiconductor diffusion furnace using a laser beam that is blocked when tube sags

Semiconductor wafers are processed in a semiconductor diffusion furnace. During processing, the semiconductor wafers are placed in a quartz tube. Also during processing, a laser beam is transmitted below a top surface of the quartz tube. While the quartz tube is not sagging, the laser beam is detected with a detector. When the top surface of the quartz tube sags so that the laser beam is obstructed by the top surface, the laser beam is no longer detected by the detector. At this point the detector will alert an operator of the system that the top surface of the quartz tube is sagging so that the laser beam is obstructed by the top surface. The operator of the semiconductor diffusion furnace then may replace the quartz tube before damage is done to the semiconductor wafers.

BACKGROUND 
The present invention concerns insitu detection of tube sagging in 
semiconductor diffusion furnace. 
Semiconductor diffusion furnaces often operate at a temperature in the 
range of twelve hundred degrees Centigrade. A semiconductor generally 
includes a quartz tube placed inside heating elements. The quartz tube is 
used as the processing chamber. 
After a period of time of processing wafers at a temperature of twelve 
hundred degrees Centigrade, the quartz robe can begin to soften. In some 
cases the top of the quartz tube will sag due to the softening and 
gravity. As time goes on the sagging tube may hit the top of silicon 
wafers that are being processed in the system. When this happens, it may 
cause some of the silicon wafers to break and may cause down time for the 
diffusion furnace. 
It is desirable, therefore, to replace the quartz tube periodically. 
However, the period of time it takes for a quartz robe to sag varies based 
on the operating temperature of the semiconductor diffusion furnace and 
differences in quartz tubes. 
SUMMARY OF THE INVENTION 
In accordance with the preferred embodiment of the present invention, 
semiconductor wafers are processed in a semiconductor diffusion furnace. 
During processing, the semiconductor wafers are placed in a quartz tube. 
Also during processing, a laser beam is transmitted below a top surface of 
the quartz tube. While the quartz tube is not sagging, the laser beam is 
detected with a detector. When the top surface of the quartz tube sags so 
that the laser beam is obstructed by the top surface, the laser beam is no 
longer detected by the detector. At this point the detector will alert an 
operator of the system that the top surface of the quartz tube is sagging 
so that the laser beam is obstructed by the top surface. The operator of 
the semiconductor diffusion furnace then may replace the quartz tube 
before damage is done to the semiconductor wafers. 
In the preferred embodiment of the present invention, the laser beam is 
transmitted with a wave length in the range of four hundred to five 
hundred nanometers. This wavelength is chosen so that the frequency of the 
signal is higher than the frequency of infra red noise, typically having a 
wavelength of six hundred to seven hundred nanometers, which is emitted 
from the quartz tube when heated. Also in the preferred embodiment, after 
the laser beam is generated by a laser generator, a plurality of prisms 
direct the laser beam from the laser generator, below a top surface of the 
tube and to the laser beam detector. 
The present invention allows in situ detection of sagging of the top 
surface of a quartz tube within a semiconductor diffusion furnace. The 
warning provided to the operator allows the operator to replace the quartz 
tube before damage is done to the semiconductor wafers.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a laser system for detecting tube sagging in a semiconductor 
diffusion furnace. A quartz oxidation tube 11 is, for example, available 
from Quartz International Corporation, having a business address of 9701 
Westward Drive, Austin, Tex. 78733. During processing, a quartz boat 13 of 
wafers 14 is placed within quartz tube 11. For example, quartz boat 13 is 
available from Quartz International Corporation. Wafers 14 are, for 
example, six inch silicon wafers. Quartz boat 13 is placed on a boat 
paddle 12. 
In order to detect sagging of a top surface 21 of quartz tube 11, a laser 
is used. In the preferred embodiment, a laser 15 generates a laser beam. 
For example, laser 15 is available as part number N39564 from Edmund 
Scientific Company, having a business address of 101 East Gloucester Pike, 
Barrington, N.J. 08007-1380. In the preferred embodiment, laser 15 
generates a laser beam with a wave length in the range of four hundred to 
five hundred nanometers. This wavelength is chosen so that the frequency 
of the signal is higher than the frequency of infra red noise, typically 
having a wavelength of six hundred to seven hundred nanometers, which is 
emitted from quartz tube 11 when heated. 
Various prisms are used to direct the laser beam, generated by laser 15, to 
a laser path immediately below top surface 21 of quartz tube 11. In the 
preferred embodiment a prism 16, a prism 17, a prism 18 and a prism 19 are 
arranged as shown. For example, each of prisms 16 through 19 can 
manufactured by a supplier such as Quartz International Corporation. A 
detector 20 is used to detect the presence of the laser beam. For example, 
detector 20 is available as part number 27-6378 from Ealing 
Electro-Optics, having a business address of 89 Doug Brown Way, Holliston 
Mass. 01746. 
In the preferred embodiment, the laser beam is initially separated from top 
surface 21 of quartz tube 11 by approximately one quarter inch. As long as 
top surface 21 of quartz tube 11 does not sag more than three-sixteenths 
of an inch, detector 20 will detect the laser beam and quartz tube 11 will 
continue to be used. 
As illustrated by FIG. 2, once top surface 21 of quartz tube 11 sags more 
than one quarter inch, top surface 21 will obstruct the path of the laser 
beam. At this point detector 20 no longer will detect the presence of the 
laser beam. An operator of the semiconductor diffusion furnace will then 
be alerted that the useful life of quartz tube 11 has been reached. The 
operator of the semiconductor diffusion furnace then may replace quartz 
tube 11 before damage is done to wafers 14. 
The foregoing discussion discloses and describes merely exemplary methods 
and embodiments of the present invention. As will be understood by those 
familiar with the art, the invention may be embodied in other specific 
forms without departing from the spirit or essential characteristics 
thereof. Accordingly, the disclosure of the present invention is intended 
to be illustrative, but not limiting, of the scope of the invention, which 
is set forth in the following claims.